Function of IRAG for cGMP kinase signalling in smooth muscle and platelets

Intracellular signalling by NO/cGMP/cGMP-dependent protein kinase type I (cGKI) regulates various physiological processes including smooth muscle contractility and platelet aggregation. An important mediator of this signalling cascade is the inositol 1,4,5-trisphosphate receptor I (IP3RI) associated protein cGMP kinase substrate (IRAG). This protein forms a trimeric complex together with the cGMP kinase Iβ (cGKIβ) and the IP3RI. Targeted deletion of exon 12 of IRAG coding for the N-terminal part of the coiled-coil domain disrupted in vivo the IRAG-IP3RI interaction. The resulting IRAG∆12/∆12 mice showed an increased mortality and a severely reduced gastrointestinal motility. The relaxation of hormone-contracted aortic and longitudinal colonic smooth muscle by cGMP was abolished in IRAG∆12/∆12 mice, whereas cAMP-mediated relaxation was not altered. In contrast to WT mice, norepinephrine-induced increases in [Ca]i were not reduced by cGMP in aortic smooth muscle cells from IRAG∆12/∆12 mice. These data suggest, that IRAG is involved in the cGMP-dependent decrease of [Ca]i in vivo and is essential for cGMP-dependent relaxation of hormone-induced vascular and colonic muscle contraction. However, cGMP-mediated relaxation of small intestinal smooth muscles was only partially affected in IRAG∆12/∆12 mice suggesting tissue specific selectivity of cGKI mechanisms.