Regulation of myelination: Schwann cell transition from a myelin-maintaining state to a quiescent state after permanent nerve transection

Glycans of myelin proteins

Human P0 is the main myelin glycoprotein of the peripheral nervous system. It can bind six different glycans, all linked to Asn(93) , the unique glycosylation site. Other myelin glycoproteins, also with a single glycosylation site (PMP22 at Asn(36) , MOG at Asn(31) ), bind only one glycan. The MAG has 10 glycosylation sites; the glycoprotein OMgp has 11 glycosylation sites. Aside from P0, no comprehensive data are available on other myelin glycoproteins. Here we review and analyze all published data on the physicochemical structure of the glycans linked to P0, PMP22, MOG, and MAG. Most data concern bovine P0, whose glycan moieties have an MW ranging from 1,294.56 Da (GP3) to 2,279.94 Da (GP5). The pI of glycosylated P0 protein varies from pH 9.32 to 9.46. The most charged glycan is MS2 containing three sulfate groups and one glucuronic acid; whereas the least charged one is the BA2 residue. All glycans contain one fucose and one galactose. The most mannose rich are the glycans MS2 and GP4, each of them has four mannoses; OPPE1 contains five N-acetylglucosamines and one sulfated glucuronic acid; GP4 contains one sialic acid. Furthermore, human P0 variants causing both gain and loss of glycosylation have been described and cause peripheral neuropathies with variable clinical severity. In particular, the substitution T(95) →M is a very common in Europe and is associated with a late-onset axonal neuropathy. Although peripheral myelin is made up largely of glycoproteins, mutations altering glycosylation have been described only in P0. This attractive avenue of research requires further study.

Homeostatic regulation of the endoneurial microenvironment during development, aging and in response to trauma, disease and toxic insult

The endoneurial microenvironment, delimited by the endothelium of endoneurial vessels and a multi-layered ensheathing perineurium, is a specialized milieu intérieur within which axons, associated Schwann cells and other resident cells of peripheral nerves function. The endothelium and perineurium restricts as well as regulates exchange of material between the endoneurial microenvironment and the surrounding extracellular space and thus is more appropriately described as a blood-nerve interface (BNI) rather than a blood-nerve barrier (BNB). Input to and output from the endoneurial microenvironment occurs via blood-nerve exchange and convective endoneurial fluid flow driven by a proximo-distal hydrostatic pressure gradient. The independent regulation of the endothelial and perineurial components of the BNI during development, aging and in response to trauma is consistent with homeostatic regulation of the endoneurial microenvironment. Pathophysiological alterations of the endoneurium in experimental allergic neuritis (EAN), and diabetic and lead neuropathy are considered to be perturbations of endoneurial homeostasis. The interactions of Schwann cells, axons, macrophages, and mast cells via cell-cell and cell-matrix signaling regulate the permeability of this interface. A greater knowledge of the dynamic nature of tight junctions and the factors that induce and/or modulate these key elements of the BNI will increase our understanding of peripheral nerve disorders as well as stimulate the development of therapeutic strategies to treat these disorders.

Polyol pathway and diabetic peripheral neuropathy

This chapter critically examines the concept of the polyol pathway and how it relates to the pathogenesis of diabetic peripheral neuropathy. The two enzymes of the polyol pathway, aldose reductase and sorbitol dehydrogenase, are reviewed. The structure, biochemistry, physiological role, tissue distribution, and localization in peripheral nerve of each enzyme are summarized, along with current informaiton about the location and structure of their genes, their alleles, and the possible links of each enzyme and its alleles to diabetic neuropathy. Inhibitors of pathway enzyme and results obtained to date with pathway inhibitors in experimental models and human neuropathy trials are updated and discussed. Experimental and clinical data are analyzed in the context of a newly developed metabolic odel of the in vivo relationship between nerve sorbitol concentration and metabolic flux through aldose reuctase. Overall, the data will be interpreted as supporting the hypothesis that metabolic flux through the polyol pathway, rather than nerve concentration of sorbitol, is the predominant polyol pathway-linked pathogeneic factor in diabetic preipheral nerve. Finally, key questions and future directions for bsic and clinical research in this area are considered. It is concluded that robust inhibition of metabolic flux through the polyol pathway in peripheral nerve will likely result in substantial clinical benefit in treating and preventing the currently intractable condition of diabetic peripheral neuropathy. To accomplish this, it is imperative to develop and test a new generation of "super-potent" polyol pathway inhibitors.

Role of axonal components during myelination

Myelination is a multistep ordered process whereby Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS), produce and extend membranous processes that envelop axons. Mechanisms that regulate this complex process are not well understood. Advances in deciphering the regulatory components of myelination have been carried out primarily in the PNS and although the mechanisms for triggering and directing myelination are not known, it is well established that myelination does not occur in the absence of axons or axon/neuron-derived factors. This appears to be true both in PNS and CNS. Progress in understanding CNS myelinogenesis has been relatively slow because of the unavailability of a suitable culture system, which, in turn, is partly due to complexity in the cellular organization of the CNS. Though the myelin composition differs between PNS and CNS, the regulation of myelination seems to parallel rather than differ between these two systems. This article reviews the regulatory role of axonal components during myelination. The first half consists of an overview of in vitro and in vivo studies carried out in the nervous system. The second half discusses the use of a cerebellar slice culture system and generation of anti-axolemma monoclonal antibodies to investigate the role of axonal membrane components that participate in myelination. It also describes the characterization of an axonal protein involved in myelination.

Neurons promote the translocation of peripheral myelin protein 22 into myelin

Schwann cells express low levels of myelin proteins in the absence of neurons. When Schwann cells and neurons are cultured together the production of myelin proteins is elevated, and myelin is formed. For peripheral myelin protein 22 (PMP22), the exact amount of protein produced is critical, because peripheral neuropathies result from its underexpression or overexpression. In this study we examined the effect of neurons on Schwann cell PMP22 production in culture and in peripheral nerve using metabolic labeling and pulse-chase studies as well as immunocytochemistry. Most of the newly synthesized PMP22 in Schwann cells is rapidly degraded in the endoplasmic reticulum. Only a small proportion of the total PMP22 acquires complex glycosylation and accumulates in the Golgi compartment. This material is translocated to the Schwann cell membrane in detectable amounts only when axonal contact and myelination occur. Myelination does not, however, alter the rapid turnover of PMP22 in Schwann cells. PMP22 may therefore be a unique myelin protein in that axonal contact promotes its insertion into the Schwann cell membrane and myelin without altering its rapid turnover rate within the cell.

Modulation of Schwann cell Po glycoprotein and galactocerebroside by the surface organization of axolemma

The nature of the axon signal for the induction of proliferation and differentiation of peripheral glial cells is still unknown. Besides the existence of interactions among surface molecules the cellular responses can also be regulated by physicochemical parameters of the membrane. We have previously reported that planar axolemma monolayers coated on glass cover-slips at different defined surface molecular packing affected the Schwann cell (SC) morphology and their proliferative response (Calderon et al.: J Neurosci Res 34:206-218, 1993). In this paper we report that relative to SC cultured on uncoated coverslips, the level of expression of both glycoprotein Po and galactocerebroside (GC) (as revealed by immunofluorescence) was increased 2-4 times in SC cultured on axolemma monolayers with either high or low molecular packing. However, the cellular distribution of these antigens was profoundly influenced by the molecular packing density of the axolemma monolayer. SC cultured on an axolemma monolayer at high molecular packing showed preferential expression of Po at the SC surface whereas GC was concentrated intracellularly. On the other hand, SC grown on an axolemma monolayer at low molecular density GC showed preferential expression at the cell surface whereas Po was concentrated intracellularly.

Expression of P0 protein mRNA along rat sciatic nerve during development

The temporal and spatial expression of P0 protein mRNA in the spinal nerve roots and in distal (popliteal trifurcation) segments of the developing rat sciatic nerve was examined using in situ hybridization. P0 mRNA was undetectable in the PNS portions at day 16 post-fertilization (E16) but present in the roots as well as in the sciatic nerve by E18. While Schwann cells were labelled by in situ hybridization at E18, it was not until birth (P1) that cells revealed the typical clusters of silver grains in a perinuclear distribution. Hybridization signal rose between P5 and P10, than decreased at P40, with no significant difference of P0 mRNA levels between the different PNS portions at each age studied. These findings demonstrate the lack of proximo-distal gradient in abundance of P0 mRNA during rat PNS development and add further insight in understanding the dynamics of axonal regulation of myelination in the PNS.

Developmental expression of P0 mRNA and P0 protein in the sciatic nerve and the spinal nerve roots of the rat

Expression of myelin P0 protein by myelinating Schwann cells in vivo is dependent on axonal influences. This report describes P0 gene expression during development of rat sciatic nerve and spinal nerve roots using Northern blotting, in situ hybridization and immunohistochemistry. We demonstrate that: (1) the appearance of P0 mRNA and P0 protein in Schwann cells during nerve development in the rat begins prenatally, at day 18 post-fertilization (E18); (2) P0 mRNA and P0 protein have essentially identical developmental profiles, and are expressed in Schwann cells that are many days prior to myelin formation; (3) initial P0 gene expression is greatest in Schwann cells at the periphery of nerve bundles and in Schwann cells in contact with motor axons; (4) the decline in P0 expression with nerve maturation is accompanied by a sharp decline in P0 message levels in most Schwann cells, but a small subpopulation of these cells continue to synthesize very high levels of P0 mRNA. This study provides data on myelin P0 protein gene expression and distribution during PNS development and adds further insights into the axonal influences controlling Schwann cell behaviour during myelination of the rat PNS.

Homophilic adhesion of the myelin P0 protein requires glycosylation of both molecules in the homophilic pair

The myelin P0 protein is glycosylated at a single site, asparagine 93, within its only immunoglobulin (Ig)-like domain. We have previously shown that P0 behaves like a homophilic adhesion molecule (Filbin, M. T., F. S. Walsh, B. D. Trapp, J. A. Pizzey, and G. I. Tennekoon. 1990. Nature (Lond.). 344:871-872). To determine if the sugar residues of this molecule contribute to its adhesiveness, the glycosylation site was eliminated by replacing asparagine 93 with an alanine, through site-directed mutagenesis of the P0 cDNA. The mutated P0 cDNA was transfected into CHO cells and surface expression of the mutated P0 was assessed by immunofluorescence, limited trypsinization and an ELISA. A cell line was chosen which expressed approximately equivalent amounts of the unglcosylated P0 (UNGP0) at the cell surface as did a cell line expressing the fully glycosylated P0 (GPo); the adhesive properties of these two cell lines were compared. It was found that when a single cell suspension of the UNGPo cells were incubated, by 60 min, unlike the GP0 cells, they had not formed large aggregates; they were indistinguishable from the control transfected cells. This suggests that the UNGP0 protein does not behave like an adhesion molecule. To establish if only one molecule in the P0:P0 homophilic pair must be glycosylated for adhesion to occur, the ability of UNGP0 cells to adhere to GP0 cells was assessed both qualitatively and quantitatively. The results of both types of assay imply that, indeed, both P0 molecules in the homophilic pair must be glycosylated for adhesion to take place.

Bergmann glia require continuous association with Purkinje cells for normal phenotype expression

Bergmann glia (Bg) respond to the early postnatal Purkinje cell (Pc) death in Lurcher (Lc) mutant mouse cerebellum by down-regulating expression of the enzyme glycerol-3-phosphate dehydrogenase (GPDH). To determine whether glial GPDH expression requires the continued presence of Pcs in adults, we used single intracerebellar injections of kainic acid to kill Pcs in wild-type mice from 7 weeks to 11 months old. Bg at all ages tested responded to Pc loss by down-regulating GPDH expression. To learn whether a high level of GPDH could be reinduced following down-regulation in Lc Bg, we grafted wild-type fetal Pcs into Lc cerebella. The influence of grafted Pcs on GPDH expression is host-age and implant-position dependent. Only Pcs implanted into hosts less than 6 weeks old were later found to be associated with GPDH-positive Bg. Grafted Pcs that migrated into the anterior folia of young hosts were more likely to be associated with GPDH-positive Bg than Pcs migrating to other positions. EM analysis showed that Bg ensheathment of grafted Pcs is thinner and more discontinuous, but qualitatively similar to normal. The results suggest that the interaction between host Bg and grafted Pcs can sustain elevated GPDH expression in Bg that have not yet down-regulated, but is not adequate to reinduce expression in those cells that have.

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.

Neuron-Schwann cell signals are conserved across species: purification and characterization of embryonic chicken Schwann cells

A monoclonal antibody, 1E8, which recognizes the peripheral myelin protein, P0, specific for chicken Schwann cells and their precursors (Bhattacharyya et al., Neuron 7:831-844, 1991), was used to immunoselect Schwann cells from embryonic day 14 (E14) chicken sciatic nerve. When cultured, these immunoselected cells displayed properties characteristic of perinatal rodent Schwann cells, including S100-immunoreactivity and O4 antigen-immunoreactivity. In addition, the purified chicken Schwann cells divided slowly when cultured alone, but when co-cultured with chicken or rat sensory neurons, they bound to axons and proliferated. Proliferation was also stimulated by the addition of bovine brain membrane extracts or chicken brain membranes. The 1E8 monoclonal antibody was also used to test the effect of axonal contact on P0 expression. Chicken Schwann cells purified using the 1E8 monoclonal antibody gradually lost P0 when cultured alone. These cells remained 1E8-negative even after prolonged co-culture with embryonic rat dorsal root ganglion neurons or chicken sensory ganglia. These results demonstrate that chicken Schwann cells behave like rodent Schwann cells in their expression of specific antigens, interactions with axons, and regulation of P0 expression. In addition, chicken Schwann cells respond to neuronal signals from the rat and cow, illustrating the cross-species conservation of these signals.

Time-dependent regenerative influence of predegenerated nerve grafts on hippocampus

Our previous studies have revealed that the predegeneration facilitated the neurite outgrowth from hippocampus following the peripheral nerve grafts implantation. The aim of the present work is to find whether the stimulative power of peripheral nerve grafts depends on the time lapse after the transection. Autologous predegenerated distal stumps of the rat sciatic nerves were implanted into the hippocampus on the 7th, 14th, 28th, and 35th day following the transection. Six weeks later, horseradish peroxidase conjugated with fluorescein isothiocyanate was injected into the graft and frozen sections of brains were made. Fluorescence microscope examination has shown that FITC-HRP labeled cells were present among the hippocampal neurons in all the brains under examination, excluding these grafted with 14-day predegenerated peripheral nerves. The FITC-HRP labeled neurons were particularly numerous when the 7- and 35-day-old predegenerated stumps were used as grafts.

Myelin protein expression in dissociated superior cervical ganglia and dorsal root ganglia cultures

Schwann cells of the adult rat superior cervical ganglia (SCG) synthesize negligible levels of the major myelin glycoprotein, P0, in vivo. This suggests that the sympathetic axons of the SCG are deficient in one of more components involved in the regulation of myelin protein expression. Here we have compared the ability of neurites from neonatal rat SCG and embryonic rat dorsal root ganglia (DRG) to induce Schwann cell expression of myelin proteins after growth in culture using a serum-free medium. Steady-state P0 mRNA levels in the SCG and DRG culture paradigms were determined with a sensitive polymerase chain reaction (PCR) assay that amplified cDNA produced by reverse transcription of mRNA. This semiquantitative assay showed a linear response to increasing amounts of P0 and actin mRNA and required substantially less cellular RNA than typical hybridization techniques. Using the PCR assay, we found that SCG cultures contained significantly lower amounts of P0 mRNA than did DRG cultures. To further confirm that SCG cultures have negligible expression of myelin proteins, immunoblot analyses were done to examine the steady-state levels of both P0 and myelin basic protein. While nonmyelinating DRG cultures had readily detectable amounts of these myelin-specific proteins, neither could be demonstrated in the SCG cultures. The data indicate that SCG neurites lack one or more signals needed to induce myelin protein expression. Employing SCG and DRG cultures in comparative biochemical studies should prove useful in identifying the axonal molecule(s) involved in the regulation of myelin protein expression.

Changes in aldose reductase after crush injury of normal rat sciatic nerve

The response of aldose reductase (AR) to crush injury was studied in normal rat sciatic nerve. Enzyme activity and immunoreactivity of AR were determined at intervals of 1, 5, 14, 28, and 35 days after crush and correlated with histologic and immunocytochemical observations. During nerve degeneration in the distal segments of crushed nerves, a significant reduction in AR activity was detected. At 5 and 14 days, coincident with Schwann cell proliferation, enzyme activity decreased by nearly two- and fourfold, respectively. Although activity of AR increased by 28 days during nerve regeneration, it was not restored to normal levels at 35 days. Similar reductions were observed with the immunoblotting of the enzyme. Quantitative analysis of immunogold labelling on electron micrographs confirmed that proliferating as well as remyelinating Schwann cells contained reduced gold particle density compared to Schwann cells of noncrushed myelinated fibers. Immunoblots of P0, a marker for the degree of Schwann cell differentiation or myelination, showed that the temporal sequence of changes in P0 paralleled that of AR. Thus expression of AR is a function of differentiated or mature Schwann cells. The putative volume regulatory role of AR in Schwann cells may become superfluous during Wallerian degeneration.

Age-dependent changes in the oligosaccharide structure of the major myelin glycoprotein, P0

The single oligosaccharide moiety of the major myelin glycoprotein, P0, resides in an immunoglobulin-like domain that appears to participate in homophilic binding. The studies presented here indicate that the structure of the P0 oligosaccharide from rat nerve changes as a function of Schwann cell age. Examination of 5-day-old nerve revealed that P0 contained predominantly endo-beta-N-acetylglucosaminidase H (endo H)-resistant, complex-type oligosaccharide. In contrast, P0 from adult rats had mostly endo H-sensitive carbohydrate, indicating the presence of appreciable high-mannose and/or hybrid-type oligosaccharide on the glycoprotein. The endo H-sensitive and -resistant P0 of adult nerve could be readily phosphorylated by protein kinase C, as could the complex-type P0 from 5-day-old nerve. This suggests that the glycoprotein progresses to the plasma membrane and myelin regardless of the type of oligosaccharide chain. Analysis of 35SO4(2-)-labeled P0 showed that the sulfate group was found on both endo H-sensitive and -resistant oligosaccharide. The endo H-sensitive P0 carbohydrate from adult nerve appears to be primarily of the hybrid type, as evidenced by (a) the elution profile of [3H]mannose-labeled P0 glycopeptides from adult nerve during concanavalin A chromatography and (b) the inability of P0 from adult nerve to interact with Galanthus nivalis agglutinin. The observed age-dependent changes of P0 oligosaccharide may modify the binding properties of this myelin glycoprotein.

Myelin sheath survival after guanethidine-induced axonal degeneration

Membrane-membrane interactions between axons and Schwann cells are required for initial myelin formation in the peripheral nervous system. However, recent studies of double myelination in sympathetic nerve have indicated that myelin sheaths continue to exist after complete loss of axonal contact (Kidd, G. J., and J. W. Heath. 1988. J. Neurocytol. 17:245-261). This suggests that myelin maintenance may be regulated either by diffusible axonal factors or by nonaxonal mechanisms. To test these hypotheses, axons involved in double myelination in the rat superior cervical ganglion were destroyed by chronic guanethidine treatment. Guanethidine-induced sympathectomy resulted in a Wallerian-like pattern of myelin degeneration within 10 d. In doubly myelinated configurations the axon, inner myelin sheath (which lies in contact with the axon), and approximately 75% of outer myelin sheaths broke down by this time. Degenerating outer sheaths were not found at later periods. It is probably that outer sheaths that degenerated were only partially displaced from the axon at the commencement of guanethidine treatment. In contrast, analysis of serial sections showed that completely displaced outer internodes remained ultrastructurally intact. These internodes survived degeneration of the axon and inner sheath, and during the later time points (2-6 wk) they enclosed only connective tissue elements and reorganized Schwann cells/processes. Axonal regeneration was not observed within surviving outer internodes. We therefore conclude that myelin maintenance in the superior cervical ganglion is not dependent on direct axonal contact or diffusible axonal factors. In addition, physical association of Schwann cells with the degenerating axon may be an important factor in precipitating myelin breakdown during Wallerian degeneration.

P0 gene expression in Schwann cells is modulated by an increase of cAMP which is dependent on the presence of axons

The role of cAMP in the regulation of P0 gene expression was investigated in Schwann cells of normal, regenerated, and permanently transected rat sciatic nerve. Forskolin treatment of endoneurial segments of rat sciatic nerve resulted in increased cAMP and P0 mRNA levels in normal and regenerated nerves but not in permanently transected nerves, where axonal regeneration is prevented. This increase of cAMP and P0 mRNA occurred within 30 and 90 min, respectively. P0 mRNA levels in the endoneurial segment of the permanently transected nerve were not increased with dibutyryl cAMP. The Schwann cells of the permanently transected nerve, however, retained the ability to myelinate 15 embryonic day (E15) dorsal root ganglia (DRG) neuron and neurite networks cultured in vitro. P0 mRNA levels increased within 4 days in transected endoneurium segments cocultured with E15 DRG neurons and neurites and further increased in 21 day myelinating cocultures. Although cAMP was not detectable in 4 day cocultures, it increased to detectable levels in 21 day cultures, suggesting that cAMP is involved in the myelinating process. These results indicate that the presence of the axon is required for the observed increase of cAMP and P0 mRNA levels and suggest that the increase of cAMP occurs within the axon which then presumably activates a different Schwann cell second messenger pathway to induce P0 gene expression.

The role of complex carbohydrates in adhesion of the myelin protein, P0

The most abundant protein of peripheral nerve myelin, a glycoprotein termed P0, is believed to be involved in the compaction of the myelin sheath and is postulated to be the closest relative to the ancestral gene for the immunoglobulin superfamily. Recently, P0 has indeed been shown to behave like a homophilic adhesion molecule via interactions of its extracellular domains. Here we demonstrate the importance of the oligosaccharide moieties of P0 in its functioning as a homophilic adhesion molecule. Expression of the complex form of P0 glycoprotein in transfected Chinese hamster ovary cells greatly increased the adhesiveness of those cells, whereas expression of the high-mannose form of P0 glycoprotein did not. This is the first step in the dissection of P0-P0 interaction at the molecular level.

Myelin maintenance by Schwann cells in the absence of axons

Formation and maintenance of myelin sheaths in the peripheral nervous system are regulated by unknown molecular interactions that are thought to depend upon physical contact between Schwann cells and axons. However, recent studies describing axons surrounded by two concentric myelin internodes in the superior cervical ganglion (SCG) of normal rodents have demonstrated that the outer myelin internodes are maintained without physical contact with the axon. To determine whether the centrally enclosed axon has a trophic effect in maintaining these remote outer internodes, we have produced axonal degeneration by surgical or chemical means. The results indicate that maintenance of myelin internodes totally displaced from axonal contact depends neither upon the presence of the axon nor on diffusible axonal factors. A further implication of these studies is that myelin breakdown during Wallerian degeneration is regulated by a positive signal which originates in degenerating nerves, rather than solely by loss of axonal trophic substances.

Blood-nerve barrier in the frog during wallerian degeneration: are axons necessary for maintenance of barrier function?

Blood-nerve barrier tissues (endoneurial blood vessels and perineurium) of the frog's sciatic nerve were studied during chronic Wallerian degeneration to determine whether barrier function depends on the presence of intact axons. Sciatic nerves of adult frogs were transected in the abdominal cavity; the ends were tied to prevent regeneration and the distal nerve stumps were examined. Vascular permeabilities to horseradish peroxidase and to [14C]sucrose increased to day 14, returned toward normal levels by 6 weeks, and continued at near normal levels to 9 months. Perineurial permeabilities to the tracers increased by day 10 and remained elevated at 9 months. Proliferation of perineurial, endothelial, and mast cells occurred between 3 days and 6 weeks, resulting in an increased vascular space (measured with [3H]dextran) and number of vascular profiles. The perineurium increased in thickness and the mast cells increased in number. This study indicates that during Wallerian degeneration of the frog's sciatic nerve there is 1) a transitory increase in vascular permeability distal to the lesion, that is related to changes within the endoneurium; 2) an irreversible increase in permeability of the perineurium, which begins later than that seen in the endoneurial blood vessels; and 3) proliferation of non-neuronal components in the absence of regenerating neuronal elements. The results indicate that maintenance of vascular integrity does not require the presence of axons in the frog's peripheral nerve, whereas perineurial integrity and barrier function are affected irreversibly by Wallerian degeneration.

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.

Recombinant peripheral myelin protein P0 confers both adhesion and neurite outgrowth-promoting properties

To probe into the functional properties of the major peripheral myelin cell surface glycoprotein P0, its ability to confer adhesion and neurite outgrowth-promoting properties was studied in cell culture. To this aim, P0 was expressed as integral membrane glycoprotein at the surface of CV-1 cells with the help of a recombinant vaccinia virus expression system. Furthermore, the immunoglobulin-like extracellular domain of P0 (P0-ED) was expressed as soluble protein in a bacterial expression system and used as substrate coated to plastic dishes or as competitor in cell adhesion and neurite outgrowth-promoting assays. The adhesion of P0-expressing CV-1 cells to P0-ED substrate was specifically inhibitable by polyclonal P0 antibodies (54% +/- 6%). In addition, the specific interaction between P0 molecules could be reduced (49% +/- 8%) by adding soluble P0-ED to the culture medium, demonstrating that the homophilic interaction between recombinant P0 molecules can be mediated, at least on one partner of interacting molecules, by the unglycosylated Ig-like domain. Substrate-coated P0-ED also conferred adhesion and neurite outgrowth ability to dorsal root ganglion neurons with neurites of a mean length of about 150 microns. This neurite outgrowth was specifically inhibitable by soluble P0 (74% +/- 14%) and P0 antibodies (65% +/- 9%). These observations indicate that P0 is capable of displaying two different types of functional roles in the myelination process of peripheral nerves: The heterophilic interaction with neurons may be responsible for the recognition between axon and myelinating Schwann cell at the onset of myelination, whereas the homophilic interaction may indicate its role in the self-recognition of the apposing loops of Schwann cell surface membranes during the myelination process and in the mature compact myelin sheath.

Expression of myelin protein gene transcripts by Schwann cells of regenerating nerve

The expression of many myelin-specific molecules in Schwann cells is profoundly decreased following denervation. This study examines the early reexpression of myelin protein genes associated with reinnervation. Following sciatic nerve crush, the distal, regenerated nerve was divided into appropriate (2.5 or 5 mm) consecutive lengths in which gene expression was monitored using Northern blotting, in situ hybridization, and immunostaining. The spatial separation of the distal axon tip and the more proximally located Schwann cells showing initial upregulation of P0 mRNA was constant over the period of 5-13 days after crush at approximately 3-4 mm in fixed, processed material. Axons associated with Schwann cells showing the initial upregulation were completely or partially enveloped in Schwann cell cytoplasm, with very few having any degree of ensheathment. It is probable that only a limited axon-Schwann cell contact is required for induction of the myelin protein genes. Myelin-associated glycoprotein mRNA was upregulated prior to those for P0 and myelin basic protein which had similar time courses. Reexpression of galactocerebroside also preceded that for P0 mRNA. Signal abundance for all myelin proteins decreased in a proximal to distal direction from the crush site, and with time the "wave" of upregulation moved distally down the nerve. In the more proximal, remyelinating zones, the signal intensity exceeded that of the contralateral normal nerve. Signal intensity also varied considerably between adjacent, expressing Schwann cells. The data provide further evidence of the strong temporospatial relationship between axons and the regulation of myelin protein genes in Schwann cells.

Axons modulate myelin protein messenger RNA levels during central nervous system myelination in vivo

Expression of myelin protein genes by myelinating Schwann cells in vivo is dependent on axonal influences. This report investigated the effect of axons on myelin protein mRNA levels in the central nervous system (CNS). In situ hybridization studies of rat spinal cord sections localized mRNAs encoding proteolipid protein (PLP) and myelin basic protein (MBP) 20 and 40 days after unilateral rhizotomy. Compared with control tissue, hybridization intensity was reduced in transected tissue, but there was little change in the number of oligodendrocytes labeled. Cellular RNA was extracted from transected and age-matched control optic nerves 5, 10, 20, and 40 days after surgery, and levels of the following mRNAs were determined by slot blot procedures: PLP, MBP, myelin-associated glycoprotein (MAG), and 2',3' cyclic nucleotide 3'-phosphodiesterase (CNP). In transected nerves, PLP and MBP mRNA levels were approximately 85%, 45%, and 25% of control values at 5, 20 and 40 days posttransection, respectively. Axonal transection had a lesser effect on CNP and MAG mRNA levels, which declined to approximately 60% of control levels at 40 days. Immunocytochemical studies indicated that the number of oligodendrocytes was not decreased 40 days after optic nerve transection. These data demonstrate that axons modulate myelin protein mRNA levels in oligodendrocytes. In contrast to Schwann cells, however, oligodendrocytes continue to express significant levels of myelin protein mRNA in vivo following loss of axonal contact.

Axonal modulation of myelin gene expression in the peripheral nerve

Myelin gene expression (P0, MBP, P2, and MAG) was investigated during Wallerian degeneration and in the presence or absence of subsequent axonal regeneration and remyelination. The steady state levels of mRNA and protein were assessed in the crushed or permanently transected rat sciatic nerve at 0, 1, 4, 7, 10, 12, 14, 21, and 35 days after injury. The mRNA and protein steady state levels of the myelin specific genes, P0 and the MBPs, decreased to low yet detectable levels during Wallerian degeneration and returned to normal levels with subsequent axonal regeneration. The steady state level of P2 protein also followed a similar pattern of expression. The steady state level of MAG mRNA decreased to undetectable levels by 4 days of injury in the permanently transected nerve. After crush injury, re-expression of MAG to levels comparable to those of normal nerves preceded that of P2 by 2 days and that of P0 and the MBPs by 3 weeks during axonal regeneration and remyelination. These results support the proposed roles for MAG in the formation of initial Schwann cell-axonal contact required for myelin assembly, for P2 in fatty acid transport during myelination, and for P0 and the MBPs in the maintenance of the integrity and compactness of the myelin sheath. In addition, these results indicate that the expression of the myelin specific genes, P0 and MBP, is constitutive and that the level of myelin specific mRNAs is modulated by axonal contact and myelin assembly.

Sulfate incorporation into peripheral nerve endoneurial glycolipids after crush and permanent transection injury

The sulfation of peripheral nerve glycolipids was examined at 35 days after both crush injury or permanent transection of the adult rat sciatic nerve by in vitro incorporation of [35S]sulfate into endoneurial slices. These experimental models of neuropathy are characterized by the presence and absence of both axonal regeneration and subsequent myelin assembly. Although the sulfo-glucuronosyl glycosphingolipids (SGGLs) were not detected by alpha-napthol reagent after HPTLC separation of the total acidic lipid extract, fluorographic analysis after sulfate incorporation revealed a 4.7-fold increase in [35S]sulfate in the sulfo-glucuronosyl paragloboside (SGPG) and a 3.5-fold increase in the sulfo-glucuronosyl-lactosaminosyl paragloboside (SGLPG) after the crush injury compared to permanent transection. These [35S]sulfate-labeled lipids were identified by comigration after HPTLC separation by immunostaining with specific IgM monoclonal antibodies from a patient with demyelinating neuropathy and plasma cell dyscrasia. Enhanced incorporation of sulfate in the crushed nerves was also observed in the sulfatides and in several unknown lipids migrating between GM2 and GM3, between GM1, and GM2, slightly above the origin, and at the origin. Since previous studies (Yao and Poduslo: J Neurochem 50:630-638, 1988) have shown [35S]sulfate incorporation, but not [3H]Gal or [3H]Glc, into sulfatides at 35 days after transection, it is possible that the sulfation observed in the present studies does not represent de novo biosynthesis but rather sulfation of an endogenous pool of glycolipids that results from the nerve injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of axons in the regulation of P0 biosynthesis by Schwann cells

The role of axons in the expression of the major myelin glycoprotein, P0, has been investigated using neuron/Schwann cell cultures. These cultures were either nonmyelinating or myelinating due to growth in defined medium or in medium containing serum and chick embryo extract, respectively. The neurons and Schwann cells used in the studies were derived from embryonic day 15 rat dorsal root ganglia (DRG), and the Schwann cells from these ganglia are shown not to synthesize appreciable levels of P0 prior to growth in culture. Myelinating cultures of Schwann cells and neurons grown together for 18-21 days synthesize P0 that is readily identified by immunoblotting. The nonmyelinating cultures, which do not assemble basal lamina, also synthesize P0 that is detectable by either [3H]mannose precursor incorporation or by immunoblotting. The steady-state level of P0 in the nonmyelinating cultures is less than that of the myelinating cultures, and the P0 that is synthesized by the former appears to be catabolized shortly after its biosynthesis. Since nonmyelinating Schwann cells synthesize P0 when in contact with neurites in vitro, we have examined the ability of such nonmyelinating cells to express the glycoprotein in vivo. Very little steady-state P0 is detected in immunoblots of the adult rat cervical sympathetic trunk (CST), a nerve in which approximately 99% of the axons are nonmyelinated. Similarly, the amounts of [3H]mannose and [3H]amino acids that are incorporated into newly synthesized P0 are much lower in the CST than in the adult sciatic nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein zero of peripheral nerve myelin: biosynthesis, membrane insertion, and evidence for homotypic interaction

Protein zero (P0), an integral membrane glycoprotein synthesized by Schwann cells, is the major glycoprotein of peripheral nerve myelin. The predicted disposition of P0 with respect to the membrane bilayer postulates the existence of extracellular and intracellular domains, that mediate compaction of the myelin lamellae. We used in vitro translations programmed with sciatic nerve mRNA and cells transfected with a P0 cDNA construct to study the biosynthesis and topology of P0 in the bilayer. The behavior of P0 at the cell surface, when expressed under physiological conditions, was also examined. We have verified the topological predictions of an earlier model, derived from analysis of a P0 cDNA, and provide evidence that the extracellular domain of P0 mediates homotypically cell-cell interactions in the transfectants.

Catabolic regulation of the expression of the major myelin glycoprotein by Schwann cells in culture

Previous studies have suggested that neonatal Schwann cell cultures deprived of axonal contact do not express components of the myelin membrane, including the major myelin glycoprotein, P0. In contrast, Schwann cells from permanently transected, adult nerve exhibit continued biosynthesis of P0 after culture, suggesting that the ability to express the myelin glycoprotein may depend on the degree of cellular differentiation. To examine further the ability of Schwann cell cultures to express P0 as a function of age, we have performed precursor incorporation studies on endoneurial explants from 4- to 12-day-old rat sciatic nerves after 5 days in culture. The data reveal that explants from 12-day-old animals synthesize detectable levels of this integral myelin protein when assayed by [3H]mannose incorporation, even though there is no apparent myelin assembly in the cultures. Pulse-chase analysis of cultures from 12-day-old rats demonstrates that [3H]mannose-labeled P0 is substantially degraded within 3 h. This catabolism largely can be prevented by the addition of swainsonine, ammonium chloride, or L-methionine methyl ester to the pulse-chase media. The former agent alters oligosaccharide processing whereas the latter two compounds inhibit lysosomal function. The P0 synthesized by the 12-day explant cultures following the addition of swainsonine is readily fucosylated, implying that the protein has progressed at least as far as the medial Golgi before its exit and subsequent catabolism. If cultures from 4-, 6-, and 8-day-old animals are analyzed for P0 biosynthesis by [3H]mannose incorporation in the presence of swainsonine, detectable levels of the glycoprotein are seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological activity of a new neuronal growth factor from injured peripheral nerve

In response to transection injury, the distal nerve segment produces a soluble neurite promoting factor (SN). In this study, the ability to support neuronal survival and differentiation have been studied. Embryonic rat dorsal root ganglion (DRG) neurons were plated out on collagen substrates and incubated in medium containing either SN or nerve growth factor (NGF). The number of surviving neurons was counted after 1, 2, 4, 7, and 15 days in vitro. After fixation and staining, the diameter of the surviving neurons was measured. During the period of observation, 60.8 +/- 5.8% of plated neurons survived in the presence of NGF and 90.5 +/- 12.9% survived with SN (P less than 0.05). The mean of median neuronal cell diameter was 28 +/- 2.7 microns with NGF and 34.2 +/- 3.7 microns with SN, (P less than 0.01). This increased diameter was due to enhanced survival of 30-50 microns diameter neurons. In parallel experiments, the degree of myelination of DRG neurons by Schwann cells was assessed morphometrically. In the presence of SN there was an 86% increased in myelination compared with NGF which indicates that not only is the survival of neurons increased but they are able to become fully differentiated in the presence of SN.

The blood-nerve barrier in Wallerian degeneration: a sequential long-term study

The blood-nerve barrier (BNB) for serum proteins was studied after a crush lesion of the murine sciatic nerve or after transsection with persistent Wallerian degeneration. Using single intraperitoneal injections of biotinylated human albumin, transferrin, IgG, and complement components as tracers, the integrity of the BNB during degeneration and regeneration was determined over time. In Wallerian degeneration induced by crush the BNB became increasingly leaky, with a maximum in the distal stump 8 days after crush (i.e., during early regeneration). When regeneration potentials could first be elicited from the small foot muscles and when thinly myelinated nerve fibers were present, the BNB gradually regained its barrier function and was nearly intact on day 30 after crush. After transsection breakdown of the BNB persisted beyond 30 days. The BNB leakage may foster repair by allowing exchange of trophic factors of large molecular size during nerve regeneration.

Regulation of UDP-galactose:ceramide galactosyltransferase and UDP-glucose:ceramide glucosyltransferase after crush and transection nerve injury

The enzyme activities of ceramide galactosyltransferase and ceramide glucosyltransferase were assayed as a function of time (0, 1, 2, 4, 7, 14, 21, 28, and 35 days) after crush injury or permanent transection of the adult rat sciatic nerve. These experimental models of neuropathy are characterized by the presence and absence of axonal regeneration and subsequent myelin assembly. Within the first 4 days after both injuries, a 50% reduction of ceramide galactosyltransferase-specific activity was observed compared to values found in the normal adult nerve. This activity remained unchanged at 7 days after injury; however, by 14 days the ceramide galactosyltransferase activity diverged in the two models. The activity increased in the crushed nerve and reached control values by 21 days, whereas a further decrease was observed in the transected nerve such that the activity was nearly immeasurable by 35 days. In contrast, the ceramide glucosyltransferase activity showed a rapid increase between 1 and 4 days, followed by a plateau that was 3.4-fold greater than that in the normal adult nerve, which persisted throughout the observation period in both the crush and transection models. [3H]Galactose precursor incorporation studies at 7, 14, 21, and 35 days after injury confirmed the previously observed shift in biosynthesis from the galactocerebrosides during myelin assembly in the crush model to the glucocerebrosides and oligohexosylceramide homologues in the absence of myelin assembly in the transection model. The transected nerves were characterized by a peak of biosynthesis of the glucocerebrosides at 14 days. Of particular interest is the biosynthesis of the glucocerebrosides and the oligohexosylceramides at 7 and 14 days after crush injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Schwann cell recruitment from intact nerve fibers

Two proximal branches of the rat facial nerve were transected and anastomosed end-to-end within a silicone tube, each of them being exposed to a massive invasion of ascending regenerating axons. The proximal nerves contained extremely large bundles of regenerated fibers, often associated with preexistent "parent fibers." The bundles showed many signs of rash and disordered cell proliferation and myelination. These included multiple Schwann cells surrounded by a common basement membrane, occurrence of different phases of myelination and even myelination of two axons by one Schwann cell. There was no evidence of mitogenic signals for fibrocytes. This model may be used for studying the mitogenic effect of axons on Schwann cells. It also suggests that so-called "groups of regenerating fibers" in neuropathy are caused by Schwann cell recruitment.

Temporal changes in PO and MBP gene expression after crush-injury of the adult peripheral nerve

The crush-injured sciatic nerve provides a model to study Schwann cell regulation of myelin gene expression during the process of demyelination and remyelination. In order to investigate the possible transcriptional regulation of myelin gene expression, the quantity, quality and translational efficiency of PO (the major myelin glycoprotein) and MBP (the myelin basic proteins) coding messages were investigated as a function of time following crush-injury of the adult rat sciatic nerve. Northern blot analysis indicated that the size of the PO and MBP transcripts remain unchanged in the distal segments of crushed sciatic nerves at 1, 2, 4, 7, 10, 14 and 21 days after crush-injury. Dot-blot analysis showed a sharp drop in levels of PO and MBP coding transcripts 1 day after crush-injury with the lowest steady-state levels at 4-7 days. Message levels were found to increase after 7 days, the highest increase in levels of message was found to be between 10 and 14 days. The highest steady-state level of both transcripts was observed at 21 days. In vitro translation and immunoprecipitation of PO-translated products from various stages of crush-injury also indicated this trend. The pattern of gene expression of PO- and MBP-coding transcripts parallel each other and follow the pattern of demyelination and remyelination. The results are also consistent with our previous interpretation which suggests that PO and MBP gene expression is regulated at the level of transcription and that these two genes might be coordinately expressed. Western blot analysis of PO protein from these stages revealed a similar decrease and then increase in the levels of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of myelin P0 glycoprotein synthesis in cultured rat Schwann cells and continuous rat PNS cell lines

We studied the effects of agents that raise intracellular cyclic AMP on synthesis of myelin components by cultured neonatal rat sciatic nerve Schwann cells and by continuous PNS cell lines derived from the fusion of neonatal rat sciatic nerve Schwann cells with rat RN22 Schwannoma. Treatment with N6,2'-O-dibutyryl cyclic AMP (dibutyryl cyclic AMP) caused a fourfold increase in Schwann cell incorporation of 35SO4 into sulfogalactosylceramide (sulfatide), and elicited a 10- to 20-fold increase in such incorporation by the continuous PNS cell lines; a similar effect on PNS cell line sulfatide radiolabelling was obtained with forskolin. Cultured Schwann cells expressed barely detectable levels of myelin P0 glycoprotein (P0) mRNA and myelin basic protein (MBP) mRNA. Treatment of the Schwann cells with axolemmal fragments or with dibutyryl cyclic AMP did not elicit a detectable increase in the levels of these mRNAs. The PNS cell lines constitutively expressed much higher levels of P0 mRNA than did the Schwann cells, and synthesized immunochemically demonstrable P0 glycoprotein, but did not express MBP. Treatment of the PNS cell lines with dibutyryl cyclic AMP markedly reduced expression of P0 mRNA and also diminished immunoreactive P0 glycoprotein. These PNS cell lines should prove useful for further studies of the control of Schwann cell differentiation.

Post-traumatic endoneurial neovascularization and nerve regeneration: a morphometric study

Neovascularization would be expected to play an important role in regeneration after nerve injury, but its mechanism is poorly understood. Quantitative investigations of endoneurial capillaries and myelinated fibers 5 and 15 mm distal to different types of nerve injury have therefore been performed. This study demonstrated that numbers of endoneurial capillaries were significantly increased at the 5 mm level 2, 4, 6 and 8 weeks after crush, transection and ischemic lesions, but not following permanent axotomy. Late neovascularization associated with delayed nerve regeneration was found following nerve ischemia. These results suggest that neovascularization following nerve injury is dependent on two variables, the degree of nerve regeneration and the severity of ischemia. Axonal outgrowth appears to be an important determinant of post-traumatic new capillary formation, while nerve ischemia causes both delayed neovascularization and nerve regeneration.

A surface protein expressed by avian myelinating and nonmyelinating Schwann cells but not by satellite or enteric glial cells

Searching for specific markers of neural crest-derived cell lineages, we immunized mice with glycoproteins purified from adult quail peripheral myelin. We obtained a monoclonal antibody that reacts with myelin and peripheral glial cells. This antibody, to Schwann cell myelin protein (SMP), is specific for the membranes of all Schwann cells, irrespective of whether they are associated with myelinated nerves. SMP persists on Schwann cells in long-term cultures in vitro, but is absent from satellite cells of peripheral ganglia, both in vivo and in vitro. The antigen (a protein doublet of Mr 75,000-80,000) is present in, but not restricted to, the myelin lamellae, since it is distributed along the whole myelinating Schwann cell membrane. In the CNS, SMP appears as a single band of Mr 80,000. SMP is first detectable by immunofluorescence at E6 in the quail, which is at least 6 days earlier than the first appearance of already described markers related to myelination.

Biosynthesis of neutral glucocerebroside homologues in the absence of myelin assembly after nerve transection

The biosynthesis of myelin-associated glycolipids during various stages of myelination was studied by in vitro incorporation of [3H]Gal, [3H]Glc, or [35S]sulfate into the endoneurium of rat sciatic nerve. In the normal adult nerve, where the level of myelin assembly is substantially reduced and Schwann cells are principally involved in maintaining the existing myelin membrane, [3H]Gal was primarily incorporated into monogalactosyl diacylglycerol (MGDG) and the galactocerebrosides (GalCe) with lower levels of incorporation into the sulfatides. Such incorporation was enhanced 35 days after crush injury of the adult rat sciatic nerve, which is characterized by active myelin assembly. In contrast, at 35 days after permanent nerve transection where there is no axonal regeneration or myelin assembly, the incorporation of [3H]Gal or [3H]Glc into GalCe was nearly undetected whereas the incorporation of [3H]Gal into MGDG was completely inhibited. Instead, the 3H-labeled glycolipids in transected nerve were identified as the glucocerebrosides (GlcCe) and oligohexosylceramide derivatives with tetrahexosylceramide being a major product. In contrast, [35S]sulfate was incorporated into endoneurial sulfatides in the transected nerve, which suggests that endogenous GalCe rather than newly synthesized GalCe served as the substrate for the sulfotransferase reaction. The GlcCe homologues are not considered as constituents of the myelin membrane but are likely plasma membrane components synthesized in the absence of myelin assembly. It is likely that the cells responsible for GlcCe biosynthesis are Schwann cells, since they comprise 90% of the total endoneurial cell area in the distal nerve segment at 35 days after transection.(ABSTRACT TRUNCATED AT 250 WORDS)

A cyclic AMP analogue induces synthesis of a myelin-specific glycoprotein by cultured Schwann cells

Neonatal rat Schwann cells, cultured with agents which increase intracellular cyclic AMP, were prompted to resume synthesis of a 170,000 Mr glycoprotein which is specific to peripheral nervous system myelin and is herein referred to as P170K. We have shown previously that similar treatment induces the synthesis by Schwann cells of the myelin lipid, galactocerebroside. In contrast to P170K and galactocerebroside, syntheses of P0 and myelin basic protein were not induced. Intracellular cyclic AMP is thus likely to be a participant in the complex system regulating myelination.

A phorbol ester-sensitive kinase catalyzes the phosphorylation of P0 glycoprotein in myelin

The proposed structural protein of peripheral nerve myelin, P0, has been shown to have several covalent modifications. In addition to being glycosylated, sulfated, and acylated, P0 is phosphorylated, with the intracellular site of this latter addition being in question. By employing nerve injury models that exhibit different levels of P0 biosynthesis in the absence and presence of myelin assembly, we have examined the cellular location of P0 phosphorylation. It is demonstrated that there is comparable P0 phosphorylation in both normal and crush-injured adult rat sciatic nerves, although the level of biosynthesis of P0 differs between these myelin maintaining and actively myelinating nerve models, respectively. The glycoprotein does not appear to be phosphorylated readily in the transected adult sciatic nerve, a preparation in which P0 biosynthesis is observed but that lacks myelin membrane. These observations suggest that the modification is not associated with the biosynthesis or maturation of P0 in the endoplasmic reticulum or Golgi, but that it instead occurs after myelin assembly. That P0 phosphorylation occurs in the normal nerve even when translation is inhibited by cycloheximide treatment lends further support to this conclusion. P0 is shown to be phosphorylated on one or more serine residues, with all or most of the phosphate group(s) being labile as evidenced by pulse-chase analysis. Addition of a biologically active phorbol ester, 12-O-tetradecanoylphorbol-13-acetate or 4 beta-phorbol 12,13-dibutyrate, substantially increases the extent of [32P]orthophosphate incorporation into the glycoprotein of normal and crushed nerve but not transected nerve. Biologically inactive 4 alpha-phorbol 12,13-didecanoate has no effect on P0 phosphorylation. Similarly, the addition of the cyclic AMP analog 8-bromo-cyclic AMP causes no appreciable changes in P0 labeling. These findings indicate that the phorbol ester-sensitive enzyme, protein kinase C, may be responsible for the phosphorylation of P0 within the myelin membrane.

Myelinated, but not unmyelinated axons, reversibly down-regulate N-CAM in Schwann cells

There is evidence from chicks and mice that N-CAM expression in Schwann cells is subject to significant regulation during development and following injury. In the present work, rat sciatic nerve and immunohistochemical methods have been used to study developmental and injury-related modulation of N-CAM in Schwann cells, using cell type specific markers to identify different Schwann cell populations, and cell counting to quantify their size. The study has sought to determine unambiguously whether immature Schwann cells in developing nerves and denervated Schwann cells in injured adult nerves express surface N-CAM, and has investigated the temporal relationship between the gradual loss of surface N-CAM and the differentiation of myelin-forming Schwann cells, monitored by the sequential appearance of the glycolipid galactocerebroside and the myelin-specific protein P0. Further points examined are whether this down-regulation of N-CAM is rapidly reversible following loss of axonal contact, and whether N-CAM reappearance in Schwann cells depends on protein synthesis. In nerves from 17- to 18-day embryos, 90% of the Schwann cells, identified with Ran-1 antibodies, expressed surface N-CAM. In nerves from newborn rats many cells are in the early stage of myelin synthesis and therefore express galactocerebroside, although they have not yet acquired P0. Suspension staining of dissociated cells from this nerve showed that 92% of the galactocerebroside-positive cells were also N-CAM positive. In suspension staining of nerves from 5-day, 10-day and adult rats, P0-positive cells were essentially N-CAM negative.(ABSTRACT TRUNCATED AT 250 WORDS)

Gene expression in the presence or absence of myelin assembly

Regulation of myelin protein gene expression in the presence and absence of myelin assembly can be assessed using crushed or permanently transected adult sciatic nerves of rats. The P0 glycoprotein and the myelin basic protein (MBP) are the major myelin-specific proteins of the peripheral nervous system. The steady-state level of P0 and MBP messenger RNA was determined by dot-blot analysis of poly(A)+ RNA from crushed and transected nerves of rats at 35 days post operation. The rat P0-specific cDNA clone, pSN63c, and mouse MBP-specific cDNA clone, pHF43, were used as probes. The level and quality of the poly(A)+ RNA was assessed by in vitro translation and immunoprecipitation of the translation products with anti-chick P0 antibody. Comparison of the steady-state level of P0 and MBP transcripts and the level of anti-P0 immunoprecipitated translation products from RNA extracts of permanently transected, crushed, adult control and 21-day-old control rat nerves indicated that the level of P0 and MBP messages was significantly reduced in the permanently transected model, whereas it was restored to normal in the crushed sciatic nerve 35 days post injury. These results suggest that regulation of P0 and MBP gene expression most likely occurs at the transcriptional or post-transcriptional level in the two models of peripheral neuropathies. Northern blot analysis indicated the absence of differential splicing of the message in crushed or transected nerves. The experiments also indicate that these two important gene products required for myelin synthesis and assembly seem to be co-regulated. However, the data do not rule out the possibility that regulation of gene expression may also occur at the level of translation or post-translational processing.

Lysosomal delivery of the major myelin glycoprotein in the absence of myelin assembly: posttranslational regulation of the level of expression by Schwann cells

The major myelin protein, P0, has been shown to have decreased levels of expression and altered oligosaccharide processing after the disruption of Schwann cell-axon interaction. We show here that lysosomal degradation of the glycoprotein shortly after its synthesis accounts for much of its decreased expression in the permanently transected adult rat sciatic nerve, a denervated preparation where there is no axonal regeneration or myelin assembly. If [3H]mannose incorporation into sciatic nerve endoneurial slices is examined in the presence of the lysosomotropic agent, NH4Cl, a marked increase in the level of newly synthesized P0 is seen. Pulse-chase analysis of [3H]mannose-labeled P0 in the presence of NH4Cl indicates that this increase is a consequence of inhibition of P0 degradation that normally occurs 1-2 h after biosynthesis in the transected nerve. P0 degradation can also be inhibited if lysosomal function is disturbed by dilation of secondary lysosomes with L-methionine methyl ester. The addition of deoxymannonojirimycin or swainsonine (SW), inhibitors of oligosaccharide-processing mannosidases I and II, respectively, also results in a decrease in P0 degradation. This inhibition is presumably caused by a blockage of transport to the lysosomes due to altered processing of the glycoprotein, although the direct inhibition of lysosomal mannosidases cannot be excluded. In contrast to the transected nerve, addition of NH4Cl or SW has no effect on P0 levels in the crushed nerve, where myelin assembly occurs. The delivery of P0 to the lysosomes of the transected nerve Schwann cells does not appear to be triggered by the mannose-6-phosphate transport system involved in acid hydrolase routing. The accumulation of a fucosylated species of P0 in the presence of SW indicates that the glycoprotein has progressed at least as far as the site of GlcNAc transferase I without lysosomal delivery, and thus differs from the typical mannose-6-phosphate-containing glycoproteins. Furthermore, there is no evidence of P0 phosphorylation either in the absence or presence of NH4Cl or SW. These findings indicate that the amount of P0 in the transected nerve is regulated at the posttranslational level as well as at the level of transcription.

Neuronal growth factors produced by adult peripheral nerve after injury

Dorsal root ganglion neurons from embryonic rats, co-cultured with endoneurial explants from transected, adult rat sciatic nerve, extended neurites in the absence of exogenous nerve growth factor (NGF). The effect was seen with endoneurial explants from normal adult sciatic nerves or from nerves which had been permanently transected up to 51 days prior to explantation. The rate of outgrowth decreased at 5 and 7 days and reached a minimum at 14 days after transection. A second phase of increased neurite-promoting activity appeared in 28-, 35-, 41- and 51-day posttransection tissue. The early phase, but not the late phase, was partially inhibited by antisera to NGF.

Studies of aldose reductase using neuronal cell culture and ligated rat sciatic nerve

Sorbinil (CP 45,634), a potent aldose reductase (AR) inhibitor, has the ability to normalize both sorbitol levels and functional parameters such as orthograde axonal transport and motor nerve conduction velocity in peripheral nerves of diabetic rats, which implicates flux through the polyol pathway in the pathophysiology of diabetic neuropathy. In order to understand more fully the role of this enzyme, it is important to determine the major cellular location of AR in peripheral nerve. Experiments were designed that have taken advantage of the observation that peripheral nerve axons degenerate distal to an injury site, while Schwann cells remain viable. One sciatic nerve in each experimental rat was chronically ligated (up to 6 weeks) before inducing diabetes by an intravenous (iv) injection of streptozotocin (STZ; 65 mg/kg). Two days after the STZ injection, both sciatic nerves were removed from each animal, and the ligated nerve was divided into proximal (Schwann cells and axons) and distal (Schwann cells only) portions before being processed for sorbitol determinations. The intact nerves and the proximal portion of the ligated nerves (having both Schwann cells and axons) retained the ability to accumulate sorbitol after STZ injection, while the distal portion (having Schwann cells only) lost this capacity 4 days after ligation. This lack of ability to accumulate sorbitol was not due to failure of the substrate (glucose) to reach the distal nerve segment. Additionally, homogenates of whole nerves and of proximal portions of ligated nerves were able to form sorbitol from glucose in the presence of NADPH while homogenates of distal portions of ligated nerves had lost approximately 85% of this activity.(ABSTRACT TRUNCATED AT 250 WORDS)