Dickkopf 1 (Dkk1) Regulates Hematopoietic Stem Cell Regeneration

Bone marrow endothelial cells (BM ECs) have been shown to regulate HSC regeneration following myelosuppression.  The role of osteolineage cells in regulating HSC regeneration remains less well understood.  Here, we show that deletion of the pro-apoptotic genes, Bak and Bax, in osterix (Osx)-expressing osteoprogenitor cells promoted HSC regeneration and hematopoietic radioprotection of mice following total body irradiation (TBI).  We identified Dkk1 to be enriched in the BM of radioprotected OsxCre;Bak1^-/-;Bax^FL/- mice and found that Bak/Bax-deficient osteolineage cells expressed increased levels of Dkk1 compared to Bax-expressing osteolineage cells (p=0.003).  Treatment of irradiated BM HSCs with DKK1 in vitro significantly increased the recovery of phenotypic HSCs (p=0.0002), colony forming cells (CFCs)(p=0.003) and long-term repopulating HSCs compared to control cultures (p=0.009).    Systemic administration of Dkk1 to lethally irradiated C57Bl6 mice accelerated the recovery of mature blood counts (p=0.008), BM HSCs (p=0.008) and progenitor cells (p=0.007).  Furthermore, survival after lethal irradiation was markedly increased in Dkk1 treated mice (93%) compared to saline controls (27%; p=0.0004).  Conversely, systemic administration of anti-Dkk1 antibody significantly delayed recovery of BM HSCs (p=0.002), peripheral white blood cells (p=0.0004), neutrophils (p<0.0001) and lymphocytes (p=0.002) in irradiated mice compared to irradiated, control mice.  Dkk1 promoted HSC regeneration via suppression of reactive oxygen species (ROS) and inhibition of caspase activation in HSCs following irradiation.  Dkk1 represents a novel, osteoprogenitor cell-derived paracrine factor which is necessary for normal hematopoietic regeneration following irradiation and can be therapeutically delivered to accelerate hematopoietic reconstitution.