Regulation of actin dynamics by WNT-5A: Implications for human airway smooth muscle contraction

An important feature of asthma is airway hyperresponsiveness in which the airway smooth muscle (ASM) is fundamentally involved. How asthmatic ASM differs from healthy is a current focus for research. We have shown previously that WNT-5A is increased in asthmatic ASM. Here, we tested the hypothesis that WNT-5A is a determinant of the contractile response of ASM. Using bovine smooth muscle strips, we found that pre-incubation with WNT-5A increased maximum tension by histamine. We used human ASM cells to study the underlying mechanisms. In ASM loaded with the calcium dye Fura-2, neither direct exposure to WNT-5A nor pre-incubation had any effect on calcium handling. Moreover, WNT-5A treatment did not significantly change IP3R, SERCA2 and RyR expression. Next, we found that exposure to WNT-5A increased abundance of filamentous actin, without affecting the total actin pool. These changes could be completely prevented by treatment with a Rho kinase inhibitor. Interestingly, ASM cells that adopted a contractile phenotype also showed greater WNT-5A protein expression compared to the proliferative phenotype. We next assessed whether TGF-β1 played an upstream role in the WNT-5A response. We found that WNT-5A siRNA not only blocked TGF-β1-induced actin polymerization, but could also abrogate TGF-β1-induced accumulation of α-SMA. In line with this contention, adding WNT-5A on top of TGF-β1 synergistically induced α-SMA abundance. In summary, WNT-5A promotes agonist-induced airway smooth muscle contraction, most likely by increasing actin polymerization and actin expression in concerted action with TGF-β1. These results may be relevant in the understanding of AHR and warrant further investigation.