Defective platelet activation in Gαq-deficient mice

Abstract

Platelets are small disc-shaped cell fragments which undergo a rapid transformation when they encounter vascular damage. They become more spherical and extrude pseudopodia, their fibrinogen receptors are activated, causing them to aggregate, they release their granule contents, and eventually form a plug which is responsible for primary haemostasis. Activation of platelets is also implicated in the pathogenesis of unstable angina, myocardial infarction and stroke. Here we show that platelets from mice deficient in the alpha-subunit of the heterotrimeric guanine-nucleotide-binding protein Gq are unresponsive to a variety of physiological platelet activators. As a result, G alpha(q)-deficient mice have increased bleeding times and are protected from collagen and adrenaline-induced thromboembolism. We conclude that G alpha(q) is essential for the signalling processes used by different platelet activators and that it cannot be replaced by G alpha(i) or the beta gamma subunits of the heterotrimeric G proteins. G alpha(q) may thus be a new target for drugs designed to block the activation of platelets.

Protein kinase C- and calcium-regulated pathways independently synergize with Gi pathways in agonist-induced fibrinogen receptor activation

“…Similar to this action, phorbol esters cause fibrinogen receptor activation without increases in intracellular calcium. Furthermore, phorbol ester-mediated platelet aggregation is unaffected in platelets from mice deficient in G q [16], suggesting that phorbol esters can stimulate fibrinogen receptor through PKC without a role for calcium. These data lead to two possible scenarios in agonist-induced fibrinogen receptor activation.…”

“…Activation of the G q pathway leads to PLC stimulation, which ultimately leads to activation of PKC and an increase in intracellular calcium. In the absence of G q stimulation, as in G q -deficient mice [16] or in human patients with a dysfunctional G q [17,18], agonist-induced platelet secretion and aggregation are affected. Similarly, events downstream of signalling, i.e.…”

A novel role for phospholipase D as an endogenous negative regulator of platelet sensitivity

“…In contrast mouse platelets express PAR-3 and PAR-4, whereby PAR-3 does not induce intercellular signaling but serves as a co-factor for PAR-4 [8]. Whereas Gα 12/13 signaling accounts for instant cytoskeletal reorganization (platelet "shape change") via calcium/calmodulin and Rho/Rho-kinase signaling [10;11], Gα q signaling induces PLC isoform β activation [12]. Activated PLC  and 2 cleave phosphatidylinositol 4,5-bisphosphate (PIP 2 ) into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP 3 ) [13].…”

“…Activated PLC  and 2 cleave phosphatidylinositol 4,5-bisphosphate (PIP 2 ) into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP 3 ) [13]. IP 3 triggers the release of Ca 2+ from intracellular stores which in concert with DAG activates protein kinase C (PKC), a protein implicated in platelet granule Elvers et al PLD as negative regulator of platelets 4 secretion [12][13][14]. Furthermore, an increase in intracellular Ca 2+ also accounts for activation of phospholipase A2 leading to the release of arachidonic acid which converted to TXA 2 in a cyclooxygenase-dependent manner.…”

A sensitized genetic system for the analysis of murine B lymphocyte signal transduction pathways dependent on Bruton's tyrosine kinase

“…Btk lo females were mated with mice deficient in each candidate gene (C) to generate pups that were C ϩ/Ϫ and either Btk Ϫ/Y , Btk Ϫ/Y tg (Btk lo ), Btk ϩ/Ϫ , or Btk ϩ/Ϫ tg. Mice were genotyped by PCR or Southern blot analysis as described (21,26,29,34,36,(40)(41)(42)48) except for CD19 (37) and CD22 (38) mutant mice that were identified by FACS analysis of peripheral blood. Males and females from these crosses were then mated to each other.…”