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IRAG is involved in NO/cGMP-mediated inhibition of platelet function

The inositol 1,4,5-trisphosphate receptor-associated cGMP kinase substrate (IRAG) is highly expressed in platelets. It is assembled in a macrocomplex together with cGMP-dependent protein kinase Iβ (cGKIβ) and the inositol 1,4,5-trisphosphate receptor type I (IP3RI). In response to cGKI activation, IRAG is phosphorylated in vivo. By mass spectrometric analysis of purified in vivo phosphorylated IRAG from cGMP-stimulated human platelets, Ser664 and Ser677 were identified as phosphorylated amino acids. Generation of phosphospecific antibodies confirmed these in vivo phosphorylation sites. To examine the role of IRAG in platelet function, we analysed the aggregation of platelets from an IRAG mouse mutant (IRAGΔ12/Δ12). In this mutant, exon 12 of the IRAG gene was deleted and thereby the IRAG-IP3RI interaction was abolished [1]. Interestingly, collagen-induced aggregation of IRAGΔ12/Δ12 platelets was not inhibited by nitric oxide (NO) and the cGMP analogue 8-pCPT-cGMP in contrast to wild type platelets. The shape change was not affected, neither in mutant nor in wild type platelets. However, iloprost and the cAMP-analogue cBIMPS still inhibited aggregation and shape change in IRAGΔ12/Δ12 and wild type platelets. Additionally, we analysed fibrinogen binding to thrombin-stimulated platelets. In wild type platelets, pre-treatment with NO and cGMP decreased fibrinogen binding by about 50%. In contrast, in IRAGΔ12/Δ12 platelets the response to NO and cGMP was nearly absent. Preincubation with iloprost and cBIMPS clearly reduced agonist-induced fibrinogen binding to both, IRAGΔ12/Δ12 and wild type platelets. These results suggest that signalling through IRAG/IP3RI is essential for NO/cGMP-dependent inhibition of platelet aggregation and activation of the fibrinogen receptor


  1. Geiselhoeringer A, Werner M, Sigl K, Smital P, Woerner R, Acheo L, Stieber J, Weinmeister P, Feil R, Feil S: IRAG is essential for relaxation of receptor-triggered smooth muscle contraction by cGMP kinase. EMBO J. 2004, 23: 4222-31. 10.1038/sj.emboj.7600440.

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Correspondence to Katja Sigl.

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Open Access This article is published under license to BioMed Central Ltd. This is an Open Access article is distributed under the terms of the Creative Commons Attribution License ( ), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Antl, M., Sigl, K., Eigelsperger, C. et al. IRAG is involved in NO/cGMP-mediated inhibition of platelet function. BMC Pharmacol 5 (Suppl 1), P2 (2005).

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