Dehydration and Cognition in Geriatrics: A Hydromolecular Hypothesis

Abstract

Dehydration is one of the ten most frequent diagnoses responsible for the hospital admission of elderly in the United States. It is associated with increased mortality, morbidity and an estimated cost of 1.14 billion per year (1-4). Older individuals are predisposed to dehydration encephalopathy as a result of decreased total body water (TBW) and diminished sensation of thirst. We hypothesize that thirst blunting in older individuals is the result of a defective microRNA-6842-3p failing to silence the expression of the vesicular GABA transporters (VGAT) and alpha 7 cholinergic nicotinic receptors in the subfornical organ (SFO) of the hypothalamus. We hypothesize further that resultant dehydration facilitates protein misfolding and aggregation, predisposing to neurocognitive disorders. We completed a search of predicted microRNA targets, utilizing the public domain tool miRDB and found that microRNA-6842-3p modulates the SLC6A1 and CHRNA7 genes both of which were previously hypothesized to inhibit the thirst sensation by their action on SFO. The primary aim of this article is to answer two questions: Can prevention and correction of dehydration in elderly lower age-related cognitive deterioration? Can exosomal miR-6842 in the peripheral blood predict dehydration encephalopathy in elderly?

Full text

HYPOTHESIS AND THEORY
published: 12 May 2016
doi: 10.3389/fmolb.2016.00018
Frontiers in Molecular Biosciences | www.frontiersin.org 1May 2016 | Volume 3 | Article 18
Edited by:
Megha Agrawal,
University of Illinois at Chicago, USA
Reviewed by:
Ravi C. Kalathur,
New York Structural Biology Center,
USA
Enno Klussmann,
Max Delbrück Center for Molecular
Medicine, Germany
*Correspondence:
Adonis Sfera
dr.sfera@gmail.com;
Michael Cummings
Michael.Cummings@DSH.ca.gov
Specialty section:
This article was submitted to
Molecular Diagnostics,
a section of the journal
Frontiers in Molecular Biosciences
Received: 28 January 2016
Accepted: 25 April 2016
Published: 12 May 2016
Citation:
Sfera A, Cummings M and Osorio C
(2016) Dehydration and Cognition in
Geriatrics: A Hydromolecular
Hypothesis. Front. Mol. Biosci. 3:18.
doi: 10.3389/fmolb.2016.00018
Dehydration and Cognition in
Geriatrics: A Hydromolecular
Hypothesis
Adonis Sfera 1, 2*, Michael Cummings 2*and Carolina Osorio 1
1Department of Psychiatry, Loma Linda University, Loma Linda, USA, 2Patton State Hospital, Patton, USA
Dehydration is one of the ten most frequent diagnoses responsible for the hospital
admission of elderly in the United States. It is associated with increased mortality,
morbidity and an estimated cost of 1.14 billion per year (Xiao et al., 2004; Schlanger et al.,
2010; Pretorius et al., 2013; Frangeskou et al., 2015). Older individuals are predisposed
to dehydration encephalopathy as a result of decreased total body water (TBW) and
diminished sensation of thirst. We hypothesize that thirst blunting in older individuals is the
result of a defective microRNA-6842-3p failing to silence the expression of the vesicular
GABA transporters (VGAT) and alpha 7 cholinergic nicotinic receptors in the subfornical
organ (SFO) of the hypothalamus. We hypothesize further that resultant dehydration
facilitates protein misfolding and aggregation, predisposing to neurocognitive disorders.
We completed a search of predicted microRNA targets, utilizing the public domain tool
miRDB and found that microRNA-6842-3p modulates the SLC6A1 and CHRNA7 genes
both of which were previously hypothesized to inhibit the thirst sensation by their action
on SFO. The primary aim of this article is to answer two questions: Can prevention
and correction of dehydration in elderly lower age-related cognitive deterioration? Can
exosomal miR-6842 in the peripheral blood predict dehydration encephalopathy in
elderly?
Keywords: dehydration, aquaporins, extracellular space, protein folding, protein conformational dynamics
HYDRATION AND COGNITION
Dehydration is one of the most common medical problems in seniors diagnosed in 6.7% of
hospitalized patients over the age of 65 (Warren et al., 1994). It leads to poor outcomes and
increased health care expenditures. Novel studies reveal that if not prevented or treated promptly,
dehydration results in longer intensive care unit (ICU) stay, higher hospital readmission rates and
placement in long term facilities (Xiao et al., 2004; Frangeskou et al., 2015). On the other hand,
preventing dehydration not only reduces healthcare expenditures, but also improves outcomes and
the elderly patients’ quality of life.
Dehydration is a contributing factor for delirium, a neurobehavioral syndrome recently
demonstrated to be a strong risk factor for dementia (Inouye, 1998; Davis et al., 2012). It is therefore
crucial to recognize and diagnose dehydration quickly, however at the present time there are no
specific biological markers for this condition. Clinical signs, plasma osmolality and urine markers
have poor specificity in elderly (George and Rockwood, 2004). For this reason potential epigenetic
markers such as microRNA-6842-3p obtained from peripheral blood exosomes may contribute not
only to early diagnosis, but also to prevention of dehydration.

Sfera et al. Biomarkers of Dehydration in Geriatrics
Water is an essential body nutrient and its homeostasis is
crucial for life. Early in the evolution, marine animals were
surrounded by water, but survival on dry land required built-in,
“portable” water (Warren et al., 1994). In humans, the muscle
tissue is a genuine fluid reservoir, carrying over 80% of TBW
(George and Rockwood, 2004).
The brain, in spite of being a highly lipophilic organ consists
of 80% water (Tait et al., 2008). Most of the CNS intracellular
water is stored in astrocytes. These cells are characterized by high
aquaporin (AQP) expression which makes them four times more
permeable to water than other brain cells, therefore true “brain
cisterns” for times of water scarcity (Thrane et al., 2014). With
the same token, because of their high AQP content, astrocytes are
prone to pathological water retention and swelling. Novel studies
demonstrate that astrocytes respond to peripheral dehydration
by up-regulation of AQP-4 proteins on their end-feet processes
probably in order to preserve water. For example, preclinical
studies demonstrate that a hyperosmotic milieu induces AQP
expression in astrocytes (Yang et al., 2013).
Overexpression of AQP-4 channels and augmented water
intake transforms these cells into genuine “sponges” resulting in
extracellular dehydration, extracellular space (ECS) hypovolemia.
If severe enough this condition may turn into a medical
emergency, dehydration encephalopathy or delirium.
The process of aging seems to undo the evolutionary
advantage of “portable water” as elderly individuals are known
to lose their fluid reservoirs by age-related decrease in both
muscle mass and astrocyte density. For example, dehydration was
demonstrated to accelerate the progression of AD which is also
known to be associated with loss of astrocytes (Ogawa et al., 2011;
Reyes-Haro et al., 2015; Rodríguez-Arellano et al., 2016).
It is well known that aging is associated with reduced
acetylcholine (ACh) in the brain, but it is perhaps less emphasized
that aging contributes to down-regulation of alpha7 nicotinic
acetylcholine receptors (alpha7nAChR) (Utsugisawa et al., 1999;
Akhmedov et al., 2013), rendering the CNS less responsive to
ACh. This is significant for the sensation of thirst which is
physiologically activated by ACh. Lower cholinergic activation
predisposes to inflammation which is also involved in cognitive
impairment. We discussed inflammation in the aging brain
elsewhere and this subject will not be brought here (Sfera and
Osorio, 2014). The alpha7nAChR are encoded by CHRNA7 gene
which is subject to microRNA epigenetic regulation, including
miR-6842.
Concerning the relationship between dehydration and
impaired cognition nutrition studies demonstrate that a
loss of only 1–2% of TBW may result in impaired cognitive
performance; in elderly this percentage was shown to be
even lower (Han and Wilber, 2013; Riebl and Davy, 2013).
Furthermore, the link between hydration and cognition can
be demonstrated by the neurocognitive disorders associated
with up-regulation of AQP-4 expression primarily on asyrocytic
end-feet (Table 1).
Novel studies demonstrate that both dehydration and
aging were associated with AQP-4 up-regulation, therefore it
should not come as a surprise that aging and water loss go
hand in hand (Trinh-Trang-Tan et al., 2003). Interestingly,
several amyloid-binding, neuroprotective compounds were
demonstrated to down-regulate AQP-4 expression, further
demonstrating the role of water in amyloid pathology (Table 2).
In addition, neuroimaging studies in dehydrated elderly, show
decrease in gray and white matter volume (Streitbürger et al.,
2012). However, it is important to keep in mind that most brain
volumetric studies rely on diffusion tensor imaging (DTI) which
detects water anisotropy and is therefore highly dependent on the
brain fluid dynamics (Meng et al., 2004; Nakamura et al., 2014).
WATER AND PROTEIN MISFOLDING
DISORDERS
Misfolded protein aggregates were shown to be involved in
many human diseases, including neurocognitive disorders and
diabetes type 2, but in spite of the increasing prevalence of
these conditions, the reason proteins misfold is not completely
understood.
Water has been known to play a major role in protein
conformational dynamics (Lemieux, 1996; Phillips, 2002; Zhao
et al., 2013). In order to become biologically active newly
transcribed proteins must fold along specific axes like paper
in the ancient Japanese art of origami (Collet, 2011; Chong
TABLE 1 | Disorders associated with cognitive deficit and AQP-4
up-regulation.
AQP-4/Cognitive deficit disorders References
Cerebral amyloid angiopathy Foglio and Fabrizio, 2010;
Moftakhar et al., 2010
Alzheimer’s disease Nagelhus and Ottersen, 2013;
Lan et al., 2015
Parkinson’s disease Subburaman and Vanisree, 2011;
Zhang et al., 2016
Multiple sclerosis Tanaka et al., 2007
Neuromyelitis optica Saji et al., 2013; Zhang et al.,
2015
Traumatic brain injury Hu et al., 2005
Cerebral ischemia Zador et al., 2009
Epilepsy Binder et al., 2012; Alvestad
et al., 2013
HIV encephalitis St. Hillaire et al., 2005
Progressive multifocal leukoencephalopathy Aoki-Yoshino et al., 2005;
Florence et al., 2012
TABLE 2 | Neuroprotective compounds associated with AQP-4
down-regulation.
AQP-4 down-regulation References
Rapamycin Guo et al., 2014
Erythropoietin Gunnarson et al., 2009; McCook et al., 2012
Curcumin Laird et al., 2010; Wang et al., 2015
Purines Morelli et al., 2010; Lee et al., 2013
Progesteron He et al., 2014
Melatonin Dehghan et al., 2013; Lin et al., 2013;
Bhattacharyaa et al., 2014
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Sfera et al. Biomarkers of Dehydration in Geriatrics
and Ham, 2015). Recently it was demonstrated that water
plays a crucial role in this process as it forms hydrogen
bonds with the amino acid chains, facilitating their collapse
into three dimensional molecular structures. In the presence
of water, folding occurs almost instantly (140 ns), resulting in
biologically active molecules available for chemical reactions
at the opportune time (Sen and Voorheis, 2014; Vajda and
Perczel, 2014). In the absence of hydration the folding process
is significantly slower and the biomolecules may miss the timing
of their reactions. This results in molecular overcrowding which
predisposes to misfolding (Gregersen et al., 2006; Stoppini
et al., 2009). Indeed, it was hypothesized by others that
biomolecular crowding relative to the fluid volume is inductive of
misfolding and aggregation (Tokuriki et al., 2004; Yerbury et al.,
2005).
Novel studies in protein conformational dynamics
demonstrate that both protein misfolding, their repair and
removal can take place in the intra and the extracellular
compartment. The chance of protein misfolding is higher in
the extracellular space (ECS) which is a rougher environment
exposing these biomolecules to a higher degree of shear and tear
(Ker and Chen, 1998; Genereux and Wiseman, 2015). For this
reason, we focus our study on the ECS where hypovolemia may
facilitate protein misfolding and aggregation.
It was hypothesized that adequate water circulation via
aquaporin (AQP) channels is essential for clearing beta amyloid
and for preventing its build-up characteristic for Alzheimer’s
disease (AD) (Figure 1). The glymphatic system paradigm
suggests that insufficient amyloid clearance and its subsequent
aggregation is the result of impaired water movement (Xie et al.,
2013). This model, however pays less attention as to why proteins
misfold in the first place.
The hydromolecular hypothesis is therefore complementary
to the glymphatic model, but also differs from it by elevating
water from an inert medium to an active participant in cognition
(via protein folding) (Levy and Onuchic, 2006). This hypothesis
raises another interesting question: do proteins participate in
information processing directly?
Novel studies in neuroscience demonstrate that proteins
participate in cognition by their ability to access logic
gates, the elementary building blocks of digital circuits (Qi
et al., 2013). These molecules are endowed with abilities to
adaptively change their shapes in Transformers-like fashion,
assembling and disassembling in response to electronic signals or
electromagnetic fields (Kidd et al., 2009; Ausländer et al., 2012).
For example, proteins were shown to assemble in the neuronal
post-synaptic membrane into heteroreceptor complexes which
may engender memory “bar codes” (Fuxe et al., 2007; Chen et al.,
2012). Calcium-calmodulin-dependent kinase III, a component
of neuronal microtubules, was hypothesized to store long term
memory by reorganizing its spatial structure in response to
synaptic activity (Smythies, 2015). Interestingly, water plays a
FIGURE 1 | Astrocyte swelling as a result of AQP-4 channels up-regulation with interstitial fluid hypovolemia and beta-amyloid misfolding.
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Sfera et al. Biomarkers of Dehydration in Geriatrics
major role in this model. Several studies revealed that dendritic
spine biomolecules may play a crucial role in associative memory
as they endow the neural circuits with Boolean logic (Craddock
et al., 2012; De Ronde et al., 2012; Qi et al., 2013). Furthermore,
proteins are endowed with Lego-like abilities to interlink,
engendering large intra and extracellular biomolecular networks
with hypothesized roles in cognition (Chen et al., 2012; Mancuso
et al., 2014). In light of this data we believe that alteration of
the normal protein conformation may impair cognition directly,
rather than indirectly by damaging synapses and neurons which
is the traditional view.
EPIGENOMIC REGULATION OF THE
SUBFORNICAL ORGAN (SFO)
Elderly individuals are prone to dehydration as a result of blunted
thirst sensation and loss of TBW as discussed above (Cowen
et al., 2013; Hooper et al., 2014). Recent preclinical data reveal
that the subfornical organ (SFO) of the hypothalamus functions
as a “thirst center” in the mammalian brain, regulating the
basic instinct of water intake (Oka et al., 2015). Since the SFO
lacks a blood-brain-barrier (BBB) it may be well positioned to
detect peripheral dehydration and respond to it by increasing
the sensation of thirst lowering water output. The SFO contains
sensitive osmoreceptors which convert peripheral changes in
osmolality into an excitatory neuronal signal, triggering both the
sensation of thirst and the release of arginine vasopressin (AVP)
by the posterior pituitary (Azizi et al., 2008).
FIGURE 2 | Water channels expressed on the SFO cells. Inhibitory
GABAergic neurons express VGAT, astrocytes AQP-9 and tanycytes AQP-4.
It was recently demonstrated that the SFO contains both
excitatory and inhibitory neurons which can be activated by
the ECS water volume and osmolality (Oka et al., 2015).
ECS hypovolemia activates the SFO excitatory neurons (which
express ETV-1 transcription factor), triggering thirst. ECS
normovolemia, on the other hand activates the SFO inhibitory
neurons (which express the vesicular GABA transporter
(VGAT)], inhibiting the sensation of thirst.
These genetically distinct neuronal groups may explain both
dehydration and psychotic polydipsia. For example, excessive
activation of excitatory, or failure to activate inhibitory SFO
neurons may result in psychotic polydipsia. The opposite may be
true in dehydration.
Several prior studies revealed that the sensation of thirst
may also be activated by the stimulation of SFO neuronal
cholinergic receptors. The SFO neurons express both nicotinic
and muscarinic cholinergic receptors, while the SFO astrocytes
express only alpha 7- nAChRs (Honda et al., 2003; Tanaka,
2003; Ono et al., 2008). Age-related paucity of these receptors
interferes with ACh activation of the thirst sensation. The glial
water channels consist of AQP-9 expressed by astrocytes and
AQP-4 expressed by tanycytes (Figure 2).
In addition to decreasing the expression of alpha 7 nAChRs,
the aging process was documented to augment the expression
of AQP channels on astrocytic end-feet as part of an
age-related senescence-associated secretory phenotype (SASP).
SASP is characterized by low grade inflammation, increased
accumulation of misfolded protein aggregates and astrocyte
swelling induced by AQP up-regulation (Picciotto and Zoli, 2002;
Salminen et al., 2011; Akhmedov et al., 2013).
FIGURE 3 | Physiologically, microRNA-6842 silences SLC6A1 and
CHRNA7 genes, activating the sensation of thirst.
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Sfera et al. Biomarkers of Dehydration in Geriatrics
Peripheral dehydration was demonstrated to alter the
expression of several SFO- related genes (Hindmarch et al.,
2008). One of these genes is SLC6A1 which expresses
VGAT on the cellular membranes of the SFO inhibitory
neurons.
Method: we conducted a search of miRDB, a public
online database for microRNA target prediction and functional
annotations. The targets in miRDB are predicted by the
bioinformatics tool, MirTarget. MirTarget was developed by
analyzing thousands of miRNA-target interactions from high-
throughput sequencing experiments. We searched the human
database for the genes of interest SLC 32A1 and CHRNA 7,
coding for VGAT and alpha 7 nicotinic cholinergic receptors
respectively. We conducted a separate search for each of the two
genes by utilizing the gene symbol SLC 32A1 and CHRNA 7.
The results revealed that 131 microRNAs modulate the SLC 32A1
gene and 57 microRNAs the CHRNA 7 gene. Analyzing this data,
miR by miR we found one common microRNA modulating both
genes, the miR-6842 (Figure 3).
A dysfunctional miR-6842 may fail to silence the SLC6A1
gene, preventing inhibition of the SFO GABAergic neurons with
resultant thirst blocking. The same is achieved via failure to
inhibit the CHRNA-7 gene, thus preventing ACh-induced thirst.
CONCLUSIONS
The hydromolecular hypothesis endeavors to explain the
relationship between dehydration and decreased cognition in
elderly as resulting from protein misfolding and aggregation in
the context of low interstitial fluid volume (ECS hypovolemia).
Defective proteins may affect cognition either directly via
impaired information processing in the brain biomolecular
networks, or indirectly via neuronal and synaptic damage, or
both.
MicroRNA-6842 may constitute a biological marker with
predictive value for dehydration encephalopathy in elderly as it
regulates two genes involved in the sensation of thirst.
AUTHOR CONTRIBUTIONS
All authors listed, have made substantial, direct and intellectual
contribution to the work, and approved it for publication.
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