Integrin-associated complexes form hierarchically with variable stoichiometry in nascent adhesions

Synergistic effect of PIP2 and PIP3 on membrane-induced phase separation of integrin complexes

The assembly of integrin adhesion complexes at the inner leaflet of the plasma membrane regulates cell adhesion to the extracellular matrix. The multivalent protein interactions within the complexes and with the cell membrane display characteristics of membrane-associated biomolecular condensates driven by liquid-liquid phase separation. The composition of lipids and the distribution of the cell membrane are crucial for forming integrin adhesion complexes. Here, we report that phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol (3,4,5)-trisphosphate (PIP3) in the model membrane synergistically regulate the formation of membrane-induced integrin adhesion condensates, which consist of β1 tails, kindlin, talin, paxillin, and focal adhesion kinase. We show that the preferential bindings of kindlin to PIP3 and talin to PIP2 enhance their recruitment to the membrane, which in turn increases the probability of membrane-associated phase separation. Our results indicate that modulating the intricate balance of membrane composition is a strategy to localize integrin adhesion complexes and optimize their density on lipid membranes.

Polydopamine Nanocomposite Hydrogel for Drug Slow-Release in Bone Defect Repair: A Review of Research Advances

In recent years, hydrogels have emerged as promising candidates for bone defect repair due to their excellent biocompatibility, high porosity, and water-retentive properties. However, conventional hydrogels face significant challenges in clinical translation, including brittleness, low mechanical strength, and poorly controlled drug degradation rates. To address these limitations, as a multifunctional polymer, polydopamine (PDA) has shown great potential in both bone regeneration and drug delivery systems. Its robust adhesive properties, biocompatibility, and responsiveness to photothermal stimulation make it an ideal candidate for enhancing hydrogel performance. Integrating PDA into conventional hydrogels not only improves their mechanical properties but also creates an environment conducive to cell adhesion, proliferation, and differentiation, thereby promoting bone defect repair. Moreover, PDA facilitates controlled drug release, offering a promising approach to optimizing treatment outcomes. This paper first explores the mechanisms through which PDA promotes bone regeneration, laying the foundation for its clinical translation. Additionally, it discusses the application of PDA-based nanocomposite hydrogels as advanced drug delivery systems for bone defect repair, providing valuable insights for both research and clinical translation.

The role and regulation of integrins in cell migration and invasion

Integrin receptors are the main molecular link between cells and the extracellular matrix (ECM) as well as mediating cell-cell interactions. Integrin-ECM binding triggers the formation of heterogeneous multi-protein assemblies termed integrin adhesion complexes (IACs) that enable integrins to transform extracellular cues into intracellular signals that affect many cellular processes, especially cell motility. Cell migration is essential for diverse physiological and pathological processes and is dysregulated in cancer to favour cell invasion and metastasis. Here, we discuss recent findings on the role of integrins in cell migration with a focus on cancer cell dissemination. We review how integrins regulate the spatial distribution and dynamics of different IACs, covering classical focal adhesions, emerging adhesion types and adhesion regulation. We discuss the diverse roles integrins have during cancer progression from cell migration across varied ECM landscapes to breaching barriers such as the basement membrane, and eventual colonization of distant organs.

Paxillin and Kindlin: Research Progress and Biological Functions

Paxillin and kindlin are essential regulatory proteins involved in cell adhesion, migration, and signal transduction. Paxillin influences cytoskeletal dynamics by interacting with multiple signaling proteins, while kindlin regulates integrin activation, affecting adhesion and motility. This review examines the structures and functions of these proteins, focusing on their roles in cancer progression, immune response, and therapeutic potential. The cooperation between paxillin and kindlin in integrin activation and focal adhesion dynamics offers valuable insights into tumor metastasis, immune function, and tissue repair.

Immobile Integrin Signaling Transit and Relay Nodes Organize Mechanosignaling through Force-Dependent Phosphorylation in Focal Adhesions

Transmembrane signaling receptors, such as integrins, organize as nanoclusters that provide several advantages, including increasing avidity, sensitivity (increasing the signal-to-noise ratio), and robustness (signaling threshold) of the signal in contrast to signaling by single receptors. Furthermore, compared to large micron-sized clusters, nanoclusters offer the advantage of rapid turnover for the disassembly of the signal. However, whether nanoclusters function as signaling hubs remains poorly understood. Here, we employ fluorescence nanoscopy combined with photoactivation and photobleaching at subdiffraction limited resolution of ∼100 nm length scale within a focal adhesion to examine the dynamics of diverse focal adhesion proteins. We show that (i) subregions of focal adhesions are enriched in an immobile population of integrin β3 organized as nanoclusters, which (ii) in turn serve to organize nanoclusters of associated key adhesome proteins-vinculin, focal adhesion kinase (FAK) and paxillin, demonstrating that signaling proceeds by formation of nanoclusters rather than through individual proteins. (iii) Distinct focal adhesion protein nanoclusters exhibit distinct protein dynamics, which is closely correlated to their function in signaling. (iv) Long-lived nanoclusters function as signaling hubs─wherein immobile integrin nanoclusters organize phosphorylated FAK to form stable nanoclusters in close proximity to them, which are disassembled in response to inactivation signal by removal of force and in turn activation of phosphatase PTPN12. (v) Signaling takes place in response to external signals such as force or geometric arrangement of the nanoclusters and when the signal is removed, these nanoclusters disassemble. We term these functional nanoclusters as integrin signaling transit and relay nodes (STARnodes). Taken together, these results demonstrate that integrin STARnodes seed signaling downstream of the integrin receptors by organizing hubs of signaling proteins (FAK, paxillin, vinculin) to relay the incoming signal intracellularly and bring about robust function.

Kindlin-2 Phase Separation in Response to Flow Controls Vascular Stability

BACKGROUND

Atheroprotective shear stress preserves endothelial barrier function, while atheroprone shear stress enhances endothelial permeability. Yet, the underlying mechanisms through which distinct flow patterns regulate EC integrity remain to be clarified. This study aimed to investigate the involvement of Kindlin-2, a key component of focal adhesion and endothelial adherens junctions crucial for regulating endothelial cell (EC) integrity and vascular stability.

METHODS

Mouse models of atherosclerosis in EC-specific Kindlin-2 knockout mice (Kindlin-2iΔEC) were used to study the role of Kindlin-2 in atherogenesis. Pulsatile shear (12±4 dynes/cm2) or oscillatory shear (0.5±4 dynes/cm2) were applied to culture ECs. Live-cell imaging, fluorescence recovery after photobleaching assay, and OptoDroplet assay were used to study the liquid-liquid phase separation (LLPS) of Kindlin-2. Co-immunoprecipitation, mutagenesis, proximity ligation assay, and transendothelial electrical resistance assay were used to explore the underlying mechanism of flow-regulated Kindlin-2 function.

RESULTS

We found that Kindlin-2 localization is altered under different flow patterns. Kindlin-2iΔEC mice showed heightened vascular permeability. Kindlin-2iΔEC were bred onto ApoE-/- mice to generate Kindlin-2iΔEC; ApoE-/- mice, which displayed a significant increase in atherosclerosis lesions. In vitro data showed that in ECs, Kindlin-2 underwent LLPS, a critical process for proper focal adhesion assembly, maturation, and junction formation. Mass spectrometry analysis revealed that oscillatory shear increased arginine methylation of Kindlin-2, catalyzed by PRMT5 (protein arginine methyltransferase 5). Functionally, arginine hypermethylation inhibits Kindlin-2 LLPS, impairing focal adhesion assembly and junction maturation. Notably, we identified R290 of Kindlin-2 as a crucial residue for LLPS and a key site for arginine methylation. Finally, pharmacologically inhibiting arginine methylation reduces EC activation and plaque formation.

CONCLUSIONS

Collectively, our study elucidates that mechanical force induces arginine methylation of Kindlin-2, thereby regulating vascular stability through its impact on Kindlin-2 LLPS. Targeting Kindlin-2 arginine methylation emerges as a promising hemodynamic-based strategy for treating vascular disorders and atherosclerosis.

Estrogen and estrogen receptors mediate the mechanobiology of bone disease and repair

It is well understood that the balance of bone formation and resorption is dependent on both mechanical and biochemical factors. In addition to cell-secreted cytokines and growth factors, sex hormones like estrogen are critical to maintaining bone health. Although the direct osteoprotective function of estrogen and estrogen receptors (ERs) has been reported extensively, evidence that estrogen signaling also has a role in mediating the effects of mechanical loading on maintenance of bone mass and healing of bone injuries has more recently emerged. Recent studies have underscored the role of estrogen and ERs in many pathways of bone mechanosensation and mechanotransduction. Estrogen and ERs have been shown to augment integrin-based mechanotransduction as well as canonical Wnt/b-catenin, RhoA/ROCK, and YAP/TAZ pathways. Estrogen and ERs also influence the mechanosensitivity of not only osteocytes but also osteoblasts, osteoclasts, and marrow stromal cells. The current review will highlight these roles of estrogen and ERs in cellular mechanisms underlying bone mechanobiology and discuss their implications for management of osteoporosis and bone fractures. A greater understanding of the mechanisms behind interactions between estrogen and mechanical loading may be crucial to addressing the shortcomings of current hormonal and pharmaceutical therapies. A combined therapy approach including high-impact exercise therapy may mitigate adverse side effects and allow an effective long-term solution for the prevention, treatment, and management of bone fragility in at-risk populations. Furthermore, future implications to novel local delivery mechanisms of hormonal therapy for osteoporosis treatment, as well as the effects on bone health of applications of sex hormone therapy outside of bone disease, will be discussed.

Ligand binding initiates single-molecule integrin conformational activation

Integrins link the extracellular environment to the actin cytoskeleton in cell migration and adhesiveness. Rapid coordination between events outside and inside the cell is essential. Single-molecule fluorescence dynamics show that ligand binding to the bent-closed integrin conformation, which predominates on cell surfaces, is followed within milliseconds by two concerted changes, leg extension and headpiece opening, to give the high-affinity integrin conformation. The extended-closed integrin conformation is not an intermediate but can be directly accessed from the extended-open conformation and provides a pathway for ligand dissociation. In contrast to ligand, talin, which links the integrin β-subunit cytoplasmic domain to the actin cytoskeleton, modestly stabilizes but does not induce extension or opening. Integrin activation is thus initiated by outside-in signaling and followed by inside-out signaling. Our results further imply that talin binding is insufficient for inside-out integrin activation and that tensile force transmission through the ligand-integrin-talin-actin cytoskeleton complex is required.

Focal Adhesion's Role in Cardiomyocytes Function: From Cardiomyogenesis to Mechanotransduction

Mechanotransduction refers to the ability of cells to sense mechanical stimuli and convert them into biochemical signals. In this context, the key players are focal adhesions (FAs): multiprotein complexes that link intracellular actin bundles and the extracellular matrix (ECM). FAs are involved in cellular adhesion, growth, differentiation, gene expression, migration, communication, force transmission, and contractility. Focal adhesion signaling molecules, including Focal Adhesion Kinase (FAK), integrins, vinculin, and paxillin, also play pivotal roles in cardiomyogenesis, impacting cell proliferation and heart tube looping. In fact, cardiomyocytes sense ECM stiffness through integrins, modulating signaling pathways like PI3K/AKT and Wnt/β-catenin. Moreover, FAK/Src complex activation mediates cardiac hypertrophic growth and survival signaling in response to mechanical loads. This review provides an overview of the molecular and mechanical mechanisms underlying the crosstalk between FAs and cardiac differentiation, as well as the role of FA-mediated mechanotransduction in guiding cardiac muscle responses to mechanical stimuli.

The RhoGAP RRC-1 is required for the assembly or stability of integrin adhesion complexes and is a member of the PIX pathway in muscle

GTPases cycle between active GTP bound and inactive GDP bound forms. Exchange of GDP for GTP is catalyzed by guanine nucleotide exchange factors (GEFs). GTPase activating proteins (GAPs) accelerate GTP hydrolysis, to promote the GDP bound form. We reported that the RacGEF, PIX-1, is required for assembly of integrin adhesion complexes (IAC) in striated muscle of Caenorhabditis elegans. In C. elegans, IACs are found at the muscle cell boundaries (MCBs), and bases of sarcomeric M-lines and dense bodies (Z-disks). Screening C. elegans mutants in proteins containing RhoGAP domains revealed that loss of function of rrc-1 results in loss of IAC components at MCBs, disorganization of M-lines and dense bodies, and reduced whole animal locomotion. RRC-1 localizes to MCBs, like PIX-1. The localization of RRC-1 at MCBs requires PIX-1, and the localization of PIX-1 requires RRC-1. Loss of function of CED-10 (Rac) shows lack of PIX-1 and RRC-1 at MCBs. RRC-1 exists in a complex with PIX-1. Transgenic rescue of rrc-1 was achieved with wild type RRC-1 but not RRC-1 with a missense mutation in a highly conserved residue of the RhoGAP domain. Our results are consistent with RRC-1 being a RhoGAP for the PIX pathway in muscle.

Nano-clusters of ligand-activated integrins organize immobile, signalling active, nano-clusters of phosphorylated FAK required for mechanosignaling in focal adhesions

Transmembrane signalling receptors, such as integrins, organise as nanoclusters that are thought to provide several advantages including, increasing avidity, sensitivity (increasing the signal-to-noise ratio) and robustness (signalling above a threshold rather than activation by a single receptor) of the signal compared to signalling by single receptors. Compared to large micron-sized clusters, nanoclusters offer the advantage of rapid turnover for the disassembly of the signal. However, if nanoclusters function as signalling hubs remains poorly understood. Here, we employ fluorescence nanoscopy combined with photoactivation and photobleaching at sub-diffraction limited resolution of ~100nm length scale within a focal adhesion to examine the dynamics of diverse focal adhesion proteins. We show that (i) subregions of focal adhesions are enriched in immobile population of integrin β3 organised as nanoclusters, which (ii) in turn serve to organise nanoclusters of associated key adhesome proteins- vinculin, focal adhesion kinase (FAK) and paxillin, demonstrating that signalling proceeds by formation of nanoclusters rather than through individual proteins. (iii) Distinct focal adhesion protein nanoclusters exhibit distinct dynamics dependent on function. (iv) long-lived nanoclusters function as signalling hubs- wherein phosphorylated FAK and paxillin formed stable nanoclusters in close proximity to immobile integrin nanoclusters which are disassembled in response to inactivation signal by phosphatase PTPN12 (v) signalling takes place in response to an external signal such as force or geometric arrangement of the nanoclusters and when the signal is removed, these nanoclusters disassemble. Taken together, these results demonstrate that signalling downstream of transmembrane receptors is organised as hubs of signalling proteins (FAK, paxillin, vinculin) seeded by nanoclusters of the transmembrane receptor (integrin).

Talin and kindlin use integrin tail allostery and direct binding to activate integrins

Integrin affinity regulation, also termed integrin activation, is essential for metazoan life. Although talin and kindlin binding to the β-integrin cytoplasmic tail is indispensable for integrin activation, it is unknown how they achieve this function. By combining NMR, biochemistry and cell biology techniques, we found that talin and kindlin binding to the β-tail can induce a conformational change that increases talin affinity and decreases kindlin affinity toward it. We also discovered that this asymmetric affinity regulation is accompanied by a direct interaction between talin and kindlin, which promotes simultaneous binding of talin and kindlin to β-tails. Disrupting allosteric communication between the β-tail-binding sites of talin and kindlin or their direct interaction in cells severely compromised integrin functions. These data show how talin and kindlin cooperate to generate a small but critical population of ternary talin-β-integrin-kindlin complexes with high talin-integrin affinity and high dynamics.

Intrinsic self-organization of integrin nanoclusters within focal adhesions is required for cellular mechanotransduction

Upon interaction with the extracellular matrix, the integrin receptors form nanoclusters as a first biochemical response to ligand binding. Here, we uncover a critical biodesign principle where these nanoclusters are spatially self-organized, facilitating effective mechanotransduction. Mouse Embryonic Fibroblasts (MEFs) with integrin β3 nanoclusters organized themselves with an intercluster distance of ∼550 nm on uniformly coated fibronectin substrates, leading to larger focal adhesions. We determined that this spatial organization was driven by cell-intrinsic factors since there was no pre-existing pattern on the substrates. Altering this spatial organization using cyclo-RGD functionalized Titanium nanodiscs (of 100 nm, corroborating to the integrin nanocluster size) spaced at intervals of 300 nm (almost half), 600 nm (normal) or 1000 nm (almost double) resulted in abrogation in mechanotransduction, indicating that a new parameter i.e., an optimal intercluster distance is necessary for downstream function. Overexpression of α-actinin, which induces a kink in the integrin tail, disrupted the establishment of the optimal intercluster distance, while simultaneous co-overexpression of talin head with α-actinin rescued it, indicating a concentration-dependent competition, and that cytoplasmic activation of integrin by talin head is required for the optimal intercluster organization. Additionally, talin head-mediated recruitment of FHOD1 that facilitates local actin polymerization at nanoclusters, and actomyosin contractility were also crucial for establishing the optimal intercluster distance and a robust mechanotransduction response. These findings demonstrate that cell-intrinsic machinery plays a vital role in organizing integrin receptor nanoclusters within focal adhesions, encoding essential information for downstream mechanotransduction signalling.

Force-dependent focal adhesion assembly and disassembly: A computational study

Cells interact with the extracellular matrix (ECM) via cell-ECM adhesions. These physical interactions are transduced into biochemical signals inside the cell which influence cell behaviour. Although cell-ECM interactions have been studied extensively, it is not completely understood how immature (nascent) adhesions develop into mature (focal) adhesions and how mechanical forces influence this process. Given the small size, dynamic nature and short lifetimes of nascent adhesions, studying them using conventional microscopic and experimental techniques is challenging. Computational modelling provides a valuable resource for simulating and exploring various "what if?" scenarios in silico and identifying key molecular components and mechanisms for further investigation. Here, we present a simplified mechano-chemical model based on ordinary differential equations with three major proteins involved in adhesions: integrins, talin and vinculin. Additionally, we incorporate a hypothetical signal molecule that influences adhesion (dis)assembly rates. We find that assembly and disassembly rates need to vary dynamically to limit maturation of nascent adhesions. The model predicts biphasic variation of actin retrograde velocity and maturation fraction with substrate stiffness, with maturation fractions between 18-35%, optimal stiffness of ∼1 pN/nm, and a mechanosensitive range of 1-100 pN/nm, all corresponding to key experimental findings. Sensitivity analyses show robustness of outcomes to small changes in parameter values, allowing model tuning to reflect specific cell types and signaling cascades. The model proposes that signal-dependent disassembly rate variations play an underappreciated role in maturation fraction regulation, which should be investigated further. We also provide predictions on the changes in traction force generation under increased/decreased vinculin concentrations, complementing previous vinculin overexpression/knockout experiments in different cell types. In summary, this work proposes a model framework to robustly simulate the mechanochemical processes underlying adhesion maturation and maintenance, thereby enhancing our fundamental knowledge of cell-ECM interactions.

GSK3 and lamellipodin balance lamellipodial protrusions and focal adhesion maturation in mouse neural crest migration

Neural crest cells are multipotent cells that delaminate from the neuroepithelium, migrating throughout the embryo. Aberrant migration causes developmental defects. Animal models are improving our understanding of neural crest anomalies, but in vivo migration behaviors are poorly understood. Here, we demonstrate that murine neural crest cells display actin-based lamellipodia and filopodia in vivo. Using neural crest-specific knockouts or inhibitors, we show that the serine-threonine kinase glycogen synthase kinase-3 (GSK3) and the cytoskeletal regulator lamellipodin (Lpd) are required for lamellipodia formation while preventing focal adhesion maturation. Lpd is a substrate of GSK3, and phosphorylation of Lpd favors interactions with the Scar/WAVE complex (lamellipodia formation) at the expense of VASP and Mena interactions (adhesion maturation and filopodia formation). This improved understanding of cytoskeletal regulation in mammalian neural crest migration has general implications for neural crest anomalies and cancer.

A Rab-bit hole: Rab40 GTPases as new regulators of the actin cytoskeleton and cell migration

The regulation of machinery involved in cell migration is vital to the maintenance of proper organism function. When migration is dysregulated, a variety of phenotypes ranging from developmental disorders to cancer metastasis can occur. One of the primary structures involved in cell migration is the actin cytoskeleton. Actin assembly and disassembly form a variety of dynamic structures which provide the pushing and contractile forces necessary for cells to properly migrate. As such, actin dynamics are tightly regulated. Classically, the Rho family of GTPases are considered the major regulators of the actin cytoskeleton during cell migration. Together, this family establishes polarity in the migrating cell by stimulating the formation of various actin structures in specific cellular locations. However, while the Rho GTPases are acknowledged as the core machinery regulating actin dynamics and cell migration, a variety of other proteins have become established as modulators of actin structures and cell migration. One such group of proteins is the Rab40 family of GTPases, an evolutionarily and functionally unique family of Rabs. Rab40 originated as a single protein in the bilaterians and, through multiple duplication events, expanded to a four-protein family in higher primates. Furthermore, unlike other members of the Rab family, Rab40 proteins contain a C-terminally located suppressor of cytokine signaling (SOCS) box domain. Through the SOCS box, Rab40 proteins interact with Cullin5 to form an E3 ubiquitin ligase complex. As a member of this complex, Rab40 ubiquitinates its effectors, controlling their degradation, localization, and activation. Because substrates of the Rab40/Cullin5 complex can play a role in regulating actin structures and cell migration, the Rab40 family of proteins has recently emerged as unique modulators of cell migration machinery.

Migration and differentiation of muscle stem cells are coupled by RhoA signalling during regeneration

Skeletal muscle is highly regenerative and is mediated by a population of migratory adult muscle stem cells (muSCs). Effective muscle regeneration requires a spatio-temporally regulated response of the muSC population to generate sufficient muscle progenitor cells that then differentiate at the appropriate time. The relationship between muSC migration and cell fate is poorly understood and it is not clear how forces experienced by migrating cells affect cell behaviour. We have used zebrafish to understand the relationship between muSC cell adhesion, behaviour and fate in vivo. Imaging of pax7-expressing muSCs as they respond to focal injuries in trunk muscle reveals that they migrate by protrusive-based means. By carefully characterizing their behaviour in response to injury we find that they employ an adhesion-dependent mode of migration that is regulated by the RhoA kinase ROCK. Impaired ROCK activity results in reduced expression of cell cycle genes and increased differentiation in regenerating muscle. This correlates with changes to focal adhesion dynamics and migration, revealing that ROCK inhibition alters the interaction of muSCs to their local environment. We propose that muSC migration and differentiation are coupled processes that respond to changes in force from the environment mediated by RhoA signalling.

Cas phosphorylation regulates focal adhesion assembly

Integrin-mediated cell attachment rapidly induces tyrosine kinase signaling. Despite years of research, the role of this signaling in integrin activation and focal adhesion assembly is unclear. We provide evidence that the Src-family kinase (SFK) substrate Cas (Crk-associated substrate, p130Cas, BCAR1) is phosphorylated and associated with its Crk/CrkL effectors in clusters that are precursors of focal adhesions. The initial phospho-Cas clusters contain integrin β1 in its inactive, bent closed, conformation. Later, phospho-Cas and total Cas levels decrease as integrin β1 is activated and core focal adhesion proteins including vinculin, talin, kindlin, and paxillin are recruited. Cas is required for cell spreading and focal adhesion assembly in epithelial and fibroblast cells on collagen and fibronectin. Cas cluster formation requires Cas, Crk/CrkL, SFKs, and Rac1 but not vinculin. Rac1 provides positive feedback onto Cas through reactive oxygen, opposed by negative feedback from the ubiquitin proteasome system. The results suggest a two-step model for focal adhesion assembly in which clusters of phospho-Cas, effectors and inactive integrin β1 grow through positive feedback prior to integrin activation and recruitment of core focal adhesion proteins.

A multi-scale clutch model for adhesion complex mechanics

Cell-matrix adhesion is a central mechanical function to a large number of phenomena in physiology and disease, including morphogenesis, wound healing, and tumor cell invasion. Today, how single cells respond to different extracellular cues has been comprehensively studied. However, how the mechanical behavior of the main individual molecules that form an adhesion complex cooperatively responds to force within the adhesion complex is still poorly understood. This is a key aspect of cell adhesion because how these cell adhesion molecules respond to force determines not only cell adhesion behavior but, ultimately, cell function. To answer this question, we develop a multi-scale computational model for adhesion complexes mechanics. We extend the classical clutch hypothesis to model individual adhesion chains made of a contractile actin network, a talin rod, and an integrin molecule that binds at individual adhesion sites on the extracellular matrix. We explore several scenarios of integrins dynamics and analyze the effects of diverse extracellular matrices on the behavior of the adhesion molecules and on the whole adhesion complex. Our results describe how every single component of the adhesion chain mechanically responds to the contractile actomyosin force and show how they control the traction forces exerted by the cell on the extracellular space. Importantly, our computational results agree with previous experimental data at the molecular and cellular levels. Our multi-scale clutch model presents a step forward not only to further understand adhesion complexes mechanics but also to impact, e.g., the engineering of biomimetic materials, tissue repairment, or strategies to arrest tumor progression.

Static and Dynamic: Evolving Biomaterial Mechanical Properties to Control Cellular Mechanotransduction

The extracellular matrix (ECM) is a highly dynamic system that constantly offers physical, biological, and chemical signals to embraced cells. Increasing evidence suggests that mechanical signals derived from the dynamic cellular microenvironment are essential controllers of cell behaviors. Conventional cell culture biomaterials, with static mechanical properties such as chemistry, topography, and stiffness, have offered a fundamental understanding of various vital biochemical and biophysical processes, such as cell adhesion, spreading, migration, growth, and differentiation. At present, novel biomaterials that can spatiotemporally impart biophysical cues to manipulate cell fate are emerging. The dynamic properties and adaptive traits of new materials endow them with the ability to adapt to cell requirements and enhance cell functions. In this review, an introductory overview of the key players essential to mechanobiology is provided. A biophysical perspective on the state-of-the-art manipulation techniques and novel materials in designing static and dynamic ECM-mimicking biomaterials is taken. In particular, different static and dynamic mechanical cues in regulating cellular mechanosensing and functions are compared. This review to benefit the development of engineering biomechanical systems regulating cell functions is expected.

The Actin Network Interfacing Diverse Integrin-Mediated Adhesions

The interface between the cellular actin network and diverse forms of integrin-mediated cell adhesions displays a unique capacity to serve as accurate chemical and mechanical sensors of the cell's microenvironment. Focal adhesion-like structures of diverse cell types, podosomes in osteoclasts, and invadopodia of invading cancer cells display distinct morphologies and apparent functions. Yet, all three share a similar composition and mode of coupling between a protrusive structure (the lamellipodium, the core actin bundle of the podosome, and the invadopodia protrusion, respectively), and a nearby adhesion site. Cytoskeletal or external forces, applied to the adhesion sites, trigger a cascade of unfolding and activation of key adhesome components (e.g., talin, vinculin, integrin), which in turn, trigger the assembly of adhesion sites and generation of adhesion-mediated signals that affect cell behavior and fate. The structural and molecular mechanisms underlying the dynamic crosstalk between the actin cytoskeleton and the adhesome network are discussed.

The βI domain promotes active β1 integrin clustering into mature adhesion sites

Modulation of integrin function is required in many physiological and pathological settings, such as angiogenesis and cancer. Integrin allosteric changes, clustering, and trafficking cooperate to regulate cell adhesion and motility on extracellular matrix proteins via mechanisms that are partly defined. By exploiting four monoclonal antibodies recognizing distinct conformational epitopes, we show that in endothelial cells (ECs), the extracellular βI domain, but not the hybrid or I-EGF2 domain of active β1 integrins, promotes their FAK-regulated clustering into tensin 1-containing fibrillar adhesions and impairs their endocytosis. In this regard, the βI domain-dependent clustering of active β1 integrins is necessary to favor fibronectin-elicited directional EC motility, which cannot be effectively promoted by β1 integrin conformational activation alone.

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.

Targeting integrin pathways: mechanisms and advances in therapy

Integrins are considered the main cell-adhesion transmembrane receptors that play multifaceted roles as extracellular matrix (ECM)-cytoskeletal linkers and transducers in biochemical and mechanical signals between cells and their environment in a wide range of states in health and diseases. Integrin functions are dependable on a delicate balance between active and inactive status via multiple mechanisms, including protein-protein interactions, conformational changes, and trafficking. Due to their exposure on the cell surface and sensitivity to the molecular blockade, integrins have been investigated as pharmacological targets for nearly 40 years, but given the complexity of integrins and sometimes opposite characteristics, targeting integrin therapeutics has been a challenge. To date, only seven drugs targeting integrins have been successfully marketed, including abciximab, eptifibatide, tirofiban, natalizumab, vedolizumab, lifitegrast, and carotegrast. Currently, there are approximately 90 kinds of integrin-based therapeutic drugs or imaging agents in clinical studies, including small molecules, antibodies, synthetic mimic peptides, antibody-drug conjugates (ADCs), chimeric antigen receptor (CAR) T-cell therapy, imaging agents, etc. A serious lesson from past integrin drug discovery and research efforts is that successes rely on both a deep understanding of integrin-regulatory mechanisms and unmet clinical needs. Herein, we provide a systematic and complete review of all integrin family members and integrin-mediated downstream signal transduction to highlight ongoing efforts to develop new therapies/diagnoses from bench to clinic. In addition, we further discuss the trend of drug development, how to improve the success rate of clinical trials targeting integrin therapies, and the key points for clinical research, basic research, and translational research.

Tension Enhances the Binding Affinity of β1 Integrin by Clamping Talin Tightly: An Insight from Steered Molecular Dynamics Simulations

Integrin activation is a predominant step for cell-cell and cell-ECM interactions. Talin and Kindlin are mechanosensitive adaptor proteins that bind to the integrin cytoplasmic tail and mediate integrin activation, cytoskeleton rearrangement, and focal adhesion assembly. However, knowledge about how Talin and Kindlin synergistically assist integrin activation remains unclear. Here, we performed so-called "ramp-clamp" SMD simulations, which modeled the mechanosignaling from Kindlin, to investigate the effect of tension on the interaction of the β1 integrin cytoplasmic tail with the Talin-F3 domain. The present results showed that mild but not excessive stretching enhanced the binding of integrin with Talin. This mechanical regulation on integrin affinity to Talin referred to an event cascade, in which under stretching, the integrin cytoplasmic tail adopted allostery in response to the mechanical stimulus, remodeling of integrin in favor of Talin-association ensued, and finally, a stable, close-knit complex was formed. In the cascade, the torsion angle transition of integrin was the cue for the stable interaction of the complex under tensile force. The present work suggested a model for Talin and Kindlin to synergistically activate integrin. It should help understand integrin activation and its mechanochemical regulation mechanism, integrin-related innate cellular immune responses, cell adhesion, cell-cell interaction, and integrin-related drug development.

Structural Basis of β2 Integrin Inside—Out Activation

β2 integrins are expressed on all leukocytes. Precise regulation of the β2 integrin is critical for leukocyte adhesion and trafficking. In neutrophils, β2 integrins participate in slow rolling. When activated by inside–out signaling, fully activated β2 integrins mediate rapid leukocyte arrest and adhesion. The two activation pathways, starting with selectin ligand engagement and chemokine receptor ligation, respectively, converge on phosphoinositide 3-kinase, talin-1, kindlin-3 and Rap1. Here, we focus on recent structural insights into autoinhibited talin-1 and autoinhibited trimeric kindlin-3. When activated, both talin-1 and kindlin-3 can bind the β2 cytoplasmic tail at separate but adjacent sites. We discuss possible pathways for talin-1 and kindlin-3 activation, recruitment to the plasma membrane, and their role in integrin activation. We propose new models of the final steps of integrin activation involving the complex of talin-1, kindlin-3, integrin and the plasma membrane.

Parvin: A hub of intracellular signalling pathways regulating cellular behaviour and disease progression

α-actinin superfamily houses the family of parvins, comprising α, β and γ isoforms in the vertebrates and a single orthologue in the invertebrates. Parvin as an adaptor protein is a member of the ternary IPP-complex including Integrin Linked Kinase (ILK) and particularly-interesting-Cys-His-rich protein (PINCH). Each of the complex proteins showed a conserved lineage and was principally used by the evolutionarily primitive integrin-adhesome machinery to regulate cellular behaviour and signalling pathways. Parvin facilitated integrin mediated integration of the extracellular matrix with cytoskeletal framework culminating in regulation of cellular adhesion and spreading, cytoskeleton reorganisation and cell survival. Studies have established role of parvin in pregnancy, lactation, matrix degradation, blood vessel formation and in several diseases such as cancer, NAFLD and cardiac diseases etc. This review narrates the history of parvin discovery, its elaborate gene structure and conservation across phyla including cellular expression, localisation and interacting partners in vertebrates as well as invertebrates. The review further discusses how parvin acts as an epicentre of signalling pathways, its associated mutants and diseased conditions.

Focal adhesion-mediated cell anchoring and migration: from in vitro to in vivo

Cell-extracellular matrix interactions have been studied extensively using cells cultured in vitro. These studies indicate that focal adhesion (FA)-based cell-extracellular matrix interactions are essential for cell anchoring and cell migration. Whether FAs play a similarly important role in vivo is less clear. Here, we summarize the formation and function of FAs in cultured cells and review how FAs transmit and sense force in vitro. Using examples from animal studies, we also describe the role of FAs in cell anchoring during morphogenetic movements and cell migration in vivo. Finally, we conclude by discussing similarities and differences in how FAs function in vitro and in vivo.

Synergistic phase separation of two pathways promotes integrin clustering and nascent adhesion formation

Integrin adhesion complexes (IACs) are integrin-based plasma-membrane-associated compartments where cells sense environmental cues. The physical mechanisms and molecular interactions that mediate initial IAC formation are unclear. We found that both p130Cas ('Cas') and Focal adhesion kinase ('FAK') undergo liquid-liquid phase separation in vitro under physiologic conditions. Cas- and FAK- driven phase separation is sufficient to reconstitute kindlin-dependent integrin clustering in vitro with recombinant mammalian proteins. In vitro condensates and IACs in mouse embryonic fibroblasts (MEFs) exhibit similar sensitivities to environmental perturbations including changes in temperature and pH. Furthermore, mutations that inhibit or enhance phase separation in vitro reduce or increase the number of IACs in MEFs, respectively. Finally, we find that the Cas and FAK pathways act synergistically to promote phase separation, integrin clustering, IAC formation and partitioning of key components in vitro and in cells. We propose that Cas- and FAK-driven phase separation provides an intracellular trigger for integrin clustering and nascent IAC formation.

Molecular motion and tridimensional nanoscale localization of kindlin control integrin activation in focal adhesions

Focal adhesions (FAs) initiate chemical and mechanical signals involved in cell polarity, migration, proliferation and differentiation. Super-resolution microscopy revealed that FAs are organized at the nanoscale into functional layers from the lower plasma membrane to the upper actin cytoskeleton. Yet, how FAs proteins are guided into specific nano-layers to promote interaction with given targets is unknown. Using single protein tracking, super-resolution microscopy and functional assays, we link the molecular behavior and 3D nanoscale localization of kindlin with its function in integrin activation inside FAs. We show that immobilization of integrins in FAs depends on interaction with kindlin. Unlike talin, kindlin displays free diffusion along the plasma membrane outside and inside FAs. We demonstrate that the kindlin Pleckstrin Homology domain promotes membrane diffusion and localization to the membrane-proximal integrin nano-layer, necessary for kindlin enrichment and function in FAs. Using kindlin-deficient cells, we show that kindlin membrane localization and diffusion are crucial for integrin activation, cell spreading and FAs formation. Thus, kindlin uses a different route than talin to reach and activate integrins, providing a possible molecular basis for their complementarity during integrin activation.

Bottom-up reconstitution of focal adhesion complexes

Focal adhesions (FA) are large macromolecular assemblies relevant for various cellular and pathological events such as migration, polarization, and metastatic cancer formation. At FA sites at the migrating periphery of a cell, hundreds of players gather and form a network to respond to extra cellular stimuli transmitted by the integrin receptor, the most upstream component within a cell, initiating the FA signaling pathway. Numerous cellular experiments have been performed to understand the FA architecture and functions; however, their intricate network formation hampers unraveling the precise molecular actions of individual players. Here, in vitro bottom‐up reconstitution presents an advantageous approach for elucidating the FA machinery and the hierarchical crosstalk of involved cellular players.

Focal adhesion dynamics in cellular function and disease

Acting as a bridge between the cytoskeleton of the cell and the extra cellular matrix (ECM), the cell-ECM adhesions with integrins at their core, play a major role in cell signalling to direct mechanotransduction, cell migration, cell cycle progression, proliferation, differentiation, growth and repair. Biochemically, these adhesions are composed of diverse, yet an organised group of structural proteins, receptors, adaptors, various enzymes including protein kinases, phosphatases, GTPases, proteases, etc. as well as scaffolding molecules. The major integrin adhesion complexes (IACs) characterised are focal adhesions (FAs), invadosomes (podosomes and invadopodia), hemidesmosomes (HDs) and reticular adhesions (RAs). The varied composition and regulation of the IACs and their signalling, apart from being an integral part of normal cell survival, has been shown to be of paramount importance in various developmental and pathological processes. This review per-illustrates the recent advancements in the research of IACs, their crucial roles in normal as well as diseased states. We have also touched on few of the various methods that have been developed over the years to visualise IACs, measure the forces they exert and study their signalling and molecular composition. Having such pertinent roles in the context of various pathologies, these IACs need to be understood and studied to develop therapeutical targets. We have given an update to the studies done in recent years and described various techniques which have been applied to study these structures, thereby, providing context in furthering research with respect to IAC targeted therapeutics.

Phosphorylation of Kindlins and the Control of Integrin Function

Integrins serve as conduits for the transmission of information between cells and their extracellular environment. Signaling across integrins is bidirectional, transducing both inside-out and outside-signaling. Integrin activation, a transition from a low affinity/avidity state to a high affinity/avidity state for cognate ligands, is an outcome of inside-signaling. Such activation is particularly important for the recognition of soluble ligands by blood cells but also influences cell-cell and cell-matrix interactions. Integrin activation depends on a complex series of interactions, which both accelerate and inhibit their interconversion from the low to the high affinity/avidity state. There are three components regarded as being most proximately involved in integrin activation: the integrin cytoplasmic tails, talins and kindlins. The participation of each of these molecules in integrin activation is highly regulated by post-translation modifications. The importance of targeted phosphorylation of integrin cytoplasmic tails and talins in integrin activation is well-established, but much less is known about the role of post-translational modification of kindlins. The kindlins, a three-member family of 4.1-ezrin-radixin-moesin (FERM)-domain proteins in mammals, bind directly to the cytoplasmic tails of integrin beta subunits. This commentary provides a synopsis of the emerging evidence for the role of kindlin phosphorylation in integrin regulation.

Pre-complexation of talin and vinculin without tension is required for efficient nascent adhesion maturation

Talin and vinculin are mechanosensitive proteins that are recruited early to integrin-based nascent adhesions (NAs). In two epithelial cell systems with well-delineated NA formation, we find these molecules concurrently recruited to the subclass of NAs maturing to focal adhesions. After the initial recruitment under minimal load, vinculin accumulates in maturing NAs at a ~ fivefold higher rate than in non-maturing NAs, and is accompanied by a faster traction force increase. We identify the R8 domain in talin, which exposes a vinculin-binding-site (VBS) in the absence of load, as required for NA maturation. Disruption of R8 domain function reduces load-free vinculin binding to talin, and reduces the rate of additional vinculin recruitment. Taken together, these data show that the concurrent recruitment of talin and vinculin prior to mechanical engagement with integrins is essential for the traction-mediated unfolding of talin, exposure of additional VBSs, further recruitment of vinculin, and ultimately, NA maturation.

Kindlin-3 recruitment to the plasma membrane precedes high-affinity β2-integrin and neutrophil arrest from rolling

Integrin-mediated neutrophil adhesion starts by arrest from rolling. Activation of integrins involves conformational changes from an inactive, bent conformation to an extended conformation (E+) with high affinity for ligand binding (H+). The cytoplasmic protein kindlin-3 is necessary for leukocyte adhesion; mutations of kindlin-3 cause leukocyte adhesion deficiency type 3. Kindlin-3 binds the β2-integrin cytoplasmic tail at a site distinct from talin-1, but the molecular mechanism by which kindlin-3 activates β2-integrins is unknown. In this study, we measured the spatiotemporal dynamics of kindlin-3 and β2-integrin conformation changes during neutrophil and HL-60 cell rolling and arrest under flow. Using high-resolution quantitative dynamic footprinting microscopy and kindlin-3-fluorescent protein (FP) fusion proteins, we found that kindlin-3 was recruited to the plasma membrane in response to interleukin-8 (IL-8) before induction of the H+ β2-integrin conformation. Intravital imaging revealed that EGFP-kindlin-3-reconstituted, kindlin-3-knockout neutrophils arrest in vivo in response to CXCL1. EGFP-kindlin-3 in primary mouse neutrophils was also recruited to the plasma membrane before arrest. Upon arrest, we found small clusters of high-affinity β2-integrin molecules within large areas of membrane-proximal kindlin-3 FP. Deletion of kindlin-3 or its pleckstrin homology (PH) domain in neutrophil-like HL-60 cells completely abolished H+ β2-integrin induction. IL-8 also triggered recruitment of the isolated kindlin-3 PH domain to the plasma membrane before arrest. In summary, we showed that the kindlin-3 PH domain is necessary for recruitment to the plasma membrane, where full-length kindlin-3 is indispensable for the induction of high-affinity β2-integrin.

Network Analysis of Integrin Adhesion Complexes

Cell-surface adhesion receptors mediate interactions with the extracellular matrix (ECM) to control many fundamental aspects of cell behavior, including cell migration, survival, and proliferation. Integrin adhesion receptors recruit structural and signaling proteins to form multimolecular adhesion complexes that link the plasma membrane to the actomyosin cytoskeleton. The assembly and turnover of adhesion complexes are tightly regulated, governed in part by the networks of physical protein interactions and functional signaling associations between components of the adhesome. Proteomic profiling of adhesion complexes has begun to reveal their molecular complexity and diversity. To interrogate the composition of cell-ECM adhesions, we detail herein an approach for the network analysis of adhesion complex proteomes. Integration of these proteomic data with adhesome databases in the context of predicted protein interactions enables the mapping of experimentally defined adhesion complex networks. Computational analysis of resultant network models can identify subnetworks of putative functionally linked adhesion protein communities. This approach provides a framework to predict functional adhesion protein relationships and generate new mechanistic hypotheses for further experimental testing.

Initiation of focal adhesion assembly by talin and kindlin: A dynamic view

Focal adhesions (FAs) are integrin-containing protein complexes regulated by a network of hundreds of protein-protein interactions. They are formed in a spatiotemporal manner upon the activation of integrin transmembrane receptors, which is crucial to trigger cell adhesion and many other cellular processes including cell migration, spreading and proliferation. Despite decades of studies, a detailed molecular level understanding on how FAs are organized and function is lacking due to their highly complex and dynamic nature. However, advances have been made on studying key integrin activators, talin and kindlin, and their associated proteins, which are major components of nascent FAs critical for initiating the assembly of mature FAs. This review will discuss the structural and functional findings of talin and kindlin and their immediate interaction network, which will shed light upon the architecture of nascent FAs and how they act as seeds for FA assembly to dynamically regulate diverse adhesion-dependent physiological and pathological responses.

Mechanical and Physical Regulation of Fibroblast-Myofibroblast Transition: From Cellular Mechanoresponse to Tissue Pathology

Fibroblasts are cells present throughout the human body that are primarily responsible for the production and maintenance of the extracellular matrix (ECM) within the tissues. They have the capability to modify the mechanical properties of the ECM within the tissue and transition into myofibroblasts, a cell type that is associated with the development of fibrotic tissue through an acute increase of cell density and protein deposition. This transition from fibroblast to myofibroblast-a well-known cellular hallmark of the pathological state of tissues-and the environmental stimuli that can induce this transition have received a lot of attention, for example in the contexts of asthma and cardiac fibrosis. Recent efforts in understanding how cells sense their physical environment at the micro- and nano-scales have ushered in a new appreciation that the substrates on which the cells adhere provide not only passive influence, but also active stimulus that can affect fibroblast activation. These studies suggest that mechanical interactions at the cell-substrate interface play a key role in regulating this phenotype transition by changing the mechanical and morphological properties of the cells. Here, we briefly summarize the reported chemical and physical cues regulating fibroblast phenotype. We then argue that a better understanding of how cells mechanically interact with the substrate (mechanosensing) and how this influences cell behaviors (mechanotransduction) using well-defined platforms that decouple the physical stimuli from the chemical ones can provide a powerful tool to control the balance between physiological tissue regeneration and pathological fibrotic response.

Recent Advances and Prospects in the Research of Nascent Adhesions

Nascent adhesions are submicron transient structures promoting the early adhesion of cells to the extracellular matrix. Nascent adhesions typically consist of several tens of integrins, and serve as platforms for the recruitment and activation of proteins to build mature focal adhesions. They are also associated with early stage signaling and the mechanoresponse. Despite their crucial role in sampling the local extracellular matrix, very little is known about the mechanism of their formation. Consequently, there is a strong scientific activity focused on elucidating the physical and biochemical foundation of their development and function. Precisely the results of this effort will be summarized in this article.

Talin-activated vinculin interacts with branched actin networks to initiate bundles

Vinculin plays a fundamental role in integrin-mediated cell adhesion. Activated by talin, it interacts with diverse adhesome components, enabling mechanical coupling between the actin cytoskeleton and the extracellular matrix. Here we studied the interactions of activated full-length vinculin with actin and the way it regulates the organization and dynamics of the Arp2/3 complex-mediated branched actin network. Through a combination of surface patterning and light microscopy experiments we show that vinculin can bundle dendritic actin networks through rapid binding and filament crosslinking. We show that vinculin promotes stable but flexible actin bundles having a mixed-polarity organization, as confirmed by cryo-electron tomography. Adhesion-like synthetic design of vinculin activation by surface-bound talin revealed that clustered vinculin can initiate and immobilize bundles from mobile Arp2/3-branched networks. Our results provide a molecular basis for coordinate actin bundle formation at nascent adhesions.

The Rho-GEF PIX-1 directs assembly or stability of lateral attachment structures between muscle cells

PIX proteins are guanine nucleotide exchange factors (GEFs) that activate Rac and Cdc42, and are known to have numerous functions in various cell types. Here, we show that a PIX protein has an important function in muscle. From a genetic screen in C. elegans, we found that pix-1 is required for the assembly of integrin adhesion complexes (IACs) at borders between muscle cells, and is required for locomotion of the animal. A pix-1 null mutant has a reduced level of activated Rac in muscle. PIX-1 localizes to IACs at muscle cell boundaries, M-lines and dense bodies. Mutations in genes encoding proteins at known steps of the PIX signaling pathway show defects at muscle cell boundaries. A missense mutation in a highly conserved residue in the RacGEF domain results in normal levels of PIX-1 protein, but a reduced level of activated Rac in muscle, and abnormal IACs at muscle cell boundaries.

The Architecture of Talin1 Reveals an Autoinhibition Mechanism

Focal adhesions (FAs) are protein machineries essential for cell adhesion, migration, and differentiation. Talin is an integrin-activating and tension-sensing FA component directly connecting integrins in the plasma membrane with the actomyosin cytoskeleton. To understand how talin function is regulated, we determined a cryoelectron microscopy (cryo-EM) structure of full-length talin1 revealing a two-way mode of autoinhibition. The actin-binding rod domains fold into a 15-nm globular arrangement that is interlocked by the integrin-binding FERM head. In turn, the rod domains R9 and R12 shield access of the FERM domain to integrin and the phospholipid PIP₂ at the membrane. This mechanism likely ensures synchronous inhibition of integrin, membrane, and cytoskeleton binding. We also demonstrate that compacted talin1 reversibly unfolds to an ∼60-nm string-like conformation, revealing interaction sites for vinculin and actin. Our data explain how fast switching between active and inactive conformations of talin could regulate FA turnover, a process critical for cell adhesion and signaling.

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The structure of the autoinhibited human full-length talin1 was analyzed by cryo-EM

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Talin1 reversibly changes between a 15-nm closed and a ∼60-nm open conformation

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Rod R9/R12 and FERM domains synchronously shield membrane and cytoskeleton binding

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F-Actin and vinculin binding to talin is regulated by the opening of talin

GIV•Kindlin Interaction Is Required for Kindlin-Mediated Integrin Recognition and Activation

Cells perceive and respond to the extracellular matrix via integrin receptors; their dysregulation has been implicated in inflammation and cancer metastasis. Here we show that a guanine nucleotide-exchange modulator of trimeric-GTPase Gαi, GIV (a.k.a Girdin), directly binds the integrin adaptor Kindlin-2. A non-canonical short linear motif within the C terminus of GIV binds Kindlin-2-FERM3 domain at a site that is distinct from the binding site for the canonical NPxY motif on the -integrin tail. Binding of GIV to Kindlin-2 allosterically enhances Kindlin-2's affinity for β1-integrin. Consequently, integrin activation and clustering are maximized, which augments cell adhesion, spreading, and invasion. Findings elucidate how the GIV•Kindlin-2 complex has a 2-fold impact: it allosterically synergizes integrin activation and enables β1-integrins to indirectly access and modulate trimeric GTPases via the complex. Furthermore, Cox proportional-hazard models on tumor transcriptomics provide trans-scale evidence of synergistic interactions between GIV•Kindlin-2•β1-integrin on time to progression to metastasis.

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GIV and Kindlin (K2), two integrin adaptors that promote metastasis, bind each other

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Binding of GIV or integrin to K2 allosterically enhances GIV•K2•integrin complexes

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Binding is required for the maximal recruitment of GIV and K2 to active integrins

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Binding facilitates integrin clustering, activation, tumor cell adhesion, invasion

Hierarchical Micro-Nano Topography Promotes Cell Adhesion and Osteogenic Differentiation via Integrin α2-PI3K-AKT Signaling Axis

Surface topography dictates important aspects of cell biological behaviors. In our study, hierarchical micro-nano topography (SLM-AHT) with micro-scale grooves and nano-scale pores was fabricated and compared with smooth topography (S) and irregular micro-scale topography (SLA) surfaces to investigate mechanism involved in cell-surface interactions. Integrin α2 had a higher expression level on SLM-AHT surface compared with S and SLA surfaces, and the expression levels of osteogenic markers icluding Runx2, Col1a1, and Ocn were concomitantly upregulated on SLM-AHT surface. Moreover, formation of mature focal adhesions were significantly enhanced in SLM-AHT group. Noticablely, silencing integrin α2 could wipe out the difference of osteogenic gene expression among surfaces with different topography, indicating a crucial role of integrin α2 in topography induced osteogenic differentiation. In addition, PI3K-AKT signaling was proved to be regulated by integrin α2 and consequently participate in this process. Taken together, our findings illustrated that integrin α2-PI3K-AKT signaling axis plays a key role in hierarchical micro-nano topography promoting cell adhesion and osteogenic differentiation.

P0-Related Protein Accelerates Human Mesenchymal Stromal Cell Migration by Modulating VLA-5 Interactions with Fibronectin

P₀-related protein (PZR), a Noonan and LEOPARD syndrome target, is a member of the transmembrane Immunoglobulin superfamily. Its cytoplasmic tail contains two immune-receptor tyrosine-based inhibitory motifs (ITIMs), implicated in adhesion-dependent signaling and regulating cell adhesion and motility. PZR promotes cell migration on the extracellular matrix (ECM) molecule, fibronectin, by interacting with SHP-2 (Src homology-2 domain-containing protein tyrosine phosphatase-2), a molecule essential for skeletal development and often mutated in Noonan and LEOPARD syndrome patients sharing overlapping musculoskeletal abnormalities and cardiac defects. To further explore the role of PZR, we assessed the expression of PZR and its ITIM-less isoform, PZRb, in human bone marrow mesenchymal stromal cells (hBM MSC), and its ability to facilitate adhesion to and spreading and migration on various ECM molecules. Furthermore, using siRNA knockdown, confocal microscopy, and immunoprecipitation assays, we assessed PZR and PZRb interactions with β1 integrins. PZR was the predominant isoform in hBM MSC. Migrating hBM MSCs interacted most effectively with fibronectin and required the association of PZR, but not PZRb, with the integrin, VLA-5(α5β1), leading to modulation of focal adhesion kinase phosphorylation and vinculin levels. This raises the possibility that dysregulation of PZR function may modify hBM MSC migratory behavior, potentially contributing to skeletal abnormalities.

Feeling Things Out: Bidirectional Signaling of the Cell-ECM Interface, Implications in the Mechanobiology of Cell Spreading, Migration, Proliferation, and Differentiation

Biophysical cues stemming from the extracellular environment are rapidly transduced into discernible chemical messages (mechanotransduction) that direct cellular activities-placing the extracellular matrix (ECM) as a potent regulator of cell behavior. Dynamic reciprocity between the cell and its associated matrix is essential to the maintenance of tissue homeostasis and dysregulation of both ECM mechanical signaling, via pathological ECM turnover, and internal mechanotransduction pathways contribute to disease progression. This review covers the current understandings of the key modes of signaling used by both the cell and ECM to coregulate one another. By taking an outside-in approach, the inherent complexities and regulatory processes at each level of signaling (ECM, plasma membrane, focal adhesion, and cytoplasm) are captured to give a comprehensive picture of the internal and external mechanoregulatory environment. Specific emphasis is placed on the focal adhesion complex which acts as a central hub of mechanical signaling, regulating cell spreading, migration, proliferation, and differentiation. In addition, a wealth of available knowledge on mechanotransduction is curated to generate an integrated signaling network encompassing the central components of the focal adhesion, cytoplasm and nucleus that act in concert to promote durotaxis, proliferation, and differentiation in a stiffness-dependent manner.

The Kindlin Outside Connection

Transmembrane integrin bridges the extracellular and intracellular environments and is activated by focal adhesion proteins, talin and kindlin. Activated integrin engages ligands from the extracellular matrix and controls intracellular responses. In this issue of Structure, Zhu et al. (2019) describe an initial step involving recruitment of paxillin by ubiquitin-like kindlin-2 domain.

Adhesions Assemble!-Autoinhibition as a Major Regulatory Mechanism of Integrin-Mediated Adhesion

The advent of cell-cell and cell-extracellular adhesion enabled cells to interact in a coherent manner, forming larger structures and giving rise to the development of tissues, organs and complex multicellular life forms. The development of such organisms required tight regulation of dynamic adhesive structures by signaling pathways that coordinate cell attachment. Integrin-mediated adhesion to the extracellular matrix provides cells with support, survival signals and context-dependent cues that enable cells to run different cellular programs. One mysterious aspect of the process is how hundreds of proteins assemble seemingly spontaneously onto the activated integrin. An emerging concept is that adhesion assembly is regulated by autoinhibition of key proteins, a highly dynamic event that is modulated by a variety of signaling events. By enabling precise control of the activation state of proteins, autoinhibition enables localization of inactive proteins and the formation of pre-complexes. In response to the correct signals, these proteins become active and interact with other proteins, ultimately leading to development of cell-matrix junctions. Autoinhibition of key components of such adhesion complexes—including core components integrin, talin, vinculin, and FAK and important peripheral regulators such as RIAM, Src, and DLC1—leads to a view that the majority of proteins involved in complex assembly might be regulated by intramolecular interactions. Autoinhibition is relieved via multiple different signals including post-translation modification and proteolysis. More recently, mechanical forces have been shown to stabilize and increase the lifetimes of active conformations, identifying autoinhibition as a means of encoding mechanosensitivity. The complexity and scope for nuanced adhesion dynamics facilitated via autoinhibition provides numerous points of regulation. In this review, we discuss what is known about this mode of regulation and how it leads to rapid and tightly controlled assembly and disassembly of cell-matrix adhesion.

Integrin nanoclusters can bridge thin matrix fibres to form cell-matrix adhesions

Integrin-mediated cell-matrix adhesions are key to sensing the geometry and rigidity of extracellular environments and influence vital cellular processes. In vivo, the extracellular matrix is composed of fibrous arrays. To understand the fibre geometries that are required for adhesion formation, we patterned nanolines of various line widths and arrangements in single, crossing or paired arrays with the integrin-binding peptide Arg-Gly-Asp. Single thin lines (width ≤30 nm) did not support cell spreading or formation of focal adhesions, despite the presence of a high density of Arg-Gly-Asp, but wide lines (>40 nm) did. Using super-resolution microscopy, we observed stable, dense integrin clusters formed on parallel (within 110 nm) or crossing thin lines (mimicking a matrix mesh) similar to those on continuous substrates. These dense clusters bridged the line pairs by recruiting activated but unliganded integrins, as verified by integrin mutants unable to bind ligands that coclustered with ligand-bound integrins when present in an active extended conformation. Thus, in a fibrous extracellular matrix mesh, stable integrin nanoclusters bridge between thin (≤30 nm) matrix fibres and bring about downstream consequences of cell motility and growth.