Growth and development in Toxoplasma gondii.

摘要

The significance of Toxoplasma gondii to human and animal health, along with its value as a model for other pathogenic protozoa, makes this microorganism an important model in the field of Apicomplexa research. It is clear from numerous studies of the diseases caused by this family of microorganisms that parasitaemia itself is key to pathogenesis, and thus, an understanding of the growth processes in these pathogens could provide better treatments. In this work, the relationship/regulation between the cell cycle and development was investigated. A G2 population arose during tachyzoite-to-bradyzoite differentiation thus, providing a mechanism of linking the cell cycle with development. To gain further insight into the regulation of the parasite cell cycle, both a reverse and forward genetic approach was used. One approach used bioinformatics and cDNA library screening to identify cell cycle related proteins. Two distinct proliferating cell nuclear antigen (PCNA) genes in T. gondii were identified and this work demonstrated that these two genes were differentially expressed during development and the tachyzoite cell cycle. Further work showed that each PCNA likely acts independently and provided evidence that TgPCNA1 potentially serves as the major replisomal PCNA. The function of PCNA2 in T. gondii remains unknown. Finally, a forward genetics approach focused on the complementation of the tachyzoite temperature sensitive (ts) mutant 11C9, which arrests within 1 to 2 divisions at the non-permissive temperature (40°C), and approximately half of the parasites arrest with a single undivided nucleus (2N) placing the defect at some point in mitosis. Complementation of ts11C9 with a homolog of the eukaryotic XPMC2 suggests that the cyclinB/cdk or wee1/chk1 pathway in this mutant is likely affected. In T. gondii, a proposed linkage between growth and development seem consistent with a developmental timer system (“clock mechanism”), yet the presence of checkpoints suggests that the domino model plays at least a partial role in regulating the tachyzoite cell cycle.