Overview of phosphoinositide signaling in platelets. (A) PIP5KI-α and PIP5KI-β synthesize the pool of PtdIns(4,5)P₂ that is hydrolyzed by PLC-β (activated by thrombin and TxA2) or PLC-γ (activated by collagen) into the second messengers Ins(1,4,5)P₃ and DAG. The Ins(1,4,5)P₃ diffuses through the cytoplasm, binds to the Ins(1,4,5)P₃ receptors on the DTS, thereby increasing the cytosolic concentration of Ca²⁺, which in turn activates multiple effector proteins. DAG is a second messenger that recruits to the plasma membrane protein kinase C (PKC). In platelets, membrane-bound PKC plays a crucial role in the secretion of granules. Both Ca²⁺ and DAG activate calcium- and diacylglycerol-regulated guanine nucleotide exchange factor (CalDAG-GEF), which then can activate a small GTPase, Rap1b. This enables Rap1b to activate the integrin αIIbβ3, a receptor that is crucial for platelet aggregation. In contrast to both PIP5KI-α and PIP5KI-β, PIP5KI-γ directly binds to talin, which helps link the integrins on the cell membrane to the underlying cytoskeleton. (B) Collagen binding to its receptor, GPVI, results in the phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) within the FcRγ chains to enable the binding of the SH2 domains within PI3K-α, PI3K-β, and PI3K-δ. On the other hand, ADP binding to the Gi-coupled P2Y₁₂ receptor triggers the release of Gβγ from the Gα subunit. Gβγ can then stimulate PI3K-γ and PI3K-β. All of the isoforms of PI3K are capable of synthesizing PtdIns(3,4,5)P₃ by phosphorylating PtdIns(4,5)P₂. PtdIns(3,4,5)P₃ can bind to and activate several effector proteins including Akt. In turn, Akt activates αIIbβ₃-mediated platelet aggregation.