Regulation of blood platelet function by the AGC family of protein kinases

Upon vascular injury, platelets aggregate at the site of blood vessel injury to form a hemostatic plug maintaining the physiological integrity of the vascular system. Platelets respond to a variety of extracellular stimuli to undergo a rapid aggregation response, releasing active granule contents and leading to a rapidly growing thrombus. During the adhesion, activation, and aggregation of platelets at an injured site, the endothelium responds by limiting the size and growth of the hemostatic plug or thrombus, or even reversing platelet reactivity. These responses are defined as endothelial thromboregulation. There are three primary (and functionally independent) pathways during the early stages of thromboregulation by which the endothelium controls platelet reactivity (1) nitric oxide (NO); (2) prostacyclin (PGI₂ ); and (3) the ectonucleotidase CD39. NO and PGI2 stimulate signalling cascades that result in the activation of the AGC family of Ser/Thr protein kinases (PKA, PKG and PKC). Once activated these kinase blunt platelet function through the phosphorylation of signalling proteins requested for activation. In this study, the role of AGC family kinases and their signaling cascades in regulating platelet function was assessed. The experimental data produced during this study demonstrate new insights in to the regulation of these kinases in platelets. More specifically it was found that1. Peroxynitrite, a derivative of NO, regulated platelet function and particularly cytoskeletal rearrangement through PKC-dependent phosphorylation of VASPSer²³⁹⁄¹⁵⁷2. NO-mediated signalling in platelets had a requirement for PKC.3. Multiple forms of PKA are present in platelets, which are differentially localised.4. The potential regulation of platelet function by PKA is mediated through Akinase anchoring proteins.5. Lipid rafts may play an important role in platelet regulation by NO and PKG.In summary, this studies present insights of the factors regulating AGC kinases in blood platelets

cGMP signaling inhibits platelet shape change through regulation of the RhoA-Rho Kinase-MLC phosphatase signaling pathway

Background: Platelet shape change, spreading and thrombus stability require activation of the actin cytoskeleton contractile machinery. The mechanisms controlling actin assembly to prevent unwanted platelet activation are unclear. Objectives: We examined the effects of nitric oxide on the signaling pathways regulating platelet actinmyosin activation. Results: S-nitrosoglutathione (GSNO) inhibited thrombin-induced platelet shape change and myosin phosphorylation of the myosin light chain (MLC). Because thrombin stimulates phospho-MLC through the RhoA/ ROCK dependent inhibition of MLC phosphatase (MLCP) we examined the effects of NO on this pathway. Thrombin caused the GTP loading and activation of RhoA, leading to the ROCK-mediated phosphorylation of MLCP on threonine 853 (thr853), which is known to inhibit phosphatase activity. Treatment of platelets with GSNO blocked ROCK-mediated increases in phosphoMLCPthr853 induced by thrombin. This effect was mimicked by the direct activator of protein kinase G, 8-pCPT-PET-cGMP, and blocked by the inhibition of guanylyl cyclase, but not inhibitors of protein kinase A. Further exploration of the mechanism demonstrated that GSNO stimulated the association of RhoA with protein kinase G (PKG) and the inhibitory phosphorylation (serine188) of RhoA in a cGMP-dependent manner. Consistent with these observations, in vitro experiments revealed that recombinant PKG caused direct phosphorylation of RhoA. The inhibition of RhoA by GSNO prevented ROCK-mediated phosphorylation and inhibition of MLCP activity. Conclusions: These data suggest novel crosstalk between the NO-cGMP-PKG and RhoA/ROCK signaling pathways to control platelet actin remodeling

Alterations in Platelet Alpha-Granule Secretion and Adhesion on Collagen under Flow in Mice Lacking the Atypical Rho GTPase RhoBTB3

Typical Rho GTPases, such as Rac1, Cdc42, and RhoA, act as molecular switches regulating various aspects of platelet cytoskeleton reorganization. The loss of these enzymes results in reduced platelet functionality. Atypical Rho GTPases of the RhoBTB subfamily are characterized by divergent domain architecture. One family member, RhoBTB3, is expressed in platelets, but its function is unclear. In the present study we examined the role of RhoBTB3 in platelet function using a knockout mouse model. We found the platelet count, size, numbers of both alpha and dense granules, and surface receptor profile in these mice were comparable to wild-type mice. Deletion of Rhobtb3 had no effect on aggregation and dense granule secretion in response to a range of agonists including thrombin, collagen, and adenosine diphosphate (ADP). By contrast, alpha-granule secretion increased in mice lacking RhoBTB3 in response to thrombin, collagen related peptide (CRP) and U46619/ADP. Integrin activation and spreading on fibrinogen and collagen under static conditions were also unimpaired; however, we observed reduced platelet accrual on collagen under flow conditions. These defects did not translate into alterations in tail bleeding time. We conclude that genetic deletion of Rhobtb3 leads to subtle alterations in alpha-granule secretion and adhesion to collagen without significant effects on hemostasis in vivo

Atherogenic Lipid Stress Induces Platelet Hyperactivity Through CD36-Mediated Hyposensitivity To Prostacyclin-; The Role Of Phosphodiesterase 3A

Prostacyclin (PGI2) controls platelet activation and thrombosis through a cyclic adenosine monophosphate (cAMP) signalling cascade. However, in patients with cardiovascular diseases this protective mechanism fails for reasons that are unclear. Using both pharmacological and genetic approaches we describe a mechanism by which oxidised low density lipoproteins (oxLDL) associated with dyslipidaemia promote platelet activation through impaired PGI2 sensitivity and diminished cAMP signalling. In functional assays using human platelets, oxLDL modulated the inhibitory effects of PGI2, but not a PDE-insensitive cAMP analogue, on platelet aggregation, granule secretion and in vitro thrombosis. Examination of the mechanism revealed that oxLDL promoted the hydrolysis of cAMP through the phosphorylation and activation of phosphodiesterase 3A (PDE3A), leading to diminished cAMP signalling. PDE3A activation by oxLDL required Src family kinases, Syk and protein kinase C. The effects of oxLDL on platelet function and cAMP signalling were blocked by pharmacological inhibition of CD36, mimicked by CD36-specific oxidised phospholipids and ablated in CD36-/- murine platelets. The injection of oxLDL into wild type mice strongly promoted FeCl3 induced carotid thrombosis in vivo, which was prevented by pharmacological inhibition of PDE3A. Furthermore, blood from dyslipidaemic mice was associated with increased oxidative lipid stress, reduced platelet sensitivity to PGI2 ex vivo and diminished PKA signalling. In contrast, platelet sensitivity to a PDE-resistant cAMP analogue remained normal. Genetic deletion of CD36, protected dyslipidaemic animals from PGI2 hyposensitivity and restored PKA signalling. These data suggest that CD36 can translate atherogenic lipid stress into platelet hyperactivity through modulation of inhibitory cAMP signalling.  

Platelet releasate promotes skeletal myogenesis by increasing muscle stem cell commitment to differentiation and accelerates muscle regeneration following acute injury

Aim: The use of platelets as biomaterials has gained intense research interest. However, the mechanisms regarding platelet-mediated skeletal myogenesis remain to be established. The aim of this study was to determine the role of platelet releasate in skeletal myogenesis and muscle stem cell fate in vitro and ex vivo respectively. Methods: We analysed the effect of platelet releasate on proliferation and differentiation of C2C12 myoblasts by means of cell proliferation assays, immunohistochemistry, gene expression and cell bioenergetics. We expanded in vitro findings on single muscle fibres by determining the effect of platelet releasate on murine skeletal muscle stem cells using protein expression profiles for key myogenic regulatory factors. Results: TRAP6 and collagen used for releasate preparation had a more pronounced effect on myoblast proliferation versus thrombin and sonicated platelets (P<0.05). In addition, platelet concentration positively correlated with myoblast proliferation. Platelet releasate increased myoblast and muscle stem cell proliferation in a dose-dependent manner, which was mitigated by VEGFR and PDGFR inhibition. Inhibition of VEGFR and PDGFR ablated MyoD expression on proliferating muscle stem cells, compromising their commitment to differentiation in muscle fibres (P<0.001). Platelet releasate was detrimental for myoblast fusion and affected differentiation of myoblasts in a temporal manner. Most importantly we show that platelet releasate promotes skeletal myogenesis through the PDGF/VEGF-Cyclin D1-MyoD-Scrib-Myogenin axis and accelerates skeletal muscle regeneration after acute injury. Conclusion: This study provides novel mechanistic insights on the role of platelet releasate in skeletal myogenesis and set the physiological basis for exploiting platelets as biomaterials in regenerative medicine

Attenuation of oxidative stress-induced lesions in skeletal muscle in a mouse model of obesity-independent hyperlipidaemia and atherosclerosis through the inhibition of Nox2 activity

Obesity leading to hyperlipidaemia and atherosclerosis is recognised to induce morphological and metabolic changes in many tissues. However, both hyperlipidaemia and atherosclerosis can occur in the absence of obesity. The impact of the latter scenario on skeletal muscle and liver is not understood sufficiently. In this regard, we used the Apolipoprotein E-deficient (ApoE-/-) mouse model, an established model of hyperlipidaemia and atherosclerosis, that does not become obese when subjected to a high-fat diet, to determine the impact of Western-type diet (WD) and ApoE deficiency on skeletal muscle morphological, metabolic and biochemical properties. To establish the potential of therapeutic targets, we further examined the impact of Nox2 pharmacological inhibition on skeletal muscle redox biology. We found ectopic lipid accumulation in skeletal muscle and the liver, and altered skeletal muscle morphology and intramuscular triacylglycerol fatty acid composition. WD and ApoE deficiency had a detrimental impact in muscle metabolome, followed by perturbed gene expression for fatty acid uptake and oxidation. Importantly, there was enhanced oxidative stress in the skeletal muscle and development of liver steatosis, inflammation and oxidative protein modifications. Pharmacological inhibition of Nox2 decreased reactive oxygen species production and protein oxidative modifications in the muscle of ApoE-/- mice subjected to a Western-type diet. This study provides key evidence to better understand the pathophysiology of skeletal muscle in the context of hyperlipidaemia and atherosclerosis and identifies Nox2 as a potential target for attenuating oxidative stress in skeletal muscle in a mouse model of obesity-independent hyperlipidaemia

Platelet function following induced hypoglycaemia in type 2 diabetes

Aim: Strict glycaemic control has been associated with an increased mortality rate in subjects with type 2 diabetes (T2DM). Here we examined platelet function immediately and 24 hours following induced hypoglycaemia in people with type 2 diabetes compared to healthy age-matched controls. Methods: Hyperinsulinaemic clamps reduced blood glucose to 2.8 mmol/L (50 mg/dl) for 1 hour. Sampling at baseline; euglycaemia 5 mmol/L (90 mg/dl); hypoglycaemia; and at 24 post clamp were undertaken. Platelet function was measured by whole blood flow cytometry. Results: 10 subjects with T2DM and 8 controls were recruited. Platelets from people with T2DM showed reduced sensitivity to prostacyclin (PGI2, 1 nM) following hypoglycaemia. The ability of PGI2 to inhibit platelet activation was significantly impaired at 24 hours compared to baseline in the T2DM group. Here, inhibition of fibrinogen binding was 29.5% (10.3–43.8) compared to 50.8% (36.8–61.1), (P < 0.05), while inhibition of P-selectin expression was 32% (16.1–47.6) vs. 54.4% (42.5–67.5) (P < 0.05). No significant changes in platelet function were noted in controls. Conclusion: Induced hypoglycaemia in T2DM enhances platelet hyperactivity through impaired sensitivity to prostacyclin at 24 hours

Prostacyclin reverses platelet stress fibre formation causing platelet aggregate instability

Prostacyclin (PGI2) modulates platelet activation to regulate haemostasis. Evidence has emerged to suggest that thrombi are dynamic structures with distinct areas of differing platelet activation. It was hypothesised that PGI2 could reverse platelet spreading by actin cytoskeletal modulation, leading to reduced capability of platelet aggregates to withstand a high shear environment. Our data demonstrates that post-flow of PGI2 over activated and spread platelets on fibrinogen, identified a significant reduction in platelet surface area under high shear. Exploration of the molecular mechanisms underpinning this effect revealed that PGI2 reversed stress fibre formation in adherent platelets, reduced platelet spreading, whilst simultaneously promoting actin nodule formation. The effects of PGI2 on stress fibres were mimicked by the adenylyl cyclase activator forskolin and prevented by inhibitors of protein kinase A (PKA). Stress fibre formation is a RhoA dependent process and we found that treatment of adherent platelets with PGI2 caused inhibitory phosphorylation of RhoA, reduced RhoA GTP-loading and reversal of myosin light chain phosphorylation. Phospho-RhoA was localised in actin nodules with PKA type II and a number of other phosphorylated PKA substrates. This study demonstrates that PGI2 can reverse key platelet functions after their initial activation and identifies a novel mechanism for controlling thrombosis

Reversal of stress fibre formation by Nitric Oxide mediated RhoA inhibition leads to reduction in the height of preformed thrombi

Evidence has emerged to suggest that thrombi are dynamic structures with distinct areas of differing platelet activation and inhibition. We hypothesised that Nitric oxide (NO), a platelet inhibitor, can modulate the actin cytoskeleton reversing platelet spreading, and therefore reduce the capability of thrombi to withstand a high shear environment. Our data demonstrates that GSNO, DEANONOate, and a PKG-activating cGMP analogue reversed stress fibre formation and increased actin nodule formation in adherent platelets. This effect is sGC dependent and independent of ADP and thromboxanes. Stress fibre formation is a RhoA dependent process and NO induced RhoA inhibition, however, it did not phosphorylate RhoA at ser188 in spread platelets. Interestingly NO and PGI2 synergise to reverse stress fibre formation at physiologically relevant concentrations. Analysis of high shear conditions indicated that platelets activated on fibrinogen, induced stress fibre formation, which was reversed by GSNO treatment. Furthermore, preformed thrombi on collagen post perfused with GSNO had a 30% reduction in thrombus height in comparison to the control. This study demonstrates that NO can reverse key platelet functions after their initial activation and identifies a novel mechanism for controlling excessive thrombosis

Regulation of blood platelet function by the AGC family of protein kinases

Upon vascular injury, platelets aggregate at the site of blood vessel injury to form a hemostatic plug maintaining the physiological integrity of the vascular system. Platelets respond to a variety of extracellular stimuli to undergo a rapid aggregation response, releasing active granule contents and leading to a rapidly growing thrombus. During the adhesion, activation, and aggregation of platelets at an injured site, the endothelium responds by limiting the size and growth of the hemostatic plug or thrombus, or even reversing platelet reactivity. These responses are defined as endothelial thromboregulation. There are three primary (and functionally independent) pathways during the early stages of thromboregulation by which the endothelium controls platelet reactivity (1) nitric oxide (NO); (2) prostacyclin (PGI₂ ); and (3) the ectonucleotidase CD39. NO and PGI2 stimulate signalling cascades that result in the activation of the AGC family of Ser/Thr protein kinases (PKA, PKG and PKC). Once activated these kinase blunt platelet function through the phosphorylation of signalling proteins requested for activation. In this study, the role of AGC family kinases and their signaling cascades in regulating platelet function was assessed. The experimental data produced during this study demonstrate new insights in to the regulation of these kinases in platelets. More specifically it was found that 1. Peroxynitrite, a derivative of NO, regulated platelet function and particularly cytoskeletal rearrangement through PKC-dependent phosphorylation of VASPSer²³⁹⁄¹⁵⁷ 2. NO-mediated signalling in platelets had a requirement for PKC. 3. Multiple forms of PKA are present in platelets, which are differentially localised. 4. The potential regulation of platelet function by PKA is mediated through Akinase anchoring proteins. 5. Lipid rafts may play an important role in platelet regulation by NO and PKG. In summary, this studies present insights of the factors regulating AGC kinases in blood platelets