Comparative and functional genomics of Toxoplasma and Hammondia hammondi

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PROJECT SUMMARY/ABSTRACT: Toxoplasma gondii is an important opportunistic pathogen of humans where it can cause severe disease in the developing fetus and those with HIV/AIDS. Parasite development is critical for its ability to cause severe disease, yet the precise mechanisms for how it does this are poorly understood. T. gondii is also modulates the cell that in infects via secretion of a multitude of effectors. Much of the work aimed at understanding how T. gondii is able to cause disease has focused exclusively on T. gondii itself. An innovative alternative to this approach is to perform functional and genetic comparisons between T. gondii and its near relatives to reveal previously unknown mechanisms of parasite virulence and developmental regulation. The parasite Hammondia hammondi is one such relative, and our work in the prior funding period has developed this organism into a powerful comparative model for probing T. gondii biology from an evolutionary context. In this renewal we follow up on two observations from the prior funding period: 1) that T. gondii is unique in its ability to facultatively regulate its conversion to the quiescent cyst stage and 2) that T. gondii is unique in its ability to alter the host cell cycle and that this may be linked to suppression of host anti-parasitic responses. In Aim 1 we use a focused RNAseq screen to identify new regulators of T. gondii development, using H. hammondi as a natural filter to focus on relevant candidate genes. Success of this Aim is facilitated by the use of new transgenic approaches for H. hammondi and preliminary data supporting the premise of our candidate based approach. In Aim 2 we determine how T. gondii and H. hammondi differentially regulate changes in the infected host cell, with a focus on underappreciated host pathways including DNA damage responses and cellular senescence. Success of this aim is facilitated by our extensive experience with T. gondii and H. hammondi sporozoites and in vitro assays, as well as transgenic approaches in H. hammondi that will permit us to perform the first ever cross-species complementation experiments in this organism. Overall these studies build on work during the prior funding period aimed at identifying important phenotypic differences between these closely related parasite species and then determining their molecular mechanisms. We expect these studies to result in new discoveries of how T. gondii regulates in growth and development and how it is able to suppress a wide variety of host defenses. Both of these lines of inquiry will identify new avenues for therapeutic intervention, whether they target the parasite itself or host responses found to be critical for its survival.