The human parasite Trypanosoma brucei does not synthesize heme de novo and instead relies entirely on heme supplied by its vertebrate host or its insect vector, the tsetse fly. In the host bloodstream T. brucei scavenges heme via haptoglobin-hemoglobin (HpHb) receptor-mediated endocytosis occurring in the flagellar pocket. However, in the procyclic developmental stage, in which T. brucei is confined to the tsetse fly midgut, this receptor is apparently not expressed, suggesting that T. brucei takes up heme by a different, unknown route. To define this alternative route, we functionally characterized heme transporter TbHrg in the procyclic stage. RNAi-induced down-regulation of TbHrg in heme-limited culture conditions resulted in slower proliferation, decreased cellular heme, and marked changes in cellular morphology so that the cells resemble mesocyclic trypomastigotes. Nevertheless, the TbHrg KO developed normally in the tsetse flies at rates comparable with wild-type cells. T. brucei cells overexpressing TbHrg displayed up-regulation of the early procyclin GPEET and down-regulation of the late procyclin EP1, two proteins coating the T. brucei surface in the procyclic stage. Light microscopy of immunostained TbHrg indicated localization to the flagellar membrane, and scanning electron microscopy revealed more intense TbHrg accumulation toward the flagellar pocket. Based on these findings, we postulate that T. brucei senses heme levels via the flagellar TbHrg protein. Heme deprivation in the tsetse fly anterior midgut might represent an environmental stimulus involved in the transformation of this important human parasite, possibly through metabolic remodeling.

• Keywords
• differentiation, flagellum, heme, import, parasite, procyclin, transporter, trypanosome,
• MeSH
• Biological Transport MeSH
• Down-Regulation MeSH
• Flagella metabolism   MeSH
• Heme metabolism   MeSH
• Real-Time Polymerase Chain Reaction MeSH
• Membrane Transport Proteins metabolism   MeSH
• Microscopy, Electron, Scanning MeSH
• Tsetse Flies parasitology   MeSH
• Cell Proliferation MeSH
• Protozoan Proteins metabolism   MeSH
• Receptors, Cell Surface metabolism   MeSH
• RNA Interference MeSH
• Amino Acid Sequence MeSH
• Life Cycle Stages MeSH
• Transgenes MeSH
• Trypanosoma brucei brucei metabolism   MeSH
• Gene Expression Regulation, Developmental MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Heme MeSH
• hemoglobin-haptoglobin receptor MeSH Browser
• Membrane Transport Proteins MeSH
• Protozoan Proteins MeSH
• Receptors, Cell Surface MeSH