ROBO4 Inhibits Irradiation-Induced Microvascular Permeability, Endothelial to Mesenchymal Transition, and Improves Hematopoietic Reconstitution

Background: Hematopoietic Stem Cells Transplantation involves irradiation preconditioning which causes bone marrow endothelial cells dysfunction. While much emphasis is on the reconstitution of an adequate number of HSCs to repopulate the hematopoietic system after the death of malignant cells, EC preservation and normal functioning are indispensable to overcome the preconditioning damages. ROBO4 is an endothelial-specific receptor that is important in maintaining the vasculature. The purpose of this study is to ascertain ROBO4’s role in regulating irradiation-induced damage of the endothelium and its subsequent effect on hematopoietic reconstruction.

Method: Microvascular endothelial cells were treated with γ-radiation to establish an endothelial cell injury model. The expression of ROBO4 in the microvascular endothelial cells was manipulated employing lentiviral mediated RNAi (shRNA) and gene overexpression technology before irradiation treatment. The permeability of endothelial cells and its mesenchymal transition (Endo-MT) was measured using qPCR, immunocytochemistry, and immunoblotting to analyze the effect on the expression and distribution of junctional molecules (CD144, Connexin 43, and Occludin, etc.), endothelial cell-specific molecules (PECAM-1, VEGFR2 and TIE2, etc.), and specific mesenchymal cell markers (Fibronectin, Vimentin, S100A4, and ACTA2, etc.). Using Transwell and wound healing migration, Matrigel tube formation, and Transwell and FITC-Dextran permeability assays, we determined the changes in endothelial functions after ROBO4 gene manipulation and irradiation. In addition, a co-culture model of endothelial cells and hematopoietic stem cells isolated from C57BL/6 mice was constructed, and the effects of irradiated endothelial cells on the proliferation, differentiation and cell cycle of hematopoietic stem cells were analyzed using flow cytometry.

Results: Ionizing radiation upregulates ROBO4 expression but disrupts endothelial junctional molecules. ROBO4 deletion enhances the irradiation-induced degradation of endothelial junctions hence increasing the permeability of the endothelium and also facilitating endothelial to mesenchymal transition (Endo-MT) by activating both the canonical (Smads) and non-canonical (PI3K/AKT/mTOR) pathways. Endothelial ROBO4 overexpression substantially reduces microvascular permeability and mitigates Endo-MT following irradiation. Even though ROBO4 overexpression does not dramatically affect endothelial functions, tube formation and migration potentials of endothelial cells significantly increase upon the knockdown of ROBO4 and irradiation. We also found that EC ROBO4 silencing inhibits the proliferation of HSPC and promotes HSC quiescence. On the other hand, ROBO4 overexpression lessened the inhibitory effects of the injured endothelium on hematopoietic stem cell expansion. Lastly, we saw that ROBO4 overexpression inhibits erythrocytosis and granulocytosis after irradiation compared to the control groups suggesting that EC ROBO4 may play a regulatory role in HSC differentiation.

Conclusion: ROBO4 maintains microvascular integrity after radiation preconditioning treatment by regulating permeability and Endo-MT, protects endothelial function, and reduces the inhibitory effect of injured endothelium on hematopoietic reconstitution.