Full-lenth cloning, expression and cellular localization of rat plasmolipin mRNA, a proteolipid of PNS and CNS

Non-coding RNAs in diabetic peripheral neuropathy: their role and mechanisms underlying their effects

Diabetic peripheral neuropathy (DPN) is a complication of diabetes with a high rate of disability. However, current clinical treatments for DPN are suboptimal. Non-coding RNAs (ncRNAs) are a type of RNAs that are not translated into proteins. NcRNAs perform functions that regulate epigenetic modifications, transcriptional or post-transcriptional regulators of proteins, and thus participate in the physiological and pathological processes of the body. NcRNAs play a role in the progress of DPN by affecting the processes of inflammation, oxidative stress, cellular autophagy or apoptosis. Therefore, ncRNAs treatment is regarded as a promising therapeutic approach for DPN. In addition, since some ncRNAs present stably in the blood of DPN patients, they are considered as potential biomarkers that contribute to early clinical diagnosis. In this paper, we review the studies on the role of ncRNAs in DPN in the last decade, and discuss the mechanisms of ncRNAs, aiming to provide a reference for the future research on the treatment and early diagnosis of DPN.

The circ_0002538/miR-138-5p/plasmolipin axis regulates Schwann cell migration and myelination in diabetic peripheral neuropathy

Circular RNAs (circRNAs) play a vital role in diabetic peripheral neuropathy. However, their expression and function in Schwann cells in individuals with diabetic peripheral neuropathy remain poorly understood. Here, we performed protein profiling and circRNA sequencing of sural nerves in patients with diabetic peripheral neuropathy and controls. Protein profiling revealed 265 differentially expressed proteins in the diabetic peripheral neuropathy group. Gene Ontology indicated that differentially expressed proteins were mainly enriched in myelination and mitochondrial oxidative phosphorylation. A real-time polymerase chain reaction assay performed to validate the circRNA sequencing results yielded 11 differentially expressed circRNAs. circ_0002538 was markedly downregulated in patients with diabetic peripheral neuropathy. Further in vitro experiments showed that overexpression of circ_0002538 promoted the migration of Schwann cells by upregulating plasmolipin (PLLP) expression. Moreover, overexpression of circ_0002538 in the sciatic nerve in a streptozotocin-induced mouse model of diabetic peripheral neuropathy alleviated demyelination and improved sciatic nerve function. The results of a mechanistic experiment showed that circ_0002538 promotes PLLP expression by sponging miR-138-5p, while a lack of circ_0002538 led to a PLLP deficiency that further suppressed Schwann cell migration. These findings suggest that the circ_0002538/miR-138-5p/PLLP axis can promote the migration of Schwann cells in diabetic peripheral neuropathy patients, improving myelin sheath structure and nerve function. Thus, this axis is a potential target for therapeutic treatment of diabetic peripheral neuropathy.

Plasmolipin and Its Role in Cell Processes

The mechanisms involved in the origin and development of malignant and neurodegenerative diseases are an important area of modern biomedicine. A crucial task is to identify new molecular markers that are associated with rearrangements of intracellular signaling and can be used for prognosis and the development of effective treatment approaches. The proteolipid plasmolipin (PLLP) is a possible marker. PLLP is a main component of the myelin sheath and plays an important role in the development and normal function of the nervous system. PLLP is involved in intracellular transport, lipid raft formation, and Notch signaling. PLLP is presumably involved in various disorders, such as cancer, schizophrenia, Alzheimer's disease, and type 2 diabetes mellitus. PLLP and its homologs were identified as possible virus entry receptors. The review summarizes the data on the PLLP structure, normal functions, and role in diseases.

■ REVIEW : Gene Expression in Nerve Regeneration

Injury of peripheral nerve in mammals leads to a complex but stereotypical pattern of histological events that comprise a highly reproducible sequence of degenerative reactions (Wallerian degeneration) succeeded by regenerative responses. These reactions are based on a corresponding sequence of cellular and mo lecular interactions that, in turn, reflect the differential expression of specific genes with functions in nerve degeneration and repair. We report on more than 60 genes and their products that show a specific pattern of regulation following peripheral nerve lesion. The group of regulated genes encoding, e.g., transcription factors, growth factors and their receptors, cytokines, neuropeptides, myelin proteins and lipid carriers, and cytoskeletal proteins as well as extracellular matrix and cell adhesion molecules. We describe and compare the distinct time-courses and cellular origin of expression and further discuss established or putative mo lecular interrelationships and functions with respect to the contribution of these genes/gene products to the molecular regeneration program of the PNS. NEUROSCIENTIST 3:112-122, 1997

Two plasmolipins from the black tiger shrimp, Penaeus monodon and their response to virus pathogens

Two isoforms of plasmolipin were initially identified from the black tiger shrimp (Penaeus monodon) EST database and completed using 50 RACE to reveal complete cDNAs of 558 bp (PmPLP1) and 537 bp(PmPLP2) with 87% nucleotide sequence identity. The deduced amino acid sequences contained four-transmembrane domains and showed the highest amino acid identity (49% and 51%, respectively) to the honey bee (Apis mellifera) chemokine-like factor (CKLF), with a very similar hydrophobic pattern to other plasmolipins. Transcripts of PmPLP1 and PmPLP2 were observed in all tested shrimp tissues with the highest expression levels in the gill and epipodite for PmPLP1 and in the hemocytes and antennal gland for PmPLP2. PmPLP1 transcript levels were significantly upregulated in hemocytes at 24 and 72 h post infection (hpi) with yellow head virus (YHV) (7.4- and 14.7- fold, respectively), but only after 72 hpi by white spot syndrome virus (WSSV). In contrast, PmPLP2 was only slightly (but statistically significant)up-regulated with YHV and WSSV. Thus, PmPLPs have the potential to be a part of viral infection mechanisms or defense response. This is the first characterization of a plasmolipin gene in crustaceans.

Loss of connexin36 increases retinal cell vulnerability to secondary cell loss

Accruing evidence indicates that gap junctions are involved in neuronal survival after brain injury. The present study was aimed at clarifying the contribution of the neuronal gap-junction protein connexin36 (Cx36) to secondary cell loss after injury in the mouse retina. A focal retinal lesion was induced by infrared laser photocoagulation. Remarkably, this model allowed spatial and temporal definition of the lesion with high reproducibility. Moreover, Cx36 is abundantly expressed in the retina and plays an essential role in the visual transmission process. Taking advantage of these features, cell death was assessed using TUNEL assay and light and electron microscopy, and the extent of Cx36 expression was studied by immunohistochemistry, Western blot, in situ hybridization and real-time RT-PCR. Secondary cell loss was most prominent between 24 and 48 h after lesioning. This peak was accompanied by an increase in Cx36 expression. When cultured explanted retinas were subjected to gap-junction blockers a significant increase in the extent of secondary cell loss after laser photocoagulation became evident. Using the same experimental paradigm we compared the incidence of cell death in wild-type and Cx36(-/-) mice. A significant increase in total number of TUNEL-positive cells occurred in the Cx36(-/-) mice compared to controls. From these data we conclude that Cx36 contributes to the survival and resistance against damage of retinal cells and thus constitutes a protective factor after traumatic injury of the retina.

Myelin tetraspan family proteins but no non-tetraspan family proteins are present in the ascidian (Ciona intestinalis) genome

Several of the proteins used to form and maintain myelin sheaths in the central nervous system (CNS) and the peripheral nervous system (PNS) are shared among different vertebrate classes. These proteins include one-to-several alternatively spliced myelin basic protein (MBP) isoforms in all sheaths, proteolipid protein (PLP) and DM20 (except in amphibians) in tetrapod CNS sheaths, and one or two protein zero (P0) isoforms in fish CNS and in all vertebrate PNS sheaths. Several other proteins, including 2', 3'-cyclic nucleotide 3'-phosphodiesterase (CNP), myelin and lymphocyte protein (MAL), plasmolipin, and peripheral myelin protein 22 (PMP22; prominent in PNS myelin), are localized to myelin and myelin-associated membranes, though class distributions are less well studied. Databases with known and identified sequences of these proteins from cartilaginous and teleost fishes, amphibians, reptiles, birds, and mammals were prepared and used to search for potential homologs in the basal vertebrate, Ciona intestinalis. Homologs of lipophilin proteins, MAL/plasmolipin, and PMP22 were identified in the Ciona genome. In contrast, no MBP, P0, or CNP homologs were found. These studies provide a framework for understanding how myelin proteins were recruited during evolution and how structural adaptations enabled them to play key roles in myelination.

Expression of neural connexins and pannexin1 in the hippocampus and inferior olive: a quantitative approach

Electrical synapses (or neuronal gap junctions) are thought to be essential for the generation of synchronous oscillatory activities in various areas of the brain. In this study, we quantified the steady state mRNA expression levels of two neuronal gap junction proteins, connexin36 (Cx36) and connexin45 (Cx45), as well as of pannexin1, a member of a novel class of communicative junction forming proteins, and of connexin47 (Cx47) which is expressed in oligodendrocytes. The expression levels of these genes were compared in two regions known for oscillatory activity and which are equipped with electrically coupled neurons. Assessment of the levels of mRNA expression in the hippocampus and the nuclear complex of the inferior olive (IO) was achieved by means of laser microdissection (LMM) in combination with real time RT-PCR. Our results demonstrate the differential expression of Cx36, Cx45, pannexin1 and Cx47 in the hippocampus, with pannexin1 showing the highest level of expression followed by Cx36, Cx47, and Cx45. In the IO, pannexin1 showed a comparable expression level as in the hippocampus, but connexin expression levels were increased. Upon direct comparison, the combination of LMM and real time RT-PCR data generated specific, robust and reproducible results consistent with recent data reported about connexin expression in the nervous system. We conclude that the analytical strategy shown here provides a technological solution to overcome the less sensitive and notoriously less specific analysis of connexin expression by in situ hybridization.

Fine-structural analysis and connexin expression in the retina of a transgenic model of Huntington's disease

Recent studies indicate that the visual system appears more frequently affected in polyglutamine diseases than expected previously. Here, we investigated retinal degenerations in adult transgenic R6/2 mice, a model for Huntington's disease (HD). Light microscopical analysis revealed retinal dystrophy all over the retina, with central areas showing major effects. Electron microscopical analysis showed strong degenerations of outer and inner photoreceptor segments, shrinkage of photoreceptor cell somata, and signs of degeneration in photoreceptor terminals in the outer plexiform layer. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling showed hints of apoptosis. Mutant huntingtin and ubiquitin were expressed in all classes of retinal neurons, the pigment epithelium, and to a minor extent in neuropil structures. For investigating possible links to functional impairments in the rod-cone pathway, expression levels of three connexins (Cx) were compared in R6/2 and wildtype mice retinae. In R6/2 mice, expression of Cx36, the major neuronal connexin in the retina, was slightly reduced in the outer plexiform layer, indicating affected photoreceptor terminals as detected at the electron microscopical level. In contrast, Cx45, a putative neuronal connexin in the retina, was remarkably reduced in the inner plexiform layer of R6/2 mice. This result corresponded to fainter signals of Cx45 mRNA as documented by in situ hybridization and to a lower level of mCx45 cDNA as obtained by polymerase chain reaction after reverse transcription, suggesting functional deficits in spatial processing of Cx45-mediated gap junction coupling due to transgene-induced retinal degenerations. Thus, it is important to clarify the meaning of visual involvement in HD.

Molecular Cloning and Functional Expression of zfCx52.6

Gap junction-mediated electrical coupling contributes to synchronous oscillatory activities of neurons, and considerable progress has been made in defining the molecular composition of gap junction channels. In particular, cloning and functional expression of gap junction proteins (connexins (Cx)) from zebrafish retina have shown that this part of the brain possesses a high degree of connexin diversity that may account for differential functional properties of electrical synapses. Here, we report the cloning and functional characterization of a new connexin, designated zebrafish Cx52.6 (zfCx52.6). This connexin shows little similarity to known connexins from fish and higher vertebrates. By combining in situ hybridization with Laser Capture Microdissection and RT-PCR, we found that this novel fish connexin is expressed in horizontal cells in the inner nuclear layer of the retina. Functional expression of zfCx52.6 in neuroblastoma cells and Xenopus oocytes led to functional gap junctional channels and, in single oocytes, induced large non-junctional membrane currents indicative of the formation of hemichannels, which were inhibited in reversible fashion by raising extracellular Ca(2+) concentrations.

Homeobox gene expression in adult dorsal root ganglia during sciatic nerve regeneration: is regeneration a recapitulation of development?

After damage of the sciatic nerve, a regeneration process is initiated. Neurons in the dorsal root ganglion regrow their axons and functional connections. The molecular mechanisms of this neuronal regenerative process have remained elusive, but a relationship with developmental processes has been conceived. This chapter discusses the applicability of the developmental hypothesis of regeneration to the dorsal root ganglion; this hypothesis states that regeneration of dorsal root ganglion neurons is a recapitulation of development. We present data on changes in gene expression upon sciatic nerve damage, and the expression and function of homeobox genes. This class of transcription factors plays a role in neuronal development. Based on these data, it is concluded that the hypothesis does not hold for dorsal root ganglion neurons, and that regeneration-specific mechanisms exist. Cytokines and the associated Jak/STAT (janus kinase/signal transducer and activator of transcription) signal transduction pathway emerge as constituents of a regeneration-specific mechanism. This mechanism may be the basis of pharmacological strategies to stimulate regeneration.

Proteolipid plasmolipin: localization in polarized cells, regulated expression and lipid raft association in CNS and PNS myelin

The proteolipid plasmolipin is member of the expanding group of tetraspan (4TM) myelin proteins. Initially, plasmolipin was isolated from kidney plasma membranes, but subsequent northern blot analysis revealed highest expression in the nervous system. To gain more insight into the functional roles of plasmolipin, we have generated a plasmolipin-specific polyclonal antibody. Immunohistochemical staining confirms our previous observation of glial plasmolipin expression and proves plasmolipin localization in the compact myelin of rat peripheral nerve and myelinated tracts of the CNS. Western blot analysis indicates a strong temporal correlation of plasmolipin expression and (re-) myelination in the PNS and CNS. However, following axotomy plasmolipin expression is also recovered in non-regenerating distal nerve stumps. In addition, we detected plasmolipin expression in distinct neuronal subpopulations of the CNS. The observed asymmetric distribution of plasmolipin in compact myelin, as well as in epithelial cells of kidney and stomach, indicates a polarized cellular localization. Therefore, we purified myelin from the CNS and PNS and demonstrated an enrichement of phosphorylated plasmolipin protein in detergent-insoluble lipid raft fractions, suggesting selective targeting of plasmolipin to the myelin membranes. The present data indicate that the proteolipid plasmolipin is a structural component of apical membranes of polarized cells and provides the basis for further functional analysis.

Proteins of peripheral myelin are associated with glycosphingolipid/cholesterol-enriched membranes

A characteristic feature of the vertebrate nervous system is the ensheathment of axons by myelin, a multilamellar membrane specialization produced by polarized glial cells. Although the main protein and lipid components of the myelin sheath are well characterized, relatively little is known about the mechanisms of their intracellular distribution to the respective sites of assembly within the myelin sheath. To analyze whether peripheral myelin protein trafficking is mediated by glycosphingolipid/cholesterol-enriched membranes (GEMs), we studied the association of established myelin proteins, peripheral myelin protein 22 (PMP22), protein zero (P0), plasmolipin, and myelin basic protein (MBP), with these membrane microdomains. To examine the association of the selected peripheral myelin proteins with detergent-insoluble GEMs, purified myelin from sciatic nerve of adult rat was extracted with Triton X-100 at 4 degrees C and 37 degrees C and, in additional experiments, was pretreated with the cholesterol chelator methyl-beta-cyclodextrin. The material was then centrifuged to equilibrium in sucrose gradients, and fractions were analyzed by Western blotting. Here we demonstrate for the first time that PMP22, P0, and plasmolipin prepared from purified peripheral myelin are associated with GEMs. To characterize whether the association of these proteins is a specialized feature of myelinating Schwann cells, we studied the distribution of PMP22, P0, and plasmolipin in transiently transfected HeLa cells. These experiments confirm the specific association of these proteins with GEMs in both neural and nonneural cell types.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

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.

Rho-dependent regulation of cell spreading by the tetraspan membrane protein Gas3/PMP22

Gas3/PMP22 plays a crucial role in regulating myelin formation and maintenance, and different genetic alterations in gas3/PMP22 are responsible for a set of human peripheral neuropathies. We have previously demonstrated that Gas3/PMP22 could regulate susceptibility to apoptosis in NIH3T3 cells but not in REF 52 cells. In this report we demonstrate that when the apoptotic response triggered by gas3/PMP22 was counteracted by Bcl-2 coexpression, morphological changes were observed. Time-lapse analysis confirmed that Gas3/PMP22 can modulate cell spreading, and this effect was strengthened after inhibition of phosphoinositide 3-kinase. Using the active form of the small GTPase RhoA, we have been able to dissect the different Gas3/PMP22 biological activities. RhoA counteracted the Gas3/PMP22-dependent morphological response but was unable to neutralize the apoptotic response. Treatment of NIH3T3 cells with cytotoxic necrotizing factor 1, which activates endogenous Rho, also counteracted Gas3/PMP22-mediated cell shape and spreading changes. Treatment of REF 52 cells, which are unresponsive to Gas3/PMP22 overexpression, with the C3 exoenzyme, inhibiting Rho activity, renders REF 52 cells responsive to Gas3/PMP22 overexpression for cell shape and spreading changes. Finally, assembly of stress fibers and focal adhesions complexes, in response to lysophosphatidic acid-induced endogenous Rho activation, was impaired in Gas3/PMP22-overexpressing cells. We hypothesize that cell shape and spreading regulated by Gas3/PMP22 through the Rho GTPase might have an important role during Schwann cells differentiation and myelinization.

rMAL is a glycosphingolipid-associated protein of myelin and apical membranes of epithelial cells in kidney and stomach

rMAL, the rat myelin and lymphocyte protein, is a small hydrophobic protein of 17 kDa with four putative transmembrane domains and is expressed in oligodendrocytes and Schwann cells, the myelinating cells of the nervous system. In addition, transcript expression has been found in kidney, spleen, and intestine. Confocal microscopy and immunoelectron microscopy with an affinity-purified antibody localized rMAL to compact myelin in a pattern similar to the structural myelin proteins: myelin basic protein and proteolipid protein. In kidney and stomach epithelia, rMAL is located almost exclusively on the apical (luminal) membranes of the cells lining distal tubuli in kidney and the glandular part of the stomach. Biochemical analysis of plasma membranes isolated from spinal cord and kidney demonstrated that rMAL is a proteolipid that is present in detergent insoluble complexes typical for proteins associated with glycosphingolipids. Lipid and protein analysis showed a co-enrichment of glycosphingolipids and rMAL protein within these complexes, indicating a close association of rMAL to glycosphingolipids in myelin and in kidney in vivo. We conclude that specific rMAL-glycosphingolipid interactions may lead to the formation and maintenance of stable protein-lipid microdomains in myelin and apical epithelial membranes. They may contribute to specific properties of these highly specialized plasma membranes.

Many facets of the peripheral myelin protein PMP22 in myelination and disease

Peripheral myelin protein 22 (PMP22) is a small, hydrophobic glycoprotein, which is most prominently expressed by Schwann cells as a component of compact myelin of the peripheral nervous system (PNS). Recent progress in molecular genetics revealed that mutations affecting the PMP22 gene including duplications, deletions, and point mutations are responsible for the most common forms of hereditary peripheral neuropathies including Charcot-Marie-Tooth disease type 1A (CMT1A), hereditary neuropathy with liability to pressure palsies (HNPP), and a subtype of Dejerine-Sottas Syndrome (DSS). Functionally, PMP22 is involved in correct myelination during development of peripheral nerves, the stability of myelin, and the maintenance of axons. While most of these functions relate to a role of PMP22 as a structural component of myelin, PMP22 has also been proposed as a regulator of Schwann cell proliferation and differentiation. In this review, we will discuss our current knowledge of PMP22 and its related proteins in the normal organism as well as in disease. In particular, we will focus on how the function of PMP22 and its regulation may be relevant to particular disease mechanisms.

Advances in Charcot-Marie-Tooth disease research: cellular function of CMT-related proteins, transgenic animal models, and pathomechanisms. The European CMT Consortium

The First Workshop of the European Consortium on Charcot-Marie-Tooth (CMT) disease brought together neuroscientists, molecular and cell biologists, neuropathologists, neurologists, and geneticists with a common interest in the understanding of the fundamental mechanisms that underlie the pathogenesis of CMT. The interdisciplinary group of 25 expert scientists discussed recent advances in (i) molecular genetics and histopathology of CMT, (ii) development of suitable animal models, (iii) understanding of the cellular function of CMT-related proteins, and (iv) studies using nerve biopsies from CMT patients. In this minireview, we summarize the key findings presented and discuss their impact on CMT research.

Myelin and lymphocyte protein (MAL/MVP17/VIP17) and plasmolipin are members of an extended gene family

An increasing number of four-transmembrane proteins has been found to be associated with CNS and PNS myelin. Some of these proteins play crucial roles in the development and maintenance of the nervous system. In the CNS, proteolipid protein (PLP) is mutated in the myelin disorder Pelizaeus-Merzbacher disease and in spastic paraplegia, while in the PNS, peripheral myelin protein 22 (PMP22) and connexin32 (C x 32) are culprit genes in the most frequent forms of hereditary peripheral neuropathies. Myelin and lymphocyte protein (MAL; also called MVP17 or VIP17) and plasmolipin are additional tetraspan proteins that are highly expressed by myelinating glial cells. However, little is known about the role of these proteins in the nervous system. As a prerequisite for functional genetic approaches in the mouse, we have isolated and characterized a mouse MAL cDNA and the corresponding structural MAL gene. Computer-aided analysis and database searches revealed that MAL belongs to a larger gene family which also includes plasmolipin, BENE and the expressed sequence tag (EST) H09290. While the overall amino acid sequence identities between mouse MAL and the related proteins are relatively low (29-37%), the conserved motif -[Q/Y-G-W-V-M-F/Y-V]- which is found at the junction of the first extracellular loop and the second membrane-associated domain serves as a fingerprint for the MAL protein family. Expression analysis of the members of the MAL gene family indicates widespread expression in various tissues, suggesting a common role of these proteins in cell biology.

Schwann cell development, differentiation and myelination

Neu-differentiation factor (glial growth factor) has been established as an important regulator of early Schwann cell development, and the lineage relationship between immature Schwann cells and the neural crest has been clarified by the identification of the Schwann cell precursor. Progress has been made in identifying transcription factors that control Schwann cell development and in defining molecules that positively and negatively regulate myelin differentiation pathways. The tetraspan group has emerged as a set of proteins with prominent functions in Schwann cell biology.