Regulation of Schwann cell proliferation: mechanisms involved in peripheral nerve development

MicroRNA Mediated Regulation of Schwann Cell Migration and Proliferation in Peripheral Nerve Injury

Schwann cells (SCs) contribute to nerve repair following injury; however, the underlying molecular mechanism is poorly understood. MicroRNAs (miRNAs), which are short noncoding RNAs, have been shown to play a role in neuronal disease. In this work, we show that miRNAs regulate the peripheral nerve system by modulating the migration and proliferation of SCs. Thus, miRNAs expressed in peripheral nerves may provide a potential therapeutic target for peripheral nerve injury or repair.

E-cadherin expression in postnatal Schwann cells is regulated by the cAMP-dependent protein kinase a pathway

Expression of E-cadherin in the peripheral nervous system is a highly regulated process that appears postnatally in concert with the development of myelinating Schwann cell lineage. As a major component of autotypic junctions, E-cadherin plays an important role in maintaining the structural integrity of noncompact myelin regions. In vivo, the appearance of E-cadherin in postnatal Schwann cell is accompanied by the disappearance of N-cadherin, suggesting reciprocal regulation of the two cadherins during Schwann cell development. The molecular signal that regulates the cadherin switch in Schwann cell is unclear. Using a neuron-Schwann cell co-culture system, here we show that E-cadherin expression is induced by components on the axonal membrane. We also show that the axonal effect is mediated through cAMP-dependent protein kinase A (cAMP-PKA) activation in the Schwann cell: (1) inhibition of cAMP-PKA blocks axon-induced E-cadherin expression and (2) cAMP elevation in the Schwann cell is sufficient to induce E-cadherin expression. In addition, cAMP-dependent E-cadherin expression is promoted by contact between adjacent Schwann cell membranes, suggesting its role in autotypic junction formation during myelination. Furthermore, cAMP-induced E-cadherin expression is accompanied by suppression of N-cadherin expression. Therefore, we propose that axon-dependent activation of cAMP-PKA serves as a signal that promotes cadherin switch during postnatal development of Schwann cells.

Neurosteroids: synthesis and functions in the central and peripheral nervous systems

Some steroids are synthesized within the central and peripheral nervous systems, mostly by glial cells. These are known as neurosteroids. In the brain, neurosteroids have been shown to act directly on membrane receptors for neurotransmitters. For example, progesterone inhibits the neuronal nicotinic acetylcholine receptor, whereas its 3 alpha,5 alpha-reduced metabolite 3 alpha,5 alpha-tetrahydroprogesterone (allopregnanolone) activates the type A gamma-aminobutyric acid receptor complex. Besides these effects, neurosteroids also regulate important glial functions, such as the synthesis of myelin proteins. Thus, in cultures of glial cells prepared from neonatal rat brain, progesterone increases the number of oligodendrocytes expressing the myelin basic protein (MBP) and the 2',3'-cyclic nucleotide-3'-phophodiesterase (CNPase). An important role for neurosteroids in myelin repair has been demonstrated in the rodent sciatic nerve, where progesterone and its direct precursor pregnenolone are synthesized by Schwann cells. After cryolesion of the male mouse sciatic nerve, blocking the local synthesis or action of progesterone impairs remyelination of the regenerating axons, whereas administration of progesterone to the lesion site promotes the formation of new myelin sheaths.

The origin and development of glial cells in peripheral nerves

During the development of peripheral nerves, neural crest cells generate myelinating and non-myelinating glial cells in a process that parallels gliogenesis from the germinal layers of the CNS. Unlike central gliogenesis, neural crest development involves a protracted embryonic phase devoted to the generation of, first, the Schwann cell precursor and then the immature Schwann cell, a cell whose fate as a myelinating or non-myelinating cell has yet to be determined. Embryonic nerves therefore offer a particular opportunity to analyse the early steps of gliogenesis from transient multipotent stem cells, and to understand how this process is integrated with organogenesis of peripheral nerves.

Constitutive Release of α4 Type V Collagen N-terminal Domain by Schwann Cells and Binding to Cell Surface and Extracellular Matrix Heparan Sulfate Proteoglycans

During peripheral nerve development, Schwann cells synthesize collagen type V molecules that contain alpha4(V) chains. This collagen subunit possesses an N-terminal domain (NTD) that contains a unique high affinity heparin binding site. The alpha4(V)-NTD is adhesive for Schwann cells and sensory neurons and is an excellent substrate for Schwann cell and axonal migration. Here we show that the alpha4(V)-NTD is released constitutively by Schwann cells both in culture and in vivo. In cultures of neonatal rat Schwann cells, alpha4(V)-NTD release is increased significantly by ascorbate treatment, which facilitates collagen post-translational modification and collagen trimer assembly. In peripheral nerve tissue, the alpha4(V)-NTD is localized to the region of the outer Schwann cell membrane and associated extracellular matrix. The released alpha4(V)-NTD binds to the cell surface and extracellular matrix heparan sulfate proteoglycans of Schwann cells. Pull-down assays and immunofluorescent staining showed that the major alpha4(V)-NTD-binding proteins are glypican-1 and perlecan. alpha4(V)-NTD binding occurs via a mechanism that requires the high affinity heparin binding site and that is blocked by soluble heparin, demonstrating that binding to proteoglycans is mediated by their heparan sulfate chains.

Heregulin and forskolin-induced cyclin D3 expression in Schwann cells: Role of a CCAAT promoter element and CCAAT enhancer binding protein

Heregulin, a polypeptide growth factor, and forskolin, an adenylyl cyclase activator, synergistically stimulate expression of cyclin D3 and cell division in Schwann cells. Heregulin induces expression in Schwann cells of a luciferase reporter gene linked to the cyclin D3 promoter. Forskolin markedly augments reporter expression in the presence of heregulin. Deletion analysis identified several promoter sites that contribute to high-level reporter expression in heregulin- and forskolin-treated Schwann cells. A promoter fragment that contains 103 bp of 5'-flanking sequence produced significant reporter expression in heregulin- and forskolin-stimulated cells. Deletion of a consensus CCAAT site within this promoter fragment caused a nearly complete loss of reporter expression. Similar results were obtained when CCAAT site mutations were introduced into the promoter. Heregulin and forskolin increased steady-state levels of CCAAT/enhancer binding protein-beta (C/EBPbeta) in Schwann cells. Mobility shift assays identified proteins in Schwann cell nuclear extracts that formed stable complexes with the cyclin D3 CCAAT promoter element and were disrupted by anti-C/EBPbeta antibody. Transfection of Schwann cells with C/EBPbeta cDNA increased cyclin D3 reporter expression. In contrast to these results, mutation of a cAMP response element in the cyclin D3 promoter had only a modest effect on heregulin- and forskolin-stimulated reporter expression. These findings demonstrate that C/EBPbeta plays a key role in the heregulin and cAMP-dependent regulation of cyclin D3 expression in Schwann cells.

Induced maturation of frog mast cells by nerve growth factor during ontogenesis

The effect of nerve growth factor (NGF) on ontogenesis of frog mast cells was investigated in vivo by histochemical, morphometric, and ultrastructural analysis. Three groups of tadpoles at various stages of development were used. In the first group, the larvae received i.p. injections of 1 ng NGF/g; the second group received 10 ng NGF/g, while the control group received only the vehicle. The first recognizable mast cells arose symmetrically in the tongue at stage 26 of Witschi's standard table. At stages 26 and 29, the mast cell number in the NGF-injected tadpoles was significantly higher than the control group. From stage 29 onward, the mast cell number rapidly increased in all groups. No significant differences in mast cell number were observed between the control group and the NGF-injected groups at stages 31 and 33. Electron microscopy revealed that at metamorphic climax (stage 33), the mast cells in the NGF-treated groups were more mature than those in the control group. Therefore, nerve growth factor at early stages of tadpole development is likely to induce differentiation of mast cell precursors, while at later stages it is likely to induce maturation of immature mast cells. The close anatomical association between mast cells and perineurium, observed during nerve development, is intriguing. Already in the early stages of nerve development, the mast cells form a network around Schwann cell-axon complexes, together with the perineurial cells. At climax, the mast cells are located between the perineurial layers, suggesting that they may play a role in the tissue-nerve barrier of the perineurium. Nerve growth factor also seems to induce perineurial cell maturation.

Microarray analysis of gene expression in proliferating Schwann cells: synergistic response of a specific subset of genes to the mitogenic action of heregulin plus forskolin

Cultured Schwann cells treated with heregulin growth factor require costimulation with a cyclic adenosine monophosphate-elevating agent to produce maximal cell proliferation. Gene chip expression analysis was used to identify genes that are induced or repressed when Schwann cells are treated with heregulin and/or forskolin. By utilizing arrays that contained 8799 probes, the expression of over 1000 genes was found to be significantly changed after 30 hr of treatment with heregulin, forskolin, or heregulin plus forskolin. Hierarchical clustering revealed groups of genes with distinct expression patterns. Of particular interest was a cluster of 140 genes that were up-regulated by heregulin plus forskolin but not by heregulin or forskolin alone. Many of the genes in this group have roles in cell division, such as cyclin B, cyclin D3, E2F-5, cdc 25B, polo-like kinase, and protein kinase C type III. These findings identify a profile of gene expression for Schwann cell proliferation.

Interferon-gamma, tumor necrosis factor-alpha, and transforming growth factor-beta inhibit cyclic AMP-induced Schwann cell differentiation

Schwann cells differentiate in vivo in response to contact with axons, and cAMP simulates some of these aspects of differentiation in vitro, particularly morphologic changes and expression of certain phenotypic molecules. Unfractionated inflammatory cytokines inhibit cAMP-induced Schwann cell expression of galactolipids (Gal). We sought to identify which cytokines were responsible for this inhibition and to determine whether other phenotypic indicators of Schwann cell differentiation were also affected. Neonatal rat Schwann cells were incubated in vitro with 1 mM 8 Bromo cAMP (8 Br cAMP) with or without the addition of interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-2, IL-6, tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), or transforming growth factor-beta (TGF-beta). Cells were then examined for morphologic changes and for expression of surface Gal and low-affinity nerve growth factor receptor (NGFRp75), employing indirect immunofluorescence. 8 Br cAMP induced Schwann cell upregulation of Gal, downregulation of NGFRp75, and the cells became enlarged and somewhat amorphous and irregular in appearance. Cells treated with IFN-gamma or TNF-alpha alone were more bipolar and more evenly distributed on coverslips than were control cells, whereas TGF-beta alone induced elongated cells often in a swirling pattern. None of the cytokines alone induced upregulation of Gal or downregulation of NGFRp75. TNF-alpha, IFN-gamma, and TGF-beta inhibited the 8 Br cAMP-induced morphologic changes, as well as the upregulation of Gal and downregulation of NGFRp75. The other cytokines had no effects on Gal or NGFRp75 expression. Thus, these three cytokines, which are present in inflammatory lesions in the peripheral nervous system, are capable of inhibiting Schwann cell differentiation.

Progesterone synthesis and myelin formation in peripheral nerves

Progesterone is synthesized in the nervous system by neurons and glial cells. Because of their simple structure, plasticity and capacity of regeneration, peripheral nerves are particularly well suited for studying the biosynthesis, mechanisms of action and effects of the hormone. Schwann cells, the myelinating glial cells in the peripheral nervous system, synthesize progesterone in response to a diffusible neuronal signal. In peripheral nerves, the local synthesis of progesterone plays an important role in the formation of myelin sheaths. This has been shown in vivo, after cryolesion of the mouse sciatic nerve, and in vitro, in cocultures of Schwann cells and sensory neurons. Schwann cells also express an intracellular receptor for progesterone, which thus functions as an autocrine signalling molecule. Progesterone may promote myelination by activating the expression of genes coding for transcription factors (Krox-20) and/or for myelin proteins (P0, PMP22). Recently, it has been proposed that progesterone may indirectly regulate myelin formation by influencing gene expression in neurons. Steroid hormones also influence the proliferation of Schwann cells: estradiol becomes a potent mitogen for Schwann cells when levels of cAMP are elevated and glucocorticosteroids have been shown to increase the mitogenic effects of peptide growth 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.

Modification of representational difference analysis applied to the isolation of forskolin-regulated genes from Schwann cells

Many aspects of the response of Schwann cells to axonal cues can be induced in vitro by the adenylyl cyclase activator forskolin, yet the role of cAMP signaling in regulating Schwann cell differentiation remains unclear. To define better the relationship between cAMP signaling and Schwann cell differentiation, we used a modification of cDNA representational difference analysis (RDA) that permits the analysis of small amounts of mRNA and identified additional genes that are differentially expressed by forskolin-treated and untreated Schwann cells. The genes that we have identified, including MKP3, a regulator of ERK signaling, and the sphingosine-1-phosphate receptor edg3/lp(B3), may play important roles in mediating Schwann cell differentiation.

Progesterone as a neuroactive neurosteroid, with special reference to the effect of progesterone on myelination

Some steroids are synthesized within the central and peripheral nervous system, mostly by glial cells. These are known as neurosteroids. In the brain, certain neurosteroids have been shown to act directly on membrane receptors for neurotransmitters. For example, progesterone inhibits the neuronal nicotinic acetylcholine receptor, whereas its 3alpha,5alpha-reduced metabolite 3alpha, 5alpha-tetrahydroprogesterone (allopregnanolone) activates the type A gamma-aminobutyric acid receptor complex. Besides these effects, neurosteroids also regulate important glial functions, such as the synthesis of myelin proteins. Thus, in cultures of glial cells prepared from neonatal rat brain, progesterone increases the number of oligodendrocytes expressing the myelin basic protein (MBP) and the 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase). An important role for neurosteroids in myelin repair has been demonstrated in the rodent sciatic nerve, where progesterone and its direct precursor pregnenolone are synthesized by Schwann cells. After cryolesion of the male mouse sciatic nerve, blocking the local synthesis or action of progesterone impairs remyelination of the regenerating axons, whereas administration of progesterone to the lesion site promotes the formation of new myelin sheaths.

Role of Schwann cells in retinal ganglion cell axon regeneration

It is a well known fact that the injured PNS can successfully regenerate, on the other hand, the CNS such as retinal ganglion cell (RGC) axons of adult mammals is incapable of regeneration. After injury, RGC axons rapidly degenerate and most cell bodies go through the process of cell death, while glial cells at the site of injury undergo a series of responses which underlie the so-called glial scar formation. However, it has become apparent that RGCs do have an intrinsic capacity to regenerate which can be elicited by experimental replacement of the inhibitory glial environment with a permissive peripheral nerve milieu. Schwann cells are a major component of the PNS and play a role in regeneration, by producing various kinds of functional substances such as diffusible neurotrophic factors, extracellular matrix and cell adhesion molecules. RGC regeneration can be induced by cooperation of these substances. The contact of RGC axons to Schwann cells based upon the structural and molecular linkages seems to be indispensable for the stable and successful regeneration. In addition to cell adhesion molecules such as NCAM and L1, data from our laboratory show that Schwann cells utilize short focal tight junctions to provide morphological stabilization of the contact with the elongating axon, as well as a small scale of gap junctions to facilitate traffic of substances between them. Moreover, our results show that modifications of functional properties in neighboring glial cells of optic nerve are induced by transplantation of Schwann cells. Astrocytes usually considered to form a glial scar guide the regenerating axons in cooperation with Schwann cells. A decrease of the oligodendrocyte marker O4 and migration of ED-1 positive macrophages is observed within the optic nerve stump. Accordingly, RGC regeneration is not a simple phenomenon of axonal elongation on the Schwann cell membrane, but is based on direct and dynamic communication between the axon and the Schwann cell, and is also accompanied by changes and responses among the glial cell populations, which may be partly associated with the mechanisms of optic nerve regeneration.

A protein with the characters of a zinc-finger is implicated in the differentiation of Schwann cells

During the development of the PNS, Schwann cells (SC) differentiate into myelinating and nonmyelinating cells, implying regulation by different transcription factors such as ZF proteins. Employing an original strategy using monoclonal antibodies specifically directed against the conserved ZF motif, we have identified a new ZF protein of 55 kDa present in rat sciatic nerve extract (SCp55). We used polyclonal antibodies and cloned cDNA to characterize the expression of SCp55 by immunohistochemistry and in situ hybridization. This protein is specific for SC and shows differential expression both during development and between the two SC phenotypes. When they differentiate the protein is first induced in myelinating SC and then in nonmyelinating SC. The nature of this protein together with its differential expression suggests that it is a transcription factor that may have a role in the development of SC.

Schwann cells proliferate at rat neuromuscular junctions during development and regeneration

Terminal Schwann cells (TSCs) cover neuromuscular junctions and are important in the repair and maintenance of these synapses. We have examined how these cells are generated at developing junctions and how their number is regulated during repair of nerve injury. At birth, approximately half of the junctions in rat soleus and extensor digitorum longus muscles have one TSC soma. Somata are absent from the remainder, although Schwann cell (SC) processes arising from somata along the preterminal axon cover almost all of these synapses. By 2 months of age, junctions have gained an additional two to three TSCs. Most of this gain occurs during the first 2 postnatal weeks and largely precedes the expansion of endplate size. Although the initial addition is caused by cell migration, mitotic labeling shows extensive division of TSCs at junctions. A slower addition of TSCs occurs in adult muscles, and TSC number in the adult is correlated with endplate size. During repair of nerve injury, TSC number is regulated by a combination of signals from motor neurons and denervated tissue. As shown previously (Connor et al., 1987), denervation of adult muscles did not, in itself, cause TSC mitosis. However, TSCs became mitotic during reinnervation. Partial denervation induced division of TSCs at innervated but not denervated endplates. A disproportionate number of these mitotic cells were found at endplates contacted by TSC processes extended from nearby denervated endplates, contacts known to promote nerve sprouting. These results show an association between TSC mitotic activity and alterations in synaptic structure during development, sprouting, and reinnervation.

Synergistic regulation of Schwann cell proliferation by heregulin and forskolin

A peptide corresponding to the epidermal growth factor homology domain of beta-heregulin stimulated autophosphorylation of the heregulin receptors erbB2 and erbB3 in Schwann cells and activation of the mitogen-activated protein (MAP) kinases ERK1 and ERK2. Heregulin-dependent activation of PAK65, a component of the stress-activated signaling pathway, ribosomal S6 kinase, and a cyclic AMP (cAMP) response element binding protein (CREB) kinase, identified as p95(RSK2), was also observed. Receptor phosphorylation and activation of these kinases in response to heregulin occurred in the absence of forskolin stimulation and were not augmented in cells treated with forskolin, a direct activator of adenylyl cyclase. Schwann cell proliferation in response to heregulin was observed only when the cells were also exposed to an agent that elevates cAMP levels. In the absence of heregulin, elevation of cAMP levels failed to stimulate Schwann cell proliferation. Forskolin significantly enhanced heregulin-stimulated expression of cyclin D and phosphorylation of the retinoblastoma gene product. In cells treated with both heregulin and forskolin there was a sustained accumulation of phospho-CREB, which was not observed in cells treated with either agent alone. Heregulin and forskolin synergistically activated transcription of a cyclin D promoter construct. These results demonstrate that heregulin-stimulated activation of MAP kinase is not sufficient to induce maximal Schwann cell proliferation. Expression of critical cell cycle regulatory proteins and cell division require activation of both heregulin and cAMP-dependent processes.

Recombinant human glial growth factor 2 (rhGGF2) improves functional recovery of crushed peripheral nerve (a double-blind study)

This in vivo double-blind study evaluated the effect of recombinant human glial growth factor 2 (rhGGF2), a Schwann cell mitogen, on the recovery of motor function of rat sciatic nerve following crush injury. Seventy three rats were divided into three groups. Group I (n=5), sham operated; Groups II (n=34) and III (n=34) received a 100 g crush load for 2 h over a 5 mm segment of the sciatic nerve. Group III was treated with 1 mg/kg rhGGF2, via subcutaneous injection one day before nerve crush and daily for the following four days. Group II received an equivalent volume of saline as a control. Motor functional recovery was assessed by calculating the sciatic functional index (SFI) and the recovery rate of tetanic contractile force of the extensor digitorum longus (EDL) muscle. Recovery of nerve function was evident at day 11 after crush in the rhGGF2-treated animals, whereas the nerves in controls were still paralyzed. The rhGGF2-treated animals showed a significant improvement of the SFI between days 11-21 postoperatively when compared to controls. The isometric tetanic contractile force was stronger in the rhGGF2-treated group than in controls, with a significant difference at 40 to 70 Hz stimulus frequencies on day 4. Correlation analysis showed that tetanic contractile force had a linear correlation with the SFI. Histologic assessment indicated that the rhGGF2-treated animals showed less severe degeneration and earlier robust remyelination of axons than controls. The results suggest that treatment with rhGGF2 is effective in promoting nerve regeneration as seen in measurements of functional recovery and qualitative assessment of nerve morphology. The mechanism of GGF's protective effect may be related to its direct action on Schwann cells, stimulating their mitosis as well as inducing neurotrophic factors essential to neuronal maintenance and repair.

TNF-alpha and TGF-beta act synergistically to kill Schwann cells

Interactions between cytokines and Schwann cells (SC) are important in development, repair, and disorders of the peripheral nervous system (PNS). Tumor necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta (TGF-beta) are two prominent cytokines which may be involved in these processes and their gene products are upregulated in some experimental neuropathies. This study focuses on the in vitro effects of these cytokines, both singly and in combination, on cultured SC. Expression of both Type I and Type II TNF-alpha receptors was demonstrated on the SC surface by immunocytochemistry. Treatment of SC with a combination of TNF-alpha plus TGF-beta causes significant detachment and cell death while treatment with each cytokine alone is not significantly cytotoxic. When compared with control cultures, SC treated with the combination of cytokines exhibit an increase in the number of cells with condensed nuclei and evidence of DNA fragmentation, characteristics consistent with cells undergoing programmed cell death. Thus, TNF-alpha plus TGF-beta induce SC loss of adhesion which is predominantly due to cell death. Apoptotic mechanisms are likely to contribute to some extent to this cell death. These findings provide in vitro evidence to support the hypothesis that cytokines can directly damage SC in PNS disorders.

Acute and chronic regulation of Na+/K+-ATPase transport activity in the RN22 Schwann cell line in response to stimulation of cyclic AMP production

Na+/K+-ATPase-dependent Rb+ uptake of RN22 Schwann cells was stimulated by cholera toxin (0.25 microg/ml), forskolin (2 mM), or 8-bromo cAMP (1 mM). At 2 h Rb+ uptake was increased by 162+/-6% (cholera toxin), 151+/-14% (forskolin), and 207+/-15% (8-bromo cAMP). Cholera toxin or 8-bromo cAMP treatment for 12-24 h resulted in a second peak of Na+/K+-ATPase-dependent Rb+ transport activity of 186+/-12 and 265+/-9% of control, respectively. Cholera toxin also transiently stimulated the activity of the Na+, K+, 2Cl- -cotransporter with a peak at 2 h (179+/-9%), returning to basal levels by 24 h. Inhibition of the Na+,K+,2Cl- -cotransporter by bumetanide (0.1 mM) or by reduction of the Na+ gradient (10 mM veratridine treatment) prevented the early peak in ATPase activity but not the second peak. These results indicated that the early transient stimulation of Na+/K+ ATPase activity by cholera toxin was due to an increase in cellular Na+, secondary to stimulation of Na+,K+,2Cl -cotransport activity. Western blot analysis of cellular homogenates and purified membrane fractions showed that the second peak of Rb+ uptake activity was a result of translocation of transport protein from an intracellular microsomal pool to the plasma membrane. Rb+ uptake by dominant negative protein kinase A mutants of the RN22 cell was not stimulated by cholera toxin treatment (acute or chronic) confirming the cAMP/protein kinase A dependency of both acute and long-term regulation of transport activity. In the absence of a change in Michaelis constants or of an increase in total transport protein of cellular homogenates, neither a change in enzyme kinetics nor an increase in de novo synthesis of transport protein could account for the increase in transport activity.

Activity of cyclic AMP phosphodiesterases and adenylyl cyclase in peripheral nerve after crush and permanent transection injuries

Recent studies demonstrate that cAMP levels are tightly controlled during demyelination and remyelination in Schwann cells as cAMP decreases to 8-10% of normal following both sciatic nerve crush or permanent transection injury and only begins to increase in the crushed nerve after remyelination (Poduslo, J. F., Walikonis, R. S., Domec, M., Berg, C. T., and Holtz-Heppelmann, C. J. (1995) J. Neurochem. 65, 149-159). To investigate the mechanisms responsible for this change in cAMP levels, cAMP phosphodiesterase (PDE) and adenylyl cyclase activities were determined before and after sciatic nerve injury. Basal cAMP PDE activity in soluble endoneurial homogenates of normal nerve was 34.9 +/- 1.9 pmol/mg of protein/min (chi +/- S.E.; n = 10). This activity increased about 3-fold within 6 days following both injuries. Basal PDE activity remained elevated in the transected nerve, but declined to 70 pmol/mg of protein/min in the crushed nerve at 21 and 35 days following injury. Isozyme-specific inhibitors and stimulators were used to identify the PDE families in the sciatic nerve. The low Km cAMP-specific (PDE4) and the Ca2+/calmodulin-stimulated (PDE1) families were found to predominate in assays using endoneurial homogenates. The PDE4 inhibitor rolipram also increased cAMP levels significantly after incubation of endoneurial tissue with various isozyme-specific inhibitors, indicating that PDE4 plays a major role in determining cAMP levels. PDE4 mRNA was localized by in situ hybridization to cells identified as Schwann cells by colabeling of S100, a Schwann cell specific protein. Adenylyl cyclase activity declined following injury, from 3.7 pmol/mg of protein/min in normal nerve to 0.70 pmol/mg/min by 7 days following injury. Both decreased synthesis and increased degradation contribute, therefore, to the reduced levels of cAMP following peripheral nerve injury and are likely critical to the process of Wallerian degeneration.

Steroid hormone receptors and steroid action in rat glial cells of the central and peripheral nervous system

The nervous system is a target for sex steroid hormones which have profound actions on the growth, maturation, differentiation and functioning of brain cells. We found that some steroids, termed "neurosteroids", are synthesized within the brain by glial cells. The term "neurosteroids" designates their site of synthesis--the nervous system, either de novo from cholesterol or from steroid hormone precursors. The biological effects of steroid hormones are mediated by specific high-affinity intracellular receptors, which, after hormone binding, function as activated transcription factors. The presence of such receptors was shown in primary cultures of oligodendrocytes and astrocytes, derived from forebrains (CNS), and in Schwann cells, derived from sciatic nerves (PNS), of newborn rats. In glial cells of the CNS, progesterone-, glucocorticoid-, estrogen and androgen-receptors (PR, GR, ER, AR) were demonstrated and of these receptors, only PR was estrogen-inducible. In glial cells of the PNS, the presence of PR and ER was shown, but the PR in Schwann cell cultures was not inducible by estrogen treatment. Different effects of steroids on glial cell growth and differentiation during primary culture were observed. In particular, a striking increase of myelin-specific proteins such as myelin basic protein (MBP) and cyclic nucleotide phosphodiesterase (CNPase) was observed when oligodendrocytes, the myelinating glial cells of the CNS, were cultured in the presence of progesterone, as determined by indirect immunofluorescence staining and immunoblotting. Insulin also increases MBP and CNP-ase in oligodendrocytes and the combined treatment (insulin + progesterone) promotes a strong synergistic stimulation (14-fold increase) of myelin protein expression. Estradiol also increases MBP- and CNPase expression in oligodendrocytes, although to a lesser extent than progesterone. In the search for optimal stimulation of myelin-protein expression, several progesterone analogues were tested and the results are discussed.

A Schwann cell mitogen accompanying regeneration of motor neurons

Motor neurons are the only adult mammalian neurons of the central nervous system to regenerate following injury. This ability is dependent on the environment of the peripheral nerve and an intrinsic capacity of motor neurons for regrowth. We report here the identification, using a technique known as messenger RNA differential display, of an extracellular signalling molecule, previously described as the pancreatic secreted protein Reg-2, that is expressed solely in regenerating and developing rat motor and sensory neurons. Axon-stimulated Schwann cell proliferation is necessary for successful regeneration, and we show that Reg-2 is a potent Schwann cell mitogen in vitro. In vivo, Reg-2 protein is transported along regrowing axons and inhibition of Reg-2 signalling significantly retards the regeneration of Reg-2-containing axons. During development, Reg-2 production by motor and sensory neurons is regulated by contact with peripheral targets. Strong candidates for peripheral factors regulating Reg-2 production are cytokines of the LIF/CNTF family, because Reg-2 is not expressed in developing motor or sensory neurons of mice carrying a targeted disruption of the LIF receptor gene, a common component of the receptor complexes for all of the LIF/CNTF family.

Multiple isoforms of neuregulin are expressed in developing rat dorsal root ganglia

Accumulating evidence suggests that neuregulin (NRG) plays special roles in the development of the mammalian nervous system. We have already identified NRG as a survival factor for Schwann cells during development. In this report, we have studied all possible NRG isoforms and expression of NRG in the developing rat dorsal root ganglia (DRG) and compared them with those of brain and spinal cord. Neural NRG isoforms comprise common immunoglobulin and epidermal growth factor domains. Various different transcripts were characterized, which arose by alternative splicing in several regions: N-terminal (exon 1 or 2), spacer (exon 5), juxtamembrane (exon 9 or 10), and cytoplasmic (exon 12, 13, or 14) domains. At least 13 novel isoforms among 16 splice variants were identified. The transmembrane isoforms of NRG are dominant forms in developing rat DRG. The mRNA expression of NRG isoforms in DRG is similar to that in spinal cord, while in brain the expression is much less. The mRNA in DRG was found at similar levels from birth to postnatal day 7 of the premyelinating stage, and it decreased afterward. Our results suggest that several NRGs, including isoforms not reported before, play a role as survival factors for Schwann cells in the premyelinating stage.

Neurosteroids, with special reference to the effect of progesterone on myelination in peripheral nerves

Some steroids are synthesized within the central and peripheral nervous system, mostly by glial cells. These are known as neurosteroids. In the brain, neurosteroids have been shown to act directly on membrane receptors for neurotransmitters. For example, progesterone inhibits the neuronal nicotinic acetylcholine receptor, whereas its 3alpha,5alpha-reduced metabolite 3alpha,5alpha-tetrahydroprogesterone (allopregnanolone) activates the A gamma-aminobutyric acid receptor complex. Besides these effects, neurosteroids also regulate important glial functions, such as the synthesis of myelin proteins. Thus, in cultures of glial cells prepared from neonatal rat brain, progesterone increases the number of oligodendrocytes expressing the myelin basic protein (MBP) and the 2',3'-cyclic nucleotide-3'-phophodiesterase (CNPase). An important role for neurosteroids in myelin repair has been demonstrated in the rodent sciatic nerve, where progesterone and its direct precursor pregnenolone are synthesized by Schwann cells. After cryolesion of the male mouse sciatic nerve, blocking the local synthesis or action of progesterone impairs remyelination of the regenerating axons, whereas administration of progesterone to the lesion site promotes the formation of new myelin sheaths.

Axonal interactions regulate Schwann cell apoptosis in developing peripheral nerve: neuregulin receptors and the role of neuregulins

Programmed cell death during development resulting from the lack of appropriate survival factors has been demonstrated in both neurons and oligodendrocytes and occurs mostly in the form of apoptosis. We now demonstrate that Schwann cells in the rat sciatic nerve undergo apoptosis during early postnatal development and that the amount of apoptosis is markedly increased by axotomy. The apoptotic Schwann cells express the low-affinity nerve growth factor receptor but not myelin-related proteins, indicating that they are in the premyelinating state. Apoptosis resulting from normal development or from axotomy can be inhibited markedly by exogenous neuregulin. Consistent with this, the neuregulin receptor components erbB2 and erbB3 are expressed and phosphorylated in developing sciatic nerve. These data suggest that Schwann cell number in developing peripheral nerve is regulated by apoptosis through competition for axonally derived neuregulin.

Cell death in the Schwann cell lineage and its regulation by neuregulin

The development of Schwann cells, the myelin-forming glial cells of the vertebrate peripheral nervous system, involves a neonatal phase of proliferation in which cells migrate along and segregate newly formed axons. Withdrawal from the cell cycle, around postnatal days 2-4 in rodents, initiates terminal differentiation to the myelinating state. During this time, Schwann cell number is subject to stringent regulation such that within the first postnatal week, axons and myelinating Schwann cells attain the one-to-one relationship characteristic of the mature nerve. The mechanisms that underly this developmental control remain largely undefined. In this report, we examine the role of apoptosis in the determination of postnatal Schwann cell number. We find that Schwann cells isolated from postnatal day 3 rat sciatic nerve undergo apoptosis in vitro upon serum withdrawal and that Schwann cell death can be prevented by beta forms of neuregulin (NRG-beta) but not by fibroblast growth factor 2 or platelet-derived growth factors AA and BB. This NRG-beta-mediated Schwann cell survival is apparently transduced through an ErbB2/ErbB3 receptor heterodimer. We also provide evidence that postnatal Schwann cells undergo developmentally regulated apoptosis in vivo. Together with other recent findings, these results suggest that Schwann cell apoptosis may play an important role in peripheral nerve development and that Schwann cell survival may be regulated by access to axonally derived NRG.

Tst-1/Oct-6/SCIP regulates a unique step in peripheral myelination and is required for normal respiration

The terminal differentiation of myelinating glia involves complex interactions that culminate in the formation of myelin. The POU domain transcription factor Tst-1/Oct-6/SCIP is expressed transiently during myelination, and we report here that it has a critical role in this developmental process. Deletion of the Tst-1/Oct-6/SCIP gene produces a severe defect in peripheral myelination by arresting Schwann cell maturation before axonal wrapping. Unexpectedly, the activation of major myelin-specific genes appears to be unaffected by the Tst-1/Oct-6/SCIP mutation, demonstrating that multiple, independently regulated events are required for terminal differentiation of Schwann cells. In addition, aberrant differentiation and migration of specific neurons in Tst-1/Oct-6/SCIP mutant homozygotes is associated with a fatal breathing defect, providing a model for investigating the regulation of pulmonary homeostasis.

Effects of gamma-IFN and NGF on subpopulations in a human neuroblastoma cell line: flow cytometric and morphological analysis

Neuroblastomas are neural crest-derived tumors that contain neuronal, melanocyte, and Schwann cell precursors. We examined the effects of treatment with gamma-interferon (gamma-IFN) and nerve growth factor (NGF), alone, and in combination, on these progenitor subpopulations in the human neuroblastoma cell line, SH-SY5Y. Using fluorescence-activated flow cytometry (FACS), changes in expression of three differentiation-specific or -associated marker proteins, the 200 kD neurofilament protein, the myelin basic protein, and the S-100 protein, were analyzed. Growth rates and morphological changes associated with each treatment over the 2-wk incubation period were noted. The greatest effects were observed with combined IFN + NGF treatment. These were significant increases in expression of all three proteins, distinctive morphological signs of differentiation, and extensive inhibition of proliferation compared to control cultures. Treatment with NGF alone resulted in increased neurofilament protein expression and in the length and number of neurite extensions, but there was no effect on the growth rate. IFN induced striking morphological changes, significant inhibition of growth, and changes in protein expression that correlated with neuronal to non-neuronal subpopulation shifts due to the death of differentiated cells. When treatment was discontinued after 15 d, the morphological changes induced by NGF were reversed within 2-3 d, while those induced by IFN +/- NGF were present up to 4 wk post-treatment. Small, neuroblastic colonies were observed throughout the treatment period and within 4-6 wk after the cessation of treatment this cell-type fully reconstituted the cultures suggesting the presence of a stem cell. Our results indicate that treatment with gamma-IFN +/- NGF can regulate growth and induce, either stem cells or progenitor neuronal, Schwann and melanocyte subpopulations in the SH-SY5Y cell line to irreversibly differentiate.

A diffusion-reaction model of nerve regeneration

The process of peripheral nerve regeneration has been modeled using 5 populations of mathematical variables to represent the biological activities of Wallerian degeneration, fibrin matrix development, Schwann cell activity, elongating neurites, and neovascularization. The mathematical model provided simulations of nerve regeneration following transection and crush injuries that correspond with growth behaviors quantified in biological experiments. Neovascularization was spatiotemporally quantified in nerve regeneration chambers and following nerve crush injury in order to test the simulations of the mathematical model. The vasculature in both the chamber and following nerve crush responded as predicted by the model, increasing beyond normal levels to a peak only to decrease back to normal. This behavior appeared as a traveling wave in the proximal-distal direction preceeding the major thrust of neuritic outgrowth suggesting that development of the vasculature is a rate-limiting step in nerve regeneration.

Spontaneously immortalized adult mouse Schwann cells secrete autocrine and paracrine growth-promoting activities

We established spontaneously immortalized Schwann cell lines from long-term cultures of adult mouse dorsal root ganglia and peripheral nerves. One of the cell lines, designated IMS32, responded to mitogenic stimuli by platelet-derived growth factor (PDGF)-BB, acidic and basic fibroblast growth factors (aFGF, bFGF), and transforming growth factors (TGF)-beta 1 and -beta 2, as determined by bromodeoxyuridine (BrdU) incorporation and double immunofluorescence for S100 and BrdU. Furthermore, conditioned media (CM) obtained from IMS32 cells showed mitogenic activity for both IMS32 cells and long-term cultured Schwann cells. Western blot analysis revealed TGF-beta-like molecule in the CM, and the activity was absorbed with anti-TGF-beta neutralizing antibody. Reverse transcription followed by polymerase chain reaction (RT-PCR) of IMS32 RNA revealed that these cells expressed TGF-beta 1, -beta 2, and -beta 3 transcripts. When rat pheochromocytoma PC12 cells were incubated with the CM, they developed neurite growth. Coculture of PC12 and IMS32 cells also showed neurite growth of PC12 cells. RNA transcripts of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), ciliary neurotrophic factor (CNTF), and glial cell line-derived neurotrophic factor (GDNF) were detected from IMS32 cells by RT-PCR. In these, we sequenced the mouse GDNF cDNA coding region and observed 97% and 90% homologies to corresponding rat and human cDNA sequences, respectively. These results indicate that the immortalized Schwann cell line mitotically responds to various growth factors and secretes autocrine and paracrine growth-promoting activities in vitro.

Calcitonin gene-related peptide promotes Schwann cell proliferation

Schwann cells in culture divide in response to defined mitogens such as PDGF and glial growth factor (GGF), but proliferation is greatly enhanced if agents such as forskolin, which increases Schwann cell intracellular cAMP, are added at the same time as PDGF or GGF (Davis, J. B., and P. Stroobant. 1990. J. Cell Biol. 110:1353-1360). The effect of forskolin is probably due to an increase in numbers of PDGF receptors (Weinmaster, G., and G. Lemke. 1990. EMBO (Eur. Mol. Biol. Organ.) J. 9:915-920. Neuropeptides and beta-adrenergic agonists have been reported to have no effect on potentiating the mitogenic response of either PDGF or GGF. We show that the neuropeptide calcitonin gene-related peptide (CGRP) increases Schwann cell cAMP levels, but the cells rapidly desensitize. We therefore stimulated the cells in pulsatile fashion to partly overcome the effects of desensitization and show that CGRP can synergize with PDGF to stimulate Schwann cell proliferation, and that CGRP is as effective as forskolin in the pulsatile regime. CGRP is a good substrate for the neutral endopeptidase 24.11. Schwann cells in vivo have this protease on their surface, so the action of CGRP could be terminated by this enzyme and desensitization prevented. We therefore suggest that CGRP may play an important role in stimulating Schwann cell proliferation by regulating the response of mitogenic factors such as PDGF.

Tenascin-C expression during wallerian degeneration in C57BL/Wlds mice: possible implications for axonal regeneration

Schwann cells in the distal stumps of lesioned peripheral nerves strongly express the extracellular matrix glycoprotein tenascin-C. To gain insights into the relationship between Wallerian degeneration, lesion induced tenascin-C upregulation and regrowth of axons we have investigated C57BL/Wlds (C57BL/Ola) mice, a mutant in which Wallerian degeneration is considerably delayed. Since we found a distinct difference in the speed of Wallerian degeneration between muscle nerves and cutaneous nerves in 16-week-old C57BL/Wlds mice, as opposed to 6-week-old animals in which Wallerian degeneration is delayed in both, we chose the older animals for closer investigation. Five days post lesion tenascin-C was upregulated in the muscle branch (quadriceps) but not in the cutaneous branch (saphenous) of the femoral nerve in 16-week-old animals. In addition myelomonocytic cells displaying endogenous peroxidase activity invaded the muscle branch readily whereas they were absent from the cutaneous branch at this time. We could further show that it is only a subpopulation of axon-Schwann cell units (mainly muscle efferents) in the muscle branch which undergo Wallerian degeneration and upregulate tenascin-C at normal speed and that the remaining axon-Schwann cell units (mainly afferents) displayed delayed Wallerian degeneration and no tenascin-C expression. Regrowing axons could only be found in the tenascin-C-positive muscle branch where they always grew in association with axon-Schwann cell units undergoing Wallerian degeneration. These observations indicate a tight relationship between Wallerian degeneration, upregulation of tenascin-C expression and regrowth of axons, suggesting an involvement of tenascin-C in peripheral nerve regeneration.

Fibroblast growth factors and insulin growth factors combine to promote survival of rat Schwann cell precursors without induction of DNA synthesis

In embryonic rat nerves, we recently identified an early cell in the Schwann cell lineage, the Schwann cell precursor. We found that when these cells were removed from contact with axons they underwent rapid apoptotic death, and that in a proportion of the cells this death could be prevented by basic fibroblast growth factor (bFGF, FGF-2). We now report that 100% of Schwann cell precursors isolated from peripheral nerves of 14-day-old-rat embryos can be rescued by a combination of insulin-like growth factor (IGF) 1 or 2 in combination with either acidic FGF (aFGF, FGF-1), bFGF or Kaposi's sarcoma FGF (K-FGF; FGF-4). The precursors display an absolute requirement for both an IGF and an FGF to achieve maximal survival. Elevation of intracellular levels of cAMP by forskolin does not result in a significant shift in the IGF/FGF dose-response curves. In contrast, the percentage of precursors rescued by FGF in the presence of insulin is dramatically increased by elevation of cAMP. These growth factor combinations did not stimulate DNA synthesis significantly in Schwann cell precursors. These findings show that cooperation between growth factors is required to suppress cell death in Schwann cell precursors, and suggest that survival and DNA synthesis are regulated by distinct growth factor combinations in these cells. The observations are consistent with the idea that survival regulation by FGFs and IGFs plays an important role in the development of glial cells in early embryonic nerves.

Mitogenic effects of platelet-derived growth factor, fibroblast growth factor, transforming growth factor-beta, and heparin-binding serum factor for adult mouse Schwann cells

Mitogenic effects of fetal calf serum (FCS), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), transforming growth factor-beta (TGF-beta), and forskolin to adult mouse Schwann cells were examined by bromodeoxyuridine (BrdU) incorporation and double immunofluorescence for S100 and BrdU. PDGF-BB, basic FGF, and TGF-beta 1 and beta 2 were all mitogenic for Schwann cells in media containing FCS. Forskolin suppressed the mitogenic activity of these factors. In serum-free media, PDGF-BB and bFGF were also mitogenic, but TGF-beta 1 and beta 2 were not. Heparin-binding fractions of FCS obtained by heparin-Sepharose chromatography synergized with TGF-beta 1 and beta 2 to produce a mitogenic response. Since PDGF-BB, acidic FGF, and basic FGF were not detected in these fractions by immunoabsorption and immunoblot assays, the presence of unidentified heparin-binding molecules in FCS bioactive for adult mouse Schwann cells is suggested.

Rat brain Na+ channel mRNAs in non-excitable Schwann cells

The expression of rat brain voltage-sensitive Na+ channel mRNAs in Schwann cells was examined using in situ hybridization cytochemistry and RT-PCR. The mRNAs of rat brain Na+ channel subtype II and III, but not subtype I, were detected in cultured Schwann cells from sciatic nerve and in intact sciatic nerve, which contains Schwann cells but not neuronal cell bodies. These results indicate that rat brain Na+ channel mRNAs, which have been considered as mainly neuronal-type messages, are also expressed in glial cells in vitro and in vivo.

The immunocytochemical demonstration of a relative lack of nerve fibres in the atrioventricular node and bundle of His in the sudden infant death syndrome (SIDS)

Whilst examining the variation with age of the nerve fibre content of the cardiac conduction system (CCS), using an immunocytochemical approach, it became evident that in two sudden infant death syndrome (SIDS) cases there was a selective lack of S100 positive nerve fibres in the atrioventricular (AV) node and His bundle. In the present study therefore, the examination of CCS with S100 was extended to a further five SIDS cases and three cases of sudden explained death. Also, in addition to S100--which selectively marks Schwann cells associated with both myelinated and non-myelinated nerves--PGP 9.5 (protein gene product) was used to reveal the presence of nerve axonal elements associated with the CCS. The results showed a uniform presence of S100 and PGP 9.5 positive nerve fibres in the sinoatrial (SA) node, the AV node and His bundle tissue of all three control cases. In contrast, five out of seven SIDS cases showed a uniform lack of staining with these markers in the AV node and His bundle tissue, whilst in the two remaining cases it was present in greatly diminished amounts. Staining in the SA node, although present in all seven cases, was reduced when compared with the control cases. This is the first time the CCS of SIDS cases has been studied with immunocytochemical markers of nerve elements. The overall results taken in conjunction with the epidemiology of SIDS suggest that the lack of AV node and His bundle innervation most probably reflects a delay in the development or maturation of the nerve elements of the CCS, similar to that noted for other parts of the central and peripheral nervous systems in SIDS.

SR13/PMP-22 expression in rat nervous system, in PC12 cells, and C6 glial cell lines

SR13/PMP-22 is a protein that was identified after screening a sciatic nerve cDNA library. Our study focused on comparing the level and pattern of expression of SR13/PMP-22 protein and RNA. Northern blot analysis revealed that although SR13/PMP-22 mRNA was present in all nervous tissues and cells studied, levels were at least seven fold higher in the sciatic nerve and the spinal cord. During sciatic nerve postnatal development and maturation, the SR13/PMP-22 mRNA was detected at 2 days after birth, reached a maximal level at day 24, and decreased to 1/3 of the maximum in adult animals. Nerve transection reduced the level of SR13/PMP-22 mRNA to less than 5% in the segment distal to the nerve injury. Experiments using in situ hybridization localized the SR13/PMP-22 mRNA in Schwann cells. Schwann cells present in the vicinity or distal to the nerve cut repressed the signal for the message. In situ hybridization experiments also demonstrated that dorsal root ganglia satellite cells contained the message for SR13/PMP-22. The SR13/PMP-22 antisera used in our study showed a complex pattern of staining. As expected, the SR13/PMP-22 antibody peptide 1 immunoreacted with the sciatic nerve sheath. However, immunocytochemistry of the dorsal root ganglia revealed that the staining was contained in the neuron's cell body and processes and also in satellite cells. We also identified immunoreactive cell bodies and fibers in the spinal cord dorsal horn. Tissue culture studies demonstrated that SR13/PMP-22 mRNA is induced in NGF treated PC12 but not in C6 glioma cell lines grown under experimental conditions that stimulated cell growth arrest. Our experiments suggest that SR13/PMP-22 may have some other function(s) in addition to its hypothesized role in peripheral myelination.

repo encodes a glial-specific homeo domain protein required in the Drosophila nervous system

We report the identification of a Drosophila locus, reversed polarity (repo). Weak repo alleles were viable but affected glia in the optic lobe, resulting in a reversal in polarity of the electrophysiological to light in the adult. Strong repo alleles caused defects in embryonic glia and resulted in embryonic lethality. Expression of repo appeared to be specific to glia throughout development. In the adult visual system, repo was expressed in laminal glia, medullar glia, and subretinal cells; in the embryo, repo was expressed in nearly all of the identified glia in the central and peripheral nervous systems except midline glia. The repo gene encoded a homeo domain protein suggesting that it might be a transcriptional regulator of genes required for glial development.

Effects of nerve segment supernatants on cultured Schwann cell proliferation and laminin production

Mouse sciatic nerves were transected and 3 hr to 16 days later proximal segments were removed and homogenized. Supernatants of these segments or of normal sciatic nerves were added to Schwann cells maintained in Dulbecco's modified Eagle's medium (DMEM) + 15% fetal calf serum (FCS). After 6 days, Schwann cells were solubilized and the protein content was measured using a Bio-Rad (Melville, NY) protein assay. Samples containing the same amounts of protein were then applied to microtiter plates and the laminin content was determined by enzyme-linked immunosorbent assay (ELISA). Lysates of cultures treated with 24 hr proximal segment supernatants contained significantly higher levels of laminin than those prepared from other intervals, from distal segments, or from control nerves. Increased surface and cytoplasmic anti-laminin immunoreactivity also was found in Schwann cells treated with 24 hr supernatants. To identify the source(s) of this effect, proximal segments removed 24 hr after transection were bisected; supernatants were prepared from each half and tested. Significant increases in laminin production were produced by supernatants from both halves. When supernatants from proximal and distal halves were compared, the latter produced significantly higher laminin levels. Electron microscopic examination of both halves showed that distal halves contained sprouting neurites and growth cones ensheathed by Schwann cells which had a basal lamina and resembled those seen during development and regeneration. Proximal halves appeared normal. Schwann cell proliferation also was compared in supernatant-treated cultures by using a bromodeoxy-uridine (BrdU) ELISA. The 24 hr and 2 day supernatants increased Schwann cell proliferation significantly; 12 hr, 4 day, and 8 day supernatants produced smaller increases. Our observations suggest that axons undergoing early regenerative changes are one of several possible sources of substance(s) in our proximal segment supernatants which increased Schwann cell proliferation and laminin production.

Actions of steroid hormones- and growth factors on glial cells of the central and peripheral nervous system

Primary cultures of oligodendrocytes and astrocytes and purified cultures of Schwann cells were prepared respectively from forebrain and sciatic nerves of newborn rats. The effects of steroid hormones and growth factors on glial cell growth and on the production of myelin-specific proteins and lipids were investigated. Progesterone (P, 100 nM) decreased the proliferation of glial cells of the central nervous system. This inhibitory effect of P was abolished by the simultaneous administration of the antagonist RU486, thus suggesting a receptor-mediated action of the hormone. The expression of myelin-specific proteins, including the myelin basic protein (MBP) and the 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase), and of a myelin-specific lipid, galactocerebroside (Gal C), was also measured during cell differentiation under different hormonal conditions. The expression of MBP in oligodendrocytes was increased by P, and this effect was not blocked by RU486. The combined application of P and insulin promoted a synergistic stimulation of MBP expression. Insulin, by itself, also increased the number of MBP-positive oligodendrocytes in culture. The effects of P and insulin appeared to be selective as dexamethasone, dehydroepiandrosterone, pregnanolone and epidermal growth factor (EGF) had no effect. Only estradiol (E2, 500 nM) increased the number of MBP-immunoreactive cells, but in contrast to P, only a small synergism between E2 and insulin on MBP expression was observed. The expression of CNPase, another myelin-specific protein, was also increased by P and, here again, a synergy between P and insulin could be observed. In contrast, the expression of Gal C, a myelin-specific lipid, was not modified by P or other steroid hormones. Moreover, the increase in Gal C-positive cells observed in response to insulin alone was not further potentiated by P. Glial cells of the peripheral nervous system, namely Schwann cells, are also sensitive to steroid hormones. Schwann cells contain estrogen receptors, and E2 stimulates their proliferation in the presence of forskolin or dibutyryl cyclic AMP (dbcAMP). The mitogenic effect of E2 was abolished by the pure antiestrogen ICI-164,384. Insulin, at micromolar concentration, also stimulated Schwann cell growth when forskolin or dbcAMP were present in the culture medium. The mitogenic effect of insulin was mediated by insulin-like growth factor I (IGF-I) receptors. Indeed, at a physiological nanomolar concentration, IGF-I but not insulin or IGF-II, increased the proliferation of Schwann cells in synergy with forskolin. In addition, Schwann cells express receptors for IGF-I.

Reciprocal Schwann cell-axon interactions

This article describes the reciprocal interactions between neurones and Schwann cells with particular reference to the role of growth factors and neurokines as signalling molecules between these cells and of the extracellular matrix as a conduit for such signalling. Major recent advances have identified molecules produced by neurones that are responsible for Schwann cell proliferation, as well as some of the Schwann cell factors regulating the expression of molecules shown to play an important role in neuronal survival and differentiation.

Changes in DNA synthesis rate in the Schwann cell lineage in vivo are correlated with the precursor--Schwann cell transition and myelination

During the development of the rat sciatic nerve extensive proliferation of glial cells occurs, and there is a very substantial rearrangement of the cytoarchitecture as axons and Schwann cells assume relationships which lead to the formation of the myelinated and unmyelinated axons characteristic of adult nerve. The maturation of Schwann cells from Schwann cell precursors and the matching of Schwann cell numbers to axons is an important part of this process. We have therefore studied the proliferation of Schwann cell precursors and Schwann cells during the development of the rat sciatic nerve from embryonic day 14 to postnatal day 28 by combining bromodeoxyuridine injections of rats with double-label immunohistochemical techniques. The results reveal that DNA synthesis occurs in both Schwann cell precursors and Schwann cells throughout early nerve development. The labelling index is already substantial at embryonic day 14, but from embryonic day 17, when essentially all the glial cells have converted from precursor to Schwann cell phenotype, it rises sharply, peaking between embryonic day 19 and 20 before declining precipitously in the early postnatal period. This rapid decline in DNA synthesis coincides with the appearance of the myelin protein P0, and in individual cells DNA synthesis is incompatible with the expression of P0 protein. Nonmyelin-forming Schwann cells, which mature later in development, continue to synthesize DNA until at least postnatal day 15, but by day 28 essentially all Schwann cells in the nerve are quiescent.

Insulin-like growth factor I: a mitogen for rat Schwann cells in the presence of elevated levels of cyclic AMP

To develop effective procedures for improving the regeneration of peripheral nerves and for preventing the formation of neurofibromas, it is necessary to identify the different mitogens that stimulate the proliferation of Schwann cells. Insulin-like growth factor I (IGF-I), which is a potent autocrine growth factor in many tissues, is synthesized by proliferating Schwann cells. However, the role of IGF-I in stimulating their division is still uncertain. Here we show that nanomolar concentrations of IGF-I stimulate the growth of Schwann cells in primary culture. IGF-I alone was uneffective, but in the presence of forskolin (5 microM) or dibutyryl cyclic AMP (dbcAMP, 10 microM), it became a potent mitogen. Neither IGF-II nor epidermal growth factor (EGF) were effective even in the presence of forskolin. Insulin also stimulated Schwann cell proliferation in the presence of forskolin, but only at micromolar concentration. Receptors for IGF-I were visualized on the Schwann cell surface by indirect immunofluorescence staining using anti-human IGF-I receptor antibodies. Their presence was also assessed by binding assays using [125I]-IGF-I as a ligand. Scatchard analysis showed a single class of high-affinity receptors (Kd = 1.5 nM). Competitions studies with unlabeled IGF-I or insulin indicated a half-maximal displacement of [125I]-IGF-I by IGF-I at about 5 nM, while insulin was about 500-fold less effective. The number of binding sites for IGF-I was increased by exposing cells for 3 days to forskolin (-forskolin: about 5,100; + forskolin: about 12,200 binding sites/cell).(ABSTRACT TRUNCATED AT 250 WORDS)