Expression profiling of sciatic nerve in a Charcot-Marie-Tooth disease type 1a mouse model

MAL overexpression leads to disturbed expression of genes that influence cytoskeletal organization and differentiation of Schwann cells

In the developing peripheral nervous system, a coordinated reciprocal signaling between Schwann cells and axons is crucial for accurate myelination. The myelin and lymphocyte protein MAL is a component of lipid rafts that is important for targeting proteins and lipids to distinct domains. MAL overexpression impedes peripheral myelinogenesis, which is evident by a delayed onset of myelination and reduced expression of the myelin protein zero (Mpz/P0) and the low-affinity neurotrophin receptor p75(NTR). This study shows that MAL overexpression leads to a significant reduction of Mpz and p75(NTR) expression in primary mouse Schwann cell cultures, which was already evident before differentiation, implicating an effect of MAL in early Schwann cell development. Their transcription was robustly reduced, despite normal expression of essential transcription factors and receptors. Further, the cAMP response element-binding protein (CREB) and phosphoinositide 3-kinase signaling pathways important for Schwann cell differentiation were correctly induced, highlighting that other so far unknown rate limiting factors do exist. We identified novel genes expressed by Schwann cells in a MAL-dependent manner in vivo and in vitro. A number of those, including S100a4, RhoU and Krt23, are implicated in cytoskeletal organization and plasma membrane dynamics. We showed that S100a4 is predominantly expressed by nonmyelinating Schwann cells, whereas RhoU was localized within myelin membranes, and Krt23 was detected in nonmyelinating as well as in myelinating Schwann cells. Their differential expression during early peripheral nerve development further underlines their possible role in influencing Schwann cell differentiation and myelination.

Peptide mimetic of the S100A4 protein modulates peripheral nerve regeneration and attenuates the progression of neuropathy in myelin protein P0 null mice

We recently found that S100A4, a member of the multifunctional S100 protein family, protects neurons in the injured brain and identified two sequence motifs in S100A4 mediating its neurotrophic effect. Synthetic peptides encompassing these motifs stimulated neuritogenesis and survival in vitro and mimicked the S100A4-induced neuroprotection in brain trauma. Here, we investigated a possible function of S100A4 and its mimetics in the pathologies of the peripheral nervous system (PNS). We found that S100A4 was expressed in the injured PNS and that its peptide mimetic (H3) affected the regeneration and survival of myelinated axons. H3 accelerated electrophysiological, behavioral and morphological recovery after sciatic nerve crush while transiently delaying regeneration after sciatic nerve transection and repair. On the basis of the finding that both S100A4 and H3 increased neurite branching in vitro, these effects were attributed to the modulatory effect of H3 on initial axonal sprouting. In contrast to the modest effect of H3 on the time course of regeneration, H3 had a long-term neuroprotective effect in the myelin protein P0 null mice, a model of dysmyelinating neuropathy (Charcot-Marie-Tooth type 1 disease), where the peptide attenuated the deterioration of nerve conduction, demyelination and axonal loss. From these results, S100A4 mimetics emerge as a possible means to enhance axonal sprouting and survival, especially in the context of demyelinating neuropathies with secondary axonal loss, such as Charcot-Marie-Tooth type 1 disease. Moreover, our data suggest that S100A4 is a neuroprotectant in PNS and that other S100 proteins, sharing high homology in the H3 motif, may have important functions in PNS pathologies.

P2X7-mediated increased intracellular calcium causes functional derangement in Schwann cells from rats with CMT1A neuropathy

Charcot-Marie-Tooth (CMT) is the most frequent inherited neuromuscular disorder, affecting 1 person in 2500. CMT1A, the most common form of CMT, is usually caused by a duplication of chromosome 17p11.2, containing the PMP22 (peripheral myelin protein-22) gene; overexpression of PMP22 in Schwann cells (SC) is believed to cause demyelination, although the underlying pathogenetic mechanisms remain unclear. Here we report an abnormally high basal concentration of intracellular calcium ([Ca(2+)](i)) in SC from CMT1A rats. By the use of specific pharmacological inhibitors and through down-regulation of expression by small interfering RNA, we demonstrate that the high [Ca(2+)](i) is caused by a PMP22-related overexpression of the P2X7 purinoceptor/channel leading to influx of extracellular Ca(2+) into CMT1A SC. Correction of the altered [Ca(2+)](i) in CMT1A SC by small interfering RNA or with pharmacological inhibitors of P2X7 restores functional parameters of SC (migration and release of ciliary neurotrophic factor), which are typically defective in CMT1A SC. More significantly, stable down-regulation of the expression of P2X7 restores myelination in co-cultures of CMT1A SC with dorsal root ganglion sensory neurons. These results establish a pathogenetic link between high [Ca(2+)](i) and impaired SC function in CMT1A and identify overexpression of P2X7 as the molecular mechanism underlying both abnormalities. The development of P2X7 inhibitors is expected to provide a new therapeutic strategy for treatment of CMT1A neuropathy.

Ciliary neurotrophic factor null alleles are not a risk factor for Charcot-Marie-Tooth disease, hereditary neuropathy with pressure palsies and amyotrophic lateral sclerosis

Growth factors, such as ciliary neurotrophic factor (CNTF), have been implicated in neuronal survival and proliferation. About 2% of the human population is homozygous for a polymorphism that induces truncated and biologically inactive CNTF but does not obviously change the phenotype. In a population of patients with hereditary neuropathy, a higher rate of the CNTF null mutation would indicate greater susceptibility for clinically significant disease, and a recent report attributes early onset and rapid deterioration in a case of familial ALS (FALS) to this mutation. We have, therefore, genotyped the CNTF polymorphism in a large group of patients with CMT 1a, HNPP, sporadic ALS, in one pedigree with FALS, and controls. All groups exhibited a similar distribution of the polymorphism. We conclude that absence of CNTF does not increase susceptibility for these disorders and confirm that it does not affect onset and course of familial and sporadic ALS.

Comparison of Schwann cell and sciatic nerve transcriptomes indicates that mouse is a valid model for the human peripheral nervous system

High-throughput gene expression analyses of murine models of the peripheral nervous system (PNS), and its cellular components, have yielded enormous amounts of expression data of the PNS in various conditions. These data provided clues for future research directions to further decipher this complex organ in relation to acquired and inherited PNS diseases. Various studies addressing the validity of mouse models for human conditions in other tissues and cell types have indicated that in many cases the mouse model only poorly represents the human situation. To determine how well the mouse can serve as model to study the biological processes occurring in the PNS, we compared the gene expression profiles that we generated for mouse and human sciatic nerve and cultured Schwann cells derived thereof. A two-way analysis based on the differentially expressed genes between the sciatic nerve and the cultured Schwann cell, and which takes into account the differential expression between mouse and man, indicates that the human PNS is well represented by that of the mouse in terms of the "biological processes" ontology.

Animal models of inherited neuropathies

PURPOSE OF REVIEW

Mutations in a number of genes have been associated with inherited neuropathies (Charcot-Marie-Tooth or CMT disease). This review highlights how animal models of demyelinating CMT have improved our understanding of disease mechanisms. Transgenic CMT models also allow therapies to be developed in a preclinical setting.

RECENT FINDINGS

Rodent models for the most common subtypes of human CMT disease are now available, and two mouse mutants modeling the rare CMT4B subform have lately extended this repertoire. In a peripheral myelin protein 22 kDa (Pmp22) transgenic rat model of CMT1A, administration of a progesterone receptor antagonist reduced Pmp22 overexpression, axon loss and clinical impairments. Dietary ascorbic acid prevented dysmyelination and premature death in a Pmp22 transgenic mouse line. Neurotrophin-3 promoted small fiber remyelination in CMT1A xenografts and sensory functions in CMT1A patients. Gene expression profiling in rodent models of CMT may identify further therapeutical targets. While original classifications distinguish the demyelinating and axonal forms of CMT, recent findings emphasize that axon loss is a common feature, possibly caused by Schwann cell defects rather than demyelination per se. This supports our model that myelination and long-term axonal support are distinct functions of all myelinating glial cells.

SUMMARY

Animal models have opened up new perspectives on the pathomechanisms and possible treatment strategies of inherited neuropathies.