Talin regulates integrin ß1-dependent and -independent cell functions in ureteric bud development.

Kidney collecting system development requires integrin-dependent cell-extracellular matrix interactions. Integrins are heterodimeric transmembrane receptors consisting of α and ß subunits; crucial integrins in the kidney collecting system express the ß1 subunit. The ß1 cytoplasmic tail has two NPxY motifs that mediate functions by binding to cytoplasmic signaling and scaffolding molecules. Talins, scaffolding proteins that bind to the membrane proximal NPxY motif, are proposed to activate integrins and to link them to the actin cytoskeleton. We have defined the role of talin binding to the ß1 proximal NPxY motif in the developing kidney collecting system in mice that selectively express a Y-to-A mutation in this motif. The mice developed a hypoplastic dysplastic collecting system. Collecting duct cells expressing this mutation had moderate abnormalities in cell adhesion, migration, proliferation and growth factor-dependent signaling. In contrast, mice lacking talins in the developing ureteric bud developed kidney agenesis and collecting duct cells had severe cytoskeletal, adhesion and polarity defects. Thus, talins are essential for kidney collecting duct development through mechanisms that extend beyond those requiring binding to the ß1 integrin subunit NPxY motif.

Loss of mouse cardiomyocyte talin-1 and talin-2 leads to ß-1 integrin reduction, costameric instability, and dilated cardiomyopathy.

Continuous contraction-relaxation cycles of the heart require strong and stable connections of cardiac myocytes (CMs) with the extracellular matrix (ECM) to preserve sarcolemmal integrity. CM attachment to the ECM is mediated by integrin complexes localized at the muscle adhesion sites termed costameres. The ubiquitously expressed cytoskeletal protein talin (Tln) is a component of muscle costameres that links integrins ultimately to the sarcomere. There are two talin genes, Tln1 and Tln2. Here, we tested the function of these two Tln forms in myocardium where Tln2 is the dominant isoform in postnatal CMs. Surprisingly, global deletion of Tln2 in mice caused no structural or functional changes in heart, presumably because CM Tln1 became up-regulated. Tln2 loss increased integrin activation, although levels of the muscle-specific ß1D-integrin isoform were reduced by 50%. With this result, we produced mice that had simultaneous loss of both CM Tln1 and Tln2 and found that cardiac dysfunction occurred by 4 wk with 100% mortality by 6 mo. ß1D integrin and other costameric proteins were lost from the CMs, and membrane integrity was compromised. Given that integrin protein reduction occurred with Tln loss, rescue of the phenotype was attempted through transgenic integrin overexpression, but this could not restore WT CM integrin levels nor improve heart function. Our results show that CM Tln2 is essential for proper ß1D-integrin expression and that Tln1 can substitute for Tln2 in preserving heart function, but that loss of all Tln forms from the heart-muscle cell leads to myocyte instability and a dilated cardiomyopathy.

LD Motif Recognition by Talin: Structure of the Talin-DLC1 Complex.

Cell migration requires coordination between integrin-mediated cell adhesion to the extracellular matrix and force applied to adhesion sites. Talin plays a key role in coupling integrin receptors to the actomyosin contractile machinery, while deleted in liver cancer 1 (DLC1) is a Rho GAP that binds talin and regulates Rho, and therefore actomyosin contractility. We show that the LD motif of DLC1 forms a helix that binds to the four-helix bundle of the talin R8 domain in a canonical triple-helix arrangement. We demonstrate that the same R8 surface interacts with the paxillin LD1 and LD2 motifs. We identify key charged residues that stabilize the R8 interactions with LD motifs and demonstrate their importance in vitro and in cells. Our results suggest a network of competitive interactions in adhesion complexes that involve LD motifs, and identify mutations that can be used to analyze the biological roles of specific protein-protein interactions in cell migration.

Vinculin controls talin engagement with the actomyosin machinery.

The link between extracellular-matrix-bound integrins and intracellular F-actin is essential for cell spreading and migration. Here, we demonstrate how the actin-binding proteins talin and vinculin cooperate to provide this link. By expressing structure-based talin mutants in talin null cells, we show that while the C-terminal actin-binding site (ABS3) in talin is required for adhesion complex assembly, the central ABS2 is essential for focal adhesion (FA) maturation. Thus, although ABS2 mutants support cell spreading, the cells lack FAs, fail to polarize and exert reduced force on the surrounding matrix. ABS2 is inhibited by the preceding mechanosensitive vinculin-binding R3 domain, and deletion of R2R3 or expression of constitutively active vinculin generates stable force-independent FAs, although cell polarity is compromised. Our data suggest a model whereby force acting on integrin-talin complexes via ABS3 promotes R3 unfolding and vinculin binding, activating ABS2 and locking talin into an actin-binding configuration that stabilizes FAs.

Talin determines the nanoscale architecture of focal adhesions.

Insight into how molecular machines perform their biological functions depends on knowledge of the spatial organization of the components, their connectivity, geometry, and organizational hierarchy. However, these parameters are difficult to determine in multicomponent assemblies such as integrin-based focal adhesions (FAs). We have previously applied 3D superresolution fluorescence microscopy to probe the spatial organization of major FA components, observing a nanoscale stratification of proteins between integrins and the actin cytoskeleton. Here we combine superresolution imaging techniques with a protein engineering approach to investigate how such nanoscale architecture arises. We demonstrate that talin plays a key structural role in regulating the nanoscale architecture of FAs, akin to a molecular ruler. Talin diagonally spans the FA core, with its N terminus at the membrane and C terminus demarcating the FA/stress fiber interface. In contrast, vinculin is found to be dispensable for specification of FA nanoscale architecture. Recombinant analogs of talin with modified lengths recapitulated its polarized orientation but altered the FA/stress fiber interface in a linear manner, consistent with its modular structure, and implicating the integrin-talin-actin complex as the primary mechanical linkage in FAs. Talin was found to be ∼97 nm in length and oriented at ∼15° relative to the plasma membrane. Our results identify talin as the primary determinant of FA nanoscale organization and suggest how multiple cellular forces may be integrated at adhesion sites.

Ligand-Occupied Integrin Internalization Links Nutrient Signaling to Invasive Migration.

Integrin trafficking is key to cell migration, but little is known about the spatiotemporal organization of integrin endocytosis. Here, we show that α5ß1 integrin undergoes tensin-dependent centripetal movement from the cell periphery to populate adhesions located under the nucleus. From here, ligand-engaged α5ß1 integrins are internalized under control of the Arf subfamily GTPase, Arf4, and are trafficked to nearby late endosomes/lysosomes. Suppression of centripetal movement or Arf4-dependent endocytosis disrupts flow of ligand-bound integrins to late endosomes/lysosomes and their degradation within this compartment. Arf4-dependent integrin internalization is required for proper lysosome positioning and for recruitment and activation of mTOR at this cellular subcompartment. Furthermore, nutrient depletion promotes subnuclear accumulation and endocytosis of ligand-engaged α5ß1 integrins via inhibition of mTORC1. This two-way regulatory interaction between mTORC1 and integrin trafficking in combination with data describing a role for tensin in invasive cell migration indicate interesting links between nutrient signaling and metastasis.

Podocyte-associated talin1 is critical for glomerular filtration barrier maintenance.

Podocytes are specialized actin-rich epithelial cells that line the kidney glomerular filtration barrier. The interface between the podocyte and the glomerular basement membrane requires integrins, and defects in either α3 or ß1 integrin, or the α3ß1 ligand laminin result in nephrotic syndrome in murine models. The large cytoskeletal protein talin1 is not only pivotal for integrin activation, but also directly links integrins to the actin cytoskeleton. Here, we found that mice lacking talin1 specifically in podocytes display severe proteinuria, foot process effacement, and kidney failure. Loss of talin1 in podocytes caused only a modest reduction in ß1 integrin activation, podocyte cell adhesion, and cell spreading; however, the actin cytoskeleton of podocytes was profoundly altered by the loss of talin1. Evaluation of murine models of glomerular injury and patients with nephrotic syndrome revealed that calpain-induced talin1 cleavage in podocytes might promote pathogenesis of nephrotic syndrome. Furthermore, pharmacologic inhibition of calpain activity following glomerular injury substantially reduced talin1 cleavage, albuminuria, and foot process effacement. Collectively, these findings indicate that podocyte talin1 is critical for maintaining the integrity of the glomerular filtration barrier and provide insight into the pathogenesis of nephrotic syndrome.

Talins and kindlins: partners in integrin-mediated adhesion.

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

Structural studies on full-length talin1 reveal a compact auto-inhibited dimer: implications for talin activation.

Talin is a large adaptor protein that activates integrins and couples them to cytoskeletal actin. Talin contains an N-terminal FERM (band 4.1, ezrin, radixin, moesin) domain (the head) linked to a flexible rod comprised of 13 amphipathic helical bundles (R1-R13) that terminate in a C-terminal helix (DD) that forms an anti-parallel dimer. We derived a three-dimensional structural model of full-length talin at a resolution of approximately 2.5nm using EM reconstruction of full-length talin and the known shapes of the individual domains and inter-domain angles as derived from small angle X-ray scattering. Talin adopts a compact conformation consistent with a dimer in which the two talin rods form a donut-shaped structure, with the two talin heads packed side by side occupying the hole at the center of this donut. In this configuration, the integrin binding site in the head domain and the actin-binding site at the carboxy-terminus of the rod are masked, implying that talin must unravel before it can support integrin activation and engage the actin cytoskeleton.

RIAM and vinculin binding to talin are mutually exclusive and regulate adhesion assembly and turnover.

Talin activates integrins, couples them to F-actin, and recruits vinculin to focal adhesions (FAs). Here, we report the structural characterization of the talin rod: 13 helical bundles (R1-R13) organized into a compact cluster of four-helix bundles (R2-R4) within a linear chain of five-helix bundles. Nine of the bundles contain vinculin-binding sites (VBS); R2R3 are atypical, with each containing two VBS. Talin R2R3 also binds synergistically to RIAM, a Rap1 effector involved in integrin activation. Biochemical and structural data show that vinculin and RIAM binding to R2R3 is mutually exclusive. Moreover, vinculin binding requires domain unfolding, whereas RIAM binds the folded R2R3 double domain. In cells, RIAM is enriched in nascent adhesions at the leading edge whereas vinculin is enriched in FAs. We propose a model in which RIAM binding to R2R3 initially recruits talin to membranes where it activates integrins. As talin engages F-actin, force exerted on R2R3 disrupts RIAM binding and exposes the VBS, which recruit vinculin to stabilize the complex.

Talin1 has unique expression versus talin 2 in the heart and modifies the hypertrophic response to pressure overload.

Integrins are adhesive, signaling, and mechanotransduction proteins. Talin (Tln) activates integrins and links it to the actin cytoskeleton. Vertebrates contain two talin genes, tln1 and tln2. How Tln1 and Tln2 function in cardiac myocytes (CMs) is unknown. Tln1 and Tln2 expression were evaluated in the normal embryonic and adult mouse heart as well as in control and failing human adult myocardium. Tln1 function was then tested in the basal and mechanically stressed myocardium after cardiomyocyte-specific excision of the Tln1 gene. During embryogenesis, both Tln forms are highly expressed in CMs, but in the mature heart Tln2 becomes the main Tln isoform, localizing to the costameres. Tln1 expression is minimal in the adult CM. With pharmacological and mechanical stress causing hypertrophy, Tln1 is up-regulated in CMs and is specifically detected at costameres, suggesting its importance in the compensatory response to CM stress. In human failing heart, CM Tln1 also increases compared with control samples from normal functioning myocardium. To directly test Tln1 function in CMs, we generated CM-specific Tln1 knock-out mice (Tln1cKO). Tln1cKO mice showed normal basal cardiac structure and function but when subjected to pressure overload showed blunted hypertrophy, less fibrosis, and improved cardiac function versus controls. Acute responses of ERK1/2, p38, Akt, and glycogen synthase kinase 3 after mechanical stress were strongly blunted in Tln1cKO mice. Given these results, we conclude that Tln1 and Tln2 have distinct functions in the myocardium. Our data show that reduction of CM Tln1 expression can lead to improved cardiac remodeling following pressure overload.

Talin1 and Rap1 are critical for osteoclast function.

To determine talin1's role in osteoclasts, we mated TLN1(fl/fl) mice with those expressing cathepsin K-Cre (CtsK-TLN1) to delete the gene in mature osteoclasts or with lysozyme M-Cre (LysM-TLN1) mice to delete TLN1 in all osteoclast lineage cells. Absence of TLN1 impairs macrophage colony-stimulating factor (M-CSF)-stimulated inside-out integrin activation and cytoskeleton organization in mature osteoclasts. Talin1-deficient precursors normally express osteoclast differentiation markers when exposed to M-CSF and receptor activator of nuclear factor κB (RANK) ligand but attach to substrate and migrate poorly, arresting their development into mature resorptive cells. In keeping with inhibited resorption, CtsK-TLN1 mice exhibit an ∼5-fold increase in bone mass. Osteoclast-specific deletion of Rap1 (CtsK-Rap1), which promotes talin/ß integrin recognition, yields similar osteopetrotic mice. The fact that the osteopetrosis of CtsK-TLN1 and CtsK-Rap1 mice is substantially more severe than that of those lacking αvß3 is likely due to added failed activation of ß1 integrins. In keeping with osteoclast dysfunction, mice in whom talin is deleted late in the course of osteoclastogenesis are substantially protected from ovariectomy-induced osteoporosis and the periarticular osteolysis attending inflammatory arthritis. Thus, talin1 and Rap1 are critical for resorptive function, and their selective inhibition in mature osteoclasts retards pathological bone loss.

Mice carrying a complete deletion of the talin2 coding sequence are viable and fertile.

Mice homozygous for several Tln2 gene targeted alleles are viable and fertile. Here we show that although the expression of talin2 protein is drastically reduced in muscle from these mice, other tissues continue to express talin2 albeit at reduced levels. We therefore generated a Tln2 allele lacking the entire coding sequence (Tln2(cd)). Tln2(cd/cd) mice were viable and fertile, and the genotypes of Tln2(cd/+) intercrosses were at the expected Mendelian ratio. Tln2(cd/cd) mice showed no major difference in body mass or the weight of the major organs compared to wild-type, although they displayed a mildly dystrophic phenotype. Moreover, Tln2(cd/cd) mouse embryo fibroblasts showed no obvious defects in cell adhesion, migration or proliferation. However, the number of Tln2(cd/cd) pups surviving to adulthood was variable suggesting that such mice have an underlying defect.

Talin contains a C-terminal calpain2 cleavage site important in focal adhesion dynamics.

Talin is a large (∼2540 residues) dimeric adaptor protein that associates with the integrin family of cell adhesion molecules in cell-extracellular matrix junctions (focal adhesions; FAs), where it both activates integrins and couples them to the actin cytoskeleton. Calpain2-mediated cleavage of talin between the head and rod domains has previously been shown to be important in FA turnover. Here we identify an additional calpain2-cleavage site that removes the dimerisation domain from the C-terminus of the talin rod, and show that an E2492G mutation inhibits calpain cleavage at this site in vitro, and increases the steady state levels of talin1 in vivo. Expression of a GFP-tagged talin1 E2492G mutant in CHO.K1 cells inhibited FA turnover and the persistence of cell protrusion just as effectively as a L432G mutation that inhibits calpain cleavage between the talin head and rod domains. Moreover, incorporation of both mutations into a single talin molecule had an additive effect clearly demonstrating that calpain cleavage at both the N- and C-terminal regions of talin contribute to the regulation of FA dynamics. However, the N-terminal site was more sensitive to calpain cleavage suggesting that lower levels of calpain are required to liberate the talin head and rod fragments than are needed to clip off the C-terminal dimerisation domain. The talin head and rod liberated by calpain2 cleavage have recently been shown to play roles in an integrin activation cycle important in FA turnover and in FAK-dependent cell cycle progression respectively. The half-life of the talin head is tightly regulated by ubiquitination and we suggest that removal of the C-terminal dimerisation domain from the talin rod may provide a mechanism both for terminating the signalling function of the talin rod and indeed for inactivating full-length talin thereby promoting FA turnover at the rear of the cell.

Distinct roles for talin-1 and kindlin-3 in LFA-1 extension and affinity regulation.

In inflammation, neutrophils and other leukocytes roll along the microvascular endothelium before arresting and transmigrating into inflamed tissues. Arrest requires conformational activation of the integrin lymphocyte function-associated antigen-1 (LFA-1). Mutations of the FERMT3 gene encoding kindlin-3 underlie the human immune deficiency known as leukocyte adhesion deficiency-III. Both kindlin-3 and talin-1, another FERM domain-containing cytoskeletal protein, are required for integrin activation, but their individual roles in the induction of specific integrin conformers are unclear. Here, we induce differential LFA-1 activation in neutrophils through engagement of the selectin ligand P-selectin glycoprotein ligand-1 or the chemokine receptor CXCR2. We find that talin-1 is required for inducing LFA-1 extension, which corresponds to intermediate affinity and induces neutrophil slow rolling, whereas both talin-1 and kindlin-3 are required for induction of the high-affinity conformation of LFA-1 with an open headpiece, which results in neutrophil arrest. In vivo, both slow rolling and arrest are defective in talin-1-deficient neutrophils, whereas only arrest is defective in kindlin-3-deficient neutrophils. We conclude that talin-1 and kindlin-3 serve distinct functions in LFA-1 activation.

New isoform-specific monoclonal antibodies reveal different sub-cellular localisations for talin1 and talin2.

Talins are adaptor proteins that connect the integrin family of cell adhesion receptors to cytoskeletal actin. Vertebrates express two closely related talins encoded by separate genes, and while it is well established that talin1 plays a key role in cell adhesion and spreading, little is known about the role of talin2. To facilitate such studies, we report the characterisation of 4 new isoform-specific talin mouse monoclonal antibodies that work in Western blotting, immuno-precipitation, immuno-fluorescence and immuno-histochemistry. Using these antibodies, we show that talin1 and talin2 do not form heterodimers, and that they are differentially localised within the cell. Talin1 was concentrated in peripheral focal adhesions while talin2 was observed in both focal and fibrillar adhesions, and knock-down of talin2 compromised fibronectin fibrillogenesis. Although differentiated human macrophages express both isoforms, only talin1 showed discrete staining and was localised to the ring structure of podosomes. However, siRNA-mediated knock-down of macrophage talin2 led to a significant reduction in podosomal matrix degradation. We have also used the antibodies to localise each isoform in tissue sections using both cryostat and paraffin-embedded material. In skeletal muscle talin2 was localised to both myotendinous junctions and costameres while talin1 was restricted to the former structure. In contrast, both isoforms co-localised in kidney with staining of the glomerulus, and the tubular epithelial and interstitial cells of the cortex and medulla. We anticipate that these antibodies will form a valuable resource for future studies on the function of the two major talin isoforms.

Subcellular localization of talin is regulated by inter-domain interactions.

Talin, which is composed of head (THD) and rod domains, plays an important role in cell adhesion events in diverse species including most metazoans and Dictyostelium discoideum. Talin is abundant in the cytosol; however, it mediates adhesion by associating with integrins in the plasma membrane where it forms a primary link between integrins and the actin cytoskeleton. Cells modulate the partitioning of talin between the plasma membrane and the cytosol to control cell adhesion. Here, we combine nuclear magnetic resonance spectroscopy (NMR) with subcellular fractionation to characterize two distinct THD-rod domain interactions that control the interaction of talin with the actin cytoskeleton or its localization to the plasma membrane. An interaction between a discrete vinculin-binding region of the rod (VBS1/2a; Tln1(482-787)), and the THD restrains talin from interacting with the plasma membrane. Furthermore, we show that vinculin binding to VBS1/2a results in talin recruitment to the plasma membrane. Thus, we have structurally defined specific inter-domain interactions between THD and the talin rod domain that regulate the subcellular localization of talin.

A conserved lipid-binding loop in the kindlin FERM F1 domain is required for kindlin-mediated αIIbß3 integrin coactivation.

The activation of heterodimeric integrin adhesion receptors from low to high affinity states occurs in response to intracellular signals that act on the short cytoplasmic tails of integrin ß subunits. Binding of the talin FERM (four-point-one, ezrin, radixin, moesin) domain to the integrin ß tail provides one key activation signal, but recent data indicate that the kindlin family of FERM domain proteins also play a central role. Kindlins directly bind integrin ß subunit cytoplasmic domains at a site distinct from the talin-binding site, and target to focal adhesions in adherent cells. However, the mechanisms by which kindlins impact integrin activation remain largely unknown. A notable feature of kindlins is their similarity to the integrin-binding and activating talin FERM domain. Drawing on this similarity, here we report the identification of an unstructured insert in the kindlin F1 FERM domain, and provide evidence that a highly conserved polylysine motif in this loop supports binding to negatively charged phospholipid head groups. We further show that the F1 loop and its membrane-binding motif are required for kindlin-1 targeting to focal adhesions, and for the cooperation between kindlin-1 and -2 and the talin head in αIIbß3 integrin activation, but not for kindlin binding to integrin ß tails. These studies highlight the structural and functional similarities between kindlins and the talin head and indicate that as for talin, FERM domain interactions with acidic membrane phospholipids as well ß-integrin tails contribute to the ability of kindlins to activate integrins.

Contact-dependent T cell activation and T cell stopping require talin1.

T cell-APC contact initiates T cell activation and is maintained by the integrin LFA-1. Talin1, an LFA-1 regulator, localizes to the immune synapse (IS) with unknown roles in T cell activation. In this study, we show that talin1-deficient T cells have defects in contact-dependent T cell stopping and proliferation. Although talin1-deficient T cells did not form stable interactions with APCs, transient contacts were sufficient to induce signaling. In contrast to prior models, LFA-1 polarized to T cell-APC contacts in talin1-deficient T cells, but vinculin and F-actin polarization at the IS was impaired. These results indicate that T cell proliferation requires sustained, talin1-mediated T cell-APC interactions and that talin1 is necessary for F-actin polarization and the stability of the IS.

Bartonella henselae engages inside-out and outside-in signaling by integrin ß1 and talin1 during invasome-mediated bacterial uptake.

The VirB/D4 type IV secretion system (T4SS) of the bacterial pathogen Bartonella henselae (Bhe) translocates seven effector proteins (BepA-BepG) into human cells that subvert host cellular functions. Two redundant pathways dependent on BepG or the combination of BepC and BepF trigger the formation of a bacterial uptake structure termed the invasome. Invasome formation is a multi-step process consisting of bacterial adherence, effector translocation, aggregation of bacteria on the cell surface and engulfment, and eventually, complete internalization of the bacterial aggregate occurs in an F-actin-dependent manner. In the present study, we show that Bhe-triggered invasome formation depends on integrin-ß1-mediated signaling cascades that enable assembly of the F-actin invasome structure. We demonstrate that Bhe interacts with integrin ß1 in a fibronectin- and VirB/D4 T4SS-independent manner and that activated integrin ß1 is essential for both effector translocation and the actin rearrangements leading to invasome formation. Furthermore, we show that talin1, but not talin2, is required for inside-out activation of integrin ß1 during invasome formation. Finally, integrin-ß1-mediated outside-in signaling by FAK, Src, paxillin and vinculin is necessary for invasome formation. This is the first example of a bacterial entry process that fully exploits the bi-directional signaling capacity of integrin receptors in a talin1-specific manner.