Trichomonas vaginalis cathepsin D-like aspartic proteinase (Tv-CatD) is regulated by iron at different levels

During infection, Trichomonas vaginalis is exposed to microenvironmental variations in pH, temperature, and the concentrations of iron, glucose, polyamines, and zinc. Environmental factors such as iron or glucose can influence the expression of some virulence factors, including proteases. Cathepsin D, an aspartic proteinase, was previously characterized and found to be positively regulated by glucose. However, we do not know whether iron can also modulate it. Therefore, the goal of this study was to analyze the effects of iron on the expression, secretion, and proteolytic activity of Tv-CatD. RT‒qPCR assays revealed no changes in tv-catd expression at the transcript level in parasites grown under iron restriction (IR) or high iron (HI) conditions. However, Western blot (WB) assays revealed that the amount of Tv-CatD under HI conditions was ~2.6-fold greater than that under IR conditions. Moreover, in vitro secretion assays followed by WB and indirect immunofluorescence assays revealed that Tv-CatD was secreted in a time-dependent manner and that its mobilization to the parasite surface appeared to occur through vesicles, mainly under HI conditions. Interestingly, the proteolytic activity of secreted Tv-CatD was ~4.7-fold lower under HI conditions than under IR conditions. These results, together with those of inhibition assays in the presence of different iron concentrations, revealed that Fe²⁺ could act as another specific inhibitor of Tv-CatD enzymatic activity in T. vaginalis. Our results suggest that iron differentially regulates Tv-CatD at the posttranscriptional level, possibly through RNA-protein complexes, and at the posttranslational level, possibly through direct binding to the catalytic domain.