Abstract 1480: The Wnt Antagonist Dickkopf-1 (Dkk-1) Targets Endosteal Cells of the Bone Marrow Stem Cell Niche and Mobilizes Endothelial Progenitor Cells
Therapeutic vasculogenesis holds promise to cure ischemic diseases. This approach involves neovascularization by endothelial progenitor cells (EPC) that are mobilized from the bone marrow (BM) for tissue regeneration. In the BM, mobilization is regulated in stem cell niches, in which osteoblasts play a key role. Since inhibition of Wnt signaling promotes late-stage osteoblast differentiation, we examined whether the Wnt signaling antagonist Dkk-1 is involved in EPC mobilization.
Methods and Results: Mice were injected with Dkk-1 (0.5 mg/kg) or G-CSF (0.25 mg/kg) as a positive control for 3 consecutive days. Dkk-1 increased the number of flk-1+sca-1+ circulating EPC (Dkk-1: 286.7 ± 46.7 %, P < 0.01 vs. PBS control) and endothelial colonies (Dkk-1: 320.4 ± 71.6 %, P < 0.01 vs. PBS). In contrast, G-CSF, but not Dkk-1 increased hematopoietic colonies (Dkk-1: 112.0 ± 9.1 %, P =0.5; G-CSF: 202.5 ± 27.3 %, P < 0.01 vs. PBS) and inflammatory cells such as mature granulocytes (Dkk-1: 90.4 ± 8.3 %, P =0.5; G-CSF: 177.2 ± 12.9 %, P < 0.01 vs. PBS). To investigate whether the EPC mobilizing effect of Dkk-1 injections may translate into enhanced neovascularization, we used a matrigel plug assay. Indeed, Dkk-1 significantly increased the number of conductance vessels in the matrigel plugs (Dkk-1: 347.6 ± 89.9 %, G-CSF: 371.8 ± 58.4 %, both P < 0.01 vs. PBS). Finally, using TOP-GAL transgenic mice carrying a β-galactosidase gene driven by a LEF/TCF/β-catenin responsive promoter, we assessed the status of the transcriptional activity of β-catenin, the product of canonical Wnt signaling after Dkk-1 administration. X-gal staining identified endosteal cells, predominantly osteoblasts but also osteoclasts, as the primary sites of canonical Wnt signaling, which was completely suppressed by Dkk-1.
Conclusion: The Wnt antagonist Dkk-1 abolished Wnt signaling in BM endosteal cells, suggesting a role for Dkk-1 in the regulation of the BM stem cell niche. Dkk-1 mobilizes EPC without concomitant recruitment of inflammatory cells as observed for G-CSF and subsequently enhances neovascularization in a matrigel plug assay. These data suggest that Dkk-1 is a specific regulator of vasculogenesis and may be of great relevance to optimize cell therapy aiming at regenerating vasculature.