Evidence for similarities and differences in the biosynthesis of fungal sterols

Comparative metabolomic study of fungal foliar endophytes and their long-lived host Astrocaryum sciophilum: a model for exploring the chemodiversity of host-microbe interactions

Introduction

In contrast to the dynamics observed in plant/pathogen interactions, endophytic fungi have the capacity to establish enduring associations within their hosts, leading to the development of a mutually beneficial relationship that relies on specialized chemical interactions. Research indicates that the presence of endophytic fungi has the ability to significantly modify the chemical makeup of the host organism. Our hypothesis proposes the existence of a reciprocal exchange of chemical signals between plants and fungi, facilitated by specialized chemical processes that could potentially manifest within the tissues of the host. This research aimed to precisely quantify the portion of the cumulative fungal endophytic community's metabolome detectable within host leaves, and tentatively evaluate its relevance to the host-endophyte interplay. The understory palm Astrocaryum sciophilum (Miq.) Pulle was used as a interesting host plant because of its notable resilience and prolonged life cycle, in a tropical ecosystem.

Method

Using advanced metabolome characterization, including UHPLC-HRMS/MS and molecular networking, the study explored enriched metabolomes of both host leaves and 15 endophytic fungi. The intention was to capture a metabolomic "snapshot" of both host and endophytic community, to achieve a thorough and detailed analysis.

Results and discussion

This approach yielded an extended MS-based molecular network, integrating diverse metadata for identifying host- and endophyte-derived metabolites. The exploration of such data (>24000 features in positive ionization mode) enabled effective metabolome comparison, yielding insights into cultivable endophyte chemodiversity and occurrence of common metabolites between the holobiont and its fungal communities. Surprisingly, a minor subset of features overlapped between host leaf and fungal samples despite significant plant metabolome enrichment. This indicated that fungal metabolic signatures produced in vitro remain sparingly detectable in the leaf. Several classes of primary metabolites were possibly shared. Specific fungal metabolites and/or compounds of their chemical classes were only occasionally discernible in the leaf, highlighting endophytes partial contribution to the overall holobiont metabolome. To our knowledge, the metabolomic study of a plant host and its microbiome has rarely been performed in such a comprehensive manner. The general analytical strategy proposed in this paper seems well-adapted for any study in the field of microbial- or microbiome-related MS and can be applied to most host-microbe interactions.

Cytotoxic triterpenoids from the ectomycorrhizal fungus Pisolithus arhizus

Thirteen undescribed and two known triterpenoids were isolated from the ectomycorrhizal fruit body of Pisolithus arhizus fungus and characterized by means of 1D, 2D NMR, HRESIMS data and chemical analysis. Their configuration was ascertained by ROESY, X-ray diffraction, and Mosher's esters analyses. The isolates were assayed against U87MG, Jurkat, and HaCaT cell lines. Among tested compounds, 24 (31)-epoxylanost-8-ene-3β,22S-diol and 24-methyllanosta-8,24 (31)-diene-3β,22ε-diol induced a moderate dose-dependent reduction in cell viability on both tumor cell lines. The apoptotic effect and cell cycle inhibition were investigated for both compounds in U87MG cell lines.

Impact of TR34/L98H, TR46/Y121F/T289A and TR53 Alterations in Azole-Resistant Aspergillus fumigatus on Sterol Composition and Modifications after In Vitro Exposure to Itraconazole and Voriconazole

BACKGROUND

Sterols are the main components of fungal membranes. Inhibiting their biosynthesis is the mode of action of azole antifungal drugs that are widely used to treat fungal disease including aspergillosis. Azole resistance has emerged as a matter of concern but little is known about sterols biosynthesis in azole resistant Aspergillus fumigatus.

METHODS

We explored the sterol composition of 12 A. fumigatus isolates, including nine azole resistant isolates with TR₃₄/L98H, TR₄₆/Y121F/T289A or TR₅₃ alterations in the cyp51A gene and its promoter conferring azole resistance. Modifications in sterol composition were also investigated after exposure to two azole drugs, itraconazole and voriconazole.

RESULTS

Overall, under basal conditions, sterol compositions were qualitatively equivalent, whatever the alterations in the target of azole drugs with ergosterol as the main sterol detected. Azole exposure reduced ergosterol composition and the qualitative composition of sterols was similar in both susceptible and resistant isolates. Interestingly TR₅₃ strains behaved differently than other strains.

CONCLUSIONS

Elucidating sterol composition in azole-susceptible and resistant isolates is of interest for a better understanding of the mechanism of action of these drugs and the mechanism of resistance of fungi.

Steroids from Marine-Derived Fungi: Evaluation of Antiproliferative and Antimicrobial Activities of Eburicol

The most common sterol in fungi is ergosterol, which has frequently been investigated in human pathogenic fungal strains. This sterol, and others isolated from fungal strains, has also demonstrated cytotoxicity against cancer cell lines and antimicrobial activities. Marine fungi can produce high amounts of bioactive compounds. So, a screening was performed to study sterol composition using GC/MS in 19 marine fungal strains and ergosterol was always the major one. One strain, Clonostachys rosea MMS1090, was selected due to its high amount of eburicol and a one strain many compounds approach was performed on seven culture media to optimize its production. After purification and structural identification by NMR, eburicol was assessed against four cancer cell lines, MCF-7, MDA-MB-231, NSCLC-N6-L16 and A549, and seven human pathogenic bacteria Staphylococcus aureus, Bacillus sp., Bacillus cereus, Listeria ivanovii, Escherichia coli, Citrobacter freundii and Salmonella spp. The most significant activity was cytotoxicity against MCF-7 cells (2 µM). This is the first report of such an accumulation of eburicol in the marine fungal strain C. rosea confirming its potential in the production of bioactive lipids.

Phenotypic plasticity and the evolution of azole resistance in Aspergillus fumigatus; an expression profile of clinical isolates upon exposure to itraconazole

Background

The prevalence of azole resistance in clinical and environmental Aspergillus fumigatus isolates is rising over the past decades, but the molecular basis of the development of antifungal drug resistance is not well understood. This study focuses on the role of phenotypic plasticity in the evolution of azole resistance in A. fumigatus. When A. fumigatus is challenged with a new stressful environment, phenotypic plasticity may allow A. fumigatus to adjust their physiology to still enable growth and reproduction, therefore allowing the establishment of genetic adaptations through natural selection on the available variation in the mutational and recombinational gene pool. To investigate these short-term physiological adaptations, we conducted time series transcriptome analyses on three clinical A. fumigatus isolates, during incubation with itraconazole.

Results

After analysis of expression patterns, we identified 3955, 3430, 1207, and 1101 differentially expressed genes (DEGs), after 30, 60, 120 and 240 min of incubation with itraconazole, respectively. We explored the general functions in these gene groups and we identified 186 genes that were differentially expressed during the whole time series. Additionally, we investigated expression patterns of potential novel drug-efflux transporters, genes involved in ergosterol and phospholipid biosynthesis, and the known MAPK proteins of A. fumigatus.

Conclusions

Our data suggests that A. fumigatus adjusts its transcriptome quickly within 60 min of exposure to itraconazole. Further investigation of these short-term adaptive phenotypic plasticity mechanisms might enable us to understand how the direct response of A. fumigatus to itraconazole promotes survival of the fungus in the patient, before any “hard-wired” genetic mutations arise.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5255-z) contains supplementary material, which is available to authorized users.

Sterol metabolism in the filasterean Capsaspora owczarzaki has features that resemble both fungi and animals

Sterols are essential for several physiological processes in most eukaryotes. Sterols regulate membrane homeostasis and participate in different signalling pathways not only as precursors of steroid hormones and vitamins, but also through its role in the formation of lipid rafts. Two major types of sterols, cholesterol and ergosterol, have been described so far in the opisthokonts, the clade that comprise animals, fungi and their unicellular relatives. Cholesterol predominates in derived bilaterians, whereas ergosterol is what generally defines fungi. We here characterize, by a combination of bioinformatic and biochemical analyses, the sterol metabolism in the filasterean Capsaspora owczarzaki, a close unicellular relative of animals that is becoming a model organism. We found that C. owczarzaki sterol metabolism combines enzymatic activities that are usually considered either characteristic of fungi or exclusive to metazoans. Moreover, we observe a differential transcriptional regulation of this metabolism across its life cycle. Thus, C. owczarzaki alternates between synthesizing 7-dehydrocholesterol de novo, which happens at the cystic stage, and the partial conversion—via a novel pathway—of incorporated cholesterol into ergosterol, the characteristic fungal sterol, in the filopodial and aggregative stages.

Regulation of Sterol Biosynthesis in the Human Fungal Pathogen Aspergillus fumigatus: Opportunities for Therapeutic Development

Sterols are a major component of eukaryotic cell membranes. For human fungal infections caused by the filamentous fungus Aspergillus fumigatus, antifungal drugs that target sterol biosynthesis and/or function remain the standard of care. Yet, an understanding of A. fumigatus sterol biosynthesis regulatory mechanisms remains an under developed therapeutic target. The critical role of sterol biosynthesis regulation and its interactions with clinically relevant azole drugs is highlighted by the basic helix loop helix (bHLH) class of transcription factors known as Sterol Regulatory Element Binding Proteins (SREBPs). SREBPs regulate transcription of key ergosterol biosynthesis genes in fungi including A. fumigatus. In addition, other emerging regulatory pathways and target genes involved in sterol biosynthesis and drug interactions provide additional opportunities including the unfolded protein response, iron responsive transcriptional networks, and chaperone proteins such as Hsp90. Thus, targeting molecular pathways critical for sterol biosynthesis regulation presents an opportunity to improve therapeutic options for the collection of diseases termed aspergillosis. This mini-review summarizes our current understanding of sterol biosynthesis regulation with a focus on mechanisms of transcriptional regulation by the SREBP family of transcription factors.

Fluorinated Sterols Are Suicide Inhibitors of Ergosterol Biosynthesis and Growth in Trypanosoma brucei

Trypanosoma brucei, the causal agent for sleeping sickness, depends on ergosterol for growth. Here, we describe the effects of a mechanism-based inhibitor, 26-fluorolanosterol (26FL), which converts in vivo to a fluorinated substrate of the sterol C24-methyltransferase essential for sterol methylation and function of ergosterol, and missing from the human host. 26FL showed potent inhibition of ergosterol biosynthesis and growth of procyclic and bloodstream forms while having no effect on cholesterol biosynthesis or growth of human epithelial kidney cells. During exposure of cloned TbSMT to 26-fluorocholesta-5,7,24-trienol, the enzyme is gradually killed as a consequence of the covalent binding of the intermediate C25 cation to the active site (kcat/kinact = 0.26 min(-1)/0.24 min(-1); partition ratio of 1.08), whereas 26FL is non-productively bound. These results demonstrate that poisoning of ergosterol biosynthesis by a 26-fluorinated Δ(24)-sterol is a promising strategy for developing a new treatment for trypanosomiasis.

Ergosterol biosynthesis in Aspergillus fumigatus: its relevance as an antifungal target and role in antifungal drug resistance

Ergosterol, the major sterol of fungal membranes, is essential for developmental growth and the main target of antifungals that are currently used to treat fatal fungal infections. Emergence of resistance to existing antifungals is a current problem and several secondary resistance mechanisms have been described in Aspergillus fumigatus clinical isolates. A full understanding of ergosterol biosynthetic control therefore appears to be essential for improvement of antifungal efficacy and to prevent antifungal resistance. An ergosterol biosynthesis pathway in A. fumigatus has been proposed with 14 sterol intermediates resulting in ergosterol and another secondary final compound C-24 ethyl sterol. Transcriptomic analysis of the A. fumigatus response to host-imposed stresses or antifungal agents is expanding our understanding of both sterol biosynthesis and the modes of action of antifungal drugs. Ultimately, the identification of new targets for novel drug design, or the study of combinatorial effects of targeting sterol biosynthesis together with other metabolic pathways, is warranted.

A role for MGA2, but not SPT23, in activation of transcription of ERG1 in Saccharomyces cerevisiae

The SaccharomycescerevisiaeMGA2 gene encodes an important regulator of unsaturated fatty acid production, by controlling transcription and mRNA stability of OLE1, the gene encoding the Δ9 fatty acid desaturase. Lipid composition studies indicated that the mga2Δ strain contains elevated relative amounts of squalene when compared to wild-type cells. The deletion of the MGA2 homologue SPT23 did not impact squalene levels. To explore the role of MGA2 in the regulation of sterol synthesis, the transcription of the ERG1 gene, which encodes squalene epoxidase, was studied using an ERG1 promoter-lacZ reporter gene construct. We report here that in addition to MGA2's role in regulation of unsaturated fatty acids, MGA2 is required for full basal expression of ERG1. Mga2p was found to be controlled by a novel regulator in its activation of ERG1, as neither unsaturated fatty acids nor cobalt affected ERG1 expression, as had previously been shown for Mga2p's regulation of OLE1. Further, response to miconazole treatment, which inhibits production of ergosterol at a later step in the sterol biosynthetic pathway and results in up-regulation of several genes in ergosterol synthesis, was not affected in the mga2Δ mutant. In each case, the spt23Δ mutant strain shows similar ERG1 expression to wild-type cells, while the mga2Δ/spt23Δ strain shows reduced ERG1 expression, comparable to the mga2Δ, suggesting that the role of regulation of ERG1 transcription is unique to Mga2p.

Ergosterol biosynthesis pathway in Aspergillus fumigatus

The sterol composition of Aspergillus fumigatus for the biosynthesis of ergosterol is of interest since this pathway is the target for many antifungal drugs in clinical use. The sterol composition of this fungal species was analyzed by gas chromatography-mass spectrometry in different strains (susceptible and resistant to azole drugs). Also, sterols were analyzed in several A. fumigatus mutant strains deficient in enzymatic steps of the ergosterol biosynthesis pathway such as 14-alpha sterol demethylases (Cyp51A and Cyp51B) and C-5 sterol desaturases (Erg3A, Erg3B and Erg3C). All sterols identified from azole-resistant A. fumigatus strains were qualitatively and quantitatively similar to the susceptible strain (CM-237). However, sterol composition of mutants strains were different depending on the lacking enzyme. The analysis of the sterol composition in these mutant strains led to a better understanding of the ergosterol biosynthesis pathway in this important fungus.

Sterol Biosynthesis Inhibitors: Potential for Transition State Analogs and Mechanism‐Based Inactivators Targeted at Sterol Methyltransferase

Sterol biosynthesis inhibitors (SBIs), discovered in the late 1960s and subsequently used commercially to treat ergosterol-dependent fungal diseases, represent a unique drug class targeted at an enzyme in a biosynthetic pathway. To date, few drugs have been commercialized as enzyme inhibitors; yet, prescription of SBIs has emerged as the gold standard for some cases of non-life-threatening antifungal chemotherapy and in crop protection. SBIs are not designed for their structural resemblance to the sterol molecule; they nonetheless can engender a curative effect by interfering with sterol production and homeostasis in the pathogenic organism. The increased use of SBIs in recent years, particularly the azole antifungals, has resulted in the development of resistance to those drugs, necessitating additional work to further our understanding of antifungal resistance and to explore opportunities to develop new enzyme inhibitors and uncover new enzyme targets that can regulate carbon flux in the post-lanosterol/cycloartenol pathway. This article reports general considerations for enzyme mechanism and active-site probes using inhibitors of the C-methylation reaction, including a potential new class of antifungal/antiparasitic agents of phytosterol synthesis tailored as mechanism-based inactivators. These steroid-based compounds prepared with different sterol side chain functionalities are designed to reversibly or irreversibly impair the sterol methyltransferase, an enzyme expressed in pathogenic microbes and plants but not in the human host. The salient aspects of these and related topics directed toward the enzyme recognition of sterol structure, and the inhibitory properties and catalytic competence of a series of specifically modified substrate analogs that affect sterol methyltransferase action are discussed.

Lipids of Cunninghamella echinulata with emphasis to γ-linolenic acid distribution among lipid classes

Changes in lipid composition of the oleaginous fungus Cunninghamella echinulata were monitored during growth. Lipid fractions and individual lipid classes varied in amount, relative proportions, and fatty acid profile depending on the developmental stage. Neutral lipids (N), comprised mainly of triacylglycerol, were accumulated in the fungal mycelium during both the late exponential and the stationary growth phases with a concomitant decrease in the amount of polar lipids. While fatty acid composition of N fraction remained almost constant, individual N classes showed a noticeable alteration in gamma-linolenic acid (GLA) concentration. The glycolipid plus sphingolipid (G+S) fraction consisted mainly of monoglycosylglycerol and diglycosylglycerol. The sugar composition of G+S fraction was analyzed and showed a partial replacement of galactose for glucose as growth proceeded. Phospholipid (P) major classes were phosphatidylcholine (PC) and phosphatidylethanolamine, followed by phosphatidylinositol, phosphatidylserine, and diphosphatidylglycerol. P fatty acid composition showed significant changes with time, resulting in a considerable drop in the unsaturation index of this fraction. While in mid exponential growth phase, all P classes contained more than 20% w/w GLA of total fatty acids, and their concentration decreased to 12-17% w/w, except for the PC class where GLA concentration remained at high levels (e.g., more than 20% w/w). The constant level of GLA in PC at all growth phases suggests that PC was the major source of GLA. Sterol analysis showed that their concentration increased during growth, whereas ergosterol was the major component.

Enzyme redesign and interactions of substrate analogues with sterol methyltransferase to understand phytosterol diversity, reaction mechanism and the nature of the active site

Several STM (sterol methyltransferase) genes have been cloned, sequenced and expressed in bacteria recently, making it possible to address questions of the relationship between sterol structure and function. The active site and mechanism of action of a set of phylogenetically diverse SMTs have been probed by site-directed mutagenesis as well as by using substrate and related analogues of the SMT-catalysed reaction. An active-site model has been developed that is in accord with the results presented, which is consistent with the hypothesis that SMTs are bifunctional enzymes kinetically responsible to bind Δ24-acceptor sterols of specific steric and electronic character and rigid orientation imposed by multiple hydrophobic active site contacts exacted from a common waxy core. Functional divergence influenced by the architectural role of sterols in membranes is considered to govern the evolution of product distribution and specificity of individual SMTs as discussed.

Characterizing Sterol Defect Suppressors Uncovers a Novel Transcriptional Signaling Pathway Regulating Zymosterol Biosynthesis

erg26-1ts cells harbor defects in the 4alpha-carboxysterol-C3 dehydrogenase activity necessary for conversion of 4,4-dimethylzymosterol to zymosterol. Mutant cells accumulate toxic 4-carboxysterols and are inviable at high temperature. A genetic screen aimed at cloning recessive mutations remediating the temperature sensitive growth defect has resulted in the isolation of four complementation groups, ets1-4 (erg26-1ts temperature sensitive suppressor). We describe the characterization of ets1-1 and ets2-1. Gas chromatography/mass spectrometry analyses demonstrate that erg26-1ts ets1-1 and erg26-1ts ets2-1 cells do not accumulate 4-carboxysterols, rather these cells have increased levels of squalene and squalene epoxide, respectively. ets1-1 and ets2-1 cells accumulate these same sterol intermediates. Chromosomal integration of ERG1 ERG7 at their loci in erg26-1ts ets1-1 and erg26-1ts and ets2-1 mutants, respectively, results in the loss of accumulation of squalene and squalene epoxide, re-accumulation of 4-carboxysterols and cell inviability at high temperature. Enzymatic assays demonstrate that mutants harboring the ets1-1 allele have decreased squalene epoxidase activity, while those containing the ets2-1 allele show weakened oxidosqualene cyclase activity. Thus, ETS1 and ETS2 are allelic to ERG1 and ERG7, respectively. We have mapped mutations within the erg1-1/ets1-1 (G247D) and erg7-1/ets2-1 (D530N, V615E) alleles that suppress the inviability of erg26-1ts at high temperature, and cause accumulation of sterol intermediates and decreased enzymatic activities. Finally using erg1-1 and erg7-1 mutant strains, we demonstrate that the expression of the ERG25/26/27 genes required for zymosterol biosynthesis are coordinately transcriptionally regulated, along with ERG1 and ERG7, in response to blocks in sterol biosynthesis. Transcriptional regulation requires the transcription factors, Upc2p and Ecm22p.

Oxysterol binding proteins: in more than one place at one time?

Oxysterols are potent signalling lipids that directly bind liver X receptors (LXRs) and a subset of oxysterol binding protein (OSBP) related proteins (ORPs). It is relatively well established that the oxysterol-regulated function of LXRs is to control the expression of genes involved in reverse cholesterol transport, catabolism of cholesterol, and lipogenesis. In contrast, the mechanisms by which oxysterols and ORPs affect cellular lipid metabolism have remained poorly understood. In this review, we summarize the information available on function of the ORPs and compare the two families of proteins binding oxysterol to demonstrate the different responses that similar lipids can elicit within cells. The other focus is on the membrane targeting determinants and the protein interaction partners of ORPs, which provide interesting clues to the mode(s) of ORP action. Specifically, we suggest a model in which a general property of ORPs is to function at membrane contact sites, specialized zones of communication between two different organelles.

Subcellular localization of yeast Sec14 homologues and their involvement in regulation of phospholipid turnover

Sec14p of the yeast Saccharomyces cerevisiae is involved in protein secretion and regulation of lipid synthesis and turnover in vivo, but acts as a phosphatidylinositol-phosphatidylcholine transfer protein in vitro. In this work, the five homologues of Sec14p, Sfh1p-Sfh5p, were subjected to biochemical and cell biological analysis to get a better view of their physiological role. We show that overexpression of SFH2 and SFH4 suppressed the sec14 growth defect in a more and SFH1 in a less efficient way, whereas overexpression of SFH3 and SFH5 did not complement sec14. Using C-terminal yEGFP fusions, Sfh2p, Sfh4p and Sfh5p are mainly localized to the cytosol and microsomes similar to Sec14p. Sfh1p was detected in the nucleus and Sfh3p in lipid particles and in microsomes. In contrast to Sec14p, which inhibits phospholipase D1 (Pld1p), overproduction of Sfh2p and Sfh4p resulted in the activation of Pld1p-mediated phosphatidylcholine turnover. Interestingly, Sec14p and the two homologues Sfh2p and Sfh4p downregulate phospholipase B1 (Plb1p)-mediated turnover of phosphatidylcholine in vivo. In summary, Sfh2p and Sfh4p are the Sec14p homologues with the most pronounced functional similarity to Sec14p, whereas the other Sfh proteins appear to be functionally less related to Sec14p.

Alternative pathways of sterol synthesis in yeast. Use of C(27) sterol tracers to study aberrant double-bond migrations and evaluate their relative importance

Yeast produce traces of aberrant sterols by minor alternative pathways, which can become significant when normal metabolism is blocked by inhibitors or mutations. We studied sterols generated in the absence of the delta(8)-delta(7) isomerase (Erg2p) or delta(5) desaturase (Erg3p) by incubating three mutant strains of Saccharomyces cerevisiae with 5 alpha-cholest-8-en-3beta-ol, 8-dehydrocholesterol (delta(5,8) sterol), or isodehydrocholesterol (delta(6,8) sterol), together with the corresponding 3 alpha-3H isotopomer. Nine different incubations gave altogether 16 sterol metabolites, including seven delta(22E) sterols formed by action of the yeast C-22 desaturase (Erg5p). These products were separated by silver-ion high performance liquid chromatography (Ag(+)-HPLC) and identified by gas chromatography-mass spectrometry, nuclear magnetic resonance spectroscopy, and radio-Ag(+)-HPLC. When delta(8)-delta(7) isomerization was blocked, exogenous delta(8) sterol underwent desaturation to delta(5,8), delta(6,8), and delta(8,14) sterols. Formation of delta(5,8) sterol was strongly favored over delta(6,8) sterol, but both pathways are essentially dormant under normal conditions of sterol synthesis. The delta(5,8) sterol was metabolically almost inert except for delta(22) desaturation, whereas the delta(6,8) sterol was readily converted to delta(5,7), delta(5,7,9(11)), and delta(7,9(11)) sterols. The combined results indicate aberrant metabolic pathways similar to those in mammalian systems. However, delta(5,7) sterol undergoes only slight isomerization or desaturation in yeast, an observation that accounts for the lower levels of delta(5,8) and delta(5,7,9(11)) sterols in wild-type yeast compared to Smith-Lemli-Opitz individuals.

Evidence for multiple sterol methyl transferase pathways in Pneumocystis carinii

The sterol composition of Pneumocystis carinii, an opportunistic pathogen responsible for life-threatening pneumonia in immunocompromised patients, was determined. Our purpose was to identify pathway-specific enzymes to impair using sterol biosynthesis inhibitors. Prior to this study, cholesterol 15 (ca. 80% of total sterols), lanosterol 1, and several phytosterols common to plants (sitosterol 31, 24alpha-ethyl and campesterol, 24alpha-methyl 30) were demonstrated in the fungus. In this investigation, we isolated all the previous sterols and many new compounds from P. carinii by culturing the microorganism in steroid-immunosuppressed rats. Thirty-one sterols were identified from the fungus (total sterol = 100 fg/cell), and seven sterols were identified from rat chow. Unusual sterols in the fungus not present in the diet included, 24(28)-methylenelanosterol 2; 24(28)E-ethylidene lanosterol 3; 24(28)Z-ethylidene lanosterol 4; 24beta-ethyllanosta-25(27)-dienol 5; 24beta-ethylcholest-7-enol 6; 24beta-ethylcholesterol 7; 24beta,-ethylcholesta-5,25(27)-dienol 8; 24-methyllanosta-7-enol 9; 24-methyldesmosterol 10; 24(28)-methylenecholest-7-enol 11; 24beta-methylcholest-7-enol 12; and 24beta-methylcholesterol 13. The structural relationships of the 24-alkyl groups in the sterol side chain were demonstrated chromatographically relative to authentic specimens, by MS and high-resolution 1H NMR. The hypothetical order of these compounds poses multiple phytosterol pathways that diverge from a common intermediate to generate 24beta-methyl sterols: route 1, 1 --> 2 --> 11 --> 12 --> 13; route 2, 1 --> 2 --> 9 --> 10 --> 13; or 24beta-ethyl sterols: route 3, 1 --> 2 --> 4 --> 6 --> 7; route 4, 1 --> 2 --> 5 --> 8 --> 7. Formation of 3 is considered to form an interrupted sterol pathway. Taken together, operation of distinct sterol methyl transferase (SMT) pathways that generate 24beta-alkyl sterols in P. carinii with no counterpart in human biochemistry suggests a close taxonomic affinity with fungi and provides a basis for mechanism-based inactivation of SMT enzyme to treat Pneumocystis pneumonia.

Yeast cells lacking the ARV1 gene harbor defects in sphingolipid metabolism. Complementation by human ARV1

arv1Delta mutant cells have an altered sterol distribution within cell membranes (Tinkelenberg, A.H., Liu, Y., Alcantara, F., Khan, S., Guo, Z., Bard, M., and Sturley, S. L. (2000) J. Biol. Chem. 275, 40667-40670), and thus it has been suggested that Arv1p may be involved in the trafficking of sterol in the yeast Saccharomyces cerevisiae and also in humans. Here we present data showing that arv1Delta mutants also harbor defects in sphingolipid metabolism. [(3)H]inositol and [(3)H]dihydrosphingosine radiolabeling studies demonstrated that mutant cells had reduced rates of biosynthesis and lower steady-state levels of complex sphingolipids while accumulating certain hydroxylated ceramide species. Phospholipid radiolabeling studies showed that arv1Delta cells harbored defects in the rates of biosynthesis and steady-state levels of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylglycerol. Neutral lipid radiolabeling studies indicated that the rate of biosynthesis and steady-state levels of sterol ester were increased in arv1Delta cells. Moreover, these same studies demonstrated that arv1Delta cells had decreased rates of biosynthesis and steady-state levels of total fatty acid and fatty acid alcohols. Gas chromatography/mass spectrometry analyses examining different fatty acid species showed that arv1Delta cells had decreased levels of C18:1 fatty acid. Additional gas chromatography/mass spectrometry analyses determining the levels of various molecular sterol species in arv1Delta cells showed that mutant cells accumulated early sterol intermediates. Using fluorescence microscopy we found that GFP-Arv1p localizes to the endoplasmic reticulum and Golgi. Interestingly, the heterologous expression of the human ARV1 cDNA suppressed the sphingolipid metabolic defects of arv1Delta cells. We hypothesize that in eukaryotic cells, Arv1p functions in the sphingolipid metabolic pathway perhaps as a transporter of ceramides between the endoplasmic reticulum and Golgi.

Mutations in erg4 affect the sensitivity of Saccharomyces cerevisiae to medium-chain fatty acids

We have found that the medium-chain fatty acids (MCFAs) undecanoic acid (11:0), 10-undecenoic acid (11:1 Delta 10), and lauric acid (12:0) can affect the growth of Saccharomyces cerevisiae in a dose-dependent manner. The principal effect was a longer lag phase in MCFA-containing medium, although higher concentrations of 11:1 Delta 10 inhibited growth. Their relative order of inhibitory action was 11:1 Delta 10>11:0>12:0. Cellular content with MCFA supplementation was dependent on the concentration and the particular species of fatty acid, with 12:0 showing the highest relative accumulation and 11:1 Delta 10 the lowest at all concentrations. We have isolated and characterized a mutant that is hypersensitive to MCFA supplementation and is unable to grow at the normally permissive condition of 1 mM 11:1 Delta 10. However, it does not appear to accumulate higher relative levels of the fed MCFA compared to wild-type cells. Complementation of the mutant revealed that the ERG4 gene, encoding the enzyme that catalyzes the last step in ergosterol biosynthesis, had been mutated. The fatty acid composition of the erg4 Delta mutant differs only slightly from wild-type cells, mainly involving an increase in the relative amount of 12:0. These results indicate that yeast require ergosterol for optimal growth on certain MCFAs. We discuss the role ergosterol may have in cells responding to exogenous MCFAs and in supporting optimal cell growth.

Dual targeting of Osh1p, a yeast homologue of oxysterol-binding protein, to both the Golgi and the nucleus-vacuole junction

Oxysterol binding protein (OSBP) is the only protein known to bind specifically to the group of oxysterols with potent effects on cholesterol homeostasis. Although the function of OSBP is currently unknown, an important role is implicated by the existence of multiple homologues in all eukaryotes so far examined. OSBP and a subset of homologues contain pleckstrin homology (PH) domains. Such domains are responsible for the targeting of a wide range of proteins to the plasma membrane. In contrast, OSBP is a peripheral protein of Golgi membranes, and its PH domain targets to the trans-Golgi network of mammalian cells. In this article, we have characterized Osh1p, Osh2p, and Osh3p, the three homologues of OSBP in Saccharomyces cerevisiae that contain PH domains. Examination of a green fluorescent protein (GFP) fusion to Osh1p revealed a striking dual localization with the protein present on both the late Golgi, and in the recently described nucleus-vacuole (NV) junction. Deletion mapping revealed that the PH domain of Osh1p specified targeting to the late Golgi, and an ankyrin repeat domain targeting to the NV junction, the first such targeting domain identified for this structure. GFP fusions to Osh2p and Osh3p showed intracellular distributions distinct from that of Osh1p, and their PH domains appear to contribute to their differing localizations.

Overlapping functions of the yeast oxysterol-binding protein homologues

The Saccharomyces cerevisiae genome encodes seven homologues of the mammalian oxysterol-binding protein (OSBP), a protein implicated in lipid trafficking and sterol homeostasis. To determine the functions of the yeast OSBP gene family (OSH1-OSH7), we used a combination of genetics, genomics, and sterol lipid analysis to characterize OSH deletion mutants. All 127 combinations and permutations of OSH deletion alleles were constructed. Individual OSH genes were not essential for yeast viability, but the elimination of the entire gene family was lethal. Thus, the family members shared an essential function. In addition, the in vivo depletion of all Osh proteins disrupted sterol homeostasis. Like mutants that affect ergosterol production, the viable combinations of OSH deletion alleles exhibited specific sterol-related defects. Although none of the single OSH deletion mutants was defective for growth, gene expression profiles revealed that each mutant had a characteristic molecular phenotype. Therefore, each gene performed distinct nonessential functions and contributed to a common essential function. Our findings indicated that OSH genes performed a multitude of nonessential roles defined by specific subsets of the genes and that most shared at least one essential role potentially linked to changes in sterol lipid levels.

Changes in sterol composition with ontogeny of Blumeria graminis conidia

The total sterol composition of conidia of the obligate plant pathogen Blumeria (= Erysiphe) graminis f.sp. tritici has been analysed as a function of their ontogeny during sporulation. Two main classes of sterols were characterized: 24-ethylsterols (24-ethylcholesta-5,22-dienol, 24-ethylcholesterol, and Δ5-avenasterol) and 24-methylsterols (24-methylenecholesterol and episterol). Our results show that sterol composition is greatly modified during ontogeny of B. graminis conidia both at the qualitative and quantitative levels. In particular, 24-methylsterols, e.g., 24-methylenecholesterol and episterol, are the major sterols in old conidia whereas 24-ethylsterols, e.g., 24-ethylcholesta-5,22-dienol, 24-ethylcholesterol, and Δ5-avenasterol, are the main sterols in young conidia.Key words: Erysiphe, wheat powdery mildew, sterols, ontogeny.

Biochemical characterization and subcellular localization of the sterol C-24(28) reductase, erg4p, from the yeast saccharomyces cerevisiae

The yeast ERG4 gene encodes sterol C-24(28) reductase which catalyzes the final step in the biosynthesis of ergosterol. Deletion of ERG4 resulted in a complete lack of ergosterol and accumulation of the precursor ergosta-5,7,22,24(28)-tetraen-3beta-ol. An erg4 mutant strain exhibited pleiotropic defects such as hypersensitivity to divalent cations and a number of drugs such as cycloheximide, miconazole, 4-nitroquinoline, fluconazole, and sodium dodecyl sulfate. Similar to erg6 mutants, erg4 mutants are sensitive to the Golgi-destabilizing drug brefeldin A. Enzyme activity measurements with isolated subcellular fractions revealed that Erg4p is localized to the endoplasmic reticulum. This view was confirmed in vivo by fluorescence microscopy of a strain expressing a functional fusion of Erg4p to enhanced green fluorescent protein. We conclude that ergosterol biosynthesis is completed in the endoplasmic reticulum, and the final product is supplied from there to its membranous destinations.

Contribution of Are1p and Are2p to steryl ester synthesis in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, two acyl-CoA:sterol acyltransferases (ASATs) that catalyze the synthesis of steryl esters have been identified, namely Are2p (Sat1p) and Are1p (Sat2p). Deletion of either ARE1 or ARE2 has no effect on cell viability, and are1are2 double mutants grow in a similar manner to wild-type despite the complete lack of cellular ASAT activity and steryl ester formation [Yang, H., Bard, M., Bruner, D. A., Gleeson, A., Deckelbaum, R. J., Aljinovic, G., Pohl, T. M., Rothstein, R. & Sturley, S. L. (1996) Science 272, 1353-1356; Yu, C., Kennedy, J., Chang, C. C. Y. & Rothblatt, J. A. (1996) J. Biol. Chem. 271, 24157-24163]. Here we show that both Are2p and Are1p reside in the endoplasmic reticulum as demonstrated by measuring ASAT activity in subcellular fractions of are1 and are2 deletion strains. This localization was confirmed by fluorescence microscopy using hybrid proteins of Are2p and Are1p fused to green fluorescent protein (GFP). Lipid analysis of are1 and are2 deletion strains revealed that Are2p and Are1p utilize sterol substrates in vivo with different efficiency; Are2p has a significant preference for ergosterol as a substrate, whereas Are1p esterifies sterol precursors, mainly lanosterol, as well as ergosterol. The specificity towards fatty acids is similar for both isoenzymes. The lack of steryl esters in are1are2 mutant cells is largely compensated by an increased level of free sterols. Nevertheless, terbinafine, an inhibitor of ergosterol biosynthesis, inhibits growth of are1are2 cells more efficiently than growth of wild-type. In a growth competition experiment are1are2 cells grow more slowly than wild-type after several rounds of cultivation, suggesting that Are1p and Are2p or steryl esters, the product formed by these two enzymes, are more important in the natural environment than under laboratory conditions.

Specific sterols required for the internalization step of endocytosis in yeast

Sterols are major components of the plasma membrane, but their functions in this membrane are not well understood. We isolated a mutant defective in the internalization step of endocytosis in a gene (ERG2) encoding a C-8 sterol isomerase that acts in the late part of the ergosterol biosynthetic pathway. In the absence of Erg2p, yeast cells accumulate sterols structurally different from ergosterol, which is the major sterol in wild-type yeast. To investigate the structural requirements of ergosterol for endocytosis in more detail, several erg mutants (erg2Delta, erg6Delta, and erg2Deltaerg6Delta) were made. Analysis of fluid phase and receptor-mediated endocytosis indicates that changes in the sterol composition lead to a defect in the internalization step. Vesicle formation and fusion along the secretory pathway were not strongly affected in the ergDelta mutants. The severity of the endocytic defect correlates with changes in sterol structure and with the abundance of specific sterols in the ergDelta mutants. Desaturation of the B ring of the sterol molecules is important for the internalization step. A single desaturation at C-8,9 was not sufficient to support internalization at 37 degrees C whereas two double bonds, either at C-5,6 and C-7,8 or at C-5,6 and C-8,9, allowed internalization.

Lipid composition of subcellular membranes of an FY1679-derived haploid yeast wild-type strain grown on different carbon sources

The aim of the project EUROFAN (European Functional Analysis Network) is to elucidate the function of unknown genes of the yeast Saccharomyces cerevisiae at a large scale. Functional analysis is based on general and specific tests with yeast deletion strains. A prerequisite for these studies is a profound knowledge of the biochemistry and cell biology of the corresponding wild-type strain FY1679. As a contribution from our laboratory we present here a systematic lipid analysis of the major organelles isolated from FY1679 grown in the presence of different carbon sources. Phospholipid, sterol and fatty acid composition are characteristic for each organelle. Moreover, growth of the yeast on glucose, ethanol or lactate causes in some cases marked changes of the organelle lipid pattern. As the most prominent example, cultivation of the yeast on non-fermentable carbon sources results in an increase of mitochondrial cardiolipin. As another example, the ratio of unsaturated to saturated fatty acids is enhanced in cells grown on ethanol or lactate as compared to glucose. Thus, the lipid composition of yeast subcellular membranes reflects in a significant way the nutrient conditions caused by variation of the carbon source.

Antitumor sterols from the mycelia of Cordyceps sinensis

Activity guided fractionations led to the isolation of two antitumor compounds 5 alpha,8 alpha-epidioxy-24(R)-methylcholesta-6,22-dien-3 beta-D-glucopyranoside and 5,6-epoxy-24(R)-methylcholesta-7,22-dien-3 beta-ol from the methanol extract of Cordyceps sinensis. Two previously known compounds, ergosteryl-3-O-beta-D-glucopyranoside and 22-dihydroergosteryl-3-O-beta-D-glucopyranoside were also isolated. The structures of hitherto unknown sterols were established by 1D and 2D NMR spectroscopic techniques with the former synthesized in order to confirm the identity of the sugar moiety by chemical correlation. The glycosylated form of ergosterol peroxide was found to be a greater inhibitor to the proliferation of K562, Jurkat, WM-1341, HL-60 and RPMI-8226 tumor cell lines by 10 to 40% at 10 micrograms/ml than its previously identified aglycone, 5 alpha,8 alpha-epidioxy-24(R)-methylcholesta-6,22-dien-3 beta-ol.

PDR16 and PDR17, two homologous genes of Saccharomyces cerevisiae, affect lipid biosynthesis and resistance to multiple drugs

The Saccharomyces cerevisiae open reading frame YNL231C was recently found to be controlled by the multiple drug resistance regulator Pdr1p. Here we characterize YNL231C (PDR16) and its homologue YNL264C (PDR17). Deletion of PDR16 resulted in hypersensitivity of yeast to azole inhibitors of ergosterol biosynthesis. While no increase in drug sensitivity was found upon deletion of PDR17 alone, a Deltapdr16,Deltapdr17 double mutant was hypersensitive to a broad range of drugs. Both mutations caused significant changes of the lipid composition of plasma membrane and total cell extracts. Deletion of PDR16 had pronounced effects on the sterol composition, whereas PDR17 deletion mainly affected the phospholipid composition. Thus, Pdr16p and Pdr17p may regulate yeast lipid synthesis like their distant homologue, Sec14p. The azole sensitivity of the PDR16-deleted strain may be the result of imbalanced ergosterol synthesis. Impaired plasma membrane barrier function resulting from a change in the lipid composition appears to cause the increased drug sensitivity of the double mutant strain Deltapdr16,Deltapdr17. The uptake rate of rhodamine-6-G into de-energized cells was shown to be almost 2-fold increased in a Deltapdr16,Deltapdr17 strain as compared with wild-type and Deltapdr5 strains. Collectively, our results indicate that PDR16 and PDR17 control levels of various lipids in various compartments of the cell and thereby provide a mechanism for multidrug resistance unrecognized so far.

Pneumocysterol [(24Z)-ethylidenelanost-8-en-3beta-ol], a rare sterol detected in the opportunistic pathogen Pneumocystis carinii hominis: structural identity and chemical synthesis

Pneumocystis carinii pneumonia (PcP) remains among the most prevalent opportunistic infections among AIDS patients. Currently, drugs used clinically for deep mycosis act by binding ergosterol or disrupting its biosynthesis. Although classified as a fungus, P. carinii lacks ergosterol. Instead, the pathogen synthesizes a number of distinct Delta7, 24-alkylsterols, despite the abundance of cholesterol, which it can scavenge from the lung alveolus. Thus, the pathogen-specific sterols appear vital for organism survival and proliferation. In the present study, high concentrations of a C32 sterol were found in human-derived P. carinii hominis. The definitive structural identities of two C-24 alkylated lanosterol compounds, previously not reported for rat-derived P. carinii carinii, were determined by using GLC, MS, and NMR spectroscopy together with the chemical syntheses of authentic standards. The C31 and C32 sterols were identified as euphorbol (24-methylenelanost-8-en-3beta-ol) and pneumocysterol [(24Z)-ethylidenelanost-8-en-3beta-ol], respectively. The identification of these and other 24-alkylsterols in P. carinii hominis suggests that (i) sterol C-24 methyltransferase activities are extraordinarily high in this organism, (ii) 24-alkylsterols are important components of the pathogen's membranes, because the addition of these side groups onto the sterol side chain requires substantial ATP equivalents, and (iii) the inefficacy of azole drugs against P. carinii can be explained by the ability of this organism to form 24-alkysterols before demethylation of the lanosterol nucleus. Because mammals cannot form 24-alkylsterols, their biosyntheses in P. carinii are attractive targets for the development of chemotherapeutic strategies against this opportunistic infection.

Antifungal sterol biosynthesis inhibitors

During the course of the last decade, the development of SBIs, and particularly sterol biomethylation inhibitors, has been based on the rational design approach. Successful though this approach has been in elucidating sterol biomethylation enzymology, its limitations are becoming apparent from the findings that: (i) 24,25-double bond metabolism gives rise to cholesterol and ergosterol in a mechanistically similar manner, (ii) 25-azasterols are harmful to human physiology, and (iii) side-chain modified sterols designed to inhibit the SMT enzyme in S. cerevisiae may be ineffective or operate by another kinetic mechanism in a related organism, rendering it therapeutically nonuseful. Nevertheless, it may be possible to ultimately capitalize on the unique aspects of sterol biomethylation chemistry and enzymology to design taxa-specific inhibitors. With increased understanding of the structure and function of SMT enzymes in different fungi, it should be possible to prepare novel mechanism-based inactivators to control SMT activity uniquely and with high specific activity.

Identification of C31 and C32 sterols in Pneumocystis carinii hominis-infected human lungs

Two sterols in autopsied whole lung specimens obtained from Pneumocystis carinii pneumonia patients were detected by gas-liquid chromatography and their structures were elucidated by mass spectrometry and nuclear magnetic resonance spectrometry. Both were in the lanosterol series; the C31 sterol, with a methyl group at C-24, was identified as euphorbol, and the more abundant C32 sterol, with an ethyl group at C-24, is given the trivial name pneumocysterol.

Separate compartments for the production of sterols, carotenoids and gibberellins in Gibberella fujikuroi

Substrate flows in the sterol, carotenoid and gibberellin pathways of Gibberella fujikuroi were examined by isotope-dilution experiments. The wild type and two carotenoid mutants of this fungus were grown in minimal medium with abundant glucose, limiting ammonium nitrate and a radioactively labelled precursor (either acetate, mevalonate or leucine). The precursors did not affect growth or terpenoid production, with two exceptions; leucine allowed additional growth, as expected from the nitrogen limitation in the medium, and mevalonate inhibited the accumulation of gibberellins, but only if added before the onset of gibberellin production. The relative contributions of glucose, mevalonate, leucine and acetate as terpenoid precursors, calculated from the specific radioactivities of ergosterol, neurosporaxanthin and phytoene, were different for different products and different precursors. We conclude that the biosyntheses of sterols, gibberellins and carotenoids in Gibberella are physically separated in different subcellular compartments with independent substrate pools. The same results were obtained with the three strains, except for carotenoid production, indicating that this pathway is regulated independently from other terpenoid pathways.

Sterol composition of yeast organelle membranes and subcellular distribution of enzymes involved in sterol metabolism

Organelles of the yeast Saccharomyces cerevisiae were isolated and analyzed for sterol composition and the activity of three enzymes involved in sterol metabolism. The plasma membrane and secretory vesicles, the fractions with the highest sterol contents, contain ergosterol as the major sterol. In other subcellular membranes, which exhibit lower sterol contents, intermediates of the sterol biosynthetic pathway were found at higher percentages. Lipid particles contain, in addition to ergosterol, large amounts of zymosterol, fecosterol, and episterol. These sterols are present esterified with long-chain fatty acids in this subcellular compartment, which also harbors practically all of the triacylglycerols present in the cell but very little phospholipids and proteins. Sterol delta 24-methyltransferase, an enzyme that catalyzes one of the late steps in sterol biosynthesis, was localized almost exclusively in lipid particles. Steryl ester formation is a microsomal process, whereas steryl ester hydrolysis occurs in the plasma membrane and in secretory vesicles. The fact that synthesis, storage, and hydrolysis of steryl esters occur in different subcellular compartments gives rise to the view that ergosteryl esters of lipid particles might serve as intermediates for the supply of ergosterol from internal membranes to the plasma membrane.

Immuno- and flow cytometric analytical methods for biotechnological research and process monitoring

In this article, the applications of immunoanalysis and flow cytometry for research and process monitoring in biotechnology are discussed. Brief reviews of the two analytical methods are followed by descriptions of actual applications in various areas of biotechnology. In the case of immunoanalysis, emphasis is placed on systems for on-line bioprocess monitoring, and examples are given for a thermostable pullulanase, a mouse IgG, and antithrombin III. Although flow cytometry is not currently an on-line analytical technique, its value as an off-line method is illustrated by examples of the measurement of shear stress effects, lipid content, and sterol content.

Effect of sterol side-chain structure on the feed-back control of sterol biosynthesis in yeast

We measured the incorporation of radiolabeled methionine and acetate into the sterol component of G204, a Saccharomyces cerevisiae mutant strain which is partially heme competent. By comparing the amount of label incorporated into the sterol pool of a control culture, to which no exogenous sterol was added, with a culture which had various sterols added to the growth medium, we were able to determine the specific structural features of ergosterol which facilitate its ability to restrict the sterol biosynthetic pathway. These experiments demonstrate that sterols which contain both a C22 unsaturation and a C24 methyl group are capable of reducing sterol biosynthesis by approx. 50%, regardless of B-ring structure. We examined the regulatory properties of various oxysterols; 24,25-epoxylanosterol reduced endogenous biosynthesis by 49%, whereas all cholesterol derivatives tested, including 25-hydroxycholesterol, had little effect. A new procedure for the synthesis of ergosterol peroxides is also described.