Astrocytes in culture synthesize and secrete a variant form of fibronectin

Abstract

Astrocytes in culture have very little cell surface fibronectin as detected by iodination or immunocytochemistry. Nonetheless, they synthesize and secrete this glycoprotein in amounts comparable with the production by fibroblasts. Astrocyte fibronectin has properties in common with other forms of the protein. It binds to gelatin- and heparin-coupled Sepharose and it is recognized by specific anti-fibronectin sera. It also exists as a dimer under non-reducing conditions. However, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), astrocyte fibronectin appears larger, under both reduced and non-reduced conditions, than other cellular fibronectins. This apparent size difference is not the result of post-translational modifications. If the cultures are treated with tunicamycin, the astrocytes produce fibronectin that is unglycosylated, as shown by [3H]glucosamine labeling, and is neither sulfated nor phosphorylated as indicated by [35S]O4 and [32P]O4 labeling studies. This astrocyte-derived, carbohydrate-free fibronectin resolves on SDS-PAGE as four bands, of which the heavier ones predominate. Fibroblasts produce a similar set of four bands, but in this form of fibronectin the less heavy bands predominate. Thus, we conclude that fibronectin is a major secreted protein of astrocytes in vitro and that these cells produce a variant form of the protein which is enriched in the higher molecular weight subunits.