Individual phosphatidylinositol transfer proteins have distinct functions that do not involve lipid transfer activity

Platelets use signal transduction pathways facilitated by class I phosphatidylinositol transfer proteins (PITPs). The 2 mammalian class I PITPs, PITPa and PITP beta, are single PITP domain soluble proteins that are encoded by different genes and share 77% sequence identity, although their individual roles in mammalian biology remain uncharacterized. These proteins are believed to shuttle phosphatidylinositol and phosphatidylcholine between separate intracellular membrane compartments, thereby regulating phosphoinositide synthesis and second messenger formation. Previously, we observed that platelet-specific deletion of PITPa, the predominantly expressed murine PITP isoform, had no effect on hemostasis but impaired tumor metastasis formation and disrupted phosphoinositide signaling. Here, we found that mice lacking the less expressed PITP beta in their platelets exhibited a similar phenotype. However, in contrast to PITPa-null platelet lysates, which have impaired lipid transfer activity, PITP beta-null platelet lysates have essentially normal lipid transfer activity, although both isoforms contribute to phosphoinositide synthesis in vitro. Moreover, we found that platelet-specific deletion of both PITPs led to ex vivo platelet aggregation/secretion and spreading defects, impaired tail bleeding, and profound tumor dissemination. Our study also demonstrated that PITP isoforms are required to maintain endogenous phosphoinositide PtdInsP2 levels and agonist-stimulated second messenger formation. The data shown here demonstrate that the 2 isoforms are functionally overlapping and that a single isoform is able to maintain the homeostasis of platelets. However, both class I PITP isoforms contribute to phosphoinositide signaling in platelets through distinct biochemical mechanisms or different subcellular domains.

Automated summary

Platelets rely on phosphatidylinositol transfer proteins (PITPs) for signal transduction pathways, with two isoforms PITPa and PITP beta playing overlapping yet distinct roles in platelet biology. Deletion of either isoform impairs tumor metastasis formation and disrupts phosphoinositide signaling, but their lipid transfer activity differs. Furthermore, platelet-specific deletion of both isoforms leads to platelet dysfunction, impaired bleeding, and enhanced tumor dissemination. These findings highlight the importance of PITP isoforms in maintaining platelet homeostasis and phosphoinositide signaling through different mechanisms.