Glial cells of mammalian brain produce a variant form of laminin

Abstract

Glial cells of mammalian brain transiently produce laminin, an extracellular matrix protein and a potent inducer of neurite outgrowth of cultured neurons. We demonstrate that the glial laminin is a variant with the 200-kDa B chains present and the 400-kDa A chain missing. Immunoprecipitation of cell extracts of purified cultured astrocytes, using antibodies to native laminin, revealed the B chains only, although the same antibody immunoprecipitated both A and B chains from the cell extracts of a basement membrane producing embryonal carcinoma cell line (PYS). This result was further obtained if cell extracts of embryonic mouse brain or the PYS cells were immunoblotted using antibodies to native laminin. Northern analysis of the poly(A) + RNA from the mouse brain revealed no A chain transcript, although the same probe detected the A chain message in a basement membrane producing F9 cell line. No differences in the sizes of the B chain transcripts in different tissues were seen. Immunostaining of embryonic mouse optic nerve and other areas of the brain, using affinity-purified chain-specific antibodies to the major subunits of laminin, showed that the glial cells expressed the B chains only. It was apparent that the punctate, extracellular deposits of laminin, found in association with neuronal migration and nerve pathway formation in vivo were due to the glial expression of the B chains. The A chain immunoreactivity in the brain was confined to basement membranes. These results indicate that the A chain is absent in the glial laminin, which may allow this variant molecule to assemble in the form of punctate deposits and have a unique role in cell-to-cell interactions during differentiation of the central nervous system.