Functional importance for developmental regulation of sterol biosynthesis in Acanthamoeba castellanii

Abstract

The sterol metabolome of Acanthamoeba castellanii (Ac) yielded 25 sterols. Substrate screening of cloned AcCYP51 revealed obtusifoliol as the natural substrate which converts to ∆^8,14-sterol (<95%). The combination of [²H₃-methyl]methionine incubation to intact cultures showing C₂₈-ergosterol incorporates 2-²H atoms and C₂₉-7-dehydroporiferasterol incorporates 5 ²H-atoms, the natural distribution of sterols, CYP51 and previously published sterol methyltransferase (SMT) data indicate separate ∆²⁴⁽²⁸⁾- and ∆²⁵⁽²⁷⁾-olefin pathways to C₂₈- and C₂₉-sterol products from the protosterol cycloartenol. In cell-based culture, we observed a marked change in sterol compositions during the growth and encystment phases monitored microscopically and by trypan blue staining; trophozoites possess C₂₈/C₂₉-∆^5,7-sterols, viable encysted cells (mature cyst) possess mostly C₂₉-∆⁵-sterol and non-viable encysted cells possess C₂₈/C₂₉-∆^5,7-sterols that turnover variably from stress to 6-methyl aromatic sterols associated with changed membrane fluidity affording lysis. An incompatible fit of steroidal aromatics in membranes was confirmed using the yeast sterol auxotroph GL7. Only viable cysts, including those treated with inhibitor, can excyst into trophozoites. 25-Azacycloartanol or voriconazole that target SMT and CYP51, respectively, are potent enzyme inhibitors in the nanomolar range against the cloned enzymes and amoeba cells. At minimum amoebicidal concentration of inhibitor amoeboid cells rapidly convert to encysted cells unable to excyst. The correlation between stage-specific sterol compositions and the physiological effects of ergosterol biosynthesis inhibitors suggests that amoeba fitness is controlled mainly by developmentally-regulated changes in the phytosterol B-ring; paired interference in the ∆^5,7-sterol biosynthesis (to ∆^5,7) - metabolism (to ∆⁵ or 6-methyl aromatic) congruence during cell proliferation and encystment could be a source of therapeutic intervention for Acanthamoeba infections.

Keywords: Acanthamoeba castellanii; Aromatic sterols; Encystment; Ergosterol biosynthesis inhibitor; Sterol evolution; Sterol methyltransferase; Trophozoite.