Mice harbouring a SCA28 patient mutation in AFG3L2 develop late-onset ataxia associated with enhanced mitochondrial proteotoxicity

Compound heterozygous mutation of AFG3L2 causes autosomal recessive spinocerebellar ataxia through mitochondrial impairment and MICU1 mediated Ca2+ overload

Autosomal recessive spinocerebellar ataxias (SCARs) are one of the most common neurodegenerative diseases characterized by progressive ataxia. Although SCARs are known to be caused by mutations in multiple genes, there are still many cases that go undiagnosed or are misdiagnosed. In this study, we presented a SCAR patient, and identified a probable novel pathogenic mutation (c.1A>G, p.M1V) in the AFG3L2 start codon. The proband's genotype included heterozygous mutations of the compound AFG3L2 (p.[M1V]; [R632X] (c.[1A>G]; [1894.C>T])), which were inherited from the father (c.1A>G, p.M1V) and mother (c.1894C>T, p.R632X). Functional studies performed on hiPSCs (human induced pluripotent stem cells) generated from the patients and HEK293T cells showed that the mutations impair mitochondrial function and the unbalanced expression of AFG3L2 mRNA and protein levels. Furthermore, this novel mutation resulted in the degradation of the protein and the reduction of the stability of the AFG3L2 protein, and MCU (mitochondrial calcium uniporter) complex mediated Ca2+ overload.

Multifaceted Roles of AFG3L2, a Mitochondrial ATPase in Relation to Neurological Disorders

AFG3L2 is a zinc metalloprotease and an ATPase localized in an inner mitochondrial membrane involved in mitochondrial quality control of several nuclear- and mitochondrial-encoded proteins. Mutations in AFG3L2 lead to diseases like slow progressive ataxia, which is a neurological disorder. This review delineates the cellular functions of AFG3L2 and its dysfunction that leads to major clinical outcomes, which include spinocerebellar ataxia type 28, spastic ataxia type 5, and optic atrophy type 12. It summarizes all relevant AFG3L2 mutations associated with the clinical outcomes to understand the detailed mechanisms attributable to its structure-related multifaceted roles in proteostasis and quality control. We face early diagnostic challenges of ataxia and optic neuropathy due to asymptomatic parents and variable clinical manifestations due to heterozygosity/homozygosity of AFG3L2 mutations. This review intends to promote AFG3L2 as a putative prognostic or diagnostic marker.

Spinocerebellar ataxias: from pathogenesis to recent therapeutic advances

Spinocerebellar ataxia is a phenotypically and genetically heterogeneous group of autosomal dominant-inherited degenerative disorders. The gene mutation spectrum includes dynamic expansions, point mutations, duplications, insertions, and deletions of varying lengths. Dynamic expansion is the most common form of mutation. Mutations often result in indistinguishable clinical phenotypes, thus requiring validation using multiple genetic testing techniques. Depending on the type of mutation, the pathogenesis may involve proteotoxicity, RNA toxicity, or protein loss-of-function. All of which may disrupt a range of cellular processes, such as impaired protein quality control pathways, ion channel dysfunction, mitochondrial dysfunction, transcriptional dysregulation, DNA damage, loss of nuclear integrity, and ultimately, impairment of neuronal function and integrity which causes diseases. Many disease-modifying therapies, such as gene editing technology, RNA interference, antisense oligonucleotides, stem cell technology, and pharmacological therapies are currently under clinical trials. However, the development of curative approaches for genetic diseases remains a global challenge, beset by technical, ethical, and other challenges. Therefore, the study of the pathogenesis of spinocerebellar ataxia is of great importance for the sustained development of disease-modifying molecular therapies.

Mitochondrial protein synthesis quality control

Human mitochondrial DNA is one of the most simplified cellular genomes and facilitates compartmentalized gene expression. Within the organelle, there is no physical barrier to separate transcription and translation, nor is there evidence that quality control surveillance pathways are active to prevent translation on faulty mRNA transcripts. Mitochondrial ribosomes synthesize 13 hydrophobic proteins that require co-translational insertion into the inner membrane of the organelle. To maintain the integrity of the inner membrane, which is essential for organelle function, requires responsive quality control mechanisms to recognize aberrations in protein synthesis. In this review, we explore how defects in mitochondrial protein synthesis can arise due to the culmination of inherent mistakes that occur throughout the steps of gene expression. In turn, we examine the stepwise series of quality control processes that are needed to eliminate any mistakes that would perturb organelle homeostasis. We aim to provide an integrated view on the quality control mechanisms of mitochondrial protein synthesis and to identify promising avenues for future research.

Ucp4 Knockdown of Cerebellar Purkinje Cells Induces Bradykinesia

Although uncoupling protein 4 (UCP4) is the most abundant protein reported in the brain, the biological function of UCP4 in cerebellum and pathological outcome of UCP4 deficiency in cerebellum remain obscure. To evaluate the role of Ucp4 in the cerebellar Purkinje cells (PCs), we generated the conditional knockdown of Ucp4 in PCs (Pcp2cre;Ucp4fl/fl mice) by breeding Ucp4fl/fl mice with Pcp2cre mice. Series results by Western blot, immunofluorescent staining, and triple RNAscope in situ hybridization confirmed the specific ablation of Ucp4 in PCs in Pcp2cre;Ucp4fl/fl mice, but did not affect the expression of Ucp2, the analog of Ucp4. Combined behavioral tests showed that Pcp2cre;Ucp4fl/fl mice displayed a characteristic bradykinesia in the spontaneous movements. The electromyogram recordings detection excluded the possibility of hypotonia in Pcp2cre;Ucp4fl/fl mice. And the electrical patch clamp recordings showed the altered properties of PCs in Pcp2cre;Ucp4fl/fl mice. Moreover, transmission electron microscope (TEM) results showed the increased mitochondrial circularity in PCs; ROS probe imaging showed the increased ROS generation in molecular layer; and finally, microplate reader assay showed the significant changes of mitochondrial functions, including ROS, ATP, and MMP in the isolated cerebellum tissue. The results suggested that the specific knockdown of mitochondrial protein Ucp4 could damage PCs possibly by attacking their mitochondrial function. The present study is the first to report a close relationship between UCP4 deletion with PCs impairment, and suggests the importance of UCP4 in the substantial support of mitochondrial function homeostasis in bradykinesia. UCP4 might be a therapeutic target for the cerebellar-related movement disorder.

Targeting Ion Channels and Purkinje Neuron Intrinsic Membrane Excitability as a Therapeutic Strategy for Cerebellar Ataxia

In degenerative neurological disorders such as Parkinson's disease, a convergence of widely varying insults results in a loss of dopaminergic neurons and, thus, the motor symptoms of the disease. Dopamine replacement therapy with agents such as levodopa is a mainstay of therapy. Cerebellar ataxias, a heterogeneous group of currently untreatable conditions, have not been identified to have a shared physiology that is a target of therapy. In this review, we propose that perturbations in cerebellar Purkinje neuron intrinsic membrane excitability, a result of ion channel dysregulation, is a common pathophysiologic mechanism that drives motor impairment and vulnerability to degeneration in cerebellar ataxias of widely differing genetic etiologies. We further propose that treatments aimed at restoring Purkinje neuron intrinsic membrane excitability have the potential to be a shared therapy in cerebellar ataxia akin to levodopa for Parkinson's disease.

Thymidine Kinase 2 and Mitochondrial Protein COX I in the Cerebellum of Patients with Spinocerebellar Ataxia Type 31 Caused by Penta-nucleotide Repeats (TTCCA)n

Spinocerebellar ataxia type 31 (SCA31), an autosomal-dominant neurodegenerative disorder characterized by progressive cerebellar ataxia with Purkinje cell degeneration, is caused by a heterozygous 2.5-3.8 kilobase penta-nucleotide repeat of (TTCCA)_n in intron 11 of the thymidine kinase 2 (TK2) gene. TK2 is an essential mitochondrial pyrimidine-deoxyribonucleoside kinase. Bi-allelic loss-of-function mutations of TK2 lead to mitochondrial DNA depletion syndrome (MDS) in humans through severe (~ 70%) reduction of mitochondrial electron-transport-chain activity, and tk2 knockout mice show Purkinje cell degeneration and ataxia through severe mitochondrial cytochrome-c oxidase subunit I (COX I) protein reduction. To clarify whether TK2 function is altered in SCA31, we investigated TK2 and COX I expression in human postmortem SCA31 cerebellum. We confirmed that canonical TK2 mRNA is transcribed from exons far upstream of the repeat site, and demonstrated that an extended version of TK2 mRNA ("TK2-EXT"), transcribed from exons spanning the repeat site, is expressed in human cerebellum. While canonical TK2 was conserved among vertebrates, TK2-EXT was specific to primates. Reverse transcription-PCR demonstrated that both TK2 mRNAs were preserved in SCA31 cerebella compared with control cerebella. The TK2 proteins, assessed with three different antibodies including our original polyclonal antibody against TK2-EXT, were detected as ~ 26 kilodalton proteins on western blot; their levels were similar in SCA31 and control cerebella. COX I protein level was preserved in SCA31 compared to nuclear DNA-encoded protein. We conclude that the expression and function of TK2 are preserved in SCA31, suggesting a mechanism distinct from that of MDS.

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.

Translation of MT-ATP6 pathogenic variants reveals distinct regulatory consequences from the co-translational quality control of mitochondrial protein synthesis

Pathogenic variants that disrupt human mitochondrial protein synthesis are associated with a clinically heterogeneous group of diseases. Despite an impairment in oxidative phosphorylation being a common phenotype, the underlying molecular pathogenesis is more complex than simply a bioenergetic deficiency. Currently, we have limited mechanistic understanding on the scope by which a primary defect in mitochondrial protein synthesis contributes to organelle dysfunction. Since the proteins encoded in the mitochondrial genome are hydrophobic and need co-translational insertion into a lipid bilayer, responsive quality control mechanisms are required to resolve aberrations that arise with the synthesis of truncated and misfolded proteins. Here, we show that defects in the OXA1L-mediated insertion of MT-ATP6 nascent chains into the mitochondrial inner membrane are rapidly resolved by the AFG3L2 protease complex. Using pathogenic MT-ATP6 variants, we then reveal discrete steps in this quality control mechanism and the differential functional consequences to mitochondrial gene expression. The inherent ability of a given cell type to recognize and resolve impairments in mitochondrial protein synthesis may in part contribute at the molecular level to the wide clinical spectrum of these disorders.

Homeostatic control of nuclear-encoded mitochondrial gene expression by the histone variant H2A.Z is essential for neuronal survival

Histone variants are crucial regulators of chromatin structure and gene transcription, yet their functions within the brain remain largely unexplored. Here, we show that the H2A histone variant H2A.Z is essential for neuronal survival. Mice lacking H2A.Z in GABAergic neurons or Purkinje cells (PCs) present with a progressive cerebellar ataxia accompanied by widespread degeneration of PCs. Ablation of H2A.Z in other neuronal subtypes also triggers cell death. H2A.Z binds to the promoters of key nuclear-encoded mitochondrial genes to regulate their expression and promote organelle function. Bolstering mitochondrial activity genetically or by organelle transplant enhances the survival of H2A.Z-ablated neurons. Changes in bioenergetic status alter H2A.Z occupancy at the promoters of nuclear-encoded mitochondrial genes, an adaptive response essential for cell survival. Our results highlight that H2A.Z fulfills a key, conserved role in neuronal survival by acting as a transcriptional rheostat to regulate the expression of genes critical to mitochondrial function.

Macrophages Impair TLR9 Agonist Antitumor Activity through Interacting with the Anti-PD-1 Antibody Fc Domain

Simple Summary

We evaluated the contribution of macrophages to the effect of combinatorial immunotherapeutic treatments based on TLR9 stimulation (with CpG-ODNs) and PD-1 blockade in an ovarian cancer preclinical model. We observed a strong reduction in the antitumor efficacy of a TLR9 agonist upon anti-PD-1 antibody administration. Specifically, we found that TLR9-stimulated macrophages, through interacting with the fragment crystallizable (Fc) domain of the anti-PD-1 antibody, acquire an immunoregulatory phenotype leading to dampening of CpG-ODN antitumor effect. Since the stimulation of macrophage TLRs can be achieved not only by synthetic agonists but also by molecules present in the tumor microenvironment, the data we are presenting may represent another possible mechanism of anti-PD-1 antibody therapy resistance. Indeed, it is possible that when delivered as a monotherapy, anti-PD-1 antibody Fc domain may interact with macrophages in which TLR signaling has already been triggered by endogenous ligands, mirroring the biological effects described in the present study.

Abstract

Background. A combination of TLR9 agonists and an anti-PD-1 antibody has been reported to be effective in immunocompetent mice but the role of innate immunity has not yet been completely elucidated. Therefore, we investigated the contribution of the innate immune system to this combinatorial immunotherapeutic regimens using an immunodeficient mouse model in which the effector functions of innate immunity can clearly emerge without any interference from T lymphocytes. Methods. Athymic mice xenografted with IGROV-1 human ovarian cells, reported to be sensitive to TLR9 agonist therapy, were treated with cytosine–guanine (CpG)-oligodeoxynucleotides (ODNs), an anti-PD-1 antibody or their combination. Results. We found that PD-1 blockade dampened CpG-ODN antitumor activity. In vitro studies indicated that the interaction between the anti-PD-1 antibody fragment crystallizable (Fc) domain and macrophage Fc receptors caused these immune cells to acquire an immunoregulatory phenotype, contributing to a decrease in the efficacy of CpG-ODNs. Accordingly, in vivo macrophage depletion abrogated the detrimental effect exerted by the anti-PD-1 antibody. Conclusion. Our data suggest that if TLR signaling is active in macrophages, coadministration of an anti-PD-1 antibody can reprogram these immune cells towards a polarization state able to negatively affect the immune response and eventually promote tumor growth.

Elovl5 is required for proper action potential conduction along peripheral myelinated fibers

Elovl5 elongates fatty acids with 18 carbon atoms and in cooperation with other enzymes guarantees the normal levels of very long‐chain fatty acids, which are necessary for a proper membrane structure. Action potential conduction along myelinated axons depends on structural integrity of myelin, which is maintained by a correct amount of fatty acids and a proper interaction between fatty acids and myelin proteins. We hypothesized that in Elovl5^−/− mice, the lack of elongation of Elovl5 substrates might cause alterations of myelin structure. The analysis of myelin ultrastructure showed an enlarged periodicity with reduced G‐ratio across all axonal diameters. We hypothesized that the structural alteration of myelin might affect the conduction of action potentials. The sciatic nerve conduction velocity was significantly reduced without change in the amplitude of the nerve compound potential, suggesting a myelin defect without a concomitant axonal degeneration. Since Elovl5 is important in attaining normal amounts of polyunsaturated fatty acids, which are the principal component of myelin, we performed a lipidomic analysis of peripheral nerves of Elovl5‐deficient mice. The results revealed an unbalance, with reduction of fatty acids longer than 18 carbon atoms relative to shorter ones. In addition, the ratio of saturated to unsaturated fatty acids was strongly increased. These findings point out the essential role of Elovl5 in the peripheral nervous system in supporting the normal structure of myelin, which is the key element for a proper conduction of electrical signals along myelinated nerves.

Recent advances in understanding hereditary spastic paraplegias and emerging therapies

Hereditary spastic paraplegias (HSPs) are a group of rare, inherited, neurological diseases characterized by broad clinical and genetic heterogeneity. Lower-limb spasticity with first motoneuron involvement is the core symptom of all HSPs. As spasticity is a syndrome and not a disease, it develops on top of other neurological signs (ataxia, dystonia, and parkinsonism). Indeed, the definition of genes responsible for HSPs goes beyond the 79 identified SPG genes. In order to avoid making a catalog of the different genes involved in HSP in any way, we have chosen to focus on the HSP with cerebellar ataxias since this is a frequent association described for several genes. This overlap leads to an intermediary group of spastic ataxias which is actively genetically and clinically studied. The most striking example is SPG7, which is responsible for HSP or cerebellar ataxia or both. There are no specific therapies against HSPs, and there is a dearth of randomized trials in patients with HSP, especially on spasticity when it likely results from other mechanisms. Thus far, no gene-specific therapy has been developed for HSP, but emerging therapies in animal models and neurons derived from induced pluripotent stem cells are potential treatments for patients.

Mitochondrial OPA1 cleavage is reversibly activated by differentiation of H9c2 cardiomyoblasts

Optic atrophy-1 (OPA1) is a dynamin-like GTPase localized to the mitochondrial inner membrane, playing key roles in inner membrane fusion and cristae maintenance. OPA1 is regulated by the mitochondrial transmembrane potential (Δψm): when Δψm is intact, long OPA1 isoforms (L-OPA1) carry out inner membrane fusion. Upon loss of Δψm, L-OPA1 isoforms are proteolytically cleaved to short (S-OPA1) isoforms by the stress-inducible OMA1 metalloprotease, causing collapse of the mitochondrial network and promoting apoptosis. Here, we show that L-OPA1 isoforms of H9c2 cardiomyoblasts are retained under loss of Δψm, despite the presence of OMA1. However, when H9c2s are differentiated to a more cardiac-like phenotype via treatment with retinoic acid (RA) in low serum media, loss of Δ ψm induces robust, and reversible, cleavage of L-OPA1 and subsequent OMA1 degradation. These findings indicate that a potent developmental switch regulates Δ ψm-sensitive OPA1 cleavage, suggesting novel developmental and regulatory mechanisms for OPA1 homeostasis.

Investigating the influence of mtDNA and nuclear encoded mitochondrial variants on high intensity interval training outcomes

Mitochondria supply intracellular energy requirements during exercise. Specific mitochondrial haplogroups and mitochondrial genetic variants have been associated with athletic performance, and exercise responses. However, these associations were discovered using underpowered, candidate gene approaches, and consequently have not been replicated. Here, we used whole-mitochondrial genome sequencing, in conjunction with high-throughput genotyping arrays, to discover novel genetic variants associated with exercise responses in the Gene SMART (Skeletal Muscle Adaptive Response to Training) cohort (n = 62 completed). We performed a Principal Component Analysis of cohort aerobic fitness measures to build composite traits and test for variants associated with exercise outcomes. None of the mitochondrial genetic variants but eight nuclear encoded variants in seven separate genes were found to be associated with exercise responses (FDR < 0.05) (rs11061368: DIABLO, rs113400963: FAM185A, rs6062129 and rs6121949: MTG2, rs7231304: AFG3L2, rs2041840: NDUFAF7, rs7085433: TIMM23, rs1063271: SPTLC2). Additionally, we outline potential mechanisms by which these variants may be contributing to exercise phenotypes. Our data suggest novel nuclear-encoded SNPs and mitochondrial pathways associated with exercise response phenotypes. Future studies should focus on validating these variants across different cohorts and ethnicities.

Losing the Beat: Contribution of Purkinje Cell Firing Dysfunction to Disease, and Its Reversal

The cerebellum is a brain structure that is highly interconnected with other brain regions. There are many contributing factors to cerebellar-related brain disease, such as altered afferent input, local connectivity, and/or cerebellar output. Purkinje cells (PC) are the principle cells of the cerebellar cortex, and fire intrinsically; that is, they fire spontaneous action potentials at high frequencies. This review paper focuses on PC intrinsic firing activity, which is altered in multiple neurological diseases, including ataxia, Huntington Disease (HD) and autism spectrum disorder (ASD). Notably, there are several cases where interventions that restore or rescue PC intrinsic activity also improve impaired behavior in these mouse models of disease. These findings suggest that rescuing PC firing deficits themselves may be sufficient to improve impairment in cerebellar-related behavior in disease. We propose that restoring PC intrinsic firing represents a good target for drug development that might be of therapeutic use for several disorders.

ATPase Domain AFG3L2 Mutations Alter OPA1 Processing and Cause Optic Neuropathy

OBJECTIVE

Dominant optic atrophy (DOA) is the most common inherited optic neuropathy, with a prevalence of 1:12,000 to 1:25,000. OPA1 mutations are found in 70% of DOA patients, with a significant number remaining undiagnosed.

METHODS

We screened 286 index cases presenting optic atrophy, negative for OPA1 mutations, by targeted next generation sequencing or whole exome sequencing. Pathogenicity and molecular mechanisms of the identified variants were studied in yeast and patient-derived fibroblasts.

RESULTS

Twelve cases (4%) were found to carry novel variants in AFG3L2, a gene that has been associated with autosomal dominant spinocerebellar ataxia 28 (SCA28). Half of cases were familial with a dominant inheritance, whereas the others were sporadic, including de novo mutations. Biallelic mutations were found in 3 probands with severe syndromic optic neuropathy, acting as recessive or phenotype-modifier variants. All the DOA-associated AFG3L2 mutations were clustered in the ATPase domain, whereas SCA28-associated mutations mostly affect the proteolytic domain. The pathogenic role of DOA-associated AFG3L2 mutations was confirmed in yeast, unraveling a mechanism distinct from that of SCA28-associated AFG3L2 mutations. Patients' fibroblasts showed abnormal OPA1 processing, with accumulation of the fission-inducing short forms leading to mitochondrial network fragmentation, not observed in SCA28 patients' cells.

INTERPRETATION

This study demonstrates that mutations in AFG3L2 are a relevant cause of optic neuropathy, broadening the spectrum of clinical manifestations and genetic mechanisms associated with AFG3L2 mutations, and underscores the pivotal role of OPA1 and its processing in the pathogenesis of DOA. ANN NEUROL 2020 ANN NEUROL 2020;88:18-32.

Mitochondrial Nascent Chain Quality Control Determines Organelle Form and Function

Proteotoxicity has long been considered a key factor in mitochondrial dysfunction and human disease. The origin of the endogenous offending toxic substrates and the regulatory pathways to deal with these insults, however, have remained unclear. Mitochondria maintain a compartmentalized gene expression system that in animals is only responsible for synthesis of 1% of the organelle proteome. Because of the relatively small contribution of the mitochondrial genome to the overall proteome, the synthesis and quality control of these nascent chains to maintain organelle proteostasis has long been overlooked. However, recent research has uncovered mechanisms by which defects to the quality control of mitochondrial gene expression are linked to a novel cellular stress response that impinges upon organelle form and function and cell fitness. In this review, we discuss the mechanisms for a key event in the response: activation of the metalloprotease OMA1. This severs the membrane tether of the dynamin-related GTPase OPA1, which is a critical determinant for mitochondrial morphology and function. We also highlight the evolutionary conservation from bacteria of these quality-control mechanisms to maintain membrane integrity, gene expression, and cell fitness.

Unique Structural Features of the Mitochondrial AAA+ Protease AFG3L2 Reveal the Molecular Basis for Activity in Health and Disease

Mitochondrial AAA+ quality-control proteases regulate diverse aspects of mitochondrial biology through specialized protein degradation, but the underlying mechanisms of these enzymes remain poorly defined. The mitochondrial AAA+ protease AFG3L2 is of particular interest, as genetic mutations localized throughout AFG3L2 are linked to diverse neurodegenerative disorders. However, a lack of structural data has limited our understanding of how mutations impact enzymatic function. Here, we used cryoelectron microscopy (cryo-EM) to determine a substrate-bound structure of the catalytic core of human AFG3L2. This structure identifies multiple specialized structural features that integrate with conserved motifs required for ATP-dependent translocation to unfold and degrade targeted proteins. Many disease-relevant mutations localize to these unique structural features of AFG3L2 and distinctly influence its activity and stability. Our results provide a molecular basis for neurological phenotypes associated with different AFG3L2 mutations and establish a structural framework to understand how different members of the AAA+ superfamily achieve specialized biological functions.

Spinocerebellar ataxia

The spinocerebellar ataxias (SCAs) are a genetically heterogeneous group of autosomal dominantly inherited progressive disorders, the clinical hallmark of which is loss of balance and coordination accompanied by slurred speech; onset is most often in adult life. Genetically, SCAs are grouped as repeat expansion SCAs, such as SCA3/Machado-Joseph disease (MJD), and rare SCAs that are caused by non-repeat mutations, such as SCA5. Most SCA mutations cause prominent damage to cerebellar Purkinje neurons with consecutive cerebellar atrophy, although Purkinje neurons are only mildly affected in some SCAs. Furthermore, other parts of the nervous system, such as the spinal cord, basal ganglia and pontine nuclei in the brainstem, can be involved. As there is currently no treatment to slow or halt SCAs (many SCAs lead to premature death), the clinical care of patients with SCA focuses on managing the symptoms through physiotherapy, occupational therapy and speech therapy. Intense research has greatly expanded our understanding of the pathobiology of many SCAs, revealing that they occur via interrelated mechanisms (including proteotoxicity, RNA toxicity and ion channel dysfunction), and has led to the identification of new targets for treatment development. However, the development of effective therapies is hampered by the heterogeneity of the SCAs; specific therapeutic approaches may be required for each disease.

Mitochondrial stress response triggered by defects in protein synthesis quality control

Quality control defects of mitochondrial nascent chain synthesis trigger a sequential stress response characterized by OMA1 activation and ribosome decay, determining mitochondrial form and function.

Mitochondria have a compartmentalized gene expression system dedicated to the synthesis of membrane proteins essential for oxidative phosphorylation. Responsive quality control mechanisms are needed to ensure that aberrant protein synthesis does not disrupt mitochondrial function. Pathogenic mutations that impede the function of the mitochondrial matrix quality control protease complex composed of AFG3L2 and paraplegin cause a multifaceted clinical syndrome. At the cell and molecular level, defects to this quality control complex are defined by impairment to mitochondrial form and function. Here, we establish the etiology of these phenotypes. We show how disruptions to the quality control of mitochondrial protein synthesis trigger a sequential stress response characterized first by OMA1 activation followed by loss of mitochondrial ribosomes and by remodelling of mitochondrial inner membrane ultrastructure. Inhibiting mitochondrial protein synthesis with chloramphenicol completely blocks this stress response. Together, our data establish a mechanism linking major cell biological phenotypes of AFG3L2 pathogenesis and show how modulation of mitochondrial protein synthesis can exert a beneficial effect on organelle homeostasis.