Functional sorting of actin isoforms in microvascular pericytes

Abstract

We characterized the form and distribution of muscle and nonmuscle actin within retinal pericytes. Antibodies with demonstrable specificities for the actin isoforms were used in localization and immunoprecipitation experiments to identify those cellular domains that were enriched or deficient in one or several actin isoforms. Living pericyte behavior was monitored with phase-contract video microscopy before fixation to identify those cellular areas that might preferentially be stained with either of the fluorescent antiactins or phallotoxins. Antibody and phallotoxin staining of pericytes revealed that nonmuscle actin is present within membrane ruffles, pseudopods, and stress fibers. In contrast, muscle actin could be convincingly localized in stress fibers, but not within specific motile areas of pericyte cytoplasm. To confirm and quantitatively extend the results obtained by fluorescence microscopy, nonionic and ionic detergents were used to selectively extract the motile or immobilized (stress fiber-containing) regions of biosynthetically labeled pericyte cytoplasm. Immunoprecipitated actins that were present within these discrete cellular domains were subjected to isoelectric focusing in urea-polyacrylamide gels before fluorographic analysis. Scanning laser densitometry of the focused actins could not reveal any detectable alpha-actin within those beta- and gamma-actin-enriched motile regions extracted with nonionic detergents. Moreover, when pericyte stress fibers are completely dissolved by ionic detergent lysis, three actin isoforms can be quantified to be present in a ratio of 1:2.75:3 (alpha:beta:gamma). These biochemical findings on biosynthetically labeled and immunoprecipitated pericyte actins confirm the fluorescent localization studies. While the regulatory events governing this actin sorting are unknown, it seems possible that such events may play important roles in controlling cell shape, adhesion, or the promotion of localized cell spreading.