Unresponsiveness of Platelets Lacking Both Gαq and Gα13

Dense but not alpha granules of platelets are required for insulin secretion from pancreatic β cells

OBJECTIVES

Platelets, originally described for their role in blood coagulation, are now also recognized as key players in modulating inflammation, tissue regeneration, angiogenesis, and carcinogenesis. Recent evidence suggests that platelets also influence insulin secretion from pancreatic β cells. The multifaceted functions of platelets are mediated by the factors stored in their alpha granules (AGs) and dense granules (DGs). AGs primarily contain proteins, while DGs are rich in small molecules, and both types of granules are released during blood coagulation. Specific components stored in AGs and DGs are implicated in various inflammatory, regenerative, and tumorigenic processes. However, the relative contributions of AGs and DGs to the regulation of pancreatic β cell function have not been previously explored.

METHODS

In this study, we utilized mouse models deficient in AG content (neurobeachin-like 2 (Nbeal2) -deficient mice) and models with defective DG release (Unc13d-deficiency in bone marrow-derived cells) to investigate the impact of platelet granules on insulin secretion from pancreatic β cells.

RESULTS

Our findings indicate that AG deficiency has little to no effect on pancreatic β cell function and glucose homeostasis. Conversely, mice with defective DG release exhibited glucose intolerance and reduced insulin secretion. Furthermore, Unc13d-deficiency in hematopoietic stem cells led to a reduction in adipose tissue gain in obese mice.

CONCLUSIONS

Obtained data suggest that DGs, but not AGs, mediate the influence of platelets on pancreatic β cells, thereby modulating glucose metabolism.

Roles of G proteins and their GTPase-activating proteins in platelets

Platelets are small anucleate blood cells supporting vascular function. They circulate in a quiescent state monitoring the vasculature for injuries. Platelets adhere to injury sites and can be rapidly activated to secrete granules and to form platelet/platelet aggregates. These responses are controlled by signalling networks that include G proteins and their regulatory guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Recent proteomics studies have revealed the complete spectrum of G proteins, GEFs, and GAPs present in platelets. Some of these proteins are specific for platelets and very few have been characterised in detail. GEFs and GAPs play a major role in setting local levels of active GTP-bound G proteins in response to activating and inhibitory signals encountered by platelets. Thus, GEFs and GAPs are highly regulated themselves and appear to integrate G protein regulation with other cellular processes. This review focuses on GAPs of small G proteins of the Arf, Rab, Ras, and Rho families, as well as of heterotrimeric G proteins found in platelets.

Platelet-derived lipids promote insulin secretion of pancreatic β cells

Hyperreactive platelets are commonly observed in diabetic patients indicating a potential link between glucose homeostasis and platelet reactivity. This raises the possibility that platelets may play a role in the regulation of metabolism. Pancreatic β cells are the central regulators of systemic glucose homeostasis. Here, we show that factor(s) derived from β cells stimulate platelet activity and platelets selectively localize to the vascular endothelium of pancreatic islets. Both depletion of platelets and ablation of major platelet adhesion or activation pathways consistently resulted in impaired glucose tolerance and decreased circulating insulin levels. Furthermore, we found platelet-derived lipid classes to promote insulin secretion and identified 20-Hydroxyeicosatetraenoic acid (20-HETE) as the main factor promoting β cells function. Finally, we demonstrate that the levels of platelet-derived 20-HETE decline with age and that this parallels with reduced impact of platelets on β cell function. Our findings identify an unexpected function of platelets in the regulation of insulin secretion and glucose metabolism, which promotes metabolic fitness in young individuals.

Fatty acids negatively regulate platelet function through formation of noncanonical 15-lipoxygenase-derived eicosanoids

The antiplatelet effect of polyunsaturated fatty acids is primarily attributed to its metabolism to bioactive metabolites by oxygenases, such as lipoxygenases (LOX). Platelets have demonstrated the ability to generate 15-LOX-derived metabolites (15-oxylipins); however, whether 15-LOX is in the platelet or is required for the formation of 15-oxylipins remains unclear. This study seeks to elucidate whether 15-LOX is required for the formation of 15-oxylipins in the platelet and determine their mechanistic effects on platelet reactivity. In this study, 15-HETrE, 15-HETE, and 15-HEPE attenuated collagen-induced platelet aggregation, and 15-HETrE inhibited platelet aggregation induced by different agonists. The observed anti-aggregatory effect was due to the inhibition of intracellular signaling including αIIbβ3 and protein kinase C activities, calcium mobilization, and granule secretion. While 15-HETrE inhibited platelets partially through activation of peroxisome proliferator-activated receptor β (PPARβ), 15-HETE also inhibited platelets partially through activation of PPARα. 15-HETrE, 15-HETE, or 15-HEPE inhibited 12-LOX in vitro, with arachidonic acid as the substrate. Additionally, a 15-oxylipin-dependent attenuation of 12-HETE level was observed in platelets following ex vivo treatment with 15-HETrE, 15-HETE, or 15-HEPE. Platelets treated with DGLA formed 15-HETrE and collagen-induced platelet aggregation was attenuated only in the presence of ML355 or aspirin, but not in the presence of 15-LOX-1 or 15-LOX-2 inhibitors. Expression of 15-LOX-1, but not 15-LOX-2, was decreased in leukocyte-depleted platelets compared to non-depleted platelets. Taken together, these findings suggest that 15-oxylipins regulate platelet reactivity; however, platelet expression of 15-LOX-1 is low, suggesting that 15-oxylipins may be formed in the platelet through a 15-LOX-independent pathway.

Functional aspects of evolution in a cluster of salivary protein genes from mosquitoes

The D7 proteins are highly expressed in the saliva of hematophagous Nematocera and bind biogenic amines and eicosanoid compounds produced by the host during blood feeding. These proteins are encoded by gene clusters expressing forms having one or two odorant-binding protein-like domains. Here we examine functional diversity within the D7 group in the genus Anopheles and make structural comparisons with D7 proteins from culicine mosquitoes in order to understand aspects of D7 functional evolution. Two domain long form (D7L) and one domain short form (D7S) proteins from anopheline and culicine mosquitoes were characterized to determine their ligand selectivity and binding pocket structures. We previously showed that a D7L protein from Anopheles stephensi, of the subgenus Cellia, could bind eicosanoids at a site in its N-terminal domain but could not bind biogenic amines in its C-terminal domain as does a D7L1 ortholog from the culicine species Aedes aegypti, raising the question of whether anopheline D7L proteins had lost their ability to bind biogenic amines. Here we find that D7L from anopheline species belonging to two other subgenera, Nyssorhynchus and Anopheles, can bind biogenic amines and have a structure much like the Ae. aegypti ortholog. The unusual D7L, D7L3, can also bind serotonin in the Cellia species An. gambiae. We also show through structural comparisons with culicine forms that the biogenic amine binding function of single domain D7S proteins in the genus Anopheles may have evolved through gene conversion of structurally similar proteins, which did not have biogenic amine binding capability. Collectively, the data indicate that D7L proteins had a biogenic amine and eicosanoid binding function in the common ancestor of anopheline and culicine mosquitoes, and that the D7S proteins may have acquired a biogenic amine binding function in anophelines through a gene conversion process.

The effect of platelet G proteins on platelet extravasation and tumor growth in the murine model of ovarian cancer

We and other investigators have shown that platelets promote metastasis and the growth of tumors. Our rationale for conducting this study is that platelets' prometastatic and progrowth effects depend on a close encounter between platelets and cancer cells. This interaction occurs inside blood vessels with circulating tumor cells and outside blood vessels with cancer cells residing in the tumor parenchyma. Our hypothesis was that platelet extravasation is required for the effect of platelets on tumor growth. Platelets respond to environmental stimuli by activation of G protein-coupled receptors on their surface. We investigated the impact of various platelet G proteins on the growth of ovarian cancer tumors and platelet extravasation. We used mice with platelet-specific deficiency of Gαi2 (Gi), Gα13 (G13), or Gαq (Gq) in a syngeneic ovarian cancer model. We measured the total weight of tumor nodules resected from tumor-bearing mice. We developed methods for automated whole-slide image acquisition and unbiased computerized image analysis to quantify extravasated platelets. We compared the number of platelets inside tumor nodules of platelet G protein-deficient tumor-bearing mice. We found that deficiency of Gi and G13, but not Gq, in platelets resulted in smaller tumors compared with those in corresponding littermates. Deficiency of Gi and G13 in platelets reduced the number of extravasated platelets by >90%, but deficiency of Gq did not reduce the number of extravasated platelets significantly. The lack of Gi or G13 in platelets reduced platelet extravasation into the tumor and tumor growth.

Arhgef1 Plays a Vital Role in Platelet Function and Thrombogenesis

Background Platelets are the cellular mediators of hemostasis and thrombosis, and their function is regulated by a number of molecular mediators, such as small GTP ases. These small GTP ases are themselves regulated by guanine nucleotide exchange factors such as Arhgefs, several of which are found in platelets, including the highly expressed Arhgef1. However, the role of Arhgef1 in platelets has not yet been investigated. Methods and Results We employed mice with genetic deletion of Arhgef1 (ie, Arhgef1-/-) and investigated their platelet phenotype by employing a host of in vivo and in vitro platelet assays. Our results indicate that Arhgef1-/- mice had prolonged carotid artery occlusion and tail bleeding times. Moreover, platelets from these mice exhibited defective aggregation, dense and α granule secretion, α II bβ3 integrin activation, clot retraction and spreading, in comparison to their wild-type littermates. Finally, we also found that the mechanism by which Arhgef1 regulates platelets is mediated in part by a defect in the activation of the RhoA-Rho-associated kinase axis, but not Rap1b. Conclusions Our data demonstrate, for the first time, that Arhgef1 plays a critical role in platelet function, in vitro and in vivo.

Significance of urinary 11-dehydro-thromboxane B2 in age-related diseases: Focus on atherothrombosis

Platelet activation plays a key role in atherogenesis and atherothrombosis. Biochemical evidence of increased platelet activation in vivo can be reliably obtained through non-invasive measurement of thromboxane metabolite (TXM) excretion. Persistent biosynthesis of TXA₂ has been associated with several ageing-related diseases, including acute and chronic cardio-cerebrovascular diseases and cardiovascular risk factors, such as cigarette smoking, type 1 and type 2 diabetes mellitus, obesity, hypercholesterolemia, hyperhomocysteinemia, hypertension, chronic kidney disease, chronic inflammatory diseases. Given the systemic nature of TX excretion, involving predominantly platelet but also extraplatelet sources, urinary TXM may reflect either platelet cyclooxygenase-1 (COX-1)-dependent TX generation or COX-2-dependent biosynthesis by inflammatory cells and/or platelets, or a combination of the two, especially in clinical settings characterized by low-grade inflammation or enhanced platelet turnover. Although urinary 11-dehydro-TXB₂ levels are largely suppressed with low-dose aspirin, incomplete TXM suppression by aspirin predicts the future risk of vascular events and death in high-risk patients and may identify individuals who might benefit from treatments that more effectively block in vivo TX production or activity. Several disease-modifying agents, including lifestyle intervention, antidiabetic drugs and antiplatelet agents besides aspirin have been shown to reduce TX biosynthesis. Taken together, these aspects may contribute to the development of promising mechanism-based therapeutic strategies to reduce the progression of atherothrombosis. We intended to critically review current knowledge on both the pathophysiological significance of urinary TXM excretion in clinical settings related to ageing and atherothrombosis, as well as its prognostic value as a biomarker of vascular events.

New Concepts and Mechanisms of Platelet Activation Signaling

Upon blood vessel injury, platelets are exposed to adhesive proteins in the vascular wall and soluble agonists, which initiate platelet activation, leading to formation of hemostatic thrombi. Pathological activation of platelets can induce occlusive thrombosis, resulting in ischemic events such as heart attack and stroke, which are leading causes of death globally. Platelet activation requires intracellular signal transduction initiated by platelet receptors for adhesion proteins and soluble agonists. Whereas many platelet activation signaling pathways have been established for many years, significant recent progress reveals much more complex and sophisticated signaling and amplification networks. With the discovery of new receptor signaling pathways and regulatory networks, some of the long-standing concepts of platelet signaling have been challenged. This review provides an overview of the new developments and concepts in platelet activation signaling.

Receptor-interacting protein kinase 3 promotes platelet activation and thrombosis

Previous studies have shown that receptor-interacting protein kinase 3 (RIP3) is involved in many important biological processes, including necroptosis, apoptosis, and inflammation. Here we show that RIP3 plays a critical role in regulating platelet functions and in vivo thrombosis and hemostasis. Tail bleeding times were significantly longer in RIP3-knockout (RIP3^-/-) mice compared with their wild-type (WT) littermates. In an in vivo model of arteriole thrombosis, mice lacking RIP3 exhibited prolonged occlusion times. WT mice repopulated with RIP3^-/- bone marrow-derived cells had longer occlusion times than RIP3^-/- mice repopulated with WT bone marrow-derived cells, suggesting a role for RIP3-deficient platelets in arterial thrombosis. Consistent with these findings, we observed that RIP3 was expressed in both human and mice platelets. Deletion of RIP3 in mouse platelets caused a marked defect in aggregation and attenuated dense granule secretion in response to low doses of thrombin or a thromboxane A₂ analog, U46619. Phosphorylation of Akt induced by U46619 or thrombin was diminished in RIP3^-/- platelets. Moreover, RIP3 interacted with Gα₁₃ Platelet spreading on fibrinogen and clot retraction were impaired in the absence of RIP3. RIP3 inhibitor dose-dependently inhibited platelet aggregation in vitro and prevented arterial thrombus formation in vivo. These data demonstrate a role for RIP3 in promoting in vivo thrombosis and hemostasis by amplifying platelet activation. RIP3 may represent a novel promising therapeutic target for thrombotic diseases.

Leukaemia-associated Rho guanine nucleotide exchange factor (LARG) plays an agonist specific role in platelet function through RhoA activation

Leukemia-Associated RhoGEF (LARG) is highly expressed in platelets, which are essential for maintaining normal haemostasis. We studied the function of LARG in murine and human megakaryocytes and platelets with Larg knockout (KO), shRNA-mediated knockdown and small molecule-mediated inhibition. We found that LARG is important for human, but not murine, megakaryocyte maturation. Larg KO mice exhibit macrothrombocytopenia, internal bleeding in the ovaries and prolonged bleeding times. KO platelets have impaired aggregation, α-granule release and integrin α2bβ3 activation in response to thrombin and thromboxane, but not to ADP. The same agonist-specific reductions in platelet aggregation occur in human platelets treated with a LARG inhibitor. Larg KO platelets have reduced RhoA activation and myosin light chain phosphorylation, suggesting that Larg plays an agonist-specific role in platelet signal transduction. Using two different in vivo assays, Larg KO mice are protected from in vivo thrombus formation. Together, these results establish that LARG regulates human megakaryocyte maturation, and is critical for platelet function in both humans and mice.

Impact of vascular thromboxane prostanoid receptor activation on hemostasis, thrombosis, oxidative stress, and inflammation

The activation of thromboxane prostanoid (TP) receptor on platelets, monocytes/macrophages, endothelial cells, and vascular smooth muscle cells (SMC) plays important roles in regulating platelet activation and vascular tone and in the pathogenesis of thrombosis and vascular inflammation. Oxidative stress and vascular inflammation increase the formation of TP receptor agonists, which promote initiation and progression of atherogenesis and thrombosis. Furthermore, TP receptor activation promotes angiogenesis and vessel wall constriction. Besides thromboxane A₂ and its endoperoxide precursors, prostaglandin G₂ and H₂, isoprostanes, and 20-hydroxyeicosatetraenoic acid also activate TP receptor as autocrine or paracrine ligands. These additional TP activators play a role in pathological conditions such as diabetes, obesity, and hypertension, and their biosynthesis is not inhibited by aspirin, at variance with that of thromboxane A₂. The understanding of TP receptor function increased our current knowledge of the pathogenesis of atherosclerosis and thrombosis, highlighting the great impact that this receptor has in cardiovascular disorders.

Chlorin e6 Prevents ADP-Induced Platelet Aggregation by Decreasing PI3K-Akt Phosphorylation and Promoting cAMP Production

A number of reagents that prevent thrombosis have been developed but were found to have serious side effects. Therefore, we sought to identify complementary and alternative medicinal materials that are safe and have long-term efficacy. In the present studies, we have assessed the ability of chlorine e6 (CE6) to inhibit ADP-induced aggregation of rat platelets and elucidated the underlying mechanism. CE6 inhibited platelet aggregation induced by 10 µM ADP in a concentration-dependent manner and decreased intracellular calcium mobilization and granule secretion (i.e., ATP and serotonin release). Western blotting revealed that CE6 strongly inhibited the phosphorylations of PI3K, Akt, c-Jun N-terminal kinase (JNK), and different mitogen-activated protein kinases (MAPKs) including extracellular signal-regulated kinase 1/2 (ERK1/2) as well as p38-MAPK. Our study also demonstrated that CE6 significantly elevated intracellular cAMP levels and decreased thromboxane A₂ formation in a concentration-dependent manner. Furthermore, we determined that CE6 initiated the activation of PKA, an effector of cAMP. Taken together, our findings indicate that CE6 may inhibit ADP-induced platelet activation by elevating cAMP levels and suppressing PI3K/Akt activity. Finally, these results suggest that CE6 could be developed as therapeutic agent that helps prevent thrombosis and ischemia.

Biased suppression of TP homodimerization and signaling through disruption of a TM GxxxGxxxL helical interaction motif

Thromboxane A2 (TXA2) contributes to cardiovascular disease (CVD) by activating platelets and vascular constriction and proliferation. Despite their preclinical efficacy, pharmacological antagonists of the TXA2 receptor (TP), a G protein-coupled receptor, have not been clinically successful, raising interest in novel approaches to modifying TP function. We determined that disruption of a GxxxGxxxL helical interaction motif in the human TP's (α isoform) fifth transmembrane (TM) domain suppressed TP agonist-induced Gq signaling and TPα homodimerization, but not its cell surface expression, ligand affinity, or Gq association. Heterodimerization of TPα with the functionally opposing prostacyclin receptor (IP) shifts TPα to signal via the IP-Gs cascade contributing to prostacyclin's restraint of TXA2 function. Interestingly, disruption of the TPα-TM5 GxxxGxxxL motif did not modify either IP-TPα heterodimerization or its Gs-cAMP signaling. Our study indicates that distinct regions of the TPα receptor direct its homo- and heterodimerization and that homodimerization is necessary for normal TPα-Gq activation. Targeting the TPα-TM5 GxxxGxxxL domain may allow development of biased TPα homodimer antagonists that avoid suppression of IP-TPα heterodimer function. Such novel therapeutics may prove superior in CVD compared with nonselective suppression of all TP functions with TXA2 biosynthesis inhibitors or TP antagonists.

Megakaryocyte-specific RhoA deficiency causes macrothrombocytopenia and defective platelet activation in hemostasis and thrombosis

Vascular injury initiates rapid platelet activation that is critical for hemostasis, but it also may cause thrombotic diseases, such as myocardial infarction or ischemic stroke. Reorganizations of the platelet cytoskeleton are crucial for platelet shape change and secretion and are thought to involve activation of the small GTPase RhoA. In this study, we analyzed the in vitro and in vivo consequences of megakaryocyte- and platelet-specific RhoA gene deletion in mice. We found a pronounced macrothrombocytopenia in RhoA-deficient mice, with platelet counts of approximately half that of wild-type controls. The mutant cells displayed an altered shape but only a moderately reduced life span. Shape change of RhoA-deficient platelets in response to G13-coupled agonists was abolished, and it was impaired in response to Gq stimulation. Similarly, RhoA was required for efficient secretion of α and dense granules downstream of G13 and Gq. Furthermore, RhoA was essential for integrin-mediated clot retraction but not for actomyosin rearrangements and spreading of activated platelets on fibrinogen. In vivo, RhoA deficiency resulted in markedly prolonged tail bleeding times but also significant protection in different models of arterial thrombosis and in a model of ischemic stroke. Together, these results establish RhoA as an important regulator of platelet function in thrombosis and hemostasis.

Platelet signaling

This chapter summarizes current ideas about the intracellular signaling that drives platelet responses to vascular injury. After a brief overview of platelet activation intended to place the signaling pathways into context, the first section considers the early events of platelet activation leading up to integrin activation and platelet aggregation. The focus is on the G protein-mediated events utilized by agonists such as thrombin and ADP, and the tyrosine kinase-based signaling triggered by collagen. The second section considers the events that occur after integrin engagement, some of which are dependent on close physical contact between platelets. A third section addresses the regulatory events that help to avoid unprovoked or excessive platelet activation, after which the final section briefly considers individual variations in platelet reactivity and the role of platelet signaling in the innate immune response and embryonic development.

Aspirin has little additional anti-platelet effect in healthy volunteers receiving prasugrel

BACKGROUND

Strong P2Y(12) blockade, as can be achieved with novel anti-platelet agents such as prasugrel, has been shown in vitro to inhibit both ADP and thromboxane A(2) -mediated pathways of platelet aggregation, calling into question the need for the concomitant use of aspirin.

OBJECTIVE

The present study investigated the hypothesis that aspirin provides little additional anti-aggregatory effect in a group of healthy volunteers taking prasugrel. STUDY PARTICIPANTS/METHODS: In all, 9 males, aged 18 to 40 years, enrolled into the 21-day study. Prasugrel was loaded at 60 mg on day 1 and maintained at 10 mg until day 21. At day 8, aspirin 75 mg was introduced and the dose increased to 300 mg on day 15. On days 0, 7, 14 and 21, platelet function was assessed by aggregometry, response to treatments was determined by VerifyNow and urine samples were collected for quantification of prostanoid metabolites.

RESULTS

At day 7, aggregation responses to a range of platelet agonists were reduced and there was only a small further inhibition of aggregation to TRAP-6, collagen and epinephrine at days 14 and 21, when aspirin was included with prasugrel. Urinary prostanoid metabolites were unaffected by prasugrel, and were reduced by the addition of aspirin, independent of dose.

CONCLUSIONS

In healthy volunteers, prasugrel produces a strong anti-aggregatory effect, which is little enhanced by the addition of aspirin. The addition of aspirin as a dual-therapy with potent P2Y(12) receptor inhibitors warrants further investigation.

Dipetalodipin, a novel multifunctional salivary lipocalin that inhibits platelet aggregation, vasoconstriction, and angiogenesis through unique binding specificity for TXA2, PGF2alpha, and 15(S)-HETE

Dipetalodipin (DPTL) is an 18 kDa protein cloned from salivary glands of the triatomine Dipetalogaster maxima. DPTL belongs to the lipocalin superfamily and has strong sequence similarity to pallidipin, a salivary inhibitor of collagen-induced platelet aggregation. DPTL expressed in Escherichia coli was found to inhibit platelet aggregation by collagen, U-46619, or arachidonic acid without affecting aggregation induced by ADP, convulxin, PMA, and ristocetin. An assay based on incubation of DPTL with small molecules (e.g. prostanoids, leukotrienes, lipids, biogenic amines) followed by chromatography, mass spectrometry, and isothermal titration calorimetry showed that DPTL binds with high affinity to carbocyclic TXA(2), TXA(2) mimetic (U-46619), TXB(2), PGH(2) mimetic (U-51605), PGD(2,) PGJ(2), and PGF(2α). It also interacts with 15(S)-HETE, being the first lipocalin described to date to bind to a derivative of 15-lipoxygenase. Binding was not observed to other prostaglandins (e.g. PGE(1), PGE(2), 8-iso-PGF(2α), prostacyclin), leukotrienes (e.g. LTB(4), LTC(4), LTD(4), LTE(4)), HETEs (e.g. 5(S)-HETE, 12(S)-HETE, 20-HETE), lipids (e.g. arachidonic acid, PAF), and biogenic amines (e.g. ADP, serotonin, epinephrine, norepinephrine, histamine). Consistent with its binding specificity, DPTL prevents contraction of rat uterus stimulated by PGF(2α) and induces relaxation of aorta previously contracted with U-46619. Moreover, it inhibits angiogenesis mediated by 15(S)-HETE and did not enhance inhibition of collagen-induced platelet aggregation by SQ29548 (TXA(2) antagonist) and indomethacin. A 3-D model for DPTL and pallidipin is presented that indicates the presence of a conserved Arg(39) and Gln(135) in the binding pocket of both lipocalins. Results suggest that DPTL blocks platelet aggregation, vasoconstriction, and angiogenesis through binding to distinct eicosanoids involved in inflammation.

The function and three-dimensional structure of a thromboxane A2/cysteinyl leukotriene-binding protein from the saliva of a mosquito vector of the malaria parasite

A salivary protein from a malaria-transmitting mosquito uses a single domain to bind to thromboxane A₂ and cysteinyl leukotrienes and prevent blood clotting and inflammation in the host on which it feeds.

The highly expressed D7 protein family of mosquito saliva has previously been shown to act as an anti-inflammatory mediator by binding host biogenic amines and cysteinyl leukotrienes (CysLTs). In this study we demonstrate that AnSt-D7L1, a two-domain member of this group from Anopheles stephensi, retains the CysLT binding function seen in the homolog AeD7 from Aedes aegypti but has lost the ability to bind biogenic amines. Unlike any previously characterized members of the D7 family, AnSt-D7L1 has acquired the important function of binding thromboxane A₂ (TXA₂) and its analogs with high affinity. When administered to tissue preparations, AnSt-D7L1 abrogated Leukotriene C₄ (LTC₄)-induced contraction of guinea pig ileum and contraction of rat aorta by the TXA₂ analog U46619. The protein also inhibited platelet aggregation induced by both collagen and U46619 when administered to stirred platelets. The crystal structure of AnSt-D7L1 contains two OBP-like domains and has a structure similar to AeD7. In AnSt-D7L1, the binding pocket of the C-terminal domain has been rearranged relative to AeD7, making the protein unable to bind biogenic amines. Structures of the ligand complexes show that CysLTs and TXA₂ analogs both bind in the same hydrophobic pocket of the N-terminal domain. The TXA₂ analog U46619 is stabilized by hydrogen bonding interactions of the ω-5 hydroxyl group with the phenolic hydroxyl group of Tyr 52. LTC₄ and occupies a very similar position to LTE₄ in the previously determined structure of its complex with AeD7. As yet, it is not known what, if any, new function has been acquired by the rearranged C-terminal domain. This article presents, to our knowledge, the first structural characterization of a protein from mosquito saliva that inhibits collagen mediated platelet activation.

When feeding, a female mosquito must inhibit the blood clotting and inflammatory responses of the host. To do this, the insect produces salivary proteins that neutralize key host molecules participating in clotting and inflammation. Here, we describe a salivary protein AnSt-D7L1 that scavenges both thomboxane A₂ and cysteinyl leukotrienes, two substances involved in blood vessel constriction, platelet aggregation, and inflammatory responses to an insect bite. We produced this protein in bacteria and showed that it tightly binds both these molecules, inhibiting the processes in which they are involved. We then determined its structure using X-ray crystallography and showed that there is a single binding site in one domain of the protein, accommodating both thromboxane A₂ and cysteinyl leukotrienes, and that this site is responsible for the scavenging effect of the protein. These studies reveal the structural features of proteins needed to bind to key molecules of potential pharmacological importance and add to our understanding of the process of mosquito blood feeding, which is essential for transmission of the malaria parasite.

Platelet physiology and antiplatelet agents

Apart from the central beneficial role platelets play in hemostasis, they are also involved in atherothrombotic diseases. Here, we review the current knowledge of platelet intracellular signal transduction pathways involved in platelet adhesion, activation, amplification of the activation signal and aggregation, as well as pathways limiting platelet aggregation. A thorough understanding of these pathways allows explanation of the mechanism of action of existing antiplatelet agents, but also helps to identify targets for novel drug development.

Differential phosphorylation of myosin light chain (Thr)18 and (Ser)19 and functional implications in platelets

BACKGROUND

Myosin IIA is an essential platelet contractile protein that is regulated by phosphorylation of its regulatory light chain (MLC) on residues (Thr)18 and (Ser)19 via the myosin light chain kinase (MLCK).

OBJECTIVE

The present study was carried out to elucidate the mechanisms regulating MLC (Ser)19 and (Thr)18 phosphorylation and the functional consequence of each phosphorylation event in platelets.

RESULTS

Induction of 2MeSADP-induced shape change occurs within 5s along with robust phosphorylation of MLC (Ser)19 with minimal phosphorylation of MLC (Thr)18. Selective activation of G(12/13) produces both slow shape change and comparably slow MLC (Thr)18 and (Ser)19 phosphorylation. Stimulation with agonists that trigger ATP secretion caused rapid MLC (Ser)19 phosphorylation while MLC (Thr)18 phosphorylation was coincident with secretion. Platelets treated with p160(ROCK) inhibitor Y-27632 exhibited a partial inhibition in secretion and had a substantial inhibition in MLC (Thr)18 phosphorylation without effecting MLC (Ser)19 phosphorylation. These data suggest that phosphorylation of MLC (Ser)19 is downstream of Gq/Ca(2+) -dependent mechanisms and sufficient for shape change, whereas MLC (Thr)18 phosphorylation is substantially downstream of G(12/13) -regulated Rho kinase pathways and necessary, probably in concert with MLC (Ser)19 phosphorylation, for full contractile activity leading to dense granule secretion. Overall, we suggest that the amplitude of the platelet contractile response is differentially regulated by a least two different signaling pathways, which lead to different phosphorylation patterns of the myosin light chain, and this mechanism results in a graded response rather than a simple on/off switch.

Thromboxane and the thromboxane receptor in cardiovascular disease

Thromboxane A(2) (TXA(2)), the primary product of COX-1-dependent metabolism of arachidonic acid, mediates its biological actions through the TXA(2) receptor, termed the TP. Irreversible inhibition of platelet COX-1-derived TXA(2) with low-dose aspirin affords protection against primary and secondary vascular thrombotic events, underscoring the central role of TXA(2) as a platelet agonist in cardiovascular disease. The limitations associated with aspirin use include significant gastrointestinal toxicity, bleeding complications, potential interindividual response variability and poor efficacy in some disease states. This, together with the broad role of TXA(2) in cardiovascular disease beyond the platelet, has refocused interest towards additional TXA(2)-associated drug targets, in particular TXA(2) synthase and the TP. The superiority of these agents over low-dose aspirin, in terms of clinical efficacy, tolerability and commercial viability, remain open questions that are the focus of ongoing research.

Serglycin proteoglycan deletion in mouse platelets: physiological effects and their implications for platelet contributions to thrombosis, inflammation, atherosclerosis, and metastasis

Serglycin is found in all nucleated hematopoietic cells and platelets, blood vessels, various reproductive and developmental tissues, and in chondrocytes. The serglycin knockout mouse has demonstrated that this proteoglycan is required for proper generation and function of secretory granules in several hematopoietic cells. The effects on platelets are profound, and include diminishing platelet aggregation responses and formation of platelet thrombi. This chapter will review cell-specific aspects of serglycin structure, its gene regulation, cell and tissue localization, and the effects of serglycin deletion on hematopoietic cell granule structure and function. The effects of serglycin knockout on platelets are described and discussed in detail. Rationales for further investigations into the contribution of serglycin to the known roles of platelets in thrombosis, inflammation, atherosclerosis, and tumor metastasis are presented.

RhoA downstream of G(q) and G(12/13) pathways regulates protease-activated receptor-mediated dense granule release in platelets

Platelet secretion is an important physiological event in hemostasis. The protease-activated receptors, PAR 1 and PAR 4, and the thromboxane receptor activate the G(12/13) pathways, in addition to the G(q) pathways. Here, we investigated the contribution of G(12/13) pathways to platelet dense granule release. 2MeSADP, which does not activate G(12/13) pathways, does not cause dense granule release in aspirin-treated platelets. However, supplementing 2MeSADP with YFLLRNP (60muM), as selective activator of G(12/13) pathways, resulted in dense granule release. Similarly, supplementing PLC activation with G(12/13) stimulation also leads to dense granule release. These results demonstrate that supplemental signaling from G(12/13) is required for G(q)-mediated dense granule release and that ADP fails to cause dense granule release because the platelet P2Y receptors, although activate PLC, do not activate G(12/13) pathways. When RhoA, downstream signaling molecule in G(12/13) pathways, is blocked, PAR-mediated dense granule release is inhibited. Furthermore, ADP activated RhoA downstream of G(q) and upstream of PLC. Finally, RhoA regulated PKCdelta T505 phosphorylation, suggesting that RhoA pathways contribute to platelet secretion through PKCdelta activation. We conclude that G(12/13) pathways, through RhoA, regulate dense granule release and fibrinogen receptor activation in platelets.

Lysophosphatidic acid induces alphavbeta6 integrin-mediated TGF-beta activation via the LPA2 receptor and the small G protein G alpha(q)

Activation of latent transforming growth factor beta (TGF-beta) by alphavbeta6 integrin is critical in the pathogenesis of lung injury and fibrosis. We have previously demonstrated that the stimulation of protease activated receptor 1 promotes alphavbeta6 integrin-mediated TGF-beta activation via RhoA, which is known to modulate cell contraction. However, whether other G protein-coupled receptors can also induce alphavbeta6 integrin-mediated TGF-beta activation is unknown; in addition, the alphavbeta6 integrin signaling pathway has not yet been fully characterized. In this study, we show that lysophosphatidic acid (LPA) induces alphavbeta6-mediated TGF-beta activation in human epithelial cells via both RhoA and Rho kinase. Furthermore, we demonstrate that LPA-induced alphavbeta6 integrin-mediated TGF-beta activity is mediated via the LPA2 receptor, which signals via G alpha(q). Finally, we show that the expression levels of both the LPA2 receptor and alphavbeta6 integrin are up-regulated and are spatially and temporally associated following bleomycin-induced lung injury. Furthermore, both the LPA2 receptor and alphavbeta6 integrin are up-regulated in the overlying epithelial areas of fibrosis in patients with usual interstitial pneumonia. These studies demonstrate that LPA induces alphavbeta6 integrin-mediated TGF-beta activation in epithelial cells via LPA2, G alpha(q), RhoA, and Rho kinase, and that this pathway might be clinically relevant to the development of lung injury and fibrosis.

Group IVA cytosolic phospholipase A2 (cPLA2alpha) and integrin alphaIIbbeta3 reinforce each other's functions during alphaIIbbeta3 signaling in platelets

Group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) catalyzes release of arachidonic acid from glycerophospholipids, leading to thromboxane A(2) (TxA(2)) production. Some platelet agonists stimulate cPLA(2)alpha, but others require fibrinogen binding to alphaIIbbeta3 to elicit TxA(2). Therefore, relationships between cPLA(2)alpha and alphaIIbbeta3 were examined. cPLA(2)alpha and a cPLA(2)alpha binding partner, vimentin, coimmunoprecipitated with alphaIIbbeta3 from platelets, independent of fibrinogen binding. Studies with purified proteins and with recombinant proteins expressed in CHO cells determined that the interaction between cPLA(2)alpha and alphaIIbbeta3 was indirect and was dependent on the alphaIIb and beta3 cytoplasmic tails. Fibrinogen binding to alphaIIbbeta3 caused an increase in integrin-associated cPLA(2)alpha activity in normal platelets, but not in cPLA(2)alpha-deficient mouse platelets or in human platelets treated with pyrrophenone, a cPLA(2)alpha inhibitor. cPLA(2)alpha activation downstream of alphaIIbbeta3 had functional consequences for platelets in that it was required for fibrinogen-dependent recruitment of activated protein kinase Cbeta to the alphaIIbbeta3 complex and for platelet spreading. Thus, cPLA(2)alpha and alphaIIbbeta3 interact to reinforce each other's functions during alphaIIbbeta3 signaling. This provides a plausible explanation for the role of alphaIIbbeta3 in TxA(2) formation and in the defective hemostatic function of mouse or human platelets deficient in cPLA(2)alpha.

G(12)/G(13)-mediated signalling in mammalian physiology and disease

The human genome encodes hundreds of G-protein-coupled receptors. Their intracellular effects, however, are mediated by only four families of heterotrimeric G proteins: G(s), G(i)/G(o), G(q)/G(11) and G(12)/G(13). Progress in the knowledge about the G(12)/G(13) family has somewhat lagged behind because their downstream effectors remained unknown for several years, and tools to specifically interfere with G(12)/G(13)-mediated signalling were, therefore, missing. However, with the identification of G(12)/G(13)-regulated signalling pathways and the recent application of new techniques, such as conditional gene inactivation, RNA interference or expression of inhibitory proteins, new insights into the in vivo functions of this G-protein family have been gained. It has become clear that this pathway regulates cellular proliferation, movement and morphology in many different organs and that it is centrally involved in various diseases including cancer and cardiovascular disorders. Here, we focus on recent progress made in the analyses of the in vivo functions of mammalian G(12)/G(13)-mediated signalling.

Rac1 is essential for phospholipase C-gamma2 activation in platelets

Platelet activation at sites of vascular injury is triggered through different signaling pathways leading to activation of phospholipase (PL) Cbeta or PLCgamma2. Active PLCs trigger Ca(2+) mobilization and entry, which is a prerequisite for adhesion, secretion, and thrombus formation. PLCbeta isoenzymes are activated downstream of G protein-coupled receptors (GPCRs), whereas PLCgamma2 is activated downstream of immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptors, such as the major platelet collagen receptor glycoprotein (GP) VI or CLEC-2. The mechanisms underlying PLC regulation are not fully understood. An involvement of small GTPases of the Rho family (Rho, Rac, Cdc42) in PLC activation has been proposed but this has not been investigated in platelets. We here show that murine platelets lacking Rac1 display severely impaired GPVI- or CLEC-2-dependent activation and aggregation. This defect was associated with impaired production of inositol 1,4,5-trisphosphate (IP(3)) and intracellular calcium mobilization suggesting inappropriate activation of PLCgamma2 despite normal tyrosine phosphorylation of the enzyme. Rac1 ( -/- ) platelets displayed defective thrombus formation on collagen under flow conditions which could be fully restored by co-infusion of ADP and the TxA(2) analog U46619, indicating that impaired GPVI-, but not G-protein signaling, was responsible for the observed defect. In line with this, Rac1 ( -/- ) mice were protected in two collagen-dependent arterial thrombosis models. Together, these results demonstrate that Rac1 is essential for ITAM-dependent PLCgamma2 activation in platelets and that this is critical for thrombus formation in vivo.

Thromboxane A2: physiology/pathophysiology, cellular signal transduction and pharmacology

Thromboxane A(2) (TXA(2)), an unstable arachidonic acid metabolite, elicits diverse physiological/pathophysiological actions, including platelet aggregation and smooth muscle contraction. TXA(2) has been shown to be involved in allergies, modulation of acquired immunity, atherogenesis, neovascularization, and metastasis of cancer cells. The TXA(2) receptor (TP) communicates mainly with G(q) and G(13), resulting in phospholipase C activation and RhoGEF activation, respectively. In addition, TP couples with G(11), G(12), G(13), G(14), G(15), G(16), G(i), G(s) and G(h). TP is widely distributed in the body, and is expressed at high levels in thymus and spleen. The second extracellular loop of TP is an important ligand-binding site, and Asp(193) is a key amino acid. There are two alternatively spliced isoforms of TP, TPalpha and TPbeta, which differ only in their C-terminals. TPalpha and TPbeta communicate with different G proteins, and undergo hetero-dimerization, resulting in changes in intracellular traffic and receptor protein conformations. TP cross-talks with receptor tyrosine kinases, such as EGF receptor, to induce cell proliferation and differentiation. TP is glycosylated in the N-terminal region for recruitment to plasma membranes. Furthermore, TP conformation is changed by coupling to G proteins, showing several states of agonist binding. Finally, several drugs modify TP-mediated events; these include cyclooxygenase inhibitors, TXA(2) synthase inhibitors and TP antagonists. Some flavonoids of natural origin also have TP receptor antagonistic activity. Recent advances in TP research have clarified TXA(2)-mediated events in detail, and further study will supply more beneficial information about TXA(2) pathophysiology.

Impaired platelet function reduces myocardial infarct size in Galphaq knock-out mice in vivo

Platelet aggregation and secretion play a crucial role in acute coronary syndromes. In Galpha(q) knock-out mice (Galpha(q)(-/-)) platelet function is eliminated in terms of aggregation and secretion of cytokines. We investigated whether restricted platelet aggregation and secretion reduces myocardial infarct size in vivo. Thirty minute regional myocardial ischemia was followed by 24 h reperfusion (I/R) in vivo. Infarct size was determined by counterstaining. Left ventricular function was measured by ultrasound. Infarct size to area at risk ratio was significantly smaller in Galpha(q)(-/-) mice (5.6+/-1.6%) compared to wild-type (WT) mice (27.2+/-3.0%, p<0.01). Fractional shortening was improved in Galpha(q)(-/-) mice compared to WT (42.2+/-1.4% versus 30.5+/-1.4%, respectively, p<0.01). WT mice, transplanted with Galpha(q)(-/-) bone marrow showed a significant reduction in infarct size compared to control (7.8+/-2.2% versus 18.4+/-2.7%, respectively, p<0.01). Platelets of Galpha(q)(-/-) mice had an impaired aggregation and secretion phenotype. In the in vivo model of ischemia and reperfusion, beyond impaired platelet aggregation, platelet secretion plays an additional role in myocardial infarct extension. Blocking platelet aggregation in combination with secretion might be a promising supplementary therapeutic strategy in acute myocardial infarction.

GDI-1 phosphorylation switch at serine 96 induces RhoA activation and increased endothelial permeability

We identified the GDI-1-regulated mechanism of RhoA activation from the Rho-GDI-1 complex and its role in mediating increased endothelial permeability. Thrombin stimulation failed to induce RhoA activation and actin stress fiber formation in human pulmonary arterial endothelial cells transduced with full-length GDI-1. Expression of a GDI-1 mutant form (C-GDI) containing the C terminus (aa 69 to 204) also prevented RhoA activation, whereas further deletions failed to alter RhoA activation. We observed that protein kinase Calpha-mediated phosphorylation of the C terminus of GDI-1 at Ser96 reduced the affinity of GDI-1 for RhoA and thereby enabled RhoA activation. Rendering GDI-1 phosphodefective with a Ser96 --> Ala substitution rescued the inhibitory activity of GDI-1 toward RhoA but did not alter the thrombin-induced activation of other Rho GTPases, i.e., Rac1 and Cdc42. Phosphodefective mutant GDI-1 also suppressed myosin light chain phosphorylation, actin stress fiber formation, and the increased endothelial permeability induced by thrombin. In contrast, expressing the phospho-mimicking mutant S96D-GDI-1 protein induced RhoA activity and increased endothelial permeability independently of thrombin stimulation. These results demonstrate the crucial role of the phosphorylation of the C terminus of GDI-1 at S96 in selectively activating RhoA. Inhibiting GDI-1 phosphorylation at S96 is a potential therapeutic target for modulating RhoA activity and thus preventing the increase in endothelial permeability associated with vascular inflammation.

In vivo thrombus formation in murine models

Platelets play a central role in hemostasis, but also in atherothrombosis, as they rapidly adhere to tissue and to one another as a response to any vascular injury. This process involves a large number of surface receptors, signaling pathways, and enzymatic cascades as well as their complex interplay. Although in vitro experiments proved successful in both identifying new receptors and pathways and developing potent and selective antithrombotic drugs, in vitro research cannot mimic the myriad hemodynamic and spatiotemporal cellular and molecular interactions that occur during the generation and propagation of thrombi in vivo. Animal models, and, with the availability of genetically modified mouse strains and of modern intravital imaging techniques, mouse models in particular, have opened new ways to identify both individual roles and the interplay of platelet proteins in complex in vivo settings. In vivo models revealed the important role of, eg, Gas6 or blood coagulation factor XII in thrombus formation, and results obtained in in vivo models raised the interesting possibility that (physiologic) hemostasis and (pathologic) thrombosis might represent 2 mechanistically different processes. This review summarizes in vivo findings that contributed significantly to our understanding of hemostatic and thrombotic processes and which may help to guide future research.

A guide to murine platelet structure, function, assays, and genetic alterations

Platelets play an important role in hemostasis, thrombosis and several other biological processes. The adaptability of mice to genetic manipulation and their genetic similarity to humans has resulted in a plethora of murine models to study platelet function. Although murine platelets differ from human platelets with regard to size, number and structure, functionally they are very similar. Thus, studies which employed these model systems have greatly improved our current understanding of the contribution of platelets to hemostasis and thrombosis. This review presents general recommendations with respect to collection, isolation and processing of murine platelets. It also describes the assays currently available to study platelet function and critically assesses their utility. The extensive literature on the effects of genetic alterations on murine platelet function is considered in detail. This review is intended to provide a convenient source of reference for platelet investigators.

Gαq-TRPC6-mediated Ca2+ Entry Induces RhoA Activation and Resultant Endothelial Cell Shape Change in Response to Thrombin

RhoA activation and increased intracellular Ca(2+) concentration mediated by the activation of transient receptor potential channels (TRPC) both contribute to the thrombin-induced increase in endothelial cell contraction, cell shape change, and consequently to the mechanism of increased endothelial permeability. Herein, we addressed the possibility that TRPC signals RhoA activation and thereby contributes in actinomyosin-mediated endothelial cell contraction and increased endothelial permeability. Transduction of a constitutively active Galphaq mutant in human pulmonary arterial endothelial cells induced RhoA activity. Preventing the increase in intracellular Ca2+ concentration by the inhibitor of Galphaq or phospholipase C and the Ca2+ chelator, BAPTA-AM, abrogated thrombin-induced RhoA activation. Depletion of extracellular Ca2+ also inhibited RhoA activation, indicating the requirement of Ca2+ entry in the response. RhoA activation could not be ascribed to storeoperated Ca2+ (SOC) entry because SOC entry induced with thapsigargin or small interfering RNA-mediated inhibition of TRPC1 expression, the predominant SOC channel in these endothelial cells, failed to alter RhoA activity. However, activation of receptor-operated Ca2+ entry by oleoyl-2-acetyl-sn-glycerol, the membrane permeable analogue of the Galphaq-phospholipase C product diacylglycerol, induced RhoA activity. Receptor-operated Ca2+ activation was mediated by TRPC6 because small interfering RNA-induced TRPC6 knockdown significantly reduced Ca2+ entry. TRPC6 knockdown also prevented RhoA activation, myosin light chain phosphorylation, and actin stress fiber formation as well as inter-endothelial junctional gap formation in response to either oleoyl-2-acetyl-sn-glycerol or thrombin. TRPC6-mediated RhoA activity was shown to be dependent on PKCalpha activation. Our results demonstrate that Galphaq activation of TRPC6 signals the activation of PKCalpha, and thereby induces RhoA activity and endothelial cell contraction.

Signaling through G(alpha)13 switch region I is essential for protease-activated receptor 1-mediated human platelet shape change, aggregation, and secretion

This study investigated the involvement of Galpha(13) switch region I (SRI) in protease-activated receptor 1 (PAR1)-mediated platelet function and signaling. To this end, myristoylated peptides representing the Galpha(13) SRI (Myr-G(13)SRI(pep)) and its random counterpart were evaluated for their effects on PAR1 activation. Initial studies demonstrated that Myr-G(13)SRI(pep) and Myr-G(13)SRI(Random-pep) were equally taken up by human platelets and did not interfere with PAR1-ligand interaction. Subsequent experiments revealed that Myr-G(13)SRI(pep) specifically bound to platelet RhoA guanine nucleotide exchange factor (p115RhoGEF) and blocked PAR1-mediated RhoA activation in platelets and human embryonic kidney cells. These results suggest a direct interaction of Galpha(13) SRI with p115RhoGEF and a mechanism for Myr-G(13)SRI(pep) inhibition of RhoA activation. Platelet function studies demonstrated that Myr-G(13)SRI(pep) specifically inhibited PAR1-stimulated shape change, aggregation, and secretion in a dose-dependent manner but did not inhibit platelet activation induced by either ADP or A23187. It was also found that Myr-G(13)SRI(pep) inhibited low dose, but not high dose, thrombin-induced aggregation. Additional experiments showed that PAR1-mediated calcium mobilization was partially blocked by Myr-G(13)SRI(pep) but not by the Rho kinase inhibitor Y-27632. Finally, Myr-G(13)SRI(pep) effectively inhibited PAR1-induced stress fiber formation and cell contraction in endothelial cells. Collectively, these results suggest the following: 1) interaction of Galpha(13) SRI with p115RhoGEF is required for G(13)-mediated RhoA activation in platelets; 2) signaling through the G(13) pathway is critical for PAR1-mediated human platelet functional changes and low dose thrombin-induced aggregation; and 3) G(13) signaling elicits calcium mobilization in human platelets through a Rho kinase-independent mechanism.

Activation of platelet function through G protein-coupled receptors

Because of their ability to become rapidly activated at places of vascular injury, platelets are important players in primary hemostasis as well as in arterial thrombosis. In addition, they are also involved in chronic pathological processes including the atherosclerotic remodeling of the vascular system. Although primary adhesion of platelets to the vessel wall is largely independent of G protein-mediated signaling, the subsequent recruitment of additional platelets into a growing platelet thrombus requires mediators such as ADP, thromboxane A(2), or thrombin, which act through G protein-coupled receptors. Platelet activation via G protein-coupled receptors involves 3 major G protein-mediated signaling pathways that are initiated by the activation of the G proteins G(q), G(13), and G(i). This review summarizes recent progress in understanding the mechanisms underlying platelet activation and thrombus extension via G protein-mediated signaling pathways.

Conditional mutagenesis of G-protein coupled receptors and G-proteins

The G-protein-coupled receptor signaling system, consisting of a huge variety of receptors as well as of many G-proteins and effectors, operates in every cell and is involved in many physiological and pathological processes. The versatility of this system and the involvement of specific components makes G-protein-coupled receptors and their signaling pathways ideal targets for pharmacological interventions. Classical mouse knockout models have often provided important preliminary insights into the biological roles of individual receptors and signaling pathways and they are routinely used in the process of target validation. The recent development of efficient conditional mutagenesis techniques now allows a much more detailed analysis of G-protein-mediated signaling transduction processes. This review summarizes some of the areas in which progress has recently been made by applying conditional mutagenesis of genes coding for G-proteins and G-protein-coupled receptors.

Lysophospholipids control integrin-dependent adhesion in splenic B cells through G(i) and G(12)/G(13) family G-proteins but not through G(q)/G(11)

Integrin-mediated adhesion is a crucial step in lymphocyte extravasation and homing. We show here that not only the chemokines CXCL12 and CXCL13 but also the lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) enhance adhesion of murine follicular and marginal zone B cells to ICAM-1 in vitro. This process involves clustering of integrin LFA-1 and is blocked by pertussis toxin, suggesting that G(i) family G-proteins are involved. In addition, lysophospholipid-induced adhesion on ICAM-1 depends on Rho and Rhokinase, indicative of an involvement of G(12)/G(13), possibly also G(q)/G(11) family G-proteins. We used G(12)/G(13)- or G(q)/G(11)-deficient B cells to study the role of these G-protein families in lysophospholipid-induced adhesion and found that the pro-adhesive effects of LPA and S1P are completely abrogated in G(12)/G(13)-deficient marginal zone B cells, reduced in G(12)/G(13)-deficient follicular B cells, and normal in G(q)/G(11)-deficient B cells. We also show that loss of lysophospholipid-induced adhesion results in disinhibition of migration in response to the follicular chemokine CXCL13, which might contribute to the abnormal localization of splenic B cell populations observed in B cell-specific G(12)/G(13)-deficient mice in vivo. Taken together, this study shows that lysophospholipids regulate integrin-mediated adhesion of splenic B cells to ICAM-1 through G(i) and G(12)/G(13) family G-proteins but not through G(q)/G(11).

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

Cell signalling diversity of the Gqalpha family of heterotrimeric G proteins

Many receptors for neurotransmitters and hormones rely upon members of the Gqalpha family of heterotrimeric G proteins to exert their actions on target cells. Galpha subunits of the Gq class of G proteins (Gqalpha, G11alpha, G14alpha and G15/16alpha) directly link receptors to activation of PLC-beta isoforms which, in turn, stimulate inositol lipid (i.e. calcium/PKC) signalling. Although Gqalpha family members share a capacity to activate PLC-beta, they also differ markedly in their biochemical properties and tissue distribution which predicts functional diversity. Nevertheless, established models suggest that Gqalpha family members are functionally redundant and that their cellular responses are a result of PLC-beta activation and downstream calcium/PKC signalling. Growing evidence, however, indicates that Gqalpha, G11alpha, G14alpha and G15/16alpha are functionally diverse and that many of their cellular actions are independent of inositol lipid signalling. Recent findings show that Gqalpha family members differ with regard to their linked receptors and downstream binding partners. Reported binding partners distinct from PLC-beta include novel candidate effector proteins, various regulatory proteins, and a growing list of scaffolding/adaptor proteins. Downstream of these signalling proteins, Gqalpha family members exhibit unexpected differences in the signalling pathways and the gene expression profiles they regulate. Finally, genetic studies using whole animal models demonstrate the importance of certain Gqalpha family members in cardiac, lung, brain and platelet functions among other physiological processes. Taken together, these findings demonstrate that Gqalpha, G11alpha, G14alpha and G15/16alpha regulate both overlapping and distinct signalling pathways, indicating that they are more functionally diverse than previously thought.

Platelets in atherothrombosis: lessons from mouse models

Platelets play a central role in hemostasis and thrombosis but also in the initiation of atherosclerosis, making platelet receptors and their intracellular signaling pathways important molecular targets for antithrombotic and anti-inflammatory therapy. Historically, much of the knowledge about hemostasis and thrombosis has been derived from patients suffering from bleeding and thrombotic disorders and the identification of the underlying molecular defects. In recent years, the availability of genetically modified mouse strains with defined defects in platelet function and the development of in vivo models to assess platelet-related physiologic and pathophysiologic processes have opened new ways to identify the individual roles and the interplay of platelet proteins in adhesion, activation, aggregation, secretion, and procoagulant activity in vitro and in vivo. This review will summarize key findings made by these approaches and discuss them in the context of human disease.