Revisiting Cysteine Protease Function in Trypanosoma cruzi: Implications for Parasite Egress and Differentiation

Chagas disease is a major global health concern affecting millions of people worldwide, with limited therapeutic options and no vaccine. The causative agent, Trypanosoma cruzi, is a unicellular eukaryotic parasite whose life cycle alternates between insect vectors and a wide range of mammalian hosts. In mammalian cells, parasite proliferation depends on iterative cycles of host cell invasion, intracellular multiplication, differentiation, and host cell rupture, releasing hundreds of infective parasites. The mechanisms governing the critical transition from replicative amastigotes to infective trypomastigotes (trypomastigogenesis) and subsequent egress remain poorly understood, largely due to the lack of robust analytical tools. Here, we combined real-time cellular impedance monitoring, stage-specific fluorescent parasites, ultrastructural expansion microscopy, and automated high-content imaging to dissect the late steps of the lytic cycle. We demonstrate that trypomastigogenesis is temporally coordinated with egress, ensuring the release of fully mature, infective trypomastigotes. Futhermore, we re-evaluate the effect of the cysteine protease inhibitor Z-Phe-Ala fluoromethyl ketone (Z-FA-FMK), previously reported as an egress inhibitor. Our results reveal that Z-FA-FMK primarily impairs trypomastigogenesis, causing an accumulation of amastigotes and blocking progression to mature trypomastigotes. This arrest delays egress and leads to the release of immature forms, highlighting the essential role of cysteine proteases in parasite differentiation. By decoupling trypomastigogenesis from egress, we provide experimental evidence that the late lytic cycle is a multi-step, tightly regulated process linking parasite differentiation with egress. Together, our work establishes a quantitative framework for dissecting lytic cycle termination and offers a versatile platform for phenotypic screening and drug discovery. ImportanceChagas disease, caused by Trypanosoma cruzi, affects millions worldwide and remains a major global health burden, causing chronic cardiac, digestive, and neurological complications. The disease, disproportionately impacting vulnerable populations, lacks effective treatments or a vaccine. Parasite replication and host cell exit are tightly linked, but the mechanisms driving the transition from intracellular replicative parasites to infective forms and their subsequent release upon host cell lysis are poorly understood. Using real-time monitoring, fluorescent parasites, and high-resolution imaging, we show that cysteine proteases are critical for parasite maturation and that their inhibition uncouples differentiation from egress, leading to the release of non-infective parasites. These findings reveal fundamental principles of parasite biology, provide a platform for drug discovery, and highlight new avenues to target Chagas disease at a critical stage of infection.