Iron-dependent regulation of transferrin receptor expression in Trypanosoma brucei

SummaryIn its mammalian host, Trypanosoma brucei covers its iron requirements by receptor-mediated uptake of host transferrin (Tf). The Tf-receptor (Tf-R) is a heterodimeric membrane protein encoded by expression site-associated gene ( ESAG ) 6 and 7 located promoter-proximal in a polycistronic expression site (ES). Each of the 20 ESs encodes a slightly different Tf-R; these differences strongly affect the binding affinity for Tfs of different hosts. The Tf-R encoded in the 221 ES has a low affinity for dog Tf. Transfer of trypanosomes with an active 221 ES to dilute dog serum leads to growth arrest, which they can overcome by switching to another ES encoding a Tf-R with higher affinity for dog Tf. Here we show that trypanosomes can also adapt to dilute dog serum without switching but by replacing the ESAG7 gene in the 221 ES by one from another ES, by deleting ESAG7 from the 221 ES with concomitant upregulation of transcription of ESAG7 in 'silent' ESs, by grossly overproducing the 221 Tf-R or by combinations of these alterations. Our results illustrate the striking genetic flexibility of trypanosomes.

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Trypanosomes change their transferrin receptor expression to allow effective uptake of host transferrin

Luenen

Kieft

Mußmann

et al. 2005

“…Switching to another expression site allows the parasite to express a more suitable TfR (Bitter et al ., 1998; Gerrits et al ., 2002). When starved for iron by treatment with an iron chelator, trypanosomes can upregulate TfR expression (Fast et al ., 1999). We show here that a more physiological form of iron starvation, induced by growth of trypanosomes in serum with a transferrin that is poorly taken up, also induces TfR upregulation.…”

Section: Discussionmentioning

confidence: 99%

The expression level determines the surface distribution of the transferrin receptor in Trypanosoma brucei

Mußmann

Janssen

Calafat

et al. 2002

SummaryThe transferrin receptor (TfR) of Trypanosoma brucei is a heterodimer attached to the surface membrane by a glycosylphosphatidylinositol (GPI) anchor. The TfR is restricted to the flagellar pocket, a deep invagination of the plasma membrane. The membrane of the flagellar pocket and the rest of the cell surface are continuous, and the mechanism that selectively retains the TfR in the pocket is unknown. Here, we report that the TfR is retained in the flagellar pocket by a specific and saturable mechanism. In bloodstream-form trypanosomes transfected with the TfR genes, TfR molecules escaped flagellar pocket retention and accumulated on the entire surface, even at modest (threefold) overproduction levels. Similar surface accumulation was observed when the TfR levels were physiologically upregulated threefold when trypanosomes were starved for transferrin. These results suggest that the TfR flagellar pocket retention mechanism is easily saturated and that control of the expression level is critical to maintain the restricted surface distribution of the receptor.

“…Expression of the ESAG 6/7 transferrin receptor can be regulated by iron levels. The mechanism involved has not been identified but differs from the mammalian iron-response system (Fast et al, 1999;Mussmann et al, 2004;van Luenen et al, 2005). The ESAG 6/7 heterodimer is attached to the membrane by a single GPI anchor on the ESAG 6 subunit.…”

Section: Introductionmentioning

confidence: 99%

Evidence that transport of iron from the lysosome to the cytosol in African trypanosomes is mediated by a mucolipin orthologue

Taylor

McLatchie

Kelly

2013

Bloodstream-form Trypanosoma brucei acquire iron by receptor-mediated endocytosis of host transferrin. However, the mechanism(s) by which iron is then transferred from the lysosome to the cytosol are unresolved. Here, we provide evidence for the involvement of a protein (TbMLP) orthologous to the mammalian endolysosomal cation channel Mucolipin 1. In T. brucei, we show that this protein is localized to the single parasite lysosome. TbMLP null mutants could only be generated in the presence of an expressed ectopic copy, suggesting that the protein is essential. RNAi-mediated ablation resulted in a growth defect in vitro and led to a sevenfold increase in susceptibility to the iron-chelators deferoxamine and salicylhydroxamic acid. Conditional null mutants remained viable when the ectopic copy was repressed, but were hypersensitive to deferoxamine and displayed a growth defect similar to that observed following RNAi. The conditional nulls also retained virulence in vivo in the absence of the doxycycline inducer. These data provide strong evidence that TbMLP has a role in import of iron into the cytosol of African trypanosomes. They also indicate that even when expression is greatly reduced, there is sufficient protein, or an alternative mechanism, to provide the parasite with an adequate supply of cytosolic iron.