Comparative analysis of BMP signaling in human ECFCs reveals vascular bed-dependent ALK2 expression

Bone morphogenetic protein (BMP) signaling pathway components play a crucial role in cardiovascular homeostasis and sprouting angiogenesis. Endothelial colony-forming cells (ECFCs) are endothelial progenitor cells with a high proliferative and angiogenic capacity and therefore are valuable candidates for angiogenic therapies and vascular regeneration. Of note, a direct comparative analysis of human umbilical cord blood-derived ECFCs (CB-ECFCs) and peripheral blood-derived ECFCs (PB-ECFCs) with a focus on BMP signaling and potential tissue-dependent effects is lacking. In this study, we characterized the BMP signaling responses in ECFCs derived from the umbilical cord and peripheral blood. Analysis of SMAD1/5 phosphorylation showed that BMP2 stimulation only leads to an activation of the BMP signaling pathway in PB-ECFCs, but not in CB-ECFCs. Analysis of gene expression levels of BMP/TGF-β type I and type II receptors demonstrated an elevated expression of ALK2 in PB-ECFCs. Evaluation of sprouting angiogenesis revealed that si-RNA-mediated silencing of ALK2 in CB-ECFCs results in hypersprouting, while si-RNA-mediated silencing of ALK3 leads to hypersprouting in PB-ECFCs. To conclude, we found a differential BMP signaling pathway activation in CB-ECFCs and PB-ECFCs, which is related to vascular bed-dependent expression of ALK2.