Ovothiol is a redundant part of a complex thiol network in promastigote Leishmania mexicana

Abstract

A gene encoding OvoA, a key enzyme involved in the biosynthesis of ovothiol, was excised form the genome of Leishmania mexicana promastigotes using CRISPR/cas mediated gene editing. The role of the enzyme in synthesising ovothiol was confirmed since both ovothiol A and ovothiol B were lost from the metabolome of the modified cells. The OvoA knockout line had similar growth kinetics to wild-type progenitor cells and, moreover, most of the changes in metabolism that accompanied the transition of log stage growth to stationary phase were mirrored in the KO line. Significant differences, however, were observed in the ratio of the reduced and oxidised forms of the other major low molecular weight thiols, glutathione and trypanothione, indicative of a role of these other thiols in maintaining reduced ovothiol and demonstrating an interconnected network of low molecular weight thiols in these cells. The OvoA knockout cells remained infective to macrophages where promastigotes transformed to amastigote forms in a manner similar to wild-type. The knockout line was tested for sensitivity to a range of current anti-leishmanial drugs and oxidative and nitrosative stresses. While generally the absence of ovothiol caused little or no change in sensitivity to these stress-inducing agents, enhanced sensitivity to amphotericin B was noted. Author summaryOvothiol is a low molecular weight histidine-derived thiol first described in sea urchin eggs, and later found in many organisms, including the protozoa of the order Kinetoplastida, that includes human pathogens such as the Leishmania species that cause leishmaniasis. Thiol metabolism in the Kinetoplastidae has been studied in some detail, particularly with regard to an unusual bis-glutathione, spermidine conjugate named trypanothione that takes on many of the roles performed by glutathione in most other organisms. Roles for ovothiol in Leishmania have not been previously defined, although potential roles in defence against oxidative stress have been hypothesised. A gene encoding the first enzyme of the pathway involved in ovothiol production, OvoA, was excised from the Leishmania mexicana genome. Its role in ovothiol synthesis was confirmed as ovothiol was absent from the mutants. Little changed, however, with respect to the phenotype of these cells, including their proliferation rate, their ability to infect macrophages or their sensitivity to a range of stress inducing agents. These included several leishmanicidal drugs, oxidative and nitrosative stresses. For amphotericin B, however, the Ovothiol lacking cells were more sensitive than wild-type indicating some role in defence against the impact of this drug.