Abstract
Actin dynamics plays an essential role in regulating airway smooth muscle contraction. The mechanisms that regulate actin dynamics in smooth muscle are not completely understood. Glia maturation factor (GMF) is a protein that has been reported to inhibit actin nucleation and to induce actin network debranching in vitro. The role of GMF in human smooth muscle cells and tissues has not been investigated. In this study, knockdown of GMF-γ by RNA interference enhanced actin polymerization and contraction in human airway smooth muscle (HASM) cells and tissues without affecting myosin phosphorylation (another important biochemical change during contractile activation). Activation of HASM cells and tissues with acetylcholine induced dissociation of GMF-γ from Arp2 of the Arp2/3 complex. Acetylcholine stimulation also increased GMF-γ phosphorylation at Tyr-104. GMF-γ phosphorylation at this residue was mediated by c-Abl tyrosine kinase. The GMF-γ mutant Y104F (phenylalanine substitution at Tyr-104) had higher association with Arp2 in HASM cells upon contractile activation. Furthermore, expression of mutant Y104F GMF-γ attenuated actin polymerization and contraction in smooth muscle. Thus, we propose a novel mechanism for the regulation of actin dynamics and smooth muscle contraction. In unstimulated smooth muscle, GMF-γ binds to the Arp2/3 complex, which induces actin disassembly and retains lower levels of F-actin. Upon contractile stimulation, phosphorylation at Tyr-104 mediated by c-Abl tyrosine kinase leads to the dissociation of GMF-γ from Arp2/3, by which GMF-γ no longer induces actin disassembly. Reduced actin disassembly renders F-actin in higher level, which facilitates smooth muscle contraction.