Peripheral myelin protein 22: facts and hypotheses

Deciphering variations, identification of marker-trait associations and candidate genes for seed oil content under terminal heat stress in Indian mustard (Brassica juncea L. Czern & Coss) germplasm stock

This research paper investigates the variability in seed oil content (SOC) in Indian mustard (Brassica juncea L.) under terminal heat stress (THS) conditions. A genetic stock of 488 genotypes of B. juncea was evaluated over two years and grouped into five classes based on the reduction in oil content under THS compared to normal sown crop. Based on heat susceptibility index (HSI), a diverse panel of 96 genotypes was selected and evaluated under THS. Twenty-two heat-tolerant donor genotypes were identified, including introgression lines derived from B. tournefortii, B. carinata and Erucastrum cardaminoides. This study is the first to report on marker-trait associations for SOC in B. juncea under THS using a GWAS approach. Furthermore, candidate genes associated with abiotic stress tolerance and lipid metabolism were identified near the significant SNPs, emphasizing their role in SOC regulation under stress. Notable candidate genes include BjuA003240 (encoding for alcohol-forming fatty acyl-CoA reductase), BjuA003242 (involving in lipid biosynthesis), BjuA003244 (associated with mitochondrial functions and stress tolerance), and BjuA003245 (related to MYB transcription factors regulating lipid biosynthesis). This study provides valuable insights into the genetic basis of SOC variation under THS in B. juncea, highlighting potential breeding targets for improved heat stress resilience in Indian mustard cultivation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-024-03985-w.

Pooled resequencing of larvae and adults reveals genomic variations associated with Ostreid herpesvirus 1 resistance in the Pacific oyster Crassostrea gigas

The Ostreid herpesvirus 1 (OsHV-1) is a lethal pathogen of the Pacific oyster (Crassostrea gigas), an important aquaculture species. To understand the genetic architecture of the defense against the pathogen, we studied genomic variations associated with herpesvirus-caused mortalities by pooled whole-genome resequencing of before and after-mortality larval samples as well as dead and surviving adults from a viral challenge. Analysis of the resequencing data identified 5,271 SNPs and 1,883 genomic regions covering 3,111 genes in larvae, and 18,692 SNPs and 28,314 regions covering 4,863 genes in adults that were significantly associated with herpesvirus-caused mortalities. Only 1,653 of the implicated genes were shared by larvae and adults, suggesting that the antiviral response or resistance in larvae and adults involves different sets of genes or differentiated members of expanded gene families. Combined analyses with previous transcriptomic data from challenge experiments revealed that transcription of many mortality-associated genes was also significantly upregulated by herpesvirus infection confirming their importance in antiviral response. Key immune response genes especially those encoding antiviral receptors such as TLRs and RLRs displayed strong association between variation in regulatory region and herpesvirus-caused mortality, suggesting they may confer resistance through transcriptional modulation. These results point to previously undescribed genetic mechanisms for disease resistance at different developmental stages and provide candidate polymorphisms and genes that are valuable for understanding antiviral immune responses and breeding for herpesvirus resistance.

Patterns of indolence in prostate cancer (Review)

Although prostate cancer is a major cause of cancer-related mortality worldwide, most patients will have a relatively indolent clinical course. Contrary to most other types of cancer, even the diagnosis of locally advanced or metastatic disease is not always lethal. The present review aimed to summarize what is known regarding the underlying mechanisms related to the indolent course of subsets of prostate cancer, at various stages. The data suggested that no specific gene alteration by itself was responsible for carcinogenesis or disease aggressiveness. However, pathway analysis identified genetic aberrations in multiple critical pathways that tend to accumulate over the course of the disease. The progression from indolence into aggressive disease is associated with a complex interplay in which genetic and epigenetic factors are involved. The effect of the immune tumor microenvironment is also very important. Emerging evidence has suggested that the upregulation of pathways related to cellular aging and senescence can identify patients with indolent disease. In addition, a number of tumors enter a long-lasting quiescent state. Further research will determine whether halting tumor evolution is a feasible option, and whether the life of patients can be markedly prolonged by inducing tumor senescence or long-term dormancy.

Colocalization Analysis of Peripheral Myelin Protein-22 and Lamin-B1 in the Schwann Cell Nuclei of Wt and TrJ Mice

Myelination of the peripheral nervous system requires Schwann cells (SC) differentiation into the myelinating phenotype. The peripheral myelin protein-22 (PMP22) is an integral membrane glycoprotein, expressed in SC. It was initially described as a growth arrest-specific (gas3) gene product, up-regulated by serum starvation. PMP22 mutations were pathognomonic for human hereditary peripheral neuropathies, including the Charcot-Marie-Tooth disease (CMT). Trembler-J (TrJ) is a heterozygous mouse model carrying the same pmp22 point mutation as a CMT1E variant. Mutations in lamina genes have been related to a type of peripheral (CMT2B1) or central (autosomal dominant leukodystrophy) neuropathy. We explore the presence of PMP22 and Lamin B1 in Wt and TrJ SC nuclei of sciatic nerves and the colocalization of PMP22 concerning the silent heterochromatin (HC: DAPI-dark counterstaining), the transcriptionally active euchromatin (EC), and the nuclear lamina (H3K4m3 and Lamin B1 immunostaining, respectively). The results revealed that the number of TrJ SC nuclei in sciatic nerves was greater, and the SC volumes were smaller than those of Wt. The myelin protein PMP22 and Lamin B1 were detected in Wt and TrJ SC nuclei and predominantly in peripheral nuclear regions. The level of PMP22 was higher, and those of Lamin B1 lower in TrJ than in Wt mice. The level of PMP22 was higher, and those of Lamin B1 lower in TrJ than in Wt mice. PMP22 colocalized more with Lamin B1 and with the transcriptionally competent EC, than the silent HC with differences between Wt and TrJ genotypes. The results are discussed regarding the probable nuclear role of PMP22 and the relationship with TrJ neuropathy.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

Treating PMP22 gene duplication-related Charcot-Marie-Tooth disease: the past, the present and the future

Charcot-Marie-Tooth (CMT) disease is the most frequent inherited neuropathy, affecting 1/1500 to 1/10000. CMT1A represents 60%-70% of all CMT and is caused by a duplication on chromosome 17p11.2 leading to an overexpression of the Peripheral Myelin Protein 22 (PMP22). PMP22 gene is under tight regulation and small changes in its expression influences myelination and affect motor and sensory functions. To date, CMT1A treatment is symptomatic and classic pharmacological options have been disappointing. Here, we review the past, present, and future treatment options for CMT1A, with a special emphasis on the highly promising potential of PMP22-targeted small interfering RNA and antisense oligonucleotides.

The Pivotal Roles of the Epithelial Membrane Protein Family in Cancer Invasiveness and Metastasis

The members of the family of epithelial membrane proteins (EMPs), EMP1, EMP2, and EMP3, possess four putative transmembrane domain structures and are composed of approximately 160 amino acid residues. EMPs are encoded by the growth arrest-specific 3 (GAS3)/peripheral myelin protein 22 kDa (PMP22) gene family. The GAS3/PMP22 family members play roles in cell migration, growth, and differentiation. Evidence indicates an association of these molecules with cancer progression and metastasis. Each EMP has pro- and anti-metastatic functions that are likely involved in the complex mechanisms of cancer progression. We have recently demonstrated that the upregulation of EMP1 expression facilitates cancer cell migration and invasion through the activation of a small GTPase, Rac1. The inoculation of prostate cancer cells overexpressing EMP1 into nude mice leads to metastasis to the lymph nodes and lungs, indicating that EMP1 contributes to metastasis. Pro-metastatic properties of EMP2 and EMP3 have also been proposed. Thus, targeting EMPs may provide new insights into their clinical utility. Here, we highlight the important aspects of EMPs in cancer biology, particularly invasiveness and metastasis, and describe recent therapeutic approaches.

PMP22 Regulates Cholesterol Trafficking and ABCA1-Mediated Cholesterol Efflux

The absence of functional peripheral myelin protein 22 (PMP22) is associated with shortened lifespan in rodents and severe peripheral nerve myelin abnormalities in several species including humans. Schwann cells and nerves from PMP22 knock-out (KO) mice show deranged cholesterol distribution and aberrant lipid raft morphology, supporting an unrecognized role for PMP22 in cellular lipid metabolism. To examine the mechanisms underlying these abnormalities, we studied Schwann cells and nerves from male and female PMP22 KO mice. Whole-cell current-clamp recordings in cultured Schwann cells revealed increased membrane capacitance and decreased membrane resistance in the absence of PMP22, which was consistent with a reduction in membrane cholesterol. Nerves from PMP22-deficient mice contained abnormal lipid droplets, with both mRNA and protein levels of apolipoprotein E (apoE) and ATP-binding cassette transporter A1 (ABCA1) being highly upregulated. Despite the upregulation of ABCA1 and apoE, the absence of PMP22 resulted in reduced localization of the transporter to the cell membrane and diminished secretion of apoE. The absence of PMP22 also impaired ABCA1-mediated cholesterol efflux capacity. In nerves from ABCA1 KO mice, the expression of PMP22 was significantly elevated and the subcellular processing of the overproduced protein was aberrant. In wild-type samples, double immunolabeling identified overlapping distribution of PMP22 and ABCA1 at the Schwann cell plasma membrane and the two proteins were coimmunoprecipitated from Schwann cell and nerve lysates. Together, these results reveal a novel role for PMP22 in regulating lipid metabolism and cholesterol trafficking through functional interaction with the cholesterol efflux regulatory protein ABCA1.SIGNIFICANCE STATEMENT Understanding the subcellular events that underlie abnormal myelin formation in hereditary neuropathies is critical for advancing therapy development. Peripheral myelin protein 22 (PMP22) is an essential peripheral myelin protein because its genetic abnormalities account for ∼80% of hereditary neuropathies. Here, we demonstrate that in the absence of PMP22, the cellular and electrophysiological properties of the Schwann cells' plasma membrane are altered and cholesterol trafficking and lipid homeostasis are perturbed. The molecular mechanisms for these abnormalities involve a functional interplay among PMP22, cholesterol, apolipoprotein E, and the major cholesterol-efflux transporter protein ATP-binding cassette transporter A1 (ABCA1). These findings establish a critical role for PMP22 in the maintenance of cholesterol homeostasis in Schwann cells.

Clinical and Molecular Characterization of PMP22 point mutations in Taiwanese patients with Inherited Neuropathy

Point mutations in the peripheral myelin protein 22 (PMP22) gene have been identified to cause demyelinating Charcot-Marie-Tooth disease (CMT) and hereditary neuropathy with liability to pressure palsy (HNPP). To investigate the mutation spectrum of PMP22 in Han-Chinese population residing in Taiwan, 53 patients with molecularly unassigned demyelinating CMT and 52 patients with HNPP-like neuropathy of unknown genetic causes were screened for PMP22 mutations by Sanger sequencing. Three point mutations were identified in four patients with demyelinating CMT, including c.256 C > T (p.Q86X) in two, and c.310delA (p.I104FfsX7) and c.319 + 1G > A in one each. One PMP22 missense mutation, c.124 T > C (p.C42R), was identified in a patient with HNPP-like neuropathy. The clinical presentations of these mutations vary from mild HNPP-like syndrome to severe infantile-onset demyelinating CMT. In vitro analyses revealed that both PMP22 p.Q86X and p.I104FfsX7 mutations result in truncated PMP22 proteins that are almost totally retained within cytosol, whereas the p.C42R mutation partially impairs cell membrane localization of PMP22 protein. In conclusion, PMP22 point mutations account for 7.5% and 1.9% of demyelinating CMT and HNPP patients with unknown genetic causes, respectively. This study delineates the clinical and molecular features of PMP22 point mutations in Taiwan, and emphasizes their roles in demyelinating CMT or HNPP-like neuropathy.

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.

Myelinating and demyelinating phenotype of Trembler-J mouse (a model of Charcot-Marie-Tooth human disease) analyzed by atomic force microscopy and confocal microscopy

The accumulation of misfolded proteins is associated with various neurodegenerative conditions. Mutations in PMP-22 are associated with the human peripheral neuropathy, Charcot-Marie-Tooth Type 1A (CMT1A). PMP-22 is a short-lived 22 kDa glycoprotein, which plays a key role in the maintenance of myelin structure and compaction, highly expressed by Schwann cells. It forms aggregates when the proteasome is inhibited or the protein is mutated. This study reports the application of atomic force microscopy (AFM) as a detector of profound topographical and mechanical changes in Trembler-J mouse (CMT1A animal model). AFM images showed topographical differences in the extracellular matrix and basal lamina organization of Tr-J/+ nerve fibers. The immunocytochemical analysis indicated that PMP-22 protein is associated with type IV collagen (a basal lamina ubiquitous component) in the Tr-J/+ Schwann cell perinuclear region. Changes in mechanical properties of single myelinating Tr-J/+ nerve fibers were investigated, and alterations in cellular stiffness were found. These results might be associated with F-actin cytoskeleton organization in Tr-J/+ nerve fibers. AFM nanoscale imaging focused on topography and mechanical properties of peripheral nerve fibers might provide new insights into the study of peripheral nervous system diseases.

F-actin distribution at nodes of Ranvier and Schmidt-Lanterman incisures in mammalian sciatic nerves

Very little is known about the function of the F-actin cytoskeleton in the regeneration and pathology of peripheral nerve fibers. The actin cytoskeleton has been associated with maintenance of tissue structure, transmission of traction and contraction forces, and an involvement in cell motility. Therefore, the state of the actin cytoskeleton strongly influences the mechanical properties of cells and intracellular transport therein. In this work, we analyze the distribution of F-actin at Schmidt-Lanterman Incisures (SLI) and nodes of Ranvier (NR) domains in normal, regenerating and pathologic Trembler J (TrJ/+) sciatic nerve fibers, of rats and mice. F-actin was quantified and it was found increased in TrJ/+, both in SLI and NR. However, SLI and NR of regenerating rat sciatic nerve did not show significant differences in F-actin, as compared with normal nerves. Cytochalasin-D and Latrunculin-A were used to disrupt the F-actin network in normal and regenerating rat sciatic nerve fibers. Both drugs disrupt F-actin, but in different ways. Cytochalasin-D did not disrupt Schwann cell (SC) F-actin at the NR. Latrunculin-A did not disrupt F-actin at the boundary region between SC and axon at the NR domain. We surmise that the rearrangement of F-actin in neurological disorders, as presented here, is an important feature of TrJ/+ pathology as a Charcot-Marie-Tooth (CMT) model.

Interaction of influenza virus NS1 protein with growth arrest-specific protein 8

NS1 protein is the only non-structural protein encoded by the influenza A virus, and it contributes significantly to disease pathogenesis by modulating many virus and host cell processes. A two-hybrid screen for proteins that interact with NS1 from influenza A yielded growth arrest-specific protein 8. Gas8 associated with NS1 in vitro and in vivo. Deletion analysis revealed that the N-terminal 260 amino acids of Gas8 were able to interact with NS1, and neither the RNA-binding domain nor the effector domain of NS1 was sufficient for the NS1 interaction. We also found that actin, myosin, and drebrin interact with Gas8. NS1 and beta-actin proteins could be co-immunoprecipitated from extracts of transfected cells. Furthermore, actin and Gas8 co-localized at the plasma membrane. These results are discussed in relation to the possible functions of Gas8 protein and their relevance in influenza virus release.

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.

Characterization of a monoclonal antibody specific for human peripheral myelin protein 22 and its use in immunohistochemical studies of the fetal and adult nervous system

We produced a mouse monoclonal antibody using cDNA and peptide immunization against the putative second extra-cellular domain of human peripheral myelin protein 22 (PMP22). It reacted specifically with human PMP22 and not with other human myelin proteins and did not react with bovine, rat, or mouse PMP22. The antibody stained the compact myelin of human peripheral nerve motor and sensory axons and did not stain central nervous system tissue. PMP22 reactivity was detected in the spinal roots of the human fetus at 19-20 weeks of gestation. The staining pattern of the PMP22 antibody resembled that of a monoclonal antibody directed against the myelin protein zero.

How to assess the pathogenicity of mutations in Charcot-Marie-Tooth disease and other diseases?

Knowledge whether a certain DNA variant is a pathogenic mutation or a harmless polymorphism is a critical issue in medical genetics, in which results of a molecular analysis may serve as a basis for diagnosis and genetic counseling. Due to its genetic heterogeneity expressed at the levels of loci, genes and mutations, Charcot-Marie-Tooth (CMT) disease can serve as a model group of clinically homogenous diseases for studying the pathogenicity of mutations. Close to a 17p11.2-p12 duplication occurring in 70% of patients with the demyelinating form of CMT disease, numerous mutations have been identified in poorly characterized genes coding for proteins of an unknown function. Functional analyses, segregation analyses of large pedigrees, and inclusion of large control groups are required to assess the potential pathogenicity of CMT mutations. Hence, the pathogenicity of numerous CMT mutations remains unclear. Some variants detected in the CMT genes and originally described as pathogenic mutations have been shown to have a polymorphic character. In contrast, polymorphisms initially considered harmless were later reclassified as pathogenic mutations. However, the process of assessing the pathogenicity of mutations, as presented in this study for CMT disorders, is a more general issue concerning all disorders with a genetic background. Since the number of DNA variants is still growing, in the near future geneticists will increasingly have to cope with the problem of pathogenicity of identified genetic variants.

Experimental Charcot-Marie-Tooth type 1A: a cDNA microarrays analysis

To reveal the spectrum of genes that are modulated in Charcot-Marie-Tooth neuropathy type 1A (CMT1A), which is due to overexpression of the gene coding for the peripheral myelin protein 22 (pmp22), we performed a cDNA microarray experiment with cDNA from sciatic nerves of a rat model of the disease. In homozygous pmp22 overexpressing animals, we found a significant down-regulation of 86 genes, while only 23 known genes were up-regulated, suggesting that the increased dosage of pmp22 induces a general down-regulation of gene expression in peripheral nerve tissue. Classification of the modulated genes into functional categories leads to the identification of some pathways altered by overexpression of pmp22. In particular, a selective down-regulation of the ciliary neurotrophic factor transcript and of genes coding for proteins involved in cell cycle regulation, for cytoskeletal components and for proteins of the extracellular matrix, was observed. Cntf expression was further studied by real-time PCR and ELISA technique in pmp22 transgenic sciatic nerves, human CMT1A sural nerve biopsies, and primary cultures of transgenic Schwann cells. According to the results of cDNA microarray analysis, a down-regulation of cntf, both at the mRNA and protein level, was found in all the conditions tested. These results are relevant to reveal the molecular function of PMP22 and the pathogenic mechanism of CMT1A. In particular, finding a specific reduction of cntf expression in CMT1A Schwann cells suggests that overexpression of pmp22 significantly affects the ability of Schwann cells to offer a trophic support to the axon, which could be a factor, among other, responsible for the development of axonal atrophy in human and experimental CMT1A.

Peripheral myelin protein 22 kDa and protein zero: domain specific trans-interactions

The peripheral myelin proteins P0 and PMP22 are associated in preparations of compact myelin and in cell cultures coexpressing both molecules. The mechanism of this interaction, however, still needs to be unravelled. We have established three different (cell-cell, cell-protein, protein-protein based) assay systems using retrovirally transduced HeLa cells that overexpressed either PMP22 or P0 and purified GST fusion oligopeptides of PMP22 and P0 to detect domain-specific interactions between these proteins. The results revealed that PMP22 and P0 are involved in both trans-homophilic and trans-heterophilic interactions. Moreover, the data clearly indicate that the heterophilic trans-interaction is mediated through the second loop of PMP22, while the first loop is involved in homophilic trans-interaction of PMP22 proteins. Both modes of interaction are due to direct protein-protein binding. In addition, we demonstrate that disease-related point mutations of P0 resulted in a decreased adhesion capability correlating with the severity of the respective disease phenotype.

Modulation of epithelial morphology, monolayer permeability, and cell migration by growth arrest specific 3/peripheral myelin protein 22

Peripheral myelin protein 22 (PMP22) is associated with a subset of hereditary peripheral neuropathies. Although predominantly recognized as a transmembrane constituent of peripheral nerve myelin, PMP22 is localized to epithelial and endothelial cell-cell junctions, where its function remains unknown. In this report, we investigated the role of PMP22 in epithelial biology. Expression of human PMP22 (hPMP22) slows cell growth and induces a flattened morphology in Madin-Darby canine kidney (MDCK) cells. The transepithelial electrical resistance (TER) and paracellular flux of MDCK monolayers are elevated by hPMP22 expression. After calcium switch, peptides corresponding to the second, but not the first, extracellular loop of PMP22 perturb the recovery of TER and paracellular flux. Finally, subsequent to wounding, epithelial monolayers expressing hPMP22 fail to migrate normally. These results indicate that PMP22 is capable of modulating several aspects of epithelial cell biology, including junctional permeability and wound closure.

Impairment of PMP22 transgenic Schwann cells differentiation in culture: implications for Charcot-Marie-Tooth type 1A disease

Charcot-Marie-Tooth type 1A (CMT1A) is a hereditary demyelinating neuropathy due to an increased genetic dosage of the peripheral myelin protein 22 (PMP22). The mechanisms leading from PMP22 overexpression to impairment of myelination are still unclear. We evaluated expression and processing of PMP22, viability, proliferation, migration, motility and shaping properties, and ability of forming myelin of PMP22 transgenic (PMP22(tg)) Schwann cells in culture. In basal conditions, PMP22(tg) Schwann cells, although expressing higher PMP22 levels than control ones, show normal motility, migration and shaping properties. Addition of forskolin to the media induces an additional stimulation of PMP22 expression and results in an impairment of cells migration and motility, and a reduction of cell area and perimeter. Similarly, co-culturing transgenic Schwann cells with neurons causes an altered cells differentiation and an impairment of myelin formation. In conclusion, exposure of PMP22(tg) Schwann to the axon or to axonal-mimicking stimuli significantly affects the transition of transgenic Schwann cells to the myelinating phenotype.

Phenotypic differences between peripheral myelin protein-22 (PMP22) and myelin protein zero (P0) mutations associated with Charcot-Marie-Tooth-related diseases

Mutations in the genes for peripheral myelin protein-22 (PMP22) and myelin protein zero (P0) cause human hereditary neuropathies with varying clinical and pathological phenotypes. In this study, we examine the effects of representative disease-causing mutations on the subcellular distribution of their corresponding PMP22- and P0-enhanced green fluorescent protein (EGFP) fusion proteins. In transiently transfected HeLa and 293 cells, we find that wild-type P0-EGFP and PMP22-EGFP are efficiently synthesized and transported through the secretory pathway to the plasma membrane. The P0-EGFP and PMP22-EGFP mutants can be classified into several groups: those that are transported to the plasma membrane as in the majority of P0 mutants; those that are retained in the endoplasmic reticulum as in the majority of PMP22 mutants; and those that are a mixture of the two. In addition, several of these disease-causing mutations are associated with the development of abnormal intracellular cytoplasmic structures that we have previously identified as either intracellular myelin figures or aggresomes. Our studies indicate that different types of PMP22 and P0 mutations are associated with specific intracellular chaperone proteins, including calnexin and BiP, and that these associations can be altered by glycosylation. These findings indicate that the various P0 and PMP22 mutants may exert their pathogenic effects in different subcellular compartments and by different mechanisms in the mammalian cell.

Association of calnexin with mutant peripheral myelin protein-22 ex vivo: a basis for "gain-of-function" ER diseases

Schwann cell-derived peripheral myelin protein-22 (PMP-22) when mutated or overexpressed causes heritable neuropathies with a previously unexplained "gain-of-function" endoplasmic reticulum (ER) retention phenotype. In wild-type sciatic nerves, PMP-22 associates in a specific, transient (t(1/2 ) approximately equal to 11 min), and oligosaccharide processing-dependent manner with the lectin chaperone calnexin (CNX), but not calreticulin nor BiP. In Trembler-J (Tr-J) sciatic nerves, prolonged association of mutant PMP-22 with CNX is found (t(1/2) > 60 min). In 293A cells overexpressing PMP-22(Tr-J), CNX and PMP-22 colocalize in large intracellular structures identified at the electron microscopy level as myelin-like figures with CNX localization in the structures dependent on PMP-22 glucosylation. Similar intracellular myelin-like figures were also present in Schwann cells of sciatic nerves from homozygous Trembler-J mice with no detectable activation of the stress response pathway as deduced from BiP and CHOP expression. Sequestration of CNX in intracellular myelin-like figures may be relevant to the autosomal dominant Charcot-Marie-Tooth-related neuropathies.

Association of the growth-arrest-specific protein Gas7 with F-actin induces reorganization of microfilaments and promotes membrane outgrowth

The growth-arrest-specific gene, Gas7, is required for neurite outgrowth in cerebellar neurons. Here we report that Gas7 can induce the formation of extended cellular processes in NIH3T3 cells by interacting with actin and mediating reorganization of microfilaments. The Gas 7 protein, which increased markedly during growth arrest of NIH3T3 cells and persisted transiently at high levels upon reentry of cells into the cell cycle, localized near the plasma membrane and selectively colocalized with microfilaments in membrane ruffles. Process extensions induced by ectopic overexpression of Gas7 were blocked by the actin-depolymerizing agent cytochalasin D, suggesting that membrane extensions produced by Gas7 require actin polymerization. Association of endogenous Gas7 protein with microfilaments was verified by F-actin affinity chromatography; direct binding of purified His-Gas7 to actin also was demonstrated and shown to be mediated by the Gas7 C-terminal domain. Similarly, localization of Gas7 in membrane ruffles was mediated by the C-terminal domain, although neither this region nor the N-terminal domain was individually sufficient to induce process formation. Biochemical studies and electron microscopy showed that both full-length Gas7 protein and its C-terminal region can promote actin assembly as well as the crosslinking of actin filaments. We propose that Gas7 localized near the plasma membrane induces the assembly of actin and the membrane outgrowth.

Hereditary neuropathy with liability to pressure palsies associated with central nervous system myelin lesions

Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominant disorder most commonly caused by a 1.5-Mb deletion in chromosome 17p11.2 which contains the peripheral myelin protein-22 (PMP22) gene. Mutations resulting in functional loss of one PMP22 gene copy are less frequent. We present a 51-year-old patient with a l.5-Mb deletion in chromosome 17p11.2 who exhibited signs of peripheral as well as central nervous system lesions. He gave a history of recurrent episodes of limb numbness and weakness with spontaneous but incomplete recovery since age 20. His father and two brothers had similar symptoms. Neurological examination revealed signs of multiple mononeuropathy associated with frontal lobe, corticospinal tract and cerebellar dysfunction, as well as signs of initial cognitive impairment. Electrophysiological investigations showed a demyelinating peripheral nerve disease with multiple conduction blocks and conduction disturbances in both optic nerves. Magnetic resonance imaging of the brain revealed multiple subcortical and periventricular foci of myelin lesions. The association of central and peripheral nervous system lesions in this patient indicates a possible role of PMP22 not only in peripheral but also in central nervous system myelin structure.

Disease mechanisms and potential therapeutic strategies in Charcot-Marie-Tooth disease

Until 10 years ago, the genetic basis of Charcot-Marie-Tooth (CMT) disease was largely unknown. With the finding of an intrachromosomal duplication on chromosome 17 in 1991, associated with the most commonly found subtype CMT1A, and the discovery of a point mutation in the peripheral myelin protein-22 (pmp22) gene in the Trembler mouse in 1992, the groundwork was laid down for a novel chapter in the elucidation of the molecular basis of this large group of peripheral neuropathies. In the meantime, several different genes have been found to be associated with different forms of demyelinating and axonal forms of CMT. In this review, we will summarize what is known today about the genetics of this group of disease which constitute the most common known monogenetic disorder affecting the nervous system in man, the animal models that have been generated, and what we have learned about the underlying disease mechanisms. Furthermore, we will review how this gain of knowledge about CMT may open new avenues to the development of novel treatment strategies.

Molecular pathogenesis of hereditary motor, sensory and autonomic neuropathies

The hereditary motor, sensory and autonomic neuropathies are a heterogeneous group of neurological diseases. The classification of such is presently in a state of change. The original classification system was based on clinical findings whose limitations are being unfurled with increasing insights into the molecular basis of these disorders. In particular, much progress has been achieved in understanding the demyelinating forms of Charcot-Marie-Tooth (type 1), for which at least a dozen loci have been delineated and six genes identified. As anticipated, these genes play predominant roles in myelin biology. Four separate loci for the axonal Charcot-Marie-Tooth neuropathies (type 2) have been identified and only now are researchers beginning to tease out the responsible genes and the underlying molecular mechanisms. Similarly, progress is being made with the pure hereditary motor neuropathies. This review presents an updated list of genes responsible for inherited peripheral neuropathies and explores the underlying molecular mechanisms actively being investigated.

Infantile demyelinating neuropathy associated with a de novo point mutation on Ser72 in PMP22 and basal lamina onion bulbs in skin biopsy

Codon 72 has been designated as a hot spot for distinct missense mutations in the peripheral myelin protein 22 (PMP22) gene. Ser72Leu substitution was associated with Dejerine-Sottas syndrome (DSS) in four patients and with congenital hypomyelination neuropathy (CHN) in one patient. Our objective was to report one other DSS patient with Ser72Leu substitution in PMP22 and to concurrently illustrate how less invasive procedures such as skin biopsy could provide a rapid and reliable alternative to conventional sural nerve biopsy for the characterization of histophenotypic features. A skin biopsy was carried out in a 2 4/12-year-old girl with muscle atrophy, hypotonia and weakness, as well as generalized areflexia and absent sensory and motor nerve responses. Standard electron microscope techniques were used. PMP22 was screened by automated direct nucleotide sequencing analysis. Morphological examination revealed basal lamina onion bulbs surrounding a de- or hypomyelinated axon in all nerve bundles. Mutation analysis demonstrated a missense point mutation in codon 72 of the PMP22 gene leading to a Ser72Leu substitution. Further genotype-phenotype correlations will have to determine whether morphologically distinct phenotypes can be correlated with specific mutations. For this purpose, cutaneous nerve bundles could serve as an alternative tool to help identify and classify subtypes in this heterogeneous syndrome.

Functional analysis for peripheral myelin protein PASII/PMP22: is it a member of claudin superfamily?

Two major glycoproteins, P0 and PASII/PMP22, are specifically expressed in peripheral myelin. Point mutations of these proteins and over or under expression of PASII/PMP22 cause various hereditary peripheral neuropathies. P0 is well characterized as a major adhesion molecule in PNS myelin, but the function of PASII/PMP22 is still unknown. Recently, an oligodendrocyte-specific protein (OSP) was identified as a member of the claudin family and as a component of tight junctions of central myelins. Since PASII/PMP22 shows similarity in structure to OSP, which is a tetraspan membrane protein, we speculated if PASII/PMP22 could be a member of claudin superfamily. The primary structure of PASII/PMP22 showed a significant homology of 48% and a 21% identity with the OSP sequence. Exogenous expression of PASII/PMP22 in C6 cells significantly inhibited BrdU incorporation to the cells. The C6 cells stably transfected with PASII/PMP22 cDNA showed no homophilic cell adhesive activity. When dorsal root ganglion (DRG) neurons were cocultured on PASII/PMP22 expressing cells, both neurite extension and branching of DRG neurons were significantly inhibited. These results indicate that PASII/PMP22 may play a role in a turning point of Schwann cell development from proliferation to differentiation. On the other hand, the cells expressing claudin family proteins are reported to show strong cell adhesive activity and an ability to form tight junctions with neighboring cells. For this reason, we currently do not have any functional data supporting that PASII/PMP22 is the member of claudin superfamily.

PMP22 carrying the trembler or trembler-J mutation is intracellularly retained in myelinating Schwann cells

Missense mutations in the murine peripheral myelin protein-22 gene (Pmp22) underly the neuropathies in the trembler (Tr) and trembler-J (Tr-J) mice and in some humans with Charcot-Marie-Tooth disease. We have generated replication-defective adenoviruses containing epitope-tagged, wild-type-, Tr-, or Tr-J-PMP22 bicistronic with the Lac-Z reporter gene. These viruses were microinjected into the sciatic nerves of 10-day-old Sprague-Dawley rats and, later, analyzed by immunohistochemistry to determine the distribution of mutant protein in infected myelinating Schwann cells. We found that epitope-tagged, wild-type PMP22 is successfully transported to compact myelin, whereas the Tr and the Tr-J mutant proteins are retained in cytoplasmic compartment, colocalizing with the endoplasmic reticulum. These results provide in vivo evidence that the pathogenesis of the Tr and Tr-J mutations are most likely a function of abnormal retention within the endoplasmic reticulum of myelinating Schwann cells.

Mutational analysis and genotype/phenotype correlation in Turkish Charcot-Marie-Tooth Type 1 and HNPP patients

The major Charcot- Marie-Tooth Type 1 (CMT1) locus, CMT1A, and Hereditary neuropathy with liability to pressure palsies (HNPP) cosegregate with a 1.5-Mb duplication and a 1.5-Mb deletion, respectively, in band 17p11.2. Point mutations in peripheral myelin gene 22 (PMP22), myelin protein zero (MPZ), and connexin 32 (Cx32) have been reported in CMT1, and in PMP22 in HNPP patients without deletion. We have screened 54 CMT1 patients, of variable clinical severity, and 25 HNPP patients from Turkey, with no duplication or deletion, for mutations in the PMP22 and Cx32 genes. A novel frameshift mutation affecting the second extracellular domain of PMP22 was found in an HNPP patient, while a point mutation in the second transmembrane domain of the protein was detected in a CMT1 patient. Two point mutations affecting different domains of Cx32 were identified in two CMTX patients. Another patient was found to carry a polymorphism in a non-conserved codon of the Cx32 gene. The clinical phenotypes of the patients correlate well with the effect of the mutation on the protein.

Membrane topology of peripheral myelin protein 22

Peripheral myelin protein 22 (PMP22) is a structural component of compact peripheral nerve myelin and is likely to play a role in the modulation of cell proliferation and cell spreading. Molecular genetics revealed that mutations affecting the PMP22 gene are responsible for the most common forms of hereditary motor and sensory neuropathies in humans. Computer analysis predicts a tetraspan-membrane structure for the PMP22 protein. We have assessed the topology of PMP22 experimentally using chimeric proteins consisting of different PMP22 domains fused to reporter genes and internally tagged molecules. Based on in vitro transcription/translation assays and immunohistochemical analysis of transfected cells, we propose that PMP22 can adopt a non-tetraspan topology that has functional implications in normal and disease processes.

Acute deterioration of Charcot-Marie-Tooth disease IA (CMT IA) following 2 mg of vincristine chemotherapy

BACKGROUND

Severe up to life-threatening neuropathy has been observed in patients with hereditary neuropathies receiving vincristine.

CASE REPORT

A 52-year-old female painter suffering from high-grade non-Hodgkin's lymphoma (stage IVB) was treated with a total of 4 mg of vincristine during two courses of CHOP chemotherapy (cyclophosphamide, vincristine, adriamycin, prednisone). At onset of treatment no neurological problems were reported. There was good lymphoma response to chemotherapy. At the same time, however, the patient gradually developed dysphagia, dysarthria, muscular weakness of both lower and upper extremities, areflexia, paraesthesia of the fingertips and bilateral sensory impairment of feet and lower legs. These symptoms continually worsened over a period of seven weeks until she was unable to walk or to perform her work. Electrophysiological studies showed peripheral axonal and demyelinative sensorimotor neuropathy in correlation to histological findings. Molecular analysis revealed 17p11.2 duplication typical for Charcot-Marie-Tooth disease IA. While continuing chemotherapy without the use of vincristine the patient's neurologic symptoms slowly recovered within six months.

CONCLUSION

Prior to administration of vincristine family and patient history as well as physical examination should be performed carefully to look for underlying hereditary neuropathy. For those patients with a clinical history or symptoms suggestive for CMT nerve conduction velocity studies and on an individual base even molecular genetic analysis are necessary to prevent serious neurologic complications. worsened significantly resulting in dependency on a wheelchair and inability to perform her work as a painter. Finally she consulted a neurologist and was admitted to hospital for further diagnostic studies and continuation of treatment for her lymphoma in March 1998 with a provisional diagnosis of severe vincristine-induced neuropathy. Medical history at time of admission included hyperthyroidism, that was currently treated with propylthiouracil, a MALT lymphoma 1983, that was treated surgically only, and a meningoencephalitis in 1968. No further medication was taken. In addition she had a history of Lyme disease since 1993 with positive IgM-titer until December 1997, when antibiotic therapy with doxycycline and ceftriaxone was administered successfully. Family history obtained on admission revealed that her mother had non-specific neuropathic symptoms as well as a poorly defined foot deformities of the mother's father. The patient's brother does not show any neurologic impairment and is in good physical health.

MAL, a proteolipid in glycosphingolipid enriched domains: functional implications in myelin and beyond

The myelin and lymphocyte protein MAL (VIP17/MVP17) is a proteolipid of 17 kD with a hydrophobicity pattern that indicates a four transmembrane domain structure. The MAL cDNA has been cloned from human T-cells, rat oligodendrocytes and the Madin-Darby canine kidney (MDCK) cell line. In the nervous system both myelinating cells, oligodendrocytes and Schwann cells, express MAL protein. MAL expression parallels myelin formation, and MAL is predominantly localized in compact myelin. Prior to myelin formation MAL is also found in immature Schwann cells. Outside the nervous system MAL expression is found in T-cells and in distinct epithelial cells, e.g. in kidney, stomach and thyroid gland, where MAL is localised in the apical plasma membrane. Specific glycosphingolipids, e.g. galactosylceramide and sulfatide, are enriched in such apical kidney and stomach membranes as well as in myelin. MAL copurifies with these glycosphingolipids in detergent insoluble domains, indicating a close association and possible functional interactions of MAL with glycosphingolipids in these tissues. Moreover, recent reports point to additional functions of MAL-glycosphingolipid complexes in signalling, cell differentiation and apical sorting. The role of MAL in the formation, stabilisation and maintenance of glycosphingolipid-enriched membrane microdomains and its contribution to specific membrane properties in myelin and epithelial cells are discussed.

The peripheral myelin protein 22 and epithelial membrane protein family

The peripheral myelin protein 22 (PMP22) and the epithelial membrane proteins (EMP-1, -2, and -3) comprise a subfamily of small hydrophobic membrane proteins. The putative four-transmembrane domain structure as well as the genomic structure are highly conserved among family members. PMP22 and EMPs are expressed in many tissues, and functions in cell growth, differentiation, and apoptosis have been reported. EMP-1 is highly up-regulated during squamous differentiation and in certain tumors, and a role in tumorigenesis has been proposed. PMP22 is most highly expressed in peripheral nerves, where it is localized in the compact portion of myelin. It plays a crucial role in normal physiological and pathological processes in the peripheral nervous system. Progress in molecular genetics has revealed that genetic alterations in the PMP22 gene, including duplications, deletions, and point mutations, are responsible for several forms of hereditary peripheral neuropathies, including Charcot-Marie-Tooth disease type 1A (CMT1A), Dejerine-Sottas syndrome (DDS), and hereditary neuropathy with liability to pressure palsies (HNPP). The natural mouse mutants Trembler and Trembler-J contain a missense mutation in different hydrophobic domains of PMP22, resulting in demyelination and Schwann cell proliferation. Transgenic mice carrying many copies of the PMP22 gene and PMP22-null mice display a variety of defects in the initial steps of myelination and/or maintenance of myelination, whereas no pathological alterations are detected in other tissues normally expressing PMP22. Further characterization of the interactions of PMP22 and EMPs with other proteins as well as their regulation will provide additional insight into their normal physiological function and their roles in disease and possibly will result in the development of therapeutic tools.

Density dependent regulation of human Schwann cell proliferation

Cessation of division is prerequisite for Schwann cell differentiation but regulation of this critical function is poorly understood. Heregulin/forskolin-induced growth of human Schwann cells (HSCs) in vitro was found to be strongly regulated by cell density and thus could model some aspects of negative growth-regulation in vivo. To better understand this phenomenon, the production of an autocrine growth-inhibitor and the role of contact-inhibition were investigated. The possible involvement of two membrane proteins, contactinhibin (CI) and peripheral myelin protein 22 (PMP22) in regulating growth was studied. Thymidine-labeling of HSCs on collagen-coated dishes was inhibited at cell densities less than one tenth of the density at maximal growth-inhibition. Medium from high density cultures did not inhibit the thymidine-labeling of HSCs at low density, a result that argues against the production of a soluble inhibitor. The expression of CI and PMP22 in nerve and HSCs, and the effect of a function-blocking antibody to CI on HSC growth, were determined. CI was detected in fresh nerve by western blotting, and could easily be detected by immunocytochemistry in cultured HSCs by five days and for several weeks thereafter. Twenty-four hour treatment with anti-CI antibody did not increase the thymidine-labeling of HSCs at any density but a significant increase in HSC number was observed in cultures treated with anti-CI for 20 days. This increase was not related to decreased cell death. PMP22, unlike other myelin proteins, was not down-regulated after nerve dissociation and by seven days nearly all HSCs were PMP22 positive. These results provide evidence for a contact-mediated mechanism of growth-regulation in HSCs and suggest that CI is involved in this mechanism.

Molecular basis of inherited neuropathies

Considerable advances in our knowledge of the most frequently encountered group of inherited neuropathies, Charcot-Marie-Tooth neurpathy (CMT) and related disorders, have recently been made by genetic studies demonstrating that these disorders are caused by duplication, deletion or point mutations of specific genes of the peripheral myelin. The present classification of CMT and related disorders is based on a combination of clinical, neurophysiological, and genetic findings, and new genes and distinct mutations responsible for different clinical phenotypes are continuously being added. The genes that encode peripheral myelin protein of 22 kDa, protein zero, connexin-32 and early growth response-2 are the genes known to be involved in the pathogenesis of inherited neuropathies. Overexpression or underexpression of peripheral myelin protein of 22 kDa are causative for the most frequent forms of CMT-CMT1A and hereditary neuropathy with liability to pressure palsies--but the mechanisms that lead to incorrect myelin formation and maintenance are still unknown. Point mutations in the myelin genes can determine a loss of function, but in some cases an aberrant protein can act through a dominant negative or a toxic gain of function mechanism, disrupting the regular and precise relationship between the different myelin genes. Animal and in-vitro models of inherited neuropathies have been developed and will probably give the information that is necessary to clarify the pathogenetic mechanisms of demyelination.

Myelin uncompaction in Charcot-Marie-Tooth neuropathy type 1A with a point mutation of peripheral myelin protein-22

BACKGROUND

The peripheral myelin protein-22 (PMP22) gene has four transmembrane domains, two extracellular loops, and a short cytoplasmic tail. Its roles in the peripheral nervous system remain unclear. The most common cause of Charcot-Marie-Tooth neuropathy type 1A (CMT1A) is a PMP22 gene duplication. Missense point mutations in the transmembrane domains are rare alternative causes that have undetermined pathogenetic mechanisms.

OBJECTIVE

To investigate the phenotype-to-genotype correlations in a pedigree with unusual CMT1A.

METHODS

We identified a pedigree with an autosomal dominant motor-sensory neuropathy and severely reduced nerve conduction velocities who did not have the PMP22 duplication. Specimens from sural nerve biopsies from two patients of different ages were evaluated morphometrically. By automated direct nucleotide sequencing we analyzed PMP22 and the gene of the major structural myelin protein zero (P0).

RESULTS

Nucleotide 159 of PMP22 showed an A-to-T heterozygous mutation, predicted to cause an aspartate-to-valine substitution at codon 37 in the first extracellular loop of the protein. The mutation co-segregated with the disease in the pedigree and was absent in 80 healthy controls. The histopathologic phenotype was a de-remyelinating neuropathy with onion bulb formations, characterized by prominent uncompaction of the myelin sheath in the majority of fibers and by frequent tomacula.

CONCLUSION

We have described a novel mutation in the first extracellular loop of PMP22 associated with an atypical CMT1A that overlaps pathologically with CMT1B caused by point mutations in the extracellular domain of P0.

Embryonic expression of epithelial membrane protein 1 in early neurons

Epithelial membrane protein 1 (EMP1) is a member of the peripheral myelin protein 22 (PMP22) family. This family is best known for the crucial contribution of PMP22 to the development and maintenance of the peripheral nervous system (PNS). PMP22 is widely expressed, with highest levels in myelinating Schwann cells, and mutations affecting the PMP22 gene lead to PNS-restricted neuropathies. We have investigated the spatio-temporal distribution of EMP1 and compared it to that of PMP22. We found that EMP1 and PMP22 mRNA are most conspicuously expressed in the prenatal mouse brain during neurogenesis. In the developing forebrain, we localized EMP1 mRNA and protein to the first set of neurons that are generated and leave the ventricular zone to form the preplate. Later in development, EMP1 was found in derivatives of the preplate, the marginal zone and the subplate. Reduced expression was observed in the newly generated cortical plate neurons. In other parts of the developing CNS and PNS, EMP1 was also detected in early neurons and along the initial fiber tracts. Furthermore, EMP1 was highly expressed by immature neurons in embryonal dorsal root ganglia-explant cultures and in neuroectodermal differentiated P19 cells. While PMP22 functions mainly in Schwann cell growth and differentiation, the spatio-temporal localization of EMP1 suggests a role in neuronal differentiation and neurite outgrowth.

Autoantibodies associated with peripheral neuropathy

High titers of serum antibodies to neural antigens occur in several forms of neuropathy. These include neuropathies associated with monoclonal gammopathy, inflammatory polyneuropathies, and paraneoplastic neuropathies. The antibodies frequently react with glycosylated cell surface molecules, including glycolipids, glycoproteins, and glycosaminoglycans, but antibodies to intracellular proteins have also been described. There are several correlations between antibody specificity and clinical symptoms, such as anti-MAG antibodies with demyelinating sensory or sensorimotor neuropathy, anti-GM1 ganglioside antibodies with motor nerve disorders, antibodies to gangliosides containing disialosyl moieties with sensory ataxic neuropathy and Miller-Fisher syndrome, and antibodies to the neuronal nuclear Hu antigens with paraneoplastic sensory neuronopathy. These correlations suggest that the neuropathies may be caused by the antibodies, but evidence for a causal relationship is stronger in some examples than others. In this review, we discuss the origins of the antibodies, evidence for and against their involvement in pathogenic mechanisms, and the implications of these findings for therapy.

Congenital hypomyelination neuropathy with Ser72Leu substitution in PMP22

We describe a patient with congenital hypomyelination neuropathy. The pathological and morphometrical findings in the sural nerve biopsy were consistent with a defect of myelin formation and maintenance. Direct sequence analysis of the genomic regions coding the peripheral myelin proteins P0 and PMP22 disclosed a heterozygous missense point mutation that leads to a Ser72Leu substitution in the second transmembrane of PMP22. Codon 72 mutations of PMP22 are associated with different phenotypes encompassing the Dejerine-Sottas syndrome and including congenital hypomyelination neuropathy.

The tmp gene, encoding a membrane protein, is a c-Myc target with a tumorigenic activity

The c-Myc oncoprotein induces cell proliferation and transformation through its activity as a transcription factor. Uncovering the genes regulated by c-Myc is an essential step for understanding these processes. We recently isolated the tumor-associated membrane protein gene, Tmp, from a c-myc-induced mouse brain tumor. Here we show that Tmp is specifically highly expressed in mammary tumors and T-cell lymphomas which develop in c-myc transgenic mice, suggesting that Tmp expression is a general characteristic of c-Myc-induced tumors. In addition, Tmp expression is induced upon serum stimulation of fibroblasts as shown in a time course closely correlated with c-myc expression. We have isolated the Tmp promoter region and identified a putative c-Myc binding element, CACGTG, located in the first intron of the gene. We show here that constructs containing the Tmp regulatory region fused to a reporter gene are activated by c-Myc through this CACGTG element and that the c-Myc-Max protein complex can bind to this element. Moreover, an inducible form of c-Myc, the MycER fusion protein, can activate the endogenous Tmp gene. We also show that Tmp-overexpressing fibroblasts induce rapidly growing tumors when injected into nude mice, suggesting that Tmp may possess a tumorigenic activity. Thus, TMP, a member of a novel family of membrane glycoproteins with a suggested role in cellular contact, is a c-Myc target and is possibly involved in c-Myc-induced transformation.

The neurobiology of Schwann cells

This selective review of Schwann cell biology focuses on questions relating to the origins, development and differentiation of Schwann cells and the signals that control these processes. The importance of neuregulins and their receptors in controlling Schwann cell precursor survival and generation of Schwann cells, and the role of these molecules in Schwann cell biology is addressed. The reciprocal signalling between peripheral glial cells and neurons in development and adult life revealed in recent years is highlighted, and the profound change in survival regulation from neuron-dependent Schwann cell precursors to adult Schwann cells that depend on autocrine survival signals is discussed. Besides providing neuronal and autocrine signals, Schwann cells signal to mesenchymal cells and influence the development of the connective tissue sheaths of peripheral nerves. The importance of Desert Hedgehog in this process is described. The control of gene expression during Schwann cell development and differentiation by transcription factors is reviewed. Knockout of Oct-6 and Krox-20 leads to delay or absence of myelination, and these results are related to morphological or physiological observations on knockout or mutation of myelin-related genes. Finally, the relationship between selected extracellular matrix components, integrins and the cytoskeleton is explored and related to disease.

Impaired intracellular trafficking is a common disease mechanism of PMP22 point mutations in peripheral neuropathies

The most common forms of hereditary motor and sensory neuropathies (HMSN) or Charcot-Marie-Tooth disease (CMT) are associated with mutations affecting myelin genes in the peripheral nervous system. A minor subgroup of CMT type 1A (CMT1A) is caused by point mutations in the gene encoding the peripheral myelin protein 22 (PMP22). To study the mechanisms by which these mutations cause the CMT pathology, we transiently transfected COS7 and Schwann cells with wild-type and PMP22 expression constructs carrying six representative dominant or de novo point mutations and one putative recessive point mutation. All but one of the first group of mutant PMP22 proteins failed to be incorporated into the plasma membrane and were retained in intracellular compartments of transfected cells. Surprisingly, the recessive PMP22 mutation produced a protein that was also mildly impaired in trafficking. Thus, our results suggest a common disease mechanism underlying the pathology of CMT1A due to PMP22 point mutations.

Post-transcriptional regulation of the peripheral myelin protein gene PMP22/gas3

The peripheral myelin protein PMP22 gene has been described as a growth arrest-specific gene gas3 and has been identified as disease gene of various demyelinating neuropathies. The gene consists of two highly conserved alternative noncoding 5'-exons la (CD25) and 1b (SR13), respectively. Differential expression patterns of these transcripts in vivo and in vitro suggest a very complex mode of PMP22 gene regulation, which cannot be explained merely by transcriptional control. In fact, the PMP22 gene is regulated on different post-transcriptional levels. While reverse transcriptase polymerase chain reaction (RT-PCR) analyses revealed no alterations in stability for both PMP22 transcripts in randomly growing Schwann cell cultures of rat sciatic nerve for at least 8 hours, in serum-induced synchronized cultures of resting cells we observed a specific cell cycle-regulated degradation of both transcripts. We further prepared diverse PMP22/CAT fusion genes to study the influence of the alternative 5'UTRs on PMP22 translation. Transient transfection of NIH3T3-fibroblasts and rat Schwann cells demonstrated that the alternative 5'UTRs (CD25 and SR13) and the 3'UTR exert differential regulatory influences on the translation efficiency.