Characterization of the receptor protein tyrosine phosphatase gamma gene product
The PTPRG gene encodes a receptor tyrosine phosphatase protein with an estimated molecular weight of 185 kDa. Using a baculovirus overexpression system and a series of anti-peptide polyclonal antibodies generated in our laboratory, we examined the activity of the protein in vitro. PTP$\gamma$ was confirmed to be a tyrosine-specific phosphatase with distinct substrate specificities. A partially purified form of the enzyme was allosterically activated by triphosphorylated nucleosides. Ptp$\gamma$ was postulated to be involved in hematopoiesis since it is expressed exclusively in chick monoblast, erythroblast and lymphoblast cell lines but not in the respective mature precursors. Interestingly, Ptp$\gamma$ demonstrates a tightly regulated peak of expression during embryonic stem cell differentiation into embryoid bodies. We perturbed ES cell expression of Ptp$\gamma$ during differentiation in transfection experiments and examined the effect on hemaotpoietic colony formation in methyl cellulose. Stable clones overexpressing Ptp$\gamma$ were blocked during an early/intermediate stage of development. A significant reduction of the level of Ptp$\gamma$ expression in ES cells prohibited hematopoietic colony formation. This suggests that Ptp$\gamma$ is a permissive factor for an early event in hematopoietic differentiation but that its downregulation is required for the completion of the program. We investigated Ptp $\gamma$ expression during mouse embryogenesis using non-radioactive in situ hybridization procedures on whole and sectioned embryos. We observed a dynamic pattern of expression in many tissues throughout development and have suggested potential roles in limb morphogenesis, muscle development and commissural neuron migration and/or differentiation. We demonstrated overlapping or complimentary spatiotemporal patterns of expression of Ptp$\gamma$ and midkine in embryos and tested the hypothesis that midkine is a candidate ligand for Ptp$\gamma.$ Our in vitro experiments did not support a role for midkine as a Ptp$\gamma$ ligand.
0307: Molecular biology