Revealing spatio-temporal dynamics with long-term trypanosomatid live-cell imaging.

Trypanosoma brucei, the causative agent of human African trypanosomiasis, is highly motile and must be able to move in all three dimensions for reliable cell division. These characteristics make long-term microscopic imaging of live T. brucei cells challenging, which has limited our understanding of important cellular events. To address this issue, we devised an imaging approach that confines cells in small volumes within cast agarose microwells that can be imaged continuously for up to 24 h. Individual T. brucei cells were imaged through multiple rounds of cell division with high spatial and temporal resolution. We developed a strategy that employs in-well "sentinel" cells to monitor potential imaging toxicity during loss-of-function experiments such as small-molecule inhibition and RNAi. Using our approach, we show that the asymmetric daughter cells produced during T. brucei division subsequently divide at different rates, with the old-flagellum daughter cell dividing first. The flagellar detachment phenotype that appears during inhibition of the Polo-like kinase homolog TbPLK occurs in a stepwise fashion, with the new flagellum initially linked by its tip to the old, attached flagellum. We probe the feasibility of a previously proposed "back-up" cytokinetic mechanism and show that cells that initiate this process do not appear to complete cell division. This live-cell imaging method will provide a novel avenue for studying a wide variety of cellular events in trypanosomatids that have previously been inaccessible.

Identification of TOEFAZ1-interacting proteins reveals key regulators of Trypanosoma brucei cytokinesis.

The protist parasite Trypanosoma brucei is an obligate extracellular pathogen that retains its highly polarized morphology during cell division and has evolved a novel cytokinetic process independent of non-muscle myosin II. The polo-like kinase homolog TbPLK is essential for transmission of cell polarity during division and for cytokinesis. We previously identified a putative TbPLK substrate named Tip of the Extending FAZ 1 (TOEFAZ1) as an essential kinetoplastid-specific component of the T. brucei cytokinetic machinery. We performed a proximity-dependent biotinylation identification (BioID) screen using TOEFAZ1 as a means to identify additional proteins that are involved in cytokinesis. Using quantitative proteomic methods, we identified nearly 500 TOEFAZ1-proximal proteins and characterized 59 in further detail. Among the candidates, we identified an essential putative phosphatase that regulates the expression level and localization of both TOEFAZ1 and TbPLK, a previously uncharacterized protein that is necessary for the assembly of a new cell posterior, and a microtubule plus-end directed orphan kinesin that is required for completing cleavage furrow ingression. The identification of these proteins provides new insight into T. brucei cytokinesis and establishes TOEFAZ1 as a key component of this essential and uniquely configured process in kinetoplastids.