Src regulates the activity of the mammalian formin protein FHOD1

Active FHOD1 promotes the formation of functional actin stress fibers

The formin FHOD1 acts as a nucleating, capping and bundling protein of actin filaments. In cells, release from the C-terminal diaphanous autoregulatory domain (DAD) of FHOD1 stimulates the protein into the active form. However, the cellular physiological relevance of active form FHOD1 and the phenotypic regulation by FHOD1 depletion are not completely understood. Here, we show that in contrast with the cytosolic diffused expression of auto-inhibited FHOD1, active FHOD1 by C-terminal truncation was recruited into all three types of actin stress fibers in human osteosarcoma cells. Notably, the recruited active FHOD1 was more incorporated with myosin II than α-actinin, and associated with both naïve and mature focal adhesions. Active FHOD1 displayed faster turnover than actin molecules on ventral stress fibers. Moreover, we witnessed the emergence of active FHOD1 from the cell periphery, which subsequently moved centripetally together with transverse arcs. Furthermore, FHOD1 knockdown resulted in defective maturation of actomyosin bundles and subsequently longer non-contractile dorsal stress fibers, whereas the turnover of both actin and myosin II were maintained normally. Importantly, the loss of FHOD1 led to slower actin centripetal flow, resulting in abnormal cell spreading and migration defects. Taken together, these results reveal a critical role of FHOD1 in temporal- and spatial- control of the morphology and dynamics of functional actin stress fibers during variable cell behavior.

Actin polymerization downstream of integrins: signaling pathways and mechanotransduction

A cell constantly adapts to its environment. Cell decisions to survive, to proliferate or to migrate are dictated not only by soluble growth factors, but also through the direct interaction of the cell with the surrounding extracellular matrix (ECM). Integrins and their connections to the actin cytoskeleton are crucial for monitoring cell attachment and the physical properties of the substratum. Cell adhesion dynamics are modulated in complex ways by the polymerization of branched and linear actin arrays, which in turn reinforce ECM-cytoskeleton connection. This review describes the major actin regulators, Ena/VASP proteins, formins and Arp2/3 complexes, in the context of signaling pathways downstream of integrins. We focus on the specific signaling pathways that transduce the rigidity of the substrate and which control durotaxis, i.e. directed migration of cells towards increased ECM rigidity. By doing so, we highlight several recent findings on mechanotransduction and put them into a broad integrative perspective that is the result of decades of intense research on the actin cytoskeleton and its regulation.

Integrin Mechano-chemical Signaling Generates Plasma Membrane Nanodomains that Promote Cell Spreading

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are a major class of lipid-anchored plasma membrane proteins. GPI-APs form nanoclusters generated by cortical acto-myosin activity. While our understanding of the physical principles governing this process is emerging, the molecular machinery and functional relevance of GPI-AP nanoclustering are unknown. Here, we first show that a membrane receptor signaling pathway directs nanocluster formation. Arg-Gly-Asp motif-containing ligands bound to the β1-integrin receptor activate src and focal adhesion kinases, resulting in RhoA signaling. This cascade triggers actin-nucleation via specific formins, which, along with myosin activity, drive the nanoclustering of membrane proteins with actin-binding domains. Concurrently, talin-mediated activation of the mechano-transducer vinculin is required for the coupling of the acto-myosin machinery to inner-leaflet lipids, thereby generating GPI-AP nanoclusters. Second, we show that these nanoclusters are functional; disruption of their formation either in GPI-anchor remodeling mutants or in vinculin mutants impairs cell spreading and migration, hallmarks of integrin function.

FHOD1 is needed for directed forces and adhesion maturation during cell spreading and migration

Matrix adhesions provide critical signals for cell growth or differentiation. They form through a number of distinct steps that follow integrin binding to matrix ligands. In an early step, integrins form clusters that support actin polymerization by an unknown mechanism. This raises the question of how actin polymerization occurs at the integrin clusters. We report here that a major formin in mouse fibroblasts, FHOD1, is recruited to integrin clusters, resulting in actin assembly. Using cell-spreading assays on lipid bilayers, solid substrates, and high-resolution force-sensing pillar arrays, we find that knockdown of FHOD1 impairs spreading, coordinated application of adhesive force, and adhesion maturation. Finally, we show that targeting of FHOD1 to the integrin sites depends on the direct interaction with Src family kinases and is upstream of the activation by Rho kinase. Thus, our findings provide insights into the mechanisms of cell migration with implications for development and disease.

The formin FHOD1 and the small GTPase Rac1 promote vaccinia virus actin-based motility

Vaccinia virus actin–based motility relies on integration of the N-WASP–ARP2/3 and Rac1–FHOD1 pathways.

Vaccinia virus dissemination relies on the N-WASP–ARP2/3 pathway, which mediates actin tail formation underneath cell-associated extracellular viruses (CEVs). Here, we uncover a previously unappreciated role for the formin FHOD1 and the small GTPase Rac1 in vaccinia actin tail formation. FHOD1 depletion decreased the number of CEVs forming actin tails and impaired the elongation rate of the formed actin tails. Recruitment of FHOD1 to actin tails relied on its GTPase binding domain in addition to its FH2 domain. In agreement with previous studies showing that FHOD1 is activated by the small GTPase Rac1, Rac1 was enriched and activated at the membrane surrounding actin tails. Rac1 depletion or expression of dominant-negative Rac1 phenocopied the effects of FHOD1 depletion and impaired the recruitment of FHOD1 to actin tails. FHOD1 overexpression rescued the actin tail formation defects observed in cells overexpressing dominant-negative Rac1. Altogether, our results indicate that, to display robust actin-based motility, vaccinia virus integrates the activity of the N-WASP–ARP2/3 and Rac1–FHOD1 pathways.

The role of the formin gene fhod-1 in C. elegans embryonic morphogenesis

During the second half of embryogenesis, the ellipsoidal Caenorhabditis elegans embryo elongates into a long, thin worm. This elongation requires a highly organized cytoskeleton composed of actin microfilaments, microtubules and intermediate filaments throughout the epidermis of the embryo. This architecture allows the embryonic epidermal cells to undergo a smooth muscle-like actin/myosin-based contraction that is redundantly controlled by LET- 502/Rho kinase and MEL-11/myosin phosphatase in one pathway and FEM-2/PP2c phosphatase and PAK-1/p21-activated kinase in a parallel pathway(s). Although actin microfilaments surround the embryo, the force for contraction is generated mainly in the lateral (seam) epidermal cells whose actin microfilaments appear qualitatively different from those in their dorsal/ventral neighbors. We have identified FHOD-1, a formin family actin nucleator, which acts in the lateral epidermis. fhod-1 mutants show microfilament defects in the embryonic lateral epidermal cells and FHOD-1 protein is detected only in those cells. fhod-1 genetic interactions with let-502, mel-11, fem-2 and pak-1 indicate that fhod-1 preferentially regulates those microfilaments acting with let-502 and mel-11, and in parallel to fem-2 and pak-1. Thus, FHOD-1 may contribute to the qualitative differences in microfilaments found in the contractile lateral epidermal cells and their non-contractile dorsal and ventral neighbors. Different microfilament populations may be involved in the different contractile pathways.

Spatial-temporal reorganization of activated integrins

Integrin receptors play important roles in cell adhesion and tumor metastasis. The coupling of mechanical sensing and biochemical ligation is known to collectively regulate the activation of integrin receptors. Recently, oligomerization of activated integrins has been considered as the primordial signature of cytoskeletal remodeling and the initiation of various downstream signals, such as focal and fibrillar adhesions. However, spatio-temporal reorganization of activated integrins and associated proteins remains poorly understood. Here, we summarized the recent discovery of sequential biophysical events of integrin activation during early adhesion formation. Using the cyclic Arg-Gly-Asp (RGD) peptide as a mobile ligand on supported lipid membranes, a series of previously unreported events were observed following integrin αvβ3 clustering and cell spreading, including a long-range lateral translocation of the integrin clusters. With initial clustering, localized actin polymerization occurred in a Src family kinase dependent manner. Clustering of liganded integrins recruits various adaptor proteins and serves as a reaction core for mechanobiological activities. In addition, there are future possibilities to investigate the role of other synergetic interactions with the activated integrin receptors.

Unleashing formins to remodel the actin and microtubule cytoskeletons

Formins are highly conserved proteins that have essential roles in remodelling the actin and microtubule cytoskeletons to influence eukaryotic cell shape and behaviour. Recent work has identified numerous cellular factors that locally recruit, activate or inactivate formins to bridle and unleash their potent effects on actin nucleation and elongation. The effects of formins on microtubules have also begun to be described, which places formins in a prime position to coordinate actin and microtubule dynamics. The emerging complexity in the mechanisms governing formins mirrors the wide range of essential functions that they perform in cell motility, cell division and cell and tissue morphogenesis.

The Diaphanous-related Formin FHOD1 associates with ROCK1 and promotes Src-dependent plasma membrane blebbing

Diaphanous-related formins (DRFs) mediate GTPase-triggered actin rearrangements to regulate central cellular processes, such as cell motility and cytokinesis. The DRF FHOD1 interacts with the Rho-GTPase Rac1 and mediates formation of actin stress fibers in its deregulated form; the physiologically relevant activities and molecular mechanisms of endogenous FHOD1, however, are still unknown. Here we report that FHOD1 physically associates via the N-terminal part of its FH2 domain with the central domain of ROCK1. Although FHOD1 does not affect the kinase activity of ROCK1, the DRF is an efficient substrate for phosphorylation by ROCK1. Co-expression of FHOD1 and ROCK1 results in the generation of nonapoptotic plasma membrane (PM) blebs, to which the DRF is efficiently recruited. Blebbing induced by FHOD1 and ROCK1 depends on F-actin integrity, the Rho-ROCK cascade, and Src activity and is reminiscent of the recently described PM blebs triggered by expression of Src homology 4 (SH4) domain PM targeting signals. Consistently, endogenous FHOD1 is required in SH4 domain expressing cells for efficient PM blebbing and rounded cell morphology in two-dimensional cultures or three-dimensional matrices, respectively. Efficient association of FHOD1 with ROCK1, as well as recruitment of the DRF to blebs, depends on Src activity, suggesting that the functional interaction between both proteins is regulated by Src. These results define a role for endogenous FHOD1 in SH4 domain-induced blebbing and suggest that its activity is regulated by ROCK1 in a Src-dependent manner.

SH4-domain-induced plasma membrane dynamization promotes bleb-associated cell motility

SH4 domains provide bipartite membrane-targeting signals for oncogenic Src family kinases. Here we report the induction of non-apoptotic plasma membrane (PM) blebbing as a novel and conserved activity of SH4 domains derived from the prototypic Src kinases Src, Fyn, Yes and Lck as well as the HASPB protein of Leishmania parasites. SH4-domain-induced blebbing is highly dynamic, with bleb formation and collapse displaying distinct kinetics. These reorganizations of the PM are controlled by Rho but not Rac or Cdc42 GTPase signalling pathways. SH4-induced membrane blebbing requires the membrane association of the SH4 domain, is regulated by the activities of Rock kinase and myosin II ATPase, and depends on the integrity of F-actin as well as microtubules. Endogenous Src kinase activity is crucial for PM blebbing in SH4-domain-expressing cells, active Src and Rock kinases are enriched in SH4-domain-induced PM blebs, and PM blebbing correlates with enhanced cell invasion in 3D matrices. These results establish a novel link between SH4 domains, Src activity and Rho signalling, and implicate SH4-domain-mediated PM dynamization as a mechanism that influences invasiveness of cells transformed by SH4-domain-containing oncoproteins.