Differential regulation of the zinc finger genes Krox-20 and Krox-24 (Egr-1) suggests antagonistic roles in Schwann cells

Phosphatidic acid mediates demyelination in Lpin1 mutant mice

Lipids play crucial roles in many aspects of glial cell biology, affecting processes ranging from myelin membrane biosynthesis to axo-glial interactions. In order to study the role of lipid metabolism in myelinating glial cells, we specifically deleted in Schwann cells the Lpin1 gene, which encodes the Mg2+-dependent phosphatidate phosphatase (PAP1) enzyme necessary for normal triacylglycerol biosynthesis. The affected animals developed pronounced peripheral neuropathy characterized by myelin degradation, Schwann cell dedifferentiation and proliferation, and a reduction in nerve conduction velocity. The observed demyelination is mediated by endoneurial accumulation of the substrate of the PAP1 enzyme, phosphatidic acid (PA). In addition, we show that PA is a potent activator of the MEK-Erk pathway in Schwann cells, and that this activation is required for PA-induced demyelination. Our results therefore reveal a surprising role for PA in Schwann cell fate determination and provide evidence of a direct link between diseases affecting lipid metabolism and abnormal Schwann cell function.

The cyclin-dependent kinase inhibitor p57kip2 is a negative regulator of Schwann cell differentiation and in vitro myelination

The p57kip2 gene encodes a member of the cyclin-dependent kinase inhibitor family, proteins known to block G(1)/S transition during the mammalian cell cycle. We observed that expression of p57kip2 in Schwann cells of the developing and injured adult peripheral nervous system is dynamically regulated. Using gene knockdown by means of vector-based RNA interference in cultured primary Schwann cells we found that reduced levels of p57kip2 lead to cell cycle exit, actin filament stabilization, altered cell morphology and growth, and down-regulation of promyelinating markers as well as induction of myelin genes and proteins. In addition, we could demonstrate that in vitro myelination is enhanced by p57kip2-suppressed Schwann cells. Using microarray technology we found that these cellular reactions are specific to lowered p57kip2 expression levels and detected a shift of the transcriptional expression program toward the pattern known from Schwann cells in developing peripheral nerves. Because in the absence of axons primary Schwann cells normally do not display differentiation-associated reactions, we conclude that we have identified a mechanism and an important intrinsic negative regulator of myelinating glia differentiation.

Disruption of Krox20-Nab interaction in the mouse leads to peripheral neuropathy with biphasic evolution

Krox20/Egr2 is a zinc finger transcription factor that plays essential roles in several developmental processes, including peripheral nervous system myelination by Schwann cells, where it acts as a master gene regulator. Krox20 is known to interact with cofactors of the Nab family and a mutation affecting isoleucine 268, which prevents this interaction, has been shown to result in congenital hypomyelinating neuropathy in humans. To further investigate the role of this interaction, we have introduced such a mutation, Krox20(I268F), in the mouse germ line. Clinical, immunohistochemical, and ultrastructural analyses of the homozygous mutants reveal that they develop a severe hypomyelination phenotype that mimics the human syndrome. Furthermore, a time-course analysis of the disease indicates that it follows a biphasic evolution, the hypomyelination phase being followed by a dramatic demyelination. Although for the regulation of most analyzed Krox20 target genes the mutation behaves as a loss of function, this is not the case for a few of them. This differential effect indicates that the molecular function of the Krox20-Nab interaction is target dependent and might explain the degradation of the residual myelin, because of imbalances in its composition. In conclusion, this work provides a novel and useful model for severe human peripheral neuropathies.

Induction of zinc-finger proliferation 1 expression in non-myelinating Schwann cells after denervation

Terminal Schwann cells (tSCs) are non-myelinating glia that wrap the nerve terminal at the neuromuscular junction. They are required for the maintenance of the neuromuscular synapse and are likely to play essential roles in the restoration of synaptic connections after nerve injury. tSCs acquire a reactive phenotype after nerve damage characterized by the extension of cellular processes that may facilitate reinnervation. The molecular signaling events underpinning the tSC reactive state remain elusive, in particular, little is known about transcription factors involved in the transcriptional reprogramming during tSC activation. Prior research implicated nine members of the zinc-finger transcription factor family in Schwann cell (SC) development and myelination, and levels of one such protein were reported increased in other non-myelinating SCs after denervation. We hypothesize that zinc-finger transcription factors could play a role during tSC activation. Because of their relative paucity, tSCs are difficult to study molecularly. Here, we used the rat cervical sympathetic trunk (CST), an autonomic nerve in which non-myelinating SCs are the predominant cell type, to isolate zinc-finger protein (ZFP) cDNAs by reverse transcriptase-polymerase chain reaction. We isolated 29 unique ZFP sequences of which zinc proliferation 1 (Zipro1) was the most abundant. We found that after CST transection, levels for Zipro1 mRNA doubled and that Zipro1 protein expression increased in non-myelinating CST SCs. We also determined that Zipro1 is expressed in tSCs and its levels increased following skeletal muscle denervation. Thus, Zipro1 is a good candidate for a transcription factor involved in activation of non-myelinating SCs in general, and tSCs in particular.

Paradoxical role of an Egr transcription factor family member, Egr2/Krox20, in learning and memory

It is well established that Egr1/zif268, a member of the Egr family of transcription factors, is critical for the consolidation of several forms of memories. Recently, the Egr3 family member has also been implicated in learning and memory. Because Egr family members encode closely related zinc-finger transcription factors sharing a highly homologous DNA binding domain that recognises the same DNA sequence, they may have related functions in brain. Another Egr family member expressed in brain, Egr2/Krox20 is known to be crucial for normal hindbrain development and has been implicated in several inherited peripheral neuropathies; however, due to Egr2-null mice perinatal lethality, its potential role in cognitive functions in the adult has not been yet explored. Here, we generated Egr2 conditional mutant mice allowing postnatal, forebrain-specific Cre-mediated Egr2 excision and tested homozygous, heterozygous and control littermates on a battery of behavioural tasks to evaluate motor capacity, exploratory behaviour, emotional reactivity and learning and memory performance in spatial and non-spatial tasks. Egr2-deficient mice had no sign of locomotor, exploratory or anxiety disturbances. Surprisingly, they also had no impairment in spatial learning and memory, taste aversion memory or fear memory using a trace conditioning paradigm. On the contrary, Egr2-deficient mice had improved performance in motor learning on a rotarod, and in object recognition memory. These results clearly do not extend the phenotypic consequences resulting from either Egr1 or Egr3 loss-of-function to Egr2. In contrast, they indicate that Egr family members may have different, and in certain circumstances antagonistic functions in the adult brain.

Differential regulation of NAB corepressor genes in Schwann cells

BACKGROUND

Myelination of peripheral nerves by Schwann cells requires not only the Egr2/Krox-20 transactivator, but also the NGFI-A/Egr-binding (NAB) corepressors, which modulate activity of Egr2. Previous work has shown that axon-dependent expression of Egr2 is mediated by neuregulin stimulation, and NAB corepressors are co-regulated with Egr2 expression in peripheral nerve development. NAB corepressors have also been implicated in macrophage development, cardiac hypertrophy, prostate carcinogenesis, and feedback regulation involved in hindbrain development.

RESULTS

To test the mechanism of NAB regulation in Schwann cells, transfection assays revealed that both Nab1 and Nab2 promoters are activated by Egr2 expression. Furthermore, direct binding of Egr2 at these promoters was demonstrated in vivo by chromatin immunoprecipitation analysis of myelinating sciatic nerve, and binding of Egr2 to the Nab2 promoter was stimulated by neuregulin in primary Schwann cells. Although Egr2 expression activates the Nab2 promoter more highly than Nab1, we surprisingly found that only Nab1 - but not Nab2 - expression levels were reduced in sciatic nerve from Egr2 null mice. Analysis of the Nab2 promoter showed that it is also activated by ETS proteins (Ets2 and Etv1/ER81) and is bound by Ets2 in vivo.

CONCLUSION

Overall, these results indicate that induction of Nab2 expression in Schwann cells involves not only Egr2, but also ETS proteins that are activated by neuregulin stimulation. Although Nab1 and Nab2 play partially redundant roles, regulation of Nab2 expression by ETS factors explains several observations regarding regulation of NAB genes. Finally, these data suggest that NAB proteins are not only feedback inhibitors of Egr2, but rather that co-induction of Egr2 and NAB genes is involved in forming an Egr2/NAB complex that is crucial for regulation of gene expression.

Glial cells: old cells with new twists

Based on their characteristics and function--migration, neural protection, proliferation, axonal guidance and trophic effects--glial cells may be regarded as probably the most versatile cells in our body. For many years, these cells were considered as simply support cells for neurons. Recently, it has been shown that they are more versatile than previously believed--as true stem cells in the nervous system--and are important players in neural function and development. There are several glial cell types in the nervous system: the two most abundant are oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Although both of these cells are responsible for myelination, their developmental origins are quite different. Oligodendrocytes originate from small niche populations from different regions of the central nervous system, while Schwann cells develop from a stem cell population (the neural crest) that gives rise to many cell derivatives besides glia and which is a highly migratory group of cells.

Noise propagation and scaling in regulation of gonadotrope biosynthesis

Reproductive physiology depends on the control of biosynthesis in the pituitary gonadotrope by hypothalamic gonadotropin-releasing hormone (GnRH). The responses to GnRH include activation of extracellular signal-regulated kinase (ERK) and induction of Egr1. Using population and single cell signaling assays, we investigated the signal and noise transmission through this signaling and gene circuit, analyzing data obtained from 43,775 individual cells in 40 experiments. After exposure to GnRH, phosphorylated ERK (pERK) is elevated in 50% of the cells at 1.7 (SD = 0.3) min. Studies of the cell-to-cell response showed that for both pERK and for Egr1 protein production the mean response (mu) and standard deviation (sigma) within individual cells were linearly related (sigma = kmu) and had similar values of k. To understand the basis for the scaling observed for noise propagation through this system, we determined the relationship between pERK and egr1 mRNA levels induced at varying concentration of GnRH. While both pERK and egr1 mRNA show a saturating sigmoidal relationship to the concentration of GnRH exposure, egr1 mRNA is linearly related to the levels of pERK. These results explain the basis for variation in cellular responses in an important mammalian signaling pathway leading to gene induction.

Interactions of Sox10 and Egr2 in myelin gene regulation

Myelination in the PNS is accompanied by a large induction of the myelin protein zero (Mpz) gene to produce the most abundant component in peripheral myelin. Analyses of knockout mice have shown that the EGR2/Krox20 and SOX10 transcription factors are required for Mpz expression. Our recent work has shown that the dominant EGR2 mutations associated with human peripheral neuropathies cause disruption of EGR2/SOX10 synergy at specific sites, including a conserved enhancer element in the first intron of the Mpz gene. Further investigation of Egr2/Sox10 interactions reveals that activation of the Mpz intron element by Egr2 requires both Sox10-binding sites. In addition, both Egr1 and Egr3 cooperate with Sox10 to activate this element, which indicates that this capacity is conserved among Egr family members. Finally, a conserved composite structure of Egr2/Sox10-binding sites in the genes encoding Mpz, myelin-associated glycoprotein and myelin basic protein genes was used to screen for similar modules in other myelin genes, revealing a potential regulatory element in the periaxin gene. Overall, these results elucidate a working model for developmental regulation of Mpz expression, several facets of which extend to regulation of other peripheral myelin genes.

Molecular physiology of pituitary development: signaling and transcriptional networks

The pituitary gland is a central endocrine organ regulating basic physiological functions, including growth, the stress response, reproduction, metabolic homeostasis, and lactation. Distinct hormone-producing cell types in the anterior pituitary arise from a common ectodermal primordium during development by extrinsic and intrinsic mechanisms, providing a powerful model system for elucidating general principles in mammalian organogenesis. The central purpose of this review is to inspect the integrated signaling and transcriptional events that affect precursor proliferation, cell lineage commitment, terminal differentiation, and physiological regulation by hypothalamic tropic factors.

An oligodendrocyte-specific zinc-finger transcription regulator cooperates with Olig2 to promote oligodendrocyte differentiation

Molecular mechanisms that control oligodendrocyte myelination during mammalian central nervous system (CNS) development are poorly understood. In this study, we identified Zfp488, an oligodendrocyte-specific zinc-finger transcription regulator, by screening for genes downregulated in the optic nerves of Olig1-null mice. The predicted primary structure of Zfp488 is evolutionarily conserved in vertebrates and invertebrates. In the developing CNS, Zfp488 is specifically expressed in oligodendrocytes but not their precursors. Its expression increases in parallel with that of major myelin genes Mbp and Plp1. Zfp488 is a nuclear protein that possesses transcriptional repression activity. In the developing chick neural tube, Zfp488 can promote oligodendrocyte precursor formation upon Notch activation. In addition, Zfp488 can interact and cooperate with the bHLH transcription factor Olig2 to promote precocious and ectopic oligodendrocyte differentiation. Furthermore, knockdown of Zfp488 via RNAi in an oligodendroglial cell line leads to the downregulation of myelin gene expression. Taken together, these data suggest that Zfp488 functions as an oligodendrocyte-specific transcription co-regulator important for oligodendrocyte maturation and that zinc-finger/bHLH cooperation can serve as a mechanism for oligodendroglial differentiation.

Mixed Analog/Digital Gonadotrope Biosynthetic Response to Gonadotropin-releasing Hormone

Mammalian reproduction requires gonadotropin-releasing hormone (GnRH)-mediated signaling from brain neurons to pituitary gonadotropes. Because the pulses of released GnRH vary greatly in amplitude, we studied the biosynthetic response of the gonadotrope to varying GnRH concentrations, focusing on extracellular-regulated kinase (ERK) phosphorylation and egr1 mRNA and protein production. The overall average level of ERK activation in populations of cells increased non-cooperatively with increasing GnRH and did not show evidence of either ultrasensitivity or bistability. However, automated image analysis of single-cell responses showed that whereas individual gonadotropes exhibited two response states, inactive and active, both the probability of activation and the average response in activated cells increased with increasing GnRH concentration. These data indicate a hybrid single-cell response having both digital (switch-like) and analog (graded) features. Mathematical modeling suggests that the hybrid response can be explained by indirect thresholding of ERK activation resulting from the distributed structure of the GnRH-modulated network. The hybrid response mechanism improves the reliability of noisy reproductive signal transmission from the brain to the pituitary.

Peripheral myelin maintenance is a dynamic process requiring constant Krox20 expression

Onset of myelination in Schwann cells is governed by several transcription factors, including Krox20/Egr2, and mutations affecting Krox20 result in various human hereditary peripheral neuropathies, including congenital hypomyelinating neuropathy (CHN) and Charcot-Marie-Tooth disease (CMT). Similar molecular information is not available on the process of myelin maintenance. We have generated conditional Krox20 mutations in the mouse that allowed us to develop models for CHN and CMT. In the latter case, specific inactivation of Krox20 in adult Schwann cells results in severe demyelination, involving rapid Schwann cell dedifferentiation and increased proliferation, followed by an attempt to remyelinate and a block at the promyelinating stage. These data establish that Krox20 is not only required for the onset of myelination but that it is also crucial for the maintenance of the myelinating state. Furthermore, myelin maintenance appears as a very dynamic process in which Krox20 may constitute a molecular switch between Schwann cell myelination and demyelination programs.

Persistent expression of Notch2 delays gonadotrope differentiation

Normal pituitary gland development requires coordination between maintenance of progenitor cell pools and selection of progenitors for differentiation. The spatial and temporal expression of Notch2 during pituitary development suggested that it could control progenitor cell differentiation in the pituitary. Consistent with this idea, Notch2 is not expressed in Prop1 mutants, and anterior pituitary progenitors in Prop1 mutants appear to be unable to transition from proliferation to differentiation properly, resulting in anterior lobe failed cell specification and evolving hypoplasia. To test the function of Notch2 directly, we used the alphaGSU subunit promoter to express activated NOTCH2 persistently in pre-gonadotropes and pre-thyrotropes of transgenic mice. At birth, there is a small reduction in the population of fully differentiated thyrotropes and almost no fully differentiated gonadotropes. The temporal and spatial expression of Hey1 suggests that it could be a mediator of this effect. Gonadotropes complete their differentiation program eventually, although expression of LH and FSH is mutually exclusive with NOTCH2 transgene expression. This demonstrates that activated Notch2 is sufficient to delay gonadotrope differentiation, and it supports the hypothesis that Notch2 regulates progenitor cell differentiation in the pituitary gland.

Regulation of Myelin-Specific Gene Expression: Relevance to CMT1

Schwann cells, the myelinating cells of the peripheral nervous system, are derived from the neural crest. Once neural crest cells are committed to the Schwann cell fate, they can take on one of two phenotypes to become myelinating or nonmyelinating Schwann cells, a decision that is determined by interactions with axons. The critical step in the differentiation of myelinating Schwann cells is the establishment of a one-to-one relationship with axons, the so-called "promyelinating" stage of Schwann cell development. The transition from the promyelinating to the myelinating stage of development is then accompanied by a number of significant changes in the pattern of gene expression, including the activation of a set of genes encoding myelin structural proteins and lipid biosynthetic enzymes, and the inactivation of a set of genes expressed only in immature or nonmyelinating Schwann cells. These changes are regulated mainly at the transcriptional level and also require continuous interaction between Schwann cells and their axons. Two transcription factors, Krox 20 (EGR2) and Oct 6 (SCIP/Tst1), are necessary for the transition from the promyelinating to the myelinating stage of Schwann cell development. Krox 20, expressed in myelinating but not promyelinating Schwann cells, is absolutely required for this transition, and myelination cannot occur in its absence. Oct 6, expressed mainly in promyelinating Schwann cells and then downregulated before myelination, is necessary for the correct timing of this transition, since myelination is delayed in its absence. Neither Krox 20 nor Oct 6, however, is required for the initial activation of myelin gene expression. Although the mechanisms of Krox 20 and Oct 6 action during myelination are not known, mutation in Krox 20 has been shown to cause CMT1, further implicating this protein in the pathogenesis of this disease. Identifying the molecular mechanisms of Krox 20 and Oct 6 action will thus be important both for understanding myelination and for designing future treatments for CMT1. Point mutations in the genes encoding the myelin proteins PMP22 and P0 cause CMT1A without a gene duplication and CMT1B, respectively. Although the clinical and pathological phenotypes of CMT1A and CMT1B are similar, their molecular pathogenesis is quite different. Point mutations in PMP22 alter the trafficking of the protein, so that it accumulates in the endoplasmic reticulum (ER) and intermediate compartment (IC). Mutant PMP22 also sequesters its normal counterpart in the ER, further reducing the amount of PMP22 available for myelin synthesis at the membrane, and accounting, at least in part, for its severe effect on myelination. Mutant PMP22 probably also activates an ER-to-nucleus signal transduction pathway associated with misfolded proteins, which may account for the decrease of myelin gene expression in Schwann cells in Trembler mutant mice. In contrast, absence of expression of the homotypic adhesion molecule, P₀, in mice in which the gene has been inactivated, produces a unique pattern of Schwann cell gene expression, demonstrating that P₀ plays a regulatory as well as a structural role in myelination. Whether this role is direct, through a P₀-mediated adhesion pathway, or indirect, through adhesion pathways mediated by cadherins or integrins, however, remains to be determined. The molecular mechanisms underlying dysmyelination in CMT1 are thus complex, with pleitropic effects on Schwann cell physiology that are determined both by the type of mutation and the protein mutated. Identifying these molecular mechanisms, however, are important both for understanding myelination and for designing future treatments for CMT1. Although demyelination is the hallmark of CMT1, the clinical signs and symptoms of this disease are probably produced by axonal degeneration, not demyelination. Interestingly, a number of recent studies have demonstrated that Schwann cells from Trembler mice or patients with CMT1A can induce local axonal abnormalities, including decreased axonal transport, and altered neurofilament phosphorylation. These data thus suggest that disability of patients with CMT1 is caused by abnormal Schwann cell-axonal interactions. Efforts both to understand the effects of myelinating Schwann cells on their axons and to prevent axonal degeneration or promote axonal regeneration are thus central for the future development of a rational molecular therapy for CMT1.

Early growth response 1 acts as a tumor suppressor in vivo and in vitro via regulation of p53

The early growth response 1 (Egr1) gene is a transcription factor that acts as both a tumor suppressor and a tumor promoter. Egr1-null mouse embryo fibroblasts bypass replicative senescence and exhibit a loss of DNA damage response and an apparent immortal growth, suggesting loss of p53 functions. Stringent expression analysis revealed 266 transcripts with >2-fold differential expression in Egr1-null mouse embryo fibroblasts, including 143 known genes. Of the 143 genes, program-assisted searching revealed 66 informative genes linked to Egr1. All 66 genes could be placed on a single regulatory network consisting of three branch points of known Egr1 target genes: TGFbeta1, IL6, and IGFI. Moreover, 19 additional genes that are known targets of p53 were identified, indicating that p53 is a fourth branch point. Electrophoretic mobility shift assay as well as chromatin immunoprecipitation confirmed that p53 is a direct target of Egr1. Because deficient p53 expression causes tumors in mice, we tested the role of Egr1 in a two-step skin carcinogenesis study (144 mice) that revealed a uniformly accelerated development of skin tumors in Egr1-null mice (P < 0.005). These studies reveal a new role for Egr1 as an in vivo tumor suppressor.

Molecular mechanisms in Schwann cell survival and death during peripheral nerve development, injury and disease

The mechanisms determining the fate of Schwann cells during disease and injury of the adult mammalian peripheral nervous system (PNS) are becoming defined by current advances in molecular neurobiology. It is now apparent that the molecular pathways which regulate the production of the mature myelinating Schwann cell during development may also apply to degenerative and regenerative mechanisms following PNS disease. This review outlines neurobiological responses of Schwann cells during development, injury and disease in order to define the molecular pathways which regulate these crucial events. These mechanisms have implications for our attempts to intervene pharmacologically during pathologies of the PNS.

Novel method for studying myelination in vivo reveals that EDTA is a potent inhibitor of myelin protein and mRNA expression during development of the rat sciatic nerve

To probe the effects of possible inhibitors or enhancers of in vivo myelination, we have modified a technique widely used in studies of the developing neuromuscular system that involves incorporation of test compounds into a silicon rubber solution, which solidifies on contact with air. U-shaped rubber implants are inserted around the sciatic nerve of 1-day-old rats and left in place for 24-48 h. Sections from the region of the nerve lying within the implant, with or without the test compound, are then immunolabeled, examined with in situ hybridization or electron microscopy. Application of EDTA (440 microg/implant) in this way strongly suppressed the levels of the myelin-associated molecules protein P0, myelin basic protein (MBP), and galactocerebroside (Galc). mRNA levels for P0 and the myelin-related transcription factor Krox-20 were also reduced, further supporting association of the EDTA-induced effect with the myelinating Schwann cells. In contrast, no obvious differences were observed in either neurofilament (NF) protein or glial fibrillary acidic protein (GFAP) expression, suggesting absence of influence on axons or nonmyelinating Schwann cells. Despite the severely altered molecular composition of myelin in the presence of EDTA, examination in the electron microscope did not reveal any apparent ultrastructural changes in the myelin sheaths or nerve development. This work introduces a novel method for studying nerve development and shows that EDTA, which chelates divalent cations such as Ca(2+) and Mg(2+), strongly and selectively reduces levels of molecules, which, on postnatal days 1-4, are expressed in myelinating cells at much higher levels than in cells not engaged in myelination.

Identification and Characterization of ZFP-57, a Novel Zinc Finger Transcription Factor in the Mammalian Peripheral Nervous System

To isolate new zinc finger genes expressed at early stages of peripheral nerve development, we have used PCR to amplify conserved zinc finger sequences. RNA from rat embryonic day 12 and 13 sciatic nerves, a stage when nerves contain Schwann cell precursors, was used to identify several genes not previously described in Schwann cells. One of them, zinc finger protein (ZFP)-57, proved to be the homologue of a mouse gene found in F9 teratocarcinoma cells. Its mRNA expression profile within embryonic and adult normal and transected peripheral nerves, and its distribution in the rest of the nervous system is described. High levels of expression are seen in embryonic nerves and spinal cord. These drop rapidly during the first few weeks after birth, a pattern mirrored in other parts of the nervous system. ZFP-57 localizes to the nucleus of Schwann and other cells. The sequence contains an N-terminal Krüppel-associated box (KRAB) domain and ZFP-57 constructs containing green fluorescent protein reveal that the protein colocalizes with heterochromatin protein 1alpha to centromeric heterochromatin in a characteristic speckled pattern in NIH3T3 cells. The KRAB domain is required for this localization, because constructs lacking it target the protein to the nucleus but not to the centromeric heterochromatin. When fused to a heterologous DNA binding domain, the KRAB domain of ZFP-57 represses transcription, and full-length ZFP-57 represses Schwann cell transcription from myelin basic protein and P(0) promoters in co-transfection assays. Zfp-57 mRNA is up-regulated in Schwann cells in response to leukemia inhibitory factor and fibroblast growth factor 2.

Rho kinase regulates schwann cell myelination and formation of associated axonal domains

The myelin sheath forms by the spiral wrapping of a glial membrane around an axon. The mechanisms involved are poorly understood but are likely to involve coordinated changes in the glial cell cytoskeleton. Because of its key role as a regulator of the cytoskeleton, we investigated the role of Rho kinase (ROCK), a major downstream effector of Rho, in Schwann cell morphology, differentiation, and myelination. Pharmacologic inhibition of ROCK activity results in loss of microvilli and stress fibers in Schwann cell cultures and strikingly aberrant myelination in Schwann cell-neuron cocultures; there was no effect on Schwann cell proliferation or differentiation. Treated Schwann cells branch aberrantly and form multiple, small, independent myelin segments along the length of axons, each with associated nodes and paranodes. This organization partially resembles myelin formed by oligodendrocytes rather than the single long myelin sheath characteristic of Schwann cells. ROCK regulates myosin light chain phosphorylation, which is robustly, but transiently, activated at the onset of myelination. These results support a key role of Rho through its effector ROCK in coordinating the movement of the glial membrane around the axon at the onset of myelination via regulation of myosin phosphorylation and actomyosin assembly. They also indicate that the molecular machinery that promotes the wrapping of the glial membrane sheath around the axon is distributed along the entire length of the internode.

Vestibular Schwann cells are a distinct subpopulation of peripheral glia with specific sensitivity to growth factors and extracellular matrix components

Vestibular nerve Schwann cells are predisposed to develop schwannoma. While knowledge concerning this condition has greatly improved, little is known about properties of normal vestibular Schwann cells. In an attempt to understand this predisposition, we evaluated cell density regulation and proliferative features of these cells taken from 6-day-old rats. Data were compared to those obtained with sciatic Schwann cells. In both vestibular and sciatic 7-day-old cultures, Schwann cells appear as bipolar or flattened cells. However, sciatic and vestibular cells greatly differ in other aspects: on poly-L-lysine coating, sciatic cells specifically synthesize myelin basic protein, while expression of P0 mRNAs is restricted to some vestibular cells. Laminin increases sciatic cell density but not that of vestibular cells. Fibronectin selectively enhances the proliferation of vestibular Schwann cells and lacks an effect on sciatic ones. Comparison of cell density changes between sciatic and vestibular cells shows that they are sensitive to two different sets of growth factors. Progesterone and FGF-2 combined with forskolin selectively enhance the cell density of sciatic glia, while IGF-1 and GDNF specifically increase vestibular cell density. Furthermore, BrdU incorporation assays indicate that GDNF is also a mitogen for vestibular cells. Altogether, vestibular Schwann cells display phenotypic features and responsiveness to exogenous signals that are significantly different from sciatic Schwann cells, suggesting that vestibular glia form a subpopulation of Schwann cells.

Early growth response 1 protein, an upstream gatekeeper of the p53 tumor suppressor, controls replicative senescence

The proliferation of most primary cells in culture is limited by replicative senescence and crisis, p53-dependent events. However, the regulation of p53 itself has not been defined. We find that deletion of the early growth response 1 (EGR1) transcription factor leads to a striking phenotype, including complete bypass of senescence and apparent immortal growth consistent with loss of a suppressor gene. EGR1-null mouse embryo fibroblasts (MEFs) exhibit decreased expression of p53, p21(Cip1/Waf1), and other p53 "marker" proteins. Precrisis WT but not EGR1-null cells exhibit irradiation-induced arrest. WT MEFs that emerge from crisis exhibit a mutated p53 (sequence confirmed), colony formation, and tumorigenicity. In contrast, high-passage EGR1-null MEFs retain the WT p53 sequence but with much reduced expression, remain untransformed, and grow continuously. An EGR1-expressing retrovirus restores p53 expression and sencescence to EGR1-null but not p53-null MEFs or postcrisis WT cells. Taken together, the results establish EGR1 as a major regulator of cell senescence and previously undescribed upstream "gatekeeper" of the p53 tumor suppressor pathway.

Mammalian achaete scute homolog 2 is expressed in the adult sciatic nerve and regulates the expression of Krox24, Mob-1, CXCR4, and p57kip2 in Schwann cells

The molecular control mechanisms and regulatory molecules involved in nerve repair are not yet well known. Schwann cells have been attributed an important role in peripheral nerve regeneration; therefore, attention has been drawn to regulatory factors expressed by these glial cells. Here, we demonstrate that Mash2, a basic helix-loop-helix (bHLH) transcription factor previously shown to be crucial for placenta development, is expressed by Schwann cells of adult peripheral nerves. We observed that this gene is downregulated after nerve lesion and, using cDNA array hybridization technology, we could demonstrate that Mash2 is a regulator of Krox24, Mob-1, and CXCR4 expression in cultured Schwann cells. In addition, we provide strong evidence that Mash2 is a negative regulator of Schwann cell proliferation. Mash2 represents a first candidate for the missing class B bHLH proteins in peripheral nerves.

Immunohistochemistry study of human vestibular nerve schwannoma differentiation

Differentiation of primary human vestibular nerve schwannomas (VS) caused by mutations of the NF2 gene was evaluated by examining the expression patterns of genes that are specifically expressed in different stages of Schwann cell lineage. In schwannoma cells that are not in contact with an axon, the expression levels of the major myelin sheath proteins, such as protein zero glycoprotein (P0), myelin basic protein (MBP), and peripheral myelin protein 22 (PMP22), were greatly reduced. However, high expression levels of nerve growth factor receptor (NGFR), neural cell adhesion molecule (N-CAM), and cell adhesion molecule L1 (L1) were observed. In addition, expression of transcription factors Krox20, Krox24, and SCIP/Oct6 was also detected in the tumor cells. These results suggest that loss of the NF2 gene was responsible for the transformation of the Schwann cells into a neoplastic stage that has a similar genetic profile to the pro-myelinating stage. Finally, the primary human vestibular schwannoma cells failed to be regulated and redifferentiated by a regenerating axon, when the human tumors were transplanted into sciatic nerve of nude rat. These results suggest that the NF2 gene might be involved in the differentiation of Schwann cells.

Immediate early genes krox-24 and krox-20 are rapidly up-regulated after wounding in the embryonic and adult mouse

Embryos show a remarkable capacity for perfect repair after injury. Wounding of embryonic skin triggers rapid activation of epithelial sweeping and mesenchymal contraction tissue movements that bear striking analogy to several naturally occurring morphogenetic tissue movements, but very little is known about the early molecular signals that might initiate such movements. Here, we describe the rapid and transient up-regulation of two immediate early genes, krox-24 and krox-20, after wounding of the embryonic mouse. Furthermore, we demonstrate that these signals are conserved, but of longer duration, in the neonate and adult wound situation. To further test the roles of these transcription factors in vivo, we performed wound healing studies on embryos lacking either Krox-24 or 20. Despite the dramatic up-regulation of these genes in response to injury, our studies reveal that neither of them on their own is essential for repair.

Characterisation of cis-acting sequences reveals a biphasic, axon-dependent regulation of Krox20 during Schwann cell development

In Schwann cells (SC), myelination is controlled by the transcription factor gene Krox20/Egr2. Analysis of cis-acting elements governing Krox20 expression in SC revealed the existence of two separate elements. The first, designated immature Schwann cell element (ISE), was active in immature but not myelinating SC, whereas the second, designated myelinating Schwann cell element (MSE), was active from the onset of myelination to adulthood in myelinating SC. In vivo sciatic nerve regeneration experiments demonstrated that both elements were activated during this process, in an axon-dependent manner. Together the activity of these elements reproduced the profile of Krox20 expression during development and regeneration. Genetic studies showed that both elements were active in a Krox20 mutant background, while the activity of the MSE, but likely not of the ISE, required the POU domain transcription factor Oct6 at the time of myelination. The MSE was localised to a 1.3 kb fragment, 35 kb downstream of Krox20. The identification of multiple Oct6 binding sites within this fragment suggested that Oct6 directly controls Krox20 transcription. Taken together, these data indicate that, although Krox20 is expressed continuously from 15.5 dpc in SC, the regulation of its expression is a biphasic, axon-dependent phenomenon involving two cis-acting elements that act in succession during development. In addition, they provide insight into the complexity of the transcription factor regulatory network controlling myelination.

Early activation of transcription factor expression in Schwann cells by progesterone

Progesterone (PROG) promotes the myelination of sciatic nerves during regeneration after cryolesion. But, little is known about the molecular mechanisms by which the hormone exerts its effects. This could be initiated by the regulation of transcription factor expression in Schwann cells, which produce the myelin sheaths in the peripheral nervous system. We investigated by RT-PCR whether PROG activated expression of transcription factors: Egr-1 (Krox-24) Egr-2 (Krox-20), Egr-3, c-jun, jun B, jun D, c-Fos, Fos B, Fra-1, Fra-2, CREB, ATF 4, SCIP and Sox-10 in cultured Schwann cells. PROG triggered a quick (visible as soon as 15 min), strong (6 to 18-fold) and transient (1-2 h) stimulation of Egr-1, Egr-2, Egr-3 and Fos B genes expression. Expression of other genes remained unaffected by PROG treatment. The same expression pattern was obtained in the MSC 80 line (mouse Schwann cells), but not in the NIH-3T3 and CHO lines. Estradiol and testosterone induced different patterns of transcription factor gene activation in Schwann cells. Serum stimulated all genes activated by PROG in addition c-fos, fra-1 and fra-2. The PROG effects were blocked by Actinomycin D and by RU 486. This suggests that the activation of these genes occurs at the transcriptional level via the interaction of the hormone with its cognate receptor. Thus, PROG can regulate Schwann cell functions and differentiation by transiently activating specific transcription factors.

Altered gene expression in Schwann cells of connexin32 knockout animals

The discovery that the dominant X-linked form of Charcot-Marie-Tooth disease (CMTX), a genetic disease of the peripheral nervous system (PNS), is associated with mutations in connexin32 (Cx32) has brought attention to the importance of connexins in glial cell biology. To gain further insight into the consequences of Cx32 deficiency, we have undertaken a detailed characterization of the gene expression profile of Schwann cells isolated from the sciatic nerve of wild-type and Cx32-null mice. Schwann cells exhibit two distinct phenotypes, myelinating and nonmyelinating, which are defined by their different morphology with respect to axons and by their unique profile of gene expression. Our findings show that, regardless of the mouse genotype, cultured Schwann cells express similar levels of messages for a number of connexins and for genes characteristic of both the myelinating and the nonmyelinating phenotypes. Furthermore, we have identified Cx36, a member of the gamma subclass of connexins, which are preferentially expressed in neuronal cells of mouse brain and retina, as an additional connexin present in Schwann cells. Mice lacking Cx32, however, exhibited a marked up-regulation of glial fibrillary acidic protein (GFAP), a cytoskeletal protein usually synthesized only by nonmyelinating Schwann cells. This observation was extended to the PNS in vivo and did not reflect a general perturbation of the expression of other nonmyelinating Schwann cell genes. These findings demonstrate that the absence of Cx32 results in a distinct pattern of gene dysregulation in Schwann cells and that Schwann cell homeostasis is critically dependent on the correct expression of Cx32 and not just any connexin. Identifying the relationship between increased GFAP expression and the absence of Cx32 could lead to the definition of specific roles for Cx32 in the control of myelin homeostasis and in the development of CMTX.

Rapid effects of triiodothyronine on immediate-early gene expression in Schwann cells

In the peripheral nervous system, triiodothyronine (T3) plays an important role in the development and regeneration of nerve fibers and in myelin formation. However, the target genes of T3 in peripheral nerves remain to be identified. We investigated whether T3 activated genes of transcription factors in Schwann cells. Expression of egr-1 (krox-24), egr-2 (krox-20), egr-3, c-jun, junB, c-fos, fosB, fra-1, fra-2, and CREB genes was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) in Schwann cells isolated from neonatal rat sciatic nerves and in the cell lines MSC-80 (mouse Schwann cells), NIH-3T3 (mouse fibroblasts), and CHO (Chinese hamster ovary cells). Some of these transcription factors have been shown to be involved in Schwann cell differentiation. T3 triggered a rapid (15-30 min), transient (1-2-h) and strong (6- to 15-fold) stimulation of Egr-1, Egr-2, Egr-3, Jun B, c-Fos, and Fos B mRNA expression in Schwann cells. In contrast, expression of c-Jun, Fra-1, Fra-2, and CREB mRNA was not affected by T3. The stimulatory effects of T3 could be abolished by adding actinomycin D. T3 triggered the same pattern of gene stimulation in the mouse Schwann cell line MSC80, but not in the NIH-3T3 and CHO cell lines. Serum activated all the genes that responded to T3 and in addition fra-1 and fra-2, but not c-jun and CREB. Immunoblotting showed that the increase in Egr-1 and c-Fos mRNA levels was accompanied by an increase in the corresponding proteins. In addition, shifts of the protein bands indicated a posttranslational modification of the two proteins. These effects of T3 are likely to be mediated by the intracellular T3 receptor, as the D-isomer RT3 and T0, which do not bind to T3 receptors, proved ineffective. The present data suggested that T3 may regulate Schwann cell functions and differentiation by transiently activating the expression of specific transcription factors.

Developmental regulation and overexpression of the transcription factor AP-2, a potential regulator of the timing of Schwann cell generation

There is now evidence from in vivo and in vitro studies that the rate of Schwann cell generation is regulated by the balance of two opposing signals, beta neuregulins and endothelins. The beta neuregulins promote the development of precursors to Schwann cells whereas endothelins retard it through an action on endothelin-B receptors. The present work has shown additional controls of this transition, and implicates AP-2 transcription factors, in particular AP-2 alpha, as negative regulators of Schwann cell generation. We found that both AP-2 alpha and AP-2 gamma are present in early embryonic nerves, whereas AP-2 beta was not. Isoform-specific analysis of AP-2 alpha showed that isoform 3 was most abundant with isoforms 1 and 2 present in lesser amounts; isoform 4 was absent. Maximal AP-2 alpha and AP-2 gamma mRNA expression occurred at embryonic day (E) 12/13 in the mouse and at E14/15 in the rat, which correlates with the presence of Schwann cell precursors in the nerve. In both rats and in mice, in vivo and in vitro, downregulation of AP-2 alpha mRNA and protein coincided with one of the main steps in Schwann cell development, the precursor-Schwann cell transition. Moreover, Schwann cell generation was delayed if this downregulation was prevented by enforced expression of AP-2 alpha in precursors. These studies suggest that AP-2 is involved in the control of the timing of Schwann cell development.

EGR2 mutations in inherited neuropathies dominant-negatively inhibit myelin gene expression

The identification of EGR2 mutations in patients with neuropathies and the phenotype Egr2/Krox20(-/-) have demonstrated that the Egr2 transcription factor is critical for peripheral nerve myelination. However, the mechanism by which these mutations cause disease remains unclear, as most patients present with disease in the heterozygous state, whereas Egr2(+/-) mice are phenotypically normal. To understand the effect of aberrant Egr2 activity on Schwann cell gene expression, we performed microarray expression profiling to identify genes regulated by Egr2 in Schwann cells. These include genes encoding myelin proteins and enzymes required for synthesis of normal myelin lipids. Using these newly identified targets, we have shown that neuropathy-associated EGR2 mutants dominant-negatively inhibit wild-type Egr2-mediated expression of essential myelin genes to levels sufficiently low to result in the abnormal myelination observed in these patients.

A distal Schwann cell-specific enhancer mediates axonal regulation of the Oct-6 transcription factor during peripheral nerve development and regeneration

The POU domain transcription factor Oct-6 is a major regulator of Schwann cell differentiation and myelination. During nerve development and regeneration, expression of Oct-6 is under the control of axonal signals. Identification of the cis-acting elements necessary for Oct-6 gene regulation is an important step in deciphering the complex signalling between Schwann cells and axons governing myelination. Here we show that a fragment distal to the Oct-6 gene, containing two DNase I-hypersensitive sites, acts as the Oct-6 Schwann cell-specific enhancer (SCE). The SCE is sufficient to drive spatially and temporally correct expression, during both normal peripheral nerve development and regeneration. We further demonstrate that a tagged version of Oct-6, driven by the SCE, rescues the peripheral nerve phenotype of Oct-6-deficient mice. Thus, our isolation and characterization of the Oct-6 SCE provides the first description of a cis-acting genetic element that responds to converging signalling pathways to drive myelination in the peripheral nervous system.

Response of Schwann cells to action potentials in development

Sensory axons become functional late in development when Schwann cells (SC) stop proliferating and differentiate into distinct phenotypes. We report that impulse activity in premyelinated axons can inhibit proliferation and differentiation of SCs. This neuron-glial signaling is mediated by adenosine triphosphate acting through P2 receptors on SCs and intracellular signaling pathways involving Ca2+, Ca2+/calmodulin kinase, mitogen-activated protein kinase, cyclic adenosine 3',5'-monophosphate response element binding protein, and expression of c-fos and Krox-24. Adenosine triphosphate arrests maturation of SCs in an immature morphological stage and prevents expression of O4, myelin basic protein, and the formation of myelin. Through this mechanism, functional activity in the developing nervous system could delay terminal differentiation of SCs until exposure to appropriate axon-derived signals.

Schwann cells and their precursors emerge as major regulators of nerve development

It is becoming ever clearer that Schwann cells and Schwann-cell precursors are an important source of developmental signals in embryonic and neonatal nerves. This article reviews experiments showing that these signals regulate the survival and differentiation of other cells in early nerves. The evidence indicates that glial-derived signals are necessary for neuronal survival at crucial periods of development, that they regulate the molecular and functional specialization of axons and that they control the maturation of the perineurial sheath that protects nerves from inflammation and unwanted macro-molecules produced in the surrounding tissues. Furthermore, an autocrine survival circuit enables Schwann cells in postnatal nerves to survive in the absence of axons, a vital requirement for successful nerve regeneration following injury. The molecular identity of these signals and their receptors is currently being determined.

Schwann cell development in embryonic mouse nerves

Previously we proposed that Schwann cell development from the neural crest is a two-step process that involves the generation of one main intermediate cell type, the Schwann cell precursor. Until now Schwann cell precursors have only been identified in the rat, and much remains to be learned about these cells and how they generate Schwann cells. Here we identify this cell in the mouse and analyze its transition to form Schwann cells in terms of timing, molecular expression, and extracellular signals and intracellular pathways involved in survival, proliferation, and differentiation. In the mouse, the transition from precursors to Schwann cells takes place 2 days earlier than in the rat, i.e., between embryo days 12/13 and 15/16, and is accompanied by the appearance of the 04 antigen and the establishment of an autocrine survival circuit. Beta neuregulins block precursor apoptosis and support Schwann cell generation in vitro, a process that is accelerated by basic fibroblast growth factor 2. The development of Schwann cells from precursors also involves a change in the intracellular survival signals utilized by neuregulins: To block precursor death neuregulins need to signal through both the mitogen-activated protein kinase and the phosphoinositide-3-kinase pathways although neuregulins support Schwann cell survival by signaling through the phosphoinositide-3-kinase pathway alone. Last, we describe the generation of precursor cultures from single 12-day-old embryos, a prerequisite for culture studies of genetically altered precursors when embryos are non-identical with respect to the transgene in question.

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.

Role of Egr-2 in up-regulation of Fas ligand in normal T cells and aberrant double-negative lpr and gld T cells

We previously identified a Fas ligand regulatory element (FLRE) in the Fas ligand (fasL) promoter that binds Egr family proteins and demonstrated that Egr-3 (PILOT) but not Egr-1 (NGFI-A, Krox-24, Tis-8, and Zif-268) induces transcription of fasL. The aberrant CD4(-)CD8(-) T cells from lpr/lpr and gld/gld mice, which have mutations in the genes encoding Fas and FasL, respectively, have an activated phenotype and constitutively express high levels of fasL mRNA, prompting us to ask what role if any the FLRE and Egr family proteins have in this aberrant expression of fasL. Unstimulated MRL-lpr/lpr and C3H-gld/gld CD4(-)CD8(-) T cells constitutively contained high levels of two proteins that bound to the FLRE. Supershift analysis revealed these proteins to be Egr-1 and Egr-2 (Krox-20); Egr-3 was not detected. Activation of normal lymph node cells resulted in increased expression of Egr-1, -2, and -3. As with egr-3, expression of egr-2 was blocked by cyclosporin A. Although overexpressed Egr-1 was ineffective, overexpressed Egr-2 was as potent as Egr-3 in inducing fasL promoter-dependent reporter constructs in T cell hybridomas and HeLa cells, and both up-regulated endogenous fasL mRNA in HeLa cells. FasL-dependent reporter constructs in MRL-lpr/lpr and C3H-gld/gld CD4(-)CD8(-) T cells were constitutively active, and this activity was largely prevented by mutation of the critical Egr family binding element. Thus, Egr-2, in addition to Egr-3, regulates FasL expression in activated normal T cells, and Egr-2 is likely to play a direct role in aberrant fasL up-regulation in lpr/lpr and gld/gld CD4(-)CD8(-) T cells.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Promyelinating Schwann cells express Tst-1/SCIP/Oct-6

Tst-1/SCIP/Oct-6, a POU domain transcription factor, is transiently expressed by developing Schwann cells and is required for their normal development into a myelinating phenotype. In tst-1/scip/oct-6-null sciatic nerves, Schwann cells are transiently arrested at the "promyelinating" stage, when they have a one-to-one relationship with an axon but before they have elaborated a myelin sheath. To determine when Schwann cells express Tst-1/SCIP/Oct-6, we examined beta-galactosidase (beta-gal) expression in heterozygous tst-1/scip/oct-6 mice, in which one copy of the tst-1/scip/oct-6 gene has been replaced with the LacZ gene. beta-Gal expression from the LacZ gene seems to parallel Tst-1/SCIP/Oct-6 expression from the endogenous tst-1/scip/oct-6 gene in developing and regenerating sciatic nerves. Furthermore, electron microscopic examination of 5bromo-4-chloro-3-indolyl-beta-D-galactopyranoside- (X-gal) and halogenated indolyl-beta-D-galactoside- (Bluo-gal) stained nerves showed that promyelinating Schwann cells express the highest levels of beta-gal, both in developing and in regenerating nerves. Thus, the expression of beta-gal, a surrogate marker of Tst-1/SCIP/Oct-6, peaks at the same stage of Schwann cell development at which development is arrested in tst-1/scip/oct-6-null mice, indicating that Tst-1/SCIP/Oct-6 has a critical role in promyelinating Schwann cells.

Origin and early development of Schwann cells

Cellular events leading to the generation of Schwann cells from the neural crest have recently been clarified and it is now possible to outline a relatively simple model of the Schwann cell lineage in the rat and mouse. Neural crest cells have to undergo three main developmental transitions to become mature Schwann cells. These are the formation of Schwann cell precursors from crest cells, the formation of immature Schwann cells from precursors and, lastly, the postnatal and reversible generation of non-myelin- and myelin-forming Schwann cells. Axonal signals involving neuregulins are important regulators of these events, in particular of the survival, proliferation, and differentiation of Schwann cell precursors. Transcription factors likely to be involved in the developmental transitions are beginning to be identified. These include Oct-6, Krox-20, and Pax-3 but also members of the basic helix-loop-helix family, Sox 10, and the cAMP response element binding protein CREB.