Roundabout 4 Is Expressed on Hematopoietic Stem Cells and Potentially Involved in the Niche-Mediated Regulation of the Side Population Phenotype§


  • Author contributions: F.S., Y.G.-K., Y.M., T.K., M.I., and Y.F.: Collection and assembly of data; J.P.H., M.T., and D.Y.L.: provision of study material; T.K.: administrative support; H.N.: conception and design, financial support, data analysis and interpretation, manuscript writing, final approval of manuscript. F.S. and Y.G.-K. contributed equally to this work.

  • Disclosure of potential conflicts of interest is found at the end of this article.

  • §

    First published online in STEM CELLSExpress October 16, 2008.


Roundabout (Robo) family proteins are immunoglobulin-type cell surface receptors that are expressed predominantly in the nervous system. The fourth member of this family, Robo4, is distinct from the other family members in that it is expressed specifically in endothelial cells. In this study, we examined the expression of Robo4 in hematopoietic stem cells (HSCs) and its possible role in HSC regulation. Robo4 mRNA was specifically expressed in murine HSCs and the immature progenitor cell fraction but not in lineage-positive cells or differentiated progenitors. Moreover, flow cytometry showed a correlation between higher expression of Robo4 and immature phenotypes of hematopoietic cells. Robo4high hematopoietic stem/progenitor cells presented higher clonogenic activity or long-term repopulating activity by colony assays or transplantation assays, respectively. A ligand for Robo4, Slit2, is specifically expressed in bone marrow stromal cells, and its expression was induced in osteoblasts in response to myelosuppressive stress. Interestingly, overexpression of Robo4 or Slit2 in HSCs resulted in their decreased residence in the c-Kit+Sca-1+Lineage-side population fraction. These results indicate that Robo4 is expressed in HSCs, and Robo4/Slit2 signaling may play a role in HSC homeostasis in the bone marrow niche. STEM CELLS2009;27:183–190