Regulation of Schwann cell nerve growth factor receptor by cyclic adenosine 3',5'-monophosphate

Influence of aging on the peripheral nerve repair process using an artificial nerve conduit

The influence of aging on the induction of nerve regeneration in artificial nerve conduits has yet to be clarified. In the present study, artificial nerve conduit transplantation and histological analysis using the sciatic nerve of young and elderly mice were performed. Using 20 male C57BL/6 mice, an artificial nerve conduit was transplanted to the sciatic nerve at 8 weeks (Young group) or 70 weeks of age (Aged group), and the sciatic nerve was evaluated histologically at 1, 4 and 12 weeks after surgery. Using hematoxylin and eosin staining, the state of induction of nerve regeneration in the artificial nerve conduit was evaluated. Additionally, immunohistochemical staining was used to investigate an angiogenic marker [vascular endothelial growth factor A (VEGFA)], Schwann cell markers [sex determining region Y-box 10 (SOX10) and S100 calcium-binding protein β (S100β)] and a nerve damage marker [nerve growth factor (NGF)]. The results revealed that the induction of nerve regeneration was significantly higher in the Young group than in the Aged group. In addition, VEGFA and SOX10 expression at 1 week, SOX10 expression at 4 weeks and SOX10, S100β and NGF expression at 12 weeks in the proximal stump were significantly higher in the Young group than in the Aged group. At the center of the artificial nerve conduit, S100β and NGF expression at 4 weeks, and VEGFA, SOX10, S100β and NGF expression at 12 weeks were significantly higher in the Young group than in the Aged group. In the distal stump, no significant difference was noted in immunostaining at any week between the two groups. The present study suggested that the nerve regeneration-inducing functions decrease due to aging.

Gpr126/Adgrg6 Has Schwann Cell Autonomous and Nonautonomous Functions in Peripheral Nerve Injury and Repair

Schwann cells (SCs) are essential for proper peripheral nerve development and repair, although the mechanisms regulating these processes are incompletely understood. We previously showed that the adhesion G protein-coupled receptor Gpr126/Adgrg6 is essential for SC development and myelination. Interestingly, the expression of Gpr126 is maintained in adult SCs, suggestive of a function in the mature nerve. We therefore investigated the role of Gpr126 in nerve repair by studying an inducible SC-specific Gpr126 knock-out mouse model. Here, we show that remyelination is severely delayed after nerve-crush injury. Moreover, we also observe noncell-autonomous defects in macrophage recruitment and axon regeneration in injured nerves following loss of Gpr126 in SCs. This work demonstrates that Gpr126 has critical SC-autonomous and SC-nonautonomous functions in remyelination and peripheral nerve repair.

SIGNIFICANCE STATEMENT

Lack of robust remyelination represents one of the major barriers to recovery of neurological functions in disease or following injury in many disorders of the nervous system. Here we show that the adhesion class G protein-coupled receptor (GPCR) Gpr126/Adgrg6 is required for remyelination, macrophage recruitment, and axon regeneration following nerve injury. At least 30% of all approved drugs target GPCRs; thus, Gpr126 represents an attractive potential target to stimulate repair in myelin disease or following nerve injury.

Adhesion GPCRs as Novel Actors in Neural and Glial Cell Functions: From Synaptogenesis to Myelination

Adhesion G-protein-coupled receptors (aGPCRs) are emerging as key regulators of nervous system development and health. aGPCRs can regulate many aspects of neural development, including cell signaling, cell-cell and cell-matrix interactions, and, potentially, mechanosensation. Here, we specifically focus on the roles of several aGPCRs in synapse biology, dendritogenesis, and myelinating glial cell development. The lessons learned from these examples may be extrapolated to other contexts in the nervous system and beyond.

New insights on Schwann cell development

In the peripheral nervous system, Schwann cells are glial cells that are in intimate contact with axons throughout development. Schwann cells generate the insulating myelin sheath and provide vital trophic support to the neurons that they ensheathe. Schwann cell precursors arise from neural crest progenitor cells, and a highly ordered developmental sequence controls the progression of these cells to become mature myelinating or nonmyelinating Schwann cells. Here, we discuss both seminal discoveries and recent advances in our understanding of the molecular mechanisms that drive Schwann cell development and myelination with a focus on cell-cell and cell-matrix signaling events.

How Schwann Cells Sort Axons: New Concepts

Peripheral nerves contain large myelinated and small unmyelinated (Remak) fibers that perform different functions. The choice to myelinate or not is dictated to Schwann cells by the axon itself, based on the amount of neuregulin I-type III exposed on its membrane. Peripheral axons are more important in determining the final myelination fate than central axons, and the implications for this difference in Schwann cells and oligodendrocytes are discussed. Interestingly, this choice is reversible during pathology, accounting for the remarkable plasticity of Schwann cells, and contributing to the regenerative potential of the peripheral nervous system. Radial sorting is the process by which Schwann cells choose larger axons to myelinate during development. This crucial morphogenetic step is a prerequisite for myelination and for differentiation of Remak fibers, and is arrested in human diseases due to mutations in genes coding for extracellular matrix and linkage molecules. In this review we will summarize progresses made in the last years by a flurry of reverse genetic experiments in mice and fish. This work revealed novel molecules that control radial sorting, and contributed unexpected ideas to our understanding of the cellular and molecular mechanisms that control radial sorting of axons.

Gpr126 functions in Schwann cells to control differentiation and myelination via G-protein activation

The myelin sheath surrounding axons ensures that nerve impulses travel quickly and efficiently, allowing for the proper function of the vertebrate nervous system. We previously showed that the adhesion G-protein-coupled receptor (aGPCR) Gpr126 is essential for peripheral nervous system myelination, although the molecular mechanisms by which Gpr126 functions were incompletely understood. aGPCRs are a significantly understudied protein class, and it was unknown whether Gpr126 couples to G-proteins. Here, we analyze Dhh(Cre);Gpr126(fl/fl) conditional mutants, and show that Gpr126 functions in Schwann cells (SCs) for radial sorting of axons and myelination. Furthermore, we demonstrate that elevation of cAMP levels or protein kinase A activation suppresses myelin defects in Gpr126 mouse mutants and that cAMP levels are reduced in conditional Gpr126 mutant peripheral nerve. Finally, we show that GPR126 directly increases cAMP by coupling to heterotrimeric G-proteins. Together, these data support a model in which Gpr126 functions in SCs for proper development and myelination and provide evidence that these functions are mediated via G-protein-signaling pathways.

Mouse schwann cells need both NRG1 and cyclic AMP to myelinate

Genetically modified mice have been a major source of information about the molecular control of Schwann-cell myelin formation, and the role of β-neuregulin 1 (NRG1) in this process in vivo. In vitro, on the other hand, Schwann cells from rats have been used in most analyses of the signaling pathways involved in myelination. To correlate more effectively in vivo and in vitro data, we used purified cultures of mouse Schwann cells in addition to rat Schwann cells to examine two important myelin-related signals, cyclic adenosine monophosphate (cAMP), and NRG1 and to determine whether they interact to control myelin differentiation. We find that in mouse Schwann cells, neither cAMP nor NRG1, when used separately, induced markers of myelin differentiation. When combined, however, they induced strong protein expression of the myelin markers, Krox-20 and P(0) . Importantly, the level of cAMP signaling was crucial in switching NRG1 from a proliferative signal to a myelin differentiation signal. Also in cultured rat Schwann cells, NRG1 promoted cAMP-induced Krox-20 and P(0) expression. Finally, we found that cAMP/NRG1-induced Schwann-cell differentiation required the activity of the cAMP response element binding family of transcription factors in both mouse and rat cells. These observations reconcile observations in vivo and on neuron-Schwann-cell cultures with studies on purified Schwann cells. They demonstrate unambiguously the promyelin effects of NRG1 in purified cells, and they show that the cAMP pathway determines whether NRG1 drives proliferation or induces myelin differentiation.

Non-antagonistic relationship between mitogenic factors and cAMP in adult Schwann cell re-differentiation

The expression of myelination-associated genes (MGs) can be induced by cyclic adenosine monophosphate (cAMP) elevation in isolated Schwann cells (SCs). To further understand the effect of known SC mitogens in the regulation of SC differentiation, we studied the response of SCs isolated from adult nerves to combined cAMP, growth factors, including neuregulin, and serum. In adult SCs, the induction of MGs by cAMP coincided with the loss of genes expressed in non-myelin-forming SCs and with a change in cell morphology from a bipolar to an expanded epithelial-like shape. Prolonged treatment with high doses of cAMP-stimulating agents, as well as low cell density, was required for the induction of SC differentiation. Stimulation with serum, neuregulin alone, or other growth factors including PDGF, IGF and FGF, increased SC proliferation but did not induce the expression of MGs or the associated morphological change. Most importantly, when these factors were administered in combination with cAMP-stimulating agents, SC proliferation was synergistically increased without reducing the differentiating activity of cAMP. Even though the initiation of DNA synthesis and the induction of differentiation were mostly incompatible events in individual cells, SCs were able to differentiate under conditions that also supported active proliferation. Overall, the results indicate that in the absence of neurons, cAMP can trigger SC re-differentiation concurrently with, but independently of, growth factor signaling.

Schwann cells express IP prostanoid receptors coupled to an elevation in intracellular cyclic AMP

We have shown previously that prostaglandin E(2) (PGE(2)) and prostaglandin I(2) (PGI(2)) are each produced in an explant model of peripheral nerve injury. We report that IP prostanoid receptor mRNA and protein are present in primary rat Schwann cells. IP prostanoid receptor stimulation using prostacyclin produced an elevation in intracellular cyclic AMP concentration ([cAMP](i)) in primary Schwann cells. Peak [cAMP](i) was observed between 5-15 min of stimulation followed by a gradual recovery toward basal level. Phosphorylation of cyclic AMP-response element binding protein (CREB) on Ser(133) was also detected after IP prostanoid receptor stimulation and CREB phosphorylation was inhibited completely by the protein kinase A inhibitor, H-89. Intracellular calcium levels were not affected by IP prostanoid receptor stimulation. Unlike forskolin, IP prostanoid receptor stimulation did not significantly augment Schwann cell proliferation in response to growth factor treatment. However, IP prostanoid receptor stimulation increased the number of Schwann cells that were able to generate a calcium transient in response to P2 purinergic receptor activation. These findings suggest that signaling via the IP prostanoid receptor may by relevant to Schwann cell biology in vivo.

Schwann cell-autonomous role of neuropilin-2

Neuropilins and group A plexins are components of receptor complexes for class 3 semaphorins, gradients of which help to guide migration of neural progenitor cells and axonal growth cones during development. We demonstrated previously that neuropilins and class 3 semaphorins are induced in sciatic nerve by crush or transection. We now report that in cultured rat Schwann cells, expression of mRNA encoding neuropilin-2 (NRP2) and plexin-A3 (PlexA3), proteins involved in semaphorin-3F (Sema3F) signal transduction, is diminished markedly by forskolin, an adenylate cyclase activator that, like axonal contact, induces Schwann cell synthesis of myelin lipids and proteins. Interestingly, Schwann cell expression of mRNA encoding NRP1, which participates in Sema3A signaling, is not downregulated by forskolin. Antibodies that recognize ectodomains of NRP2 but not control antibodies prevented cultured Schwann cells from aligning in parallel and forming columns. These results are consistent with the view that in nerves undergoing Wallerian degeneration, Schwann cell NRP2 facilitates assembly of Schwann cells into the tubular aggregates (bands of Büngner) that guide regenerating axons.

Nerve Growth Factor Regulates Expression of the Nerve Growth Factor Receptor Gene in Adult Sensory Neurons

Sensory neurons of the adult rat dorsal root ganglion (DRG) can be maintained in culture in the absence of nerve growth factor (NGF). We have thus used dissociated cultures of these neurons to study effects of NGF on the regulation of expression of mRNA encoding the nerve growth factor receptor (NGF-R). In the absence of NGF, levels of NGF-R mRNA remained constant for 7 days in cultures of adult rat DRG neurons. In the presence of NGF, NGF-R mRNA levels rose two - three-fold after 2 days, reaching plateau levels (five - six-fold elevation) after 5 days. This NGF-induced up-regulation could be demonstrated even after prior NGF-deprivation for 3 - 4 days. NGF had no effect upon NGF-R mRNA levels in DRG non-neuronal cells. Epidermal growth factor (EGF), fibroblast growth factor (FGF) and ciliary neurotrophic factor (CNTF) were without effect on NGF-R mRNA levels, but 8-bromo-cAMP decreased NGF-R mRNA levels by 65% after 2 days. NGF also induced a rapid (30 min) rise in expression of c-fos in DRG neurons, but not in non-neuronal cells. Our results suggest that endogenous levels of NGF may regulate the expression of NGF-R in vivo.

Axon-Schwann cell interactions regulate the expression of fibroblast growth factor-5 (FGF-5)

We screened for genes whose expression is significantly up- or downregulated during Wallerian degeneration in adult rat sciatic nerve with cDNA arrays. Fibroblast growth factor-5 (FGF-5) mRNA seemed to be induced. This was confirmed by northern blotting and in situ hybridization, as well as Western blotting for FGF-5 in axotomized nerve. Axon-Schwann cell interactions decreased the steady-state level of FGF-5 mRNA in regenerating sciatic nerves, and forskolin diminished its expression in cultured Schwann cells. We conclude that denervated Schwann cells synthesize FGF-5, which is a secreted, neuronotrophic member of the FGF family.

Absence of P0 leads to the dysregulation of myelin gene expression and myelin morphogenesis

P0, the major peripheral nervous system (PNS) myelin protein, is a member of the immunoglobulin supergene family of membrane proteins and can mediate homotypic adhesion. P0 is an essential structural component of PNS myelin; mice in which P0 expression has been eliminated by homologous recombination (P0-/-) develop a severe dysmyelinating neuropathy with predominantly uncompacted myelin. Although P0 is thought to play a role in myelin compaction by promoting adhesion between adjacent extracellular myelin wraps, as an adhesion molecule it could also have a regulatory function. Consistent with this hypothesis, Schwann cells in adult P0-/- mice display a novel molecular phenotype: PMP22 expression is down-regulated, MAG and PLP expression are up-regulated, and MBP expression is unchanged. As in quaking viable mutant mice (qk(v)), which have uncompacted myelin morphologically similar to that found in P0-/- mice, neither the qKI-6 or qKI-7 proteins are expressed in P0-/- peripheral nerve. In addition to these changes in gene expression in the P0 knockout, PLP/DM-20 accumulates in the endoplasmic reticulum of P0-/- Schwann cells, whereas MAG accumulates in redundant loops of uncompacted myelin, not at nodes of Ranvier or Schmidt-Lantermann incisures. Taken together, these results demonstrate that P0 is involved, either directly or indirectly, in the regulation of both myelin gene expression and myelin morphogenesis.

Nerve injury and inflammatory cytokines modulate gap junctions in the peripheral nervous system

In the peripheral nervous system (PNS), myelinating Schwann cells express the gap junction protein connexin32 (Cx32) and lower levels of connexin43 (Cx43). Although the function of Cx43 in Schwann cells is not yet known, in adult mammals Cx32 is thought to form reflexive contacts within individual myelinating glial cells and provide direct pathways for intracellular ionic and metabolic exchange from the cell body to the innermost periaxonal cytoplasmic regions. In response to nerve injury, Schwann cells in the degenerating region down-regulate expression of Cx32 and there is increased expression of connexin46 (Cx46) mRNA and protein. The cascade of Schwann cell responses seen after the injury-induced decrease in Cx32, and the observation that dividing Schwann cells express Cx46, and possibly other connexins, and are coupled through gap junction channels, raise the intriguing possibility that there are coordinated changes in Schwann cell proliferation and connexin expression. Moreover, intercellular junctional coupling among cells in general may be important during injury responses. Consistent with this hypothesis, dividing Schwann cells are preferentially coupled through junctional channels as compared to non-dividing cells, which are generally uncoupled. Moreover, the strength of junctional coupling among cultured Schwann cells is modulated by a number of cytokines to which Schwann cells are exposed to in vivo after nerve injury, and Cx46 mRNA and protein levels correlate with the degree of coupling. Other injury-induced cellular changes in connexin expression that may be functionally important during injury responses include a transient increase in Cx43 in endoneurial fibroblasts. This paper reviews what is known about connexin expression and function in the adult mammalian PNS, and focuses on some of the changes that occur following nerve injury. Moreover, evidence that inflammatory cytokines released after injury modulate connexin expression and junctional coupling strength is presented.

Immunocytochemical expression of human muscle cell p75 neurotrophin receptor is down-regulated by cyclic adenosine 3',5'-monophosphate

To investigate whether the immunocytochemical expression of low affinity neurotrophin receptor (p75) in human muscle is modulated by increased levels of intracellular cyclic adenosine 3',5'-monophosphate (cAMP), human cultured myogenic cells were treated with cAMP analogues dibutyryl cAMP (dbcAMP 0.5-1 mM) and 8-bromo cAMP (1 mM) or the adenylate cyclase activator forskolin (10-100 microM). Cultures were processed for indirect immunofluorescence microscopy using an anti-human p75 mAb. The treatment of cultured muscle cells with cAMP analogues or forskolin for two days induced a decrease of immunoreactivity for p75 and a reduction of both myotube formation and morphological cell differentiation. Removal of cAMP derivatives from the medium resulted in a return of immunoreactive cells to the levels of untreated controls. These data indicate that adenylate cyclase is involved in the regulation of human muscle p75.

Age-related differences in mouse Schwann cell response to cyclic AMP

An elevation of intracellular cyclic adenosine 3', 5'-monophosphate (cAMP) is known to stimulate proliferation and expression of surface galactocerebroside (galC) in cultured Schwann cells from newborn rats and mice. We investigated the age-related differences in proliferative and differentiating responses of Schwann cells to forskolin, an adenylate cyclase activator, in mice at postnatal days (P) 3, 10 and 30. The proportion of surface galC-positive cells decreased progressively with time in all ages during the initial three days in vitro (DIV) but the proportion of Schwann cells expressing surface galC at P30 was lower than that at P3 or P10. Administration of 50 microM forskolin in the media on the four DIV induced surface galC expression in Schwann cells from P3 and P10 mice but not from P30 mice. Forskolin also stimulated the proliferation of Schwann cells from P3 and P10 mice in media containing 2% fetal bovine serum (FBS) but not those from P30 mice, which, however, proliferated in media containing 10% FBS. These results suggest that Schwann cells in suckling mice are more sensitive to cAMP in both proliferative and differentiating responses than in adult. These different responses in vitro may reflect differences in the intrinsic metabolic status of Schwann cells in vivo since myelin-forming Schwann cells proliferate actively prior to myelination but cease or become less active in proliferation once myelination progresses.

Axon-Schwann cell interactions regulate the expression of c-jun in Schwann cells

The transcription factor c-jun is selectively expressed by non-myelinating Schwann cells in normal peripheral nerve, and be "denervated," previously myelinatng Schwann cells, after axotomy. When axons regenerate into the distal nerve-stump, the expression of c-jun declines as Schwann cells remyelinate axons. Treating cultured Schwann cells with forskolin, a drug that mimics many of the effects of axon-Schwann cell interactions, decreases the expression of myelin-specific genes. Overexpressing c-jun in cultured Schwann cells, however, does not decrease the expression of a myelin basic protein promoter-reporter construct, indicating that c-jun expression may not directly regulate myelin-specific gene expression. These data suggest that c-jun is invloved in regulating the phenotype of non-myelinating and denervated Schwann cells.

Low-affinity nerve growth factor receptor expression in sciatic nerve during P2-peptide induced experimental allergic neuritis

Northern blot, immunocytochemistry, and single nerve fiber immunostaining were used to determine the expression of low-affinity nerve growth factor (NGF) receptor (p75NGFR) in the peripheral nervous system of Lewis rats during the course of experimental allergic neuritis (EAN) induced with 100 micrograms of 'SP26' synthetic peptide, corresponding to residues 53-78 of bovine P2 myelin protein. This severe reversible polyneuropathy, characterized by extensive multifocal demyelination in nerve roots, and axonal degeneration within sciatic nerve, represents a useful tool to investigate Schwann cell gene expression in these different pathological conditions. Our results showed that both p75NGFR mRNA and protein were induced within sciatic nerve by day 18 and 23 after 'SP26' immunization. By this time, however, p75NGFR was not detectable in roots of cauda equina. These data demonstrate that, during EAN induced by immunization with 100 micrograms of 'SP26', p75NGFR is up-regulated in Schwann cells and in nerve fibers distally along the nerve. This suggests that the induction of the receptor in sciatic nerve during P2-peptide EAN is related to the loss of axonal contact by Schwann cells.

cAMP-dependent differential regulation of extracellular matrix (ECM) gene expression in cultured rat Schwann cells

cAMP-dependent regulation of the steady-state mRNA levels for the ECM components, laminin A, B1 and B2 chains, collagen types I, III and IV were examined by Northern blot analysis in cultured rat Schwann cells. ECM mRNAs of laminin B1 chain and collagen types I and IV were expressed at high levels in the control Schwann cells, while laminin B2 chain and collagen type III mRNA levels were low, and laminin A chain mRNA was not detectable. When Schwann cells were treated with forskolin or cAMP derivatives, the gene expression for the ECM molecules constituting the Schwann cell basement membrane, laminin B1 and B2 chains, and collagen type IV, was enhanced in time- and dose-dependent manners for exogenously administered forskolin or cAMP derivatives, while the mRNA levels for the ECM molecules, which are not the major components of the basement membrane, fibrillary collagen types I and III were significantly suppressed. This cAMP-dependent differential regulation of Schwann cell ECM gene expression may be related to the role of each ECM molecule in the peripheral nerve development and regeneration.

Expression of growth-associated protein-43 kD in Schwann cells is regulated by axon-Schwann cell interactions and cAMP

We have examined the regulation of growth-associated protein 43 kD (GAP-43) in rat Schwann cells. In unlesioned adult nerves, GAP-43-immunoreactivity was restricted to non-myelinating Schwann cells and unmyelinated axons. When adult nerves were transected to cause permanent axotomy, previously myelinating Schwann cells expressed progressively more GAP-43-immunoreactivity over 3 weeks, and GAP-43 mRNA levels increased over a similar time course. The peak level of GAP-43 mRNA occurred at least 2 weeks later than that of nerve growth factor receptor, another marker of denervated Schwann cells. In contrast, after nerve-crush, which allows axonal regeneration, many fewer Schwann cells had GAP-43-immunoreactivity, and the amount of GAP-43 mRNA was markedly lower than in transected nerves. Forskolin, a drug that activates adenylate cyclase and mimics many effects of axon-Schwann cell interactions, markedly reduced GAP-43-immunoreactivity and mRNA expression in cultured Schwann cells, whereas interleukin-1 had no effect. These data demonstrate that axon-Schwann cell interactions inhibit the expression of GAP-43 in Schwann cells and that this effect is mimicked by forskolin.

Antibodies to interleukin-6 inhibit Schwann cell proliferation induced by unfractionated cytokines

We have previously demonstrated that unfractionated cytokines induce proliferation of neonatal rat Schwann cells in vitro. Incubation of Schwann cells with natural or recombinant human interleukin-1 (IL-1) does not induce Schwann cell proliferation, but antibodies to IL-1 inhibit Schwann cell proliferation induced by unfractionated cytokines of human origin. In this study we investigated the effect of interleukin-6 (IL-6), another monokine, on proliferation of rat Schwann cells. Incubation of Schwann cells with recombinant (r) mouse IL-6 and two types of human rIL-6 did not induce proliferation of Schwann cells or of endoneurial fibroblasts. However, antibodies to rhIL-6 inhibited Schwann cell proliferation induced by unfractionated cytokines but had no effect on fibroblast proliferation. Addition of excess rhIL-6 to a mixture of unfractionated cytokines plus anti-rhIL-6 abrogated the inhibitory effect of the anti-rhIL-6. Therefore, IL-6 is a co-mitogen for neonatal Schwann cells in vitro.

Neurons regulate Schwann cell genes by diffusible molecules

Successful peripheral nerve regeneration and functional recovery require the reestablishment of the neuron-Schwann cell relationship in the regenerating rat sciatic nerve, neurons differentially regulate Schwann cell genes. The message for the low-affinity NGF receptor, p75NGFR, is induced in Schwann cells distal to the injury and is repressed as regenerating axons make contact with these cells. The inverse is true for mRNA of the myelin gene P0; expression decreases distal to injury and increases as new axons contact Schwann cells and a program of myelination is initiated. Using an in vitro co-culture paradigm in which primary neurons and adult Schwann cells are separated by a microporous membrane, we show that axon contact is not an absolute requirement for neuronal regulation of Schwann cell genes. In this system neurons but not other cell types, repress the expression of Schwann cell p75NGFR while inducing the expression of the POU domain transcription factor, suppressed cAMP inducible POU, and myelin P0. These results demonstrate that regenerating axons can direct the Schwann cell genetic program from a distance through diffusible molecules.

Expression of low-affinity NGF receptor and trkB mRNA in human SH-SY5Y neuroblastoma cells

We have used the human neuroblastoma cell line SH-SY5Y as a model system to investigate the expression and regulation of the receptors for brain-derived neurotrophic factor (BDNF), a member of the nerve growth factor (NGF) family of neurotrophins. We demonstrate that SH-SY5Y cells express transcripts encoding the low-affinity NGF receptor (LNGFR) and trkB, the signal transducing receptor unit for BDNF. Interaction of BDNF with SH-SY5Y cells increased the transcription of the c-fos gene, showing that these molecules encode functional BDNF receptors. Our findings that differentiating agents such as retinoids and cAMP analogs increased the expression of LNGFR, but decreased trkB mRNA levels, suggest that LNGFR and trkB have different roles during neuronal differentiation.

Control of receptor sensitivity at the mRNA level

Neurons are able to adjust the sensitivity of receptor-mediated processes according to the level of receptor activation. Extrapolating from our knowledge of other cellular proteins, regulation of receptor mRNA availability would provide a highly economical means of achieving this objective. Epidermal growth factor is able to induce long-lasting increases in its receptor binding by increasing receptor mRNA levels, and similar effects have been shown for other growth factors. Studies on G-protein-coupled receptors, in particular using adrenoceptor clones transfected into cultured cell lines, have shown that changes in receptor number are generally associated with an alteration in receptor mRNA content. At the neuromuscular junction, dramatic increases in nicotinic acetylcholine receptor number are achieved by activating receptor subunit gene transcription. Less information is available concerning the regulation of ligand-gated ion channels in the brain. Overall, the evidence suggests that receptor mRNA levels are frequently controlled by the degree of receptor stimulation. Receptor mRNA levels are therefore likely to be one of the most important control points for both homologous and heterologous regulation of receptor sensitivity.

Gene expression of high- (p140trk) and low-affinity nerve growth factor receptor (LNGFR) in the adult and aged human peripheral nervous system

Steady-state mRNA levels and immunoreactive proteins for high- (p140trk) and low-affinity nerve growth factor receptor (LNGFR) in the adult and aged human peripheral nervous system (PNS) were examined in autopsied material. trk mRNA expression was observed only in the sympathetic and dorsal root ganglia, while LNGFR mRNA was expressed widely through the PNS as well as non-neural tissues. Immunoreactive trk proto-oncogene product (p140trk) and LNGFR occurred in the perikarya of the subset of the sympathetic and dorsal root ganglion neurons, but only LNGFR immunoreactivity also occurred in the perineurium and the outer layer of the vessels. The spatial patterns of the trk and LNGFR gene expression in the adult human PNS were similar to those observed in the rat, mouse and chick, and their expression was well preserved in the aged.

In vitro studies of axonally-regulated Schwann cell genes during Wallerian degeneration

Wallerian degeneration in vivo is associated with marked downregulation of myelin protein genes such as P(o) and upregulation of other genes such as nerve growth factor receptor (NGF-R), glial fibrillary acidic protein (GFAP) and neural cell adhesion molecule (N-CAM). This study examines the expression of these genes during Wallerian degeneration in vitro and how manipulating Ca2+ affects this response. Small explants of sciatic nerve from normal young adult rats cultured for five days show similar reversal of the myelinating phenotype as found in vivo. If Ca++ is removed from the culture medium through the addition of EGTA, expression of the nerve growth factor receptor and glial fibrillary acidic protein genes is inhibited but downregulation of the P(o) gene still occurs. Explants cultured in medium containing EGTA are still capable of expressing nerve growth factor receptor if the medium is replaced by one containing Ca2+. Supplementation of normal medium with drugs modulating Ca2+, such as Bepridil which blocks the Na+Ca2+ exchanger or compound 48/80 which inhibits calmodulin, also prevent the expression of the nerve growth factor receptor gene during Wallerian degeneration in vitro. Treatment of the cervical sympathetic trunk with Bepridil leads to loss of the nerve growth factor receptor immunoreactivity which is normally present. The results indicate that Ca2+ may play a role in the expression of the nerve growth factor receptor gene during Wallerian degeneration and provide some indication that this effect may be directly on the Schwann cell rather than operating indirectly via the axon.

Modulation of Schwann cell phenotype by TGF-beta 1: inhibition of P0 mRNA expression and downregulation of the low affinity NGF receptor

The phenotype of a fully differentiated, mature Schwann cell is apparently largely determined by Schwann cell-axon interactions. In vitro, elevation of intracellular cAMP levels in Schwann cells induces a phenotype which resembles that of a mature, i.e., axon-related, Schwann cell. Therefore, an important role for cAMP as a second messenger of axon-Schwann cell interactions in vivo is assumed. However, the effects of cAMP on Schwann cells are not restricted to induction of features of a mature phenotype. For example, elevation of intracellular cAMP levels results also in a markedly increased synthesis of nerve growth factor (NGF) mRNA, which is barely detectable in intact sciatic nerves of adult animals. Furthermore, since cAMP induces myelin gene expression in cultured Schwann cells, additional regulatory mechanisms have to be postulated for the induction and maintenance of a mature non-myelinating Schwann cell phenotype. Here we show that a soluble protein "growth factor" can partially induce a non-myelinating nature Schwann cell phenotype in vitro. Treatment with transforming growth factor beta 1 (TGF-beta 1) results in a marked and rapid downregulation of the low affinity NGF receptor (NGFR) on cultured Schwann cells without induction of P0 gene expression. In contrast, in agreement with previous studies, an increase in P0 mRNA levels and a reduction in NGFR mRNA levels are observed after cAMP elevation. Downregulation of NGFR mRNA after cAMP elevation is much slower when compared with the effect of TGF-beta 1, suggesting the involvement of different intracellular mechanisms. Consistent with this hypothesis, we did not observe an induction of mRNA coding for TGF-beta isoforms after cAMP elevation in cultured Schwann cells which constitutively synthesize TGF-beta 1 mRNA.

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and low-affinity nerve growth factor receptor (LNGFR) mRNA levels in cultured rat Schwann cells; differential time- and dose-dependent regulation by cAMP

Steady-state mRNA levels for NGF, BDNF, and NT-3, and their receptors, LNGFR, trk, trkB, and trkC, were examined in cultured rat Schwann cells. NGF, BDNF, and LNGFR mRNAs were expressed in control Schwann cells, but NT-3, trk, trkB and trkC mRNAs were not. As previously reported, when Schwann cells were treated with forskolin or 8-bromo-cAMP, NGF mRNA expression was enhanced, while BDNF and LNGFR mRNA levels were suppressed. The optimal concentration of forskolin for NGF mRNA enhancement and BDNF mRNA suppression was similar to that for LNGFR mRNA suppression, which coincided with the optimal concentration for Schwann cells to differentiate. However, the lag-time for forskolin-induces down-regulation of LNGFR mRNA was far greater than that for NGF mRNA increase and BDNF mRNA suppression.

Structural domains of the extracellular domain of human nerve growth factor receptor detected by partial proteolysis

Using partial proteolytic cleavage, the nerve growth factor (NGF) binding site and the epitopes for two anti-NGF receptor (NGFR) monoclonal antibodies were localized on the recombinant extracellular domain (RED) of the NGFR. The RED was prepared in the baculovirus-insect cell system and was purified by immunoaffinity and ion-exchange chromatography. The four cysteine-rich repeat domains and some additional C-terminal sequences were resistant to proteolysis with papain or proteinase K. The Mr 32,000 papain-resistant fragment (P32) and the Mr 30,000 proteinase K-resistant fragment (K30) share the same N terminus as the intact RED and have C termini in the vicinity of residue 170. Even though P32 and K30 have the same N terminus and probably differ by only a small number of amino acids at the C terminus, P32, but not K30, binds 125I-NGF. As judged by Western blot analysis, two anti-NGFR antibodies (ME20.4 and NGFR5) bind to P32 but have a lesser affinity for K30. Since antibody ME20.4 inhibits NGF binding but antibody NGFR5 does not, these antibodies bind to distinct epitopes. However, these epitopes apparently are closely spaced since these antibodies compete with each other for binding to biotinylated RED. NGF, but not the control protein cytochrome c, protects RED from papain digestion. Therefore, the P32 C terminus is important for the expression of the NGF binding site and the antibody-defined epitopes, even though the NGF binding site and antibody-defined epitopes probably are not encoded by the P32 C terminus. These data suggest that complex interactions occur between different regions of the RED, and that optimum NGF binding requires the integrity of multiple RED domains, including a short sequence to the C terminus of residue 170.

The nerve growth factor receptor: a multicomponent system that mediates the actions of the neurotrophin family of proteins

Nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 (NT-3) are members of a family of structurally related proteins termed neurotrophins that promote the growth and survival of neurons in the central and peripheral nervous systems. Each of these proteins bind to at least two membrane receptors. One is the low affinity nerve growth factor receptor (p75), which binds each member of the neurotrophin family. The other is one of a family of tyrosine kinase receptors--trkA binds only NGF, the related trkB receptor binds BDNF and NT-3, and trkC binds NT-3 alone. This article reviews kinetic and biochemical information on p75 and its relationship to the trk gene products.

Platelet-derived growth factor and regulation of Schwann cell proliferation in vivo

To examine the role of platelet-derived growth factor (PDGF) in the in vivo regulation of Schwann cell proliferation, steady-state levels of mRNAs encoding PDGF A and B chains, and PDGF alpha and beta receptors were measured in immature and adult rat sciatic nerves and in cultured rat Schwann cells. PDGF B chain and PDGF beta receptor mRNAs are present in immature rat sciatic nerves and to a lesser extent in adult rat nerves. Short-term cultures of neonatal rat Schwann cells express PDGF beta receptor mRNA, but not PDGF B chain mRNA, and are stimulated to synthesize DNA by addition of PDGF BB to the medium. These data indicate that PDGF BB is a developmentally regulated paracrine growth factor for rat Schwann cells. Very long-term cultures of rat Schwann cells, which have lost normal dependence on exogenous growth factors, express PDGF B chain mRNA as well as mRNAs encoding the PDGF alpha and beta receptors, suggesting that, under these circumstances, PDGF BB also act as an autocrine growth factor. PDGF A chain mRNA is present in both immature and adult rat sciatic nerves and is expressed by primary and secondary cultures of rat Schwann cells as well. However, because the abundance of PDGF alpha receptor mRNA is very low in rat Schwann cells, PDGF AA is not likely to be a significant autocrine growth factor for rat Schwann cells.

P0 gene expression in cultured Schwann cells

This study examines the expression of the major myelin protein gene P0 in cultured Schwann cells, grown on their own or in association with neurons. Many freshly dissociated Schwann cells from actively myelinating nerves express Po mRNA in high abundance. If neurons are not present, signal intensity falls markedly with time so that by 7 days in culture only a basal expression is evident which is negligible compared to the level in vivo. Dorsal root ganglia from embryo day 16 (E16) rats contain no significant levels of Po mRNA but when grown in full myelinating medium (containing serum and embryo extract) increasing expression is seen from 4 to 5 days onward even though myelination does not occur until after the second week. In this intervening period the intensity of P0 mRNA expression is lower than that found in the actively myelinating cell. Neurons from sympathetic ganglia are also capable of inducing P0 mRNA expression. Schwann cells in dorsal root ganglia explants grown in serum-free defined medium do not assemble a basal lamina and will not wrap or myelinate axons. Nevertheless P0 mRNA, but not protein, is expressed in levels similar to those found in full myelinating medium prior to myelination. Such Schwann cells also exhibit galactocerebroside and the sulphatide recognised by the 04 antibody. It appears that in defined medium or in myelinating medium prior to myelination axonal signals can induce P0 mRNA expression to a certain degree. However, full up-regulation is usually associated with the rapid membrane expansion accompanying myelination. Whether this augmented up-regulation is due to further axonal signalling or events in the Schwann cell is unknown, but the results suggest that P0 expression can be regulated at several stages of synthesis.

Axons induce differentiation of neurofibroma Schwann-like cells

Neurofibromatosis type 1 (NF-1, von Recklinghausen's disease) is characterized by the focal accumulation of Schwann-like cells (SLC) to form subcutaneous and plexiform neurofibromas and schwannomas. The aim of the present study was to determine whether NF-SLC are competent to differentiate in the presence of axons. Five dermal neurofibromas from five patients with NF-type 1 were enzymatically dissociated and the resultant cells were co-cultured with fetal rat dorsal root ganglion neurons. The cultures were studied by indirect immunofluorescence microscopy using antibodies against galactocerebroside (galC), P0 glycoprotein, human nerve growth factor receptor (NGFR) and human myelin-associated glycoprotein (MAG). SLC were strongly NGFR+ but galC- and MAG-SLC for the 2 weeks of coculture. After 3 weeks in vitro, SLC-NGFR was down-regulated but some of the spindle shaped cells had become galC+. MAG-SLC first appeared after 5 weeks in vitro but P0 glycoprotein was never detected when studied up to 6 weeks. Our data demonstrate that axons induce SLC to down-regulate surface NGFR and to express some myelin components in a qualitatively normal fashion.

The effects of cAMP on differentiation of cultured Schwann cells: progression from an early phenotype (04+) to a myelin phenotype (P0+, GFAP-, N-CAM-, NGF-receptor-) depends on growth inhibition

The present experiments were designed to clarify the relationship between cAMP elevation, proliferation and differentiation in Schwann cells. They were carried out on short-term cultures of cells obtained from neonatal rat sciatic nerves. It was found that the myelin-related phenotype was expressed in response to agents that elevate or mimic intracellular cAMP (forskolin, cholera toxin, cAMP analogues), provided cell division was absent. This phenotype included upregulation of the major myelin protein P0 and downregulation of GFAP, N-CAM, A5E3, and NGF receptor. In contrast, when cells were cultured in conditions where cell division occurred, elevation of intracellular cAMP produced an alternative response, characterized by DNA synthesis and absence of myelin-related differentiation. The cAMP mediated induction of an early Schwann cell antigen, 04, followed a different pattern since it was induced equally in dividing and nondividing cells. These observations are consistent with the proposal that during development of the rat sciatic nerve: (a) cAMP elevation, possibly induced by axon-associated factors, is a primary signal responsible for the induction of 04 expression in proliferating Schwann cells during the premyelination period; (b) subsequent withdrawal of cells associated with the larger axons from the cell cycle acts as a permissive secondary signal for induction of myelination, since in quiescent cells the ongoing cAMP elevation will trigger myelination.

Cyclic nucleotide-dependent protein kinases

The actions of several hormones and neurotransmitters evoke signal transduction pathways which rapidly elevate the cytosolic concentrations of the intracellular messengers, cAMP and cGMP. The cyclic-nucleotide dependent protein kinases, cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG), are the major intracellular receptors of cAMP and cGMP. These enzymes become active upon binding respective cyclic nucleotides and modulate a diverse array of biochemical events through the phosphorylation of specific substrate proteins. The focus of this review is to describe the progress made in understanding the structure and function of both PKA and PKG.

Characterization of two neuroblastoma cell lines expressing recombinant nerve growth factor receptors

In earlier studies, a 75,000-dalton glycoprotein (gp75) has been identified as a component of both low- and high-affinity nerve growth factor (NGF) receptors (NGFRs). Using an amphoteric expression vector, we have introduced the cDNA encoding the human gp75 into two neuroblastoma cell lines. SHEP is a human neuroblastoma cell line that lacks most neuronal characteristics and does not express NGFRs. The transformant line SHEP/NGFR expressed a single affinity class of NGF binding sites, did not display NGF-induced up-regulation of fos oncogene expression, and did not efficiently internalize NGF. LAN5 is a neuroblastoma cell line with neuronal characteristics, including expression of neurofilament and display of short neurites. This cell line expresses a small number of high-affinity NGFRs but no detectable low-affinity sites. The transformant line LAN5/NGFR expressed both high- and low-affinity NGFRs, displayed NGF-induced up-regulation of fos oncogene, and efficiently internalized NGF. The number of high-affinity NGF binding sites was nearly the same for LAN5 and LAN5/NGFR, a finding suggesting that there is a limiting number of some separately coded factor or subunit that is required for high-affinity NGFRs. Because NGF induction of fos oncogene expression correlated with expression of high-affinity NGFRs, the putative second factor may also limit NGF responsiveness.

Regulation of Schwann cell surface and truncated nerve growth factor receptors in vitro by axonal components

A tissue culture model was designed to characterize the ability of axonal membranes and detergent extracts of membranes to down regulate nerve growth factor (NGF) receptors on the Schwann cell surface. The findings establish that (i) axons from intact neurons, as well as axonal membrane fragments, and extracts of these axons can down regulate Schwann cell NGF receptor number, (ii) central, as well as peripheral, axons have the ability to affect Schwann cell NGF receptor expression, (iii) cross-species reactivity is evident with this phenomenon, (iv) physical contact is necessary for axons to down regulate Schwann cell NGF receptor, (v) the response of axons is glial specific, and (vi) soluble, truncated NGF receptors elaborated by Schwann cells are also down-regulated by axon components.

Neuronal modulation of Schwann cell glial fibrillary acidic protein (GFAP)

Adult rat sciatic nerves contain cytoskeletal peptides that resemble CNS glial fibrillary acidic protein (GFAP) in immunoreactivity and molecular weight. Immunohistological examination of teased nerve fascicles indicated that these peptides are expressed selectively by Schwann cells related to small axons. Radiolabelled mouse and rat CNS GFAP cDNA probes hybridized with a single, 2.7 kb RNA band in Northern blots prepared from total RNA from both rat sciatic nerve and rat brain. Sciatic nerve GFAP mRNA was detectable by this means in adult, 2 month, or 21 day postnatal rats, but not in 3,6, or 10 day postnatal rats. Sciatic nerve transection caused a marked reduction in the level of GFAP mRNA in the axotomized distal stump. We conclude that Schwann cell synthesis of GFAP is developmentally regulated and that Schwann cells, unlike astroglia, require continued trophic input from small axons in order to express GFAP.