Assembly of non-contractile dorsal stress fibers requires α-actinin-1 and Rac1 in migrating and spreading cells

Abstract

SummaryCell migration and spreading is driven by actin polymerization and actin stress fibers. Actin stress fibers are considered to contain aactinin crosslinkers and nonmuscle myosin II motors. Although several actin stress fiber subtypes have been identified in migrating and spreading cells, the degree of molecular diversity of their composition and the signaling pathways regulating fiber subtypes remain largely uncharacterized. In the present study we identify that dorsal stress fiber assembly requires a-actinin-1. Loss of dorsal stress fibers in a-actinin-1-depleted cells results in defective maturation of leading edge focal adhesions. This is accompanied by a delay in early cell spreading and slower cell migration without noticeable alterations in myosin light chain phosphorylation. In agreement with the unaltered myosin II activity, dorsal stress fiber trunks lack myosin II and are resistant to myosin II ATPase inhibition. Furthermore, the non-contractility of dorsal stress fibers is supported by the finding that Rac1 induces dorsal stress fiber assembly whereas contractile ventral stress fibers are induced by RhoA. Loss of dorsal stress fibers either by depleting a-actinin-1 or Rac1 results in a b-actin accumulation at the leading edge in migrating and spreading cells. These findings molecularly specify dorsal stress fibers from other actin stress fiber subtypes. Furthermore, we propose that non-contractile dorsal stress fibers promote cell migration and early cell spreading through Rac1-induced actin polymerization.

The microtubule end-binding protein EB2 is a central regulator of microtubule reorganisation in apico-basal epithelial differentiation

“…5D) resemble dorsal/radial filaments associated with lamellipodia that are activated by Rac1 and assembled by mDia formins. In addition, expression of the FH1/FH2 domains of several formins have been shown to induce microtubule stability as well as co-alignment with actin filaments (Bartolini and Gundersen, 2010;Gasteier et al, 2005;Ishizaki et al, 2001;Kobielak et al, 2004;Kovac et al, 2013;Oakes et al, 2012;Ryu et al, 2009;Thurston et al, 2012;Yang et al, 2007). The role of formins in microtubule straightness, co-alignment with actin filaments and EB1 and ACF7 lattice association in EB2-depleted cells was therefore investigated.…”

Geometry and network connectivity govern the mechanics of stress fibers

“…As such, SFs are capable of acting both locally and globally through their networked interactions with other cellular structures to impose tensile loads within the cell (9). This notion is a critical yet largely unexplored linchpin in current models of cell motility, where specific SF subsets are thought to coordinate tensile activities to direct remodeling of ECM adhesions and sculpt migratory processes (1,10,11).…”

The role of small GTPase Rac1 in stress signaling

“…In focal complexes, lamellipodium extension at the leading edge involves actin polymerization which is controlled in the long run by Rac1 (Clainche & Carlier 2008;Pollard & Cooper 2009;Parri & Chiarugi 2010). As one of the central regulators of actin dynamics Rac1 coordinates stress fiber assemblies (Kovac et al 2013;Tojkander et al 2012). Stress fiber components are linked to cellular signaling pathways, resulting in a variety of intracellular responses including the phosphorylation dependent recruitment of signaling proteins (Clainche & Carlier 2008).…”

“…Rac1 has numerous roles in normal physiology and disease state of cell functionality such as cell cycle regulation (Michaelson et al 2008), lamellipodia formation, membrane ruffling (Steffen et al 2013), regulation of NADPH oxidase activity in NADPH complex (Flinder et al 2011), cellular adhesion (Lawson & Burridge 2014), proliferation (Woodcock et al 2010), survival, differentiation and malignant transformation (Wertheimer et al 2012), cell migration, actin polymerization, spreading (Kovac et al 2013). Rac1 is also important in various oncogenesis pathways including initiation, progression, invasion and metastasis (Davis et al 2013) and stress sensing (Han et al 2001).…”

“…Having a fundamental role in wide variety of cellular processes such as migration, spreading and morphogenesis, an understanding of the roles of the actin cytoskeleton and stress fibers are getting important (Tojkander et al 2012;Kovac et al 2013;Burridge & Wittchen 2013). The process of stress fiber dynamics is currently being rigorously studied and is not yet well understood.…”