Secreted frizzled related-protein 2(Sfrp2)deficiency decreases adult skeletal stem cell function in mice

In a previous transcriptomic study of human bone marrow stromal cells(BMSCs,also known as bone marrow-derived^mesenchymal stem cells#,)/SFRP2 was highly over-represented in a subset of multipotent BMSCs(skeletal stem cells,SSCs),which recreate a bone/marrow organ in an in vivo ectopic bone formation assay.SFRPs modulate WNT signaling,which is essential to maintain skeletal homeostasis,but the specific role of SFRP2 in BMSCs/SSCs is unclear.Here,we evaluated Sfrp2 deficiency on BMSC/SSC function in models of skeletal organogenesis and regeneration.The skeleton of Sfrp2-def\c\ent(KO)mice is overtly normal;but their BMSCs/SSCs exhibit reduced colony-forming efficiency,reflecting low SSC self-renewal/abundancy.Sfrp2 KO BMSCs/SSCs formed less trabecular bone than those from WT littermates in the ectopic bone formation assay.Moreover,regeneration of a cortical drilled hole defect was dramatically impaired in Sfrp2 KO mice.Sfrp2-deficient BMSCs/SSCs exhibited poor in vitro osteogenic differentiation as measured by Runx2 and Osterix expression and calcium accumulation.Interestingly,activation of the Wnt co-receptor;Lrp6,and expression of Wnt target genes,Axin2,C-myc and Cyclin Dl,were reduced in Sfrp2-deficient BMSCs/SSCs.Addition of recombinant Sfrp2 restored most of these activities,suggesting that Sfrp2 acts as a Wnt agonist.We demonstrate that Sfrp2 plays a role in self-renewal of SSCs and in the recruitment and differentiation of adult SSCs during bone healing.SFRP2 is also a useful marker of BMSC/SSC multipotency,and a factor to potentially improve the quality of ex vivo expanded BMSC/SSC products.

Erythropoietin modulates bone marrow stromal cell differentiation

Erythropoietin is essential for bone marrow erythropoiesis and erythropoietin receptor on non-erythroid cells including bone marrow stromal cells suggests systemic effects of erythropoietin. Tg6 mice with chronic erythropoietin overexpression have a high hematocrit, reduced trabecular and cortical bone and bone marrow adipocytes, and decreased bone morphogenic protein 2 driven ectopic bone and adipocyte formation. Erythropoietin treatment(1 200 IU·kg–1) for 10 days similarly exhibit increased hematocrit,reduced bone and bone marrow adipocytes without increased osteoclasts, and reduced bone morphogenic protein signaling in the bone marrow. Interestingly, endogenous erythropoietin is required for normal differentiation of bone marrow stromal cells to osteoblasts and bone marrow adipocytes.ΔEpoR_E mice with erythroid restricted erythropoietin receptor exhibit reduced trabecular bone, increased bone marrow adipocytes, and decreased bone morphogenic protein 2 ectopic bone formation. Erythropoietin treated ΔEpoR_E mice achieved hematocrit similar to wild-type mice without reduced bone, suggesting that bone reduction with erythropoietin treatment is associated with non-erythropoietic erythropoietin response. Bone marrow stromal cells from wild-type,Tg6, and ΔEpoR_E-mice were transplanted into immunodeficient mice to assess development into a bone/marrow organ. Like endogenous bone formation, Tg6 bone marrow cells exhibited reduced differentiation to bone and adipocytes indicating that high erythropoietin inhibits osteogenesis and adipogenesis, while ΔEpoR_E bone marrow cells formed ectopic bones with reduced trabecular regions and increased adipocytes, indicating that loss of erythropoietin signaling favors adipogenesis at the expense of osteogenesis. In summary, endogenous erythropoietin signaling regulates bone marrow stromal cell fate and aberrant erythropoietin levels result in their impaired differentiation.