Lyn and PKCδ order SHIP1 embargo

Upon vascular injury, platelets adhere to collagen and release dense granule content to recruit more circulating platelets to an injured site. This process is initiated by signaling through glycoprotein VI (GPVI), a major platelet collagen receptor, leading to intracellular calcium mobilization and the activation of protein kinase C (PKC) isoforms. Of the 7 PKC isoforms expressed in platelets, PKCδ differentially regulates dense granule release in platelets; it positively regulates dense granule release by thrombin and negatively regulates it by collagen.¹ 

Chari et al provide convincing evidence regarding the negative regulation of dense granule release downstream of GPVI signaling and reveal a new association of PKCδ with Lyn and SHIP1.¹  Lyn is an important member of the Src tyrosine kinase family that functions downstream of GPVI, whereas SHIP1 is a lipid phosphatase that regulates PIP3 levels in platelets.²  SHIP1 associates with members of the Src family of protein kinases and is phosphorylated on Y1020, leading to its activation.³ 

Activation of GPVI results in activation of Syk, which activates LAT, leading to the generation of diacylglycerol (DAG) and IP3 through activation of PLCγ. IP3 induces release of Ca²⁺ from intracellular stores whereas DAG activates several isoforms of PKC in a Ca²⁺-dependent manner, leading to dense granule release.⁴ 

Using both pharmacologic agents and PKCδ-null murine platelets, it has been previously shown that PKCδ, which is also activated by DAG, exerts a negative effect on dense granule release induced by collagen.^5,–7  Here, Chari et al show that PKCδ selectively associates with tyrosine-phosphorylated SHIP1 downstream of GPVI, but not with thrombin receptors.¹  This association is absent in Lyn knockout murine platelets, suggesting a role for Lyn-mediated tyrosine phosphorylation of SHIP1. Furthermore, SHIP1 tyrosine phosphorylation is diminished in PKCδ-null mice, suggesting a role for PKCδ in bringing Lyn and SHIP1 together. Most interestingly, when this ternary interaction among Lyn, SHIP1, and PKCδ fails due to the absence of any one of the members, collagen-induced dense granule release is potentiated. Thus, the tight association through phosphorylation events among Lyn, SHIP1, and PKCδ puts an embargo on dense granule release induced through GPVI signaling (outlined in the figure). As SHIP1 is known to reduce levels of PIP3, an important enhancer of platelet functions, it is possible that the negative regulation is achieved by hydrolysis of PIP3 by SHIP1 that is associated with and possibly regulated by PKCδ.

Future studies should answer some of these intriguing questions: Does PKCδ, a serine threonine kinase, phosphorylate the serine residues of SHIP1? Does the phosphorylation of serine residues of SHIP1 affect its catalytic activity? Does Lyn directly phosphorylate SHIP1 or is some other tyrosine kinase involved in this process? Does PKCδ serve as an adapter protein to bring Lyn and SHIP1 together? What is the mechanism of the association of PKCδ with SHIP1? Why does SHIP1 only associate with PKCδ downstream of GPVI? Are there other targets of PKCδ that regulate dense granule release? Thus, using a combination of biochemical, pharmacologic, and genetic approaches, Chari et al provide an intriguing avenue for further studies that will enhance our knowledge about platelet dense granule release. These insights may also aid in design and development of novel therapeutic agents for the treatment of thrombotic disorders.