Pathomechanisms of mutant proteins in Charcot-Marie-Tooth disease

Epigenetic regulation of the human GDAP1 gene

Mutations in the ganglioside-induced differentiation-associated protein 1 (GDAP1) gene are linked to Charcot-Marie-Tooth (CMT) disease, a hereditary neurodegenerative condition. The protein encoded by this gene is involved in mitochondrial fission and calcium homeostasis. Recently, GDAP1 has also been implicated in the survival of patients with certain cancers. Despite its significant role in specific cellular processes and associated diseases, the mechanisms regulating GDAP1 expression are largely unknown. Here, we show for the first time that methylation of the CpG island in the proximal promoter of the GDAP1 gene inhibits its activity. Treating cells with low GDAP1 expression using methyltransferase and HDAC inhibitors induced the expression of this gene and its encoded protein. This induction was associated with promoter demethylation and increased association of acetylated histones with the GDAP1 promoter. Thus, we identified a mechanism that could be used to manipulate GDAP1 expression.

Adaptive introgression reveals the genetic basis of a sexually selected syndrome in wall lizards

The joint expression of particular colors, morphologies, and behaviors is a common feature of adaptation, but the genetic basis for such "phenotypic syndromes" remains poorly understood. Here, we identified a complex genetic architecture associated with a sexually selected syndrome in common wall lizards, by capitalizing on the adaptive introgression of coloration and morphology into a distantly related lineage. Consistent with the hypothesis that the evolution of phenotypic syndromes in vertebrates is facilitated by developmental linkage through neural crest cells, most of the genes associated with the syndrome are involved in neural crest cell regulation. A major locus was a ~400-kb region, characterized by standing structural genetic variation and previously implied in the evolutionary innovation of coloration and beak size in birds. We conclude that features of the developmental and genetic architecture contribute to maintaining trait integration, facilitating the extensive and rapid introgressive spread of suites of sexually selected characters.

Structural bases for the Charcot-Marie-Tooth disease induced by single amino acid substitutions of myelin protein zero

Myelin protein zero (MPZ or P0) is a transmembrane protein which functions to glue membranes in peripheral myelin. Inter-membrane adhesion is mediated by homophilic interactions between the extracellular domains (ECDs) of MPZ. Single amino acid substitutions in an ECD cause demyelinating neuropathy, Charcot-Marie-Tooth disease (CMT), with unknown mechanisms. In this study, by using a novel assay system "nanomyelin," we revealed that a stacked-rings-like ECD-8-mer is responsible for membrane adhesion. Two inter-ECD interactions, cis and head-to-head, are essential to constituting the 8-mer and to gluing the membranes. This result was reinforced by the observation that the CMT-related N87H substitution at the cis interface abolished membrane-adhesion activity. In contrast, the CMT-related D32G and E68V variants retained membrane-stacking activity, whereas their thermal stability was lower than that of the WT. Reduced thermal stability may lead to impairment of the long-term stability of ECD and the layered membranes of myelin.

On the path to evidence-based therapy in neuromuscular disorders

Neuromuscular disorders encompass a diverse group of acquired and genetic diseases characterized by loss of motor functionality. Although cure is the goal, many therapeutic strategies have been envisioned and are being studied in randomized clinical trials and entered clinical practice. As in all scientific endeavors, the successful clinical translation depends on the quality and translatability of preclinical findings and on the predictive value and feasibility of the clinical models. This chapter focuses on five exemplary diseases: childhood spinal muscular atrophy (SMA), Charcot-Marie-Tooth (CMT) disorders, chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), acquired autoimmune myasthenia gravis (MG), and Duchenne muscular dystrophy (DMD), to illustrate the progress made on the path to evidenced-based therapy.

Neurogenetic motor disorders

Advances in the field of neurogenetics have practical applications in rapid diagnosis on blood and body fluids to extract DNA, obviating the need for invasive investigations. The ability to obtain a presymptomatic diagnosis through genetic screening and biomarkers can be a guide to life-saving disease-modifying therapy or enzyme replacement therapy to compensate for the deficient disease-causing enzyme. The benefits of a comprehensive neurogenetic evaluation extend to family members in whom identification of the causal gene defect ensures carrier detection and at-risk counseling for future generations. This chapter explores the many facets of the neurogenetic evaluation in adult and pediatric motor disorders as a primer for later chapters in this volume and a roadmap for the future applications of genetics in neurology.

Missense mutations in inositol 1,4,5-trisphosphate receptor type 3 result in leaky Ca2+ channels and activation of store-operated Ca2+ entry

Mutations in all subtypes of the inositol 1,4,5-trisphosphate receptor Ca2+ release channel are associated with human diseases. In this report, we investigated the functionality of three neuropathy-associated missense mutations in IP3R3 (V615M, T1424M, and R2524C). The mutants only exhibited function when highly over-expressed compared to endogenous hIP3R3. All variants resulted in elevated basal cytosolic Ca2+ levels, decreased endoplasmic reticulum Ca2+ store content, and constitutive store-operated Ca2+ entry in the absence of any stimuli, consistent with a leaky IP3R channel pore. These variants differed in channel function; when stably over-expressed the R2524C mutant was essentially dead, V615M was poorly functional, and T1424M exhibited activity greater than that of the corresponding wild-type following threshold stimulation. These results demonstrate that a common feature of these mutations is decreased IP3R3 function. In addition, these mutations exhibit a novel phenotype manifested as a constitutively open channel, which inappropriately gates SOCE in the absence of stimulation.

Mitochondrial membrane proteins and VPS35 orchestrate selective removal of mtDNA

Understanding the mechanisms governing selective turnover of mutation-bearing mtDNA is fundamental to design therapeutic strategies against mtDNA diseases. Here, we show that specific mtDNA damage leads to an exacerbated mtDNA turnover, independent of canonical macroautophagy, but relying on lysosomal function and ATG5. Using proximity labeling and Twinkle as a nucleoid marker, we demonstrate that mtDNA damage induces membrane remodeling and endosomal recruitment in close proximity to mitochondrial nucleoid sub-compartments. Targeting of mitochondrial nucleoids is controlled by the ATAD3-SAMM50 axis, which is disrupted upon mtDNA damage. SAMM50 acts as a gatekeeper, influencing BAK clustering, controlling nucleoid release and facilitating transfer to endosomes. Here, VPS35 mediates maturation of early endosomes to late autophagy vesicles where degradation occurs. In addition, using a mouse model where mtDNA alterations cause impairment of muscle regeneration, we show that stimulation of lysosomal activity by rapamycin, selectively removes mtDNA deletions without affecting mtDNA copy number, ameliorating mitochondrial dysfunction. Taken together, our data demonstrates that upon mtDNA damage, mitochondrial nucleoids are eliminated outside the mitochondrial network through an endosomal-mitophagy pathway. With these results, we unveil the molecular players of a complex mechanism with multiple potential benefits to understand mtDNA related diseases, inherited, acquired or due to normal ageing.

Genetic spectrum of inherited neuropathies in India

Background and Objectives:

Charcot-Marie-Tooth (CMT) disease is the commonest inherited neuromuscular disorder and has heterogeneous manifestations. Data regarding genetic basis of CMT from India is limited. This study aims to report the variations by using high throughput sequencing in Indian CMT cohort.

Methods:

Fifty-five probands (M:F 29:26) with suspected inherited neuropathy underwent genetic testing (whole exome: 31, clinical exome: 17 and targeted panel: 7). Their clinical and genetic data were analysed.

Results:

Age at onset ranged from infancy to 54 years. Clinical features included early-onset neuropathy (n=23), skeletal deformities (n=45), impaired vision (n=8), impaired hearing (n=6), facial palsy (n=8), thickened nerves (n=4), impaired cognition (n=5), seizures (n=5), pyramidal signs (n=7), ataxia (n=8) and vocal cord palsy, slow tongue movements and psychosis in one patient each. Twenty-eight patients had demyelinating electrophysiology. Abnormal visual and auditory evoked potentials were noted in 60.60% and 37.5% respectively. Sixty two variants were identified in 37 genes including variants of uncertain significance (n=34) and novel variants (n=45). Eleven patients had additional variations in genes implicated in CMTs/ other neurological disorders. Ten patients did not have variations in neuropathy associated genes, but had variations in genes implicated in other neurological disorders. In seven patients, no variations were detected.

Conclusion:

In this single centre cohort study from India, genetic diagnosis could be established in 87% of patients with inherited neuropathy. The identified spectrum of genetic variations adds to the pool of existing data and provides a platform for validation studies in cell culture or animal model systems.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

Mutations in the Amyloid-β Protein Precursor Reduce Mitochondrial Function and Alter Gene Expression Independent of 42-Residue Amyloid-β Peptide

BACKGROUND

Dominant missense mutations in the amyloid-β protein precursor (AβPP) cause early-onset familial Alzheimer's disease (FAD) and are associated with changes in the production or properties of the amyloid-β peptide (Aβ), particularly of the 42-residue variant (Aβ 42) that deposits in the Alzheimer's disease (AD) brain. Recent findings, however, show that FAD mutations in AβPP also lead to increased production of longer Aβ variants of 45-49 residues in length.

OBJECTIVE

We aimed to test neurotoxicity of Aβ 42 vis-á-vis longer variants, focusing specifically on mitochondrial function, as dysfunctional mitochondria are implicated in the pathogenesis of AD.

METHODS

We generated SH-SY5Y human neuroblastoma cells stably expressing AβPP mutations that lead to increased production of long Aβ peptides with or without Aβ 42. These AβPP-expressing cells were tested for oxygen consumption rates (OCR) under different conditions designed to interrogate mitochondrial function. These cell lines were also examined for expression of genes important for mitochondrial or neuronal structure and function.

RESULTS

The mutant AβPP-expressing cells showed decreased basal OCRs as well as decreased OCRs associated with mitochondrial ATP production, even more so in the absence of Aβ 42 production. Moreover, mutant AβPP-expressing cells producing longer forms of Aβ displayed altered expression of certain mitochondrial- and neuronal-associated genes, whether or not Aβ 42 was produced.

CONCLUSION

These findings suggest that mutant AβPP can cause mitochondrial dysfunction that is associated with long Aβ but not with Aβ 42.

Nanoscale Model System for the Human Myelin Sheath

Neurodegenerative disorders are among the most common diseases in modern society. However, the molecular bases of diseases such as multiple sclerosis or Charcot-Marie-Tooth disease remain far from being fully understood. Research in this field is limited by the complex nature of native myelin and by difficulties in obtaining good in vitro model systems of myelin. Here, we introduce an easy-to-use model system of the myelin sheath that can be used to study myelin proteins in a native-like yet well-controlled environment. To this end, we present myelin-mimicking nanodiscs prepared through one of the amphiphilic copolymers styrene/maleic acid (SMA), diisobutylene/maleic acid (DIBMA), and styrene/maleimide sulfobetaine (SMA-SB). These nanodiscs were tested for their lipid composition using chromatographic (HPLC) and mass spectrometric (MS) methods and, utilizing spin probes within the nanodisc, their comparability with liposomes was studied. In addition, their binding behavior with bovine myelin basic protein (MBP) was scrutinized to ensure that the nanodiscs represent a suitable model system of myelin. Our results suggest that both SMA and SMA-SB are able to solubilize the myelin-like (cytoplasmic) liposomes without preferences for specific lipid headgroups or fatty acyl chains. In nanodiscs of both SMA and SMA-SB (called SMA(-SB)-lipid particles, short SMALPs or SMA-SBLPs, respectively), the polymers restrict the lipids' motion in the hydrophobic center of the bilayer. The headgroups of the lipids, however, are sterically less hindered in nanodiscs when compared with liposomes. Myelin-like SMALPs are able to bind bovine MBP, which can stack the lipid bilayers like in native myelin, showing the usability of these simple, well-controlled systems in further studies of protein-lipid interactions of native myelin.

Demise of nociceptive Schwann cells causes nerve retraction and pain hyperalgesia

ABSTRACT

Recent findings indicate that nociceptive nerves are not "free", but similar to touch and pressure sensitive nerves, terminate in an end-organ in mice. This sensory structure consists of the nociceptive nerves and specialized nociceptive Schwann cells forming a mesh-like organ in subepidermis with pain transduction initiated at both these cellular constituents. The intimate relation of nociceptive nerves with nociceptive Schwann cells in mice raises the question whether defects in nociceptive Schwann cells can by itself contribute to pain hyperalgesia, nerve retraction, and peripheral neuropathy. We therefore examined the existence of nociceptive Schwann cells in human skin and their possible contribution to neuropathy and pain hyperalgesia in mouse models. Similar to mouse, human skin contains SOX10+/S100B+/AQP1+ Schwann cells in the subepidermal border that have extensive processes, which are intimately associated with nociceptive nerves projecting into epidermis. The ablation of nociceptive Schwann cells in mice resulted in nerve retraction and mechanical, cold, and heat hyperalgesia. Conversely, ablating the nociceptive nerves led to a retraction of epidermal Schwann cell processes, changes in nociceptive Schwann cell soma morphology, heat analgesia, and mechanical hyperalgesia. Our results provide evidence for a nociceptive sensory end-organ in the human skin and using animal models highlight the interdependence of the nerve and the nociceptive Schwann cell. Finally, we show that demise of nociceptive Schwann cells is sufficient to cause neuropathic-like pain in the mouse.

The Complex Clinical and Genetic Landscape of Hereditary Peripheral Neuropathy

Hereditary peripheral neuropathy (HPN) is a complex group of neurological disorders caused by mutations in genes expressed by neurons and Schwann cells. The inheritance of a single mutation or multiple mutations in several genes leads to disease phenotype. Patients exhibit symptoms during development, at an early age or later in adulthood. Most of the mechanistic understanding about these neuropathies comes from animal models and histopathological analyses of postmortem human tissues. Diagnosis is often very complex due to the heterogeneity and overlap in symptoms and the frequent overlap between various genes and different mutations they possess. Some symptoms in HPN are common through different subtypes such as axonal degeneration, demyelination, and loss of motor and sensory neurons, leading to similar physiologic abnormalities. Recent advances in gene-targeted therapies, genetic engineering, and next-generation sequencing have augmented our understanding of the underlying pathogenetic mechanisms of HPN.

A new mutation in DNM2 gene in a large Italian family

The Charcot-Marie-Tooth (CMT) disease is the most common inherited peripheral neuropathy with great clinical and genetic heterogeneity. Mutations in DNM2 have been associated with CMT dominant intermediate B (CMTDIB). However, mutations in the same gene are known to induce also axonal CMT (CMT2M) or centronuclear myopathy. Moreover, the ability of effectively and simultaneously sequencing different CMT-related genes by next-generation sequencing approach makes it possible to detect even the presence of modifier genes that sometimes give reason of clinical variability in the context of complex phenotypes. Here, we describe an Italian family with very variable severity of phenotype among members harboring a novel DNM2 gene mutation which caused a prevalent CMT2M phenotype. The contemporary presence of a de novo variant in PRX gene in the most severely affected family member suggests a possible modulator effect of the PRX variant thus highlighting the possible impact of modifier genes in CMT.

Developmental Neurotoxicity of the Harmful Algal Bloom Toxin Domoic Acid: Cellular and Molecular Mechanisms Underlying Altered Behavior in the Zebrafish Model

BACKGROUND

Harmful algal blooms (HABs) produce potent neurotoxins that threaten human health, but current regulations may not be protective of sensitive populations. Early life exposure to low levels of the HAB toxin domoic acid (DomA) produces long-lasting behavioral deficits in rodent and primate models; however, the mechanisms involved are unknown. The zebrafish is a powerful in vivo vertebrate model system for exploring cellular processes during development and thus may help to elucidate mechanisms of DomA developmental neurotoxicity.

OBJECTIVES

We used the zebrafish model to investigate how low doses of DomA affect the developing nervous system, including windows of susceptibility to DomA exposure, structural and molecular changes in the nervous system, and the link to behavioral alterations.

METHODS

To identify potential windows of susceptibility, DomA (0.09-0.18 ng) was delivered to zebrafish through caudal vein microinjection during distinct periods in early neurodevelopment. Following exposure, structural and molecular targets were identified using live imaging of transgenic fish and RNA sequencing. To assess the functional consequences of exposures, we quantified startle behavior in response to acoustic/vibrational stimuli.

RESULTS

Larvae exposed to DomA at 2 d postfertilization (dpf), but not at 1 or 4 dpf, showed consistent deficits in startle behavior at 7 dpf, including lower responsiveness and altered kinematics. Similarly, myelination in the spinal cord was disorganized after exposure at 2 dpf but not 1 or 4 dpf. Time-lapse imaging revealed disruption of the initial stages of myelination. DomA exposure at 2 dpf down-regulated genes required for maintaining myelin structure and the axonal cytoskeleton.

DISCUSSION

These results in zebrafish reveal a developmental window of susceptibility to DomA-induced behavioral deficits and identify altered gene expression and disrupted myelin structure as possible mechanisms. The results establish a zebrafish model for investigating the mechanisms of developmental DomA toxicity, including effects with potential relevance to exposed sensitive human populations. https://doi.org/10.1289/EHP6652.

Metabolomics markers in Neurology: current knowledge and future perspectives for therapeutic targeting

INTRODUCTION

Metabolomics is an emerging approach providing new insights into the metabolic changes and underlying mechanisms involved in the pathogenesis of neurological disorders.

AREAS COVERED

Here, the authors present an overview of the current knowledge of metabolic profiling (metabolomics) to provide critical insight on the role of biochemical markers and metabolic alterations in neurological diseases.

EXPERT OPINION

Elucidation of characteristic metabolic alterations in neurological disorders is crucial for a better understanding of their pathogenesis, and for identifying potential biomarkers and drug targets. Nevertheless, discrepancies in diagnostic criteria, sample handling protocols, and analytical methods still affect the generalizability of current study results.

The ubiquitous role of mitochondria in Parkinson and other neurodegenerative diseases

Orderly mitochondrial life cycle, plays a key role in the pathology of neurodegenerative diseases. Mitochondria are ubiquitous in neurons as they respond to an ever-changing demand for energy supply. Mitochondria constantly change in shape and location, feature of their dynamic nature, which facilitates a quality control mechanism. Biological studies in mitochondria dynamics are unveiling the mechanisms of fission and fusion, which essentially arrange morphology and motility of these organelles. Control of mitochondrial network homeostasis is a critical factor for the proper function of neurons. Disease-related genes have been reported to be implicated in mitochondrial dysfunction. Increasing evidence implicate mitochondrial perturbation in neuronal diseases, such as AD, PD, HD, and ALS. The intricacy involved in neurodegenerative diseases and the dynamic nature of mitochondria point to the idea that, despite progress toward detecting the biology underlying mitochondrial disorders, its link to these diseases is difficult to be identified in the laboratory. Considering the need to model signaling pathways, both in spatial and temporal level, there is a challenge to use a multiscale modeling framework, which is essential for understanding the dynamics of a complex biological system. The use of computational models in order to represent both a qualitative and a quantitative structure of mitochondrial homeostasis, allows to perform simulation experiments so as to monitor the conformational changes, as well as the intersection of form and function.

[Associations of single nucleotide polymorphisms of KIF1B gene with the severity of clinical manifestations of multiple sclerosis]

BACKGROUND

Multiple sclerosis (MS) is one of the most common neurological diseases, which development and features depend on various factors, including genetic ones.

AIM

To identify associations of single nucleotide polymorphisms rs10492972 and rs117525287 of the KIF1B gene with the development and severity of clinical manifestations of MS patients in the Perm krai.

MATERIAL AND METHODS

The study group consisted of 197 patients with MS, the control group included 80 healthy volunteers. Clinical characteristics were assessed by DDS, EDSS, SCRIPPS, FSS and Arm-index. A genetic study was carried out by real-time polymerase chain reaction using a CFX96 detection amplifier (Biorad, USA) and the CFX Manager 2.1 software package (Biorad, USA).

RESULTS AND CONCLUSION

The study group was characterized by a moderate degree of disability: EDSS 4 [3; 6], SCRIPPS 62 [50; 79] points and moderate progression rate 0.42 [0,28; 0,67] points /year. A significant prevalence of the heterozygous genotype T/C (rs10492972) in the group of healthy people was established. The influence of the dominant T allele of rs10492972 on the severity of clinical manifestations on EDSS and SCRIPPS was found. No associations of the polymorphisms with the clinical parameters assessed by EDSS, SCRIPPS, FSS and Arm-index, as well as with the rate of disease progression were obtained. Significant differences in the frequencies of rs10492972 genotypes in groups of MS patients and healthy people were identified; their effect on the severity of MS was not determined.

In silico and in vitro effects of the I30T mutation on myelin protein zero instability in the cell membrane

Charcot-Marie-Tooth (CMT) diseases are a heterogeneous group of genetic peripheral neuropathies caused by mutations in a variety of genes, which are involved in the development and maintenance of peripheral nerves. Myelin protein zero (MPZ) is expressed by Schwann cells, and MPZ mutations can lead to primarily demyelinating polyneuropathies including CMT type 1B. Different mutations demonstrate various forms of disease pathomechanisms, which may be beneficial in understanding the disease cellular pathology. Our molecular dynamics simulation study on the possible impacts of I30T mutation on the MPZ protein structure suggested a higher hydrophobicity and thus lower stability in the membranous structures. A study was also conducted to predict native/mutant MPZ interactions. To validate the results of the simulation study, the native and mutant forms of the MPZ protein were separately expressed in a cellular model, and the protein trafficking was chased down in a time course pattern. In vitro studies provided more evidence on the instability of the MPZ protein due to the mutation. In this study, qualitative and quantitative approaches were adopted to confirm the instability of mutant MPZ in cellular membranes.

Charcot-Marie-Tooth type 4B2 demyelinating neuropathy in miniature Schnauzer dogs caused by a novel splicing SBF2 (MTMR13) genetic variant: a new spontaneous clinical model

Background

Charcot-Marie-Tooth (CMT) disease is the most common neuromuscular disorder in humans affecting 40 out of 100,000 individuals. In 2008, we described the clinical, electrophysiological and pathological findings of a demyelinating motor and sensory neuropathy in Miniature Schnauzer dogs, with a suspected autosomal recessive mode of inheritance based on pedigree analysis. The discovery of additional cases has followed this work and led to a genome-wide association mapping approach to search for the underlying genetic cause of the disease.

Methods

For genome wide association screening, genomic DNA samples from affected and unaffected dogs were genotyped using the Illumina CanineHD SNP genotyping array. SBF2 and its variant were sequenced using primers and PCRs. RNA was extracted from muscle of an unaffected and an affected dog and RT-PCR performed. Immunohistochemistry for myelin basic protein was performed on peripheral nerve section specimens.

Results

The genome-wide association study gave an indicative signal on canine chromosome 21. Although the signal was not of genome-wide significance due to the small number of cases, the SBF2 (also known as MTMR13) gene within the region of shared case homozygosity was a strong positional candidate, as 22 genetic variants in the gene have been associated with demyelinating forms of Charcot-Marie-Tooth disease in humans. Sequencing of SBF2 in cases revealed a splice donor site genetic variant, resulting in cryptic splicing and predicted early termination of the protein based on RNA sequencing results.

Conclusions

This study reports the first genetic variant in Miniature Schnauzer dogs responsible for the occurrence of a demyelinating peripheral neuropathy with abnormally folded myelin. This discovery establishes a genotype/phenotype correlation in affected Miniature Schnauzers that can be used for the diagnosis of these dogs. It further supports the dog as a natural model of a human disease; in this instance, Charcot-Marie-Tooth disease. It opens avenues to search the biological mechanisms responsible for the disease and to test new therapies in a non-rodent large animal model. In particular, recent gene editing methods that led to the restoration of dystrophin expression in a canine model of muscular dystrophy could be applied to other canine models such as this before translation to humans.

Enhanced mitochondrial autophagy (mitophagy) in oligodendrocytes might play a role in white matter pathology in schizophrenia

Disrupted white matter integrity is a typical feature of brain pathologic alterations in schizophrenia, which includes impaired myelination, decreased oligodendrocyte densities, distortion of their spatial distribution and deviations from normal oligodendrocyte cell morphology. While most genes expressed "in the remaining" oligodendrocytes are downregulated in schizophrenia, only a few are upregulated. To the latter group belong prohibitin 2 and DISC 1, which were recently identified as mitochondria-located mitophagy receptors. Their overexpression, together with greatly reduced numbers and densities of oligodendroglial mitochondria and the structurally "normal appearance of the remaining mitochondria" in these cells as reported by Uranova's group (Uranova et al., 2001, 2004, 2018), point to enhanced mitophagy in oligodendrocytes in schizophrenia, which is possibly even cell protective by preventing apoptosis. Since massive loss of white matter oligodendrocytes is a characteristic feature of schizophrenia, we assume that increased mitophagy is a late event in the development and/or further progression of white matter pathologic changes. Moreover, altered oligodendroglial mitophagy might in part result from antipsychotic treatment. Further studies are clearly needed to substantiate our hypothesis on enhanced mitochondrial autophagy in schizophrenia, whereby the "drug-naïve state" and the possible influence of antipsychotic treatment could be elegantly simulated using animal models of the disease.

Clinical and Neuroimaging Spectrum of Peroxisomal Disorders

Peroxisomes play vital roles in a broad spectrum of cellular metabolic pathways. Defects in genes encoding peroxisomal proteins can result in a wide array of disorders, depending upon the metabolic pathways affected. These disorders can be broadly classified into 2 main groups; peroxisome biogenesis disorders (PBDs) and single peroxisomal enzyme deficiencies. Peroxisomal enzyme deficiencies are result of dysfunction of a specific metabolic pathway, while PBDs are due to generalized peroxisomal dysfunction. Mutations in PEX1 gene are the most common cause of PBDs, accounting for two-thirds of cases. Peroxisomal fission defects is a recently recognized entity, included under the subgroup of PBDs. The aim of this article is to provide a comprehensive review on the clinical and neuroimaging spectrum of peroxisomal disorders.

BIG1/Arfgef1 and Arf1 regulate the initiation of myelination by Schwann cells in mice

During development of the peripheral nervous system in mammals, Schwann cells wrap their plasma membranes around neuronal axons, forming multiple myelin sheaths. A mature myelin sheath insulates axons and increases nerve conduction velocity while protecting nerve fibers from various stresses such as physical ones. Despite this functional importance, the molecular units that underlie dynamic morphological changes in formation of myelin sheaths are not sufficiently understood. Arf1 is a small guanosine triphosphate-binding protein that plays multiple roles in intracellular trafficking and related signaling, both of which are processes involved in cell morphogenesis. We demonstrate that the Arf1 guanine nucleotide exchange factor, brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1)/Arfgef1, and the effector Arf1 regulate the initiation of myelination of axons by Schwann cells. Schwann cell-specific BIG1 conditional knockout mice, which have been generated here, exhibit reduced myelin thickness and decreased localization of myelin protein zero in the myelin membrane, compared with their littermate controls. BIG1 knockout mouse nerves specifically decrease the amounts of Arf1 in the AP1 clathrin adaptor protein subunits but not the Arf1 binding to GGA1 (Golgi-localized, gamma-adaptin ear-containing, Arf-binding protein 1) transporting proteins. The amounts of Arf1 in the COPI coatomer protein subunits were comparable in the knockout mice and controls. Similar results in myelin thickness are observed in Arf1 conditional knockout mice, which have also been generated here. Thus, the BIG1 and Arf1 unit plays a key role in Schwann cell myelination, newly adding it to the list of molecular units controlling myelination.

Involvement of Aryl hydrocarbon receptor in myelination and in human nerve sheath tumorigenesis

Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor involved in xenobiotic metabolism. Plexiform neurofibromas (PNFs) can transform into malignant peripheral nerve sheath tumors (MPNSTs) that are resistant to existing therapies. These tumors are primarily composed of Schwann cells. In addition to neurofibromatosis type 1 (NF1) gene inactivation, further genetic lesions are required for malignant transformation. We have quantified the mRNA expression levels of AHR and its associated genes in 38 human samples. We report that AHR and the biosynthetic enzymes of its endogenous ligand are overexpressed in human biopsies of PNFs and MPNSTs. We also detect a strong nuclear AHR staining in MPNSTs. The inhibition of AHR by siRNA or antagonists, CH-223191 and trimethoxyflavone, induces apoptosis in human MPNST cells. Since AHR dysregulation is observed in these tumors, we investigate AHR involvement in Schwann cell physiology. Hence, we studied the role of AHR in myelin structure and myelin gene regulation in Ahr^-/- mice during myelin development. AHR ablation leads to locomotion defects and provokes thinner myelin sheaths around the axons. We observe a dysregulation of myelin gene expression and myelin developmental markers in Ahr^-/- mice. Interestingly, AHR does not directly bind to myelin gene promoters. The inhibition of AHR in vitro and in vivo increased β-catenin levels and stimulated the binding of β-catenin on myelin gene promoters. Taken together, our findings reveal an endogenous role of AHR in peripheral myelination and in peripheral nerve sheath tumors. Finally, we suggest a potential therapeutic approach by targeting AHR in nerve tumors.

Port-to-port delivery: Mobilization of toxic sphingolipids via extracellular vesicles

The discovery that most cells produce extracellular vesicles (EVs) and release them in the extracellular milieu has spurred the idea that these membranous cargoes spread pathogenic mechanisms. In the brain, EVs may have multifold and important physiological functions, from deregulating synaptic activity to promoting demyelination to changes in microglial activity. The finding that small EVs (exosomes) contain α-synuclein and β-amyloid, among other pathogenic proteins, is an example of this notion, underscoring their potential role in the brains of patients with Parkinson's and Alzheimer's diseases. Given that they are membranous vesicles, we speculate that EVs also have an intrinsic capacity to incorporate sphingolipids. In conditions under which these lipids are elevated to toxic levels, such as in Krabbe's disease and metachromatic leukodystrophy, EVs may contribute to spread disease from sick to healthy cells. In this essay, we discuss a working hypothesis that brain cells in sphingolipidoses clear some of the accumulated lipid material to attempt restoring cell homeostasis via EV secretion. We hypothesize that secreted sphingolipid-loaded EVs shuttle pathogenic lipids to cells that are not intrinsically affected, contributing to establishing non-cell-autonomous defects. © 2016 Wiley Periodicals, Inc.

Unwrapping the unappreciated: recent progress in Remak Schwann cell biology

Schwann cells (SCs) are specialized glial cells that myelinate and protect axons in the peripheral nervous system (PNS). Although myelinating SCs are more commonly studied, the PNS also contains a variety of non-myelinating SCs, including but not limited to Remak SCs (RSCs), terminal SCs, enteric glia. Although the field currently lacks many robust tools for interrogating the functions of non-myelinating SCs, recent evidence suggests that, like their myelinating counterparts, non-myelinating SCs are critical for proper PNS function. In this review, we focus specifically on RSCs and highlight recent advances in understanding regulators of RSC development, function, and participation in PNS regeneration.

Molecular mechanisms of Charcot-Marie-Tooth neuropathy linked to mutations in human myelin protein P2

Charcot-Marie-Tooth (CMT) disease is one of the most common inherited neuropathies. Recently, three CMT1-associated point mutations (I43N, T51P, and I52T) were discovered in the abundant peripheral myelin protein P2. These mutations trigger abnormal myelin structure, leading to reduced nerve conduction velocity, muscle weakness, and distal limb atrophy. P2 is a myelin-specific protein expressed by Schwann cells that binds to fatty acids and membranes, contributing to peripheral myelin lipid homeostasis. We studied the molecular basis of the P2 patient mutations. None of the CMT1-associated mutations alter the overall folding of P2 in the crystal state. P2 disease variants show increased aggregation tendency and remarkably reduced stability, T51P being most severe. In addition, P2 disease mutations affect protein dynamics. Both fatty acid binding by P2 and the kinetics of its membrane interactions are affected by the mutations. Experiments and simulations suggest opening of the β barrel in T51P, possibly representing a general mechanism in fatty acid-binding proteins. Our findings demonstrate that altered biophysical properties and functional dynamics of P2 may cause myelin defects in CMT1 patients. At the molecular level, a few malformed hydrogen bonds lead to structural instability and misregulation of conformational changes related to ligand exchange and membrane binding.

Glutathione-conjugating and membrane-remodeling activity of GDAP1 relies on amphipathic C-terminal domain

Mutations in the ganglioside-induced differentiation associated protein 1 (GDAP1) cause severe peripheral motor and sensory neuropathies called Charcot-Marie-Tooth disease. GDAP1 expression induces fission of mitochondria and peroxisomes by a currently elusive mechanism, while disease causing mutations in GDAP1 impede the protein's role in mitochondrial dynamics. In silico analysis reveals sequence similarities of GDAP1 to glutathione S-transferases (GSTs). However, a proof of GST activity and its possible impact on membrane dynamics are lacking to date. Using recombinant protein, we demonstrate for the first time theta-class-like GST activity for GDAP1, and it's activity being regulated by the C-terminal hydrophobic domain 1 (HD1) of GDAP1 in an autoinhibitory manner. Moreover, we show that the HD1 amphipathic pattern is required to induce membrane dynamics by GDAP1. As both, fission and GST activities of GDAP1, are critically dependent on HD1, we propose that GDAP1 undergoes a molecular switch, turning from a pro-fission active to an auto-inhibited inactive conformation.

Toll-Like Receptor 2, Toll-Like Receptor 4, Myeloid Differentiation Response Gene 88, and Toll-IL-1 Receptor Domain-Containing Adaptor-Inducing Interferon-γ (TRIF) Selectively Regulate Susceptibility of P0106-125-Induced Murine Experimental Autoimmune Neuritis

The functional relevance of the innate immune system has not yet been dissected in P0_106-125-induced murine experimental autoimmune neuritis. Therefore, the role of Toll-like receptor (TLR) 2, TLR4, myeloid differentiation response gene 88, and Toll-IL-1 receptor domain-containing adaptor-inducing interferon-γ (TRIF), factors critically involved in the TLR signaling pathway, was studied in experimental autoimmune neuritis. In the absence of TLR2, TLR4, myeloid differentiation response gene 88, or TRIF, the clinical course was significantly attenuated compared to wild-type mice. This could be attributed to impaired NF-κB activation, as shown by the absence of nuclear translocation of RelA with a decreased expression of IL-6, IL-12p40, and IL-17A. Remarkably, P0_106-125-immunized TLR2^0/0 mice exhibited a delayed recovery as compared to TLR4^0/0 mice, which was because of an impaired T helper cell 2 polarization. Immunized TLR2^0/0 mice were unable to induce OX40 and OX40L by matrix metalloproteinase-2 on splenic dendritic cells. Subsequently, M2 polarization was impaired and macrophages were unable to sufficiently induce T regulatory cells (T_regs). Thus, in the recovery phase, T_regs were significantly increased in TLR4^0/0 mice as compared to wild-type mice, whereas T_regs in immunized TLR2^0/0 mice were only slightly increased. Our data highlight the relevance of innate immunity and, especially, the tight interaction between the innate and the adaptive immune system, which should be considered for therapeutic approaches of autoimmune diseases.

Evaluation of the Acquisition of the Aerobic Metabolic Capacity by Myelin, during its Development

Our previous reports indicate that the electron transfer chain and F_oF₁-ATP synthase are functionally expressed in myelin sheath, performing an extra-mitochondrial oxidative phosphorylation (OXPHOS), which would provide energy to the nerve axon. This supports the idea that myelin plays a trophic role for the axon. Although the four ETC complexes and ATP synthase are considered exquisite mitochondrial proteins, they are found ectopically expressed in several membranous structures. This study was designed to understand when and how the mitochondrial OXPHOS machinery is embedded in myelin, following myelinogenesis in the rat, which starts at birth and continues until the first month of age. Rats were sacrificed at different time points (from day 5 to 90 post birth). Western blot, immunofluorescence microscopy, luminometric, and oximetric analyses show that the isolated myelin starts to show OXPHOS components around the 11th day after birth and increases proportionally to the rat age, becoming similar to those of adult rat around the 30-third day. Interestingly, WB data show the same temporal relationship between myelinogenesis and appearance of proteins involved in mitochondrial fusion and cellular trafficking. It may be speculated that the OXPHOS complexes may be transferred to the endoplasmic reticulum membrane (known to interact with mitochondria) and from there through the Golgi apparatus to the forming myelin membrane.

Dynamin-2 mutations associated with centronuclear myopathy are hypermorphic and lead to T-tubule fragmentation

Skeletal muscle requires adequate membrane trafficking and remodeling to maintain its normal structure and functions. Consequently, many human myopathies are caused by mutations in membrane trafficking machinery. The large GTPase dynamin-2 (Dyn2) is best known for catalyzing membrane fission during clathrin-mediated endocytosis (CME), which is critical for cell signaling and survival. Despite its ubiquitous expression, mutations of Dyn2 are associated with two tissue-specific congenital disorders: centronuclear myopathy (CNM) and Charcot-Marie-Tooth (CMT) neuropathy. Several disease models for CNM-Dyn2 have been established to study its pathogenic mechanism; yet the cellular and biochemical effects of these mutations are still not fully understood. Here we comprehensively compared the biochemical activities of disease-associated Dyn2 mutations and found that CNM-Dyn2 mutants are hypermorphic with enhanced membrane fission activity, whereas CMT-Dyn2 is hypomorphic. More importantly, we found that the expression of CNM-Dyn2 mutants does not impair CME in myoblast, but leads to T-tubule fragmentation in both C2C12-derived myotubes and Drosophila body wall muscle. Our results demonstrate that CNM-Dyn2 mutants are gain-of-function mutations, and their primary effect in muscle is T-tubule disorganization, which explains the susceptibility of muscle to Dyn2 hyperactivity.

Peroxisome-mitochondria interplay and disease

Peroxisomes and mitochondria are ubiquitous, highly dynamic organelles with an oxidative type of metabolism in eukaryotic cells. Over the years, substantial evidence has been provided that peroxisomes and mitochondria exhibit a close functional interplay which impacts on human health and development. The so-called "peroxisome-mitochondria connection" includes metabolic cooperation in the degradation of fatty acids, a redox-sensitive relationship, an overlap in key components of the membrane fission machineries and cooperation in anti-viral signalling and defence. Furthermore, combined peroxisome-mitochondria disorders with defects in organelle division have been revealed. In this review, we present the latest progress in the emerging field of peroxisomal and mitochondrial interplay in mammals with a particular emphasis on cooperative fatty acid β-oxidation, redox interplay, organelle dynamics, cooperation in anti-viral signalling and the resulting implications for disease.

Late onset neuropathy with spontaneous clinical remission in mice lacking the POZ domain of the transcription factor Myc-interacting zinc finger protein 1 (Miz1) in Schwann cells

The transcription factor Miz1 (Myc-interacting zinc finger 1) is a known regulator of the cell cycle but also has cell cycle-independent functions. Here we analyzed the role of Miz1 in the peripheral nervous system, using an early embryonic conditional knock-out model in which the Miz1 POZ domain is ablated in Schwann cells. Although the development of myelinated nerve fibers was not impaired, Miz1ΔPOZ mice acquired behavioral signs of a peripheral neuropathy at the age of 3 months. At this time, ultrastructural analysis of the sciatic nerve showed de- and dysmyelination of fibers, with massive outfoldings and a focal infiltration of macrophages. Although the expression of genes encoding structural myelin proteins, such as periaxin, myelin basic protein, and myelin protein zero, was decreased, genes associated with a negative regulation of myelination, including c-Jun, Sox2, and Id2, were up-regulated in Miz1ΔPOZ mice compared with controls. In animals older than 4 months, the motor disabilities vanished, and the ultrastructure of the sciatic nerve exhibited numerous tomacula and remyelinated fibers, as indicated by thinner myelin. No second acute attack was observed up to the age of 1 year. Thus, the deletion of the Miz1 POZ domain in Schwann cells induces an acute neuropathy with a subsequent regeneration in which there is ongoing balancing between de- and remyelination. Miz1ΔPOZ mice are impaired in the maintenance of myelinated fibers and are a promising model for studying remyelination in adult peripheral nerves.

Intermediate Charcot-Marie-Tooth disease

Charcot-Marie-Tooth (CMT) disease is a common neurogenetic disorder and its heterogeneity is a challenge for genetic diagnostics. The genetic diagnostic procedures for a CMT patient can be explored according to the electrophysiological criteria: very slow motor nerve conduction velocity (MNCV) (<15 m/s), slow MNCV (15-25 m/s), intermediate MNCV (25-45 m/s), and normal MNCV (>45 m/s). Based on the inheritance pattern, intermediate CMT can be divided into dominant (DI-CMT) and recessive types (RI-CMT). GJB1 is currently considered to be associated with X-linked DI-CMT, and MPZ, INF2, DNM2, YARS, GNB4, NEFL, and MFN2 are associated with autosomal DI-CMT. Moreover, GDAP1, KARS, and PLEKHG5 are associated with RI-CMT. Identification of these genes is not only important for patients and families but also provides new information about pathogenesis. It is hoped that this review will lead to a better understanding of intermediate CMT and provide a detailed diagnostic procedure for intermediate CMT.

Rab proteins: the key regulators of intracellular vesicle transport

Vesicular/membrane trafficking essentially regulates the compartmentalization and abundance of proteins within the cells and contributes in many signalling pathways. This membrane transport in eukaryotic cells is a complex process regulated by a large and diverse array of proteins. A large group of monomeric small GTPases; the Rabs are essential components of this membrane trafficking route. Most of the Rabs are ubiquitously expressed proteins and have been implicated in vesicle formation, vesicle motility/delivery along cytoskeleton elements and docking/fusion at target membranes through the recruitment of effectors. Functional impairments of Rabs affecting transport pathways manifest different diseases. Rab functions are accompanied by cyclical activation and inactivation of GTP-bound and GDP-bound forms between the cytosol and membranes which is regulated by upstream regulators. Rab proteins are characterized by their distinct sub-cellular localization and regulate a wide variety of endocytic, transcytic and exocytic transport pathways. Mutations of Rabs affect cell growth, motility and other biological processes.

The Gdap1 knockout mouse mechanistically links redox control to Charcot-Marie-Tooth disease

Mutations in the mitochondrial fission factor GDAP1 are associated with severe peripheral neuropathies, but why the CNS remains unaffected is unclear. Using a Gdap1^−/− mouse, Niemann et al. demonstrate that a CNS-expressed Gdap1 paralogue changes its subcellular localisation under oxidative stress conditions to also act as a mitochondrial fission factor.

The ganglioside-induced differentiation-associated protein 1 (GDAP1) is a mitochondrial fission factor and mutations in GDAP1 cause Charcot–Marie–Tooth disease. We found that Gdap1 knockout mice (Gdap1^−/−), mimicking genetic alterations of patients suffering from severe forms of Charcot–Marie–Tooth disease, develop an age-related, hypomyelinating peripheral neuropathy. Ablation of Gdap1 expression in Schwann cells recapitulates this phenotype. Additionally, intra-axonal mitochondria of peripheral neurons are larger in Gdap1^−/− mice and mitochondrial transport is impaired in cultured sensory neurons of Gdap1^−/− mice compared with controls. These changes in mitochondrial morphology and dynamics also influence mitochondrial biogenesis. We demonstrate that mitochondrial DNA biogenesis and content is increased in the peripheral nervous system but not in the central nervous system of Gdap1^−/− mice compared with control littermates. In search for a molecular mechanism we turned to the paralogue of GDAP1, GDAP1L1, which is mainly expressed in the unaffected central nervous system. GDAP1L1 responds to elevated levels of oxidized glutathione by translocating from the cytosol to mitochondria, where it inserts into the mitochondrial outer membrane. This translocation is necessary to substitute for loss of GDAP1 expression. Accordingly, more GDAP1L1 was associated with mitochondria in the spinal cord of aged Gdap1^−/− mice compared with controls. Our findings demonstrate that Charcot–Marie–Tooth disease caused by mutations in GDAP1 leads to mild, persistent oxidative stress in the peripheral nervous system, which can be compensated by GDAP1L1 in the unaffected central nervous system. We conclude that members of the GDAP1 family are responsive and protective against stress associated with increased levels of oxidized glutathione.

Genetics of Charcot-Marie-Tooth (CMT) Disease within the Frame of the Human Genome Project Success

Charcot-Marie-Tooth (CMT) neuropathies comprise a group of monogenic disorders affecting the peripheral nervous system. CMT is characterized by a clinically and genetically heterogeneous group of neuropathies, involving all types of Mendelian inheritance patterns. Over 1,000 different mutations have been discovered in 80 disease-associated genes. Genetic research of CMT has pioneered the discovery of genomic disorders and aided in understanding the effects of copy number variation and the mechanisms of genomic rearrangements. CMT genetic study also unraveled common pathomechanisms for peripheral nerve degeneration, elucidated gene networks, and initiated the development of therapeutic approaches. The reference genome, which became available thanks to the Human Genome Project, and the development of next generation sequencing tools, considerably accelerated gene and mutation discoveries. In fact, the first clinical whole genome sequence was reported in a patient with CMT. Here we review the history of CMT gene discoveries, starting with technologies from the early days in human genetics through the high-throughput application of modern DNA analyses. We highlight the most relevant examples of CMT genes and mutation mechanisms, some of which provide promising treatment strategies. Finally, we propose future initiatives to accelerate diagnosis of CMT patients through new ways of sharing large datasets and genetic variants, and at ever diminishing costs.

Connexin 32 and 43 mutations: do they play a role in chronic rhinosinusitis?

OBJECTIVE

Dysfunction of the sinonasal epithelium may contribute to the pathogenesis of chronic rhinosinusitis (CRS) including recurrent acute rhinosinusitis (RARS). Mutations in connexin 32 and 43 proteins have been associated with a number of human diseases. The objective of this study is to investigate the role of mutations in connexin 32 or connexin 43 genes in CRS and RARS.

METHODS

Prospective case series of 19 patients with CRS and /or RARS. Clinical and demographic factors were noted and buccal swabs were collected for DNA sequencing of connexin 32 and connexin 43 genes.

RESULTS

One patient was found to have a conservative V193I mutation in the connexin 32 gene. Connexin 43 mutations were found in two patients - a silent R239R mutation and an AAA insertion after the stop codon in the 3' UTR. None of these mutations are associated with any known diseases or predicted to lead to protein dysfunction.

CONCLUSION

Mutations in connexin 32 or 43 genes in patients with CRS, including RARS, appear to be rare. The etiologic role of connexin mutations in chromic rhinosinusitis is suspect, and routine sequencing for connexin mutations in patients with RARS or CRS is not cost effective.

Genetic testing practices for Charcot-Marie-Tooth type 1A disease

INTRODUCTION

Charcot-Marie-Tooth disease type 1A (CMT1A) is caused by a PMP22 gene duplication. CMT1A has a robust electrical phenotype that can be used to direct genetic testing. We compared specialty CMT center CMT1A diagnosis rates to those of outside physicians.

METHODS

Charts were reviewed for 102 patients with CMT1A seen at a specialty CMT clinic between 2001 and 2009. Nerve conduction studies, family history, date of genetic testing, and type of genetic testing (single gene vs. panel) were collected.

RESULTS

Although the specialty clinic ordered more PMP22 duplication testing alone beginning at an earlier year, thereby reducing costs, both the specialty clinic and outside physicians began the decade doing panel testing and ended the decade looking at only PMP22.

CONCLUSIONS

Specialty centers adapt earlier to changes in testing practice than non-specialty centers. As the landscape of genetic testing changes, the algorithms for testing will also likely change.

Mitochondrial dysfunction in neuromuscular disorders

This review deciphers aspects of mitochondrial (mt) dysfunction among nosologically, pathologically, and genetically diverse diseases of the skeletal muscle, lower motor neuron, and peripheral nerve, which fall outside the traditional realm of mt cytopathies. Special emphasis is given to well-characterized mt abnormalities in collagen VI myopathies (Ullrich congenital muscular dystrophy and Bethlem myopathy), megaconial congenital muscular dystrophy, limb-girdle muscular dystrophy type 2 (calpainopathy), centronuclear myopathies, core myopathies, inflammatory myopathies, spinal muscular atrophy, Charcot-Marie-Tooth neuropathy type 2, and drug-induced peripheral neuropathies. Among inflammatory myopathies, mt abnormalities are more prominent in inclusion body myositis and a subset of polymyositis with mt pathology, both of which are refractory to corticosteroid treatment. Awareness is raised about instances of phenotypic mimicry between cases harboring primary mtDNA depletion, in the context of mtDNA depletion syndrome, and established neuromuscular disorders such as spinal muscular atrophy. A substantial body of experimental work, derived from animal models, attests to a major role of mitochondria (mt) in the early process of muscle degeneration. Common mechanisms of mt-related cell injury include dysregulation of the mt permeability transition pore opening and defective autophagy. The therapeutic use of mt permeability transition pore modifiers holds promise in various neuromuscular disorders, including muscular dystrophies.

Curcumin facilitates a transitory cellular stress response in Trembler-J mice

We have previously shown that oral administration of curcumin significantly decreases the percentage of apoptotic Schwann cells and partially mitigates the severe neuropathy phenotype of the Trembler-J (Tr-J) mouse model in a dose-dependent manner. Here we compared the gene expression in sciatic nerves of 2-week-old pups and adult Tr-J with the same age groups of wild-type mice and found a significant increase in gene expression for hypoxia, inflammatory response and heat-shock proteins, the latter specifically the Hsp70 family, in Tr-J mice. We also detected an activation of different branches of unfolded protein responses (UPRs) in Tr-J mice. Administering curcumin results in lower expression of UPR markers suggesting it relieves endoplasmic reticulum (ER) cell stress sensors in sciatic nerves of Tr-J mice while the level of heat-shock proteins stays comparable to untreated Tr-J mice. We further tested if Hsp70 levels could influence the severity of the Tr-J neuropathy. Notably, reduced dosage of the Hsp70 strongly potentiates the severity of the Tr-J neuropathy, though the absence of Hsp70 had little effect in wild-type mice. In aggregate, these data provide further insights into the pathological disease mechanisms caused by myelin gene mutations and further support the exploration of curcumin as a therapeutic approach for selected forms of inherited neuropathy and potentially for other genetic diseases due to ER-retained mutants.

Charcot-Marie-Tooth disease-associated mutants of GDAP1 dissociate its roles in peroxisomal and mitochondrial fission

Mitochondria and peroxisomes can be fragmented by the process of fission. The fission machineries of both organelles share a set of proteins. GDAP1 is a tail-anchored protein of mitochondria and induces mitochondrial fragmentation. Mutations in GDAP1 lead to Charcot-Marie-Tooth disease (CMT), an inherited peripheral neuropathy, and affect mitochondrial dynamics. Here, we show that GDAP1 is also targeted to peroxisomes mediated by the import receptor Pex19. Knockdown of GDAP1 leads to peroxisomal elongation that can be rescued by re-expressing GDAP1 and by missense mutated forms found in CMT patients. GDAP1-induced peroxisomal fission is dependent on the integrity of its hydrophobic domain 1, and on Drp1 and Mff, as is mitochondrial fission. Thus, GDAP1 regulates mitochondrial and peroxisomal fission by a similar mechanism. However, our results reveal also a more critical role of the amino-terminal GDAP1 domains, carrying most CMT-causing mutations, in the regulation of mitochondrial compared to peroxisomal fission.

Aberrant Schwann cell lipid metabolism linked to mitochondrial deficits leads to axon degeneration and neuropathy

Mitochondrial dysfunction is a common cause of peripheral neuropathy. Much effort has been devoted to examining the role played by neuronal/axonal mitochondria, but how mitochondrial deficits in peripheral nerve glia (Schwann cells [SCs]) contribute to peripheral nerve diseases remains unclear. Here, we investigate a mouse model of peripheral neuropathy secondary to SC mitochondrial dysfunction (Tfam-SCKOs). We show that disruption of SC mitochondria activates a maladaptive integrated stress response (ISR) through the actions of heme-regulated inhibitor (HRI) kinase, and causes a shift in lipid metabolism away from fatty acid synthesis toward oxidation. These alterations in SC lipid metabolism result in depletion of important myelin lipid components as well as in accumulation of acylcarnitines (ACs), an intermediate of fatty acid β-oxidation. Importantly, we show that ACs are released from SCs and induce axonal degeneration. A maladaptive ISR as well as altered SC lipid metabolism are thus underlying pathological mechanisms in mitochondria-related peripheral neuropathies.