The CUL5 E3 ligase complex negatively regulates central signaling pathways in CD8+ T cells

CD8+ T cells play an important role in anti-tumor immunity. Better understanding of their regulation could advance cancer immunotherapies. Here we identify, via stepwise CRISPR-based screening, that CUL5 is a negative regulator of the core signaling pathways of CD8+ T cells. Knocking out CUL5 in mouse CD8+ T cells significantly improves their tumor growth inhibiting ability, with significant proteomic alterations that broadly enhance TCR and cytokine signaling and their effector functions. Chemical inhibition of neddylation required by CUL5 activation, also enhances CD8 effector activities with CUL5 validated as a major target. Mechanistically, CUL5, which is upregulated by TCR stimulation, interacts with the SOCS-box-containing protein PCMTD2 and inhibits TCR and IL2 signaling. Additionally, CTLA4 is markedly upregulated by CUL5 knockout, and its inactivation further enhances the anti-tumor effect of CUL5 KO. These results together reveal a negative regulatory mechanism for CD8+ T cells and have strong translational implications in cancer immunotherapy.

Intracellular tension sensor reveals mechanical anisotropy of the actin cytoskeleton

The filamentous actin (F-actin) cytoskeleton is a composite material consisting of cortical actin and bundled F-actin stress fibers, which together mediate the mechanical behaviors of the cell, from cell division to cell migration. However, as mechanical forces are typically measured upon transmission to the extracellular matrix, the internal distribution of forces within the cytoskeleton is unknown. Likewise, how distinct F-actin architectures contribute to the generation and transmission of mechanical forces is unclear. Therefore, we have developed a molecular tension sensor that embeds into the F-actin cytoskeleton. Using this sensor, we measure tension within stress fibers and cortical actin, as the cell is subject to uniaxial stretch. We find that the mechanical response, as measured by FRET, depends on the direction of applied stretch relative to the cell's axis of alignment. When the cell is aligned parallel to the direction of the stretch, stress fibers and cortical actin both accumulate tension. By contrast, when aligned perpendicular to the direction of stretch, stress fibers relax tension while the cortex accumulates tension, indicating mechanical anisotropy within the cytoskeleton. We further show that myosin inhibition regulates this anisotropy. Thus, the mechanical anisotropy of the cell and the coordination between distinct F-actin architectures vary and depend upon applied load.

Setdb1 -loss induces type-I interferons and immune clearance of melanoma

Despite recent advances in the treatment of melanoma, many patients with metastatic disease still succumb to their disease. To identify tumor-intrinsic modulators of immunity to melanoma, we performed a whole-genome CRISPR screen in melanoma and identified multiple components of the HUSH complex, including Setdb1 , as hits. We found that loss of Setdb1 leads to increased immunogenicity and complete tumor clearance in a CD8+ T-cell dependent manner. Mechanistically, loss of Setdb1 causes de-repression of endogenous retroviruses (ERVs) in melanoma cells and triggers tumor-cell intrinsic type-I interferon signaling, upregulation of MHC-I expression, and increased CD8+ T-cell infiltration. Furthermore, spontaneous immune clearance observed in Setdb1 ^-/- tumors results in subsequent protection from other ERV-expressing tumor lines, supporting the functional anti-tumor role of ERV-specific CD8+ T-cells found in the Setdb1 ^-/- microenvironment. Blocking the type-I interferon receptor in mice grafted with Setdb1 ^-/- tumors decreases immunogenicity by decreasing MHC-I expression, leading to decreased T-cell infiltration and increased melanoma growth comparable to Setdb1 ^wt tumors. Together, these results indicate a critical role for Setdb1 and type-I interferons in generating an inflamed tumor microenvironment, and potentiating tumor-cell intrinsic immunogenicity in melanoma. This study further emphasizes regulators of ERV expression and type-I interferon expression as potential therapeutic targets for augmenting anti-cancer immune responses.

Fibroblasts secrete fibronectin under lamellipodia in a microtubule- and myosin II-dependent fashion

Fibronectin (FN) is an essential structural and regulatory component of the extracellular matrix (ECM), and its binding to integrin receptors supports cell adhesion, migration, and signaling. Here, using live-cell microscopy of fibroblasts expressing FN tagged with a pH-sensitive fluorophore, we show that FN is secreted predominantly at the ventral surface of cells in an integrin-independent manner. Locally secreted FN then undergoes β1 integrin-dependent fibrillogenesis. We find that the site of FN secretion is regulated by cell polarization, which occurs in bursts under stabilized lamellipodia at the leading edge. Moreover, analysis of FN secretion and focal adhesion dynamics suggest that focal adhesion formation precedes FN deposition and that deposition continues during focal adhesion disassembly. Lastly, we show that the polarized FN deposition in spreading and migrating cells requires both intact microtubules and myosin II-mediated contractility. Thus, while FN secretion does not require integrin binding, the site of exocytosis is regulated by membrane and cytoskeletal dynamics with secretion occurring after new adhesion formation.

Organization, dynamics and mechanoregulation of integrin-mediated cell-ECM adhesions

The ability of animal cells to sense, adhere to and remodel their local extracellular matrix (ECM) is central to control of cell shape, mechanical responsiveness, motility and signalling, and hence to development, tissue formation, wound healing and the immune response. Cell-ECM interactions occur at various specialized, multi-protein adhesion complexes that serve to physically link the ECM to the cytoskeleton and the intracellular signalling apparatus. This occurs predominantly via clustered transmembrane receptors of the integrin family. Here we review how the interplay of mechanical forces, biochemical signalling and molecular self-organization determines the composition, organization, mechanosensitivity and dynamics of these adhesions. Progress in the identification of core multi-protein modules within the adhesions and characterization of rearrangements of their components in response to force, together with advanced imaging approaches, has improved understanding of adhesion maturation and turnover and the relationships between adhesion structures and functions. Perturbations of adhesion contribute to a broad range of diseases and to age-related dysfunction, thus an improved understanding of their molecular nature may facilitate therapeutic intervention in these conditions.

Use of Ecto-Tagged Integrins to Monitor Integrin Exocytosis and Endocytosis

Controlled exocytosis and endocytosis of integrin adhesion receptors is required for normal cell adhesion, migration, and signaling. In this chapter, we describe the design of functional β1 integrins carrying extracellular fluorescent or chemically traceable tags (ecto-tag) and methods for their use to image β1 integrin trafficking in cells. We provide approaches to generate cells in which endogenous β1 integrins are replaced by ecto-tagged integrins containing a pH-sensitive fluorophore pHluorin or a HaloTag and describe strategies using photobleaching, selective extracellular/intracellular labeling, and chase, quenching, and blocking to reveal β1 integrin exocytosis, endocytosis, and recycling by live total internal reflection fluorescence (TIRF) microscopy.

Tousled-like kinase 2 targets ASF1 histone chaperones through client mimicry

Tousled-like kinases (TLKs) are nuclear serine-threonine kinases essential for genome maintenance and proper cell division in animals and plants. A major function of TLKs is to phosphorylate the histone chaperone proteins ASF1a and ASF1b to facilitate DNA replication-coupled nucleosome assembly, but how TLKs selectively target these critical substrates is unknown. Here, we show that TLK2 selectivity towards ASF1 substrates is achieved in two ways. First, the TLK2 catalytic domain recognizes consensus phosphorylation site motifs in the ASF1 C-terminal tail. Second, a short sequence at the TLK2 N-terminus docks onto the ASF1a globular N-terminal domain in a manner that mimics its histone H3 client. Disrupting either catalytic or non-catalytic interactions through mutagenesis hampers ASF1 phosphorylation by TLK2 and cell growth. Our results suggest that the stringent selectivity of TLKs for ASF1 is enforced by an unusual interaction mode involving mutual recognition of a short sequence motifs by both kinase and substrate.

Tousled-like kinase 2 (TLK2) phosphorylates ASF1 histone chaperones to promote nucleosome assembly in S phase. Here, the authors show that TLK2 targets ASF1 by simulating its client protein histone H3, exploiting a primordial protein interaction surface for regulatory control.

A Small-Scale shRNA Screen in Primary Mouse Macrophages Identifies a Role for the Rab GTPase Rab1b in Controlling Salmonella Typhi Growth

Salmonella Typhi is a human-restricted bacterial pathogen that causes typhoid fever, a life-threatening systemic infection. A fundamental aspect of S. Typhi pathogenesis is its ability to survive in human macrophages but not in macrophages from other animals (i.e. mice). Despite the importance of macrophages in establishing systemic S. Typhi infection, the mechanisms that macrophages use to control the growth of S. Typhi and the role of these mechanisms in the bacterium's adaptation to the human host are mostly unknown. To facilitate unbiased identification of genes involved in controlling the growth of S. Typhi in macrophages, we report optimized experimental conditions required to perform loss-of function pooled shRNA screens in primary mouse bone-marrow derived macrophages. Following infection with a fluorescent-labeled S. Typhi, infected cells are sorted based on the intensity of fluorescence (i.e. number of intracellular fluorescent bacteria). shRNAs enriched in the fluorescent population are identified by next-generation sequencing. A proof-of-concept screen targeting the mouse Rab GTPases confirmed Rab32 as important to restrict S. Typhi in mouse macrophages. Interestingly and rather unexpectedly, this screen also revealed that Rab1b controls S. Typhi growth in mouse macrophages. This constitutes the first report of a Rab GTPase other than Rab32 involved in S. Typhi host-restriction. The methodology described here should allow genome-wide screening to identify mechanisms controlling the growth of S. Typhi and other intracellular pathogens in primary immune cells.

PPP6C negatively regulates oncogenic ERK signaling through dephosphorylation of MEK

Flux through the RAF-MEK-ERK protein kinase cascade is shaped by phosphatases acting on the core components of the pathway. Despite being an established drug target and a hub for crosstalk regulation, little is known about dephosphorylation of MEK, the central kinase within the cascade. Here, we identify PPP6C, a phosphatase frequently mutated or downregulated in melanoma, as a major MEK phosphatase in cells exhibiting oncogenic ERK pathway activation. Recruitment of MEK to PPP6C occurs through an interaction with its associated regulatory subunits. Loss of PPP6C causes hyperphosphorylation of MEK at activating and crosstalk phosphorylation sites, promoting signaling through the ERK pathway and decreasing sensitivity to MEK inhibitors. Recurrent melanoma-associated PPP6C mutations cause MEK hyperphosphorylation, suggesting that they promote disease at least in part by activating the core oncogenic pathway driving melanoma. Collectively, our studies identify a key negative regulator of ERK signaling that may influence susceptibility to targeted cancer therapies.

Through an shRNA screen, Cho et al. identify PPP6C as a phosphatase that inactivates the kinase MEK, sensitizing tumor cells to clinical MEK inhibitors. This study suggests that cancer-associated loss-of-function PPP6C mutations prevalent in melanoma serve to activate the core oncogenic RAF-MEK-ERK pathway that drives the disease.

Signalling through cerebral cavernous malformation protein networks

Cerebral cavernous malformations (CCMs) are neurovascular abnormalities characterized by thin, leaky blood vessels resulting in lesions that predispose to haemorrhages, stroke, epilepsy and focal neurological deficits. CCMs arise due to loss-of-function mutations in genes encoding one of three CCM complex proteins, KRIT1, CCM2 or CCM3. These widely expressed, multi-functional adaptor proteins can assemble into a CCM protein complex and (either alone or in complex) modulate signalling pathways that influence cell adhesion, cell contractility, cytoskeletal reorganization and gene expression. Recent advances, including analysis of the structures and interactions of CCM proteins, have allowed substantial progress towards understanding the molecular bases for CCM protein function and how their disruption leads to disease. Here, we review current knowledge of CCM protein signalling with a focus on three pathways which have generated the most interest—the RhoA–ROCK, MEKK3–MEK5–ERK5–KLF2/4 and cell junctional signalling pathways—but also consider ICAP1-β1 integrin and cdc42 signalling. We discuss emerging links between these pathways and the processes that drive disease pathology and highlight important open questions—key among them is the role of subcellular localization in the control of CCM protein activity.

Differences in self-association between kindlin-2 and kindlin-3 are associated with differential integrin binding

The integrin family of transmembrane adhesion receptors coordinates complex signaling networks that control the ability of cells to sense and communicate with the extracellular environment. Kindlin proteins are a central cytoplasmic component of these networks, directly binding integrin cytoplasmic domains and mediating interactions with cytoskeletal and signaling proteins. The physiological importance of kindlins is well established, but how the scaffolding functions of kindlins are regulated at the molecular level is still unclear. Here, using a combination of GFP nanotrap association assays, pulldown and integrin-binding assays, and live-cell imaging, we demonstrate that full-length kindlins can oligomerize (self-associate) in mammalian cells, and we propose that this self-association inhibits integrin binding and kindlin localization to focal adhesions. We show that both kindlin-2 and kindlin-3 can self-associate and that kindlin-3 self-association is more robust. Using chimeric mapping, we demonstrate that the F2PH and F3 subdomains are important for kindlin self-association. Through comparative sequence analysis of kindlin-2 and kindlin-3, we identify kindlin-3 point mutations that decrease self-association and enhance integrin binding, affording mutant kindlin-3 the ability to localize to focal adhesions. Our results support the notion that kindlin self-association negatively regulates integrin binding.

Chapter 22: Structural and signaling functions of integrins

The integrin family of transmembrane adhesion receptors is essential for sensing and adhering to the extracellular environment. Integrins are heterodimers composed of non-covalently associated α and β subunits that engage extracellular matrix proteins and couple to intracellular signaling and cytoskeletal complexes. Humans have 24 different integrin heterodimers with differing ligand binding specificities and non-redundant functions. Complex structural rearrangements control the ability of integrins to engage ligands and to activate diverse downstream signaling networks, modulating cell adhesion and dynamics, processes which are crucial for metazoan life and development. Here we review the structural and signaling functions of integrins focusing on recent advances which have enhanced our understanding of how integrins are activated and regulated, and the cytoplasmic signaling networks downstream of integrins.

Serine phosphorylation of the small phosphoprotein ICAP1 inhibits its nuclear accumulation

Nuclear accumulation of the small phosphoprotein integrin cytoplasmic domain-associated protein-1 (ICAP1) results in recruitment of its binding partner, Krev/Rap1 interaction trapped-1 (KRIT1), to the nucleus. KRIT1 loss is the most common cause of cerebral cavernous malformation, a neurovascular dysplasia resulting in dilated, thin-walled vessels that tend to rupture, increasing the risk for hemorrhagic stroke. KRIT1's nuclear roles are unknown, but it is known to function as a scaffolding or adaptor protein at cell-cell junctions and in the cytosol, supporting normal blood vessel integrity and development. As ICAP1 controls KRIT1 subcellular localization, presumably influencing KRIT1 function, in this work, we investigated the signals that regulate ICAP1 and, hence, KRIT1 nuclear localization. ICAP1 contains a nuclear localization signal within an unstructured, N-terminal region that is rich in serine and threonine residues, several of which are reportedly phosphorylated. Using quantitative microscopy, we revealed that phosphorylation-mimicking substitutions at Ser-10, or to a lesser extent at Ser-25, within this N-terminal region inhibit ICAP1 nuclear accumulation. Conversely, phosphorylation-blocking substitutions at these sites enhanced ICAP1 nuclear accumulation. We further demonstrate that p21-activated kinase 4 (PAK4) can phosphorylate ICAP1 at Ser-10 both in vitro and in cultured cells and that active PAK4 inhibits ICAP1 nuclear accumulation in a Ser-10-dependent manner. Finally, we show that ICAP1 phosphorylation controls nuclear localization of the ICAP1-KRIT1 complex. We conclude that serine phosphorylation within the ICAP1 N-terminal region can prevent nuclear ICAP1 accumulation, providing a mechanism that regulates KRIT1 localization and signaling, potentially influencing vascular development.

The subcellular localization of type I p21-activated kinases is controlled by the disordered variable region and polybasic sequences

p21-activated kinases (PAKs) are serine/threonine kinase effectors of the small GTPases Rac and Cdc42 and major participants in cell adhesion, motility, and survival. Type II PAKs (PAK4, -5, and -6) are recruited to cell-cell boundaries, where they regulate adhesion dynamics and colony escape. In contrast, the type I PAK, PAK1, does not localize to cell-cell contacts. We have now found that the other type I PAKs (PAK2 and PAK3) also fail to target to cell-cell junctions. PAKs contain extensive similarities in sequence and domain organization; therefore, focusing on PAK1 and PAK6, we used chimeras and truncation mutants to investigate their differences in localization. We observed that a weakly conserved sequence region (the variable region), located between the Cdc42-binding CRIB domain and the kinase domain, inhibits PAK1 targeting to cell-cell junctions. Accordingly, substitution of the PAK1 variable region with that from PAK6 or removal of this region of PAK1 resulted in its localization to cell-cell contacts. We further show that Cdc42 binding is required, but not sufficient, to direct PAKs to cell-cell contacts and that an N-terminal polybasic sequence is necessary for PAK1 recruitment to cell-cell contacts, but only if the variable region-mediated inhibition is released. We propose that all PAKs contain cell-cell boundary-targeting motifs but that the variable region prevents type I PAK accumulation at junctions. This highlights the importance of this poorly conserved, largely disordered region in PAK regulation and raises the possibility that variable region inhibition may be released by cellular signals.

Filamin A mediates isotropic distribution of applied force across the actin network

In this work, Kumar et al. use their previously developed talin tension sensor to study the immediate response of cells to uniaxial stretch. Tension measurements together with high-resolution electron microscopy reveal a novel role for the actin cross-linking protein filamin A in mediating tensional symmetry within the F-actin network.

Cell sensing of externally applied mechanical strain through integrin-mediated adhesions is critical in development and physiology of muscle, lung, tendon, and arteries, among others. We examined the effects of strain on force transmission through the essential cytoskeletal linker talin. Using a fluorescence-based talin tension sensor (TS), we found that uniaxial stretch of cells on elastic substrates increased tension on talin, which was unexpectedly independent of the orientation of the focal adhesions relative to the direction of strain. High-resolution electron microscopy of the actin cytoskeleton revealed that stress fibers (SFs) are integrated into an isotropic network of cortical actin filaments in which filamin A (FlnA) localizes preferentially to points of intersection between SFs and cortical actin. Knockdown (KD) of FlnA resulted in more isolated, less integrated SFs. After FlnA KD, tension on talin was polarized in the direction of stretch, while FlnA reexpression restored tensional symmetry. These data demonstrate that a FlnA-dependent cortical actin network distributes applied forces over the entire cytoskeleton–matrix interface.

Coarse-Grained Simulation of Full-Length Integrin Activation

Integrin conformational dynamics are critical to their receptor and signaling functions in many cellular processes, including spreading, adhesion, and migration. However, assessing integrin conformations is both experimentally and computationally challenging because of limitations in resolution and dynamic sampling. Thus, structural changes that underlie transitions between conformations are largely unknown. Here, focusing on integrin αvβ₃, we developed a modified form of the coarse-grained heterogeneous elastic network model (hENM), which allows sampling conformations at the onset of activation by formally separating local fluctuations from global motions. Both local fluctuations and global motions are extracted from all-atom molecular dynamics simulations of the full-length αvβ₃ bent integrin conformer, but whereas the former are incorporated in the hENM as effective harmonic interactions between groups of residues, the latter emerge by systematically identifying and treating weak interactions between long-distance domains with flexible and anharmonic connections. The new hENM model allows integrins and single-point mutant integrins to explore various conformational states, including the initiation of separation between α- and β-subunit cytoplasmic regions, headpiece extension, and legs opening.

Kindlin-2 interacts with a highly conserved surface of ILK to regulate focal adhesion localization and cell spreading

The integrin-associated adaptor proteins integrin-linked kinase (ILK) and kindlin-2 play central roles in integrin signaling and control of cell morphology. A direct ILK-kindlin-2 interaction is conserved across species and involves the F2PH subdomain of kindlin-2 and the pseudokinase domain (pKD) of ILK. However, complete understanding of the ILK-kindlin-2 interaction and its role in integrin-mediated signaling has been impeded by difficulties identifying the binding site for kindlin-2 on ILK. We used conservation-guided mapping to dissect the interaction between ILK and kindlin-2 and identified a previously unknown binding site for kindlin-2 on the C-lobe of the pKD of ILK. Mutations at this site inhibit binding to kindlin-2 while maintaining structural integrity of the pKD. Importantly, kindlin-binding-defective ILK mutants exhibit impaired focal adhesion localization and fail to fully rescue the spreading defects seen in ILK knockdown cells. Furthermore, kindlin-2 mutants with impaired ILK binding are also unable to fully support cell spreading. Thus, the interaction between ILK and kindlin-2 is critical for cell spreading and focal adhesion localization, representing a key signaling axis downstream of integrins.This article has an associated First Person interview with the first author of the paper.

Structural basis of the filamin A actin-binding domain interaction with F-actin

Actin-cross-linking proteins assemble actin filaments into higher-order structures essential for orchestrating cell shape, adhesion, and motility. Missense mutations in the tandem calponin homology domains of their actin-binding domains (ABDs) underlie numerous genetic diseases, but a molecular understanding of these pathologies is hampered by the lack of high-resolution structures of any actin-cross-linking protein bound to F-actin. Here, taking advantage of a high-affinity, disease-associated mutant of the human filamin A (FLNa) ABD, we combine cryo-electron microscopy and functional studies to reveal at near-atomic resolution how the first calponin homology domain (CH1) and residues immediately N-terminal to it engage actin. We further show that reorientation of CH2 relative to CH1 is required to avoid clashes with actin and to expose F-actin-binding residues on CH1. Our data explain localization of disease-associated loss-of-function mutations to FLNaCH1 and gain-of-function mutations to the regulatory FLNaCH2. Sequence conservation argues that this provides a general model for ABD-F-actin binding.

Novel ecto-tagged integrins reveal their trafficking in live cells

Integrins are abundant heterodimeric cell-surface adhesion receptors essential in multicellular organisms. Integrin function is dynamically modulated by endo-exocytic trafficking, however, major mysteries remain about where, when, and how this occurs in living cells. To address this, here we report the generation of functional recombinant β1 integrins with traceable tags inserted in an extracellular loop. We demonstrate that these ‘ecto-tagged’ integrins are cell-surface expressed, localize to adhesions, exhibit normal integrin activation, and restore adhesion in β1 integrin knockout fibroblasts. Importantly, β1 integrins containing an extracellular pH-sensitive pHluorin tag allow direct visualization of integrin exocytosis in live cells and revealed targeted delivery of integrin vesicles to focal adhesions. Further, using β1 integrins containing a HaloTag in combination with membrane-permeant and -impermeant Halo dyes allows imaging of integrin endocytosis and recycling. Thus, ecto-tagged integrins provide novel powerful tools to characterize integrin function and trafficking.

Integrins are cell-surface adhesion receptors that are modulated by endo-exocytic trafficking, but existing tools to study this process can interfere with function. Here the authors develop β1 integrins carrying traceable tags in the extracellular domain; a pH-sensitive pHlourin tag or a HaloTag to facilitate dye attachment.

Nuclear Localization of Integrin Cytoplasmic Domain-associated Protein-1 (ICAP1) Influences β1 Integrin Activation and Recruits Krev/Interaction Trapped-1 (KRIT1) to the Nucleus

Binding of ICAP1 (integrin cytoplasmic domain-associated protein-1) to the cytoplasmic tails of β1 integrins inhibits integrin activation. ICAP1 also binds to KRIT1 (Krev interaction trapped-1), a protein whose loss of function leads to cerebral cavernous malformation, a cerebrovascular dysplasia occurring in up to 0.5% of the population. We previously showed that KRIT1 functions as a switch for β1 integrin activation by antagonizing ICAP1-mediated inhibition of integrin activation. Here we use overexpression studies, mutagenesis, and flow cytometry to show that ICAP1 contains a functional nuclear localization signal and that nuclear localization impairs the ability of ICAP1 to suppress integrin activation. Moreover, we find that ICAP1 drives the nuclear localization of KRIT1 in a manner dependent upon a direct ICAP1/KRIT1 interaction. Thus, nuclear-cytoplasmic shuttling of ICAP1 influences both integrin activation and KRIT1 localization, presumably impacting nuclear functions of KRIT1.

PAK6 targets to cell-cell adhesions through its N-terminus in a Cdc42-dependent manner to drive epithelial colony escape

The six serine/threonine kinases in the p21-activated kinase (PAK) family are important regulators of cell adhesion, motility and survival. PAK6, which is overexpressed in prostate cancer, was recently reported to localize to cell-cell adhesions and to drive epithelial cell colony escape. Here we report that PAK6 targeting to cell-cell adhesions occurs through its N-terminus, requiring both its Cdc42/Rac interactive binding (CRIB) domain and an adjacent polybasic region for maximal targeting efficiency. We find PAK6 localization to cell-cell adhesions is Cdc42-dependent, as Cdc42 knockdown inhibits PAK6 targeting to cell-cell adhesions. We further find the ability of PAK6 to drive epithelial cell colony escape requires kinase activity and is disrupted by mutations that perturb PAK6 cell-cell adhesion targeting. Finally, we demonstrate that all type II PAKs (PAK4, PAK5 and PAK6) target to cell-cell adhesions, albeit to differing extents, but PAK1 (a type I PAK) does not. Notably, the ability of a PAK isoform to drive epithelial colony escape correlates with its targeting to cell-cell adhesions. We conclude that PAKs have a broader role in the regulation of cell-cell adhesions than previously appreciated.

Regulation of integrin-mediated adhesions

Integrins are heterodimeric transmembrane adhesion receptors that couple the actin cytoskeleton to the extracellular environment and bidirectionally relay signals across the cell membrane. These processes are critical for cell attachment, migration, differentiation, and survival, and therefore play essential roles in metazoan development, physiology, and pathology. Integrin-mediated adhesions are regulated by diverse factors, including the conformation-specific affinities of integrin receptors for their extracellular ligands, the clustering of integrins and their intracellular binding partners into discrete adhesive structures, mechanical forces exerted on the adhesion, and the intracellular trafficking of integrins themselves. Recent advances shed light onto how the interaction of specific intracellular proteins with the short cytoplasmic tails of integrins controls each of these activities.

CCM2-CCM3 interaction stabilizes their protein expression and permits endothelial network formation

CCM2–CCM3 interactions protect CCM2 and CCM3 from proteasomal degradation, and both CCM2 and CCM3 are required for normal endothelial cell network formation.

Mutations in the essential adaptor proteins CCM2 or CCM3 lead to cerebral cavernous malformations (CCM), vascular lesions that most frequently occur in the brain and are strongly associated with hemorrhagic stroke, seizures, and other neurological disorders. CCM2 binds CCM3, but the molecular basis of this interaction, and its functional significance, have not been elucidated. Here, we used x-ray crystallography and structure-guided mutagenesis to show that an α-helical LD-like motif within CCM2 binds the highly conserved “HP1” pocket of the CCM3 focal adhesion targeting (FAT) homology domain. By knocking down CCM2 or CCM3 and rescuing with binding-deficient mutants, we establish that CCM2–CCM3 interactions protect CCM2 and CCM3 proteins from proteasomal degradation and show that both CCM2 and CCM3 are required for normal endothelial cell network formation. However, CCM3 expression in the absence of CCM2 is sufficient to support normal cell growth, revealing complex-independent roles for CCM3.

Direct interactions with the integrin β1 cytoplasmic tail activate the Abl2/Arg kinase

Integrins are heterodimeric α/β extracellular matrix adhesion receptors that couple physically to the actin cytoskeleton and regulate kinase signaling pathways to control cytoskeletal remodeling and adhesion complex formation and disassembly. β1 integrins signal through the Abl2/Arg (Abl-related gene) nonreceptor tyrosine kinase to control fibroblast cell motility, neuronal dendrite morphogenesis and stability, and cancer cell invasiveness, but the molecular mechanisms by which integrin β1 activates Arg are unknown. We report here that the Arg kinase domain interacts directly with a lysine-rich membrane-proximal segment in the integrin β1 cytoplasmic tail, that Arg phosphorylates the membrane-proximal Tyr-783 in the β1 tail, and that the Arg Src homology domain then engages this phosphorylated region in the tail. We show that these interactions mediate direct binding between integrin β1 and Arg in vitro and in cells and activate Arg kinase activity. These findings provide a model for understanding how β1-containing integrins interact with and activate Abl family kinases.

The talin head domain reinforces integrin-mediated adhesion by promoting adhesion complex stability and clustering

Talin serves an essential function during integrin-mediated adhesion in linking integrins to actin via the intracellular adhesion complex. In addition, the N-terminal head domain of talin regulates the affinity of integrins for their ECM-ligands, a process known as inside-out activation. We previously showed that in Drosophila, mutating the integrin binding site in the talin head domain resulted in weakened adhesion to the ECM. Intriguingly, subsequent studies showed that canonical inside-out activation of integrin might not take place in flies. Consistent with this, a mutation in talin that specifically blocks its ability to activate mammalian integrins does not significantly impinge on talin function during fly development. Here, we describe results suggesting that the talin head domain reinforces and stabilizes the integrin adhesion complex by promoting integrin clustering distinct from its ability to support inside-out activation. Specifically, we show that an allele of talin containing a mutation that disrupts intramolecular interactions within the talin head attenuates the assembly and reinforcement of the integrin adhesion complex. Importantly, we provide evidence that this mutation blocks integrin clustering in vivo. We propose that the talin head domain is essential for regulating integrin avidity in Drosophila and that this is crucial for integrin-mediated adhesion during animal development.

Cells are the building blocks of our bodies. How do cells rearrange to form three-dimensional body plans and maintain specific tissue structures? Specialized adhesion molecules on the cell surface mediate attachment between cells and their surrounding environment to hold tissues together. Our work uses the developing fruit fly embryo to demonstrate how such connections are regulated during tissue growth. Since the genes and molecules involved in this process are highly similar between flies and humans, we can also apply our findings to our understanding of how human tissues form and are maintained. We observe that, in late developing muscles, clusters of cell adhesion molecules concentrate together to create stronger attachments between muscle cells and tendon cells. This strengthening mechanism allows the fruit fly to accommodate increasing amounts of force imposed by larger, more active muscles. We identify specific genetic mutations that disrupt these strengthening mechanisms and lead to severe developmental defects during fly development. Our results illustrate how subtle fine-tuning of the connections between cells and their surrounding environment is important to form and maintain normal tissue structure across the animal kingdom.

Up-regulation of thrombospondin-2 in Akt1-null mice contributes to compromised tissue repair due to abnormalities in fibroblast function

Vascular remodeling is essential for tissue repair and is regulated by multiple factors, including thrombospondin-2 (TSP2) and hypoxia/VEGF-induced activation of Akt. In contrast to TSP2 knock-out (KO) mice, Akt1 KO mice have elevated TSP2 expression and delayed tissue repair. To investigate the contribution of increased TSP2 to Akt1 KO mice phenotypes, we generated Akt1/TSP2 double KO (DKO) mice. Full-thickness excisional wounds in DKO mice healed at an accelerated rate when compared with Akt1 KO mice. Isolated dermal Akt1 KO fibroblasts expressed increased TSP2 and displayed altered morphology and defects in migration and adhesion. These defects were rescued in DKO fibroblasts or after TSP2 knockdown. Conversely, the addition of exogenous TSP2 to WT cells induced cell morphology and migration rates that were similar to those of Akt1 KO cells. Akt1 KO fibroblasts displayed reduced adhesion to fibronectin with manganese stimulation when compared with WT and DKO cells, revealing an Akt1-dependent role for TSP2 in regulating integrin-mediated adhesions; however, this effect was not due to changes in β1 integrin surface expression or activation. Consistent with these results, Akt1 KO fibroblasts displayed reduced Rac1 activation that was dependent upon expression of TSP2 and could be rescued by a constitutively active Rac mutant. Our observations show that repression of TSP2 expression is a critical aspect of Akt1 function in tissue repair.

Differences in binding to the ILK complex determines kindlin isoform adhesion localization and integrin activation

Kindlins are essential FERM-domain-containing focal adhesion (FA) proteins required for proper integrin activation and signaling. Despite the widely accepted importance of each of the three mammalian kindlins in cell adhesion, the molecular basis for their function has yet to be fully elucidated, and the functional differences between isoforms have generally not been examined. Here, we report functional differences between kindlin-2 and -3 (also known as FERMT2 and FERMT3, respectively); GFP-tagged kindlin-2 localizes to FAs whereas kindlin-3 does not, and kindlin-2, but not kindlin-3, can rescue α5β1 integrin activation defects in kindlin-2-knockdown fibroblasts. Using chimeric kindlins, we show that the relatively uncharacterized kindlin-2 F2 subdomain drives FA targeting and integrin activation. We find that the integrin-linked kinase (ILK)-PINCH-parvin complex binds strongly to the kindlin-2 F2 subdomain but poorly to that of kindlin-3. Using a point-mutated kindlin-2, we establish that efficient kindlin-2-mediated integrin activation and FA targeting require binding to the ILK complex. Thus, ILK-complex binding is crucial for normal kindlin-2 function and differential ILK binding contributes to kindlin isoform specificity.

TRIM15 is a focal adhesion protein that regulates focal adhesion disassembly

Focal adhesions are macromolecular complexes that connect the actin cytoskeleton to the extracellular matrix. Dynamic turnover of focal adhesions is crucial for cell migration. Paxillin is a multi-adaptor protein that plays an important role in regulating focal adhesion dynamics. Here, we identify TRIM15, a member of the tripartite motif protein family, as a paxillin-interacting factor and a component of focal adhesions. TRIM15 localizes to focal contacts in a myosin-II-independent manner by an interaction between its coiled-coil domain and the LD2 motif of paxillin. Unlike other focal adhesion proteins, TRIM15 is a stable focal adhesion component with restricted mobility due to its ability to form oligomers. TRIM15-depleted cells display impaired cell migration and reduced focal adhesion disassembly rates, in addition to enlarged focal adhesions. Thus, our studies demonstrate a cellular function for TRIM15 as a regulatory component of focal adhesion turnover and cell migration.

Dynamin 2 regulation of integrin endocytosis, but not VEGF signaling, is crucial for developmental angiogenesis

Here we show that dynamin 2 (Dnm2) is essential for angiogenesis in vitro and in vivo. In cultured endothelial cells lacking Dnm2, vascular endothelial growth factor (VEGF) signaling and receptor levels are augmented whereas cell migration and morphogenesis are impaired. Mechanistically, the loss of Dnm2 increases focal adhesion size and the surface levels of multiple integrins and reduces the activation state of β1 integrin. In vivo, the constitutive or inducible loss of Dnm2 in endothelium impairs branching morphogenesis and promotes the accumulation of β1 integrin at sites of failed angiogenic sprouting. Collectively, our data show that Dnm2 uncouples VEGF signaling from function and coordinates the endocytic turnover of integrins in a manner that is crucially important for angiogenesis in vitro and in vivo.

Podocyte-associated talin1 is critical for glomerular filtration barrier maintenance

Podocytes are specialized actin-rich epithelial cells that line the kidney glomerular filtration barrier. The interface between the podocyte and the glomerular basement membrane requires integrins, and defects in either α3 or β1 integrin, or the α3β1 ligand laminin result in nephrotic syndrome in murine models. The large cytoskeletal protein talin1 is not only pivotal for integrin activation, but also directly links integrins to the actin cytoskeleton. Here, we found that mice lacking talin1 specifically in podocytes display severe proteinuria, foot process effacement, and kidney failure. Loss of talin1 in podocytes caused only a modest reduction in β1 integrin activation, podocyte cell adhesion, and cell spreading; however, the actin cytoskeleton of podocytes was profoundly altered by the loss of talin1. Evaluation of murine models of glomerular injury and patients with nephrotic syndrome revealed that calpain-induced talin1 cleavage in podocytes might promote pathogenesis of nephrotic syndrome. Furthermore, pharmacologic inhibition of calpain activity following glomerular injury substantially reduced talin1 cleavage, albuminuria, and foot process effacement. Collectively, these findings indicate that podocyte talin1 is critical for maintaining the integrity of the glomerular filtration barrier and provide insight into the pathogenesis of nephrotic syndrome.

Cerebral cavernous malformation proteins at a glance

Loss-of-function mutations in genes encoding KRIT1 (also known as CCM1), CCM2 (also known as OSM and malcavernin) or PDCD10 (also known as CCM3) cause cerebral cavernous malformations (CCMs). These abnormalities are characterized by dilated leaky blood vessels, especially in the neurovasculature, that result in increased risk of stroke, focal neurological defects and seizures. The three CCM proteins can exist in a trimeric complex, and each of these essential multi-domain adaptor proteins also interacts with a range of signaling, cytoskeletal and adaptor proteins, presumably accounting for their roles in a range of basic cellular processes including cell adhesion, migration, polarity and apoptosis. In this Cell Science at a Glance article and the accompanying poster, we provide an overview of current models of CCM protein function focusing on how known protein-protein interactions might contribute to cellular phenotypes and highlighting gaps in our current understanding.

Integrin cytoplasmic tail interactions

Integrins are heterodimeric cell surface adhesion receptors essential for multicellular life. They connect cells to the extracellular environment and transduce chemical and mechanical signals to and from the cell. Intracellular proteins that bind the integrin cytoplasmic tail regulate integrin engagement of extracellular ligands as well as integrin localization and trafficking. Cytoplasmic integrin-binding proteins also function downstream of integrins, mediating links to the cytoskeleton and to signaling cascades that impact cell motility, growth, and survival. Here, we review key integrin-interacting proteins and their roles in regulating integrin activity, localization, and signaling.

Kindlin binds migfilin tandem LIM domains and regulates migfilin focal adhesion localization and recruitment dynamics

Focal adhesions (FAs), sites of tight adhesion to the extracellular matrix, are composed of clusters of transmembrane integrin adhesion receptors and intracellular proteins that link integrins to the actin cytoskeleton and signaling pathways. Two integrin-binding proteins present in FAs, kindlin-1 and kindlin-2, are important for integrin activation, FA formation, and signaling. Migfilin, originally identified in a yeast two-hybrid screen for kindlin-2-interacting proteins, is a LIM domain-containing adaptor protein found in FAs and implicated in control of cell adhesion, spreading, and migration. By binding filamin, migfilin provides a link between kindlin and the actin cytoskeleton. Here, using a combination of kindlin knockdown, biochemical pulldown assays, fluorescence microscopy, fluorescence resonance energy transfer (FRET), and fluorescence recovery after photobleaching (FRAP), we have established that the C-terminal LIM domains of migfilin dictate its FA localization, shown that these domains mediate an interaction with kindlin in vitro and in cells, and demonstrated that kindlin is important for normal migfilin dynamics in cells. We also show that when the C-terminal LIM domain region is deleted, then the N-terminal filamin-binding region of the protein, which is capable of targeting migfilin to actin-rich stress fibers, is the predominant driver of migfilin localization. Our work details a correlation between migfilin domains that drive kindlin binding and those that drive FA localization as well as a kindlin dependence on migfilin FA recruitment and mobility. We therefore suggest that the kindlin interaction with migfilin LIM domains drives migfilin FA recruitment, localization, and mobility.

Substrate and inhibitor specificity of the type II p21-activated kinase, PAK6

The p21-activated kinases (PAKs) are important effectors of Rho-family small GTPases. The PAK family consists of two groups, type I and type II, which have different modes of regulation and signaling. PAK6, a type II PAK, influences behavior and locomotor function in mice and has an ascribed role in androgen receptor signaling. Here we show that PAK6 has a peptide substrate specificity very similar to the other type II PAKs, PAK4 and PAK5 (PAK7). We find that PAK6 catalytic activity is inhibited by a peptide corresponding to its N-terminal pseudosubstrate. Introduction of a melanoma-associated mutation, P52L, into this peptide reduces pseudosubstrate autoinhibition of PAK6, and increases phosphorylation of its substrate PACSIN1 (Syndapin I) in cells. Finally we determine two co-crystal structures of PAK6 catalytic domain in complex with ATP-competitive inhibitors. We determined the 1.4 Å co-crystal structure of PAK6 with the type II PAK inhibitor PF-3758309, and the 1.95 Å co-crystal structure of PAK6 with sunitinib. These findings provide new insights into the structure-function relationships of PAK6 and may facilitate development of PAK6 targeted therapies.

ASB2α, an E3 ubiquitin ligase specificity subunit, regulates cell spreading and triggers proteasomal degradation of filamins by targeting the filamin calponin homology 1 domain

Filamins are actin-binding and cross-linking proteins that organize the actin cytoskeleton and anchor transmembrane proteins to the cytoskeleton and scaffold signaling pathways. During hematopoietic cell differentiation, transient expression of ASB2α, the specificity subunit of an E3-ubiquitin ligase complex, triggers acute proteasomal degradation of filamins. This led to the proposal that ASB2α regulates hematopoietic cell differentiation by modulating cell adhesion, spreading, and actin remodeling through targeted degradation of filamins. Here, we show that the calponin homology domain 1 (CH1), within the filamin A (FLNa) actin-binding domain, is the minimal fragment sufficient for ASB2α-mediated degradation. Combining an in-depth flow cytometry analysis with mutagenesis of lysine residues within CH1, we find that arginine substitution at each of a cluster of three lysines (Lys-42, Lys-43, and Lys-135) renders FLNa resistant to ASB2α-mediated degradation without altering ASB2α binding. These lysines lie within previously predicted actin-binding sites, and the ASB2α-resistant filamin mutant is defective in targeting to F-actin-rich structures in cells. However, by swapping CH1 with that of α-actinin1, which is resistant to ASB2α-mediated degradation, we generated an ASB2α-resistant chimeric FLNa with normal subcellular localization. Notably, this chimera fully rescues the impaired cell spreading induced by ASB2α expression. Our data therefore reveal ubiquitin acceptor sites in FLNa and establish that ASB2α-mediated effects on cell spreading are due to loss of filamins.

Talins and kindlins: partners in integrin-mediated adhesion

Integrin receptors provide a dynamic, tightly-regulated link between the extracellular matrix (or cellular counter-receptors) and intracellular cytoskeletal and signalling networks, enabling cells to sense and respond to their chemical and physical environment. Talins and kindlins, two families of FERM-domain proteins, bind the cytoplasmic tail of integrins, recruit cytoskeletal and signalling proteins involved in mechanotransduction and synergize to activate integrin binding to extracellular ligands. New data reveal the domain structure of full-length talin, provide insights into talin-mediated integrin activation and show that RIAM recruits talin to the plasma membrane, whereas vinculin stabilizes talin in cell-matrix junctions. How kindlins act is less well-defined, but disease-causing mutations show that kindlins are also essential for integrin activation, adhesion, cell spreading and signalling.

Mechanism for KRIT1 release of ICAP1-mediated suppression of integrin activation

KRIT1 (Krev/Rap1 Interaction Trapped-1) mutations are observed in ∼40% of autosomal-dominant cerebral cavernous malformations (CCMs), a disease occurring in up to 0.5% of the population. We show that KRIT1 functions as a switch for β1 integrin activation by antagonizing ICAP1 (Integrin Cytoplasmic Associated Protein-1)-mediated modulation of "inside-out" activation. We present cocrystal structures of KRIT1 with ICAP1 and ICAP1 with integrin β1 cytoplasmic tail to 2.54 and 3.0 Å resolution (the resolutions at which I/σI = 2 are 2.75 and 3.0 Å, respectively). We find that KRIT1 binds ICAP1 by a bidentate surface, that KRIT1 directly competes with integrin β1 to bind ICAP1, and that KRIT1 antagonizes ICAP1-modulated integrin activation using this site. We also find that KRIT1 contains an N-terminal Nudix domain, in a region previously designated as unstructured. We therefore provide insights to integrin regulation and CCM-associated KRIT1 function.

Structural and functional characterization of the kindlin-1 pleckstrin homology domain

Inside-out activation of integrins is mediated via the binding of talin and kindlin to integrin β-subunit cytoplasmic tails. The kindlin FERM domain is interrupted by a pleckstrin homology (PH) domain within its F2 subdomain. Here, we present data confirming the importance of the kindlin-1 PH domain for integrin activation and its x-ray crystal structure at a resolution of 2.1 Å revealing a C-terminal second α-helix integral to the domain but found only in the kindlin protein family. An isoform-specific salt bridge occludes the canonical phosphoinositide binding site, but molecular dynamics simulations display transient switching to an alternative open conformer. Molecular docking reveals that the opening of the pocket would enable potential ligands to bind within it. Although lipid overlay assays suggested the PH domain binds inositol monophosphates, surface plasmon resonance demonstrated weak affinities for inositol 3,4,5-triphosphate (Ins(3,4,5)P(3); K(D) ∼100 μM) and no monophosphate binding. Removing the salt bridge by site-directed mutagenesis increases the PH domain affinity for Ins(3,4,5)P(3) as measured by surface plasmon resonance and enables it to bind PtdIns(3,5)P(2) on a dot-blot. Structural comparison with other PH domains suggests that the phosphate binding pocket in the kindlin-1 PH domain is more occluded than in kindlins-2 and -3 due to its salt bridge. In addition, the apparent affinity for Ins(3,4,5)P(3) is affected by the presence of PO(4) ions in the buffer. We suggest the physiological ligand of the kindlin-1 PH domain is most likely not an inositol phosphate but another phosphorylated species.

Zasp regulates integrin activation

Integrins are heterodimeric adhesion receptors that link the extracellular matrix (ECM) to the cytoskeleton. Binding of the scaffold protein, talin, to the cytoplasmic tail of β-integrin causes a conformational change of the extracellular domains of the integrin heterodimer, thus allowing high-affinity binding of ECM ligands. This essential process is called integrin activation. Here we report that the Z-band alternatively spliced PDZ-motif-containing protein (Zasp) cooperates with talin to activate α5β1 integrins in mammalian tissue culture and αPS2βPS integrins in Drosophila. Zasp is a PDZ-LIM-domain-containing protein mutated in human cardiomyopathies previously thought to function primarily in assembly and maintenance of the muscle contractile machinery. Notably, Zasp is the first protein shown to co-activate α5β1 integrins with talin and appears to do so in a manner distinct from known αIIbβ3 integrin co-activators.

Cell adhesion: a FERM grasp of the tail sorts out integrins

As well as modulating integrin activation, a conserved NPxY motif in integrin cytoplasmic tails that binds the FERM-domain-containing proteins kindlin and sorting nexin 17 plays pivotal roles in integrin recycling and degradation.

Nanopatterning reveals an ECM area threshold for focal adhesion assembly and force transmission that is regulated by integrin activation and cytoskeleton tension

Integrin-based focal adhesions (FA) transmit anchorage and traction forces between the cell and the extracellular matrix (ECM). To gain further insight into the physical parameters of the ECM that control FA assembly and force transduction in non-migrating cells, we used fibronectin (FN) nanopatterning within a cell adhesion-resistant background to establish the threshold area of ECM ligand required for stable FA assembly and force transduction. Integrin-FN clustering and adhesive force were strongly modulated by the geometry of the nanoscale adhesive area. Individual nanoisland area, not the number of nanoislands or total adhesive area, controlled integrin-FN clustering and adhesion strength. Importantly, below an area threshold (0.11 µm(2)), very few integrin-FN clusters and negligible adhesive forces were generated. We then asked whether this adhesive area threshold could be modulated by intracellular pathways known to influence either adhesive force, cytoskeletal tension, or the structural link between the two. Expression of talin- or vinculin-head domains that increase integrin activation or clustering overcame this nanolimit for stable integrin-FN clustering and increased adhesive force. Inhibition of myosin contractility in cells expressing a vinculin mutant that enhances cytoskeleton-integrin coupling also restored integrin-FN clustering below the nanolimit. We conclude that the minimum area of integrin-FN clusters required for stable assembly of nanoscale FA and adhesive force transduction is not a constant; rather it has a dynamic threshold that results from an equilibrium between pathways controlling adhesive force, cytoskeletal tension, and the structural linkage that transmits these forces, allowing the balance to be tipped by factors that regulate these mechanical parameters.

Structural basis for paxillin binding and focal adhesion targeting of β-parvin

β-Parvin is a cytoplasmic adaptor protein that localizes to focal adhesions where it interacts with integrin-linked kinase and is involved in linking integrin receptors to the cytoskeleton. It has been reported that despite high sequence similarity to α-parvin, β-parvin does not bind paxillin, suggesting distinct interactions and cellular functions for these two closely related parvins. Here, we reveal that β-parvin binds directly and specifically to leucine-aspartic acid repeat (LD) motifs in paxillin via its C-terminal calponin homology (CH2) domain. We present the co-crystal structure of β-parvin CH2 domain in complex with paxillin LD1 motif to 2.9 Å resolution and find that the interaction is similar to that previously observed between α-parvin and paxillin LD1. We also present crystal structures of unbound β-parvin CH2 domain at 2.1 Å and 2.0 Å resolution that show significant conformational flexibility in the N-terminal α-helix, suggesting an induced fit upon paxillin binding. We find that β-parvin has specificity for the LD1, LD2, and LD4 motifs of paxillin, with K(D) values determined to 27, 42, and 73 μM, respectively, by surface plasmon resonance. Furthermore, we show that proper localization of β-parvin to focal adhesions requires both the paxillin and integrin-linked kinase binding sites and that paxillin is important for early targeting of β-parvin. These studies provide the first molecular details of β-parvin binding to paxillin and help define the requirements for β-parvin localization to focal adhesions.

Filamin A controls matrix metalloproteinase activity and regulates cell invasion in human fibrosarcoma cells

Filamins are an important family of actin-binding proteins that, in addition to bundling actin filaments, link cell surface adhesion proteins, signaling receptors and channels to the actin cytoskeleton, and serve as scaffolds for an array of intracellular signaling proteins. Filamins are known to regulate the actin cytoskeleton, act as mechanosensors that modulate tissue responses to matrix density, control cell motility and inhibit activation of integrin adhesion receptors. In this study, we extend the repertoire of filamin activities to include control of extracellular matrix (ECM) degradation. We show that knockdown of filamin increases matrix metalloproteinase (MMP) activity and induces MMP2 activation, enhancing the ability of cells to remodel the ECM and increasing their invasive potential, without significantly altering two-dimensional random cell migration. We further show that within filamin A, the actin-binding domain is necessary, but not sufficient, to suppress the ECM degradation seen in filamin-A-knockdown cells and that dimerization and integrin binding are not required. Filamin mutations are associated with neuronal migration disorders and a range of congenital malformations characterized by skeletal dysplasia and various combinations of cardiac, craniofacial and intestinal anomalies. Furthermore, in breast cancers loss of filamin A has been correlated with increased metastatic potential. Our data suggest that effects on ECM remodeling and cell invasion should be considered when attempting to provide cellular explanations for the physiological and pathological effects of altered filamin expression or filamin mutations.

Structural basis for small G protein effector interaction of Ras-related protein 1 (Rap1) and adaptor protein Krev interaction trapped 1 (KRIT1)

Cerebral cavernous malformations (CCMs) affect 0.1-0.5% of the population resulting in leaky vasculature and severe neurological defects. KRIT1 (Krev interaction trapped-1) mutations associate with ∼40% of familial CCMs. KRIT1 is an effector of Ras-related protein 1 (Rap1) GTPase. Rap1 relocalizes KRIT1 from microtubules to cell membranes to impact integrin activation, potentially important for CCM pathology. We report the 1.95 Å co-crystal structure of KRIT1 FERM domain in complex with Rap1. Rap1-KRIT1 interaction encompasses an extended surface, including Rap1 Switch I and II and KRIT1 FERM F1 and F2 lobes. Rap1 binds KRIT1-F1 lobe using a GTPase-ubiquitin-like fold interaction but binds KRIT1-F2 lobe by a novel interaction. Point mutagenesis confirms the interaction. High similarity between KRIT1-F2/F3 and talin is revealed. Additionally, the mechanism for FERM domains acting as GTPase effectors is suggested. Finally, structure-based alignment of each lobe suggests classification of FERM domains as ERM-like and TMFK-like (talin-myosin-FAK-KRIT-like) and that FERM lobes resemble domain "modules."

FAK promotes recruitment of talin to nascent adhesions to control cell motility

An alternative linkage is shown whereby FAK brings talin to nascent adhesions independent of talin binding to β1 integrins.

Cell migration is a dynamic process that involves the continuous formation, maturation, and turnover of matrix–cell adhesion sites. New (nascent) adhesions form at the protruding cell edge in a tension-independent manner and are comprised of integrin receptors, signaling, and cytoskeletal-associated proteins. Integrins recruit focal adhesion kinase (FAK) and the cytoskeletal protein talin to nascent adhesions. Canonical models support a role for talin in mediating FAK localization and activation at adhesions. Here, alternatively, we show that FAK promotes talin recruitment to nascent adhesions occurring independently of talin binding to β1 integrins. The direct binding site for talin on FAK was identified, and a point mutation in FAK (E1015A) prevented talin association and talin localization to nascent adhesions but did not alter integrin-mediated FAK recruitment and activation at adhesions. Moreover, FAK E1015A inhibited cell motility and proteolytic talin cleavage needed for efficient adhesion dynamics. These results support an alternative linkage for FAK–talin interactions within nascent adhesions essential for the control of cell migration.

Filamins in mechanosensing and signaling

Filamins are essential, evolutionarily conserved, modular, multidomain, actin-binding proteins that organize the actin cytoskeleton and maintain extracellular matrix connections by anchoring actin filaments to transmembrane receptors. By cross-linking and anchoring actin filaments, filamins stabilize the plasma membrane, provide cellular cortical rigidity, and contribute to the mechanical stability of the plasma membrane and the cell cortex. In addition to binding actin, filamins interact with more than 90 other binding partners including intracellular signaling molecules, receptors, ion channels, transcription factors, and cytoskeletal and adhesion proteins. Thus, filamins scaffold a wide range of signaling pathways and are implicated in the regulation of a diverse array of cellular functions including motility, maintenance of cell shape, and differentiation. Here, we review emerging structural and functional evidence that filamins are mechanosensors and/or mechanotransducers playing essential roles in helping cells detect and respond to physical forces in their local environment.