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Abstract and Figures

Absorption of photon energy by neuronal mitochondria leads to numerous downstream neuroprotective effects. Red and near infrared (NIR) light are associated with significantly less safety concerns than light of shorter wavelengths and they are therefore, the optimal choice for irradiating the retina. Potent neuroprotective effects have been demonstrated in various models of retinal damage, by red/NIR light, with limited data from human studies showing its ability to improve visual function. Improved neuronal mitochondrial function, increased blood flow to neural tissue, upregulation of cell survival mediators and restoration of normal microglial function have all been proposed as potential underlying mechanisms of red/NIR light.
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Mitochondrion
journal homepage: www.elsevier.com/locate/mito
Photostimulation of mitochondria as a treatment for retinal
neurodegeneration
Kathy Beirne
a,b
, Malgorzata Rozanowska
a,b
, Marcela Votruba
a,b,c,
a
School of Optometry and Vision Sciences, CardiUniversity, Cardi,UK
b
CardiInstitute for Tissue Engineering and Repair, CardiUniversity, Cardi,UK
c
CardiEye Unit, University Hospital of Wales, Cardi,UK
ARTICLE INFO
Keywords:
Red light
Near infrared light
Retina
Neurodegeneration
Photostimulation
Low-level light therapy
Photobiomodulation
ABSTRACT
Absorption of photon energy by neuronal mitochondria leads to numerous downstream neuroprotective eects.
Red and near infrared (NIR) light are associated with signicantly less safety concerns than light of shorter
wavelengths and they are therefore, the optimal choice for irradiating the retina. Potent neuroprotective eects
have been demonstrated in various models of retinal damage, by red/NIR light, with limited data from human
studies showing its ability to improve visual function. Improved neuronal mitochondrial function, increased
blood ow to neural tissue, upregulation of cell survival mediators and restoration of normal microglial function
have all been proposed as potential underlying mechanisms of red/NIR light.
1. Introduction
The therapeutic properties of light have been known since antiquity,
as far back as 1400 BCE, where it was used by Hindus to treat skin
disorders (Roelandts, 2002). The ancient Egyptians, Greeks and Romans
were also reportedly aware of the benecial eects of sunlight which
they used to treat various ailments (McDonagh, 2001). The evidence for
the use of phototherapy in those time, however, is purely anecdotal. It
was not until 1903 that the therapeutic power of light gained scientic
recognition, when Niels Finsen was awarded the Nobel Prize in
medicine for the discovery of UV light as a treatment for skin
tuberculosis (lupus vulgaris)(Finsen, 1901).
Red light was, later, found to have biostimulatory eects; an
unintentional discovery made by Endre Mester, in 1967, who wanted
to assess the ability of 694 nm lasers to cause carcinogenesis in mice
(Mester et al., 1971). The mice in both the light-treated and untreated
groups were shaved prior to laser exposure. The results found that the
light-treated group did not develop cancer, but more intriguingly, the
hair grew back on the laser treated mice at a faster rate than the
untreated group.
In more recent times, there has been a surge in the use of red and
near infrared (NIR) lasers and LEDs in clinical and preclinical research
(Desmet et al., 2006). As red and NIR light have relatively long
wavelengths, they have the advantage of a greater penetration depth
over shorter wavelengths, making them an ideal choice for the
treatment of neural tissue (Hartwig and Van Veen, 1979). In addition
to light being able to penetrate into the tissue of interest, another
requirement is that the photon energy corresponds to the absorption
characteristics of the chromophores responsible for triggering the
benecial eects upon photoexcitation. It appears that red and NIR
light correspond to the absorption maxima of such chromophores as
will be discussed later. For various reasons, LEDs are most commonly
used as the light source in these studies. Most importantly, red/NIR LED
therapy has been approved for use in humans and has been deemed as a
non-signicant risk by the U.S. Food and Drug Administration.
Although shorter wavelengths of visible light and UV light are also
employed for therapeutic purposes, their safety for use in humans,
especially for the eye, is less clear (Rozanowska et al., 2009;
Rozanowska, 2012). With the ultimate objective of exploring the
ecacy of phototherapy as a treatment for neurodegeneration in the
human retina, this review will focus only on the use of wavelengths that
are least likely to cause adverse eects, that is, red and NIR light
(Barolet, 2008).
http://dx.doi.org/10.1016/j.mito.2017.05.002
Received 15 July 2016; Received in revised form 15 February 2017; Accepted 8 May 2017
Corresponding author at: School of Optometry and Vision Sciences, CardiUniversity, Cardi, UK.
E-mail addresses: BeirneK@cardi.ac.uk (K. Beirne), RozanowskaMB@cardi.ac.uk (M. Rozanowska), VotrubaM@cardi.ac.uk (M. Votruba).
Abbreviations: NIR, near infrared; LED, light emitting diode; FDA, the U.S. Food and Drug Administration; COX, cytochrome coxidase; RGC, retinal ganglion cell; ERG,
electroretinogram; NO, nitric oxide; ROS, reactive oxygen species; RNS, reactive nitrogen species; NF, nuclear factor; IL, interleukin
Mitochondrion 36 (2017) 85–95
Available online 09 May 2017
1567-7249/ © 2017 Elsevier B.V. and Mitochondria Research Society. All rights reserved.
MARK
2. The potential of red/NIR light as a treatment for
neurodegeneration
2.1. Evidence from in vitro studies
Red and NIR light have been shown to provide protection against
the deleterious eects of mitochondrial electron transport chain
inhibitors and excitotoxic cell death in neurons in vitro (Wong-Riley
et al., 2005; Ying et al., 2008; Huang et al., 2014). Since impaired
mitochondrial function and excitotoxicity are common causes of cell
death in neurodegenerative conditions, the ability of red/NIR light to
protect against these challenges in vitro has emphasized the potential of
this therapy in various neurodegenerative conditions.
2.2. The eects of red/NIR light in models of neurodegeneration
Red/NIR light therapy has shown great potential in the treatment of
acute neurodegenerative conditions, showing neuroprotective eects in
rodent models of spinal cord injury, traumatic brain injury and stroke
(Byrnes et al., 2005; Wu et al., 2012; Xuan et al., 2015; Dong et al.,
2015; Oron et al., 2006; Giacci et al., 2014).
Furthermore, red light has been shown to have benecial eects in
animal models of some of the most prevalent neurodegenerative
diseases. A reduction in cell loss and other markers of disease severity
was seen with red/NIR light treatment, in rodent models of multiple
sclerosis, Alzheimer's and Parkinson's disease (Muili et al., 2012, 2013;
Purushothuman et al., 2013, 2015; Oueslati et al., 2015; Johnstone
et al., 2014; Peoples et al., 2012; Shaw et al., 2010).
2.3. The potential of red/NIR light as a successful treatment for
neurodegeneration in humans
While transcranial red/NIR light therapy is yielding remarkable
results in numerous rodent models of neurodegeneration, the real
question is how well these results will translate when applying this
therapy to human patients.
Interestingly, in a neurotoxin-induced monkey model of Parkinson's
disease, 670 nm light was delivered directly to the macaque midbrain
using an implanted optical bre which was activated over the period of
time of 57 days when the neurotoxin precursor, 1-methyl-4-phenyl-
1,2,3,6-tetrahydropyridine (MPTP), was injected (Darlot et al., 2016).
The study found a reduction in clinically-assessed behavioural impair-
ment with this method of red light delivery in this primate model as
well as neuroprotection to the dopaminergic neurons of the substantia
nigra. Although a more invasive method of delivery than transcranial
red light treatment, no major adverse eects were observed following
surgical implantation of the optical bre. However, it would have been
of great interest if the eects of transcranial light delivery were also
tested in this model, for comparison.
3. Red light treatment in retinal degenerative diseases
Since the retina is an extension of the CNS, the neuroprotective
eects of red/NIR light, as discussed above, should also be observed in
this tissue. In fact, irradiating the retina with red or NIR light seems
more likely to be successful as a non-invasive treatment for human
patients as the issue of tissue penetration is avoided.
3.1. The safety of light treatment on the retina
The greatest concern arising when aspiring to use red light therapy
to treat retinal degeneration, is the potential retinal damage that may
occur upon direct exposure of the retina to light with high levels of
irradiance. The dangers of high levels of irradiance on the retina is
highlighted in a study on anesthetised monkeys (Friedman and
Kuwabara, 1968). It was found that white light with a retinal irradiance
of 270 mW/cm
2
caused irreversible damage to the photoreceptors and
retinal pigment epithelium. White light is made up of light of all
wavelengths in the visible light spectrum, with light of shorter
wavelengths and higher frequencies having a greater damaging eect
on photoreceptors. Blue light, with a relatively short wavelength, was
found to cause irreversible damage to S cones (Harwerth and Sperling,
1975). While exposure to green and red light caused damage to M and L
cones, respectively, the damage to these cones was reversible, with a
full recovery of function seen after a few weeks. More recent studies on
macaque monkey, however, have demonstrated that yellow light of
568 nm wavelength can cause retinal damage manifested as disruption
of the retinal pigment epithelium at the dose below the Maximal
Permissible Exposure established by the American National Standard
Institute's (ANSI) as a standard for the safe use of lasers (Hunter et al.,
2012).
Albeit transient and less severe than light of shorter wavelengths,
damage to L cones upon exposures to high levels of red light would be a
cause for concern when considering red light as a treatment for retinal
degeneration. This concern has been addressed with numerous in vivo
studies. These studies have shown that therapeutic eects were
achieved, in the absence of retinal damage, when rodent retinas were
exposed to 670 nm light with a therapeutically eective irradiance and
exposure times (Albarracin et al., 2011, 2013; Giacci et al., 2014).
Further, this included irradiance of 60 mW/cm
2
which is the highest
irradiance level found in studies of in vivo models of retinal degenera-
tion where positive results were achieved using 670 nm light. This
demonstrates the safety of using 670 nm light as a treatment for retinal
degeneration.
In addition to photoreceptor damage, the possibility of photother-
mal damage to the retina and surrounding ocular structures evokes
further concern when considering using light to treat retinal degenera-
tion (Youssef et al., 2011). Comparing the eects of green, red and NIR
laser light exposure on the temperature rise in the human choroid, it
was found that the longer wavelengths led to a smaller degree of
choroidal heating, due to the decrease in absorption by melanin with
increasing wavelength (Vogel and Birngruber, 1992). The variation in
choroidal heating between green and red wavelengths was minor
compared with the dierence between green and NIR wavelengths.
Still the question remains as to which would be the optimal
wavelength for use as a neuroprotective agent in the retina.
Addressing this, the ecacy of red and NIR light was compared in a
model of partial optic nerve transection (Giacci et al., 2014). It was
found that although protective eects were seen in retinas treated with
both red and NIR light, 670 nm light was more eective in improving
visual function compared with 830 nm light. However, in a rat model of
light induced retinal degeneration protective eects were observed
with 670 nm light treatment, but no protection was seen with 830 nm
light (Giacci et al., 2014). It is therefore not surprising that most studies
testing the eectiveness of phototherapy on neurodegeneration in the
retina use red light at 670 nm.
3.2. 670 nm light therapy in models of photoreceptor damage
As discussed above, exposing the retina to bright light can cause
photoreceptor damage, an event that can occur with excessive sunlight
exposure or accidental exposure to high intensity articial light sources.
Models of light induced photoreceptor damage are also used to simulate
retinal degenerative diseases, involving photoreceptor specic death.
Emphasizing the vast and diverse eects of light on biological tissue,
irradiating the retina with red light provided protection against the
structural damage to the outer retina and loss in photoreceptor function
in a rat model of light induced photoreceptor degeneration (Albarracin
et al., 2011). Methanol can also induce damaging eects on the retina
causing photoreceptor toxicity due to the ability of its metabolite,
formic acid, to inhibit cytochrome coxidase, the terminal enzyme of the
electron transport chain. In a rat model of methanol induced retinal
K. Beirne et al. Mitochondrion 36 (2017) 85–95
86
toxicity, red light treatment brought about a signicant recovery of rod
and cone mediated function, in addition to preventing methanol
induced changes to outer retinal morphology (Eells et al., 2003). These
studies show the ability of red light to protect against loss of photo-
receptor function, an event that would cause severe visual impairment
and would otherwise be irreversible.
3.3. The eects of red light on inammation in the outer retina
Inammation in the retina has been implicated in many retinal
diseases including age related macular degeneration (AMD) and
diabetic retinopathy, and as such, looking at ways to alleviate
inammation in these conditions is thought to reduce the severity of
symptoms associated with these diseases (Whitcup et al., 2013). Also,
an upregulation in inammatory proteins has been observed following
light damage in rats, demonstrating pathological features similar to
dryAMD (Rutar et al., 2012). In addition to its ability to protect
against light induced photoreceptor damage, red light treatment was
found to reduce the complement propagation that occurs in the retina
following light damage. Red light treatment was also tested in an aged
genetic mouse model of AMD, the complement factor H knockout,
which, likewise, presents with reduced retinal function and increased
inammation (Begum et al., 2013). In contrast to most studies with
red/NIR light, where the light source was held directly in front of the
animal, the environmental light was supplemented with red light, for
this study. Even though the red light exposure was indirect, the levels
reaching the retina were sucient to reduce inammation in the outer
retina, in this model.
In addition to inammation associated with disease pathology,
inammation increases in the retina with age. Red light was eective
in reducing proinammatory cytokines and a chronic marker of
inammation in the aged mouse retina, demonstrating the potential
of red light to also alleviate the visual decline associated with normal
aging (Kokkinopoulos et al., 2013). These studies draw attention to the
ability of red light to produce anti-inammatory eects associated with
outer retinal degeneration, whether by induced damage, disease
pathology or normal aging.
3.4. The eects of red light on neurodegeneration in the inner retina
Retinal ganglion cell (RGC) death and optic nerve degeneration are
hallmarks of optic neuropathy, a frequent cause of vision loss, of which
the causes are many. Red light therapy has been trialled in a rat model
of diabetic retinopathy and positive outcomes have been reported.
There was a signicant reduction in the diabetes induced RGC death
and a 50% improvement in the diabetes-induced reduction in ERG
amplitude with exposure to red light (Tang et al., 2013). Highlighting
the benecial eects of red light on the retinal vasculature, red light
also prevented the diabetes-induced increase in leukostasis in the
retinal vasculature, an event which is implicated in the pathogenesis
of diabetic retinopathy. Further, the diabetes-induced increase in
retinal expression of an adhesion molecule, essential for leukostasis,
ICAM-1, was also prevented with red light. Similarly, in a model of
secondary degeneration of RGCs, resulting from traumatic injury to the
optic nerve, red light was found to be protective (Fitzgerald et al.,
2010). The secondary damage, following partial optic nerve transec-
tion, normally leads to further loss of RGCs and visual function;
however, normal visual function was restored with red light treatment
(Fitzgerald et al., 2010). Furthermore, treatment with 670 nm light in a
rat model of partial optic nerve transection, resulted in improved
vision, 7 days post injury (Giacci et al., 2014). Additionally, dendro-
pathy of retinal ganglion cells, an event found to be associated with
visual loss in experimental models of glaucoma and autosomal domi-
nant optic atrophy, was partially prevented, in an axotomy model of
neurodegeneration, with red light treatment (Beirne et al., 2016).
Protection against RGC and optic nerve degeneration, arising from
dierent conditions, appears to be possible with red light treatment.
Since RGC dysfunction and optic nerve degeneration are common
features among numerous other types of optic neuropathies red light
has the potential to provide protection in these conditions also.
However, the successful outcomes achieved in the discussed experi-
mental models were seen when red light was administered immediately
after induced optic nerve injury, a treatment strategy that would be
unachievable in a real life clinical setting. Although the therapeutic
window of opportunity for red/NIR light therapy in models of traumatic
optic neuropathy has not been explored, it has been assessed in other
CNS injury models. NIR light therapy improved motor function in a
rabbit model of embolic stroke when treatment was administered
6 hour post-embolization but was ineective when administered
12 hour post embolization (Lapchak et al., 2007). The ndings show
that neuroprotection can be achieved when red/NIR light is adminis-
tered for up to 6 h after the induced injury, showing the therapy to be
applicable to a clinical setting. Other studies on animals demonstrated
eectiveness of near-infrared light in a mouse model of traumatic brain
injury when administered 4 h after injury, with additional treatments
administered at one day and two days post injury (Xuan et al., 2016;
Thunshelle and Hamblin, 2016). The transcranial treatment upregu-
lated brain-derived neurotrophic factor (BDNF), improved neurological
functions, reduced the size of the lesion, stimulated formation of new
neurons and synaptogenesis. There is also a growing body of evidence
suggesting that people aected by chronic traumatic brain injury or
after stroke can benet from transcranial irradiation with red/near-
infrared light (Naeser et al., 2016; Hamblin et al., 2016).
3.5. Red/NIR light as a therapy for patients with retinal disease
The protective eects seen with red light treatment, in the absence
of adverse eects, in numerous in vivo models of retinal degeneration,
strongly suggest this non-invasive treatment should be trialled in
patients with retinal degeneration. Progress to this end has commenced
with trials of red and NIR light therapy yielding promising results in
patients with AMD. In one such study, a brief exposure of NIR light
(780 nm), from a semiconductor laser diode, to AMD patients, twice per
week for two weeks, resulted in a signicant improvement in their
visual acuity (Ivandic and Ivandic, 2008). This improvement in vision
was seen in patients with both wet and dry AMD and was maintained
for 336 months after treatment. Moreover, no adverse eects of the
treatment were seen. In this study, the laser was applied transconjunc-
tivally, to the macula, when the eye was in adduction. In another study
looking at the eects of photobiomodulation on patients with dry AMD,
the retina was irradiated through the pupil with red light (670 nm),
from the FDA approved Warp 10 LED light source, 3 times per week for
6 weeks (Merry et al., 2013). The visual acuity and contrast sensitivity
remained signicantly improved for 12 months after treatment; how-
ever, the improvement in visual acuity began to decline after 4 months.
The results provide vital information on the time at which patients may
benet from re-treatment, in addition to providing pilot data on the
safety and eectiveness of 670 nm light from an LED source. Red/NIR
light has also been trialled in diabetic macular edema and Leber's
hereditary optic neuropathy, however, there are no results available
from these studies. A summary of all the studies using red/NIR light as a
treatment intervention in conditions associated with neurodegeneration
in the retina is available in Table 1. From all the studies listed, red/NIR
light is showing the greatest potential as a treatment for AMD. Since
positive outcomes have been observed in patients with AMD, this paves
the way for the application of this therapy in other retinal degenerative
conditions, particularly those where positive outcomes have been seen
in preclinical studies.
4. The eects of red/NIR light on mitochondrial dysfunction
Although the therapeutic benets of red/NIR light therapy have
K. Beirne et al. Mitochondrion 36 (2017) 85–95
87
been demonstrated in a number of dierent disease models, in addition
to AMD patients, the underlying molecular mechanisms are less well
understood (Desmet et al., 2006). The question is no longer whether or
not light has biological eects, it is rather how these eects are
mediated at a cellular and molecular level (Hamblin and Demidova,
2006).
4.1. Cytochrome c oxidase: the photoacceptor for red/NIR light
Endeavors to uncover the underlying molecular mechanisms suggest
a major role for cytochrome coxidase (COX) which is the terminal
enzyme of the electron transport chain, transferring electrons from
cytochrome cto molecular oxygen (Chung et al., 2012; Karu, 1999).
COX is a large multicomponent protein, containing two copper
centers (Cu
A
and Cu
B
) and two heme iron containing centers (heme a
and heme a3), which absorbs photons in the red to NIR region of the
electromagnetic spectrum (Karu, 1999). These transition metals are
also the intermediate redox sites in the electron transfer pathway from
cytochrome cto oxygen, a process which is coupled to the pumping of
protons across the inner mitochondrial membrane. The electrons pass
from cytochrome cto Cu
A
then passed to heme a, from heme ato heme
a3-Cu
B
and nally to molecular oxygen.
4.2. The absorption of photons by photoacceptors in COX
One theory proposed to explain how photon energy is absorbed by
COX, centers on its heme molecules (Zielke, 2014). Heme is comprised
of a porphyrin ring with an iron atom at its center that can continuously
switch its oxidation states between ferrous (Fe
2+
) and ferric (Fe
3+
)by
accepting or donating an electron. The porphyrin ring is made up of
four pyrrole rings that are connected through their carbon atoms via π
bonds. The electrons in these πbonds are delocalised, moving back and
forth from one conguration to another, creating resonance. Electrons,
like photons, have a dual nature, behaving like particles or electro-
magnetic waves, thereby creating a resonating electromagnetic cloud in
the porphyrin ring. Photons with similar wavelengths are absorbed by
this cloud, increasing its energy. The energy from these photons causes
photoexcitation of electrons of Fe
2+
, bringing them to an unstable
higher energy level. Upon absorption of sucient energy, these
electrons are released from the orbitals of Fe
2+
causing the oxidation
of Fe
2+
to Fe
3+
. The oxidised iron atom can then accept electrons from
cytochrome c, thus increasing the electron ux through the electron
transport chain.
Experimental evidence shows that red/NIR light has the ability to
upregulate the enzymatic activity of complex IV, increase the mito-
chondrial membrane potential and increase ATP production (Begum
et al., 2013; Kokkinopoulos et al., 2013; Hamblin and Demidova, 2006;
Karu, 2008; Ferraresi et al., 2015a,b; Tina Karu et al., 2013). These
mitochondrial speciceects of photobiomodulation may oer a
partial explanation for its benecial eects in neurodegenerative
diseases associated with mitochondrial dysfunction. However, in genet-
ic or toxin induced models of Parkinson's disease, where loss of
dopaminergic cells was triggered by complex I dysfunction, neuropro-
tective eects were seen with red/NIR light treatment. The absorption
of photon energy by the heme group in complex IV may explain how
photobiomodulation can increase ATP production when complex IV is
inhibited but fails to explain how this eect can be achieved in models
with complex I inhibition.
It has been hypothesised by Zielke et al. that the electrons released
in this oxidation process are free to reduce NAD
+
and FAD, providing
substrates for complex I and II, respectively, creating a closed circuit of
electron transfer (Zielke, 2014). In the case of aberrant functioning of
COX the ux of electrons through this closed circuit would maintain the
proton pumping functions of complex I and III, thereby maintaining the
electrochemical gradient across the mitochondrial membrane required
for ATP synthesis. However, since oxygen is very electronegative it
would be most likely that the released electrons would be readily
accepted by the oxygen molecule bound to the reduced heme a3-Cu
B
component of COX, reducing it to water. In the event of aberrant COX
activity the released electron would most likely react with unbound
oxygen molecules, forming reactive oxygen species. Therefore, how
red/NIR provides neuroprotection against complex I dysfunction is not
explained by its direct action on the COX.
5. The nitric oxide theory of red/NIR light therapy
5.1. The role of nitric oxide in mitochondrial respiration
Nitric oxide (·NO) has important roles in the regulation of blood
pressure and vasculature tone, however, excessive ·NO production, as
seen in neurodegenerative diseases, can cause impairment of mitochon-
drial respiration and apoptosis (Zhao, 2005). Mitochondria harbor
nitric oxide synthase (NOS) to produce NO, which they use to hinder
respiration, as an intrinsic mechanism to prevent oxygen from reaching
precariously low levels (Barolet, 2008). ·NO, at low concentrations,
competes with oxygen to bind to the reduced heme a3-Cu
B
component
of COX (Chung et al., 2012). This prevents COX from reducing
molecular oxygen, thus impairing the proton pumping abilities of the
enzyme and essentially the energy production ability of the mitochon-
dria. Additionally, ·NO was found to cause inhibition of complex I, II
and IV-dependent respiration in mitochondrial suspensions where the
inhibitory action of ·NO was found to be more profound at lower oxygen
tensions (Cassina and Radi, 1996). Interestingly, this inhibitory action
of ·NO was overcome by reoxygenation of mitochondrial suspensions
for a mere 10 s, resulting in complete recovery of complex I and IV
dependent oxygen consumption and a 50% recovery of complex II
dependent respiration. This shows that the inhibitory actions of ·NO are
almost completely reversible upon restoration of normal oxygen levels.
Table 1
A summary of the completed, planned and terminated clinical trials, using red/NIR light as a treatment intervention in conditions associated with retinal neurodegeneration.
Responsible party Last veried on
ClinicalTrials.gov
Condition Duration of
treatment period
Number of
patients
Duration of improved
vision after treatment
Status
Harry T Whelan 01/09/2013 Diabetic macular edema 3 months 20 No results available Completed
Ivandic and Ivandic
(2008)
N/A exudative and non-
exudative AMD
2 weeks 203 Up to 36 months Completed and published
(Ivandic and Ivandic, 2008)
Merry et al. (2013) 01/11/2011 Non-exudative age-related
macular degeneration
6 weeks 9 Up to 12 months Completed and published
(Merry et al., 2013)
Merry et al. (2016) N/A Non-exudative age-related
macular degeneration
3 weeks 24 3 months Completed and published
(Merry, 2016)
University of Sydney 01/06/2014 Diabetic retinopathy 4 weeks N/A N/A Planned
LumiThera, Inc. 01/04/2016 Age-related macular
degeneration
3 weeks 30 N/A Planned
Harry T Whelan 01/09/2014 Leber's hereditary optic
neuropathy
3 months 4 N/A Terminated (0/4 patients
completed the study)
K. Beirne et al. Mitochondrion 36 (2017) 85–95
88
Therefore, this intrinsic mechanism would protect a tissue if the
depleted oxygen supply were temporary, by putting the mitochondria
in a state of conservation until the return of normal oxygen levels.
However, prolonged inhibition of respiration would deplete ATP levels
and bring about cell death (Kalogeris et al., 2012).
5.2. The eects of red/NIR light on ·NO-mediated mitochondrial
dysfunction
Red/NIR light has been proposed to inuence the photodissociation
of ·NO from COX, thereby allowing oxygen to reclaim its binding site,
permitting the ATP production process to resume (Chung et al., 2012).
This would be most benecial in pathological situations where ·NO
levels are higher than normal physiological amounts, favouring the
binding of ·NO rather than O
2
to COX. Experiments have shown that
irradiating cells with red/NIR light increases COX activity in normal
healthy neurons and restores the activity in COX inhibited neurons
(Wong-Riley et al., 2005). However, the increase in COX activity
appears to be mediated by an increase in the expression of the proteins
in the COX complex, suggesting that a mechanism, additional to the
disinhibition of COX, may also be involved (Lim et al., 2010).
Furthermore, exposure of cells to an irreversible COX inhibitor was
overcome by NIR light treatment, providing further support to the claim
that restoration of activity is mediated by upregulating the expression
of COX proteins (Wong-Riley et al., 2005).
An indirect eect of red/NIR light on COX activity helps explain
how benecial eects are also seen in neurodegenerative conditions
that are associated with impairment of other electron transport chain
complexes. The absorption of photon energy from red/NIR light by COX
may indeed have direct eects on the enzyme itself, however, this
initial event may trigger further downstream events, which may have
more far-reaching eects. As ·NO is widely known for its function as an
intercellular signaling molecule, the release of ·NO upon red/NIR light
exposure would increase its bioavailability allowing it to function as a
signaling molecule (Lohr et al., 2009). It has been suggested that ·NO
intracellular signaling could play a role in the upregulation of COX
proteins upon red/NIR light exposure. Intracellular signaling from the
mitochondria to the nucleus may be triggered by other byproducts of
mitochondrial respiration, levels of which may be altered by red/NIR
light, therefore, will be discussed in more detail later in this review.
5.3. The indirect eects of red/NIR light on mitochondrial dysfunction
The ndings by Cassina and Radi suggest that increasing the
delivery of oxygen to the mitochondria, as would occur with increased
blood ow to the tissue, would improve mitochondrial function in
tissues with ·NO-mediated mitochondrial electron transport chain
inhibition (Cassina and Radi, 1996). It has been found that the exposure
of blood vessels to red light from an LED source can induce photo-
relaxation of blood vessels, an event that would increase blood ow
and, in turn, oxygen delivery to the irradiated tissue (Plass et al., 2012).
Exposure of porcine coronary arteries to red light caused their
vasodilation, as measured by wall tension in the exposed vessels. Since
nitric oxide is known to have a primary role in the regulation of
vasculature tone, and NOS is activated upon absorption of visible light,
the vasodilation eect seen upon red light exposure is thought to be
mediated through nitric oxide (Samoilova et al., 2008). Additionally,
red/NIR light can trigger the photodissociation of ·NO from nitrosyl
hemoglobin and nitrosyl myoglobin (Lohr et al., 2009). The ·NO
released from hemoglobin in the blood would contribute to the
vasodilation eects of red/NIR light. Depending on the physiological
situation, photobiomodulation can either reduce or increase ·NO levels,
however, the molecular events determining whether the eect will be
an inhibitory or a stimulatory one are yet to be identied (Gavish et al.,
2008). An increase in ·NO upon red light exposure would provide
benecial eects in conditions such as TBI, where increased cerebral
blood ow could increase mitochondrial function in hypoxic cells
(Fig. 1)(Naeser et al., 2014). As mentioned earlier in the review,
hypoxia can trigger inhibition of respiration through the binding of ·NO
to COX. The photodissociation of ·NO from COX in hypoxic tissue may
not cause a signicant improvement in mitochondrial function since
there would be limited oxygen available to reclaim the binding site on
Fig. 1. The direct and indirect actions of red/NIR light on nitric oxide improve mitochondrial function in neurons vulnerable to degeneration, in acute neurodegenerative conditions such
as TBI.
K. Beirne et al. Mitochondrion 36 (2017) 85–95
89
COX. However, if combined with an increase in cerebral blood ow, the
associated increase in oxygen levels would lead to a more substantial
improvement in mitochondrial function. This proposes a partial ex-
planation for the neuroprotective eects seen upon red/NIR light
exposure.
5.4. The eects of red/NIR light on cellular function
As mentioned above, the ability of red/NIR light to increase free ·NO
can have benecial eects in cells where mitochondria are dysfunc-
tional. Providing further support to this theory, it was found that red/
NIR light was protective against hypoxia and re-oxygenation injury in
cultured cardiomyocytes. The observed protection was dependent on an
increase in ·NO as the protective eects were abolished in the presence
of ·NO scavengers (Zhang et al., 2009). Further, it was observed that the
increase in ·NO seen upon red/NIR exposure was partially prevented by
the non-selective inhibition of all isoforms of NOS. Of note, there are
three isoforms of NOS: Neuronal NOS (nNOS), which is expressed in the
central nervous system and plays a role in synaptic plasticity and
central regulation of blood pressure, endothelial NOS (eNOS), which is
mostly expressed in endothelial cells and primarily functions in
controlling blood pressure, and inducible NOS (iNOS), which can be
expressed in many cell types in response to cytokines and other agents
to generate large amounts of ·NO (Förstermann and Sessa, 2012). The
ndings show that the increase in ·NO by red/NIR light is mediated in
part by its action on NOS, however, the exact isoform of NOS
responsible for the protective eects has not been determined. The
source of the remaining NO could be that which is released during the
photodissociation of ·NO from COX in the mitochondria as discussed
above.
This in vitro model of cardiac ischemia provides useful insight into
how red/NIR light mediates its eects via ·NO in ischemic conditions.
Since the experiment was done in vitro, the observed protective eects
most likely arose from the local eects of an increase in intracellular
·NO rather than an indirect eect of ·NO by increasing blood ow to the
ischemic tissue. Although reperfusion is essential to limit cell death
after hypoxia, paradoxically, this event itself causes further cell death
due to excess ROS production (Keszler et al., 2014). The increase in free
·NO upon red/NIR light exposure would increase its availability to bind
to COX and cause the reversible S-nitrosation of complex I, events
which would slow down the reactivation of the electron transport chain
during the crucial initial stages of reperfusion (Chouchani et al., 2013).
In this particular pathological situation red/NIR light, administered
upon reoxygenation, could reduce the harmful levels of ROS produced
during reperfusion injury by reversibly inhibiting the electron transport
chain.
Contrastingly, in an in vivo model of cerebral ischemia, where excess
·NO production is said to be associated with neurotoxic eects, red/NIR
light has been shown to have the ability to reduce the levels of ·NO by
down-regulating the activity and expression of all isoforms of NOS
(Leung et al., 2002). Both studies focus on ·NO to explore the under-
lying mechanism responsible for the protective eect of red/NIR light
in models of ischemia, yet, in these examples, the eects on ·NO were
found to be conicting. The respective increase and decrease in ·NO
levels seen upon red/NIR light irradiation in the discussed models, was
dependent on the respective activation and inhibition of NOS. Although
theories have been proposed to explain the increase in NOS activity in
response to red/NIR light irradiation, how red/NIR light inhibits NOS
activity is less clear. Since the expression of the three isoforms of NOS
showed a similar trend to the specic activities of the NOS enzymes in
response to red/NIR light, red/NIR light must be somehow suppressing
the expression of NOS, but the mechanism responsible for this eect is
unknown (Leung et al., 2002).
This observed dual eect of red/NIR light on intracellular ·NO levels
has great relevance in the eld of neurodegenerative conditions. ·NO at
physiological amounts confers neuroprotection, however, if produced
in excess, ·NO has neurotoxic eects (Calabrese et al., 2007). In the in
vivo model of cerebral ischemia mentioned above the light was
administered immediately after middle cerebral artery occlusion but
the levels of NOS activity were not measured until 4 days post injury,
the time at which the NOS levels peaked before returning to pre-injury
levels. In the in vitro model of cardiac ischemia the light was also
administered immediately after hypoxia but the ·NO levels were
measured after just 2 h of reoxygenation. It is possible, therefore, that
red/NIR light triggers an initial increase in ·NO levels, sucient to
reduce ROS production and bring about the observed cytoprotective
eects. Furthermore, as the cell is then in a state of elevated ·NO levels
and reduced ROS levels this may be sucient to switch othe
Fig. 2. A proposed explanation for the observed dual eect of red/NIR light on NO in models of ischemia.
K. Beirne et al. Mitochondrion 36 (2017) 85–95
90
endogenous trigger that induces the increased expression of NOS and
the subsequent delayed surge in ·NO levels, which only contribute to the
toxic eects at that late stage of ischemia. However, as shown in Fig. 2,
low levels of ·NO are produced during the early stages of ischemia to
induce neuroprotective eects in the absence of red/NIR light, yet an
increase in ·NO is responsible for the neuroprotective eects achieved
with red/NIR light in the in vitro model of ischemia. Therefore, how
does the ·NO produced by red/NIR light provide further neuroprotec-
tive eects in the early stages of ischemia? Also unanswered is how the
increase in ·NO by red/NIR light and the associated reduction in ROS
production would downregulate the delayed surge in NO production in
the later stages of ischemia when the ·NO produced in the absence of
red/NIR light fails to do so.
There is much evidence to show that ·NO acts as a neuroprotective
agent through its various cellular eects. One such eect is the
induction of the signaling molecule cyclic guanosine 3,5-monopho-
sphate (cGMP), a molecule with a key role in vasodilation and
mitochondrial biogenesis (Tengan et al., 2012). Mitochondrial biogen-
esis has been found to occur in response to red/NIR light irradiation
(Nguyen et al., 2014). Diseases with a mitochondrial origin such as
Leber's hereditary optic neuropathy, retinitis pigmentosa and autosomal
dominant optic atrophy would benet from the associated increase in
mitochondrial biogenesis, as a way of supporting neuronal survival.
5.5. The eects of reactive nitrogen species on mitochondrial function
There is much evidence to show that ·NO, at higher doses, is toxic to
neurons; however, this NO-mediated toxicity is not produced by ·NO
alone, but by the formation of reactive nitrogen species (RNS) (Lipton
et al., 1993). As a by-product of mitochondrial respiration the super-
oxide radical anion (O
2
·
) is formed when electrons from complex I or
III are transferred to oxygen molecules instead of their respective
substrates: ubiquinone and cytochrome c(Lenaz, 2001). If the amount
of superoxide produced in the cell increases to a level that would
exceeds the antioxidant capacity of the cell, oxidative stress will result.
In that pro-oxidant state, the production of ·NO can cause the genera-
tion of additional cytotoxic compounds. The reaction of NO with the
superoxide radical anion (O
2
·
) leads to the generation of the powerful
oxidant peroxynitrite (ONOO
), which causes detrimental eects in the
cell, through its interactions with lipids, proteins and DNA (Lipton
et al., 1993).
Nitric oxide, superoxide and peroxynitrite are often generated in
excess during inammatory and pathological conditions, contributing
to the associated toxic eects (Rubbo et al., 1994). Peroxynitrite can
induce mitochondrial dysfunction and cell death in neurons by its
ability to inhibit many mitochondrial proteins including complex I, II
and IV, and ATP synthase. It can also increase mitochondrial proton
permeability, an eect which may be caused by lipid peroxidation
(Brown and Borutaite, 2004). Impairment of normal mitochondrial
function causes depletion of ATP and generation of free radicals,
causing cellular dysfunction and further oxidative stress. Peroxynitrite
has been found to be involved in the pathogenesis of many neurode-
generative disorders (Torreilles et al., 1999). Since the formation of
peroxynitrite depends on the availability of ·NO, inhibition of nitric
oxide synthase activity, as was found to occur in cerebral ischemic-rats
irradiated with red light, may reduce the amount of peroxynitrite
produced and the associated deleterious eects in neurodegenerative
disorders (Leung et al., 2002).
6. Red/NIR light in the mitochondrial signaling pathway
Causing further controversy in the eorts to uncover the underlying
molecular mechanism of red/NIR light therapy, is the eect that it has
on ROS and RNS production. It is also uncertain whether an increase or
a decrease in these molecules, in the cell, in response to red/NIR light
exposure would be the most therapeutically benecial. In some
physiological situations red light mediates its therapeutic eects by
increasing levels of free radicals but in other circumstances by reducing
the levels of free radicals. Consequently, a further look at the molecular
eects of free radicals in neurodegeneration is required to uncover the
underlying mechanism of red/NIR therapy.
6.1. The role of ROS and RNS in neurodegeneration
Postmortem analysis of the brains of patients with various neuro-
degenerative diseases shows an increase in ROS and RNS in the aected
brain regions (Andersen, 2004; Aslan and Ozben, 2004). It is known
that these reactive species can cause oxidative and nitrative damage to
cellular components thereby having toxic eects on the cell (Tafur and
Mills, 2008). It may be deduced from this association that the increase
in ROS and RNS is contributing to the cellular death and that an
antioxidant may be benecial in such conditions. Since red/NIR light
has been found to be protective in models of such conditions it could,
therefore, be possible that the therapeutic benets of red/NIR light
could be due, in part, to an antioxidant eect. One possible mechanism
of providing the antioxidant protection is the photodissociation of ·NO,
which at low concentrations can exert an antioxidant eect (Niziolek
et al., 2003a,b, 2005, 2006).
Because red/NIR light has been found to increase the activity of the
electron transport chain, and ROS/RNS production is a byproduct of
such activity, sometimes it is presumed that red/NIR light increases
ROS production. It can be argued, however, that by removing NO-
mediated inhibition of COX, the electron ow is restored and therefore
the likelihood of donating an electron from complex I or III to oxygen
(which results in generation of superoxide radical anion) is reduced
(Lenaz, 2001). Thus by enabling the electron ow in the electron
transport chain, the risk of formation of superoxide is decreased.
Investigation into the eects of red/NIR light on ROS production by
various groups provides inconsistent results as some found a reduction
in ROS upon irradiation while others found that ROS was, in fact,
upregulated (Tafur and Mills, 2008). Regardless of the eect of red/NIR
light on ROS levels, it is unclear whether an antioxidant or pro-oxidant
eect would be most benecial when employing red/NIR light as a
neuroprotective agent. Regulated ROS production could trigger signal-
ing pathways involved in cell protection, but unregulated ROS produc-
tion could result in cellular damage and cell death (Zorov et al., 2014).
In addition, when ROS levels are too low, this also has detrimental
eects for the cell. The concept that lower, non-toxic levels of ROS are
essential for promoting cell survival by inducing an adaptive responses
is called mitochondrial hormesis or mitohormesis (Ristow and
Schmeisser, 2014). Exposure to red/NIR light may trigger a transient
increase in ROS levels, sucient to induce this adaptive response and
provide neuroprotective eects. This explanation seems plausible in
situations where light is administered as a pre-treatment as the cells
could employ this adaptive response to protect against a subsequent
injury-induced increase in ROS levels. However, if the cell is already in
a state of elevated ROS levels, as in many pathological situations, it is
unclear as to how an additional increase in ROS by red/NIR light would
provide neuroprotective eects. From the current literature, it is clear
that the mechanism for the eect of red/NIR light on ROS production,
in addition to the cellular mechanisms responsible for balancing the
ROS levels required for maintaining optimal mitochondrial and cellular
function are not fully understood. However, since maintaining redox
homeostasis is paramount for the optimal functioning of the cell and
neuronal survival, we suggest that the exposure to red/NIR light may
restore redox homeostasis in pathological conditions where it is
perturbed. Therefore, in cells with elevated ROS levels, red/NIR light
may cause mild cellular stress by an unknown mechanism that may
induce an adaptive response, which includes the upregulation of genes
with a role in redox homeostasis. We suggest that the observed increase
in ·NO upon red/NIR light irradiation, by the various mechanisms
discussed in Section 5, may facilitate the production of peroxynitrite
K. Beirne et al. Mitochondrion 36 (2017) 85–95
91
through the reaction of ·NO with ROS. The elevated RNS levels may
induce an adaptive response by triggering a dierent signaling pathway
to that which is triggered by ROS.
6.2. ROS/RNS as signaling intermediates
The upregulation of ROS and RNS, such as peroxynitrite, are
thought to promote cell viability and increase proliferation owing to
their ability to function as signal intermediates (Fig. 3). ROS, which is
produced by the ETC complexes as a byproduct of cellular respiration
permits communication from the mitochondria to the rest of the cell.
This mitochondrial signal transduction can activate various signaling
pathways resulting in the expression of a plethora of genes including
those involved either directly or indirectly in the suppression of
apoptosis, cell survival or cell proliferation. Curiously, among the genes
aected by red/NIR light irradiation, were those with roles in anti-
oxidation (Tafur and Mills, 2008).
6.3. The mitochondrial signaling pathway
ROS is thought to mediate its protective eect via the activation of
the redox sensitive transcription factor nuclear factor (NF)-ĸB which is
proposed to be sensitive to an increase in ROS generation. In support of
this theory it was found that an increase in ROS production by
mitochondrial inhibitors brought about a concurrent increase in
NFĸB, while the exposure to antioxidants reduced the NFĸB activation
(Chen et al., 2011). NFĸB can both induce and repress gene expression
by binding to ĸB elements in the promoter and enhancer regions of the
gene. This transcription factor has been shown to induce the expression
of numerous genes with functions in cell survival, the stress response
and inammation (Chen et al., 2011). When superoxides are produced
in the mitochondria they are metabolized to H
2
0
2
which is thought to
activate NF-ĸB by triggering the dissociation of inhibitor of ĸB kinase
from NFĸB. This allows NF-ĸB to translocate to the nucleus where it can
alter gene expression (Fig. 3)(Gloire et al., 2006).
The redox sensitive signaling pathways however dier with cell
type making it dicult to predict the outcome of red/NIR light therapy
if attempting to nd a general cellular mechanism (Gloire et al., 2006).
It is no surprise that the results obtained appear to be inconsistent with
varying cell type. This may provide some explanation for the dier-
ential results that have been observed with the use of red/NIR light
therapy in dierent cell types. For example, irradiation of traumatized
muscle tissue with red light therapy reduced the levels of ROS and NFĸB
activation and the associated increase in the expression of proinam-
matory genes (Rizzi et al., 2006). On the other hand NIR light has been
shown to induce ROS production and NFĸB activation in murine
embryonic broblasts (Chen et al., 2011).
In a similar fashion, peroxynitrite production triggers this stress
response, causing the up or downregulation of cell signaling cascades in
a cell dependent manner (Liaudet et al., 2009). Peroxynitrite signaling
is mediated by tyrosine nitration of proteins, particularly those involved
in phosphotyrosine-dependent signaling. The oxidant is also involved in
signaling via mitogen activated protein kinase, protein kinase B and C,
NFĸB and the insulin receptor. Of note, these pathways which converge
on the upregulation of mediators of cell survival, growth and prolifera-
tion are also activated by other stress stimuli such as ROS.
In support of the theory that red/NIR light causes upregulation in
the expression of genes associated with cell survival, is the increased
neuroprotective eects observed when red/NIR light is administered
before the induced injury in models of neurodegeneration, compared to
when administered after injury (Ying et al., 2008). This suggests that
red/NIR light could be mediating its protective eects by instigating the
production of ROS/RNS at levels sucient to cause the upregulation of
stress response genes, with negligible damage to the irradiated tissue. If
the same tissue was subsequently exposed to a toxic agent, the cell
would be equipped with the appropriate defense mechanisms to cope
with such an insult. The destructive eects to the tissue in such an
instance would be remarkably less, such as is seen in various pretreat-
ment models (Albarracin et al., 2013; Rutar et al., 2012; Albarracin and
Valter, 2012). This knowledge could be exploited in neurodegenerative
diseases with a pre-symptomatic phase. The observation demonstrates
that red/NIR light therapy would be most eective as a preventative
therapeutic treatment, administered before the onset of clinical symp-
toms and irreversible damage. With respect to retinal degenerative
disease this preventative treatment approach would be particularly
relevant in inherited optic neuropathies such as Leber's hereditary optic
Fig. 3. The upregulation of ROS or RNS by red/NIR light triggers the translocation of the transcription factor NFĸB to the nucleus, enabling NFĸB to alter gene expression.
K. Beirne et al. Mitochondrion 36 (2017) 85–95
92
neuropathy. Individuals with a LHON-causing mtDNA pathogenic
variant could be monitored so that treatment with red/NIR light could
begin upon detection of pre-symptomatic abnormalities as a pre-
treatment for the clinical symptoms. In the event of a more acute
neurodegenerative condition such as TBI, exploiting the enhanced
therapeutic eects of red/NIR light observed when used as a pretreat-
ment would not be feasible. Therefore, the neuroprotective potential of
red/NIR light therapy, as seen in the pre-treated animal models, may be
somewhat limited in a real life clinical setting.
7. The eects of red/NIR light on the anti-inammatory response
Peroxynitrite and ROS are also produced by macrophages during
inammation, which are responsible for the cytotoxic eects (Kang
et al., 2002). As mentioned above, red/NIR light has shown the ability
to reduce the presence of these reactive species in irradiated tissues,
thereby attenuating some of the cytotoxic eects of the inammatory
response. Furthermore, modulation of the immune response, itself, was
seen in many studies with red/NIR light treatment (Begum et al., 2013;
Kokkinopoulos et al., 2013; Gavish et al., 2008). Yet, how red/NIR light
mediates this anti-inammatory eect is poorly understood.
Mitochondria play a major role in the activation of the inamma-
some, a molecular platform that activates proapoptotic proteins,
cytokines and other mediators of inammation upon detection of
infectious agents or cellular damage (Green et al., 2011). ROS can
similarly activate the inammasome. The production of mitochond-
rially-derived ROS is known to increase when mitochondria are
dysfunctional. Decreased electron ux through the electron transport
chain reduces the ability of COX to fully reduce oxygen to water,
thereby increasing the formation of ROS. The action of red/NIR light in
improving the functions of the mitochondrial electron transport chain
could reduce the amount of ROS generated, which may reduce the
activation of the inammasome. Alternatively, the mitochondrial signal
transduction triggered by an increase in ROS/RNS as discussed above,
could also upregulate the expression of anti-inammatory or antiox-
idants proteins, to either directly or indirectly dampen the inamma-
tory response. However, it has been found that low level laser
irradiation (LLLI) can reduce the gene expression of anti-inammatory
cytokines as well as pro-inammatory cytokines (Gavish et al., 2008).
In this particular situation LPS was used to elicit an inammatory like
phenotype, triggering the expression of pro-inammatory cytokines in
addition to the anti-inammatory cytokine, IL-10. The anti-inamma-
tory cytokine is also triggered during the inammatory response to
limit the host immune response, thereby minimising the damage caused
to the aected tissue during the inammatory response (Iyer and
Cheng, 2012). The ndings demonstrate that LLLI is able to dampen
the entire inammatory response as oppose to specically down-
regulating proinammatory cytokines, suggesting that red/NIR light
may be acting further upstream, inuencing factors governing the
inammatory response. Also, there appears to be a correlation between
the inammatory level in the tissue and the extent of the inhibition of
the inammatory response (Gavish et al., 2008). It was found that LLLI
had little eect on the inammatory response at low levels of
inammation, but, produced a potent anti-inammatory eect at high
levels of inammation (Gavish et al., 2008). This shows an ability of
red/NIR light to restore homeostasis in the tissue. In fact, ROS is
produced by the microglia itself, and plays an important role in the
induction of pro-inammatory genes (Innamorato et al., 2009). There-
fore, restoring the redox balance in microglia may facilitate the
transition of the microglia from an activated to a resting state.
It has been suggested that the mitochondrial dysfunction seen in
many neurodegenerative diseases not only aects neurons, but also the
microglia (Ferger et al., 2010). Experimental ndings have shown that
complex I inhibition in microglia inhibited the IL-4 mediated reduction
in pro-inammatory cytokines and the secretion of the neuroprotective
insulin-like growth factor-1. The inammatory response triggered by
pathogens or damaged neurons functions to protect the neural tissue,
but if this response is not attenuated the response would switch from a
protective one to a deleterious one (Ramesh et al., 2013). When
activated, microglia secrete anti-inammatory cytokines, in addition
to pro-inammatory cytokines to control the inammatory response,
preventing unnecessary damage to neural tissue. Electron transport
deciencies in microglia, appear to perturb the anti-inammatory arm
of the immune response (Ferger et al., 2010). Since red/NIR light has
been shown to improve the function of the electron transport chain in
neurons, it can be suggested that a similar eect would be seen in
microglia with mitochondrial dysfunction.
8. Conclusion
Wavelengths of light in the red to NIR region of the electromagnetic
spectrum are optimal for the photostimulation of mitochondria as a
treatment for neurodegeneration in the retina. The photon energy of
red/NIR light appears to correspond to the absorption maxima of
chromophores present in complex IV of the electron transport chain,
triggering biostimulatory eects. Additionally, red and NIR light have
an increased ability to penetrate tissue and are associated with
signicantly less safety concerns than light of shorter wavelengths.
The potential for this treatment to be a success in patients is high. This
is supported, rstly, by the potent neuroprotective eects demonstrated
in various models of retinal damage. Secondly, data from human
studies, albeit limited, shows the ability of red and NIR light to improve
visual function. Furthermore, no adverse eects were observed in these
previously published studies. The ndings suggest that red/NIR light
therapy is safe and eective as a non-invasive treatment for retinal
neurodegeneration.
Much evidence has been gathered in eorts to elucidate the under-
lying mechanism responsible for the neuroprotective eects of red/NIR
light, but the mechanism remains unclear. Experimental ndings
suggest that there are many possible molecular and cellular eects of
red/NIR light, which could all contribute to the observed neuroprotec-
tive eects, when explored separately, but when taken collectively
some eects appear to contradict others. Improved neuronal mitochon-
drial function, increased blood ow to neural tissue, an increase in ·NO
levels, slowing down of the reactivation of the mitochondrial electron
transport chain during reperfusion, increased mitochondrial biogenesis,
reduction in ROS levels, a reduction in ·NO levels, upregulation of cell
survival mediators and restoration of normal microglial function have
all been proposed as potential underlying mechanisms of red/NIR light.
The increased ability of the neuron to survive during challenging
conditions may be due to the resulting net eect of a number of red/
NIR light induced molecular and cellular events. The eects that prevail
in a particular cell/tissue may depend on the state of the cell/tissue at
the time of irradiation and the subsequent challenges to which the cell
is exposed to.
Aside from the fact that the mechanism of action is unclear, the lack
of consensus surrounding the optimal parameters of red/NIR light, such
as irradiance, radiant exposure and wavelength, for dierent conditions
is a cause for concern. Although benecial eects have been found
using several dierent parameter combinations, eorts to nd the best
possible eect with the least possible risk are, for the most part, not
done. This unconventional experimental approach has been established
based on the assumption that red/NIR light therapy is safe, but this
approach would not be tolerated for any other therapeutic interven-
tions. In order for advances to be made in this eld a much more
detailed collection of experiments needs to be done to establish the
optimal parameters for each condition with a potential for treatment
with red/NIR light.
Acknowledgements
This research was funded by Fight for Sight UK (505025).
K. Beirne et al. Mitochondrion 36 (2017) 85–95
93
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... PBM occurs as a result of low-intensity photochemical reactions in the cell in contrast to thermal photo-ablation produced by highintensity lasers [8]. PBM has been shown to act on mitochondria-mediated signaling pathways to preserve mitochondrial function, attenuate oxidative stress, stimulate the production of cytoprotective factors, preventing cell death in in vitro and in vivo experimental models [9][10][11]. In retinal cell culture and animal models of DR, 670 nm light therapy resulted in decreased reactive oxygen species (ROS) production [12,13] and slowed disease progression [10,14,15]. ...
... The mechanisms by which PBM exerts a beneficial effect have not been clearly elucidated. Studies have shown that 670 nm light is absorbed by cytochrome-c-oxidase promoting mitochondrial bioenergetic function and increasing ATP synthesis [9,10,12,14], however other pathways have been mechanistically implicated including alteration of transcription [10,16], release of nitric oxide [17,18], and activation of kinases such as Akt [19,20]. Regardless of the exact mechanism, studies in both mice and humans have shown improved vision with PBM treatment [11,13]. ...
... Novel to the current study, we report this increase in ROS is mitigated in one hour by a single, short light treatment (670nm, 180s). These findings are consistent with others where PBM has been shown to attenuate oxidative stress in many disease models, including DR [9,10,[12][13][14]44]. However, unlike the current study, the majority of these studies have investigated the effects of 670 nm PBM on oxidative stress and ROS production several days after exposure to cytotoxic stressors including high glucose. ...
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Diabetic retinopathy (DR), the most common complication of diabetes mellitus, is associated with oxidative stress, nuclear factor-κB (NFκB) activation, and excess production of vascular endothelial growth factor (VEGF) and intracellular adhesion molecule-1 (ICAM-1). Muller glial cells, spanning the entirety of the retina, are involved in DR inflammation. Mitigation of DR pathology currently occurs via invasive, frequently ineffective therapies which can cause adverse effects. The application of far-red to near-infrared (NIR) light (630-1000nm) reduces oxidative stress and inflammation in vitro and in vivo . Thus, we hypothesize that 670nm light treatment will diminish oxidative stress preventing downstream inflammatory mechanisms associated with DR initiated by Muller cells. In this study, we used an in vitro model system of rat Müller glial cells grown under normal (5 mM) or high (25 mM) glucose conditions and treated with a 670 nm light emitting diode array (LED) (4.5 J/cm ² ) or no light (sham) daily. We report that a single 670 nm light treatment diminished reactive oxygen species (ROS) production and preserved mitochondrial integrity in this in vitro model of early DR. Furthermore, treatment for 3 days in culture reduced NFκB activity to levels observed in normal glucose and prevented the subsequent increase in ICAM-1. The ability of 670nm light treatment to prevent early molecular changes in this in vitro high glucose model system suggests light treatment could mitigate early deleterious effects modulating inflammatory signaling and diminishing oxidative stress.
... This study is focused on the early pathological changes associated with DR and a potential mitigation strategy with treatment with far-red to near-infrared light, or photobiomodulation (PBM). PBM has been shown to act on mitochondriamediated signaling pathways to preserve mitochondrial function, attenuate oxidative stress, stimulate the production of cytoprotective factors, preventing cell death in in vitro and in vivo experimental models [8][9][10] . In retinal cell culture and animal models of DR, 670 nm light therapy resulted in decreased ROS production 11,12 and slowed disease progression 9,13,14 . ...
... Studies have shown that 670 nm light is absorbed by cytochrome-c-oxidase promoting mitochondrial bioenergetic function and increasing ATP synthesis 8,9,11,13 , however other pathways have been mechanistically implicated including alteration of transcription 9,15 , release of nitric oxide 16,17 , and activation of kinases such as Akt 18,19 . Regardless of the exact mechanism, studies in both mice and humans have shown improved vision with PBM treatment 10,12 . ...
... This increase in ROS is mitigated in one hour by one, short light treatment (670nm, 180s). These ndings are consistent with others where PBM has been shown to attenuate oxidative stress in many disease models, including DR 8,9,[11][12][13]33 . However, unlike the current study, the majority of these studies have investigated the effects of 670 nm PBM on oxidative stress and ROS production several days after exposure to cytotoxic stressors including high glucose. ...
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Full-text available
Diabetic retinopathy (DR), the most common complication of diabetes mellitus, is associated with oxidative stress, nuclear factor-kB (NFkB) activation, and excess production of vascular endothelial growth factor (VEGF) and intracellular adhesion molecule-1 (ICAM-1). Current therapies are invasive, frequently ineffective, and have adverse effects. The application of far-red to near-infrared (NIR) light (630-1000nm) reduces oxidative stress and inflammation in vitro and in vivo . Thus, we hypothesize that 670nm light treatment will dimish oxidative stress preventing downstream inflammatory mechanisms associated with DR. We used an in vitro model system of rat Müller glial cells grown under normal (5 mM) or high (25 mM) glucose conditions and treated with a 670 nm light emitting diode array (LED) (4.5 J/cm ² ) or no light (sham) daily. We report that a single 670 nm light treatment diminished ROS production and preserved mitochondrial integrity and ATP production in this in vitro model of diabetic retinopathy. Furthermore, treatment for 3 days in culture reduced NFkB activity to levels observed in normal glucose and prevented the subsequent increase in ICAM-1. The ability of 670nm light treatment to prevent early molecular changes in this established DR model system suggests light treatment could become an early therapeutic option for DR.
... However, all these factors are essential mainly for the cells of the albino animals' eye or for weakly pigmented eyes. In contrast, pigmented animals are extremely resistant to the action of various prooxidants [16,21,45,[58][59][60][61][62][63][64][65][66][67][68]. ...
... A similar pattern with respect to light damage is also observed for invertebrates: animals containing large amounts of screening pigments are less sensitive to various pro-oxidant factors [63,67,68]. We investigated two genetically similar populations of the Baltic shrimp of the Mysis relicta species. ...
... It can be seen that the eye homogenate of the marine population is much more resistant to peroxidation than the homogenate of the lake population. We established [68] that the effect of the low sensitivity of the cells of the complex eye of the marine population shrimp to the action of pro-oxidants is not associated with a higher content of low-molecular antioxidants and antioxidant enzymes in them, but is due precisely to the presence of a large number of ommochrome granules. ...
... In vivo measurements in the retina with broadband near-infrared spectroscopy show a consistent progressive increase in cytochrome c oxidase 5 oxidation after 670 nm exposure for 5 minutes [224]. Photobiomodulation increases blood flow to neural tissue, increases nitric oxide levels, slows reactivation of the mitochondrial electron transport chain during reperfusion reduction in ROS levels, reduces nitric oxide levels, increases expression of cell survival mediators, and improves microglial function [225]. levels [221]. ...
... In vivo measurements in the retina with broadband near-infrared spectroscopy show a consistent progressive increase in cytochrome c oxidase 5 oxidation after 670 nm exposure for 5 minutes [224]. Photobiomodulation increases blood flow to neural tissue, increases nitric oxide levels, slows reactivation of the mitochondrial electron transport chain during reperfusion reduction in ROS levels, reduces nitric oxide levels, increases expression of cell survival mediators, and improves microglial function [225]. ...
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The tightly structured neural retina has a unique vascular network comprised of three interconnected plexuses in the inner retina (and choroid for outer retina), which provide oxygen and nutrients to neurons to maintain normal function. Clinical and experimental evidence suggests that neuronal metabolic needs control both normal retinal vascular development and pathological aberrant vascular growth. Particularly, photoreceptors, with the highest density of mitochondria in the body, regulate retinal vascular development by modulating angiogenic and inflammatory factors. Photoreceptor metabolic dysfunction, oxidative stress, and inflammation may cause adaptive but ultimately pathological retinal vascular responses, leading to blindness. Here we focus on the factors involved in neurovascular interactions, which are potential therapeutic targets to decrease energy demand and/or to increase energy production for neovascular retinal disorders.
... Photobiomodulation (PBM) involves the use of red or near-infrared light as a medical intervention and has been proposed as a treatment for retinal degeneration due to its putative neuroprotective effects [15]. Irradiation with red light ameliorated retinal dysfunction and/or degeneration in a number of models of ocular dysfunction, including age-related macular degeneration [16,17], retinopathy of prematurity [18], diabetic retinopathy [19], age-related deterioration [20], retinal ischemia [21], and following physical or light-induced retinal damage [22,23]. ...
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Retinal ganglion cells (RGCs) undergo dendritic pruning in a variety of neurodegenerative diseases, including glaucoma and autosomal dominant optic atrophy (ADOA). Axotomising RGCs by severing the optic nerve generates an acute model of RGC dendropathy, which can be utilized to assess the therapeutic potential of treatments for RGC degeneration. Photobiomodulation (PBM) with red light provided neuroprotection to RGCs when administered ex vivo to wild-type retinal explants. In the current study, we used aged (13–15-month-old) wild-type and heterozygous B6;C3-Opa1Q285STOP (Opa1+/−) mice, a model of ADOA exhibiting RGC dendropathy. These mice were pre-treated with 4 J/cm2 of 670 nm light for five consecutive days before the eyes were enucleated and the retinas flat-mounted into explant cultures for 0-, 8- or 16-h ex vivo. RGCs were imaged by confocal microscopy, and their dendritic architecture was quantified by Sholl analysis. In vivo 670 nm light pretreatment inhibited the RGC dendropathy observed in untreated wild-type retinas over 16 h ex vivo and inhibited dendropathy in ON-center RGCs in wild-type but not Opa1+/− retinas. Immunohistochemistry revealed that aged Opa1+/− RGCs exhibited increased nitrosative damage alongside significantly lower activation of NF-κB and upregulation of DJ-1. PBM restored NF-κB activation in Opa1+/− RGCs and enhanced DJ-1 expression in both genotypes, indicating a potential molecular mechanism priming the retina to resist future oxidative insult. These data support the potential of PBM as a treatment for diseases involving RGC degeneration.
... During PBM, cytochrome oxidase, a primary photoreceptor of NIR light, plays a critical role in a constructive role of recoveries of injured eye and brain tissues because the enzyme in mitochondria is critical for oxidation. For example, NIR light has robust neuroprotective effects in improvement and functional restoration of damaged retina by increase of mitochondrial function, improvement of blood flow to damaged neurons, increase of cell survival factors [87]. ...
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Near infrared (NIR) light therapy, or photobiomodulation therapy (PBMT), has gained persistent worldwide attention in recent years as a new novel scientific approach for therapeutic applications in ophthalmology. This ongoing therapeutic adoption of NIR therapy is largely propelled by significant advances in the fields of photobiology and bioenergetics, such as the discovery of photoneuromodulation by cytochrome c oxidase and the elucidation of therapeutic biochemical processes. Upon transcranial delivery, NIR light has been shown to significantly increase cytochrome oxidase and superoxide dismutase activities which suggests its role in inducing metabolic and antioxidant beneficial effects. Furthermore, NIR light may also boost cerebral blood flow and cognitive functions in humans without adverse effects. In this review, we highlight the value of NIR therapy as a novel paradigm for treatment of visual and neurological conditions, and provide scientific evidence to support the use of NIR therapy with emphasis on molecular and cellular mechanisms in eye diseases.
... In a prior section of this review, we discussed how low-wavelength visible blue light is thought to contribute to ROS production and could exacerbate LHON. On the other hand, red or near-infrared light therapy has demonstrated significant neuroprotective effects in models of retinal damage by increased mitochondrial membrane potential, complex IV activity, and ATP production [116]. Eyeglasses that filter blue light but transmit red light could be helpful in either prophylaxis or slowing the progression of LHON. ...
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Leber's hereditary optic neuropathy (LHON) is a mitochondrial disease mainly affecting retinal ganglion cells (RGCs). The pathogenesis of LHON remains ill-characterized due to a historic lack of effective disease models. Promising models have recently begun to emerge; however, less effective models remain popular. Many such models represent LHON using non-neuronal cells or assume that mutant mtDNA alone is sufficient to model the disease. This is problematic because context-specific factors play a significant role in LHON pathogenesis, as the mtDNA mutation itself is necessary but not sufficient to cause LHON. Effective models of LHON should be capable of demonstrating processes that distinguish healthy carrier cells from diseased cells. In light of these considerations, we review the pathophysiology of LHON as it relates to old, new and future models. We further discuss treatments for LHON and unanswered questions that might be explored using these new model systems.
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... R ed light plays roles in anti-inflammation and biological regulation of mitochondrial electron chain transfer in biological tissues [1][2][3][4] . In recent years, its use in clinical practice has gradually increased, promoting wound repair and tissue cell regeneration, improving blood circulation, treating peripheral nerve injury and chronic pain, and reducing damage to the retina, optic nerve, and other fields [5][6][7][8][9][10] . ...
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Цель исследования - выяснение влияния синего (450 нм) и красного (630 нм) светодиодного облучения различной энергетической экспозиции на активность митохондриальной цитохром с-оксидазы. Методики. Объектом исследования служили митохондрии, изолированные из печени японского перепела Coturnix japonica . Цитохром с-оксидазную активность измеряли по скорости окисления восстановленного тетраметил-р-фенилендиамина в присутствии ротенона. Результаты. Показано, что облучение митохондрий синим светом с энергетическими экспозициями, не превышающими 4 Дж/см вызывает примерно 5-15-процентную стимуляцию активности цитохром с-оксидазы, а дозы больше 5 Дж/см приводят к ингибированию активности фермента. Облучение митохондрий красным светом также оказывает стимулирующее действие (10-20%) на цитохром с-оксидазную активность по сравнению с необлученными образцами. Заключение. Предполагается, что низкодозовое облучение синим светом может иметь сходное с красным светом терапевтическое действие при фотобиомодуляции. The aim was to study the effect of blue (450 nm) and red (630 nm) LED irradiation with different energy exposures on activity of mitochondrial cytochrome c oxidase. Methods. The study was performed on mitochondria isolated from the liver of Japanese quail Coturnix japonica . Cytochrome c oxidase activity was measured by the rate of oxidation of reduced tetramethyl-p-phenylenediamine in the presence of rotenone. Results. Irradiation of mitochondria with blue light at doses not exceeding 4 J/cm caused approximately 5-15% stimulation of cytochrome c oxidase activity while doses higher than 5 J/cm led to inhibition of this enzyme. Irradiation of mitochondria with red light also exerted a slight stimulating effect (10-20%) on cytochrome c oxidase activity compared to unirradiated samples. Conclusion. The study suggested that low-dose irradiation with blue light may produce a therapeutic effect similar to red light in photobiomodulation.
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Objective: We review the general topic of traumatic brain injury (TBI) and our research utilizing transcranial photobiomodulation (tPBM) to improve cognition in chronic TBI using red/near-infrared (NIR) light-emitting diodes (LEDs) to deliver light to the head. tPBM improves mitochondrial function increasing oxygen consumption, production of adenosine triphosphate (ATP), and improving cellular energy stores. Nitric oxide is released from the cells increasing regional blood flow in the brain. Review of published studies: In our previously published study, 11 chronic TBI patients with closed-head TBI caused by different accidents (motor vehicle accident, sports-related, improvised explosive device blast injury) and exhibiting long-lasting cognitive dysfunction received 18 outpatient treatments (Monday, Wednesday, Friday for 6 weeks) starting at 10 months to 8 years post-TBI. LED therapy is nonthermal, painless, and noninvasive. An LED-based device classified as nonsignificant risk (FDA cleared) was used. Each LED cluster head (5.35 cm diameter, 500mW, 22.2 mW/cm2) was applied for 9 min 45 sec (13 J/cm2) using 11 locations on the scalp: midline from front-to-back hairline and bilaterally on frontal, parietal, and temporal areas. Testing was performed before and after transcranial LED (tLED; at 1 week, 1 month, and at 2 months after the 18th treatment) and showed significant improvements in executive function and verbal memory. There were also fewer post-traumatic stress disorder (PTSD) symptoms reported. Ongoing studies: Ongoing, current studies involve TBI patients who have been treated with tLED using either 26 J/cm2 per LED location on the head or treated with intranasal only (iLED) using red (633 nm) and NIR (810 nm) diodes placed into the nostrils. The NIR iLED is hypothesized to deliver photons to the hippocampus, and the red 633 nm iLED is believed to increase melatonin. Results have been similar to the previously published tLED study. Actigraphy sleep data showed increased time asleep (on average one additional hour per night) after the 18th tLED or iLED treatment. LED treatments may be performed in the home. Sham-controlled studies with veterans who have cognitive dysfunction from Gulf War Illness, blast TBI, and TBI/PTSD are currently ongoing.
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Background: Low-level laser therapy (LLLT) or photobiomodulation (PBM) is a possible treatment for brain injury, including traumatic brain injury (TBI). Methods: We review the fundamental mechanisms at the cellular and molecular level and the effects on the brain are discussed. There are several contributing processes that have been proposed to lead to the beneficial effects of PBM in treating TBI such as stimulation of neurogenesis, a decrease in inflammation, and neuroprotection. Both animal and clinical trials for ischemic stroke are outlined. A number of articles have shown how transcranial LLLT (tLLLT) is effective at increasing memory, learning, and the overall neurological performance in rodent models with TBI. Results: Our laboratory has conducted three different studies on the effects of tLLLT on mice with TBI. The first studied pulsed against continuous laser irradiation, finding that 10 Hz pulsed was the best. The second compared four different wavelengths, discovering only 660 and 810 nm to have any effectiveness, whereas 732 and 980 nm did not. The third looked at varying regimens of daily laser treatments (1, 3, and 14 days) and found that 14 laser applications was excessive. We also review several studies of the effects of tLLLT on neuroprogenitor cells, brain-derived neurotrophic factor and synaptogenesis, immediate early response knockout mice, and tLLLT in combination therapy with metabolic inhibitors. Conclusions: Finally, some clinical studies in TBI patients are covered.
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Purpose: To evaluate the efficacy of photobiomodulation (PBM) treatment for patients with dry age-related macular degeneration (AMD). Methods: Assessments on 42 eyes with dry AMD (age related eye disease study (AREDS) 2-4) were conducted. Multiwavelength light emitting diode (LED) light comprising of yellow (590 nm), red (670 nm) and near-infrared (790 nm) bandwidths was applied to subjects' eyes for a treatment course of 3 weeks. Outcome measures were changes in best-corrected visual acuity (BCVA), contrast sensitivity (CS), drusen volume and central drusen thickness. Results: Significant improvement in mean BCVA of 5.90 letters (p < 0.001) was seen on completion of the 3-week treatment and 5.14 letters (p < 0.001) after 3 months. Contrast sensitivity improved significantly (log unit improvement of 0.11 (p = 0.02) at 3 weeks and 3 months (log unit improvement of 0.16 (p = 0.02) at three cycles per degree. Drusen volume decreased by 0.024 mm(3) (p < 0.001) and central drusen thickness was significantly reduced by a mean of 3.78 μm (p < 0.001), while overall central retinal thickness and retinal volume remained stable. Conclusion: This is the first study demonstrating improvements in functional and anatomical outcomes in dry AMD subjects with PBM therapy. These findings corroborate an earlier pilot study that looked at functional outcome measures. The addition of anatomical evidence contributes to the basis for further development of a non-invasive PBM treatment for dry AMD.
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Objective: To evaluate if LED Photobiomodulation (PBM) can affect vision in patients with dry Age- Related Macular Degeneration (AMD). Methods: Prospective interventional case series. Near Infra Red (NIR) and yellow wavelengths of low powered LED light were applied to eyes with AMD in serial consecutive treatments. Included were patients with dry AMD, 50 years or older and with visual acuity between 20/20-20/200. Primary outcome measures selected were change in visual acuity, contrast sensitivity and fixation stability. Results: The treatment protocol was completed in 18 eyes (9 patients). Changes in visual acuity (p<0.0001) and contrast sensitivity (p<0.0001 at 3 cycles/degree and p<0.0032 at 1.5 cycles/degree) were positive and significant. There were no significant changes in fixation stability parameters. Conclusions: LED PBM proves to be beneficial for improvement of vision and contrast sensitivity as well as a safe treatment for dry AMD in this pilot study. Larger studies are warranted to validate the findings from this study.
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Converging lines of evidence indicate that near-infrared light treatment, also known as photobiomodulation (PBM), may exert beneficial effects and protect against cellular toxicity and degeneration in several animal models of human pathologies, including neurodegenerative disorders. In the present study, we report that chronic PMB treatment mitigates dopaminergic loss induced by unilateral overexpression of human α-synuclein (α-syn) in the substantia nigra of an AAV-based rat genetic model of Parkinson's disease (PD). In this model, daily exposure of both sides of the rat's head to 808-nm near-infrared light for 28 consecutive days alleviated α-syn-induced motor impairment, as assessed using the cylinder test. This treatment also significantly reduced dopaminergic neuronal loss in the injected substantia nigra and preserved dopaminergic fibers in the ipsilateral striatum. These beneficial effects were sustained for at least 6 weeks after discontinuing the treatment. Together, our data point to PBM as a possible therapeutic strategy for the treatment of PD and other related synucleinopathies.
Chapter
Low-level laser (light) therapy (LLLT) for traumatic events that cause brain damage is currently an experimental concept. The broad goals for clinical utilization are the prevention of brain damage, relief of symptoms, and stimulation of the repair process. Experimental studies have tested and continue to test these goals by investigating LLLT in animal models of stroke and traumatic brain injury (TBI). Animal studies of tLLLT for ischemic stroke have been summarized in this chapter. Studies by other groups on LLLT in mouse TBI models have been discussed. Four studies from our laboratory have been described. The first study looked at pulsed versus continuous wave (CW) laser irradiation in a controlled cortical impact (CCI)-TBI model and found pulsed 810 nm laser at 10 Hz to be superior. The second study looked at four different laser wavelengths (660, 730, 810, and 980 nm) in a closed-head TBI model; only 660 nm and 810 nm were effective. The third study looked at different treatment repetition regimens (1, 3, and 14 daily laser treatments). The last study used immunofluorescence techniques to show that neurogenesis, brain-derived neurotrophic factor (BDNF), and synaptogenesis were upregulated after transcranial LLLT (tLLLT) for TBI. Limitations in knowledge are still apparent, such as the optimal wavelength, light source, doses, pulsed or CW, polarization state, treatment timing, and repetition frequency. Collaborative efforts between clinicians and basic researchers will likely increase the usage and understanding of effective laser-based therapies in the central nervous system (CNS).
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Red light has been shown to provide neuroprotective effects. Axotomising the optic nerve initiates retinal ganglion cell (RGC) degeneration, and an early marker of this is dendritic pruning. We hypothesised that 670 nm light can delay axotomy induced dendritic pruning in the retinal explant. To test this hypothesis, we monitored the effects of 670 nm light (radiant exposure of 31.7 J/cm(2) ), on RGC dendritic pruning in retinal explants from C57BL/6J mice, at 40 minutes, 8 hours and 16 hours post axotomy. For sham-treated retinae, area under the Sholl curve, peak of the Sholl curve and dendritic length at 8 hours post axotomy showed statistically significant reductions by 42.3% (p=0.008), 29.8% (p=0.007) and 38.4% (p=0.038), respectively, which were further reduced after 16 hours by 40.56% (p<0.008), 33.9% (p<0.007), 45.43% (p<0.006), respectively. Dendritic field area was also significantly reduced after 16 hours, by 44.23% (p<0.019). Such statistically significant reductions were not seen in light-treated RGCs at 8 or 16 hours post axotomy. The results demonstrate the ability of 670 nm light to partially prevent ex vivo dendropathy in the mouse retina, suggesting that it is worth exploring as a treatment option for dendropathy associated neurodegenerative diseases, including glaucoma and Alzheimer's disease. This article is protected by copyright. All rights reserved.
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We previously showed that near-infrared laser photobiomodulation (PBM) (810 nm, CW, 18 J/cm(2) , 25 mW/cm(2) ) delivered to the mouse daily for 3-days after a controlled cortical impact traumatic brain injury (TBI) gave a significant improvement in neurological/cognitive function. However the same parameters delivered 14X daily gave significantly less benefit. This biphasic dose response intrigued us, and we decided to follow the mice that received 3X or 14X laser treatments out to 56-days post-TBI. We found the 14X group showed worse neurological function than the no-treatment TBI group at 2-weeks, but started to improve steadily during the next 6-weeks, and by 56-days were significantly better than the no-treatment TBI mice, but still worse than the 3X mice. A marker of activated glial cells (GFAP) was significantly increased in the brain regions (compared to both untreated TBI and 3X groups) at 4-weeks in the 14X group, but the GFAP had fallen to low levels in both 3X and 14X groups by 8-weeks. We conclude that an excessive number of laser-treatments delivered to mice can temporarily inhibit the process of brain repair stimulated by tPBM, but then the inhibitory effect ceases, and brain repair can resume. The mechanism may be temporary induction of reactive gliosis.
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Temperature profiles in human retina and choroid during laser coagulation were calculated for five different laser wavelengths: 514, 580, 620, 647 and 810 nm. For this, the spectral absorption of melanin and hemoglobin, the distribution of these chromophores in retina and choroid in vivo, and heat conduction were taken into account. The coagulation parameters chosen were a spot size of 500 μm and an exposure time of 100 msec. All curves were normalized to the same temperature rise at the retinal pigment epithelium. It turned out that the temperature profiles in the retina and inner choroid were very similar for all wavelengths, because their shape depends mainly on heat conduction from the strongly absorbing RPE and from the choroid. In the mid and outer choroid, however, a much higher temperature rise was observed at long wavelengths, especially at 810 nm, than at 514 and 580 nm. Discussing the therapeutic consequences of these findings, the conclusion is drawn that green laser wavelengths are best suited for most clinical applications of photocoagulation.