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Pharmacological approaches and optical recordings have shown that Schwann cells of a myelinating phenotype are activated by 5-HT upon its interaction with the 5-HT(2A) receptor (5-HT(2A)R). In order to further characterize the expression and distribution of this receptor in Schwann cells, we examined rat sciatic nerve and cultured rat Schwann cells using probes specific to 5-HT(2A)R protein mRNA. We also examined the endogenous sources of 5-HT in rat sciatic nerve by employing both histochemical stains and an antibody that specifically recognizes 5-HT. Rat Schwann cells of a myelinating phenotype contained both 5-HT(2A)R protein and mRNA. In the healthy adult rat sciatic nerve, 5-HT(2A)Rs were evenly distributed along the outermost portion of the Schwann cell plasma membrane and within the cytoplasm. The most prominent source of 5-HT was within granules of the endoneurial mast cells, closely juxtaposed to Schwann cells within myelinating sciatic nerves. These results support the hypothesis that the 5-HT receptors expressed by rat Schwann cells in vivo are activated by the release of 5-HT from neighboring mast cells.

This study aimed to generate a probe for perisynaptic Schwann cells (PSCs) to investigate the emerging role of these synapse-associated glial cells in the formation and maintenance of the neuromuscular junction (NMJ). We have obtained a novel monoclonal antibody, 2A12, which labels the external surface of PSC membranes at the frog NMJ. The antibody reveals PSC fine processes or "fingers" that are interposed between nerve terminal and muscle membrane, interdigitating with bands of acetylcholine receptors. This antibody also labels PSCs at the avian neuromuscular junction and recognizes a 200 kDa protein in Torpedo electric organs. In frog muscles, axotomy induces sprouting of PSC processes beyond clusters of acetylcholine receptors and acetylcholinesterase at denervated junctional branches. PSC branches often extend across several muscle fibers. At some junctions, PSC sprouts join the tips of neighboring branches. The average length of PSC sprouts is approximately 156 microm at 3-week denervated NMJs. PSC sprouting is accompanied by a significant increase in the number of Schwann cell bodies per NMJ. Following nerve regeneration, nerve terminals reinnervate the junction along the PSC processes. In vivo observations of normal frog muscles also show PSC processes longer than nerve terminals at some junctional branches. The results suggest that nerve injury induces profuse PSC sprouting that may play a role in guiding nerve terminal regeneration at frog NMJs. In addition, antibody 2A12 reveals the fine morphology of PSCs in relation to other synaptic elements and is a useful probe in elucidating the function of these synapse-associated glial cells in vivo.

Previous studies, showing that cultured rat Schwann cells could be induced to express MHC class II molecules, raised the possibility that Schwann cells in living nerves might, under some conditions, express MHC class II molecules and take part in activation of T lymphocytes. In the present work, the ability of myelin- and non-myelin-forming Schwann cells in vivo to express MHC class II molecules was investigated. Lymphokines or bacterial antigens were injected into the living sciatic nerve of adult rats. Examination of the nerves three days after injection of interferon-gamma or six days after injection of either tumour necrosis factor, antigens from mycobacterium leprae or whole mycobacteria leprae, revealed strong MHC class II immunostaining on some myelin-forming Schwann cells in the vicinity of the injection site. Very few non-myelin-forming cells expressed MHC class II molecules. MHC class II positive mononuclear cells were present in the injected nerves and endothelial cells of capillaries expressed high levels of MHC class II antigens. Crushing the sciatic nerve without injection of factors also induced MHC class II molecules on a few Schwann cells. Thus rat Schwann cells can be induced to express MHC class II molecules in vivo as in vitro. This strengthens the possibility that in living nerves Schwann cells are able to function as accessory cells in the initiation or augmentation of T cell-mediated immune responses.

Little is known about the molecular properties of the nonmyelin-forming glia of the P.N.S. In addition, the relationship between these cells and C.N.S. glia is poorly understood, in contrast with the well-documented biochemical and functional similarities between myelin-forming Schwann cells and oligodendrocytes. We have shown elsewhere that two cell surface proteins found on astrocytes, Ran-2 and A5E3 antigen, and the intermediate filament protein, glial fibrillary acidic protein (GFAP), are all widely expressed in situ by nonmyelin-forming Schwann cells in various nerves from adult rats, but not by myelin-forming Schwann cells. Here we investigate whether these three proteins are exclusively coexpressed by the same population of Schwann cells in situ in adult nerves, or alternatively whether subpopulations of nonmyelin-forming Schwann cells can be defined that express one or two, but not all three of these antigens. The distribution of two surface proteins, Ran-2 and A5E3 antigen, both defined by monoclonal antibodies, is compared with the distribution of the intracellular intermediate filament protein GFAP in peripheral nerve trunks of adult rats using double label immunofluorescence. It is clear that the expression of all three proteins is confined to the same population of Schwann cells, namely, the Schwann cells surrounding the unmyelinated axons. The myelin-forming Schwann cells surrounding the larger axons do not express immunohistochemically detectable amounts of any of the proteins. In teased nerve preparations from the preganglionic sympathetic trunk, sciatic nerve, brachial plexus, dorsal and ventral roots, a complete correspondence is found between expression of Ran-2 and GFAP, and between A5E3 and GFAP in nonmyelin-forming Schwann cells. No Schwann cells positive for Ran-2 alone, A5E3 antigen alone, or GFAP alone were found. The same constellation of antigens is also expressed by astrocytes in the adult rat C.N.S. and enteric glia in the P.N.S. and not by other cell types. The phenotype A5E3+, Ran-2+, GFAP+ is thus characteristic of major categories of adult rat P.N.S. and C.N.S. glial cells, suggesting that this may reflect some common function for these three cell types.

Schwann cells within the distal portion of a transected nerve undergo a series of poorly understood events in response to injury and loss of axonal contact. These events may influence the regeneration of PNS neurons. In this study we examined the alteration of antigens located in the basal lamina, plasma membrane and cytoplasm of Schwann cells within the distal nerve stump: (a) after a complete transection of the sciatic nerve, and (b) subsequent to reestablished contact between regenerating axons and dedifferentated Schwann cells separated from contact with neurons. Visualization of laminin and heparan sulphate proteoglycan molecules at various intervals after nerve transection always revealed intact basal lamina channels. In response to loss of axonal contact, vimentin expression by Schwann cells within the distal nerve stump increased, becoming a predominant intermediate filament protein of the cytoskeleton while glial fibrillary acid protein (GFAP) expression decreased. This reversal in the prominence of intermediate filament protein was maintained until the onset of axonal reinnervation, at which point expression of GFAP increased and vimentin decreased. Expression of the Schwann cell plasma membrane associated protein, C4, closely mimicked GFAP expression during axon degeneration and subsequent reinnervation. In the normal uninjured nerve, tissue plasminogen activator (tPA) and S-100 were localized in the region near the Schwann cell-axon interface and the outer Schwann cell plasma membrane. In response to loss of axonal contact, the S-100 and tPA immunoreactivity associated with the Schwann cell-axon interface was lost while that localized around the outer Schwann cell plasma membrane remained unchanged. The results of this study demonstrate that Schwann cells modulate a portion of their antigenic repertoire in response to a loss of axonal contact and after contact with regenerating axons.

In order to investigate the possible role of Schwann cells in immune reactions, and in particular their involvement in the response to infection with Mycobacterium leprae, it was determined under what conditions Schwann cells express major histocompatibility complex class II (MHC class II) antigens, since these molecules are thought to have a key role in antigen presentation during cellular immune responses. In situ and in vitro preparations from newborn and adult rat sciatic nerves were used as a model system to examine this question. Schwann cells in dissociated cell cultures did not express immunohistochemically detectable amounts of MHC class II antigens. Teased nerve preparations from the sciatic nerves of healthy adult rats showed no detectable immunolabelling of either myelin-forming or non-myelin-forming Schwann cells. When dissociated Schwann cell cultures derived from the sciatic nerves of either neonatal or adult rats were treated with 10, 50 or 100 units of gamma interferon, MHC class II antigens were detectable on the surface of some Schwann cells 48 h after addition of the interferon. By 72 h, 32.29 +/- 3.9% of Schwann cells in the cultures from neonatal rats and 53.32 +/- 5.4% of Schwann cells in cultures from adult rats, identified by the presence of intracellular S-100, were clearly MHC class II-positive, especially at doses of 50 and 100 units per ml of gamma interferon. Some, but not all, of the fibroblastic cells were very weakly MHC class II-positive. Infection of the cultures with Mycobacterium leprae did not induce MHC class II antigen expression in either Schwann cells or fibroblasts.(ABSTRACT TRUNCATED AT 250 WORDS)