Loss of PIP5KIβ demonstrates that PIP5KI isoform-specific PIP2 synthesis is required for IP3 formation

The three isoforms of PIP5KI (alpha, beta, and gamma) synthesize PI4,5P(2) (PIP2) by phosphorylating PI4P. Therefore, it is not clear why platelets, like all eukaryotic cells, have more than one isoform. To test the hypothesis that PIP5KI isoforms have nonoverlapping functions, we generated a murine line containing a null mutation of PIP5KI beta and analyzed the effect on platelet signaling. PIP5KI beta-null mice had normal platelet counts. In contrast to platelets lacking PIP5KI alpha, platelets lacking PIP5KI beta exhibited impaired aggregation accompanied by disaggregation. Although platelets lacking PIP5KI beta had only a moderate deficiency of PIP2 under basal conditions, they had a striking deficiency in PIP2 synthesis and IP3 formation after thrombin stimulation. We have also observed that platelets lacking both PIP5KI alpha and PIP5KI beta have a complete loss of thrombin-induced IP3 synthesis even though they still contain PIP5KI gamma, the predominant PIP5KI isoform in platelets. These results demonstrate that PIP5KI beta, like PIP5KI alpha, contributes to the rapid synthesis of a pool of PIP2 that is required for second-messenger formation, whereas the pool of PIP2 synthesized by PIP5KI gamma does not contribute to this process. Additionally, we found that PIP5KI beta-null platelets failed to form arterial thrombi properly in vivo. Together, these data demonstrate that PIP5KI beta is required for rapid PIP2 synthesis, second-messenger production, and stable platelet adhesion under shear in vivo. These results also demonstrate that after stimulation of a G protein-coupled receptor, IP3 is completely derived from a rapidly synthesized discrete pool Of PIP2 synthesized by PIP5KI alpha and PIP5KI beta.