Mitochondrial Regulatory Pathways in the Pathogenesis of Alzheimer’s Disease

INTRODUCTION

Alzheimer’s disease (AD), first described over ten decades (in 1907) ago by Dr. Alois Alzheimer after whom the disease was named, is an age-associated neurodegenerative disease affecting up to 15% of the population 65 years and older [1–3]. It is the fourth leading cause of death in Canada afflicting approximately 500,000 Canadians with significant human and economic burden at a cost of over 12 billion dollars a year and an individual lifetime cost of about 170,000 dollars [4]. AD affects over 5 million people in the United States and interestingly this number will increase remarkably as the human population ages [5]. This disease is the most common cause of age-related, incurable neurodegenerative disorders that leads to terminal dementia [6]. It is characterized by progressive memory decline to profound cognitive failure, behavioral insufficiencies, and language abnormalities [2, 3]. Inflammatory responses have been implicated in the pathogenesis of AD not only by the identification of activated microglia and astrogliosis expressing proinflammatory modulators and markers of glial activation but also by the increased involvement of cytokines and chemokines-mediated inflammatory changes [7–9]. The chemokines/cytokines implicated in AD include transforming growth factor-β (TGF-β) and interleukin-1α (IL-1α), while the proinflammatory modulators and glial activation markers are IL-6, IL-1β, tumor necrosis factor-α (TNF-α), major histocompatibility complex class II (MHC class II), cyclooxygenase-2 (Cox 2), and MCP-1 [8, 9].

Significant knowledge and confirmation of clinical diagnoses of AD have been achieved at postmortem, histopathologically, by identification of neocortical,hippocampal, and subcortical amyloid-beta (Aβ) accumulation, neurofibrillary tangles (NFT), and varying degrees of synaptic and neuronal losses and deaths [10, 11]. These predilection sites are crucial and are recognized as centers for active cognitive function. As re-enforced in the current consensus criteria (National Institute on Aging/Reagan Institute of the Alzheimer’s Association Consensus Recommendations (NIA-Reagan Criteria) for postmortem diagnosis of AD) for neuropathologic diagnosis of AD, based on morphologic techniques, amyloid accumulation, neurofibrillary changes, and neuritic plaques remain the gold standards for neuropathologic evaluation and diagnosis of AD following death among several other features of AD neuropathologic changes. These changes include clinical history of cognitive impairment and dementia [12]. The NIA-Reagan Criteria recognizes stages such as the pre-clinical stage of AD that is associated with or without AD-induced dementia where there are no obvious neuropathologic changes, coupled with soluble forms of Aβ and tau, which have been linked to AD pathologies but could not be detected by morphologic methods [12]. Further, the NIA-Reagan Criteria suggests the possibility that other processes and pathologies may be important contributors to the well-known established neuropathology of AD. Clearly, biochemical and mitochondrial changes are on the top list of the rule outs as part of the processes and pathologies in AD.

For decades, the amyloid-β protein precursor (AβPP) hypothesis has dominated studies to better understand the pathogenesis of AD and to identify potential targets for preventive and therapeutic intervention. Little attention has been paid to the mitochondrial molecular and biochemical pathways in the pathogenesis of AD, which are pivotal in the regulation of neuronal energy homeostasis and second messenger signaling molecules such as calcium ions (Ca^2 +) and reactive oxygen (nitrogen) species [RO(N)S] as well as inflammatory responses [13–18]. The importance of Ca regulation in neuronal firing and transmission, secretion of neurotransmitter across synaptic junctions (exocytosis), and ionic homeostasis cannot be overemphasized [19–21]. Also, oxidative damage to neuronal macromolecules orchestrated by ROS and RNS during mitochondrial dysregulation poses great danger to neuronal survival. These molecules include lipids of cell membranes and myelin sheath, proteins, and nuclear (nDNA) as well as mitochondrial DNA (mtDNA)[22, 23].

Cellular investigation to fully understand the molecular mechanisms of AD could not have been timely as a growing body of evidence links mitochondrial dysfunction to neurodegenerative disorders with recent advances in the ability to determine metabolism and bioenergetics properties in cultured neural cells, isolated synaptic nerve terminals, and intact brain slices in transgenic animal models.

Being the largest single consumer of oxygen in the body, the brain and neuronal function are highly dependent and extremely sensitive to fluctuations in mitochondrial function and bioenergetics load. AD is associated with mitochondrial dysfunction, and it has been suggested that cerebral energy metabolism is impaired in AD coupled with defective glucose sensing and utilization [24–26]. The mechanisms of agents that positively modify the bioenergetics defects are essential for the development of therapeutic interventions for effective AD management and treatment.

This review is a proof of concept for research proposal to elucidate the involvement of mitochondria and the molecular and biochemical mechanisms of mitochondrial dysfunction in AD with the objective of identifying potential therapeutic targets and early biomarkers that indicate reliably whether a person who is still alive and healthy is destined for AD. Because AD has a long asymptomatic period, when pathophysiological processes are already operational, followed by a phase of symptomatic predementia with progressive cognitive impairment, ultimately leading to the onset of clinical dementia [27], the need for bioindicators of an early onset of the disease is important to institute intervention procedures that may delay, modify, or truncate the progression of the disease.

NEUROPATHOLOGY OF AD

Histopthologically, lesions of AD are localized and affect mainly the neocortex and the hippocampal formation. Neurons of zone CA1, subiculum, and the entorhinal cortex are more susceptible to AD-induced neuronal lesions and losses than those of CA3 and the dentate gyrus of the hippocampal formation [28]. In the neocortex, the frontal, parietal, and temporal lobes consisting of the primary motor and sensory cortical areas experience neuronal death and the other pathognomonic lesions (Aβ neuritic plaques and NFTs) of AD [29, 30]. The entorhinal cortex-dentate gyrus circuit, also referred to as the perforant pathway, is most often affected with the resultant tissue damage, structural and synaptic disconnection between the archicortex (hippocampal formation) and neocortex, and this has significant implication to memory relay and behavioral and learning attributes [31–33].

MITOCHONDRIAL INVOLVEMENT IN THE PATHOGENESIS OF AD

Mitochondria respond to extra- and intra-neuronal stressors in order to maintain the neuronal and metabolic needs of the CNS. Dysregulation of energy metabolism has long been recognized as a remarkable event that characterizes AD, and this is supported by studies associating dysfunction of mitochondrial ETC with the pathophysiological outcomes of AD [34, 35]. The consequences of mitochondrial impairment include neuronal energy deficit, oxidative damage to cellular macromolecules, and necrosis or apoptosis in severe cases. Other metabolic insufficiencies in mitochondrial dysfunction include impaired heme regulation, cardiolipin, copper, iron-sulfur cluster metabolism, and Ca homeostasis [36–40]. Maintenance of optimum neuronal energy (ATP) balance is confined to mitochondrial turnover as well as the proteins of mitochondrial ETC complex I–IV and those of tricarboxylic acid cycle (TCA), some of which are depleted in AD [41, 42].

It has been postulated that reduction of neuronal energy status precedes and sometimes coincides with onset of cognitive deficit in AD following or in association with reduced glucose uptake and utilization in brain areas known to be involved in memory processing [43–45]. This notion is buttressed by a significant reduction in the activity of mitochondrial inner membrane enzymes including cytochrome c oxidase [41, 46–49] and ATP synthase (complex V) [50], as well as mitochondrial matrix enzymes such as α-ketoglutarate dehydrogenase [51] and pyruvate dehydrogenase [52] from mitochondria isolated from AD hemi-brains and patients.

Furthermore, an association has been established between defective energy metabolism and increased Aβ deposition and increased intracellular AβPP processing and fragments [53, 54]. Since intraneuronal and intramitochondrial localization of Aβ are remarkable in AD [55, 56], it is highly possible that AβPP and AβPP fragments may bind to neuronal mitochondrial components and subsequently inhibit electron transfer along the ETC, which generates the proton motive force that is required for oxidative phosphorylation and ATP production [57–60]. Because inhibition of electron transfer across the ETC results in ROS generation from the ETC complexes, it is highly suggestive that mitochondrial impairment and oxidative stress are attributive, inextricable culprits in the reduction of neuronal energy status and in AD [55, 60]. Taking these together, the evidence suggests that inhibition of mitochondrial ETC complexes that results in the reduction of mitochondrial energy status will eventually cause impairment of memory and learning processes as seen in rats with inhibited cytochrome c oxidase [61, 62].

Morphologically in a cell line model of AD, mitochondria are enlarged and swollen with pale patchy matrix and fewer distorted cristae, resulting in a significant decrease in mitochondrial cytochrome c oxidase (complex IV) activity [63, 64]. The outer mitochondrial membranes are incomplete or absent and there are occasional intramitochondrial inclusion bodies [64]. The consequence of these structural abnormalities includes reduction in mitochondrial membrane potential, mitochondrial bioenergetics, ETC function, and increased production of free radicals and ROS, which is partly a consequence of defect or mutation in mtDNA [63, 64]. Limited availability of AD biopsy brain samples and the fact that few willing biopsy donors were mainly obtained in the late stage of the disease when mitochondrial morphological changes of early stages of the AD may not be demonstrable resulted in scarcity of data from human biopsy samples. This is also true of autopsy brain samples that may have lost ultrastructural details during preservation and fixation [64]. However, available reports reinforce the alteration of mitochondrial morphology in autopsy and biopsy samples of patients suffering from AD [65, 66]. Moreover, mitochondrial morphological alterations have also been correlated with metabolic and energy deficits in neurons in AD patients [66].

INVOLVEMENT OF OXIDATIVE STRESS IN AD

Human brains account for one fifth (20%) of total body O₂ consumption. This high O₂ concentration is used to generate energy by oxidative phosphorylation in the mitochondrial ETC for neuronal function and to maintain ionic gradient for impulse transmission [67]. The requirement for energy is so huge that slight deficit or outright depletion of ATP results in neuronal damage and death [68]. However, the synthesis of energy by the mitochondrial ETC inadvertently releases potentially damaging endogenous ROS and RNS. These include the superoxide anion radical (O₂.) and its subsequent transformations by divalent metals-induced catalysis to hydroxyl ion (HO.), hydrogen peroxide (H₂O₂), and as well as nitrogen oxide (NO) and peroxynitrite (ONOO^–) [68]. All of these free radicals are potentially damaging to cellular components. Interestingly, neurons and neuroglia are highly susceptible to oxidative damage because of the inherent inefficiencies of brain antioxidants to metabolize the free radicals and render them harmless [69]. Moreover, abnormal concentrations of divalent metals such as iron (Fe), copper (Cu), and zinc (Zn) as seen during inflammation, demyelination, and degeneration may contribute to metal-induced catalysis for ROS production and tissue damage [69].

Further, Aβ protein has been demonstrated to be toxic to cultured neuronal cell lines by promoting free radical production, mediating oxidative damage and subsequent accumulation of peroxidized lipids [70, 71]. This indicates that Aβ aggregation acting in consonance with oxidative stress contributes to the toxic modifications and neuronal death in AD (Fig. 1) [72]. Being the center of O₂ metabolism, the mitochondria mediate oxidative stress by generating ROS from redox centers of the ETC that potentially cause membrane peroxidation, protein oxidation, and DNA damage. The downstream effect of this is oxidative damage to the energy dependent membrane Ca-ATPase that regulates intraneuronal Ca concentration [73], Ca dysregulation, and DNA splicing, deletion, or gene mutation. There is also evidence that Aβ impairs the mitochondria ETC complexes that consequently result in the inhibition of oxidative phosphorylation and ATP synthesis (Fig. 1) [74, 75]. Mitochondrial ATP deficit invariably impairs energy-dependent mitochondrial Ca^2 + ATPase and further dysregulates other Ca handling mechanisms [76]. The pathophysiological changes seen in AD are attributable to mitochondrial responses because blockade of mitochondrial energy metabolism result in the alteration of the metabolism of AβPP switching it to the synthesis of a rather more amyloidogenic protein form of Aβ [53]. This eventually results in neuronal death and dementia [77, 78].

NEURONAL CALCIUM SIGNALING IN AD

Mitochondria together with endoplasmic reticulum (ER) interact with intracellular Ca to maintain Ca homeostasis. This cross-talk involves mitochondria taking up Ca when there is cytosolic Ca overload that would exceed mitochondrial Ca handling capacity and cause downstream reduction in mitochondrial membrane potential (MMP), mitochondrial permeability transition pore (MPTP) formation, oxidative stress, impaired ATP synthesis, and mitochondria demise. Calcium-induced mitochondrial dysfunction in turn causes unregulated Ca trafficking and signaling between the mitochondrial and the cytosol, which is critical to cell survival and death [79–81]. Additionally, intracellular and mitochondrial Aβ/tau binding and induced mitochondrial dysfunction result in decreased MMP, MPTP formation, oxidative stress, impaired ATP synthesis [82, 83], and invariably Ca dyshomeostasis.

Mitochondria are one of the main intracellular calcium storage depots, next to the neuronal ER (100–500μM). To regulate intraneuronal Ca to physiological levels during fluctuating Ca tides, mitochondria act as cellular sink that accumulate excess Ca in order to maintain intracellular Ca homeostasis (50–300 nM) [80]. Interestingly, Ca regulates various neuronal functions including impulse transmission, synaptic plasticity, and neuronal death. Further, impaired intraneuronal Ca signaling is known to promote abnormal aggregation of Aβ peptide that is considered a pathological hallmark of AD [84]. Further, mutant gene in the membrane localized AβPP and presenilin-1 has been observed during abnormal Ca signaling and further suggests that abnormal neuronal Ca signaling may predispose and destine the brain to AD [85–88].

Calcium plays key signaling roles in the CNS including gene transcription induction and neuronal plasticity as well as neuronal excitability and apoptosis [80, 89–91]. Intracellular sources of Ca not only include influx from the extracellular compartment via plasma membrane voltage-gated Ca channels but also through receptor (inositol triphosphate receptor and ryanodine receptor) mediated cytosolic calcium-induced Ca efflux from the ER and Ca channels of mitochondria [81, 92]. The intracellular efflux mechanism includes ER Ca influx via the energy-dependent pump, Ca^2 + ATPase the mediate ER sequestration of Ca [92]. These, together with a set of resident ER Ca buffer systems including calnexin, calsequestrin, and calreticulin, contribute to the maintenance of intracellular Ca homeostasis [92].

Previously, the Aβ hypothesis dominated AD research but increasing evidence suggests that dysregulation of Ca signaling is a major event in the pathophysiology of AD [81, 94]. Regulation of the intraneuronal Ca concentration is very precise in concentration and time with a fine line between normal and abnormal concentrations. Normal Ca levels maintain physiological function but abnormal concentrations lead to neuronal demise and loss in designated areas of cognitive and memory functions in AD, especially in the neocortex (frontal, parietal, and temporal), hippocampus, and amygdala [79].

MITOCHONDRIAL DNA DAMAGE IN AD

Other than the nucleus (nDNA), mitochondria are the only intraneuronal organelles that constitutively have their own DNA (mtDNA) and encode 13 subunits of the mitochondrial ETC polypeptides [95, 96]. Due to close proximity with the main intraneuronal sources of ROS and lack of protective role of histone, mtDNA is highly sensitive and susceptible to oxidative stress-induced DNA damage since mitochondria can act both as targets of oxidative damage and source of ROS [67]. However, the limited ability of mtDNA to undergo repair and functional restoration offers further vulnerability that renders mtDNA prone to destruction by free radicals [97–99]. This hypothesis is reinforced by the increased accumulation of DNA oxidized adduct, 8-hydroxy-2’-deoxyguanosine (⁸OHdG) of mtDNA than nDNA in AD compared to the control brain [100]. Mutations in AβPP, presenilin-1, and presenilin-2 have been implicated as major risk factors in AD and typically associated with early onset familial AD isoform [77, 101]. However, the apolipoprotein E (APOE) gene is more or less associated with increased genetic risk of late onset sporadic AD, modifying the onset of active AD earlier than anticipated and this involves dysregulation of Aβ accumulation/clearance and or imbalance in cholesterol metabolism [77, 103].

Among several polymorphic risk variants that present neuronal vulnerability to damage and death, the APOE4 (E4) gene is one of the strongest recognized risk factor in AD [101, 105]. Meta-analysis of clinical and autopsy-based studies defining APOE-related AD phenotypes demonstrates an increased risk of AD in individuals with one (E2/E4, E3/E4) or two (E4/E4) copies of the APOE4 allele compared with APOE3 (E3), while APOE2 (E2) is known to be protective [106]. Also oxidative stress is reported to be elevated in individuals who are homozygous for APOE4 compared to APOE3/APOE3 AD patients, suggesting that APOE4 plays a critical role in modulating AD pathology through mitochondrial mechanistic pathways [107]. These modifications include altering lipid metabolism and mobilization, accelerating oxidative processes, altering membrane formation, arborization, and synaptic remodeling, promoting hyperphosphorylation of tau protein and NFT formation as well as induction of neuronal death [101, 108], all of which directly or indirectly involves the mitochondria. Furthermore, APOE4 has been reported to undergo proteolysis and translocation of N-terminal fragment into the mitochondria and subsequently interacts with NFTs, promotes amyloid aggregation, impairs oxidative phosphorylation, and increased ROS [109]. Additionally, mitochondrial enzyme (α-ketoglutarate dehydrogenase complex) activity is reported to be decreased in AD brain and correlates in individuals carrying the APOE4 phenotype [110]. Reduction in brain metabolism prior to the onset of cerebral atrophy in AD has been shown to be independent of pretangles, NFTs, or neuritic plaques, an indication that mitochondrial indices could be an early warning signal in AD pathogenesis [111, 112].

Aβ-INDUCED NEUROTOXICITY IN AD

A defective intraneuronal protein clearance sequel to an apparent ineffective primary ubiquitin-proteasome and or secondary autophagy-lysosome clearance pathways of protein degradation results in Aβ aggregation in AD brains. Several reports suggest that Aβ accumulation mediates its neurodegenerative effects in AD by microglia activation via receptor for advanced glycation end-products and toll-like receptors and subsequently orchestrate microglia-mediated inflammatory response on the one hand and induction of factors that mediate neuronal death on the other (Fig. 1) [8, 113]. These include induction of oxidative stress mediated by reactive species such as NO and ROS; inflammation through the release of proinflammatory cytokines including IL-1β, IL-6, and TNF-α; and chemokines, e.g., IL-8 (Fig. 1) [8, 113].

Inhibition of a mitochondrial enzyme such as Aβ-binding alcohol dehydrogenase (ABAD), which is one of the main enzymes expressed in the mitochondria, by Aβ binding has been implicated in Aβ-induced mitochondrial dysfunction, oxidative stress, and cell death in AD [114–116]. Evidence of this specific intramitochondrial binding has been demonstrated by colocalization of ABAD and Aβ in the mitochondria of AD brains and in the brains of transgenic mice expressing mutant amyloid precursor protein (mAPP) singly or in co-expression with ABAD [114, 115]. This binding and subsequent mitochondrial toxicity has been suggested to be the cause of hippocampal-associated learning and memory insufficiency observed in transgenic mAPP/ABAD mice [115, 117]. Additional support for ABAD-Aβ binding-induced mitochondrial liability was demonstrated in neuronal cell cultures from transgenic mAPP/ABAD mice in an enriched Aβ environment resulting in the release of mitochondrial enzymes such as cytochrome c, lactate dehydrogenase, and induction of oxidative stress, which are indicative of mitochondrial impairment [115].

HYPERPHOSPHORYLATION OF TAU MICROTUBULE-ASSOCIATED PROTEIN IN AD

NFTs found in AD are induced abnormally phosphorylated or hyperphosphorylation of tau microtubule-associated proteins, derivative of protolytic AβPP processing from a high energy mitochondrial derived phosphate molecule, ATP [118]. Glycogen synthase kinase-3β and protein phosphatase 2A are two main serine/threonine kinases and phosphatases that mediate and regulate each other in hyperphosphorylation of neuronal microtubule-associated protein, tau [119]. This results in the loss of microtubule biological function and reorganization of the microfilament by inhibiting microtubule assembly, destabilizing its network, and leading to the formation of NFTs [120–122]. These two enzymes account for a significant amount of tau phosphate activity in the brain and hyperactivity vis-a-vis downregulation of the enzymes resulting in hyperphosphorylation, respectively, and subsequently late onset AD [123, 124]. It has been suggested that inflammation supports the formation of NFTs by tau kinase mediated activation [119, 125]. Additional support to this is shown by the fact that mutation in the microtubule-associated protein tau result in frontotemporal dementia [77].

MITOCHONDRIAL DYNAMICS AND BIOGENESIS IN Aβ/TAU INDUCED MITOCHONDRIAL DYSFUNCTION IN AD

A more interesting mitochondria-AD subject of discussion is energy dependent mitochondrial dynamic and biogenesis which are impaired in Aβ/tau-induced mitochondrial dysfunction in AD resulting in loss of MMP, abnormal NAD⁺/NADH ratio, tubulin cytoskeleton network deformities, and impairment of retrograde axonal transport system [126, 127]. These bioenergetics defects are restored by pharmacologically-mediated microtubule stabilization, decreased degradation, promotion of Aβ clearance, and prevention of apoptosis using Taxol, a tubulin stabilizer by re-establishing microtubule dynamics [127]. Microtubule dynamics and assembly depend on mitochondria distribution, number, and energy supplies that are drastically reduced during defective mitochondrial energy metabolism [128]. Neuronal microtubule growth, maintenance, and functionality including microtubule transport system are accentuated by tau protein [129, 130]. Mitochondrial dysfunction is known to be an early event in the pathogenesis of AD. Previous reports indicate that Aβ-mediated energy metabolic failure promotes increased intraneuronal free tubulin levels and decreased acetylated tubulin suggesting microtubule disruption [127, 131]. Acetylation of tubulin confers stability and restores its functionality. Additionally, interference with the autophagy-lysosome pathway is seen in Aβ-mediated mitochondrial toxicity because clearance of autophagic vacuoles in dysfunctional mitochondria (mitophagy) relies on morphologically stable microtubule network and ATP availability [127]. The consequence of lack of clearance results in intracellular and mitochondria accumulation of Aβ peptides, induced tau phosphorylation, decreased autophagy, reduced mitochondrial and lysosomal mobility, and finally contributes in the pathogenesis of AD [127, 133].

Further, mitochondrial dynamics, a balance of interactive processes of mitochondrial proteins between fission and fusion, are continually regulating mitochondria turnover and mass [82, 134–136]. Mitochondrial biogenesis plays a critical role in cell survival and during this process over 90% of these mitochondrial proteins, encoded in the nucleus and translated in the cytosolic ribosome, are imported into the mitochondria via specialized trans-mitochondrial membrane pathways [82, 134–136]. Accumulation of Aβ, therefore, rather than leading to the generation of new mitochondrial population, instead tilts the equilibrium toward mitochondrial disintegration [83, 137–139]. Additionally, upregulation of mitochondrial fission proteins and simultaneous downregulation of fusion proteins have previously been reported in human AD patients’ brain tissues [138, 139] as well as a neuronal cell line [137], supporting the impairment of mitochondrial dynamics in AD [140]. Additionally, experimental evidence in neuronal cell cultures from AβPP transgenic mice shows that genes and proteins associated with mitochondrial dynamics/biogenesis were regulated in favor of mitochondrial fission compared with the control wild type (WT) cultures, suggesting increased mitochondrial disintegration in the Aβ enriched condition [114]. Fragmentation and morphological abnormalities of mitochondria in AβPP transgenic neuronal cultures were confirmed by transmission electron microscopy [114]. Mitochondrial fission-mediated Fis1 gene and protein were reported to be significantly upregulated, while fusion-associated Mfn1 gene and protein remarkable decreased in AβPP cultures indicative of tipped fission/fusion balance in favor of fission [114].

Three groups of mitochondrial proteins that play critical roles in the mitochondrial biogenesis and dynamics are severely affected. The antioxidant enzymes involved in oxygen metabolism such as superoxide dismutase, catalase, and glutathione; proteins of the ETC of the inner mitochondrial membrane including complexes I–IV, in charge of oxidative phosphorylation and ATP synthesis; and enzymes of the Kreb cycle such as pyruvate dehydrogenase, malate dehydrogenase, and aconitase, responsible for the generation of proton motive force, are all impaired by Aβ binding and inactivation as reported in animal model of AD [82]. These indicate that mitochondrial biogenesis/turnover is impaired and implicated in the pathogenesis of AD.

ASSOCIATION OF MITOCHONDRIA WITH NON-AD DISEASE PATHOLOGIES THAT CAUSE DEMENTIA

AD, nevertheless, could exist entirely in its original form, however it is often co-associated with other diseases, which may stand as individual disease entities, exhibiting neuropathologic changes and clinical signs similar to AD [141]. These diseases include Lewy body disease, hippocampal sclerosis, vascular brain injury, argyrophilic grain disease, and transactivation DNA-binding protein 43 (TDP-43) inclusions [141, 142]. Lewy body disease is recognized as a subset of other diseases such as Parkinson’s disease and dementia due to the presence and accumulation of α-synuclein [143]. α-synuclein, being a mitochondrial lipid component, gives functional activity to mitochondrial inner membrane proteins such as complex I and III of the mitochondrial ETC. It has been demonstrated that complex I and III activities are decreased in mice brain lacking α-synuclein, suggesting that α-synuclein is critical in the maintenance of mitochondrial function [143, 144]. A common association in the brain of patients with AD is the presence of cerebrovascular and vascular brain injury, and the challenge is for neuroscientists to delineate the contribution of these diseases to the cognitive impairment [145]. Interestingly, cerebrovascular insults are commonly associated with defective oxygen metabolism and oxidative stress as a result of extended hypotension and or hypoxia, thereby causing injuries mediated via the mitochondrial pathway [145, 146]. The link between mitochondrial and TDP-43 is seen in the hyperphosphorylation and N-terminal truncation in the protein molecule, which is a hallmark of TDP-43 proteinapathy. Additionally, there are abnormal clusters of mitochondria in spinal motor neurons of sympatometic, dendrites, and axons of TDP-43_prp mice, though the mechanism underlying this mitochondrial abnormality is not well understood [147, 148].

ROLE OF MITOCHONDRIA IN NEURAL RESERVE IN AD

Neural or cognitive reserve in AD explains the susceptibility of different patients to brain damage relative to the degree of the damage and the measure of brain recovery or brain reserve capacity, which includes the viability of neurons, synaptic transmission, and functional brain mass [149]. These are antecedents to the variations in cognitive decline responses of different individual to AD-induced brain pathology. Though the mechanisms underlying cognitive reserve are not fully understood, neural reserve is not unconnected with the peculiar resilience of individual patients to the existing networks mediating cognitive reserve and the ability of others to recruit compensatory mechanisms [150, 151].

Cognitive reserve has been attributed to a well-developed and nourished brain with abundance of synapses and healthy neurons in the sections of the brain responsible for cognition and its interconnections [152]. Neuropathology alone is unlikely to account fully for cognitive ability, but a measure of neuroanatomical and biochemical indices prior to pathology may correlate deficit in cognitive ability [153]. Therefore, structural degeneration such as neuronal and synaptic loss is as crucial as accumulated neuropathology [30]. It has been suggested the plastic reorganization of neurocognitive networks such as dendritic arborization, synaptic remodeling, neurite extension, axonal sprouting, neurogenesis, and synaptogenesis reverses cognitive decline [154]. The role of mitochondria in neural reserve is very critical here in regulation synaptic plasticity and neurogenesis. The mobility of mitochondria between subcellular compartments involved in neuroplasticity including synaptic terminal, dendrites, axons, and cell bodies supplies ATP and NAD⁺ and regulates Ca^2 + and oxygen metabolism, thereby playing a significant role in neural differentiation, neural arborization and outgrowth, neurotransmitter release, and dendritic remodeling [155–157]. Neural stem cell proliferation being proportional to low mitochondrial oxygen consumption and high level of glycolytic activity add additional evidence on the role of mitochondria in neural development [158, 159]. Further, neuronal differentiation has been linked to increase in mitochondrial mass, and inhibition of mitochondrial translation prevents neural differentiation suggesting the involvement of mitochondrial genome and mitochondrial protein synthesis in neuronal differentiation [160].

DIAGNOSIS OF AD

Mitochondrial dysfunction is noted to be a striking feature of AD; however, mitochondrial markers have not been recognized as specific endpoints of AD neuropathology and caution should be exhibited in using those in definitive diagnosis of AD. A definitive diagnosis of AD disease is only possible after death at autopsy, by correlation of clinical indicators with pathological findings of the brain tissue. To arrive at a diagnosis, identification of biomarkers such as intra- and extracellular Aβ lesions, NFT (tau), and neuronal losses in the brain will have to be established and matched with cognitive impairment. These biomarkers give information on the processes that are going on in living human brain including aspects of changes in brain structure and Aβ conformation [5].

Two popular biomedical imaging techniques are used in AD diagnosis. Magnetic resonance imaging is key to measurement of these changes in brain structure over time and can give early indication of the onset of AD. Positron emission tomography (PET) focuses on functional biomarker measurement of Aβ aggregation and is used to observed early changes in the brain and brain energy metabolism in AD using glucose [5]. Measurement of tau, mainly seen in late changes (over 60–65 years) in AD, is currently being developed and has not given very good results previously [5]. These neuropathological findings at autopsy together with clinical signs of cognitive deficit are quite conclusive to arrive at a definitive diagnosis of AD [81, 94].

THERAPEUTIC INTERVENTION IN AD

Mitochondrial therapeutic intervention offers potential treatment and/or a disease-modifying strategy that would be beneficial in the treatment of AD and needs to be vigorously explored. Treatment of AD, presently, provides symptomatic benefit to clinically active AD patients. Current drugs in development are focusing on different aspects of Aβ metabolism in the brain and these processes can be identified in the brain or in cerebrospinal fluid using PET [5]. Preventive trials in presymptomatic subjects who are likely going to develop dementia as a result of AD are currently ongoing using anti-Aβ agents. High-risk individuals such as genetic mutation carriers are especially enrolled in these trials and the focus is on Aβ because it shows up early (40–50 years) in life of potential AD patients [130]. These individuals display brain changes that can be measure early on before clinical signs and symptoms appear [124].

CONCLUSIONS

As medical research on AD swells, increasing evidence suggests that mitochondrial dysfunction plays a critical role in the pathogenesis of AD. Oxidative stress is a known factor in AD induction and may result in increased levels of genetic mutations seen in these patients. Molecular and biochemical analysis of the mitochondrial pathways are launching pads for understanding the mechanisms leading to AD. Similarly, detailed understanding of the mitochondrial basis will be key and very crucial in the development of effective intervention strategies for prediction, identification, prevention, and treatment of AD. Clearly, more research and attention need to be devoted to the mitochondrial pathways to provide solution to unanswered questions in AD research and treatment.