Abstract
Trypanosoma brucei, the causative agent of African sleeping sickness, is transmitted to its mammalian host by the tsetse. In the fly, the parasite’s surface is covered with invariant procyclin, while in the mammal it resides extracellularly in its bloodstream form (BF) and is densely covered with highly immunogenic Variant Surface Glycoprotein (VSG). In the BF, the parasite varies this highly immunogenic surface VSG using a repertoire of ~2500 distinct VSG genes. Recent reports in mammalian systems point to a role for histone acetyl-lysine recognizing bromodomain proteins in the maintenance of stem cell fate, leading us to hypothesize that bromodomain proteins may maintain the BF cell fate in trypanosomes. Using small-molecule inhibitors and genetic mutants for individual bromodomain proteins, we performed RNA-seq experiments that revealed changes in the transcriptome similar to those seen in cells differentiating from the BF to the insect stage. This was recapitulated at the protein level by the appearance of insect-stage proteins on the cell surface. Furthermore, bromodomain inhibition disrupts two major BF-specific immune evasion mechanisms that trypanosomes harness to evade mammalian host antibody responses. First, monoallelic expression of the antigenically varied VSG is disrupted. Second, rapid internalization of antibodies bound to VSG on the surface of the trypanosome is blocked. Thus, our studies reveal a role for trypanosome bromodomain proteins in maintaining bloodstream stage identity and immune evasion. Importantly, bromodomain inhibition leads to a decrease in virulence in a mouse model of infection, establishing these proteins as potential therapeutic drug targets for trypanosomiasis. Our 1.25Å resolution crystal structure of a trypanosome bromodomain in complex with I-BET151 reveals a novel binding mode of the inhibitor, which serves as a promising starting point for rational drug design.
Highlights
Trypanosoma brucei is a unicellular, protozoan parasite and the causative agent of Human African Trypanosomiasis
T. brucei lives in the bloodstream of the mammalian host before migrating to the insect through the bite of its insect vector, the tsetse fly
Bromodomain proteins bind to DNA that is wrapped around histone proteins, acting as mediators that interact with the transcription machinery to determine which genes are turned on and which are kept repressed
Summary
Trypanosoma brucei is a unicellular, protozoan parasite and the causative agent of Human African Trypanosomiasis (sleeping sickness). The life cycle of Trypanosoma brucei requires adaptation to two distinct habitats: the fly (tsetse) and the bloodstream of its mammalian hosts. Within these habitats, the parasite assumes a succession of proliferative and quiescent developmental forms, which vary widely in metabolism, motility, and composition of the surface coat that covers the plasma membrane. The parasite relies on two strategies to evade the mammalian host antibody response It varies (“switches”) its highly immunogenic surface antigen, using a repertoire of ~2,500 distinct VSG genes [1]. There is a drastic change in metabolism as the trypanosomes leave the glucose-rich environment of the blood and transition to the fly midgut, where they are more reliant on mitochondrion Krebs cycle enzymes and respiratory chain and oxidative phosphorylation enzymes (reviewed in [5])
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