ABC transporter implicated in parasite drug resistance

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.

Slide1
Leishmania have two distinct life cycle stages, one within their mammalian hosts and one within the sand fly vector. The parasites are taken up during an infected blood meal and replicate within mammalian cells before being transferred to the sand fly during the next meal. Adapted from CDC (http://www.cdc.gov/parasites/leishmaniasis/biology.html).

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.

Rebecca Hall

Twitter: @RebeccaJHall13

Blog https://ananxiousscientist.wordpress.com/

Ivacaftor, the Miracle Drug?

 

Ivacaftor has been hailed as a miracle drug for the treatment of Cystic fibrosis (CF). Although Ivacaftor is not designed for every CF sufferer, it is life changing for those who benefit from it.

CF is a genetic disease that affects 70,000 people worldwide and is characterised by an overly viscous mucus lining of the airways, resulting in difficulty in clearing the airways by coughing, and an increase in infections from opportunistic pathogens.  CF is caused by different mutations in the Cystic fibrosis transmembrane conductance regulator (CFTR), an ABC transporter ion channel, which results in an imbalance in ion concentration and the observed phenotype of highly viscous mucus. The CFTR conducts chloride and as well as regulating other ion channels, such as chloride channels and glutathione transport. There are approximately 1900 known mutations within the CFTR, which is primarily expressed within the airway submucosal glands in the lungs.

Until now all CF care has been supportive rather than curative. However a recent breakthrough with the drug Ivacaftor, which treats the underlying problem rather than just the symptoms of CF, could change the CF care landscape. Ivacaftor is used to treat CF patients with one of a set of specific mutations including G551D, which affects approximately 4% of CF patients and is the third most common mutation. The G551D mutation is characterised by correct positioning of the CFTR on the epithelial cell surface but incorrect function, the CFTR is unable to transport chloride ions.

Slide1
Predicted structure of the CFTR. TMD; Transmembrane Domain, NBD; Nucleotide Binding Domain. The G551D mutation occurs in the NBD2 region and prevents ATP dependent gating. Adapted from Kim and Skach, 2012 (http://dx.doi.org/10.3389/fphar.2012.00201).

Ivacaftor, developed by Vertex Pharmaceuticals together with the Cystic fibrosis Foundation, assists with the function of the mutant CFTR by directly binding the CFTR channel. Binding increases the probability of channel opening by inducing opening of the ion channel independent of ATP binding and hydrolysis. This reduces the imbalance in the ion concentration and lowers the viscosity of the mucus in the airways allowing easier breathing of the CF patient. CF sufferers who have had access to Ivacaftor have been able to “run without coughing” and “take deep breaths”.

The FDA approved Ivacaftor in January 2012 and the combination drug with lumacaftor gained FDA approval in July 2015. The inclusion of lumacaftor in the combination drug enables treatment of CF patients with a slightly different mutation in addition to those treated by Ivacaftor.

However there is a catch, Ivacaftor treatment costs $300,000 per patient per year. Patients are likely to be taking Ivacaftor for the rest of their lives and so the cost becomes astronomical. On the other hand Vertex has stated that they will make the drug available free of charge for those patients in the US without medical insurance and with a household income of less than $150,000 a year.

The real question surrounding Ivacaftor is; how to pay for the miracle?

 

Source: Kotha and Clancy (2013). Ivacaftor treatment of cystic fibrosis patients with the G551D mutation: a review of the evidence. Therapeutic Advances in Respiratory Disease, 7(5)288-296.

Bryony Ackroyd

Twitter: @BryonyAckroyd