SFRP2 FROM THE NICHE IS REQUIRED TO MAINTAIN THE REGENERATION OF THE HEMATOPOIETIC STEM CELL POOL

We previously found that Sfrp2 was overexpressed in stromal cells, which maintain hematopoietic stem cells (HSCs) in in vitro culture. Here, we determined the relevance of Sfrp2 expression by niche cells for the maintenance of hematopoiesis under stress conditions in vivo. To determine the role of Sfrp2 in HSC responses under stress conditions, we studied in vitro cultures, 5-FU treatment in vivo, and HSC engraftment in transplantation experiments. In in-vitro co-cultures with shSfrp2 stromal cells, the number lineage-negative Kit+ Sca-1+ (LSK) and progenitors increased. The LSK cells showed higher levels of Ki-67 expression, BrdU incorporation, and catenin-dependent Wnt signaling. Yet, total repopulating activity of these cultures was diminished, suggesting exhaustion of HSCs. These in vitro results were mirrored in in vivo models of stress, such as aging, 5-FU treatment and hematopoietic regeneration in Sfrp2-/- recipients. In all three in vivo situations of stress, we noted an increase of LSK cells, which in accompanied by increased levels of - catenin and cyclin D1. In the transplantation experiments, the increase in LSK cells was accociated with a progressive loss of HSCs in serial transplantation recipients. Similarly, genotoxic stress in 5-FU-treated Sfrp2-/- mice showed LSK cells with a higher cycling activity than wild-type (WT) littermates, as shown by higher levels BrdU incorporation, and a higher expression of Ki-67 and canonical Wnt signaling mediators. Importantly, increased cycling of LSKs was accompanied with stress-induced senescence-associated DNA damage response as indicated by H2A.X staining and depolarized localization of acetylated H4K16. Our experiments are consistent with the view that Sfrp2 expression in the niche is required to limit stress-induced DNA damage and canonical Wnt-mediated HSC activation, thus preventing HSC exhaustion.