Syndecan-2 Surface Expression Identifies Hematopoietic Stem Cells with Increased Repopulating Capacity

Syndecans are transmembrane glycoproteins, which can regulate cell proliferation, growth, and adhesion through interactions with neighboring proteins within the plasma membrane or at the cytoplasmic interface. Although syndecans have been described to regulate aberrant signaling in hematological malignances, the role of syndecans in regulating normal hematopoietic stem cell (HSC) proliferation, differentiation, and self-renewal is largely unknown. We demonstrate that syndecan-1 and syndecan-3 are expressed on the surface of < 10% of murine hematopoietic stem and progenitor cells, whereas, syndecan-4 is expressed on 50% of lineage negative (Lin^-) progenitor cells and 98% of c-Kit⁺Sca-1⁺Lin^- (KSL) hematopoietic stem/progenitor cells, KSL CD34^-CD48^-CD150^+/- short-term HSCs, and 100% of CD34^-CD48^-CD150⁺ long-term HSCs. Interestingly, we find that syndecan-2 is expressed by 11% of KSL CD34^-CD48^-CD150^+/- short-term HSCs and 36% of CD34^-CD48^-CD150⁺ long-term HSCs. More specifically, our data demonstrate a 15-fold increase in syndecan-2 surface expression on KSL CD34^-CD48^-CD150⁺ HSCs compared to Lin^- progenitor cells (p<.0001, ****). Collectively, these data suggest that syndecan-2 may be a marker for long-term HSCs. In keeping with this hypothesis, we found that syndecan-2⁺ CD34^- KSL cells produce two-fold more granulocyte, erythrocyte, monocyte, megakaryocyte (GEMM) colonies compared to syndecan-2^- CD34^- KSL cells (p=.0017, **). Cell cycle analyses revealed a significant increase in BrdU incorporation in syndecan-2⁺ KSL cells compared to syndecan-2^- KSL cells (90% versus 40%, p<.0001, ****). Competitive repopulation assays comparing syndecan-2⁺ or syndecan-2^- CD34^- KSL bone marrow cells demonstrated that mice transplanted with syndecan-2⁺ CD34^- KSL cells displayed threefold increased donor multilineage hematopoietic cell repopulation compared to mice transplanted with syndecan-2^- CD34^- KSL cells. These data suggest that syndecan-2 expression marks a highly proliferative population of HSCs with increased multilineage repopulating capacity and that syndecan-2⁺ HSCs can be readily isolated to enhance the efficacy of hematopoietic cell transplantation.