Journal of Neuroscience, Vol 5, 2205-2211, Copyright © 1985 by Society for Neuroscience
Astrocytes in culture synthesize and secrete a variant form of fibronectin
J Price and RO Hynes
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.
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