Animal models of inherited neuropathies

How vitamins act as novel agents for ameliorating diabetic peripheral neuropathy: A comprehensive overview

Diabetic peripheral neuropathy (DPN) is a pervasive and incapacitating sequela of diabetes, affecting a significant proportion of those diagnosed with the disease, yet an effective treatment remains elusive. Vitamins have been extensively studied, emerging as a promising target for diagnosing and treating various systemic diseases, but their role in DPN is not known. This review collates and synthesizes knowledge regarding the interplay between vitamins and DPN, drawing on bibliographies from prior studies and relevant articles, and stratifying the therapeutic strategies from prophylactic to interventional. In addition, the clinical evidence supporting the use of vitamins to ameliorate DPN is also evaluated, underscoring the potential of vitamins as putative therapeutic agents. We anticipate that this review will offer novel insights for developing and applying vitamin-based therapies for DPN.

A novel mouse model of CMT1B identifies hyperglycosylation as a new pathogenetic mechanism

Mutations in the Myelin Protein Zero gene (MPZ), encoding P0, the major structural glycoprotein of peripheral nerve myelin, are the cause of Charcot-Marie-Tooth (CMT) type 1B neuropathy, and most P0 mutations appear to act through gain-of-function mechanisms. Here, we investigated how misglycosylation, a pathomechanism encompassing several genetic disorders, may affect P0 function. Using in vitro assays, we showed that gain of glycosylation is more damaging for P0 trafficking and functionality as compared with a loss of glycosylation. Hence, we generated, via CRISPR/Cas9, a mouse model carrying the MPZD61N mutation, predicted to generate a new N-glycosylation site in P0. In humans, MPZD61N causes a severe early-onset form of CMT1B, suggesting that hyperglycosylation may interfere with myelin formation, leading to pathology. We show here that MPZD61N/+ mice develop a tremor as early as P15 which worsens with age and correlates with a significant motor impairment, reduced muscular strength and substantial alterations in neurophysiology. The pathological analysis confirmed a dysmyelinating phenotype characterized by diffuse hypomyelination and focal hypermyelination. We find that the mutant P0D61N does not cause significant endoplasmic reticulum stress, a common pathomechanism in CMT1B, but is properly trafficked to myelin where it causes myelin uncompaction. Finally, we show that myelinating dorsal root ganglia cultures from MPZD61N mice replicate some of the abnormalities seen in vivo, suggesting that they may represent a valuable tool to investigate therapeutic approaches. Collectively, our data indicate that the MPZD61N/+ mouse represents an authentic model of severe CMT1B affirming gain-of-glycosylation in P0 as a novel pathomechanism of disease.

Long-term outcome of Miniature Schnauzers with genetically confirmed demyelinating polyneuropathy: 12 cases

BACKGROUND

A demyelinating polyneuropathy with focally folded myelin sheaths was reported in 3 Miniature Schnauzers in France in 2008 and was predicted to represent a naturally occurring canine homologue of Charcot-Marie-Tooth (CMT) disease. A genetic variant of MTRM13/SBF2 has been identified as causative in affected Miniature Schnauzers with this polyneuropathy.

OBJECTIVE

To provide data on the long-term progression in affected Miniature Schnauzers from Spain confirmed with the MTRM13/SBF2 genetic variant.

ANIMALS

Twelve Miniature Schnauzers presented between March 2013 and June 2019.

METHODS

Only dogs presented with consistent clinical signs and homozygous for the MTRM13/SBF2 genetic variant were included. Clinical signs, age of onset and presentation, time from onset to presentation, treatment, outcome, and time from diagnosis to final follow-up were retrospectively reviewed.

RESULTS

The hallmark clinical signs at the time of presentation were regurgitation with radiologically confirmed megaesophagus (11/12) and aphonic bark (11/12) with or without obvious neuromuscular weakness despite electrodiagnostic evidence of appendicular demyelinating polyneuropathy. Age of onset and clinical presentation were 3-18 and 4-96 months, respectively. Treatment was mostly symptomatic and consisted of head elevation during meals, antacids, prokinetics, bethanechol, sildenafil, mirtazapine, or some combination of these. During the follow-up period (7-73 months), clinical signs were unchanged in (11/12) cases with aspiration pneumonia developing occasionally (6/12) and being the cause of death in 1 dog.

CONCLUSIONS AND CLINICAL IMPORTANCE

Demyelinating polyneuropathy of Miniature Schnauzers tends to remain stable over the long term leading to a good prognosis with preventive feeding measures and symptomatic treatment to control aspiration pneumonia.

Cellular Signal-Regulated Schwann Cell Myelination and Remyelination

Increasing studies have demonstrated multiple signaling molecules responsible for oligodendrocytes and Schwann cells development such as migration, differentiation, myelination, and axo-glial interaction. However, complicated roles in these events are still poorly understood. This chapter focuses on well established intracellular signaling transduction and recent topics that control myelination and are elucidated from accumulating evidences. The underlying molecular mechanisms, which involved in membrane trafficking through small GTPase Arf6 and its activator cytohesins, demonstrate the crosstalk between well established intracellular signaling transduction and a new finding signaling pathway in glial cells links to physiological phenotype and essential role in peripheral nerve system (PNS). Since Arf family proteins affect the expression levels of myelin protein zero (MPZ) and Krox20, which is a transcription factor regulatory factor in early developmental stages of Schwann cells, Arf proteins likely to be key regulator for Schwann cells development. Herein, we discuss how intracellular signaling transductions in Schwann cells associate with myelination in CNS and PNS.

Comparison of three congruent patient-specific cell types for the modelling of a human genetic Schwann-cell disorder

Patient-specific human induced pluripotent stem cells (hiPSCs) hold great promise for the modelling of genetic disorders. However, these cells display wide intra-individual and inter-individual variations in gene expression, making it challenging to distinguish true-positive and false-positive phenotypes. Also, data from hiPSC phenotypes and from human embryonic stem cells (hESCs) harbouring the same disease mutation are lacking. Here, we report a comparison of molecular, cellular and functional characteristics of three congruent patient-specific cell types ― hiPSCs, hESCs, and direct lineage-converted cells ― derived from currently available differentiation and direct-reprogramming technologies, for the modelling of Charcot Marie Tooth 1A, a human genetic Schwann-cell disorder featuring a 1.4 megabase chromosomal duplication. In particular, we find that the chemokines CXCL1 and MCP1 are commonly upregulated in all three congruent models and in clinical patient samples. The development of congruent models of a single genetic disease by using somatic cells from a common patient will facilitate the search for convergent phenotypes.

Implications of Schwann Cells Biomechanics and Mechanosensitivity for Peripheral Nervous System Physiology and Pathophysiology

The presence of bones around the central nervous system (CNS) provides it with highly effective physiologically crucial mechanical protection. The peripheral nervous system (PNS), in contrast, lacks this barrier. Consequently, the long held belief is that the PNS is mechanically vulnerable. On the other hand, the PNS is exposed to a variety of physiological mechanical stresses during regular daily activities. This fact prompts us to question the dogma of PNS mechanical vulnerability. As a matter of fact, impaired mechanics of PNS nerves is associated with neuropathies with the liability to mechanical stresses paralleled by significant impairment of PNS physiological functions. Our recent biomechanical integrity investigations on nerve fibers from wild-type and neuropathic mice lend strong support in favor of natural mechanical protection of the PNS and demonstrate a key role of Schwann cells (SCs) therein. Moreover, recent works point out that SCs can sense mechanical properties of their microenvironment and the evidence is growing that SCs mechanosensitivity is important for PNS development and myelination. Hence, SCs exhibit mechanical strength necessary for PNS mechanoprotection as well as mechanosensitivity necessary for PNS development and myelination. This mini review reflects on the intriguing dual ability of SCs and implications for PNS physiology and pathophysiology.

Locomotor and skeletal muscle abnormalities in trembler J neuropathic mice

INTRODUCTION

Patients with hereditary peripheral neuropathies exhibit characteristic deformities of the hands and feet and have difficulty ambulating. To examine to what extent neuropathic animals recapitulate these deficits, we studied trembler J (TrJ) mice, which model early-onset demyelinating neuropathy.

METHODS

A cohort of 4-month-old female wild type and neuropathic mice were evaluated for locomotor measurements, neuromuscular function, and skeletal muscle proteolysis and morphometry.

RESULTS

Utilizing the DigiGait imaging system, we identified pronounced alterations in forepaw and hindpaw angles and a decrease in hindpaw area on the treadmill in neuropathic rodents. Torque production by the tibialis anterior (TA) muscle was significantly weakened and was paralleled by a decrease in myofiber cross-sectional area and an increase in muscle tissue proteolysis.

DISCUSSION

Our findings in TrJ mice reflect the phenotypic presentation of the human neuropathy in which patients exhibit weakness of the TA muscle resulting in foot drop and locomotor abnormalities. Muscle Nerve 57: 664-671, 2018.

Nano-scale Biophysical and Structural Investigations on Intact and Neuropathic Nerve Fibers by Simultaneous Combination of Atomic Force and Confocal Microscopy

The links between neuropathies of the peripheral nervous system (PNS), including Charcot-Marie-Tooth1A and hereditary neuropathy with liability to pressure palsies, and impaired biomechanical and structural integrity of PNS nerves remain poorly understood despite the medical urgency. Here, we present a protocol describing simultaneous structural and biomechanical integrity investigations on isolated nerve fibers, the building blocks of nerves. Nerve fibers are prepared from nerves harvested from wild-type and exemplary PNS neuropathy mouse models. The basic principle of the designed experimental approach is based on the simultaneous combination of atomic force microscopy (AFM) and confocal microscopy. AFM is used to visualize the surface structure of nerve fibers at nano-scale resolution. The simultaneous combination of AFM and confocal microscopy is used to perform biomechanical, structural, and functional integrity measurements at nano- to micro-scale. Isolation of sciatic nerves and subsequent teasing of nerve fibers take ~45 min. Teased fibers can be maintained at 37°C in a culture medium and kept viable for up to 6 h allowing considerable time for all measurements which require 3–4 h. The approach is designed to be widely applicable for nerve fibers from mice of any PNS neuropathy. It can be extended to human nerve biopsies.

Endoplasmic reticulum quality control and dysmyelination

Dysmyelination contributes to several human diseases including multiple sclerosis, Charcot-Marie-Tooth, leukodystrophies, and schizophrenia and can result in serious neurological disability. Properly formed, compacted myelin sheaths are required for appropriate nerve conduction velocities and the health and survival of neurons. Many different molecular mechanisms contribute to dysmyelination and many of these mechanisms originate at the level of the endoplasmic reticulum. The endoplasmic reticulum is a critical organelle for myelin biosynthesis and maintenance as the site of myelin protein folding quality control, Ca2+ homeostasis, cholesterol biosynthesis, and modulation of cellular stress. This review paper highlights the role of the endoplasmic reticulum and its resident molecules as an upstream and dynamic contributor to myelin and myelin pathologies.

An ARHGEF10 deletion is highly associated with a juvenile-onset inherited polyneuropathy in Leonberger and Saint Bernard dogs

An inherited polyneuropathy (PN) observed in Leonberger dogs has clinical similarities to a genetically heterogeneous group of peripheral neuropathies termed Charcot-Marie-Tooth (CMT) disease in humans. The Leonberger disorder is a severe, juvenile-onset, chronic, progressive, and mixed PN, characterized by exercise intolerance, gait abnormalities and muscle atrophy of the pelvic limbs, as well as inspiratory stridor and dyspnea. We mapped a PN locus in Leonbergers to a 250 kb region on canine chromosome 16 (P_raw = 1.16×10⁻¹⁰, P_{genome, corrected} = 0.006) utilizing a high-density SNP array. Within this interval is the ARHGEF10 gene, a member of the rho family of GTPases known to be involved in neuronal growth and axonal migration, and implicated in human hypomyelination. ARHGEF10 sequencing identified a 10 bp deletion in affected dogs that removes four nucleotides from the 3′-end of exon 17 and six nucleotides from the 5′-end of intron 17 (c.1955_1958+6delCACGGTGAGC). This eliminates the 3′-splice junction of exon 17, creates an alternate splice site immediately downstream in which the processed mRNA contains a frame shift, and generates a premature stop codon predicted to truncate approximately 50% of the protein. Homozygosity for the deletion was highly associated with the severe juvenile-onset PN phenotype in both Leonberger and Saint Bernard dogs. The overall clinical picture of PN in these breeds, and the effects of sex and heterozygosity of the ARHGEF10 deletion, are less clear due to the likely presence of other forms of PN with variable ages of onset and severity of clinical signs. This is the first documented severe polyneuropathy associated with a mutation in ARHGEF10 in any species.

Leonberger dogs are a breed originally produced by crossing large-bodied dogs, including Saint Bernards and Newfoundlands. A peripheral neuropathy has been described in Leonbergers that is similar to a group of inherited polyneuropathies known as Charcot-Marie-Tooth disease in humans. We collected a cohort of well-characterized Leonberger polyneuropathy cases and controls, conducted a genome-wide association study, and ultimately identified a highly associated and likely causative mutation in the AHGEF10 gene. This sequence variant is a 10-bp deletion encompassing a splice site, which forces use of a downstream splice site to create a processed mRNA with a premature stop codon, and represents a loss-of-function mutation. The identical mutation was also found in several polyneuropathy-affected Saint Bernards. When homozygous, this deletion results in the onset of clinical signs before four years of age. ARHGEF10 has not previously been associated with severe CMT, but comes from a family of genes shown to be involved in neuron morphology. This first-documented severe polyneuropathy associated with an ARHGEF10 mutation in any species provides an opportunity to gain further insights into the pathobiology of diseases associated with this gene. The ARHGEF10 mutation does not, however, by itself account for all cases of polyneuropathy in Leonbergers or Saint Bernards.

A novel transgenic mouse model of Chinese Charcot-Marie-Tooth disease type 2L

We previously found that the K141N mutation in heat shock protein B8 (HSPB8) was responsible for Charcot-Marie-Tooth disease type 2L in a large Chinese family. The objective of the present study was to generate a transgenic mouse model bearing the K141N mutation in the human HSPB8 gene, and to determine whether this ^K141NHSPB8 transgenic mouse model would manifest the clinical phenotype of Charcot-Marie-Tooth disease type 2L, and consequently be suitable for use in studies of disease pathogenesis. Transgenic mice overexpressing ^K141NHSPB8 were generated using K141N mutant HSPB8 cDNA cloned into a pCAGGS plasmid driven by a human cytomegalovirus expression system. PCR and western blot analysis confirmed integration of the ^K141NHSPB8 gene and widespread expression in tissues of the transgenic mice. The ^K141NHSPB8 transgenic mice exhibited decreased muscle strength in the hind limbs and impaired motor coordination, but no obvious sensory disturbance at 6 months of age by behavioral assessment. Electrophysiological analysis showed that the compound motor action potential amplitude in the sciatic nerve was significantly decreased, but motor nerve conduction velocity remained normal at 6 months of age. Pathological analysis of the sciatic nerve showed reduced myelinated fiber density, notable axonal edema and vacuolar degeneration in ^K141NHSPB8 transgenic mice, suggesting axonal involvement in the peripheral nerve damage in these animals. These findings indicate that the ^K141NHSPB8 transgenic mouse successfully models Charcot-Marie-Tooth disease type 2L and can be used to study the pathogenesis of the disease.

A molecular signature predictive of indolent prostate cancer

Many newly diagnosed prostate cancers present as low Gleason score tumors that require no treatment intervention. Distinguishing the many indolent tumors from the minority of lethal ones remains a major clinical challenge. We now show that low Gleason score prostate tumors can be distinguished as indolent and aggressive subgroups on the basis of their expression of genes associated with aging and senescence. Using gene set enrichment analysis, we identified a 19-gene signature enriched in indolent prostate tumors. We then further classified this signature with a decision tree learning model to identify three genes--FGFR1, PMP22, and CDKN1A--that together accurately predicted outcome of low Gleason score tumors. Validation of this three-gene panel on independent cohorts confirmed its independent prognostic value as well as its ability to improve prognosis with currently used clinical nomograms. Furthermore, protein expression of this three-gene panel in biopsy samples distinguished Gleason 6 patients who failed surveillance over a 10-year period. We propose that this signature may be incorporated into prognostic assays for monitoring patients on active surveillance to facilitate appropriate courses of treatment.

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

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

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

Dominant Charcot-Marie-Tooth syndrome and cognate disorders

Charcot-Marie-Tooth neuropathy (CMT) is a group of genetically heterogeneous disorders sharing a similar phenotype, characterized by wasting and weakness mainly involving the distal muscles of lower and upper limbs, variably associated with distal sensory loss and skeletal deformities. This chapter deals with dominantly transmitted CMT and related disorders, namely hereditary neuropathy with liability to pressure palsies (HNPP) and hereditary neuralgic amyotrophy (HNA). During the last 20 years, several genes have been uncovered associated with CMT and our understanding of the underlying molecular mechanisms has greatly improved. Consequently, a precise genetic diagnosis is now possible in the majority of cases, thus allowing proper genetic counseling. Although, unfortunately, treatment is still unavailable for all types of CMT, several cellular and animal models have been developed and some compounds have proved effective in these models. The first trials with ascorbic acid in CMT type 1A have been completed and, although negative, are providing relevant information on disease course and on how to prepare for future trials.

Murine therapeutic models for Charcot-Marie-Tooth (CMT) disease

INTRODUCTION OR BACKGROUND

Charcot-Marie-Tooth (CMT) disease represents a broad group of inherited motor and sensory neuropathies which can originate from various genetic aberrations, e.g. mutations, deletions and duplications.

SOURCES OF DATA

We performed a literature review on murine animal models of CMT disease with regard to experimental therapeutic approaches. Hereby, we focussed on the demyelinating subforms of CMT (CMT1). PubMed items were CMT, animal model, demyelination and therapy.

AREAS OF AGREEMENT

Patients affected by CMT suffer from slowly progressive, distally pronounced muscle atrophy caused by an axonal loss. The disease severity is highly variable and impairments may result in wheelchair boundness. No therapy is available yet.

AREAS OF CONTROVERSY

Numerous rodent models for the various CMT subtypes are available today. The selection of the correct animal model for the specific CMT subtype provides an important prerequisite for the successful translation of experimental findings in patients.

GROWING POINTS

Despite more than 20 years of remarkable progress in CMT research, the disease is still left untreatable. There is a growing number of experimental therapeutic strategies that may be translated into future clinical trials in patients with CMT.

AREAS TIMELY FOR DEVELOPING RESEARCH

The slow disease progression and insensitive outcome measures hamper clinical therapy trials in CMT. Biomarkers may provide powerful tools to monitor therapeutic efficacy. Recently, we have shown that transcriptional profiling can be utilized to assess and predict the disease severity in a transgenic rat model and in affected humans.

Genetic disruption of Pten in a novel mouse model of tomaculous neuropathy

‘Tomacula’ and myelin outfoldings are striking neuropathological features of a diverse group of inherited demyelinating neuropathies. Whereas the underlying genetic defects are well known, the molecular mechanisms of tomacula formation have remained obscure. We hypothesized that they are caused by uncontrolled, excessive myelin membrane growth, a process, which is regulated in normal development by neuregulin-1/ErbB2, PI3 Kinase signalling and ERK/MAPK signalling. Here, we demonstrate by targeted disruption of Pten in Schwann cells that hyperactivation of the endogenous PI3 Kinase pathway causes focal hypermyelination, myelin outfoldings and tomacula, even when induced in adult animals by tamoxifen, and is associated with progressive peripheral neuropathy. Activated AKT kinase is associated with PtdIns(3,4,5)P₃ at paranodal loops and Schmidt–Lanterman incisures. This striking myelin pathology, with features of human CMT type 4B1 and HNPP, is dependent on AKT/mTOR signalling, as evidenced by a significant amelioration of the pathology in mice treated with rapamycin. We suggest that regions of non-compact myelin are under lifelong protection by PTEN against abnormal membrane outgrowth, and that dysregulated phosphoinositide levels play a critical role in the pathology of tomaculous neuropathies.

Aging of myelinating glial cells predominantly affects lipid metabolism and immune response pathways

Both the central and the peripheral nervous systems are prone to multiple age-dependent neurological deficits, often attributed to still unknown alterations in the function of myelinating glia. To uncover the biological processes affected in glial cells by aging, we analyzed gene expression of the Schwann cell-rich mouse sciatic nerve at 17 time points throughout life, from day of birth until senescence. By combining these data with the gene expression data of myelin mouse mutants carrying deletions of either Pmp22, SCAP, or Lpin1, we found that the majority of age-related transcripts were also affected in myelin mutants (54.4%) and were regulated during PNS development (59.5%), indicating a high level of overlap in implicated molecular pathways. The expression profiles in aging copied the direction of transcriptional changes observed in neuropathy models; however, they had the opposite direction when compared with PNS development. The most significantly altered biological processes in aging involved the inflammatory/immune response and lipid metabolism. Interestingly, both these pathways were comparably changed in the aging optic nerve, suggesting that similar biological processes are affected in aging of glia-rich parts of the central and peripheral nervous systems. Our comprehensive comparison of gene expression in three distinct biological conditions including development, aging, and myelin disease thus revealed a previously unanticipated relationship among themselves and identified lipid metabolism and inflammatory/immune response pathways as potential therapeutical targets to prevent or delay so far incurable age-related and inherited forms of neuropathies.

Behavioral and molecular exploration of the AR-CMT2A mouse model Lmna (R298C/R298C)

In 2002, we identified LMNA as the first gene responsible for an autosomal recessive axonal form of Charcot-Marie-Tooth disease, AR-CMT2A. All patients were found to be homozygous for the same mutation in the LMNA gene, p.Arg298Cys. In order to investigate the physiopathological mechanisms underlying AR-CMT2A, we have generated a knock-in mouse model for the Lmna p.Arg298Cys mutation. We have explored these mice through an exhaustive series of behavioral tests and histopathological analyses, but were not able to find any peripheral nerve phenotype, even at 18 months of age. Interestingly at the molecular level, however, we detect a downregulation of the Lmna gene in all tissues tested from the homozygous knock-in mouse Lmna (R298C/R298C) (skeletal muscle, heart, peripheral nerve, spinal cord and cerebral trunk). Importantly, we further reveal a significant upregulation of Pmp22, specifically in the sciatic nerves of Lmna (R298C/R298C) mice. These results indicate that, despite the absence of a perceptible phenotype, abnormalities exist in the peripheral nerves of Lmna (R298C/R298C) mice that are absent from other tissues. Although the mechanisms leading to deregulation of Pmp22 in Lmna (R298C/R298C) mice are still unclear, our results support a relation between Lmna and Pmp22 and constitute a first step toward understanding AR-CMT2A physiopathology.

A mutation in the gene encoding mitochondrial Mg²+ channel MRS2 results in demyelination in the rat

The rat demyelination (dmy) mutation serves as a unique model system to investigate the maintenance of myelin, because it provokes severe myelin breakdown in the central nervous system (CNS) after normal postnatal completion of myelination. Here, we report the molecular characterization of this mutation and discuss the possible pathomechanisms underlying demyelination. By positional cloning, we found that a G-to-A transition, 177 bp downstream of exon 3 of the Mrs2 (MRS2 magnesium homeostasis factor (Saccharomyces cerevisiae)) gene, generated a novel splice acceptor site which resulted in functional inactivation of the mutant allele. Transgenic rescue with wild-type Mrs2-cDNA validated our findings. Mrs2 encodes an essential component of the major Mg²⁺ influx system in mitochondria of yeast as well as human cells. We showed that the dmy/dmy rats have major mitochondrial deficits with a markedly elevated lactic acid concentration in the cerebrospinal fluid, a 60% reduction in ATP, and increased numbers of mitochondria in the swollen cytoplasm of oligodendrocytes. MRS2-GFP recombinant BAC transgenic rats showed that MRS2 was dominantly expressed in neurons rather than oligodendrocytes and was ultrastructurally observed in the inner membrane of mitochondria. Our observations led to the conclusion that dmy/dmy rats suffer from a mitochondrial disease and that the maintenance of myelin has a different mechanism from its initial production. They also established that Mg²⁺ homeostasis in CNS mitochondria is essential for the maintenance of myelin.

The myelin sheath that surrounds the axon of a neuron acts as a biological insulator. Its major function is to increase the speed at which impulses propagate along myelinated fibers in the central nervous system, as well as the peripheral nervous system. Alterations or damage affecting this structure (demyelination) result in the disruption of signals between the brain and other parts of the body. In the rat, mutations producing demyelination have been frequently identified and characterized and have contributed to a better understanding of the genetics of myelin development, physiology, and pathology. This paper reports the molecular characterization of a recessive allele responsible for the progressive disruption of myelin that was initially observed in mutant rats, previously named demyelination (dmy). This mutation generates an additional splicing acceptor site in an intron of the mitochondrial Mg²⁺ transporter gene (Mrs2), resulting in the insertion of a 83-bp genomic DNA segment into the Mrs2 transcript and complete functional inactivation of the mutant allele. We firstly defined the biological function of MRS2 in mammals and demonstrated the crucial and unexpected role of MRS2 in myelin physiology. Our findings might be helpful in the development of new therapeutic strategies for demyelinating syndromes.

The Wlds transgene reduces axon loss in a Charcot-Marie-Tooth disease 1A rat model and nicotinamide delays post-traumatic axonal degeneration

Charcot-Marie-Tooth disease (CMT) is the most common inherited neuropathy and a duplication of the peripheral myelin protein of 22 kDa (PMP22) gene causes the most frequent subform CMT1A. Clinical impairments are determined by the amount of axonal loss. Axons of the spontaneous mouse mutant Wallerian degeneration slow (Wlds) show markedly reduced degeneration following various types of injuries. Protection is conferred by a chimeric Wlds gene encoding an N-terminal part of ubiquitination factor Ube4b and full length nicotinamide mononucleotide adenylyl transferase 1 (Nmnat1). Nmnat1 enzyme generates nicotinamide adenine dinucleotide (NAD) from nicotinamide mononucleotide. Here, in a Pmp22 transgenic animal model of Charcot-Marie-Tooth disease type 1A (CMT rat), the Wlds transgene reduced axonal loss and clinical impairments without altering demyelination. Furthermore, nicotinamide - substrate precursor of the Nmnat1 enzyme - transiently delayed posttraumatic axonal degeneration in an in vivo model of acute peripheral nerve injury, but to a lower extent than Wlds. In contrast, 8 weeks of nicotinamide treatment did not influence axonal loss or clinical manifestations in the CMT rat. Therefore, nicotinamide can partially substitute for the protective Wlds effect in acute traumatic, but not in chronic secondary axonal injury. Future studies are needed to develop axon protective therapy in CMT1A which may be combined with therapeutic strategies aimed at downregulation of toxic PMP22 overexpression.

A deletion in the N-myc downstream regulated gene 1 (NDRG1) gene in Greyhounds with polyneuropathy

The polyneuropathy of juvenile Greyhound show dogs shows clinical similarities to the genetically heterogeneous Charcot-Marie-Tooth (CMT) disease in humans. The pedigrees containing affected dogs suggest monogenic autosomal recessive inheritance and all affected dogs trace back to a single male. Here, we studied the neuropathology of this disease and identified a candidate causative mutation. Peripheral nerve biopsies from affected dogs were examined using semi-thin histology, nerve fibre teasing and electron microscopy. A severe chronic progressive mixed polyneuropathy was observed. Seven affected and 17 related control dogs were genotyped on the 50k canine SNP chip. This allowed us to localize the causative mutation to a 19.5 Mb interval on chromosome 13 by homozygosity mapping. The NDRG1 gene is located within this interval and NDRG1 mutations have been shown to cause hereditary motor and sensory neuropathy-Lom in humans (CMT4D). Therefore, we considered NDRG1 a positional and functional candidate gene and performed mutation analysis in affected and control Greyhounds. A 10 bp deletion in canine NDRG1 exon 15 (c.1080_1089delTCGCCTGGAC) was perfectly associated with the polyneuropathy phenotype of Greyhound show dogs. The deletion causes a frame shift (p.Arg361SerfsX60) which alters several amino acids before a stop codon is encountered. A reduced level of NDRG1 transcript could be detected by RT-PCR. Western blot analysis demonstrated an absence of NDRG1 protein in peripheral nerve biopsy of an affected Greyhound. We thus have identified a candidate causative mutation for polyneuropathy in Greyhounds and identified the first genetically characterized canine CMT model which offers an opportunity to gain further insights into the pathobiology and therapy of human NDRG1 associated CMT disease. Selection against this mutation can now be used to eliminate polyneuropathy from Greyhound show dogs.

Reversal of neuropathy phenotypes in conditional mouse model of Charcot-Marie-Tooth disease type 2E

Mutations in the gene encoding for the neurofilament light subunit (NF-L) are responsible for Charcot-Marie-Tooth (CMT) neuropathy type 2E. To address whether CMT2E disease is potentially reversible, we generated a mouse model with conditional doxycycline-responsive gene system that allows repression of mutant hNF-LP22S transgene expression in adult neurons. The hNF-LP22S;tTa transgenic (tg) mice recapitulated key features of CMT2E disease, including aberrant hindlimb posture, motor deficits, hypertrophy of muscle fibres and loss of muscle innervation without neuronal loss. Remarkably, a 3-month treatment of hNF-LP22S;tTa mice with doxycycline after onset of disease efficiently down-regulated expression of hNF-LP22S and it caused reversal of CMT neurological phenotypes with restoration of muscle innervation and of neurofilament protein distribution along the sciatic nerve. These data suggest that therapeutic approaches aimed at abolishing expression or neutralizing hNF-L mutants might not only halt the progress of CMT2E disease, but also revert the disabilities.

Concurrent Lpin1 and Nrcam mouse mutations result in severe peripheral neuropathy with transitory hindlimb paralysis

Peripheral neuropathy is a broad category of disorders with a diverse etiology, grouped together by their common pathogenic effect on the peripheral nervous system (PNS). Because of the heterogeneity observed to be responsible for these disorders, a forward genetics method of gene discovery was used to identify additional affected pathways. In this report, we describe the mutant mouse line 20884, generated by N-ethyl-N-nitrosourea mutagenesis, which is characterized by adult-onset transitory hindlimb paralysis. Linkage mapping revealed that two point mutations are responsible for the phenotype: a partial loss-of-function mutation in the gene for phosphatidate phosphatase Lpin1 and a truncation mutation in the gene that encodes the neuronal cell adhesion molecule NrCAM. To investigate how the 20884 Lpin1 and Nrcam mutations interact to produce the paralysis phenotype, the double mutant and both single mutants were analyzed by quantitative behavioral, histological, and electrophysiological means. The Lpin1(20884) mutant and the double mutant are characterized by similar levels of demyelination and aberrant myelin structures. Nevertheless, the double mutant exhibits more severe electrophysiological abnormalities than the Lpin1(20884) mutant. The Nrcam(20884) mutant is characterized by normal sciatic nerve morphology and a mild electrophysiological defect. Comparison of the double mutant phenotype with the two single mutants does not point to an additive relationship between the two defects; rather, the Lpin1(20884) and Nrcam(20884) defects appear to act synergistically to produce the 20884 phenotype. It is proposed that the absence of NrCAM in a demyelinating environment has a deleterious effect, possibly by impairing the process of remyelination.

Functional and comparative genomics analyses of pmp22 in medaka fish

Background

Pmp22, a member of the junction protein family Claudin/EMP/PMP22, plays an important role in myelin formation. Increase of pmp22 transcription causes peripheral neuropathy, Charcot-Marie-Tooth disease type1A (CMT1A). The pathophysiological phenotype of CMT1A is aberrant axonal myelination which induces a reduction in nerve conduction velocity (NCV). Several CMT1A model rodents have been established by overexpressing pmp22. Thus, it is thought that pmp22 expression must be tightly regulated for correct myelin formation in mammals. Interestingly, the myelin sheath is also present in other jawed vertebrates. The purpose of this study is to analyze the evolutionary conservation of the association between pmp22 transcription level and vertebrate myelin formation, and to find the conserved non-coding sequences for pmp22 regulation by comparative genomics analyses between jawed fishes and mammals.

Results

A transgenic pmp22 over-expression medaka fish line was established. The transgenic fish had approximately one fifth the peripheral NCV values of controls, and aberrant myelination of transgenic fish in the peripheral nerve system (PNS) was observed. We successfully confirmed that medaka fish pmp22 has the same exon-intron structure as mammals, and identified some known conserved regulatory motifs. Furthermore, we found novel conserved sequences in the first intron and 3'UTR.

Conclusion

Medaka fish undergo abnormalities in the PNS when pmp22 transcription increases. This result indicates that an adequate pmp22 transcription level is necessary for correct myelination of jawed vertebrates. Comparison of pmp22 orthologs between distantly related species identifies evolutionary conserved sequences that contribute to precise regulation of pmp22 expression.

Small Rho GTPases are key regulators of peripheral nerve biology in health and disease

A thorough knowledge of the cellular and molecular basis of the structure and function of peripheral nerves is of paramount importance not only for a better understanding of the fascinating biology of the peripheral nervous system but also for providing critical insights into the various diseases affecting peripheral nerves as the firm foundation of potential treatments. Genetic approaches in model organisms, in combination with research on hereditary forms of neuropathies, have contributed significantly to our progress in this field. In this review, we will focus on recent advances using these synergistic approaches that led to the identification of small Rho GTPases and their regulators as crucial functional players in proper development and function of myelinated peripheral nerves, with a particular emphasis on the cell biology of Schwann cells in health and disease.

Analysis of peripheral nerve expression profiles identifies a novel myelin glycoprotein, MP11

The myelin sheath insulates axons and allows for rapid salutatory conduction in the nervous system of all vertebrates. The formation of peripheral myelin requires expression of the transcription factor Egr2, which is responsible for inducing such essential myelin-associated genes as Mpz, Mbp, Pmp22, and Mag. Using microarray analysis to compare gene expression patterns in peripheral nerve during development, during remyelination after nerve injury, and in a congenital hypomyelinating mouse model, we identified an evolutionarily conserved novel component of myelin called Mp11 (myelin protein of 11 kDa). The Mp11 genomic locus contains multiple conserved Egr binding sites, and Mp11 induction is regulated by the expression of Egr2. Similar to other Egr2-dependent genes, it is induced during developmental myelination and remyelination after nerve injury. Mp11 is a glycoprotein expressed preferentially in the myelin of the peripheral nervous system versus CNS and is specifically localized to the Schmidt-Lanterman incisures and paranodes of peripheral nerve. The Mp11 protein contains no identifiable similarity to other known protein domains or motifs. However, like other myelin genes, strict Mp11 expression levels are a requirement for the in vitro myelination of DRG neurons, indicating that this previously uncharacterized gene product is a critical component of peripheral nervous system myelin.

Remodeling of motor nerve terminals in demyelinating axons of periaxin-null mice

Myelin formation around axons increases nerve conduction velocity and influences both the structure and function of the myelinated axon. In the peripheral nervous system, demyelinating forms of hereditary Charcot-Marie-Tooth (CMT) diseases cause reduced nerve conduction velocity initially and ultimately axonal degeneration. Several mouse models of CMT diseases have been generated, allowing the study of the consequences of disrupting Schwann cell function on peripheral nerve fibers. Nevertheless, the effect of demyelination at the level of the neuromuscular synapse has been largely overlooked. Here we show that in mice lacking functional Periaxin (Prx) genes, a model of a recessive type of CMT disease known as CMT4F, neuromuscular junctions (NMJs) develop profound morphological changes in the preterminal region of motor axons. These changes include extensive preterminal branches that originate in demyelinated regions of the nerve fiber and axonal swellings associated with residually-myelinated regions of the fiber. Using intracellular recording from muscle fibers we detected asynchronous failure of action potential transmission at high but not low stimulation frequencies, a phenomenon consistent with branch point failure. Taken together, our morphological and electrophysiological findings suggest that preterminal branching due to segmental demyelination near the neuromuscular synapse in Periaxin KO mice may underlie some characteristics of disabilities, including coordination deficits, present in this mouse model of CMT disease. These results reveal the importance of studying how demyelinating diseases might influence NMJ function and contribute to clinical disability.

Mouse forward genetics in the study of the peripheral nervous system and human peripheral neuropathy

Forward genetics, the phenotype-driven approach to investigating gene identity and function, has a long history in mouse genetics. Random mutations in the mouse transcend bias about gene function and provide avenues towards unique discoveries. The study of the peripheral nervous system is no exception; from historical strains such as the trembler mouse, which led to the identification of PMP22 as a human disease gene causing multiple forms of peripheral neuropathy, to the more recent identification of the claw paw and sprawling mutations, forward genetics has long been a tool for probing the physiology, pathogenesis, and genetics of the PNS. Even as spontaneous and mutagenized mice continue to enable the identification of novel genes, provide allelic series for detailed functional studies, and generate models useful for clinical research, new methods, such as the piggyBac transposon, are being developed to further harness the power of forward genetics.

CNS-derived glia ensheath peripheral nerves and mediate motor root development

Motor function requires that motor axons extend from the spinal cord at regular intervals and that they are myelinated by Schwann cells. Little attention has been given to another cellular structure, the perineurium, which ensheaths the motor nerve, forming a flexible, protective barrier. Consequently, the origin of perineurial cells and their roles in motor nerve formation are poorly understood. Using time-lapse imaging in zebrafish, we show that perineurial cells are born in the CNS, arising as ventral spinal-cord glia before migrating into the periphery. In embryos lacking perineurial glia, motor neurons inappropriately migrated outside of the spinal cord and had aberrant axonal projections, indicating that perineurial glia carry out barrier and guidance functions at motor axon exit points. Additionally, reciprocal signaling between perineurial glia and Schwann cells was necessary for motor nerve ensheathment by both cell types. These insights reveal a new class of CNS-born glia that critically contributes to motor nerve development.

New implications for the QUAKING RNA binding protein in human disease

The use of spontaneously occurring mouse models has proved to be a valuable tool throughout the years to delineate the signals required for nervous system development. This is especially true in the field of myelin biology, with a large number of different models available. The quaking viable mouse models dysmyelination in the nervous system and links the QUAKING RNA binding proteins to myelination and cell fate decisions. In this Mini-Review, we highlight the biological functions attributed to this KH-type RNA binding protein and the recent achievements linking it to human disorders.

Hereditary neuropathies

PURPOSE OF REVIEW

The purpose of this review is to help neurologists understand new concepts in hereditary neuropathies, from the clinician's point of view, in the molecular era after the burst of information regarding peripheral nerve biology.

RECENT FINDINGS

Recent studies have focused on understanding the pathomechanisms involved in hereditary neuropathies. In the past year identification of new genes has slowed down since scientists have concentrated more on the function of genes causing Charcot-Marie-Tooth disease and Schwann cell-axon interactions to reveal the molecular cell biology of the disease. Animal models for the most common subtypes of human Charcot-Marie-Tooth disease are now available.

SUMMARY

Rapid advances in the molecular genetics and cell biology of hereditary neuropathies have highlighted the great genetic complexity of Charcot-Marie-Tooth disease. The evolution from a simple clinical classification to a complex molecular one has not facilitated our understanding of the disease. Moreover, the new molecular classification is not simple to use as different mutations of the same gene produce a range of phenotypes. The clinicians have to look for specific clinical and electrophysiological clues to direct the patient to appropriate genetic testing.

Charcot-Marie-Tooth type 4B demyelinating neuropathy: deciphering the role of MTMR phosphatases

Charcot-Marie-Tooth type 4B (CMT4B) is a severe autosomal recessive neuropathy with demyelination and myelin outfoldings of the nerve. This disorder is genetically heterogeneous, but thus far, mutations in myotubularin-related 2 (MTMR2) and MTMR13 genes have been shown to underlie CMT4B1 and CMT4B2, respectively. MTMR2 and MTMR13 belong to a family of ubiquitously expressed proteins sharing homology with protein tyrosine phosphatases (PTPs). The MTMR family, which has 14 members in humans, comprises catalytically active proteins, such as MTMR2, and catalytically inactive proteins, such as MTMR13. Despite their homology with PTPs, catalytically active MTMR phosphatases dephosphorylate both PtdIns3P and PtdIns(3,5)P2 phosphoinositides. Thus, MTMR2 and MTMR13 may regulate vesicular trafficking in Schwann cells. Loss of these proteins could lead to uncontrolled folding of myelin and, ultimately, to CMT4B. In this review, we discuss recent findings on this interesting protein family with the main focus on MTMR2 and MTMR13 and their involvement in the biology of Schwann cell and CMT4B neuropathies.

Mechanisms of disease: inherited demyelinating neuropathies--from basic to clinical research

The hereditary motor and sensory neuropathies (also known as Charcot-Marie-Tooth disease or CMT) are characterized by a length-dependent loss of axonal integrity in the PNS, which leads to progressive muscle weakness and sensory deficits. The 'demyelinating' neuropathies (CMT disease types 1 and 4) are genetically heterogeneous, but their common feature is that the primary defect perturbs myelination. As we discuss in this Review, several new genes associated with CMT1 and CMT4 have recently been identified. The emerging view is that a range of different subcellular defects in Schwann cells can cause axonal loss, which represents the final common pathway of all CMT disease and is independent of demyelination. We propose that Schwann cells provide a first line of axonal neuroprotection. A better understanding of axon-glia interactions should open the way to therapeutic interventions for demyelinating neuropathies. Transgenic animal models have become essential for dissecting CMT disease mechanisms and exploring novel therapies.

The immunocompetence of Schwann cells

Schwann cells are the myelinating glial cells of the peripheral nervous system that support and ensheath axons with myelin to enable rapid saltatory signal propagation in the axon. Immunocompetence, however, has only recently been recognized as an important feature of Schwann cells. An autoimmune response against components of the peripheral nervous system triggers disabling inflammatory neuropathies in patients and corresponding animal models. The immune system participates in nerve damage and disease manifestation even in non-inflammatory hereditary neuropathies. A growing body of evidence suggests that Schwann cells may modulate local immune responses by recognizing and presenting antigens and may also influence and terminate nerve inflammation by secreting cytokines. This review summarizes current knowledge on the interaction of Schwann cells with the immune system, which is involved in diseases of the peripheral nervous system.

Antiprogesterone therapy uncouples axonal loss from demyelination in a transgenic rat model of CMT1A neuropathy

OBJECTIVE

Charcot-Marie-Tooth disease (CMT) is the most common inherited neuropathy, and a duplication of the Pmp22 gene causes the most frequent subform CMT1A. Using a transgenic rat model of CMT1A, we tested the hypothesis that long-term treatment with anti-progesterone (Onapristone) reduces Pmp22 overexpression and improves CMT disease phenotype of older animals, thereby extending a previous proof-of-concept observation in a more clinically relevant setting.

METHODS

We applied placebo-controlled progesterone-antagonist therapy to CMT rats for 5 months and performed grip-strength analysis to assess the motor phenotype. Quantitative Pmp22 RT-PCR and complete histological analysis of peripheral nerves and skin biopsies were performed.

RESULTS

Anti-progesterone therapy significantly increased muscle strength and muscle mass of CMT rats and reduced the performance difference to wildtype rats by about 50%. Physical improvements can be explained by the prevention of axon loss. Surprisingly, the effects of anti-progesterone were not reflected by improved myelin sheath thickness. Electrophysiology confirmed unaltered NCV, but less reduced CMAP recordings in the treatment group. Moreover, the reduction of Pmp22 mRNA, as quantified in cutaneous nerves, correlated with the clinical phenotype at later stages.

INTERPRETATION

Progesterone-antagonist long-term therapy reduces [corrected] Pmp22 overexpression to a degree at which the axonal support function of Schwann cells is better maintained than myelination. This suggests that axonal loss in CMT1A is not caused by demyelination, but rather by a Schwann cell defect that has been partially uncoupled by anti-progesterone treatment. Pmp22 expression analysis in skin may provide a prognostic marker for disease severity and for monitoring future clinical trials.

Axonal regulation of myelination by neuregulin 1

Neuregulins comprise a family of epidermal growth factor-like ligands that interact with ErbB receptor tyrosine kinases to control many aspects of neural development. One of the most dramatic effects of neuregulin-1 is on glial cell differentiation. The membrane-bound neuregulin-1 type III isoform is an axonal ligand for glial ErbB receptors that regulates the early Schwann cell lineage, including the generation of precursors. Recent studies have shown that the amount of neuregulin-1 type III expressed on axons also dictates the glial phenotype, with a threshold level triggering Schwann cell myelination. Remarkably, neuregulin-1 type III also regulates Schwann cell membrane growth to adjust myelin sheath thickness to match axon caliber precisely. Whether this signaling system operates in central nervous system myelination remains an open question of major importance for human demyelinating diseases.