Lack of GDAP1 induces neuronal calcium and mitochondrial defects in a knockout mouse model of charcot-marie-tooth neuropathy

Conformational analysis of membrane-proximal segments of GDAP1 in a lipidic environment using synchrotron radiation suggests a mode of assembly at the mitochondrial outer membrane

The mitochondrial outer membrane creates a diffusion barrier between the cytosol and the mitochondrial intermembrane space, allowing the exchange of metabolic products, important for efficient mitochondrial function in neurons. The ganglioside-induced differentiation-associated protein 1 (GDAP1) is a mitochondrial outer membrane protein with a critical role in mitochondrial dynamics and metabolic balance in neurons. Missense mutations in the GDAP1 gene are linked to the most common human peripheral neuropathy, Charcot-Marie-Tooth disease (CMT). GDAP1 is a distant member of the glutathione-S-transferase (GST) superfamily, with unknown enzymatic properties or functions at the molecular level. The structure of the cytosol-facing GST-like domain has been described, but there is no consensus on how the protein interacts with the mitochondrial outer membrane. Here, we describe a model for GDAP1 assembly on the membrane using peptides vicinal to the GDAP1 transmembrane domain. We used oriented circular dichroism spectroscopy (OCD) with synchrotron radiation to study the secondary structure and orientation of GDAP1 segments at the outer and inner surfaces of the outer mitochondrial membrane. These experiments were complemented by small-angle X-ray scattering, providing the first experimental structural models for full-length human GDAP1. The results indicate that GDAP1 is bound into the membrane via a single transmembrane helix, flanked by two peripheral helices interacting with the outer and inner leaflets of the mitochondrial outer membrane in different orientations. Impairment of these interactions could be a mechanism for CMT in the case of missense mutations affecting these segments instead of the GST-like domain.

Intravenous Administration of an AAV9 Vector Ubiquitously Expressing C1orf194 Gene Improved CMT-Like Neuropathy in C1orf194-/- Mice

Charcot-Marie-Tooth (CMT) disease, also known as hereditary motor sensory neuropathy, is a group of rare genetically heterogenous diseases characterized by progressive muscle weakness and atrophy, along with sensory deficits. Despite extensive pre-clinical and clinical research, no FDA-approved therapy is available for any CMT type. We previously identified C1ORF194, a novel causative gene for CMT, and found that both C1orf194 knock-in (I121N) and knockout mice developed clinical phenotypes similar to those in patients with CMT. Encouraging results of adeno-associated virus (AAV)-mediated gene therapy for spinal muscular atrophy have stimulated the use of AAVs as vehicles for CMT gene therapy. Here, we present a gene therapy approach to restore C1orf194 expression in a knockout background. We used C1orf194-/- mice treated with AAV serotype 9 (AAV9) vector carrying a codon-optimized WT human C1ORF194 cDNA whose expression was driven by a ubiquitously expressed chicken β-actin promoter with a CMV enhancer. Our preclinical evaluation demonstrated the efficacy of AAV-mediated gene therapy in improving sensory and motor abilities, thus achieving largely normal gross motor performance and minimal signs of neuropathy, on the basis of neurophysiological and histopathological evaluation in C1orf194-/- mice administered AAV gene therapy. Our findings advance the techniques for delivering therapeutic interventions to individuals with CMT.

Rapid degeneration of iPSC-derived motor neurons lacking Gdap1 engages a mitochondrial-sustained innate immune response

Charcot-Marie-Tooth disease is a chronic hereditary motor and sensory polyneuropathy targeting Schwann cells and/or motor neurons. Its multifactorial and polygenic origin portrays a complex clinical phenotype of the disease with a wide range of genetic inheritance patterns. The disease-associated gene GDAP1 encodes for a mitochondrial outer membrane protein. Mouse and insect models with mutations in Gdap1 have reproduced several traits of the human disease. However, the precise function in the cell types affected by the disease remains unknown. Here, we use induced-pluripotent stem cells derived from a Gdap1 knockout mouse model to better understand the molecular and cellular phenotypes of the disease caused by the loss-of-function of this gene. Gdap1-null motor neurons display a fragile cell phenotype prone to early degeneration showing (1) altered mitochondrial morphology, with an increase in the fragmentation of these organelles, (2) activation of autophagy and mitophagy, (3) abnormal metabolism, characterized by a downregulation of Hexokinase 2 and ATP5b proteins, (4) increased reactive oxygen species and elevated mitochondrial membrane potential, and (5) increased innate immune response and p38 MAP kinase activation. Our data reveals the existence of an underlying Redox-inflammatory axis fueled by altered mitochondrial metabolism in the absence of Gdap1. As this biochemical axis encompasses a wide variety of druggable targets, our results may have implications for developing therapies using combinatorial pharmacological approaches and improving therefore human welfare. A Redox-immune axis underlying motor neuron degeneration caused by the absence of Gdap1. Our results show that Gdap1^-/- motor neurons have a fragile cellular phenotype that is prone to degeneration. Gdap1^-/- iPSCs differentiated into motor neurons showed an altered metabolic state: decreased glycolysis and increased OXPHOS. These alterations may lead to hyperpolarization of mitochondria and increased ROS levels. Excessive amounts of ROS might be the cause of increased mitophagy, p38 activation and inflammation as a cellular response to oxidative stress. The p38 MAPK pathway and the immune response may, in turn, have feedback mechanisms, leading to the induction of apoptosis and senescence, respectively. CAC, citric acid cycle; ETC, electronic transport chain; Glc, glucose; Lac, lactate; Pyr, pyruvate.

Differential effects of Mendelian GDAP1 clinical variants on mitochondria-lysosome membrane contacts sites

GDAP1 pathogenic variants cause Charcot-Marie-Tooth (CMT) disease, the most common hereditary motor and sensory neuropathy. CMT-GDAP1 can be axonal or demyelinating, with autosomal dominant or recessive inheritance, leading to phenotypic heterogeneity. Recessive GDAP1 variants cause a severe phenotype, whereas dominant variants are associated with a milder disease course. GDAP1 is an outer mitochondrial membrane protein involved in mitochondrial membrane contact sites (MCSs) with the plasmatic membrane, the endoplasmic reticulum (ER), and lysosomes. In GDAP1-deficient models, the pathophysiology includes morphological defects in mitochondrial network and ER, impaired Ca2+ homeostasis, oxidative stress, and mitochondrial MCSs defects. Nevertheless, the underlying pathophysiology of dominant variants is less understood. Here, we study the effect upon mitochondria-lysosome MCSs of two GDAP1 clinical variants located in the α-loop interaction domain of the protein. p.Thr157Pro dominant variant causes the increase in these MCSs that correlates with a hyper-fissioned mitochondrial network. In contrast, p.Arg161His recessive variant, which is predicted to significantly change the contact surface of GDAP1, causes decreased contacts with more elongated mitochondria. Given that mitochondria-lysosome MCSs regulate Ca2+ transfer from the lysosome to mitochondria, our results support that GDAP1 clinical variants have different consequences for Ca2+ handling and that could be primary insults determining differences in severity between dominant and recessive forms of the disease.

The genetic basis of endometriosis and comorbidity with other pain and inflammatory conditions

Endometriosis is a common condition associated with debilitating pelvic pain and infertility. A genome-wide association study meta-analysis, including 60,674 cases and 701,926 controls of European and East Asian descent, identified 42 genome-wide significant loci comprising 49 distinct association signals. Effect sizes were largest for stage 3/4 disease, driven by ovarian endometriosis. Identified signals explained up to 5.01% of disease variance and regulated expression or methylation of genes in endometrium and blood, many of which were associated with pain perception/maintenance (SRP14/BMF, GDAP1, MLLT10, BSN and NGF). We observed significant genetic correlations between endometriosis and 11 pain conditions, including migraine, back and multisite chronic pain (MCP), as well as inflammatory conditions, including asthma and osteoarthritis. Multitrait genetic analyses identified substantial sharing of variants associated with endometriosis and MCP/migraine. Targeted investigations of genetically regulated mechanisms shared between endometriosis and other pain conditions are needed to aid the development of new treatments and facilitate early symptomatic intervention.

Neuroprotective Mechanisms of Amylin Receptor Activation, Not Antagonism, in the APP/PS1 Mouse Model of Alzheimer's Disease

BACKGROUND

Amylin, a pancreatic amyloid peptide involved in energy homeostasis, is increasingly studied in the context of Alzheimer's disease (AD) etiology. To date, conflicting pathogenic and neuroprotective roles for this peptide and its analogs for AD pathogenesis have been described.

OBJECTIVE

Whether the benefits of amylin are associated with peripheral improvement of metabolic tone/function or directly through the activation of central amylin receptors is also unknown and downstream signaling mechanisms of amylin receptors are major objectives of this study.

METHODS

To address these questions more directly we delivered the amylin analog pramlintide systemically (IP), at previously identified therapeutic doses, while centrally (ICV) inhibiting the receptor using an amylin receptor antagonist (AC187), at doses known to impact CNS function.

RESULTS

Here we show that pramlintide improved cognitive function independently of CNS receptor activation and provide transcriptomic data that highlights potential mechanisms. Furthermore, we show than inhibition of the amylin receptor increased amyloid-beta pathology in female APP/PS1 mice, an effect than was mitigated by peripheral delivery of pramlintide. Through transcriptomic analysis of pramlintide therapy in AD-modeled mice we found sexual dimorphic modulation of neuroprotective mechanisms: oxidative stress protection in females and membrane stability and reduced neuronal excitability markers in males.

CONCLUSION

These data suggest an uncoupling of functional and pathology-related events and highlighting a more complex receptor system and pharmacological relationship that must be carefully studied to clarify the role of amylin in CNS function and AD.

The regulatory mechanism between lysosomes and mitochondria in the aetiology of cardiovascular diseases

Coordinated action among various organelles maintains cellular functions. For instance, mitochondria and lysosomes are the main organelles contributing to cellular metabolism and provide energy for cardiomyocyte contraction. They also provide essential signalling platforms in the cell that regulate many key processes such as autophagy, apoptosis, oxidative stress, inflammation and cell death. Often, abnormalities in mitochondrial or lysosomal structures and functions bring about cardiovascular diseases (CVDs). Although the communication between mitochondria and lysosomes throughout the cardiovascular system is intensely studied, the regulatory mechanisms have not been completely understood. Thus, we summarize the most recent studies related to mitochondria and lysosomes' role in CVDs and their potential connections and communications under cardiac pathophysiological conditions. Further, we discuss limitations and future perspectives regarding diagnosis, therapeutic strategies and drug discovery in CVDs.

Endoplasmic reticulum-mitochondria signaling in neurons and neurodegenerative diseases

Recent advances have revealed common pathological changes in neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis with related frontotemporal dementia (ALS/FTD). Many of these changes can be linked to alterations in endoplasmic reticulum (ER)-mitochondria signaling, including dysregulation of Ca2+ signaling, autophagy, lipid metabolism, ATP production, axonal transport, ER stress responses and synaptic dysfunction. ER-mitochondria signaling involves specialized regions of ER, called mitochondria-associated membranes (MAMs). Owing to their role in neurodegenerative processes, MAMs have gained attention as they appear to be associated with all the major neurodegenerative diseases. Furthermore, their specific role within neuronal maintenance is being revealed as mutant genes linked to major neurodegenerative diseases have been associated with damage to these specialized contacts. Several studies have now demonstrated that these specialized contacts regulate neuronal health and synaptic transmission, and that MAMs are damaged in patients with neurodegenerative diseases. This Review will focus on the role of MAMs and ER-mitochondria signaling within neurons and how damage of the ER-mitochondria axis leads to a disruption of vital processes causing eventual neurodegeneration.

Distinct roles for the Charcot-Marie-tooth disease-causing endosomal regulators Mtmr5 and Mtmr13 in axon radial sorting and Schwann cell myelination

The form of Charcot-Marie-Tooth type 4B (CMT4B) disease caused by mutations in myotubularin-related 5 (MTMR5; also called SET Binding Factor 1; SBF1) shows a spectrum of axonal and demyelinating nerve phenotypes. This contrasts with the CMT4B subtypes caused by MTMR2 or MTMR13 (SBF2) mutations, which are characterized by myelin outfoldings and classic demyelination. Thus, it is unclear whether MTMR5 plays an analogous or distinct role from that of its homolog, MTMR13, in the peripheral nervous system (PNS). MTMR5 and MTMR13 are pseudophosphatases predicted to regulate endosomal trafficking by activating Rab GTPases and binding to the phosphoinositide 3-phosphatase MTMR2. In the mouse PNS, Mtmr2 was required to maintain wild type levels of Mtmr5 and Mtmr13, suggesting that these factors function in discrete protein complexes. Genetic elimination of both Mtmr5 and Mtmr13 in mice led to perinatal lethality, indicating that the two proteins have partially redundant functions during embryogenesis. Loss of Mtmr5 in mice did not cause CMT4B-like myelin outfoldings. However, adult Mtmr5-/- mouse nerves contained fewer myelinated axons than control nerves, likely as a result of axon radial sorting defects. Consistently, Mtmr5 levels were highest during axon radial sorting and fell sharply after postnatal day seven. Our findings suggest that Mtmr5 and Mtmr13 ensure proper axon radial sorting and Schwann cell myelination, respectively, perhaps through their direct interactions with Mtmr2. This study enhances our understanding of the non-redundant roles of the endosomal regulators MTMR5 and MTMR13 during normal peripheral nerve development and disease.

Animal Models as a Tool to Design Therapeutical Strategies for CMT-like Hereditary Neuropathies

Since ancient times, animal models have provided fundamental information in medical knowledge. This also applies for discoveries in the field of inherited peripheral neuropathies (IPNs), where they have been instrumental for our understanding of nerve development, pathogenesis of neuropathy, molecules and pathways involved and to design potential therapies. In this review, we briefly describe how animal models have been used in ancient medicine until the use of rodents as the prevalent model in present times. We then travel along different examples of how rodents have been used to improve our understanding of IPNs. We do not intend to describe all discoveries and animal models developed for IPNs, but just to touch on a few arbitrary and paradigmatic examples, taken from our direct experience or from literature. The idea is to show how strategies have been developed to finally arrive to possible treatments for IPNs.

GDAP1 Involvement in Mitochondrial Function and Oxidative Stress, Investigated in a Charcot-Marie-Tooth Model of hiPSCs-Derived Motor Neurons

Mutations in the ganglioside-induced differentiation associated protein 1 (GDAP1) gene have been associated with demyelinating and axonal forms of Charcot-Marie-Tooth (CMT) disease, the most frequent hereditary peripheral neuropathy in humans. Previous studies reported the prevalent GDAP1 expression in neural tissues and cells, from animal models. Here, we described the first GDAP1 functional study on human induced-pluripotent stem cells (hiPSCs)-derived motor neurons, obtained from normal subjects and from a CMT2H patient, carrying the GDAP1 homozygous c.581C>G (p.Ser194*) mutation. At mRNA level, we observed that, in normal subjects, GDAP1 is mainly expressed in motor neurons, while it is drastically reduced in the patient’s cells containing a premature termination codon (PTC), probably degraded by the nonsense-mediated mRNA decay (NMD) system. Morphological and functional investigations revealed in the CMT patient’s motor neurons a decrease of cell viability associated to lipid dysfunction and oxidative stress development. Mitochondrion is a key organelle in oxidative stress generation, but it is also mainly involved in energetic metabolism. Thus, in the CMT patient’s motor neurons, mitochondrial cristae defects were observed, even if no deficit in ATP production emerged. This cellular model of hiPSCs-derived motor neurons underlines the role of mitochondrion and oxidative stress in CMT disease and paves the way for new treatment evaluation.

Effective therapeutic strategies in a pre-clinical mouse model of Charcot-Marie-tooth disease

Charcot–Marie–Tooth (CMT) disease is a neuropathy that lacks effective therapy. CMT patients show degeneration of peripheral nerves, leading to muscle weakness and loss of proprioception. Loss of mitochondrial oxidative phosphorylation proteins and enzymes of the antioxidant response accompany degeneration of nerves in skin biopsies of CMT patients. Herein, we followed a drug-repurposing approach to find drugs in a Food and Drug Administration-approved library that could prevent development of CMT disease in the Gdap1-null mouse model. We found that the antibiotic florfenicol is a mitochondrial uncoupler that prevents the production of reactive oxygen species and activates respiration in human GDAP1-knockdown neuroblastoma cells and in dorsal root ganglion neurons of Gdap1-null mice. Treatment of CMT-affected Gdap1-null mice with florfenicol has no beneficial effect in the course of the disease. However, administration of florfenicol, or the antioxidant MitoQ, to pre-symptomatic GDAP1-null mice prevented weight gain and ameliorated the motor coordination deficiencies that developed in the Gdap1-null mice. Interestingly, both florfenicol and MitoQ halted the decay in mitochondrial and redox proteins in sciatic nerves of Gdap1-null mice, supporting that oxidative damage is implicated in the etiology of the neuropathy. These findings support the development of clinical trials for translation of these drugs for treatment of CMT patients.

Selective Neuron Vulnerability in Common and Rare Diseases-Mitochondria in the Focus

Mitochondrial dysfunction is a central feature of neurodegeneration within the central and peripheral nervous system, highlighting a strong dependence on proper mitochondrial function of neurons with especially high energy consumptions. The fitness of mitochondria critically depends on preservation of distinct processes, including the maintenance of their own genome, mitochondrial dynamics, quality control, and Ca²⁺ handling. These processes appear to be differently affected in common neurodegenerative diseases, such as Alzheimer’s and Parkinson’s disease, as well as in rare neurological disorders, including Huntington’s disease, Amyotrophic Lateral Sclerosis and peripheral neuropathies. Strikingly, particular neuron populations of different morphology and function perish in these diseases, suggesting that cell-type specific factors contribute to the vulnerability to distinct mitochondrial defects. Here we review the disruption of mitochondrial processes in common as well as in rare neurological disorders and its impact on selective neurodegeneration. Understanding discrepancies and commonalities regarding mitochondrial dysfunction as well as individual neuronal demands will help to design new targets and to make use of already established treatments in order to improve treatment of these diseases.

Genetic Neuropathy Due to Impairments in Mitochondrial Dynamics

Mitochondria are dynamic organelles capable of fusing, dividing, and moving about the cell. These properties are especially important in neurons, which in addition to high energy demand, have unique morphological properties with long axons. Notably, mitochondrial dysfunction causes a variety of neurological disorders including peripheral neuropathy, which is linked to impaired mitochondrial dynamics. Nonetheless, exactly why peripheral neurons are especially sensitive to impaired mitochondrial dynamics remains somewhat enigmatic. Although the prevailing view is that longer peripheral nerves are more sensitive to the loss of mitochondrial motility, this explanation is insufficient. Here, we review pathogenic variants in proteins mediating mitochondrial fusion, fission and transport that cause peripheral neuropathy. In addition to highlighting other dynamic processes that are impacted in peripheral neuropathies, we focus on impaired mitochondrial quality control as a potential unifying theme for why mitochondrial dysfunction and impairments in mitochondrial dynamics in particular cause peripheral neuropathy.

Molecular Mechanisms behind Inherited Neurodegeneration of the Optic Nerve

Inherited neurodegeneration of the optic nerve is a paradigm in neurology, as many forms of isolated or syndromic optic atrophy are encountered in clinical practice. The retinal ganglion cells originate the axons that form the optic nerve. They are particularly vulnerable to mitochondrial dysfunction, as they present a peculiar cellular architecture, with axons that are not myelinated for a long intra-retinal segment, thus, very energy dependent. The genetic landscape of causative mutations and genes greatly enlarged in the last decade, pointing to common pathways. These mostly imply mitochondrial dysfunction, which leads to a similar outcome in terms of neurodegeneration. We here critically review these pathways, which include (1) complex I-related oxidative phosphorylation (OXPHOS) dysfunction, (2) mitochondrial dynamics, and (3) endoplasmic reticulum-mitochondrial inter-organellar crosstalk. These major pathogenic mechanisms are in turn interconnected and represent the target for therapeutic strategies. Thus, their deep understanding is the basis to set and test new effective therapies, an urgent unmet need for these patients. New tools are now available to capture all interlinked mechanistic intricacies for the pathogenesis of optic nerve neurodegeneration, casting hope for innovative therapies to be rapidly transferred into the clinic and effectively cure inherited optic neuropathies.

Mitochondria and calcium defects correlate with axonal dysfunction in GDAP1-related Charcot-Marie-Tooth mouse model

Ganglioside-induced differentiation associated protein 1 (GDAP1) gene encodes a protein of the mitochondrial outer membrane and of the mitochondrial membrane contacts with the endoplasmic reticulum (MAMs) and lysosomes. Since mutations in GDAP1 cause Charcot-Marie-Tooth, an inherited motor and sensory neuropathy, its function is essential for peripheral nerve physiology. Our previous studies showed structural and functional defects in mitochondria and their contacts when GDAP1 is depleted. Nevertheless, the underlying axonal pathophysiological events remain unclear. Here, we have used embryonic motor neurons (eMNs) cultures from Gdap1 knockout (Gdap1^-/-) mice to investigate in vivo mitochondria and calcium homeostasis in the axons. We imaged mitochondrial axonal transport and we found a defective pattern in the Gdap1^-/- eMNs. We also detected pathological and functional mitochondria membrane abnormalities with a drop in ATP production and a deteriorated bioenergetic status. Another consequence of the loss of GDAP1 in the soma and axons of eMNs was the in vivo increase calcium levels in both basal conditions and during recovery after neuronal stimulation with glutamate. Further, we found that glutamate-stimulation of respiration was lower in Gdap1^-/- eMNs showing that the basal bioenergetics failure jeopardizes a full respiratory response and prevents a rapid return of calcium to basal levels. Together, our results demonstrate that the loss of GDAP1 critically compromises the morphology and function of mitochondria and its relationship with calcium homeostasis in the soma and axons, offering important insight into the cellular mechanisms associated with axonal degeneration of GDAP1-related CMT neuropathies and the relevance that axon length may have.

Impaired Mitochondrial Mobility in Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is a progressive, peripheral neuropathy and the most commonly inherited neurological disorder. Clinical manifestations of CMT mutations are typically limited to peripheral neurons, the longest cells in the body. Currently, mutations in at least 80 different genes are associated with CMT and new mutations are regularly being discovered. A large portion of the proteins mutated in axonal CMT have documented roles in mitochondrial mobility, suggesting that organelle trafficking defects may be a common underlying disease mechanism. This review will focus on the potential role of altered mitochondrial mobility in the pathogenesis of axonal CMT, highlighting the conceptional challenges and potential experimental and therapeutic opportunities presented by this "impaired mobility" model of the disease.

Structure of the Complete Dimeric Human GDAP1 Core Domain Provides Insights into Ligand Binding and Clustering of Disease Mutations

Charcot-Marie-Tooth disease (CMT) is one of the most common inherited neurological disorders. Despite the common involvement of ganglioside-induced differentiation-associated protein 1 (GDAP1) in CMT, the protein structure and function, as well as the pathogenic mechanisms, remain unclear. We determined the crystal structure of the complete human GDAP1 core domain, which shows a novel mode of dimerization within the glutathione S-transferase (GST) family. The long GDAP1-specific insertion forms an extended helix and a flexible loop. GDAP1 is catalytically inactive toward classical GST substrates. Through metabolite screening, we identified a ligand for GDAP1, the fatty acid hexadecanedioic acid, which is relevant for mitochondrial membrane permeability and Ca²⁺ homeostasis. The fatty acid binds to a pocket next to a CMT-linked residue cluster, increases protein stability, and induces changes in protein conformation and oligomerization. The closest homologue of GDAP1, GDAP1L1, is monomeric in its full-length form. Our results highlight the uniqueness of GDAP1 within the GST family and point toward allosteric mechanisms in regulating GDAP1 oligomeric state and function.

Mitochondria-lysosome membrane contacts are defective in GDAP1-related Charcot-Marie-Tooth disease

Mutations in the GDAP1 gene cause Charcot-Marie-Tooth (CMT) neuropathy. GDAP1 is an atypical glutathione S-transferase (GST) of the outer mitochondrial membrane and the mitochondrial membrane contacts with the endoplasmic reticulum (MAMs). Here, we investigate the role of this GST in the autophagic flux and the membrane contact sites (MCSs) between mitochondria and lysosomes in the cellular pathophysiology of GDAP1 deficiency. We demonstrate that GDAP1 participates in basal autophagy and that its depletion affects LC3 and PI3P biology in autophagosome biogenesis and membrane trafficking from MAMs. GDAP1 also contributes to the maturation of lysosome by interacting with PYKfyve kinase, a pH-dependent master lysosomal regulator. GDAP1 deficiency causes giant lysosomes with hydrolytic activity, a delay in the autophagic lysosome reformation, and TFEB activation. Notably, we found that GDAP1 interacts with LAMP-1, which supports that GDAP1-LAMP-1 is a new tethering pair of mitochondria and lysosome membrane contacts. We observed mitochondria-lysosome MCSs in soma and axons of cultured mouse embryonic motor neurons and human neuroblastoma cells. GDAP1 deficiency reduces the MCSs between these organelles, causes mitochondrial network abnormalities, and decreases levels of cellular glutathione (GSH). The supply of GSH-MEE suffices to rescue the lysosome membranes and the defects of the mitochondrial network, but not the interorganelle MCSs nor early autophagic events. Overall, we show that GDAP1 enables the proper function of mitochondrial MCSs in both degradative and nondegradative pathways, which could explain primary insults in GDAP1-related CMT pathophysiology, and highlights new redox-sensitive targets in axonopathies where mitochondria and lysosomes are involved.

Mutations in GDAP1 Influence Structure and Function of the Trans-Golgi Network

Charcot-Marie-Tooth disease (CMT) is a heritable neurodegenerative disease that displays great genetic heterogeneity. The genes and mutations that underlie this heterogeneity have been extensively characterized by molecular genetics. However, the molecular pathogenesis of the vast majority of CMT subtypes remains terra incognita. Any attempts to perform experimental therapy for CMT disease are limited by a lack of understanding of the pathogenesis at a molecular level. In this study, we aim to identify the molecular pathways that are disturbed by mutations in the gene encoding GDAP1 using both yeast and human cell, based models of CMT-GDAP1 disease. We found that some mutations in GDAP1 led to a reduced expression of the GDAP1 protein and resulted in a selective disruption of the Golgi apparatus. These structural alterations are accompanied by functional disturbances within the Golgi. We screened over 1500 drugs that are available on the market using our yeast-based CMT-GDAP1 model. Drugs were identified that had both positive and negative effects on cell phenotypes. To the best of our knowledge, this study is the first report of the Golgi apparatus playing a role in the pathology of CMT disorders. The drugs we identified, using our yeast-based CMT-GDAP1 model, may be further used in translational research.

USP30 sets a trigger threshold for PINK1-PARKIN amplification of mitochondrial ubiquitylation

A new inhibitor of the deubiquitylase USP30, an actionable target relevant to Parkinson’s Disease, is introduced and characterised for parameters related to mitophagy.

The mitochondrial deubiquitylase USP30 negatively regulates the selective autophagy of damaged mitochondria. We present the characterisation of an N-cyano pyrrolidine compound, FT3967385, with high selectivity for USP30. We demonstrate that ubiquitylation of TOM20, a component of the outer mitochondrial membrane import machinery, represents a robust biomarker for both USP30 loss and inhibition. A proteomics analysis, on a SHSY5Y neuroblastoma cell line model, directly compares the effects of genetic loss of USP30 with chemical inhibition. We have thereby identified a subset of ubiquitylation events consequent to mitochondrial depolarisation that are USP30 sensitive. Within responsive elements of the ubiquitylome, several components of the outer mitochondrial membrane transport (TOM) complex are prominent. Thus, our data support a model whereby USP30 can regulate the availability of ubiquitin at the specific site of mitochondrial PINK1 accumulation following membrane depolarisation. USP30 deubiquitylation of TOM complex components dampens the trigger for the Parkin-dependent amplification of mitochondrial ubiquitylation leading to mitophagy. Accordingly, PINK1 generation of phospho-Ser65 ubiquitin proceeds more rapidly in cells either lacking USP30 or subject to USP30 inhibition.

Cigarette smoke exposure has region-specific effects on GDAP1 expression in mouse hippocampus

Early detection markers for substance use disorders are urgently needed. Recently, an association between the methylation of Ganglioside-induced differentiation-associated protein 1 (GDAP1) and alcohol addiction was found in a US and German population. In this study, we investigate whether GDAP1 expression might be affected by cigarette smoke as well and thus might be a marker of substance addiction in general. 11 adult female C57BL/6 J mice (6 wildtype and 5 lacking the NO-sensitive guanylyl cyclase1 (NO-GC1 KO)) were exposed to cigarette smoke over a period of 5 weeks, their brains immunohistochemically stained and compared to 11 non exposed mice (5 WT and 6 KO). The deletion of NO-GC1 results in a complete loss of synaptic plasticity, therefore, addiction-related alterations might become more obvious. Co-staining of anti-GDAP1 and DAPI revealed protein in the stratum granulare of the hippocampus. Three randomized frames for dentate gyrus (DG) and three for Cornu Ammonis region 1 (CA1) were used to count GDAP1. Cigarette smoke exposure significantly influenced GDAP1 expression depending on the hippocampal region but was not influenced by guanyl cyclase. In conclusion, cigarette smoke exposure alone had an effect on GDAP1 amount in both regions. Therewith, GDAP1might be a biomarker for substance addiction in general.

The role of mitochondrial dynamics in human cancers

Mitochondria are crucial cellular organelles. Under extracellular stimulations, mitochondria undergo constant fusion and fission dynamics to meet different cellular demands. Mitochondrial dynamics is regulated by specialized proteins and lipids. Dysregulated mitochondrial dynamics has been linked to the initiation and progression of diverse human cancers, affecting aspects such as cancer metastasis, drug resistance and cancer stem cell survival, suggesting that targeting mitochondrial dynamics is a potential therapeutic strategy. In the present review, we summarize the molecular mechanisms underlying fusion and fission dynamics and discuss the effects of mitochondrial dynamics on the development of human cancers.

Oxidative Stress, a Crossroad Between Rare Diseases and Neurodegeneration

Oxidative stress is an imbalance between production and accumulation of oxygen reactive species and/or reactive nitrogen species in cells and tissues, and the capacity of detoxifying these products, using enzymatic and non-enzymatic components, such as glutathione. Oxidative stress plays roles in several pathological processes in the nervous system, such as neurotoxicity, neuroinflammation, ischemic stroke, and neurodegeneration. The concepts of oxidative stress and rare diseases were formulated in the eighties, and since then, the link between them has not stopped growing. The present review aims to expand knowledge in the pathological processes associated with oxidative stress underlying some groups of rare diseases: Friedreich’s ataxia, diseases with neurodegeneration with brain iron accumulation, Charcot-Marie-Tooth as an example of rare neuromuscular disorders, inherited retinal dystrophies, progressive myoclonus epilepsies, and pediatric drug-resistant epilepsies. Despite the discrimination between cause and effect may not be easy on many occasions, all these conditions are Mendelian rare diseases that share oxidative stress as a common factor, and this may represent a potential target for therapies.

Structural and functional divergence of GDAP1 from the glutathione S-transferase superfamily

Mutations in ganglioside-induced differentiation-associated protein 1 (GDAP1) alter mitochondrial morphology and result in several subtypes of the inherited peripheral neuropathy Charcot-Marie-Tooth disease; however, the mechanism by which GDAP1 functions has remained elusive. GDAP1 contains primary sequence homology to the GST superfamily; however, the question of whether GDAP1 is an active GST has not been clearly resolved. Here, we present biochemical evidence, suggesting that GDAP1 has lost the ability to bind glutathione without a loss of substrate binding activity. We have revealed that the α-loop, located within the H-site motif is the primary determinant for substrate binding. Using structural data of GDAP1, we have found that critical residues and configurations in the G-site which canonically interact with glutathione are altered in GDAP1, rendering it incapable of binding glutathione. Last, we have found that the overexpression of GDAP1 in HeLa cells results in a mitochondrial phenotype which is distinct from oxidative stress-induced mitochondrial fragmentation. This phenotype is dependent on the presence of the transmembrane domain, as well as a unique hydrophobic domain that is not found in canonical GSTs. Together, we data point toward a non-enzymatic role for GDAP1, such as a sensor or receptor.

Loss-of-function of EBP50 is a new cause of hereditary peripheral neuropathy: EBP50 functions in peripheral nerve system

Finding causative genetic mutations is important in the diagnosis and treatment of hereditary peripheral neuropathies. This study was conducted to find new genes involved in the pathophysiology of hereditary peripheral neuropathy. We identified a new mutation in the EBP50 gene, which is co-segregated with neuropathic phenotypes, including motor and sensory deficit in a family with Charcot-Marie-Tooth disease. EBP50 is known to be important for the formation of microvilli in epithelial cells, and the discovery of this gene mutation allowed us to study the function of EBP50 in the nervous system. EBP50 was strongly expressed in the nodal and paranodal regions of sciatic nerve fibers, where Schwann cell microvilli contact the axolemma, and at the growth tips of primary Schwann cells. In addition, EBP50 expression was decreased in mouse models of peripheral neuropathy. Knockout mice were used to study EBP50 function in the peripheral nervous system. Interestingly motor function deficit and abnormal histology of nerve fibers were observed in EBP50^+/- heterozygous mice at 12 months of age, but not 3 months. in vitro studies using Schwann cells showed that NRG1-induced AKT activation and migration were significantly reduced in cells overexpressing the I325V mutant of EBP50 or cells with knocked-down EBP50 expression. In conclusion, we show for the first time that loss of function due to EBP50 gene deficiency or mutation can cause peripheral neuropathy.

Much More Than a Scaffold: Cytoskeletal Proteins in Neurological Disorders

Recent observations related to the structure of the cytoskeleton in neurons and novel cytoskeletal abnormalities involved in the pathophysiology of some neurological diseases are changing our view on the function of the cytoskeletal proteins in the nervous system. These efforts allow a better understanding of the molecular mechanisms underlying neurological diseases and allow us to see beyond our current knowledge for the development of new treatments. The neuronal cytoskeleton can be described as an organelle formed by the three-dimensional lattice of the three main families of filaments: actin filaments, microtubules, and neurofilaments. This organelle organizes well-defined structures within neurons (cell bodies and axons), which allow their proper development and function through life. Here, we will provide an overview of both the basic and novel concepts related to those cytoskeletal proteins, which are emerging as potential targets in the study of the pathophysiological mechanisms underlying neurological disorders.

Gene therapies for axonal neuropathies: Available strategies, successes to date, and what to target next

Nearly one-hundred loci in the human genome have been associated with different forms of Charcot-Marie-Tooth disease (CMT) and related inherited neuropathies. Despite this wealth of gene targets, treatment options are still extremely limited, and clear "druggable" pathways are not obvious for many of these mutations. However, recent advances in gene therapies are beginning to circumvent this challenge. Each type of CMT is a monogenic disorder, and the cellular targets are usually well-defined and typically include peripheral neurons or Schwann cells. In addition, the genetic mechanism is often also clear, with loss-of-function mutations requiring restoration of gene expression, and gain-of-function or dominant-negative mutations requiring silencing of the mutant allele. These factors combine to make CMT a good target for developing genetic therapies. Here we will review the state of relatively established gene therapy approaches, including viral vector-mediated gene replacement and antisense oligonucleotides for exon skipping, altering splicing, and gene knockdown. We will also describe earlier stage approaches for allele-specific knockdown and CRIPSR/Cas9 gene editing. We will next describe how these various approaches have been deployed in clinical and preclinical studies. Finally, we will evaluate various forms of CMT as candidates for gene therapy based on the current understanding of their genetics, cellular/tissue targets, validated animal models, and availability of patient populations and natural history data.

The role of mitochondria-associated membranes in cellular homeostasis and diseases

Mitochondria and endoplasmic reticulum (ER) are fundamental in the control of cell physiology regulating several signal transduction pathways. They continuously communicate exchanging messages in their contact sites called MAMs (mitochondria-associated membranes). MAMs are specific microdomains acting as a platform for the sorting of vital and dangerous signals. In recent years increasing evidence reported that multiple scaffold proteins and regulatory factors localize to this subcellular fraction suggesting MAMs as hotspot signaling domains. In this review we describe the current knowledge about MAMs' dynamics and processes, which provided new correlations between MAMs' dysfunctions and human diseases. In fact, MAMs machinery is strictly connected with several pathologies, like neurodegeneration, diabetes and mainly cancer. These pathological events are characterized by alterations in the normal communication between ER and mitochondria, leading to deep metabolic defects that contribute to the progression of the diseases.

Targeting the programmed axon degeneration pathway as a potential therapeutic for Charcot-Marie-Tooth disease

The programmed axon degeneration pathway has emerged as an important process contributing to the pathogenesis of several neurological diseases. The most crucial events in this pathway include activation of the central executioner SARM1 and NAD⁺ depletion, which leads to an energetic failure and ultimately axon destruction. Given the prevalence of this pathway, it is not surprising that inhibitory therapies are currently being developed in order to treat multiple neurological diseases with the same therapy. Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of neurological diseases that may also benefit from this therapeutic approach. To evaluate the appropriateness of this strategy, the contribution of the programmed axon degeneration pathway to the pathogenesis of different CMT subtypes is being actively investigated. The subtypes CMT1A, CMT1B and CMT2D are the first to have been examined. Based on the results from these studies and advances in developing therapies to block the programmed axon degeneration pathway, promising therapeutics for CMT are now on the horizon.

Analyzing chemotherapy-induced peripheral neuropathy in vivo using non-mammalian animal models

Non-mammalian models of CIPN remain relatively sparse, but the knowledge gained from the few published studies suggest that these species have great potential to serve as a discovery platform for new pathways and underlying genetic mechanisms of CIPN. These models permit large-scale genetic and pharmacological screening, and they are highly suitable for in vivo imaging. CIPN phenotypes described in rodents have been confirmed in those models, and conversely, genetic players leading to axon de- and regeneration under conditions of chemotherapy treatment identified in these non-mammalian species have been validated in rodents. Given the need for non-traditional approaches with which to identify new CIPN mechanisms, these models bear a strong potential due to the conservation of basic mechanisms by which chemotherapeutic agents induce neurotoxicity.

Roles for the Endoplasmic Reticulum in Regulation of Neuronal Calcium Homeostasis

By influencing Ca²⁺ homeostasis in spatially and architecturally distinct neuronal compartments, the endoplasmic reticulum (ER) illustrates the notion that form and function are intimately related. The contribution of ER to neuronal Ca²⁺ homeostasis is attributed to the organelle being the largest reservoir of intracellular Ca²⁺ and having a high density of Ca²⁺ channels and transporters. As such, ER Ca²⁺ has incontrovertible roles in the regulation of axodendritic growth and morphology, synaptic vesicle release, and neural activity dependent gene expression, synaptic plasticity, and mitochondrial bioenergetics. Not surprisingly, many neurological diseases arise from ER Ca²⁺ dyshomeostasis, either directly due to alterations in ER resident proteins, or indirectly via processes that are coupled to the regulators of ER Ca²⁺ dynamics. In this review, we describe the mechanisms involved in the establishment of ER Ca²⁺ homeostasis in neurons. We elaborate upon how changes in the spatiotemporal dynamics of Ca²⁺ exchange between the ER and other organelles sculpt neuronal function and provide examples that demonstrate the involvement of ER Ca²⁺ dyshomeostasis in a range of neurological and neurodegenerative diseases.

A nanoscale, multi-parametric flow cytometry-based platform to study mitochondrial heterogeneity and mitochondrial DNA dynamics

Mitochondria are well-characterized regarding their function in both energy production and regulation of cell death; however, the heterogeneity that exists within mitochondrial populations is poorly understood. Typically analyzed as pooled samples comprised of millions of individual mitochondria, there is little information regarding potentially different functionality across subpopulations of mitochondria. Herein we present a new methodology to analyze mitochondria as individual components of a complex and heterogeneous network, using a nanoscale and multi-parametric flow cytometry-based platform. We validate the platform using multiple downstream assays, including electron microscopy, ATP generation, quantitative mass-spectrometry proteomic profiling, and mtDNA analysis at the level of single organelles. These strategies allow robust analysis and isolation of mitochondrial subpopulations to more broadly elucidate the underlying complexities of mitochondria as these organelles function collectively within a cell.

Neuroinflammation in the pathogenesis of axonal Charcot-Marie-Tooth disease caused by lack of GDAP1

Mutations in the GDAP1 mitochondrial outer membrane gene cause Charcot-Marie-Tooth (CMT) neuropathy. Reduction or absence of GDAP1 has been associated with abnormal changes in the mitochondrial morphology and dynamics, oxidative stress and changes in calcium homeostasis. Neuroinflammation has been described in rodent models of genetic demyelinating CMT neuropathies but not in CMT primarily associated with axonopathy. Inflammatory processes have also been related to mitochondrial changes and oxidative stress in central neurodegenerative disorders. Here we investigated the presence of neuroinflammation in the axonal neuropathy of the Gdap1^-/- mice. We showed by transcriptome profile of spinal cord and the in vivo detection of activated phagocytes that the absence of GDAP1 is associated with upregulation of inflammatory pathways. We observed reactive gliosis in spinal cord with increase of the astroglia markers GFAP and S100B, and the microglia marker IBA1. Additionally, we found significant increase of inflammatory mediators such as TNF-α and pERK, and C1qa and C1qb proteins of the complement system. Importantly, we observed an increased expression of CD206 and CD86 as M2 and M1 microglia and macrophage response markers, respectively, in Gdap1^-/- mice. These inflammatory changes were also associated with abnormal molecular changes in synapses. In summary, we demonstrate that inflammation in spinal cord and sciatic nerve, but not in brain and cerebellum, is part of the pathophysiology of axonal GDAP1-related CMT.

[Analysis of GDAP1 gene mutation in a pedigree with autosomal dominant Charcot-Marie-Tooth disease]

OBJECTIVE

To investigate the molecular genetic mechanism of Charcot- Marie-Tooth (CMT) disease in a pedigree.

METHODS

Genomic DNA was extracted from the peripheral blood of the family members of a pedigree with autosomal dominant CMT disease, and 65 candidate genes of the proband were screened using target exon capture and the next generation sequencing, and the suspicious genes were verified using Sanger sequencing. PolyPhen-2, PROVEAN and SIFT software were used to predict the function of the mutant genes, and PyMOL-1 software was used to simulate the mutant protein structure.

RESULTS

A heterozygous missense mutation [c.371A>G (p.Y124C)] was detected in exon 3 of GDAP1 gene of the proband. This heterozygous mutation was also detected in both the proband's mother and her brother, but not in her father. Multiple sequence alignment analysis showed that tyrosine at codon 124 of GDAP1 protein was highly conserved. All the 3 prediction software predicted that the mutation was harmful. Molecular structure simulation showed a weakened interaction force between the amino acid residues at codon 124 and the surrounding amino acid residues to affect the overall stability of the protein.

CONCLUSIONS

The mutation of GDAP1 gene may be related to the pathogenesis of autosomal dominant AD-CMT in this pedigree. The newly discovered c.371A>G mutation (p.Y124C) expands the mutation spectrum of GDAP1 gene, but further study is needed to clarify the underlying pathogenesis.

Calcium Deregulation and Mitochondrial Bioenergetics in GDAP1-Related CMT Disease

The pathology of Charcot-Marie-Tooth (CMT), a disease arising from mutations in different genes, has been associated with an impairment of mitochondrial dynamics and axonal biology of mitochondria. Mutations in ganglioside-induced differentiation-associated protein 1 (GDAP1) cause several forms of CMT neuropathy, but the pathogenic mechanisms involved remain unclear. GDAP1 is an outer mitochondrial membrane protein highly expressed in neurons. It has been proposed to play a role in different aspects of mitochondrial physiology, including mitochondrial dynamics, oxidative stress processes, and mitochondrial transport along the axons. Disruption of the mitochondrial network in a neuroblastoma model of GDAP1-related CMT has been shown to decrease Ca²⁺ entry through the store-operated calcium entry (SOCE), which caused a failure in stimulation of mitochondrial respiration. In this review, we summarize the different functions proposed for GDAP1 and focus on the consequences for Ca²⁺ homeostasis and mitochondrial energy production linked to CMT disease caused by different GDAP1 mutations.

Altered interplay between endoplasmic reticulum and mitochondria in Charcot-Marie-Tooth type 2A neuropathy

Mutations in the MFN2 gene encoding Mitofusin 2 lead to the development of Charcot-Marie-Tooth type 2A (CMT2A), a dominant axonal form of peripheral neuropathy. Mitofusin 2 is localized at both the outer membrane of mitochondria and the endoplasmic reticulum and is particularly enriched at specialized contact regions known as mitochondria-associated membranes (MAM). We observed that expression of MFN2^R94Q induces distal axonal degeneration in the absence of overt neuronal death. The presence of mutant protein leads to reduction in endoplasmic reticulum and mitochondria contacts in CMT2A patient-derived fibroblasts, in primary neurons and in vivo, in motoneurons of a mouse model of CMT2A. These changes are concomitant with endoplasmic reticulum stress, calcium handling defects, and changes in the geometry and axonal transport of mitochondria. Importantly, pharmacological treatments reinforcing endoplasmic reticulum-mitochondria cross-talk, or reducing endoplasmic reticulum stress, restore the mitochondria morphology and prevent axonal degeneration. These results highlight defects in MAM as a cellular mechanism contributing to CMT2A pathology mediated by mutated MFN2.

Hereditary sensory neuropathy type 1-associated deoxysphingolipids cause neurotoxicity, acute calcium handling abnormalities and mitochondrial dysfunction in vitro

Hereditary sensory neuropathy type 1 (HSN-1) is a peripheral neuropathy most frequently caused by mutations in the SPTLC1 or SPTLC2 genes, which code for two subunits of the enzyme serine palmitoyltransferase (SPT). SPT catalyzes the first step of de novo sphingolipid synthesis. Mutations in SPT result in a change in enzyme substrate specificity, which causes the production of atypical deoxysphinganine and deoxymethylsphinganine, rather than the normal enzyme product, sphinganine. Levels of these abnormal compounds are elevated in blood of HSN-1 patients and this is thought to cause the peripheral motor and sensory nerve damage that is characteristic of the disease, by a largely unresolved mechanism. In this study, we show that exogenous application of these deoxysphingoid bases causes dose- and time-dependent neurotoxicity in primary mammalian neurons, as determined by analysis of cell survival and neurite length. Acutely, deoxysphingoid base neurotoxicity manifests in abnormal Ca2+ handling by the endoplasmic reticulum (ER) and mitochondria as well as dysregulation of cell membrane store-operated Ca2+ channels. The changes in intracellular Ca2+ handling are accompanied by an early loss of mitochondrial membrane potential in deoxysphingoid base-treated motor and sensory neurons. Thus, these results suggest that exogenous deoxysphingoid base application causes neuronal mitochondrial dysfunction and Ca2+ handling deficits, which may play a critical role in the pathogenesis of HSN-1.

Mortal engines: Mitochondrial bioenergetics and dysfunction in neurodegenerative diseases

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

AMPK activation negatively regulates GDAP1, which influences metabolic processes and circadian gene expression in skeletal muscle

Objective

We sought to identify AMPK-regulated genes via bioinformatic analysis of microarray data generated from skeletal muscle of animal models with genetically altered AMPK activity. We hypothesized that such genes would play a role in metabolism. Ganglioside-induced differentiation-associated protein 1 (GDAP1), a gene which plays a role in mitochondrial fission and peroxisomal function in neuronal cells but whose function in skeletal muscle is undescribed, was identified and further validated. AMPK activation reduced GDAP1 expression in skeletal muscle. GDAP1 expression was elevated in skeletal muscle from type 2 diabetic patients but decreased after acute exercise.

Methods

The metabolic impact of GDAP1 silencing was determined in primary skeletal muscle cells via siRNA-transfections. Confocal microscopy was used to visualize whether silencing GDAP1 impacted mitochondrial network morphology and membrane potential.

Results

GDAP1 silencing increased mitochondrial protein abundance, decreased palmitate oxidation, and decreased non-mitochondrial respiration. Mitochondrial morphology was unaltered by GDAP1 silencing. GDAP1 silencing and treatment of cells with AMPK agonists altered several genes in the core molecular clock machinery.

Conclusion

We describe a role for GDAP1 in regulating mitochondrial proteins, circadian genes, and metabolic flux in skeletal muscle. Collectively, our results implicate GDAP1 in the circadian control of metabolism.

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Transcriptomic studies reveal GDAP1 mRNA is inversely associated with AMPK activity.

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GDAP1 silencing increases mitochondrial protein abundance in skeletal muscle.

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GDAP1 silencing influences expression of core molecular clock machinery.

•

GDAP1 is a AMPK target involved in metabolism and circadian gene expression.

Myelinated axons fail to develop properly in a genetically authentic mouse model of Charcot-Marie-Tooth disease type 2E

We have analyzed a mouse model of Charcot-Marie-Tooth disease 2E (CMT2E) harboring a heterozygous p.Asn98Ser (p.N98S) Nefl mutation, whose human counterpart results in a severe, early-onset neuropathy. Behavioral, electrophysiological, and pathological analyses were done on separate cohorts of Nefl^N98S/+ mutant mice and their wild type Nefl^+/+ littermates between 8 and 48 weeks of age. The motor performance of Nefl^N98S/+ mice, as evidenced by altered balance and gait measures, was impaired at every age examined (from 6 to 25 weeks of age). At all times examined, myelinated axons were smaller and contained markedly fewer neurofilaments in Nefl^N98S/+ mice, in all examined aspects of the PNS, from the nerve roots to the distal ends of the sciatic and caudal nerves. Similarly, the myelinated axons in the various tracts of the spinal cord and in the optic nerves were smaller and contained fewer neurofilaments in mutant mice. The myelinated axons in both the PNS and the CNS of mutant mice had relatively thicker myelin sheaths. The amplitude and the nerve conduction velocity of the caudal nerves were reduced in proportion with the diminished sizes of myelinated axons. Conspicuous aggregations of neurofilaments were only seen in primary sensory and motor neurons, and were largely confined to the cell bodies and proximal axons. There was evidence of axonal degeneration and regeneration of myelinated axons, mostly in distal nerves. In summary, the p.N98S mutation causes a profound reduction of neurofilaments in the myelinated axons of the PNS and CNS, resulting in substantially reduced axonal diameters, particularly of large myelinated axons, and distal axon loss in the PNS.

On the Role of Store-Operated Calcium Entry in Acute and Chronic Neurodegenerative Diseases

In both excitable and non-excitable cells, calcium (Ca²⁺) signals are maintained by a highly integrated process involving store-operated Ca²⁺ entry (SOCE), namely the opening of plasma membrane (PM) Ca²⁺ channels following the release of Ca²⁺ from intracellular stores. Upon depletion of Ca²⁺ store, the stromal interaction molecule (STIM) senses Ca²⁺ level reduction and migrates from endoplasmic reticulum (ER)-like sites to the PM where it activates the channel proteins Orai and/or the transient receptor potential channels (TRPC) prompting Ca²⁺ refilling. Accumulating evidence suggests that SOCE dysregulation may trigger perturbation of intracellular Ca²⁺ signaling in neurons, glia or hematopoietic cells, thus participating to the pathogenesis of diverse neurodegenerative diseases. Under acute conditions, such as ischemic stroke, neuronal SOCE can either re-establish Ca²⁺ homeostasis or mediate Ca²⁺ overload, thus providing a non-excitotoxic mechanism of ischemic neuronal death. The dualistic role of SOCE in brain ischemia is further underscored by the evidence that it also participates to endothelial restoration and to the stabilization of intravascular thrombi. In Parkinson's disease (PD) models, loss of SOCE triggers ER stress and dysfunction/degeneration of dopaminergic neurons. Disruption of neuronal SOCE also underlies Alzheimer's disease (AD) pathogenesis, since both in genetic mouse models and in human sporadic AD brain samples, reduced SOCE contributes to synaptic loss and cognitive decline. Unlike the AD setting, in the striatum from Huntington's disease (HD) transgenic mice, an increased STIM2 expression causes elevated synaptic SOCE that was suggested to underlie synaptic loss in medium spiny neurons. Thus, pharmacological inhibition of SOCE is beneficial to synapse maintenance in HD models, whereas the same approach may be anticipated to be detrimental to cortical and hippocampal pyramidal neurons. On the other hand, up-regulation of SOCE may be beneficial during AD. These intriguing findings highlight the importance of further mechanistic studies to dissect the molecular pathways, and their corresponding targets, involved in synaptic dysfunction and neuronal loss during aging and neurodegenerative diseases.

Endoplasmic reticulum and mitochondria in diseases of motor and sensory neurons: a broken relationship?

Recent progress in the understanding of neurodegenerative diseases revealed that multiple molecular mechanisms contribute to pathological changes in neurons. A large fraction of these alterations can be linked to dysfunction in the endoplasmic reticulum (ER) and mitochondria, affecting metabolism and secretion of lipids and proteins, calcium homeostasis, and energy production. Remarkably, these organelles are interacting with each other at specialized domains on the ER called mitochondria-associated membranes (MAMs). These membrane structures rely on the interaction of several complexes of proteins localized either at the mitochondria or at the ER interface and serve as an exchange platform of calcium, metabolites, and lipids, which are critical for the function of both organelles. In addition, recent evidence indicates that MAMs also play a role in the control of mitochondria dynamics and autophagy. MAMs thus start to emerge as a key element connecting many changes observed in neurodegenerative diseases. This review will focus on the role of MAMs in amyotrophic lateral sclerosis (ALS) and hereditary motor and sensory neuropathy, two neurodegenerative diseases particularly affecting neurons with long projecting axons. We will discuss how defects in MAM signaling may impair neuronal calcium homeostasis, mitochondrial dynamics, ER function, and autophagy, leading eventually to axonal degeneration. The possible impact of MAM dysfunction in glial cells, which may affect the capacity to support neurons and/or axons, will also be described. Finally, the possible role of MAMs as an interesting target for development of therapeutic interventions aiming at delaying or preventing neurodegeneration will be highlighted.

Mitochondrial deficits and abnormal mitochondrial retrograde axonal transport play a role in the pathogenesis of mutant Hsp27-induced Charcot Marie Tooth Disease

Mutations in the small heat shock protein Hsp27, encoded by the HSPB1 gene, have been shown to cause Charcot Marie Tooth Disease type 2 (CMT-2) or distal hereditary motor neuropathy (dHMN). Protein aggregation and axonal transport deficits have been implicated in the disease. In this study, we conducted analysis of bidirectional movements of mitochondria in primary motor neuron axons expressing wild type and mutant Hsp27. We found significantly slower retrograde transport of mitochondria in Ser135Phe, Pro39Leu and Arg140Gly mutant Hsp27 expressing motor neurons than in wild type Hsp27 neurons, although anterograde movement velocities remained normal. Retrograde transport of other important cargoes, such as the p75 neurotrophic factor receptor was minimally altered in mutant Hsp27 neurons, implicating that axonal transport deficits primarily affect mitochondria and the axonal transport machinery itself is less affected. Investigation of mitochondrial function revealed a decrease in mitochondrial membrane potential in mutant Hsp27 expressing motor axons, as well as a reduction in mitochondrial complex 1 activity, increased vulnerability of mitochondria to mitochondrial stressors, leading to elevated superoxide release and reduced mitochondrial glutathione (GSH) levels, although cytosolic GSH remained normal. This mitochondrial redox imbalance in mutant Hsp27 motor neurons is likely to cause low level of oxidative stress, which in turn will contribute to, and indeed may be the underlying cause of the deficits in mitochondrial axonal transport. Together, these findings suggest that the mitochondrial abnormalities in mutant Hsp27-induced neuropathies may be a primary cause of pathology, leading to further deficits in the mitochondrial axonal transport and onset of disease.

Common and Divergent Mechanisms in Developmental Neuronal Remodeling and Dying Back Neurodegeneration

Cell death is an inherent process that is required for the proper wiring of the nervous system. Studies over the last four decades have shown that, in a parallel developmental pathway, axons and dendrites are eliminated without the death of the neuron. This developmentally regulated 'axonal death' results in neuronal remodeling, which is an essential mechanism to sculpt neuronal networks in both vertebrates and invertebrates. Studies across various organisms have demonstrated that a conserved strategy in the formation of adult neuronal circuitry often involves generating too many connections, most of which are later eliminated with high temporal and spatial resolution. Can neuronal remodeling be regarded as developmentally and spatially regulated neurodegeneration? It has been previously speculated that injury-induced degeneration (Wallerian degeneration) shares some molecular features with 'dying back' neurodegenerative diseases. In this opinion piece, we examine the similarities and differences between the mechanisms regulating neuronal remodeling and those being perturbed in dying back neurodegenerative diseases. We focus primarily on amyotrophic lateral sclerosis and peripheral neuropathies and highlight possible shared pathways and mechanisms. While mechanistic data are only just beginning to emerge, and despite the inherent differences between disease-oriented and developmental processes, we believe that some of the similarities between these developmental and disease-initiated degeneration processes warrant closer collaborations and crosstalk between these different fields.

Motion estimation of subcellular structures from fluorescence microscopy images

We present an automatic image processing framework to study moving intracellular structures from live cell fluorescence microscopy. The system includes the identification of static and dynamic structures from time-lapse images using data clustering as well as the identification of the trajectory of moving objects with a probabilistic tracking algorithm. The method has been successfully applied to study mitochondrial movement in neurons. The approach provides excellent performance under different experimental conditions and is robust to common sources of noise including experimental, molecular and biological fluctuations.

Plasma metabolome and skin proteins in Charcot-Marie-Tooth 1A patients

Objective

Charcot-Marie-Tooth 1A (CMT1A) disease is the most common inherited neuropathy that lacks of therapy and of molecular markers to assess disease severity. Herein, we have pursued the identification of potential biomarkers in plasma samples and skin biopsies that could define the phenotype of CMT1A patients at mild (Mi), moderate (Mo) and severe (Se) stages of disease as assessed by the CMT neuropathy score to contribute to the understanding of CMT pathophysiology and eventually inform of the severity of the disease.

Methods

We have used: (i) a high-throughput untargeted metabolomic approach of plasma samples in a cohort of 42 CMT1A patients and 15 healthy controls (CRL) using ultrahigh liquid chromatography coupled to mass spectrometry and (ii) reverse phase protein microarrays to quantitate the expression of some proteins of energy metabolism and of the antioxidant response in skin biopsies of a cohort of 70 CMT1A patients and 13 healthy controls.

Results

The metabolomic approach identified 194 metabolites with significant differences among the four groups (Mi, Mo, Se, CRL) of samples. A multivariate Linear Discriminant Analysis model using 12 metabolites afforded the correct classification of the samples. These metabolites indicate an increase in protein catabolism and the mobilization of membrane lipids involved in signaling inflammation with severity of CMT1A. A concurrent depletion of leucine, which is required for the biogenesis of the muscle, is also observed in the patients. Protein expression in skin biopsies indicates early loss of mitochondrial and antioxidant proteins in patients’ biopsies.

Conclusion

The findings indicate that CMT1A disease is associated with a metabolic state resembling inflammation and sarcopenia suggesting that it might represent a potential target to prevent the nerve and muscle wasting phenotype in these patients. The observed changes in metabolites could be useful as potential biomarkers of CMT1A disease after appropriate validation in future longitudinal studies.