Generation of contractile actomyosin bundles depends on mechanosensitive actin filament assembly and disassembly

Abstract

Adhesion and morphogenesis of many non-muscle cells are guided by contractile actomyosin bundles called ventral stress fibers. While it is well established that stress fibers are mechanosensitive structures, physical mechanisms by which they assemble, align, and mature have remained elusive. Here we show that arcs, which serve as precursors for ventral stress fibers, undergo lateral fusion during their centripetal flow to form thick actomyosin bundles that apply tension to focal adhesions at their ends. Importantly, this myosin II-derived force inhibits vectorial actin polymerization at focal adhesions through AMPK-mediated phosphorylation of VASP, and thereby halts stress fiber elongation and ensures their proper contractility. Stress fiber maturation additionally requires ADF/cofilin-mediated disassembly of non-contractile stress fibers, whereas contractile fibers are protected from severing. Taken together, these data reveal that myosin-derived tension precisely controls both actin filament assembly and disassembly to ensure generation and proper alignment of contractile stress fibers in migrating cells.DOI: http://dx.doi.org/10.7554/eLife.06126.001

Tension regulates myosin dynamics during Drosophila embryonic wound repair

“…We find that dynamic motor activity is crucial for the stabilization of myosin at the cortex, consistent with recent work showing that myosin phosphorylation regulates contractility in epithelial morphogenesis (Kasza et al, 2014;Vasquez et al, 2014;Vasquez et al, 2016). Several actin regulators are sensitive to load, including formins (Courtemanche et al, 2013;Higashida et al, 2013;Jégou et al, 2013), and the filament-severing protein cofilin (Hayakawa et al, 2011;Tojkander et al, 2015). The mechanical dependence of myosin dynamics could thus be a result of direct effects on the motor, indirect effects on the actin filaments bound by the motor, or a combination of both.…”

The inner workings of stress fibers − from contractile machinery to focal adhesions and back

“…They appear to be attached solely to the ventral membrane through focal adhesions at a single, discrete end, usually near the cell edges. From there, they extend towards the cell center through formin-and VASP-driven actin polymerization (Hotulainen and Lappalainen, 2006;Skau et al, 2015;Tojkander et al, 2015), apparently to serve as tracks on which transverse arcs can slide centripetally (Hotulainen and Lappalainen, 2006;Tee et al, 2015). Transverse arcs, however, are long, curved actin structures along which alternating and repetitive bands of α-actinin and myosin are arranged.…”

Cellular functions of the ADF/cofilin family at a glance

“…The same modification has been reported to influence cofilin activities during apoptosis (see Apoptosis section below). Lastly, mechanosensitive regulation of cofilin actin binding and severing activity has been reported (Hayakawa et al, 2011;Tojkander et al, 2015). Cofilin appears to preferentially bind to less-tensile actin filaments and to mediate their degradation, whereas filaments under tension are protected from cofilinmediated fragmentation.…”

“…Cofilin appears to preferentially bind to less-tensile actin filaments and to mediate their degradation, whereas filaments under tension are protected from cofilinmediated fragmentation. This mechano-regulation has been shown to be important for the maturation of contractile stress fibres in cells (Tojkander et al, 2015).…”