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

Supported lipid bilayer platforms to probe cell mechanobiology

Mammalian and bacterial cells sense and exert mechanical forces through the process of mechanotransduction, which interconverts biochemical and physical signals. This is especially important in contact-dependent signaling, where ligand-receptor binding occurs at cell-cell or cell-ECM junctions. By virtue of occurring within these specialized junctions, receptors engaged in contact-dependent signaling undergo oligomerization and coupling with the cytoskeleton as part of their signaling mechanisms. While our ability to measure and map biochemical signaling within cell junctions has advanced over the past decades, physical cues remain difficult to map in space and time. Recently, supported lipid bilayer (SLB) technologies have emerged as a flexible platform to mimic and perturb cell-cell and cell-ECM junctions, allowing one to study membrane receptor mechanotransduction. Changing the lipid composition and underlying substrate tunes bilayer fluidity, and lipid and ligand micro- and nano-patterning spatially control positioning and clustering of receptors. Patterning metal gridlines within SLBs confines lipid mobility and introduces mechanical resistance. Here we review fundamental SLB mechanics and how SLBs can be engineered as tunable cell substrates for mechanotransduction studies. Finally, we highlight the impact of this work in understanding the biophysical mechanisms of cell adhesion. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

EGFR and HER2 activate rigidity sensing only on rigid matrices

Epidermal growth factor receptor (EGFR) interacts with integrins during cell spreading and motility, but little is known about the role of EGFR in these mechanosensing processes. Here we show, using two different cell lines, that in serum- and EGF-free conditions, EGFR or HER2 activity increase spreading and rigidity-sensing contractions on rigid, but not soft, substrates. Contractions peak after 15-20 min, but diminish by tenfold after 4 h. Addition of EGF at that point increases spreading and contractions, but this can be blocked by myosin-II inhibition. We further show that EGFR and HER2 are activated through phosphorylation by Src family kinases (SFK). On soft surfaces, neither EGFR inhibition nor EGF stimulation have any effect on cell motility. Thus, EGFR or HER2 can catalyse rigidity sensing after associating with nascent adhesions under rigidity-dependent tension downstream of SFK activity. This has broad implications for the roles of EGFR and HER2 in the absence of EGF both for normal and cancerous growth.

αV-class integrins exert dual roles on α5β1 integrins to strengthen adhesion to fibronectin

Upon binding to the extracellular matrix protein, fibronectin, αV-class and α5β1 integrins trigger the recruitment of large protein assemblies and strengthen cell adhesion. Both integrin classes have been functionally specified, however their specific roles in immediate phases of cell attachment remain uncharacterized. Here, we quantify the adhesion of αV-class and/or α5β1 integrins expressing fibroblasts initiating attachment to fibronectin (≤120 s) by single-cell force spectroscopy. Our data reveals that αV-class integrins outcompete α5β1 integrins. Once engaged, αV-class integrins signal to α5β1 integrins to establish additional adhesion sites to fibronectin, away from those formed by αV-class integrins. This crosstalk, which strengthens cell adhesion, induces α5β1 integrin clustering by RhoA/ROCK/myosin-II and Arp2/3-mediated signalling, whereas overall cell adhesion depends on formins. The dual role of both fibronectin-binding integrin classes commencing with an initial competition followed by a cooperative crosstalk appears to be a basic cellular mechanism in assembling focal adhesions to the extracellular matrix.

FHOD1 formin is upregulated in melanomas and modifies proliferation and tumor growth

The functional properties of actin-regulating formin proteins are diverse and in many cases cell-type specific. FHOD1, a formin expressed predominantly in cells of mesenchymal lineage, bundles actin filaments and participates in maintenance of cell shape, migration and cellular protrusions. FHOD1 participates in cancer-associated epithelial to mesenchymal transition (EMT) in oral squamous cell carcinoma and breast cancer. The role of FHOD1 in melanomas has not been characterized. Here, we show that FHOD1 expression is typically strong in cutaneous melanomas and cultured melanoma cells while the expression is low or absent in benign nevi. By using shRNA to knockdown FHOD1 in melanoma cells, we discovered that FHOD1 depleted cells are larger, rounder and have smaller focal adhesions and inferior migratory capacity as compared to control cells. Importantly, we found FHOD1 depleted cells to have reduced colony-forming capacity and attenuated tumor growth in vivo, a finding best explained by the reduced proliferation rate caused by cell cycle arrest. Unexpectedly, FHOD1 depletion did not prevent invasive growth at the tumor margins. These results suggest that FHOD1 participates in key cellular processes that are dysregulated in malignancy, but may not be essential for melanoma cell invasion.

Integrin and cadherin clusters: A robust way to organize adhesions for cell mechanics

Recent studies at the nanometer scale have revealed that relatively uniform clusters of adhesion proteins (50-100 nm) constitute the modular units of cell adhesion sites in both cell-matrix and cell-cell adhesions. Super resolution microscopy and membrane protein diffusion studies both suggest that even large focal adhesions are formed of 100 nm clusters that are loosely aggregated. Clusters of 20-50 adhesion molecules (integrins or cadherins) can support large forces through avidity binding interactions but can also be disassembled or endocytosed rapidly. Assembly of the clusters of integrins is force-independent and involves gathering integrins at ligand binding sites where they are stabilized by cytoplasmic adhesion proteins that crosslink the integrin cytoplasmic tails plus connect the clusters to the cell cytoskeleton. Cooperative-signaling events can occur in a single cluster without cascading to other clusters. Thus, the clusters appear to be very important elements in many cellular processes and can be considered as a critical functional module.

Computational simulation of formin-mediated actin polymerization predicts homologue-dependent mechanosensitivity

Many actin structures are nucleated and assembled by the barbed-end tracking polymerase formin family, including filopodia, focal adhesions, the cytokinetic ring and cell cortex. These structures respond to forces in distinct ways. Formins typically have profilin-actin binding sites embedded in highly flexible disordered FH1 domains, hypothesized to diffusively explore space to rapidly capture actin monomers for delivery to the barbed end. Recent experiments demonstrate that formin-mediated polymerization accelerates when under tension. The acceleration has been attributed to modifying the state of the FH2 domain of formin. Intriguingly, the same acceleration is reported when tension is applied to the FH1 domains, ostensibly pulling monomers away from the barbed end. Here we develop a mesoscale coarse-grain model of formin-mediated actin polymerization, including monomer capture and delivery by FH1, which sterically interacts with actin along its entire length. The binding of actin monomers to their specific sites on FH1 is entropically disfavored by the high disorder. We find that this penalty is attenuated when force is applied to the FH1 domain by revealing the binding site, increasing monomer capture efficiency. Overall polymerization rates can decrease or increase with increasing force, depending on the length of FH1 domain and location of binding site. Our results suggest that the widely varying FH1 lengths and binding site locations found in known formins could be used to differentially respond to force, depending on the actin structure being assembled. © 2016 Wiley Periodicals, Inc.

Integrin-mediated mechanotransduction

Sun, Guo, and Fässler review the function and regulation of integrin-mediated mechanotransduction and discuss how its dysregulation impacts cancer progession.

Cells can detect and react to the biophysical properties of the extracellular environment through integrin-based adhesion sites and adapt to the extracellular milieu in a process called mechanotransduction. At these adhesion sites, integrins connect the extracellular matrix (ECM) with the F-actin cytoskeleton and transduce mechanical forces generated by the actin retrograde flow and myosin II to the ECM through mechanosensitive focal adhesion proteins that are collectively termed the “molecular clutch.” The transmission of forces across integrin-based adhesions establishes a mechanical reciprocity between the viscoelasticity of the ECM and the cellular tension. During mechanotransduction, force allosterically alters the functions of mechanosensitive proteins within adhesions to elicit biochemical signals that regulate both rapid responses in cellular mechanics and long-term changes in gene expression. Integrin-mediated mechanotransduction plays important roles in development and tissue homeostasis, and its dysregulation is often associated with diseases.

The sarcomeric cytoskeleton: from molecules to motion

Highly ordered organisation of striated muscle is the prerequisite for the fast and unidirectional development of force and motion during heart and skeletal muscle contraction. A group of proteins, summarised as the sarcomeric cytoskeleton, is essential for the ordered assembly of actin and myosin filaments into sarcomeres, by combining architectural, mechanical and signalling functions. This review discusses recent cell biological, biophysical and structural insight into the regulated assembly of sarcomeric cytoskeleton proteins and their roles in dissipating mechanical forces in order to maintain sarcomere integrity during passive extension and active contraction. α-Actinin crosslinks in the Z-disk show a pivot-and-rod structure that anchors both titin and actin filaments. In contrast, the myosin crosslinks formed by myomesin in the M-band are of a ball-and-spring type and may be crucial in providing stable yet elastic connections during active contractions, especially eccentric exercise.

Matrix mechanics controls FHL2 movement to the nucleus to activate p21 expression

Substrate rigidity affects many physiological processes through mechanochemical signals from focal adhesion (FA) complexes that subsequently modulate gene expression. We find that shuttling of the LIM domain (domain discovered in the proteins, Lin11, Isl-1, and Mec-3) protein four-and-a-half LIM domains 2 (FHL2) between FAs and the nucleus depends on matrix mechanics. In particular, on soft surfaces or after the loss of force, FHL2 moves from FAs into the nucleus and concentrates at RNA polymerase (Pol) II sites, where it acts as a transcriptional cofactor, causing an increase in p21 gene expression that will inhibit growth on soft surfaces. At the molecular level, shuttling requires a specific tyrosine in FHL2, as well as phosphorylation by active FA kinase (FAK). Thus, we suggest that FHL2 phosphorylation by FAK is a critical, mechanically dependent step in signaling from soft matrices to the nucleus to inhibit cell proliferation by increasing p21 expression.

Kank2 activates talin, reduces force transduction across integrins and induces central adhesion formation

Integrin-based adhesions play critical roles in cell migration. Talin activates integrins and flexibly connects integrins to the actomyosin cytoskeleton, thereby serving as a 'molecular clutch' that transmits forces to the extracellular matrix to drive cell migration. Here we identify the evolutionarily conserved Kank protein family as novel components of focal adhesions (FAs). Kank proteins accumulate at the lateral border of FAs, which we term the FA belt, and in central sliding adhesions, where they directly bind the talin rod domain through the Kank amino-terminal (KN) motif and induce talin and integrin activation. In addition, Kank proteins diminish the talin-actomyosin linkage, which curbs force transmission across integrins, leading to reduced integrin-ligand bond strength, slippage between integrin and ligand, central adhesion formation and sliding, and reduced cell migration speed. Our data identify Kank proteins as talin activators that decrease the grip between the integrin-talin complex and actomyosin to regulate cell migration velocity.

In-situ coupling between kinase activities and protein dynamics within single focal adhesions

The dynamic activation of oncogenic kinases and regulation of focal adhesions (FAs) are crucial molecular events modulating cell adhesion in cancer metastasis. However, it remains unclear how these events are temporally coordinated at single FA sites. Therefore, we targeted fluorescence resonance energy transfer (FRET)-based biosensors toward subcellular FAs to report local molecular events during cancer cell adhesion. Employing single FA tracking and cross-correlation analysis, we quantified the dynamic coupling characteristics between biochemical kinase activities and structural FA within single FAs. We show that kinase activations and FA assembly are strongly and sequentially correlated, with the concurrent FA assembly and Src activation leading focal adhesion kinase (FAK) activation by 42.6 ± 12.6 sec. Strikingly, the temporal coupling between kinase activation and individual FA assembly reflects the fate of FAs at later stages. The FAs with a tight coupling tend to grow and mature, while the less coupled FAs likely disassemble. During FA disassembly, however, kinase activations lead the disassembly, with FAK being activated earlier than Src. Therefore, by integrating subcellularly targeted FRET biosensors and computational analysis, our study reveals intricate interplays between Src and FAK in regulating the dynamic life of single FAs in cancer cells.

Endothelial Cells Use a Formin-Dependent Phagocytosis-Like Process to Internalize the Bacterium Listeria monocytogenes

Vascular endothelial cells act as gatekeepers that protect underlying tissue from blood-borne toxins and pathogens. Nevertheless, endothelial cells are able to internalize large fibrin clots and apoptotic debris from the bloodstream, although the precise mechanism of such phagocytosis-like uptake is unknown. We show that cultured primary human endothelial cells (HUVEC) internalize both pathogenic and non-pathogenic Listeria bacteria comparably, in a phagocytosis-like process. In contrast with previously studied host cell types, including intestinal epithelial cells and hepatocytes, we find that endothelial internalization of Listeria is independent of all known pathogenic bacterial surface proteins. Consequently, we exploited the internalization and intracellular replication of L. monocytogenes to identify distinct host cell factors that regulate phagocytosis-like uptake in HUVEC. Using siRNA screening and subsequent genetic and pharmacologic perturbations, we determined that endothelial infectivity was modulated by cytoskeletal proteins that normally modulate global architectural changes, including phosphoinositide-3-kinase, focal adhesions, and the small GTPase Rho. We found that Rho kinase (ROCK) is acutely necessary for adhesion of Listeria to endothelial cells, whereas the actin-nucleating formins FHOD1 and FMNL3 specifically regulate internalization of bacteria as well as inert beads, demonstrating that formins regulate endothelial phagocytosis-like uptake independent of the specific cargo. Finally, we found that neither ROCK nor formins were required for macrophage phagocytosis of L. monocytogenes, suggesting that endothelial cells have distinct requirements for bacterial internalization from those of classical professional phagocytes. Our results identify a novel pathway for L. monocytogenes uptake by human host cells, indicating that this wily pathogen can invade a variety of tissues by using a surprisingly diverse suite of distinct uptake mechanisms that operate differentially in different host cell types.

F-actin waves, actin cortex disassembly and focal exocytosis driven by actin-phosphoinositide positive feedback

Actin polymerization is controlled by the phosphoinositide composition of the plasma membrane. However, the molecular mechanisms underlying the spatiotemporal regulation of actin network organization over extended length scales are still unclear. To observe phosphoinositide-dependent cytoskeletal dynamics we combined the model system of frustrated phagocytosis, total internal reflection microscopy and manipulation of the buffer tonicity. We found that macrophages interacting with IgG-coated glass substrates formed circular F-actin waves on their ventral surface enclosing a region of plasma membrane devoid of cortical actin. Plasma membrane free of actin cortex was strongly depleted of PI(4,5)P2 , but enriched in PI(3,4)P2 and displayed a fivefold increase in exocytosis. Wave formation could be promoted by application of a hypotonic shock. The actin waves were characteristic of a bistable wavefront at the boundary between the regions of membrane containing and lacking cortical actin. Phosphoinositide modifiers and RhoGTPase activities dramatically redistributed with respect to the wavefronts, which often exhibited spatial oscillations. Perturbation of either lipid or actin cytoskeleton-related pathways led to rapid loss of both the polarized lipid distribution and the wavefront. As waves travelled over the plasma membrane, wavefront actin was seen to rapidly polymerize and depolymerize at pre-existing clusters of FcγRIIA, coincident with rapid changes in lipid composition. Thus the potential of receptors to support rapid F-actin polymerization appears to depend acutely on the local concentrations of multiple lipid species. We propose that interdependence through positive feedback from the cytoskeleton to lipid modifiers leads to coordinated local cortex remodeling, focal exocytosis, and organizes extended actin networks.

Adhesion protein networks reveal functions proximal and distal to cell-matrix contacts

Cell adhesion to the extracellular matrix is generally mediated by integrin receptors, which bind to intracellular adhesion proteins that form multi-molecular scaffolding and signalling complexes. The networks of proteins, and their interactions, are dynamic, mechanosensitive and extremely complex. Recent efforts to characterise adhesions using a variety of technologies, including imaging, proteomics and bioinformatics, have provided new insights into their composition, organisation and how they are regulated, and have also begun to reveal unexpected roles for so-called adhesion proteins in other cellular compartments (for example, the nucleus or centrosomes) in diseases such as cancer. We believe this is opening a new chapter on understanding the wider functions of adhesion proteins, both proximal and distal to cell-matrix contacts.

The FAK-Arp2/3 interaction promotes leading edge advance and haptosensing by coupling nascent adhesions to lamellipodia actin

Focal adhesion kinase (FAK) is an important regulator of focal adhesion dynamics during cell migration. Distinct functions of FAK—kinase activation and Arp2/3 binding—enable cells to mechanosense or haptotax during spreading and migration.

Cell migration is initiated in response to biochemical or physical cues in the environment that promote actin-mediated lamellipodial protrusion followed by the formation of nascent integrin adhesions (NAs) within the protrusion to drive leading edge advance. Although FAK is known to be required for cell migration through effects on focal adhesions, its role in NA formation and lamellipodial dynamics is unclear. Live-cell microscopy of FAK^−/− cells with expression of phosphorylation deficient or a FERM-domain mutant deficient in Arp2/3 binding revealed a requirement for FAK in promoting the dense formation, transient stabilization, and timely turnover of NA within lamellipodia to couple actin-driven protrusion to adhesion and advance of the leading edge. Phosphorylation on Y397 of FAK promotes dense NA formation but is dispensable for transient NA stabilization and leading edge advance. In contrast, transient NA stabilization and advance of the cell edge requires FAK–Arp2/3 interaction, which promotes Arp2/3 localization to NA and reduces FAK activity. Haptosensing of extracellular matrix (ECM) concentration during migration requires the interaction between FAK and Arp2/3, whereas FAK phosphorylation modulates mechanosensing of ECM stiffness during spreading. Taken together, our results show that mechanistically separable functions of FAK in NA are required for cells to distinguish distinct properties of their environment during migration.

Conformational coupling of integrin and Thy-1 regulates Fyn priming and fibroblast mechanotransduction

Lateral associations between inactive α_v integrin and Thy-1 glycoprotein control integrin avidity to extracellular matrix ligand, the localization and kinetics of downstream signal activity, and mechanosensitive remodeling of the cytoskeleton.

Progressive fibrosis is characterized by excessive deposition of extracellular matrix (ECM), resulting in gross alterations in tissue mechanics. Changes in tissue mechanics can further augment scar deposition through fibroblast mechanotransduction. In idiopathic pulmonary fibrosis, a fatal form of progressive lung fibrosis, previous work has shown that loss of Thy-1 (CD90) expression in fibroblasts correlates with regions of active fibrogenesis, thus representing a pathologically relevant fibroblast subpopulation. We now show that Thy-1 is a regulator of fibroblast rigidity sensing. Thy-1 physically couples to inactive α_vβ₃ integrins via its RGD-like motif, altering baseline integrin avidity to ECM ligands and also facilitating preadhesion clustering of integrin and membrane rafts via Thy-1’s glycophosphatidylinositol tether. Disruption of Thy-1–α_vβ₃ coupling altered recruitment of Src family kinases to adhesion complexes and impaired mechanosensitive, force-induced Rho signaling, and rigidity sensing. Loss of Thy-1 was sufficient to induce myofibroblast differentiation in soft ECMs and may represent a physiological mechanism important in wound healing and fibrosis.

Specification of Architecture and Function of Actin Structures by Actin Nucleation Factors

The actin cytoskeleton is essential for diverse processes in mammalian cells; these processes range from establishing cell polarity to powering cell migration to driving cytokinesis to positioning intracellular organelles. How these many functions are carried out in a spatiotemporally regulated manner in a single cytoplasm has been the subject of much study in the cytoskeleton field. Recent work has identified a host of actin nucleation factors that can build architecturally diverse actin structures. The biochemical properties of these factors, coupled with their cellular location, likely define the functional properties of actin structures. In this article, we describe how recent advances in cell biology and biochemistry have begun to elucidate the role of individual actin nucleation factors in generating distinct cellular structures. We also consider how the localization and orientation of actin nucleation factors, in addition to their kinetic properties, are critical to their ability to build a functional actin cytoskeleton.

α5β1 integrin recycling promotes Arp2/3-independent cancer cell invasion via the formin FHOD3

Rab-coupling protein–mediated integrin trafficking promotes filopodia formation via RhoA-ROCK-FHOD3, generating non-lamellipodial actin spike protrusions that drive cancer cell migration in 3D extracellular matrix and in vivo.

Invasive migration in 3D extracellular matrix (ECM) is crucial to cancer metastasis, yet little is known of the molecular mechanisms that drive reorganization of the cytoskeleton as cancer cells disseminate in vivo. 2D Rac-driven lamellipodial migration is well understood, but how these features apply to 3D migration is not clear. We find that lamellipodia-like protrusions and retrograde actin flow are indeed observed in cells moving in 3D ECM. However, Rab-coupling protein (RCP)-driven endocytic recycling of α5β1 integrin enhances invasive migration of cancer cells into fibronectin-rich 3D ECM, driven by RhoA and filopodial spike-based protrusions, not lamellipodia. Furthermore, we show that actin spike protrusions are Arp2/3-independent. Dynamic actin spike assembly in cells invading in vitro and in vivo is regulated by Formin homology-2 domain containing 3 (FHOD3), which is activated by RhoA/ROCK, establishing a novel mechanism through which the RCP–α5β1 pathway reprograms the actin cytoskeleton to promote invasive migration and local invasion in vivo.

Nascent Integrin Adhesions Form on All Matrix Rigidities after Integrin Activation

Integrin adhesions assemble and mature in response to ligand binding and mechanical factors, but the molecular-level organization is not known. We report that ∼100-nm clusters of ∼50 β3-activated integrins form very early adhesions under a wide variety of conditions on RGD surfaces. These adhesions form similarly on fluid and rigid substrates, but most adhesions are transient on rigid substrates. Without talin or actin polymerization, few early adhesions form, but expression of either the talin head or rod domain in talin-depleted cells restores early adhesion formation. Mutation of the integrin binding site in the talin rod decreases cluster size. We suggest that the integrin clusters constitute universal early adhesions and that they are the modular units of cell matrix adhesions. They require the association of activated integrins with cytoplasmic proteins, in particular talin and actin, and cytoskeletal contraction on them causes adhesion maturation for cell motility and growth.

Integrin-beta3 clusters recruit clathrin-mediated endocytic machinery in the absence of traction force

The turnover of integrin receptors is critical for cell migration and adhesion dynamics. Here we find that force development at integrins regulates adaptor protein recruitment and endocytosis. Using mobile RGD (Arg-Gly-Asp) ligands on supported lipid membranes (RGD membranes) and rigid RGD ligands on glass (RGD-glass), we find that matrix force-dependent integrin signals block endocytosis. Dab2, an adaptor protein of clathrin-mediated endocytosis, is not recruited to activated integrin-beta3 clusters on RGD-glass; however, it is recruited to integrin-mediated adhesions on RGD membranes. Further, when force generation is inhibited on RGD-glass, Dab2 binds to integrin-beta3 clusters. Dab2 binding to integrin-beta3 excludes other adhesion-related adaptor proteins, such as talin. The clathrin-mediated endocytic machinery combines with Dab2 to facilitate the endocytosis of RGD-integrin-beta3 clusters. From these observations, we propose that loss of traction force on ligand-bound integrin-beta3 causes recruitment of Dab2/clathrin, resulting in endocytosis of integrins.

Micropatterning of TCR and LFA-1 ligands reveals complementary effects on cytoskeleton mechanics in T cells

The formation of the immunological synapse between a T cell and the antigen-presenting cell (APC) is critically dependent on actin dynamics, downstream of T cell receptor (TCR) and integrin (LFA-1) signalling. There is also accumulating evidence that mechanical forces, generated by actin polymerization and/or myosin contractility regulate T cell signalling. Because both receptor pathways are intertwined, their contributions towards the cytoskeletal organization remain elusive. Here, we identify the specific roles of TCR and LFA-1 by using a combination of micropatterning to spatially separate signalling systems and nanopillar arrays for high-precision analysis of cellular forces. We identify that Arp2/3 acts downstream of TCRs to nucleate dense actin foci but propagation of the network requires LFA-1 and the formin FHOD1. LFA-1 adhesion enhances actomyosin forces, which in turn modulate actin assembly downstream of the TCR. Together our data shows a mechanically cooperative system through which ligands presented by an APC modulate T cell activation.

Formin-like 2 Promotes β1-Integrin Trafficking and Invasive Motility Downstream of PKCα

Regulated turnover of integrin receptors is essential for cell adhesion and migration. Pathways selectively regulating β1-integrin recycling are implicated in cancer invasion and metastasis, yet proteins required for the internalization of this pro-invasive integrin remain to be identified. Here, we uncover formin-like 2 (FMNL2) as a critical regulator of β1-integrin internalization downstream of protein kinase C (PKC). PKCα associates with and phosphorylates FMNL2 at S1072 within its Diaphanous autoregulatory region, leading to the release of formin autoinhibition. Phosphorylation of FMNL2 triggers its rapid relocation and promotes its interaction with the cytoplasmic tails of the α-integrin subunits for β1-integrin endocytosis. FMNL2 drives β1-integrin internalization and invasive motility in a phosphorylation-dependent manner, while a FMNL2 mutant defective in actin assembly interferes with β1-integrin endocytosis and cancer cell invasion. Our data establish a role for FMNL2 in the regulation of β1-integrin and provide a mechanistic understanding of the function of FMNL2 in cancer invasiveness.

FHOD proteins in actin dynamics--a formin' class of its own

Eukaryotic cells have evolved a variety of actin-binding proteins to regulate the architecture and the dynamics of the actin cytoskeleton in time and space. The Diaphanous-related formins (DRF) represent a diverse group of Rho-GTPase-regulated actin regulators that control a range of actin structures composed of tightly-bundled, unbranched actin filaments as found in stress fibers and in filopodia. Under resting conditions, DRFs are auto-inhibited by an intra-molecular interaction between the C-terminal and the N-terminal domains. The auto-inhibition is thought to be released by binding of an activated RhoGTPase to the N-terminal GTPase-binding domain (GBD). However, there is growing evidence for more sophisticated variations from this simplified linear activation model. In this review we focus on the formin homology domain-containing proteins (FHOD), an unconventional group of DRFs. Recent findings on the molecular control and cellular functions of FHOD proteins in vivo are discussed in the light of the phylogeny of FHOD proteins.

Functional hierarchy of redundant actin assembly factors revealed by fine-grained registration of intrinsic image fluctuations

Highly redundant pathways often contain components whose functions are difficult to decipher from the responses induced by genetic or molecular perturbations. Here, we present a statistical approach that samples and registers events observed in images of intrinsic fluctuations in unperturbed cells to establish the functional hierarchy of events in systems with redundant pathways. We apply this approach to study the recruitment of actin assembly factors involved in the protrusion of the cell membrane. We find that the formin mDia1, along with nascent adhesion components, is recruited to the leading edge of the cell before protrusion onset, initiating linear growth of the lamellipodial network. Recruitment of Arp2/3, VASP, cofilin, and the formin mDia2 then promotes sustained exponential growth of the network. Experiments changing membrane tension suggest that Arp2/3 recruitment is mechano-responsive. These results indicate that cells adjust the overlapping contributions of multiple factors to actin filament assembly during protrusion on a ten-second timescale and in response to mechanical cues.

Reinforcing the LINC complex connection to actin filaments: the role of FHOD1 in TAN line formation and nuclear movement

Positioning the nucleus is critical for many cellular processes including cell division, migration and differentiation. The linker of nucleoskeleton and cytoskeleton (LINC) complex spans the inner and outer nuclear membranes and has emerged as a major factor in connecting the nucleus to the cytoskeleton for movement and positioning. Recently, we discovered that the diaphanous formin family member FHOD1 interacts with the LINC complex component nesprin-2 giant (nesprin-2G) and that this interaction plays essential roles in the formation of transmembrane actin-dependent nuclear (TAN) lines and nuclear movement during cell polarization in fibroblasts. We found that FHOD1 strengthens the connection between nesprin-2G and rearward moving dorsal actin cables by providing a second site of interaction between nesprin-2G and the actin cable. These results indicate that the LINC complex connection to the actin cytoskeleton can be enhanced by cytoplasmic factors and suggest a new model for TAN line formation. We discuss how the nesprin-2G-FHOD1 interaction may be regulated and its possible functional significance for development and disease.

SMIFH2 has effects on Formins and p53 that perturb the cell cytoskeleton

Formin proteins are key regulators of the cytoskeleton involved in developmental and homeostatic programs, and human disease. For these reasons, small molecules interfering with Formins' activity have gained increasing attention. Among them, small molecule inhibitor of Formin Homology 2 domains (SMIFH2) is often used as a pharmacological Formin blocker. Although SMIFH2 inhibits actin polymerization by Formins and affects the actin cytoskeleton, its cellular mechanism of action and target specificity remain unclear. Here we show that SMIFH2 induces remodelling of actin filaments, microtubules and the Golgi complex as a result of its effects on Formins and p53. We found that SMIFH2 triggers alternated depolymerization-repolymerization cycles of actin and tubulin, increases cell migration, causes scattering of the Golgi complex, and also cytotoxicity at high dose. Moreover, SMIFH2 reduces expression and activity of p53 through a post-transcriptional, proteasome-independent mechanism that influences remodelling of the cytoskeleton. As the action of SMIFH2 may go beyond Formin inhibition, only short-term and low-dose SMIFH2 treatments minimize confounding effects induced by loss of p53 and cytotoxicity.

The formin FHOD1 in cardiomyocytes

Members of the formin family are known to be involved in the regulation of the actin cytoskeleton. We have recently identified a muscle specific splice variant of the formin FHOD3 and demonstrated its role in the maintenance of the contractile filaments of cardiomyocytes. Here, we characterize the expression and subcellular localization of FHOD3's closest relative, FHOD1, in the heart. Confocal microscopy shows that FHOD1 is mainly located at the intercalated disc, the special type of cell-cell contact between cardiomyocytes, but also partially associated with the myofibrils. Subcellular targeting of FHOD1 is probably mediated by its N-terminal domain, since expression constructs lacking this domain show aberrant localization in primary cultures of neonatal rat cardiomyocytes. Finally, we show that in contrast to FHOD3, FHOD1 shows increased expression levels in dilated cardiomyopathy, suggesting that the two formins play distinct roles and are differentially regulated in cardiomyocytes.

Distribution of formins in cardiac muscle: FHOD1 is a component of intercalated discs and costameres

The formin homology domain-containing protein1 (FHOD1) suppresses actin polymerization by inhibiting nucleation, but bundles actin filaments and caps filament barbed ends. Two polyclonal antibodies against FHOD1 were generated against (i) its N-terminal sequence (residues 1-339) and (ii) a peptide corresponding the sequence from position 358-371, which is unique for FHOD1 and does not occur in its close relative FHOD3. After affinity purification both antibodies specifically stain purified full length FHOD1 and a band of similar molecular mass in homogenates of cardiac muscle. The antibody against the N-terminus of FHOD1 was used for immunostaining cells of established lines, primary neonatal (NRC) and adult (ARC) rat cardiomyocytes and demonstrated the presence of FHOD1 in HeLa and fibroblastic cells along stress fibers and within presumed lamellipodia and actin arcs. In NRCs and ARCs we observed a prominent staining of presumed intercalated discs (ICD). Immunostaining of sections of hearts with both anti-FHOD1 antibodies confirmed the presence of FHOD1 in ICDs and double immunostaining demonstrated its colocalisation with cadherin, plakoglobin and a probably slightly shifted localization to connexin43. Similarly, immunostaining of isolated mouse or pig ICDs corroborated the presence of FHOD1 and its colocalisation with the mentioned cell junctional components. Anti-FHOD1 immunoblots of isolated ICDs demonstrated the presence of an immunoreactive band comigrating with purified FHOD1. Conversely, an anti-peptide antibody specific for FHOD3 with no cross-reactivity against FHOD1 immunostained on sections of cardiac muscle and ARCs the myofibrils in a cross-striated pattern but not the ICDs. In addition, the anti-peptide-FHOD1 antibody stained the lateral sarcolemma of ARCs in a banded pattern. Double immunostaining with anti-cadherin and -integrin-ß1 indicated the additional localization of FHOD1 in costameres. Immunostaining of cardiac muscle sections or ARCs with antibodies against mDia3-FH2-domain showed colocalisation with cadherin along the lateral border of cardiomyocytes suggesting also its presence in costameres.

Early events in cell spreading as a model for quantitative analysis of biomechanical events

In this review, we focus on the early events in the process of fibroblast spreading on fibronectin matrices of different rigidities. We present a focused position piece that illustrates the many different tests that a cell makes of its environment before it establishes mature matrix adhesions. When a fibroblast is placed on fibronectin-coated glass surfaces at 37°C, it typically spreads and polarizes within 20-40 min primarily through αvβ3 integrin binding to fibronectin. In that short period, the cell goes through three major phases that involve binding, integrin activation, spreading, and mechanical testing of the surface. The advantage of using the model system of cell spreading from the unattached state is that it is highly reproducible and the stages that the cell undergoes can thus be studied in a highly quantitative manner, in both space and time. The mechanical and biochemical parameters that matter in this example are often surprising because of both the large number of tests that occur and the precision of the tests. We discuss our current understanding of those tests, the decision tree that is involved in this process, and an extension to the behavior of the cells at longer time periods when mature adhesions develop. Because many other matrices and integrins are involved in cell-matrix adhesion, this model system gives us a limited view of a subset of cellular behaviors that can occur. However, by defining one cellular process at a molecular level, we know more of what to expect when defining other processes. Because each cellular process will involve some different proteins, a molecular understanding of multiple functions operating within a given cell can lead to strategies to selectively block a function.

Appreciating force and shape—the rise of mechanotransduction in cell biology

Although the shapes of organisms are encoded in their genome, the developmental processes that lead to the final form of vertebrates involve a constant feedback between dynamic mechanical forces, and cell growth and motility. Mechanobiology has emerged as a discipline dedicated to the study of the effects of mechanical forces and geometry on cell growth and motility—for example, during cell-matrix adhesion development—through the signalling process of mechanotransduction.

Formins as effector proteins of Rho GTPases

Formin proteins were recognized as effectors of Rho GTPases some 15 years ago. They contribute to different cellular actin cytoskeleton structures by their ability to polymerize straight actin filaments at the barbed end. While not all formins necessarily interact with Rho GTPases, a subgroup of mammalian formins, termed Diaphanous-related formins or DRFs, were shown to be activated by small GTPases of the Rho superfamily. DRFs are autoinhibited in the resting state by an N- to C-terminal interaction that renders the central actin polymerization domain inactive. Upon the interaction with a GTP-bound Rho, Rac, or Cdc42 GTPase, the C-terminal autoregulation domain is displaced from its N-terminal recognition site and the formin becomes active to polymerize actin filaments. In this review we discuss the current knowledge on the structure, activation, and function of formin-GTPase interactions for the mammalian formin families Dia, Daam, FMNL, and FHOD. We describe both direct and indirect interactions of formins with GTPases, which lead to formin activation and cytoskeletal rearrangements. The multifaceted function of formins as effector proteins of Rho GTPases thus reflects the diversity of the actin cytoskeleton in cells.

FHOD1 interaction with nesprin-2G mediates TAN line formation and nuclear movement

Active positioning of the nucleus is integral to division, migration, and differentiation of mammalian cells¹. Fibroblasts polarizing for migration orient their centrosomes by actin-dependent nuclear movement². This nuclear movement depends on nesprin-2 giant (N2G), a large, actin-binding outer nuclear membrane component of transmembrane actin-associated (TAN) lines that couple nuclei to moving actin cables³. Here, we identify the diaphanous formin FHOD1 as an interaction partner of N2G. Silencing FHOD1 expression or expression of fragments containing binding sites of N2G or FHOD1 disrupted nuclear movement and centrosome orientation in polarizing fibroblasts. Unexpectedly, silencing of FHOD1 expression did not affect the formation or rearward flow of dorsal actin cables required for nuclear positioning. Rather, N2G-FHOD1 interaction provided a second connection to actin cables essential for TAN line formation and thus nuclear movement. These results reveal a unique function for a formin in coupling an organelle to actin filaments for translocation and suggest that TAN lines require multi-point attachments to actin cables to resist the large forces necessary to move the nucleus.

A formin-g role during development and disease

Several different protein families were shown to be involved in the regulation of actin filament formation and have been studied extensively in processes such as cell migration. Among them are members of the formin family, which tend to promote the formation of linear actin filaments. Studies in recent years, often using loss of function animal models, have indicated that formin family members play roles beyond cell motility in vitro and are involved in processes ranging from tissue morphogenesis and cell differentiation to diseases such as cancer and cardiomyopathy. Therefore the aim of this review is to discuss these findings and to start putting them into a subcellular context.

The Drosophila FHOD1-like formin Knittrig acts through Rok to promote stress fiber formation and directed macrophage migration during the cellular immune response

A tight spatiotemporal control of actin polymerization is important for many cellular processes that shape cells into a multicellular organism. The formation of unbranched F-actin is induced by several members of the formin family. Drosophila encodes six formin genes, representing six of the seven known mammalian subclasses. Knittrig, the Drosophila homolog of mammalian FHOD1, is specifically expressed in the developing central nervous system midline glia, the trachea, the wing and in macrophages. knittrig mutants exhibit mild tracheal defects but survive until late pupal stages and mainly die as pharate adult flies. knittrig mutant macrophages are smaller and show reduced cell spreading and cell migration in in vivo wounding experiments. Rescue experiments further demonstrate a cell-autonomous function of Knittrig in regulating actin dynamics and cell migration. Knittrig localizes at the rear of migrating macrophages in vivo, suggesting a cellular requirement of Knittrig in the retraction of the trailing edge. Supporting this notion, we found that Knittrig is a target of the Rho-dependent kinase Rok. Co-expression with Rok or expression of an activated form of Knittrig induces actin stress fibers in macrophages and in epithelial tissues. Thus, we propose a model in which Rok-induced phosphorylation of residues within the basic region mediates the activation of Knittrig in controlling macrophage migration.