Mice Deficient in Rap1A GTPase Display Reduced Adhesion of Hematopoietic and Mesenchymal Stromal Cells but Intact Hematopoietic Repopulation Potential

Abstract

Background: The Guanine Triphosphatase (GTPase) Rap1A has been implicated in cell proliferation and migration in different tissues. Its role in the production, adhesion and migration of bone marrow-derived hematopoietic cells is however incompletely understood. Methods: We analysed mice lacking one or both Rap1A alleles and measured their adhesion function and their hematopoietic repopulation capacity using competitive bone marrow transplantation in the CD45.1/CD45.2 transplant model. We further employed flow cytometry and shear stress-dependent flow chamber adhesion assays for phenotypic characterization and analysis of adhesion functions. Results: Rap1A-/- mice showed decreased numbers of lymphocytes and hematopoietic colony forming progenitors in the blood, while the numbers of granulocytes, monocytes or platelets were not significantly altered compared to controls. Chemokine CXCL12-induced adhesion of lineage marker depleted (Lin-) bone marrow cells to Vascular Cell Adhesion Molecule (VCAM)-1 and to Intercellular Adhesion Molecule (ICAM)-1 was significantly reduced in Rap1A-/- mice. However, competitive repopulation by Rap1A deficient bone marrow cells was not inferior to wild type bone marrow 9 months after transplantation. No outgrowth of Mesenchymal Stromal Cells (MSCs) was observed from bone marrow of Rap1A-/- mice, whereas Rap1A+/- MSCs displayed reduced expression of adhesion molecules involved in progenitor homing and impaired adhesion to primary endothelial cells and VCAM-1 under shear stress in vitro, as compared to their wild-type counterparts. Conclusions: Rap1A regulates adhesion of immature hematopoietic cells and mesenchymal stromal cells, but appears to be redundant for hematopoietic engraftment.