The role of folate transport in antifolate drug action in Trypanosoma brucei

The aim of this study was to identify and characterise mechanisms of resistance to antifolate drugs in African trypanosomes. Genome-wide RNAi library screens were undertaken in bloodstream form Trypanosoma brucei exposed to the antifolates methotrexate and raltitrexed. RNAi knockdown, in conjunction with drug susceptibly and folate transport studies, were used to validate the functions of the putative folate transporters. The transport kinetics of folate and methotrexate were further characterised in whole cells. RNA interference target sequencing (RIT-seq) experiments identified a tandem array of genes encoding a folate transporter family, TbFT1-3, as major contributors to antifolate drug uptake. RNAi knockdown of TbFT1-3 substantially reduced folate transport into trypanosomes and reduced the parasite's susceptibly to the classical antifolates methotrexate and raltitrexed. In contrast, knockdown of TbFT1-3 increased susceptibly to the non-classical antifolates pyrimethamine and nolatrexed. Both folate and methotrexate transport were inhibited by classical antifolates, but not by non-classical antifolates or biopterin. Thus, TbFT1-3 mediate the uptake of folate and classical antifolates in trypanosomes and TbFT1-3 loss-of-function is a mechanism of anti-folate drug resistance.