tPA expands PαS-MSCs in a Plg- and MMP-9–dependent manner and results in the activation of MSC growth factors. (A) Representative FACS blots and (B) the absolute numbers of PαS-MSCs per femur from WT B6 mice, Plg^−/−, MMP-9^−/−, and tPA^−/− mice treated for 2 days with recombinant tPA or vehicle as determined by FACS analysis (n = 3/group). (C) Expression of genes involved in MSC maintenance/proliferation: platelet derived growth factor-A, PDGF-BB, PDGF-CC, PDGF-DD, vascular endothelial growth factor, TGFβ1, eEGF, insulin-like growth factor-1, hepatocyte growth factor, FGF2, BMP-2, and BMP-3 in total BMNCs retrieved from rec. tPA or vehicle-treated mice at day 2. The mRNA levels are standardized to glyceraldehyde-3-phosphate dehydrogenase (GAPDH; n = 3/group). (D) Fold increase of the number of cultured PαS-MSCs compared with initial cell input after treatment with recombinant PDGF-AA, PDGF-BB, PDGF-CC, PDGF-DD, EGF, FGF2, and tPA for 3 days (n = 3/group). (E) PDGF-BB serum level was measured by ELISA on day 2 from tPA-treated WT B6, tPA^−/−, Plg^−/−, and MMP-9^−/− mice (n = 3/group). (F) Relative mRNA expression of KitL in MSCs (P4) cultured in the presence of vehicle, tPA, FGF2, and PDGF-BB for 2 days. Data are normalized to GAPDH (n = 4/group). (G) Immunohistologic staining for KitL (green) and PDGFRα (red) in BM sections of tPA-treated mice. Arrows indicates perivascularly localized PDGFRα⁺ KitL coexpressing cells. Scale bar, 10 µm. (H) Serum KitL levels of WT B6 mice treated with vehicle or rec. tPA for 2 days as determined by ELISA (n = 3/group). Mean ± SEM. *P < .05; **P < .01; ***P < .001 by Student t test or ANOVA with Tukey HSD tests for multigroup analysis. Data are representative of 2 independent experiments.