An ABC transporter in Leishmania potentially confers resistance to the antimony used in leishmanicidal drugs by sequestering the compound in vesicles and exporting them via the parasite’s flagellar pocket.
Leishmaniasis is a neglected tropical disease (NTD) caused by the protozoan parasite Leishmania. It is responsible for 20 000 – 30 000 deaths every year in countries including India, Bangladesh and South Sudan. The World Health Organisation (WHO) estimates that 310 million are at risk of developing visceral leishmaniasis.
Leishmania has two distinct life cycles, one in its mammalian host and one in its sandfly vector. The sandfly injects promastigotes into the skin during a blood meal. These promastigotes are then taken up by macrophages where they transform into amastigotes and multiply. They are eventually released from the infected cell into the bloodstream from where they may be taken up by another sandfly during its next blood meal.
Current treatments for leishmaniasis, including amphotericin B, miltefosine and pentavalent antimonials, can be both toxic and expensive. This coupled with the ever-increasing issue of drug resistance means that the disease is in danger of reaching crisis point. Scientists have been attempting to elucidate the various ways in which resistance could arise in the hope of curtailing some of the problems facing Leishmania control.
A team from Spain have done just that, identifying an ATP-binding cassette (ABC) transporter in Leishmania which they believe might be involved in resistance to antimony. Leishmania has 42 ABC genes yet few have been characterised. The team led by Ana Perea looked at SbV, an antimony-based drug which is taken up by the amastigote (intracellular) form of the parasite. It becomes reduced to SbIII and activated once inside the macrophages. Leishmania encodes enzymes that are capable of reducing SbV to SbIII, which then combines with thiols that are effluxed from the parasite.
The transporter in question is LABCG2. It was chosen as related transporters LABCG4 and LABCG6 had previously been implicated in resistance to the drug miltefosine. LABCG2 is involved in phosphatidylserine (PS) externalisation during infection of the host macrophages. They found that overexpressing LABCG2 resulting in the promastigotes becoming 7-fold more resistant to the antimony-based compound. This resistance was however not seen in other leishmanicidal drugs such as miltefosine.
The team then delved into exactly what was behind the resistance to SbIII. The parasites were incubated in antimony and after 60 minutes the accumulation of the compound was measured. The mutants which overexpressed LABCG2 were found to have accumulated 76% of the total amount of SbIII that the controls had. They interpreted this as an indication that the LABCG2 transporter mediates the elimination of antimony from the parasite.
Finally, they looked to establish whether thiols, which bind to and export heavy metals, could play a role in Leishmania antimony resistance. They found that thiol efflux from the parasites was greater in the presence of antimony and, following tagging by green fluorescent protein (GFP) discovered that the transporter does localise at the plasma membrane.
Overexpressing the LABCG2 ABC transporter might therefore protect Leishmania against otherwise toxic antimonic drugs by effluxing them as a complex bound to thiols. They believe that this could be a mechanism by which Leishmania may become drug resistant, although emphasise the need for LABCG2 knockout mutants to really establish what role the transporter plays in the parasite.
Source: Perea et al. (2016). The LABCG2 transporter from the protozoan parasite Leishmania is involved in antimony resistance. Antimicrobial Agents and Chemotherapy, 60, 3489 – 3496.