Overexpression of mutant HSP27 causes axonal neuropathy in mice

Morc2a variants cause hydroxyl radical-mediated neuropathy and are rescued by restoring GHKL ATPase

Mutations in the Microrchidia CW-type zinc finger 2 (MORC2) GHKL ATPase module cause a broad range of neuropathies, such as Charcot-Marie-Tooth disease type 2Z; however, the aetiology and therapeutic strategy are not fully understood. Previously, we reported that the Morc2a p.S87L mouse model exhibited neuropathy and muscular dysfunction through DNA damage accumulation. In the present study, we analysed the gene expression of Morc2a p.S87L mice and designated the primary causing factor. We investigated the pathological pathway using Morc2a p.S87L mouse embryonic fibroblasts and human fibroblasts harbouring MORC2 p.R252W. We subsequently assessed the therapeutic effect of gene therapy administered to Morc2a p.S87L mice. This study revealed that Morc2a p.S87L causes a protein synthesis defect, resulting in the loss of function of Morc2a and high cellular apoptosis induced by high hydroxyl radical levels. We considered the Morc2a GHKL ATPase domain as a therapeutic target because it simultaneously complements hydroxyl radical scavenging and ATPase activity. We used the adeno-associated virus (AAV)-PHP.eB serotype, which has a high CNS transduction efficiency, to express Morc2a or Morc2a GHKL ATPase domain protein in vivo. Notably, AAV gene therapy ameliorated neuropathy and muscular dysfunction with a single treatment. Loss-of-function characteristics due to protein synthesis defects in Morc2a p.S87L were also noted in human MORC2 p.S87L or p.R252W variants, indicating the correlation between mouse and human pathogenesis. In summary, CMT2Z is known as an incurable genetic disorder, but the present study demonstrated its mechanisms and treatments based on established animal models. This study demonstrates that the Morc2a p.S87L variant causes hydroxyl radical-mediated neuropathy, which can be rescued through AAV-based gene therapy.

Charcot-Marie-Tooth type 2A in vivo models: Current updates

Charcot-Marie-Tooth type 2A (CMT2A) is an inherited sensorimotor neuropathy associated with mutations within the Mitofusin 2 (MFN2) gene. These mutations impair normal mitochondrial functioning via different mechanisms, disturbing the equilibrium between mitochondrial fusion and fission, of mitophagy and mitochondrial axonal transport. Although CMT2A disease causes a significant disability, no resolutive treatment for CMT2A patients to date. In this context, reliable experimental models are essential to precisely dissect the molecular mechanisms of disease and to devise effective therapeutic strategies. The most commonly used models are either in vitro or in vivo, and among the latter murine models are by far the most versatile and popular. Here, we critically revised the most relevant literature focused on the experimental models, providing an update on the mammalian models of CMT2A developed to date. We highlighted the different phenotypic, histopathological and molecular characteristics, and their use in translational studies for bringing potential therapies from the bench to the bedside. In addition, we discussed limitations of these models and perspectives for future improvement.

ATP1A1-linked diseases require a malfunctioning protein product from one allele

Heterozygous germline variants in ATP1A1, the gene encoding the α1 subunit of the Na+/K+-ATPase (NKA), have been linked to diseases including primary hyperaldosteronism and the peripheral neuropathy Charcot-Marie-Tooth disease (CMT). ATP1A1 variants that cause CMT induce loss-of-function of NKA. This heterodimeric (αβ) enzyme hydrolyzes ATP to establish transmembrane electrochemical gradients of Na+ and K+ that are essential for electrical signaling and cell survival. Of the 4 catalytic subunit isoforms, α1 is ubiquitously expressed and is the predominant paralog in peripheral axons. Human population sequencing datasets indicate strong negative selection against both missense and protein-null ATP1A1 variants. To test whether haploinsufficiency generated by heterozygous protein-null alleles are sufficient to cause disease, we tested the neuromuscular characteristics of heterozygous Atp1a1+/- knockout mice and their wildtype littermates, while also evaluating if exercise increased CMT penetrance. We found that Atp1a1+/- mice were phenotypically normal up to 18 months of age. Consistent with the observations in mice, we report clinical phenotyping of a healthy adult human who lacks any clinical features of known ATP1A1-related diseases despite carrying a plasma-membrane protein-null early truncation variant, p.Y148*. Taken together, these results suggest that a malfunctioning gene product is required for disease induction by ATP1A1 variants and that if any pathology is associated with protein-null variants, they may display low penetrance or high age of onset.

A novel HSPB1S139F mouse model of Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth Disease (CMT) is a commonly inherited peripheral polyneuropathy. Clinical manifestations for this disease include symmetrical distal polyneuropathy, altered deep tendon reflexes, distal sensory loss, foot deformities, and gait abnormalities. Genetic mutations in heat shock proteins have been linked to CMT2. Specifically, mutations in the heat shock protein B1 (HSPB1) gene encoding for heat shock protein 27 (Hsp27) have been linked to CMT2F and distal hereditary motor and sensory neuropathy type 2B (dHMSN2B) subtype. The goal of the study was to examine the role of an endogenous mutation in HSPB1 in vivo and to define the effects of this mutation on motor function and pathology in a novel animal model. As sphingolipids have been implicated in hereditary and sensory neuropathies, we examined sphingolipid metabolism in central and peripheral nervous tissues in 3-month-old HspS139F mice. Though sphingolipid levels were not altered in sciatic nerves from HspS139F mice, ceramides and deoxyceramides, as well as sphingomyelins (SMs) were elevated in brain tissues from HspS139F mice. Histology was utilized to further characterize HspS139F mice. HspS139F mice exhibited no alterations to the expression and phosphorylation of neurofilaments, or in the expression of acetylated α-tubulin in the brain or sciatic nerve. Interestingly, HspS139F mice demonstrated cerebellar demyelination. Locomotor function, grip strength and gait were examined to define the role of HspS139F in the clinical phenotypes associated with CMT2F. Gait analysis revealed no differences between HspWT and HspS139F mice. However, both coordination and grip strength were decreased in 3-month-old HspS139F mice. Together these data suggest that the endogenous S139F mutation in HSPB1 may serve as a mouse model for hereditary and sensory neuropathies such as CMT2F.

ATP1A1 -linked diseases require a malfunctioning protein product from one allele

Heterozygous germline variants in ATP1A1 , the gene encoding the α1 subunit of the Na + /K + -ATPase (NKA), have been linked to diseases including primary hyperaldosteronism and the peripheral neuropathy Charcot-Marie-Tooth disease (CMT). ATP1A1 variants that cause CMT induce loss-of-function of NKA. This heterodimeric (αβ) enzyme hydrolyzes ATP to establish transmembrane electrochemical gradients of Na + and K + that are essential for electrical signaling and cell survival. Of the 4 catalytic subunit isoforms, α1 is ubiquitously expressed and is the predominant paralog in peripheral axons. Human population sequencing datasets indicate strong negative selection against both missense and protein-null ATP1A1 variants. To test whether haploinsufficiency generated by heterozygous protein-null alleles are sufficient to cause disease, we tested the neuromuscular characteristics of heterozygous Atp1a1 +/- knockout mice and their wildtype littermates, while also evaluating if exercise increased CMT penetrance. We found that Atp1a1 +/- mice were phenotypically normal up to 18 months of age. Consistent with the observations in mice, we report clinical phenotyping of a healthy adult human who lacks any clinical features of known ATP1A1 -related diseases despite carrying a protein-null early truncation variant, p.Y148*. Taken together, these results suggest that a malfunctioning gene product is required for disease induction by ATP1A1 variants and that if any pathology is associated with protein-null variants, they may display low penetrance or high age of onset.

Insights on Human Small Heat Shock Proteins and Their Alterations in Diseases

The family of the human small Heat Shock Proteins (HSPBs) consists of ten members of chaperones (HSPB1-HSPB10), characterized by a low molecular weight and capable of dimerization and oligomerization forming large homo- or hetero-complexes. All HSPBs possess a highly conserved centrally located α-crystallin domain and poorly conserved N- and C-terminal domains. The main feature of HSPBs is to exert cytoprotective functions by preserving proteostasis, assuring the structural maintenance of the cytoskeleton and acting in response to cellular stresses and apoptosis. HSPBs take part in cell homeostasis by acting as holdases, which is the ability to interact with a substrate preventing its aggregation. In addition, HSPBs cooperate in substrates refolding driven by other chaperones or, alternatively, promote substrate routing to degradation. Notably, while some HSPBs are ubiquitously expressed, others show peculiar tissue-specific expression. Cardiac muscle, skeletal muscle and neurons show high expression levels for a wide variety of HSPBs. Indeed, most of the mutations identified in HSPBs are associated to cardiomyopathies, myopathies, and motor neuropathies. Instead, mutations in HSPB4 and HSPB5, which are also expressed in lens, have been associated with cataract. Mutations of HSPBs family members encompass base substitutions, insertions, and deletions, resulting in single amino acid substitutions or in the generation of truncated or elongated proteins. This review will provide an updated overview of disease-related mutations in HSPBs focusing on the structural and biochemical effects of mutations and their functional consequences.

Morc2a p.S87L mutant mice develop peripheral and central neuropathies associated with neuronal DNA damage and apoptosis

The microrchidia (MORC)-family CW-type zinc finger 2 (MORC2) gene is related to DNA repair, adipogenesis and epigenetic silencing via the human silencing hub (HUSH) complex. MORC2 missense mutation is known to cause peripheral neuropathy of Charcot-Marie-Tooth disease type 2 Z (CMT2Z). However, there have been reports of peripheral and central neuropathy in patients, and the disease has been co-categorized with developmental delay, impaired growth, dysmorphic facies and axonal neuropathy (DIGFAN). The etiology of MORC2 mutation-mediated neuropathy remains uncertain. Here, we established and analyzed Morc2a p.S87L mutant mice. Morc2a p.S87L mice displayed the clinical symptoms expected in human CMT2Z patients, such as axonal neuropathy and skeletal muscle weakness. Notably, we observed severe central neuropathy with cerebella ataxia, cognition disorder and motor neuron degeneration in the spinal cord, and this seemed to be evidence of DIGFAN. Morc2a p.S87L mice exhibited an accumulation of DNA damage in neuronal cells, followed by p53/cytochrome c/caspase 9/caspase 3-mediated apoptosis. This study presents a new mouse model of CMT2Z and DIGFAN with a Morc2a p.S87L mutation. We suggest that neuronal apoptosis is a possible target for therapeutic approach in MORC2 missense mutation. This article has an associated First Person interview with the first author of the paper.

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.

Gait parameters as tools for analyzing phenotypic alterations of a mouse model of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT), a genetically heterogeneous group of diseases in the peripheral nervous system, is characterized by progressive and symmetrical distal weakness resulting in gait abnormality. The necessity of the diagnostic and prognostic biomarkers has been raised for both basic research and clinical practice in CMT. Since biomarkers for animal study of CMT are limited, we evaluated the feasibility of gait parameters as tool for measuring disease phenotype of CMT mouse model. Using a Trembler-J (Tr-J) mouse, a CMT type 1 (CMT1) mouse model, we analyzed kinematic parameters such as angles of hip, knee and ankle (sagittal plane), and spatial parameters including step width and stride length (transverse plane). Regarding of kinematic parameters, Tr-J mice exhibited less plantarflexed ankle during the swing phase and more dorsiflexed ankle at the terminal stance compared to control mice. The range of motion in ankle angle of Tr-J mice was significantly greater than that of control mice. In spatial parameter, Tr-J mice exhibited wider step width compared to control mice. These results are similar to previously reported gait patterns of CMT1 patients. In comparison with other markers such as nerve conduction study and rotarod test, gait parameters dynamically reflected the disease progression of CMT1 mice. Therefore, these data imply that gait parameters can be used as useful tools to analyzed the disease phenotype and progression during preclinical study of peripheral neuropathy such as CMT.

Addressing the Need of a Translational Approach in Peripheral Neuropathy Research: Morphology Meets Function

Peripheral neuropathies (PNs) are a type of common disease that hampers the quality of life of affected people. Treatment, in most cases, is just symptomatic and often ineffective. To improve drug discovery in this field, preclinical evidence is warranted. In vivo rodent models allow a multiparametric approach to test new therapeutic strategies, since they can allow pathogenetic and morphological studies different from the clinical setting. However, human readouts are warranted to promptly translate data from the bench to the bedside. A feasible solution would be neurophysiology, performed similarly at both sides. We describe a simple protocol that reproduces the standard clinical protocol of a neurophysiology hospital department. We devised the optimal montage for sensory and motor recordings (neurography) in mice, and we also implemented F wave testing and a short electromyography (EMG) protocol at rest. We challenged this algorithm by comparing control animals (BALB/c mice) with a model of mild neuropathy to grasp even subtle changes. The neurophysiological results were confirmed with neuropathology. The treatment group showed all expected alterations. Moreover, the neurophysiology matched the neuropathological analyses. Therefore, our protocol can be suggested to promptly translate data from the bench to the bedside and vice versa.

The Neurochaperonopathies: Anomalies of the Chaperone System with Pathogenic Effects in Neurodegenerative and Neuromuscular Disorders

The chaperone (or chaperoning) system (CS) constitutes molecular chaperones, co-chaperones, and chaperone co-factors, interactors and receptors, and its canonical role is protein quality control. A malfunction of the CS may cause diseases, known as the chaperonopathies. These are caused by qualitatively and/or quantitatively abnormal molecular chaperones. Since the CS is ubiquitous, chaperonopathies are systemic, affecting various tissues and organs, playing an etiologic-pathogenic role in diverse conditions. In this review, we focus on chaperonopathies involved in the pathogenic mechanisms of diseases of the central and peripheral nervous systems: the neurochaperonopathies (NCPs). Genetic NCPs are linked to pathogenic variants of chaperone genes encoding, for example, the small Hsp, Hsp10, Hsp40, Hsp60, and CCT-BBS (chaperonin-containing TCP-1- Bardet–Biedl syndrome) chaperones. Instead, the acquired NCPs are associated with malfunctional chaperones, such as Hsp70, Hsp90, and VCP/p97 with aberrant post-translational modifications. Awareness of the chaperonopathies as the underlying primary or secondary causes of disease will improve diagnosis and patient management and open the possibility of investigating and developing chaperonotherapy, namely treatment with the abnormal chaperone as the main target. Positive chaperonotherapy would apply in chaperonopathies by defect, i.e., chaperone insufficiency, and consist of chaperone replacement or boosting, whereas negative chaperonotherapy would be pertinent when a chaperone actively participates in the initiation and progression of the disease and must be blocked and eliminated.

Animal Models of CMT2A: State-of-art and Therapeutic Implications

Charcot-Marie-Tooth disease type 2A (CMT2A), arising from mitofusin 2 (MFN2) gene mutations, is the most common inherited axonal neuropathy affecting motor and sensory neurons. The cellular and molecular mechanisms by which MFN2 mutations determine neuronal degeneration are largely unclear. No effective treatment exists for CMT2A, which has a high degree of genetic/phenotypic heterogeneity. The identification of mutations in MFN2 has allowed the generation of diverse transgenic animal models, but to date, their ability to recapitulate the CMT2A phenotype is limited, precluding elucidation of its pathogenesis and discovery of therapeutic strategies. This review will critically present recent progress in in vivo CMT2A disease modeling, discoveries, drawbacks and limitations, current challenges, and key reflections to advance the field towards developing effective therapies for these patients.

Homeostatic Roles of the Proteostasis Network in Dendrites

Cellular protein homeostasis, or proteostasis, is indispensable to the survival and function of all cells. Distinct from other cell types, neurons are long-lived, exhibiting architecturally complex and diverse multipolar projection morphologies that can span great distances. These properties present unique demands on proteostatic machinery to dynamically regulate the neuronal proteome in both space and time. Proteostasis is regulated by a distributed network of cellular processes, the proteostasis network (PN), which ensures precise control of protein synthesis, native conformational folding and maintenance, and protein turnover and degradation, collectively safeguarding proteome integrity both under homeostatic conditions and in the contexts of cellular stress, aging, and disease. Dendrites are equipped with distributed cellular machinery for protein synthesis and turnover, including dendritically trafficked ribosomes, chaperones, and autophagosomes. The PN can be subdivided into an adaptive network of three major functional pathways that synergistically govern protein quality control through the action of (1) protein synthesis machinery; (2) maintenance mechanisms including molecular chaperones involved in protein folding; and (3) degradative pathways (e.g., Ubiquitin-Proteasome System (UPS), endolysosomal pathway, and autophagy. Perturbations in any of the three arms of proteostasis can have dramatic effects on neurons, especially on their dendrites, which require tightly controlled homeostasis for proper development and maintenance. Moreover, the critical importance of the PN as a cell surveillance system against protein dyshomeostasis has been highlighted by extensive work demonstrating that the aggregation and/or failure to clear aggregated proteins figures centrally in many neurological disorders. While these studies demonstrate the relevance of derangements in proteostasis to human neurological disease, here we mainly review recent literature on homeostatic developmental roles the PN machinery plays in the establishment, maintenance, and plasticity of stable and dynamic dendritic arbors. Beyond basic housekeeping functions, we consider roles of PN machinery in protein quality control mechanisms linked to dendritic plasticity (e.g., dendritic spine remodeling during LTP); cell-type specificity; dendritic morphogenesis; and dendritic pruning.

Mutations in HspB1 and hereditary neuropathies

Charcot-Marie-Tooth (CMT) disease is major hereditary neuropathy. CMT has been linked to mutations in a range of proteins, including the small heat shock protein HspB1. Here we review the properties of several HspB1 mutants associated with CMT. In vitro, mutations in the N-terminal domain lead to a formation of larger HspB1 oligomers when compared with the wild-type (WT) protein. These mutants are resistant to phosphorylation-induced dissociation and reveal lower chaperone-like activity than the WT on a range of model substrates. Mutations in the α-crystallin domain lead to the formation of yet larger HspB1 oligomers tending to dissociate at low protein concentration and having variable chaperone-like activity. Mutations in the conservative IPV motif within the C-terminal domain induce the formation of very large oligomers with low chaperone-like activity. Most mutants interact with a partner small heat shock protein, HspB6, in a manner different from that of the WT protein. The link between the altered physico-chemical properties and the pathological CMT phenotype is a subject of discussion. Certain HspB1 mutations appear to have an effect on cytoskeletal elements such as intermediate filaments and/or microtubules, and by this means damage the axonal transport. In addition, mutations of HspB1 can affect the metabolism in astroglia and indirectly modulate the viability of motor neurons. While the mechanisms of pathological mutations in HspB1 are likely to vary greatly across different mutations, further in vitro and in vivo studies are required for a better understanding of the CMT disease at molecular level.

Neuromuscular Diseases Due to Chaperone Mutations: A Review and Some New Results

Skeletal muscle and the nervous system depend on efficient protein quality control, and they express chaperones and cochaperones at high levels to maintain protein homeostasis. Mutations in many of these proteins cause neuromuscular diseases, myopathies, and hereditary motor and sensorimotor neuropathies. In this review, we cover mutations in DNAJB6, DNAJB2, αB-crystallin (CRYAB, HSPB5), HSPB1, HSPB3, HSPB8, and BAG3, and discuss the molecular mechanisms by which they cause neuromuscular disease. In addition, previously unpublished results are presented, showing downstream effects of BAG3 p.P209L on DNAJB6 turnover and localization.

Novel HDAC6 Inhibitors Increase Tubulin Acetylation and Rescue Axonal Transport of Mitochondria in a Model of Charcot-Marie-Tooth Type 2F

Disruption of axonal transport causes a number of rare, inherited axonopathies and is heavily implicated in a wide range of more common neurodegenerative disorders, many of them age-related. Acetylation of α-tubulin is one important regulatory mechanism, influencing microtubule stability and motor protein attachment. Of several strategies so far used to enhance axonal transport, increasing microtubule acetylation through inhibition of the deacetylase enzyme histone deacetylase 6 (HDAC6) has been one of the most effective. Several inhibitors have been developed and tested in animal and cellular models, but better drug candidates are still needed. Here we report the development and characterization of two highly potent HDAC6 inhibitors, which show low toxicity, promising pharmacokinetic properties, and enhance microtubule acetylation in the nanomolar range. We demonstrate their capacity to rescue axonal transport of mitochondria in a primary neuronal culture model of the inherited axonopathy Charcot-Marie-Tooth Type 2F, caused by a dominantly acting mutation in heat shock protein beta 1.

Targeted PMP22 TATA-box editing by CRISPR/Cas9 reduces demyelinating neuropathy of Charcot-Marie-Tooth disease type 1A in mice

Charcot-Marie-Tooth 1A (CMT1A) is the most common inherited neuropathy without a known therapy, which is caused by a 1.4 Mb duplication on human chromosome 17, which includes the gene encoding the peripheral myelin protein of 22 kDa (PMP22). Overexpressed PMP22 protein from its gene duplication is thought to cause demyelination and subsequently axonal degeneration in the peripheral nervous system (PNS). Here, we targeted TATA-box of human PMP22 promoter to normalize overexpressed PMP22 level in C22 mice, a mouse model of CMT1A harboring multiple copies of human PMP22. Direct local intraneural delivery of CRISPR/Cas9 designed to target TATA-box of PMP22 before the onset of disease, downregulates gene expression of PMP22 and preserves both myelin and axons. Notably, the same approach was effective in partial rescue of demyelination even after the onset of disease. Collectively, our data present a proof-of-concept that CRISPR/Cas9-mediated targeting of TATA-box can be utilized to treat CMT1A.

Small Heat Shock Proteins and Human Neurodegenerative Diseases

The review discusses the role of small heat shock proteins (sHsps) in human neurodegenerative disorders, such as Charcot-Marie-Tooth disease (CMT), Parkinson's and Alzheimer's diseases, and different forms of tauopathies. The effects of CMT-associated mutations in two small heat shock proteins (HspB1 and HspB8) on the protein stability, oligomeric structure, and chaperone-like activity are described. Mutations in HspB1 shift the equilibrium between different protein oligomeric forms, leading to the alterations in its chaperone-like activity and interaction with protein partners, which can induce damage of the cytoskeleton and neuronal death. Mutations in HspB8 affect its interaction with the adapter protein Bag3, as well as the process of autophagy, also resulting in neuronal death. The impact of sHsps on different forms of amyloidosis is discussed. Experimental studies have shown that sHsps interact with monomers or small oligomers of amyloidogenic proteins, stabilize their structure, prevent their aggregation, and/or promote their specific proteolytic degradation. This effect might be due to the interaction between the β-strands of sHsps and β-strands of target proteins, which prevents aggregation of the latter. In cooperation with the other heat shock proteins, sHsps can promote disassembly of oligomers formed by amyloidogenic proteins. Despite significant achievements, further investigations are required for understanding the role of sHsps in protection against various neurodegenerative diseases.

Charcot-Marie-Tooth 2F (Hsp27 mutations): A review

Charcot-Marie-Tooth disease is a commonly inherited form of neuropathy. Although named over 100 years ago, identification of subtypes of Charcot-Marie-Tooth has rapidly expanded in the preceding decades with the advancement of genetic sequencing, including type 2F (CMT2F), due to mutations in heat shock protein 27 (Hsp27). However, despite CMT being one of the most common inherited neurological diseases, definitive mechanistic models of pathology and effective treatments for CMT2F are lacking. This review extensively profiles the published literature on CMT2F and distal hereditary motor neuropathy II (dHMN II), a similar neuropathy with exclusively motor symptoms that is also due to mutations in Hsp27. This includes a review of case reports and sequencing studies detailing disease course. Included are tables listing of all known published mutations of Hsp27 that cause symptoms of CMT2F and dHMN II. Furthermore, pathological mechanisms are assessed. While many groups have established pathologies relating to defective chaperone function, cellular neurofilament and microtubule structure and function, and mitochondrial and metabolic dysfunction, there are still discrepancies in results between different model systems. Moreover, initial mouse models have also produced promising results with similar phenotypes to humans, however discrepancies still exist. Both patient-focused and scientific studies have demonstrated variability in phenotypes even considering specific mutations. Given the clinical heterogeneity in presentation, CMT2F and dHMN II likely result from similar pathological mechanisms of the same general disease process that may present distinctly due to other genetic and environment influences. Determining how these influences exert their effects to produce pathology contributing to the disease phenotype will be a major future challenge ahead in the field.

miR-381 Attenuates Peripheral Neuropathic Phenotype Caused by Overexpression of PMP22

Charcot-Marie Tooth disease type 1A (CMT1A), the major type of CMT, is caused by duplication of peripheral myelin protein 22 (PMP22) gene whose overexpression causes structural and functional abnormalities in myelination. We investigated whether miRNA-mediated regulation of PMP22 expression could reduce the expression level of PMP22, thereby alleviating the demyelinating neuropathic phenotype of CMT1A. We found that several miRNAs were down-regulated in C22 mouse, a CMT1A mouse model. Among them, miR-381 could target 3′ untranslated region (3′UTR) of PMP22 in vitro based on Western botting and quantitative Real Time-PCR (qRT-PCR) results. In vivo efficacy of miR-381 was assessed by administration of LV-miR-381, an miR-381 expressing lentiviral vector, into the sciatic nerve of C22 mice by a single injection at postnatal day 6 (p6). Administration of LV-miR-381 reduced expression level of PMP22 along with elevated level of miR-381 in the sciatic nerve. Rotarod performance analysis revealed that locomotor coordination of LV-miR-381 administered C22 mice was significantly enhanced from 8 weeks post administration. Electrophysiologically, increased motor nerve conduction velocity was observed in treated mice. Histologically, toluidine blue staining and electron microscopy revealed that structural abnormalities of myelination were improved in sciatic nerves of LV-miR-381 treated mice. Therefore, delivery of miR-381 ameliorated the phenotype of peripheral neuropathy in CMT1A mouse model by down-regulating PMP22 expression. These data suggest that miRNA can be used as a potent therapeutic strategy to control diseases with copy number variations such as CMT1A.

Restoring mitofusin balance prevents axonal degeneration in a Charcot-Marie-Tooth type 2A model

Mitofusin-2 (MFN2) is a mitochondrial outer-membrane protein that plays a pivotal role in mitochondrial dynamics in most tissues, yet mutations in MFN2, which cause Charcot-Marie-Tooth disease type 2A (CMT2A), primarily affect the nervous system. We generated a transgenic mouse model of CMT2A that developed severe early onset vision loss and neurological deficits, axonal degeneration without cell body loss, and cytoplasmic and axonal accumulations of fragmented mitochondria. While mitochondrial aggregates were labeled for mitophagy, mutant MFN2 did not inhibit Parkin-mediated degradation, but instead had a dominant negative effect on mitochondrial fusion only when MFN1 was at low levels, as occurs in neurons. Finally, using a transgenic approach, we found that augmenting the level of MFN1 in the nervous system in vivo rescued all phenotypes in mutant MFN2R94Q-expressing mice. These data demonstrate that the MFN1/MFN2 ratio is a key determinant of tissue specificity in CMT2A and indicate that augmentation of MFN1 in the nervous system is a viable therapeutic strategy for the disease.

Small heat shock proteins and neurodegeneration: recent developments

Members of the small heat shock protein (sHSP) family are molecular chaperones with a critical role in the maintenance of cellular homeostasis under unfavorable conditions. The chaperone properties of sHSPs prevent protein aggregation, and sHSP deregulation underlies the pathology of several diseases, including neurodegenerative disorders. Recent evidence suggests that the clientele of sHSPs is broad, and the mechanisms of sHSP-mediated neuroprotection diverse. Nonetheless, the crosstalk of sHSPs with the neurodegeneration-promoting signaling pathways remains poorly understood. Here, we survey recent findings on the role and regulation of sHSPs in neurodegenerative diseases.

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.

Decreased ceramide underlies mitochondrial dysfunction in Charcot-Marie-Tooth 2F

Charcot-Marie-Tooth (CMT) disease is the most commonly inherited neurologic disorder, but its molecular mechanisms remain unclear. One variant of CMT, 2F, is characterized by mutations in heat shock protein 27 (Hsp27). As bioactive sphingolipids have been implicated in neurodegenerative diseases, we sought to determine if their dysregulation is involved in CMT. Here, we show that Hsp27 knockout mice demonstrated decreases in ceramide in peripheral nerve tissue and that the disease-associated Hsp27 S135F mutant demonstrated decreases in mitochondrial ceramide. Given that Hsp27 is a chaperone protein, we examined its role in regulating ceramide synthases (CerSs), an enzyme family responsible for catalyzing generation of the sphingolipid ceramide. We determined that CerSs colocalized with Hsp27, and upon the presence of S135F mutants, CerS1 lost its colocalization with mitochondria suggesting that decreased mitochondrial ceramides result from reduced mitochondrial CerS localization rather than decreased CerS activity. Mitochondria in mutant cells appeared larger with increased interconnectivity. Furthermore, mutant cell lines demonstrated decreased mitochondrial respiratory function and increased autophagic flux. Mitochondrial structural and functional changes were recapitulated by blocking ceramide generation pharmacologically. These results suggest that mutant Hsp27 decreases mitochondrial ceramide levels, producing structural and functional changes in mitochondria leading to neuronal degeneration.-Schwartz, N. U., Linzer, R. W., Truman, J.-P., Gurevich, M., Hannun, Y. A., Senkal, C. E., Obeid, L. M. Decreased ceramide underlies mitochondrial dysfunction in Charcot-Marie-Tooth 2F.

HSPB1 mutations causing hereditary neuropathy in humans disrupt non-cell autonomous protection of motor neurons

Heat shock protein beta-1 (HSPB1), is a ubiquitously expressed, multifunctional protein chaperone. Mutations in HSPB1 result in the development of a late-onset, distal hereditary motor neuropathy type II (dHMN) and axonal Charcot-Marie Tooth disease with sensory involvement (CMT2F). The functional consequences of HSPB1 mutations associated with hereditary neuropathy are unknown. HSPB1 also displays neuroprotective properties in many neuronal disease models, including the motor neuron disease amyotrophic lateral sclerosis (ALS). HSPB1 is upregulated in SOD1-ALS animal models during disease progression, predominately in glial cells. Glial cells are known to contribute to motor neuron loss in ALS through a non-cell autonomous mechanism. In this study, we examined the non-cell autonomous role of wild type and mutant HSPB1 in an astrocyte-motor neuron co-culture model system of ALS. Astrocyte-specific overexpression of wild type HSPB1 was sufficient to attenuate SOD1(G93A) astrocyte-mediated toxicity in motor neurons, whereas, overexpression of mutHSPB1 failed to ameliorate motor neuron toxicity. Expression of a phosphomimetic HSPB1 mutant in SOD1(G93A) astrocytes also reduced toxicity to motor neurons, suggesting that phosphorylation may contribute to HSPB1 mediated-neuroprotection. These data provide evidence that astrocytic HSPB1 expression may play a central role in motor neuron health and maintenance.

Small heat shock proteins in ageing and age-related diseases

Small heat shock proteins (sHSPs) are gatekeepers of cellular homeostasis across species, preserving proteome integrity under stressful conditions. Nonetheless, recent evidence suggests that sHSPs are more than molecular chaperones with merely auxiliary role. In contrast, sHSPs have emerged as central lifespan determinants, and their malfunction has been associated with the manifestation of neurological disorders, cardiovascular disease and cancer malignancies. In this review, we focus on the role of sHSPs in ageing and age-associated diseases and highlight the most prominent paradigms, where impairment of sHSP function has been implicated in human pathology.

Pmp22 mutant allele-specific siRNA alleviates demyelinating neuropathic phenotype in vivo

Charcot-Marie-Tooth disease (CMT) is a genetic disorder that can be caused by aberrations in >80 genes. CMT has heterogeneous modes of inheritance, including autosomal dominant, autosomal recessive, X-linked dominant, and X-linked recessive. Over 95% of cases are dominantly inherited. In this study, we investigated whether regulation of a mutant allele by an allele-specific small interfering RNA (siRNA) can alleviate the demyelinating neuropathic phenotype of CMT. We designed 19 different allele-specific siRNAs for Trembler J (Tr-J) mice harboring a naturally occurring mutation (Leu16Pro) in Pmp22. Using a luciferase assay, we identified an siRNA that specifically and selectively reduced the expression level of the mutant allele and reversed the low viability of Schwann cells caused by mutant Pmp22 over-expression in vitro. The in vivo efficacy of the allele-specific siRNA was assessed by its intraperitoneal injection to postnatal day 6 of Tr-J mice. Administration of the allele-specific siRNA to Tr-J mice significantly enhanced motor function and muscle volume, as assessed by the rotarod test and magnetic resonance imaging analysis, respectively. Increases in motor nerve conduction velocity and compound muscle action potentials were also observed in the treated mice. In addition, myelination, as evidenced by toluidine blue staining and electron microscopy, was augmented in the sciatic nerves of the mice after allele-specific siRNA treatment. After validating suppression of the Pmp22 mutant allele at the mRNA level in the Schwann cells of Tr-J mice, we observed increased expression levels of myelinating proteins such as myelin basic protein and myelin protein zero. These data indicate that selective suppression of the Pmp22 mutant allele by non-viral delivery of siRNA alleviates the demyelinating neuropathic phenotypes of CMT in vivo, implicating allele-specific siRNA treatment as a potent therapeutic strategy for dominantly inherited peripheral neuropathies.

Pilot phenotype and natural history study of hereditary neuropathies caused by mutations in the HSPB1 gene

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Mutations in HSPB1 result in a motor predominant neuropathy.

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The mean age of disease onset was in the 4th decade.

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HSPB1 neuropathy is characterised by early plantar flexion weakness.

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Muscle MRI demonstrates selective denervation of gastrocnemius and soleus.

Mutations in HSPB1 are one of the commonest causes of distal Hereditary Motor Neuropathy (dHMN). Transgenic mouse models of the disease have identified HDAC6 inhibitors as promising treatments for the condition paving the way for human trials. A detailed phenotype and natural history study of HSPB1 neuropathy is therefore required in order to inform the duration and outcome measures of any future trials. Clinical and neurophysiological data and lower limb muscle MRI were collected both prospectively and retrospectively from patients with mutations in HSPB1. The natural history was assessed by recording the weighted Charcot–Marie–Tooth Examination Score (CMTES) at annual intervals in a subset of patients. 20 patients from 14 families were recruited into the study. The average age of onset was in the 4th decade. Patients presented with a length dependent neuropathy but with early ankle plantar flexion weakness. Neurophysiology confirmed a motor neuropathy but also showed sensory nerve involvement in most patients. Cross sectional muscle MRI revealed soleus and medial gastrocnemius fat infiltration as an early signature of mutant HSPB1 disease. In this study neither semi quantitative muscle MRI, the CMTES nor neurophysiology were able to detect disease progression in HSPB1 neuropathy over 1 or 2 years. Further studies are therefore required to identify a suitable biomarker before clinical trials in HSPB1 neuropathy can be undertaken.

HSPB3 protein is expressed in motoneurons and induces their survival after lesion-induced degeneration

The human small heat shock proteins (HSPBs) form a family of molecular chaperones comprising ten members (HSPB1-HSPB10), whose functions span from protein quality control to cytoskeletal dynamics and cell death control. Mutations in HSPBs can lead to human disease and particularly point mutations in HSPB1 and HSPB8 are known to lead to peripheral neuropathies. Recently, a missense mutation (R7S) in yet another member of this family, HSPB3, was found to cause an axonal motor neuropathy (distal hereditary motor neuropathy type 2C, dHMN2C). Until now, HSPB3 protein localization and function in motoneurons (MNs) have not yet been characterized. Therefore, we studied the endogenous HSPB3 protein distribution in the spinal cords of chicken and mouse embryos and in the postnatal nervous system (central and peripheral) of chicken, mouse and human. We further investigated the impact of wild-type and mutated HSPB3 on MN cell death via overexpressing these genes in ovo in an avian model of MN degeneration, the limb-bud removal. Altogether, our findings represent a first step for a better understanding of the cellular and molecular mechanisms leading to dHMN2C.

Late onset dHMN II caused by c.404C>G mutation in HSPB1 gene

Distal hereditary motor neuropathy (dHMN) type II is genetically heterogeneous. We report three siblings of a German family with late onset distal motor neuropathy due to the c.404C>G mutation in heat-shock 27-kDa protein 1 gene (HSPB1/HSP27). A 36-year-old mutation carrier, daughter of one sibling, did not present any clinical or electrophysiological abnormalities. The index patient (oldest brother) developed weakness of the distal lower limbs and nocturnal muscle cramps at the age of 54. After 5 years this patient developed an l-DOPA-responsive hypokinetic rigid syndrome, establishing a diagnosis of Parkinson's disease. Although none of the three other mutation carriers displayed Parkinsonian signs, a pathogenic relationship with Parkinson's disease remains a possibility, based on the known molecular pathology of HSPB1. The rare pathogenic HSPB1 c.404C>G mutation may predispose for late-onset of dHMN type II.

A Mutation in PMP2 Causes Dominant Demyelinating Charcot-Marie-Tooth Neuropathy

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of peripheral neuropathies with diverse genetic causes. In this study, we identified p.I43N mutation in PMP2 from a family exhibiting autosomal dominant demyelinating CMT neuropathy by whole exome sequencing and characterized the clinical features. The age at onset was the first to second decades and muscle atrophy started in the distal portion of the leg. Predominant fatty replacement in the anterior and lateral compartment was similar to that in CMT1A caused by PMP22 duplication. Sural nerve biopsy showed onion bulbs and degenerating fibers with various myelin abnormalities. The relevance of PMP2 mutation as a genetic cause of dominant CMT1 was assessed using transgenic mouse models. Transgenic mice expressing wild type or mutant (p.I43N) PMP2 exhibited abnormal motor function. Electrophysiological data revealed that both mice had reduced motor nerve conduction velocities (MNCV). Electron microscopy revealed that demyelinating fibers and internodal lengths were shortened in both transgenic mice. These data imply that overexpression of wild type as well as mutant PMP2 also causes the CMT1 phenotype, which has been documented in the PMP22. This report might expand the genetic and clinical features of CMT and a further mechanism study will enhance our understanding of PMP2-associated peripheral neuropathy.

Isolation of causative mutation is still challenging in genetic diseases with a variety of genetic causes. We discovered a mutation in a novel gene from a family exhibiting a peripheral neuropathy by virtue of next-generation sequencing. Although the family shows characteristic clinical features of hereditary motor and sensory neuropathy, we could not find a mutation from well-known genes. To demonstrate the clinical relevance of the novel gene, we generated transgenic mice, which carry the patients’ mutation within their chromosome. The transgenic mice exhibited the same phenotype as the patients including peripheral neuropathic symptoms and reduced locomotor function. We also observed the affected peripheral nervous system through electron microscopy. It seems that the expression of the mutant protein impairs the myelin of peripheral nervous system. These data might expand the genetic, clinical, and pathophysiological features of the peripheral neuropathy and a further investigation will enhance our understanding of disease in the peripheral nervous system.