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Talin (herein referring to the talin-1 form), is a cytoskeletal adapter protein that binds integrin receptors and F-actin, and is a key factor in the formation and regulation of integrin-dependent cell-matrix adhesions. Talin forms the mechanical link between the cytoplasmic domain of integrins and the actin cytoskeleton. Through this linkage, talin is at the origin of mechanosignaling occurring at the plasma membrane-cytoskeleton interface. Despite its central position, talin is not able to fulfill its tasks alone, but requires help from kindlin and paxillin to detect and transform the mechanical tension along the integrin-talin-F-actin axis into intracellular signaling. The talin head forms a classical FERM domain, which is required to bind and regulate the conformation of the integrin receptor, as well as to induce intracellular force sensing. The FERM domain allows the strategic positioning of protein-protein and protein-lipid interfaces, including the membrane-binding and integrin affinity-regulating F1 loop, as well as the interaction with lipid-anchored Rap1 (Rap1a and Rap1b in mammals) GTPase. Here, we summarize the structural and regulatory features of talin and explain how it regulates cell adhesion and force transmission, as well as intracellular signaling at integrin-containing cell-matrix attachment sites., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

Focal adhesions are composed of transmembrane integrins, linking the extracellular matrix to the actomyosin cytoskeleton, via cytoplasmic proteins. Adhesion depends on the activation of integrins. Talin and kindlin proteins are intracellular activators of integrins that bind to β-integrin cytoplasmic tails. Integrin activation and clustering through extracellular ligands guide the organization of adhesion complexes. However, the roles of talin and kindlin in this process are poorly understood. To determine the contribution of talin, kindlin, lipids and actomyosin in integrin clustering, we used a biomimetic in vitro system, made of giant unilamellar vesicles, containing transmembrane integrins (herein αIIbβ3), with purified talin (talin-1), kindlin (kindlin-2, also known as FERMT2) and actomyosin. Here, we show that talin and kindlin individually have the ability to cluster integrins. Talin and kindlin synergize to induce the formation of larger integrin clusters containing the three proteins. Comparison of protein density reveals that kindlin increases talin and integrin density, whereas talin does not affect kindlin and integrin density. Finally, kindlin increases integrin-talin-actomyosin coupling. Our study unambiguously demonstrates how kindlin and talin cooperate to induce integrin clustering, which is a major parameter for cell adhesion., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

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.

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., (© 2020 The Protein Society.)

Integrin receptors enable cells to sense and respond to their chemical and physical environment. As a class of membrane receptors, they provide a dynamic, tightly regulated link between the extracellular matrix or cellular counter-receptors and intracellular cytoskeletal and signaling networks. They enable transmission of mechanical force across the plasma membrane, and particularly for cardiomyocytes, may sense the mechanical load placed on cells. Talins and Kindlins are two families of FERM-domain proteins which bind the cytoplasmic tail of integrins, recruit cytoskeletal and signaling proteins involved in mechano-transduction, and those which synergize to activate integrins, allowing the integrins to physically change and bind to extracellular ligands. In this review, we will discuss the roles of talin and kindlin, particularly as integrin activators, with a focus on cardiac myocytes.

Kindlins play an important role in supporting integrin activation by cooperating with talin; however, the mechanistic details remain unclear. Here, we show that kindlins interacted directly with paxillin and that this interaction could support integrin αIIbβ3 activation. An exposed loop in the N-terminal F 0 subdomain of kindlins was involved in mediating the interaction. Disruption of kindlin binding to paxillin by structure-based mutations significantly impaired the function of kindlins in supporting integrin αIIbβ3 activation. Both kindlin and talin were required for paxillin to enhance integrin activation. Interestingly, a direct interaction between paxillin and the talin head domain was also detectable. Mechanistically, paxillin, together with kindlin, was able to promote the binding of the talin head domain to integrin, suggesting that paxillin complexes with kindlin and talin to strengthen integrin activation. Specifically, we observed that crosstalk between kindlin-3 and the paxillin family in mouse platelets was involved in supporting integrin αIIbβ3 activation and in vivo platelet thrombus formation. Taken together, our findings uncover a novel mechanism by which kindlin supports integrin αIIbβ3 activation, which might be beneficial for developing safer anti-thrombotic therapies., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2017. Published by The Company of Biologists Ltd.)

Integrins require an activation step prior to ligand binding and signaling. How talin and kindlin contribute to these events in non-hematopoietic cells is poorly understood. Here we report that fibroblasts lacking either talin or kindlin failed to activate β1 integrins, adhere to fibronectin (FN) or maintain their integrins in a high affinity conformation induced by Mn(2+). Despite compromised integrin activation and adhesion, Mn(2+) enabled talin- but not kindlin-deficient cells to initiate spreading on FN. This isotropic spreading was induced by the ability of kindlin to directly bind paxillin, which in turn bound focal adhesion kinase (FAK) resulting in FAK activation and the formation of lamellipodia. Our findings show that talin and kindlin cooperatively activate integrins leading to FN binding and adhesion, and that kindlin subsequently assembles an essential signaling node at newly formed adhesion sites in a talin-independent manner.

Integrin function is regulated by activation involving conformational changes that modulate ligand-binding affinity and downstream signaling. Activation is regulated through inside-out signaling which is controlled by many signaling pathways via a final common pathway through kindlin and talin, which bind to the intracellular tail of beta integrins. Previous studies have shown that the axon growth inhibitory molecules NogoA and chondroitin sulfate proteoglycans (CSPGs) inactivate integrins. Overexpressing kindlin-1 in dorsal root ganglion (DRG) neurons activates integrins, enabling their axons to overcome inhibitory molecules in the environment, and promoting regeneration in vivo following dorsal root crush. Other studies have indicated that expression of the talin head alone or with kindlin can enhance integrin activation. Here, using adult rat DRG neurons, we investigate the effects of overexpressing various forms of talin on axon growth and integrin signaling. We found that overexpression of the talin head activated axonal integrins but inhibited downstream signaling via FAK, and did not promote axon growth. Similarly, co-expression of the talin head and kindlin-1 prevented the growth-promoting effect of kindlin-1, suggesting that the talin head acts as a form of dominant negative for integrin function. Using full-length talin constructs in PC12 cells we observed that neurite growth was enhanced by the expression of wild-type talin and more so by two 'activated' forms of talin produced by point mutation (on laminin and aggrecan-laminin substrates). Nevertheless, co-expression of full-length talin with kindlin did not promote neurite growth more than either molecule alone. In vivo, we find that talin is present in PNS axons (sciatic nerve), and also in CNS axons of the corticospinal tract., (Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.)

Simple Summary: Neurons are biological wires that send signals not only across the brain, but throughout the body. Like almost all cells in the human body, neurons adhere to their surroundings through receptors known as integrins. As the nervous system develops, integrins become essential for neuronal wires, known as axons, to adhere within their environment and pull themselves forward as they grow towards their target. Following injury to the nervous system, the damaged axons must regrow towards their target to restore communication. The integrin receptor therefore plays an important role in this process of regeneration. Integrins mediate their effects through an intricate scaffold of proteins inside the cell, which must be coordinated by a specific central protein, called focal adhesion kinase. This review discusses the signalling pathway of integrin receptors and focal adhesion kinase, specifically in neurons, and how this signalling pathway mediates neuronal growth. We also discuss how integrins and focal adhesion kinase could contribute to neuronal repair following nervous system injury. Integrin receptors are essential contributors to neurite outgrowth and axon elongation. Activated integrins engage components of the extracellular matrix, enabling the growth cone to form point contacts, which connect the extracellular substrate to dynamic intracellular protein complexes. These adhesion complexes facilitate efficient growth cone migration and neurite extension. Major signalling pathways mediated by the adhesion complex are instigated by focal adhesion kinase (FAK), whilst axonal guidance molecules present in vivo promote growth cone turning or retraction by local modulation of FAK activity. Activation of FAK is marked by phosphorylation following integrin engagement, and this activity is tightly regulated during neurite outgrowth. FAK inhibition slows neurite outgrowth by reducing point contact turnover; however, mutant FAK constructs with enhanced activity stimulate aberrant outgrowth. Importantly, FAK is a major structural component of maturing adhesion sites, which provide the platform for actin polymerisation to drive leading edge advance. In this review, we discuss the coordinated signalling of integrin receptors and FAK, as well as their role in regulating neurite outgrowth and axon elongation. We also discuss the importance of the integrin–FAK axis in vivo, as integrin expression and activation are key determinants of successful axon regeneration following injury. [ABSTRACT FROM AUTHOR]

In multicellular organisms, cell adhesion is a pivotal physiological process which is essential for cell–cell communications, cell migration, and interactions with extracellular matrix. Integrins, a family of large hetero-dimeric type I membrane proteins, are known for driving cell adhesion functions. Among 24 different integrins, four β2 integrins, αL β2, αM β2, αX β2 and αD β2, are specific for cell adhesion and migration of leukocytes. Many cytosolic proteins interact with short cytosolic tails (CTs) of β2 and other integrins which are essential in bi-directional signaling processes. Further, phosphorylation of CTs of integrins regulates binding of intra-cellular proteins and signaling systems. In this review, recent advances in structures and interactions of multi-protein complexes of integrin tails, with a focus on β2 integrin, and cytosolic proteins are discussed along with a proposed future direction. [ABSTRACT FROM AUTHOR]

Neutrophils are the most abundant leukocytes in humans and are critical for innate immunity and inflammation. Integrins are critical for neutrophil functions, especially for their recruitment to sites of inflammation or infections. Integrin conformational changes during activation have been heavily investigated but are still not fully understood. Many regulators, such as talin, Rap1-interacting adaptor molecule (RIAM), Rap1, and kindlin, are critical for integrin activation and might be potential targets for integrin-regulating drugs in treating inflammatory diseases. In this review, we outline integrin activation regulators in neutrophils with a focus on the above critical regulators, as well as newly discovered modulators that are involved in integrin activation. [ABSTRACT FROM AUTHOR]

Cell adhesion is essential for the formation of organs, cellular migration, and interaction with target cells and the extracellular matrix. Integrins are large protein α/β-chain heterodimers and form a major family of cell adhesion molecules. Recent research has dramatically increased our knowledge of how integrin phosphorylations regulate integrin activity. Phosphorylations determine the signaling complexes formed on the cytoplasmic tails, regulating downstream signaling. α-Chain phosphorylation is necessary for inducing β-chain phosphorylation in LFA-1, and the crosstalk from one integrin to another activating or inactivating its function is in part mediated by phosphorylation of β-chains. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus receptor angiotensin-converting enzyme 2 (ACE2) and possible integrin coreceptors may crosstalk and induce a phosphorylation switch and autophagy. Integrins are transmembrane proteins involved in cell–cell and cell–matrix communication. Recent studies show how integrin phosphorylations regulate integrin activity. Phosphorylation of both integrin α- and β-chains are emerging as being critical for activity. Phosphorylation of integrin β-chains enables kindlin binding by the assistance of talin, resulting in cell adhesion. Crosstalk between integrins and other receptors may occur by phosphorylation switches. [ABSTRACT FROM AUTHOR]

Abstract Integrins are a family of transmembrane glycoprotein signaling receptors that can transmit bioinformation bidirectionally across the plasma membrane. Integrin αIIbβ3 is expressed at a high level in platelets and their progenitors, where it plays a central role in platelet functions, hemostasis, and arterial thrombosis. Integrin αIIbβ3 also participates in cancer progression, such as tumor cell proliferation and metastasis. In resting platelets, integrin αIIbβ3 adopts an inactive conformation. Upon agonist stimulation, the transduction of inside-out signals leads integrin αIIbβ3 to switch from a low- to high-affinity state for fibrinogen and other ligands. Ligand binding causes integrin clustering and subsequently promotes outside-in signaling, which initiates and amplifies a range of cellular events to drive essential platelet functions such as spreading, aggregation, clot retraction, and thrombus consolidation. Regulation of the bidirectional signaling of integrin αIIbβ3 requires the involvement of numerous interacting proteins, which associate with the cytoplasmic tails of αIIbβ3 in particular. Integrin αIIbβ3 and its signaling pathways are considered promising targets for antithrombotic therapy. This review describes the bidirectional signal transduction of integrin αIIbβ3 in platelets, as well as the proteins responsible for its regulation and therapeutic agents that target integrin αIIbβ3 and its signaling pathways.

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. [ABSTRACT FROM AUTHOR]

Integrins are heterodimeric transmembrane receptors that connect the extracellular matrix environment to the actin cytoskeleton via adaptor molecules through assembly of a range of adhesion structures. Recent advances in biochemical, imaging and biophysical methods have enabled a deeper understanding of integrin signalling and their associated regulatory processes. The identification of the consensus integrin-based 'adhesomes' within the last 5 years has defined common core components of adhesion complexes and associated partners. These approaches have also uncovered unexpected adhesion protein behaviour and molecules recruited to adhesion sites that have expanded our understanding of the molecular and physical control of integrin signalling. [ABSTRACT FROM AUTHOR]

Integrin activation is essential for creating functional transmembrane receptors capable of inducing downstream cellular effects such as cell migration, cell spreading, neurite outgrowth and axon regeneration. Integrins are bidirectional signalling molecules that mediate their effects by 'inside--out' and 'outside--in' signalling. This review will provide a detailed overview of integrin activation focusing on intracellular activation in neurons and discussing direct implications in the regulation of neurite outgrowth and axon regeneration. [ABSTRACT FROM AUTHOR]

Kindlins play an important role in supporting integrin activation by cooperating with talin; however, the mechanistic details remain unclear. Here, we show that kindlins interacted directly with paxillin and that this interaction could support integrin αIIbβ3 activation. An exposed loop in the N-terminal F0 subdomain of kindlins was involved in mediating the interaction. Disruption of kindlin binding to paxillin by structure-based mutations significantly impaired the function of kindlins in supporting integrin αIIbβ3 activation. Both kindlin and talin were required for paxillin to enhance integrin activation. Interestingly, a direct interaction between paxillin and the talin head domain was also detectable. Mechanistically, paxillin, together with kindlin, was able to promote the binding of the talin head domain to integrin, suggesting that paxillin complexes with kindlin and talin to strengthen integrin activation. Specifically, we observed that crosstalk between kindlin-3 and the paxillin family in mouse platelets was involved in supporting integrin αIIbβ3 activation and in vivo platelet thrombus formation. Taken together, our findings uncover a novel mechanism by which kindlin supports integrin αIIbβ3 activation, which might be beneficial for developing safer anti-thrombotic therapies. [ABSTRACT FROM AUTHOR]

Integrins mediate leukocyte accumulation to the sites of inflammation, thereby enhancing their potential as an important therapeutic target for inflammatory disorders. Integrin activation triggered by inflammatory mediators or signaling pathway is a key step to initiate leukocyte migration to inflamed tissues; however, an appropriately regulated integrin deactivation is indispensable for maintaining productive leukocyte migration. While typical integrin antagonists that block integrin activation target the initiation of leukocyte migration, a novel class of experimental compounds has been designed to block integrin deactivation, thereby perturbing the progression of cell migration. Current review discusses the mechanisms by which integrin is activated and subsequently deactivated by focusing on its structure-function relationship. [ABSTRACT FROM AUTHOR]

Studies on the mechanism of integrin inside-out activation have been focused on the role of β-integrin cytoplasmic tails, which are relatively conserved and bear binding sites for the intracellular activators including talin and kindlin. Cytoplasmic tails for α-integrins share a conserved GFFKR motif at the membrane-proximal region and this forms a specific interface with the β-integrin membrane-proximal region to keep the integrin inactive. The α-integrin membrane-distal regions, after the GFFKR motif, are diverse both in length and sequence and their roles in integrin activation have not been well-defined. In this study, we report that the α-integrin cytoplasmic membrane-distal region contributes to maintaining integrin in the resting state and to integrin inside-out activation. Complete deletion of the α-integrin membrane-distal region diminished talin-and kindlin-mediated integrin ligand binding and conformational change. A proper length and suitable amino acids in α-integrin membrane-distal region was found to be important for integrin inside-out activation. Our data establish an essential role for the α-integrin cytoplasmic membrane-distal region in integrin activation and provide new insights into how talin and kindlin induce the high-affinity integrin conformation that is required for fully functional integrins. [ABSTRACT FROM AUTHOR]

Abstract: The integrin family of heterodimeric cell adhesion molecules exists in both low- and high-affinity states, and integrin activation requires binding of the talin FERM (four-point-one, ezrin, radixin, moesin) domain to membrane-proximal sequences in the β-integrin cytoplasmic domain. However, it has recently become apparent that the kindlin family of FERM domain proteins is also essential for talin-induced integrin activation. FERM domains are typically composed of F1, F2, and F3 domains, but the talin FERM domain is atypical in that it contains a large insert in F1 and is preceded by a previously unrecognized domain, F0. Initial sequence alignments showed that the kindlin FERM domain was most similar to the talin FERM domain, but the homology appeared to be restricted to the F2 and F3 domains. Based on a detailed characterization of the talin FERM domain, we have reinvestigated the sequence relationship with kindlins and now show that kindlins do indeed contain the same domain structure as the talin FERM domain. However, the kindlin F1 domain contains an even larger insert than that in talin F1 that disrupts the sequence alignment. The insert, which varies in length between different kindlins, is not conserved and, as in talin, is largely unstructured. We have determined the structure of the kindlin-1 F0 domain by NMR, which shows that it adopts the same ubiquitin-like fold as the talin F0 and F1 domains. Comparison of the kindlin-1 and talin F0 domains identifies the probable interface with the kindlin-1 F1 domain. Potential sites of interaction of kindlin F0 with other proteins are discussed, including sites that differ between kindlin-1, kindlin-2, and kindlin-3. We also demonstrate that F0 is required for the ability of kindlin-1 to support talin-induced αIIbβ3 integrin activation and for the localization of kindlin-1 to focal adhesions. [Copyright &y& Elsevier]

Integrins represent the biologically and medically significant family of cell adhesion molecules that govern a wide range of normal physiology. The activities of integrins in cells are dynamically controlled via activation-dependent conformational changes regulated by the balance of intracellular activators, such as talin and kindlin, and inactivators, such as Shank-associated RH domain interactor (SHARPIN) and integrin cytoplasmic domain-associated protein 1 (ICAP-1). The activities of integrins are alternatively controlled by homotypic lateral association with themselves to induce integrin clustering and/or by heterotypic lateral engagement with tetraspanin and syndecan in the same cells to modulate integrin adhesiveness. It has recently emerged that integrins are expressed not only in cells but also in exosomes, important entities of extracellular vesicles secreted from cells. Exosomal integrins have received considerable attention in recent years, and they are clearly involved in determining the tissue distribution of exosomes, forming premetastatic niches, supporting internalization of exosomes by target cells and mediating exosome-mediated transfer of the membrane proteins and associated kinases to target cells. A growing body of evidence shows that tumor and immune cell exosomes have the ability to alter endothelial characteristics (proliferation, migration) and gene expression, some of these effects being facilitated by vesicle-bound integrins. As endothelial metabolism is now thought to play a key role in tumor angiogenesis, we also discuss how tumor cells and their exosomes pleiotropically modulate endothelial functions in the tumor microenvironment. [ABSTRACT FROM AUTHOR]

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. • Integrins couple the extracellular environment to intracellular signals. • Thermodynamic regulation of integrin conformation controls integrin activity. • Integrin activity is tightly regulated by binding partners (e.g. talin and kindlin). • Integrins play essential roles in epithelial cell adhesion and polarity. [ABSTRACT FROM AUTHOR]

Integrins are a family of transmembrane glycoprotein signaling receptors that can transmit bioinformation bidirectionally across the plasma membrane. Integrin αIIbβ3 is expressed at a high level in platelets and their progenitors, where it plays a central role in platelet functions, hemostasis, and arterial thrombosis. Integrin αIIbβ3 also participates in cancer progression, such as tumor cell proliferation and metastasis. In resting platelets, integrin αIIbβ3 adopts an inactive conformation. Upon agonist stimulation, the transduction of inside-out signals leads integrin αIIbβ3 to switch from a low- to high-affinity state for fibrinogen and other ligands. Ligand binding causes integrin clustering and subsequently promotes outside-in signaling, which initiates and amplifies a range of cellular events to drive essential platelet functions such as spreading, aggregation, clot retraction, and thrombus consolidation. Regulation of the bidirectional signaling of integrin αIIbβ3 requires the involvement of numerous interacting proteins, which associate with the cytoplasmic tails of αIIbβ3 in particular. Integrin αIIbβ3 and its signaling pathways are considered promising targets for antithrombotic therapy. This review describes the bidirectional signal transduction of integrin αIIbβ3 in platelets, as well as the proteins responsible for its regulation and therapeutic agents that target integrin αIIbβ3 and its signaling pathways. [ABSTRACT FROM AUTHOR]