Hypertonia-linked protein Trak1 functions with mitofusins to promote mitochondrial tethering and fusion

Hypertonia is a neurological dysfunction associated with a number of central nervous system disorders, including cerebral palsy, Parkinson’s disease, dystonia, and epilepsy. Genetic studies have identified a homozygous truncation mutation in Trak1 that causes hypertonia in mice. Moreover, elevated Trak1 protein expression is associated with several types of cancersand variants in Trak1 are linked to childhood absence epilepsy in humans. Despite the importance of Trak1 in health and disease, the mechanisms of Trak1 action remain unclear and the pathogenic effects of Trak1 mutation are unknown. Here we report that Trak1 has a crucial function in regulation of mitochondrial fusion. Depletion of Trak1 inhibits mitochondrial fusion, resulting in mitochondrial fragmentation, whereas overexpression of Trak1 elongates and enlarges mitochondria. Our analyses revealed that Trak1 interacts and colocalizes with mitofusins on the outer mitochondrial membrane and functions with mitofusins to promote mitochondrial tethering and fusion. Furthermore, Trak1 is required for stress-induced mitochondrial hyperfusion and pro-survival response. We found that hypertonia-associated mutation impairs Trak1 mitochondrial localization and its ability to facilitate mitochondrial tethering and fusion. Our findings uncover a novel function of Trak1 as a regulator of mitochondrial fusion and provide evidence linking dysregulated mitochondrial dynamics to hypertonia pathogenesis.     

Betaine enhances the cellular survival via mitochondrial fusion and fission factors,MFN2 and DRP1

Betaine is a key metabolite of the methionine cycle and known for attenuating alcoholic steatosis in the liver. Recent studies have focused on the protection effect of betaine in mitochondrial regulation through the enhanced oxidative phosphorylation system. However, the mechanisms of its beneficial effects have not been clearly identified yet. Mitochondrial dynamics is important for the maintenance of functional mitochondria and cell homeostasis. A defective mitochondrial dynamics and oxidative phosphorylation system have been closely linked to several pathologies, raising the possibility that novel drugs targeting mitochondrial dynamics may present a therapeutic potential to restore the cellular homeostasis. In this study, we investigated betaine’s effect on mitochondrial morphology and physiology and demonstrated that betaine enhances mitochondrial function by increasing mitochondrial fusion and improves cell survival. Furthermore, it rescued the unbalance of the mitochondrial dynamics from mitochondrial oxidative phosphorylation dysfunction induced by oligomycin and rotenone. The elongation properties by betaine were accompanied by lowering DRP1 and increasing MFN2 expression. These data suggest that betaine could play an important role in remodeling mitochondrial dynamics to enhance mitochondrial function and cell viability.     

Respiratory supercomplexes: structure,function and assembly

The mitochondrial respiratory chain consists of 5 enzyme complexes that are responsible for ATP generation. The paradigm of the electron transport chain as discrete enzymes diffused in the inner mitochondrial membrane has been replaced by the solid state supercomplex model wherein the respiratory complexes associate with each other to form supramolecular complexes. Defects in these supercomplexes, which have been shown to be functionally active and required for forming stable respiratory complexes, have been associated with many genetic and neurodegenerative disorders demonstrating their biomedical significance. In this review, we will summarize the functional and structural significance of supercomplexes and provide a comprehensive review of their assembly and the assembly factors currently known to play a role in this process.     

Brain Region-Specific, Age-Related, Alterations in Mitochondrial Responses to Elevated Calcium

An age-related Ca(2+) dysregulation and increased production of reactive oxygen species (ROS) may contribute to late-onset neurodegenerative disorders. These alterations are often attributed to impaired mitochondrial function yet few studies have directly examined mitochondria isolated from various regions of the aged brain. The purpose of this study was to examine Ca(2+)-buffering and ROS production in mitochondria isolated from Fischer 344 rats ranging in age from 4 to 25 months. Mitchondria isolated from the cortex of the 25 month rat brain exhibited greater rates of ROS production and mitochondrial swelling in response to increasing Ca(2+) loads as compared to mitochondria isolated from younger (4, 13 month) animals. The increased swelling is indicative of opening of the mitochondrial permeability transition pore indicating impaired Ca(2+) buffering/cycling in aged animals. These age-related differences were not observed in mitochondria isolated from cerebellum. Together, these results demonstrate region specific, age-related, alterations in mitochondrial responses to Ca(2+).     

Mitochondrial proteomics as a selective tool for unraveling Parkinson’s disease pathogenesis

Parkinson’s disease (PD) is a neurodegenerative disease characterized by the large-scale loss of dopaminergic neurons in the substantia nigra and the formation of protein aggregates that accumulate in the cytoplasm of the remaining dopaminergic neurons. Most cases arise sporadically, while the precise cause remains obscure. This lack of understanding as to the etiology of PD continues to serve as a major barrier for delivering effective therapeutics. Mitochondria are potent integrators and coordinators of apoptosis, necrosis and cell survival. Neurotoxin-based and genetically modified animals, which mimic aspects of the core pathologies seen in human PD, support a role for oxidative stress, production of reactive oxygen species in excess and mitochondrial dysfunction in PD pathogenesis. This and other similar discoveries provide a convergence point for an explosion of morphological, biochemical, molecular, cell and animal model studies for investigating the contribution made by mitochondrial dysfunction to PD pathology. Proteomics screening technologies have proved to be a valuable aid in the investigator’s tool bag, by which to confirm a prominent role for mitochondrial proteins in PD pathology. Here, we discuss how an improved understanding of the mitochondrial proteome through the application of high-throughput proteomics, combined with genetic studies and pharmacological manipulations to influence mitochondrial dynamics and functions, promises to give insights into PD’s underlying disease mechanisms. Ultimately, such insights may pave the way towards designing novel strategies for providing symptomatic, neuroprotective and restorative therapeutic options to PD patients.     

线粒体功能障碍与孤独症

孤独症谱系障碍（ASDs）患儿中约有5%伴有线粒体功能紊乱.线粒体功能紊乱会损害对能量高度依赖的生理进程,如神经发育和神经可塑性,从而导致孤独症.本文综述了孤独症个体中线粒体过量的活性氧（reactive oxygen species,ROS）产生及其抗氧化系统减弱、呼吸链复合物异常、线粒体基因突变及与线粒体功能相关的基因组DNA编码的蛋白质异常等方面的研究,旨在阐述线粒体系统多方面的紊乱在孤独症个体中均有所体现,希望能够对孤独症的发病机制和治疗提供帮助.     

Evaluation of Ectopic Mitochondrial DNA in HeLa Cells

The presence of ectopic DNA in the cytoplasm induces inflammation and cell death. It has been widely reported that leakage of nuclear DNA into the cytoplasm can mainly be sensed by cyclic GMP-AMP synthase (cGAS). We recently reported that mitochondria-derived cytoplasmic double-stranded DNA (dsDNA) that has escaped lysosomal degradation induces significant cytotoxicity in cultured cells and in vivo. Cytoplasmic mitochondrial DNA is assumed to be involved in various diseases and disorders, and more and more papers have been published confirming this. On the other hand, the current method for evaluating mitochondrial DNA in the cytoplasm may not be quantitative. Here, we introduce in detail a method to evaluate ectopic mitochondrial DNA in cells. This method is useful in basic research as well as in the study of aging, Parkinson’s disease, Alzheimer’s disease, heart failure, autoimmune diseases, cancer, and other conditions.     

线粒体相关的疾病治疗及干预策略

线粒体疾病是一种累及不同的组织和器官的复杂异质性疾病,由核基因或线粒体基因的遗传缺陷导致,同时也受环境因素的影响。近十年来,有关线粒体疾病的诊断及发生机制的研究进展迅速,而疾病的治疗方法却研究较少。着重介绍线粒体疾病的相关治疗方法和干预策略。     

Phylogeny of the green lacewing Chrysoperla nipponensis species‐complex (Neuroptera: Chrysopidae) in China,based on mitochondrial sequences and AFLP data

Abstract Several studies have indicated that the green lacewing, Chrysoperla nipponensis (Neuroptera: Chrysopidae) may include more than one valid species. We investigated the phylogenetic status of Chrysoperla nipponensis s.l. in China and Japan using mitochondrial sequences and AFLP data. The molecular phylogenetic analyses based on mitochondrial genes showed that the C. nipponensis species‐complex comprises four clades, each having high support values. In addition, the phylogenetic tree based on AFLP data indicates that the species‐complex comprises three groups. These results confirm that C. nipponensis s.l. comprises at least three genetically distinct clades and suggests that two of these clades may be closely related to populations of C. nipponensis in Japan. However, these clades cannot be recognized as species until analysis of their courtship songs has been completed.     

Redox regulation of mitochondrial fission,protein misfolding,synaptic damage,and neuronal cell death: potential implications for Alzheimer’s and Parkinson’s diseases

Normal mitochondrial dynamics consist of fission and fusion events giving rise to new mitochondria, a process termed mitochondrial biogenesis. However, several neurodegenerative disorders manifest aberrant mitochondrial dynamics, resulting in morphological abnormalities often associated with deficits in mitochondrial mobility and cell bioenergetics. Rarely, dysfunctional mitochondrial occur in a familial pattern due to genetic mutations, but much more commonly patients manifest sporadic forms of mitochondrial disability presumably related to a complex set of interactions of multiple genes (or their products) with environmental factors (G × E). Recent studies have shown that generation of excessive nitric oxide (NO), in part due to generation of oligomers of amyloid-β (Aβ) protein or overactivity of the NMDA-subtype of glutamate receptor, can augment mitochondrial fission, leading to frank fragmentation of the mitochondria. S-Nitrosylation, a covalent redox reaction of NO with specific protein thiol groups, represents one mechanism contributing to NO-induced mitochondrial fragmentation, bioenergetic failure, synaptic damage, and eventually neuronal apoptosis. Here, we summarize our evidence in Alzheimer’s disease (AD) patients and animal models showing that NO contributes to mitochondrial fragmentation via S-nitrosylation of dynamin-related protein 1 (Drp1), a protein involved in mitochondrial fission. These findings may provide a new target for drug development in AD. Additionally, we review emerging evidence that redox reactions triggered by excessive levels of NO can contribute to protein misfolding, the hallmark of a number of neurodegenerative disorders, including AD and Parkinson’s disease. For example, S-nitrosylation of parkin disrupts its E3 ubiquitin ligase activity, and thereby affects Lewy body formation and neuronal cell death.     

Mitochondrial behaviour,morphology, and animal performance

We have a limited understanding of the proximate mechanisms that are responsible for the development of variation in animal performance and life‐history strategies. Provided that components of an organism's successful life history – for example, mate competition, gestation, lactation, etc. – are energetically demanding, increased energy production within mitochondria is likely the foundation from which organisms are able to perform these tasks. Mitochondrial behaviour (positioning within the cell and communication between mitochondria) and morphology affect variation in energy production at the molecular, cellular, and organismal levels. Therefore, adaptations in mitochondrial behaviour and morphology that favour efficient energy production likely influence variation in animal performance. Previous work has linked greater proportions of inter‐mitochondrial junctions and density of the inner mitochondrial membrane, among other traits, with increased energetic demand. Future research should focus on how inter‐mitochondrial junctions and morphology of the inner mitochondrial membrane, in particular, influence animal performance in accordance with mitochondrial density, fission, and fusion.     

Diagnosis and molecular basis of mitochondrial respiratory chain disorders: Exome sequencing for disease gene identification

Mitochondrial disorders have the highest incidence among congenital metabolic diseases, and are thought to occur at a rate of 1 in 5000 births. About 25% of the diseases diagnosed as mitochondrial disorders in the field of pediatrics have mitochondrial DNA abnormalities, while the rest occur due to defects in genes encoded in the nucleus. The most important function of the mitochondria is biosynthesis of ATP. Mitochondrial disorders are nearly synonymous with mitochondrial respiratory chain disorder, as respiratory chain complexes serve a central role in ATP biosynthesis. By next-generation sequencing of the exome, we analyzed 104 patients with mitochondrial respiratory chain disorders. The results of analysis to date were 18 patients with novel variants in genes previously reported to be disease-causing, and 27 patients with mutations in genes suggested to be associated in some way with mitochondria, and it is likely that they are new disease-causing genes in mitochondrial disorders. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.     

线粒体转运蛋白质家族

线粒体转运蛋白质家族（mitochondrial transporter family）等可溶性物质载体（solute carrier,SLC）,主要包括SLC25,广泛存在于真核生物线粒体中,负责可溶性物质跨线粒体内膜的转运。SLC25家族成员拥有相似的结构特征、种类繁多的转运底物,并与细胞的多种生理功能密切相关。有研究表明,SLC25家族蛋白质的缺失或突变可导致多种代谢性疾病或神经系统疾病的发生。     

Mitochondrial alteration in type 2 diabetes and obesity: An epigenetic link

The growing epidemic of type 2 diabetes mellitus (T2DM) and obesity is largely attributed to the current lifestyle of over-consumption and physical inactivity. As the primary platform controlling metabolic and energy homeostasis, mitochondria show aberrant changes in T2DM and obese subjects. While the underlying mechanism is under extensive investigation, epigenetic regulation is now emerging to play an important role in mitochondrial biogenesis, function, and dynamics. In line with lifestyle modifications preventing mitochondrial alterations and metabolic disorders, exercise has been shown to change DNA methylation of the promoter of PGC1α to favor gene expression responsible for mitochondrial biogenesis and function. In this article we discuss the epigenetic mechanism of mitochondrial alteration in T2DM and obesity, and the effects of lifestyle on epigenetic regulation. Future studies designed to further explore and integrate the epigenetic mechanisms with lifestyle modification may lead to interdisciplinary interventions and novel preventive options for mitochondrial alteration and metabolic disorders.     

Mitochondrial quality control pathways as determinants of metabolic health

Mitochondrial function is key for maintaining cellular health, while mitochondrial failure is associated with various pathologies, including inherited metabolic disorders and age‐related diseases. In order to maintain mitochondrial quality, several pathways of mitochondrial quality control have evolved. These systems monitor mitochondrial integrity through antioxidants, DNA repair systems, and chaperones and proteases involved in the mitochondrial unfolded protein response. Additional regulation of mitochondrial function involves dynamic exchange of components through mitochondrial fusion and fission. Sustained stress induces a selective autophagy – termed mitophagy – and ultimately leads to apoptosis. Together, these systems form a network that acts on the molecular, organellar, and cellular level. In this review, we highlight how these systems are regulated in an integrated context‐ and time‐dependent network of mitochondrial quality control that is implicated in healthy aging.     

Preparation of Mitochondrial Enriched Fractions for Metabolic Analysis in Drosophila

Since mitochondria play roles in amino acid metabolism, carbohydrate metabolism and fatty acid oxidation, defects in mitochondrial function often compromise the lives of those who suffer from these complex diseases. Detecting mitochondrial metabolic changes is vital to the understanding of mitochondrial disorders and mitochondrial responses to pharmacological agents. Although mitochondrial metabolism is at the core of metabolic regulation, the detection of subtle changes in mitochondrial metabolism may be hindered by the overrepresentation of other cytosolic metabolites obtained using whole organism or whole tissue extractions. Here we describe an isolation method that detected pronounced mitochondrial metabolic changes in Drosophila that were distinct between whole-fly and mitochondrial enriched preparations. To illustrate the sensitivity of this method, we used a set of Drosophila harboring genetically diverse mitochondrial DNAs (mtDNA) and exposed them to the drug rapamycin. Using this method we showed that rapamycin modifies mitochondrial metabolism in a mitochondrial-genotype-dependent manner. However, these changes are much more distinct in metabolomics studies when metabolites were extracted from mitochondrial enriched fractions. In contrast, whole tissue extracts only detected metabolic changes mediated by the drug rapamycin independently of mtDNAs.     

Sex and age differences in AMPK phosphorylation,mitochondrial homeostasis,and inflammation in hearts from inflammatory cardiomyopathy patients

Linked to exacerbated inflammation, myocarditis is a cardiovascular disease, which may lead to dilated cardiomyopathy. Although sex and age differences in the development of chronic myocarditis have been postulated, underlying cellular mechanisms remain poorly understood. In the current study, we aimed to investigate sex and age differences in mitochondrial homeostasis, inflammation, and cellular senescence. Cardiac tissue samples from younger and older patients with inflammatory dilated cardiomyopathy (DCMI) were used. The expression of Sirt1, phosphorylated AMPK, PGC-1α, Sirt3, acetylated SOD2, catalase, and several mitochondrial genes was analyzed to assess mitochondrial homeostasis. The expression of NF-κB, TLR4, and interleukins was used to examine the inflammatory state in the heart. Finally, several senescence markers and telomere length were investigated. Cardiac AMPK expression and phosphorylation were significantly elevated in male DCMI patients, whereas Sirt1 expression remained unchanged in all groups investigated. AMPK upregulation was accompanied by a preserved expression of all mitochondrial proteins/genes investigated in older male DCMI patients, whereas the expression of TOM40, TIM23, and the mitochondrial oxidative phosphorylation genes was significantly reduced in older female patients. Mitochondrial homeostasis in older male patients was further supported by the reduced acetylation of mitochondrial proteins as indicated by acetylated SOD2. The inflammatory markers NF-κB and TLR4 were downregulated in older male DCMI patients, whereas the expression of IL-18 was increased in older female patients. This was accompanied by progressed senescence in older DCMI hearts. In conclusion, older women experience more dramatic immunometabolic disorders on the cellular level than older men.     

A peep into mitochondrial disorder:multifaceted from mitochondrial DNAmutations to nuclear gene modulation

Mitochondrial genome is responsible for multiple human diseases in a maternal inherited pattern, yet phenotypes of patients in a same pedigree frequently vary largely. Genes involving in epigenetic modification, RNA processing, and other biological pathways, rather than “threshold effect” and environmental factors, provide more specific explanation to the aberrant phenotype. Thus, the double hit theory, mutations both in mitochondrial DNA and modifying genes aggravating the symptom, throws new light on mitochondrial dysfunction processes. In addition, mitochondrial retrograde signaling pathway that leads to reconfiguration of cell metabolism to adapt defects in mitochondria may as well play an active role. Here we review selected examples of modifier genes and mitochondrial retrograde signaling in mitochondrial disorders, which refine our understanding and will guide the rational design of clinical therapies.