Chamberlain, James W.
(2018)
Essential or redundant? Disentangling the (GR)ESAG families of Trypanosoma brucei.
PhD thesis, University of Nottingham.
Abstract
The protozoan parasite Trypanosoma brucei escapes its mammalian host’s immune response by switching expression of its major surface antigen, variant surface glycoprotein (VSG), which forms a protective coat around the cell. The actively expressed VSG is transcribed from one of 14 bloodstream expression sites (BESs), which also encode variable repertoires of expression site associated genes (ESAGs), most of which localise to the parasite surface. These genes form distinct families, are co-transcribed with VSG, and the repertoire expressed changes upon in situ switch. Specific ESAGs are known to protect the parasite from human serum lytic effects, modulate the host’s innate immune response, mediate uptake of essential nutrients, and may adapt the parasite to the serum of different mammalian hosts. In addition to BES-linked ESAGs, these families also encompass non-BES encoded genes related to ESAGs (GRESAGs), some of which, unlike the ESAGs, are expressed in both mammalian and insect lifecycle stages.
To investigate the functional relationship between BES and non-BES (GR)ESAGs, I established a system to specifically ablate only BES-derived transcript or multiple (GR)ESAG family members. The former was carried out by tagging BES-linked ESAGs with GFP, and then targeting the GFP ORF by RNAi. The latter involved pan-family RNAi constructs, designed using regions with high identity across individual (GR)ESAG families. Pan-family, but not BES-specific, knockdown induced growth defects, showing that transcript from the active BES itself is not essential for parasite survival in vitro. To investigate this further, a strategy based on Cre recombinase genome engineering was developed to specifically reduce the ESAG repertoire contained within the active BES. Stable cell lines ready for recombination were obtained, and will be used for analysis of modified BESs.
Together, the results achieved show that (GR)ESAG families, as opposed to the actively-expressed ESAG copy, are important for parasite survival in vitro, and there is redundancy between BES-linked and non-BES transcripts. Therefore, the often-overlooked GRESAGs may play more significant roles than originally thought.
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