CHCHD10 mutations promote loss of mitochondrial cristae junctions with impaired mitochondrial genome maintenance and inhibition of apoptosis

Decoding mitochondrial quality control mechanisms: Identifying treatment targets for enhanced cellular health

Mitochondria are singular cell organelles essential for many cellular functions, which includes responding to stress, regulating calcium levels, maintaining protein homeostasis, and coordinating apoptosis response. The vitality of cells, therefore, hinges on the optimal functioning of these dynamic organelles. Mitochondrial Quality Control Mechanisms (MQCM) play a pivotal role in ensuring the integrity and functionality of mitochondria. Perturbations in these mechanisms have been closely associated with the pathogenesis of neurodegenerative disorders such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and amyotrophic lateral sclerosis. Compelling evidence suggests that targeting specific pathways within the MQCM could potentially offer a therapeutic avenue for rescuing mitochondrial integrity and mitigating the progression of neurodegenerative diseases. The intricate interplay of cellular stress, protein misfolding, and impaired quality control mechanisms provides a nuanced understanding of the underlying pathology. Consequently, unravelling the specific MQCM dysregulation in neurodegenerative disorders becomes paramount for developing targeted therapeutic strategies. This review delves into the impaired MQCM pathways implicated in neurodegenerative disorders and explores emerging therapeutic interventions. By shedding light on pharmaceutical and genetic manipulations aimed at restoring MQCM efficiency, the discussion aims to provide insights into novel strategies for ameliorating the progression of neurodegenerative diseases. Understanding and addressing mitochondrial quality control mechanisms not only underscore their significance in cellular health but also offer a promising frontier for advancing therapeutic approaches in the realm of neurodegenerative disorders.

Aberrant protein aggregation in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal disease. As its pathological mechanisms are not well understood, there are no efficient therapeutics for it at present. While it is highly heterogenous both etiologically and clinically, it has a common salient hallmark, i.e., aberrant protein aggregation (APA). The upstream pathogenesis and the downstream effects of APA in ALS are sophisticated and the investigation of this pathology would be of consequence for understanding ALS. In this paper, the pathomechanism of APA in ALS and the candidate treatment strategies for it are discussed.

Amyloid fibril structures link CHCHD10 and CHCHD2 to neurodegeneration

CHCHD10 is mutated in rare cases of FTD and ALS and aggregates in mouse models of disease. Here we show that the disordered N-terminal domain of CHCHD10 forms amyloid fibrils and report their cryoEM structure. Disease-associated mutations cannot be accommodated by the WT fibril structure, while sequence differences between CHCHD10 and CHCHD2 are tolerated, explaining the co-aggregation of the two proteins and linking CHCHD10 and CHCHD2 amyloid fibrils to neurodegeneration.

Synaptopathy: presynaptic convergence in frontotemporal dementia and amyotrophic lateral sclerosis

Frontotemporal dementia and amyotrophic lateral sclerosis are common forms of neurodegenerative disease that share overlapping genetics and pathologies. Crucially, no significantly disease-modifying treatments are available for either disease. Identifying the earliest changes that initiate neuronal dysfunction is important for designing effective intervention therapeutics. The genes mutated in genetic forms of frontotemporal dementia and amyotrophic lateral sclerosis have diverse cellular functions, and multiple disease mechanisms have been proposed for both. Identification of a convergent disease mechanism in frontotemporal dementia and amyotrophic lateral sclerosis would focus research for a targetable pathway, which could potentially effectively treat all forms of frontotemporal dementia and amyotrophic lateral sclerosis (both familial and sporadic). Synaptopathies are diseases resulting from physiological dysfunction of synapses, and define the earliest stages in multiple neuronal diseases, with synapse loss a key feature in dementia. At the presynapse, the process of synaptic vesicle recruitment, fusion and recycling is necessary for activity-dependent neurotransmitter release. The unique distal location of the presynaptic terminal means the tight spatio-temporal control of presynaptic homeostasis is dependent on efficient local protein translation and degradation. Recently, numerous publications have shown that mutations associated with frontotemporal dementia and amyotrophic lateral sclerosis present with synaptopathy characterized by presynaptic dysfunction. This review will describe the complex local signalling and membrane trafficking events that occur at the presynapse to facilitate neurotransmission and will summarize recent publications linking frontotemporal dementia/amyotrophic lateral sclerosis genetic mutations to presynaptic function. This evidence indicates that presynaptic synaptopathy is an early and convergent event in frontotemporal dementia and amyotrophic lateral sclerosis and illustrates the need for further research in this area, to identify potential therapeutic targets with the ability to impact this convergent pathomechanism.

Mitochondrial membrane synthesis, remodelling and cellular trafficking

Mitochondria are dynamic cellular organelles with complex roles in metabolism and signalling. Primary mitochondrial disorders are a group of approximately 400 monogenic disorders arising from pathogenic genetic variants impacting mitochondrial structure, ultrastructure and/or function. Amongst these disorders, defects of complex lipid biosynthesis, especially of the unique mitochondrial membrane lipid cardiolipin, and membrane biology are an emerging group characterised by clinical heterogeneity, but with recurrent features including cardiomyopathy, encephalopathy, neurodegeneration, neuropathy and 3-methylglutaconic aciduria. This review discusses lipid synthesis in the mitochondrial membrane, the mitochondrial contact site and cristae organising system (MICOS), mitochondrial dynamics and trafficking, and the disorders associated with defects of each of these processes. We highlight overlapping functions of proteins involved in lipid biosynthesis and protein import into the mitochondria, pointing to an overarching coordination and synchronisation of mitochondrial functions. This review also focuses on membrane interactions between mitochondria and other organelles, namely the endoplasmic reticulum, peroxisomes, lysosomes and lipid droplets. We signpost disorders of these membrane interactions that may explain the observation of secondary mitochondrial dysfunction in heterogeneous pathological processes. Disruption of these organellar interactions ultimately impairs cellular homeostasis and organismal health, highlighting the central role of mitochondria in human health and disease.

The MICOS Complex Regulates Mitochondrial Structure and Oxidative Stress During Age-Dependent Structural Deficits in the Kidney

The kidney filters nutrient waste and bodily fluids from the bloodstream, in addition to secondary functions of metabolism and hormone secretion, requiring an astonishing amount of energy to maintain its functions. In kidney cells, mitochondria produce adenosine triphosphate (ATP) and help maintain kidney function. Due to aging, the efficiency of kidney functions begins to decrease. Dysfunction in mitochondria and cristae, the inner folds of mitochondria, is a hallmark of aging. Therefore, age-related kidney function decline could be due to changes in mitochondrial ultrastructure, increased reactive oxygen species (ROS), and subsequent alterations in metabolism and lipid composition. We sought to understand if there is altered mitochondrial ultrastructure, as marked by 3D morphological changes, across time in tubular kidney cells. Serial block facing-scanning electron microscope (SBF-SEM) and manual segmentation using the Amira software were used to visualize murine kidney samples during the aging process at 3 months (young) and 2 years (old). We found that 2-year mitochondria are more fragmented, compared to the 3-month, with many uniquely shaped mitochondria observed across aging, concomitant with shifts in ROS, metabolomics, and lipid homeostasis. Furthermore, we show that the mitochondrial contact site and cristae organizing system (MICOS) complex is impaired in the kidney due to aging. Disruption of the MICOS complex shows altered mitochondrial calcium uptake and calcium retention capacity, as well as generation of oxidative stress. We found significant, detrimental structural changes to aged kidney tubule mitochondria suggesting a potential mechanism underlying why kidney diseases occur more readily with age. We hypothesize that disruption in the MICOS complex further exacerbates mitochondrial dysfunction, creating a vicious cycle of mitochondrial degradation and oxidative stress, thus impacting kidney health.

Apolipoprotein O modulates cholesterol metabolism via NRF2/CYB5R3 independent of LDL receptor

Apolipoprotein O (APOO) plays a critical intracellular role in regulating lipid metabolism. Here, we investigated the roles of APOO in metabolism and atherogenesis in mice. Hepatic APOO expression was increased in response to hyperlipidemia but was inhibited after simvastatin treatment. Using a novel APOO global knockout (Apoo-/-) model, it was found that APOO depletion aggravated diet-induced obesity and elevated plasma cholesterol levels. Upon crossing with low-density lipoprotein receptor (LDLR) and apolipoprotein E (APOE) knockout hyperlipidemic mouse models, Apoo-/- Apoe-/- and Apoo-/- Ldlr-/- mice exhibited elevated plasma cholesterol levels, with more severe atherosclerotic lesions than littermate controls. This indicated the effects of APOO on cholesterol metabolism independent of LDLR and APOE. Moreover, APOO deficiency reduced cholesterol excretion through bile and feces while decreasing phospholipid unsaturation by inhibiting NRF2 and CYB5R3. Restoration of CYB5R3 expression in vivo by adeno-associated virus (AAV) injection reversed the reduced degree of phospholipid unsaturation while decreasing blood cholesterol levels. This represents the first in vivo experimental validation of the role of APOO in plasma cholesterol metabolism independent of LDLR and elucidates a previously unrecognized cholesterol metabolism pathway involving NRF2/CYB5R3. APOO may be a metabolic regulator of total-body cholesterol homeostasis and a target for atherosclerosis management. Apolipoprotein O (APOO) regulates plasma cholesterol levels and atherosclerosis through a pathway involving CYB5R3 that regulates biliary and fecal cholesterol excretion, independently of the LDL receptor. In addition, down-regulation of APOO may lead to impaired mitochondrial function, which in turn aggravates diet-induced obesity and fat accumulation.

Super-resolution microscopies, technological breakthrough to decipher mitochondrial structure and dynamic

Mitochondria are complex organelles with an outer membrane enveloping a second inner membrane that creates a vast matrix space partitioned by pockets or cristae that join the peripheral inner membrane with several thin junctions. Several micrometres long, mitochondria are generally close to 300 nm in diameter, with membrane layers separated by a few tens of nanometres. Ultrastructural data from electron microscopy revealed the structure of these mitochondria, while conventional optical microscopy revealed their extraordinary dynamics through fusion, fission, and migration processes but its limited resolution power restricted the possibility to go further. By overcoming the limits of light diffraction, Super-Resolution Microscopy (SRM) now offers the potential to establish the links between the ultrastructure and remodelling of mitochondrial membranes, leading to major advances in our understanding of mitochondria's structure-function. Here we review the contributions of SRM imaging to our understanding of the relationship between mitochondrial structure and function. What are the hopes for these new imaging approaches which are particularly important for mitochondrial pathologies?

CHCHD10S59L/+ mouse model: Behavioral and neuropathological features of frontotemporal dementia

CHCHD10-related disease causes a spectrum of clinical presentations including mitochondrial myopathy, cardiomyopathy, amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). We generated a knock-in mouse model bearing the p.Ser59Leu (S59L) CHCHD10 variant. Chchd10S59L/+ mice have been shown to phenotypically replicate the disorders observed in patients: myopathy with mtDNA instability, cardiomyopathy and typical ALS features (protein aggregation, neuromuscular junction degeneration and spinal motor neuron loss). Here, we conducted a comprehensive behavioral, electrophysiological and neuropathological assessment of Chchd10S59L/+ mice. These animals show impaired learning and memory capacities with reduced long-term potentiation (LTP) measured at the Perforant Pathway-Dentate Gyrus (PP-DG) synapses. In the hippocampus of Chchd10S59L/+ mice, neuropathological studies show the involvement of protein aggregates, activation of the integrated stress response (ISR) and neuroinflammation in the degenerative process. These findings contribute to decipher mechanisms associated with CHCHD10 variants linking mitochondrial dysfunction and neuronal death. They also validate the Chchd10S59L/+ mice as a relevant model for FTD, which can be used for preclinical studies to test new therapeutic strategies for this devastating disease.

Aberrant CHCHD2-associated mitochondriopathy in Kii ALS/PDC astrocytes

Amyotrophic Lateral Sclerosis/Parkinsonism-Dementia Complex (ALS/PDC), a rare and complex neurological disorder, is predominantly observed in the Western Pacific islands, including regions of Japan, Guam, and Papua. This enigmatic condition continues to capture medical attention due to affected patients displaying symptoms that parallel those seen in either classical amyotrophic lateral sclerosis (ALS) or Parkinson's disease (PD). Distinctly, postmortem examinations of the brains of affected individuals have shown the presence of α-synuclein aggregates and TDP-43, which are hallmarks of PD and classical ALS, respectively. These observations are further complicated by the detection of phosphorylated tau, accentuating the multifaceted proteinopathic nature of ALS/PDC. The etiological foundations of this disease remain undetermined, and genetic investigations have yet to provide conclusive answers. However, emerging evidence has implicated the contribution of astrocytes, pivotal cells for maintaining brain health, to neurodegenerative onset, and likely to play a significant role in the pathogenesis of ALS/PDC. Leveraging advanced induced pluripotent stem cell technology, our team cultivated multiple astrocyte lines to further investigate the Japanese variant of ALS/PDC (Kii ALS/PDC). CHCHD2 emerged as a significantly dysregulated gene when disease astrocytes were compared to healthy controls. Our analyses also revealed imbalances in the activation of specific pathways: those associated with astrocytic cilium dysfunction, known to be involved in neurodegeneration, and those related to major neurological disorders, including classical ALS and PD. Further in-depth examinations revealed abnormalities in the mitochondrial morphology and metabolic processes of the affected astrocytes. A particularly striking observation was the reduced expression of CHCHD2 in the spinal cord, motor cortex, and oculomotor nuclei of patients with Kii ALS/PDC. In summary, our findings suggest a potential reduction in the support Kii ALS/PDC astrocytes provide to neurons, emphasizing the need to explore the role of CHCHD2 in maintaining mitochondrial health and its implications for the disease.

C. elegans as a model to study mitochondrial biology and disease

Mitochondria perform a myriad of essential functions that ensure organismal homeostasis, including maintaining bioenergetic capacity, sensing and signalling the presence of pathogenic threats, and determining cell fate. Their function is highly dependent on mitochondrial quality control and the appropriate regulation of mitochondrial size, shape, and distribution during an entire lifetime, as well as their inheritance across generations. The roundworm Caenorhabditis elegans has emerged as an ideal model organism through which to study mitochondria. The remarkable conservation of mitochondrial biology has allowed C. elegans researchers to investigate complex processes that are challenging to study in higher organisms. In this review, we explore the key recent contributions of C. elegans to mitochondrial biology through the lens of mitochondrial dynamics, organellar removal, and mitochondrial inheritance, as well as their involvement in immune responses, various types of stress, and transgenerational signalling.

Disruption of Mitophagy Flux through the PARL-PINK1 Pathway by CHCHD10 Mutations or CHCHD10 Depletion

Coiled-coil-helix-coiled-coil-helix domain-containing 10 (CHCHD10) is a nuclear-encoded mitochondrial protein which is primarily mutated in the spectrum of familial and sporadic amyotrophic lateral sclerosis (ALS)-frontotemporal dementia (FTD). Endogenous CHCHD10 levels decline in the brains of ALS-FTD patients, and the CHCHD10S59L mutation in Drosophila induces dominant toxicity together with PTEN-induced kinase 1 (PINK1), a protein critical for the induction of mitophagy. However, whether and how CHCHD10 variants regulate mitophagy flux in the mammalian brain is unknown. Here, we demonstrate through in vivo and in vitro models, as well as human FTD brain tissue, that ALS/FTD-linked CHCHD10 mutations (R15L and S59L) impair mitophagy flux and mitochondrial Parkin recruitment, whereas wild-type CHCHD10 (CHCHD10WT) normally enhances these measures. Specifically, we show that CHCHD10R15L and CHCHD10S59L mutations reduce PINK1 levels by increasing PARL activity, whereas CHCHD10WT produces the opposite results through its stronger interaction with PARL, suppressing its activity. Importantly, we also demonstrate that FTD brains with TAR DNA-binding protein-43 (TDP-43) pathology demonstrate disruption of the PARL-PINK1 pathway and that experimentally impairing mitophagy promotes TDP-43 aggregation. Thus, we provide herein new insights into the regulation of mitophagy and TDP-43 aggregation in the mammalian brain through the CHCHD10-PARL-PINK1 pathway.

3D reconstruction of murine mitochondria reveals changes in structure during aging linked to the MICOS complex

During aging, muscle gradually undergoes sarcopenia, the loss of function associated with loss of mass, strength, endurance, and oxidative capacity. However, the 3D structural alterations of mitochondria associated with aging in skeletal muscle and cardiac tissues are not well described. Although mitochondrial aging is associated with decreased mitochondrial capacity, the genes responsible for the morphological changes in mitochondria during aging are poorly characterized. We measured changes in mitochondrial morphology in aged murine gastrocnemius, soleus, and cardiac tissues using serial block-face scanning electron microscopy and 3D reconstructions. We also used reverse transcriptase-quantitative PCR, transmission electron microscopy quantification, Seahorse analysis, and metabolomics and lipidomics to measure changes in mitochondrial morphology and function after loss of mitochondria contact site and cristae organizing system (MICOS) complex genes, Chchd3, Chchd6, and Mitofilin. We identified significant changes in mitochondrial size in aged murine gastrocnemius, soleus, and cardiac tissues. We found that both age-related loss of the MICOS complex and knockouts of MICOS genes in mice altered mitochondrial morphology. Given the critical role of mitochondria in maintaining cellular metabolism, we characterized the metabolomes and lipidomes of young and aged mouse tissues, which showed profound alterations consistent with changes in membrane integrity, supporting our observations of age-related changes in muscle tissues. We found a relationship between changes in the MICOS complex and aging. Thus, it is important to understand the mechanisms that underlie the tissue-dependent 3D mitochondrial phenotypic changes that occur in aging and the evolutionary conservation of these mechanisms between Drosophila and mammals.

Genomic and transcriptomic advances in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder and the most common motor neuron disease. ALS shows substantial clinical and molecular heterogeneity. In vitro and in vivo models coupled with multiomic techniques have provided important contributions to unraveling the pathomechanisms underlying ALS. To date, despite promising results and accumulating knowledge, an effective treatment is still lacking. Here, we provide an overview of the literature on the use of genomics, epigenomics, transcriptomics and microRNAs to deeply investigate the molecular mechanisms developing and sustaining ALS. We report the most relevant genes implicated in ALS pathogenesis, discussing the use of different high-throughput sequencing techniques and the role of epigenomic modifications. Furthermore, we present transcriptomic studies discussing the most recent advances, from microarrays to bulk and single-cell RNA sequencing. Finally, we discuss the use of microRNAs as potential biomarkers and promising tools for molecular intervention. The integration of data from multiple omic approaches may provide new insights into pathogenic pathways in ALS by shedding light on diagnostic and prognostic biomarkers, helping to stratify patients into clinically relevant subgroups, revealing novel therapeutic targets and supporting the development of new effective therapies.

Mitochondria, a Key Target in Amyotrophic Lateral Sclerosis Pathogenesis

Mitochondrial dysfunction occurs in numerous neurodegenerative diseases, particularly amyotrophic lateral sclerosis (ALS), where it contributes to motor neuron (MN) death. Of all the factors involved in ALS, mitochondria have been considered as a major player, as secondary mitochondrial dysfunction has been found in various models and patients. Abnormal mitochondrial morphology, defects in mitochondrial dynamics, altered activities of respiratory chain enzymes and increased production of reactive oxygen species have been described. Moreover, the identification of CHCHD10 variants in ALS patients was the first genetic evidence that a mitochondrial defect may be a primary cause of MN damage and directly links mitochondrial dysfunction to the pathogenesis of ALS. In this review, we focus on the role of mitochondria in ALS and highlight the pathogenic variants of ALS genes associated with impaired mitochondrial functions. The multiple pathways demonstrated in ALS pathogenesis suggest that all converge to a common endpoint leading to MN loss. This may explain the disappointing results obtained with treatments targeting a single pathological process. Fighting against mitochondrial dysfunction appears to be a promising avenue for developing combined therapies in the future.

Nutritional and metabolic factors in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a complex neurodegenerative disease that is classically thought to impact the motor system. Over the past 20 years, research has started to consider the contribution of non-motor symptoms and features of the disease, and how they might affect ALS prognosis. Of the non-motor features of the disease, nutritional status (for example, malnutrition) and metabolic balance (for example, weight loss and hypermetabolism) have been consistently shown to contribute to more rapid disease progression and/or earlier death. Several complex cellular changes observed in ALS, including mitochondrial dysfunction, are also starting to be shown to contribute to bioenergetic failure. The resulting energy depletion in high energy demanding neurons makes them sensitive to apoptosis. Given that nutritional and metabolic stressors at the whole-body and cellular level can impact the capacity to maintain optimal function, these factors present avenues through which we can identify novel targets for treatment in ALS. Several clinical trials are now underway evaluating the effectiveness of modifying energy balance in ALS, making this article timely in reviewing the evidence base for metabolic and nutritional interventions.

Mitochondrial heterogeneity in diseases

As key organelles involved in cellular metabolism, mitochondria frequently undergo adaptive changes in morphology, components and functions in response to various environmental stresses and cellular demands. Previous studies of mitochondria research have gradually evolved, from focusing on morphological change analysis to systematic multiomics, thereby revealing the mitochondrial variation between cells or within the mitochondrial population within a single cell. The phenomenon of mitochondrial variation features is defined as mitochondrial heterogeneity. Moreover, mitochondrial heterogeneity has been reported to influence a variety of physiological processes, including tissue homeostasis, tissue repair, immunoregulation, and tumor progression. Here, we comprehensively review the mitochondrial heterogeneity in different tissues under pathological states, involving variant features of mitochondrial DNA, RNA, protein and lipid components. Then, the mechanisms that contribute to mitochondrial heterogeneity are also summarized, such as the mutation of the mitochondrial genome and the import of mitochondrial proteins that result in the heterogeneity of mitochondrial DNA and protein components. Additionally, multiple perspectives are investigated to better comprehend the mysteries of mitochondrial heterogeneity between cells. Finally, we summarize the prospective mitochondrial heterogeneity-targeting therapies in terms of alleviating mitochondrial oxidative damage, reducing mitochondrial carbon stress and enhancing mitochondrial biogenesis to relieve various pathological conditions. The possibility of recent technological advances in targeted mitochondrial gene editing is also discussed.

Loss of mitochondrial Chchd10 or Chchd2 in zebrafish leads to an ALS-like phenotype and Complex I deficiency independent of the mitochondrial integrated stress response

Mutations in CHCHD10 and CHCHD2, encoding two paralogous mitochondrial proteins, have been identified in cases of amyotrophic lateral sclerosis, frontotemporal lobar degeneration, and Parkinson's disease. Their role in disease is unclear, though both have been linked to mitochondrial respiration and mitochondrial stress responses. Here, we investigated the biological roles of these proteins during vertebrate development using knockout (KO) models in zebrafish. We demonstrate that loss of either or both proteins leads to motor impairment, reduced survival and compromised neuromuscular junction integrity in larval zebrafish. Compensation by Chchd10 was observed in the chchd2^-/- model, but not by Chchd2 in the chchd10^-/- model. The assembly of mitochondrial respiratory chain Complex I was impaired in chchd10^-/- and chchd2^-/- zebrafish larvae, but unexpectedly not in a double chchd10^-/- and chchd2^-/- model, suggesting that reduced mitochondrial Complex I cannot be solely responsible for the observed phenotypes, which are generally more severe in the double KO. We observed transcriptional activation markers of the mitochondrial integrated stress response (mt-ISR) in the double chchd10^-/- and chchd2^-/- KO model, suggesting that this pathway is involved in the restoration of Complex I assembly in our double KO model. The data presented here demonstrates that the Complex I assembly defect in our single KO models arises independently of the mt-ISR. Furthermore, this study provides evidence that both proteins are required for normal vertebrate development.

Single cell analysis of docosahexaenoic acid suppression of sequential LPS-induced proinflammatory and interferon-regulated gene expression in the macrophage

Preclinical and clinical studies suggest that consumption of long chain omega-3 polyunsaturated fatty acids (PUFAs) reduces severity of chronic inflammatory and autoimmune diseases. While these ameliorative effects are conventionally associated with downregulated expression of proinflammatory cytokine and chemokine genes, our laboratory has recently identified Type 1 interferon (IFN1)-regulated gene expression to be another key target of omega-3 PUFAs. Here we used single cell RNA sequencing (scRNAseq) to gain new mechanistic perspectives on how the omega-3 PUFA docosahexaenoic acid (DHA) influences TLR4-driven proinflammatory and IFN1-regulated gene expression in a novel self-renewing murine fetal liver-derived macrophage (FLM) model. FLMs were cultured with 25 µM DHA or vehicle for 24 h, treated with modest concentration of LPS (20 ng/ml) for 1 and 4 h, and then subjected to scRNAseq using the 10X Chromium System. At 0 h (i.e., in the absence of LPS), DHA increased expression of genes associated with the NRF2 antioxidant response (e.g. Sqstm1, Hmox1, Chchd10) and metal homeostasis (e.g.Mt1, Mt2, Ftl1, Fth1), both of which are consistent with DHA-induced polarization of FLMs to a more anti-inflammatory phenotype. At 1 h post-LPS treatment, DHA inhibited LPS-induced cholesterol synthesis genes (e.g. Scd1, Scd2, Pmvk, Cyp51, Hmgcs1, and Fdps) which potentially could contribute to interference with TLR4-mediated inflammatory signaling. At 4 h post-LPS treatment, LPS-treated FLMs reflected a more robust inflammatory response including upregulation of proinflammatory cytokine (e.g. Il1a, Il1b, Tnf) and chemokine (e.g.Ccl2, Ccl3, Ccl4, Ccl7) genes as well as IFN1-regulated genes (e.g. Irf7, Mx1, Oasl1, Ifit1), many of which were suppressed by DHA. Using single-cell regulatory network inference and clustering (SCENIC) to identify gene expression networks, we found DHA modestly downregulated LPS-induced expression of NF-κB-target genes. Importantly, LPS induced a subset of FLMs simultaneously expressing NF-κB- and IRF7/STAT1/STAT2-target genes that were conspicuously absent in DHA-pretreated FLMs. Thus, DHA potently targeted both the NF-κB and the IFN1 responses. Altogether, scRNAseq generated a valuable dataset that provides new insights into multiple overlapping mechanisms by which DHA may transcriptionally or post-transcriptionally regulate LPS-induced proinflammatory and IFN1-driven responses in macrophages.

A CHCHD6-APP axis connects amyloid and mitochondrial pathology in Alzheimer's disease

The mechanistic relationship between amyloid-beta precursor protein (APP) processing and mitochondrial dysfunction in Alzheimer's disease (AD) has long eluded the field. Here, we report that coiled-coil-helix-coiled-coil-helix domain containing 6 (CHCHD6), a core protein of the mammalian mitochondrial contact site and cristae organizing system, mechanistically connects these AD features through a circular feedback loop that lowers CHCHD6 and raises APP processing. In cellular and animal AD models and human AD brains, the APP intracellular domain fragment inhibits CHCHD6 transcription by binding its promoter. CHCHD6 and APP bind and stabilize one another. Reduced CHCHD6 enhances APP accumulation on mitochondria-associated ER membranes and accelerates APP processing, and induces mitochondrial dysfunction and neuronal cholesterol accumulation, promoting amyloid pathology. Compensation for CHCHD6 loss in an AD mouse model reduces AD-associated neuropathology and cognitive impairment. Thus, CHCHD6 connects APP processing and mitochondrial dysfunction in AD. This provides a potential new therapeutic target for patients.

Intracellular energy controls dynamics of stress-induced ribonucleoprotein granules

Energy metabolism and membraneless organelles have been implicated in human diseases including neurodegeneration. How energy deficiency regulates ribonucleoprotein particles such as stress granules (SGs) is still unclear. Here we identified a unique type of granules induced by energy deficiency under physiological conditions and uncovered the mechanisms by which the dynamics of diverse stress-induced granules are regulated. Severe energy deficiency induced the rapid formation of energy deficiency-induced stress granules (eSGs) independently of eIF2α phosphorylation, whereas moderate energy deficiency delayed the clearance of conventional SGs. The formation of eSGs or the clearance of SGs was regulated by the mTOR-4EBP1-eIF4E pathway or eIF4A1, involving assembly of the eIF4F complex or RNA condensation, respectively. In neurons or brain organoids derived from patients carrying the C9orf72 repeat expansion associated with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), the eSG formation was enhanced, and the clearance of conventional SGs was impaired. These results reveal a critical role for intracellular energy in the regulation of diverse granules and suggest that disruptions in energy-controlled granule dynamics may contribute to the pathogenesis of relevant diseases.

Stress granules are associated with neurodegenerative diseases. Here, Wang et al. found intracellular energy deficiencies trigger a unique type of granules and disrupt granule disassembly through 4EBP1/eIF4A.

Sam50 exerts neuroprotection by maintaining the mitochondrial structure during experimental cerebral ischemia/reperfusion injury in rats

AIM

To investigate the role of Sam50, a barrel protein on the surface of the mitochondrial outer membrane, in cerebral ischemia-reperfusion (I/R) injury and its underlying mechanisms.

METHODS

A middle cerebral artery occlusion/reperfusion (MCAO/R) model in adult male Sprague-Dawley rats was established in vivo, and cultured neurons were exposed to oxygen-glucose deprivation/reoxygenation (OGD/R) to simulate I/R injury in vitro. Lentiviral vector encoding Sam50 or Sam50 shRNA was constructed and administered to rats by intracerebroventricular injection to overexpress and knockdown Sam50, respectively.

RESULTS

First, after MCAO/R induction, the mitochondrial structure was damaged, and Sam50 protein levels were increased responsively both in vivo and in vitro. Then, it was found that Sam50 overexpression could reduce infarction size, inhibit neuronal cell death, improve neurobehavioral disability, protect mitochondrial structure integrity, and ameliorate mitochondrial dysfunction, which was induced by I/R injury both in vivo and in vitro. However, Sam50 downregulation showed the opposite results and aggravated I/R injury by inducing neuronal cell death, neurobehavioral disability, and mitochondrial dysfunction. Moreover, we found that the interaction between Sam50 and Mic19 was broken off after OGD/R, showing that the Sam50-Mic19-Mic60 axis was breakage in neurons, which would be a reason for mitochondrial structure and function abnormalities induced by I/R injury.

CONCLUSION

Sam50 played a vital role in the protection of neurons and mitochondria in cerebral I/R injury, which could be a novel target for mitochondrial protection and ameliorating I/R injury.

CHCHD10 Modulates Thermogenesis of Adipocytes by Regulating Lipolysis

Brown and beige adipocytes dissipate energy in a nonshivering thermogenesis manner, exerting beneficial effects on metabolic homeostasis. CHCHD10 is a nuclear-encoded mitochondrial protein involved in cristae organization; however, its role in thermogenic adipocytes remains unknown. We identify CHCHD10 as a novel regulator for adipocyte thermogenesis. CHCHD10 is dramatically upregulated during thermogenic adipocyte activation by PPARγ-PGC1α and positively correlated with UCP1 expression in adipose tissues from humans and mice. We generated adipocyte-specific Chchd10 knockout mice (Chchd10-AKO) and found that depleting CHCHD10 leads to impaired UCP1-dependent thermogenesis and energy expenditure in the fasting state, with no effect in the fed state. Lipolysis in adipocytes is disrupted by CHCHD10 deficiency, while augmented lipolysis through ATGL overexpression recovers adipocyte thermogenesis in Chchd10-AKO mice. Consistently, overexpression of Chchd10 activates thermogenic adipocytes. Mechanistically, CHCHD10 deficiency results in the disorganization of mitochondrial cristae, leading to impairment of oxidative phosphorylation complex assembly in mitochondria, which in turn inhibits ATP generation. Decreased ATP results in downregulation of lipolysis by reducing nascent protein synthesis of ATGL, thereby suppressing adipocyte thermogenesis. As a result, Chchd10-AKO mice are prone to develop high-fat diet-induced metabolic disorders. Together, our findings reveal an essential role of CHCHD10 in regulating lipolysis and the thermogenic program in adipocytes.

Oxidative Stress in Amyotrophic Lateral Sclerosis: Synergy of Genetic and Environmental Factors

Amyotrophic lateral sclerosis (ALS) is a grievous neurodegenerative disease whose survival is limited to only a few years. In spite of intensive research to discover the underlying mechanisms, the results are fairly inconclusive. Multiple hypotheses have been regarded, including genetic, molecular, and cellular processes. Notably, oxidative stress has been demonstrated to play a crucial role in ALS pathogenesis. In addition to already recognized and exhaustively studied genetic mutations involved in oxidative stress production, exposure to various environmental factors (e.g., electromagnetic fields, solvents, pesticides, heavy metals) has been suggested to enhance oxidative damage. This review aims to describe the main processes influenced by the most frequent genetic mutations and environmental factors concurring in oxidative stress occurrence in ALS and the potential therapeutic molecules capable of diminishing the ALS related pro-oxidative status.

CHCHD2 maintains mitochondrial contact site and cristae organizing system stability and protects against mitochondrial dysfunction in an experimental model of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's dementia. Mitochondrial dysfunction is involved in the pathology of PD. Coiled-coil-helix-coiled-coil-helix domain-containing 2 (CHCHD2) was identified as associated with autosomal dominant PD. However, the mechanism of CHCHD2 in PD remains unclear.

METHODS

Short hairpin RNA (ShRNA)-mediated CHCHD2 knockdown or lentivirus-mediated CHCHD2 overexpression was performed to investigate the impact of CHCHD2 on mitochondrial morphology and function in neuronal tumor cell lines represented with human neuroblastoma (SHSY5Y) and HeLa cells. Blue-native polyacrylamide gel electrophoresis (PAGE) and two-dimensional sodium dodecyl sulfate-PAGE analysis were used to illustrate the role of CHCHD2 in mitochondrial contact site and cristae organizing system (MICOS). Co-immunoprecipitation and immunoblotting were used to address the interaction between CHCHD2 and Mic10. Serotype injection of adeno-associated vector-mediated CHCHD2 and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration were used to examine the influence of CHCHD2 in vivo.

RESULTS

We found that the overexpression of CHCHD2 can protect against methyl-4-phenylpyridinium (MPP+)-induced mitochondrial dysfunction and inhibit the loss of dopaminergic neurons in the MPTP-induced mouse model. Furthermore, we identified that CHCHD2 interacted with Mic10, and overexpression of CHCHD2 can protect against MPP+-induced MICOS impairment, while knockdown of CHCHD2 impaired the stability of MICOS.

CONCLUSION

This study indicated that CHCHD2 could interact with Mic10 and maintain the stability of the MICOS complex, which contributes to protecting mitochondrial function in PD.

Modulation of synaptic plasticity, motor unit physiology, and TDP-43 pathology by CHCHD10

Mutations in CHCHD10, a gene coding for a mitochondrial intermembrane space protein, are associated with Frontotemporal dementia (FTD)-Amyotrophic lateral sclerosis (ALS) spectrum disorders, which are pathologically characterized by cytoplasmic inclusions containing TDP-43. FTD/ALS-linked CHCHD10 mutations and TDP-43 inclusions similarly induce mitochondrial defects in respiration, fusion/fission, mtDNA stability, and cristae structure, while sizeable amounts of cytoplasmic TDP-43 aggregates are found in mitochondria. However, the mechanistic link between CHCHD10 and TDP-43 pathogenesis remains unclear. In this study, we present immunohistochemical and biochemical evidence demonstrating that insoluble CHCHD10 aggregates accumulate and colocalize with phospho-TDP-43 inclusions in brains of FTLD-TDP and AD patients, and that insoluble CHCHD10 levels tightly correlate with insoluble TDP-43 levels in control and FTLD-TDP brains. In an experimental exploration of this pathological phenotype, transgenic mice neuronally expressing FTD/ALS-linked CHCHD10R15L or CHCHDS59L mutations but not CHCHD10WT transgenic mice exhibit significantly increased CHCHD10 aggregation and phospho-TDP-43 pathology, which often colocalize within the same inclusions. Such pathologies are reflected in poor functional outcomes in long-term synaptic plasticity, motor unit physiology, and behavior in CHCHD10R15L and CHCHDS59L transgenic mice. In contrast, expression of CHCHD10WT in hTDP-43 transgenic mice (TAR4;CHCHD10WT) significantly mitigates phospho-TDP-43 pathology and rescues TDP-43-induced impairments in synaptic integrity and long-term synaptic plasticity. In isolated mitochondria, the S59L mutation induces the aggregation of resident CHCHD10S59L protein as well as the aggregation and slower turnover of recombinant TDP-43 imported into mitochondria. Likewise, in an in vitro cell-free system, the S59L mutation induces the aggregation of CHCHD10S59L protein while simultaneously enhancing the aggregation of recombinant TDP-43, as evidenced by filter trap assays and atomic force microscopy. In contrast, recombinant CHCHD10WT inhibits the growth of TDP-43 aggregates. These results in human brains, transgenic mice, and in vitro systems substantiate the role of wild type and mutant CHCHD10 in modulating mitochondrial CHCHD10 and TDP-43 pathogenesis together with associated phenotypes in long-term synaptic plasticity and motor unit physiology in mice and humans.

CHCHD10 and SLP2 control the stability of the PHB complex: a key factor for motor neuron viability

CHCHD10 is an amyotrophic lateral sclerosis/frontotemporal dementia (ALS/FTD) gene that encodes a mitochondrial protein whose precise function is unclear. Here we show that CHCHD10 interacts with the Stomatin-Like Protein 2 (SLP2) and participates to the stability of the Prohibitin (PHB) complex in the inner mitochondrial membrane. By using patient fibroblasts and mouse models expressing the same CHCHD10 variant (p.Ser59Leu), we show that SLP2 forms aggregates with prohibitins, found in vivo in the hippocampus and as aggresome-like inclusions in spinal motor neurons of Chchd10S59L/+ mice. Affected cells and tissues display instability of the PHB complex which participates at least in part to the activation of the OMA1 cascade with OPA1 processing leading to mitochondrial fragmentation, abnormal mitochondrial cristae morphogenesis and neuronal death found in spinal cord and the hippocampus of Chchd10S59L/+ animals. Destabilization of the PHB complex leads to the instability of the mitochondrial contact site and cristae organizing system (MICOS) complex, likely via the disruption of OPA1/Mitofilin interaction. Thus, SLP2/PHB aggregates and destabilization of the PHB complex are critical in the sequence of events leading to motor neuron death in CHCHD10S59L-related disease.

Heteroplasmic mitochondrial DNA mutations in frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) is a common cause of young onset dementia and is characterised by focal neuropathology. The reasons for the regional neuronal vulnerability are not known. Mitochondrial mechanisms have been implicated in the pathogenesis of FTLD, raising the possibility that frontotemporal regional mutations of mitochondrial DNA (mtDNA) are contributory causes. Here we applied dual sequencing of the entire mtDNA at high depth to identify high-fidelity single nucleotide variants (mtSNVs) and mtDNA rearrangements in post mortem brain tissue of people affected by FTLD and age-matched controls. Both mtSNVs and mtDNA rearrangements were elevated in the temporal lobe, with the greatest burden seen in FTLD. mtSNVs found in multiple brain regions also reached a higher heteroplasmy levels in the temporal lobe. The temporal lobe of people with FTLD had a higher burden of ribosomal gene variants predicted to affect intra-mitochondrial protein synthesis, and a higher proportion of missense variants in genes coding for respiratory chain subunits. In conclusion, heteroplasmic mtDNA variants predicted to affect oxidative phosphorylation are enriched in FTLD temporal lobe, and thus may contribute to the regional vulnerability in pathogenesis.

Structures of the Wild-Type and S59L Mutant CHCHD10 Proteins Important in Amyotrophic Lateral Sclerosis-Frontotemporal Dementia

The S59L genetic mutation of the mitochondrial coiled-coil-helix-coiled-coil-helix domain-containing protein 10 (CHCHD10) is involved in the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The wild-type and mutant forms of this protein contain intrinsically disordered regions, and their structural characterization has been facing challenges. Here, for the first time in the literature, we present the structural ensemble properties of the wild-type and S59L mutant form of CHCHD10 in an aqueous solution environment at the atomic level with dynamics. Even though available experiments suggested that the S59L mutation may not change the structure of the CHCHD10 protein, our structural analysis clearly shows that the structure of this protein is significantly affected by the S59L mutation. We present here the secondary structure components with their abundances per residue, the tertiary structure properties, the free energy surfaces based on the radius of gyration and end-to-end distance values, the Ramachandran plots, the quantity of intramolecular hydrogen bonds, and the principal component analysis results. These results may be crucial in designing more efficient treatment for ALS and FTD diseases.

The pathogenesis of amyotrophic lateral sclerosis: Mitochondrial dysfunction, protein misfolding and epigenetics

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with multiple complex mechanisms involved. Among them, mitochondrial dysfunction plays an important role in ALS. Multiple studies have shown that mitochondria are closely associated with reactive oxygen species production and oxidative stress and exhibit different functional states in different genetic backgrounds. In this review we explored the roles of Ca²⁺, autophagy, mitochondrial quality control in the regulation of mitochondrial homeostasis and their relationship with ALS. In addition, we also summarized and analyzed the roles of protein misfolding and abnormal aggregation in the pathogenesis of ALS. Moreover, we also discussed how epigenetic mechanisms such as DNA methylation and protein post-translational modification affect initiation and progression of ALS. Nevertheless, existing events still cannot fully explain the pathogenesis of ALS at present, more studies are required to explore pathological mechanisms of ALS.

Chchd10 is dispensable for myogenesis but critical for adipose browning

The Chchd10 gene encodes a coiled-coil-helix-coiled-coil-helix-domain containing protein predicted to function in the mitochondrion and nucleus. Mutations of Chchd10 are associated with ALS, dementia and myopathy in humans and animal models, but how knockout of Chchd10 (Chchd10^KO) affects various tissues especially skeletal muscle and adipose tissues remains unclear. Here we show that Chchd10 expression increases as myoblasts and preadipocytes differentiate. During myogenesis, CHCHD10 interacts with TAR DNA binding protein 43 (TDP-43) in regenerating myofibers in vivo and in newly differentiated myotubes ex vivo. Surprisingly, Chchd10^KO mice had normal skeletal muscle development, growth and regeneration, with moderate defects in grip strength and motor performance. Chchd10^KO similarly had no effects on development of brown and white adipose tissues (WAT). However, Chchd10^KO mice had blunted response to acute cold and attenuated cold-induced browning of WAT, with markedly reduced UCP1 levels. Together, these results demonstrate that Chchd10 is dispensable for normal myogenesis and adipogenesis but is required for normal motility and cold-induced, mitochondrion-dependent browning of adipocytes. The data also suggest that human CHCHD10 mutations cause myopathy through a gain-of-function mechanism.

Therapeutic Approaches to Amyotrophic Lateral Sclerosis from the Lab to the Clinic

Amyotrophic Lateral Sclerosis (ALS) is a terminal neuro-degenerative disorder that is clinically recognized as a gradual degeneration of the upper and lower motor neurons, with an average duration of 3 to 5 years from initiation of symptoms to death. The mechanisms underlying the pathogenesis and progression of the disease are multifactorial. Therefore, to find effective treatments, it is necessary to understand this heterogeneity underlying the progression of ALS. Recent developments in gene therapy have opened a new avenue to treat this condition, especially for the characterized genetic types. Gene therapy methods have been studied in a variety of pre-clinical settings and clinical trials, and they may be a promising path for developing an effective and safe ALS cure. A growing body of evidence demonstrates abnormalities in energy metabolism at the cellular and whole-body level in animal models and in people living with ALS. The use and incorporation of high-throughput "omics" methods has radically transformed our thought about ALS, strengthening our understanding of the disease's dynamic molecular architecture, differentiating distinct patient subtypes, and creating a reasonable basis for the identification of biomarkers and novel individualised treatments. Future clinical and laboratory trials would also focus on the diverse relationships between metabolism and ALS to address the issue of whether targeting deficient metabolism in ALS is an effective way to change disease progression. In this review, we focus on the detailed pathogenesis of ALS and highlight principal genes, i.e., SOD1, TDP-43, C9orf72, and FUS, targeted therapeutic approaches of ALS. An attempt is made to provide up-to-date information on clinical outcomes, including various biomarkers which are thought to be important players in early ALS detection.

Serum Creatine, Not Neurofilament Light, Is Elevated in CHCHD10-Linked Spinal Muscular Atrophy

Objective

To characterize serum biomarkers in mitochondrial CHCHD10-linked spinal muscular atrophy Jokela (SMAJ) type for disease monitoring and for the understanding of pathogenic mechanisms.

Methods

We collected serum samples from a cohort of 49 patients with SMAJ, all carriers of the heterozygous c.197G>T p.G66V variant in CHCHD10. As controls, we used age- and sex-matched serum samples obtained from Helsinki Biobank. Creatine kinase and creatinine were measured by standard methods. Neurofilament light (NfL) and glial fibrillary acidic protein (GFAP) were measured with single molecule array (Simoa), fibroblast growth factor 21 (FGF-21), and growth differentiation factor 15 (GDF-15) with an enzyme-linked immunosorbent assay. For non-targeted plasma metabolite profiling, samples were analyzed with liquid chromatography high-resolution mass spectrometry. Disease severity was evaluated retrospectively by calculating a symptom-based score.

Results

Axon degeneration marker, NfL, was unexpectedly not altered in the serum of patients with SMAJ, whereas astrocytic activation marker, GFAP, was slightly decreased. Creatine kinase was elevated in most patients, particularly men. We identified six metabolites that were significantly altered in serum of patients with SMAJ in comparison to controls: increased creatine and pyruvate, and decreased creatinine, taurine, N-acetyl-carnosine, and succinate. Creatine correlated with disease severity. Altered pyruvate and succinate indicated a metabolic response to mitochondrial dysfunction; however, lactate or mitochondrial myopathy markers FGF-21 or GDF-15 was not changed.

Conclusions

Biomarkers of muscle mass and damage are altered in SMAJ serum, indicating a role for skeletal muscle in disease pathogenesis in addition to neurogenic damage. Despite the minimal mitochondrial pathology in skeletal muscle, signs of a metabolic shift can be detected.

CHCHD2 and CHCHD10 regulate mitochondrial dynamics and integrated stress response

Mitochondrial dysfunction is becoming one of the main pathology factors involved in the etiology of neurological disorders. Recently, mutations of the coiled-coil-helix-coiled-coil-helix domain containing 2 (CHCHD2) and 10 (CHCHD10) which encode two homologous proteins that belong to the mitochondrial CHCH domain protein family, are linked to Parkinson's disease and amyotrophic lateral sclerosis (ALS)/frontotemporal dementia (FTD), respectively. However, the physiological and pathological roles of these twin proteins have not been well elaborated. Here, we show that, in physiological conditions, CHCHD2 and CHCHD10 interact with OMA1 and suppress its enzyme activity, which not only restrains the initiation of the mitochondrial integrated response stress (mtISR), but also suppresses the processing of OPA1 for mitochondrial fusion. Further, during mitochondria stress-induced by carbonyl cyanide m-chlorophenylhydrazone (CCCP) treatment, CHCHD2 and CHCHD10 translocate to the cytosol and interacte with eIF2a, which attenuates mtISR overactivation by suppressing eIF2a phosphorylation and its downstream response. As such, knockdown of CHCHD2 and CHCHD10 triggers mitochondrial ISR, and such cellular response is enhanced by CCCP treatment. Therefore, our findings demonstrate the first "mtISR suppressor" localized in mitochondria for regulating stress responses in mammalian cells, which has a profound pathological impact on the CHCH2/CHCH10-linked neurodegenerative disorder.

Mitochondrial Membrane Remodeling

Mitochondria are key regulators of many important cellular processes and their dysfunction has been implicated in a large number of human disorders. Importantly, mitochondrial function is tightly linked to their ultrastructure, which possesses an intricate membrane architecture defining specific submitochondrial compartments. In particular, the mitochondrial inner membrane is highly folded into membrane invaginations that are essential for oxidative phosphorylation. Furthermore, mitochondrial membranes are highly dynamic and undergo constant membrane remodeling during mitochondrial fusion and fission. It has remained enigmatic how these membrane curvatures are generated and maintained, and specific factors involved in these processes are largely unknown. This review focuses on the current understanding of the molecular mechanism of mitochondrial membrane architectural organization and factors critical for mitochondrial morphogenesis, as well as their functional link to human diseases.

Cristae junction as a fundamental switchboard for mitochondrial ion signaling and bioenergetics

OPA1 and MICU1 are both involved in the regulation of mitochondrial Ca2+ uptake and the stabilization of the cristae junction, which separates the inner mitochondrial membrane into the interboundary membrane and the cristae membrane. In this mini-review, we focus on the synergetic control of OPA1 and MICU1 on the cristae junction that serves as a fundamental regulator of multiple mitochondrial functions. In particular, we point to the critical role of an adaptive cristae junction permeability in mitochondrial Ca2+ signaling, spatial H+ gradients and mitochondrial membrane potential, metabolic activity, and apoptosis. These characteristics bear on a distinct localization of the oxidative phosphorylation machinery, the FoF1-ATPase, and mitochondrial Ca2+uniporter (MCU) within sections of the inner mitochondrial membrane isolated by the cristae junction and regulated by proteins like OPA1 and MICU1. We specifically focus on the impact of MICU1-regulated cristae junction on the activity and distribution of MCU within the complex ultrastructure of mitochondria.

Genetic architecture of motor neuron diseases

Motor neuron diseases (MNDs) are rare and frequently fatal neurological disorders in which motor neurons within the brainstem and spinal cord regions slowly die. MNDs are primarily caused by genetic mutations, and > 100 different mutant genes in humans have been discovered thus far. Given the fact that many more MND-related genes have yet to be discovered, the growing body of genetic evidence has offered new insights into the diverse cellular and molecular mechanisms involved in the aetiology and pathogenesis of MNDs. This search may aid in the selection of potential candidate genes for future investigation and, eventually, may open the door to novel interventions to slow down disease progression. In this review paper, we have summarized detailed existing research findings of different MNDs, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal bulbar muscle atrophy (SBMA) and hereditary spastic paraplegia (HSP) in relation to their complex genetic architecture.

Quantitative high-confidence human mitochondrial proteome and its dynamics in cellular context

Mitochondria are key organelles for cellular energetics, metabolism, signaling, and quality control and have been linked to various diseases. Different views exist on the composition of the human mitochondrial proteome. We classified >8,000 proteins in mitochondrial preparations of human cells and defined a mitochondrial high-confidence proteome of >1,100 proteins (MitoCoP). We identified interactors of translocases, respiratory chain, and ATP synthase assembly factors. The abundance of MitoCoP proteins covers six orders of magnitude and amounts to 7% of the cellular proteome with the chaperones HSP60-HSP10 being the most abundant mitochondrial proteins. MitoCoP dynamics spans three orders of magnitudes, with half-lives from hours to months, and suggests a rapid regulation of biosynthesis and assembly processes. 460 MitoCoP genes are linked to human diseases with a strong prevalence for the central nervous system and metabolism. MitoCoP will provide a high-confidence resource for placing dynamics, functions, and dysfunctions of mitochondria into the cellular context.

The Role of Mitochondria in Optic Atrophy With Autosomal Inheritance

Optic atrophy (OA) with autosomal inheritance is a form of optic neuropathy characterized by the progressive and irreversible loss of vision. In some cases, this is accompanied by additional, typically neurological, extra-ocular symptoms. Underlying the loss of vision is the specific degeneration of the retinal ganglion cells (RGCs) which form the optic nerve. Whilst autosomal OA is genetically heterogenous, all currently identified causative genes appear to be associated with mitochondrial organization and function. However, it is unclear why RGCs are particularly vulnerable to mitochondrial aberration. Despite the relatively high prevalence of this disorder, there are currently no approved treatments. Combined with the lack of knowledge concerning the mechanisms through which aberrant mitochondrial function leads to RGC death, there remains a clear need for further research to identify the underlying mechanisms and develop treatments for this condition. This review summarizes the genes known to be causative of autosomal OA and the mitochondrial dysfunction caused by pathogenic mutations. Furthermore, we discuss the suitability of available in vivo models for autosomal OA with regards to both treatment development and furthering the understanding of autosomal OA pathology.

Mitochondrial Quality Control Strategies: Potential Therapeutic Targets for Neurodegenerative Diseases?

Many lines of evidence have indicated the therapeutic potential of rescuing mitochondrial integrity by targeting specific mitochondrial quality control pathways in neurodegenerative diseases, such as Parkinson's disease, Huntington's disease, and Alzheimer's disease. In addition to ATP synthesis, mitochondria are critical regulators of ROS production, lipid metabolism, calcium buffering, and cell death. The mitochondrial unfolded protein response, mitochondrial dynamics, and mitophagy are the three main quality control mechanisms responsible for maintaining mitochondrial proteostasis and bioenergetics. The proper functioning of these complex processes is necessary to surveil and restore mitochondrial homeostasis and the healthy pool of mitochondria in cells. Mitochondrial dysfunction occurs early and causally in disease pathogenesis. A significant accumulation of mitochondrial damage resulting from compromised quality control pathways leads to the development of neuropathology. Moreover, genetic or pharmaceutical manipulation targeting the mitochondrial quality control mechanisms can sufficiently rescue mitochondrial integrity and ameliorate disease progression. Thus, therapies that can improve mitochondrial quality control have great promise for the treatment of neurodegenerative diseases. In this review, we summarize recent progress in the field that underscores the essential role of impaired mitochondrial quality control pathways in the pathogenesis of neurodegenerative diseases. We also discuss the translational approaches targeting mitochondrial function, with a focus on the restoration of mitochondrial integrity, including mitochondrial dynamics, mitophagy, and mitochondrial proteostasis.

Loss of CHCHD2 and CHCHD10 activates OMA1 peptidase to disrupt mitochondrial cristae phenocopying patient mutations

Dominant mutations in the mitochondrial paralogs coiled-helix-coiled-helix (CHCHD) domain 2 (C2) and CHCHD10 (C10) were recently identified as causing Parkinson's disease and amyotrophic lateral sclerosis/frontotemporal dementia/myopathy, respectively. The mechanism by which they disrupt mitochondrial cristae, however, has been uncertain. Using the first C2/C10 double knockout (DKO) mice, we report that C10 pathogenesis and the normal function of C2/C10 are intimately linked. Similar to patients with C10 mutations, we found that C2/C10 DKO mice have disrupted mitochondrial cristae, because of cleavage of the mitochondrial-shaping protein long form of OPA1 (L-OPA1) by the stress-induced peptidase OMA1. OMA1 was found to be activated similarly in affected tissues of mutant C10 knock-in (KI) mice, demonstrating that L-OPA1 cleavage is a novel mechanism for cristae abnormalities because of both C10 mutation and C2/C10 loss. Using OMA1 activation as a functional assay, we found that C2 and C10 are partially functionally redundant, and some but not all disease-causing mutations have retained activity. Finally, C2/C10 DKO mice partially phenocopied mutant C10 KI mice with the development of cardiomyopathy and activation of the integrated mitochondrial integrated stress response in affected tissues, tying mutant C10 pathogenesis to C2/C10 function.

DNA damage as a mechanism of neurodegeneration in ALS and a contributor to astrocyte toxicity

Increasing evidence supports the involvement of DNA damage in several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Elevated levels of DNA damage are consistently observed in both sporadic and familial forms of ALS and may also play a role in Western Pacific ALS, which is thought to have an environmental cause. The cause of DNA damage in ALS remains unclear but likely differs between genetic subgroups. Repeat expansion in the C9ORF72 gene is the most common genetic cause of familial ALS and responsible for about 10% of sporadic cases. These genetic mutations are known to cause R-loops, thus increasing genomic instability and DNA damage, and generate dipeptide repeat proteins, which have been shown to lead to DNA damage and impairment of the DNA damage response. Similarly, several genes associated with ALS including TARDBP, FUS, NEK1, SQSTM1 and SETX are known to play a role in DNA repair and the DNA damage response, and thus may contribute to neuronal death via these pathways. Another consistent feature present in both sporadic and familial ALS is the ability of astrocytes to induce motor neuron death, although the factors causing this toxicity remain largely unknown. In this review, we summarise the evidence for DNA damage playing a causative or secondary role in the pathogenesis of ALS as well as discuss the possible mechanisms involved in different genetic subtypes with particular focus on the role of astrocytes initiating or perpetuating DNA damage in neurons.

MICOS and the mitochondrial inner membrane morphology - when things get out of shape

Mitochondria play a key role in cellular signalling, metabolism and energetics. Proper architecture and remodelling of the inner mitochondrial membrane are essential for efficient respiration, apoptosis and quality control in the cell. Several protein complexes including mitochondrial contact site and cristae organizing system (MICOS), F₁ F_O -ATP synthase, and Optic Atrophy 1 (OPA1), facilitate formation, maintenance and stability of cristae membranes. MICOS, the F₁ F_O -ATP synthase, OPA1 and inner membrane phospholipids such as cardiolipin and phosphatidylethanolamine interact with each other to organize the inner membrane ultra-structure and remodel cristae in response to the cell's demands. Functional alterations in these proteins or in the biosynthesis pathway of cardiolipin and phosphatidylethanolamine result in an aberrant inner membrane architecture and impair mitochondrial function. Mitochondrial dysfunction and abnormalities hallmark several human conditions and diseases including neurodegeneration, cardiomyopathies and diabetes mellitus. Yet, they have long been regarded as secondary pathological effects. This review discusses emerging evidence of a direct relationship between protein- and lipid-dependent regulation of the inner mitochondrial membrane morphology and diseases such as fatal encephalopathy, Leigh syndrome, Parkinson's disease, and cancer.

Emerging Roles of the MICOS Complex in Cristae Dynamics and Biogenesis

Simple Summary

Mitochondria possess an outer and inner membrane. The part of the inner membrane parallel to the outer membrane is termed the inner boundary membrane, while the cristae membrane folds towards the mitochondrial matrix and houses the respiratory chain complexes. Crista junctions are located at the interface of the inner boundary membrane and the cristae membrane and contain the important ‘mitochondrial contact site and cristae organizing system’ complex. Despite the growing evidence that the mitochondrial inner membrane could remodel, cristae membranes were largely considered static for nearly seventy years, as the observations were mostly based on electron microscopy and tomography. Recently, using fluorescence super-resolution techniques, several studies showed that cristae membranes undergo dynamic remodeling in living cells, and probably even fission and fusion of the inner membrane. In this review, we discuss the important recent literature conveying the emerging role of the MICOS complex in cristae dynamics and its relation to cristae biogenesis. As the aberrant inner membrane architecture is connected to various pathologies such as cardiomyopathies, neurodegeneration and diabetes, understanding the roles of various molecules connected with cristae biogenesis and dynamics would shed light on the pathophysiology, probably leading to therapeutics in the near future.

Abstract

Mitochondria are double membrane-enclosed organelles performing important cellular and metabolic functions such as ATP generation, heme biogenesis, apoptosis, ROS production and calcium buffering. The mitochondrial inner membrane (IM) is folded into cristae membranes (CMs) of variable shapes using molecular players including the ‘mitochondrial contact site and cristae organizing system’ (MICOS) complex, the dynamin-like GTPase OPA1, the F₁F_O ATP synthase and cardiolipin. Aberrant cristae structures are associated with different disorders such as diabetes, neurodegeneration, cancer and hepato-encephalopathy. In this review, we provide an updated view on cristae biogenesis by focusing on novel roles of the MICOS complex in cristae dynamics and shaping of cristae. For over seven decades, cristae were considered as static structures. It was recently shown that cristae constantly undergo rapid dynamic remodeling events. Several studies have re-oriented our perception on the dynamic internal ambience of mitochondrial compartments. In addition, we discuss the recent literature which sheds light on the still poorly understood aspect of cristae biogenesis, focusing on the role of MICOS and its subunits. Overall, we provide an integrated and updated view on the relation between the biogenesis of cristae and the novel aspect of cristae dynamics.

Mitochondrial CHCHD2: Disease-Associated Mutations, Physiological Functions, and Current Animal Models

Rare mutations in the mitochondrial protein coiled-coil-helix-coiled-coil-helix domain containing 2 (CHCHD2) are associated with Parkinson's disease (PD) and other Lewy body disorders. CHCHD2 is a bi-organellar mediator of oxidative phosphorylation, playing crucial roles in regulating electron flow in the mitochondrial electron transport chain and acting as a nuclear transcription factor for a cytochrome c oxidase subunit (COX4I2) and itself in response to hypoxic stress. CHCHD2 also regulates cell migration and differentiation, mitochondrial cristae structure, and apoptosis. In this review, we summarize the known disease-associated mutations of CHCHD2 in Asian and Caucasian populations, the physiological functions of CHCHD2, how CHCHD2 mutations contribute to α-synuclein pathology, and current animal models of CHCHD2. Further, we discuss the necessity of continued investigation into the divergent functions of CHCHD2 and CHCHD10 to determine how mutations in these similar mitochondrial proteins contribute to different neurodegenerative diseases.

TDP-43 and PINK1 mediate CHCHD10S59L mutation-induced defects in Drosophila and in vitro

Mutations in coiled-coil-helix-coiled-coil-helix domain containing 10 (CHCHD10) can cause amyotrophic lateral sclerosis and frontotemporal dementia (ALS-FTD). However, the underlying mechanisms are unclear. Here, we generate CHCH10^S59L-mutant Drosophila melanogaster and HeLa cell lines to model CHCHD10-associated ALS-FTD. The CHCHD10^S59L mutation results in cell toxicity in several tissues and mitochondrial defects. CHCHD10^S59L independently affects the TDP-43 and PINK1 pathways. CHCHD10^S59L expression increases TDP-43 insolubility and mitochondrial translocation. Blocking TDP-43 mitochondrial translocation with a peptide inhibitor reduced CHCHD10^S59L-mediated toxicity. While genetic and pharmacological modulation of PINK1 expression and activity of its substrates rescues and mitigates the CHCHD10^S59L-induced phenotypes and mitochondrial defects, respectively, in both Drosophila and HeLa cells. Our findings suggest that CHCHD10^S59L-induced TDP-43 mitochondrial translocation and chronic activation of PINK1-mediated pathways result in dominant toxicity, providing a mechanistic insight into the CHCHD10 mutations associated with ALS-FTD.

Multi-OMICS study of a CHCHD10 variant causing ALS demonstrates metabolic rewiring and activation of endoplasmic reticulum and mitochondrial unfolded protein responses

Mutations in CHCHD10, coding for a mitochondrial intermembrane space protein, are a rare cause of autosomal dominant amyotrophic lateral sclerosis. Mutation-specific toxic gain of function or haploinsufficiency models have been proposed to explain pathogenicity. To decipher the metabolic dysfunction associated with the haploinsufficient p.R15L variant, we integrated transcriptomic, metabolomic and proteomic data sets in patient cells subjected to an energetic stress that forces the cells to rely on oxidative phosphorylation for ATP production. Patient cells had a complex I deficiency that resulted in an increased NADH/NAD+ ratio, diminished TCA cycle activity, a reorganization of one carbon metabolism and an increased AMP/ATP ratio leading to phosphorylation of AMPK and inhibition of mTORC1. These metabolic changes activated the unfolded protein response (UPR) in the ER through the IRE1/XBP1 pathway, upregulating downstream targets including ATF3, ATF4, CHOP and EGLN3, and two cytokine markers of mitochondrial disease, GDF15 and FGF21. Activation of the mitochondrial UPR was mediated through an upregulation of the transcription factors ATF4 and ATF5, leading to increased expression of mitochondrial proteases and heat shock proteins. There was a striking transcriptional up regulation of at least seven dual specific phosphatases, associated with an almost complete dephosphorylation of JNK isoforms, suggesting a concerted deactivation of MAP kinase pathways. This study demonstrates that loss of CHCHD10 function elicits an energy deficit that activates unique responses to nutrient stress in both the mitochondria and ER, which may contribute to the selective vulnerability of motor neurons.

Amyotrophic Lateral Sclerosis and Frontotemporal Lobar Degenerations: Similarities in Genetic Background

Amyotrophic lateral sclerosis (ALS) is a devastating, uniformly lethal progressive degenerative disorder of motor neurons that overlaps with frontotemporal lobar degeneration (FTLD) clinically, morphologically, and genetically. Although many distinct mutations in various genes are known to cause amyotrophic lateral sclerosis, it remains poorly understood how they selectively impact motor neuron biology and whether they converge on common pathways to cause neuronal degeneration. Many of the gene mutations are in proteins that share similar functions. They can be grouped into those associated with cell axon dynamics and those associated with cellular phagocytic machinery, namely protein aggregation and metabolism, apoptosis, and intracellular nucleic acid transport. Analysis of pathways implicated by mutant ALS genes has provided new insights into the pathogenesis of both familial forms of ALS (fALS) and sporadic forms (sALS), although, regrettably, this has not yet yielded definitive treatments. Many genes play an important role, with TARDBP, SQSTM1, VCP, FUS, TBK1, CHCHD10, and most importantly, C9orf72 being critical genetic players in these neurological disorders. In this mini-review, we will focus on the molecular mechanisms of these two diseases.

Human microsporidian pathogen Encephalitozoon intestinalis impinges on enterocyte membrane trafficking and signaling

Microsporidia are a large phylum of obligate intracellular parasites. Approximately a dozen species of microsporidia infect humans, where they are responsible for a variety of diseases and occasionally death, especially in immunocompromised individuals. To better understand the impact of microsporidia on human cells, we infected human colonic Caco2 cells with Encephalitozoon intestinalis, and showed that these enterocyte cultures can be used to recapitulate the life cycle of the parasite, including the spread of infection with infective spores. Using transmission electron microscopy, we describe this lifecycle and demonstrate nuclear, mitochondrial and microvillar alterations by this pathogen. We also analyzed the transcriptome of infected cells to reveal host cell signaling alterations upon infection. These high-resolution imaging and transcriptional profiling analysis shed light on the impact of the microsporidial infection on its primary human target cell type.This article has an associated First Person interview with the first authors of the paper.