Identification of a protein phosphatase 2A family member that regulates cell cycle progression in Trypanosoma brucei.

The cell cycle consists of an orderly sequence of events, whose purpose is to faithfully replicate and segregate cellular components. Many events in the cell cycle are triggered by protein kinases and counteracting phosphoprotein phosphatases (PPP). In Trypanosoma brucei, RNAi has been used to characterize numerous regulatory kinases, while the role of protein phosphatases has primarily been deduced with inhibitors such as okadaic acid and calyculin. In the present study, we identify for the first time a protein phosphatase 2A family member (TbPP2A-1) whose knockdown with RNAi phenocopies the effects of okadaic acid (OKA). In bloodstream forms (BF) and insect stage procyclic forms (PF) RNAi of TbPP2A-1 generates a cell population characterized by: an inhibition of cell growth, a block in cytokinesis; continued synthesis of nuclear DNA leading to aneuploidy; continued mitosis leading to cells with N>2, and an unusual phenotype where number of kinetoplasts (and flagella) is less than the number of nuclei. An engineered cell line was constructed to further study TbPP2A-1 and to facilitate the discovery of other cell cycle regulatory genes.

The human Aurora kinase inhibitor danusertib is a lead compound for anti-trypanosomal drug discovery via target repurposing.

New drugs for neglected tropical diseases such as human African trypanosomiasis (HAT) are needed, yet drug discovery efforts are not often focused on this area due to cost. Target repurposing, achieved by the matching of essential parasite enzymes to those human enzymes that have been successfully inhibited by small molecule drugs, provides an attractive means by which new drug optimization programs can be pragmatically initiated. In this report we describe our results in repurposing an established class of human Aurora kinase inhibitors, typified by danusertib (1), which we have observed to be an inhibitor of trypanosomal Aurora kinase 1 (TbAUK1) and effective in parasite killing in vitro. Informed by homology modeling and docking, a series of analogs of 1 were prepared that explored the scope of the chemotype and provided a nearly 25-fold improvement in cellular selectivity for parasite cells over human cells.

The cell cycle as a therapeutic target against Trypanosoma brucei: Hesperadin inhibits Aurora kinase-1 and blocks mitotic progression in bloodstream forms.

Aurora kinase family members co-ordinate a range of events associated with mitosis and cytokinesis. Anti-cancer therapies are currently being developed against them. Here, we evaluate whether Aurora kinase-1 (TbAUK1) from pathogenic Trypanosoma brucei might be targeted in anti-parasitic therapies as well. Conditional knockdown of TbAUK1 within infected mice demonstrated its essential contribution to infection. An in vitro kinase assay was developed which used recombinant trypanosome histone H3 as a substrate. Tandem mass spectroscopy identified a novel phosphorylation site in the carboxyl-tail of recombinant trypanosome histone H3. Hesperadin, an inhibitor of human Aurora B, prevented the phosphorylation of substrate with IC(50) of 40 nM. Growth of cultured bloodstream forms was also sensitive to Hesperadin (IC(50) of 50 nM). Hesperadin blocked nuclear division and cytokinesis but not other aspects of the cell cycle. Consequently, growth arrested cells accumulated multiple kinetoplasts, flagella and nucleoli, similar to the effects of RNAi-dependent knockdown of TbAUK1 in cultured bloodstream forms cells. Molecular models predicted high-affinity binding of Hesperadin to both conserved and novel sites in TbAUK1. Collectively, these data demonstrate that cell cycle progression is essential for infections with T. brucei and that parasite Aurora kinases can be targeted with small-molecule inhibitors.

The RACK1 signal anchor protein from Trypanosoma brucei associates with eukaryotic elongation factor 1A: a role for translational control in cytokinesis.

RACK1 is a WD-repeat protein that forms signal complexes at appropriate locations in the cell. RACK1 homologues are core components of ribosomes from yeast, plants and mammals. In contrast, a cryo-EM analysis of trypanosome ribosomes failed to detect RACK1, thus eliminating an important translational regulatory mechanism. Here we report that TbRACK1 from Trypanosoma brucei associates with eukaryotic translation elongation factor-1a (eEF1A) as determined by tandem MS of TAP-TbRACK1 affinity eluates, co-sedimentation in a sucrose gradient, and co-precipitation assays. Consistent with these observations, sucrose gradient purified 80S monosomes and translating polysomes each contained TbRACK1. When RNAi was used to deplete cells of TbRACK1, a shift in the polysome profile was observed, while the phosphorylation of a ribosomal protein increased. Under these conditions, cell growth became hypersensitive to the translational inhibitor anisomycin. The kinetoplasts and nuclei were misaligned in the postmitotic cells, resulting in partial cleavage furrow ingression during cytokinesis. Overall, these findings identify eEF1A as a novel TbRACK1 binding partner and establish TbRACK1 as a component of the trypanosome translational apparatus. The synergy between anisomycin and TbRACK1 RNAi suggests that continued translation is required for complete ingression of the cleavage furrow.

The RACK1 homologue from Trypanosoma brucei is required for the onset and progression of cytokinesis.

The receptor for activated C kinase 1 (RACK1) is a conserved scaffold protein that helps regulate a range of cell activities including cell growth, shape, and protein translation. We report that a homologue of RACK1 is required for cytokinesis in pathogenic Trypanosoma brucei. The protein, referred to as TRACK, is comprised of WD repeat elements and can complement cpc2 null mutants of Schizosaccharomyces pombe. TRACK is expressed throughout the trypanosome life cycle and is distributed predominantly in a perinuclear region and the cytoplasm but not along the endoplasmic reticulum, mitochondrion, or cleavage furrow of dividing cells. When tetracycline-inducible RNA interference (RNAi) is used to deplete the cellular content of TRACK, the cells remain metabolically active, but growth is inhibited. In bloodstream forms, growth arrest is due to a delay in the onset of cytokinesis. By contrast, procyclic forms are able to initiate cytokinesis in the absence of TRACK but arrest midway through cell cleavage. The RNAi cells undergo multiple rounds of partial cytokinesis and accumulate nuclei and cytoplasmic extensions with attached flagella. The TRACK RNAi construct is also inducible within infected mice. Under these conditions parasites are eliminated from peripheral blood within 3 days post-infection. Taken as a whole, these data indicate that trypanosomes utilize a RACK1 homologue to regulate the final stages of mitosis. Moreover, disrupting the interaction between TRACK and its partners might be targeted in the design of novel therapies.