Abnormalities in the axonal cytoskeleton induced by a connexin32 mutation in nerve xenografts

Gait Pattern in Charcot-Marie-Tooth Disease Type 1A According to Disease Severity

The aim of this study was to evaluate the characteristics of gait patterns in Charcot-Marie-Tooth disease type 1A (CMT1A) patients according to disease severity. Twenty-two CMT1A patients were enrolled and classified into two groups, according to the disease severity. The healthy control group consisted of 22 subjects with no gait impairment. Full barefoot three-dimensional gait analysis with temporospatial, kinematic, and kinetic data was performed among the mild and moderate CMT1A group and the control group. Minimal hip abduction, maximal hip extension generation, peak knee flexion moment at stance, ankle dorsiflexion at initial contact, maximal ankle plantarflexion at push-off and maximal ankle rotation moment at stance in the CMT1A group showed a significant difference compared to the control group (p < 0.05). In the moderate group, there were greater maximal hip flexion angles in swing, and smaller dorsiflexion angles at initial contact compared to the control group and mild group. CMT patients had typical gait characteristics and their gait patterns were different according to severity. The analysis of gait patterns in patients with CMT1A will help to understand gait function and provide important information for the treatment of patients with CMT in the future.

New evidence for secondary axonal degeneration in demyelinating neuropathies

Development of peripheral nervous system (PNS) myelin involves a coordinated series of events between growing axons and the Schwann cell (SC) progenitors that will eventually ensheath them. Myelin sheaths have evolved out of necessity to maintain rapid impulse propagation while accounting for body space constraints. However, myelinating SCs perform additional critical functions that are required to preserve axonal integrity including mitigating energy consumption by establishing the nodal architecture, regulating axon caliber by organizing axonal cytoskeleton networks, providing trophic and potentially metabolic support, possibly supplying genetic translation materials and protecting axons from toxic insults. The intermediate steps between the loss of these functions and the initiation of axon degeneration are unknown but the importance of these processes provides insightful clues. Prevalent demyelinating diseases of the PNS include the inherited neuropathies Charcot-Marie-Tooth Disease, Type 1 (CMT1) and Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) and the inflammatory diseases Acute Inflammatory Demyelinating Polyneuropathy (AIDP) and Chronic Inflammatory Demyelinating Polyneuropathy (CIDP). Secondary axon degeneration is a common feature of demyelinating neuropathies and this process is often correlated with clinical deficits and long-lasting disability in patients. There is abundant electrophysiological and histological evidence for secondary axon degeneration in patients and rodent models of PNS demyelinating diseases. Fully understanding the involvement of secondary axon degeneration in these diseases is essential for expanding our knowledge of disease pathogenesis and prognosis, which will be essential for developing novel therapeutic strategies.

Gene replacement therapy after neuropathy onset provides therapeutic benefit in a model of CMT1X

X-linked Charcot-Marie-Tooth disease (CMT1X), one of the commonest forms of inherited demyelinating neuropathy, results from GJB1 gene mutations causing loss of function of the gap junction protein connexin32 (Cx32). The aim of this study was to examine whether delayed gene replacement therapy after the onset of peripheral neuropathy can provide a therapeutic benefit in the Gjb1-null/Cx32 knockout model of CMT1X. After delivery of the LV-Mpz.GJB1 lentiviral vector by a single lumbar intrathecal injection into 6-month-old Gjb1-null mice, we confirmed expression of Cx32 in lumbar roots and sciatic nerves correctly localized at the paranodal myelin areas. Gjb1-null mice treated with LV-Mpz.GJB1 compared with LV-Mpz.Egfp (mock) vector at the age of 6 months showed improved motor performance at 8 and 10 months. Furthermore, treated mice showed increased sciatic nerve conduction velocities, improvement of myelination and reduced inflammation in lumbar roots and peripheral nerves at 10 months of age, along with enhanced quadriceps muscle innervation. Plasma neurofilament light (NEFL) levels, a clinically relevant biomarker, were also ameliorated in fully treated mice. Intrathecal gene delivery after the onset of peripheral neuropathy offers a significant therapeutic benefit in this disease model, providing a proof of principle for treating patients with CMT1X at different ages.

Gene therapy to promote regeneration in Charcot-Marie-Tooth disease

The molecular pathogenesis underlying Charcot-Marie-Tooth (CMT) neuropathy subtypes is becoming increasingly variable and identification of common approaches for treatment, independently of the disease causing gene defect, is therefore much desirable. Gene therapy approach from the clinical translational view point is particularly challenging for the most common "demyelinating" CMT1 subtypes, caused by primary Schwann cell genetic defects. Studies have shown that impaired regenerative capacity of distal axons is major contributing factor to distal axonal loss in primary Schwann cell genetic defects and neurotrophin 3 (NT-3) improves impaired regeneration in CMT1 mouse models. This review surveys the evidence supporting the rationale for AAV1.NT-3 surrogate gene therapy to improve nerve regeneration in CMT1A. The translational process, from proof of principal studies to the design of the phase I/IIa trial evaluating scAAV1.tMCK.NTF3 gene therapy for treatment of CMT1A is summarized.

Efficacy of exogenous pyruvate in TremblerJ mouse model of Charcot-Marie-Tooth neuropathy

INTRODUCTION

Classic Charcot-Marie-Tooth (CMT) neuropathies including those with Schwann cell genetic defects exhibit a length-dependent process affecting the distal axon. Energy deprivation in the distal axon has been the proposed mechanism accounting for length-dependent distal axonal degeneration. We hypothesized that pyruvate, an intermediate glycolytic product, could restore nerve function, supplying lost energy to the distal axon.

METHODS

To test this possibility, we supplied pyruvate to the drinking water of the Trembler-J (TrJ ) mouse and assessed efficacy based on histology, electrophysiology, and functional outcomes. Pyruvate outcomes were compared with untreated TrJ controls alone or adeno-associated virus mediated NT-3 gene therapy (AAV1.NT-3)/pyruvate combinatorial approach.

RESULTS

Pyruvate supplementation resulted increased myelinated fiber (MF) densities and myelin thickness in sciatic nerves. Combining pyruvate with proven efficacy from AAV1.tMCK.NT-3 gene therapy provided additional benefits showing improved compound muscle action potential amplitudes and nerve conduction velocities compared to pyruvate alone cohort. The end point motor performance of both the pyruvate and the combinatorial therapy cohorts was better than untreated TrJ controls. In a unilateral sciatic nerve crush paradigm, pyruvate supplementation improved myelin-based outcomes in both regenerating and the contralateral uncrushed nerves.

CONCLUSIONS

This proof of principle study demonstrates that exogenous pyruvate alone or as adjunct therapy in TrJ may have clinical implications and is a candidate therapy for CMT neuropathies without known treatment.

GJB1-associated X-linked Charcot-Marie-Tooth disease, a disorder affecting the central and peripheral nervous systems

Charcot-Marie-Tooth disease (CMT) is a group of inherited diseases characterized by exclusive or predominant involvement of the peripheral nervous system. Mutations in GJB1, the gene encoding Connexin 32 (Cx32), a gap-junction channel forming protein, cause the most common X-linked form of CMT, CMT1X. Cx32 is expressed in Schwann cells and oligodendrocytes, the myelinating glia of the peripheral and central nervous systems, respectively. Thus, patients with CMT1X have both central and peripheral nervous system manifestations. Study of the genetics of CMT1X and the phenotypes of patients with this disorder suggest that the peripheral manifestations of CMT1X are likely to be due to loss of function, while in the CNS gain of function may contribute. Mice with targeted ablation of Gjb1 develop a peripheral neuropathy similar to that seen in patients with CMT1X, supporting loss of function as a mechanism for the peripheral manifestations of this disorder. Possible roles for Cx32 include the establishment of a reflexive gap junction pathway in the peripheral and central nervous system and of a panglial syncitium in the central nervous system.

Gene expression profiling studies in regenerating nerves in a mouse model for CMT1X: uninjured Cx32-knockout peripheral nerves display expression profile of injured wild type nerves

X-linked Charcot-Marie-Tooth disease (CMT1X) is an inherited peripheral neuropathy caused by mutations in GJB1, the human gene for Connexin32 (Cx32). This present study uses Ilumina Ref8-v2 BeadArray to examine the expression profiles of injured and uninjured sciatic nerves at 5, 7, and 14 days post-crush injury (dpi) from Wild Type (WT) and Cx32-knockout (Cx32KO) mice to identify the genes and signaling pathways that are dysregulated in the absence of Schwann cell Cx32. Given the assumption that loss of Schwann cell Cx32 disrupts the regeneration and maintenance of myelinated nerve leading to a demyelinating neuropathy in CMT1X, we initially hypothesized that nerve crush injury would result in significant increases in differential gene expression in Cx32KO mice relative to WT nerves. However, microarray analysis revealed a striking collapse in the number of differentially expressed genes at 5 and 7 dpi in Cx32KO nerves relative to WT, while uninjured and 14 dpi time points showed large numbers of differentially regulated genes. Further comparisons within each genotype showed limited changes in Cx32KO gene expression following crush injury when compared to uninjured Cx32KO nerves. By contrast, WT nerves exhibited robust changes in gene expression at 5 and 7 dpi with no significant differences in gene expression by 14dpi relative to uninjured WT nerve samples. Taken together, these data suggest that the gene expression profile in uninjured Cx32KO sciatic nerve strongly resembles that of a WT nerve following injury and that loss of Schwann cell Cx32 leads to a basal state of gene expression similar to that of an injured WT nerve. These findings support a role for Cx32 in non-myelinating and regenerating populations of Schwann cells in normal axonal maintenance in re-myelination, and regeneration of peripheral nerve following injury. Disruption of Schwann cell-axonal communication in CMT1X may cause dysregulation of signaling pathways that are essential for the maintenance of intact myelinated peripheral nerves and to establish the necessary conditions for successful regeneration and remyelination following nerve injury.

AAV1.NT-3 gene therapy for charcot-marie-tooth neuropathy

Charcot–Marie–Tooth (CMT) neuropathies represent a heterogeneous group of peripheral nerve disorders affecting 1 in 2,500 persons. One variant, CMT1A, is a primary Schwann cell (SC) disorder, and represents the single most common variant. In previous studies, we showed that neurotrophin-3 (NT-3) improved the trembler^J (Tr^J) mouse and also showed efficacy in CMT1A patients. Long-term treatment with NT-3 was not possible related to its short half-life and lack of availability. This led to considerations of NT-3 gene therapy via adenoassociated virus (AAV) delivery to muscle, acting as secretory organ for widespread distribution of this neurotrophic agent. In the Tr^J model of demyelinating CMT, rAAV1.NT-3 therapy resulted in measurable NT-3 secretion levels in blood sufficient to provide improvement in motor function, histopathology, and electrophysiology of peripheral nerves. Furthermore, we showed that the compound muscle action potential amplitude can be used as surrogate for functional improvement and established the therapeutic dose and a preferential muscle-specific promoter to achieve sustained NT-3 levels. These studies of intramuscular (i.m.) delivery of rAAV1.NT-3 serve as a template for future CMT1A clinical trials with a potential to extend treatment to other nerve diseases with impaired nerve regeneration.

How do mutations in GJB1 cause X-linked Charcot-Marie-Tooth disease?

The X-linked form of Charcot-Marie-Tooth disease (CMT1X) is the second most common form of hereditary motor and sensory neuropathy. The clinical phenotype is characterized by progressive weakness, atrophy, and sensory abnormalities that are most pronounced in the distal extremities. Some patients have CNS manifestations. Affected males have moderate to severe symptoms, whereas heterozygous females are usually less affected. Neurophysiology shows intermediate slowing of conduction and length-dependent axonal loss. Nerve biopsies show more prominent axonal degeneration than de/remyelination. Mutations in GJB1, the gene that encodes the gap junction (GJ) protein connexin32 (Cx32) cause CMT1X; more than 400 different mutations have been described. Many Cx32 mutants fail to form functional GJs, or form GJs with abnormal biophysical properties. Schwann cells and oligodendrocytes express Cx32, and the GJs formed by Cx32 play an important role in the homeostasis of myelinated axons. Animal models of CMT1X demonstrate that loss of Cx32 in myelinating Schwann cells causes a demyelinating neuropathy. Effective therapies remain to be developed. This article is part of a Special Issue entitled Electrical Synapses.

Water translocation from the axial cylinder to myelin sheath structures of the nerve fiber

We studied isolated myelinated nerve fibers from frog sciatic nerve surviving in Ringer solution or in water-free liquid perfluorodecalin immiscible with water or mineral oil. Swelling of incisures and perikaryon, loosening of myelin in the node, and formation of the axial cylinder varicosities were found in the fibers surviving in Ringer solution after 5-7 h. The same process, swelling of Schmidt-Lantermann myelin incisures, Schwann cell perikaryon, and loosening of myelin lamellae in the Ranvier nodes was found in water-free perfluorodecalin medium. However, swelling of the perikaryon and incisures spread along the axial cylinder and the reaction of the fiber developed in perfluorodecalin much later and unfolded slower than in the control. These changes developed much sooner and progressed much more rapidly than in perfluorodecalin in fibers surviving in mineral oil. Swelling of the myelin sheath structures in water-free medium indicated an uncommon new form of the neuron-glia relationships: water translocation from the axial cylinder to Schwann cell under unfavorable conditions.

Reliability of the CMT neuropathy score (second version) in Charcot-Marie-Tooth disease

The Charcot-Marie-Tooth neuropathy score (CMTNS) is a reliable and valid composite score comprising symptoms, signs, and neurophysiological tests, which has been used in natural history studies of CMT1A and CMT1X and as an outcome measure in treatment trials of CMT1A. Following an international workshop on outcome measures in Charcot-Marie-Tooth disease (CMT), the CMTNS was modified to attempt to reduce floor and ceiling effects and to standardize patient assessment, aiming to improve its sensitivity for detecting change over time and the effect of an intervention. After agreeing on the modifications made to the CMTNS (CMTNS2), three examiners evaluated 16 patients to determine inter-rater reliability; one examiner evaluated 18 patients twice within 8 weeks to determine intra-rater reliability. Three examiners evaluated 63 patients using the CMTNS and the CMTNS2 to determine how the modifications altered scoring. For inter- and intra-rater reliability, intra-class correlation coefficients (ICCs) were ≥0.96 for the CMT symptom score and the CMT examination score. There were small but significant differences in some of the individual components of the CMTNS compared with the CMTNS2, mainly in the components that had been modified the most. A longitudinal study is in progress to determine whether the CMTNS2 is more sensitive than the CMTNS for detecting change over time.

Foot deformity in charcot marie tooth disease according to disease severity

Objective

To investigate the characteristics of foot deformities in patients with Charcot-Marie-Tooth (CMT) disease compared with normal persons according to severity of disease.

Method

Sixty-two patients with CMT disease were recruited for this study. The normal control group was composed of 28 healthy people without any foot deformity. Patients were classified into a mild group and a moderate group according to the CMT neuropathy score. Ten typical radiological angles representing foot deformities such as pes equinus and pes varus were measured. The CMT group angles were compared with those of the normal control group, and those of the mild group were also compared with those of the moderate group.

Results

The lateral (Lat.) talo-first metatarsal angle, anteroposterior talo-first metatarsal angle, Lat. calcaneal-first metatarsal angle, Lat. naviocuboid overlap, Lat. calcaneal pitch, Lat. tibiocalcaneal angle, and Lat. talocalcaneal angle in the CMT group showed a significant difference compared to the normal control group (p<0.05). These findings revealed CMT patients have pes cavus, forefoot adduction, midfoot supination and pes varus deformity. Compared to the mild group, the moderate group significantly showed an increased Lat. calcaneal pitch and decreased Lat. calcaneal-first metatarsal angle, Lat. tibiocalcaneal angle, Lat. talocalcaneal angle, and Lat. talo-first metatarsal angle (p<0.05). These findings revealed that the pes cavus deformity of CMT patients tend to be worse with disease severity.

Conclusion

The characteristic equinovarus foot deformity patterns in CMT patients were revealed and these deformities tended to be worse with disease severity. Radiographic measures may be useful for the investigation of foot deformities in CMT patients.

Arabidopsis thaliana, a plant model organism for the neuronal microtubule cytoskeleton?

The microtubule cytoskeleton is an important component of both neuronal cells and plant cells. While there are large differences in the function of microtubules between the two groups of organisms, for example plants coordinate the ordered deposition of cellulose through the microtubule cytoskeleton, there are also some notable similarities. It is suggested that Arabidopsis thaliana, with its superior availability of knockout lines, may be a suitable model organism for some aspects of the neuronal microtubule cytoskeleton. Some cellular processes that involve the neuronal microtubule cytoskeleton including neurotransmitter signalling and neurotrophic support may have homologous processes in plant cells. A number of microtubule-associated proteins (MAPs) are conserved, including katanin, EB1, CLASP, spastin, gephyrin, CRIPT, Atlastin/RHD3, and ELP3. As a demonstration of the usefulness of a plant model system for neuronal biology, an analysis of plant tubulin-binding proteins was used to show that Charcot-Marie-Tooth disease type 2D and spinal muscular atrophy may be due to microtubule dysfunction and suggest that indeed the plant microtubule cytoskeleton may be particularly similar to that of motor neurons as both are heavily reliant upon motor proteins.

Axonal pathology precedes demyelination in a mouse model of X-linked demyelinating/type I Charcot-Marie Tooth neuropathy

The X-linked demyelinating/type I Charcot-Marie-Tooth neuropathy (CMT1X) is an inherited peripheral neuropathy caused by mutations in GJB1, the gene that encodes the gap junction protein connexin32. Connexin32 is expressed by myelinating Schwann cells and forms gap junctions in noncompact myelin areas, but axonal involvement is more prominent in X-linked compared with other forms of demyelinating Charcot-Marie-Tooth disease. To clarify the cellular and molecular mechanisms of axonal pathology in CMT1X, we studied Gjb1-null mice at early stages (i.e. 2-4 months old) of the neuropathy, when there is minimal or no demyelination. The diameters of large myelinated axons were progressively reduced in Gjb1-null mice compared with those in wild-type littermates. Furthermore, neurofilaments were relatively more dephosphorylated and more densely packed starting at 2 months of age. Increased expression of β-amyloid precursor protein, a marker of axonal damage, was also detected in Gjb1-null nerves. Finally, fast axonal transport, assayed by sciatic nerve ligation experiments, was slower in distal axons of Gjb1-null versus wild-type animals with reduced accumulation of synaptic vesicle-associated proteins. These findings demonstrate that axonal abnormalities including impaired cytoskeletal organization and defects in axonal transport precede demyelination in this mouse model of CMT1X.

CMT1X phenotypes represent loss of GJB1 gene function

OBJECTIVE

To investigate possible genotype-phenotype correlations and to evaluate the natural history of patients with Charcot-Marie-Tooth disease type 1X (CMT1X).

BACKGROUND

CMT1X is caused by over 260 distinct mutations in the gap junction beta 1 (GJB1) gene, located on the X chromosome, which encodes the gap junction protein connexin 32 (Cx32). The natural history of CMT1X is poorly understood, and it remains unknown whether particular mutations cause more severe neuropathies through abnormal gain-of-function mechanisms.

METHODS

We evaluated 73 male patients with CMT1X, who each have 1 of 28 different GJB1 mutations predicted to affect nearly all domains of Cx32. Disability was evaluated quantitatively by the CMT Neuropathy Score (CMTNS) as well as by the CMT Symptom Score (CMTSS) and the CMT Examination Score (CMTES), which are both based on the CMTNS. Patients were also evaluated by neurophysiology.

RESULTS

In all patients, disability increased with age, and the degree of disability was comparable with that observed in patients with a documented GJB1 deletion. Disability correlated with a loss of motor units as assessed by motor unit number estimates.

CONCLUSIONS

Taken together, these data suggest that most GJB1 mutations cause neuropathy by a loss of normal connexin 32 function. Therefore, treatment of male patients with Charcot-Marie-Tooth disease type 1X may prove amenable to gene replacement strategies.

The Inherited Neuropathies

Neuropathy is one of the most common referrals to neurologic clinics. Patients often undergo extensive testing for acquired etiologies; inherited causes are common. Increasingly, genetic causes are becoming known and commercial testing available. The rate of recent discovery has been rapid and relates to the extent of single gene disorders of nerve, the ease of peripheral nervous system functional examination, and readily accessible pathologic tissue. Foremost in the rate of recent discoveries is the work and tools of the human genome project. the rapidity of the ongoing discovery requires clinicians to be familiar with molecular biologic discoveries and consider wisely which testing should be performed.

Neurotrophins and peripheral neuropathies

Peripheral neuropathy is a common disorder seen in general neurology and neuromuscular specialty clinics. Treatment options directed at the underlying cause can only be offered in a handful of conditions, such as those with possible autoimmune etiology. The remainder fall into the idiopathic or genetic category with no known treatment. This review surveys the evidence supporting the rationale for the therapeutic use of neurotrophins and other neurotrophic factors in these disorders in relationship to the underlying pathobiological process. Previous clinical trials are assessed, and increasingly better understood and appreciated therapeutic potential of neurotrophins is emphasized.

The neurotrophin receptor p75NTR in Schwann cells is implicated in remyelination and motor recovery after peripheral nerve injury

The function of the p75(NTR) neurotrophin receptor (p75(NTR)) in nervous system regeneration is still controversial. Part of that controversy may be due to the fact that p75(NTR) is expressed by both neuronal and glial cell types and may have very distinct and even contradictory roles in each population. In this study, to elucidate the in vivo function of p75(NTR) in Schwann cells during remyelination after peripheral nerve injury, we established a new animal model for p75(NTR)-deficient Schwann cell transplantation in nude mice. We performed quantitative assessments of the functional, histological, and electrophysiological recovery after sciatic nerve injury, and compared them with those of the p75(NTR)(+/+) Schwann cell transplanted animals. At 7-10 weeks after injury, the motor recovery in the p75(NTR)(-/-) Schwann cell transplanted animals was significantly impaired compared with that in the p75(NTR)(+/+) Schwann cell transplanted animals. The lower number of the retrogradely labeled motoneurons and the hypomyelination in the p75(NTR)(-/-) Schwann cell transplanted animals were evident at 6 and 10 weeks after injury. At 10 weeks after injury, the radial growth in the axon caliber was also impaired in the p75(NTR)(-/-) Schwann cell transplanted animals. Measurement of the amount of myelin proteins and the nerve conduction velocity at 10 weeks after injury reflected these results. In summary, the p75(NTR) expression in Schwann cells is important for remyelination process, and the motor recovery after injury is impaired due to impaired axonal growth, remyelination, and radial growth in the axon calibers.

The myelinated axon is dependent on the myelinating cell for support and maintenance: molecules involved

The myelin-forming cells, oligodendrocytes and Schwann cells, extend processes that spirally wrap axons and provide the insulation that allows rapid saltatory conduction. Recent data suggest a further role for the myelin-forming cells in axonal support and maintenance. This Mini-Review summarises some of the data that support this view and highlights the molecules involved.

Pathogenesis of X-linked Charcot-Marie-Tooth disease: differential effects of two mutations in connexin 32

X-linked Charcot-Marie-Tooth disease is an inherited peripheral neuropathy arising in patients with mutations in the gene encoding connexin 32 (Cx32). Cx32 is expressed at the paranodes and Schmidt-Lantermann incisures of myelinating Schwann cells in which it is believed to form a reflexive pathway between the abaxonal and adaxonal cytoplasmic domains. Patients with the Val181Ala (V181A) mutation have a severe peripheral neuropathy. Experiments using a nude mouse xenograft system show that Schwann cells expressing only this mutant form of Cx32 are profoundly impaired in their ability to support the earliest stages of regeneration of myelinated fibers. Coupling between paired Xenopus oocytes expressing V181A is reduced compared with the coupling between oocytes expressing wild-type human Cx32 (32WT), and protein levels assayed by Western blot are substantially lower. Immunocytochemisty shows that Neuro2a cells expressing the V181A mutant have very few gap junction plaques compared with cells expressing 32WT; Cx32 protein levels are lower in these cells than in those expressing 32WT. Because failure of normal regeneration is evident before formation of myelin, loss of function of Cx32 may impact on the function of precursors of the myelinating Schwann cell before the formation of the hypothesized reflexive pathway. The Glu102Gly (E102G) mutation leads to a milder phenotype. Early regeneration is normal in grafts with Schwann cells expressing the E102G mutant. The only abnormality detected in the behavior of its channel is increased sensitivity to acidification-induced closure, a property that may lead to reduced gap junction coupling during periods of metabolic stress. This restricted functional abnormality may explain the relatively mild phenotype seen in the xenograft model and in E102G patients.

Evidence for impaired axonal regeneration in PMP22 duplication: studies in nerve xenografts

Whether axonal regeneration in Charcot-Marie-Tooth (CMT) neuropathies is impaired has not been addressed in detail. Our studies in nude mice harboring xenografts from patients with different primary Schwann cell (SC) genetic defects suggested an intimate association between the onset of myelination and impairment in the growth capacity of nude mice axons engulfed by the mutant SCs. To assess the effects of peripheral myelin protein 22 (PMP22) gene duplication on the regeneration process, we conducted morphometric studies to generate temporal growth profiles of myelinated axons within the xenografts obtained from CMT1A patients and from healthy controls. Axon size distribution histograms in controls at different time intervals revealed that size differentiation of myelinated fibers within the grafts is established as early as 2 weeks, and that the temporal pattern of myelination of different sized axons has striking similarities to myelination during development. In PMP22 duplication grafts, the onset of myelination is delayed and the regeneration capacity of all fiber sizes is impaired. This defect, however, is most pronounced for the large diameter axons. In addition, significant large fiber loss occurred after 12 weeks with a concomitant new cycle of regeneration of small size axons. These studies show that the PMP22 duplication in SCs have profound effects on the regeneration process, which might be a contributing factor to preferential distal axonal loss.

Phenotypic expression of a Pro 87 to Leu mutation in the connexin 32 gene in a large Swiss family with Charcot-Marie-Tooth neuropathy

BACKGROUND

The clinical manifestations of CMTX have been well described but the natural history has not yet been studied in detail. We studied phenotype variability in a family with a Pro 87 to Leu mutation of the connexin 32 (Cx32) gene.

METHODS

A total of 32 family members, of which 19 patients were affected, underwent clinical, electrophysiological, and genetic studies.

RESULTS

Onset was in the second decade. Clinical features were similar in both sexes when quantitative scores were compared, but more males had a steppage gait and skeletal deformities. All adult patients had a predominant involvement of the thenar muscles. The median values of nerve conduction velocities (NCVs) were not statistically different in men and in women. The correlation coefficients were low between motor NCVs within the same extremities, indicating nonuniform slowing between nerves, the ulnar nerve being the least affected. When disability was rated, a strong correlation was seen in male patients between severity of motor axonal loss and duration of the disease. The main pathological features were axonal loss, clusters of regenerating fibers and paranodal demyelination, the hallmark of a Schwann cell pathology.

CONCLUSIONS

Our data support the hypothesis that clinical disability in CMTX is caused by loss of large myelinated axons in men. Furthermore, this study shows that the nerves are not uniformly affected in terms of axonal loss. Preventing axonal degeneration and promoting axonal regeneration in the most affected nerves might be the best therapeutic approaches to ameliorate disability in CMTX.

Mouse models of Charcot-Marie-Tooth disease

A common peripheral neuropathy, Charcot-Marie-Tooth disease, progressively develops with distal muscle atrophy. Several genes expressed in Schwann cells and neurons have been identified to be responsible for this hereditary disease, and used in generating transgenic and knockout mice. Such mice are good disease models for cell biological and therapeutic studies.

Secondary axon atrophy and neurological dysfunction in demyelinating neuropathies

PURPOSE OF THE REVIEW

Secondary axonal atrophy is common in most if not all demyelinating neuropathies and is likely responsible for the majority of clinical symptoms. We review clinical, electrophysiological and morphological evidence for secondary axonal atrophy in demyelinating neuropathies and summarize recent hypotheses on possible pathomechanisms.

RECENT FINDINGS

Elucidation of genetic defects responsible for hereditary demyelinating neuropathies and insights into axon-Schwann cell interactions have allowed longitudinal studies of genetically defined demyelinating neuropathies and research into the pathomechanism of secondary axonal atrophy.

SUMMARY

There is ample clinical electrophysiological and electropathological evidence that secondary axonal atrophy is found in hereditary and demyelinating neuropathies. Recognizing secondary axonal atrophy as a main cause for clinical disability in demyelinating neuropathies is important for the clinician and may reveal a therapeutic target common to all different forms of demyelinating neuropathies.

Connexin29 expression, immunocytochemistry and freeze-fracture replica immunogold labelling (FRIL) in sciatic nerve

The recently discovered connexin29 (Cx29) was reported to be present in the central and peripheral nervous systems (CNS and PNS), and its mRNA was found in particular abundance in peripheral nerve. The expression and localization of Cx29 protein in sciatic nerve were investigated using an antibody against Cx29. The antibody recognized Cx29 in HeLa cells transfected with Cx29 cDNA, while nontransfected HeLa cells were devoid of Cx29. Immunoblotting of sciatic nerve homogenate revealed monomeric and possibly higher molecular weight forms of Cx29. These were distinguished from connexin32 (Cx32), which also is expressed in peripheral nerve. Double immunofluorescence labelling for Cx29 and Cx32 revealed only partial colocalization of the two connexins, with codistribution at intermittent, conical-shaped striations along nerve fibers. By freeze-fracture replica immunogold labelling (FRIL), Cx32 was found in gap junctions in the outermost layers of myelin, whereas Cx29-immunogold labelling was found only in the innermost layer of myelin in close association with hexagonally arranged intramembrane particle (IMP) 'rosettes' and gap junction-like clusters of IMPs. Although both Cx32 and Cx29 were detected in myelin of normal mice, only Cx29 was present in Schwann cell membranes in Cx32 knockout mice. The results confirm that Cx29 is a second connexin expressed in Schwann cells of sciatic nerve. In addition, Cx29 is present in distinctive IMP arrays in the inner most layer of myelin, adjacent to internodal axonal plasma membranes, where this connexin may have previously unrecognized functions.

Anti-myelin-associated glycoprotein antibodies alter neurofilament spacing

Axon calibre is crucial to efficient impulse transmission in the peripheral nervous system. Neurofilament numbers determine gross axonal diameter, but intra-axonal distribution depends on the phosphorylation status of neurofilament sidearms. Myelin-associated glycoprotein (MAG) has been implicated in the signalling cascade controlling neurofilament phosphorylation and hence in the control of axon calibre. In an electron microscopic morphometric study we measured nearest neighbour neurofilament distances (NNND) in the axons of sural nerves from patients with anti-MAG paraproteinaemic neuropathies and compared these with normal human sural nerves and those from patients with Guillain-Barré syndrome or chronic inflammatory demyelinating polyradiculoneuropathy. Axon calibre was similar in all groups. In normal human sural nerves, axonal NNND was correlated with axonal diameter (r = 0.56). In diseased axons this correlation did not exist. The NNND was significantly reduced in demyelinated axons (30.5+/-2.2 nm) and those with widely spaced myelin (28.9+/-1.3 nm) from patients with anti-MAG antibodies compared with normal axons from normal patients (39.8+/- 3.2 nm) or those with demyelinating neuropathy (35.8+/-4.6 nm). This reinforces the hypothesis that MAG is involved in the control of neurofilament spacing through sidearm phosphorylation and demonstrates a MAG-mediated pathogenic effect of the anti-MAG antibody in peripheral nerves.

Progress in clinical neurosciences: Charcot-Marie-Tooth disease and related inherited peripheral neuropathies

The classification of Charcot-Marie-Tooth disease and related hereditary motor and sensory neuropathies has evolved to incorporate clinical, electrophysiological and burgeoning molecular genetic information that characterize the many disorders. For several inherited neuropathies, the gene product abnormality is known and for others, candidate genes have been identified. Genetic testing can pinpoint a specific inherited neuropathy for many patients. However, clinical and electrophysiological assessments continue to be essential tools for diagnosis and management of this disease group. This article reviews clinical, electrophysiological, pathological and molecular aspects of hereditary motor and sensory neuropathies.

The effect of myelinating Schwann cells on axons

Myelinating Schwann cells control the number of neurofilaments and elevate the phosphorylation state of neurofilaments in the axon, eventually leading to the typical large axon caliber. Conversely, absence of myelin leads to lower amounts of neurofilaments, reduced phosphorylation levels, and smaller axon diameters. In addition, myelinating Schwann cells mediate the spacing of Na(+) channel clusters during development of the node of Ranvier. When axons are associated with mutant Schwann cells in inherited neuropathies, their calibers are reduced and their neurofilaments are less phosphorylated and more closely spaced. Also, axonal transport is reduced and axons degenerate at the distal ends of long nerves. Myelin-associated glycoprotein may mediate some aspects of Schwann cell-axon communication, but much remains to be learned about the molecular bases of Schwann cell-axon communication.

Electrophysiological features of inherited demyelinating neuropathies: A reappraisal in the era of molecular diagnosis

The observation that inherited demyelinating neuropathies have uniform conduction slowing and that acquired disorders have nonuniform or multifocal slowing was made prior to the identification of mutations in myelin-specific genes which cause many of the inherited disorders involving peripheral nerve myelin. It is now clear that the electrophysiological aspects of these disorders are more complex than previously realized. Specifically, certain mutations appear to induce nonuniform slowing of conduction which resemble the findings in acquired demyelinating neuropathies. It is clinically important to recognize the different electrodiagnostic patterns of the various inherited demyelinating neuropathies. In addition, an understanding of the relationship between mutations of specific genes and their associated neurophysiological findings is likely to facilitate understanding of the role of these myelin proteins in peripheral nerve function and of how abnormalities in myelin proteins lead to neuropathy. We therefore review the current information on the electrophysiological features of the inherited demyelinating neuropathies in hopes of clarifying their electrodiagnostic features and to shed light on the physiological consequences of the different genetic mutations.

Mutations in connexin 32: the molecular and biophysical bases for the X-linked form of Charcot-Marie-Tooth disease

The connexins are a family of homologous integral membrane proteins that form channels that provide a low resistance pathway for the transmission of electrical signals and the diffusion of small ions and non-electrolytes between coupled cells. Individuals carrying mutations in the gene encoding connexin 32 (Cx32), a gap junction protein expressed in the paranodal loops and Schmidt-Lantermann incisures of myelinating Schwann cells, develop a peripheral neuropathy - the X-linked form of Charcot-Marie-Tooth disease (CMTX). Over 160 different mutations in Cx32 associated with CMTX have been identified. Some mutations will lead to complete loss of function with no possibility of expression of functional channels. Some mutations in Cx32 lead to the abnormal accumulation of Cx32 proteins in the cytoplasm, particularly in the Golgi apparatus; CMTX may arise due to incorrect trafficking of Cx32 or to interference with trafficking of other proteins. On the other hand, many mutant forms of Cx32 can form functional channels. Some functional mutants have conductance voltage relationships that are disrupted to a degree which would lead to a substantial reduction in the available gap junction mediated communication pathway. Others have essentially normal steady-state g-V relations. In one of these cases (Ser26Leu), the only change introduced by the mutation is a reduction in the pore diameter from 7 A for the wild-type channel to less than 3 A for Ser26Leu. This reduction in pore diameter may restrict the passage of important signaling molecules. These findings suggest that in some, if not all cases of CMTX, loss of function of normal Cx32 is sufficient to cause CMTX.

Connexin channels in Schwann cells and the development of the X-linked form of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease comprises a group of genetically heterogenous disorders of the peripheral nervous system. The X-linked form of Charcot-Marie-Tooth (CMTX) is associated with mutations in the gene encoding the gap junction protein connexin32 (Cx32), which is expressed in Schwann cells. Immunocytochemical evidence suggests that Cx32 is localized to the incisures of Schmidt-Lanterman and the paranodes of myelinating Schwann cells, where it appears to form reflexive gap junctions. It is currently thought that this cytoplasmic continuity provides a much shorter diffusion pathway for the transport of ions, metabolites and second messenger molecules through intracellular channels between the adaxonal and peri-nuclear regions of Schwann cells, across the myelin sheath. This review summarizes our current understanding of the role of connexins in Schwann cells and focuses on the lessons for channel function and disease pathophysiology derived from the functional analysis of Cx32 mutations. One of the most intriguing aspects emerging from this work is that several mutations retain functional competence, although the mutated channels exhibit altered gating properties. This suggests that partial and/or selective disruption of the radial communication pathway formed by Cx32 is sufficient to cause a functional deficit and lead to the development of CMTX. The next challenge will be to define, at the molecular level, the sequence of events involved in the disease process. The presence of a group of functional mutations should help understand the cellular basis of CMTX, by allowing the identification of the specific molecules that need to be exchanged through Cx32 channels, but are excluded from the mutated ones.

Demyelinating X-linked Charcot-Marie-Tooth disease: unusual electrophysiological findings

X-linked Charcot-Marie-Tooth disease (CMT-X) is caused by mutations of connexin-32 (Cx-32), which encodes a gap-junction protein. Whether the neuropathy is primarily demyelinative or axonal remains to be established. We report findings of prominent demyelination in a 71-year-old woman with late-onset disease. Electrophysiological studies revealed a nonuniform slowing of motor conduction velocities and dispersion of compound action potentials indicative of a demyelinating process which was confirmed by nerve biopsy. Such electrophysiological features are unusual in hereditary neuropathies and are more commonly found with acquired chronic demyelinating neuropathies. A systematic search confirmed the molecular genomic diagnosis of CMT-X, illustrating the value of such tests in sporadic cases. Severity of clinical symptoms and signs may vary with age and sex of the patient. The pathology of CMT-X in other reported cases has been variably interpreted as axonal, demyelinating, or showing both features. Our observations emphasize the demyelinative nature.

Heterophilic binding of L1 on unmyelinated sensory axons mediates Schwann cell adhesion and is required for axonal survival

This study investigated the function of the adhesion molecule L1 in unmyelinated fibers of the peripheral nervous system (PNS) by analysis of L1- deficient mice. We demonstrate that L1 is present on axons and Schwann cells of sensory unmyelinated fibers, but only on Schwann cells of sympathetic unmyelinated fibers. In L1-deficient sensory nerves, Schwann cells formed but failed to retain normal axonal ensheathment. L1-deficient mice had reduced sensory function and loss of unmyelinated axons, while sympathetic unmyelinated axons appeared normal. In nerve transplant studies, loss of axonal-L1, but not Schwann cell-L1, reproduced the L1-deficient phenotype. These data establish that heterophilic axonal-L1 interactions mediate adhesion between unmyelinated sensory axons and Schwann cells, stabilize the polarization of Schwann cell surface membranes, and mediate a trophic effect that assures axonal survival.

X-linked dominant Charcot-Marie-Tooth neuropathy: clinical, electrophysiological, and morphological phenotype in four families with different connexin32 mutations(1)

The sensorimotor neuropathy of the Charcot-Marie-Tooth type (CMT) is the most common hereditary disorder of the peripheral nervous system. The X-linked dominant form of CMT (CMTX) is associated with mutations in the gene for the gap junction protein connexin32. We examined four CMTX pedigrees two of which had potentially novel mutations in the only coding exon of connexin32. One previously unreported missense mutation, Ala39Val, was found in a family displaying a CMT phenotype with additional upper limb postural tremor reminiscent of a Roussy-Lévy syndrome. A novel single base insertion, 679insT, is among the first mutations found in the fourth transmembrane domain of connexin32. Frameshift and premature stop of translation are supposed to result in a non-functional carboxy-terminus. Two further families had the known missense mutations Arg15Trp and Arg22Gln. Several female carriers were found normal on clinical presentation, however, the genotype was paralleled by decreased nerve conduction velocities (NCV) and slowed central conduction of brain stem auditory evoked responses (BAER). Median motor NCVs showed mild (in women) to intermediate (in males) reduction, indicating a peripheral neuropathy with a predominating axonal component. Nerve biopsy findings were consistent with the electrophysiological data showing a marked loss of large myelinated fibres and clusters of regenerating axons. Electron microscopy revealed various alterations of the axoglial attachment zone. This suggests defective axon-Schwann cell interactions which may induce the axonopathy in CMTX.

Loss of distal axons and sensory Merkel cells and features indicative of muscle denervation in hindlimbs of P0-deficient mice

Mice lacking the major Schwann cell myelin component P0 show a severe dysmyelination with pathological features reminiscent of the Déjérine-Sottas syndrome in humans. Previous morphological and electrophysiological studies on these mice did not only demonstrate a compromised myelination and myelin maintenance, but were suggestive of an impairment of axons as well. Here, we studied the axonal pathology in P0-deficient mice by quantitative electron microscopy. In addition, we investigated epidermal receptor end organs by immunocytochemistry and muscle pathology by histochemistry. In proximal sections of facial and femoral nerves, axon calibers were significantly reduced, whereas the number of myelin-competent axons was not diminished in 5- and 17-month-old P0-deficient mice. However, in distal branches of the femoral and sciatic nerve (digital nerves innervating the skin of the first toe) the numbers of myelin-competent axons were reduced by 70% in 6-month-old P0-deficient mice. Immunolabeling of foot pads revealed a corresponding loss of Merkel cells by 75%, suggesting that survival of these cells is dependent on the presence or maintenance of their innervating myelinated axons. In addition, quadriceps and gastrocnemius muscles showed pathological features indicative of denervation and axonal sprouting. These findings demonstrate that loss of an important myelin component can initiate degenerative mechanisms not only in the Schwann cell but also in the distal portions of myelinated axons, leading to the degeneration of specialized receptor end organs and impairment of muscle innervation.

Effects of PMP22 duplication and deletions on the axonal cytoskeleton

Axonal loss in Charcot-Marie-Tooth type 1A (CMT1A) is an important feature correlated with the functional disability in affected individuals. It is not known, however, how the most common genetic defect in Schwann cells (PMP22 duplication) causes the CMT1A phenotype and results in axonal loss. In this study, sural nerve segments from individuals with PMP22 duplications or deletions, causing the reciprocal disorder hereditary neuropathy with pressure palsies (HNPP), were grafted into the cut ends of the sciatic nerve of nude mice. The xenografts and host segments were studied at 2, 4, 6, 8, 12, and 16 weeks after grafting and compared with the controls from healthy volunteers. Within the CMT1A xenografts, the nude mice axons in the proximal part of the graft showed a significant increase in axonal area with an increase in the neurofilament and membranous organelle (mitochondria) density, compared with distal graft and distal host segments. A preferential distal axonal loss, associated with a perpetual axonal atrophy, degeneration, and axonal sprouting was observed over time, with increasing intensity at 8 to 16 weeks. These alterations were seen to a lesser extent in HNPP xenografts and were not observed in controls. In addition, the onset of regeneration-associated myelination was delayed, more significantly in HNPP xenografts than those of CMT1A. Our findings indicate that the PMP22 duplication in Schwann cells results in an impairment in the normal axonal cytoskeletal organization, resulting in distal axonal degeneration and fiber loss, and the affect of PMP22 deletion on axonal cytoskeleton is less deleterious.