Laminin is associated with the "neurite outgrowth-promoting factors" found in conditioned media

Conditioned media (CMs) from many cell types contain a factor that can adsorb to a polycationic substratum and cause neurons plated on that substratum to extend neurites rapidly and profusely. The extracellular matrix glycoprotein laminin, when bound to a substratum, elicits a similar response by neurons. In this report, six CMs that contain a "neurite outgrowth-promoting factor" were studied. Immunoprecipitation with affinity-purified anti-laminin antibodies demonstrated that laminin is present in all of them, and when it was selectively removed, there was a corresponding loss of neurite outgrowth-promoting activity in each CM. Antibodies to purified laminin failed, however, to block the outgrowth-promoting activity of five of the CMs tested, even though these antibodies blocked the outgrowth-promoting activity of purified laminin in the same assay. This result could reflect differences in amino acid sequence or protein modification between CM-derived laminin and the purified laminin used in generating antibodies. Alternatively, it could reflect the fact that other molecules bind to laminin in CMs and could interfere with the binding of antibodies to sites on laminin that are important for biological activity.