Platelets play a central role in mediating atherothrombosis, characterized by an unpredictable atherosclerotic plaque disruption, and are therefore the target of numerous therapies aimed at reducing their activity, particularly in the prevention of coronary artery thrombosis in heart attacks 
. PKC has been established, largely by pharmacological studies, as a major regulator of multiple platelet activities 
, and it is increasingly clear that the different isozymes of PKC expressed in platelets perform distinct functions.
On activation, platelets release a multitude of active substances from secretory granules, essential for platelet pro-aggregatory responses and development of a stable thrombus in arteries. Yoshioka et al.
have analyzed the mechanisms governing Ca2+
-induced secretion of α-granules and dense-core granules in permeabilized human platelets, and identified PKCα as an essential component of the mechanism. Using similar biochemical approaches, the same group demonstrated that PKCα is also involved in the regulation of Ca2+-
induced platelet aggregation 
. Pula et al.
have shown that two tyrosine kinases, Syk and Src, physically interact with PKCα, leading to distinct functional consequences. Although they found that PKCα activity was dependent on Syk, Syk activity was not regulated by PKCα; however, Src activity was negatively regulated by PKCα. These results suggest that PKCα is an important factor in the complex interaction with tyrosine kinases for regulation of functional activities in platelets. Additionally, in a recent elegant study reconstructing the signalling pathway regulating platelet integrin αIIb
in a heterologous cell system, it was shown that PKCα expression is required for activation of the integrin through the Rap1 pathway 
. Key molecular functions of platelet activation are therefore mediated by PKCα.
On the basis of pharmacological data, the specificity of the role played by PKCα has been a contentious issue, due to the lack of selectivity of the reagents available. We have therefore developed a genetic approach using PKCα knockout (Prkca−/−
) mice to determine the role of PKCα in regulating platelet function and thrombus formation 
. With regard to the suggested role of PKCα in regulating secretion of dense- and α-granules 
, we were able to confirm definitively that secretion of these granules was substantially diminished in Prkca−/−
platelets showed significantly reduced functionally relevant phosphorylation on Ser95 of synaptosomal associated protein SNAP-23, an essential component of the membrane fusion machinery and an important regulator of vesicle docking and fusion 
. This observation therefore may potentially explain the observed secretion defect in PKCα-deficient platelets.
In addition, Prkca−/− platelets show a marked knockdown in activation of integrin αIIbβ3 in response either to thrombin or collagen-related peptide, paralleling a deficit in their ability to undergo aggregation at submaximal concentrations of agonist. This is also consistent with the impaired ability of Prkca−/− platelets to form a thrombus in vitro in blood flowing over a collagen-coated surface and in an in vivo laser-induced model of thrombus formation (). The defect in secretion was shown, however, to be the central significant event, because addition of exogenous ADP (adenosine diphosphate), the major constituent released from platelet dense granules, rescued the deficits in responses seen in Prkca−/− platelets, confirming that granule secretion is likely to be the primary function for PKCα.
Figure 2 PKCα plays a critical role regulating platelet function and thrombosis in arteries. Platelets are the smallest cellular component of the blood, and flow close to the endothelial lining of arteries continually surveying the vessel for breaches (more ...)
These findings, together with the fact that Prkca−/− mice do not demonstrate any evidence of overt bleeding, have revealed that PKCα is a potential drug target for antithrombotic therapy, because selective inhibitors would be expected to exert a major effect on thrombus formation while sparing primary platelet adhesive functions.