Rap1 In Platelets Research Articles

The Small Gtpase Rap1 in Platelets Is Critical for Arterial but Not Venous Thrombosis in Mice

Background: Platelets and their main adhesion receptors, integrins, are critical in hemostasis and arterial thrombosis, i.e., in situations involving severe insult to the vasculature and elevated shear stress, respectively. We recently demonstrated that integrin activation under both of these conditions depends on the small GTPase Rap1 directly activating the integrin adapter protein, Talin1. Our studies further suggested that the Rap1-talin1 axis is less important for platelet function at sites of inflammation, i.e., in situations of mild endothelial insult and low shear stress.Aim: To investigate the contribution of the platelet Rap1-Talin1-integrin signaling axis in the pathogenesis of venous thrombosis (VT).Methods: The following mice with documented deficiencies in platelets were subjected to the inferior vena cava (IVC) stenosis VT model: Rap1amKO (Rap1a fl/fl pf4-Cre+), Rap1bmKO (Rap1b fl/fl pf4-Cre+), or both isoforms (Rap1a fl/flRap1b fl/fl pf4-Cre+ [Rap1dKO]), and Talin1mKO (Tln1 fl/flpf4-Cre+). Thrombus weight was determined 48 hours after flow restriction. Clot contraction and tissue plasminogen activator (tPA)-mediated lysis of whole blood clots were studied ex vivo to characterize effects on clot consolidation and stability .Results: Compared to control mice, thrombus weight was markedly reduced in Talin1mKO mice (13.1±2 and 2.2±1.2 mg, p<0.05), but not Rap1dKO or single knockout mice. Ex vivo clot contraction was significantly impaired in whole blood from Talin1mKO (clot weight: control 13.35±3.4 mg vs. 34.63±4.2 mg), Rap1amKO (control 18.0±3.9 mg vs. 25.6±6.0 mg), Rap1bmKO (control 18.0±3.9 vs 23.0±6.6 mg) and in Rap1dKO (control 18.03±3.9 mg vs. 30.02±4.6 mg). Clot weights were not significantly different between Rap1dKO and Talin1mKO mice. However, ex vivo clot lysis assays revealed that compared to controls, clots formed in whole blood from Talin1mKO mice lysed faster, whereas clots from Rap1dKO did not.Conclusions: Platelet Talin1 is critical for VT in mice, and this role is partially independent of its best-known upstream regulator, Rap1. Future studies will identify the alternative mechanism allowing for Rap1-independent Talin1 signaling and platelet function in VT and how these signaling affect various phases of VT. DisclosuresWolberg: Takeda: Research Funding; Bristol Myers Squibb: Research Funding; CSL Behring: Consultancy.

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

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

Functional redundancy between RAP1 isoforms in murine platelet production and function

AbstractRAP GTPases, important regulators of cellular adhesion, are abundant signaling molecules in the platelet/megakaryocytic lineage. However, mice lacking the predominant isoform, RAP1B, display a partial platelet integrin activation defect and have a normal platelet count, suggesting the existence of a RAP1-independent pathway to integrin activation in platelets and a negligible role for RAP GTPases in megakaryocyte biology. To determine the importance of individual RAP isoforms on platelet production and on platelet activation at sites of mechanical injury or vascular leakage, we generated mice with megakaryocyte-specific deletion (mKO) of Rap1a and/or Rap1b. Interestingly, Rap1a/b-mKO mice displayed a marked macrothrombocytopenia due to impaired proplatelet formation by megakaryocytes. In platelets, RAP isoforms had redundant and isoform-specific functions. Deletion of RAP1B, but not RAP1A, significantly reduced α-granule secretion and activation of the cytoskeleton regulator RAC1. Both isoforms significantly contributed to thromboxane A2 generation and the inside-out activation of platelet integrins. Combined deficiency of RAP1A and RAP1B markedly impaired platelet aggregation, spreading, and clot retraction. Consistently, thrombus formation in physiological flow conditions was abolished in Rap1a/b-mKO, but not Rap1a-mKO or Rap1b-mKO, platelets. Rap1a/b-mKO mice were strongly protected from experimental thrombosis and exhibited a severe defect in hemostasis after mechanical injury. Surprisingly, Rap1a/b-mKO platelets were indistinguishable from controls in their ability to prevent blood–lymphatic mixing during development and hemorrhage at sites of inflammation. In summary, our studies demonstrate an essential role for RAP1 signaling in platelet integrin activation and a critical role in platelet production. Although important for hemostatic/thrombotic plug formation, platelet RAP1 signaling is dispensable for vascular integrity during development and inflammation.

Protein kinase C signaling dysfunction in von Willebrand disease (p.V1316M) type 2B platelets

Abstractvon Willebrand disease (VWD) type 2B is characterized by gain-of-function mutations in von Willebrand factor (VWF), enhancing its binding affinity for the platelet receptor glycoprotein (GP)Ibα. VWD type 2B patients display a bleeding tendency associated with loss of high-molecular-weight VWF multimers and variable thrombocytopenia. We recently demonstrated that a marked defect in agonist-induced activation of the small GTPase, Rap1, and integrin αIIbβ3 in VWD (p.V1316M) type 2B platelets also contributes to the bleeding tendency. Here, we investigated the molecular mechanisms underlying impaired platelet Rap1 signaling in this disease. Two distinct pathways contribute to Rap1 activation in platelets: rapid activation mediated by the calcium-sensing guanine nucleotide exchange factor CalDAG–GEF-I (CDGI) and sustained activation that is dependent on signaling by protein kinase C (PKC) and the adenosine 5′-diphosphate receptor P2Y12. To investigate which Rap1 signaling pathway is affected, we expressed VWF/p.V1316M by hydrodynamic gene transfer in wild-type and Caldaggef1−/− mice. Using αIIbβ3 integrin activation as a read-out, we demonstrate that platelet dysfunction in VWD (p.V1316M) type 2B affects PKC-mediated, but not CDGI-mediated, activation of Rap1. Consistently, we observed decreased PKC substrate phosphorylation and impaired granule release in stimulated VWD type 2B platelets. Interestingly, the defect in PKC signaling was caused by a significant increase in baseline PKC substrate phosphorylation in circulating VWD (p.V1316M) type 2B platelets, suggesting that the VWF–GPIbα interaction leads to preactivation and exhaustion of the PKC pathway. Consistent with PKC preactivation, VWD (p.V1316M) type 2B mice also exhibited marked shedding of platelet GPIbα. In summary, our studies identify altered PKC signaling as the underlying cause of platelet hypofunction in p.V1316M-associated VWD type 2B.

CGMP and cAMP pathways rearrange ARHGAP17 and ARHGEF6 protein complexes to control Rac1 in platelets

Endothelial cells inhibit blood platelets by releasing nitric oxide (NO) and prostacyclin (PGI2). NO and PGI2 activate cGMP and cAMP synthesis, leading to the activation of cGMP- and cAMP-dependent protein kinases (PKG, PKA), phosphorylation of substrate proteins, and inhibition and reversal of platelet activation and aggregation. A known target of PKA and PKG is the small G-protein Rac1. The active form of Rac1, Rac1-GTP, contributes to cytoskeletal reorganization, platelet spreading, adhesion, granule release, and aggregation. PKG and PKA activation have been shown to reduce Rac1-GTP levels. Rac1-GTP is controlled by GTPase-activating proteins (GAPs - reduce Rac1-GTP) and Guanine-nucleotide exchange factors (GEFs - elevate Rac1-GTP). We hypothesized that specific GAPs and/or GEFs might mediate the inhibitory effects of PKG and PKA on Rac1. Screening for potential substrates among the GAPs and GEFs expressed in platelets resulted in the identification of the GAP ARHGAP17 (also known as Rich1 or Nadrin) as well as the GEF ARHGEF6 (also known as alpha-PIX or Cool2) as new substrates of PKG and PKA in human platelets. We mapped the phosphorylation sites on ARHGAP17 and ARHGEF6, and we investigated the functional consequences of phosphorylation. Phosphorylation of ARHGAP17 by PKA resulted in a dissociation of ARHGAP17 and its interaction partner CIP4. Phosphorylation of ARHGEF6 by both kinases resulted in the association of the ARHGEF6/GIT1 complex with the phospho-serine/threonine binding protein 14-3-3. Our data suggest that PKG- and PKA-mediated phosphorylations of ARHGAP17 and ARHGEF6 lead to a reorganization of protein complexes involved in the control of Rac1. These events are likely to contribute to Rac1 inhibition by cyclic nucleotides. Similar findings regarding the regulation of a critical small G-protein by GAP and GEF phosphorylation have been described for Rap1 in platelets before. We conclude that phosphorylation of GAPs and GEFs of small G-proteins might represent a general principle of platelet regulation by cGMP and cAMP.

Abstract 130: Platelet Rap1 Signaling, Mediated by CalDAG-GEFI and P2Y12, Contributes to Atherosclerotic Lesion Development in Mice

Background and Objective The development and progression of atherosclerotic lesions is mediated by various blood cell types, including platelets and neutrophils. We recently identified CalDAG-GEFI (CDGI) as a critical component of integrin-mediated adhesion of platelets and neutrophils at sites of vascular injury. In platelets, signaling by CDGI mediates the early activation of Rap1, while receptor for ADP, P2Y12, the target of the clinically used drug Plavix, is required for sustained Rap1 activation and thrombus stability. In this study, we evaluated lesion formation in atherosclerosis-prone low-density lipoprotein receptor deficient (Ldlr-/-) mice lacking CDGI and/or P2Y12 in hematopoietic cells. Methods and Results Ldlr-/- mice were irradiated and reconstituted with bone-marrow from wild-type (WT), CalDAG-GEFI-/- (CdgI), P2y12-/-, or CdgI-/- P2y12-/- (DKO) mice. Reconstituted mice were fed a fat and cholesterol enriched diet (21% fat, 0.2% cholesterol) for 12 weeks, ad libitum. Atherosclerotic lesions in the aortic sinus of Ldlr-/-;CdgI-/- chimeras were 42% smaller than those in Ldlr-/-;WT controls (0.18 ± 0.02 mm2 vs 0.31 ± 0.05 mm2, respectively; n = 13 each group, p &lt; 0.001). Lesions in Ldlr-/-;P2y12-/- chimeras were also significantly smaller compared to controls (0.22 ± 0.10 mm2; n = 13, p &lt; 0.05). Lesions in Ldlr-/-;DKO were reduced by 48% compared to WT controls (0.16 ± 0.02 mm2; n = 13, p &lt; 0.001 ), but they were not statistically different from Ldlr-/-;CdgI-/- chimeras. Platelet adhesion and activation on collagen under flow was markedly impaired in Ldlr-/-;CdgI-/- and Ldlr-/-;DKO blood, but only partially impaired in Ldlr-/-;P2y12-/- blood. Importantly, firm neutrophil adhesion to collagen-bound platelets was significantly reduced in Ldlr-/-;CdgI-/- and Ldlr-/-;DKO blood, but not in Ldlr-/-;P2y12-/- blood. Total cholesterol and triglyceride levels were similar among groups. Conclusion Our findings reveal a critical role for CDGI and P2Y12, and Rap1-dependent platelet activation, in promoting atherosclerotic lesion development in hypercholesterolemic mice. Further studies are required to determine if the observed protection in Ldlr-/-;CdgI-/- chimeras is due to impaired neutrophil function.

Synaptotagmin-like protein 1 interacts with the GTPase-activating protein Rap1GAP2 and regulates dense granule secretion in platelets

AbstractThe small guanine-nucleotide–binding protein Rap1 plays a key role in platelet aggregation and hemostasis, and we recently identified Rap1GAP2 as the only GTPase-activating protein of Rap1 in platelets. In search of Rap1GAP2-associated proteins, we performed yeast-2-hybrid screening and found synaptotagmin-like protein 1 (Slp1) as a new binding partner. We confirmed the interaction of Rap1GAP2 and Slp1 in transfected COS-1 and HeLa cells and at endogenous level in human platelets. Mapping studies showed that Rap1GAP2 binds through amino acids T524-K525-X-T527 within its C-terminus to the C2A domain of Slp1. Slp1 contains a Rab27-binding domain, and we demonstrate that Rap1GAP2, Slp1, and Rab27 form a trimeric complex in transfected cells and in platelets. Purified Slp1 dose-dependently decreased dense granule secretion in streptolysin-O–permeabilized platelets stimulated with calcium or guanosine 5′-O-[gamma-thio] triphosphate. The isolated C2A domain of Slp1 had a stimulatory effect on granule secretion and reversed the inhibitory effect of full-length Slp1. Purified Rap1GAP2 augmented dense granule secretion of permeabilized platelets, whereas deletion of the Slp1-binding TKXT motif abolished the effect of Rap1GAP2. We conclude that Slp1 inhibits dense granule secretion in platelets and that Rap1GAP2 modulates secretion by binding to Slp1.

Cyclic Nucleotide-dependent Protein Kinases Inhibit Binding of 14-3-3 to the GTPase-activating Protein Rap1GAP2 in Platelets

GTPase-activating proteins are required to terminate signaling by Rap1, a small guanine nucleotide-binding protein that controls integrin activity and cell adhesion. Recently, we identified Rap1GAP2, a GTPase-activating protein of Rap1 in platelets. Here we show that 14-3-3 proteins interact with phosphorylated serine 9 at the N terminus of Rap1GAP2. Platelet activation by ADP and thrombin enhances serine 9 phosphorylation and increases 14-3-3 binding to endogenous Rap1GAP2. Conversely, inhibition of platelets by endothelium-derived factors nitric oxide and prostacyclin disrupts 14-3-3 binding. These effects are mediated by cGMP- and cAMP-dependent protein kinases that phosphorylate Rap1GAP2 at serine 7, adjacent to the 14-3-3 binding site. 14-3-3 binding does not change the GTPase-activating function of Rap1GAP2 in vitro. However, 14-3-3 binding attenuates Rap1GAP2 mediated inhibition of cell adhesion. Our findings define a novel crossover point of activatory and inhibitory signaling pathways in platelets.

CGMP-dependent protein kinase regulates Rap1 signaling in platelets

cGMPand cAMP-dependent protein kinases (cGK and cAK) mediate the inhibitory effects of endotheliumderived messenger molecules nitric oxide and prostacyclin on platelets. To understand the mechanisms involved in platelet inhibition we searched for new substrates of cGK and cAK. We identified Rap1GAP2, the only GTPase-activating protein of Rap1 in platelets. Rap1 is a guaninenucleotide binding protein that controls integrin activity, platelet adhesion and aggregation. Rap1GAP2 is required to turn over Rap1-GTP to Rap1-GDP resulting in the inactivation of integrins and reduced cellular adhesion. Using phospho-specific antibodies we demonstrate phosphorylation of endogenous Rap1GAP2 on serine 7 by cGK and cAK in intact platelets. Yeast-two-hybrid screening revealed an interaction of the phosphoserine/-threonine binding adapter protein 14-3-3 with Rap1GAP2, and we mapped the 14-3-3 binding site to the N-terminus of Rap1GAP2 close to the cGK/cAK phosphorylation site. We could show that 14-3-3 binding to Rap1GAP2 requires phosphorylation of serine 9. Platelet activation by ADP and thrombin treatment induces Rap1GAP2 serine 9 phosphorylation and enhances the attachment of 14-3-3 to Rap1GAP2. In contrast, phosphorylation of serine 7 by cGK/cAK leads to the detachment of 14-3-3. Furthermore, Rap1GAP2 serine 7 phosphorylation correlates with the inhibition of Rap1-GTP formation by cGMP and cAMP in platelets. Cell adhesion experiments provide additional evidence that Rap1GAP2 is activated by the detachment of 14-3-3. Point mutants of Rap1GAP2 deficient in 14-3-3 binding inhibit Rap1-mediated cell adhesion significantly stronger than a Rap1GAP2 mutant that binds 14-3-3 constitutively. Our findings define a novel regulatory mechanism that might contribute to both platelet activation and endothelial inhibition of platelet adhesion and aggregation. from 3rd International Conference on cGMP Generators, Effectors and Therapeutic Implications Dresden, Germany. 15–17 June 2007

Analyzing the mechanism of Rap1 activation in platelets: Rap1 activation is related to the release reaction mediated through the collagen receptor GPVI

The abundant Rap1 in platelets becomes activated when these cells are stimulated by various agonists, but its function has remained unknown. In view of this, we developed an assay to quantitatively measure activated Rap1 and used it to determine relationships between Rap1 activation and several platelet functions: integrin α 2β 1 activation, tyrosine phosphorylation, and the release reaction. We looked at how these processes are affected by the protein kinase C inhibitor BIMI, tyrosine kinase inhibitor PP2, PI 3-kinase inhibitor wortmannin, and ADP scavenger apyrase. In CRP (collagen related peptide)-activated platelets, all the inhibitors severely inhibited Rap1 activation, but had little effect on integrin α 2β 1 activation, indicating that the integrin activation mechanism is different from the Rap1 activation mechanism, at least in GPVI-dependent activation. With p85α-null mouse platelets, we demonstrated that Rap1 activation involves PI 3-kinase p85α-dependent tyrosine phosphorylation. All the inhibitors similarly decreased Rap1 activation and the serotonin release reaction, and the inhibition of Rap1 activation was not due to the lack of released ADP. Our results indicate that platelet Rap1 activation is closely related to the release reaction and not to integrin α 2β 1 activation in GPVI-mediated platelet activation.

Identification of P2Y12-dependent and -independent mechanisms of glycoprotein VI–mediated Rap1 activation in platelets

Glycoprotein (GP) VI is a critical platelet collagen receptor, yet the steps involved in GPVI-mediated platelet activation remain incompletely understood. Because activation of Rap1, an abundant small guanosine triphosphatase (GTPase) in platelets, contributes to integrin alpha(IIb)beta(3) activation, we asked whether and how GPVI signaling activates Rap1 in platelets. Here we show that platelet Rap1 is robustly activated upon addition of convulxin, a GPVI-specific agonist. Using a reconstituted system in RBL-2H3 cells, we found that GPVI-mediated Rap1 activation is dependent on FcRgamma but independent of another platelet collagen receptor, alpha(2)beta(1). Interestingly, GPVI-mediated Rap1 activation in human platelets is largely dependent on adenosine diphosphate (ADP) signaling through the P2Y(12) and not the P2Y(1) receptor. However, experiments with specific ADP receptor antagonists and platelets from knockout mice deficient in P2Y(1) or the P2Y(12)-associated G-protein, Galphai(2), indicate that human and murine platelets also have a significant P2Y(12)-independent component of GPVI-mediated Rap1 activation. The P2Y(12)-independent component is dependent on phosphatidylinositol 3-kinase and is augmented by epinephrine-mediated signaling. P2Y(12)-dependent and -independent components are also observed in GPVI-mediated platelet aggregation, further supporting a role for Rap1 in aggregation. These results define mechanisms of GPVI-mediated platelet activation and implicate Rap1 as a key signaling protein in GPVI-induced platelet signaling.

The protein kinase A in platelets from patients with panic disorder

Although previous studies suggested that dysfunctions in the protein kinase A (PKA) and in some of its substrates are associated with several psychiatric disorders, there is no evidence regarding the possible involvement of such components in panic disorder (PD). Thus, the aim of the present study was to investigate the levels of PKA and Rap1 in platelets from patients with such disorder. Twenty-four drug free patients with PD and 24 healthy volunteers participated to the study. Employing the Western Blot analysis, immunostaining and computer-assisted imaging, the levels of the regulatory (R, type I and type II) and the catalytic (C) subunits of PKA, and those of Rap1 were assessed in platelets from the two groups. The data show that patients with PD have significantly higher levels of platelet RI and C subunits of PKA than controls, whereas the levels of RII were unchanged. No significant differences were found in the immunolabelling of Rap1 between groups. These findings may provide clues toward understanding the involvement of cAMP signalling in anxiety disorders.

Activation of Rap1B by Gi Family Members in Platelets

It has become increasingly appreciated that receptors coupled to G(alpha)(i) family members can stimulate platelet aggregation, but the mechanism for this has remained unclear. One possible mediator is the small GTPase, Rap1, which has been shown to contribute to integrin activation in several cell lines and to be activated by a calcium-dependent mechanism in platelets. Here, we demonstrate that Rap1 is also activated by G(alpha)(i) family members in platelets. First, we show that platelets from mice lacking the G(alpha)(i) family member G(alpha)(z) (which couples to the alpha(2A) adrenergic receptor) are deficient in epinephrine-stimulated Rap1 activation. We also show that platelets from mice lacking G(alpha)(i2), which couples to the ADP receptor, P2Y12, exhibit reduced Rap1 activation in response to ADP. In contrast, platelets from mice that lack G(alpha)(q) show no decrease in the ability to activate Rap1 in response to epinephrine but show a partial reduction in ADP-stimulated Rap1 activation. This result, combined with studies of human platelets treated with ADP receptor-selective inhibitors, indicates that ADP-stimulated Rap1 activation in human platelets is dependent on both the G(alpha)(i)-coupled P2Y12 receptor and the G(alpha)(q)-coupled P2Y1 receptor. G(alpha)(i)-dependent activation of Rap1 in platelets does not appear to be mediated by enhanced intracellular calcium release because no increase in intracellular calcium concentration was detected in response to epinephrine and because the calcium response to ADP was not diminished in platelets from the G(alpha)(i2)-/- mouse. Finally, using human platelets treated with selective inhibitors of phosphatidylinositol 3-kinase (PI3K) and mouse platelets selectively lacking the G(beta)(gamma)-activated form of his enzyme (PI3Kgamma), we show that G(i)-mediated Rap1 activation is PI3K-dependent. In summary, activation of Rap1 can be stimulated by G(alpha)(i)- and PI3K-dependent mechanisms in platelets and by G(q)- and Ca(2+)-dependent mechanisms, both of which may play a role in promoting platelet activation.

The cAMP-dependent protein kinase substrate Rap1 in platelets from patients with obsessive compulsive disorder or schizophrenia

Previous studies have reported that the cAMP-dependent protein kinase and one of its substrates, namely Rap1, are altered in patients with affective disorders. Abnormalities in the cAMP-dependent protein kinase have also been reported in platelets of patients with obsessive compulsive disorder and schizophrenia. However, it remains to be determined whether abnormalities in Rap1 are specifically related to affective disorders or may also be present in schizophrenia and obsessive compulsive disorder. Thus, we investigated Rap1 in platelets from 12 drug-free patients with obsessive compulsive disorder, ten drug-free patients with schizophrenia, and 20 healthy subjects. While no difference was observed in the levels of Rap1 between groups, the phosphorylation state of Rap1 was significantly lower in patients with obsessive compulsive disorder than in schizophrenic patients and controls. These data further support the idea that abnormalities of cAMP signalling pathway could be associated, albeit in a somewhat different way, with several psychiatric disorders.

Altered Rap1 endogenous phosphorylation and levels in platelets from patients with bipolar disorder

Previous studies have reported abnormalities either in the cAMP-dependent endogenous phosphorylation or in the levels of Rap1 in platelets from bipolar patients. One limitation of these findings was that they come from different groups of patients in independent studies. To overcome this limitation, we designed the present study in which both these biochemicals parameters were assessed in the same cohort of euthymic bipolar patients and healthy subjects. The results showed that the cAMP-dependent phosphorylation of Rap1 was significantly higher in platelets of bipolar patients with respect to healthy subjects. Furthermore, immunoblotting experiments revealed that also the levels of Rap1 were significantly higher in bipolar patients than in control subjects, thus supporting that the abnormal phosphorylation can be ascribed to the increased levels of Rap1. Taken together the results of the present study further support that downstream components of the cAMP signal cascade could be involved in the pathophysiology of bipolar disorders.

Rapid Ca2+-mediated activation of Rap1 in human platelets.

Rap1 is a small, Ras-like GTPase whose function and regulation are still largely unknown. We have developed a novel assay to monitor the active, GTP-bound form of Rap1 based on the differential affinity of Rap1GTP and Rap1GDP for the Rap binding domain of RalGDS (RBD). Stimulation of blood platelets with alpha-thrombin or other platelet activators caused a rapid and strong induction of Rap1 that associated with RBD in vitro. Binding to RBD increased from undetectable levels in resting platelets to >50% of total Rap1 within 30 s after stimulation. An increase in the intracellular Ca2+ concentration is both necessary and sufficient for Rap1 activation since it was induced by agents that increase intracellular Ca2+ and inhibited by a Ca2+-chelating agent. Neither inhibition of translocation of Rap1 to the cytoskeleton nor inhibition of platelet aggregation affected thrombin-induced activation of Rap1. In contrast, prostaglandin I2 (PGI2), a strong negative regulator of platelet function, inhibited agonist-induced as well as Ca2+-induced activation of Rap1. From our results, we conclude that Rap1 activation in platelets is an important common event in early agonist-induced signalling, and that this activation is mediated by an increased intracellular Ca2+ concentration.

Carboxyl methylation of platelet rap1 proteins is stimulated by guanosine 5'-(3-O-thio)triphosphate

Carboxyl methylation of platelet ras-related proteins, known as rap proteins, was investigated in this study. Platelet membrane proteins of Mr 23,000 incorporated radioactivity in the presence of S-[methyl-3H]adenosylmethionine and platelet cytosol. About 97% of the radioactivity present in the Mr 23,000 proteins was liberated as volatile methanol under basic (1 M sodium hydroxide) conditions. Cycloheximide, an inhibitor of protein synthesis, inhibited incorporation of S-[methyl-3H]adenosylmethionine by 25%. These results suggest that at least 75% of the radioactivity present in the Mr 23,000 proteins is due to carboxyl methylation and not due to the incorporation of S-[methyl-3H]adenosylmethionine into proteins or due to the incorporation of base-stable methyl groups into side chains of arginine, histidine, or lysine residues. Protein methylation did not occur if membranes or cytosol alone was incubated with S-[methyl-3H]adenosylmethionine. Guanosine 5'(3-O-thio)triphosphate increased methylation of the Mr 23,000 proteins in a time- and concentration-dependent manner. Acetyl-farnesylcysteine, a synthetic substrate for carboxyl methyltransferases, completely blocked methylation of the Mr 23,000 membrane proteins. On the basis of one- and two-dimensional Western blots using rap-specific antisera, the Mr 23,000 methylated proteins were identified as rap1 proteins. The existence of the carboxyl-terminal CAAX motif in rap1 proteins, similar to the CAAX motif present in p21ras as well as in the yeast mating factors, leads us to suggest that methylation of rap1 proteins possibly occurs at the alpha-carboxyl-terminal cysteine.