Biological Consequences of Thrombin Receptor Deficiency in Mice

Abstract

SummaryThe thrombin receptor (ThrR) is a membrane-bound, G-protein-coupled receptor for the serine protease thrombin. This receptor is expressed in a wide variety of cells and tissues, and elicits a range of physiological responses associated with tissue injury, inflammation, and wound repair. To achieve a better understanding of the physiological role of the ThrR, we have employed homologous recombination to create mice with a disrupted ThrR gene. Following heterozygous (+/-) intercrosses, a total of 351 surviving offspring were genotyped. Only 7% of these offspring were identified as homozygous (-/-) for the disrupted allele, indicating a profound effect on embryonic development. Paradoxically, adult ThrR-/- mice appeared to be normal by anatomical and histological analysis, including their platelet number and function. Similarly, ThrR deficiency had no detectable effect in adult ThrR-/- mice on basal heart rate, arterial blood pressure, vasomotor responses to angiotensin II and acetylcholine, and coagulation parameters, even though the ThrR is expressed in many cardiovascular tissue types. In addition, the loss of ThrR function in the peripheral vasculature of adult ThrR-/- mice was confirmed by the absence of various standard hemodynamic effects of the ThrR-activating peptides SFLLRN-NH2 and TFLLRNPNDK-NH2 Our results indicate that ThrR deficiency has a strong impact on fetal development; however, ThrR-/- mice that proceed to full development display surprisingly little change in phenotype compared to the wild-type

Altered Vascular Injury Responses in Mice Deficient in Protease-Activated Receptor-1

“…34 As has been reported previously, 34,35 these mice appear phenotypically normal on both gross and histological examinations. However, matings between PAR-1 Ϫ/Ϫ mice occur less frequently and generally produce much smaller litters.…”

“…This finding was not unexpected, since thrombin-induced platelet activation in mice and coagulation in general do not involve PAR-1. 34,35 Early occlusive and nonocclusive thrombosis resolved over time. The rate of resolution was not different in WT and PAR-1-deficient mice.…”

“…In addition, the accentuated cellular response to platelet-derived growth factor in fibroblasts from PAR-1-deficient mice also represents a secondary compensation that may mask the absence of PAR-1. 34 The increased vascular smooth muscle cell density in PAR-1-deficient mice, which may be a consequence of the lack of PAR-1 during development, will tend to alter the vascular injury response. Thus, the apparent inhibition of positive remodeling in PAR-1-deficient mice may be a consequence of the higher baseline cell density rather than the absence of PAR-1.…”

Protease activated receptors: theme and variations

“…In both humans and mice, PAR1 appears to be the most widely-expressed of the three thrombin-responsive PAR family members. Despite this, the`knockout' of PAR1 in mice was not uniformly lethal (Connolly et al, 1996;Darrow et al, 1996;Damiano et al, 1999). Instead, half the mice developed normally in the absence of PAR1, while the other half died at embryonic day 9, apparently from the absence of the normal contribution of PAR1 to the development of a stable vasculature.…”

Thrombin's faces revealed: cellular effects include induction of neoangiogenesis

“…The ®rst clue came from PAR1 À/À knockout mice, where disruption of the murine gene resulted in approximately 50% lethality of homozygous (PAR1 À/À ) mice at embryonic (E) day E9±E10 [8,9]. These embryonic deaths corresponded to development of the circulatory system, and the growth de®cits of PAR1 À/À mice displayed delayed vascularization of the yolk sac, possibly intimating a role for PAR1 in vasculogenesis.…”