Small GTPase Rho regulates thrombin-induced platelet aggregation

Rho-Proteins and Downstream Pathways as Potential Targets in Sepsis and Septic Shock: What Have We Learned from Basic Research

Sepsis and septic shock are associated with acute and sustained impairment in the function of the cardiovascular system, kidneys, lungs, liver, and brain, among others. Despite the significant advances in prevention and treatment, sepsis and septic shock sepsis remain global health problems with elevated mortality rates. Rho proteins can interact with a considerable number of targets, directly affecting cellular contractility, actin filament assembly and growing, cell motility and migration, cytoskeleton rearrangement, and actin polymerization, physiological functions that are intensively impaired during inflammatory conditions, such as the one that occurs in sepsis. In the last few decades, Rho proteins and their downstream pathways have been investigated in sepsis-associated experimental models. The most frequently used experimental design included the exposure to bacterial lipopolysaccharide (LPS), in both in vitro and in vivo approaches, but experiments using the cecal ligation and puncture (CLP) model of sepsis have also been performed. The findings described in this review indicate that Rho proteins, mainly RhoA and Rac1, are associated with the development of crucial sepsis-associated dysfunction in different systems and cells, including the endothelium, vessels, and heart. Notably, the data found in the literature suggest that either the inhibition or activation of Rho proteins and associated pathways might be desirable in sepsis and septic shock, accordingly with the cellular system evaluated. This review included the main findings, relevance, and limitations of the current knowledge connecting Rho proteins and sepsis-associated experimental models.

RhoA and Rac1 GTPases Differentially Regulate Agonist-Receptor Mediated Reactive Oxygen Species Generation in Platelets

Agonist induced generation of reactive oxygen species (ROS) by NADPH oxidases (NOX) enhances platelet aggregation and hence the risk of thrombosis. RhoA and Rac1 GTPases are involved in ROS generation by NOX in a variety of cells, but their roles in platelet ROS production remain unclear. In this study we used platelets from RhoA and Rac1 conditional knockout mice as well as human platelets treated with Rhosin and NSC23767, rationally designed small molecule inhibitors of RhoA and Rac GTPases, respectively, to better define the contributions of RhoA and Rac1 signaling to ROS generation and platelet activation. Treatment of platelets with Rhosin inhibited: (a) U46619 induced activation of RhoA; (b) phosphorylation of p47phox, a critical component of NOX; (c) U46619 or thrombin induced ROS generation; (d) phosphorylation of myosin light chain (MLC); (e) platelet shape change; (f) platelet spreading on immobilized fibrinogen; and (g) release of P-selectin, secretion of ATP and aggregation. Conditional deletion of RhoA or Rac1 gene inhibited thrombin induced ROS generation in platelets. Addition of Y27632, a RhoA inhibitor, NSC23766 or Phox-I, an inhibitor of Rac1-p67phox interaction, to human platelets blocked thrombin induced ROS generation. These data suggest that: (a) RhoA/ROCK/p47phox signaling axis promotes ROS production that, at least in part, contributes to platelet activation in conjunction with or independent of the RhoA/ROCK mediated phosphorylation of MLC; and (b) RhoA and Rac1 differentially regulate ROS generation by inhibiting phosphorylation of p47phox and Rac1-p67phox interaction, respectively.

Platelets Are Highly Activated in Patients of Chronic Thromboembolic Pulmonary Hypertension

Objective—

Chronic thromboembolic pulmonary hypertension (CTEPH) is a fatal disease that is distinct from pulmonary arterial hypertension (PAH). Although CTEPH is characterized by obstruction of major pulmonary artery because of chronic thrombus, it remains unclear whether CTEPH is associated with prothrombotic condition.

Approach and Results—

In addition to conventional markers, GTP-bound levels of Rap1, RhoA, RalA, Rac1, and Ras in platelets, which are implicated for platelet activation, were measured in patients without pulmonary hypertension (non-PH, n=15), patients with PAH (n=19), and patients with CTEPH (n=25). Furthermore, the responsiveness to ex vivo thrombin stimulation was also evaluated. The ratios of the P-selectin positive platelets in the non-PH patients, patients with PAH, and patients with CTEPH were 1.40% (median and interquartile range, 0.83–1.82), 2.40% (1.80–3.39), and 2.63% (1.90–8.22), respectively (non-PH versus CTEPH, P <0.01). The activated GPIIb/IIIa-positive platelets were 6.01% (1.34–7.87), 11.39% (5.69–20.86), and 9.74% (7.83–24.01), respectively (non-PH versus CTEPH, P =0.01). GTP-bound RhoA was 1.79% (0.94–2.83), 4.03% (2.01–5.14), and 2.01% (1.22–2.48), respectively (non-PH versus PAH, P =0.04), and GTP-bound RalA was 1.58% (1.08–2.11), 3.02% (2.03–3.54), and 2.64% (1.42–4.28), respectively (non-PH versus PAH, P =0.023; non-PH versus CTEPH, P =0.048). In contrast, Rac1, Rap1, or Ras was not activated in any groups. The platelets of patients with CTEPH exhibited hyperresponsiveness to ex vivo thrombin stimulation compared with those of non-PH patients when evaluated for the surface markers. Either D-dimer or fibrin degradation product level was not increased in patients with CTEPH.

Conclusions—

These results provide the first direct evidence that platelets of patients with CTEPH are highly activated and exhibit hyperresponsiveness to thrombin stimulation.

Rho GTPases in platelet function

The Rho family of GTP binding proteins, also commonly referred to as the Rho GTPases, are master regulators of the platelet cytoskeleton and platelet function. These low-molecular-weight or 'small' GTPases act as signaling switches in the spatial and temporal transduction, and amplification of signals from platelet cell surface receptors to the intracellular signaling pathways that drive platelet function. The Rho GTPase family members RhoA, Cdc42 and Rac1 have emerged as key regulators in the dynamics of the actin cytoskeleton in platelets and play key roles in platelet aggregation, secretion, spreading and thrombus formation. Rho GTPase regulators, including GEFs and GAPs and downstream effectors, such as the WASPs, formins and PAKs, may also regulate platelet activation and function. In this review, we provide an overview of Rho GTPase signaling in platelet physiology. Previous studies of Rho GTPases and platelets have had a shared history, as platelets have served as an ideal, non-transformed cellular model to characterize Rho function. Likewise, recent studies of the cell biology of Rho GTPase family members have helped to build an understanding of the molecular regulation of platelet function and will continue to do so through the further characterization of Rho GTPases as well as Rho GAPs, GEFs, RhoGDIs and Rho effectors in actin reorganization and other Rho-driven cellular processes.

Effect of supplementation with an 80:20 cis9,trans11 conjugated linoleic acid blend on the human platelet proteome

SCOPE

The dietary fatty acid cis9,trans11 conjugated linoleic acid (cis9,trans11 CLA) has been shown to modify the function of endothelial cells, monocytes, and platelets, all of which are involved in the development of atherosclerosis. Potential mechanisms for the platelet effects have not been assessed previously. In this study, we assessed how supplementation of the diet with an 80:20 cis9,trans11 CLA blend affects the platelet proteome.

METHODS AND RESULTS

In a double-blind, randomized, placebo-controlled, parallel-group trial, 40 overweight but apparently healthy adults received either 4 g per day of cis9,trans11 CLA-enriched oil or placebo oil, consisting of palm oil and soybean oil, for 3 months. Total platelet proteins were extracted from washed platelets, separated using two-dimensional gel electrophoresis and differentially regulated protein spots were identified by LC-ESI-MS/MS. Supplementation with the CLA blend, compared with placebo, resulted in significant alterations in levels of 46 spots (p < 0.05), of which 40 were identified. Network analysis revealed that the majority of these proteins participate in regulation of the cytoskeleton and platelet structure, as well as receptor action, signaling, and focal adhesion.

CONCLUSION

The platelet proteomics approach revealed novel insights into regulation of cellular biomarkers of atherogenic and thrombotic pathways by an 80:20 cis9,trans11 CLA blend.

The Rho-kinase inhibitor Y-27632 inhibits cholestasis-induced platelet interactions in the hepatic microcirculation

Bile duct obstruction is associated with hepatic accumulation of leukocytes and liver injury. Emerging data suggest that platelets may play an important role in tissue damage and inflammation. Herein, we characterized the platelet response in cholestatic liver injury and evaluated the role of Rho-kinase signaling. For this purpose, C57BL/6 mice were treated with the Rho-kinase inhibitor Y-27632 (10 mg/kg) and vehicle before undergoing bile duct ligation (BDL) for 12 h. Platelet rolling and adhesion, formation of platelet aggregates as well as microvascular perfusion in the liver were analyzed using intravital fluorescence microscopy. Liver damage was monitored by measuring serum levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Administration of Y-27632 reduced the BDL-associated increase of ALT and AST by 95% and 89%, respectively. The inhibition of Rho-kinase also reduced cholestasis-induced platelet rolling and adhesion by more than 46% and 73% in postsinusoidal venules and platelet adhesion in sinusoids by 60%. In addition, Y-27632 decreased platelet aggregation in hepatic sinusoids and postsinusoidal venules by 69% and 81%. BDL caused a significant reduction of hepatic microvascular perfusion. Importantly, pretreatment with Y-27632 restored sinusoidal perfusion in cholestatic animals. Our findings demonstrate that Rho-kinase regulates multiple aspects of platelet interaction in the microcirculation of cholestatic animals. Moreover, inhibition of Rho-kinase signaling not only attenuates platelet responses but also maintains microvascular perfusion and protects against hepatocellular injury in cholestasis. Thus, targeting Rho-kinase signaling may be an effective way to protect against platelet-mediated liver injury in obstructive jaundice.

Biochemical characterization of the Rho GTPase-regulated actin assembly by diaphanous-related formins, mDia1 and Daam1, in platelets

The diaphanous-related formins are actin nucleating and elongating factors. They are kept in an inactive state by an intramolecular interaction between the diaphanous inhibitory domain (DID) and the diaphanous-autoregulatory domain (DAD). It is considered that the dissociation of this autoinhibitory interaction upon binding of GTP-bound Rho to the GTPase binding domain next to DID induces exposure of the FH1-FH2 domains, which assemble actin filaments. Here, we isolated two diaphanous-related formins, mDia1 and Daam1, in platelet extracts by GTP-RhoA affinity column chromatography. We characterized them by a novel assay, where beads coated with the FH1-FH2-DAD domains of either mDia1 or Daam1 were incubated with platelet cytosol, and the assembled actin filaments were observed after staining with rhodamine-phalloidin. Both formins generated fluorescent filamentous structures on the beads. Quantification of the fluorescence intensity of the beads revealed that the initial velocity in the presence of mDia1 was more than 10 times faster than in the presence of Daam1. The actin assembly activities of both FH1-FH2-DADs were inhibited by adding cognate DID domains. GTP-RhoA, -RhoB, and -RhoC, but not GTP-Rac1 or -Cdc42, bound to both mDia1 and Daam1 and efficiently neutralized the inhibition by the DID domains. The association between RhoA and Daam1 was induced by thrombin stimulation in platelets, and RhoA-bound endogenous formins induced actin assembly, which was inhibited by the DID domains of Daam1 and mDia1. Thus, mDia1 and Daam1 are platelet actin assembly factors having distinct efficiencies, and they are directly regulated by Rho GTPases.

Supervised membrane swimming: small G-protein lifeguards regulate PIPK signalling and monitor intracellular PtdIns(4,5)P2 pools

Regulation of PIPK (phosphatidylinositol phosphate kinase) and PtdIns(4,5)P2 signalling by small G-proteins and their effectors is key to many biological functions. Through selective recruitment and activation of different PIPK isoforms, small G-proteins such as Rho, Rac and Cdc42 modulate actin dynamics and cytoskeleton-dependent cellular events in response to extracellular signalling. These activities affect a number of processes, including endocytosis, bacterial penetration into host cells and cytolytic granule-mediated targeted cell killing. Small G-proteins and their modulators are also regulated by phosphoinositides through translocation and conformational changes. Arf family small G-proteins act at multiple sites as regulators of membrane trafficking and actin cytoskeletal remodelling, and regulate a feedback loop comprising phospholipase D, phosphatidic acid, PIPKs and PtdIns(4,5)P2, contributing to enhancement of PtdIns(4,5)P2-mediated cellular events and receptor signalling. Na+, Kir (inwardly rectifying K+), Ca2+ and TRP (transient receptor potential) ion channels are regulated by small G-proteins and membrane pools of PtdIns(4,5)P2. Yeast phosphatidylinositol 4-phosphate 5-kinases Mss4 and Its3 are involved in resistance against disturbance of sphingolipid biosynthesis and maintenance of cell integrity through the synthesis of PtdIns(4,5)P2 and downstream signalling through the Rom2/Rho2 and Rgf1/Rho pathways. Here, we review models for regulated intracellular targeting of PIPKs by small G-proteins and other modulators in response to extracellular signalling. We also describe the spatial and temporal cross-regulation of PIPKs and small G-proteins that is critical for a number of cellular functions.

Staphylococcal enterotoxin B initiates protein kinase C translocation and eicosanoid metabolism while inhibiting thrombin-induced aggregation in human platelets

Staphylococcal enterotoxin (SE) B, a heat-stable toxin secreted by Staphylococcus aureus, has been implicated in the pathogenesis and exacerbation of several critical illnesses. It has been hypothesized that enterotoxins may interact with blood products such as platelets, in addition to T-lymphocytes and renal proximal tubule cells. The aim of this present study was to elucidate whether SEB directly alters human platelet function. Human platelet rich plasma (PRP) was pre-incubated with SEA, SEB, SEC or TSST-1, (at various concentrations and incubation times). After incubation, PRP was exposed to thrombin and aggregation was assessed. Incubation with all toxins tested resulted in decreased aggregation, specifically; exposure to 10mu g/ml of SEB for 30 min caused a 20% decrease and a 49% decrease at 90 min. A similar reduction in aggregation was seen in samples incubated with phorbol myristate acetate, a known stimulator of protein kinase C (PKC). Further, platelets exposed to SEB exhibited an increased plasma membrane PKC activity. Sphingosine, an inhibitor of PKC proved to block the SEB-induced reduction in aggregation. SEB effects on platelet metabolism were investigated using high performance liquid chromatography showing up to a 2-fold increase of active metabolites lipoxin A4 and 12-HETE, as compared to control. These data indicate that SEB is able to induce platelet dysfunction, and these effects may be mediated through activation of PKC.

Rho A participates in the regulation of phosphatidylserine-dependent procoagulant activity at the surface of megakaryocytic cells

Once exposed at the external surface of activated platelets or apoptotic cells, phosphatidylserine, an anionic phospholipid mostly sequestered in the inner leaflet of the plasma membrane, plays essential roles in hemostasis and phagocytosis. The mechanism governing the migration of the phosphatidylserine to the exoplasmic leaflet is not yet fully understood. We have proposed that store-operated calcium entry (SOCE) constitutes a key step of this process. ERK pathway is among the elements modulating SOCE and phosphatidylserine externalization in megakaryocytic HEL cells. Here, we investigated the role of small GTPase Rho A, which may interact with the ERK pathway. Specific inhibitors of Rho A (exoenzyme C3 and toxin B) reduced both SOCE and phosphatidylserine-dependent procoagulant activity. Simultaneous inhibition of Rho A and extracellular signal-regulated kinase (ERK) pathways did not elicit further reduction with respect to each individual one. Rho A can regulate SOCE and phosphatidylserine exposure through the reorganization of actin cytoskeleton, but not through ROCK pathway. Hence, Rho A is another regulatory element for the completion of SOCE-induced phosphatidylserine transmembrane redistribution in HEL cells.

Role of protease-activated receptors in the vascular system

Thrombin is one of the key molecules involved in the development of vascular diseases. Thrombin does not only serve as a coagulation factor, but it also exerts cellular effects by activating protease (proteinase)-activated receptors (PARs), a family of seven-transmembrane G protein-coupled receptors. This study focused on the role of PARs in the vascular system. Among the four members so far identified, PAR-1 and PAR-2 were found to play an important role in the vascular system, while the functional roles of PAR-3 and PAR-4 appear to be mostly limited to platelets. The endothelial cells play a primary role in mediating the vascular effects of PARs under physiological conditions, while PARs of the smooth muscle cells can be induced under pathological conditions, and therefore play a more pathophysiological role. PAR-1 and PAR-2 mediate various vascular effects including regulation of vascular tone, proliferation and hypertrophy of smooth muscle and angiogenesis. Since proteases are activated under pathological conditions such as hemorrhage, tissue damage, and inflammation, PARs are suggested to play a critical role in the development of functional and structural abnormality in the vascular lesion. Understanding the functional role of PARs in the vascular system can thus help in the development of new strategies for the prevention and therapy of vascular diseases.

Serotonylation of small GTPases is a signal transduction pathway that triggers platelet alpha-granule release

Serotonin is a neurotransmitter in the central nervous system. In the periphery, serotonin functions as a ubiquitous hormone involved in vasoconstriction and platelet function. Serotonin is synthesized independently in peripheral tissues and neurons by two different rate-limiting tryptophan hydroxylase (TPH) isoenzymes. Here, we show that mice selectively deficient in peripheral TPH and serotonin exhibit impaired hemostasis, resulting in a reduced risk of thrombosis and thromboembolism, although the ultrastructure of the platelets is not affected. While the aggregation of serotonin-deficient platelets in vitro is apparently normal, their adhesion in vivo is reduced due to a blunted secretion of adhesive alpha-granular proteins. In elucidating the mechanism further, we demonstrate that serotonin is transamidated to small GTPases by transglutaminases during activation and aggregation of platelets, rendering these GTPases constitutively active. Our data provides evidence for a receptor-independent signaling mechanism, termed herein as "serotonylation," which leads to alpha-granule exocytosis from platelets.

Direct demonstration of involvement of protein kinase Calpha in the Ca2+-induced platelet aggregation

Platelets play critical roles in hemostasis and thrombosis through their aggregation following activation of integrin alphaIIbbeta3. However, the molecular mechanism of the integrin activation inside platelets remains largely unknown. Pharmacological experiments have demonstrated that protein kinase C (PKC) plays an important role in platelet aggregation. Because PKC inhibitors can have multiple substrates and given that non-PKC-phorbol ester-binding signaling molecules have been demonstrated to play important roles, the precise involvement of PKC in cellular functions requires re-evaluation. Here, we have established an assay for analyzing the Ca2+-induced aggregation of permeabilized platelets. The aggregation of platelets was inhibited by the addition of the arginine-glycine-aspartate-serine peptide, an integrin-binding peptide inhibitor of alphaIIbbeta3, suggesting that the aggregation was mediated by the integrin. The aggregation was also dependent on exogenous ATP and platelet cytosol, indicating the existence of essential cytosolic factors required for the aggregation. To examine the role of PKC in the aggregation assay, we immunodepleted PKCalpha and beta from the cytosol. The PKC-depleted cytosol lost the aggregation-supporting activity, which was recovered by the addition of purified PKCalpha. Furthermore, the addition of purified PKCalpha in the absence of cytosol did not support the aggregation, whereas the cytosol containing less PKC supported it efficiently, suggesting that additional factors besides PKC would also be required. Thus, we directly demonstrated that PKCalpha is involved in the regulation of Ca2+-induced platelet aggregation.

Rho GTPase/Rho kinase negatively regulates endothelial nitric oxide synthase phosphorylation through the inhibition of protein kinase B/Akt in human endothelial cells

Endothelial nitric oxide synthase (eNOS) is an important regulator of cardiovascular homeostasis by production of nitric oxide (NO) from vascular endothelial cells. It can be activated by protein kinase B (PKB)/Akt via phosphorylation at Ser-1177. We are interested in the role of Rho GTPase/Rho kinase (ROCK) pathway in regulation of eNOS expression and activation. Using adenovirus-mediated gene transfer in human umbilical vein endothelial cells (HUVECs), we show here that both active RhoA and ROCK not only downregulate eNOS gene expression as reported previously but also inhibit eNOS phosphorylation at Ser-1177 and cellular NO production with concomitant suppression of PKB activation. Moreover, coexpression of a constitutive active form of PKB restores the phosphorylation but not gene expression of eNOS in the presence of active RhoA. Furthermore, we show that thrombin inhibits eNOS phosphorylation, as well as expression via Rho/ROCK pathway. Expression of the active PKB reverses eNOS phosphorylation but has no effect on downregulation of eNOS expression induced by thrombin. Taken together, these data demonstrate that Rho/ROCK pathway negatively regulates eNOS phosphorylation through inhibition of PKB, whereas it downregulates eNOS expression independent of PKB.

RhoA sustains integrin alpha IIbbeta 3 adhesion contacts under high shear

The small GTPase RhoA modulates the adhesive nature of many cell types; however, despite high levels of expression in platelets, there is currently limited evidence for an important role for this small GTPase in regulating platelet adhesion processes. In this study, we have examined the role of RhoA in regulating the adhesive function of the major platelet integrin, alpha(IIb)beta(3). Our studies demonstrate that activation of RhoA occurs as a general feature of platelet activation in response to soluble agonists (thrombin, ADP, collagen), immobilized matrices (von Willebrand factor (vWf), fibrinogen) and high shear stress. Blocking the ligand binding function of integrin alpha(IIb)beta(3), by pretreating platelets with c7E3 Fab, demonstrated the existence of integrin alpha(IIb)beta(3)-dependent and -independent mechanisms regulating RhoA activation. Inhibition of RhoA (C3 exoenzyme) or its downstream effector Rho kinase had no effect on integrin alpha(IIb)beta(3) activation induced by soluble agonists or adhesive substrates, however, both inhibitors reduced shear-dependent platelet adhesion on immobilized vWf and shear-induced platelet aggregation in suspension. Detailed analysis of the sequential adhesive steps required for stable platelet adhesion on a vWf matrix under shear conditions revealed that RhoA did not regulate platelet tethering to vWf or the initial formation of integrin alpha(IIb)beta(3) adhesion contacts but played a major role in sustaining stable platelet-matrix interactions. These studies define a critical role for RhoA in regulating the stability of integrin alpha(IIb)beta(3) adhesion contacts under conditions of high shear stress.

Differential regulation of Rho and Rac through heterotrimeric G-proteins and cyclic nucleotides

Platelets were used to study the activation of Rho and Rac through G-protein-coupled receptors and its regulation by cyclic nucleotides. The thromboxane A(2) (TXA(2)) mimetic rapidly activated both small GTPases independently of integrin alpha(IIb)beta(3) activation., which leads to the activation of G(12)/G(13) and G(q) did not induce Rac activation in G alpha(q)-deficient platelets but was able to activate Rho, to stimulate actin polymerization and phosphatidylinositol 4,5-bisphosphate formation, and to induce shape change. Rac activation by in wild-type platelets could be blocked by chelation of intracellular Ca(2+) and was partially sensitive to apyrase and AR-C69931MX, an antagonist of the G(i)-coupled ADP receptor. Cyclic AMP, which completely blocks platelet function, inhibited the -induced activation of G(q) and G(12)/G(13) as well as of Rac and Rho. In contrast, cGMP, which has no effect on platelet shape change blocked only activation of G(q) and Rac. These data demonstrate that Rho and Rac are differentially regulated through heterotrimeric G-proteins. The G(12)/G(13)-mediated Rho activation is involved in the shape change response, whereas Rac is activated through G(q) and is not required for shape change. Cyclic AMP and cGMP differentially interfere with -induced Rho and Rac activation at least in part by selective effects on the regulation of individual G-proteins through the TXA(2) receptor.