Focal adhesion size controls tension-dependent recruitment of α-smooth muscle actin to stress fibers

TLDR

α‑smooth muscle actin (α‑SMA) expression makes fibroblasts highly contractile and is a hallmark of myofibroblast differentiation. The study proposes that the distinct molecular composition and higher phosphorylation of supermature focal adhesions compared to classical ones underlie their greater stress resistance. High‑tension recruitment of α‑SMA requires anchoring stress fibers at 8–30‑μm supermature focal adhesions, which exert ~12 nN/μm² stress, and can be induced by stretching classical 6‑μm adhesions to supermature size on extendable membranes. α‑SMA is recruited to stress fibers only under high tension, and preventing supermature focal adhesion formation by culturing on compliant (≤11 kPa) or rigid 6‑μm classical FA patterns retains α‑SMA in the cytosol.

Abstract

Expression of α-smooth muscle actin (α-SMA) renders fibroblasts highly contractile and hallmarks myofibroblast differentiation. We identify α-SMA as a mechanosensitive protein that is recruited to stress fibers under high tension. Generation of this threshold tension requires the anchoring of stress fibers at sites of 8–30-μm-long “supermature” focal adhesions (suFAs), which exert a stress approximately fourfold higher (∼12 nN/μm2) on micropatterned deformable substrates than 2–6-μm-long classical FAs. Inhibition of suFA formation by growing myofibroblasts on substrates with a compliance of ≤11 kPa and on rigid micropatterns of 6-μm-long classical FA islets confines α-SMA to the cytosol. Reincorporation of α-SMA into stress fibers is established by stretching 6-μm-long classical FAs to 8.1-μm-long suFA islets on extendable membranes; the same stretch producing 5.4-μm-long classical FAs from initially 4-μm-long islets is without effect. We propose that the different molecular composition and higher phosphorylation of FAs on supermature islets, compared with FAs on classical islets, accounts for higher stress resistance.