PROJECT 3: Protein Tyrosine Phosphatase-Sigma, A Novel Target for Mitigation of Acute Radiation Injury PROJECT SUMMARY The threat of terrorism using radiological/nuclear weapons is a major national security and public health concern in the United States. The potentially devastating effects of acute radiation exposure on the hematopoietic, gastrointestinal and other vital organs are well chronicled, but few effective mitigators exist which are capable of ameliorating radiation-induced vital organ damage. My laboratory has discovered that cells within the BM microenvironment, specifically vascular endothelial cells (ECs), secrete growth factors capable of promoting hematopoietic stem cell (HSC) regeneration following radiation injury. We showed further that 2 EC-derived proteins, pleiotrophin (PTN) and epidermal growth factor (EGF), produced marked improvements in mice survival when administered after lethal dose irradiation. PTN treatment significantly improved the survival of irradiated mice when administered as late as 4 days after radiation exposure, highlighting the practical utility of PTN as a deliverable mitigator of radiation injury. Based on our discovery that PTN promotes HSC regeneration via inhibition of protein tyrosine phosphatase receptor ? zeta (PTP?), we screened HSCs for expression of other members of the PTP receptor family. We found that PTP-Sigma (PTP?) was more than 100-fold overexpressed compared to PTP? on BM HSCs. Interestingly, HSCs from PTP?-/- mice displayed 8-fold increased long term repopulating capacity compared to HSCs from PTP?+/+ mice and loss-of- function studies suggested that this was mediated by the RhoGTPase, Rac1. Importantly, PTP?-/- mice displayed significantly increased recovery of BM colony forming cells (CFCs) at day +10 following 600 cGy total body irradiation (TBI) compared to identically irradiated PTP?+/+ mice. Based upon these preliminary data, we hypothesize that PTP? regulates HSC regeneration following irradiation and that inhibition of PTP? will promote HSC regeneration and improve survival following radiation injury. In keeping with this hypothesis, we propose the following Specific Aims: 1. Determine whether genetic or pharmacologic modulation of PTP? can promote hematopoietic regeneration and improve the survival of irradiated mice. 2. Determine the mechanism through which PTP? regulates HSC regeneration following radiation injury. 3. Determine whether PTP? inhibition can improve human HSC regeneration and non-human primate survival. The broad objective of this proposal is to interrogate PTP? and Rac proteins as novel targets for radiation mitigation and to develop a PTP?- or Rac-targeted therapeutic for victims of radiation injury and for dual use to accelerate hematopoietic reconstitution in myelosuppressed or myeloablated patients.