Inhibition of intracellular granule movement by microinjection of monoclonal antibodies against caldesmon.

Monoclonal antibodies, C2, C9, C18, and C21, against chicken gizzard caldesmon (called high molecular weight isoform) were shown to crossreact with a low molecular weight isoform of caldesmon in chicken embryo fibroblasts (CEF). These antibodies were used in a microinjection study to investigate the in vivo function of caldesmon in nonmuscle cell motility. Injected cells did not appear to change their morphology significantly; the cells displayed a flat appearance and were able to ruffle and locomote normally. However, in the C21 injected cells, saltatory movements of granules and organelles appeared to be greatly inhibited. This inhibition of granule movement was reversible, so that by 3 hr after injection, granules in injected cells had already recovered to normal speed. The inhibition of granule movement in cells injected with C2, C9, or C18 antibody, or with C21 antibody preabsorbed with caldesmon, were not significantly different from that in uninjected cells. In a previous epitope study, we demonstrated that, of the antibodies used in this study, only C21 antibody was able to compete with the binding of caldesmon to Ca++/calmodulin and to F-actin, although both C21 and C2 antibodies recognized the same carboxyl-terminal 10K fragment of gizzard caldesmon [Lin et al., 1991: Cell Motil. Cytoskeleton 20:95-108]. The caldesmon distribution in C21 injected cells changed from stress-fiber localization to a more diffuse appearance, when the injection was performed at 10-30 mg/ml of C21 antibody. We have previously shown that a monoclonal anti-tropomyosin antibody exhibited motility-dependent recognition of an epitope, and that microinjection of this antibody specifically inhibited intracellular granule movements of CEF cells [Hegmann et al., 1989: J. Cell Biol. 109:1141-1152]. Therefore, it is likely that tropomyosin and caldesmon may both function in intracellular granule movement by regulating the contractile system in response to [Ca++] change inside nonmuscle cells.