1. Regulation of platelet plug formation by phosphoinositide metabolism
by Sang H. Min, and Charles S. Abrams
Blood
Volume 122(8):
August 22, 2013
©2013 by American Society of Hematology

2. Metabolism of phosphoinositides by phosphoinositide-metabolizing enzymes.
Metabolism of phosphoinositides by phosphoinositide-metabolizing enzymes. Shown is the relationship between different phosphoinositides and their metabolizing lipid kinases (red arrows), lipid phosphatases (blue arrows), and PLC (green arrows). In this review, we focus on the signal transduction mediated by the lipid kinases PIP5KI and PI3K, and the PLC in platelets. PIP5KI, phosphatidylinositol-4-phosphate-5-kinase type I; PI3K, phosphatidylinositol-3-kinases; PLC, phospholipase C.
Sang H. Min, and Charles S. Abrams Blood 2013;122:
©2013 by American Society of Hematology

3. Isoform-specific functional roles of PIP5KI in megakaryocytes and platelets.
Isoform-specific functional roles of PIP5KI in megakaryocytes and platelets. (A) PIP5KI-α/β double knockout platelets have impaired generation of the second messenger Ins(1,4,5)P3 following activation with the platelet agonist, thrombin. (B) In contrast, PIP5KI-γ knockout platelets produce normal amounts of Ins(1,4,5)P3 after thrombin stimulation. (C) However, megakaryocytes differentiated from progenitor cells of PIP5KI-γ knockout mice display defective anchoring of their cellular membrane to their underlying cytoskeleton. This is shown in the confocal images of megakaryocytes bound to immobilized fibrinogen and stained with green fluorescence protein (GFP) fused to the PLC-δ pleckstrin homology domain. (D) Schematic cartoon illustrating defective plasma membrane attachment to the cytoskeleton in the PIP5KI-γ knockout megakaryocytes compared with the wild-type megakaryocytes when analyzed by an optical trap (laser tweezers).
Sang H. Min, and Charles S. Abrams Blood 2013;122:
©2013 by American Society of Hematology

4. Overview of phosphoinositide signaling in platelets.
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.
Sang H. Min, and Charles S. Abrams Blood 2013;122:
©2013 by American Society of Hematology