Critical roles for the COOH-terminal NITY and RGT sequences of the integrin beta3 cytoplasmic domain in inside-out and outside-in signaling

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.

The VWF/LRP4/αVβ3-axis represents a novel pathway regulating proliferation of human vascular smooth muscle cells

AIMS

Von Willebrand factor (VWF) is a plasma glycoprotein involved in primary haemostasis, while also having additional roles beyond haemostasis namely in cancer, inflammation, angiogenesis, and potentially in vascular smooth muscle cell (VSMC) proliferation. Here, we addressed how VWF modulates VSMC proliferation and investigated the underlying molecular pathways and the in vivo pathophysiological relevance.

METHODS AND RESULTS

VWF induced proliferation of human aortic VSMCs and also promoted VSMC migration. Treatment of cells with a siRNA against αv integrin or the RGT-peptide blocking αvβ3 signalling abolished proliferation. However, VWF did not bind to αvβ3 on VSMCs through its RGD-motif. Rather, we identified the VWF A2 domain as the region mediating binding to the cells. We hypothesized the involvement of a member of the LDL-related receptor protein (LRP) family due to their known ability to act as co-receptors. Using the universal LRP-inhibitor receptor-associated protein, we confirmed LRP-mediated VSMC proliferation. siRNA experiments and confocal fluorescence microscopy identified LRP4 as the VWF-counterreceptor on VSMCs. Also co-localization between αvβ3 and LRP4 was observed via proximity ligation analysis and immuno-precipitation experiments. The pathophysiological relevance of our data was supported by VWF-deficient mice having significantly reduced hyperplasia in carotid artery ligation and artery femoral denudation models. In wild-type mice, infiltration of VWF in intimal regions enriched in proliferating VSMCs was found. Interestingly, also analysis of human atherosclerotic lesions showed abundant VWF accumulation in VSMC-proliferating rich intimal areas.

CONCLUSION

VWF mediates VSMC proliferation through a mechanism involving A2 domain binding to the LRP4 receptor and integrin αvβ3 signalling. Our findings provide new insights into the mechanisms that drive physiological repair and pathological hyperplasia of the arterial vessel wall. In addition, the VWF/LRP4-axis may represent a novel therapeutic target to modulate VSMC proliferation.

The 14-3-3ζ-c-Src-integrin-β3 complex is vital for platelet activation

Several adaptor molecules bind to cytoplasmic tails of β-integrins and facilitate bidirectional signaling, which is critical in thrombosis and hemostasis. Interfering with integrin-adaptor interactions spatially or temporally to inhibit thrombosis without affecting hemostasis is an attractive strategy for the development of safe antithrombotic drugs. We show for the first time that the 14-3-3ζ-c-Src-integrin-β3 complex is formed during platelet activation. 14-3-3ζ-c-Src interaction is mediated by the -PIRLGLALNFSVFYYE- fragment (PE16) on the 14-3-3ζ and SH2-domain on c-Src, whereas the 14-3-3ζ-integrin-β3 interaction is mediated by the -ESKVFYLKMKGDYYRYL- fragment (EL17) on the 14-3-3ζ and -KEATSTF- fragment (KF7) on the β3-integrin cytoplasmic tail. The EL17-motif inhibitor, or KF7 peptide, interferes with the formation of the 14-3-3ζ-c-Src-integrin-β3 complex and selectively inhibits β3 outside-in signaling without affecting the integrin-fibrinogen interaction, which suppresses thrombosis without causing significant bleeding. This study characterized a previously unidentified 14-3-3ζ-c-Src-integrin-β3 complex in platelets and provided a novel strategy for the development of safe and effective antithrombotic treatments.

The Role of BEHAB/Brevican in the Tumor Microenvironment: Mediating Glioma Cell Invasion and Motility

Malignant gliomas are the most common tumors in the central nervous system (CNS) and, unfortunately, are also the most deadly. The lethal nature of malignant gliomas is due in large part to their unique and distinctive ability to invade the surrounding neural tissue. The invasive and dispersive nature of these tumors makes them particularly challenging to treat, and currently there are no effective therapies for malignant gliomas. The brain tumor microenvironment plays a particularly important role in mediating the invasiveness of gliomas, and, therefore, understanding its function is key to developing novel therapies to treat these deadly tumors. A defining aspect of the tumor microenvironment of gliomas is the unique composition of the extracellular matrix that enables tumors to overcome the typically inhibitory environment found in the CNS. One conspicuous component of the glioma tumor microenvironment is the neural-specific ECM molecule, brain-enriched hyaluronan binding (BEHAB)/brevican (B/b). B/b is highly overexpressed in gliomas, and its expression in these tumors contributes importantly to the tumor invasiveness and aggressiveness. However, B/b is a complicated protein with multiple splice variants, cleavage products, and glycoforms that contribute to its complex functions in these tumors and provide unique targets for tumor therapy. Here we review the role of B/b in glioma tumor microenvironment and explore targeting of this protein for glioma therapy.

Post-translational modification-regulated leukocyte adhesion and migration

Leukocytes undergo frequent phenotypic changes and rapidly infiltrate peripheral and lymphoid tissues in order to carry out immune responses. The recruitment of circulating leukocytes into inflamed tissues depends on integrin-mediated tethering and rolling of these cells on the vascular endothelium, followed by transmigration into the tissues. This dynamic process of migration requires the coordination of large numbers of cytosolic and transmembrane proteins whose functional activities are typically regulated by post-translational modifications (PTMs). Our recent studies have shown that the lysine methyltransferase, Ezh2, critically regulates integrin signalling and governs the adhesion dynamics of leukocytes via direct methylation of talin, a key molecule that controls these processes by linking integrins to the actin cytoskeleton. In this review, we will discuss the various modes of leukocyte migration and examine how PTMs of cytoskeletal/adhesion associated proteins play fundamental roles in the dynamic regulation of leukocyte migration. Furthermore, we will discuss molecular details of the adhesion dynamics controlled by Ezh2-mediated talin methylation and the potential implications of this novel regulatory mechanism for leukocyte migration, immune responses, and pathogenic processes, such as allergic contact dermatitis and tumorigenesis.

Functional Effect of the Mutations Similar to the Cleavage during Platelet Activation at Integrin β3 Cytoplasmic Tail when Expressed in Mouse Platelets

Previous studies in Chinese hamster ovary cells showed that truncational mutations of β3 at sites of F⁷⁵⁴ and Y⁷⁵⁹ mimicking calpain cleavage regulate integrin signaling. The roles of the sequence from F⁷⁵⁴ to C-terminus and the conservative N⁷⁵⁶ITY⁷⁵⁹ motif in platelet function have yet to be elaborated. Mice expressing β3 with F⁷⁵⁴ and Y⁷⁵⁹ truncations, or NITY deletion (β3-ΔTNITYRGT, β3-ΔRGT, or β3-ΔNITY) were established through transplanting the homozygous β3-deficient mouse bone marrow cells infected by the GFP tagged MSCV MigR1 retroviral vector encoding different β3 mutants into lethally radiated wild-type mice. The platelets were harvested for soluble fibrinogen binding and platelet spreading on immobilized fibrinogen. Platelet adhesion on fibrinogen- and collagen-coated surface under flow was also tested to assess the ability of the platelets to resist hydrodynamic drag forces. Data showed a drastic inhibition of the β3-ΔTNITYRGT platelets to bind soluble fibrinogen and spread on immobilized fibrinogen in contrast to a partially impaired fibrinogen binding and an almost unaffected spreading exhibited in the β3-ΔNITY platelets. Behaviors of the β3-ΔRGT platelets were consistent with the previous observations in the β3-ΔRGT knock-in platelets. The adhesion impairment of platelets with the β3 mutants under flow was in different orders of magnitude shown as: β3-ΔTNITYRGT>β3-ΔRGT>β3-ΔNITY to fibrinogen-coated surface, and β3-ΔTNITYRGT>β3-ΔNITY>β3-ΔRGT to collagen-coated surface. To evaluate the interaction of the β3 mutants with signaling molecules, GST pull-down and immunofluorescent assays were performed. Results showed that β3-ΔRGT interacted with kindlin but not c-Src, β3-ΔNITY interacted with c-Src but not kindlin, while β3-ΔTNITYRGT did not interact with both proteins. This study provided evidence in platelets at both static and flow conditions that the calpain cleavage-related sequences of integrin β3, i.e. T⁷⁵⁵NITYRGT⁷⁶², R⁷⁶⁰GT⁷⁶², and N⁷⁵⁶ITY⁷⁵⁹ participate in bidirectional, outside-in, and inside-out signaling, respectively and the association of c-Src or kindlin with β3 integrin may regulate these processes.

The interaction of Gα13 with integrin β1 mediates cell migration by dynamic regulation of RhoA

Heterotrimeric G protein Gα13 is known to transmit G protein-coupled receptor (GPCR) signals leading to activation of RhoA and plays a role in cell migration. The mechanism underlying the role of Gα13 in cell migration, however, remains unclear. Recently we found that Gα13 interacts with the cytoplasmic domain of integrin β3 subunits in platelets via a conserved ExE motif. Here we show that a similar direct interaction between Gα13 and the cytoplasmic domain of the integrin β1 subunit plays a critical role in β1-dependent cell migration. Point mutation of either glutamic acid in the Gα13-binding (767)EKE motif in β1 or treatment with a peptide derived from the Gα13-binding sequence of β1 abolished Gα13-β1 interaction and inhibited β1 integrin-dependent cell spreading and migration. We further show that the Gα13-β1 interaction mediates β1 integrin-dependent Src activation and transient RhoA inhibition during initial cell adhesion, which is in contrast to the role of Gα13 in mediating GPCR-dependent RhoA activation. These data indicate that Gα13 plays dynamic roles in both stimulating RhoA via a GPCR pathway and inhibiting RhoA via an integrin signaling pathway. This dynamic regulation of RhoA activity is critical for cell migration on β1 integrin ligands.

Evaluation of targeting c-Src by the RGT-containing peptide as a novel antithrombotic strategy

Background

Interaction of integrin β3 with c-Src plays critical roles in cellular signaling which is heavily implicated in platelet adhesion and aggregation, as well as in tumor cell proliferation and metastasis or in osteoclastic bone resorption. Selectively blocking integrin αIIbβ3 outside-in signaling in platelets has been a focus of attention because of its effective antithrombotic potential together with a sufficient hemostatic capacity. The myristoylated RGT peptide has been shown to achieve this blockade by targeting the association of c-Src with the integrin β3 tail, but the lack of key information regarding the mechanisms of action prevents this strategy from being further developed into practical antithrombotics. Therefore, in-depth knowledge of the precise mechanisms for RGT peptide in regulating platelet function is needed to establish the basis for a potential antithrombotic therapy by targeting c-Src.

Methods

The reduction-sensitive peptides were applied to rule out the membrane anchorage after cytoplasmic delivery. The c-Src activity was assayed at living cell or at protein levels to assess the direct effect of RGT targeting on c-Src. Thrombus formation under flow in the presence of cytoplasmic RGT peptide was observed by perfusing whole blood through the collagen-coated micro-chamber.

Results

The RGT peptide did not depend on the membrane anchorage to inhibit outside-in signaling in platelets. The myr-AC ~ CRGT peptide readily blocked agonist-induced c-Src activation by disrupting the Src/β3 association and inhibited the RhoA activation and collagen-induced platelet aggregation in addition to the typical outside-in signaling events. The myr-AC ~ CRGT had no direct effect on the kinase activity of c-Src in living cells as evidenced by its inability to dissociate Csk from c-Src or to alter the phosphorylation level of c-Src Y⁴¹⁶ and Y⁵²⁷, consistent results were also from in vitro kinase assays. Under flow conditions, the myr-AC ~ CRGT peptide caused an inhibition of platelet thrombus formation predominantly at high shear rates.

Conclusions

These findings provide novel insights into the molecular mechanisms by which the RGT peptide regulates integrin signaling and platelet function and reinforce the potential of the RGT peptide-induced disruption of Src/β3 association as a druggable target that would finally enable in vivo and clinical studies using the structure-based small molecular mimetics.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-015-0159-8) contains supplementary material, which is available to authorized users.

Direct interaction of kindlin-3 with integrin αIIbβ3 in platelets is required for supporting arterial thrombosis in mice

OBJECTIVE

Kindlin-3 is a critical supporter of integrin function in platelets. Lack of expression of kindlin-3 protein in patients impairs integrin αIIbβ3-mediated platelet aggregation. Although kindlin-3 has been categorized as an integrin-binding partner, the functional significance of the direct interaction of kindlin-3 with integrin αIIbβ3 in platelets has not been established. Here, we evaluated the significance of the binding of kindlin-3 to integrin αIIbβ3 in platelets in supporting integrin αIIbβ3-mediated platelet functions.

APPROACH AND RESULTS

We generated a strain of kindlin-3 knockin (K3KI) mice that express a kindlin-3 mutant that carries an integrin-interaction defective substitution. K3KI mice could survive normally and express integrin αIIbβ3 on platelets similar to their wild-type counterparts. Functional analysis revealed that K3KI mice exhibited defective platelet function, including impaired integrin αIIbβ3 activation, suppressed platelet spreading and platelet aggregation, prolonged tail bleeding time, and absence of platelet-mediated clot retraction. In addition, whole blood drawn from K3KI mice showed resistance to in vitro thrombus formation and, as a consequence, K3KI mice were protected from in vivo arterial thrombosis.

CONCLUSIONS

These observations demonstrate that the direct binding of kindlin-3 to integrin αIIbβ3 is involved in supporting integrin αIIbβ3 activation and integrin αIIbβ3-dependent responses of platelets and consequently contributes significantly to arterial thrombus formation.

A directional switch of integrin signalling and a new anti-thrombotic strategy

Integrins are critical in thrombosis and hemostasis¹. Antagonists of the platelet integrin α_IIbβ₃ are potent anti-thrombotic drugs, but also have the life-threatening adverse effect of bleeding^2,3. It is thus desirable to develop new antagonists that do not cause bleeding. Integrins transmit signals bidirectionally^4,5. Inside-out signaling activates integrins via a talin-dependent mechanism^6,7. Integrin ligation mediates thrombus formation and outside-in signaling^8,9, which requires Gα₁₃ and greatly expands thrombi. Here we show that Gα₁₃ and talin bind to mutually exclusive, but distinct sites within the integrin β₃ cytoplasmic domain in opposing waves. The first talin binding wave mediates inside-out signaling and also “ligand-induced integrin activation”, but is not required for outside-in signaling. Integrin ligation induces transient talin dissociation and Gα₁₃ binding to an ExE motif, which selectively mediates outside-in signaling and platelet spreading. The second talin binding wave is associated with clot retraction. An ExE motif-based inhibitor of Gα₁₃-integrin interaction selectively abolishes outside-in signaling without affecting integrin ligation, and suppresses occlusive arterial thrombosis without affecting bleeding time. Thus, we have discovered a novel mechanism for the directional switch of integrin signaling and, based on this mechanism, we designed a potent new anti-thrombotic that does not cause bleeding.

Mitochondrially mediated integrin αIIbβ3 protein inactivation limits thrombus growth

When platelets are strongly stimulated, a procoagulant platelet subpopulation is formed that is characterized by phosphatidylserine (PS) exposure and epitope modulation of integrin αIIbβ3 or a loss of binding of activation-dependent antibodies. Mitochondrial permeability transition pore (mPTP) formation, which is essential for the formation of procoagulant platelets, is impaired in the absence of cyclophilin D (CypD). Here we investigate the mechanisms responsible for these procoagulant platelet-associated changes in integrin αIIbβ3 and the physiologic role of procoagulant platelet formation in the regulation of platelet aggregation. Among strongly stimulated adherent platelets, integrin αIIbβ3 epitope changes, mPTP formation, PS exposure, and platelet rounding were closely associated. Furthermore, platelet mPTP formation resulted in a decreased ability to recruit additional platelets. In the absence of CypD, integrin αIIbβ3 function was accentuated in both static and flow conditions, and, in vivo, a prothrombotic phenotype occurred in mice with a platelet-specific deficiency of CypD. CypD-dependent proteolytic events, including cleavage of the integrin β3 cytoplasmic domain, coincided closely with integrin αIIbβ3 inactivation. Calpain inhibition blocked integrin β3 cleavage and inactivation but not mPTP formation or PS exposure, indicating that integrin inactivation and PS exposure are mediated by distinct pathways subsequent to mPTP formation. mPTP-dependent alkalinization occurred in procoagulant platelets, suggesting a possible alternative mechanism for enhancement of calpain activity in procoagulant platelets. Together, these results indicate that, in strongly stimulated platelets, mPTP formation initiates the calpain-dependent cleavage of integrin β3 and associated regulatory proteins, resulting in integrin αIIbβ3 inactivation, and demonstrate a novel CypD-dependent negative feedback mechanism that limits platelet aggregation and thrombotic occlusion.

Missense mutations in the ABCB6 transporter cause dominant familial pseudohyperkalemia

Familial Pseudohyperkalemia (FP) is a dominant red cell trait characterized by increased serum [K(+)] in whole blood stored at or below room temperature, without additional hematological abnormalities. Functional gene mapping and sequencing analysis of the candidate genes within the 2q35-q36 critical interval identified-in 20 affected individuals among three multigenerational FP families-two novel heterozygous missense mutations in the ABCB6 gene that cosegregated with disease phenotype. The two genomic substitutions altered two adjacent nucleotides within codon 375 of ABCB6, a porphyrin transporter that, in erythrocyte membranes, bears the Langereis blood group antigen system. The ABCB6 R375Q mutation did not alter the levels of mRNA or protein, or protein localization in mature erythrocytes or erythroid precursor cells, but it is predicted to modestly alter protein structure. ABCB6 mRNA and protein levels increase during in vitro erythroid differentiation of CD34(+) erythroid precursors and the erythroleukemia cell lines HEL and K562. These data suggest that the two missense mutations in residue 375 of the ABCB6 polypeptide found in affected individuals of families with chromosome 2-linked FP could contribute to the red cell K(+) leak characteristic of this condition.

Spatial coordination of kindlin-2 with talin head domain in interaction with integrin β cytoplasmic tails

Both talin head domain and kindlin-2 interact with integrin β cytoplasmic tails, and they function in concert to induce integrin activation. Binding of talin head domain to β cytoplasmic tails has been characterized extensively, but information on the interaction of kindin-2 with this integrin segment is limited. In this study, we systematically examine the interactions of kindlin-2 with integrin β tails. Kindlin-2 interacted well with β(1) and β(3) tails but poorly with the β(2) cytoplasmic tail. This binding selectivity was determined by the non-conserved residues, primarily the three amino acids at the extreme C terminus of the β(3) tail, and the sequence in β(2) was non-permissive. The region at the C termini of integrin β(1) and β(3) tails recognized by kindlin-2 was a binding core of 12 amino acids. Kindlin-2 and talin head do not interact with one another but can bind simultaneously to the integrin β(3) tail without enhancing or inhibiting the interaction of the other binding partner. Kindlin-2 itself failed to directly unclasp integrin α/β tail complex, indicating that kindlin-2 must cooperate with talin to support the integrin activation mechanism.

A mutation in the β3 cytoplasmic tail causes variant Glanzmann thrombasthenia by abrogating transition of αIIb β3 to an active state

BACKGROUND

The cytoplasmic tails of α(IIb) and β(3) regulate essential α(IIb) β(3) functions. We previously described a variant Glanzmann thrombasthenia mutation in the β(3) cytoplasmic tail, IVS14: -3C>G, which causes a frameshift with an extension of β(3) by 40 residues.

OBJECTIVES

The aim of this study was to characterize the mechanism by which the mutation abrogates transition of α(IIb) β(3) from a resting state to an active state.

METHODS

 We expressed the natural mutation, termed 742ins, and three artificial mutations in baby hamster kidney (BHK) cells along with wild-type (WT) α(IIb) as follows: β(3) -742stop, a truncated mutant to evaluate the effect of deleted residues; β(3) -749stop, a truncated mutant that preserves the NPLY conserved sequence; and β(3) -749ins, in which the aberrant tail begins after the conserved sequence. Flow cytometry was used to determine ligand binding to BHK cells.

RESULTS AND CONCLUSIONS

Surface expression of α(IIb) β(3) of all four mutants was at least 60% of WT expression, but there was almost no binding of soluble fibrinogen following activation with activating antibodies (anti-ligand-induced-binding-site 6 [antiLIBS6] or PT25-2). Activation of the α(IIb) β(3) mutants was only achieved when both PT25-2 and antiLIBS6 were used together or following treatment with dithiothreitol. These data suggest that the ectodomain of the four mutants is tightly locked in a resting conformation but can be forced to become active by strong stimuli. These data and those of others indicate that the middle part of the β(3) tail is important for maintaining α(IIb) β(3) in a resting conformation.

LFA-1 fine-tuning by cathepsin X

The adhesion molecule lymphocyte function-associated antigen (LFA)-1 plays a key role in immune surveillance and response. Its conformation is spatially and temporally regulated, enabling adhesion and deadhesion during T-cell migration. LFA-1 adhesion to its major ligand intercellular adhesion molecule 1 is controlled by adaptor proteins which bind the cytoplasmic tail of the β (2) subunit. Cathepsin X, a cysteine carboxypeptidase, promotes T-cell migration and morphological changes by cleaving the β (2) cytoplasmic tail of LFA-1. In this way, it modulates the affinity of LFA-1 for structural adaptors talin-1 and α-actinin-1 and enables the stepwise transition between intermediate and high-affinity conformations of LFA-1, an event that is necessary for effective T-cell function. Cathepsin X regulation that would allow precise modulation of LFA-1 affinity has a great potential for anti-LFA-1 therapy.

Platelet adhesion and activation mechanisms in arterial thrombosis and ischaemic stroke

Platelet adhesion, activation and aggregation on the exposed subendothelial extracellular matrix (ECM) are essential for haemostasis, but may also lead to occlusion of diseased vessels. Binding of the glycoprotein (GP)Ib-V-IX complex to immobilised von Willebrand factor (VWF) initiates adhesion of flowing platelets to the ECM, and thereby enables the collagen receptor GPVI to interact with its ligand and to mediate platelet activation. This process is reinforced by locally produced thrombin and platelet-derived secondary mediators, such as adenosine diphosphate (ADP) and thromboxane A(2) (TxA(2)). Together, these events promote a shift of β1 and β3 integrins from a low to a high affinity state for their ligands through 'inside-out' signalling allowing firm platelet adhesion and aggregation. Formed platelet aggregates are stabilised by fibrin formation and signalling events between adjacent platelets involving multiple platelet receptors, such as the newly discovered C-type lectin-like receptor 2 (CLEC-2). While occlusive thrombus formation is the principal pathogenic event in myocardial infarction, the situation is more complex in ischaemic stroke where infarct development often progresses despite sustained early reperfusion of previously occluded major intracranial arteries, a process referred to as 'reperfusion injury'. Increasing experimental evidence now suggests that early platelet adhesion and activation events, orchestrate a 'thrombo-inflammatory' cascade in this setting, whereas platelet aggregation and thrombus formation are not required. This review summarises recent developments in understanding the principal platelet adhesion receptor systems with a focus on their involvement in arterial thrombosis and ischaemic stroke models.

Calpain-mediated proteolysis of paxillin negatively regulates focal adhesion dynamics and cell migration

The dynamic turnover of integrin-mediated adhesions is important for cell migration. Paxillin is an adaptor protein that localizes to focal adhesions and has been implicated in cell motility. We previously reported that calpain-mediated proteolysis of talin1 and focal adhesion kinase mediates adhesion disassembly in motile cells. To determine whether calpain-mediated paxillin proteolysis regulates focal adhesion dynamics and cell motility, we mapped the preferred calpain proteolytic site in paxillin. The cleavage site is between the paxillin LD1 and LD2 motifs and generates a C-terminal fragment that is similar in size to the alternative product paxillin delta. The calpain-generated proteolytic fragment, like paxillin delta, functions as a paxillin antagonist and impairs focal adhesion disassembly and migration. We generated mutant paxillin with a point mutation (S95G) that renders it partially resistant to calpain proteolysis. Paxillin-deficient cells that express paxillin S95G display increased turnover of zyxin-containing adhesions using time-lapse microscopy and also show increased migration. Moreover, cancer-associated somatic mutations in paxillin are common in the N-terminal region between the LD1 and LD2 motifs and confer partial calpain resistance. Taken together, these findings suggest a novel role for calpain-mediated proteolysis of paxillin as a negative regulator of focal adhesion dynamics and migration that may function to limit cancer cell invasion.

Inhibition of platelet activation by peptide analogs of the beta(3)-intracellular domain of platelet integrin alpha(IIb)beta(3) conjugated to the cell-penetrating peptide Tat(48-60)

Activation of the platelet integrin-receptor alpha(IIb)beta(3) is the final pathway of platelet aggregation, regardless of the initiating stimulus. Many studies suggest that there are several cytoplasmic proteins such as talin and beta(3)-endonexin that bind to N(744)PLY(747) and N(756)ITY(759) motif of the beta(3) cytoplasmic tail and play the major role in the receptor activation. In this study, we investigated the role of the membrane distal region of human beta(3) cytoplasmic tail and specifically the N(743)NPLYKEA(750) and T(755)NITYRGT(762) sequence that contains an NXXY motif, in platelet aggregation, secretion, alpha(IIb)beta(3) activation (PAC-1 binding) and fibrinogen binding. We synthesized two peptides corresponding to the above sequences as well as their conjugates with the Tat(48-60) cell-penetrating peptide. The capability of conjugates to penetrate the platelet membrane was investigated with confocal laser scanning microscopy using carboxyfluorescein (CF)-labeled peptides. Our results showed that the conjugated with the Tat(48-60) sequence peptides penetrate the platelet membrane and inhibit platelet aggregation in both PRP and washed platelets in a dose-dependent manner. The Tat-beta(3)743-750 conjugate exhibited similar inhibitory activity in PRP and in washed platelets whereas the Tat-beta(3)755-762 conjugate was more potent inhibitor of aggregation in washed platelets than in PRP. Both conjugated peptides were also able to inhibit P-selectin membrane expression as well as PAC-1 and fibrinogen binding to the platelets, the Tat-beta(3)755-762 conjugate being more potent than Tat-beta(3)743-750. The Tat(48-60) peptide and the peptides beta(3)743-750 and beta(3)755-762, which were not conjugated to the Tat(48-60) sequence, did not exhibit any inhibitory effect on the above parameters. In conclusion, the present study shows for the first time that the peptide analogs of the intracellular domain of the beta(3) subunit beta(3)743-750 and beta(3)755-762 conjugated to the cell-penetrating peptide Tat(48-60) are capable of penetrating the platelet membrane and expressing biological activity by inhibiting the activation of alpha(IIb)beta(3), the fibrinogen binding to the activated receptor as well as platelet aggregation. Further studies are necessary to support whether such conjugated peptides may be useful tools for the development of potent antiplatelet agents acting intracellularly through the platelet integrin alpha(IIb)beta(3).

Calpain-mediated degradation of reversibly oxidized protein-tyrosine phosphatase 1B

Protein-tyrosine phosphatases (PTPs) are regulated by reversible inactivating oxidation of the catalytic-site cysteine. We have previously shown that reversible oxidation upon UVA irradiation is followed by calpain-mediated PTP degradation. Here, we address the mechanism of regulated cleavage and the physiological function of PTP degradation. Reversible oxidation of PTP1B in vitro strongly facilitated the association with calpain and led to greatly increased calpain-dependent inactivating cleavage. Both oxidation-induced association and cleavage depended exclusively on the presence of the catalytic (reversibly oxidized) cysteine residue 215. A major cleavage site was identified preceding amino acid position Ala77. In calpain-deficient cells, insulin signaling was apparently diminished, consistent with a possible role for calpain in removing a negative regulator of insulin signaling. Reversibly oxidized PTP1B may be a target of calpain in this context.

The tail of integrins, talin, and kindlins

Integrins are transmembrane cell-adhesion molecules that carry signals from the outside to the inside of the cell and vice versa. Like other cell surface receptors, integrins signal in response to ligand binding; however, events within the cell can also regulate the affinity of integrins for ligands. This feature is important in physiological situations such as those in blood, in which cells are always in close proximity to their ligands, yet cell-ligand interactions occur only after integrin activation in response to specific external cues. This review focuses on the mechanisms whereby two key proteins, talin and the kindlins, regulate integrin activation by binding the tails of integrin-beta subunits.

IP-10 induces dissociation of newly formed blood vessels

The signals that prune the exuberant vascular growth of tissue repair are still ill defined. We demonstrate that activation of CXC chemokine receptor 3 (CXCR3) mediates the regression of newly formed blood vessels. We present evidence that CXCR3 is expressed on newly formed vessels in vivo and in vitro. CXCR3 is expressed on vessels at days 7-21 post-wounding, and is undetectable in unwounded or healed skin. Treatment of endothelial cords with CXCL10 (IP-10), a CXCR3 ligand present during the resolving phase of wounds, either in vitro or in vivo caused dissociation even in the presence of angiogenic factors. Consistent with this, mice lacking CXCR3 express a greater number of vessels in wound tissue compared to wild-type mice. We then hypothesized that signaling from CXCR3 not only limits angiogenesis, but also compromises vessel integrity to induce regression. We found that activation of CXCR3 triggers micro-calpain activity, causing cleavage of the cytoplasmic tail of beta3 integrins at the calpain cleavage sites c'754 and c'747. IP-10 stimulation also activated caspase 3, blockage of which prevented cell death but not cord dissociation. This is the first direct evidence for an extracellular signaling mechanism through CXCR3 that causes the dissociation of newly formed blood vessels followed by cell death.

Increased extracellular pressure enhances cancer cell integrin-binding affinity through phosphorylation of beta1-integrin at threonine 788/789

Increased extracellular pressure stimulates beta1-integrin-dependent cancer cell adhesion. We asked whether pressure-induced adhesion is mediated by changes in beta1-integrin binding affinity or avidity and whether these changes are phosphorylation dependent. We evaluated integrin affinity and clustering in human SW620 colon cancer cells by measuring differences in binding between soluble Arg-Gly-Asp (RGD)-Fc ligands and RGD-Fc-F(ab')2 multimeric complexes under ambient and 15-mmHg increased pressures. Phosphorylation of beta1-integrin S785 and T788/9 residues in SW620 and primary malignant colonocytes was assessed in parallel. We further used GD25-beta1-integrin-null murine fibroblasts stably transfected with either wild-type beta1A-integrin, S785A, TT788/9AA, or T788D mutants to investigate the role of beta1-integrin site-specific phosphorylation. SW620 binding of RGD-Fc-F(ab')2 multimeric complexes, but not soluble RGD-Fc ligands, was sensitive to integrin clustering. RGD-Fc ligand binding was significantly increased under elevated pressure, suggesting that pressure modulates beta1-integrin affinity. Pressure stimulated both beta1-integrin S785 and T788/9 phosphorylation. GD25-beta1A-integrin wild-type and S785A cells displayed an increase in adhesion to fibronectin under elevated pressure, an effect absent in beta1-integrin-null and TT788/9AA cells. T788D substitution significantly elevated basal cell adhesion but displayed no further increase under pressure. These results suggest pressure-induced cell adhesion is mediated by beta1-integrin T788/9 phosphorylation-dependent changes in integrin binding affinity.

Antithrombotic effects of targeting alphaIIbbeta3 signaling in platelets

alphaIIbbeta3 interaction with fibrinogen promotes Src-dependent platelet spreading in vitro. To determine the consequences of this outside-in signaling pathway in vivo, a "beta3(Delta760-762)" knockin mouse was generated that lacked the 3 C-terminal beta3 residues (arginine-glycine-threonine [RGT]) necessary for alphaIIbbeta3 interaction with c-Src, but retained beta3 residues necessary for talin-dependent fibrinogen binding. beta3(Delta760-762) mice were compared with wild-type beta3(+/+) littermates, beta3(+/-) heterozygotes, and knockin mice where beta3 RGT was replaced by beta1 C-terminal cysteine-glycine-lysine (EGK) to potentially enable signaling by Src kinases other than c-Src. Whereas beta3(+/+), beta3(+/-) and beta3/beta1(EGK) platelets spread and underwent tyrosine phosphorylation normally on fibrinogen, beta3(Delta760-762) platelets spread poorly and exhibited reduced tyrosine phosphorylation of c-Src substrates, including beta3 (Tyr(747)). Unlike control mice, beta3(Delta760-762) mice were protected from carotid artery thrombosis after vessel injury with FeCl(3). Some beta3(Delta760-762) mice exhibited prolonged tail bleeding times; however, none demonstrated spontaneous bleeding, excess bleeding after surgery, fecal blood loss, or anemia. Fibrinogen binding to beta3(Delta760-762) platelets was normal in response to saturating concentrations of protease-activated receptor 4 or glycoprotein VI agonists, but responses to adenosine diphosphate were impaired. Thus, deletion of beta3 RGT disrupts c-Src-mediated alphaIIbbeta3 signaling and confers protection from arterial thrombosis. Consequently, targeting alphaIIbbeta3 signaling may represent a feasible antithrombotic strategy.

Kindlin-2 controls bidirectional signaling of integrins

Control of integrin activation is required for cell adhesion and ligand-induced signaling. Here we report that loss of the focal adhesion protein Kindlin-2 in mice results in peri-implantation lethality caused by severe detachment of the endoderm and epiblast from the basement membrane. We found that Kindlin-2-deficient cells were unable to activate their integrins and that Kindlin-2 is required for talin-induced integrin activation. Furthermore, we demonstrate that Kindlin-2 is required for integrin outside-in signaling to enable firm adhesion and spreading. Our findings provide evidence that Kindlin-2 is a novel and essential element of bidirectional integrin signaling.

The proteoglycan brevican binds to fibronectin after proteolytic cleavage and promotes glioma cell motility

The adult neural parenchyma contains a distinctive extracellular matrix that acts as a barrier to cell and neurite motility. Nonneural tumors that metastasize to the central nervous system almost never infiltrate it and instead displace the neural tissue as they grow. In contrast, invasive gliomas disrupt the extracellular matrix and disperse within the neural tissue. A major inhibitory component of the neural matrix is the lectican family of chondroitin sulfate proteoglycans, of which brevican is the most abundant member in the adult brain. Interestingly, brevican is also highly up-regulated in gliomas and promotes glioma dispersion by unknown mechanisms. Here we show that brevican secreted by glioma cells enhances cell adhesion and motility only after proteolytic cleavage. At the molecular level, brevican promotes epidermal growth factor receptor activation, increases the expression of cell adhesion molecules, and promotes the secretion of fibronectin and accumulation of fibronectin microfibrils on the cell surface. Moreover, the N-terminal cleavage product of brevican, but not the full-length protein, associates with fibronectin in cultured cells and in surgical samples of glioma. Taken together, our results provide the first evidence of the cellular and molecular mechanisms that may underlie the motility-promoting role of brevican in primary brain tumors. In addition, these results underscore the important functional implications of brevican processing in glioma progression.

Kindlin-2 (Mig-2): a co-activator of beta3 integrins

Integrin activation is essential for dynamically linking the extracellular environment and cytoskeletal/signaling networks. Activation is controlled by integrins' short cytoplasmic tails (CTs). It is widely accepted that the head domain of talin (talin-H) can mediate integrin activation by binding to two sites in integrin β's CT; in integrin β₃ this is an NPLY⁷⁴⁷ motif and the membrane-proximal region. Here, we show that the C-terminal region of integrin β₃ CT, composed of a conserved TS⁷⁵²T region and NITY⁷⁵⁹ motif, supports integrin activation by binding to a cytosolic binding partner, kindlin-2, a widely distributed PTB domain protein. Co-transfection of kindlin-2 with talin-H results in a synergistic enhancement of integrin α_IIbβ₃ activation. Furthermore, siRNA knockdown of endogenous kindlin-2 impairs talin-induced α_IIbβ₃ activation in transfected CHO cells and blunts α_vβ₃-mediated adhesion and migration of endothelial cells. Our results thus identify kindlin-2 as a novel regulator of integrin activation; it functions as a coactivator.

RGT, a synthetic peptide corresponding to the integrin beta 3 cytoplasmic C-terminal sequence, selectively inhibits outside-in signaling in human platelets by disrupting the interaction of integrin alpha IIb beta 3 with Src kinase

Mutational analysis has established that the cytoplasmic tail of the integrin beta 3 subunit binds c-Src (termed as Src in this study) and is critical for bidirectional integrin signaling. Here we show in washed human platelets that a cell-permeable, myristoylated RGT peptide (myr-RGT) corresponding to the integrin beta 3 C-terminal sequence dose-dependently inhibited stable platelet adhesion and spreading on immobilized fibrinogen, and fibrin clot retraction as well. Myr-RGT also inhibited the aggregation-dependent platelet secretion and secretion-dependent second wave of platelet aggregation induced by adenosine diphosphate, ristocetin, or thrombin. Thus, myr-RGT inhibited integrin outside-in signaling. In contrast, myr-RGT had no inhibitory effect on adenosine diphosphate-induced soluble fibrinogen binding to platelets that is dependent on integrin inside-out signaling. Furthermore, the RGT peptide induced dissociation of Src from integrin beta 3 and dose-dependently inhibited the purified recombinant beta 3 cytoplasmic domain binding to Src-SH3. In addition, phosphorylation of the beta 3 cytoplasmic tyrosines, Y(747) and Y(759), was inhibited by myr-RGT. These data indicate an important role for beta 3-Src interaction in outside-in signaling. Thus, in intact human platelets, disruption of the association of Src with beta 3 and selective blockade of integrin alpha IIb beta 3 outside-in signaling by myr-RGT suggest a potential new antithrombotic strategy.

Fibrinogen interaction of CHO cells expressing chimeric alphaIIb/alphavbeta3 integrin

AIM

The molecular mechanisms of the affinity regulation of alphavbeta3 integrin are important in tumor development, wound repairing, and angiogenesis. It has been established that the cytoplasmic domains of alphavbeta3 integrin play an important role in integrin-ligand affinity regulation. However, the relationship of structure-function within these domains remains unclear.

METHODS

The extracellular and transmembrane domain of alphaIIb was fused to the alphav integrin cytoplasmic domain, and the chimeric alpha subunit was coexpressed in Chinese hamster ovary (CHO) cells with the wild-type beta3 subunit or with 3 mutant beta3 sequences bearing truncations at the positions of T741, Y747, and F754, respectively. The CHO cells expressing these recombinant integrins were tested for soluble fibrinogen binding and the cell adhesion and spreading on immobilized fibrinogen.

RESULTS

All 4 types of integrins bound soluble fibrinogen in the absence of agonist stimulation, and only the cells expressing the chimeric alpha subunit with the wild-type beta3 subunit, but not those with truncated beta3, could adhere to and spread on immobilized fibrinogen.

CONCLUSION

The substitution alphaIIb at the cytoplasmic domain with the alphav cytoplasmic sequence rendered the extracellular alphaIIbbeta3 a constitutively activated conformation for ligands without the need of pinside-outq signals. Our results also indicated that the COOH-terminal sequence of beta3 might play a key role in integrin alphaIIb/alphavbeta3-mediated cell adhesion and spreading on immobilized fibrinogen. The cells expressing alphaIIb/alphavbeta3 have enormous potential for facilitating drug screening for antagonists either to alphavbeta3 intracellular interactions or to alphaIIbbeta3 receptor functions.

Cooperative role of the membrane-proximal and -distal residues of the integrin beta3 cytoplasmic domain in regulation of talin-mediated alpha IIb beta3 activation

Integrin cytoplasmic tails regulate integrin activation that is required for high affinity binding with ligands. The interaction of the integrin beta subunit tail with a cytoplasmic protein, talin, largely contributes to integrin activation. Here we report the cooperative interaction of the beta3 membrane-proximal and -distal residues in regulation of talin-mediated alpha IIb beta3 activation. Because a chimeric integrin, alpha IIb beta3/beta1, in which the beta3 tail was replaced with the beta1 tail was constitutively active, we searched for the residues responsible for integrin activation among the residues that differed between the beta3 and beta1 tails. Single amino acid substitutions of Ile-719 and Glu-749 in the beta3 membrane-proximal and -distal regions, respectively, with the corresponding beta1 residues or alanine rendered alphaIIbbeta3 constitutively active. The I719M/E749S double mutant had the same ligand binding activity as alpha IIb beta3/beta1. These beta3 mutations also induced alphaVbeta3 activation. Conversely, substitution of Met-719 or Ser-749 in the beta1 tail with the corresponding beta3 tail residue (M719I or S749E) inhibited alpha IIb beta3/beta1 activation, and the M719I/S749E double mutant inhibited ligand binding to a level comparable with that of the wild-type alpha IIb beta3. Knock down of talin by short hairpin RNA inhibited the I719M- and E749S-induced alpha IIb beta3 activation. These results suggest that the beta3 membrane-proximal and -distal residues cooperatively regulate talin-mediated alpha IIb beta3 activation.

A molecular switch that controls cell spreading and retraction

Integrin-dependent cell spreading and retraction are required for cell adhesion, migration, and proliferation, and thus are important in thrombosis, wound repair, immunity, and cancer development. It remains unknown how integrin outside-in signaling induces and controls these two opposite processes. This study reveals that calpain cleavage of integrin β₃ at Tyr⁷⁵⁹ switches the functional outcome of integrin signaling from cell spreading to retraction. Expression of a calpain cleavage–resistant β₃ mutant in Chinese hamster ovary cells causes defective clot retraction and RhoA-mediated retraction signaling but enhances cell spreading. Conversely, a calpain-cleaved form of β₃ fails to mediate cell spreading, but inhibition of the RhoA signaling pathway corrects this defect. Importantly, the calpain-cleaved β₃ fails to bind c-Src, which is required for integrin-induced cell spreading, and this requirement of β₃-associated c-Src results from its inhibition of RhoA-dependent contractile signals. Thus, calpain cleavage of β₃ at Tyr⁷⁵⁹ relieves c-Src–mediated RhoA inhibition, activating the RhoA pathway that confines cell spreading and causes cell retraction.

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.

Programmed autologous cleavage of platelet receptors

Platelet adhesion receptors play a critical role in vascular pathophysiology, and control platelet adhesion, activation and aggregation in hemostasis, thrombotic disease and atherogenesis. One of the key emerging mechanisms for regulating platelet function is the programmed autologous cleavage of platelet receptors. Induced by ligand binding or platelet activation, proteolysis at extracellular (ectodomain shedding) or intracellular (cytoplasmic domain deactivation) sites down-regulates the adheso-signaling function of receptors, thereby controlling not only platelet responsiveness, but in the case of ectodomain shedding, liberating soluble ectodomain fragments into plasma where they constitute potential modulators or markers. This review discusses the underlying mechanisms for dual proteolytic pathways of receptor regulation, and the impact of these pathways on thrombus formation and stability in vivo.

Platelet Receptor Proteolysis

The platelet plasma membrane is literally at the cutting-edge of recent research into proteolytic regulation of the function and surface expression of platelet receptors, revealing new mechanisms for how the thrombotic propensity of platelets is controlled in health and disease. Extracellular proteolysis of receptors irreversibly inactivates receptor-mediated adhesion and signaling, as well as releasing soluble fragments into the plasma where they act as potential markers or modulators. Platelet-surface sheddases, particularly of the metalloproteinase-disintegrin (ADAM) family, can be regulated by many of the same mechanisms that control receptor function, such as calmodulin association or activation of signaling pathways. This provides layers of regulation (proteinase and receptor), and a higher order of control of cellular function. Activation of pathways leading to extracellular shedding is concomitant with activation of intracellular proteinases such as calpain, which may also irreversibly deactivate receptors. In this review, platelet receptor shedding will be discussed in terms of (1) the identity of proteinases involved in receptor proteolysis, (2) key platelet receptors regulated by proteolytic pathways, and (3) how shedding might be regulated in normal physiology or future therapeutics. In particular, a focus on proteolytic regulation of the platelet collagen receptor, glycoprotein (GP)VI, illustrates many of the key biochemical, cellular, and clinical implications of current research in this area.

Blockade of integrin beta3-FAK signaling pathway activated by osteopontin inhibits neointimal formation after balloon injury

BACKGROUND

Osteopontin (OPN) promotes the migration and adhesion of vascular smooth muscle cells (VSMCs) through cell surface receptor, integrin beta3. In order to elucidate the signaling pathway by which OPN is involved in neointimal formation, we focused on integrin beta3-focal adhesion kinase (FAK) upon VSMC migration.

METHODS

The integrin beta3 and FAK expression in VSMC and in neointima was detected by Western blot and immunohistochemistry staining. FAK phosphorylation induced by OPN was verified using a linear OPN 13 peptide containing RGD motif and anti-OPN antibody. The role of integrin beta3-FAK pathway in VSMC adhesion and migration induced with OPN was tested by the overexpression of FAK-related nonkinase and integrin beta3 cytoplasmic domain.

RESULTS

The results showed that OPN increased integrin beta3 expression and induced rapid and transient FAK phosphorylation. Inhibition of the phosphorylation of FAK significantly suppressed VSMC migration induced by OPN. Similarly, blockade of the interaction of integrin beta3 with OPN inhibited VSMC adhesion induced by OPN. The experiment, in vivo, demonstrated that OPN expression level was consistent with neointimal thickening. Administration of anti-OPN antibody for blocking OPN function suppressed integrin beta3 and FAK expression induced by balloon injury, and neointimal thickening was inhibited.

CONCLUSIONS

These data indicate that integrin beta3-FAK signaling modulates OPN-induced VSMC migration during neointimal formation.

Pathological shear stress directly regulates platelet αIIbβ3signaling

Integrin mechanotransduction is a ubiquitous biological process. Mechanical forces are transduced transmembranously by an integrin's ligand-bound extracellular domain through its β-subunit's cytoplasmic domain connected to the cytoskeleton. This often culminates in the activation of tyrosine kinases directing cell responses. The delicate balance between hemostasis and thrombosis requires exquisitely fine-tuned integrin function, and balance is maintained in vivo despite that the major platelet integrin αIIbβ3is continuously subjected to frictional or shearing forces generated by laminar blood flow. To test the hypothesis that platelet function is regulated by the direct effects of mechanical forces on αIIbβ3, we examined αIIbβ3/cytoskeletal interactions in human platelets exposed to shear stress in a cone-plate viscometer. We observed that α-actinin, myosin heavy chain, and Syk coimmunoprecipitate with αIIbβ3in resting platelets and that 120 dyn/cm2shear stress leads to their disassociation from αIIbβ3. Shear-induced disassociation of α-actinin and myosin heavy chain from the β3tail is unaffected by blocking von Willebrand factor (VWF) binding to glycoprotein (Gp) Ib-IX-V but abolished by blocking VWF binding to αIIbβ3. Syk's disassociation from β3is inhibited when VWF binding to either GpIb-IX-V or αIIbβ3is blocked. Shear stress-induced phosphorylation of SLP-76 and its association with tyrosine-phosphorylated adhesion and degranulation-promoting adapter protein are inhibited by blocking ligand binding to αIIbβ3but not by blocking ligand binding to GpIb-IX-V. Chinese hamster ovary cells expressing αIIbβ3with β3truncated of its cytoskeletal binding domains demonstrate diminished shear-dependent adhesion and cohesion. These results support the hypothesis that shear stress directly modulates αIIbβ3function and suggest that shear-induced αIIbβ3-mediated signaling contributes to the regulation of platelet aggregation by directing the release of constraining cytoskeletal elements from the β3-tail.

Tyrosine phosphorylation of the integrin beta 3 subunit regulates beta 3 cleavage by calpain

Outside-in signaling of beta(3) integrins induces and requires phosphorylation at tyrosine 747 (Tyr(747)) and tyrosine 759 (Tyr(759)) of the beta(3) subunit, but the mechanism for this requirement is unclear. On the other hand, a key consequence of integrin signaling, cell spreading, is inhibited by calpain cleavage of beta(3) cytoplasmic domain. Here we show that beta(3) tyrosine phosphorylation inhibits calpain cleavage. Mutating both tyrosines to phenylalanine sensitizes beta(3) to calpain cleavage. Furthermore, phosphorylation at Tyr(747) and Tyr(759) of beta(3) in the focal adhesion sites and the leading edge of spreading platelets was differentially regulated. Selective dephosphorylation of Tyr(759) is associated with calpain cleavage at Tyr(759). Thus, one mechanism by which tyrosine phosphorylation promotes integrin signaling and cell spreading is its inhibition of calpain cleavage of the beta(3) cytoplasmic domain.

Differential regulation of platelet aggregation and aminophospholipid exposure by calpain

Aggregation, exposure of procoagulant phospholipids and shedding of microparticles are platelet responses that depend on activating conditions. To determine how these different responses are interconnected, we simultaneously measured fibrinogen (Fg) binding and aminophospholipid exposure on activated platelets by means of flow cytometry. Low calcium ionophore (A23187) concentrations induced Fg binding but not annexin V binding. In contrast, high A23187 concentrations induced annexin V binding but not Fg binding. Collagen, both alone and in the presence of thrombin, induced both Fg and annexin V binding. Dual labelling was found on 38 +/- 9% of platelets stimulated by thrombin plus collagen. The regulatory role of calpain in these platelet functions was investigated. When calpain was partially inhibited by 2 microg/ml calpeptin, Fg binding still occurred but aminophospholipid exposure was limited. By contrast, complete inhibition of calpain by 100 microg/ml calpeptin or E64d decreased Fg binding but enhanced aminophospholipid exposure. In these latter conditions, cytosolic calcium-extruding systems were inhibited. The results suggest that (i) conditions that favour aminophospholipid exposure tend to decrease the aggregation process and (ii) calpain determines the switch to either aggregation or aminophospholipid exposure by controlling intracellular calcium.

Structure and function of the platelet integrin alphaIIbbeta3

The platelet integrin alpha(IIb)beta(3) is required for platelet aggregation. Like other integrins, alpha(IIb)beta(3) resides on cell surfaces in an equilibrium between inactive and active conformations. Recent experiments suggest that the shift between these conformations involves a global reorganization of the alpha(IIb)beta(3) molecule and disruption of constraints imposed by the heteromeric association of the alpha(IIb) and beta(3) transmembrane and cytoplasmic domains. The biochemical, biophysical, and ultrastructural results that support this conclusion are discussed in this Review.

Calpain is required for normal osteoclast function and is down-regulated by calcitonin

Osteoclast motility is thought to depend on rapid podosome assembly and disassembly. Both mu-calpain and m-calpain, which promote the formation and disassembly of focal adhesions, were observed in the podosome belt of osteoclasts. Calpain inhibitors disrupted the podosome belt, blocked the constitutive cleavage of the calpain substrates filamin A, talin, and Pyk2, which are enriched in the podosome belt, induced osteoclast retraction, and reduced osteoclast motility and bone resorption. The motility and resorbing activity of mu-calpain(-/-) osteoclast-like cells were also reduced, indicating that mu-calpain is required for normal osteoclast activity. Histomorphometric analysis of tibias from mu-calpain(-/-) mice revealed increased osteoclast numbers and decreased trabecular bone volume that was apparent at 10 weeks but not at 5 weeks of age. In vitro studies suggested that the increased osteoclast number in the mu-calpain(-/-) bones resulted from increased osteoclast survival, not increased osteoclast formation. Calcitonin disrupted the podosome ring, induced osteoclast retraction, and reduced osteoclast motility and bone resorption in a manner similar to the effects of calpain inhibitors and had no further effect on these parameters when added to osteoclasts pretreated with calpain inhibitors. Calcitonin inhibited the constitutive cleavage of a fluorogenic calpain substrate and transiently blocked the constitutive cleavage of filamin A, talin, and Pyk2 by a protein kinase C-dependent mechanism, demonstrating that calcitonin induces the inhibition of calpain in osteoclasts. These results indicate that calpain activity is required for normal osteoclast activity and suggest that calcitonin inhibits osteoclast bone resorbing activity in part by down-regulating calpain activity.

Integrin alphaIIb-subunit cytoplasmic domain mutations demonstrate a requirement for tyrosine phosphorylation of beta3-subunits in actin cytoskeletal organization

Using truncated or mutated alphaIIb integrin cytoplasmic domains fused to the alphaV extracellular domain and expressed with the beta3 integrin subunit, we demonstrate that the double mutation of proline residues 998 and 999 to alanine (PP998/999AA), previously shown to disturb the C-terminal conformation of the alphaIIb integrin cytoplasmic domain, prevents tyrosine phosphorylation of beta3 integrin induced by Arg-Gly-Asp peptide ligation. This mutation also inhibits integrin mediated actin assembly and cell adhesion to vitronectin. In contrast, progressive truncation of the alphaIIb-subunit cytoplasmic domain did not reproduce these effects. Interestingly, the PP998/999AA mutations of alphaIIb did not affect beta3 tyrosine phosphorylation, cell adhesion, or actin polymerization induced by manganese. Exogenous addition of manganese was sufficient to rescue beta3 phosphorylation, cell adhesion, and actin assembly in cells expressing the PP998/999AA mutation when presented with a vitronectin substrate. Further, induction of the high affinity conformation of this mutant beta3 integrin by incubation with either Arg-Gly-Asp peptide or exogenous manganese was equivalent. These results suggest that the extracellular structure of beta3 integrins in the high affinity conformation is not directly related to the structure of the cytoplasmic face of the integrin. Moreover, the requirement for beta3 phosphorylation is demonstrated without mutation of the beta3 subunit. In support of our previous hypothesis of a role for beta3 phosphorylation in adhesion, these studies demonstrate a strong correlation between beta3 tyrosine phosphorylation and assembly of a cytoskeleton competent to support firm cell adhesion.

Sequential activation of p38 and ERK pathways by cGMP-dependent protein kinase leading to activation of the platelet integrin alphaIIb beta3

Integrin activation (inside-out signaling) in platelets can be initiated by agonists such as von Willebrand factor (VWF) and thrombin. Here we show that a mitogen-activated protein kinase (MAPK), p38, plays an important role in the activation of integrin alphaIIb beta3 induced by VWF and thrombin. A dominant-negative mutant of p38, p38AF, inhibits alphaIIb beta3 activation induced by VWF binding to its receptor, the platelet glycoprotein Ib-IX (GPIb-IX), and p38 inhibitors diminish platelet aggregation induced by VWF or low-dose thrombin. The inhibitory effect of p38 inhibitor is unlikely to be caused by the previous suggested effect on cyclo-oxygenase, as inhibition also was observed in the presence of high concentrations of cyclo-oxygenase inhibitor, aspirin. VWF or thrombin induces p38 activation, which is inhibited in cGMP-dependent protein kinase (PKG)-knockout mouse platelets and PKG inhibitor-treated human platelets, indicating that activation of p38 is downstream from PKG in the signaling pathway. p38AF or p38 inhibitors diminish PKG-induced phosphorylation of extracellular stimuli-responsive kinase (ERK), which also is important in integrin activation. Thus, p38 plays an important role in mediating PKG-dependent activation of ERK. These data delineate a novel signaling pathway in which platelet agonists sequentially activate PKG, p38, and ERK pathways leading to integrin activation.

Specification of the direction of adhesive signaling by the integrin beta cytoplasmic domain

Integrin adhesion receptors can signal in two directions: first, they can regulate cellular behaviors by modulating cellular signaling enzymes ("outside-in signaling"); second, cells can regulate the affinity of integrins ("inside-out signaling") by such pathways. Integrin beta cytoplasmic domains (tails) mediate both types of signaling, and Src family kinases (SFKs) and talin, which bind to beta tails, are important for integrin signaling. Here, we utilized "homology scanning" mutagenesis to identify beta tail mutants selectively defective in c-Src binding and found that amino acid exchanges affecting a combination of an Arg and Thr residue in the integrin beta3 tail control the binding specificity for SFKs but have no effect on talin binding. Using beta tail mutants at these residues, we found that SFK binding to integrin beta tails is dispensable for inside-out signaling but is obligatory for cell spreading, a marker of outside-in signaling. Conversely, we found that point mutations that disrupt talin binding abolish integrin activation, but they do not inhibit SFK binding to the beta3 tail or the initiation of outside-in signaling once the integrins are in a high affinity form. Thus, we show that inside-out and outside-in integrin signaling are mediated by distinct and separable interactions of the integrin beta tails. Furthermore, based on our results, it is possible to discern the relative contributions of the direction of integrin signaling on biological functions in cell culture and, ultimately, in vivo.

A tripeptide mimetic of von Willebrand factor residues 981-983 enhances platelet adhesion to fibrinogen by signaling through integrin alpha(IIb)beta3

BACKGROUND

RGD is a major recognition sequence for ligands of platelet alpha(IIb)beta3.

OBJECTIVE AND METHODS

To identify potential binding sites for alpha(IIb)beta3 apart from RGD, we screened phage display libraries by blocking the enrichment of RGD-containing phages with a GRGDS peptide and identified a novel integrin recognition tripeptide sequence, VPW.

RESULTS

Platelets adhered to an immobilized cyclic VPW containing peptide in a alpha(IIb)beta3-dependent manner; platelets and alpha(IIb)beta3-expressing CHO cells adhered faster to immobilized alpha(IIb)beta3-ligands in the presence of soluble VPW. In platelets adhering to fibrinogen, VPW accelerated the activation of the tyrosine kinase Syk which controls cytoskeletal rearrangements. In alpha(IIb)beta3-expressing CHO cells, VPW induced a faster formation of stress fibers. Sequence alignment positioned VPW to V980-P981-W982 in the von Willebrand factor (vWf) A-3 domain. In blood from a vWf-deficient individual, VPW increased platelet adhesion to fibrinogen but not to collagen under flow and rescued the impaired adhesion to vWf deficient in A-3.

CONCLUSION

These data reveal a VPW sequence that contributes to alpha(IIb)beta3 activation in in vitro experiments. Whether the V980-P981-W982 sequence in vWf shows similar properties under in vivo conditions remains to be established.

Integrin activation

The ability of cells to regulate dynamically their adhesion to one another and to the extracellular matrix (ECM) that surrounds them is essential in multicellular organisms. The integrin family of transmembrane adhesion receptors mediates both cell-cell and cell-ECM adhesion. One important, rapid and reversible mechanism for regulating adhesion is by increasing the affinity of integrin receptors for their extracellular ligands (integrin activation). This is controlled by intracellular signals that, through their action on integrin cytoplasmic domains, induce conformational changes in integrin extracellular domains that result in increased affinity for ligand. Recent studies have shed light on the final intracellular steps in this process and have revealed a vital role for the cytoskeletal protein talin.

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.