A mitochondrial bottleneck hypothesis of Alzheimer's disease

Dysfunction of Mitochondria in Alzheimer’s Disease: ANT and VDAC Interact with Toxic Proteins and Aid to Determine the Fate of Brain Cells

Alzheimer’s disease (AD), certainly the most widespread proteinopathy, has as classical neuropathological hallmarks, two groups of protein aggregates: senile plaques and neurofibrillary tangles. However, the research interest is rapidly gaining ground in a better understanding of other pathological features, first, of all the mitochondrial dysfunctions. Several pieces of evidence support the hypothesis that abnormal mitochondrial function may trigger aberrant processing of amyloid progenitor protein or tau and thus neurodegeneration. Here, our aim is to emphasize the role played by two ‘bioenergetic’ proteins inserted in the mitochondrial membranes, inner and outer, respectively, that is, the adenine nucleotide translocator (ANT) and the voltage-dependent anion channel (VDAC), in the progression of AD. To perform this, we will magnify the ANT and VDAC defects, which are measurable hallmarks of mitochondrial dysfunction, and collect all the existing information on their interaction with toxic Alzheimer’s proteins. The pathological convergence of tau and amyloid β-peptide (Aβ) on mitochondria may finally explain why the therapeutic strategies used against the toxic forms of Aβ or tau have not given promising results separately. Furthermore, the crucial role of ANT-1 and VDAC impairment in the onset/progression of AD opens a window for new therapeutic strategies aimed at preserving/improving mitochondrial function, which is suspected to be the driving force leading to plaque and tangle deposition in AD.

Mitochondrial targeting of potent nanoparticulated drugs in combating diseases

Mitochondrial dysfunction, characterized by the electron transport chain (ETC) leakage and reduced adenosine tri-phosphate synthesis, occurs primarily due to free radicals -induced mutations in either the mitochondrial deoxyribonucleic acid (mtDNA) or nuclear (n) DNA caused by pathogenic infections, toxicant exposures, adverse drug-effects, or other environmental exposures, leading to secondary dysfunction affecting ischemic, diabetic, cancerous, and degenerative diseases. In these concerns, mitochondria-targeted remedies may include a significant role in the protection and treatment of mitochondrial function to enhance its activity. Coenzyme Q10 pyridinol and pyrimidinol antioxidant analogues and other potent drug-compounds for their multifunctional radical quencher and other anti-toxic activities may take a significant therapeutic effectivity for ameliorating mitochondrial dysfunction. Moreover, the encapsulation of these bioactive ligands-attached potent compounds in vesicular system may enable them a superb biological effective for the treatment of mitochondria-targeted dysfunction-related diseases with least side effects. This review depicts mainly on mitochondrial enzymatic dysfunction and their amelioration by potent drugs with the usages of nanoparticulated delivery system against mitochondria-affected diseases.

Naringin Exhibits Mas Receptor-Mediated Neuroprotection Against Amyloid Beta-Induced Cognitive Deficits and Mitochondrial Toxicity in Rat Brain

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with clinical manifestation of loss in cognitive functions in an individual. Though several drug candidates have been developed in the management of AD, an alternative option is still required due to serious adverse effects of the former. Recently, naringin exerts therapeutic benefits through rennin angiotensin system in experimental animals. However, its report  on Mas receptor-mediated action against amyloid beta (Aβ)-induced mitochondrial dysfunction in AD-like animals is lacking. The experimental dementia was induced in the male rats by intracerebroventricular administration of Aβ_(1-42) on day 1 (D-1) of the experimental schedule of 14 days. Naringin treatment for 14 days attenuated Aβ-induced cognitive impairments of the animals in Morris water maze (MWM) and Y-maze tests. Further, naringin ameliorated the Aβ-induced cholinergic dysfunction in terms of decrease in the activity of choline acetyl transferase (ChAT) and level of acetylcholine (ACh) and increase in the activity of acetylcholine esterase (AChE) in rat hippocampus, prefrontal cortex, and amygdala. Furthermore, naringin attenuated Aβ-induced decrease in mitochondrial function, integrity, and bioenergetics in all the brain regions. Naringin also attenuated Aβ-induced increase in mitochondrial and cytosolic calcium level in all the brain regions. Moreover, naringin reversed Aβ-induced increase in apoptosis and level of mitochondrial calcium uniporter and decrease in the level of hemeoxygenase-1 in all the brain regions. On the contrary, A779 significantly abolished the therapeutic potential of naringin on Aβ-induced alteration in behavioral, biochemical, and molecular observations in these experimental animals. Thus, these observations indicate that naringin could be potential alternative in the management of AD.

Mitochondria as a promising target for developing novel agents for treating Alzheimer's disease

The mitochondria-targeting drugs can be conventionally divided into the following groups: those compensating for the energy deficit involved in neurodegeneration, including stimulants of mitochondrial bioenergetics and activators of mitochondrial biogenesis; and neuroprotectors, that are compounds increasing the resistance of mitochondria to opening of mitochondrial permeability transition (MPT) pores. Although compensating for the energy deficit and inhibition of MPT are obvious targets for drugs used in the very early stages of Alzheimer-like pathology, but their use as the monotherapy for patients with severe symptoms is unlikely to be sufficiently effective. It would be optimal to combine targets that would provide the cognitive-stimulating, the neuroprotective effects and the ability to affect specific disease-forming mechanisms. In the design of such drugs, assessment of their potential mitochondrial-targeted effects is of particular importance. The possibility of targeted drug design for simultaneous action on mitochondrial and neurotransmitter's receptors targets is, in particularly, based on the known interplay of various cellular pathways and the presence of common structural components. Of particular interest is directed search for multitarget drugs that would act simultaneously on mitochondrial calcium-dependent functions, the targets (receptors, enzymes, etc.) facilitating neurotransmission, and the molecular targets related to the action of so-called disease-modifying factors, in particular, the formation and overcoming of the toxicity of β-amyloid or hyperphosphorylated tau protein. The examples of such approaches realized on the level of preclinical and clinical trials are presented below.

Central and Peripheral Metabolic Defects Contribute to the Pathogenesis of Alzheimer's Disease: Targeting Mitochondria for Diagnosis and Prevention

Significance: Epidemiological studies indicate that metabolic disorders are associated with an increased risk for Alzheimer's disease (AD). Metabolic remodeling occurs in the central nervous system (CNS) and periphery, even in the early stages of AD. Mitochondrial dysfunction has been widely accepted as a molecular mechanism underlying metabolic disorders. Therefore, focusing on early metabolic changes, especially from the perspective of mitochondria, could be of interest for early AD diagnosis and intervention. Recent Advances: We and others have identified that the levels of several metabolites are fluctuated in the periphery before their accumulation in the CNS, which plays an important role in the pathogenesis of AD. Mitochondrial remodeling is likely one of the earliest signs of AD, linking nutritional imbalance to cognitive deficits. Notably, by improving mitochondrial function, mitochondrial nutrients efficiently rescue cellular metabolic dysfunction in the CNS and periphery in individuals with AD. Critical Issues: Peripheral metabolic disorders should be intensively explored and evaluated for the early diagnosis of AD. The circulating metabolites derived from mitochondrial remodeling represent novel potential diagnostic biomarkers for AD that are more readily detected than CNS-oriented biomarkers. Moreover, mitochondrial nutrients provide a promising approach to preventing and delaying AD progression. Future Directions: Abnormal mitochondrial metabolism in the CNS and periphery is involved in AD pathogenesis. More clinical studies provide evidence for the suitability and reliability of circulating metabolites and cytokines for the early diagnosis of AD. Targeting mitochondria to rewire cellular metabolism is a promising approach to preventing AD and ameliorating AD-related metabolic disorders.

A review on mitochondrial restorative mechanism of antioxidants in Alzheimer's disease and other neurological conditions

Neurodegenerative diseases are intricate in nature because of the involvement of the multiple pathophysiological events including mitochondrial dysfunction, neuroinflammation and oxidative stress. Alzheimer’s disease (AD) is a neurodegenerative disease explained by extracellular amyloid β deposits, intracellular neurofibrillary tangles and mitochondrial dysfunction. Increasing evidence has indicated that mitochondrial dysfunction displays significant role in the pathophysiological processes of AD. Mitochondrial dysfunction involves alterations in mitochondrial respiratory enzyme complex activities, oxidative stress, opening of permeability transition pore, and enhanced apoptosis. Various bioenergetics and antioxidants have been tried or under different investigational phase against AD and other neurodegenerative disorders (Parkinson’s disease, Huntington’s disease, and Amyotrophic lateral sclerosis) because of their complex and multiple site of action. These mitochondrial-targeting bioenergetics and antioxidant compounds such as coenzyme Q10, idebenone, creatine, mitoQ, mitovitE, MitoTEMPOL, latrepirdine, methylene blue, triterpenoids, SS peptides, curcumin, Ginkgo biloba, and omega-3 polyunsaturated fatty acids with potential efficacy in AD have been identified. Present review is intent to discuss mitochondrial restorative mechanisms of these bioenergetics and antioxidants as a potential alternative drug strategy for effective management of AD.

Dissecting Complex and Multifactorial Nature of Alzheimer's Disease Pathogenesis: a Clinical, Genomic, and Systems Biology Perspective

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by loss of memory and other cognitive functions. AD can be classified into familial AD (FAD) and sporadic AD (SAD) based on heritability and into early onset AD (EOAD) and late onset AD (LOAD) based on age of onset. LOAD cases are more prevalent with genetically complex architecture. In spite of significant research focused on understanding the etiological mechanisms, search for diagnostic biomarker(s) and disease-modifying therapy is still on. In this article, we aim to comprehensively review AD literature on established etiological mechanisms including role of beta-amyloid and apolipoprotein E (APOE) along with promising newer etiological factors such as epigenetic modifications that have been associated with AD suggesting its multifactorial nature. As genomic studies have recently played a significant role in elucidating AD pathophysiology, a systematic review of findings from genome-wide linkage (GWL), genome-wide association (GWA), genome-wide expression (GWE), and epigenome-wide association studies (EWAS) was conducted. The availability of multi-dimensional genomic data has further coincided with the advent of computational and network biology approaches in recent years. Our review highlights the importance of integrative approaches involving genomics and systems biology perspective in elucidating AD pathophysiology. The promising newer approaches may provide reliable means of early and more specific diagnosis and help identify therapeutic interventions for LOAD.

Tau facilitates Aβ-induced loss of mitochondrial membrane potential independent of cytosolic calcium fluxes in mouse cortical neurons

Alzheimer's disease (AD) is defined by presence of two pathological hallmarks, the intraneuronal neurofibrillary tangle (NFT) formed by abnormally processed tau, and the extracellular amyloid plaques formed primarily by the amyloid beta peptide (Aβ). In AD it is likely that these two proteins act in concert to impair neuronal function, and there is evidence to suggest that one of the key targets on which they converge is the mitochondria. For example, overexpression of a pathologic form of tau in rat primary cortical neurons exacerbates Aβ-induced mitochondrial membrane potential (ΔΨm) loss due to impairment of the calcium (Ca(2+)) buffering capability of mitochondria. However the role of physiological levels of tau in mediating Aβ-induced mitochondrial dysfunction was not examined. Therefore in this present study we used primary neurons from wild type (WT) and tau knockout (tau(-/-)) mice to investigate whether endogenous tau facilitates Aβ-induced ΔΨm loss and alterations in cytosolic calcium (Ca(2+)cyt). Knocking out tau significantly protected mouse primary cortical neurons from loss of ΔΨm caused by low concentrations of Aβ42, which supports our previous findings. However, the absence of tau resulted in significantly greater increases in Ca(2+)cyt in response to Aβ treatment when compared to those observed in WT mouse primary cortical neurons. This unexpected outcome may be explained by findings that suggest tau(-/-) neurons display certain phenotypic abnormalities associated with alterations in Ca(2+)cyt. Overall, data indicate that tau facilitates Aβ-induced mitochondrial dysfunction and this effect is independent of Aβ-induced alterations in Ca(2+)cyt.(1).

Mitochondria and cell bioenergetics: increasingly recognized components and a possible etiologic cause of Alzheimer's disease

SIGNIFICANCE

Mitochondria and brain bioenergetics are increasingly thought to play an important role in Alzheimer's disease (AD).

RECENT ADVANCES

Data that support this view are discussed from the perspective of the amyloid cascade hypothesis, which assumes beta-amyloid perturbs mitochondrial function, and from an opposite perspective that assumes mitochondrial dysfunction promotes brain amyloidosis. A detailed review of cytoplasmic hybrid (cybrid) studies, which argue mitochondrial DNA (mtDNA) contributes to sporadic AD, is provided. Recent AD endophenotype data that further suggest an mtDNA contribution are also summarized.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Biochemical, molecular, cybrid, biomarker, and clinical data pertinent to the mitochondria-bioenergetics-AD nexus are synthesized and the mitochondrial cascade hypothesis, which represents a mitochondria-centric attempt to conceptualize sporadic AD, is discussed.

Alzheimer's disease: diagnostics, prognostics and the road to prevention

Alzheimer's disease (AD) presents one of the leading healthcare challenges of the 21st century, with a projected worldwide prevalence of >107 million cases by 2025. While biomarkers have been identified, which may correlate with disease progression or subtype for the purpose of disease monitoring or differential diagnosis, a biomarker for reliable prediction of late onset disease risk has not been available until now. This deficiency in reliable predictive biomarkers, coupled with the devastating nature of the disease, places AD at a high priority for focus by predictive, preventive and personalized medicine. Recent data, discovered using phylogenetic analysis, suggest that a variable length poly-T sequence polymorphism in the TOMM40 gene, adjacent to the APOE gene, is predictive of risk of AD age-of-onset when coupled with a subject's current age. This finding offers hope for reliable assignment of disease risk within a 5-7 year window, and is expected to guide enrichment of clinical trials in order to speed development of preventative medicines.

Genetic basis of Alzheimer's dementia: role of mtDNA mutations

Alzheimer's disease (AD) is the most common neurodegenerative disorder associated to dementia in late adulthood. Amyloid precursor protein, presenilin 1 and presenilin 2 genes have been identified as causative genes for familial AD, whereas apolipoprotein E epsilon4 allele has been associated to the risk for late onset AD. However, mutations on these genes do not explain the majority of cases. Mitochondrial respiratory chain (MRC) impairment has been detected in brain, muscle, fibroblasts and platelets of Alzheimer's patients, indicating a possible involvement of mitochondrial DNA (mtDNA) in the aetiology of the disease. Several reports have identified mtDNA mutations in Alzheimer's patients, suggesting the existence of related causal factors probably of mtDNA origin, thus pointing to the involvement of mtDNA in the risk contributing to dementia, but there is no consensual opinion in finding the cause for impairment. However, mtDNA mutations might modify age of onset, contributing to the neurodegenerative process, probably due to an impairment of MRC and/or translation mechanisms.

Nicotine protects rat brain mitochondria against experimental injuries

Epidemiological studies have reported that cigarette smoking may protect from neurodegenerative diseases such as Parkinson's disease. These protective effects are thought to be mediated by nicotine. Recent data showed that nicotine significantly decreases respiratory control ratio (RCR) and superoxide anion generation of brain mitochondria. Thus, we investigated nicotine effects on rat brain in two experimental models: first, an in vitro anoxia/reoxygenation experiment and secondly, an in vivo rotenone-induced Parkinson-like syndrome. Anoxia/reoxygenation impaired mitochondrial respiration by 43.68% whereas in the presence of nicotine, it was less impaired, by 31.1% at 10(-7) M. In rats chronically administered rotenone (3 mg/kg/day), we observed profound mitochondrial damage: the RCR decreased by 50.36% and the superoxide anion generation and the membrane anisotropy increased by 56.03 and 13.43%, respectively. All of these indications of mitochondrial damage were limited by chronic administration of nicotine. Nicotine developed mitochondrial effects in vivo and in vitro at very low concentration. All these results were in accordance with epidemiological studies, which report a protective effect of nicotine in neurodegenerative diseases. Thus, we propose that one effect of nicotine is to preserve mitochondrial functions of the rat central nervous system.

Down-regulation of H tau 40 protein expression by minor groove binders

The DNA minor groove binders SN6999, SN6570, and SN6113, structurally related to netropsin and distamycin, were investigated for sequence-specific interactions with the 154 base pair cDNA fragment of the human tau 40 protein, involved in pathology of Alzheimer's disease. Footprinting results indicated that both the former compounds displayed a pronounced AT-preference, while the latter SN-derivative bound to DNA in a non-sequence specific manner. The influence of these ligands on the protein synthesis was analysed using monoclonal antibodies against h tau protein. Both sequence specific binders markedly impeded protein synthesis. The non-specific binder, however, did not affect protein biosynthesis.

An evaluation of the role of mitochondria in neurodegenerative diseases: mitochondrial mutations and oxidative pathology, protective nuclear responses, and cell death in neurodegeneration

There is mounting evidence for mitochondrial involvement in neurodegenerative diseases including Alzheimer's and Parkinson's disease and amyotrophic lateral sclerosis. Mitochondrial DNA mutations, whether inherited or acquired, lead to impaired electron transport chain (ETC) functioning. Impaired electron transport, in turn, leads to decreased ATP production, formation of damaging free-radicals, and altered calcium handling. These toxic consequences of ETC dysfunction lead to further mitochondrial damage including oxidation of mitochondrial DNA, proteins, and lipids, and opening of the mitochondrial permeability transition pore, an event linked to cell death in numerous model systems. Although protective nuclear responses such as antioxidant enzymes and bcl-2 may be induced to combat these pathological changes, such a vicious cycle of increasing oxidative damage may insidiously damage neurons over a period of years, eventually leading to neuronal cell death. This hypothesis, a synthesis of the mitochondrial mutations and oxidative stress hypotheses of neurodegeneration, is readily tested experimentally, and clearly points out many potential therapeutic targets for preventing or ameliorating these diseases.

Modification of swelling-contraction-aggregation processes in rat muscle mitochondria by the 1,4-dihydropyridines, cerebrocrast and glutapyrone, themselves and in the presence of azidothymidine

The influence of the 1,4-dihydropyridines (DHPs), water-soluble glutapyrone available as sodium, potassium and ammonium salts of 2-(2,6-dimethyl-3,5-diethoxycarbonyl-1,4-DHP-4-carboxamide)glutaric acid, from one side, and a lipophylic cerebrocrast, 2-propoxyethyl 2,6-dimethyl-4-(2-difluoromethoxyphenyl)-1,4-DHP-3,5-dicarboxylate, from the other side, on partially damaged mitochondria of the Wistar rat hindlimb muscle was also studied. The following tests were made: (1) rates of endogenous respiration and substrate (succinate) oxidation and oxidative phosphorylation; (2) rates and amplitudes of high-amplitude swelling and contraction after the addition of ATP, ADP and succinate to the previously swollen mitochondria and (3) rate of reversible self-aggregation of mitochondria isolated in salt media after ATP-induced contraction without and in the presence of azidothymidine (AZT). Cerebrocrast (10-100 microM) partially normalized the endogenous respiration rate and slightly augmented the respiration rate after the addition of succinate and to lesser extent ADP. Cerebrocrast in a concentration-dependent manner (2.5-50 microM) increased (two-fold at 20-50 microM) the active contraction amplitude of swollen mitochondria, induced by single or repeated additions of ATP. The influence of cerebrocrast on the ADP- and succinate-induced contractions was less obvious. Unlike cerebrocrast glutapyrone caused a reduction of the ATP-induced contraction amplitude (two-fold at 0.5-5.0 mM), not impairing the mitochondrial contraction ability in response to ATP or succinate. Pre-exposure to 2.5 mM glutapyrone resulted in at least a 10-fold inhibition of the reversible aggregation rate in the presence of 99 and 198 microM AZT. The results suggest the usefulness of further study of cerebrocrast and glutapyrone in preventing AZT-induced and some other mitochondrial myopathies.