Disruption of stem cell niche-confined R-spondin 3 expression leads to impaired hematopoiesis.

Self-renewal and differentiation of stem and progenitor cells are tightly regulated to ensure tissue homeostasis. This regulation is enabled both remotely by systemic circulating cues, such as cytokines and hormones, and locally by various niche-confined factors. R-spondin 3 (RSPO3) is one of the most potent enhancers of Wnt signaling and its expression is usually restricted to the stem cell niche where it provides localized enhancement of Wnt signaling to regulate stem cell expansion and differentiation. Disruption of this niche-confined expression can disturb proper tissue organization and lead to cancers. Here, we investigate the consequences of disrupting the niche restricted expression of RSPO3 in various tissues including the hematopoietic system. We show that normal Rspo3 expression is confined to the perivascular niche in the bone marrow. Induction of increased systemic levels of circulating RSPO3 outside of the niche results in prominent loss of early-B cell progenitors and anemia but surprisingly has no effect on hematopoietic stem cells (HSCs). Using molecular, pharmacological and genetic approaches, we demonstrate that these RSPO3-induced hematopoietic phenotypes are Wnt and RSPO3 dependent and mediated through non-canonical Wnt signaling. Our study highlights a distinct role for a Wnt/RSPO3 signaling axis in the regulation of hematopoiesis, as well as possible challenges related to therapeutic usage of R-spondins for regenerative medicine.