A variety of cell surface markers such as c-Kit, Sca-1, CD34 and Flt-3 have been utilized to prospectively isolate murine or human hematopoietic stem cells (HSCs). While murine HSCs were shown to be highly enriched in CD34−c-Kit+Sca-1+Lineage- (CD34−KSL) fraction, this population is still not homogeneous for long-term HSCs. In human, CD34+ cells are regarded as crude HSC fraction and used for clinical applications. However, quiescent human HSCs are also found in CD34− fraction, indicating that CD34 is not a bona fide marker for human HSC. Thus, novel surface markers that can be used to purify human or murine HSCs to homogeneity need to be identified.

Roundabout (Robo) family proteins are immunoglobulin-type cell surface receptors that are predominantly expressed in nervous system. Slit2, a ligand for Robo, is a large leucine-rich repeat-containing secreted protein that is also expressed in brain. By binding with Robo, Slit2 acts as a repellant for axon guidance of developing neurons and they are critical for correct wiring of neuronal network. Robo family comprises four family members, Robo1 – Robo4, and Robo4 is distinct in that it is expressed specifically in endothelial cells, but not in brain. In this study, we investigated Robo4 for its possible application for HSC identification in murine and human hematopoietic system.

By RT-PCR, Robo4 was specifically expressed in murine KSL fraction, and was not expressed in lineage positive cells and various progenitors such as common myeloid progenitor (CMP), granulocyte-monocyte progenitor (GMP), megakaryocyte/erythroid progenitor (MEP) and common lymphoid progenitor (CLP). Moreover, the expression of Robo4 was highest in side population of KSL cells (KSL-SP), and moderate in KSL-main population (KSL-MP) cells. Monoclonal antibody raised against Robo4 identified its high expression in KSL cells by FACS. FACS analysis of human cord blood cells revealed that Robo4 is highly expressed in CD34+ cells, and CD34+Robo4high population fell into CD38− fraction, which enriches human HSCs. Bone marrow transplantation experiments revealed that Robo4+ fraction of murine KSL cells had long-term repopulating activity, while Robo4−KSL cells not. Although both Robo4+ and Robo4− CD34−KSL cells repopulated murine hematopoietic system for long-term, Robo4+CD34−KSL cells achieved higher chimerism after repopulation compared with Robo4−CD34−KSL.

To investigate the physiological role of Robo4 in HSC homeostasis, we next examined the expression of Slit2 in hematopoietic system. Interestingly, Slit2 is specifically expressed in bone marrow stromal cells, but not in hematopoietic cells. Moreover, Slit2 is induced in osteoblasts, a critical cellular component composing HSC niche, in response to myelosuppressive stress such as 5FU treatment.

These results indicate that

  1. Robo4 is expressed in murine and human hematopoietic HSCs and useful for HSC purification, and

  2. Robo4 - Slit2 system may play a role in HSC physiology in niche environment under hematopoietic stress.

Disclosure: No relevant conflicts of interest to declare.

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