Regulation of the Development of Trypanosoma brucei
The parasitic protozoa, Trypanosoma brucei spp., are the causative pathogens of African sleeping sickness in humans and nagana in cattle. They are a family of deeply branched eukaryotes that exhibit many unique features during their cell cycle progression. Due to the presence of a single mitochondrion in each cell, the nuclear cycle and mitochondrial cycle in T. brucei have to be coordinated for a fruitful cell division, suggesting also that cytokinesis could be controlled by either of the two cycles. Our studies indicated that cytokinesis is primarily regulated by the mitochondrion in the insect (procyclic) form but by the nucleus in the bloodstream form of T. brucei. We also observed that a block of mitosis achieves genuine mitotic arrest in the procylic form but not in the bloodstream form, which continues with re-entries into a new G1 phase and nonstop nuclear DNA synthesis. The bloodstream form is thus apparently lacking the spindle assembly checkpoint, which is present and functioning in the procyclic form. These unusual changes in command, never observed among any other eukaryotes previously, prompted us to examine some of the protein kinases in T. brucei known to control both mitosis and cytokinesis in other eukaryotes. The Polo-like kinase (TbPLK) turns out regulating only cytokinesis and localizing to the flagellum attachment zone (FAZ) in T. brucei. The Aurora-like kinase (TbAUK1) resides in a novel chromosomal passenger complex (CPC) that trans-localizes from the central spindle midzone in late anaphase across the nuclear envelope to the FAZ and then transcends down from the anterior to the posterior end along the FAZ to separate the cell into two. This is a most unusual mode of cytokinesis. We plan to pursue it further by tracing the CPC trans-localization with time-lapse imaging and dissecting the specific roles of TbPLK and TbAUK1 and their potential interactions during this fascinating process with the eventual goal of establishing the cell regulation in T. brucei a useful model system for advanced basic research as well as a potential target for anti-trypanosomiasis chemotherapy. African trypanosomes, the Trypanosoma brucei spp., are parasitic protozoa causing sleeping sickness in humans and nagana in cattle in Sub-Sahara Africa. They are ancient organisms bearing unique features in cell division. A single mitochondrion in each trypanosome cell requires a coordinated mitochondrial and nuclear division for successful cell growth. Theoretically, either the mitochondrion or the nucleus alone could signal the cell to divide. In our studies, we found that the insect form of T. brucei divides under the control of mitochondrion whereas the bloodstream form is regulated by its nucleus. This ready change of command on cell division in the same organism has never been observed previously, because cell division is always tightly controlled by mitosis in other organisms. We have since examined in T. brucei the Polo-like kinase (TbPLK) and Aurora-like kinase (TbAUK1), known to play pivotal roles in controlling both mitosis and cell division in other organisms, to find out how they function in T. brucei. TbPLK turns out playing no role at all in mitosis. It controls only cell division and localizes outside the nucleus. TbAUK1 resides in a novel protein complex and trans-localizes at the completion of mitosis from the nuclear spindle to the dorsal side of the cell and transcends down from the anterior to the posterior end of the cell to divide it into two. This is a novel pattern of cell division. We plan to look into the detailed mechanism behind it, delineate the functions of TbPLK and TbAUK1 in conducting this unique event and develop it into a potential target for anti-sleeping sickness chemotherapy.