Protein Tyrosine Phosphatase-Sigma (PTPσ) Regulates Hematopoietic Stem Cell Repopulating Capacity

Hematopoietic stem cell (HSC) proliferation, differentiation and self-renewal are regulated by signaling through receptor tyrosine kinases (RTKs) such as c-kit, Flt-3 and Tie2. The functions of receptor protein tyrosine phosphatases (RPTPs) in counterbalancing RTK signaling in HSCs remain incompletely understood. Among 9 examined RPTPs, we found that PTP-sigma (PTPσ) was significantly overexpressed in mouse and human HSCs compared to more mature hematopoietic cells. PTPσ^-/- mice displayed no difference in mature blood counts or phenotypic HSC frequency compared to PTPσ^+/+ mice. However, competitive transplantation of BM cells from PTPσ^-/- mice resulted in greater than 8-fold increased multilineage hematopoietic repopulation in primary and secondary recipient mice compared to mice transplanted with BM from PTPσ^+/+ mice. While HSCs from PTPσ^-/- mice displayed no differences in cell cycle status or homing capability compared to PTPσ^+/+ mice, PTPσ^-/- HSCs expressed significantly increased levels of activated Rac1, a RhoGTPase which regulates HSC engraftment capacity, compared to PTPσ^+/+ BM cells. PTPσ^-/- BM cells also displayed significantly increased transendothelial migration capacity and cobblestone area forming cells (CAFCs), consistent with increased Rac1 activation. Furthermore, Rac inhibition abrogated the increased migration capacity of PTPσ^-/- BM cells, suggesting that the augmented engraftment capacity of PTPσ^-/- BM cells was mediated via Rac activation. Translationally, we demonstrate that negative selection of human cord blood CD34⁺CD38^-CD45RA^-Lin^- cells for PTPσ expression yielded a 15-fold enrichment for human long-term repopulating HSCs compared to CD34⁺CD38^-CD45RA^-Lin^- cells or CD34⁺CD38^-CD45RA^-lin^-PTPσ⁺ cells. These data suggest that PTPσ regulates HSC repopulating capacity via inhibition of Rac1 and that selection for human PTPσ - negative HSCs is a translatable strategy to significantly enrich human HSCs for transplantation.