| The Plasmodium life cycle is complex and consists of multiple morphologically and antigenically distinct stages that alternate between the mosquito vector and the vertebrate host. Sporozoites are unique among the invasive stages of Plasmodium in that they must invade two different cell types during the course of the life cycle, the salivary glands of the mosquito and the liver of the mammalian host.; The stable transfection of Plasmodium blood stages has paved the way for a genetic approach for the functional analysis of proteins in the parasite life cycle. The studies presented in this thesis describe the development of methodology and protocols for gene targeting in P. berghei, a rodent malaria model that permits the in vivo analysis of parasite-host interactions. The functions of two sporozoite proteins, circumsporozoite protein (CS) and thrombospondin-related anonymous protein (TRAP), are studied using the molecular genetic tools developed in P. berghei. CS is the major surface protein of sporozoites, whereas TRAP is transiently expressed on the sporozoite surface. Inactivation of the TRAP gene demonstrates that TRAP is critical for sporozoite gliding motility and host cell invasion. In a previous study, disruption of the CS gene was shown to profoundly inhibit sporozoite formation within oocysts. We have confirmed by complementation that the null phenotype is due to the lack of CS. To study the function of CS in sporozoite formation, we have analyzed the ultrastructure of sporogonic development in wild-type, CS(-) and low-CS producer lines of P. berghei. We present evidence that CS controls an early step in sporozoite budding from the oocyst cytoplasm. |
