Functional redundancy between RAP1 isoforms in murine platelet production and function

Stefanini, L., Lee, R. H., Paul, D. S., O'Shaughnessy, E. C., Ghalloussi, D., Jones, C. I. ORCID: https://orcid.org/0000-0001-7537-1509, Boulaftali, Y., Poe, K. O., Piatt, R., Kechele, D. O., Caron, K. M., Hahn, K. M., Gibbins, J. M. ORCID: https://orcid.org/0000-0002-0372-5352 and Bergmeier, W. (2018) Functional redundancy between RAP1 isoforms in murine platelet production and function. Blood, 132 (18). pp. 1951-1962. ISSN 0006-4971 doi: 10.1182/blood-2018-03-838714

Abstract/Summary

RAP GTPases, important regulators of cellular adhesion, are abundant signaling molecules in the platelet/megakaryocytic lineage. However, mice lacking the predominant isoform, RAP1B, display a partial platelet integrin activation defect and have a normal platelet count, suggesting the existence of a RAP1-independent pathway to integrin activation in platelets and a negligible role for RAP GTPases in megakaryocyte biology. To determine the importance of individual RAP isoforms on platelet production and on platelet activation at sites of mechanical injury or vascular leakage, we conditionally deleted Rap1a and/or Rap1b in the megakaryocytic lineage (mKO). Interestingly, Rap1a/b-mKO mice displayed a marked macrothrombocytopenia due to impaired pro- platelet formation by megakaryocytes. In platelets, RAP isoforms had both redundant and isoform-ˇspecific functions. Deletion of RAP1B, but not RAP1A, significantly reduced α- granule secretion and activation of the cytoskeleton regulator RAC1. Both isoforms significantly contributed to thromboxane A2 generation and the inside-out activation of platelet integrins. Combined deficiency of RAP1A and RAP1B markedly impaired platelet aggregation, spreading and clot retraction. Consistently, thrombus formation in physiological flow conditions was abolished in Rap1a/b-mKO, but not Rap1a-mKO or Rap1b-mKO platelets. Rap1a/b-mKO mice were strongly protected from experimental thrombosis and exhibited a severe defect in hemostasis after mechanical injury. Surprisingly, Rap1a/b-mKO platelets were indistinguishable from controls in their ability to prevent blood-lymphatic mixing during development and hemorrhage at sites of inflammation. In summary, our studies demonstrate an essential role for RAP1 signaling in platelet integrin activation and a critical role in platelet production. While important for hemostatic/thrombotic plug formation, platelet RAP1 signaling is dispensable for vascular integrity during development and inflammation.

Item Type	Article
URI	https://reading-clone.eprints-hosting.org/id/eprint/78896
Identification Number/DOI	10.1182/blood-2018-03-838714
Refereed	Yes
Divisions	Interdisciplinary centres and themes > Institute for Cardiovascular and Metabolic Research (ICMR) Life Sciences > School of Biological Sciences > Biomedical Sciences
Publisher	American Society of Hematology
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