Characterization of a family of protein tyrosine phosphatases from Microplitis demolitor bracovirus

Abstract

Among the most virulent pathogens of insects are symbiotic polydnaviruses (PDVs) which are associated with parasitoid wasps. Characterization of the virulence molecules PDVs produce and functional studies are essential to understanding how PDVs kill insect pests. Many gene products encoded by these pathogens target conserved signaling pathways, making them valuable tools for understanding processes regulating the insect immune system. My dissertation project focused on the parasitoid Microplitis demolitor, its associated PDV named M. demolitor bracovirus (MdBV), and a family of viral genes called protein tyrosine phosphatases (PTPs). Specific aims of my project were to: 1) characterize the structure, function and expression of MdBV PTPs, 2) assess whether any PTP family members play a role in immunosuppression, and 3) understand how MdBV infection inhibits insect growth and pupation. Analysis of PTP expression indicated that most members of this family are expressed in hemocytes (immune cells) of its insect host. MdBV-infected hemocytes also exhibited higher levels of tyrosine phosphatase activity than non-infected hemocytes. Functional studies indicated that two PTPs have tyrosine phosphatase activity, whereas two other did not. The catalytically active PTPs co-localize to focal adhesions and inhibit phagocytosis of several foreign targets in hemocyte-like cell lines, suggesting that focal adhesion kinase or related proteins are candidate substrates for these viral PTPs. I also found that the catalytically active PTPs caused apoptosis when expressed in Sf-21 cells. My findings suggest that the anti-phagocytic activity of PTP-H2 is conserved but that its apoptosis-inducing activity is restricted and depends in part on intrinsic differences between cell types. The results of my endocrine studies indicate that MdBV blocks pupation by disabling production of ecdysteroids by prothoracic glands (PGs) and the production or release of brain hormones that trigger PGs to produce ecdysteroids. My metabolic studies also indicate that infected larvae undergo a shift in carbohydrate metabolism that resembles the shift observed during starvation. In summary, my dissertation work provides important insights into the mechanisms by which PDVs cause disease in insects and provides evidence for a role of two PTP family members in immunsuppression.