Trypanosoma brucei is the protozoan parasite causing African trypanosomiasis, a neurological disease that affects humans and farm-stock in the impoverished sub-Saharan areas where tsetse fly transmission vector is endemic. Although it has great impact on public health and local economies, it has been neglected in drug discovery for almost a century. Current treatments are either toxic or of difficult administration, besides having serious risks of inducing resistance. Protein kinases are the primary set of signaling proteins in eukaryotes, including Trypanosoma brucei. Their druggability has been widely exploited in cancer research, and has been established in the parasite too. A recent kinome-wide RNAi screen with 176 individual cell lines of mammalian infective bloodstream forms of Trypanosoma brucei identified protein kinases required for proliferation in vitro. In order to investigate which protein kinases are also essential virulence factors in vivo, lines were pooled, inoculated into mice and screened for loss of fitness after 48 hours RNAi compared to uninduced controls. The presence of trypanosomes in the bloodstream was assessed using RNAi target sequencing (RITseq) and compared to an in vitro control. This revealed 49 protein kinases with a significant loss of fitness in vivo in two independent experiments, and a strong correlation between in vitro and in vivo loss of fitness for the majority. However, depletion of nine protein kinases affected more pronouncedly the growth in vivo than in vitro. Amongst these protein kinases were several with putative functions related with stress responses mediated through the PI3K/TOR or MAPK signaling cascades including CK2A2, a promiscuous protein kinase whose activity can be stress-induced; two MAP3Ks, involved in cell integrity upon osmotic shock; VPS15, component of the PI3K complex with roles in autophagosome formation and vesicular trafficking; BUD32, transducer of the PI3K/TOR pathway involved in translational regulation; and FAZ20, a parasite-specific pseudo-kinase localizing to the flagellum attachment zone. The other three have been implicated in repair of alkylation-induced cellular damage: SRPK1, a stress response RNA splicing regulator; AUK2, which acts during mitosis; and CAMKL, an AMPK with calcium-binding domains putatively involved in metabolic regulation. Identification of these virulence-associated protein kinases provides new insights in T. brucei-host interaction and reveals novel potential drug targets for protein kinase inhibitors. This RNAi screen revealed that the evolutionary divergent NEK kinase Repressor of Differentiation Kinase 2 (RDK2) has severe loss of fitness both in vitro and in vivo. Depletion of RDK2 had been shown previously to promote differentiation from bloodstream to procyclic-like forms causing the parasite’s death. Further investigation showed RDK2 to be an active protein kinase capable both of phosphorylating a substrate and to autophosphorylate. Protein kinase activity could be ablated by mutation of lysine 70 to methionine. Mutation of both serine residues (195 and 197), identified as sites of phosphorylation by phosphoproteomics, to alanine or glutamic acid, preventing and mimicking phosphorylation respectively, had no effect on protein kinase activity, suggesting they do not have a direct regulatory role on protein kinase activity. Introducing in the RNAi line a recoded RDK2 whose transcript eluded interference, permitted to some extent the rescue of the induced phenotype, while introducing a recoded inactive mutant did not. This may suggest that the lack of kinase activity was responsible for the RNAi phenotype and not depletion of the protein alone. RDK2 RNAi-differentiated cells could be maintained in conditioned procyclic form media for more than a week. However, they were unable to proliferate. Overexpression of RDK2 blocked the differentiation mediated by sequential treatment with 8-pCPT-cAMP and citrate/cis-aconitate (CCA). RNAi experiments in combination with known differentiation cues, suggested that when differentiation is triggered by the CCA signalling pathway, RDK2 inactivation happens downstream of the phosphatase TbPTP1. Differentiation caused by RDK2 inactivation could be traced in flow cytometry by the detection of EP procyclin expression. This was exploited in a cell-based mechanism-directed phenotypic screen for RDK2 inhibitors. A preliminary run with 518 drug-like molecules that had shown protein kinase inhibition, trypanocidal activity and/or activation of the EP procyclin promoter, unveiled 6 compounds triggering EP procyclin expression and parasite death in the low micromolar range. These compounds can be investigated further to assess whether RDK2 is their in vivo target.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:716917 |
| Date | January 2017 |
| Creators | Fernandez-Cortes, Fernando |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/8208/ |
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