Fascin Plays a Role in Stress Fiber Organization and Focal Adhesion Disassembly

Abstract

Migrating cells nucleate focal adhesions (FAs) at the cell front and disassemble them at the rear to allow cell translocation. FAs are made of a multiprotein complex, the adhesome, which connects integrins to stress fibers made of mixed-polarity actin filaments [1-5]. Myosin II-driven contraction of stress fibers generates tensile forces that promote adhesion growth [6-9]. However, tension must be tightly controlled, because if released, FAs disassemble [3, 10-12]. Conversely, excess tension can cause abrupt cell detachment resulting in the loss of a major part of the adhesion [9, 12]. Thus, both adhesion growth and disassembly depend on tensile forces generated by stress fiber contraction, but how this contractility is regulated remains unclear. Here, we show that the actin-bundling protein fascin crosslinks the actin filaments into parallel bundles at the stress fibers' termini. Fascin prevents myosin II entry at this region and inhibits its activity in vitro. In fascin-depleted cells, polymerization of actin filaments at the stress fiber termini is slower, the actin cytoskeleton is reorganized into thicker stress fibers with a higher number of myosin II molecules, FAs are larger and less dynamic, and consequently, traction forces that cells exert on their substrate are larger. We also show that fascin dissociation from stress fibers is required to allow their severing by cofilin, leading to efficient disassembly of FAs.

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

“…Sarcomeres near the stress fiber center, for example, are slightly longer than their counterparts closer to the termini (Elkhatib et al, 2014;Peterson et al, 2004). However, when stress fiber contractility is stimulated by calyculin A or lipoprotein-a (LPA), this small difference becomes quite pronounced.…”

“…As the pointed end of an actin filament always grows away from the nucleation site, these filaments all possess the same uniform polarity. Accordingly, the actin crosslinker fascin, which binds to unipolar, bundled actin filaments, is only found in focal adhesions and at stress fiber termini (Elkhatib et al, 2014). In contrast, towards the stress fiber center, actin polarity appears to be bi-directional, as is the case in myofibrils (Cramer et al, 1997) ( Fig.…”

“…Note that finer sub-resolution connections can also exist between neighboring stress fibers [e.g. actin fibers (Xu et al, 2012) expand over time compared to those located closer to the focal adhesions (Elkhatib et al, 2014;Peterson et al, 2004). Another possibility is that, as well-spread cells have multiple interconnections between stress fibers that together form dynamic 2D branched networks (Fig.…”

Human Spermatozoa Quantitative Proteomic Signature Classifies Normo- and Asthenozoospermia

“…Ninein is a protein that helps assemble the centrosomes (21,22), which regulate the axoneme activation (23) which, in turn, is responsible for sperm motility (24). Fascin-3 is an actin bundling protein and together they regulate actin stress fiber contraction (25), which is important for axoneme activation converging to sperm motility. Plexin-B2 is a cell surface receptor (downregulated in AZS sperm cells in our data), which, upon ligand based activation, activates the RhoA protein (26) leading to focal adhesion assembly and contraction of actin stress fibers (27), once again converging to axoneme activation and sperm motility pathway.…”

Reconstituting the actin cytoskeleton at or near surfaces in vitro

“…These biomimetic studies show that cross-linking may play a role in controlling how the network contracts. Indeed cross-linking proteins are abundant in the acto-myosin cell cortex [107], and myosin-regulatory roles for the actin-binding proteins fascin and ADF/cofilin, sometimes contradictory in the latter case, have been recently reported in cells [31,[108][109][110]. These issues will be one of the many questions to address in the future with biomimetics,…”