ROLE AND REGULATION OF INTERMEDIATE FILAMENT PROTEIN VIMENTIN IN SMOOTH MUSCLE

Intermediate filament protein vimentin connects to the membrane at desmosomes and to dense bodies in the cytoplasm. However, the role of vimentin in smooth muscle function is not well understood. We depleted canine tracheal smooth muscle strips of vimentin by antisense oligodeoxynucleotides. Downregulation of vimentin attenuated tension development without affecting myosin light chain phosphorylation. In addition, the distribution of desmosomal protein plakoglobin on the membrane was decreased in cells freshly dissociated from vimentin‐deficient tissues. We also found that acetylcholine (ACh) stimulation induced vimentin phosphorylation on Ser‐56 and increased the ratio of soluble/insoluble vimentin in tracheal smooth muscle strips. Downregulation of p21‐activated kinase (PAK) by antisense inhibited vimentin phosphorylation and the increase in soluble/insoluble vimentin. Treatment of muscle strips with the Rho kinase inhibitor Y27632, or PKC inhibitors calphostin C and BIS, or the actin polymerization inhibitor cytochalasin D did not depress vimentin phosphorylation in response to ACh activation. In conclusion, our results suggest that vimentin is necessary for force development and desmosomal formation, but not contractile protein activation. PAK, but not Rho kinase, PKC and the actin cytoskeleton, regulates vimentin phosphorylation at Ser‐56 and vimentin remodeling in smooth muscle.