Sterols of Saccharomyces cerevisiae erg6 Knockout Mutant Expressing the Pneumocystis carinii S-Adenosylmethionine:Sterol C-24 Methyltransferase

The sterol C-24 methyltransferase encoding gene, erg6, is essential for viability of Aspergillus species

Triazoles, the most widely used class of antifungal drugs, inhibit the biosynthesis of ergosterol, a crucial component of the fungal plasma membrane. Inhibition of a separate ergosterol biosynthetic step, catalyzed by the sterol C-24 methyltransferase Erg6, reduces the virulence of pathogenic yeasts, but its effects on filamentous fungal pathogens like Aspergillus fumigatus remain unexplored. Here, we show that the lipid droplet-associated enzyme Erg6 is essential for the viability of A. fumigatus and other Aspergillus species, including A. lentulus, A. terreus, and A. nidulans. Downregulation of erg6 causes loss of sterol-rich membrane domains required for apical extension of hyphae, as well as altered sterol profiles consistent with the Erg6 enzyme functioning upstream of the triazole drug target, Cyp51A/Cyp51B. Unexpectedly, erg6-repressed strains display wild-type susceptibility against the ergosterol-active triazole and polyene antifungals. Finally, we show that erg6 repression results in significant reduction in mortality in a murine model of invasive aspergillosis. Taken together with recent studies, our work supports Erg6 as a potentially pan-fungal drug target.

Lipid-Centric Approaches in Combating Infectious Diseases: Antibacterials, Antifungals and Antivirals with Lipid-Associated Mechanisms of Action

One of the global challenges of the 21st century is the increase in mortality from infectious diseases against the backdrop of the spread of antibiotic-resistant pathogenic microorganisms. In this regard, it is worth targeting antibacterials towards the membranes of pathogens that are quite conservative and not amenable to elimination. This review is an attempt to critically analyze the possibilities of targeting antimicrobial agents towards enzymes involved in pathogen lipid biosynthesis or towards bacterial, fungal, and viral lipid membranes, to increase the permeability via pore formation and to modulate the membranes' properties in a manner that makes them incompatible with the pathogen's life cycle. This review discusses the advantages and disadvantages of each approach in the search for highly effective but nontoxic antimicrobial agents. Examples of compounds with a proven molecular mechanism of action are presented, and the types of the most promising pharmacophores for further research and the improvement of the characteristics of antibiotics are discussed. The strategies that pathogens use for survival in terms of modulating the lipid composition and physical properties of the membrane, achieving a balance between resistance to antibiotics and the ability to facilitate all necessary transport and signaling processes, are also considered.

The sterol C-24 methyltransferase encoding gene, erg6, is essential for viability of Aspergillus species

Ergosterol is a critical component of fungal plasma membranes. Although many currently available antifungal compounds target the ergosterol biosynthesis pathway for antifungal effect, current knowledge regarding ergosterol synthesis remains incomplete for filamentous fungal pathogens like Aspergillus fumigatus. Here, we show for the first time that the lipid droplet-associated sterol C-24 methyltransferase, Erg6, is essential for A. fumigatus viability. We further show that this essentiality extends to additional Aspergillus species, including A. lentulus, A. terreus, and A. nidulans. Neither the overexpression of a putative erg6 paralog, smt1, nor the exogenous addition of ergosterol could rescue erg6 deficiency. Importantly, Erg6 downregulation results in a dramatic decrease in ergosterol and accumulation in lanosterol and is further characterized by diminished sterol-rich plasma membrane domains (SRDs) at hyphal tips. Unexpectedly, erg6 repressed strains demonstrate wild-type susceptibility against the ergosterol-active triazole and polyene antifungals. Finally, repressing erg6 expression reduced fungal burden accumulation in a murine model of invasive aspergillosis. Taken together, our studies suggest that Erg6, which shows little homology to mammalian proteins, is potentially an attractive antifungal drug target for therapy of Aspergillus infections.

Characterization of the cholesterol biosynthetic pathway in Dioscorea transversa

Cholesterol is the precursor of bioactive plant metabolites such as steroidal saponins. An Australian plant, Dioscorea transversa, produces only two steroidal saponins: 1β-hydroxyprotoneogracillin and protoneogracillin. Here, we used D. transversa as a model in which to elucidate the biosynthetic pathway to cholesterol, a precursor to these compounds. Preliminary transcriptomes of D. transversa rhizome and leaves were constructed, annotated, and analyzed. We identified a novel sterol side chain reductase (SSR) as a key initiator of cholesterol biosynthesis in this plant. By complementation in yeast, we determine that this SSR reduces Δ^24,28 double bonds required for phytosterol biogenesis, as well as Δ^24,25 double bonds. The latter function is believed to initiate cholesterogenesis by reducing cycloartenol to cycloartanol. Through heterologous expression, purification and enzymatic reconstitution we also demonstrate that the D. transversa sterol demethylase (CYP51) effectively demethylates obtusifoliol, an intermediate of phytosterol biosynthesis and 4-desmethyl-24,25-dihydrolanosterol, a postulated downstream intermediate of cholesterol biosynthesis. In summary, we investigated specific steps of the cholesterol biosynthetic pathway, providing further insight into the downstream production of bioactive steroidal saponin metabolites.

Yeast as a promising heterologous host for steroid bioproduction

With the rapid development of synthetic biology and metabolic engineering technologies, yeast has been generally considered as promising hosts for the bioproduction of secondary metabolites. Sterols are essential components of cell membrane, and are the precursors for the biosynthesis of steroid hormones, signaling molecules, and defense molecules in the higher eukaryotes, which are of pharmaceutical and agricultural significance. In this mini-review, we summarize the recent engineering efforts of using yeast to synthesize various steroids, and discuss the structural diversity that the current steroid-producing yeast can achieve, the challenge and the potential of using yeast as the bioproduction platform of various steroids from higher eukaryotes.

Discovery of Novel Piperidinylthiazole Derivatives As Broad-Spectrum Fungicidal Candidates

Oxathiapiprolin is one of the best active fungicides discovered for oomycetes control. To develop a fungicide candidate with a broad spectrum of activity, 22 new piperidinylthiazole derivatives were designed and synthesized. Compound 5l showed the best activity against Pseudoperonospora cubensis (Berk. et Curt.) Rostov and Phytophthora infestans in vivo with 100% and 80% of inhibition, respectively, at 1 mg/L, and 72.87% of field efficacy against P. cubensis at 1 g ai/667 m² validated these results. Compound 5i exhibited a broad spectrum of excellent activity against Sclerotinia sclerotiorum with EC₅₀ = 0.30 mg/L (>10 times more active than oxathiapiprolin and azoxystrobin in vitro), good activity against Botrytis cinerea, Cercospora arachidicola, and Gibberella zeae with EC₅₀ of 14.54, 5.57, and 14.03 mg/L in vitro and against P. cubensis and P. infestans with 60% and 30% inhibition rates, respectively, at 1 mg/L in vivo. Mode of action studies by RNA sequencing analysis discovered oxysterol-binding protein (OSBP), chitin synthase (CHS1), and (1,3)-β-glucan synthase (FKS2) as the potent target of 5i against S. sclerotiorum. Quenching studies validated that OSBP was the same target of both 5i and oxathiapiprolin; it was quenched by both of them. Our studies discovered isothiazole-containing piperidinylthiazole as an OSBP target-based novel lead for fungicide development.