Robo4 Cooperates with Cxcr4 to Specify Hematopoietic Stem Cell Localization to Bone Marrow Niches

Abstract

Summary Specific bone marrow (BM) niches are critical for hematopoietic stem cell (HSC) function during both normal hematopoiesis and in stem cell transplantation therapy. We demonstrate that the guidance molecule Robo4 functions to specifically anchor HSCs to BM niches. Robo4-deficient HSCs displayed poor localization to BM niches and drastically reduced long-term reconstitution capability while retaining multilineage potential. Cxcr4, a critical regulator of HSC location, is upregulated in Robo4−/− HSCs to compensate for Robo4 loss. Robo4 deletion led to altered HSC mobilization efficiency, revealing that inhibition of both Cxcr4- and Robo4-mediated niche interactions are necessary for efficient HSC mobilization. Surprisingly, we found that WT HSCs express very low levels of Cxcr4 and respond poorly to Cxcr4 manipulation relative to other hematopoietic cells. We conclude that Robo4 cooperates with Cxcr4 to endow HSCs with competitive access to limited stem cell niches, and we propose Robo4 as a therapeutic target in HSC transplantation therapy.

Slit-Robo signaling

“…Robo4 and Cxcr4 are both important for anchoring hematopoietic stem cells to bone marrow niches, an adhesive effect that needs to be overcome for effective recruitment of these cells into the bloodstream (Smith-Berdan et al, 2011). A physical interaction between the receptors however, was not shown in this study, even though Cxcr4 is compensatorily upregulated in Robo4 −/− mutant mice.…”

Wnt-inhibitory factor 1 dysregulation of the bone marrow niche exhausts hematopoietic stem cells

“…Modulation of this axis of stem cell regulation has been observed previously with cytoxic agents in stem cell mobilization. 49 Because Wif1 mice have more HSPCs in their spleens at steady state, it will be interesting to examine stem cell mobilization in the future. In addition, these data may suggest that Wif1 HSCs are displaced from the quiescent stem cell niche anatomically and that this displacement aids and abets the dysregulation seen in both compartments.…”

Cell cycle regulation in hematopoietic stem cells

“…Moreover, disruption of HSC quiescence leads to defects in HSC self-renewal and often results in HSC exhaustion (Orford and Scadden 2008), hence underscoring the critical importance Sanchez-Aguilera et al, 2011). Other factors also contribute to HSC localization to the BM either in conjunction with CXCR4, such as prostaglandin E2 (PGE2) and the neuronal guidance protein Robo4 (Hoggatt et al, 2009;Smith-Berdan et al, 2011), or independently from CXCR4 like c-Kit, the calcium-sensing receptor (CaR), and the transcription factor Egr1 (Christensen et al, 2004;Adams et al, 2006;Min et al, 2008). Thereafter, HSCs remain anchored in the BM niche by complex integrindependent mechanisms (Scott et al, 2003;Forsberg and SmithBerdan 2009), though small numbers of HSCs will periodically migrate from the BM into the circulation and back for short periods of time under homeostatic conditions, perhaps as a form of immunosurveillance (Massberg et al, 2007;Bhattacharya et al, 2009).…”