Regulation of gastric smooth muscle contraction via Ca(2+)-dependent and Ca(2+)-independent actin polymerization

In gastrointestinal smooth muscle, acetylcholine induced muscle contraction is biphasic, initial peak followed by sustained contraction. Contraction is regulated by phosphorylation of 20 kDa myosin light chain (MLC) at Ser(19), interaction of actin and myosin, and actin polymerization. The present study characterized the signaling mechanisms involved in actin polymerization during initial and sustained muscle contraction in response to muscarinic M3 receptor activation in gastric smooth muscle cells by targeting the effectors of initial (phospholipase C (PLC)-β/Ca(2+) pathway) and sustained (RhoA/focal adhesion kinase (FAK)/Rho kinase pathway) contraction. The initial Ca(2+) dependent contraction and actin polymerization is mediated by sequential activation of PLC-β1 via Gα(q), IP(3) formation, Ca(2+) release and Ca(2+) dependent phosphorylation of proline-rich-tyrosine kinase 2 (Pyk2) at Tyr(402). The sustained Ca(2+) independent contraction and actin polymerization is mediated by activation of RhoA, and phosphorylation of FAK at Tyr(397). Both phosphorylation of Pyk2 and FAK leads to phosphorylation of paxillin at Tyr(118) and association of phosphorylated paxillin with the GEF proteins p21-activated kinase (PAK) interacting exchange factor α, β (α and β PIX) and DOCK 180. These GEF proteins stimulate Cdc42 leading to the activation of nucleation promoting factor N-WASP (neuronal Wiskott-Aldrich syndrome protein), which interacts with actin related protein complex 2/3 (Arp2/3) to induce actin polymerization and muscle contraction. Acetylcholine induced muscle contraction is inhibited by actin polymerization inhibitors. Thus, our results suggest that a novel mechanism for the regulation of smooth muscle contraction is mediated by actin polymerization in gastrointestinal smooth muscle which is independent of MLC(20) phosphorylation.