RSPO/LGR signaling mediates astrocyte-induced proliferation of adult hippocampal neural stem cells

In the dentate gyrus of the adult hippocampus, neurogenesis from neural stem cells (NSCs) is regulated by Wnt signals from the local microenvironment. The Wnt/{beta}-catenin pathway is active in NSCs, where it regulates proliferation and fate commitment, and subsequently its activity is strongly attenuated. The mechanisms controlling this pattern of activity are poorly understood. In stem cells from adult peripheral tissues, secreted R-spondin proteins (RSPO1-4) interact with LGR4-6 receptors and control Wnt signaling strength. Here, we found that RSPO1-3 and LGR4-6 are expressed in the adult dentate gyrus and in cultured NSCs isolated from the adult mouse hippocampus. The expression of LGR4-5 decreased in NSCs upon differentiation, concomitantly with the reported decrease in Wnt activity. Treatment with RSPO1-3 increased hippocampal NSCs proliferation and the expression of the Wnt target gene Cyclin D1. Moreover, RSPO1-3 were expressed by primary cultures of dentate gyrus astrocytes, a crucial component of the neurogenic niche able to induce NSC proliferation and neurogenesis. In co-culture experiments, astrocyte-induced proliferation of NSCs was prevented by RSPO2 knockdown in astrocytes, and by LGR5 knockdown in hippocampal NSCs. Altogether, our results indicate that RSPO/LGR signaling is present in the dentate niche, where it could control Wnt activity and proliferation of NSCs.