Absence of the Wnt Antagonist Sclerostin Adversely Affects B Cell Development in the Bone Marrow Niche

Abstract 220

Hematopoietic cell fate decisions are dependent on their localized microenvironmental niche. In the bone, endosteal osteoblasts have been shown to support hematopoietic stem cells (HSC) self-renewal, as demonstrated by transgenic and knockout mouse models in which osteoblast populations were increased or decreased. In addition, Wnt signaling and the Wnt antagonist Dkk-1 have been implicated in various aspects of hematopoiesis and HSC self-renewal. Sclerostin (Sost) is a secreted protein that is primarily expressed by fully mature osteocytes and acts on osteoblasts as a negative regulator of bone growth, by antagonizing Wnt signaling by its binding to the Wnt co-receptors Lrp4, Lrp5, and/or Lrp6. Here, we investigated the role of Sost on hematopoiesis in the bone marrow niche. Increased osteoblast activity in sclerostin-knockout (Sost^−/−) mice results in hypermineralized bones with small bone marrow cavities. As such, Sost^−/− mice contain markedly reduced numbers of CD45⁺hematopoietic cells in the bone marrow. Since hematopoietic stem cell activity is dependent on osteoblast function, we examined whether the hyperactive osteoblast activity in Sost^−/− mice influences the numbers of hematopoietic stem cells, lymphoid progenitor cells and myeloid progenitor cells in the bone marrow. Surprisingly, no differences were observed in hematopoietic stem and progenitor cell frequency and cell number. However, we found the bone marrow of Sost^−/− mice to be depleted of B cells, and this reduction can be attributed to premature apoptosis beginning at the pre-pro-B cell stage. Examination of Sost expression showed that no hematopoietic cells expressed Sost, however, pre-pro, immature and recirculating B cells expressed Lrp5 and Lrp6. These gene expression patterns suggested that the defect in B cell development in Sost^−/− mice is non-cell autonomous and that absence of Sost could affect Wnt signaling in these populations. We observed that the expression of Wnt target genes CCND1 and Lef-1 were not affected by the absence of Sost, but c-Myc was significantly upregulated in recirculating B cells in the bone marrow. We also observed a significant decrease in CXCL12 expression in the bone marrow stroma in Sost^−/− mice, consistent with their inability to adequately support B cell development. Taken together, our results indicate that the B cell developmental defects in Sost^−/− mice are non-cell autonomous, and we are currently performing reciprocal bone marrow transplantation experiments to further support this hypothesis. Our studies demonstrate a novel role for Sost in the regulation of B cell development in the bone marrow, and demonstrate that distinct Wnt antagonists play specific roles in the regulation of hematopoiesis.