Forward Genetics in <i>Toxoplasma gondii</i> Reveals a Family of Rhoptry Kinases That Mediates Pathogenesis

Toxoplasma gondii is a widespread protozoan parasite that commonly infects domestic, wild, and companion animals (8). Like related tissue coccidians, T. gondii has a complex life cycle alternating between a sexual cycle, which occurs only within enterocytes of the small intestines of cats (all members of the Felidae appear to be susceptible), and asexual propagation in a variety of warm-blooded vertebrate hosts (8). The sexual phase culminates with fecal shedding of a spore-like stage called the oocyst, which is diploid and undergoes meiosis in the environment. During the asexual cycle, the parasite interconverts between a fast-growing lytic form known as the tacyhzoite and a slow-growing, semidormant form called the bradyzoite. Tachyzoites propagate rapidly in virtually all types of nucleated cells, including macrophages, while differentiation to bradyzoites is favored in long-lived, terminally differentiated host cells (49). Tachyzoites are adept at direct migration across cellular barriers and also disseminate rapidly within leukocytes, thereby reaching sites of immune privilege such as the central nervous system and the developing fetus, where they are more likely to cause disease (2). The life cycle shows remarkable flexibility between lytic and dormant states, thus facilitating asexual transmission between intermediate hosts. This adaptation may account for the recent spread and emergence of a few dominant clonal groups within North America and Europe (46). Toxoplasma is well adapted to mammalian hosts, being transmitted by ingestion of undercooked meat harboring tissue cysts and through food and water supplies contaminated with oocysts shed from cats (8). Humans are accidental hosts of T. gondii, yet seroprevalence rates indicate high rates of chronic infection in many countries of Europe and Central and South America (16). Infections are often mild or subclinical in healthy adults; however, toxoplasmosis can present as a clinically important infection in immunocompromised patients and the developing fetus (32). In many regions of the world, toxoplasmosis remains a frequent problem in patients infected with human immunodeficiency virus, due to lack of access to effective antiviral therapy (29). Additionally, in regions such as southern Brazil, ocular toxoplasmosis often presents as a clinically severe infection in otherwise healthy adults (18). Although T. gondii is primarily an opportunistic pathogen, it has emerged as a model for study of the biology of apicomplexan parasites, a group that contains Plasmodium spp. (malaria parasites), Cryptosporidium spp., and a variety of animal pathogens. Although the life cycles of these parasites differ substantially, they share common pathways for actin-myosin-based motility, calcium-dependent secretion, and active cell invasion (41). Toxoplasma offers excellent tools for studying molecular and cell biology, forward and reverse genetics, and animal models. Hence, exploration of the molecular basis of complex traits such as pathogenesis has been feasible in this system. This review summarizes recent advances using forward genetics to identify genes involved in pathogenesis, as well as the use of reverse genetics to validate the roles of prospective candidate genes.

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