Role of Plasmin and uPAR in Bone Marrow HSC/HPC Retention and Mobilization.

We previously identified the plasmin protease family as a critical determinant of the mobilization of hematopoietic stem and progenitor cells (HSC/HPC), but the role of the urokinase receptor uPAR remained unclear. uPAR is a membrane-anchored glycoprotein, which not only localizes its ligand urokinase (uPA) to the cell surface via its GPI-anchor but also regulates β1-integrin dependent cell adhesion and migration.

Following 5-FU myeloablation or G-CSF treatment, mice lacking uPAR (uPAR^−/−) had impaired hematopoietic recovery and HSC/HPC mobilization as compared to wild type (WT) mice. However, this phenotype was not mimicked in mice lacking uPA, suggesting a role of uPAR in mobilization independent of uPA-mediated proteolysis.

The impaired mobilization in uPAR^−/− mice was reversed upon pre-transplantation with WT BM cells (BMC), suggesting functional expression of uPAR on transplantable BMCs. Conversely, loss or inhibition of uPAR on transplanted BMCs impaired homing to the BM but not to the spleen, and compromised survival of myeloablated WT recipients. In vitro experiments revealed that loss or inhibition of uPAR impaired BMC adhesion to stromal cells and fibronectin. Anti-α4-β1 antibodies blocked adhesion of WT but not uPAR^−/− BMCs. Thus, uPAR appears to regulate BM homing and α4-β1 dependent retention of transplantable BMCs, possibly HSC/HPCs.

If uPAR mediates retention of HSC/HPCs, then this signal should be inactivated upon mobilization. Indeed, in 5-FU or G-CSF-treated WT mice, we found increased uPAR cleavage, and elevated levels of soluble uPAR (suPAR) in BM plasma. These processes failed to occur in mice lacking plasminogen, suggesting that plasmin cleaves uPAR during mobilization. Cleavage of uPAR appeared critical as the inactivation of the retention signals membrane-bound Kit ligand and SDF-1α was normal in uPAR^−/− mice. Moreover, the generated suPAR may also affect the BM, as administration of recombinant suPAR in WT mice enhanced hematopoietic recovery and HSC/HPC mobilization after 5-FU or G-CSF. In vitro and transplantation experiments revealed that suPAR blocked α4-β1 dependent adhesion.

Thus, in steady state, membrane-anchored uPAR appears to function as a BM retention signal for transplantable BMCs, possibly HSC/HPCs. In conditions of mobilization, the uPAR retention signal is cleaved, which weakens α4-β1 dependent adhesion and allows mobilization out of the BM. Soluble uPAR may then additionally amplify mobilization, in part by further attenuating α4-β1 dependent adhesion to the BM. Currently, we are investigating the role of uPAR on subsets of HSC/HPCs, and in the different BM niches. We are also performing long-term competitive repopulation experiments to further delineate the therapeutic potential of uPAR.