The mechanisms of bone marrow (BM) haematopoietic progenitor cell (HPC) retention, engraftment and mobilization remain incompletely identified. Though a family member of the established HPC marker Sca-1, a role of membrane-anchored urokinase receptor (uPAR) in HPC biology remains unknown. We therefore studied its importance in HPC retention and mobilization by phenotyping mice lacking uPAR.

We found that membrane-anchored uPAR marks various subpopulations of HPCs and that, in steady-state conditions, cell-autonomous loss of uPAR partially depletes the HPC pool in the BM. Loss of uPAR on HPCs impaired their cell cycle quiescence, chemoprotection, and short-term homing and engraftment in the BM niche, indicating that uPAR is a cell-autonomous retention signal for HPCs. Importantly, in response to 5-FU myeloablation or G-CSF stimulation, the membrane-anchored uPAR retention signal on HPCs is inactivated by plasmin via proteolytic cleavage and converted into a soluble uPAR cleavage product (suPAR). Further studies highlighted that suPAR is not merely a waste product, but actively amplifies the mobilization of HPCs. Consistent herewith, mobilization of HPCs was impaired in the absence of uPAR or plasminogen. Membrane-anchored uPAR enhanced, while suPAR inhibited alpha4-beta1 integrin-dependent adhesion on BM stromal cells, thus suggesting that plasmin converts uPAR from a retention to a mobilization signal. Interestingly, membrane-anchored uPAR is also expressed on Lin^–Sca-1 ⁺ cKit⁺ BM cells while cell-autonomous loss of uPAR on donor BM cells reduces their long-term engraftment and multilineage repopulation of primary and secondary myeloablated recipients, suggesting that uPAR might also play a role in haematopoietic stem cells (HSCs).

Hence, our findings indicate that expression of membrane-anchored uPAR on HPCs functions as a novel retention signal promoting their maintenance, cell cycle quiescence, chemoprotection, homing, engraftment and mobilization, but they also unveil an unprecedented mobilization pathway, involving the inactivation of the membrane-anchored uPAR retention signal on HPCs by plasmin.