In-depth PtdIns(3,4,5)P 3 signalosome analysis identifies DAPP1 as a negative regulator of GPVI-driven platelet function

The class I phosphoinositide 3-kinase (PI3K) isoforms play important roles in platelet priming, activation, and stable thrombus formation. Class I PI3Ks predominantly regulate cell function through their catalytic product, the signaling phospholipid phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P 3 ], which coordinates the localization and/or activity of a diverse range of binding proteins. Notably, the complete repertoire of these class I PI3K effectors in platelets remains unknown, limiting mechanistic understanding of class I PI3K–mediated control of platelet function. We measured robust agonist-driven PtdIns(3,4,5)P 3 generation in human platelets by lipidomic mass spectrometry (MS), and then used affinity-capture coupled to high-resolution proteomic MS to identify the targets of PtdIns(3,4,5)P 3 in these cells. We reveal for the first time a diverse platelet PtdIns(3,4,5)P 3 interactome, including kinases, signaling adaptors, and regulators of small GTPases, many of which are previously uncharacterized in this cell type. Of these, we show dual adaptor for phosphotyrosine and 3-phosphoinositides (DAPP1) to be regulated by Src-family kinases and PI3K, while platelets from DAPP1-deficient mice display enhanced thrombus formation on collagen in vitro. This was associated with enhanced platelet α/δ granule secretion and α IIb β 3 integrin activation downstream of the collagen receptor glycoprotein VI. Thus, we present the first comprehensive analysis of the PtdIns(3,4,5)P 3 signalosome of human platelets and identify DAPP1 as a novel negative regulator of platelet function. This work provides important new insights into how class I PI3Ks shape platelet function.