Mitochondrial loss, dysfunction and altered dynamics in Huntington's disease

The Role of Mitochondrial Impairment in Alzheimer´s Disease Neurodegeneration: The Tau Connection

Accumulative evidence has shown that mitochondrial dysfunction plays a pivotal role in the pathogenesis of Alzheimer's disease (AD). Mitochondrial impairment actively contributes to the synaptic and cognitive failure that characterizes AD. The presence of soluble pathological forms of tau like hyperphosphorylated at Ser396 and Ser404 and cleaved at Asp421 by caspase 3, negatively impacts mitochondrial bioenergetics, transport, and morphology in neurons. These adverse effects against mitochondria health will contribute to the synaptic impairment and cognitive decline in AD. Current studies suggest that mitochondrial failure induced by pathological tau forms is likely the result of the opening of the mitochondrial permeability transition pore (mPTP). mPTP is a mitochondrial mega-channel that is activated by increases in calcium and is associated with mitochondrial stress and apoptosis. This structure is composed of different proteins, where Ciclophilin D (CypD) is considered to be the primary mediator of mPTP activation. Also, new studies suggest that mPTP contributes to Aβ pathology and oxidative stress in AD. Further, inhibition of mPTP through the reduction of CypD expression prevents cognitive and synaptic impairment in AD mouse models. More importantly, tau protein contributes to the physiological regulation of mitochondria through the opening/interaction with mPTP in hippocampal neurons. Therefore, in this paper, we will discuss evidence that suggests an important role of pathological forms of tau against mitochondrial health. Also, we will discuss the possible role of mPTP in the mitochondrial impairment produced by the presence of tau pathology and its impact on synaptic function present in AD.

Pathological mitochondria in neurons and perivascular astrocytic endfeet of idiopathic normal pressure hydrocephalus patients

BACKGROUND

A growing body of evidence suggests that the accumulation of amyloid-β and tau (HPτ) in the brain of patients with the dementia subtype idiopathic normal pressure hydrocephalus (iNPH) is associated with delayed extravascular clearance of metabolic waste. Whether also clearance of intracellular debris is affected in these patients needs to be examined. Hypothetically, defective extra- and intra-cellular clearance of metabolites may be instrumental in the neurodegeneration and dementia characterizing iNPH. This study explores whether iNPH is associated with altered mitochondria phenotype in neurons and astrocytes.

METHODS

Cortical brain biopsies of 9 reference (REF) individuals and 30 iNPH patients were analyzed for subcellular distribution and morphology of mitochondria using transmission electron microscopy. In neuronal soma of REF and iNPH patients, we identified normal, pathological and clustered mitochondria, mitochondria-endoplasmic reticulum contact sites and autophagic vacuoles. We also differentiated normal and pathological mitochondria in pre- and post-synaptic nerve terminals, as well as in astrocytic endfoot processes towards vessels.

RESULTS

We found a high prevalence of pathological mitochondria in neuronal soma and pre- and post-synaptic terminals, as well as increased mitochondrial clustering, and altered number of mitochondria-endoplasmic reticulum contact sites in iNPH. Non-fused autophagic vacuoles were more abundant in neuronal soma of iNPH patients, suggestive of cellular clearance failure. Moreover, the length of postsynaptic densities was reduced in iNPH, potentially related to reduced synaptic activity. In astrocytic endfoot processes, we also found increased number, area and area fraction of pathological mitochondria in iNPH patients. The proportion of pathological mitochondria correlated significantly with increasing degree of astrogliosis and reduced perivascular expression of aquaporin-4 (AQP4), assessed by light microscopy immunohistochemistry.

CONCLUSION

Our results provide evidence of mitochondrial pathology and signs of impaired cellular clearance in iNPH patients. The results indicate that iNPH is a neurodegenerative disease with close similarity to Alzheimer's disease.

Integrative Characterization of the R6/2 Mouse Model of Huntington's Disease Reveals Dysfunctional Astrocyte Metabolism

Huntington's disease is a fatal neurodegenerative disease, where dysfunction and loss of striatal and cortical neurons are central to the pathogenesis of the disease. Here, we integrated quantitative studies to investigate the underlying mechanisms behind HD pathology in a systems-wide manner. To this end, we used state-of-the-art mass spectrometry to establish a spatial brain proteome from late-stage R6/2 mice and compared this with wild-type littermates. We observed altered expression of proteins in pathways related to energy metabolism, synapse function, and neurotransmitter homeostasis. To support these findings, metabolic ¹³C labeling studies confirmed a compromised astrocytic metabolism and regulation of glutamate-GABA-glutamine cycling, resulting in impaired release of glutamine and GABA synthesis. In recent years, increasing attention has been focused on the role of astrocytes in HD, and our data support that therapeutic strategies to improve astrocytic glutamine homeostasis may help ameliorate symptoms in HD.

SIRT3 Regulation of Mitochondrial Quality Control in Neurodegenerative Diseases

Neurodegenerative diseases are disorders that are characterized by a progressive decline of motor and/or cognitive functions caused by the selective degeneration and loss of neurons within the central nervous system. The most common neurodegenerative diseases are Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD). Neurons have high energy demands, and dysregulation of mitochondrial quality and function is an important cause of neuronal degeneration. Mitochondrial quality control plays an important role in maintaining mitochondrial integrity and ensuring normal mitochondrial function; thus, defects in mitochondrial quality control are also significant causes of neurodegenerative diseases. The mitochondrial deacetylase SIRT3 has been found to have a large effect on mitochondrial function. Recent studies have also shown that SIRT3 has a role in mitochondrial quality control, including in the refolding or degradation of misfolded/unfolded proteins, mitochondrial dynamics, mitophagy, and mitochondrial biogenesis, all of which are affected in neurodegenerative diseases.

Early Neurodegeneration in R6/2 Mice Carrying the Huntington's Disease Mutation with a Super-Expanded CAG Repeat, Despite Normal Lifespan

The threshold of CAG repeat expansion in the HTT gene that causes HD is 36 CAG repeats, although 'superlong' expansions are found in individual neurons in postmortem brains. Previously, we showed that, compared to mice with <250 CAG repeats, onset of disease in R6/2 mice carrying superlong (>440) CAG repeat expansions was delayed, and disease progression was slower. Inclusion pathology also differed from 250 CAG repeat mice, being dominated by a novel kind of extranuclear neuronal inclusion (nENNI) that resembles a class of aggregate seen in patients with the adult onset form of HD. Here, we characterised neuropathology in R6/2 mice with >400 CAG repeats using light and electron microscopy. nENNIs were found with increased frequency and wider distribution with age. Some nENNIs appear to 'mature' as the disease develops, developing a multi-layered cored structure. Mice with superlong CAG repeats do not develop clinical signs until they are around 30-40 weeks of age, and they attain a normal life span (>2 years). Nevertheless, they show brain atrophy and unequivocal neuron loss from the striatum and cortex by 22 weeks of age, an age at which similar pathology is seen in 250 CAG repeat mice. Since this time-point is 'end stage' for a 250 CAG mouse, but very far (at least 18 months) from end stage for a >  440 CAG repeat mouse, our data confirm that the appearance of clinical signs, the formation of inclusions, and neurodegeneration are processes that progress independently. A better understanding of the relationship between CAG repeat length, neurodegenerative pathways, and clinical behavioural signs is essential, if we are to find strategies to delay or reverse the course of this disease.

PGC-1α, Sirtuins and PARPs in Huntington's Disease and Other Neurodegenerative Conditions: NAD+ to Rule Them All

In this review, we summarize the available published information on the neuroprotective effects of increasing nicotinamide adenine dinucleotide (NAD+) levels in Huntington's disease models. We discuss the rationale of potential therapeutic benefit of administering nicotinamide riboside (NR), a safe and effective NAD+ precursor. We discuss the agonistic effect on the Sirtuin1-PGC-1α-PPAR pathway as well as Sirtuin 3, which converge in improving mitochondrial function, decreasing ROS production and ameliorating bioenergetics deficits. Also, we discuss the potential synergistic effect of increasing NAD+ combined with PARPs inhibitors, as a clinical therapeutic option not only in HD, but other neurodegenerative conditions.

Cell-Autonomous and Non-cell-Autonomous Pathogenic Mechanisms in Huntington's Disease: Insights from In Vitro and In Vivo Models

Huntington's disease (HD) is an autosomal dominant disorder caused by an expansion in the trinucleotide CAG repeat in exon-1 in the huntingtin gene, located on chromosome 4. When the number of trinucleotide CAG exceeds 40 repeats, disease invariably is manifested, characterized by motor, cognitive, and psychiatric symptoms. The huntingtin (Htt) protein and its mutant form (mutant huntingtin, mHtt) are ubiquitously expressed but although multiple brain regions are affected, the most vulnerable brain region is the striatum. Striatal medium-sized spiny neurons (MSNs) preferentially degenerate, followed by the cortical pyramidal neurons located in layers V and VI. Proposed HD pathogenic mechanisms include, but are not restricted to, excitotoxicity, neurotrophic support deficits, collapse of the protein degradation mechanisms, mitochondrial dysfunction, transcriptional alterations, and disorders of myelin. Studies performed in cell type-specific and regionally selective HD mouse models implicate both MSN cell-autonomous properties and cell-cell interactions, particularly corticostriatal but also with non-neuronal cell types. Here, we review the intrinsic properties of MSNs that contribute to their selective vulnerability and in addition, we discuss how astrocytes, microglia, and oligodendrocytes, together with aberrant corticostriatal connectivity, contribute to HD pathophysiology. In addition, mHtt causes cell-autonomous dysfunction in cell types other than MSNs. These findings have implications in terms of therapeutic strategies aimed at preventing neuronal dysfunction and degeneration.

Energy Metabolism and Mitochondrial Superoxide Anion Production in Pre-symptomatic Striatal Neurons Derived from Human-Induced Pluripotent Stem Cells Expressing Mutant Huntingtin

In the present study, we investigated whether mutant huntingtin (mHTT) impairs mitochondrial functions in human striatal neurons derived from induced pluripotent stem cells (iPSCs). Striatal neurons and astrocytes derived from iPSCs from unaffected individuals (Ctrl) and Huntington's disease (HD) patients with HTT gene containing increased number of CAG repeats were used to assess the effect of mHTT on bioenergetics and mitochondrial superoxide anion production. The human neurons were thoroughly characterized and shown to express MAP2, DARPP32, GABA, synapsin, and PSD95. In human neurons and astrocytes expressing mHTT, the ratio of mHTT to wild-type huntingtin (HTT) was 1:1. The human neurons were excitable and could generate action potentials, confirming successful conversion of iPSCs into functional neurons. The neurons and astrocytes from Ctrl individuals and HD patients had similar levels of ADP and ATP and comparable respiratory and glycolytic activities. The mitochondrial mass, mitochondrial membrane potential, and superoxide anion production in human neurons appeared to be similar regardless of mHTT presence. The present results are in line with the results obtained in our previous studies with isolated brain mitochondria and cultured striatal neurons from YAC128 and R6/2 mice, in which we demonstrated that mutant huntingtin at early stages of HD pathology does not deteriorate mitochondrial functions. Overall, our results argue against bioenergetic deficits as a factor in HD pathogenesis and suggest that other detrimental processes might be more relevant to the development of HD pathology.

Role of PGC-1α in Mitochondrial Quality Control in Neurodegenerative Diseases

As one of the major cell organelles responsible for ATP production, it is important that neurons maintain mitochondria with structural and functional integrity; this is especially true for neurons with high metabolic requirements. When mitochondrial damage occurs, mitochondria are able to maintain a steady state of functioning through molecular and organellar quality control, thus ensuring neuronal function. And when mitochondrial quality control (MQC) fails, mitochondria mediate apoptosis. An apparently key molecule in MQC is the transcriptional coactivator peroxisome proliferator activated receptor γ coactivator-1α (PGC-1α). Recent findings have demonstrated that upregulation of PGC-1α expression in neurons can modulate MQC to prevent mitochondrial dysfunction in certain in vivo and in vitro aging or neurodegenerative encephalopathy models, such as Huntington's disease, Alzheimer's disease, and Parkinson's disease. Because mitochondrial function and quality control disorders are the basis of pathogenesis in almost all neurodegenerative diseases (NDDs), the role of PGC-1α may make it a viable entry point for the treatment of such diseases. This review focuses on multi-level MQC in neurons, as well as the regulation of MQC by PGC-1α in these major NDDs.

Reviewing Biochemical Implications of Normal and Mutated Huntingtin in Huntington's Disease

Huntingtin (Htt) is a multi-function protein of the brain. Normal Htt shows a common alpha-helical structure but conformational changes in the form with beta strands are the principal cause of Huntington's disease. Huntington's disease is a genetic neurological disorder caused by a repeated expansion of the CAG trinucleotide, causing instability in the N-terminal of the gene coding for the Huntingtin protein. The mutation leads to the abnormal expansion of the production of the polyglutamine tract (polyQ) resulting in the form of an unstable Huntingtin protein commonly referred to as mutant Huntingtin. Mutant Huntingtin is the cause of the complex neurological metabolic alteration of Huntington's disease, resulting in both the loss of all the functions of normal Huntingtin and the genesis of abnormal interactions due to the presence of this mutation. One of the problems arising from the misfolded Huntingtin is the increase in oxidative stress, which is common in many neurological diseases such as Alzheimer's, Parkinson's, Amyotrophic Lateral Sclerosis and Creutzfeldt-Jakob disease. In the last few years, the use of antioxidants had a strong incentive to find valid therapies for defence against neurodegenerations. Although further studies are needed, the use of antioxidant mixtures to counteract neuronal damages seems promising.

Mitochondrial dynamics and their potential as a therapeutic target

Mitochondrial dynamics shape the mitochondrial network and contribute to mitochondrial function and quality control. Mitochondrial fusion and division are integrated into diverse cellular functions and respond to changes in cell physiology. Imbalanced mitochondrial dynamics are associated with a range of diseases that are broadly characterized by impaired mitochondrial function and increased cell death. In various disease models, modulating mitochondrial fusion and division with either small molecules or genetic approaches has improved function. Although additional mechanistic understanding of mitochondrial fusion and division will be critical to inform further therapeutic approaches, mitochondrial dynamics represent a powerful therapeutic target in a wide range of human diseases.

Current progress of mitochondrial transplantation that promotes neuronal regeneration

Background

Mitochondria are the major source of intracellular adenosine triphosphate (ATP) and play an essential role in a plethora of physiological functions, including the regulation of metabolism and the maintenance of cellular homeostasis. Mutations of mitochondrial DNA, proteins and impaired mitochondrial function have been implicated in the neurodegenerative diseases, stroke and injury of the central nervous system (CNS). The dynamic feature of mitochondrial fusion, fission, trafficking and turnover have also been documented in these diseases.

Perspectives

A major bottleneck of traditional approach to correct mitochondria-related disorders is the difficulty of drugs or gene targeting agents to arrive at specific sub-compartments of mitochondria. Moreover, the diverse nature of mitochondrial mutations among patients makes it impossible to develop one drug for one disease. To this end, mitochondrial transplantation presents a new paradigm of therapeutic intervention that benefits neuronal survival and regeneration for neurodegenerative diseases, stroke, and CNS injury. Supplement of healthy mitochondria to damaged neurons has been reported to promote neuronal viability, activity and neurite re-growth. In this review, we provide an overview of the recent advance and development on mitochondrial therapy.

Conclusion

Key parameters for the success of mitochondrial transplantation depend on the source and quality of isolated mitochondria, delivery protocol, and cellular uptake of supplemented mitochondria. To expedite clinical application of the mitochondrial transplantation, current isolation protocol needs optimization to obtain high percentage of functional mitochondria, isolated mitochondria may be packaged by biomaterials for successful delivery to brain allowing for efficient neuronal uptake.

Age-Related Oxidative Changes in Primary Porcine Fibroblasts Expressing Mutated Huntingtin

BACKGROUND

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by CAG triplet expansions in the huntingtin gene. Oxidative stress is linked to HD pathology, although it is not clear whether this is an effect or a mediator of disease. The transgenic (TgHD) minipig expresses the N-terminal part of human-mutated huntingtin and represents a unique model to investigate therapeutic strategies towards HD. A more detailed characterization of this model is needed to fully utilize its potential.

METHODS

In this study, we focused on the molecular and cellular features of fibroblasts isolated from TgHD minipigs and the wild-type (WT) siblings at different ages, pre-symptomatic at the age of 24-36 months and with the onset of behavioural symptoms at the age of 48 months. We measured oxidative stress, the expression of oxidative stress-related genes, proliferation capacity along with the expression of cyclin B1 and D1 proteins, cellular permeability, and the integrity of the nuclear DNA (nDNA) and mitochondrial DNA in these cells.

RESULTS

TgHD fibroblasts isolated from 48-month-old animals showed increased oxidative stress, which correlated with the overexpression of SOD2 encoding mitochondrial superoxide dismutase 2, and the NEIL3 gene encoding DNA glycosylase involved in replication-associated repair of oxidized DNA. TgHD cells displayed an abnormal proliferation capacity and permeability. We further demonstrated increased nDNA damage in pre-symptomatic TgHD fibroblasts (isolated from animals aged 24-36 months).

CONCLUSIONS

Our results unravel phenotypic alterations in primary fibroblasts isolated from the TgHD minipig model at the age of 48 months. Importantly, nDNA damage appears to precede these phenotypic alterations. Our results highlight the impact of fibroblasts from TgHD minipigs in studying the molecular mechanisms of HD pathophysiology that gradually occur with age.

Morphological control of mitochondria as the novel mechanism of Gastrodia elata in attenuating mutant huntingtin-induced protein aggregations

BACKGROUND

According to Compendium of Materia Medica, Gastrodia elata (GE) Blume as a top grade and frequently prescribed herbal medicine has been used in treating dizziness, headaches, and epilepsy, indicating a neuroprotective effect. Because GE is capable of suppressing a hyperactive liver and thus calming endogenous wind, and because Huntington's disease (HD) can be classified as a phenomenon of disturbed liver wind, it is suggested that GE might be beneficial in treating HD. However, although current studies support GE for the prevention of diverse neurodegenerations such as HD, its detailed mechanisms remain elusive.

PURPOSE

To investigate the molecular mechanism of GE in preventing HD by focusing on mitochondrial morphology, which is highly associated with HD etiology and thus proposed as a therapeutic target of neurodegenerations.

STUDY DESIGN/METHODS

The overexpression of the mutant huntingtin (mHTT) gene in rat pheochromocytoma (PC12) cells was used as an in vitro cell model of HD. A filter retardation assay was applied to measure protein aggregations during HTT expression. Cotransfection with mitochondrial fusion and fission genes was used to test their relationships with HTT aggregates by monitoring with a confocal laser scanning microscope and filter retardation assay. Western blot analysis was used to estimate protein expression under different drug treatments or cotransfections with other related genes.

RESULTS

The overexpression of mutant but not normal HTT genes significantly resulted in protein aggregations in PC12 cells. GE dose-dependently attenuated mHTT-induced protein aggregations and free radical formations. GE significantly reversed mHTT-induced mitochondrial fragmentation and dysregulation of mitochondrial fusion and fission molecules. The overexpression of mitochondrial fusion genes attenuated mHTT-induced protein aggregations. Further, Mdivi-1, a DRP1 fission molecule inhibitor, significantly reversed mHTT-induced protein aggregations and mitochondrial fragmentation.

CONCLUSION

GE attenuated mHTT aggregations through the control of mitochondrial fusion and the fission pathway.

Enhanced cerebral branched-chain amino acid metabolism in R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a hereditary and fatal disease causing profound neurodegeneration. Deficits in cerebral energy and neurotransmitter metabolism have been suggested to play a central role in the neuronal dysfunction and death associated with HD. The branched-chain amino acids (BCAAs), leucine, isoleucine and valine, are important for cerebral nitrogen homeostasis, neurotransmitter recycling and can be utilized as energy substrates in the tricarboxylic acid (TCA) cycle. Reduced levels of BCAAs in HD have been validated by several reports. However, it is still unknown how cerebral BCAA metabolism is regulated in HD. Here we investigate the metabolism of leucine and isoleucine in the R6/2 mouse model of HD. Acutely isolated cerebral cortical and striatal slices of control and R6/2 mice were incubated in media containing 15N- or 13C-labeled leucine or isoleucine and slice extracts were analyzed by gas chromatography-mass spectrometry (GC-MS) to determine isotopic enrichment of derived metabolites. Elevated BCAA transamination was found from incubations with [15N]leucine and [15N]isoleucine, in both cerebral cortical and striatal slices of R6/2 mice compared to controls. Metabolism of [U-13C]leucine and [U-13C]isoleucine, entering oxidative metabolism as acetyl CoA, was maintained in R6/2 mice. However, metabolism of [U-13C]isoleucine, entering the TCA cycle as succinyl CoA, was elevated in both cerebral cortical and striatal slices of R6/2 mice, suggesting enhanced metabolic flux via this anaplerotic pathway. To support the metabolic studies, expression of enzymes in the BCAA metabolic pathway was assessed from a proteomic resource. Several enzymes related to BCAA metabolism were found to exhibit augmented expression in the R6/2 brain, particularly related to isoleucine metabolism, suggesting an increase in the BCAA metabolic machinery. Our results show that the capacity for cerebral BCAA metabolism, predominantly of isoleucine, is amplified in the R6/2 brain and indicates that perturbations in cerebral BCAA homeostasis could have functional consequences for HD pathology.

The Role of the Antioxidant Response in Mitochondrial Dysfunction in Degenerative Diseases: Cross-Talk between Antioxidant Defense, Autophagy, and Apoptosis

The mitochondrion is an essential organelle important for the generation of ATP for cellular function. This is especially critical for cells with high energy demands, such as neurons for signal transmission and cardiomyocytes for the continuous mechanical work of the heart. However, deleterious reactive oxygen species are generated as a result of mitochondrial electron transport, requiring a rigorous activation of antioxidative defense in order to maintain homeostatic mitochondrial function. Indeed, recent studies have demonstrated that the dysregulation of antioxidant response leads to mitochondrial dysfunction in human degenerative diseases affecting the nervous system and the heart. In this review, we outline and discuss the mitochondrial and oxidative stress factors causing degenerative diseases, such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, and Friedreich's ataxia. In particular, the pathological involvement of mitochondrial dysfunction in relation to oxidative stress, energy metabolism, mitochondrial dynamics, and cell death will be explored. Understanding the pathology and the development of these diseases has highlighted novel regulators in the homeostatic maintenance of mitochondria. Importantly, this offers potential therapeutic targets in the development of future treatments for these degenerative diseases.

Amelioration of Huntington's disease phenotype in astrocytes derived from iPSC-derived neural progenitor cells of Huntington's disease monkeys

Huntington’s disease (HD) is a devastating monogenic, dominant, hereditary, neurodegenerative disease. HD is caused by the expansion of CAG repeats in exon 1 of the huntingtin (HTT) gene, IT15, resulting in an expanded polyglutamine (polyQ) residue in the N-terminus of the HTT protein. HD is characterized by the accumulation of mutant HTT (mHTT) in neural and somatic cells. Progressive brain atrophy occurs initially in the striatum and extends to different brain regions with progressive decline in cognitive, behavioral and motor functions. Astrocytes are the most abundant cell type in the brain and play an essential role in neural development and maintaining homeostasis in the central nervous system (CNS). There is increasing evidence supporting the involvement of astrocytes in the development of neurodegenerative diseases such as Parkinson’s disease (PD), Huntington’s disease (HD), Alzheimer’s disease (AD), and amyotrophic lateral sclerosis (ALS). We have generated neural progenitor cells (NPCs) from induced pluripotent stem cells (iPSCs) of transgenic HD monkeys as a model for studying HD pathogenesis. We have reported that NPCs can be differentiated in vitro into mature neural cells, such as neurons and glial cells, and are an excellent tool to study the pathogenesis of HD. To better understand the role of astrocytes in HD pathogenesis and discover new therapies to treat HD, we have developed an astrocyte differentiation protocol and evaluated the efficacy of RNAi to ameliorate HD phenotypes in astrocytes. The resultant astrocytes expressed canonical astrocyte-specific markers examined by immunostaining and real-time PCR. Flow cytometry (FACS) analysis showed that the majority of the differentiated NPCs (95.7%) were positive for an astrocyte specific marker, glial fibrillary acidic protein (GFAP). Functionalities of astrocytes were evaluated by glutamate uptake assay and electrophysiology. Expression of mHTT in differentiated astrocytes induced cytosolic mHTT aggregates and nuclear inclusions, suppressed the expression of SOD2 and PGC1, reduced ability to uptake glutamate, decreased 4-aminopyridine (4-AP) response, and shifted I/V plot measured by electrophysiology, which are consistent with previous reports on HD astrocytes and patient brain samples. However, expression of small-hairpin RNA against HTT (shHD) ameliorated and reversed aforementioned HD phenotypes in astrocytes. This represents a demonstration of a novel non-human primate (NHP) astrocyte model for studying HD pathogenesis and a platform for discovering novel HD treatments.

Mitochondrial Dysfunction in Huntington's Disease; Interplay Between HSF1, p53 and PGC-1α Transcription Factors

Huntington’s disease (HD) is a neurodegenerative disease caused by an expanded CAG repeat in the huntingtin (HTT) gene, causing the protein to misfold and aggregate. HD progression is characterized by motor impairment and cognitive decline associated with the preferential loss of striatal medium spiny neurons (MSNs). The mechanisms that determine increased susceptibility of MSNs to mutant HTT (mHTT) are not fully understood, although there is abundant evidence demonstrating the importance of mHTT mediated mitochondrial dysfunction in MSNs death. Two main transcription factors, p53 and peroxisome proliferator co-activator PGC-1α, have been widely studied in HD for their roles in regulating mitochondrial function and apoptosis. The action of these two proteins seems to be interconnected. However, it is still open to discussion whether p53 and PGC-1α dependent responses directly influence each other or if they are connected via a third mechanism. Recently, the stress responsive transcription factor HSF1, known for its role in protein homeostasis, has been implicated in mitochondrial function and in the regulation of PGC-1α and p53 levels in different contexts. Based on previous reports and our own research, we discuss in this review the potential role of HSF1 in mediating mitochondrial dysfunction in HD and propose a unifying mechanism that integrates the responses mediated by p53 and PGC-1α in HD via HSF1.

Inhibition of Drp1/Fis1 interaction slows progression of amyotrophic lateral sclerosis

Bioenergetic failure and oxidative stress are common pathological hallmarks of amyotrophic lateral sclerosis (ALS), but whether these could be targeted effectively for novel therapeutic intervention needs to be determined. One of the reported contributors to ALS pathology is mitochondrial dysfunction associated with excessive mitochondrial fission and fragmentation, which is predominantly mediated by Drp1 hyperactivation. Here, we determined whether inhibition of excessive fission by inhibiting Drp1/Fis1 interaction affects disease progression. We observed mitochondrial excessive fragmentation and dysfunction in several familial forms of ALS patient‐derived fibroblasts as well as in cultured motor neurons expressing SOD1 mutant. In both cell models, inhibition of Drp1/Fis1 interaction by a selective peptide inhibitor, P110, led to a significant reduction in reactive oxygen species levels, and to improvement in mitochondrial structure and functions. Sustained treatment of mice expressing G93A SOD1 mutation with P110, beginning at the onset of disease symptoms at day 90, produced an improvement in motor performance and survival, suggesting that Drp1 hyperactivation may be an attractive target in the treatment of ALS patients.

Peroxiredoxin6, a Multitask Antioxidant Enzyme Involved in the Pathophysiology of Chronic Noncommunicable Diseases

SIGNIFICANCE

Chronic noncommunicable diseases (NCDs) are the leading causes of disability and death worldwide. NCDs mainly comprise diabetes mellitus, cardiovascular diseases, chronic obstructive pulmonary disease, cancer, and neurological degenerative diseases, which kill more than 80% of population, especially the elderly, worldwide. Recent Advances: Several recent theories established NCDs as multifactorial diseases, where a combination of genetic, epigenetic, and environmental factors contributes to their pathogenesis. Nevertheless, recent findings suggest that the common factor linking all these pathologies is an increase in oxidative stress and the age-related loss of the antioxidant mechanisms of defense against it. Impairment in mitochondrial homeostasis with consequent deregulation in oxidative stress balance has also been suggested.

CRITICAL ISSUES

Therefore, antioxidant proteins deserve particular attention for their potential role against NCDs. In particular, peroxiredoxin(Prdx)6 is a unique antioxidant enzyme, belonging to the Prdx family, with double properties, peroxidase and phospholipase activities. Through these activities, Prdx6 has been shown to be a powerful antioxidant enzyme, implicated in the pathogenesis of different NCDs. Recently, we described a phenotype of diabetes mellitus in Prdx6 knockout mice, suggesting a pivotal role of Prdx6 in the pathogenesis of cardiometabolic diseases.

FUTURE DIRECTIONS

Increasing awareness on the role of antioxidant defenses in the pathogenesis of NCDs may open novel therapeutic approaches to reduce the burden of this pandemic phenomenon. However, knowledge of the role of Prdx6 in NCD prevention and pathogenesis is still not clarified.

Modulation of Mitochondrial Dynamics in Neurodegenerative Diseases: An Insight Into Prion Diseases

Mitochondrial dysfunction is a common and prominent feature of prion diseases and other neurodegenerative disorders. Mitochondria are dynamic organelles that constantly fuse with one another and subsequently break apart. Defective or superfluous mitochondria are usually eliminated by a form of autophagy, referred to as mitophagy, to maintain mitochondrial homeostasis. Mitochondrial dynamics are tightly regulated by processes including fusion and fission. Dysfunction of mitochondrial dynamics can lead to the accumulation of abnormal mitochondria and contribute to cellular damage. Neurons are among the cell types that consume the most energy, have a highly complex morphology, and are particularly dependent on mitochondrial functions and dynamics. In this review article, we summarize the molecular mechanisms underlying the mitochondrial dynamics and the regulation of mitophagy and discuss the dysfunction of these processes in the progression of prion diseases and other neurodegenerative disorders. We have also provided an overview of mitochondrial dynamics as a therapeutic target for neurodegenerative diseases.

Meldonium improves Huntington's disease mitochondrial dysfunction by restoring peroxisome proliferator-activated receptor γ coactivator 1α expression

Mitochondrial dysfunction seems to play a fundamental role in the pathogenesis of neurodegeneration in Huntington's disease (HD). We assessed possible neuroprotective actions of meldonium, a small molecule affecting mitochondrial fuel metabolism, in in vitro and in vivo HD models. We found that meldonium was able to prevent cytotoxicity induced by serum deprivation, to reduce the accumulation of mutated huntingtin (mHtt) aggregates, and to upregulate the expression of peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) in mHTT-expressing cells. The PGC-1α increase was accompanied by the increment of mitochondrial mass and by the rebalancing of mitochondrial dynamics with a promotion of the mitochondrial fusion. Meldonium-induced PGC-1α significantly alleviated motor dysfunction and prolonged the survival of a transgenic HD Drosophila model in which mHtt expression in the nervous system led to progressive motor performance deficits. Our study strongly suggests that PGC-1α, as a master coregulator of mitochondrial biogenesis, energy homeostasis, and antioxidant defense, is a potential therapeutic target in HD.

SIRT1 is increased in affected brain regions and hypothalamic metabolic pathways are altered in Huntington disease

AIMS

Metabolic dysfunction is involved in modulating the disease process in Huntington disease (HD) but the underlying mechanisms are not known. The aim of this study was to investigate if the metabolic regulators sirtuins are affected in HD.

METHODS

Quantitative real-time polymerase chain reactions were used to assess levels of SIRT1-3 and downstream targets in post mortem brain tissue from HD patients and control cases as well as after selective hypothalamic expression of mutant huntingtin (HTT) using recombinant adeno-associated viral vectors in mice.

RESULTS

We show that mRNA levels of the metabolic regulator SIRT1 are increased in the striatum and the cerebral cortex but not in the less affected cerebellum in post mortem HD brains. Levels of SIRT2 are only increased in the striatum and SIRT3 is not affected in HD. Interestingly, mRNA levels of SIRT1 are selectively increased in the lateral hypothalamic area (LHA) and ventromedial hypothalamus (VMH) in HD. Further analyses of the LHA and VMH confirmed pathological changes in these regions including effects on SIRT1 downstream targets and reduced mRNA levels of orexin (hypocretin), prodynorphin and melanin-concentrating hormone (MCH) in the LHA and of brain-derived neurotrophic factor (BDNF) in the VMH. Analyses after selective hypothalamic expression of mutant HTT suggest that effects on BDNF, orexin, dynorphin and MCH are early and direct, whereas changes in SIRT1 require more widespread expression of mutant HTT.

CONCLUSIONS

We show that SIRT1 expression is increased in HD-affected brain regions and that metabolic pathways are altered in the HD hypothalamus.

Mortal engines: Mitochondrial bioenergetics and dysfunction in neurodegenerative diseases

Mitochondria are best known for their role in ATP generation. However, studies over the past two decades have shown that mitochondria do much more than that. Mitochondria regulate both necrotic and apoptotic cell death pathways, they store and therefore coordinate cellular Ca²⁺ signaling, they generate and metabolize important building blocks, by-products and signaling molecules, and they also generate and are targets of free radical species that modulate many aspects of cell physiology and pathology. Most estimates suggest that although the brain makes up only 2 percent of body weight, utilizes about 20 percent of the body's total ATP. Thus, mitochondrial dysfunction greatly impacts brain functions and is indeed associated with numerous neurodegenerative diseases. Furthermore, a number of abnormal disease-associated proteins have been shown to interact directly with mitochondria, leading to mitochondrial dysfunction and subsequent neuronal cell death. Here, we discuss the role of mitochondrial dynamics impairment in the pathological processes associated with neurodegeneration and suggest that a therapy targeting mitochondrialdysfunction holds a great promise.

PGC-1α sparks the fire of neuroprotection against neurodegenerative disorders

Recently, growing evidence has demonstrated that peroxisome proliferator activated receptor γ (PPARγ) coactivator-1α (PGC-1α) is a superior transcriptional regulator that acts via controlling the expression of anti-oxidant enzymes and uncoupling proteins and inducing mitochondrial biogenesis, which plays a beneficial part in the central nervous system (CNS). Given the significance of PGC-1α, we summarize the current literature on the molecular mechanisms and roles of PGC-1α in the CNS. Thus, in this review, we first briefly introduce the basic characteristics regarding PGC-1α. We then depict some of its important cerebral functions and discuss upstream modulators, partners, and downstream effectors of the PGC-1α signaling pathway. Finally, we highlight recent progress in research on the involvement of PGC-1α in certain major neurodegenerative disorders (NDDs), including Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Collectively, the data presented here may be useful for supporting the future potential of PGC-1α as a therapeutic target.

The Role of Adenosine Tone and Adenosine Receptors in Huntington's Disease

Huntington's disease (HD) is a hereditary neurodegenerative disorder caused by a mutation in the IT15 gene that encodes for the huntingtin protein. Mutated hungtingtin, although widely expressed in the brain, predominantly affects striato-pallidal neurons, particularly enriched with adenosine A2A receptors (A2AR), suggesting a possible involvement of adenosine and A2AR is the pathogenesis of HD. In fact, polymorphic variation in the ADORA2A gene influences the age at onset in HD, and A2AR dynamics is altered by mutated huntingtin. Basal levels of adenosine and adenosine receptors are involved in many processes critical for neuronal function and homeostasis, including modulation of synaptic activity and excitotoxicity, the control of neurotrophin levels and functions, and the regulation of protein degradation mechanisms. In the present review, we critically analyze the current literature involving the effect of altered adenosine tone and adenosine receptors in HD and discuss why therapeutics that modulate the adenosine system may represent a novel approach for the treatment of HD.

Mitochondrial Quality Control in Neurodegenerative Diseases: Focus on Parkinson's Disease and Huntington's Disease

In recent years, several important advances have been made in our understanding of the pathways that lead to cell dysfunction and death in Parkinson's disease (PD) and Huntington's disease (HD). Despite distinct clinical and pathological features, these two neurodegenerative diseases share critical processes, such as the presence of misfolded and/or aggregated proteins, oxidative stress, and mitochondrial anomalies. Even though the mitochondria are commonly regarded as the "powerhouses" of the cell, they are involved in a multitude of cellular events such as heme metabolism, calcium homeostasis, and apoptosis. Disruption of mitochondrial homeostasis and subsequent mitochondrial dysfunction play a key role in the pathophysiology of neurodegenerative diseases, further highlighting the importance of these organelles, especially in neurons. The maintenance of mitochondrial integrity through different surveillance mechanisms is thus critical for neuron survival. Mitochondria display a wide range of quality control mechanisms, from the molecular to the organellar level. Interestingly, many of these lines of defense have been found to be altered in neurodegenerative diseases such as PD and HD. Current knowledge and further elucidation of the novel pathways that protect the cell through mitochondrial quality control may offer unique opportunities for disease therapy in situations where ongoing mitochondrial damage occurs. In this review, we discuss the involvement of mitochondrial dysfunction in neurodegeneration with a special focus on the recent findings regarding mitochondrial quality control pathways, beyond the classical effects of increased production of reactive oxygen species (ROS) and bioenergetic alterations. We also discuss how disturbances in these processes underlie the pathophysiology of neurodegenerative disorders such as PD and HD.

MFN2 agonists reverse mitochondrial defects in preclinical models of Charcot-Marie-Tooth disease type 2A

Mitofusins (MFNs) promote fusion-mediated mitochondrial content exchange and subcellular trafficking. Mutations in Mfn2 cause neurodegenerative Charcot-Marie-Tooth disease type 2A (CMT2A). We showed that MFN2 activity can be determined by Met376 and His380 interactions with Asp725 and Leu727 and controlled by PINK1 kinase-mediated phosphorylation of adjacent MFN2 Ser378 Small-molecule mimics of the peptide-peptide interface of MFN2 disrupted this interaction, allosterically activating MFN2 and promoting mitochondrial fusion. These first-in-class mitofusin agonists overcame dominant mitochondrial defects provoked in cultured neurons by CMT2A mutants MFN2 Arg94→Gln94 and MFN2 Thr105→Met105, as demonstrated by amelioration of mitochondrial dysmotility, fragmentation, depolarization, and clumping. A mitofusin agonist normalized axonal mitochondrial trafficking within sciatic nerves of MFN2 Thr105→Met105 mice, promising a therapeutic approach for CMT2A and other untreatable diseases of impaired neuronal mitochondrial dynamism and/or trafficking.

Warning SINEs: Alu elements, evolution of the human brain, and the spectrum of neurological disease

Alu elements are a highly successful family of primate-specific retrotransposons that have fundamentally shaped primate evolution, including the evolution of our own species. Alus play critical roles in the formation of neurological networks and the epigenetic regulation of biochemical processes throughout the central nervous system (CNS), and thus are hypothesized to have contributed to the origin of human cognition. Despite the benefits that Alus provide, deleterious Alu activity is associated with a number of neurological and neurodegenerative disorders. In particular, neurological networks are potentially vulnerable to the epigenetic dysregulation of Alu elements operating across the suite of nuclear-encoded mitochondrial genes that are critical for both mitochondrial and CNS function. Here, we highlight the beneficial neurological aspects of Alu elements as well as their potential to cause disease by disrupting key cellular processes across the CNS. We identify at least 37 neurological and neurodegenerative disorders wherein deleterious Alu activity has been implicated as a contributing factor for the manifestation of disease, and for many of these disorders, this activity is operating on genes that are essential for proper mitochondrial function. We conclude that the epigenetic dysregulation of Alu elements can ultimately disrupt mitochondrial homeostasis within the CNS. This mechanism is a plausible source for the incipient neuronal stress that is consistently observed across a spectrum of sporadic neurological and neurodegenerative disorders.

The mitochondrial transcription factor TFAM in neurodegeneration: emerging evidence and mechanisms

The mitochondrial transcription factor A, or TFAM, is a mitochondrial DNA (mtDNA)-binding protein essential for genome maintenance. TFAM functions in determining the abundance of the mitochondrial genome by regulating packaging, stability, and replication. More recently, TFAM has been shown to play a central role in the mtDNA stress-mediated inflammatory response. Emerging evidence indicates that decreased mtDNA copy number is associated with several aging-related pathologies; however, little is known about the association of TFAM abundance and disease. In this Review, we evaluate the potential associations of altered TFAM levels or mtDNA copy number with neurodegeneration. We also describe potential mechanisms by which mtDNA replication, transcription initiation, and TFAM-mediated endogenous danger signals may impact mitochondrial homeostasis in Alzheimer, Huntington, Parkinson, and other neurodegenerative diseases.

Increased Degradation Rates in the Components of the Mitochondrial Oxidative Phosphorylation Chain in the Cerebellum of Old Mice

Brain structures differ in the magnitude of age-related neuron loss with the cerebellum being more affected. An underlying cause could be an age-related decline in mitochondrial bioenergetics. Successful aging of mitochondria reflects a balanced turnover of proteins involved in mitochondrial biogenesis and mitophagy. Thus, an imbalance in mitochondrial turnover can contribute to the diminution of cellular function seen during aging. Mitochondrial biogenesis and mitophagy are mediated by a set of proteins including MFN1, MFN2, OPA1, DRP1, FIS1 as well as DMN1l and DNM1, all of which are required for mitochondrial fission. Using N15 labeling, we report that the turnover rates for DMN1l and FIS1 go in opposite directions in the cerebellum of 22-month-old C57BL6j mice as compared to 3-month-old mice. Previous studies have reported decreased turnover rates for the mitochondrial respiratory complexes of aged rodents. In contrast, we found increased turnover rates for mitochondrial proteins of the oxidative phosphorylation chain in the aged mice as compared to young mice. Furthermore, the turnover rate of the components that are most affected by aging –complex III components (ubiquinol cytochrome C oxidoreductase) and complex IV components (cytochrome C oxidase)– was significantly increased in the senescent cerebellum. However, the turnover rates of proteins involved in mitophagy (i.e., the proteasomal and lysosomal degradation of damaged mitochondria) were not significantly altered with age. Overall, our results suggest that an age-related imbalance in the turnover rates of proteins involved in mitochondrial biogenesis and mitophagy (DMN1l, FIS1) in conjunction with an age-related imbalance in the turnover rates of proteins of the complexes III and IV of the electron transfer chain, might impair cerebellar mitochondrial bioenergetics in old mice.

HACE1 is essential for astrocyte mitochondrial function and influences Huntington disease phenotypes in vivo

Oxidative stress is a prominent feature of Huntington disease (HD), and we have shown previously that reduced levels of hace1 (HECT domain and Ankyrin repeat containing E3 ubiquitin protein ligase 1) in patient striatum may contribute to the pathogenesis of HD. Hace1 promotes the stability of Nrf2 and thus plays an important role in antioxidant response mechanisms, which are dysfunctional in HD. Moreover, hace1 overexpression mitigates mutant huntingtin (mHTT)-induced oxidative stress in vitro through promotion of the Nrf2 antioxidant response. Here, we show that the genetic ablation of hace1 in the YAC128 mouse model of HD accelerates motor deficits and exacerbates cognitive and psychiatric phenotypes in vivo. We find that both the expression of mHTT and the ablation of hace1 alone are sufficient to cause deficits in astrocytic mitochondrial respiration. We confirm the crucial role of hace1 in astrocytes in vivo, since its ablation is sufficient to cause dramatic astrogliosis in wild-type FVB/N mice. Astrogliosis is not observed in the presence of mHTT but a strong dysregulation in the expression of astrocytic markers in HACE1-/- x YAC128 striatum suggests an additive effect of mHTT expression and hace1 loss on this cell type. HACE1-/- x YAC128 mice and primary cells derived from these animals therefore provide model systems that will allow for the further dissection of Nrf2 pathways and astrocyte dysfunction in the context of HD.

Nonmammalian Models of Huntington's Disease

Flies, worms, yeast and more recently zebra fish have all been engineered to express expanded polyglutamine repeat versions of Huntingtin with various resulting pathologies including early death, neurodegeneration, and loss of motor function. Each of these models present particular features that make it useful in studying the mechanisms of polyglutamine pathology. However, one particular unbiased readout of mHTT pathology is functional loss of motor control. Loss of motor control is prominent in patients, but it remains unresolved whether pathogenic symptoms in patients result from overt degeneration and loss of neurons or from malfunctioning of surviving neurons as the pathogenic insult builds up. This is why a functional assay such as motor control can be uniquely powerful in revealing early as well as late neurological deficits and does not rely on assumptions such as that the level of inclusions or the degree of neuronal loss can be equated with the level of pathology. Drosophila is well suited for such assays because it contains a functioning nervous system with many parallels to the human condition. In addition, the ability to readily express mHTT transgenes in different tissues and subsets of neurons allows one the possibility of isolating a particular effect to a subset of neurons where one can correlate subcellular events in response to mHTT challenge with pathology at both the cellular and organismal levels. Here we describe methods to monitor the degree of motor function disruption in Drosophila models of HD and we include a brief summary of other nonmammalian models of HD and discussion of their unique strengths.

Mitochondrial-Based Therapeutics for the Treatment of Spinal Cord Injury: Mitochondrial Biogenesis as a Potential Pharmacological Target

Spinal cord injury (SCI) is characterized by an initial trauma followed by a progressive cascade of damage referred to as secondary injury. A hallmark of secondary injury is vascular disruption leading to vasoconstriction and decreased oxygen delivery, which directly reduces the ability of mitochondria to maintain homeostasis and leads to loss of ATP-dependent cellular functions, calcium overload, excitotoxicity, and oxidative stress, further exacerbating injury. Restoration of mitochondria dysfunction during the acute phases of secondary injury after SCI represents a potentially effective therapeutic strategy. This review discusses the past and present pharmacological options for the treatment of SCI as well as current research on mitochondria-targeted approaches. Increased antioxidant activity, inhibition of the mitochondrial permeability transition, alternate energy sources, and manipulation of mitochondrial morphology are among the strategies under investigation. Unfortunately, many of these tactics address single aspects of mitochondrial dysfunction, ultimately proving largely ineffective. Therefore, this review also examines the unexplored therapeutic efficacy of pharmacological enhancement of mitochondrial biogenesis, which has the potential to more comprehensively improve mitochondrial function after SCI.

Structure, function, and regulation of mitofusin-2 in health and disease

Mitochondria are highly dynamic organelles that constantly migrate, fuse, and divide to regulate their shape, size, number, and bioenergetic function. Mitofusins (Mfn1/2), optic atrophy 1 (OPA1), and dynamin-related protein 1 (Drp1), are key regulators of mitochondrial fusion and fission. Mutations in these molecules are associated with severe neurodegenerative and non-neurological diseases pointing to the importance of functional mitochondrial dynamics in normal cell physiology. In recent years, significant progress has been made in our understanding of mitochondrial dynamics, which has raised interest in defining the physiological roles of key regulators of fusion and fission and led to the identification of additional functions of Mfn2 in mitochondrial metabolism, cell signalling, and apoptosis. In this review, we summarize the current knowledge of the structural and functional properties of Mfn2 as well as its regulation in different tissues, and also discuss the consequences of aberrant Mfn2 expression.

Transplantation of wild-type mouse hematopoietic stem and progenitor cells ameliorates deficits in a mouse model of Friedreich's ataxia

Friedreich's ataxia (FRDA) is an incurable autosomal recessive neurodegenerative disease caused by reduced expression of the mitochondrial protein frataxin due to an intronic GAA-repeat expansion in the FXN gene. We report the therapeutic efficacy of transplanting wild-type mouse hematopoietic stem and progenitor cells (HSPCs) into the YG8R mouse model of FRDA. In the HSPC-transplanted YG8R mice, development of muscle weakness and locomotor deficits was abrogated as was degeneration of large sensory neurons in the dorsal root ganglia (DRGs) and mitochondrial capacity was improved in brain, skeletal muscle, and heart. Transplanted HSPCs engrafted and then differentiated into microglia in the brain and spinal cord and into macrophages in the DRGs, heart, and muscle of YG8R FRDA mice. We observed the transfer of wild-type frataxin and Cox8 mitochondrial proteins from HSPC-derived microglia/macrophages to FRDA mouse neurons and muscle myocytes in vivo. Our results show the HSPC-mediated phenotypic rescue of FRDA in YG8R mice and suggest that this approach should be investigated further as a strategy for treating FRDA.

Accurate and comprehensive analysis of single nucleotide variants and large deletions of the human mitochondrial genome in DNA and single cells

Massive parallel sequencing (MPS) can accurately quantify mitochondrial DNA (mtDNA) single nucleotide variants (SNVs), but no MPS methods are currently validated to simultaneously and accurately establish the breakpoints and frequency of large deletions at low heteroplasmic loads. Here we present the thorough validation of an MPS protocol to quantify the load of very low frequency, large mtDNA deletions in bulk DNA and single cells, along with SNV calling by standard methods. We used a set of well-characterized DNA samples, DNA mixes and single cells to thoroughly control the study. We developed a custom script for the detection of mtDNA rearrangements that proved to be more accurate in detecting and quantifying deletions than pre-existing tools. We also show that PCR conditions and primersets must be carefully chosen to avoid biases in the retrieved variants and an increase in background noise, and established a lower detection limit of 0.5% heteroplasmic load for large deletions, and 1.5 and 2% for SNVs, for bulk DNA and single cells, respectively. Finally, the analysis of different single cells provided novel insights into mtDNA cellular mosaicism.

Protective effects of reduced dynamin-related protein 1 against amyloid beta-induced mitochondrial dysfunction and synaptic damage in Alzheimer's disease

The purpose of our study was to understand the protective effects of reduced expression of dynamin-related protein (Drp1) against amyloid beta (Aβ) induced mitochondrial and synaptic toxicities in Alzheimer's disease (AD) progression and pathogenesis. Our recent molecular and biochemical studies revealed that impaired mitochondrial dynamics-increased mitochondrial fragmentation and decreased fusion-in neurons from autopsy brains of AD patients and from transgenic AD mice and neurons expressing Aβ, suggesting that Aβ causes mitochondrial fragmentation in AD. Further, our recent co-immunoprecipitation and immunostaining analysis revealed that the mitochondrial fission protein Drp1 interacted with Aβ, and this interaction increased as AD progressed. Based on these findings, we hypothesize that a partial deficiency of Drp1 inhibits Drp1-Aβ interactions and protects Aβ-induced mitochondrial and synaptic toxicities, and maintains mitochondrial dynamics and neuronal function in AD neurons. We crossed Drp1+/- mice with APP transgenic mice (Tg2576 line) and created double mutant (APPXDrp1+/-) mice. Using real-time RT-PCR and immunoblotting analyses, we measured mRNA expressions and protein levels of genes related to the mitochondrial dynamics, mitochondrial biogenesis and synapses from 6-month-old Drp1+/-, APP, APPXDrp1+/- and wild-type (WT) mice. Using biochemical methods, we also studied mitochondrial function and measured soluble Aβ in brain tissues from all lines of mice in our study. Decreased mRNA expressions and protein levels of Drp1 and Fis1 (fission) and CypD (matrix) genes, and increased levels of Mfn1, Mfn2 and Opa1 (fusion), Nrf1, Nrf2, PGC1α, TFAM (biogenesis) and synaptophysin, PSD95, synapsin 1, synaptobrevin 1, neurogranin, GAP43 and synaptopodin (synaptic) were found in 6-month-old APPXDrp1+/- mice relative to APP mice. Mitochondrial functional assays revealed that mitochondrial dysfunction is reduced in APPXDrp1+/- mice relative to APP mice, suggesting that reduced Drp1enhances mitochondrial function in AD neurons. Sandwich ELISA assay revealed that soluble Aβ levels were significantly reduced in APPXDrp1+/- mice relative to APP mice, indicating that reduced Drp1 decreases soluble Aβ production in AD progression. These findings suggest that a partial reduction of Drp1 reduces Aβ production, reduces mitochondrial dysfunction, and maintains mitochondrial dynamics, enhances mitochondrial biogenesis and synaptic activity in APP mice. These findings may have implications for the development of Drp1 based therapeutics for AD patients.

Huntington's Disease and Mitochondria

Huntington's disease (HD) as an inherited neurodegenerative disorder leads to neuronal loss in striatum. Progressive motor dysfunction, cognitive decline, and psychiatric disturbance are the main clinical symptoms of the HD. This disease is caused by expansion of the CAG repeats in exon 1 of the huntingtin which encodes Huntingtin protein (Htt). Various cellular and molecular events play role in the pathology of HD. Mitochondria as important organelles play crucial roles in the most of neurodegenerative disorders like HD. Critical roles of the mitochondria in neurons are ATP generation, Ca²⁺ buffering, ROS generation, and antioxidant activity. Neurons as high-demand energy cells closely related to function, maintenance, and dynamic of mitochondria. In the most neurological disorders, mitochondrial activities and dynamic are disrupted which associate with high ROS level, low ATP generation, and apoptosis. Accumulation of mutant huntingtin (mHtt) during this disease may evoke mitochondrial dysfunction. Here, we review recent findings to support this hypothesis that mHtt could cause mitochondrial defects. In addition, by focusing normal huntingtin functions in neurons, we purpose mitochondria and Huntingtin association in normal condition. Moreover, mHtt affects various cellular signaling which ends up to mitochondrial biogenesis. So, it could be a potential candidate to decline ATP level in HD. We conclude how mitochondrial biogenesis plays a central role in the neuronal survival and activity and how mHtt affects mitochondrial trafficking, maintenance, integrity, function, dynamics, and hemostasis and makes neurons vulnerable to degeneration in HD.

Mitochondrial Metabolism in a Large-Animal Model of Huntington Disease: The Hunt for Biomarkers in the Spermatozoa of Presymptomatic Minipigs

BACKGROUND

Huntington disease (HD) is a fatal neurodegenerative disorder involving reduced muscle coordination, mental and behavioral changes, and testicular degeneration. In order to further clarify the decreased fertility and penetration ability of the spermatozoa of transgenic HD minipig boars (TgHD), we applied a set of mitochondrial metabolism (MM) parameter measurements to this promising biological material, which can be collected noninvasively in longitudinal studies.

OBJECTIVE

We aimed to optimize methods for MM measurements in spermatozoa and to establish possible biomarkers of HD in TgHD spermatozoa expressing the N-terminal part of mutated human huntingtin.

METHODS

Semen samples from 12 TgHD and wild-type animals, aged 12-65 months, were obtained repeatedly during the study. Respiration was measured by polarography, MM was assessed by the detection of oxidation of radiolabeled substrates (mitochondrial energy-generating system; MEGS), and the content of the oxidative phosphorylation system subunits was detected by Western blot. Three possibly interfering factors were statistically analyzed: the effect of HD, generation and aging.

RESULTS

We found 5 MM parameters which were significantly diminished in TgHD spermatozoa and propose 3 specific MEGS incubations and complex I-dependent respiration as potential biomarkers of HD in TgHD spermatozoa.

CONCLUSIONS

Our results suggest a link between the gain of toxic function of mutated huntingtin in TgHD spermatozoa and the observed MM and/or glycolytic impairment. We determined 4 biomarkers useful for HD phenotyping and experimental therapy monitoring studies in TgHD minipigs.

[Mitophagy and nervous system disease]

Mitophagy is a process during which the cell selectively removes the mitochondria via the mechanism of autophagy. It is crucial to the functional completeness of the whole mitochondrial network and determines cell survival and death. On the one hand, the damaged mitochondria releases pro-apoptotic factors which induce cell apoptosis; on the other hand, the damaged mitochondria eliminates itself via autophagy, which helps to maintain cell viability. Mitophagy is of vital importance for the development and function of the nervous system. Neural cells rely on autophagy to control protein quality and eliminate the damaged mitochondria, and under normal circumstances, mitophagy can protect the neural cells. Mutations in genes related to mitophagy may cause the development and progression of neurodegenerative diseases. An understanding of the role of mitophagy in nervous system diseases may provide new theoretical bases for clinical treatment. This article reviews the research advances in the relationship between mitophagy and different types of nervous system diseases.

Comparison of Sirtuin 3 Levels in ALS and Huntington's Disease-Differential Effects in Human Tissue Samples vs. Transgenic Mouse Models

Neurodegenerative diseases are characterized by distinct patterns of neuronal loss. In amyotrophic lateral sclerosis (ALS) upper and lower motoneurons degenerate whereas in Huntington’s disease (HD) medium spiny neurons in the striatum are preferentially affected. Despite these differences the pathophysiological mechanisms and risk factors are remarkably similar. In addition, non-neuronal features, such as weight loss implicate a dysregulation in energy metabolism. Mammalian sirtuins, especially the mitochondrial NAD+ dependent sirtuin 3 (SIRT3), regulate mitochondrial function and aging processes. SIRT3 expression depends on the activity of the metabolic master regulator peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), a modifier of ALS and HD in patients and model organisms. This prompted us to systematically probe Sirt3 mRNA and protein levels in mouse models of ALS and HD and to correlate these with patient tissue levels. We found a selective reduction of Sirt3 mRNA levels and function in the cervical spinal cord of end-stage ALS mice (superoxide dismutase 1, SOD1^G93A). In sharp contrast, a tendency to increased Sirt3 mRNA levels was found in the striatum in HD mice (R6/2). Cultured primary neurons express the highest levels of Sirt3 mRNA. In primary cells from PGC-1α knock-out (KO) mice the Sirt3 mRNA levels were highest in astrocytes. In human post mortem tissue increased mRNA and protein levels of Sirt3 were found in the spinal cord in ALS, while Sirt3 levels were unchanged in the human HD striatum. Based on these findings we conclude that SIRT3 mediates the different effects of PGC-1α during the course of transgenic (tg) ALS and HD and in the human conditions only partial aspects Sirt3 dysregulation manifest.

Abnormalities of Mitochondrial Dynamics in Neurodegenerative Diseases

Neurodegenerative diseases are incurable and devastating neurological disorders characterized by the progressive loss of the structure and function of neurons in the central nervous system or peripheral nervous system. Mitochondria, organelles found in most eukaryotic cells, are essential for neuronal survival and are involved in a number of neuronal functions. Mitochondrial dysfunction has long been demonstrated as a common prominent early pathological feature of a variety of common neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington’s disease (HD). Mitochondria are highly dynamic organelles that undergo continuous fusion, fission, and transport, the processes of which not only control mitochondrial morphology and number but also regulate mitochondrial function and location. The importance of mitochondrial dynamics in the pathogenesis of neurodegenerative diseases has been increasingly unraveled after the identification of several key fusion and fission regulators such as Drp1, OPA1, and mitofusins. In this review, after a brief discussion of molecular mechanisms regulating mitochondrial fusion, fission, distribution, and trafficking, as well as the important role of mitochondrial dynamics for neuronal function, we review previous and the most recent studies about mitochondrial dynamic abnormalities observed in various major neurodegenerative diseases and discuss the possibility of targeting mitochondrial dynamics as a likely novel therapeutic strategy for neurodegenerative diseases.