Biochemical deficits and cognitive decline in brain aging: Intervention by dietary supplements

Accepted Manuscript Title: Biochemical Deficits and Cognitive Decline in Brain Aging: Intervention by Dietary Supplements Authors: Jit Poddar, Munmun Pradhan, Gargi Ganguly, Sasanka Chakrabarti PII: DOI: Reference:

S0891-0618(17)30202-8 https://doi.org/10.1016/j.jchemneu.2018.04.002 CHENEU 1567

To appear in: Received date: Revised date: Accepted date:

13-9-2017 28-2-2018 13-4-2018

Please cite this article as: Poddar J, Pradhan M, Ganguly G, Chakrabarti S, Biochemical Deficits and Cognitive Decline in Brain Aging: Intervention by Dietary Supplements, Journal of Chemical Neuroanatomy (2010), https://doi.org/10.1016/j.jchemneu.2018.04.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Review Article Biochemical Deficits and Cognitive Decline in Brain Aging: Intervention by Dietary Supplements.

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Jit Poddar1, Munmun Pradhan1, Gargi Ganguly2, Sasanka Chakrabarti1* Department of Biochemistry, ICARE Institute of Medical Sciences and Research, Haldia 721645, India. 2

Department of Pathology,Institute of Post-graduate Medical Education and Research, Kolkata

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700020, India.

*Corresponding author at:

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: Prof. Sasanka Chakrabarti

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Name

Postal Address: Department of Biochemistry, ICARE Institute of Medical Sciences and

: [email protected]

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E-mail

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Research, Haldia 721645, India

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Mobile

: +919874489805

Highlights

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 Brain aging causes cognitive deficits.  Mitochondrial dysfunction and oxidative damage occur in the aged brain.  Impaired glucose metabolism and neuroinflammation are typical of aging brain.  The damage mechanisms in the aged brain resemble those in Alzheimer's disease.  Nutraceuticals, vitamins and antioxidants restore the deficits of the aging brain.

Abstract:

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The aging of brain in the absence of neurodegenerative diseases, usually called non-pathological

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brain aging or normal cognitive aging, is characterized by an impairment of memory and cognitive functions. The underlying cellular and molecular changes in the aging brain that include oxidative mitochondrial

impairment,

changes

in

glucose-energy

metabolism

and

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damage,

neuroinflammation have been reported widely from animal experiments and human studies. The cognitive deficit of non-pathological brain aging is the resultant of such inter-dependent and

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reinforcing molecular pathologies which have striking similarities with those operating in Alzheimer's disease which causes progressive, irreversible and a devastating form of dementia and

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cognitive decline in the elderly people. Further, this article has described elaborately how nutraceuticals present in a wide variety of plants, fruits and seeds, natural vitamins or their analogues, synthetic antioxidants and other compounds taken with the diet can ameliorate the cognitive decline of brain aging by correcting the biochemical alterations at multiple levels. The clinical usefulness of such dietary supplements should be examined both for normal brain aging and Alzheimer's disease through randomized controlled trials.

Keywords: Oxidative stress; Mitochondria; Neuroinflammation; Microglia; Cognitive deficit; Alzheimer's disease; Nutraceutical.

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1. Introduction:

Across the globe there has been a steady increase in the lifespan of general population which has

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brought into focus the various socio-economic and medical problems of geriatric population. This has led to an upswing in aging research both at the clinical and basic levels, and brain aging, not

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surprisingly, has occupied the center stage of research activity in this context. Brain aging is

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associated with structural and biochemical alterations and cognitive deficits which progress slowly

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over the years during old age. This is usually termed as non-pathological brain aging or normal cognitive aging that impairs the day-to-day quality of life and social interactions of the aged persons with increased morbidity and the need for institutional care [Deary et al., 2009]. The

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normal cognitive aging is affected by various psycho-social, environmental and physiological

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factors which are being actively investigated at present [Deary et al., 2009, Hurst et al., 2013]. In addition to this the aging brain may set a backdrop for the development of pathological states like

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Alzheimer's disease (AD) which presents with devastating loss of memory and cognition along with diffuse neuronal loss and characteristic histopathological signatures [Serrano-Pozo et al.,

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2011; Serrano-Pozo et al., 2013]. The molecular damage mechanisms of non-pathological brain aging have many commonalities with those of AD implying that a detailed analysis of the former

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may ultimately lead to a better understanding of AD pathogenesis. As a corollary, it is also likely 1

Abbreviations:

RAM: Radial Arm Maze MWM: Morris Water Maze ROS: Reactive Oxygen Species, 4-HNE: 4Hydroxynonenal PET: Positron Emission Tomography FDG:18F-2-Deoxyglucose FDG-PET: 18F2Deoxyglucose Positron Emission Tomography LDH: Lactate Dehydrogenase CSF: Cerebrospinal Fluid IL6: Interleukin 6 ILI β: Interleukin 1β TNFα: Tumor Necrosis Factor α RAGE: Advanced Glycation End Products TLRs: Toll-Like Receptors LPS: Lipopolysaccharide BBB: Blood Brain Barrier PUFAs: Polyunsaturated Fatty Acids APP: Amyloid Precursor Protein.

that drugs or other measures that ameliorate the brain deficit associated with non-pathological aging may also prove to be beneficial in rescuing the brain damage in clinical cases of AD. The first part of this review will analyze the major pathophysiologic features of normal brain aging such as cognitive decline, oxidative damage, inflammatory reactions, altered glucose-energy

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metabolism and mitochondrial dysfunctions indicating briefly their similitude with those of AD [Lister and Barnes, 2009; Mariani et al., 2005; Cunnane et al., 2011; Yin et al., 2016; Swerdlow, 2011; Chakrabarti et al., 2011]. Subsequently, we will examine the various interventions by nutraceuticals, vitamins, synthetic and natural antioxidants and other compounds that may ameliorate the damage mechanisms associated with brain aging in experimental models or clinical trials.

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2. Aging brain and cognition:

Aging is associated with impairment in certain cognitive domains in humans. In particular, the

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long-term memory such as the declarative memory is impaired during non-pathological aging with

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episodic memory much more affected than the semantic memory [Caroline et al., 2013; Glisky, 2007]. On the other hand, non- declarative memory such as the procedural memory and implicit

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memory are not much affected [Caroline et al., 2013; Glisky, 2007]. The working memory which

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is a kind of short term memory requiring active manipulation and processing of information also declines during normal aging which also impairs attention especially performing attentional tasks requiring divided attention and attention shifting [Glisky, 2007]. The other cognitive domains that

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are affected by non-pathological brain aging is processing speed, executive functions, reasoning and problem-solving ability [Caroline et al., 2013; Deary et al., 2009]. The acquisition and

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retention of new spatial learning is adversely affected during non-pathological brain aging as assessed by route learning test, but performance of aged person is comparable with young subjects

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on mental navigation test with a known environment learnt earlier [Rosenbaum et al., 2009] In experimental animals especially in rodents, working and reference memory have been analyzed employing T- Maze and Radial Arm Maze (RAM), and the role of hippocampus in this process has been well established [Wenk, 2001; Vorhees and Williams, 2014]. During normal aging, a significant impairment in both working and reference memory with a marked deterioration of

acquisition and retention in learning tasks in aged rats has been observed [Ohta et al., 1991; Barreto et al., 2010]. Morris Water Maze (MWM) has been extensively used to study spatial learning and reference memory in rats, and various variations of the original technique to assess working spatial memory, discrimination learning, latent learning etc. have been developed [Gallagher et al., 1993; Vorhees and Williams, 2006]. Many studies with MWM have indicated significant impairment of

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spatial learning as well as reference and working spatial memory in aged rodents [ Frick et al.,1995; Zyzak et al., 1995]. Long-term spatial memory has also been shown to be impaired in aged mice as assessed by object-location memory task, while object-recognition memory is unaffected by aging [Wimmer et al., 2012]. However, many studies have shown that the aged animals are heterogeneous with respect to memory impairment and in their response to learning tasks implying that age-related cognitive deficit progresses at different rates in different animals [Beas et al.,

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2013].

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Overall, the age-related cognitive decline across all species including human beings are similar in

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nature, and the cognitive domains that are impaired in aging are dependent on intact hippocampal

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and pre-frontal cortical functions [Samson and Barnes, 2013]. In hippocampus of aging brain, ultrastructural changes with diminished neurogenesis have been observed, although it is not

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established that a hippocampal neurogenesis is responsible for cognitive decline [Lister and Barnes, 2009; Bizon et al., 2004, Morrison and Baxter, 2013]. On the other hand, loss of spine

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density and dendritic arborizations have been noted in hippocampus, prefrontal cortex and other neocortical areas in different species including human beings [Lister and Barnes, 2009; Dickstein

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et al., 2013]. In hippocampus the significant loss occurs in large complex synapses in the regions of CA1 and dentate gyrus, but in pre-frontal cortex simple axospinous synapses involving thin

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spines are lost [Morrison and Baxter, 2013]. The hippocampal long-term potentiation (LTP) and synaptic plasticity are affected without major changes in the electrophysiological properties of

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neurons, and further significant neuronal loss is not seen in hippocampus or other areas of the brain in non-pathological aging [Lister and Barnes, 2009; Samson and Barnes, 2013; Long et al., 1999]. 3. Biochemical alterations in aging brain: 3.1Oxidative damage:

The brain is highly vulnerable to oxidative damage because of high oxygen consumption rate, abundant content of polyunsaturated fatty acids, regional enrichment with transition metals like iron and a deficient antioxidant defence system [Chakrabarti et al., 2013, Halliwell, 2006]. Oxidative damage is mediated by a variety of reactive oxygen species (ROS) which includes superoxide radical (O2.-), hydrogen peroxide (H2O2). The detailed chemistry of formation and

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inter-conversions of ROS has been elucidated comprehensively, and these highly reactive free radicals are well-known to cause cellular damage through oxidation of phospholipid, proteins and DNA [Chakrabarti et al., 2013; Halliwell and Gutteridge, 2007, Halliwell, 2006, Poon et al., 2004]. However, these also take part in redox signaling pathways affecting cellular functions through altered activities of different protein kinases and transcription factors [Chakrabarti et al., 2013; Schieber and Chandel, 2014, Yin et al., 2014]. Oxidative stress is long considered as a major

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mechanism of aging, and thus an accumulation of markers of oxidative damage in the aging brain

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such as malonaldehyde, 4-hydroxynonenal (4-HNE), fluorescent lipid peroxidation products, protein carbonyls, HNE-protein adducts, protein -nitrotyrosines etc., has been reported in multiple

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studies in experimental animals [Poon et al., 2004, Halliwell and Gutteridge, 2007; Grimm et al.,

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2011; Sahoo et al., 2014; Perluigi et al., 2010; Viani et al., 1991; Chakraborty et al., 2003, Head et al., 2002]. Apart from such accumulation of oxidative damage products, inactivation of soluble

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and membrane-bound enzymes, ion channels etc. and altered fluidity and permeability of

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membranes have been reported in aging brain. Several membrane-bound transport enzymes like Na+, K+ -ATPase and Ca2+ -ATPase and the metabolic enzyme glutamine synthetase are inactivated presumably by oxidative process in the aged brain [Head et al., 2002, Chakraborty et

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al., 2003; Zaidi et al., 2015]. Such oxidative inactivation of specific proteins and enzymes in the aged brain has been identified by using the techniques of redox proteomics. In brief, redox identifies

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proteomics

the

profile

of

oxidized

proteins

through

2D-electrophoresis,

immunodetection and mass spectrometric analysis [Butterfield and Dalle-Donne, 2012]. Recent

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redox proteomics data have identified the oxidatively modified enzymes of energy metabolism such as pyruvate kinase, enolase, creatine kinase, aldolase etc. in the aged rat brain [Perluigi et al., 2010]. Redox-proteomics based analysis of oxidized proteins in the post-mortem brains of old human subjects who clinically had no cognitive impairment has revealed that cytoskeletal proteins and those involved in energy metabolism and neurotransmission were mostly affected by oxidative damage [Dominguez et al., 2016]. This study has also shown regional variability in protein

oxidation in the brain of old subjects, and further the most highly oxidized proteins could be detected in the post-mortem brain tissue of even middle-aged persons [Dominguez et al., 2016]. In AD brain, the accumulation of oxidative damage markers of DNA, lipid and carbohydrate like 8-hydroxy-2-deoxyguanosine (8-OHdG), 4-HNE

and its adducts, advanced glycation end

products (AGEs) is extensive and redox proteomics data further indicate that the degree of

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oxidative damage to specific proteins is more severe compared to that in non-pathological aged brain [Smith et al., 2000, Gella and Durany, 2009, Perluigi et al., 2009].

The ion channels that are well-documented to be inactivated by oxidation and plausibly involved in normal brain aging and AD are the plasma membrane associated voltage-gated, delayed rectifier and Ca2+ activated K+ channels [Sesti et al., 2010; Sesti, 2016]. Several receptors for

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neurotransmitter are inactivated by oxidative modification. For example, specific binding of 3Hspiperone and 3Hserotonin is significantly decreased in peroxidized rat brain membranes in vitro

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[MuakkassahKelly et al., 1982]. Similarly, exposure of PC12 cells to 4-HNE, a reactive aldehyde

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derived from lipid peroxidation, diminishes ligand binding to cholinergic muscarinic,

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benzodiazepine and serotonin receptors [Siddiqui et al., 2008]. The impairment of neurotransmitter binding to the specific receptors as a result of oxidative damage is a distinct possibility in the aging

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brain, but this aspect has not been explored thoroughly. The altered fluidity and permeability of membrane components of aged brain reported in multiple studies in animals could again be the

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result of oxidative damage [Viani et al., 1991, Choi and Yu, 1995, Urano et al., 1998, Thakurta et al., 2013]. The inactivation of ion channels and altered ionic fluxes across membranes may bring

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in subtle changes in neuronal electrophysiology within the aged brain. The other aspect of oxidative stress is related to altered redox signalling mediated by a large variety of redox-sensitive

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transcription factors and soluble and membrane-bound kinases affecting inflammation, apoptosis, cell differentiation and proliferation [ Finkel, 2011, Chakrabarti et al., 2013, Kaur et al., 2015]. In

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the specific context of brain aging, the role of redox signalling has been implicated in alterations of mitochondrial functions and biogenesis, inflammatory response and changes in gene expressions in the brain [Chakrabarti et al., 2011, Bishop et al., 2010, Yin et al., 2014, von Bernhardi et al., 2015]. There are several reasons for increased oxidative stress in the aging brain which may include enhancement of ROS production, impaired anti-oxidant defence, and increased availability of

transition metals like iron. Several studies have demonstrated enhanced production of ROS from the mitochondria as well as from NADPH oxidase in the aged brain [Sasaki et al., 2008; Thakurta et al., 2012]. Both mitochondrial dysfunction and the pro-inflammatory state existing in the aged brain as discussed subsequently in this review probably contribute to increased ROS production. The antioxidant status of aged brain has also been the subject of substantial investigations. The

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antioxidant defence in the brain depends on many small molecular weight compounds like reduced glutathione (GSH), vitamin C, tocopherols, melatonin etc. as well as several enzymes and proteins like superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx), peroxiredoxin, thioredoxin and thioredoxin reductase, and most of these exist as several isoforms [Reiter, 1995, Halliwell, 2006, Halliwell and Gutteridge, 2007]. However, the most important antioxidant defence in the brain both within mitochondria and cytosol is maintained by GSH which can remove

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ROS non-enzymatically or more importantly with the help of several isoenzymes of GPx. Thus,

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GPx can remove H2O2 and reactive lipid hydroperoxides with the help of GSH forming oxidized glutathione (GSSG) as a by-product. The two forms of glutathione, GSH and GSSG, are inter-

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convertible by the enzyme glutathione reductase with the help of NADPH. Thus, the ratio of GSH

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/ GSSG and the levels of GPx and glutathione reductase are important parameters of assessing oxidative stress in the tissue. Moreover, the synthesis of GSH is regulated by the availability of

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cysteine and the activity of γ-glutamylcysteine synthetase, and thus these could be important in

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determining the antioxidant status of the brain. The factors affecting synthesis of glutathione and its exchange between neurons and astrocytes in the brain has been studied in great details [ Halliwell, 2006, Aoyama et al., 2008, Aoyama and Nakaki, 2014]. Multiple studies have shown a

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decline in the brain GSH content with altered glutathione metabolism during aging in experimental animals. Thus, a decrease in GSH content, but not GSSG or cysteine, in different brain regions of

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aged mice has been demonstrated [Chen et al., 1989]. Further, in the brain of aged rats a decrease in the GSH content and GSH / GSSG ratio and diminished activity of γ-glutamylcysteine

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synthetase have been observed [Iantomasi et al., 1993, Zhu et al., 2006]. Likewise, another study has shown decreased levels of GSH, γ-glutamylcysteine synthetase and glutathione reductase in the brain of aged rats than that in young controls [Sandhu and Kaur, 2002]. The age-dependent decline of GSH has also been demonstrated in human subjects under in vivo condition by proton magnetic resonance spectroscopy [Emir et al., 2011]. A moderately large autopsy study recently has, however, failed to validate the age-related decline in brain GSH content [Tong et al., 2016].

Age-related decreases of GSH / GSSG ratio and GSH content have been noticed in the brain of AD transgenic mice [Zhang et al., 2012]. Post-mortem studies have also indicated significantly lower levels of brain GSH and glutathione- related antioxidant enzymes in patients of AD and MCI than in age-matched controls [Ansari and Scheff, 2010, Aoyama and Nakaki, 2014]. Further, using proton magnetic resonance spectroscopy significantly decreased levels of GSH in the hippocampus

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and frontal cortex have been observed in vivo in patients of MCI and AD in comparison to that in age-matched control subjects [Mandal, 2015]. It is important to mention in the context of oxidative stress that GSH not only scavenges ROS, but also protects the critical thiol residues from oxidative damage by reversible S-glutathionylation [Aoyama and Nakaki, 2014]. The activation status of different redox signalling proteins could also be affected by reversible S-glutathionylation [ Kaur

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et al., 2015, Finkel, 2011].

As to the activities of antioxidant enzymes in the aged brain, varied reports are available. For

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example, the activities of SOD, GPx, and catalase have been reported to be diminished in the

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aging brain in some studies while others have claimed either no alteration or an increase in the

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activity of any of these enzymes and furthermore the pattern of changes are not uniform in all regions of the aging brain [Zhu et al., 2006, Sandhu and Kaur, 2002, Leutner et al., 2001; Tsay et

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al., 2000, Alper et al., 1998, Hussain et al., 1995, Iantomasi et al., 1993, Rao et al., 1990, Tayarani et al., 1989]. However, there are more consistent reports of increased accumulation of transition

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metals like Fe, both total and chelatable iron, as well as ferritin in the rodent brain during aging with regional iron enrichment in mid-brain areas like substantia nigra, globus pallidus and red

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nucleus [Roskams and Connor, 1994; Focht et al., 1997]. In humans, the age-related increase and regional enrichment in non-heme iron in post-mortem brain was originally reported in a classic

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paper long ago, and now the same has been corroborated in ante-mortem brain by magnetic resonance imaging [Hallgren and Sourander, 1958; Drayer et al., 1986]. In pathological brain aging

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like AD, the iron accumulation is far more conspicuous and takes place both within neurons as well as near the amyloid plaques [Chakrabarti et al., 2013; Smith et al., 1997]. 3.2Glucose uptake and metabolism in the aged brain. The primary metabolic fuel for the brain is glucose, but the metabolism of glucose in neurons and glia is somewhat different which is maintained by differential expressions of key metabolic

enzymes and their isoforms as well as several transport proteins [Itoh et al., 2003; Camandola and Mattson, 2017]. In general, within neurons glucose is oxidized via glycolysis followed by TCA cycle and mitochondrial oxidative phosphorylation, but in astrocytes glucose is primarily oxidized in glycolytic pathway to produce lactate which is substantially transported to neurons for further oxidation [Itoh et al., 2003; Camandola and Mattson, 2017]. Further, lactate is now considered as

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an important energy source of the brain by providing the substrates for Citric Acid Cycle possibly both in the neurons and astrocytes and under aerobic and anaerobic conditions [Schurr, 2006; Gallagher et al., 2009]. A number of studies have clearly demonstrated a decreased glucose utilization of the brain in AD by employing positron emission tomography (PET) scan after

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F-

2deoxyglucose (FDG) administration [Mosconi, 2013; Mosconi, 2008]. Furthermore, the decreased glucose utilization of the brain is well-correlated with the degree of cognitive decline at

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different stages of AD, and initially it begins in the hippocampus [ Mosconi, 2013; Mosconi et al.,

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2008, Cunnane et al., 2011]. In non-pathological aging of the brain, glucose hypometabolism as determined 18F-2-deoxyglucose positron emission tomography (FDGPET) scan has been shown to

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be milder with regional variations and even some studies have failed to observe any significant

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change in cerebral glucose utilization rate with aging and any correlation with cognitive score of the person [ Mosconi, 2013; Duara et al., 1984; Loessner et al., 1995]. However, a moderately

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large PET scan study has revealed a diminished brain glucose utilization in frontal and temporal

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cortices in both male and females, but this study also reported considerable regional and gender variations in brain glucose utilization among the aged population [Shen et al., 2012]. In experimental animals, age-dependent decline in glucose oxidation has been generally observed in

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multiple studies, but there are important variations that are to be remembered [Yin et al., 2016; Camandola and Mattson, 2017]. For example, the impairment of glucose metabolism in aging brain

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has been observed, but incorporation of 14-C in to different amino acids in the brain after administration of U-14C-glucose has been variable [Matsumoto et al., 1985]. Likewise, another

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study demonstrated impaired glucose metabolism in the brain of aged rats compared to young controls, but the energy state as measured by the levels of ATP, ADP and creatine phosphate was reported to be similar in two groups [Hoffman et al., 1985]. A significant regional variation in cerebral glucose metabolism in aged rats and variations in behavioral test performance and brain glucose metabolism within the aged group have been reported in an earlier study [Gage et al., 1984]. More recently, using PET scan, 18F-2 deoxyglucose uptake has been shown to be decreased

in the brain of aged brown Norway rats which is well correlated with the impairment of spatial learning in this group [Awasthi et al., 2011]. Using nuclear magnetic resonance spectroscopy, in situ hybridization, enzyme assay by spectrophotometry and analysis of isoenzyme patterns, a recent study has shown in aging brains of normal mice as well as mutated prematurely aging mice a significant increase in lactate level, increased transcriptional activity of Ldh-A gene and

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consequent increased levels of those LDH (lactate dehydrogenase) isoenzymes favouring pyruvate to lactate conversion [Ross et al., 2010]. In conformity with this study, an increased cerebrospinal fluid (CSF) level of lactate has been reported in aged humans implying an age-dependent alteration in glucose metabolism [Yesavage et al., 1982]. The changes in glucose metabolism in aged brain may be partly accounted by a diminished number of glucose transporters and decreased activities of key glycolytic enzymes [Camandola and Mattson, 2017; Yin et al., 2016]. A significant loss of

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glucose transporters like GLUT1 and GLUT3 has also been documented in the brain of AD

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transgenic mice and AD subjects, but the activities of several key glycolytic enzymes like pyruvate kinase, LDH and phosphofructokinase I are actually increased in AD post-mortem brain

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presumably because of reactive astrocytosis associated with this disease [Bigl et al., 1999,

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Camandola and Mattson, 2017; Yin et al., 2016].

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3.2Mitochondrial dysfunction in aging brain.

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There is a growing body of evidence indicating different types of mitochondrial functional alterations in aging brain of experimental animals and human beings which has been well reviewed

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by several authors [Chakrabarti et al., 2011; Navarro and Boveris, 2010; Grimm and Eckert, 2017]. Thus, mitochondrial bioenergetic alterations with membrane depolarization, decreased oxygen

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consumption, decreased activities of respiratory complexes and decline in ATP synthesis have been reported by multiple groups [Chakrabarti et al., 2011; Navarro and Boveris, 2010; Boveris and Navarro, 2008; Bowling et al., 1993; Ferrándiz et al., 1994]. Ultrastructural changes with

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altered fission-fusion dynamics leading to appearance of abnormal interconnected mitochondria have been demonstrated in the brain of aged wild-type mice and transgenic AD mice as well as in the post-mortem brain tissue of AD patients [Zhang et al., 2016]. The reasons for mitochondrial dysfunction in aging brain is not entirely clear, but accumulation of mitochondrial DNA mutations during aging could be an important contributing factor [ Bender et al., 2006; Nakanishi and Zhou Wu, 2009]. The susceptibility of mitochondrial DNA to age-dependent accumulation of mutations

possibly results from the proximity of the former to mitochondrial ROS-generating enzyme complexes as well as to defective DNA repair mechanisms in mitochondria [Nakanishi and Wu, 2009; Chakrabarti et al., 2011]. While the general view holds that an accumulation of mtDNA mutations occurs during aging as a result of repeated damage by ROS or other agents, it has also been suggested that replication errors and spontaneous base hydrolysis are probably more

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important for the high level of somatic mtDNA mutations in aged animals [Bratic and Larsson, 2013; Kennedy et al., 2013]. Apart from mitochondrial DNA, increased ROS in the aging brain may also directly damage the mitochondrial proteins including those of respiratory chain complexes as well as mitochondrial membrane phospholipids especially cardiolipin, and these processes may be responsible for the impaired mitochondrial functions in brain aging [Chakrabarti et al., 2011; Boveris and Navarro, 2008; Mecocci et al., 1997; Navarro and Boveris, 2004; Paradies

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et al., 2011; Sen et al., 2007]. This may lead to a vicious cycle because impaired mitochondrial

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functions may further enhance increased ROS production. A recent mitochondrial lipidomic study has revealed that brain mitochondria in aged mice have significantly lower levels of

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polyunsaturated fatty acids, and it will be interesting to study if this is related to functional

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alterations of mitochondria in the aged brain of animals [Pollard et al., 2017]. In contrast to nonpathological aging of brain, more extensive structural and functional alterations of brain

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mitochondria have been reported in AD patients which include small abnormal mitochondria with

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broken cristae, diminished levels of respiratory complex subunits, decreased activities of several TCA cycle enzymes, accumulation of oxidative damage markers of mtDNA, altered mitochondrial fusion-fission processes and accumulation of APP (amyloid precursor protein) [Wang et al., 2014;

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Chakrabarti and Sinha, 2012, Devi et al., 2006]. A multitude of functional defects of mitochondria have been reported in different types of AD transgenic animals [Chakrabarti and Sinha, 2012;

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Rönnbäck et al., 2016].

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3.3Inflammatory Response in the Aging Brain. Inflammatory response in the brain is principally mediated by the resident macrophages of the brain called microglia, although astrocytes can contribute to this process as well [Cherry et al., 2014; Wyss-Coray and Rogers, 2012]. It is thought that during inflammatory response the normal microglial cells with typical ramified morphology follow several activation pathways to perform distinctive functions [Cherry et al., 2014; Wyss-Coray and Rogers, 2012]. Either of two activation

pathways can attain classical M1 activation leads to tissue injury causing neuronal death and inflammation while the alternative M2 phenotype ushers in repair and neuroprotection [Cherry et al., 2014; Wyss-Coray and Rogers, 2012; Kaur etal., 2015]. When microglia are activated to M1 phenotype, they secrete proinflammatory cytokines like interleukin 6(IL6), interleukin 1β (IL1β), tumour necrosis factor α (TNFα), several chemokines, complement components and ROS, and also

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express surface antigens like CD40, CD45, CD86, Fcγ receptors, MHC class II, receptors for advanced glycation end products (RAGE), formyl peptide receptors, etc. [Cherry et al., 2014; Wyss-Coray and Rogers, 2012; Kaur etal., 2015]. In the aging brain of experimental animals, a state of chronic inflammation exists as typified by microglial changes towards M1 activation state with deramified morphology with shorter and less complex branching of processes and increased production of pro-inflammatory cytokines and surface markers like MHC class II, CD86, CD68,

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CIITA, CD11b, Toll-like receptors (TLRs) etc. [von Bernhardi et al., 2015; Sierra et al., 2007;

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Frank et al., 2006; Norden and Godbout, 2013]. Further, the aged brain microglia exhibit altered phagocytosis and both increased basal and lipopolysaccharide (LPS)-stimulated production of

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proinflammatory cytokines [von Bernhardi et al., 2015; Sierra et al., 2007; Barrientos et al., 2010].

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The enhanced response of aged microglia to LPS presumably accounts for the increased production of pro-inflammatory cytokines in the brain and increased 'sickness behavior' of aged animals after

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peripheral administration of LPS [Barrientos et al., 2010; Wynne et al., 2009]. Most of these age-

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related changes in microglia have been observed in different species like mouse, rat, dog and gerbil, and increased levels of pro-inflammatory markers examined by qRT-PCR, immunostaining and immunoassays [von Bernhardi et al., 2015; Sierra et al., 2007; Frank et al., 2006; Norden and

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Godbout, 2013; Barrientos et al., 2010]. In a recent study with white matter microglia from mice and post-mortem human brains, it has been reported that upregulation of genes related to

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inflammation and phagocytosis occurs in aged mice and analogous changes occur in human brain from middle-aged onwards [Raj et al., 2017]. Using the labelled ligand ([11C] -(R)-PK11195)

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specific for activated microglia followed by PET-scan, such inflammatory changes in the brain have also been demonstrated in middle-aged human beings [Raj et al., 2017]. Because of microglial sensitization the aging brain has a steady-state level of increased pro-inflammatory cytokines, and additionally there is a decrease in anti-inflammatory cytokines like IL-10 [Sparkman and Johnson, 2008]. The receptors for proinflammatory cytokines in the CNS have a wide distribution, and these cytokines regulate synaptic plasticity, neurogenesis, neurotransmitter turnover and functions in

different brain regions affecting memory, learning, mood and behavior [Sparkman and Johnson, 2008; Chakrabarti et al., 2015; Donzis and Tronson, 2014]. The raised pro-inflammatory cytokines in the aging brain partially accounts for the impaired memory and learning as observed in multiple studies in experimental animals [Barrientos et al., 2010; Sparkman and Johnson, 2008; Barrientos et al., 2009; Simen et al., 2011]. The other important observation in this context is that a rise in

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peripheral circulating pro-inflammatory cytokines can activate the microglia in brain through neural mechanisms or chemical mediators or directly by entering the organ through leaky regions in blood brain barrier (BBB) leading to neuroinflammation in CNS, and this may have a role in cognitive decline of aging both in experimental animals and human subjects [Barrientos et al., 2010; Lim et al., 2013]. This is of special importance because of general association of various systemic inflammatory conditions in aged subjects. In contrast to non-pathological brain aging, the

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role of neuroinflammation is far more conspicuous in AD where amyloid beta protein is generally

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held as a major contributor to microglial activation to proinflammatory state [ Chakrabarti et al.,

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2015; Hensley, 2010; Gomez-Nicola and Boche, 2015].

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4. Comparison of pathophysiology between non-pathological brain aging and AD. We have already indicated that neuroinflammation, oxidative damage, glucose hypometabolism

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and mitochondrial dysfunction occur in the AD brain in more varied and aggravated forms than

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those in non-pathological brain aging. More importantly, however, in contrast to non-pathological brain aging, extensive loss of neurons and synapses and abnormal accumulations of various

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proteins are the distinctive pathological features of AD affected brain. Although earlier studies suggested substantial loss of neurons in non-pathological brain aging, later accurate estimations

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using unbiased stereological methods have confirmed that neuronal loss is minimal during normal brain aging in rats and primates including humans [Morrison and Hof, 1997, Rapp et al., 2002, Merril et al., 2001, Peters, 2002, Freeman et al., 2008]. However, the question of neuronal death

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in normal brain aging is not fully settled and many studies do indicate some neuronal loss in different brain regions during aging [Mortera and Herculano-Houzel, 2012, West, 1993, Smith et al., 1999, Siwak-Tapp et al., 2008]. On the other hand, extensive loss of neurons, degeneration of neuronal processes and synaptic degeneration affecting entorhinal cortex, hippocampus, amygdala and different neocortical areas occur in AD brain [Perl, 2010, Serrano-Pozo et al., 2011]. The other typical feature of AD pathology is the wide-spread accumulation of various protein aggregates,

especially amyloid beta peptide or Aβ (Aβ42 predominantly and also Aβ40) and hyperphosphorylated tau. The peptide Aβ42 is derived from Amyloid Precursor Protein (APP) by sequential hydrolysis by beta-site APP cleavage enzyme 1 (BACE1) or β-secretase and γsecretase in the amyloidogenic pathway, and it

undergoes oligomerization to produce a

heterogeneous group of soluble aggregates of different sizes and shapes which finally form well-

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structured mature fibrils composed of parallel or anti-parallel β-sheets [Hane and Leonenko, 2014]. The post-mortem histopathology of AD brain exhibits characteristic

deposition of Aβ42

extraneuronally forming the amyloid plaques and neuritic plaques, while the hyperphosphorylated tau protein accumulates to form intracellular neurofibrillary tangles [Nelson et al., 2009, Perl, 2010, Serrano-Pozo et al., 2011]. The deposition of Aβ begins long before the development of clinical dementia and can be followed ante-mortem in the brain of AD subjects by PET-amyloid

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imaging [Serrano-Pozo et al., 2011, Rowe and Villemagne, 2011, Rodrigue et al., 2009]. It is

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important to note that brain autopsy of persons non-demented at death often show extensive amyloid plaques and PET-amyloid imaging indicates diffuse deposition of Aβ oligomers in the

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brain of healthy aged individuals without MCI or AD [Morris et al., 2014, Aizenstein et al., 2008,

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Rodrigue et al., 2009]. In the brain of aged experimental animals like dog, Octodon degu and nonhuman primates typical amyloid plaques could be seen with other histopathological features of AD

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pathological brain aging.

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[Braidy et al., 2015]. Table 1 summarizes the pathophysiological features of AD and non-

5.Interventions in age-related brain changes: Role of Dietary Supplements.

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The different pathophysiological pathways in the aging brain described so far form an interdependent and complex network of damage involving

multiple cell types like neurons,

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microglia and astrocytes which have very different but specific functions [Yin et al., 2016]. For example, mitochondrial bioenergetic impairment leads to enhanced production of oxygen free radicals which in turn inflict further damage to mitochondrial DNA and proteins leading to more production of ROS [Nakanishi and Wu, 2009.]. Furthermore, mitochondrial redox status may depend on the bioenergetic processes of the organelle through conversion of NADH to NADPH by nicotinamide nucleotide transhydrogenase, and conversely the bioenergetic efficiency may be

regulated by oxidative damage to respiratory complexes or the enzymes of the TCA cycle by ROS [Yin et al., 2016].

Similarly, activated microglia cells produce and release ROS and

proinflammatory cytokines, and ROS, in turn, may lead to further production and secretion of proinflammatory cytokines through activation of the transcription factor NF- kB and the multiprotein complex called the inflammasome [Yin et al., 2016; Kaur et al., 2015]. It is logical

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to assume that cognitive decline observed in non-pathological aging is because of a combined effect of mitochondrial dysfunction, oxidative stress, inflammation and glucose hypo metabolism (Fig.1) , and the cross-talk among these processes implies that any therapeutic intervention in one damage pathway may inhibit the other pathways. Conversely, a combination of drugs causing multiple interventions in these damage mechanisms could be beneficial in combating age-related cognitive decline. The possibility of therapeutic interventions in brain aging should be examined

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because of the shared molecular damage mechanisms.

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from these angles, and in addition it is necessary to explore if these could be beneficial in AD also

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A plethora of reports are already available which have shown that nutraceuticals like fruit

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polyphenols, both flavonoids and non-flavonoid polyphenols, vitamins, synthetic and natural antioxidants, polyunsaturated fatty acids (PUFAs) and other compounds have beneficial effects on

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the impairment of learning and memory associated with non-pathological aging in animals or experimental models of AD or in human subjects with cognitive deficits and dementia [Joseph et

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al., 2009; Domenico and Giudetti, 2017; Araujo et al., 2008; Lau et al., 2005, Arsenault et al., 2011, Ashrafpour et al., 2015, Mecocci et al., 2014, Laye et al., 2015]. Some studies have added

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the supplements as fruits or seeds or plants or their extract directly in the diet, but others have used purified components extracted from the plants making it easier for the identification of active

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components and their molecular mechanisms. Thus, diet supplemented with berries or walnut or grape juice has been reported to ameliorate age-related decline of object recognition memory,

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spatial and working memory or other behavioral tasks in rodents [Malin et al., 2011; Bensalem et al., 2016; Joseph et al., 2009, Willis et al., 2009; Galli et al., 2002, Morris et al., 2015]. Purified flavonoids like anthocyanin, catechin and epicatechin or non-flavonoid polyphenols such as resveratrol and curcumin have been shown in multiple studies to prevent memory and learning deficits in experimental animals during normal or accelerated aging [Rendiero et al., 2013; Dong et al., 2012; Kodali et al., 2015]. Similarly, the flavonoid quercetin prevents AD pathology in

sporadic or transgenic AD models [Ashrafpour et al., 2015, Sabogal‐Guáqueta et al., 2015]. A large-scale epidemiological study has shown that aged women (70 years or more) who were having diets rich in flavonoids from berries for a very long time had slower cognitive decline [Devore et al., 2012]. Another prospective study comprising of 1640 aged subjects has shown that persons with high dietary intake of flavonoids maintained better cognitive functions at the baseline and

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showed slower cognitive decline over a period of ten years [Letenneur et al., 2007]. Several systematic reviews of various interventional studies with different sources of dietary flavonoids for variable short periods in healthy adults or post-menopausal women or cognitively healthy aged or aged with mild cognitive impairment have been published using different measures of cognitive and executive functions [Lamport et al., 2012, Macready et al., 2009]. The beneficial effects of flavonoids on cognitive functions were reported in some studies while in a significant number of

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studies no such effect could be observed [Lamport et al., 2012, Macready et al., 2009]. However,

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the studies included in these systematic reviews differed from each other significantly in design and methodology, and the results are difficult to compare. Further, randomized clinical trials of

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flavonoids in AD patients have failed to produce discernible clinical benefit in most cases, but

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more well-designed and large-scale studies are probably necessary before a final verdict can be given [Mecocci and Polidori, 2012]. The beneficial effects of flavonoids on age-dependent

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cognitive deficits are related to their multiple actions such as anti-oxidative functions, prevention

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of neuroinflammation, neurotrophic actions through interaction with ERK and Akt signalling pathways, inhibition of apoptosis and stimulation of neuronal survival and synaptic plasticity [Lau et al., 2005, Vauzour et al., 2008; Spencer et al., 2009, Kelsey et al., 2010, Rendeiro et al., 2013].

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The flavonoids have additional effects in increasing cerebral blood flow, hippocampal neurogenesis and altered expressions of genes related to neuronal development, neurotransmission

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and synaptic functions in cortex and hippocampus [Vauzour et al., 2008, Dong et al., 2012]. Antioxidants like vitamin E and its analogues like tocotrienols, mitochondrial coenzymes

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(coenzyme Q10) and nutrients like α-lipoic acid and acetyl-L-carnitine given alone or in combination, are effective in improving age-dependent loss of cognition and memory in experimental animals [Thakurta et al., 2013, Takatsu et al., 2009; Kaneai et al., 2016; Liu, 2008; Ando et al., 2001]. A combination of N-acetylcysteine, α-lipoic acid and α-tocopherol administered with the diet over a prolonged period of time has been shown to prevent

mitochondrial dysfunction, oxidative damage and altered amyloid homeostasis in the aged brain along with improved spatial and reference memory of the animals [Bagh et al., 2011; Thakurta et al., 2013; Sinha et al., 2016]. The mitochondrial coenzyme Q10 supplementation in old mice for a prolonged period prevents protein oxidation in the brain and age-related decline of spatial memory [Shetty et al., 2013]. A combination of N-acetylcysteine and α-lipoic acid prevents oxidative stress

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and memory impairment in aged SAMP8 mice [Farr et al., 2003]. Acetyl-L-carnitine improves memory and learning capacity in aged rats with improved hippocampal synaptic functions [Kobayashi et al., 2010]. Other studies have shown that oxidative stress, neuroinflammation or mitochondrial dysfunction in aging brain could be attenuated by α-lipoic acid, vitamin E and tocotrienol, and these may account for the ability of the latter to prevent age-dependent cognitive decline [Kaneai et al., 2016; Liu, 2008; Navarro et al., 2011]. The spin-trapping agent N-tert-butyl-

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α-phenylnitrone which scavenges free radicals has been used in multiple studies to prevent

et al., 1996].

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oxidative brain damage and cognitive aging in different species animals [Carney et al., 1991, Sack The effects of these dietary supplements on the accumulation of amyloid beta

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protein in the brain during normal aging are particularly important. An accumulation of amyloid

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beta protein (Aβ42 in particular) with increased activity of β-secretase and overexpression of amyloid precursor protein (APP) during normal aging is well documented in differen species

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[Sinha et al., 2016; Fukumoto et al., 2004; Rodrigue et al., 2009]. In turn Aβ42 could initiate

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oxidative stress, inflammatory response and mitochondrial dysfunction creating a vicious cycle of damage, and thus the process is an obvious link between non-pathological aging and AD. It is interesting to note that prolonged dietary administration of a combination of vitamin E, N-

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acetylcysteine and α-lipoic acid to rats significantly prevents the age-related increase of β-secretase activity and APP and amyloid beta peptide formation in the brain [Sinha et al., 2016]. Likewise,

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early administration of vitamin E prevents amyloid beta deposition in transgenic AD mice [Sung

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et al., 2014].

Based on such studies in animals, population based observational studies and randomized controlled trials have been conducted in elderly subjects with these antioxidants and nutraceuticals. In a longitudinal population based study high vitamin E intake through diet is associated with better cognitive performance during aging [Morris et al., 2002]. However, in another large-scale study of elderly women long- term dietary intake of high vitamin E, calculated from responses to food

frequency questionnaire, failed to produce any effect on cognitive decline [Devore et al., 2013]. Likewise, in a randomized controlled trial, vitamin E supplementation over a long time has not shown any beneficial effect on the cognitive decline of elderly women [Kang et al., 2006]. AcetylL-carnitine has similarly been proved to be ineffective in providing clinical benefit to AD patients in double-blind randomized contolled studies [ Hudson and Tabet, 2003]. Other antioxidants,

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mitochondrial nutrients and nutraceuticals like vitamin E, vitamin C, N-acetylcysteine, α-lipoic acid, coenzyme Q and others have also failed in randomized placebo controlled clinical trials for the therapy of AD [ Mecocci and Polidori, 2012, Galasko et al., 2012].

Vitamin D of late has generated a considerable interest in the context of cognitive aging and pathological dementia like AD. This vitamin in its active form as 1α,25 dihydroxyvitamin D3 acts

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as a steroid hormone with pleotropic actions mediated by genomic and non-genomic mechanisms in many target organs including the brain. Vitamin D prevents cognitive impairment in aged rats

N

with improvement of synaptic functions in the hippocampus [Latimer et al., 2004]. In transgenic

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AD mice, vitamin D supplementation decreases the level of amyloid beta peptide and the number

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of amyloid plaques in the brain [ Yu et al., 2011]. In several prospective studies, low levels of vitamin D in elderly subjects have been associated with greater risk of cognitive decline [Llewellyn

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et al., 2010, Matchar et al., 2016, Slinin et al., 2012]. Cross-sectional studies have also indicated that low circulating levels of vitamin D are associated with AD or AD with dementia [Annweiler

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et al., 2013, Banerjee et al., 2017].

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Other dietary components like ω-3 and ω-6 fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) presentin fish oil can improve cognitive function and coping skill in

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aged mice or prevents cognitive deficits and electrophysiological alterations in entorhinal cortical neurons in transgenic AD models [Cutuli et al., 2016; Arsenault et al., 2011]. Dietary intake of walnuts which are rich sources of linoleic (ω-3) and α-linolenic (ω-6) acids as well as flavonoids

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has been shown to prevent cognitive impairment in aged rats in experimental studies or elderly human subjects in an interventional study [Willis et al., 2009, Carey et al., 2012, Rajaram et al., 2017]. In several cross-sectional and prospective studies in aged human subjects, increased dietary intake of EPA and DHA as fish or fish products is associated with better maintenance of cognitive functions as measured by MMSE or other neuropsychological tests [ Nurk et al., 2007, van Gelder et al., 2007, Fotuhi et al., 2009]. However, in randomized controlled studies in elderly individual

with no cognitive impairment in the beginning of the trial, supplementation with ω-3 fatty acid shows no beneficial effects on cognitive performance [van de Rest et al., 2008, Fothui et al., 2009, Sydenham et al., 2012]. A randomised placebo controlled interventional trial in 39 AD cases, ωfatty acids plus α-lipoic acid slowed down the cognitive decline and functional impairment over a period of 1 year [Shinto et al., 2014]. In another randomised controlled study, supplementation

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with only ω-fatty acids provided clinical benefit in cognitive measures in MCI but not in AD patients [Chiu et al., 2010]. Similar results were reported earlier in a larger randomised clinical study where supplementation with ω-3 fatty acids had no effect on cognitive outcome in mild and moderate AD cases [Freund-Levi et al., 2006]. At the experimental level, however, there are multiple presumable cellular mechanisms of beneficial effects of ω-3 or ω-6 fatty acids on agerelated cognitive decline which include inhibition of microglial activation and neuroinflammation,

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stimulation of neurite growth, neurigenesis and synaptic plasticity, modulation of neuronal

N

membrane phospholipid composition and functions, increased production of neurotrophic factors and neuroprotective and anti-inflammatory metabolites of EPA and DHA [Cole et al., 2009; Dyall,

M

A

2015].

Phosphatidylserine, a naturally occurring phospholipid in biological system, after oral

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administration has been shown to cross the blood-brain barrier and ameliorate memory and cognitive deficits in aged rats and dogs presumably by enhancing cholinergic neurotransmission [

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Osella et al., 2008; Lee et al., 2010]. The chronic administration of phosphatidylserine has been shown to restore partially the loss of cholinergic muscarinic receptors in the brain of 18 months

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old mice [Gelbmann and Müller, 1992]. This phospholipid is considered as a general enhancer of cognitive functions in humans also, and in several large placebo-controlled double-blind studies

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phosphatidylserines has been shown to improve behavioral and cognitive functions in geriatric populations [Glade and Smith, 2015; Cenacchi et al., 1993, Kato-Kataoka et al., 2010]. A cocktail

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of phosphatidylserine, vitamin E, Ginkgo biloba and pyridoxine has been shown to improve shortterm visuo-spatial memory performance in aged beagles [Araujo, 2008]. In summary, it may be stated that extensive experimental studies in animals and many prospective and cross-sectional epidemiological studies in humans have strongly suggested the beneficial role of nutraceuticals and similar compounds in preventing memory and cognitive loss in nonpathological brain aging as well as AD. However, randomized controlled interventional studies

with these compounds have generally met with failures in improving cognitive performance either in normal brain aging or AD. The reasons for this discrepancy may include short duration and small sample size of such interventional studies vis-a-vis the large scale and long-term prospective studies. Moreover, there are uncertainties regarding doses, combinations and bioavailability of the nutraceuticals employed in such interventional trials. Further and especially in case of AD, the

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initiation of such intervention when the disease is moderately advanced may negatively affect the final outcome measures. Although many studies have corrected the confounding effects of various factors on the outcome measures, there are likely to be others including the effects of gender, APOE polymorphism, hormonal status etc.that may have affected the results of human observational and interventional studies of dietary supplements on cognitive impairment of aging. We, however, will not explore further these issues in this review because of the contentious and

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indeterminate effects of these factors on cognition and cognitive aging [Barrett-Connor and Kritz-

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Silverstein, 1999, Gur and Gur., 2002, Li and Singh, 2014, Hogervorst et al., 2005, Luine, 2014,

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Chu et al., 2014, Zhen et al., 2017].

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Conclusion:

This review has clearly highlighted the proposition that an understanding of the molecular

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mechanisms of normal brain aging will help us to explain why aging is the dominant risk factor

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for AD. It also indicates that it is necessary to look beyond amyloid beta protein or phosphorylated tau to understand the pathogenesis of AD especially in sporadic cases. This discourse also raises

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the hope for some preventive or disease-modifying interventions through the use of a variety of commonly available dietary supplements for normal cognitive aging as well as clinical cases of

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MCI or AD in future. This will not be a small gain in view of the growing population of elderly persons in the world and the repeated failures of the pharmaceutical industries to develop effective and curative drugs for these conditions. However, the randomized controlled trials with such

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dietary supplements should be designed, conducted and evaluated critically in this context.

Acknowledgment:

The authors are alone responsible for the content and writing of this paper. The authors wish to acknowledge the continued support to SC from the Department of Science and Technology and Department of Biotechnology, Government of India for research on brain aging and

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neurodegenerative diseases.

Conflict of Interest:

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The authors declare that there is no conflict of interest.

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Legends:

Fig. 1. Molecular damage mechanisms in non-pathological brain aging. Interacting damage mechanisms of oxidative stress, neuroinflammation, mitochondrial dysfunction and glucose hypometabolism lead to cognitive decline in non-pathological brain aging. Synaptic loss and subtle changes in neuronal functions are plausible causes of cognitive impairment. HP, hippocampus; PFC, pre-frontal cortex.

Neuroinflammation

Oxidative Stress ROS

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Activated Microglia Pro-inflammatory Cytokines, Surface Markers, Chemokines

GSH

Oxidative Damage Markers

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Cognitive Decline

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Inactivation of ion channels, pumps, enzymes; Energy depletion Altered LTP, impaired synaptic plasticity in HP; Synaptic loss in HP, PFC; Diminished neurigenesis in HP; ? Neuronal loss Altered gene expression

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Glucose Uptake Glycolytic Enzymes Mitochondrial Dysfunction

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Table 1. Molecular and Cellular Alterations in Non-pathological Aging and AD

Non-pathological Aging

Alzheimer's Disease

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Pathophysiology in the Brain

Accumulation of oxidative Similar changes but more damage markers; extensive and severe. Decreased GSH / GSSG Accumulation of ratio; Region-specific transition metals increased levels of especially in the transition metals; amyloid plaques. Oxidative damage to specific proteins of cytoskeleton, energy metabolism, neurotransmission etc.identified by redoxproteomics.

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Oxidative Damage

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Mitochondrial Dysfunction.

Impaired respiratory chain Small and fragmented complex activities and mitochondria with ATP synthesis. Loss of broken cristae. Altered transmembrane fusion-fission potential; Altered processes. Diminished fusion-fission activities of respiratory processes. chain complexes and Accumulation of TCA cycle enzymes. mtDNA mutations. Accumulation of APP within mitochondria.

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Abnormal Accumulation

Generally increased levels of Extensive deposition of Aβ as Aβ. Amyloid plaques amyloid and neuritic Protein seen in a few species plaques. Wide-spread and some post-mortem intraneuronal human brains. accumulation of phosphorylated Tau protein.

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Inflammatory Response and Neurodegeneration.

Activated microgliawith Similar but much more deramified morphology aggressive and characteristic inflammatory response. surface markers. Microglial activation by Enhanced production of proAβ is important in inflammatory triggering cytokines, other neuroinflammation. chemokines, ROS etc. Wide-spread neuronal death Increased sensitization and loss of synapses to LPS. and neuronal processes Neuronal death is probably in entorhinal cortex, minimal. hippocampus, Some loss of synapses and amygdala, different dendritic processes in neocortical areas. hippocampus and prefrontal cortex.