A Small Molecule Inhibitor of Protein Tyrosine Phosphatase-Sigma (PTPσ) Promotes Hematopoietic Stem Cell (HSC) Regeneration
Authors: Zhang Yurun , Quarmyne Mamle , Himburg Heather A , Yan Xiao , McBride William , Jung Michael , Chute John P. 
Published: October 2019

Abstract
Receptor tyrosine kinases (RTKs), such as c-kit, Flt-3 and Tie2, regulate hematopoietic stem cell (HSC) proliferation, differentiation and maintenance. Substantially less is known regarding the function of receptor protein tyrosine phosphatases (PTPs) in regulating HSC fate. We recently discovered that receptor protein tyrosine phosphatase sigma (PTPσ) is highly expressed by murine and human HSCs. Interestingly, constitutive deletion of PTPσ caused a marked increase in HSC repopulating capacity as measured in primary and secondary competitive repopulation assays (J Clin Invest 2015;125:177-182). Further, negative selection of human cord blood (CB) HSCs for PTPσ surface expression (PTPσ - negative CB HSCs) conferred more than 10-fold increased human CB hematopoietic engraftment through 20 weeks in transplanted NSG mice. Additionally, PTPσ-deficient mice displayed significantly augmented recovery of phenotypic bone marrow (BM) HSCs and colony forming cells at day +10 following 550 cGy total body irradiation. Based on these observations, we hypothesized that PTPσ may function as a negative regulator of HSC self-renewal and regeneration. We sought to develop pharmacologic strategies to inhibit PTPσ function as a means to augment HSC functional capacity. Based on structure-activity-relationship analysis of PTPσ, we screened candidate small molecules for ability to modulate PTPσ function on BM HSCs. We identified a small molecule, 5483071 (Chembridge), as a candidate PTPσ inhibitor. In silico simulation indicated that 5483071 rigidly docked into the binding site of PTPσ intracellular domain through hydrogen bonding and electrostatic interactions. In a model of HSC injury and regeneration, we irradiated C57Bl6 mice with 700 cGy TBI and treated mice systemically with either 10 μcg of 5483071 or water subcutaneously every other day from day +1 to day +14. Irradiated mice treated with 5483071 displayed significantly increased 60 day survival compared to controls (P=0.0007). Irradiated mice treated with 5483071 showed accelerated recovery of BM SLAM+kit+sca-1+lin- HSCs (P=0.02), BM KSL cells (P=0.01), and colony forming cells (CFCs) (P=0.0003). In vitro culture of BM KSL cells with 5483071 significantly increased the levels of activated Rac1 (P=0.0004), which recapitulated the effects of PTPσ deletion on Rac1 activation in HSCs. Importantly, treatment of BM KSL cells from PTPσ -/- mice with 5483071 caused no change in Rac1 activation, suggesting that 5483071 acted specifically on PTPσ and was not mediating effects via inhibition of other phosphatases. Systemic administration of 5483071 to irradiated mice caused an increase in BM KSL cell cycling at 72 hours compared to irradiated control mice (p=0.02), while also decreasing BM KSL cell apoptosis at 24 hours after TBI (p=0.02). Subsequent to these findings, we have generated several new small molecule inhibitors of PTPσ with novel composition of matter and have taken a lead compound into pre-clinical studies for investigational new drug (IND) development. PTPσ represents a novel receptor tyrosine phosphatase that regulates HSC self-renewal and regeneration. Targeted inhibition of PTPσ has high therapeutic potential to promote hematopoietic regeneration in patients receiving myelosuppressive chemotherapy and/or radiotherapy or undergoing myeloablative hematopoietic cell transplantation.

Disclosures
No relevant conflicts of interest to declare.