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Exercise Builds Brain Health: Key Roles of Growth Factor Cascades and Inflammation

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Human and other animal studies demonstrate that exercise targets many aspects of brain function and has broad effects on overall brain health. The benefits of exercise have been best defined for learning and memory, protection from neurodegeneration and alleviation of depression, particularly in elderly populations. Exercise increases synaptic plasticity by directly affecting synaptic structure and potentiating synaptic strength, and by strengthening the underlying systems that support plasticity including neurogenesis, metabolism and vascular function. Such exercise-induced structural and functional change has been documented in various brain regions but has been best-studied in the hippocampus - the focus of this review. A key mechanism mediating these broad benefits of exercise on the brain is induction of central and peripheral growth factors and growth factor cascades, which instruct downstream structural and functional change. In addition, exercise reduces peripheral risk factors such as diabetes, hypertension and cardiovascular disease, which converge to cause brain dysfunction and neurodegeneration. A common mechanism underlying the central and peripheral effects of exercise might be related to inflammation, which can impair growth factor signaling both systemically and in the brain. Thus, through regulation of growth factors and reduction of peripheral and central risk factors, exercise ensures successful brain function.
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Exercise builds brain health: key roles
of growth factor cascades and
inflammation
Carl W. Cotman, Nicole C. Berchtold and Lori-Ann Christie
University of California, Irvine Institute for Brain Aging and Dementia, 1113 Gillespie Building, Irvine, CA 92617-4540, USA
Human and other animal studies demonstrate that
exercise targets many aspects of brain function and has
broad effects on overall brain health. The benefits of
exercise have beenbest defined for learning and memory,
protection from neurodegeneration and alleviation of
depression, particularly in elderly populations. Exercise
increases synaptic plasticity by directly affecting synaptic
structure and potentiating synaptic strength, and by
strengthening the underlying systems that support
plasticity including neurogenesis, metabolism and vas-
cular function. Such exercise-induced structural and func-
tional change has been documented in various brain
regions but has been best-studied in the hippocampus
– the focus of this review. A key mechanism mediating
these broad benefits of exercise on the brain is induction
of central and peripheral growth factors and growth
factor cascades, which instruct downstream structural
and functional change. In addition, exercise reduces per-
ipheral risk factors such as diabetes, hypertension and
cardiovascular disease, which converge to cause brain
dysfunction and neurodegeneration. A common mechan-
ism underlying the central and peripheral effects of exer-
cise might be related to inflammation, which can impair
growth factor signaling both systemically and in the
brain. Thus, through regulation of growth factors and
reduction of peripheral and central risk factors, exercise
ensures successful brain function.
Introduction
Much evidence is converging on the concept that lifestyle
factors such as exercise can improve learning and memory,
delay age-related cognitive decline, reduce risk of neuro-
degeneration, and play a part in alleviating depression. As
we delineate in the first part of this review, the evidence
that exercise can affect these endpoints has become better
established in the past few years, and provides a founda-
tion for elucidating more precisely the mechanisms
through which exercise modulates brain function. In the
subsequent two sections, by focusing primarily on the
hippocampus, we discuss how exercise can affect brain
structure, from increased neurogenesis and angiogenesis
to greater dendritic complexity, and we define the under-
lying mechanisms. It is increasingly clear that a central
mechanism is exercise-dependent peripheral and central
regulation of growth factors, which operate in unique
cascades to orchestrate structural and functional change.
In turn, mechanisms that interfere with growth factor
signaling – specifically inflammation – are modulated by
exercise in the periphery and in the central nervous system
(CNS), as outlined in the last section. We propose that
reduction of inflammation by exercise is a common means
by which exercise reduces peripheral risk factors for cog-
nitive decline and neurodegeneration. We conclude with a
brief analysis of future directions and approaches to opti-
mize the impact of exercise on brain function.
Various functional modalities are improved by
exercise
Exercise enhances learning and plasticity
In humans, robust effects of exercise have been most clearly
demonstrated in aging populations, where sustained exer-
cise participation enhances learning and memory, improves
executive function, counteracts age-related and disease-
related mental decline, and protects against age-related
atrophy in brain areas crucial for higher cognitive processes
[1–3]. Interestingly, a dose–response relationship between
exercise duration/intensity and health-related quality of life
has been reported, whereby the best outcomes are associ-
ated with moderate exercise [4]. Consistent with research in
humans, rodent studies demonstrate that exercise can facili-
tate both acquisition and retention in young and aged
animals in various hippocampus-dependent tasks including
the Morris water maze [5,6], the radial arm maze [7], passive
avoidance [8] and object recognition [9]. Not all studies,
however, have consistently demonstrated improvements
in both acquisitionand retention: some have shown benefits
in acquisition or retention only. This variability is probably
related to differences in the exercise protocol (voluntary
versus forced), in combination with the intensity (in forced
exercise models) and duration of exercise exposure. Alth-
ough both forced exercise and voluntary exercise benefit
acquisition and/or learning, voluntary exercise seems to
produce benefits more reliably, especially after shorter
exercise duration. In addition, although some studies show
improvements after 1 week of exercise [6,10], most benefits
have been associated with longer-term exercise (3–12
weeks) [5,7–9].
Along with improved behavioral performance, exercise
facilitates synaptic plasticity in the hippocampus, a key
structure for spatial learning. Facilitated plasticity is most
Review TRENDS in Neurosciences Vol.30 No.9
Corresponding author: Cotman, C.W. (cwcotman@uci.edu).
Available online 31 August 2007.
www.sciencedirect.com 0166-2236/$ see front matter . Published by Elsevier Ltd. doi:10.1016/j.tins.2007.06.011
evident in the dentate gyrus (DG), where exercise
enhances both short-term potentiation and long-term
potentiation (LTP) [11] – synaptic analogs of learning.
In particular, exercise enhances potentiation in response
to theta [11] and high-frequency [9,12] stimulation, and
reduces the threshold of theta stimulation required for
LTP induction in the perforant path [11]. Exercise-facili-
tated LTP in the DG is paralleled by altered cytoarchitec-
ture in the DG, including increases in dendritic length,
dendritic complexity, spine density and neural progenitor
proliferation [13]. Interestingly, no potentiation in res-
ponse to high-frequency stimulation has been reported
in the CA1 after exercise [12]; however, exercise effects
in the CA1 have been studied less extensively than those in
the DG. In parallel with the effects of exercise on hippo-
campal cytoarchitecture and electrophysiological proper-
ties, exercise increases the levels of synaptic proteins
(synapsin and synaptophysin [14]), glutamate receptors
(NR2b and GluR5 [11]) and the availability of several
classes of growth factor including brain-derived neuro-
trophic factor (BDNF) [15] and insulin-like growth fac-
tor-1 (IGF-1) [16], which can enhance plasticity. The
potential central role of growth factors in exercise-depend-
ent benefits in brain maintenance, health and function is
explored in more detail below.
Although strong evidence supports the idea that
exercise can facilitate learning in humans and other
animals, there is a gap in our knowledge regarding the
types of learning that are improved with exercise. For
example, human studies on exercise-dependent effects
on cognition have focused on frontal-brain-dependent
tasks (executive function), whereas animal studies have
assessed effects primarily on hippocampus-dependent
learning and plasticity. A key area of future research will
be to refine animal studies investigating the cognitive
effects of exercise to increase their relevance and translat-
ability to humans.
Exercise is neuroprotective
In addition to benefitting learning and memory, extensive
research demonstrates that exercise has neuroprotective
effects. These effects have been best defined with respect to
reducing brain injury, and to delaying onset of and decline
in several neurodegenerative diseases. For example,
engaging individuals affected by stroke in post-stroke thera-
peutic exercise programs accelerates functional rehabilita-
tion (reviewed in Ref. [17]). Clinical trials assessing the
efficacy of post-stroke exercise typically combine cardiovas-
cular training (treadmill or exercise bike) with weight train-
ing or targeted movement therapy, and the improvements
are probably due to the combination of interventions.
Animal models of ischemia (middle cerebral artery occlu-
sions) suggest, however, that cardiovascular training thera-
pies alone can reduce stroke damage and improve recovery.
Notably, reduced infarct volume and improved function
have been observed when animals engage in either forced
[18] or voluntary [19] running, and both pre-stroke [18] and
post-stroke [19] exercise shows efficacy. An essential future
goal will be to define the type, timing and intensity of
exercise interventions to determine how exercise will aid
in post-stroke rehabilitation.
In addition to the benefits of exercise in stroke,
retrospective and cross-sectional studies suggest that
participation in physical activity delays onset of and
reduces risk for Alzheimer disease (AD), Huntington’s
disease and Parkinson’s disease, and can even slow func-
tional decline after neurodegeneration has begun [2–4,20].
Intervention studies demonstrate that individuals with
AD who exercise show improved function on the daily
living scale, slowed rate of decline in cognitive tests,
improved physical function and decreased depressive
symptoms, as compared with non-exercisers who show
continued decline [21,22]. Recent evidence suggests that
exercise might have the most cognitive benefits in individ-
uals with the ApoE4 genotype (a risk factor for AD) [23],
although this area remains controversial [20]. Similar to
studies on AD, clinical intervention studies in individuals
with Parkinson’s disease demonstrate that aerobic train-
ing improves movement initiation and aerobic capacity
[24], and improves activities of daily living [25]. In parallel
with clinical studies, exercise has been shown to improve
function in several animal models of neurodegenerative
diseases by, for example, delaying symptom onset and
slowing cognitive decline in mice transgenic for Hunting-
ton’s disease [26], and improving spatial learning and
memory in transgenic mouse models of AD [27].
Mechanisms underlying the benefits of exercise in
neurodegeneration are in the early stages of investigation
in animal models such as transgenic mouse models of AD. In
these models, exercise reduces the load of amyloid-b(Ab)
plaques in the hippocampus and cortex, possibly by regulat-
ing processing of the amyloid precursor protein and/or
increasing degradation and clearance of Ab[27,28].Impor-
tantly, exercising animals show improved hippocampus-
dependent learning [27], indicating that the benefits of
exercise are functionally significant in this neurodegenera-
tive condition.
Exercise is therapeutic and protective in depression
Emerging evidence suggests that exercise has therapeutic
and preventative effects on depression. The prevention and
treatment of depression are important areas to define:
depression is linked to cognitive decline [29] and is con-
sidered to cause a worldwide health burden greater than
that of ischemic heart disease, cerebrovascular disease or
tuberculosis [30]. Therapeutic effects of exercise on depres-
sion have been most clearly established in human studies.
Randomized and crossover clinical trials demonstrate the
efficacy of aerobic or resistance training exercise (2–4
months) as a treatment for depression in both young
[31] and older [32,33] individuals. The benefits are similar
to those achieved with anti-depressants [32]. They are also
dose dependent: greater improvements are seen with
higher levels of exercise [33].
Furthermore, therapeutic effects of exercise on
depressive symptoms have been demonstrated in con-
ditions of neurodegeneration in humans. Specifically, in
a randomized clinical trial, 3 months of exercise interven-
tion improved depressive symptoms in individuals with
AD, whereas non-exercising subjects showed worsening of
depressive symptoms [21]. In addition to a therapeutic
effect, evidence from human studies shows that exercise
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can provide some protection from the development of
depression [34], although further studies are needed to
resolve inconsistent findings. A protective effect of sus-
tained exercise (>2 weeks) has been clearly demonstrated
in animal models of depression, including stress-induced
learned helplessness [35,36]. In addition, a therapeutic
effect of exercise on exiting depression has been recently
established in an animal model [37]; this therapeutic effect
parallels that observed in human studies.
Although exercise seems to have both preventative and
therapeutic effects on the course of depression, the under-
lying mechanisms are poorly understood. Protective effects
of exercise from stress have focused on the hippocampus,
where exercise-induced neurogenesis [38] and growth fac-
tor expression [39] have been proposed as potential
mediators, although not without controversy [40]. Other
proposed mechanisms include exercise-driven changes in
the hypothalamic–pituitary–adrenal axis that regulates
the stress response [31], and altered activity of dorsal
raphe serotonin neurons implicated in mediating learned
helplessness behaviors [36]. It is important to note that
the translatability of animal studies is dependent on the
animal model of depression and how well it parallels
the human condition – an area that remains under active
investigation.
Mechanisms of exercise effects on brain health
In parallel with its benefits in learning and depression,
exercise modulates a range of supporting systems for brain
maintenance and plasticity including neurogenesis,
enhanced CNS metabolism and angiogenesis. Neurogen-
esis and other exercise-induced alterations in neuronal
circuitry and function must be met by an adequate nutrient
and energy supply, which in turn is supported by changes
in metabolic function and blood flow.
Enhanced hippocampal neurogenesis is one of the
most reproducible effects of exercise in the rodent brain
[12,16,41], and might be a key mechanism mediating
exercise-related improvements in learning and memory
and resistance to depression (although the role of neuro-
genesis in these functions is controversial at present). In
both young and old animals, exercise stimulates prolifer-
ation of the neural progenitor population, increases
the number of new neurons, and promotes survival of
these new cells [12,16,41]. These new neurons become
functionally integrated into the hippocampal architec-
ture [42], but they are unique from mature granule cells
in that they have a lower threshold of excitability [43].
This feature makes these new neurons well suited to
mediate exercise-stimulated enhanced plasticity, such
as facilitated perforant-path LTP [11].Hippocampal
neurogenesis has been linked to learning and memory
[44,45] and might be related to the therapeutic effects
of antidepressants ([46];butseeRef.[40])–twofunc-
tional endpoints that are improved by exercise. The
functional consequences of hippocampal neurogenesis
remain under intense debate, and it will be important
to determine whether the enhancement of hippocampal
neurogenesis with exercise contributes to facilitated
plasticity, improved learning and memory, or protection
from stress.
To support exercise-induced changes in brain function
such as enhanced plasticity, neurogenesis and resilience to
insult, the brain must meet increased nutrient and energy
needs. Such demands are met with higher expression of
enzymes involved in glucose use and metabolism in the
hippocampus [47,48] and in other brain regions. In
addition, exercise leads to widespread growth of blood
vessels in the hippocampus [5], cortex [49] and cerebellum
[50]; these blood vessels provide increased nutrient and
energy supply. Indeed, a recent in vivo imaging study in
humans (ages 21–45) has shown that 12 weeks of cardio-
vascular training increases blood flow in the DG, and this
increase is correlated with improved rate of learning in a
hippocampus-dependent task [51]. In turn, these changes
ensure that the enhanced brain function stimulated by
exercise can be supported and maintained. In addition,
exercise-induced increases in microglia and astrocytes
[52], observed in several brain regions, also might help
to maintain enhanced brain health and function with
exercise. The significance of changes in glia and astrocytes
in response to exercise has not been defined and merits
further study.
Growth factors are central to the benefits of exercise
for the brain
Exercise modulates both plasticity and various supporting
systems that participate in maintaining brain function and
health. To understand how exercise achieves these effects,
the regulatory mechanisms underlying these changes need
to be defined. At first glance, it would seem unlikely that
common mechanisms could mediate the varied effects of
exercise on learning, depression, neurogenesis, angiogen-
esis and overall brain health. An emerging overarching
concept, however, is that exercise increases brain avail-
ability of several classes of growth factors that modulate
nearly all of the functional endpoints enhanced by exercise.
At present, BDNF, IGF-1 and vascular endothelial-
derived growth factor (VEGF) are the principal growth
factors known to mediate the effects of exercise on the
brain. These growth factors work in concert to produce
complementary functional effects, modulating both over-
lapping and unique aspects of exercise-related benefits in
brain plasticity, function and health. Effects of exercise on
learning and depression are predominantly regulated by
IGF-1 and BDNF, whereas exercise-dependent stimu-
lation of angiogenesis and hippocampal neurogenesis
seems to be regulated by IGF-1 and VEGF (Figure 1).
Role of growth factors in exercise-induced benefits in
learning and plasticity
Abundant evidence from animal and human research
supports the idea that BDNF is essential for hippocampal
function, synaptic plasticity, learning, and modulation of
depression [53]. In animal studies, exercise increases
BDNF in several brain regions, and the most robust and
enduring response occurs in the hippocampus [54]. After
several days of exercise, BDNF gene and protein pro-
duction by neurons is increased in all hippocampal sub-
fields, and remains higher for weeks with sustained
exercise [15]. Regulation of hippocampal BDNF by exercise
is mediated by neurotransmitter systems (reviewed in Refs
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[54,55]), by neuroendocrine systems [54], and by IGF-1
[56]. Like BDNF, IGF-1 gene expression is increased in
hippocampal neurons in response to exercise, occurring
several days after exercise onset [56]. In addition, periph-
eral circulating levels of IGF-1 are rapidly increased in
response to exercise (within 1 h) [57], and the peripheral
increase in IGF-1 seems to be essential for exercise-
induced neurogenesis [16] and improved memory [56].
Both BDNF signaling and IGF-1 signaling are crucial
mechanisms underlying improved learning in response to
exercise, as has been established by using blocking anti-
bodies in combination with exercise. BDNF signaling can
be blocked with antibodies to TrkB (anti-TrkB), the re-
ceptor for BDNF. Intra-hippocampal injection of anti-TrkB
attenuates the beneficial effects of exercise on hippo-
campus-dependent learning, specifically blocking improve-
ments in both the acquisition and the retention of a spatial
learning task [6,14]. In addition, anti-TrkB attenuates the
exercise-dependent induction of synaptic proteins (e.g.
synaptophysin and synapsin) in the hippocampus [6,14].
These results demonstrate that BDNF signaling must be
active for the effects of exercise on hippocampal plasticity
to manifest.
In parallel, function-blocking antibodies to IGF-1
(anti-IGF-1) also demonstrate that IGF-1 signaling has
an essential role in exercise effects on hippocampus-depend-
ent learning and plasticity. Intra-hippocampal injection of
anti-IGF-1 prevents enhancement of spatial recall, but
not acquisition [56]. In addition, anti-IGF-1 attenuates
exercise-dependent induction of synapsin I and blocks
exercise-induced activation of the calmodulin kinase II
and mitogen-activated protein kinase II (MAPKII) path-
ways [56] – effects that seem to be mediated by IGF-1-
dependent regulation of BDNF signaling [56].Thislast
study could not differentiate whether the effects were due
to a block of peripherally derived or centrally produced IGF-
1, and IGF-1 from both sources potentially could be involved.
Much evidence indicates that there are points of
convergence between IGF-1 and BDNF signaling. First,
IGF-1 increases BDNF signaling in response to exercise.
Blocking IGF-1 signaling in vivo prevents the induction of
hippocampal BDNF in response to exercise and, in paral-
lel, attenuates the exercise-dependent induction of synap-
tic proteins (e.g. synapsin I) downstream from TrkB
signaling [56]. Second, IGF-1 increases neuronal levels
of TrkB in hippocampal cultures, thereby increasing
BDNF signaling [58] – an effect that might also occur in
vivo. Third, BDNF, but not IGF-1, modulates the exercise-
dependent enhancement of synaptic plasticity mechan-
isms that are thought to underlie learning and memory.
For example, BDNF, similar to exercise, facilitates LTP
(reviewed in Ref. [59]) and activates MAPK [60] – a signal
transduction pathway that is important for LTP. By con-
trast, a direct role for IGF-1 in LTP has not been shown,
and IGF-1 is only a weak activator of the MAPK pathway in
comparison to BDNF [60]. These results suggest that IGF-
1 and BDNF work in concert, and that there is a conver-
gence on BDNF signaling as a final common downstream
mechanism mediating exercise effects on hippocampal
plasticity and learning.
Figure 1. Exercise regulates learning, neurogenesis and angiogenesis through growth factor cascades. Insulin growth factor-1 (IGF-1), brain-derived neurotrophic factor
(BDNF) and vascular endothelial growth factor (VEGF) derived from central and peripheral sources act in concert to modulate exercise-dependent effects on the brain.
(a) Exercise enhances learning by induction of BDNF and IGF-1. Neurotransmitters, including NMDA receptors and the noradrenergic (NE) system [54,55], peripheral IGF-1
and possibly centrally derived IGF-1, mediate the induction of hippocampal BDNF with exercise. In turn, BDNF signaling is likely to be a hub for effects of exercise on
learning, including acquisition, retention and LTP. (b) Exercise stimulates neurogenesis in the hippocampus through the interactive effects of IGF-1 with VEGF. Peripheral
IGF-1 and VEGF cross the blood–brain barrier (BBB) and drive enhanced proliferation and survival. (c) Exercise stimulates angiogenesis through the effects of IGF-1 and
VEGF on endothelial cell proliferation and vessel growth. Peripheral sources of the growth factors (and possibly also central sources) mediate the effects. The role of BDNF
in exercise-mediated neurogenesis and angiogenesis has not been directly tested.
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Role of growth factors in exercise-induced benefits in
depression
The hippocampus is one brain region implicated in the
pathophysiology of depression, and exercise-dependent
induction of BDNF in the hippocampus might be a mech-
anism contributing to the protective and therapeutic effect
of exercise on this disorder. This idea is based on the
observation that hippocampal infusion of BDNF or over-
expression of TrkB receptors produces antidepressant-like
effects in preclinical models of behavioral despair [61,62],
whereas mice lacking BDNF show impaired antidepress-
ant responses [63]. Furthermore, human genetic studies
demonstrate that impaired BDNF availability is associ-
ated with susceptibility to depression and other mood
disorders [64]. Lastly, evidence indicates that BDNF-
mediated TrkB signaling is both sufficient and necessary
for antidepressant-like effects in rodents [65]. These data
suggest that exercise-dependent induction of hippocampal
BDNF might contribute to protective or therapeutic effects
of exercise on depression. In addition, exercise and phar-
maceutical antidepressants seem to act synergistically to
upregulate BDNF in the hippocampus, suggesting that
there is a convergent mechanism between these thera-
peutic interventions [66].
Similar to BDNF, antidepressant effects have been
reported for IGF-1: ventricular IGF-1 injection produces
antidepressant-like (anxiolytic-like) effects that endure for
a week or more [67]. Although the evidence for IGF-1 is not
as compelling as that for BDNF, increases in both of these
growth factors in the CNS might contribute to anxiolytic or
anti-depressant benefits of exercise. The mechanism by
which growth factors might have antidepressant effects is
largely unknown. It has been recently proposed, however,
that neurotrophic factors themselves do not control mood,
but rather they facilitate the activity-dependent modu-
lation of networks that are required to induce antidepress-
ant effects [39]. If BDNF signaling does play a central part
in exercise-induced benefits in depression, it will be
important to determine whether exercise interacts with
BDNF polymorphisms, particularly the valine–methionine
polymorphism that causes impaired BDNF transport and
release [68].
Role of growth factors in exercise effects on
neurogenesis and angiogenesis
Whereas IGF-1 and BDNF mediate behavioral
improvements with exercise, the interactive effects of
IGF-1 with VEGF seem to orchestrate exercise-induced
neurogenesis and angiogenesis. Both IGF-1 and VEGF are
increased in the periphery by exercise and cross the blood–
brain barrier to enter the brain [16,41,69]. Peripheral
sources of IGF-1 and VEGF mediate stimulation of neu-
rogenesis and angiogenesis with exercise, as has been
demonstrated by using blocking antibodies. For example,
blocking either IGF-1 [16] or VEGF [41] signaling (by
blocking peripheral growth factor entry to the brain) pre-
vents exercise-induced proliferation of neural precursors in
the hippocampus, and blocking IGF-1 partially blocks the
survival-promoting effect of exercise on newly generated
neural precursors [16] (the effects of anti-VEGF on survival
have not been assessed).
In addition to a role in neurogenesis, peripheral IGF-1 is
necessary for exercise-induced vessel remodeling in the
brain [69], an effect that might be mediated in part by
induction of VEGF. Exercise-induced angiogenesis is associ-
ated with an increase in brain VEGF mRNA and protein
[49]; this increase has potent mitotic activity specific to
vascular endothelial cells, affecting proliferation, survival,
adhesion, migration and capillary tube formation [70].A
role for BDNF in exercise-dependent neurogenesis or angio-
genesis has not been directly tested. We can predict, how-
ever, that induction of BDNF participates in increasing
proliferation and survival of new neurons because BDNF
regulates baseline neurogenesis in vivo [71].
Downstream regulation of signal transduction, gene
transcription and protein expression
Although it is clear that growth factors and growth factor
signaling cascades are central regulatory mechanisms
underlying the effects of exercise in the CNS, there is less
information on the mechanisms by which these growth
factors and other effectors regulate the structural, meta-
bolic and functional endpoints.
It is known that exercise controls signal transduction
pathways and gene expression, which then effect down-
stream change. For example, exercise can activate the
MAPK and phosphatidylinositol 3-kinase (PI3K) pathways
in neurons [56]; these pathways can augment LTP and
production of additional growth factors. In addition, exer-
cise regulates activity of transcription factors such as
CREB [72], which is crucial for learning and memory.
Furthermore, proteomic and microarray analyses have
shown that many classes of proteins, in addition to growth
factors, are regulated by exercise [47,48], including those
involved in metabolism, inflammation and synaptic
plasticity. Lastly, as described above, exercise – through
gene and protein expression – controls proliferation of
various types of cell in the CNS, including neural progeni-
tors, glia and epithelial cells.
Growth factors orchestrate most, if not all, of the brain
responses to exercise through either direct or indirect
effects. As the field evolves, these and other downstream
effects will be further defined as probable mechanisms that
mediate the neuroprotective, structural, metabolic and
functional changes elicited by exercise.
Systemic mechanisms: exercise reduces peripheral
risk factors
An emerging fundamental concept is that brain health and
cognitive function are modulated by the interplay of var-
ious central and peripheral factors. Specifically, brain
function is compromised by the presence of peripheral risk
factors for cognitive decline, including hypertension,
hyperglycemia, insulin insensitivity and dyslipidemia –
a cluster of features that have been conceptualized as
the ‘metabolic syndrome’ [73]. Of the various aspects of
the metabolic syndrome, the most crucial for cognitive
function are hypertension and glucose intolerance [73].
A common feature of many of these conditions is systemic
inflammation, which contributes to most if not all of the
conditions of the metabolic syndrome. Furthermore,
systemic inflammation exacerbates CNS inflammation
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[74] and correlates with cognitive decline [75,76].
Remarkably, exercise reduces all of these peripheral risk
factors, improving cardiovascular health, lipid–cholesterol
balance, energy metabolism, glucose use, insulin sensi-
tivity and inflammation [77,78]. Exercise is thus uniquely
positioned to improve brain health and function by redu-
cing the peripheral (indirect) risk factors for cognitive
decline and, in parallel, by directly enhancing brain health
and cognitive function.
The central and peripheral effects of exercise that
improve brain health and cognitive function might be
mediated through common mechanisms that converge on
modulating growth factor signaling. Specifically, exercise
can improve growth factor signaling by directly increasing
growth factor levels (see above) and by reducing pro-
inflammatory conditions, which impair growth factor sig-
naling. The effects on peripheral and central IGF-1 sig-
naling are one example. The presence of pro-inflammatory
cytokines impairs insulin–IGF-1 signal transduction and
is a mechanism of insulin resistance [79,80]. Peripheral
IGF-1 is essential in glucose metabolism, tissue mainten-
ance [57] and cerebrovascular function [81], and a low level
of IGF-1 places individuals at risk for cognitive impairment
[82]. Exercise increases peripheral IGF-1, leading to
improved insulin sensitivity [83], restored insulin–IGF-1
signaling [84] and improved brain health and cognitive
function [85]. Furthermore, pro-inflammatory cytokines
impair IGF-1 signal transduction in neurons [86,87]. Exer-
cise might counteract the negative effects of this inflam-
mation by acting to restore IGF-1 signaling, because it
reduces circulating pro-inflammatory cytokines [88].In
addition to effects on IGF-1 signal transduction, reduction
of inflammation by exercise could also improve BDNF
signaling in the brain. Inflammation and pro-inflamma-
tory cytokines impair BDNF signaling in neurons, leading
to a condition referred to as ‘neurotrophin resistance’,
which is conceptually similar to insulin resistance [87].
Recent data indicate that exercise improves the overall
immune condition of the brain, for example, by reducing
brain IL-1b(a pro-inflammatory cytokine) in a mouse
model of AD [89], and by reducing brain inflammation in
response to stroke [90] or peripheral infection [91].In
addition, exercise could attenuate levels of pro-inflamma-
tory cytokine in the brain of individuals with AD by redu-
cing the load of Ab, which itself has pro-inflammatory
effects [92]. Thus, the reduction of peripheral and central
inflammation by exercise can serve as a common mechan-
ism to reduce the risk for both diabetes and cognitive
decline.
Conclusion and future directions
Human and animal studies indicate that exercise targets
many aspects of brain function and has broad effects on
overall brain health, resilience, learning and memory, and
depression, particularly in elderly populations. Exercise
sets into motion an interactive cascade of growth factor
signaling that has the net effect of stimulating plasticity,
enhancing cognitive function, attenuating the mechanisms
driving depression, stimulating neurogenesis and improv-
ing cerebrovascular perfusion. IGF-1 signaling converges
on BDNF signaling, which might be a hub for effects of
exercise on learning and depression. In addition to central
mechanisms, exercise reduces several peripheral risk fac-
tors for cognitive decline. A common mechanism between
many of these peripheral risk factors is inflammation,
which interferes with growth factor signaling in the per-
iphery and in the brain. Exercise might improve growth
factor signaling by both reducing pro-inflammatory con-
ditions and directly increasing growth factor levels. A
unifying concept is that exercise mobilizes growth factor
Figure 2. Exercise induces growth factor cascades, a central mechanism mediating exercise-dependent benefits in cognition, synaptic plasticity, neurogenesis and vascular
function. In addition, exercise reduces peripheral risk factors for cognitive decline such as hypertension and insulin resistance, components of the metabolic syndrome that
converge to increase the risk for brain dysfunction and neurodegeneration. Inflammation, which can impair growth factor signaling, exacerbate the metabolic syndrome
and accelerate cognitive decline, is reduced by exercise. Overall, exercise induces growth factor cascades and reduces peripheral risk factors for cognitive decline, all of
which converge to improve brain health and function, and to delay the onset of and slow the decline in neurodegenerative diseases including Alzheimer disease (AD) and
Parkinson’s disease (PD).
Review TRENDS in Neurosciences Vol.30 No.9 469
www.sciencedirect.com
cascades – both peripherally and centrally – that
act synergistically and drive exercise-mediated brain
responses (Figure 2).
Although much progress has been made in animal
studies, there is a need for rigorous clinical intervention
trials on exercise that are guided by this knowledge from
animal models. We can identify three areas where
additional research is needed to facilitate translation to
clinical trials. First, findings from animal behavioral stu-
dies must be translated to humans and, conversely, animal
studies must be refined to increase their relevance to
humans. Second, the extent, frequency and types of exer-
cise that result in functional benefits must be defined.
Third, we need to identify and to target mechanisms by
which exercise might act synergistically with key pharma-
ceuticals to augment improvements observed with either
exercise or medication alone. Overall, exercise increases
brain health – just as it improves body health – and thus
represents an exciting lifestyle intervention technique to
improve brain plasticity, function and resistance to neu-
rodegenerative diseases.
Acknowledgements
Support provided in part by grant NIA AG00538 and a donation from
Rich Muth.
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... Physical activity increases the availability of different classes of growth factors that stimulate hippocampal neurogenesis (21), neuronal plasticity (22), and the reduction of symptoms of depression (23). Several studies show, especially in the early stages of the disease, that stimulation and participation in various types of activities help to counterbalance the cognitive changes related to the pathology, thanks to cognitive plasticity (24,25). ...
... Several studies show, especially in the early stages of the disease, that stimulation and participation in various types of activities help to counterbalance the cognitive changes related to the pathology, thanks to cognitive plasticity (24,25). Although in a recent review it emerged that the results of studies on the subject are inconsistent and of poor quality (26), other studies suggest that physical activity is able not only to delay the onset and reduce the risk of Alzheimer's disease but also to slow the functional decline after the onset of neurodegeneration (23,27). ...
... /fneur. . physical activity (23,28). The WHO guidelines on reducing the risk of cognitive decline and dementia suggest that healthy elderly people perform at least 150 min of moderate-intensity aerobic physical activity per week (29). ...
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... The exercise goal has many features that can modulate the brain function and have wide effects on total brain health [4]. The important mechanism mediating these benefits of exercise on the brain is influence of the growth factors and cascades of this factor, common mechanism subjacent the systemic effects of exercise that might be related to inflammation. ...
... This fact can impair growth factor signaling both systematically as in the brain. The exercise ensures both successful brain function as help in homeostasis maintenance [4]. ...
... Acute as well as chronic physical exercise may lead to an increase in the BDNF concentration 29,32 . These exercise-related increases may support a reduction in mood disorders and the protection and regeneration of various tissues, resulting in the facilitation of cognitive function 27,29,48 . It is worth adding that exercise-related upregulation of BDNF may help to compensate for age-dependent reductions in neurogenesis, synaptogenesis, synaptic plasticity, and learning and memory, leading to a more resilient brain in the context of age-related structural and functional changes 9,12,27,49 . ...
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... It is likely that PA may influence neural systems (e.g., attention, learning and memory) [34]. It is also possible that moderate to vigorous PA may increase molecular mediators (e.g., brain-derived neurotrophic factor) and that PA may induce the development of the cellular environment and promote neurogenesis and improved vascular condition, thus improving cognition [32,35]. ...
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... According to a study, as compared to their wild-type littermates' hippocampi, Nrf2 deletion mice were more vulnerable to the inflammation caused by lipopolysaccharide (LPS), which was shown to increase microglial cells and the inflammatory markers iNOS, IL-6 and TNF-α [62]. The increase in peripheral IGF-1 that flows into the brain after exercise may also activate Nrf2 [63,64]. Rojo et al. recently demonstrated that Nrf2-deficient animals displayed increased astrogliosis and microgliosis [65]. ...
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Cited By (since 1996): 24, Export Date: 23 March 2012, Source: Scopus, CODEN: GHIRF, doi: 10.1016/j.ghir.2006.11.001, PubMed ID: 17208483, Language of Original Document: English, Correspondence Address: Landi, F.; Department of Gerontology and Geriatrics, Catholic University of Sacred Heart, 00168 Roma, Italy; email: francesco_landi@rm.unicatt.it, Chemicals/CAS: growth hormone, 36992-73-1, 37267-05-3, 66419-50-9, 9002-72-6; somatomedin C, 67763-96-6; IGFBP3 protein, human; Insulin-Like Growth Factor Binding Proteins; Insulin-Like Growth Factor I, 67763-96-6, References: Fratiglioni, L., De Ronchi, D., Aguero-Torres, H., Worldwide prevalence and incidence of dementia (1999) Drug. Aging, 15, pp. 365-375;