Differential Protein Expression and Adaptative Mechanisms in Sleeping Sickness

Recently, researchers have turned to proteomics to measure variable protein expression in the proteomes of infectious organisms that cause debilitating disease with limited or poor drug-intervention choices. A single-cell organism, Trypanosoma brucei, causes sleeping sickness in humans. The organism has a unique life cycle. The tsetse fly is the primary vector, spreading trypanosomes with their saliva when they bite a victim and acquiring them in the same manner. In an infected human host, trypanosomes travel through lymphatic fluid, then enter the blood stream, and finally enter the cerebrospinal fluid where they can cross the blood-brain barrier. Damage to the brain is irreversible and ends almost inevitably in death. The disease is called sleeping sickness because, in its final stages, it causes sleep disruption wherein victims are insomniac at night and sleep during the day.¹

Trypanosomes are able to survive and adapt to the massive environmental differences between blood, saliva, lymph, and cerebrospinal fluid, as well as overcome many host defense mechanisms during their life cycle.² To survive in different environments, they are uniquely able to rapidly alter many fundamental biological processes, including metabolism, cell-cycle regulation, organelle activity, and endosome function. As a result, they have been the focus of intense study; however, the underlying mechanisms involved in the parasite’s remarkable ability to adapt continue to be elusive.

To better understand how trypanosomes adjust to differential host environments, a team from the Max Planck Institute for Biochemistry used stable isotope labeling by amino acids and mass spectrometry to look for big differences in the proteome of trypanosomes. They compared protein expression differences between bloodstream and insect phases. The investigators found that several proteins are strongly stage-specific, without significant changes in corresponding mRNA.²

For example, levels of a GPI inositol deacylase precursor, which is involved in GPI anchoring, are more than 30 times higher in the bloodstream phase compared to their presence in the insect phase. Similarly, an ATP-dependent DEAD-box RNA helicase is upregulated by 25 times in the bloodstream phase proteome. They also found an upregulated oxidoreductase specific to the bloodstream phase, which they speculate may be involved in stage-specific metabolism. ²

Several methyltransferases and ribosomal proteins are also differentially abundant. In addition, stage-specifically expressed nucleic proteins may explain previously noted differences in nuclear architecture and chromatin structure. Similarly, the team found differentially expressed proteins that are involved in RNA binding, translation, and protein modification — such as ubiquitination — that may provide clues to the differentiation mechanisms.²

Because the host adaptations that trypanosomes undergo are so manifold, any one of, or a combination of, stage-specific proteins may present new potential drug targets. By blocking certain proteins from expressing at a given stage, drug developers may prevent the protozoa from surviving, thus ending its lifecycle and ridding the world of sleeping sickness.

References

1. World Health Organization (2012) ‘Trypanosomiasis, Human African (sleeping sickness) fact sheet. http://www.who.int/mediacentre/factsheets/fs259/en/

2. Butter, F., et al. (2012) ‘Comparative proteomics of two life cycle stages of stable isotope-labeled trypanosoma brucei reveals novel components of the parasite’s host adaptation machinery‘, Molecular and Cellular Proteomics, 12 (1), (pp. 172-179)