The intracellular amastigote of Trypanosoma cruzi maintains an actively beating flagellum

Throughout its complex life cycle, the uniflagellate parasitic protist, Trypanosoma cruzi, adapts to different host environments by transitioning between elongated motile extracellular forms and non-motile intracellular amastigote forms that replicate in the cytoplasm of mammalian host cells. Despite their name, intracellular T. cruzi amastigotes retain a short flagellum that extends beyond the opening of the flagellar pocket with access to the extracellular milieu. Contrary to the long-held view that the T. cruzi amastigote flagellum is inert, we now report that this organelle is motile and displays quasiperiodic beating inside mammalian host cells. Kymograph analysis determined an average flagellar beat frequency of ~0.7 Hz for intracellular amastigotes. Similar beat frequencies were measured in extracellular amastigotes following their isolation from host cells. Inhibitor studies reveal roles for parasite mitochondrial respiration and intracellular calcium availability in modulating flagellar beat in T. cruzi amastigotes. Together, these findings demonstrate that flagellar motility is an intrinsic property of T. cruzi amastigotes and suggest that this organelle may play an active role in the parasite infection process. To our knowledge, this is the first record of an intracellular eukaryotic flagellum beating within another eukaryotic cell.