By Zandile Nare, University of Edinburgh
The process of transcriptional regulation, DNA recombination, condensation and replication depends on alterations in chromatin structure that do not involve changes in the DNA sequence, but are still inherited. This type of regulation is said to be beyond genetics as denoted by the term epigenetics.
In the nucleus of all eukaryotic cells exists chunks of DNA of approximately 170 base pairs wrapped onto a histone octamer. This forms a nucleosome, the basic repetitive unit of chromatin which aids the compaction of DNA allowing it to fit into cell nuclei (see image 1). Histone 1 (H1) binds the DNA loop between two adjacent nucleosomes. Nucleosomes also act as a dynamic binding surface for proteins involved in gene regulation and transcriptional control.
A few things I have learned about Trypanosoma brucei and African sleeping sickness
T. brucei is a parasite which is responsible for causing African sleeping sickness and nagana in cattle in sub-Saharan Africa. T. brucei infection of a mammalian host is achieved when the vector, an infected tsetse fly, takes a blood meal from the host. Following infection, T. brucei initially occupies the bloodstream (see image 2), lymph system and interstitial spaces until it eventually crosses the blood brain barrier. During this stage, clinical symptoms include sleeping disturbances to which the disease owes its name. An estimated 61 million people across 36 countries in sub-Saharan Africa are at risk of developing sleeping sickness, which is fatal if left untreated. Furthermore, the effects of nagana in livestock result in significant economic losses for the affected countries.
Trypanosoma brucei life cycle and coat switching
Throughout its life cycle T. brucei must change its structure in a tightly regulated manner in order to adapt to the different host environments. Although the exact mechanism is still unclear, we know that in the mammalian host some of the slender bloodstream form parasites differentiate into quiescent stumpy forms. Once ingested and in the midgut of the tsetse fly, parasites sense a chemical signal and the decrease in temperature which initiates differentiation into a proliferative procyclic form.
Procyclic parasites do not express variant surface glycoproteins (VSGs), the most abundant protein coat found on the surface of bloodstream and metacyclic form T. brucei parasites. Instead, procyclic forms of T. brucei are covered by glycosylphosphatidylinositol anchored proteins known as procyclins; EP or GPEET After migration to the salivary glands, T. brucei again differentiates and proliferates as epimastigotes. The surface of these epimastigotes is covered in BARP, an alanine rich protein. BARP is replaced by VSGs when epimastigotes differentiate into non-replicative metacyclic forms. When the tsetse fly takes a blood meal these metacyclic parasites are transmitted to the mammal where they differentiate into the bloodstream forms and the cycle starts again. Remarkably, these parasites have a large collection of VSG genes which they use to express a variety of immunologically distinct cell surface coats allowing for antigenic variation. Trypanosomes use antigenic variation as a major mechanism of immune evasion which is central to its pathogenicity.
Unconventional uses of conventional RNA polymerases in T. brucei & host immune evasion
In the majority of eukaryotes, transcription is the main regulatory mechanism for controlling the levels of gene products. In general, the mechanism of eukaryotic gene expression involves a catalogue of events resulting in the recruitment of a specific polymerase complex resulting in mRNA synthesis. Interestingly, this theory is not true for leishmanial or trypanosomal protozoa.
Cell differentiation in T. brucei is associated with the upregulation and downregulation of several genes some of which seem to be regulated by epigenetic mechanisms. T. brucei as well as Trypanosoma cruzi and Leishmania major genomes encode all five subunits common to the three groups of RNA polymerases. Uniquely, T. brucei utilises these conventional polymerases for unconventional functions. For example, in other organisms, RNA polymerase I (pol I) is used exclusively for the transcription of ribosomal DNA (rDNA). However, in T. brucei, pol I not only transcribes rDNA genes, but it also transcribes the genes encoding for the cell surface proteins EP, GPEET and VSG which are essential for the survival of the parasite.
RNA polymerase II (Pol II) transcribes most of the polycistronic transcription units in the genome of T. brucei. Most genes in a polycistronic unit in T. brucei are not functionally related and gene regulation occurs mainly at the post-transcriptional level. It has therefore thought that unlike in higher eukaryotes, there will fewer histone post-transcriptional modifications and chromatin-modifying and chromatin-remodelling enzymes with the role of repressing Pol II transcription. As a result, T. brucei is a useful model system for the study of the epigenetic phenomena. In recent years, a lot of useful data for the identification and characterisation of histone post translational modifications has been obtained from studies in trypanosomes. T. brucei could be a key tool in advancing knowledge in this field.
I am an Eastbio funder PhD student within the Institute of Immunology and Infection Research at the University of Edinburgh. My research focuses on understanding the structure activity relationships of RNA editing ligase 1 (REL1), a validated drug target in T. brucei, and REL2. My goal is to understand how these ligases function and utilise this knowledge to identify lead compounds for anti-trypanosomatid drug discovery. Outside of the lab I enjoy engaging in science communication via social media and through writing blog posts like this. These are both great ways to communicate science to a wider audience and foster collaborations between scientists.
This post is the fourth in our epigenetics series. The first, Epigenetics: past and present by David Hornby, and the second, Epigenetic tags for personalized diagnoses by Sarah Kearns, were published in October; and the third post, Why bacteria are smarter than we think they are by Megan De Ste Croix in November. If you are interested in reading more on this topic, you can also check out the October issue of The Biochemist magazine on the theme of epigenetics.