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Talin-1 is the principal platelet Rap1 effector of integrin activation.

ABSTRACT: Ras-related protein 1 (Rap1) is a major convergence point of the platelet-signaling pathways that result in talin-1 binding to the integrin β cytoplasmic domain and consequent integrin activation, platelet aggregation, and effective hemostasis. The nature of the connection between Rap1 and talin-1 in integrin activation is an important remaining gap in our understanding of this process. Previous work identified a low-affinity Rap1-binding site in the talin-1 F0 domain that makes a small contribution to integrin activation in platelets. We recently identified an additional Rap1-binding site in the talin-1 F1 domain that makes a greater contribution than F0 in model systems. Here we generated mice bearing point mutations, which block Rap1 binding without affecting talin-1 expression, in either the talin-1 F1 domain (R118E) alone, which were viable, or in both the F0 and F1 domains (R35E,R118E), which were embryonic lethal. Loss of the Rap1-talin-1 F1 interaction in platelets markedly decreases talin-1-mediated activation of platelet β1- and β3-integrins. Integrin activation and platelet aggregation in mice whose platelets express only talin-1(R35E, R118E) are even more impaired, resembling the defect seen in platelets lacking both Rap1a and Rap1b. Although Rap1 is important in thrombopoiesis, platelet secretion, and surface exposure of phosphatidylserine, loss of the Rap1-talin-1 interaction in talin-1(R35E, R118E) platelets had little effect on these processes. These findings show that talin-1 is the principal direct effector of Rap1 GTPases that regulates platelet integrin activation in hemostasis.

SUBMITTER: Lagarrigue F

PROVIDER: S-EPMC7472713 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Rap1 and membrane lipids cooperatively recruit talin to trigger integrin activation.

Project description:Recruitment and tethering of talin to the plasma membrane initiate the process of integrin activation. Multiple factors including the Rap1 proteins, RIAM (also known as APBB1IP) and PIP2 bind talin proteins and have been proposed to regulate these processes, but not systematically analyzed. By expressing specific talin mutants into talin-null fibroblasts, we show that binding of the talin F0 domain to Rap1 synergizes with membrane lipid binding of the talin F2 domain during talin membrane targeting and integrin activation, whereas the interaction of the talin rod with RIAM was dispensable. We also characterized a second Rap1-binding site within the talin F1 domain by detailed NMR analysis. Interestingly, while talin F1 exhibited significantly weaker Rap1-binding affinity than talin F0, expression of a talin F1 Rap1-binding mutant inhibited cell adhesion, spreading, talin recruitment and integrin activation similarly to the talin F0 Rap1-binding mutant. Moreover, the defects became significantly stronger when both Rap1-binding sites were mutated. In conclusion, our data suggest a model in which cooperative binding of Rap1 to the talin F0 and F1 domains synergizes with membrane PIP2 binding to spatiotemporally position and activate talins to regulate integrin activity.

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Rap1 binding and a lipid-dependent helix in talin F1 domain promote integrin activation in tandem.

Project description:Rap1 GTPases bind effectors, such as RIAM, to enable talin1 to induce integrin activation. In addition, Rap1 binds directly to the talin1 F0 domain (F0); however, this interaction makes a limited contribution to integrin activation in CHO cells or platelets. Here, we show that talin1 F1 domain (F1) contains a previously undetected Rap1-binding site of similar affinity to that in F0. A structure-guided point mutant (R118E) in F1, which blocks Rap1 binding, abolishes the capacity of Rap1 to potentiate talin1-induced integrin activation. The capacity of F1 to mediate Rap1-dependent integrin activation depends on a unique loop in F1 that has a propensity to form a helix upon binding to membrane lipids. Basic membrane-facing residues of this helix are critical, as charge-reversal mutations led to dramatic suppression of talin1-dependent activation. Thus, a novel Rap1-binding site and a transient lipid-dependent helix in F1 work in tandem to enable a direct Rap1-talin1 interaction to cause integrin activation.

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ADAP interactions with talin and kindlin promote platelet integrin αIIbβ3 activation and stable fibrinogen binding.

Project description:ADAP is a hematopoietic-restricted adapter protein that promotes integrin activation and is a carrier for other adapter proteins, Src kinase-associated phosphoprotein 1 (SKAP1) and SKAP2. In T lymphocytes, SKAP1 is the ADAP-associated molecule that activates integrins through direct linkages with Rap1 effectors (regulator of cell adhesion and polarization enriched in lymphoid tissues; Rap1-interacting adapter molecule). ADAP also promotes integrin αIIbβ3 activation in platelets, which lack SKAP1, suggesting an ADAP integrin-regulatory pathway different from those in lymphocytes. Here we characterized a novel association between ADAP and 2 essential integrin-β cytoplasmic tail-binding proteins involved in αIIbβ3 activation, talin and kindlin-3. Glutathione S-transferase pull-downs identified distinct regions in ADAP necessary for association with kindlin or talin. ADAP was physically proximal to talin and kindlin-3 in human platelets, as assessed biochemically, and by immunofluorescence microscopy and proximity ligation. Relative to wild-type mouse platelets, ADAP-deficient platelets exhibited reduced co-localization of talin with αIIbβ3, and reduced irreversible fibrinogen binding in response to a protease activated receptor 4 (PAR4) thrombin receptor agonist. When ADAP was heterologously expressed in Chinese hamster ovary cells co-expressing αIIbβ3, talin, PAR1, and kindlin-3, it associated with an αIIbβ3/talin complex and enabled kindlin-3 to promote agonist-dependent ligand binding to αIIbβ3. Thus, ADAP uniquely promotes activation of and irreversible fibrinogen binding to platelet αIIbβ3 through interactions with talin and kindlin-3.

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Rap1-Rac1 circuits potentiate platelet activation.

Project description:ObjectiveThe goal of this study was to investigate the potential crosstalk between Rap1 and Rac1, 2 small GTPases central to platelet activation, particularly downstream of the collagen receptor GPVI.Methods and resultsWe compared the activation response of platelets with impaired Rap signaling (double knock-out; deficient in both the guanine nucleotide exchange factor, CalDAG-GEFI, and the Gi-coupled receptor for ADP, P2Y12), to that of wild-type platelets treated with a small-molecule Rac inhibitor, EHT 1864 (wild-type /EHT). We found that Rac1 is sequentially activated downstream of Rap1 on stimulation via GPVI. In return, Rac1 provides important feedback for both CalDAG-GEFI- and P2Y12-dependent activation of Rap1. When analyzing platelet responses controlled by Rac1, we observed (1) impaired lamellipodia formation, clot retraction, and granule release in both double knock-out and EHT 1864-treated wild-type platelets; and (2) reduced calcium store release in EHT 1864-treated wild-type but not double knock-out platelets. Consistent with the latter finding, we identified 2 pools of Rac1, one activated immediately downstream of GPVI and 1 activated downstream of Rap1.ConclusionsWe demonstrate important crosstalk between Rap1 and Rac1 downstream of GPVI. Whereas Rap1 signaling directly controls sustained Rac1 activation, Rac1 affects CalDAG-GEFI- and P2Y12-dependent Rap1 activation via its role in calcium mobilization and granule/ADP release, respectively.

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Kindlin Assists Talin to Promote Integrin Activation.

Project description:Integrin αIIbβ3 is a predominant type of integrin abundantly expressed on the surface of platelets and its activation regulates the process of thrombosis. Talin and kindlin are cytoplasmic proteins that bind to integrin and modulate its affinity for extracellular ligands. Although the molecular details of talin-mediated integrin activation are known, the mechanism of kindlin involvement in this process remains elusive. Here, we demonstrate that the interplay between talin and kindlin promotes integrin activation. Our all-atomic molecular dynamics simulations on complete transmembrane and cytoplasmic domains of integrin αIIbβ3, talin1 F2/F3 subdomains, and the kindlin2 FERM domain in an explicit lipid-water environment over a microsecond timescale unraveled the role of kindlin as an enhancer of the talin interaction with the membrane proximal region of β-integrin. The cooperation of kindlin with talin results in a complete disruption of salt bridges between R995 on αIIb and D723/E726 on β3. Furthermore, kindlin modifies the molecular mechanisms of inside-out activation by decreasing the crossing angle between transmembrane helices of integrin αIIbβ3, which eventually results in parallelization of integrin dimer. In addition, our control simulation featuring integrin in complex with kindlin reveals that kindlin binding is not sufficient for unclasping the inner-membrane and outer-membrane interactions of integrin dimer, thus ruling out the possibility of solitary action of kindlin in integrin activation.

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Rap1 binding to the talin 1 F0 domain makes a minimal contribution to murine platelet GPIIb-IIIa activation.

Project description:Activation of platelet glycoprotein IIb-IIIa (GPIIb-IIIa; integrin ?IIb?3) leads to high-affinity fibrinogen binding and platelet aggregation during hemostasis. Whereas GTP-bound Rap1 GTPase promotes talin 1 binding to the ?3 cytoplasmic domain to activate platelet GPIIb-IIIa, the Rap1 effector that regulates talin association with ?3 in platelets is unknown. Rap1 binding to the talin 1 F0 subdomain was proposed to forge the talin 1-Rap1 link in platelets. Here, we report a talin 1 point mutant (R35E) that significantly reduces Rap1 affinity without a significant effect on its structure or expression. Talin 1 head domain (THD) (R35E) was of similar potency to wild-type THD in activating ?IIb?3 in Chinese hamster ovary cells. Coexpression with activated Rap1b increased activation, and coexpression with Rap1GAP1 reduced activation caused by transfection of wild-type THD or THD(R35E). Furthermore, platelets from Tln1R35E/R35E mice showed similar GPIIb-IIIa activation to those from wild-type littermates in response to multiple agonists. Tln1R35E/R35E platelets exhibited slightly reduced platelet aggregation in response to low doses of agonists; however, there was not a significant hemostatic defect, as judged by tail bleeding times. Thus, the Rap1-talin 1 F0 interaction has little effect on platelet GPIIb-IIIa activation and hemostasis and cannot account for the dramatic effects of loss of Rap1 activity on these platelet functions.

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Talin-driven inside-out activation mechanism of platelet αIIbβ3 integrin probed by multimicrosecond, all-atom molecular dynamics simulations.

Project description:Platelet aggregation is the consequence of the binding of extracellular bivalent ligands such as fibrinogen and von Willebrand factor to the high affinity, active state of integrin αIIbβ3. This state is achieved through a so-called "inside-out" mechanism characterized by the membrane-assisted formation of a complex between the F2 and F3 subdomains of intracellular protein talin and the integrin β3 tail. Here, we present the results of multi-microsecond, all-atom molecular dynamics simulations carried on the complete transmembrane (TM) and C-terminal (CT) domains of αIIbβ3 integrin in an explicit lipid-water environment, and in the presence or absence of the talin-1 F2 and F3 subdomains. These large-scale simulations provide unprecedented molecular-level insights into the talin-driven inside-out activation of αIIbβ3 integrin. Specifically, they suggest a preferred conformation of the complete αIIbβ3 TM/CT domains in a lipid-water environment, and testable hypotheses of key intermolecular interactions between αIIbβ3 integrin and the F2/F3 domains of talin-1. Notably, not only do these simulations give support to a stable left-handed reverse turn conformation of the αIIb juxtamembrane motif rather than a helical turn, but they raise the question as to whether TM helix separation is required for talin-driven integrin activation.

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Talin is required for integrin-mediated platelet function in hemostasis and thrombosis.

Project description:Integrins are critical for hemostasis and thrombosis because they mediate both platelet adhesion and aggregation. Talin is an integrin-binding cytoplasmic adaptor that is a central organizer of focal adhesions, and loss of talin phenocopies integrin deletion in Drosophila. Here, we have examined the role of talin in mammalian integrin function in vivo by selectively disrupting the talin1 gene in mouse platelet precursor megakaryocytes. Talin null megakaryocytes produced circulating platelets that exhibited normal morphology yet manifested profoundly impaired hemostatic function. Specifically, platelet-specific deletion of talin1 led to spontaneous hemorrhage and pathological bleeding. Ex vivo and in vitro studies revealed that loss of talin1 resulted in dramatically impaired integrin alphaIIbbeta3-mediated platelet aggregation and beta1 integrin-mediated platelet adhesion. Furthermore, loss of talin1 strongly inhibited the activation of platelet beta1 and beta3 integrins in response to platelet agonists. These data establish that platelet talin plays a crucial role in hemostasis and provide the first proof that talin is required for the activation and function of mammalian alpha2beta1 and alphaIIbbeta3 integrins in vivo.

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Direct Rap1/Talin1 interaction regulates platelet and neutrophil integrin activity in mice.

Project description:Targeting Talin1 to the plasma membrane is a crucial step in integrin activation, which in leukocytes is mediated by a Rap1/RIAM/Talin1 pathway, whereas in platelets, it is RIAM independent. Recent structural, biochemical, and cell biological studies have suggested direct Rap1/Talin1 interaction as an alternative mechanism to recruit Talin1 to the membrane and induce integrin activation. To test whether this pathway is of relevance in vivo, we generated Rap1 binding-deficient Talin1 knockin (Tln13mut) mice. Although Tln13mut mice showed no obvious abnormalities, their platelets exhibited reduced integrin activation, aggregation, adhesion, and spreading, resulting in prolonged tail-bleeding times and delayed thrombus formation and vessel occlusion in vivo. Surprisingly, neutrophil adhesion to different integrin ligands and ?2 integrin-dependent phagocytosis were also significantly impaired, which caused profound leukocyte adhesion and extravasation defects in Tln13mut mice. In contrast, macrophages exhibited no defect in adhesion or spreading despite reduced integrin activation. Taken together, our findings suggest that direct Rap1/Talin1 interaction is of particular importance in regulating the activity of different integrin classes expressed on platelets and neutrophils, which both depend on fast and dynamic integrin-mediated responses.

| S-EPMC6307989 | biostudies-literature

Beta integrin tyrosine phosphorylation is a conserved mechanism for regulating talin-induced integrin activation.

Project description:Integrins are large membrane-spanning receptors fundamental to cell adhesion and migration. Integrin adhesiveness for the extracellular matrix is activated by the cytoskeletal protein talin via direct binding of its phosphotyrosine-binding-like F3 domain to the cytoplasmic tail of the beta integrin subunit. The phosphotyrosine-binding domain of the signaling protein Dok1, on the other hand, has an inactivating effect on integrins, a phenomenon that is modulated by integrin tyrosine phosphorylation. Using full-length tyrosine-phosphorylated (15)N-labeled beta3, beta1A, and beta7 integrin tails and an NMR-based protein-protein interaction assay, we show that talin1 binds to the NPXY motif and the membrane-proximal portion of beta3, beta1A, and beta7 tails, and that the affinity of this interaction is decreased by integrin tyrosine phosphorylation. Dok1 only interacts weakly with unphosphorylated tails, but its affinity is greatly increased by integrin tyrosine phosphorylation. The Dok1 interaction remains restricted to the integrin NPXY region, thus phosphorylation inhibits integrin activation by increasing the affinity of beta integrin tails for a talin competitor that does not form activating membrane-proximal interactions with the integrin. Key residues governing these specificities were identified by detailed structural analysis, and talin1 was engineered to bind preferentially to phosphorylated integrins by introducing the mutation D372R. As predicted, this mutation affects talin1 localization in live cells in an integrin phosphorylation-specific manner. Together, these results indicate that tyrosine phosphorylation is a common mechanism for regulating integrin activation, despite subtle differences in how these integrins interact with their binding proteins.

| S-EPMC2794784 | biostudies-literature

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