Hematopoietic stem cell fate decisions are regulated by Wnt antagonists: comparisons and current controversies.

Wingless and int (Wnt) proteins are secreted proteins that are important for regulating hematopoietic stem cell self-renewal and differentiation in the bone marrow microenvironment in mice. The mechanisms by which Wnt signaling regulates these hematopoietic cell fate decisions are not fully understood. Secreted Wnt antagonists, which are expressed in bone and bone marrow stromal cells, either bind to Wnt ligands directly or block Wnt receptors and co-receptors to halt Wnt-mediated signal transduction in both osteolineage and hematopoietic cell types. Secreted frizzled related proteins-1 and -2, Wnt inhibitory factor-1, Dickkopf-1, and Sclerostin are Wnt antagonists that influence hematopoietic cell fate decisions in the bone marrow niche. In this review, we compare and contrast the roles of these Wnt antagonists and their effects on hematopoietic development in mice, and also discuss the clinical significance of targeting Wnt antagonists within the context of hematopoietic disease.