Blood coagulation - GPCRs in platelet aggregation

GPCRs in platelet aggregation

Platelets circulate in the blood and adhere at places of vascular injury to the vessel wall resulted in the formation of platelet plug that is responsible for primary hemostasis. Platelet activation is initiated by their interaction with adhesive macromolecules like Collagen and von Willebrand factor at the subendothelial surface inducing the recruitment of additional platelets via diffusible mediators [1].

These mediators include ADP, Thromboxane A2, ADP and Serotonin that are secreted or released from activated platelets as well as Thrombin produced on the surface of activated platelets. These platelet stimuli are commonly transduced via G protein-coupled receptors (GPCRs). Prostaglandin I2, in contrast to ADP, Thromboxane A2, Thrombin, ADP and Serotonin, inhibits platelet function [2].

Activation of G-protein alpha-13 by TBXA2R and Thrombin receptors (PAR1 and PAR4) stimulates activation of the small GTAase RhoA, likely through the activation of the Rho guanine nucleotide exchange factors ARHGEF1 (p115RhoGEF) and LARG [2], [3], [4]. RhoA activates ROCK that inhibits Myosin light chain phosphatase (MLCP) activity via phosphorylation of the MLCP (reg). Attenuation of dephosphorylation of the regulatory subunits of non-muscle Myosin II, MELC and MRLC, leads to formation of actomyosin fibrils [2], [5], [6].

ROCK kinases also phosphorylate LIMK1 and LIMK2, which in turn phosphorylate actin-modulating protein Cofilin followed by reorganization of Actin cytoskeletal [7], [8]. Cytoskeleton rearrangement results in platelet shape change that is necessary for proper platelet aggregation.

G-protein alpha-q is activated by ADP via P2Y1, Thromboxane A2 via TBXA2R, Serotonin via HTR2A, and Thrombin via PAR1 and PAR4 [1].

G-protein alpha-q activates PLC-beta2 and PLC-beta3, which stimulate the production of 1,2-Diacyglycerol and IP3, with the consequent release of Ca("2+) from the dense tubular system and stimulation of Protein kinase C, namely PKC-alpha [1], [9], [10].

PKC-alpha regulates platelet aggregation by mediating the secondary secretion of mediators (such as ADP, ADP and Serotonin, etc.) from dense granules, thereby promoting platelet stimulation and contributing to alpha-IIb/beta-3 integrin activation [10], [11], [12].

alpha-IIb/beta-3 integrin is the major platelet integrin. It is required for platelet interactions with plasma proteins (such as Fibrinogen (fibrin)) and is essential for platelet adhesion and aggregation. alpha-IIb/beta-3 integrin is a protein complex composed of ITGA2B and ITGB3 subunits. The process of transforming integrin into a competent receptor for Fibrinogen (fibrin) is the final step in platelet activation [12].

PKC-alpha has been shown to activate RAS-related protein RAP-1A that induces the formation of an "integrin activation complex" containing the RAP-1A effector, PREL1, and a cytoskeletal protein Talin. Talin binds to ITGB3 cytoplasmic domains and positively regulates alpha-IIb/beta-3 integrin affinity [13].

Calcium binding protein Calmyrin binds directly to ITGA2B and inhibits Talin binding to ITGB3 thereby limiting the extent of ligand binding to alpha-IIb/beta-3 integrin [14].

Another ITGB3 binding protein, NRIF3, increases the affinity state of alpha-IIb/beta-3 integrin [15].

G-protein alpha-i family, which is activated by PAR1, Purinergic receptor P2Y12 (another ADP receptor) and Alpha-2A adrenergic receptor (ADP receptor), inhibits the production of Cyclic AMP by Adenylate cyclase and stimulates Phosphoinositide-3-kinase (PI3K)-dependent pathways.

Stimulation of G-protein alpha-i family coupled receptors results in formation of free G-protein beta/gamma dimers that stimulate PI3K cat class IB (p110-gamma) and PI3K cat class IA (p110-beta) activity. Both these lipid-metabolizing enzymes have been shown to activate RAS-related protein RAP-1B that binds to alpha-IIb/beta-3 integrin and positively regulates its affinity [11], [16], [17], [18], [19], [20], [21]. Ca("2+) and 1,2-Diacyglycerol -regulated Rap1 exchange factor CALDAG-GEFI is also involved in the activation of RAP-1A and RAP-1B [22], [23].

Prostacyclin receptor, which inhibits platelet function, is coupled to G-protein alpha-s that stimulates Adenylate cyclase. High levels of cytosolic Cyclic AMP suppress platelet activation, in part by PKA-cat (cAMP-dependent)-mediated suppression of the IP3R1, which triggers Ca("2+) release from the dense tubular system [24].

Thereby, each of the agonists binds to GPCRs and regulates platelet functions. Activation of platelets by strong agonists (such as ADP, Thromboxane A2 and Thrombin) initiates rapid shape change, enhances secretion of platelet granule contents and triggers Fibrinogen (fibrin) binding to alpha-IIb/beta-3 integrin. Weak agonists (such as ADP and Serotonin) potentiate these signaling pathways. Fibrinogen (fibrin) can simultaneously bind two adjacent platelets. This binding initiates the process of platelet aggregation that can lead to formation of very large aggregates at sites of vascular injury. Platelets can also become activated under pathological conditions, e.g., in ruptured atherosclerotic plaques, leading to arterial thrombosis [9], [24], [25].

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References:

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