The cell biology of Trypanosoma brucei differentiation

Abstract

Developmental events in the life-cycle of the sleeping sickness parasite comprise integrated changes in cell morphology, metabolism, gene expression and signalling pathways. In each case these processes differ from the eukaryotic norm. In the past three years, understanding of these developmental processes has progressed from a description of the cytological events of differentiation to a discovery of its underlying molecular controls. With an expanding set of reagents for the identification of distinct parasite life-cycle stages in the tsetse, trypanosome differentiation is being studied from the molecular to the organismal and population level. Interestingly, the new molecular discoveries provide insights into the biology of the parasite in the field.

Figure 1. A schematic diagram of the life-cycle of Trypanosoma brucei.

Mammalian infection is initiated when metacyclic (‘Meta’) forms are inoculated by the bite of an infected tsetse fly. These develop to proliferative bloodstream slender (SL) forms that multiply in the blood to establish the infection. As numbers increase, a parasite-derived factor, stumpy induction factor (SIF), promotes division arrest and the generation of stumpy (ST) forms, commitment to this transition occurring while the parasite is still morphologically slender (this intermediate form has been termed Stumpy*; [25••]). The experimental induction of cells with stumpy characteristics can also be induced by various chemical treatments (8-pCPTcAMP, hydrolysable cAMP and troglitazone) although whether these act via the SIF signalling pathway is unclear. Once stumpy cells are up taken by tsetse flies they transform to procyclic (PC) forms, this being induced in vitro by exposure to citrate/cis-aconitate (CCA), mild acid or pronase treatment. Procyclic forms develop initially as early forms (which express GPEET procyclin), this being down regulated as they transform to late procyclic forms. Asymmetric division of proventricular (PV) forms generates a long and short form, the short form of which is believed to attach to the salivary gland and transform to proliferative epimastigote (‘Epi’) forms, which express BARP and are believed to be capable of sexual exchange. These eventually mature to detached metacyclic forms that express a VSG coat and are ready for transmission to a new mammalian host. Mitochondrial status is indicated inside the circle in boxes, surface antigen expression is indicated outside the circle.

Figure 2. Cytological events leading from slender to procyclic forms.

Slender cells proliferate in the mammalian bloodstream and produce SIF, triggering the density-dependent production of stumpy forms. This differentiation is inhibited by the action of signalling pathway(s) involving TbMAPK5 and ZFK. Once stumpy forms are generated, they are held poised for differentiation to procyclic forms by the action of TbPTP1 [25••]. These cells also express, at low level, a CCA receptor/transmitter for the signal to differentiate, which has access to the cell surface (shown as a ‘star’). Upon uptake by tsetse flies, Engstler and Boshart [16••] propose that a reduction in temperature of >15 °C promotes super-induction of the CCA receptor/transmitter that is trafficked to the stumpy cell surface, sensitising these cells to the inductive signal. Slender cells, in contrast, do not traffic the receptor/transmitter to the surface. Exposure to differentiation signals in the fly inactivates TbPTP1 and initiates transformation to procyclic forms, this being induced by CCA, proteolytic attack of the parasite surface or pH stress in the tsetse midgut and/or in vitro.