Nonmyelin-forming Schwann cells coexpress surface proteins and intermediate filaments not found in myelin-forming cells: a study of Ran-2, A5E3 antigen and glial fibrillary acidic protein.

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.