Mitochondrial abnormalities in cybrid cell models of sporadic Alzheimer's disease worsen with passage in culture

Interactions between amyloid, amyloid precursor protein, and mitochondria

Mitochondrial dysfunction and Aβ accumulation are hallmarks of Alzheimer's disease (AD). Decades of research describe a relationship between mitochondrial function and Aβ production. Amyloid precursor protein (APP), of which Aβ is generated from, is found within mitochondria. Studies suggest Aβ can be generated in mitochondria and imported into mitochondria. APP and Aβ alter mitochondrial function, while mitochondrial function alters Aβ production from APP. The role these interactions contribute to AD pathology and progression are unknown. Here, we discuss prior research, the rigor of those studies, and the critical knowledge gaps of relationships between APP, Aβ, and mitochondria.

The Alzheimer's Disease Mitochondrial Cascade Hypothesis: A Current Overview

Viable Alzheimer's disease (AD) hypotheses must account for its age-dependence; commonality; association with amyloid precursor protein, tau, and apolipoprotein E biology; connection with vascular, inflammation, and insulin signaling changes; and systemic features. Mitochondria and parameters influenced by mitochondria could link these diverse characteristics. Mitochondrial biology can initiate changes in pathways tied to AD and mediate the dysfunction that produces the clinical phenotype. For these reasons, conceptualizing a mitochondrial cascade hypothesis is a straightforward process and data accumulating over decades argue the validity of its principles. Alternative AD hypotheses may yet account for its mitochondria-related phenomena, but absent this happening a primary mitochondrial cascade hypothesis will continue to evolve and attract interest.

Targeting cellular batteries for the therapy of neurological diseases

The mitochondria, apart from being known as the cell's "powerhouse," are crucial in the viability of nerve cells. Any damage to these cellular organelles can result in their cellular level dysfunction which includes rapidly multiplying reactive oxygen species (ROS) from the mitochondrial membrane, impaired calcium ion homeostasis, and disturbed mitochondrial dynamics by the formation of permeability transition pore in mitochondria. All these impaired biochemical changes lead to various neurological disorders such as progressive supranuclear palsy (PSP), Parkinson's disease (PD), and Alzheimer's disease (AD). Moreover, impaired mitochondrial functions are particularly prone to damage owing to prolonged lifespan and stretched length of the neurons. At the same time, neurons are highly dependent on ATP, and thus, the mitochondria play a central role in the pathogenesis pertaining to neuronal disorders. Dysfunction in the mitochondria is an early pathological hallmark of neurological disorders, and its early detection with the help of suitable biomarkers can lead to promising treatment in this area. Thus, the drugs which are targeting mitochondrial dysfunctions are the emerging area of research in connection with neurological disorders. This can be evidenced by the great opportunities for mitigation, diagnosis, and treatment of numerous human disorders that entail mitochondrial dysfunction at the nexus of their pathogenesis. Here, we throw light at the mitochondrial pathologies and indications of dysfunctional mitochondria in PD, AD, and PSP. There is also an insight into the possible therapeutic strategies highlighting the need for mitochondria-based medicine and made an attempt for claiming the prerequisite for the therapy of neurological diseases.

Regulation of neuronal bioenergetics as a therapeutic strategy in neurodegenerative diseases

Neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis are a heterogeneous group of debilitating disorders with multifactorial etiologies and pathogeneses that manifest distinct molecular mechanisms and clinical manifestations with abnormal protein dynamics and impaired bioenergetics. Mitochondrial dysfunction is emerging as an important feature in the etiopathogenesis of these age-related neurodegenerative diseases. The prevalence and incidence of these diseases is on the rise with the increasing global population and average lifespan. Although many therapeutic approaches have been tested, there are currently no effective treatment routes for the prevention or cure of these diseases. We present the current status of our knowledge and understanding of the involvement of mitochondrial dysfunction in these diseases and highlight recent advances in novel therapeutic strategies targeting neuronal bioenergetics as potential approach for treating these diseases.

Mitophagy and the Brain

Stress mechanisms have long been associated with neuronal loss and neurodegenerative diseases. The origin of cell stress and neuronal loss likely stems from multiple pathways. These include (but are not limited to) bioenergetic failure, neuroinflammation, and loss of proteostasis. Cells have adapted compensatory mechanisms to overcome stress and circumvent death. One mechanism is mitophagy. Mitophagy is a form of macroautophagy, were mitochondria and their contents are ubiquitinated, engulfed, and removed through lysosome degradation. Recent studies have implicated mitophagy dysregulation in several neurodegenerative diseases and clinical trials are underway which target mitophagy pathways. Here we review mitophagy pathways, the role of mitophagy in neurodegeneration, potential therapeutics, and the need for further study.

Alzheimer's and Parkinson's brain tissues have reduced expression of genes for mtDNA OXPHOS Proteins, mitobiogenesis regulator PGC-1α protein and mtRNA stabilizing protein LRPPRC (LRP130)

We used RNA sequencing (RNA-seq) to quantitate gene expression in total RNA extracts of vulnerable brain tissues from Alzheimer's disease (AD, frontal cortical ribbon) and Parkinson's disease (PD, ventral midbrain) subjects and phenotypically negative control subjects. Paired-end sequencing files were processed with HISAT2 aligner/Cufflinks quantitation against the hg38 human genome. We observed a significant decrease in gene expression of all mtDNA OXPHOS genes in AD and PD tissues. Gene expression of the master mitochondrial biogenesis regulator PGC-1α (PPARGC1A) was significantly reduced in AD; expression of genes for mitochondrial transcription factors A (TFAM) and B1/B2 (TFB1M/TFB2M) were not significantly changed in AD and PD tissues. 2-way ANOVAs showed significant reduction in AD brain Complex I subunits' expressions and nearly significant reductions in PD brain. We found a significant reduction in both AD and PD brain samples of expression of genes for leucine-rich pentatricopeptide repeat containing (LRPPRC, a.k.a. LRP130), a known mtRNA-stabilizing protein. Our findings suggest that AD and PD brain tissues have a reduction in mitochondrial ATP production derived from a reduction of mitobiogenesis and mtRNA stability. If true, increased brain expression of PGC-1α and/or LRPPRC may improve bioenergetics of AD and PD and alter the course of neurodegeneration in both conditions. (201 words).

Mitochondria and Mitochondrial Cascades in Alzheimer's Disease

Decades of research indicate mitochondria from Alzheimer's disease (AD) patients differ from those of non-AD individuals. Initial studies revealed structural differences, and subsequent studies showed functional deficits. Observations of structure and function changes prompted investigators to consider the consequences, significance, and causes of AD-related mitochondrial dysfunction. Currently, extensive research argues mitochondria may mediate, drive, or contribute to a variety of AD pathologies. The perceived significance of these mitochondrial changes continues to grow, and many currently believe AD mitochondrial dysfunction represents a reasonable therapeutic target. Debate continues over the origin of AD mitochondrial changes. Some argue amyloid-β (Aβ) induces AD mitochondrial dysfunction, a view that does not challenge the amyloid cascade hypothesis and that may in fact help explain that hypothesis. Alternatively, data indicate mitochondrial dysfunction exists independent of Aβ, potentially lies upstream of Aβ deposition, and suggest a primary mitochondrial cascade hypothesis that assumes mitochondrial pathology hierarchically supersedes Aβ pathology. Mitochondria, therefore, appear at least to mediate or possibly even initiate pathologic molecular cascades in AD. This review considers studies and data that inform this area of AD research.

New Therapeutics to Modulate Mitochondrial Function in Neurodegenerative Disorders

BACKGROUND

Mitochondrial function and energy metabolism are impaired in neurodegenerative diseases. There is evidence for these functional declines both within the brain and systemically in Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral Sclerosis. Due to these observations, therapeutics targeted to alter mitochondrial function and energy pathways are increasingly studied in pre-clinical and clinical settings.

METHODS

The goal of this article was to review therapies with specific implications on mitochondrial energy metabolism published through May 2016 that have been tested for treatment of neurodegenerative diseases.

RESULTS

We discuss implications for mitochondrial dysfunction in neurodegenerative diseases and how this drives new therapeutic initiatives.

CONCLUSION

Thus far, treatments have achieved varying degrees of success. Further investigation into the mechanisms driving mitochondrial dysfunction and bioenergetic failure in neurodegenerative diseases is warranted.

Aβ42 oligomers impair the bioenergetic activity in hippocampal synaptosomes derived from APP-KO mice

Employing hippocampal synaptosomes from amyloid precursor protein (APP)-deleted mice we analyzed the immediate effects of amyloid beta peptide 42 (Aβ42) peptide in its oligomeric or fibrillar assembly or of soluble amyloid precursor protein alpha (sAPPα) protein on their bioenergetic activity. Upon administration of oligomeric Aβ42 peptide for 30 min we observed a robust decrease both in mitochondrial activity and in mitochondrial membrane potential (MMP). In contrast the respective fibrillary or scrambled peptides showed no effect, indicating that inhibition strictly depends on the oligomerization status of the peptide. Hippocampal synaptosomes from old APP-KO mice revealed a further reduction of their already impaired bioenergetic activity upon incubation with 10 μm Aβ42 peptide. In addition we evaluated the influence of the sAPPα protein on mitochondrial activity of hippocampal synaptosomes derived from young or old APP-KO animals. In neither case 20 nm nor 200 nm sAPPα protein had an effect on mitochondrial metabolic activity. Our findings demonstrate that hippocampal synaptosomes derived from APP-KO mice are a most suitable model system to evaluate the impact of Aβ42 peptide on its bioenergetic activity and to further elucidate the molecular mechanisms underlying the impairments by oligomeric Aβ42 on mitochondrial function. Our data demonstrate that extracellular Aβ42 peptide is taken up into synaptosomes where it immediately attenuates mitochondrial activity.

Mitochondria, Cybrids, Aging, and Alzheimer's Disease

Mitochondrial and bioenergetic function change with advancing age and may drive aging phenotypes. Mitochondrial and bioenergetic changes are also documented in various age-related neurodegenerative diseases, including Alzheimer's disease (AD). In some instances AD mitochondrial and bioenergetic changes are reminiscent of those observed with advancing age but are greater in magnitude. Mitochondrial and bioenergetic dysfunction could, therefore, link neurodegeneration to brain aging. Interestingly, mitochondrial defects in AD patients are not brain-limited, and mitochondrial function can be linked to classic AD histologic changes including amyloid precursor protein processing to beta amyloid. Also, transferring mitochondria from AD subjects to cell lines depleted of endogenous mitochondrial DNA (mtDNA) creates cytoplasmic hybrid (cybrid) cell lines that recapitulate specific biochemical, molecular, and histologic AD features. Such findings have led to the formulation of a "mitochondrial cascade hypothesis" that places mitochondrial dysfunction at the apex of the AD pathology pyramid. Data pertinent to this premise are reviewed.

Relationships Between Mitochondria and Neuroinflammation: Implications for Alzheimer's Disease

Mitochondrial dysfunction and neuroinflammation occur in Alzheimer's disease (AD). The causes of these pathologic lesions remain uncertain, but links between these phenomena are increasingly recognized. In this review, we discuss data that indicate mitochondria or mitochondrial components may contribute to neuroinflammation. While mitochondrial dysfunction could cause neuroinflammation, neuroinflammation could also cause mitochondrial dysfunction. However, based on the systemic nature of AD mitochondrial dysfunction as well as data from experiments we discuss, the former possibility is perhaps more likely. If correct, then manipulation of mitochondria, either directly or through manipulations of bioenergetic pathways, could prove effective in reducing metabolic dysfunction and neuroinflammation in AD patients. We also review some potential approaches through which such manipulations may be achieved.

The Level of ALR is Regulated by the Quantity of Mitochondrial DNA

Augmenter of liver regeneration (ALR) contributes to mitochondrial biogenesis, maintenance and to the physiological operation of mitochondria. The depletion of ALR has been widely studied and had serious consequences on the mitochondrial functions. However the inverse direction, the effect of the depletion of mitochondrial electron transfer chain and mtDNA on ALR expression has not been investigated yet. Thus mtDNA depleted, ρ(0) cell line was prepared to investigate the role of mitochondrial electron transfer chain and mtDNA on ALR expression. The depletion of mtDNA has not caused any difference at mRNA level, but at protein level the expression of ALR has been markedly increased. The regulatory role of ATP and ROS levels could be ruled out because the treatment of the parental cell line with different respiratory inhibitors and uncoupling agent could not provoke any changes in the protein level of ALR. The effect of mtDNA depletion on the protein level of ALR has been proved not to be liver specific, since the phenomenon could be observed in the case of two other, non-hepatic cell lines. It seems the level of mtDNA and/or its products may have regulatory role on the protein level of ALR. The up-regulation of ALR can be a part of the adaptive response in ρ(0) cells that preserves the structural integrity and the transmembrane potential despite the absence of protein components encoded by the mtDNA.

Cytoplasmic hybrid (cybrid) cell lines as a practical model for mitochondriopathies

Cytoplasmic hybrid (cybrid) cell lines can incorporate human subject mitochondria and perpetuate its mitochondrial DNA (mtDNA)-encoded components. Since the nuclear background of different cybrid lines can be kept constant, this technique allows investigators to study the influence of mtDNA on cell function. Prior use of cybrids has elucidated the contribution of mtDNA to a variety of biochemical parameters, including electron transport chain activities, bioenergetic fluxes, and free radical production. While the interpretation of data generated from cybrid cell lines has technical limitations, cybrids have contributed valuable insight into the relationship between mtDNA and phenotype alterations. This review discusses the creation of the cybrid technique and subsequent data obtained from cybrid applications.

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The cytoplasmic hybrid (cybrid) model can be used to determine mitochondrial DNA (mtDNA) contributions to phenotypic alterations.

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Cybrids are used to study mitochondriopathies such as Parkinson’s disease and Alzheimer’s disease.

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mtDNA heteroplasmy threshold and nuclear DNA-mtDNA compatibility can be determined using cybrid models.

Mitochondrial oxygen consumption deficits in skeletal muscle isolated from an Alzheimer's disease-relevant murine model

Background

Age is considered a primary risk factor for neurodegenerative diseases including Alzheimer’s disease (AD). It is also now well understood that mitochondrial function declines with age. Mitochondrial deficits have been previously assessed in brain from both human autopsy tissue and disease-relevant transgenic mice. Recently it has been recognized that abnormalities of muscle may be an intrinsic aspect of AD and might contribute to the pathophysiology. However, deficits in mitochondrial function have yet to be clearly assessed in tissues outside the central nervous system (CNS). In the present study, we utilized a well-characterized AD-relevant transgenic mouse strain to assess mitochondrial respiratory deficits in both brain and muscle. In addition to mitochondrial function, we assessed levels of transgene-derived amyloid precursor protein (APP) in homogenates isolated from brain and muscle of these AD-relevant animals.

Results

We now demonstrate that skeletal muscles isolated from these animals have differential levels of mutant full-length APP depending on muscle type. Additionally, isolated muscle fibers from young transgenic mice (3 months) have significantly decreased maximal mitochondrial oxygen consumption capacity compared to non-transgenic, age-matched mice, with similar deficits to those previously described in brain.

Conclusions

This is the first study to directly examine mitochondrial function in skeletal muscle from an AD-relevant transgenic murine model. As with brain, these deficits in muscle are an early event, occurring prior to appearance of amyloid plaques.

Mitochondrial diseases of the brain

Neurodegenerative disorders are debilitating diseases of the brain, characterized by behavioral, motor and cognitive impairments. Ample evidence underpins mitochondrial dysfunction as a central causal factor in the pathogenesis of neurodegenerative disorders including Parkinson's disease, Huntington's disease, Alzheimer's disease, Amyotrophic lateral sclerosis, Friedreich's ataxia and Charcot-Marie-Tooth disease. In this review, we discuss the role of mitochondrial dysfunction such as bioenergetics defects, mitochondrial DNA mutations, gene mutations, altered mitochondrial dynamics (mitochondrial fusion/fission, morphology, size, transport/trafficking, and movement), impaired transcription and the association of mutated proteins with mitochondria in these diseases. We highlight the therapeutic role of mitochondrial bioenergetic agents in toxin and in cellular and genetic animal models of neurodegenerative disorders. We also discuss clinical trials of bioenergetics agents in neurodegenerative disorders. Lastly, we shed light on PGC-1α, TORC-1, AMP kinase, Nrf2-ARE, and Sirtuins as novel therapeutic targets for neurodegenerative disorders.

Mutations that affect mitochondrial functions and their association with neurodegenerative diseases

Mitochondria are essential for mammalian and human cell function as they generate ATP via aerobic respiration. The proteins required in the electron transport chain are mainly encoded by the circular mitochondrial genome but other essential mitochondrial proteins such as DNA repair genes, are coded in the nuclear genome and require transport into the mitochondria. In this review we summarize current knowledge on the association of point mutations and deletions in the mitochondrial genome that are detrimental to mitochondrial function and are associated with accelerated ageing and neurological disorders including Alzheimer's, Parkinson's, Huntington's and Amyotrophic lateral sclerosis (ALS). Mutations in the nuclear encoded genes that disrupt mitochondrial functions are also discussed. It is evident that a greater understanding of the causes of mutations that adversely affect mitochondrial metabolism is required to develop preventive measures against accelerated ageing and neurological disorders caused by mitochondrial dysfunction.

Bioenergetic flux, mitochondrial mass and mitochondrial morphology dynamics in AD and MCI cybrid cell lines

Bioenergetic dysfunction occurs in Alzheimer's disease (AD) and mild cognitive impairment (MCI), a clinical syndrome that frequently precedes symptomatic AD. In this study, we modeled AD and MCI bioenergetic dysfunction by transferring mitochondria from MCI, AD and control subject platelets to mtDNA-depleted SH-SY5Y cells. Bioenergetic fluxes and bioenergetics-related infrastructures were characterized in the resulting cytoplasmic hybrid (cybrid) cell lines. Relative to control cybrids, AD and MCI cybrids showed changes in oxygen consumption, respiratory coupling and glucose utilization. AD and MCI cybrids had higher ADP/ATP and lower NAD+/NADH ratios. AD and MCI cybrids exhibited differences in proteins that monitor, respond to or regulate cell bioenergetic fluxes including HIF1α, PGC1α, SIRT1, AMPK, p38 MAPK and mTOR. Several endpoints suggested mitochondrial mass increased in the AD cybrid group and probably to a lesser extent in the MCI cybrid group, and that the mitochondrial fission-fusion balance shifted towards increased fission in the AD and MCI cybrids. As many of the changes we observed in AD and MCI cybrid models are also seen in AD subject brains, we conclude reduced bioenergetic function is present during very early AD, is not brain-limited and induces protean retrograde responses that likely have both adaptive and mal-adaptive consequences.

Role of platelets in neurodegenerative diseases: a universal pathophysiology

Platelets play an important role in a variety of disorders, namely, cardiovascular, psychosomatic, psychiatric, thrombosis, HIV/AIDS in addition to various neurodegenerative diseases (NDDs). Recent evidence indicates that platelet react to diverse stressors, thereby offering an interesting vantage point for understanding their potential role in contemporary medical research. This review addresses the possible role of platelets as a systemic probe in various NDDs, such as amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease, Alzheimer's disease, multiple sclerosis, etc. The current review based on published literature, describes a probable link between platelets and pathophysiology of various NDDs. It also discusses how platelets epitomize ultrastructural, morphological, biochemical and molecular changes, highlighting their emerging role as systemic tools in different NDDs.

Mitochondrial metabolism in Parkinson's disease impairs quality control autophagy by hampering microtubule-dependent traffic

Abnormal presence of autophagic vacuoles is evident in brains of patients with Parkinson's disease (PD), in contrast to the rare detection of autophagosomes in a normal brain. However, the actual cause and pathological significance of these observations remain unknown. Here, we demonstrate a role for mitochondrial metabolism in the regulation of the autophagy-lysosomal pathway in ex vivo and in vitro models of PD. We show that transferring mitochondria from PD patients into cells previously depleted of mitochondrial DNA is sufficient to reproduce the alterations in the autophagic system observed in PD patient brains. Although the initial steps of this pathway are not compromised, there is an increased accumulation of autophagosomes associated with a defective autophagic activity. We prove that this functional decline was originated from a deficient mobilization of autophagosomes from their site of formation toward lysosomes due to disruption in microtubule-dependent trafficking. This contributed directly to a decreased proteolytic flux of α-synuclein and other autophagic substrates. Our results lend strong support for a direct impact of mitochondria in autophagy as defective autophagic clearance ability secondary to impaired microtubule trafficking is driven by dysfunctional mitochondria. We uncover mitochondria and mitochondria-dependent intracellular traffic as main players in the regulation of autophagy in PD.

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.

A mitochondrial etiology of Alzheimer and Parkinson disease

BACKGROUND

The genetics and pathophysiology of Alzheimer Disease (AD) and Parkinson Disease (PD) appears complex. However, mitochondrial dysfunction is a common observation in these and other neurodegenerative diseases.

SCOPE OF REVIEW

We argue that the available data on AD and PD can be incorporated into a single integrated paradigm based on mitochondrial genetics and pathophysiology.

MAJOR CONCLUSIONS

Rare chromosomal cases of AD and PD can be interpreted as affecting mitochondrial function, quality control, and mitochondrial DNA (mtDNA) integrity. mtDNA lineages, haplogroups, such haplogroup H5a which harbors the mtDNA tRNA(Gln) A8336G variant, are important risk factors for AD and PD. Somatic mtDNA mutations are elevated in AD, PD, and Down Syndrome and Dementia (DSAD) both in brains and also systemically. AD, DS, and DSAD brains also have reduced mtDNA ND6 mRNA levels, altered mtDNA copy number, and perturbed Aβ metabolism. Classical AD genetic changes incorporated into the 3XTg-AD (APP, Tau, PS1) mouse result in reduced forebrain size, life-long reduced mitochondrial respiration in 3XTg-AD males, and initially elevated respiration and complex I and IV activities in 3XTg-AD females which markedly declines with age.

GENERAL SIGNIFICANCE

Therefore, mitochondrial dysfunction provides a unifying genetic and pathophysiology explanation for AD, PD, and other neurodegenerative diseases. This article is part of a Special Issue entitled Biochemistry of Mitochondria.

The role of mitochondria in neurodegenerative diseases

Mitochondria are implicated in several metabolic pathways including cell respiratory processes, apoptosis, and free radical production. Mitochondrial abnormalities have been documented in neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's diseases, and amyotrophic lateral sclerosis. Several studies have demonstrated that mitochondrial impairment plays an important role in the pathogenesis of this group of disorders. In this review, we discuss the role of mitochondria in the main neurodegenerative diseases and review the updated knowledge in this field.

Mitochondrial dysfunction and Alzheimer's disease

To date, one of the most discussed hypotheses for Alzheimer's disease (AD) etiology implicates mitochondrial dysfunction and oxidative stress as one of the primary events in the course of AD. In this review we focus on the role of mitochondria and mitochondrial DNA (mtDNA) variation in AD and discuss the rationale for the involvement of mitochondrial abnormalities in AD pathology. We summarize the current data regarding the proteins involved in mitochondrial function and pathology observed in AD, and discuss the role of somatic mutations and mitochondrial haplogroups in AD development.

May "mitochondrial eve" and mitochondrial haplogroups play a role in neurodegeneration and Alzheimer's disease?

Mitochondria, the powerhouse of the cell, play a critical role in several metabolic processes and apoptotic pathways. Multiple evidences suggest that mitochondria may be crucial in ageing-related neurodegenerative diseases. Moreover, mitochondrial haplogroups have been linked to multiple area of medicine, from normal ageing to diseases, including neurodegeneration. Polymorphisms within the mitochondrial genome might lead to impaired energy generation and to increased amount of reactive oxygen species, having either susceptibility or protective role in several diseases. Here, we highlight the role of the mitochondrial haplogroups in the pathogenetic cascade leading to diseases, with special attention to Alzheimer's disease.

Aberrant subcellular neuronal calcium regulation in aging and Alzheimer's disease

In this mini-review/opinion article we describe evidence that multiple cellular and molecular alterations in Alzheimer's disease (AD) pathogenesis involve perturbed cellular calcium regulation, and that alterations in synaptic calcium handling may be early and pivotal events in the disease process. With advancing age neurons encounter increased oxidative stress and impaired energy metabolism, which compromise the function of proteins that control membrane excitability and subcellular calcium dynamics. Altered proteolytic cleavage of the β-amyloid precursor protein (APP) in response to the aging process in combination with genetic and environmental factors results in the production and accumulation of neurotoxic forms of amyloid β-peptide (Aβ). Aβ undergoes a self-aggregation process and concomitantly generates reactive oxygen species that can trigger membrane-associated oxidative stress which, in turn, impairs the functions of ion-motive ATPases and glutamate and glucose transporters thereby rendering neurons vulnerable to excitotoxicity and apoptosis. Mutations in presenilin-1 that cause early-onset AD increase Aβ production, but also result in an abnormal increase in the size of endoplasmic reticulum calcium stores. Some of the events in the neurodegenerative cascade can be counteracted in animal models by manipulations that stabilize neuronal calcium homeostasis including dietary energy restriction, agonists of glucagon-like peptide 1 receptors and drugs that activate mitochondrial potassium channels. Emerging knowledge of the actions of calcium upstream and downstream of Aβ provides opportunities to develop novel preventative and therapeutic interventions for AD. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Nerve growth factor attenuates oxidant-induced β-amyloid neurotoxicity in sporadic Alzheimer's disease cybrids

Although mitochondrial dysfunction has been linked to Alzheimer's disease (AD), it is not fully understood how this dysfunction may induce neuronal death. In this study, we show that transmitochondrial hybrid cells (cybrids) expressing mitochondrial genes from patients with sporadic AD (SAD) have substantial alterations in basal upstream tyrosine kinase signaling and downstream serine-threonine kinase signaling that are mediated by intracellular free radicals. This is associated with reduced tropomyocin receptor kinase (TrkA) and p75 neurotrophin receptor receptor expression that profoundly alters nerve growth factor signaling, increases generation of Aβ and decreases viability. Many of these observed effects in SAD cybrids would be predicted to increase risk of premature neuronal death and reduce resistance to stressors and add further support for the pathogenic role of mtDNA expression in the pathogenesis of SAD.

Is there a primary role of the mitochondrial genome in Alzheimer's disease?

The "mitochondrial cascade hypothesis" could explain many of the biochemical, genetic and pathological features of sporadic Alzheimer's disease (AD). Somatic mutations in mitochondrial DNA (mtDNA) could cause energy failure, increased oxidative stress and accumulation of amyloid beta, which in a vicious cycle reinforces mtDNA damage and oxidative stress. Despite the evidence of mitochondrial dysfunction in AD, and despite the cognitive impairment frequently reported in patients with mtDNA mutation, no causative mutation in the mtDNA have been linked to AD. Indeed, results of studies on the role of mtDNA polymorphisms or haplogroups in AD are controversial. In this minireview, we summarize the actual knowledge about the involvement of mtDNA in AD pathology.

Abnormal mitochondrial dynamics--a novel therapeutic target for Alzheimer's disease?

Mitochondria are dynamic organelles that undergo continuous fission and fusion, which could affect all aspects of mitochondrial function. Mitochondrial dysfunction has been well documented in Alzheimer's disease (AD). In the past few years, emerging evidence indicates that an imbalance of mitochondrial dynamics is involved in the pathogenesis of AD. In this review, we discuss in detail the abnormal mitochondrial dynamics in AD and how such abnormal dynamics may impact mitochondrial and neuronal function and contribute to the course of disease. Based on this discussion, we propose that mitochondrial dynamics could be a potential therapeutic target for AD.

Mitochondrial medicine for neurodegenerative diseases

Mitochondrial dysfunction has been reported in a wide array of neurological disorders ranging from neuromuscular to neurodegenerative diseases. Recent studies on neurodegenerative diseases have revealed that mitochondrial pathology is generally found in inherited or sporadic neurodegenerative diseases and is believed to be involved in the pathophysiological process of these diseases. Commonly seen types of mitochondrial dysfunction in neurodegenerative diseases include excessive free radical generation, lowered ATP production, mitochondrial permeability transition, mitochondrial DNA lesions, perturbed mitochondrial dynamics and apoptosis. Mitochondrial medicine as an emerging therapeutic strategy targeted to mitochondrial dysfunction in neurodegenerative diseases has been proven to be of value, though this area of research is still at in its early stage. In this article, we report on recent progress in the development of several mitochondrial therapies including antioxidants, blockade of mitochondrial permeability transition, and mitochondrial gene therapy as evidence that mitochondrial medicine has promise in the treatment of neurodegenerative diseases.

Mitochondrial permeability transition pore in Alzheimer's disease: cyclophilin D and amyloid beta

Amyloid beta (Abeta) plays a critical role in the pathophysiology of Alzheimer's disease. Increasing evidence indicates mitochondria as an important target of Abeta toxicity; however, the effects of Abeta toxicity on mitochondria have not yet been fully elucidated. Recent biochemical studies in vivo and in vitro implicate mitochondrial permeability transition pore (mPTP) formation involvement in Abeta-mediated mitochondrial dysfunction. mPTP formation results in severe mitochondrial dysfunction such as reactive oxygen species (ROS) generation, mitochondrial membrane potential dissipation, intracellular calcium perturbation, decrease in mitochondrial respiration, release of pro-apoptotic factors and eventually cell death. Cyclophilin D (CypD) is one of the more well-known mPTP components and recent findings reveal that Abeta has significant impact on CypD-mediated mPTP formation. In this review, the role of Abeta in the formation of mPTP and the potential of mPTP inhibition as a therapeutic strategy in AD treatment are examined.

Insulin and Insulin-Sensitizing Drugs in Neurodegeneration: Mitochondria as Therapeutic Targets

Insulin, besides its glucose lowering effects, is involved in the modulation of lifespan, aging and memory and learning processes. As the population ages, neurodegenerative disorders become epidemic and a connection between insulin signaling dysregulation, cognitive decline and dementia has been established. Mitochondria are intracellular organelles that despite playing a critical role in cellular metabolism are also one of the major sources of reactive oxygen species. Mitochondrial dysfunction, oxidative stress and neuroinflammation, hallmarks of neurodegeneration, can result from impaired insulin signaling. Insulin-sensitizing drugs such as the thiazolidinediones are a new class of synthetic compounds that potentiate insulin action in the target tissues and act as specific agonists of the peroxisome proliferator-activated receptor gamma (PPAR-γ). Recently, several PPAR agonists have been proposed as novel and possible therapeutic agents for neurodegenerative disorders. Indeed, the literature shows that these agents are able to protect against mitochondrial dysfunction, oxidative damage, inflammation and apoptosis. This review discusses the role of mitochondria and insulin signaling in normal brain function and in neurodegeneration. Furthermore, the potential protective role of insulin and insulin sensitizers in Alzheimer´s, Parkinson´s and Huntington´s diseases and amyotrophic lateral sclerosis will be also discussed.

Adverse stress, hippocampal networks, and Alzheimer's disease

Recent clinical data have implicated chronic adverse stress as a potential risk factor in the development of Alzheimer's disease (AD) and data also suggest that normal, physiological stress responses may be impaired in AD. It is possible that pathology associated with AD causes aberrant responses to chronic stress, due to potential alterations in the hypothalamic-pituitary-adrenal (HPA) axis. Recent study in rodent models of AD suggests that chronic adverse stress exacerbates the cognitive deficits and hippocampal pathology that are present in the AD brain. This review summarizes recent findings obtained in experimental AD models regarding the influence of chronic adverse stress on the underlying cellular and molecular disease processes including the potential role of glucocorticoids. Emerging findings suggest that both AD and chronic adverse stress affect hippocampal neural networks in a similar fashion. We describe alterations in hippocampal plasticity, which occur in both chronic stress and AD including dendritic remodeling, neurogenesis, and long-term potentiation. Finally, we outline potential roles for oxidative stress and neurotrophic factor signaling as the key determinants of the impact of chronic stress on the plasticity of neural networks and AD pathogenesis.

The Alzheimer's disease mitochondrial cascade hypothesis: an update

In 2004 we proposed the mitochondrial cascade hypothesis of sporadic Alzheimer's disease (AD). Our hypothesis assumed sporadic and autosomal dominant AD are not etiologically homogeneous, considered evidence that AD pathology is not brain-limited, and incorporated aging theory. The mitochondrial cascade hypothesis asserted: (1) inheritance determines mitochondrial baseline function and durability; (2) mitochondrial durability influences how mitochondria change with age; and (3) when mitochondrial change reaches a threshold, AD histopathology and symptoms ensue. We now review the reasoning used to formulate the hypothesis, discuss pertinent interim data, and update its tenants. Readers are invited to consider the conceptual strengths and weaknesses of this hypothesis.

Mitochondria, cognitive impairment, and Alzheimer's disease

To date, the beta amyloid (Abeta) cascade hypothesis remains the main pathogenetic model of Alzheimer's disease (AD), but its role in the majority of sporadic AD cases is unclear. The "mitochondrial cascade hypothesis" could explain many of the biochemical, genetic, and pathological features of sporadic AD. Somatic mutations in mitochondrial DNA (mtDNA) could cause energy failure, increased oxidative stress, and accumulation of Abeta, which in a vicious cycle reinforce the mtDNA damage and the oxidative stress. Despite the evidence of mitochondrial dysfunction in AD, no causative mutations in the mtDNA have been detected so far. Indeed, results of studies on the role of mtDNA haplogroups in AD are controversial. In this review we discuss the role of the mitochondria, and especially of the mtDNA, in the cascade of events leading to neurodegeneration, dementia, and AD.

Stable and nondisruptive in vitro/in vivo labeling of mesenchymal stem cells by internalizing quantum dots

Progress in stem cell research has prioritized the refinement of cell-labeling techniques for in vitro and in vivo basic and therapeutic studies. Although quantum dots, because of their optical properties, are emerging as favorable nanoparticles for bioimaging, substantial refinements or modifications that would improve their biocompatibility are still required. We report here that internalizing quantum dots (i-QDs) generated by their conjugation with an internalizing antibody against a heat shock protein-70 family stress chaperone, mortalin, offered an efficient, genetically noninvasive, nontoxic, and functionally inert way to label mesenchymal stem cells (MSCs). The i-QD-labeled MSCs underwent normal adipocyte, osteocyte, and chondrocyte differentiation in vitro and in vivo, suggesting the potential application of i-QDs in in vivo diagnostics, regenerative and therapeutic medicine.

Mitochondrial DNA sequence variation and neurodegeneration

Mitochondria, the powerhouse of the cell, play a critical role in several metabolic processes and apoptotic pathways. Many lines of evidence suggest that mitochondria have a central role in ageing-related neurodegenerative diseases. Moreover, there is a long history of investigations on mitochondria aimed at identifying genetic markers relating to ageing and neurodegenerative diseases. In this review, some of the major neurodegenerative disorders are highlighted and the role of mitochondrial haplogroups in the pathogenetic cascade leading to these diseases is discussed.

Oxidative stress signaling in Alzheimer's disease

Multiple lines of evidence demonstrate that oxidative stress is an early event in Alzheimer's disease (AD), occurring prior to cytopathology, and therefore may play a key pathogenic role in AD. Oxidative stress not only temporally precedes the pathological lesions of the disease but also activates cell signaling pathways, which, in turn, contribute to lesion formation and, at the same time, provoke cellular responses such as compensatory upregulation of antioxidant enzymes found in vulnerable neurons in AD. In this review, we provide an overview of the evidence of oxidative stress and compensatory responses that occur in AD, particularly focused on potential sources of oxidative stress and the roles and mechanism of activation of stress-activated protein kinase pathways.

A mitochondrial etiology of neurodegenerative diseases: evidence from Parkinson's disease

Evidence continues to accrue implicating mitochondrial dysfunction in the etiology of a number of neurodegenerative diseases. For example, Parkinson's disease (PD) can be induced by mitochondrial toxins, and nuclear DNA (nDNA) loci linked to PD have been associated with mitochondrial dysfunction. Although conclusions about the role of mitochondrial DNA (mtDNA) variants in PD vary, we argue here that this is attributable to the novel genetics of the mtDNA and the fact that clinically relevant mtDNA variation encompasses ancient adaptive polymorphisms, recent deleterious mutations, and somatic mutations. An mtDNA association with PD is supported by an analysis of the Russian Tatar population which revealed that polymorphisms associated with haplogroup H mtDNAs increased PD risk (odds ratio [OR]= 2.58, P= 0.0001), whereas those associated with haplogroup UK cluster mtDNAs were protective (OR = 0.38, P= 0.003). Moreover, mtDNA sequencing revealed that PD patients with either haplogroup H or UK cluster mtDNAs can harbor additional recent variants that might further modulate PD risk. Therefore, the complexity of PD genetics may reflect the complex mitochondrial genetics.

Protein stability and aggregation in Parkinson's disease

Parkinson's disease (PD), the second most common age-related neurodegenerative disease, results in abnormalities in motor functioning. Many fundamental questions regarding its aetiology remain unanswered. Pathologically, it is not until 70-80% of the dopaminergic neurons from the substantia nigra pars compacta are lost before clinical symptoms are observed. Thus research into PD is complicated by this apparent paradox in that what appears to be the beginning of the disease at the clinical level is really the end point neurochemically. Consequently, we can only second guess when the disease started and what initiated it. The causation is probably complex, with contributions from both genetic and environmental factors. Intracellular proteinaceous inclusions, Lewy bodies and Lewy neurites, found in surviving dopaminergic neurons, are the key pathological characteristic of PD. Their presence points to an inability within these terminally differentiated cells to deal with aggregating proteins. Recent advances in our knowledge of the underlying disease process have come about from studies on models based on genes associated with rare hereditary forms of PD, and mitochondrial toxins that mimic the behavioural effects of PD. The reason that dopaminergic neurons are particularly sensitive may be due to the additional cellular stress caused by the breakdown of the inherently chemically unstable neurotransmitter, dopamine. In the present review, I discuss the proposal that in sporadic disease, interlinked problems of protein processing and inappropriate mitochondrial activity seed the foundation for age-related increased levels of protein damage, and a reduced ability to deal with the damage, leading to inclusion formation and, ultimately, cell toxicity.

Mitochondria in cybrids containing mtDNA from persons with mitochondriopathies

The cytoplasmic hybrid (cybrid) technique allows investigators to express selected mitochondrial DNA (mtDNA) sequences against fixed nuclear DNA (nDNA) backgrounds. Cybrids have been used to study the effects of known mtDNA mutations on mitochondrial biochemistry, mtDNA-nDNA inter-species compatibility, and mtDNA integrity in persons without mtDNA mutations defined previously. This review discusses events leading up to creation of the cybrid technique, as well as data obtained via application of the cybrid strategies listed above. Although interpreting cybrid data requires awareness of technique limitations, valuable insights into mtDNA genotype-functional phenotype relationships are suggested.

Treating neurodegeneration by modifying mitochondria: potential solutions to a "complex" problem

Mitochondria function differently in aged brains than they do in young brains. Consistently reported changes include reduced electron transport chain (ETC) enzyme activities, reduced phosphorylation of ADP, and increased reactive oxygen species (ROS) production. Various neurodegenerative diseases are also associated with changes in mitochondrial function, and these changes both recapitulate and extend those seen in "normal" aging. Unfortunately, attempts to treat neurodegenerative diseases by treating mitochondria-related pathology have thus far minimally impacted affected patients. A better understanding of how mitochondrial function changes in aging and neurodegenerative diseases, though, now suggests new approaches to mitochondrial therapy may prove more efficacious. Increasing ETC capacity, increasing oxidative phosphorylation, or decreasing mitochondrial ROS may yet prove useful for the treatment of brain aging and neurodegenerative diseases, and accomplishing this seems increasingly feasible. This review will discuss the role of mitochondrial function and dysfunction in aging and neurodegenerative diseases, and will focus on potential treatment strategies.

Mitochondrial cascade hypothesis of Alzheimer's disease: myth or reality?

Mitochondria are recognized to play a pivotal role in neuronal cell survival or death because they are regulators of both energy metabolism and apoptotic pathways. Morphologic, biochemical, and molecular genetic studies suggest that mitochondria might be a convergence point for neurodegeneration, including Alzheimer's disease (AD). The functions and properties of mitochondria might render subsets of selectively vulnerable neurons intrinsically susceptible to cellular aging and stress. However, the question, "Is mitochondrial dysfunction a necessary step in neurodegeneration?" is still unanswered. This review presents the ways in which malfunctioning mitochondria and oxidative stress might contribute to neuronal death in AD.

Chronic exposure to sub-lethal beta-amyloid (Abeta) inhibits the import of nuclear-encoded proteins to mitochondria in differentiated PC12 cells

Studies on amyloid beta (Abeta|), the peptide thought to play a crucial role in the pathogenesis of Alzheimer's disease, have implicated mitochondria in Abeta-mediated neurotoxicity. We used differentiated PC12 cells stably transfected with an inducible green fluorescent protein (GFP) fusion protein containing an N'-terminal mitochondrial targeting sequence (mtGFP), to examine the effects of sub-lethal Abeta on the import of nuclear-encoded proteins to mitochondria. Exposure to sub-lethal Abeta(25-35) (10 mumol/L) for 48 h inhibited mtGFP import to mitochondria; average rates decreased by 20 +/- 4%. Concomitant with the decline in mtGFP, cytoplasmic mtGFP increased significantly while mtGFP expression and intramitochondrial mtGFP turnover were unchanged. Sub-lethal Abeta(1-42) inhibited mtGFP import and increased cytoplasmic mtGFP but only after 96 h. The import of two endogenous nuclear-encoded mitochondrial proteins, mortalin/mtHsp70 and Tom20 also declined. Prior to the decline in import, mitochondrial membrane potential (mmp), and reactive oxygen species levels were unchanged in Abeta-treated cells versus reverse phase controls. Sustained periods of decreased import were associated with decreased mmp, increased reactive oxygen species, increased vulnerability to oxygen-glucose deprivation and altered mitochondrial morphology. These findings suggest that an Abeta-mediated inhibition of mitochondrial protein import, and the consequent mitochondrial impairment, may contribute to Alzheimer's disease.