Identification of shc as the primary protein binding to the tyrosine-phosphorylated beta 3 subunit of alpha IIbbeta 3 during outside-in integrin platelet signaling

Mudanpioside C Discovered from Paeonia suffruticosa Andr. Acts as a Protein Disulfide Isomerase Inhibitor with Antithrombotic Activities

Paeonia suffruticosa Andr. is a well-known landscape plant worldwide and also holds significant importance in China due to its medicinal and dietary properties. Previous studies have found that Cortex Moutan (CM), the dried root bark of P. suffruticosa, showed antiplatelet and cardioprotective effects, although the underlying mechanism and active compounds remain to be revealed. In this study, protein disulfide isomerase (PDI) inhibitors in CM were identified using a ligand-fishing method combined with the UHPLC-Q-TOF-MS assay. Further, their binding sites and inhibitory activities toward PDI were validated. The antiplatelet aggregation and antithrombotic activity were investigated. The results showed that two structurally similar compounds in CM were identified as the inhibitor for PDI with IC50 at 3.22 μM and 16.73 μM; among them Mudanpioside C (MC) is the most effective PDI inhibitor. Molecular docking, site-directed mutagenesis, and MST assay unequivocally demonstrated the specific binding of MC to the b'-x domain of PDI (Kd = 3.9 μM), acting as a potent PDI inhibitor by interacting with key amino acids K263, D292, and N298 within the b'-x domain. Meanwhile, MC could dose-dependently suppress collagen-induced platelet aggregation and interfere with platelet activation, adhesion, and spreading. Administration of MC can significantly inhibit thrombosis formation without disturbing hemostasis in mice. These findings present a promising perspective on the antithrombotic properties of CM and highlight the potential application of MC as lead compounds for targeting PDI in thrombosis therapy.

Posttranslational modifications of platelet adhesion receptors

Platelets play a key role in normal hemostasis, whereas pathological platelet adhesion is involved in various cardiovascular events. The underlying cause in cardiovascular events involves plaque rupture leading to subsequent platelet adhesion, activation, release, and eventual thrombosis. Traditional antithrombotic drugs often target the signal transduction process of platelet adhesion receptors by influencing the synthesis of some key molecules, and their effects are limited. Posttranslational modifications (PTMs) of platelet adhesion receptors increase the functional diversity of the receptors and affect platelet physiological and pathological processes. Antithrombotic drugs targeting PTMs of platelet adhesion receptors may represent a new therapeutic idea. In this review, various PTMs, including phosphorylation, glycosylation, ubiquitination, nitrosylation, methylation, lipidation, and proteolysis, of three platelet adhesion receptors, glycoprotein Ib-IX-V (GPIb-IX-V), glycoprotein VI (GPVI), and integrin α_IIbβ_3, are reviewed. It is important to comprehensively understand the PTMs process of platelet adhesion receptors.

PEAK1 Y635 phosphorylation regulates cell migration through association with Tensin3 and integrins

Integrins mediate cell adhesion by connecting the extracellular matrix to the intracellular cytoskeleton and orchestrate signal transduction in response to chemical and mechanical stimuli by interacting with many cytoplasmic proteins. We used BioID to interrogate the interactomes of β1 and β3 integrins in epithelial cells and identified PEAK1 as an interactor of the RGD-binding integrins α5β1, αVβ3, and αVβ5 in focal adhesions. We demonstrate that the interaction between integrins and PEAK1 occurs indirectly through Tensin3, requiring both the membrane-proximal NPxY motif on the integrin β tail and binding of the SH2 domain of Tensin3 to phosphorylated Tyr-635 on PEAK1. Phosphorylation of Tyr-635 is mediated by Src and regulates cell migration. Additionally, we found that Shc1 localizes in focal adhesions in a PEAK1 phosphorylated Tyr-1188-dependent fashion. Besides binding Shc1, PEAK1 also associates with a protein cluster that mediates late EGFR/Shc1 signaling. We propose a model in which PEAK1 binds Tensin3 and Shc1 to converge integrin and growth factor receptor signal transduction.

Intrinsic disorder in integral membrane proteins

The well-defined roles and specific protein-protein interactions of many integral membrane proteins (IMPs), such as those functioning as receptors for extracellular matrix proteins and soluble growth factors, easily align with considering IMP structure as a classical "lock-and-key" concept. Nevertheless, continued advances in understanding protein conformation, such as those which established the widespread existence of intrinsically disordered proteins (IDPs) and especially intrinsically disordered regions (IDRs) in otherwise three-dimensionally organized proteins, call for ongoing reevaluation of transmembrane proteins. Here, we present basic traits of IDPs and IDRs, and, for some select single-span IMPs, consider the potential functional advantages intrinsic disorder might provide and the possible conformational impact of disease-associated mutations. For transmembrane proteins in general, we highlight several investigational approaches, such as biophysical and computational methods, stressing the importance of integrating them to produce a more-complete mechanistic model of disorder-containing IMPs. These procedures, when synergized with in-cell assessments, will likely be key in translating in silico and in vitro results to improved understanding of IMP conformational flexibility in normal cell physiology as well as disease, and will help to extend their potential as therapeutic targets.

Cytoplasmic short linear motifs in ACE2 and integrin β3 link SARS-CoV-2 host cell receptors to mediators of endocytosis and autophagy

SARS-CoV-2, the virus that causes COVID-19, enters cells through endocytosis upon binding to the cell surface receptor ACE2 and potentially others, including integrins. Using bioinformatics, Mészáros et al. predicted the presence of short amino acid sequences, called short linear motifs (SLiMs), in the cytoplasmic tails of ACE2 and various integrins that may engage the endocytic and autophagic machinery. Using affinity binding assays, Kliche et al. not only confirmed that many of these predicted SLiMs interacted with target peptides in various components of the endocytosis and autophagy machinery, but also found that these interactions were regulated by the phosphorylation of SLiM-adjacent amino acids. Together, these findings have identified a potential link between autophagy and integrin signaling and could lead to new ways to prevent viral infection.

The spike protein of SARS-CoV-2 binds the angiotensin-converting enzyme 2 (ACE2) on the host cell surface and subsequently enters host cells through receptor-mediated endocytosis. Additional cell receptors may be directly or indirectly involved, including integrins. The cytoplasmic tails of ACE2 and integrins contain several predicted short linear motifs (SLiMs) that may facilitate internalization of the virus as well as its subsequent propagation through processes such as autophagy. Here, we measured the binding affinity of predicted interactions between SLiMs in the cytoplasmic tails of ACE2 and integrin β₃ with proteins that mediate endocytic trafficking and autophagy. We validated that a class I PDZ-binding motif mediated binding of ACE2 to the scaffolding proteins SNX27, NHERF3, and SHANK, and that a binding site for the clathrin adaptor AP2 μ2 in ACE2 overlaps with a phospho-dependent binding site for the SH2 domains of Src family tyrosine kinases. Furthermore, we validated that an LC3-interacting region (LIR) in integrin β₃ bound to the ATG8 domains of the autophagy receptors MAP1LC3 and GABARAP in a manner enhanced by LIR-adjacent phosphorylation. Our results provide molecular links between cell receptors and mediators of endocytosis and autophagy that may facilitate viral entry and propagation.

Serpina3c Regulates Adipogenesis by Modulating Insulin Growth Factor 1 and Integrin Signaling

Preadipocyte differentiation can be induced upon a hormonal treatment, and various factors secreted by the cells may contribute to adipogenesis. In this study, RNA-seq revealed Serpina3c as a critical factor regulating the signaling network during adipogenesis. Serpina3c is a secretory protein and is highly expressed in fat tissues. Knockdown of Serpina3c decreased adipogenesis by attenuating the mitotic clonal expansion of 3T3-L1 cells. These cells exhibited decreases in integrin α5, which abolished the phosphorylation of integrin β3. We found that Serpina3c inhibits a serine protease that regulates integrin α5 degradation. Knockdown of Serpina3c disrupted integrin-mediated insulin growth factor 1 (IGF-1) signaling and ERK activation. Serpina3c-mediated regulation of integrin-IGF-1 signaling is also associated with AKT activation, which affects the nuclear translocation of GSK3β. Altogether, our results indicate that Serpina3c secreted from differentiating adipocytes inhibits serine proteases to modulate integrin/IGF-1-mediated ERK and AKT signaling and thus is a critical factor contributing to adipogenesis.

•

RNA-seq revealed Serpina3c as a critical factor regulating adipogenesis

•

Knockdown of Serpina3c attenuated the mitotic clonal expansion of 3T3-L1 cells

•

Knockdown of Serpina3c leads to the degradation of integrin α5

•

Serpina3c regulates integrin-mediated IGF-1 signaling and ERK/AKT activation

Cell Adhesion by Integrins

Integrins are heterodimeric cell surface receptors ensuring the mechanical connection between cells and the extracellular matrix. In addition to the anchorage of cells to the extracellular matrix, these receptors have critical functions in intracellular signaling, but are also taking center stage in many physiological and pathological conditions. In this review, we provide some historical, structural, and physiological notes so that the diverse functions of these receptors can be appreciated and put into the context of the emerging field of mechanobiology. We propose that the exciting journey of the exploration of these receptors will continue for at least another new generation of researchers.

Peptides derived from the integrin β cytoplasmic tails inhibit angiogenesis

BACKGROUND

Integrins are essential regulators of angiogenesis. However, the antiangiogenic potential of peptides derived from the integrin cytoplasmic tails (CT) remains mostly undetermined.

METHODS

Here we designed a panel of membrane-penetrating peptides (termed as mβCTPs), each comprising a C-terminal NxxY motif from one of the conserved integrin β CTs, and evaluated their antiangiogenic ability using both in vitro and in vivo approaches.

RESULTS

We found that mβ3CTP, mβ5CTP and mβ6CTP, derived respectively from the integrin β3, β5 and β6 CTs, but not others, exhibit antiangiogenic ability. Interestingly, we observed that the integrin β3, β5 and β6 CTs but not others are able to interact with β₃-endonexin. In addition, the antiangiogenic core in mβ3CTP is identical to a previously identified β₃-endonexin binding region in the integrin β3 CT, indicating that the antiangiogenic mβCTPs may function via their binding to β₃-endonexin. Consistently, knockdown of endogenous β₃-endonexin in HUVECs significantly suppresses tube formation, suggesting that β₃-endonexin is proangiogenic. However, neither treatment with the antiangiogenic mβCTPs nor knockdown of endogenous β₃-endonexin affects integrin-mediated HUVEC adhesion and migration, indicating that their antiangiogenic effect may not rely on directly regulating integrin activity. Importantly, both treatment with the antiangiogenic mβCTPs and knockdown of endogenous β₃-endonexin in HUVECs inhibit VEGF expression and cell proliferation, thereby providing mechanistic explanations for the functional consequences.

CONCLUSION

Our results suggest that the antiangiogenic mβCTPs can interact with β₃-endonexin in vascular endothelial cells and suppress its function in regulating VEGF expression and cell proliferation, thus disclosing a unique pathway that may be useful for developing novel antiangiogenic strategies.

p66Shc Couples Mechanical Signals to RhoA through Focal Adhesion Kinase-Dependent Recruitment of p115-RhoGEF and GEF-H1

Tissue cells respond to changes in tensional forces with proliferation or death through the control of RhoA. However, the response coupling mechanisms that link force with RhoA activation are poorly understood. We found that tension applied to fibronectin-coated microbeads caused recruitment of all three isoforms of the Shc adapter (p66^Shc, p52^Shc, and p46^Shc) to adhesion complexes. The Shc PTB domain was necessary and sufficient for this recruitment, and screening studies revealed the direct interactions with the FERM domain of focal adhesion kinase (FAK) that were required for Shc translocation to adhesion complexes. The FAK/p66^Shc complex specifically bound and activated the Rho guanyl exchange factors (GEFs) p115-RhoGEF and GEF-H1, leading to tension-induced RhoA activation. In contrast, the FAK/p52^Shc complex bound SOS1 but not the Rho GEFs to mediate tension-induced Ras activation. Nuclear translocation and activation of the YAP/TAZ transcription factors on firm substrates required the FAK/p66^Shc/Rho GEF complex, and both proliferation on firm substrates and anoikis in suspension required signaling through p66^Shc and its associated Rho GEFs. These studies reveal the binary and exclusive assignment of p66^Shc and p52^Shc to tension-induced Rho or Ras signals, respectively, and suggest an integrated role for the two Shc isoforms in coordinating the cellular response to mechanical stimuli.

Proplatelet formation is selectively inhibited by collagen type I through Syk-independent GPVI signaling

Collagen receptors GPVI (also known as GP6) and integrin α2β1 are highly expressed on blood platelets and megakaryocytes, their immediate precursors. After vessel injury, subendothelial collagen becomes exposed and induces platelet activation to prevent blood loss. Collagen types I and IV are thought to have opposite effects on platelet biogenesis, directing proplatelet formation (PPF) towards the blood vessels to prevent premature release within the marrow cavity. We used megakaryocytes lacking collagen receptors or treated megakaryocytes with blocking antibodies, and could demonstrate that collagen-I-mediated inhibition of PPF is specifically controlled by GPVI. Other collagen types competed for binding and diminished the inhibitory signal, which was entirely dependent on receptor-proximal Src family kinases, whereas Syk and LAT were dispensable. Adhesion assays indicate that megakaryocyte binding to collagens is mediated by α2β1, and that collagen IV at the vascular niche might displace collagen I from megakaryocytes and thus contribute to prevention of premature platelet release into the marrow cavity and thereby directionally promote PPF at the vasculature.

Beta 1-integrin-c-Met cooperation reveals an inside-in survival signalling on autophagy-related endomembranes

Receptor tyrosine kinases (RTKs) and integrins cooperate to stimulate cell migration and tumour metastasis. Here we report that an integrin influences signalling of an RTK, c-Met, from inside the cell, to promote anchorage-independent cell survival. Thus, c-Met and β1-integrin co-internalize and become progressively recruited on LC3B-positive 'autophagy-related endomembranes' (ARE). In cells growing in suspension, β1-integrin promotes sustained c-Met-dependent ERK1/2 phosphorylation on ARE. This signalling is dependent on ATG5 and Beclin1 but not on ATG13, suggesting ARE belong to a non-canonical autophagy pathway. This β1-integrin-dependent c-Met-sustained signalling on ARE supports anchorage-independent cell survival and growth, tumorigenesis, invasion and lung colonization in vivo. RTK-integrin cooperation has been assumed to occur at the plasma membrane requiring integrin 'inside-out' or 'outside-in' signalling. Our results report a novel mode of integrin-RTK cooperation, which we term 'inside-in signalling'. Targeting integrin signalling in addition to adhesion may have relevance for cancer therapy.

Interaction of kindlin-2 with integrin β3 promotes outside-in signaling responses by the αVβ3 vitronectin receptor

The bidirectional signaling and hemostatic functions of platelet αIIbβ3 are regulated by kindlin-3 through interactions with the β3 cytoplasmic tail. Little is known about kindlin regulation of the related "vitronectin receptor," αVβ3. These relationships were investigated in endothelial cells, which express αVβ3 and kindlin-2 endogenously. "β3ΔRGT" knock-in mice lack the 3 C-terminal β3 tail residues, whereas in "β3/β1(EGK)" mice, RGT is replaced by the corresponding residues of β1. The wild-type β3 tail pulled down kindlin-2 and c-Src in vitro, whereas β3ΔRGT bound neither protein and β3/β1(EGK) bound kindlin-2, but not c-Src. β3ΔRGT endothelial cells, but not β3/β1(EGK) endothelial cells, exhibited migration and spreading defects on vitronectin and reduced sprouting in 3-dimensional fibrin. Short hairpin RNA silencing of kindlin-2, but not c-Src, blocked sprouting by β3 wild-type endothelial cells. Moreover, defective sprouting by β3ΔRGT endothelial cells could be rescued by conditional, forced interaction of αVβ3ΔRGT with kindlin-2. Stimulation of β3ΔRGT endothelial cells led to normal extracellular ligand binding to αVβ3, pin-pointing their defect to one of outside-in αVβ3 signaling. β3ΔRGT mice, but not β3/β1(EGK) mice, exhibited defects in both developmental and tumor angiogenesis, responses that require endothelial cell function. Thus, the β3/kindlin-2 interaction promotes outside-in αVβ3 signaling selectively, with biological consequences in vivo.

Phospho-Tyrosine(s) vs. Phosphatidylinositol Binding in Shc Mediated Integrin Signaling

The Shc adaptor protein, particularly its p52 isoform, has been identified as a primary signaling partner for the tyrosine(s)-phosphorylated cytoplasmic tails of activated β₃ integrins. Inspired by our recent structure of the Shc PTB domain in complex with a bi-phosphorylated peptide derived from β₃ cytoplasmic tail, we have initiated the investigation of Shc interaction with phospholipids of the membrane. We are particularly focused on PtdIns and their effects on Shc mediated integrin signaling in vitro. Here we present thermodynamic profiles and molecular details of the interactions between Shc, integrin, and PtdIns, all of which have been studied by ITC and solution NMR methods. A model of p52 Shc interaction with phosphorylated β₃ integrin cytoplasmic tail at the cytosolic face of the plasma membrane is proposed based on these data.

Structural basis of protein-protein interactions

Regulated interactions between proteins govern signaling pathways within and between cells. Structural studies on protein complexes formed reversibly and/or transiently illustrate the remarkable diversity of interactions, both in terms of interfacial size and nature. In recent years, "domain-peptide" interactions have gained much greater recognition and may be viewed as both pre-translational and posttranslational-dependent functional switches. Our understanding of the multistep regulation of auto-inhibited multidomain proteins has also grown. Their activity may be understood as the "combinatorial" output of multiple input signals, including phosphorylation, location, and mechanical force. The prospects for bridging the gap between the new "systems biology" data and the traditional "reductionist" data are also discussed.

The G protein-coupled estrogen receptor-1, GPER-1, promotes fibrillogenesis via a Shc-dependent pathway resulting in anchorage-independent growth

The G protein-coupled estrogen receptor-1, GPER-1, coordinates fibronectin (FN) matrix assembly and release of heparan-bound epidermal growth factor (HB-EGF). This mechanism of action results in the recruitment of FN-engaged integrin α5β1 to fibrillar adhesions and the formation of integrin α5β1-Shc adaptor protein complexes. Here, we show that GPER-1 stimulation of murine 4 T1 or human SKBR3 breast cancer cells with 17β-estradiol (E2β) promotes the formation of focal adhesions and actin stress fibers and results in increased cellular adhesion and haptotaxis on FN, but not collagen. These actions are also induced by the xenoestrogen, bisphenol A, and the estrogen receptor (ER) antagonist, ICI 182, 780, but not the inactive stereoisomer, 17α-estradiol (E2α). In addition, we show that GPER-1 stimulation of breast cancer cells allows for FN-dependent, anchorage-independent growth and FN fibril formation in "hanging drop" assays, indicating that these GPER-1-mediated actions occur independently of adhesion to solid substrata. Stable expression of Shc mutant Y317F lacking its primary tyrosyl phosphorylation site disrupts E2β-induced focal adhesion and actin stress fiber formation and abolishes E2β-enhanced haptotaxis on FN and anchorage-dependent growth. Collectively, these data demonstrate that E2β action via GPER-1 enhances cellular adhesivity and FN matrix assembly and allows for anchorage-independent growth, cellular events that may allow for cellular survival, and tumor progression.

P66Shc mediates increased platelet activation and aggregation in hypercholesterolemia

BACKGROUND AND HYPOTHESIS

Hypercholesterolemia leads to a prothrombotic phenotype. Platelet hyperactivity associated with hypercholesterolemia has been attributed, in part, to oxidative stress. P66Shc is a well-known determinant of cellular and organismal oxidative stress. However, its role in platelet biology is not known. We hypothesized that p66Shc mediates platelet hyperactivation and hyperaggregation in hypercholesterolemia.

METHODS AND RESULTS

P66Shc was expressed in both human and mouse platelets, as determined by qRT-PCR and immunoblotting. Mouse platelet p66Shc expression was upregulated by hypercholesterolemia induced by high-fat diet feeding. Compared to wild-type mice, high-fat diet-induced p66Shc expression in platelets was suppressed in transgenic mice expressing a short hairpin RNA targeting p66Shc (p66ShcRNAi). High-fat diet feeding of wild-type mice amplified surface P-selectin expression on platelets stimulated by the thrombin receptor agonist protease-activated receptor-4 (PAR4), and increased aggregation of platelets induced by thrombin. These exaggerated platelet responses induced by high-fat diet feeding were significantly blunted in p66ShcRNAi mice. Finally, thrombin-stimulated platelet reactive oxygen species were suppressed in p66ShcRNAi mice.

CONCLUSIONS

Hypercholesterolemia stimulates p66Shc expression in platelets, promoting platelet oxidative stress, hyperreactivity and hyperaggregation via p66Shc.

Integrin β3 crosstalk with VEGFR accommodating tyrosine phosphorylation as a regulatory switch

Integrins mediate cell adhesion, migration, and survival by connecting intracellular machinery with the surrounding extracellular matrix. Previous studies demonstrated the importance of the interaction between β₃ integrin and VEGF type 2 receptor (VEGFR2) in VEGF-induced angiogenesis. Here we present in vitro evidence of the direct association between the cytoplasmic tails (CTs) of β₃ and VEGFR2. Specifically, the membrane-proximal motif around ⁸⁰¹YLSI in VEGFR2 mediates its binding to non-phosphorylated β₃CT, accommodating an α-helical turn in integrin bound conformation. We also show that Y⁷⁴⁷ phosphorylation of β₃ enhances the above interaction. To demonstrate the importance of β₃ phosphorylation in endothelial cell functions, we synthesized β₃CT-mimicking Y⁷⁴⁷ phosphorylated and unphosphorylated membrane permeable peptides. We show that a peptide containing phospho-Y⁷⁴⁷ but not F⁷⁴⁷ significantly inhibits VEGF-induced signaling and angiogenesis. Moreover, phospho-Y⁷⁴⁷ peptide exhibits inhibitory effect only in WT but not in β₃ integrin knock-out or β₃ integrin knock-in cells expressing β₃ with two tyrosines substituted for phenylalanines, demonstrating its specificity. Importantly, these peptides have no effect on fibroblast growth factor receptor signaling. Collectively these data provide novel mechanistic insights into phosphorylation dependent cross-talk between integrin and VEGFR2.

Integrin {beta}3 phosphorylation dictates its complex with the Shc phosphotyrosine-binding (PTB) domain

Adaptor protein Shc plays a key role in mitogen-activated protein kinase (MAPK) signaling pathway, which can be mediated through a number of different receptors including integrins. By specifically recognizing the tyrosine-phosphorylated integrin β(3), Shc has been shown to trigger integrin outside-in signaling, although the structural basis of this interaction remains nebulous. Here we present the detailed structural analysis of Shc phosphotyrosine-binding (PTB) domain in complex with the bi-phosphorylated β(3)integrin cytoplasmic tail (CT). We show that this complex is primarily defined by the phosphorylation state of the integrin C-terminal Tyr(759), which fits neatly into the classical PTB pocket of Shc. In addition, we have identified a novel binding interface which concurrently accommodates phosphorylated Tyr(747) of the highly conserved NPXY motif of β(3). The structure represents the first snapshot of an integrin cytoplasmic tail bound to a target for mediating the outside-in signaling. Detailed comparison with the known Shc PTB structure bound to a target TrkA peptide revealed some significant differences, which shed new light upon the PTB domain specificity.

p21-activated kinase 4 phosphorylation of integrin beta5 Ser-759 and Ser-762 regulates cell migration

Modulation of integrin alphavbeta5 regulates vascular permeability, angiogenesis, and tumor dissemination. In addition, we previously found a role for p21-activated kinase 4 (PAK4) in selective regulation of integrin alphavbeta5-mediated cell motility (Zhang, H., Li, Z., Viklund, E. K., and Strömblad, S. (2002) J. Cell Biol. 158, 1287-1297). This report focuses on the molecular mechanisms of this regulation. We here identified a unique PAK4-binding membrane-proximal integrin beta5-SERS-motif involved in controlling cell attachment and migration. We also mapped the integrin beta5-binding site within PAK4. We found that PAK4 binding to integrin beta5 was not sufficient to promote cell migration, but that PAK4 kinase activity was required for PAK4 promotion of cell motility. Importantly, PAK4 specifically phosphorylated the integrin beta5 subunit at Ser-759 and Ser-762 within the beta5-SERS-motif. Point mutation of these two serine residues abolished the PAK4-induced cell migration, indicating a functional role for these phosphorylations in migration. Our results may give important leads to the functional regulation of integrin alphavbeta5, with implications for vascular permeability, angiogenesis, and cancer dissemination.

The PTB domain of ShcA couples receptor activation to the cytoskeletal regulator IQGAP1

Adaptor proteins respond to stimuli and recruit downstream complexes using interactions conferred by associated protein domains and linear motifs. The ShcA adaptor contains two phosphotyrosine recognition modules responsible for binding activated receptors, resulting in the subsequent recruitment of Grb2 and activation of Ras/MAPK. However, there is evidence that Grb2-independent signalling from ShcA has an important role in development. Using mass spectrometry, we identified the multidomain scaffold IQGAP1 as a ShcA-interacting protein. IQGAP1 and ShcA co-precipitate and are co-recruited to membrane ruffles induced by activated receptors of the ErbB family, and a reduction in ShcA protein levels inhibits the formation of lamellipodia. We used NMR to characterize a direct, non-canonical ShcA PTB domain interaction with a helical fragment from the IQGAP1 N-terminal region that is pTyr-independent. This interaction is mutually exclusive with binding to a more conventional PTB domain peptide ligand from PTP-PEST. ShcA-mediated recruitment of IQGAP1 may have an important role in cytoskeletal reorganization downstream of activated receptors at the cell surface.

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

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.

Proteomic and phospho-proteomic profile of human platelets in basal, resting state: insights into integrin signaling

During atherogenesis and vascular inflammation quiescent platelets are activated to increase the surface expression and ligand affinity of the integrin αIIbβ3 via inside-out signaling. Diverse signals such as thrombin, ADP and epinephrine transduce signals through their respective GPCRs to activate protein kinases that ultimately lead to the phosphorylation of the cytoplasmic tail of the integrin αIIbβ3 and augment its function. The signaling pathways that transmit signals from the GPCR to the cytosolic domain of the integrin are not well defined. In an effort to better understand these pathways, we employed a combination of proteomic profiling and computational analyses of isolated human platelets. We analyzed ten independent human samples and identified a total of 1507 unique proteins in platelets. This is the most comprehensive platelet proteome assembled to date and includes 190 membrane-associated and 262 phosphorylated proteins, which were identified via independent proteomic and phospho-proteomic profiling. We used this proteomic dataset to create a platelet protein-protein interaction (PPI) network and applied novel contextual information about the phosphorylation step to introduce limited directionality in the PPI graph. This newly developed contextual PPI network computationally recapitulated an integrin signaling pathway. Most importantly, our approach not only provided insights into the mechanism of integrin αIIbβ3 activation in resting platelets but also provides an improved model for analysis and discovery of PPI dynamics and signaling pathways in the future.

Redox control of platelet function

There has recently been a dramatic expansion in research in the area of redox biology with systems that utilize thiols to perform redox chemistry being central to redox control. Thiol-based reactions occur in proteins involved in platelet function, including extracellular platelet proteins. The alphaIIbbeta3 fibrinogen receptor contains free thiols that are required for the activation of this receptor to a fibrinogen-binding conformation. This process is under enzymatic control, with protein disulfide isomerase playing a central role in the activation of alphaIIbbeta3. Other integrins, such as the alpha2beta1 collagen receptor on platelets, are also regulated by protein disulfide isomerase and thiol metabolism. Low molecular weight thiols that are found in blood regulate these processes by converting redox sensitive disulfide bonds to thiols and by providing the appropriate redox potential for these reactions. Additional mechanisms of redox control of platelets involve nitric oxide that inhibits platelet responses, and reactive oxygen species that potentiate platelet thrombus formation. Specific nitrosative or oxidative modifications of thiol groups in platelets may modulate platelet function. Since many biologic processes are regulated by redox reactions that involve surface thiols, the extracellular redox state can have an important influence on health and disease status and may be a target for therapeutic intervention.

Coordinate regulation of estrogen-mediated fibronectin matrix assembly and epidermal growth factor receptor transactivation by the G protein-coupled receptor, GPR30

Estrogen promotes changes in cytoskeletal architecture not easily attributed to the biological action of estrogen receptors, ERalpha and ERbeta. The Gs protein-coupled transmembrane receptor, GPR30, is linked to specific estrogen binding and rapid estrogen-mediated release of heparin-bound epidermal growth factor. Using marker rescue and dominant interfering mutant strategies, we show that estrogen action via GPR30 promotes fibronectin (FN) matrix assembly by human breast cancer cells. Stimulation with 17beta-estradiol or the ER antagonist, ICI 182, 780, results in the recruitment of FN-engaged integrin alpha5beta1 conformers to fibrillar adhesions and the synthesis of FN fibrils. Concurrent with this cellular response, GPR30 promotes the formation of Src-dependent, Shc-integrin alpha5beta1 complexes. Function-blocking antibodies directed against integrin alpha5beta1 or soluble Arg-Gly-Asp peptide fragments derived from FN specifically inhibited GPR30-mediated epidermal growth factor receptor transactivation. Estrogen-mediated FN matrix assembly and epidermal growth factor receptor transactivation were similarly disrupted in integrin beta1-deficient GE11 cells, whereas reintroduction of integrin beta1 into GE11 cells restored these responses. Mutant Shc (317Y/F) blocked GPR30-induced FN matrix assembly and tyrosyl phosphorylation of erbB1. Interestingly, relative to recombinant wild-type Shc, 317Y/F Shc was more readily retained in GPR30-induced integrin alpha5beta1 complexes, yet this mutant did not prevent endogenous Shc-integrin alpha5beta1 complex formation. Our results suggest that GPR30 coordinates estrogen-mediated FN matrix assembly and growth factor release in human breast cancer cells via a Shc-dependent signaling mechanism that activates integrin alpha5beta1.

The mechanical integrin cycle

Cells govern tissue shape by exerting highly regulated forces at sites of matrix adhesion. As the major force-bearing adhesion-receptor protein, integrins have a central role in how cells sense and respond to the mechanics of their surroundings. Recent studies have shown that a key aspect of mechanotransduction is the cycle by which integrins bind to the matrix at the leading cell edge, attach to the cytoskeleton, transduce mechanical force, aggregate in the plasma membrane as part of increasingly strengthened adhesion complexes, unbind and, ultimately, are recycled. This mechanical cycle enables the transition from early complexes to larger, more stable adhesions that can then rapidly release. Within this mechanical cycle, integrins themselves exhibit intramolecular conformational change that regulates their binding affinity and may also be dependent upon force. How the cell integrates these dynamic elements into a rigidity response is not clear. Here, we focus on the steps in the integrin mechanical cycle that are sensitive to force and closely linked to integrin function, such as the lateral alignment of integrin aggregates and related adhesion components.

Expression of RAC2 in endothelial cells is required for the postnatal neovascular response

Herein, we describe an obligate role for the hematopoietic specific GTPase, RAC2 in endothelial integrin signaling and the postnatal neovascularization response in vivo. Using a Rac2 knockout mouse model, we discovered that despite the presence of both RAC1 and RAC2 protein in endothelial cells, RAC2 is obligately required for the postnatal neovascular response and alphavbeta3/ alpha4beta1/alpha5beta1 integrin-directed migration on vitronectin, H296 and CH271, fibronectin fragments, respectively. The molecular basis for RAC2 specificity was explored. A genetic analysis of Syk -/+ or Syk-/+;Rac2 -/+ mice revealed that SYK kinase is required for the integrin induced activation of RAC2. The analysis of endothelial cells from Rac2-/+ versus Syk-/+;Rac2-/+ mice provided genetic evidence that SYK-RAC2 signaling axis regulates integrin (alphavbeta3, alpha4beta1 and alpha5beta1) dependent migration. Our results provide evidence that a specific region of the nonreceptor protein tyrosine kinase, SYK, the B linker region containing Y342 and Y346 is required for SYK's regulation of RAC2 and integrin dependent migration. Moreover, the capacity of mice to vascularize the ischemic hindlimb following femoral artery ligation or matrigel plugs was markedly reduced in mice homozygous deficient for the Rac2 gene. These findings identify a novel signaling axis for the induction and potential modulation of postnatal angiogenesis.

Platelet integrin alpha(IIb)beta(3): activation mechanisms

Integrin alpha(IIb)beta(3) plays a critical role in platelet aggregation, a central response in hemostasis and thrombosis. This function of alpha(IIb)beta(3) depends upon a transition from a resting to an activated state such that it acquires the capacity to bind soluble ligands. Diverse platelet agonists alter the cytoplasmic domain of alpha(IIb)beta(3) and initiate a conformational change that traverses the transmembrane region and ultimately triggers rearrangements in the extracellular domain to permit ligand binding. The membrane-proximal regions of alpha(IIb) and beta(3) cytoplasmic tails, together with the transmembrane segments of the subunits, contact each other to form a complex which restrains the integrin in the resting state. It is unclasping of this complex that induces integrin activation. This clasping/unclasping process is influenced by multiple cytoplasmic tail binding partners. Among them, talin appears to be a critical trigger of alpha(IIb)beta(3) activation, but other binding partners, which function as activators or suppressors, are likely to act as co-regulators of integrin activation.

Saxatilin, a Snake Venom Disintegrin, Regulates Platelet Activation Associated with Human Vascular Endothelial Cell Migration and Invasion

BACKGROUND

Platelet activation results in platelet aggregation and the secretion of granules, which contain a variety of constituents including nonprotein molecules, adhesive proteins and hydrolases. The platelet-derived supernatant (PDS), which contains these granules, is known to trigger the activation of endothelium and chemotaxis of monocytes.

METHODS AND RESULTS

PDS derived from collagen-activated platelets stimulated human umbilical vein endothelial cell (HUVEC) migration and invasion, as measured through the use of a Boyden chamber. This collagen-induced PDS also triggered integrin alpha(v)beta(3) upregulation in HUVECs. The inclusion of a neutralizing antibody to platelet-derived growth factor (PDGF)-B abolished HUVEC migration/invasion and integrin alpha(v)beta(3) upregulation, showing that PDGF-AB mediates the proangiogenic effects of collagen-activated PDS. Saxatilin, a snake venom disintegrin known to interrupt platelet aggregation by antagonizing integrin alpha(IIb)beta(3), inhibited the collagen-induced platelet activation and abolished the angiogenic properties of PDS. Saxatilin also inhibited the collagen-induced phosphorylation of Syk, a key mediator of inside-out signaling in platelet activation.

CONCLUSION

Saxatilin inhibits platelet activation, platelet PDGF-AB release as well as subsequent endothelial cell migration and invasion.

A pseudosymmetric cell adhesion regulatory domain in the beta7 tail of the integrin alpha4beta7 that interacts with focal adhesion kinase and src

The beta7 integrins alpha4beta7 and alphaEbeta7 play key roles in forming the gut-associated lymphoid tissue, and contribute to chronic inflammation. The alpha4beta7 integrin-mediated adhesion of activated lymphocytes is largely due to a transient increase in avidity from ligand-induced clustering of alpha4beta7 at the cell-surface. Here, we report that L and D enantiomers of a cell-permeable peptide YDRREY encompassing residues 735-740 of the cytoplasmic tail of the beta7 subunit inhibit the adhesion of T cells to beta7 integrin ligands. The YDRREY peptide abrogated mucosal addressin cell adhesion molecule-1-induced clustering of alpha4beta7 on the surface of activated T cells. A mutated form of the YDRREY peptide carrying either single or double conservative mutations at Tyr(735)Phe and Tyr(740)Phe was unable to inhibit T cell adhesion, suggesting that both tandem tyrosines are critical for activity. The YDRREY peptide was bound and phosphorylated by focal adhesion kinase and src, which may serve to sequester cytoskeletal proteins to the cytoplasmic domain of alpha4beta7. The quasi-palindromic sequence YDRREY within the beta7 cytoplasmic tail constitutes a cell adhesion regulatory domain that modulates the interaction of beta7-expressing leukocytes with their endothelial and epithelial ligands. Cell-permeable peptidomimetics based on this motif have utility as anti-inflammatory reagents for the treatment of chronic inflammatory disease.

Genetic analysis of beta1 integrin "activation motifs" in mice

Akey feature of integrins is their ability to regulate the affinity for ligands, a process termed integrin activation. The final step in integrin activation is talin binding to the NPXY motif of the integrin beta cytoplasmic domains. Talin binding disrupts the salt bridge between the alpha/beta tails, leading to tail separation and integrin activation. We analyzed mice in which we mutated the tyrosines of the beta1 tail and the membrane-proximal aspartic acid required for the salt bridge. Tyrosine-to-alanine substitutions abolished beta1 integrin functions and led to a beta1 integrin-null phenotype in vivo. Surprisingly, neither the substitution of the tyrosines with phenylalanine nor the aspartic acid with alanine resulted in an obvious defect. These data suggest that the NPXY motifs of the beta1 integrin tail are essential for beta1 integrin function, whereas tyrosine phosphorylation and the membrane-proximal salt bridge between alpha and beta1 tails have no apparent function under physiological conditions in vivo.

Novel targets for antithrombotic drug discovery

Platelet aggregation is a dynamic entity, capable of directing its own growth and stability via the activation of signaling cascades that lead to the expression and secretion of various secondary agonists. Recent data using proteomics and genomics strategies have established that signaling pathways during platelet aggregation are triggered by two homophilic adhesion molecules, CD84 and CD150 (SLAM), and by a novel EGF-containing receptor, PEAR1, which are tyrosine-phosphorylated in a platelet-aggregation-dependent fashion (N. Nanda, P. Andre, M. Bao et al., Platelet aggregation induces platelet aggregate stability via SLAM family receptor signaling, Blood 106 (2005) 3028-3034, N. Nanda, M. Bao, H. Lin et al., Platelet Endothelial Aggregation Receptor 1 (PEAR1), a novel epidermal growth factor repeat-containing transmembrane receptor, participates in platelet contact-induced activation, J. Biol. Chem. 280 (2005) 24680-24689). Analysis of SLAM-deficient mice revealed an overall defect in platelet aggregation in vitro and a delayed arterial thrombotic process in vivo. The data indicate that these aggregation co-receptors may function in a "platelet synapse" and may be novel targets for antithrombotic drug discovery.

Contact-dependent signaling events that promote thrombus formation

There is increasing evidence that formation of a stable hemostatic plug requires adhesive and signaling events that continue beyond the onset of platelet aggregation. These events are facilitated and, in some cases, made possible, by the persistent close contacts between platelets that can only occur when platelets begin to aggregate. Participants include integrins and other cell adhesion molecules, secreted agonists, receptor tyrosine kinases, and protein fragments that are shed from the surface of activated platelets. Collectively, these molecules promote the continued growth and stability of the hemostatic plug.

Minding the gaps to promote thrombus growth and stability

Efforts to understand the role of platelets in hemostasis and thrombosis have largely focused on the earliest events of platelet activation, those that lead to aggregation. Although much remains to be learned about those early events, this Review examines a later series of events: the interactions between platelets that can only occur once aggregation has begun, bringing platelets into close contact with each other, creating a protected environment in the gaps between aggregated platelets, and fostering the continued growth and stability of the hemostatic plug.

Intracellular signaling in platelets

PURPOSE OF REVIEW

Over the past few years, a large portion of platelet research has focused on intracellular signaling events that contribute to stable platelet adhesion and aggregation.

RECENT FINDINGS

Studies of knockout mice have suggested critical roles for several previously unappreciated signaling molecules including phosphatidylinositol 3-kinase, the exchange factor CalDAG-GEFI, and the small GTPase Rap1b. These proteins may function to remodel the platelet cytoskeleton and thereby regulate both adhesion and aggregation. The abundant cytoskeletal protein talin appears to be a key regulator of the platelet integrin alphaIIbbeta3. Recent evidence suggests that talin binding to the cytoplasmic tail of beta3 promotes integrin oligomerization, thereby increasing the binding avidity the alphaIIbbeta3 complex for fibrinogen.

SUMMARY

The identification of platelet signaling pathways not only has clinical implications for diagnosis, but perhaps more importantly for rationale drug design. Aspirin, dipyridamole (Persantine), and thienopyridines (ticlopidine and clopidogrel) are all examples of agents that specifically target discrete platelet signaling pathways. These drugs have already been proven to be beneficial in the treatment of cardiovascular disease. Novel agents that target newly identified signaling pathways hold promise of greater specificity and efficacy.

GPVI and integrin alphaIIb beta3 signaling in platelets

This review summarizes recent developments in our understanding of the molecular basis of platelet activation by two distinct types of surface receptor, the immunoglobulin GPVI, and the integrin alphaIIb beta3 (also known as GPIIbIIIa). These two classes of receptor signal through similar yet distinct tyrosine kinase-based signaling cascades leading to activation of phospholipase C gamma2. The significance of these signaling cascades in platelet adhesion and platelet aggregation at arterial rates of shear is discussed.

Integrin activation by talin

The development and integrity of the cardiovascular system depends on integrins, a family of adhesion receptors, vitally important for homeostasis of animal species from fruit fly to man. Integrins are critical players in cell migration, cell adhesion, cell cycle progression, differentiation, and apoptosis. Consequently, integrins have a major impact on the patterning and functions of the blood and cardiovascular system. Integrins undergo conformational changes, which alter their affinity for ligands through a process operationally defined as integrin activation. Integrin activation is important for platelet aggregation, leukocyte extravasation, and cell adhesion and migration, thus influencing such processes as hemostasis, inflammation and angiogenesis. Recently, a series of studies have begun to define the mechanism of integrin activation by demonstrating that binding of a cytoskeletal protein, talin, to integrin beta subunit cytoplasmic tail is a last common step in integrin activation. These findings indicate that talin is likely to be at the center of converging signaling pathways regulating integrin activation.

Platelet Endothelial Aggregation Receptor 1 (PEAR1), a Novel Epidermal Growth Factor Repeat-containing Transmembrane Receptor, Participates in Platelet Contact-induced Activation

The present study was designed to identify novel membrane proteins that signal during platelet aggregation. Because one putative mechanism for signaling by a membrane protein involves phosphorylation, we used oligonucleotide-based microarray analyses and mass spectrometric proteomics techniques to specifically discover membrane proteins and also identify those proteins that become phosphorylated on tyrosine, threonine, or serine residues upon platelet aggregation. Surprisingly, both techniques converged to identify a novel membrane protein we have termed PEAR1 (platelet endothelial aggregation receptor 1). Sequence analysis of PEAR1 predicts a type-1 membrane protein, 15 extracellular epidermal growth factor-like repeats, and multiple cytoplasmic tyrosines. Analysis of the tissue distribution of PEAR1 showed that it was most highly expressed in platelets and endothelial cells. Upon platelet aggregation induced by physiological agonists, PEAR1 became phosphorylated on tyrosine (Tyr-925), and serine (Ser-953 and Ser-1029) residues. PEAR1 tyrosine phosphorylation was blocked by eptifibatide, an alpha(IIb)beta(3) antagonist, which inhibits platelet aggregation. Immune clustering of PEAR1 resulted in PEAR1 phosphorylation. Aggregation-induced PEAR1 tyrosine phosphorylation lead to the subsequent association with the ShcB adaptor protein. Platelet proximity induced by centrifugation also induced PEAR1 tyrosine phosphorylation, a reaction not inhibited by eptifibatide. These data suggest that PEAR1 is a novel platelet receptor that signals secondary to alpha(IIb)beta(3)-mediated platelet-platelet contacts.

Role of SHPS-1 in the regulation of insulin-like growth factor I-stimulated Shc and mitogen-activated protein kinase activation in vascular smooth muscle cells

Insulin-like growth factor I (IGF-I) stimulates smooth muscle cell (SMC) proliferation, and the mitogen-activated protein kinase (MAPK) pathway plays an important role in mediating IGF-I-induced mitogenic signaling. Our prior studies have shown that recruitment of Src homology 2 domain tyrosine phosphatase (SHP-2) to the membrane scaffolding protein Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1) is required for IGF-I-dependent MAPK activation. The current studies were undertaken to define the upstream signaling components that are required for IGF-I-stimulated MAPK activation and the role of SHPS-1 in regulating this process. The results show that IGF-I-induced Shc phosphorylation and its subsequent binding to Grb2 is required for sustained phosphorylation of MAPK and increased cell proliferation in SMCs. Furthermore, for Shc to be phosphorylated in response to IGF-I requires that Shc must associate with SHPS-1 and this association is mediated in part by SHP-2. Preincubation of cells with a peptide that contains a phospho-tyrosine binding motif sequence derived from SHPS-1 inhibited IGF-I-stimulated SHP-2 transfer to SHPS-1, the association of Shc with SHPS-1, and IGF-I-dependent Shc phosphorylation. Expression of an SHPS-1 mutant that did not bind to Shc or SHP-2 resulted in decreased Shc and MAPK phosphorylation in response to IGF-I. In addition, SMCs expressing a mutant form of the beta3 subunit of the alphaVbeta3, which results in impairment of SHP-2 transfer to SHPS-1, also showed attenuated IGF-I-dependent Shc and MAPK phosphorylation. Further analysis showed that Shc and SHP-2 can be coimmunoprecipitated after IGF-I stimulation. A cell-permeable peptide that contained a polyproline sequence from Shc selectively inhibited Shc/SHP-2 association and impaired Shc but not SHP-2 binding to SHPS-1. Exposure to this peptide also inhibited IGF-I-stimulated Shc and MAPK phosphorylation. Cells expressing a mutant form of Shc with the four prolines substituted with alanines showed no Shc/SHPS-1 association in response to IGF-I. We conclude that SHPS-1 functions as an anchor protein that recruits both Shc and SHP-2 and that their recruitment is necessary for IGF-I-dependent Shc phosphorylation, which is required for an optimal mitogenic response in SMCs.

The Pro33 isoform of integrin beta3 enhances outside-in signaling in human platelets by regulating the activation of serine/threonine phosphatases

Integrin beta(3) is polymorphic at residue 33 (Leu(33) or Pro(33)), and the Pro(33)-positive platelets display enhanced aggregation, P-selectin secretion, and shorter bleeding times. Because outside-in signaling is critical for platelet function, we hypothesized that the Pro(33) variant provides a more efficient signaling than the Leu(33) isoform. When compared with Pro(33)-negative platelets, Pro(33)-positive platelets demonstrated significantly greater serine/threonine phosphorylation of extracellular signal-regulated kinase (ERK2) and myosin light chain (MLC) but not cytoplasmic phospholipase A2 upon thrombin-induced aggregation. Tyrosine phosphorylation of integrin beta(3) and the adaptor protein Shc was no different in the fibrinogen-engaged platelets from both genotypes. The addition of Integrilin (alpha(IIb)beta(3)-fibrinogen blocker) or okadaic acid (serine/threonine phosphatase inhibitor) dramatically enhanced ERK2 and MLC phosphorylation in the Pro(33)-negative platelets when compared with Pro(33)-positive platelets, suggesting that integrin engagement during platelet aggregation activates serine/threonine phosphatases. The phosphatase activity of myosin phosphatase (MP) that dephosphorylates MLC is inactivated by phosphorylation of the myosin binding subunit of MP at Thr(696), and aggregating Pro(33)-positive platelets exhibited an increased Thr(696) phosphorylation of MP. These studies highlight a role for the dephosphorylation events via the serine/threonine phosphatases during the integrin outside-in signaling mechanism, and the Leu(33) --> Pro polymorphism regulates this process. Furthermore, these findings support a mechanism whereby the reported enhanced alpha granule secretion in the Pro(33)-positive platelets could be mediated by an increased phosphorylation of MLC, which in turn is caused by an increased phosphorylation and subsequent inactivation of myosin phosphatase.

Integrins: versatile integrators of extracellular signals

Extracellular matrix (ECM) molecules and growth factors have a crucial role in the signalling that controls cell behaviour during development. Integrins, which are cell-surface receptors for ECM molecules, and growth factor receptors cooperate with each other to regulate this signalling by several mechanisms. In particular, direct interactions between the integrin and growth factor receptors themselves, which often occur within a single macromolecular complex, amplify signalling by mechanisms that include posttranslational modifications and integrin shape changes that are related to activation. As a result, growth factor concentrations in the physiological range, which are too low to initiate signalling alone, do so in the presence of the ECM, enabling integrins to control the time and space of growth factor signalling.

Direct demonstration of involvement of the adaptor protein ShcA in the regulation of Ca2+-induced platelet aggregation

Platelet aggregation is mediated by conformational change of integrin alpha(IIb)beta(3). Tyrosine-phosphorylation of cytoplasmic domain of beta(3) upon platelet activation has been demonstrated to play a critical role in this process. Recently, the adaptor protein ShcA has been shown to bind to the tyrosine-phosphorylated beta(3), while it remains open whether ShcA plays any role in platelet aggregation. Here, we show that ShcA bound to tyrosine-phosphorylated beta(3)-tail peptide through its phosphotyrosine-binding domain in vitro. Then, we examined the involvement of ShcA in platelet aggregation by a previously established in vitro assay using platelets permeabilized with streptolysin-O, where exogenous addition of platelet cytosol is required for reconstitution of the Ca(2+)-induced aggregation. When ShcA was specifically depleted with anti-ShcA antibody from the cytosol, this ShcA-depleted cytosol lost the aggregation-supporting activity, which was rescued by addition of purified recombinant ShcA. Thus, ShcA is essential for the Ca(2+)-induced platelet aggregation.

The talin-tail interaction places integrin activation on FERM ground

Integrins are essential receptors for the development and functioning of multicellular animals because they mediate cell migration and cell adhesion, and regulate cell proliferation and apoptosis. Cellular regulation of the affinity of integrins for ligands - so-called 'integrin activation' - is a central property of these receptors. Integrin activation controls cell adhesion, migration and extracellular matrix assembly, thereby contributing to processes such as angiogenesis, tumor cell metastasis, inflammation, the immune response and hemostasis. Recent studies indicate that a crucial, final step in integrin activation is the binding of talin, a cytoskeletal protein, to the cytoplasmic domain of the integrin beta subunit. These results provide a focus for unraveling the many biochemical pathways implicated in integrin activation and suggest a general structural model for the connections between integrins and diverse cellular signal transduction pathways.

Real-time Analysis of Very Late Antigen-4 Affinity Modulation by Shear

Shear promotes endothelial recruitment of leukocytes, cell activation, and transmigration. Mechanical stress on cells caused by shear can induce a rapid integrin conformational change and activation, followed by an increase in binding to the extracellular matrix. The molecular mechanism of increased avidity is unknown. We have shown previously that the affinity of the alpha(4)beta(1) integrin, very late antigen-4 (VLA-4), measured with an LDV-containing small molecule, varies with cellular avidity, measured from cell disaggregation rates. In this study, we measured in real time affinity changes of VLA-4 in response to shear. The resulting affinity was comparable with the state mediated by receptor signaling and corresponded in time with intracellular Ca(2+) responses. Ca(2+) ionophores and N,N'-[1,2-ethanediyl-bis(oxy-2,1-phenylene)]bis[N-[2-[(acetyloxy)methoxy]-2-oxoethyl]]-, bis[(acetyloxy)methyl]ester demonstrate that the affinity regulation of VLA-4 in the presence of shear was related to Ca(2+) signaling. Pertussis toxin treatment implicates G(i) in an unknown pathway that connects shear, Ca(2+) elevation, VLA-4 affinity, and cell avidity.

Interaction of filamin A with the integrin beta 7 cytoplasmic domain: role of alternative splicing and phosphorylation

Integrin-filamin binding plays an important role in adhesion-mediated control of the actin cytoskeleton. Here, using the interaction between recombinant fragments from the C-terminus of filamin A and the cytoplasmic tail of integrin beta 7 as a model, we report a negative regulatory role for filamin alternative splicing. Splice variant forms of filamin A lacking a 41-amino acid segment interacted more strongly than full-length fragments. In addition, we provide evidence that phosphorylation of the splice variant region is unlikely to represent the mechanism by which binding is reduced.

Integrins: dynamic scaffolds for adhesion and signaling in platelets

The major platelet integrin, alphaIIbbeta3, is required for platelet interactions with proteins in plasma and the extracellular matrices (ECMs) that are essential for platelet adhesion and aggregation during hemo stasis and arterial thrombosis. Lig and binding to alphaIIbbeta3 is controlled by inside-out signals that modulate receptor conformation and clustering. In turn, ligand binding triggers outside-in signals through alphaIIbbeta3 that, when disrupted, can cause a bleeding diathesis. In the past 5 years there has been an explosion of knowledge about the structure and function ofalphaIIbbeta3 and the related integrin, alphaVbeta3. These developments are discussed here, and current models of bidirectional alphaIIbbeta3 signaling are presented as frameworks for future investigations. An understanding that alphaIIbbeta3 functions as a dynamic molecular scaffold for extracellular and intracellular proteins has translated into diagnostic and therapeutic insights relevant to hematology and cardiovascular medicine, and further advances can be anticipated.

PKD1/PKCmu promotes alphavbeta3 integrin recycling and delivery to nascent focal adhesions

To identify kinases that regulate integrin recycling, we have immunoprecipitated alphavbeta3 integrin from NIH 3T3 fibroblasts in the presence and absence of primaquine (a drug that inhibits receptor recycling and leads to accumulation of integrins in endosomes) and screened for co-precipitating kinases. Primaquine strongly promoted association of alphavbeta3 integrin with PKD1, and fluorescence microscopy indicated that integrin and PKD1 associate at a vesicular compartment that is downstream of a Rab4-dependent transport step. PKD1 association was mediated by the C-terminal region of the beta3 integrin cytodomain, and mutants of beta3 that were unable to recruit PKD1 did not recycle in a PDGF-dependent fashion. Furthermore, suppression of endogenous PKD1 levels by RNAi, or overexpression of catalytically inactive PKD1 inhibited PDGF-dependent recycling of alphavbeta3 from early endosomes to the plasma membrane and blocked recruitment of alphavbeta3 to newly formed focal adhesions during cell spreading. These data indicate that PKD1 influences cell migration by directing vesicular transport of the alphavbeta3 integrin heterodimer.