Structural analysis of a human glial variant laminin

Astrocytes secrete laminin-like molecules in culture and may represent a major source of laminin in the developing central nervous system, yet these laminins have not been extensively characterized. We previously reported the presence of an astrocyte-derived variant laminin in media conditioned by human U251 MG astrocytoma cells. This laminin was partially purified in a highly anionic Mono Q fraction with strong adhesion activity for fibroblasts and glial cells (Aukhil et al. (1990) Matrix 10: 98-111). We now show that glial laminin could be dissociated from an anionic species, perhaps an ~400- kDa keratan sulfate proteoglycan present in the preparation, by a second round of Mono Q anion exchange chromatography in the presence of 6 M urea. Cell adhesion activity remained tightly associated with laminin-containing fractions, suggesting that glial laminin was responsible for the adhesion activity in the original preparation. Immunochemical and SDS-PAGE gel analyses of laminin heterotrimers demonstrated that glial laminin contained the β2 and γ1 chains in disulfide-bonded heterotrimeric complexes with a 360-kDa chain, a 320-kDa chain, or a postulated ~200-kDa chain. While these chains were not recognized by antibodies directed against the α1-, α2-, or α3-related laminin chains, rotary shadowed glial laminin molecules appeared to contain α chains, as judged by the presence of an apparent G-domain terminating the long arm of each laminin molecule. These findings suggest that glial laminin contains one or more variant α chains, perhaps related to one of the more recently described α chains, α3B, α4, or α5. Together our results implicate human U251 MG glial laminin as a previously uncharacterized laminin isoform with strong adhesive activity for fibroblasts and glial cells.