Use of l-Proline and ATP Production by Trypanosoma cruzi Metacyclic Forms as Requirements for Host Cell Invasion

The process of host cell invasion by Trypanosoma cruzi depends on parasite energy. What source of energy is used for that event is not known. To address this and other questions related to T. cruzi energy requirements and cell invasion, we analyzed metacyclic trypomastigote forms of the phylogenetically distant CL and G strains. For both strains, the nutritional stress experienced by cells starved for 24, 36, or 48 h in phosphate-buffered saline reduced the ATP content and the ability of the parasite to invade HeLa cells proportionally to the starvation time. Inhibition of ATP production by treating parasites with rotenone plus antimycin A also diminished the infectivity. Nutrient depletion did not alter the expression of gp82, the surface molecule that mediates CL strain internalization, but increased the expression of gp90, the negative regulator of cell invasion, in the G strain. When l-proline was given to metacyclic forms starved for 36 h, the ATP levels were restored to those of nonstarved controls for both strains. Glucose had no such effect, although this carbohydrate and l-proline were transported in similar fashions. Recovery of infectivity promoted by l-proline treatment of starved parasites was restricted to the CL strain. The profile of restoration of ATP content and gp82-mediated invasion capacity by l-proline treatment of starved Y-strain parasites was similar to that of the CL strain, whereas the Dm28 and Dm30 strains, whose infectivity is downregulated by gp90, behaved like the G strain. l-Proline was also found to increase the ability of the CL strain to traverse a gastric mucin layer, a property important for the establishment of T. cruzi infection by the oral route. Efficient translocation of parasites through gastric mucin toward the target epithelial cells in the stomach mucosa is an essential requirement for subsequent cell invasion. By relying on these closely associated ATP-driven processes, the metacyclic trypomastigotes effectively accomplish their internalization.Host cell invasion by Trypanosoma cruzi, which is critical for the establishment of infection in mammalian hosts, is a multistep process involving various parasite and host cell molecules that, in a concerted series of events, leads to intracellular Ca²⁺ mobilization in both types of cells (5, 12, 32). The first step of this process, namely, T. cruzi attachment to target cells, requires parasite energy (23). The main source of energy for this event is unknown. Also unknown is whether there are differences in energy requirements among different T. cruzi strains that use distinct mechanisms to enter target cells.Epimastigotes, the proliferative and noninfective developmental forms of T. cruzi, use mainly glucose, l-proline, and l-glutamic acid as carbon sources and to obtain energy through respiration (27). In these parasite forms, the active transport of glucose, l-proline, or l-glutamic acid has been previously demonstrated (2, 24, 25, 29). With regard to l-proline, which is one of the prominent constituents of the hemolymph and tissue fluids of hematophagous insect vectors (1, 6), its uptake by active proline transport systems has been found in epimastigotes (24) and in tissue culture trypomastigotes that correspond to the bloodstream parasites and intracellular epimastigotes and amastigotes (30). Concerning the metacyclic trypomastigotes, which are found in the terminal portions of the digestive tract of triatomine insects and constitute the parasite forms responsible for the initial interaction with host cells, there is no information regarding whether they transport and utilize as an energy source the compounds referred to above.The replicative epimastigotes and the infective metacyclic trypomastigotes may differentially use the resources available in the extracellular milieu for the production of energy required for diverse biological processes. Epimastigotes were shown to transform into metacyclic trypomastigotes within 48 h when l-proline or l-glutamate was added to the medium (15). Another study, using a different T. cruzi strain, showed a high rate of differentiation of epimastigotes to metacyclic forms in a medium composed of artificial triatomine urine supplemented with proline (7). l-Proline was also shown to induce transformation of the intracellular epimastigote-like stage into the trypomastigote stage (30), and it seems to be a main carbon source in the intracellular stages, since its development throughout the intracellular life cycle stages is dependent on the presence of proline and since glucose is not taken up by amastigotes (A. M. Silber, R. R. Tonelli, G. C. Lopes, N. L. Cunha e Silva, A. C. T. Torrecilhas, R. I. Schumacher, W. Colli, and M. J. M. Alves, submitted for publication). The most prominent function of metacyclic forms is to invade host cells, and different T. cruzi strains may use distinct strategies to accomplish that function, as illustrated by studies conducted with the CL and G strains, which belong to two highly divergent genetic subgroups (4). Metacyclic forms of the CL strain engage the stage-specific surface gp82 glycoprotein to bind to host cells and induce actin cytoskeleton disruption and parasite internalization (9). On the other hand, G strain metacyclic forms preferentially use the mucin-like molecules gp35 and gp50 to induce the recruitment of target cell actin to the site of parasite entry (13). In addition, G strain infectivity is influenced by gp90, a metacyclic-stage-specific molecule that downregulates host cell invasion (17). We addressed here issues related to energy requirements in T. cruzi infection by analyzing CL and G strains and, based on the findings with these strains, by extending the study to include other CL-like or G-like parasite strains.