Platelets are central players in atherothrombosis development in coronary artery disease. The PKC family provides important intracellular mechanisms for regulating platelet activity, and platelets express several members of this family, including the classical isoforms PKCα and PKCβ and novel isoforms PKCδ and PKCθ. Here, we used a genetic approach to definitively demonstrate the role played by PKCα in regulating thrombus formation and platelet function. Thrombus formation in vivo was attenuated in Prkca -/-mice, and PKCα was required for thrombus formation in vitro, although this PKC isoform did not regulate platelet adhesion to collagen. The ablation of in vitro thrombus formation in Prkca -/-platelets was rescued by the addition of ADP, consistent with the key mechanistic finding that dense-granule biogenesis and secretion depend upon PKCα expression. Furthermore, defective platelet aggregation in response to either collagen-related peptide or thrombin could be overcome by an increase in agonist concentration. Evidence of overt bleeding, including gastrointestinal and tail bleeding, was not seen in Prkca -/-mice. In summary, the effects of PKCα ablation on thrombus formation and granule secretion may implicate PKCα as a drug target for antithrombotic therapy.

To cite this article: Harper MT, Poole AW. Diverse functions of protein kinase C isoforms in platelet activation and thrombus formation. J Thromb Haemost 2010; 8: 454-62.Summary. Platelet activation is a complex balance of positive and negative signaling pathways. The protein kinase C (PKC) family is a major regulator of platelet granule secretion, integrin activation, aggregation, spreading and procoagulant activity. As broad-spectrum PKC inhibitors reduce secretion and aggregation, the PKC family is generally considered to be a positive regulator of platelet activation. However, the individual members of the PKC family that are expressed in platelets are regulated in different ways, and an increasing body of evidence indicates that they have distinct, and often opposing, roles. Many of the recent advances in understanding the contributions of individual PKC isoforms have come from mouse gene knockout studies. PKCa, a classic isoform, is an essential positive regulator of granule secretion and thrombus formation, both in vitro and in vivo. Mice lacking PKCa show much reduced thrombus formation in vivo but do not have a bleeding defect, suggesting that PKCa could be an attractive antithrombotic target. Important, apparently non-redundant, roles, both positive and negative, for the novel PKC isoforms d, h and e in granule secretion have also been proposed, indicating highly complex regulation of this essential process. Similarly, PKCb, PKCd and PKCh have non-redundant roles in platelet spreading, as absence of either PKCb or PKCh reduces spreading, whereas PKCd negatively regulates filopodial formation. This negative signaling by PKCd may reduce platelet aggregation and so restrict thrombus formation. In this review, we discuss the current understanding of the regulation and functions of individual PKC isoforms in platelet activation and thrombus formation.

Arterial thrombosis, a major cause of myocardial infarction and stroke, is initiated by activation of blood platelets by subendothelial collagen. The protein kinase C (PKC) family centrally regulates platelet activation, and it is becoming clear that the individual PKC isoforms play distinct roles, some of which oppose each other. Here, for the first time, we address all four of the major platelet-expressed PKC isoforms, determining their comparative roles in regulating platelet adhesion to collagen and their subsequent activation under physiological flow conditions. Using mouse gene knock-out and pharmacological approaches in human platelets, we show that collagen-dependent α-granule secretion and thrombus formation are mediated by the conventional PKC isoforms, PKCα and PKCβ, whereas the novel isoform, PKCθ, negatively regulates these events. PKCδ also negatively regulates thrombus formation but not α-granule secretion. In addition, we demonstrate for the first time that individual PKC isoforms differentially regulate platelet calcium signaling and exposure of phosphatidylserine under flow. Although platelet deficient in PKCα or PKCβ showed reduced calcium signaling and phosphatidylserine exposure, these responses were enhanced in the absence of PKCθ. In summary therefore, this direct comparison between individual subtypes of PKC, by standardized methodology under flow conditions, reveals that the four major PKCs expressed in platelets play distinct non-redundant roles, where conventional PKCs promote and novel PKCs inhibit thrombus formation on collagen.

Blood platelet aggregation must be tightly controlled to promote clotting at injury sites but avoid inappropriate occlusion of blood vessels. Thrombin, which cleaves and activates Gq-coupled protease-activated receptors, and collagen-related peptide, which activates the receptor glycoprotein VI, stimulate platelets to aggregate and form thrombi. Coincident activation by these two agonists synergizes, causing the exposure of phosphatidylserine on the cell surface, which is a marker of cell death in many cell types. Phosphatidylserine exposure is also essential to produce additional thrombin on platelet surfaces, which contributes to thrombosis. We found that activation of either thrombin receptors or glycoprotein VI alone produced a calcium signal that was largely dependent only on store-operated Ca(2+) entry. In contrast, experiments with platelets from knockout mice showed that the presence of both ligands activated nonselective cation channels of the transient receptor potential C (TRPC) family, TRPC3 and TRPC6. These channels principally allowed entry of Na(+), which coupled to reverse-mode Na(+)/Ca(2+) exchange to allow calcium influx and thereby contribute to Ca(2+) signaling and phosphatidylserine exposure. Thus, TRPC channels act as coincidence detectors to coordinate responses to multiple signals in cells, thereby indirectly mediating in platelets an increase in intracellular calcium concentrations and exposure of prothrombotic phosphatidylserine.

Wnts regulate important intracellular signaling events, and dysregulation of the Wnt pathway has been linked to human disease. Here, we uncover numerous Wnt canonical effectors in human platelets where Wnts, their receptors, and downstream signaling components have not been previously described. We demonstrate that the Wnt3a ligand inhibits platelet adhesion, activation, dense granule secretion, and aggregation. Wnt3a also altered platelet shape change and inhibited the activation of the small GTPase RhoA. In addition, we found the Wnt-␤-catenin signaling pathway to be functional in platelets. Finally, disruption of the Wnt Frizzled 6 receptor in the mouse resulted in a hyperactivatory platelet phenotype and a reduced sensitivity to Wnt3a. Taken together our studies reveal a novel functional role for Wnt signaling in regulating anucleate platelet function and may provide a tractable target for future antiplatelet therapy.Wnt-␤-catenin pathway ͉ integrin ␣IIb␤3 ͉ frizzled 6 knockout mice A nucleate platelets are the principle effectors of haemostasis and are found circulating in a nonadhesive, quiescent state. At sites of vascular damage, platelets adhere to various exposed subendothelial matrix proteins and are activated, converting from a resting, discoid shape into larger, flattened structures with extended pseudopodia (1). Such activated platelets secrete and synthesize further agonists, inflammatory mediators, and vasoactive substances and through conformational changes in their major integrin receptor, ␣IIb␤3, aggregate to other platelets via fibrinogen (Fb) to form a haemostatic plug (2). Aberrant platelet activation can cause pathological thrombus formation, leading to thrombosis and ultimately vessel occlusion and tissue ischemia, the processes underlying myocardial infarction and stroke. Understanding the regulation of platelet activity is thus fundamental to comprehending thrombotic disorders and developing therapeutic strategies.The mammalian Wnt gene family is comprised of 19 secreted Wnt glycoproteins, which play essential roles in cell proliferation, cell-fate determination, and cell-fate differentiation during embryonic development and adult homeostasis (3, 4). These Wnt ligands activate a number of different signaling pathways via distinct receptors and downstream effectors to mediate effects on gene transcription and cell adhesion/migration (5, 6). For the Wnt-␤-catenin (␤-cat) signaling pathway (Fig. 1A), traditionally referred to as the ''canonical'' pathway, Wnts bind to a surface receptor complex comprised of a Frizzled (Fzd) receptor and the Lipoprotein Receptor-related Protein 5/6 (LRP5/6) coreceptor (5, 7). The signal is then transduced to the cytoplasmic protein Dishevelled (Dvl) where downstream pathways regulate the stability of ␤-cat (5, 7). In the absence of Wnt, ␤-cat is phosphorylated by a destruction complex containing Casein Kinase 1 (CK1), Glycogen Synthase Kinase 3␤ (GSK3␤), Axin-1, FRAT-1, and Adenomatous Polyposis Coli (APC), which targets ␤-cat for degradation via ubiquitina...