Hematopoietic Stem Cell Regeneration through Paracrine Regulation of the Wnt5a-Prox1 Signaling Axis

The crosstalk between bone marrow (BM) microenvironment (niche) and hematopoietic stem cells (HSCs) is critical for HSC regeneration after injury. Here we show that deletion of the genes encoding the DNA repair-deficient syndrome Fanconi anemia (FA), Fanca and Fancc, in mice dampens HSC regeneration through both direct effects on HSCs and indirect effects on BM niche cells. Specifically, Fanca- or Fancc-deficiency compromises hematologic recovery and dampens HSC regeneration following irradiation. FA HSCs show persistent upregulation of the Wnt target Prox1, a homeobox transcription factor, in response to total body irradiation (TBI). Accordingly, lineage-specific deletion of Prox1 improves long-term repopulation of the irradiated FA HSCs. Forced expression of Prox1 in wild-type (WT) HSC mimics the defective repopulation phenotype of FA HSCs. By analyzing paracrine factors in Wnt signaling, we found that WT mice, but not FA mice, show significant induction by TBI of BM stromal Wnt5a protein, which is produced in LepR ⁺CXCL12 ⁺ BM stromal cells. Wnt5a treatment of irradiated FA mice enhances hematopoietic recovery and HSC regeneration. Conversely, Wnt5a neutralization in co-cultured LepR ⁺ BM stromal cells inhibits HSC regeneration and hematopoietic recovery following TBI. Mechanistically, Wnt5a secreted by LepR ⁺CXCL12 ⁺ BM stromal cells inhibits b-catenin accumulation, thereby repressing Prox1 transcription in irradiated HSPCs. The detrimental effect of deregulated Wnt5a-Prox1 signaling on HSC regeneration and hematopoietic recovery is also observed in aged mice. Irradiation induces upregulation of Prox1 in the HSCs of aged mice, and deletion of Prox1 in aged HSCs improves HSC regeneration and hematopoietic recovery after irradiation. Finally, treatment of aged mice with Wnt5a enhances hematopoietic repopulation. Collectively, these findings identify the novel paracrine Wnt5a-Prox1 signaling axis in regulating HSC regeneration under conditions of injury and aging.