Endogenous Sterol Synthesis Is Dispensable for Epimastigote Growth but Not Stress Tolerance.

In addition to scavenging exogenous cholesterol, the parasitic kinetoplastid can endogenously synthesize sterols. Similar to fungal species, synthesizes ergostane type sterols and is sensitive to a class of azole inhibitors of ergosterol biosynthesis that target the enzyme lanosterol 14α-demethylase (CYP51). In the related kinetoplastid parasite , CYP51 is essential, yet in , the cognate enzyme is dispensable for growth; but not heat resistance. The essentiality of CYP51 and the specific role of ergostane-type sterol products in has not been established. To better understand the importance of this pathway, we have disrupted the gene in epimastigotes (). Disruption of leads to accumulation of 14-methylated sterols and a concurrent absence of the final sterol product ergosterol. While epimastigotes have slowed proliferation compared to wild type parasites, the enzyme is not required for growth; however, epimastigotes exhibit sensitivity to elevated temperature, an elevated mitochondrial membrane potential and fail to establish growth as intracellular amastigotes . Further genetic disruption of squalene epoxidase () results in the absence of all endogenous sterols and sterol auxotrophy, yet failed to rescue tolerance to stress in parasites, suggesting the loss of ergosterol and not accumulation of 14-methylated sterols modulates stress tolerance.