Cloning and characterisation of the Equilibrative Nucleoside Transporter family of Trypanosoma cruzi: ultra-high affinity and selectivity to survive in the intracellular niche.

BACKGROUND

Trypanosoma cruzi, the causative agent of Chagas' disease is unable to synthesise its own purines and relies on salvage from the host. In other protozoa, purine uptake has been shown to be mediated by Equilibrative Nucleoside Transporters (ENTs).

METHODS

To investigate the functionality of T. cruzi-encoded ENT transporters, its four putative ENT genes (TcrNB1, TcrNB2, TcrNT1 and TcrNT2) were cloned and expressed in genetically adapted Trypanosoma brucei procyclic cells from which the nucleobase transporter locus was deleted.

RESULTS

TcrNB1 displayed very high affinity for hypoxanthine (K 93.8 ± 4.7 nM for) and guanine, and moderate affinity for adenine. TcrNT1 was found to be a high-affinity guanosine/inosine transporter (inosine K is 1.0 ± 0.03 μM; guanosine K is 0.92 ± 0.2 μM). TcrNT2 encoded a high-affinity thymidine transporter (K = 223.5 ± 7.1 nM) with a clear preference for 2'-deoxypyrimidines. TcrNB2, whose activity could not be determined in our system, could be a low-affinity purine nucleobase transporter, given its sequence and predicted structural similarities to Leishmania major NT4. All 4 transporter genes were highly expressed in the amastigote forms, with much lower expression in the non-dividing stages.

CONCLUSIONS

The data appear to show that, surprisingly, T. cruzi has a preference for oxopurines over aminopurines and efficiently transports 2'-deoxypyrimidines. The T. cruzi ENTs display exceptionally high substrate affinity as an adaptation to their intracellular localisation.

GENERAL SIGNIFICANCE

This study reports the first cloning of T. cruzi purine and pyrimidine transporters, including the first gene encoding a pyrimidine-selective protozoan transporter.