Platelet signaling

Abstract

This chapter summarizes current ideas about the intracellular signaling that drives platelet responses to vascular injury. After a brief overview of platelet activation intended to place the signaling pathways into context, the first section considers the early events of platelet activation leading up to integrin activation and platelet aggregation. The focus is on the G protein-mediated events utilized by agonists such as thrombin and ADP, and the tyrosine kinase-based signaling triggered by collagen. The second section considers the events that occur after integrin engagement, some of which are dependent on close physical contact between platelets. A third section addresses the regulatory events that help to avoid unprovoked or excessive platelet activation, after which the final section briefly considers individual variations in platelet reactivity and the role of platelet signaling in the innate immune response and embryonic development.

Fig. 1

An overview of some of the pathways that support platelet activation. Targets for antiplatelet agents that are currently in clinical use or in clinical trials are indicated in blue. PLC, phospholipase C; PKC, protein kinase C; IP₃, inositol-1,4,5-trisphosphate; TxA₂, thromboxane A₂; GP, glycoprotein; IP and TP, PGI₂ and TxA₂ receptors

Fig. 2

G_q signaling in platelets. Agonists whose receptors are coupled to G_q are able to activate PLCβ via G_qα. The potency with which the activation occurs varies with the agonist, with thrombin and TxA₂ providing a stronger stimulus for PLCβ-mediated phosphoinositide hydrolysis than ADP. Thrombin activates two G_q-coupled receptors on human platelets, PAR1 and PAR4, which differ somewhat in the kinetics of PLC activation. Abbreviations: AA, arachidonic acid; ADP, adenosine diphosphate; COX-1, cyclooxygenase 1; DAG, diacylglycerol; GDP, guanosine 5′-diphosphate; GTP, guanosine 5′-triphosphate; IP3 R, IP₃ receptor; MLCK, myosin light chain kinase; PI3K, phosphatidylinositol 3-kinase; PAK, p21-activated kinase; PAR, protease-activated receptor; PG, prostaglandin; PIP₂, phosphatidylinositol-4,5-bisphosphate; PKC, protein kinase C; PLA₂, phospholipase A₂; PLCβ, phospholipase Cβ; TxA₂, thromboxane A₂

Fig. 3

G_i signaling in platelets. G_i2 is the predominant G_i family member expressed in human platelets. In addition to inhibiting adenylyl cyclase (alleviating the repressive effects of cAMP), G_i2 couples P2Y₁₂ ADP receptors to PI 3-kinase, Akt phosphorylation and Rap1B activation. Other effectors may exist as well. Abbreviations: AC, adenylyl cyclase; ADP, adenosine diphosphate; ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; DAG, diacylglycerol; PI3K, phosphatidylinositol 3-kinase; PLA₂, phospholipase A₂; PLCβ, phospholipase Cβ; GDP, guanosine 5′-diphosphate; GTP, guanosine 5′-triphosphate; PIP₂, phosphatidylinositol-4,5-bisphosphate; PKC, protein kinase C

Fig. 4

G_12/13 signaling in platelets. Agonists whose receptors are coupled to the G₁₂ family members expressed in platelets are able to trigger shape change, in part by Rho-dependent activation of kinases that include the Rho-activated kinase, p160 ROCK, and the downstream kinases, MLCK and LIM-K. Although G₁₂ and G₁₃ are both expressed, based on knockout studies, G₁₃ is the dominant G₁₂ family member in mouse platelets. Y27632 inhibits p160 ROCK. Abbreviations: GDP, guanosine 5′-diphosphate; GTP, guanosine 5′-triphosphate; IP3 R, receptor for 1,4,5-IP₃; MLCK, myosin light chain kinase; PAR, protease-activated receptor; TxA₂, thromboxane A₂

Fig. 5

Platelet activation by collagen. Platelets use several different molecular complexes to support platelet activation by collagen. These include (1) VWF-mediated binding of collagen to the GPIb-IX-V complex and integrin α_IIbβ₃, (2) a direct interaction between collagen and both the integrin α₂β₁ and the GP VI/FcRγ-chain complex. Clustering of GP VI results in the phosphorylation of tyrosine residues in the FcRγ cytoplasmic domain, followed by the binding and activation of the tyrosine kinase, Syk. One consequence of Syk activation is the phosphorylation and activation of phospholipase Cγ, leading to phosphoinositide hydrolysis, secretion of ADP and the production and release of TxA₂

Fig. 6

Regulatory events that impact platelet activation. Although platelets are primed to respond rapidly to injury, a number of regulatory events have been described that can limit the rate and/or extent of the response. Examples shown in the figure include (1) PECAM and CEACAM, which reduce Syk activation downstream of the collagen receptor, GP VI, by recruiting the tyrosine phosphatase, SHP2, (2) RGS10 and RGS18 which shorten the duration of G protein-dependent signaling in platelets and (3) PGI₂, which stimulates cAMP formation in platelets and dampens platelet responsiveness via protein kinase A (PKA)