Trypanosomes are also an emerging model system for cellular, molecular and developmental biology. The list of original discoveries includes GPI-anchoring, trans-splicing, RNAi (in parallel to Fire and Mellow), polycistronic transcription in eukaryotes or hydrodynamic protein sorting. Here are some basic facts about the trypanosome cell:
In the posterior half of the unicellular parasites, between flagellar pocket and nucleus, the endoplasmic reticulum features an exit site that is intimately connected to the localization of the single trypanosome Golgi. This site is characterized by an extensive network of tubular structures and abundant vesicles of variable size. In trypanosomes the assignment of cis- and trans-sites of the Golgi apparatus is rather straightforward. The single lysosome is frequently found close to the nucleus, while the mitochondrion stretches throughout the cell.
The trypanosome flagellar pocket is a flask-shaped invagination around the emerging flagellum. It is continuous with the pellicular and flagellar membrane, representing about 5% of the total cellular membrane area. A hemidesmosome-like zone closes the flagellar pocket making it a secluded, yet extracellular compartment that contains an electron-dense, carbohydrate-rich matrix. Molecules entering the flagellar pocket have to pass through this semipermeable adhesion zone. There are no conclusive data available on the exclusion limit or the rates of diffusion in either direction. At the moment we have very little understanding of the factors controlling access to the flagellar pocket, or of the physical properties or biochemical constituents of the lumen of the pocket. However, it appears that all communication between the mammalian host environment and the parasite is confined to this microdomain. Endocytosis and secretion in bloodstream forms of T. brucei occur solely in this region. The lack of pellicular microtubules in the pocket area might be one explanation for this exclusivity.
The diploid genome of T. brucei contains several hundred different VSG genes whereas only one of them is expressed at a time. In the trypanosome bloodstream stage, the active VSG gene is always located close to a telomer in a long polycistronic transcription unit known as expression site. Mono-allelic expression of VSGs, driven by RNA polymerase I, is performed in an extranucleolar expression site body. Trypanosomes can switch the expressed VSG gene by either replacing the VSG gene in an active expression site, or by silencing the active expression site and activating an alternate one. There are approximately 20 telomeric expression sites. Several genes are expressed together with the VSG gene in a 40-60 kb polycistronic transcription unit. These additional genes are termed expression site-associated genes and encode i.e. for a variety of receptors. Depending on the expression site they are transcribed from, these feature differences in affinities for their substrates. Thus, multiple expression sites appear to help the parasite to adapt to the diversity of macromolecules that are encountered in different mammalian hosts. Replacement of a VSG gene in the active expression site is accomplished by one of two different mechanisms. By gene conversion, one copy of a silent VSG gene from the large chromosome-internal repertoire replaces the active one. Alternatively, by reciprocal recombination, the transcribed VSG gene is transferred to a silent expression site and a previously silent VSG gene recombines into the active expression site.
Proliferating ‘slender’ cells of Trypanosoma brucei evade the host’s immune attack by antigenic variation, i.e. by switching the expression of the predominant cell surface antigen. The growth of the parasite population, however, is not only limited by the host’s immune response, but also by the parasite itself. T. brucei responds to increased cell density by differentiation to the cell cycle arrested ‘stumpy’ stage. Stumpy cells have lost a major line of defense, the capability of undergoing antigenic variation. However, the stumpy stage is essential for a crucial part of the parasite life cycle, the successful infection of the transmitting insect, the tsetse fly. Certainly, T. brucei stumpy cells are not stranded in the mammalian bloodstream. They integrate in part unique molecular machineries that allow temporary survival in the host as well as successful infection of the insect vector.