Preliminary evidence for a glucan acceptor in the yeast Candida albicans

Stress-Induced Changes in the Lipid Microenvironment of β-(1,3)-d-Glucan Synthase Cause Clinically Important Echinocandin Resistance in Aspergillus fumigatus

Resistance to first-line triazole antifungal agents among Aspergillus species has prompted the use of second-line therapy with echinocandins. As the number of Aspergillus-infected patients treated with echinocandins is rising, clinical observations of drug resistance are also increasing, indicating an emerging global health threat. Our knowledge regarding the development of clinical echinocandin resistance is largely derived from Candida spp., while little is known about resistance in Aspergillus. Therefore, it is important to understand the specific cellular responses raised by A. fumigatus against echinocandins. We discovered a new mechanism of resistance in A. fumigatus that is independent of the well-characterized FKS mutation mechanism observed in Candida. This study identified an off-target effect of CAS, i.e., ROS production, and integrated oxidative stress and sphingolipid alterations into a novel mechanism of resistance. This stress-induced response has implications for drug resistance and/or tolerance mechanisms in other fungal pathogens.

Aspergillus fumigatus is a leading cause of invasive fungal infections. Resistance to first-line triazole antifungals has led to therapy with echinocandin drugs. Recently, we identified several high-minimum-effective-concentration (MEC) A. fumigatus clinical isolates from patients failing echinocandin therapy. Echinocandin resistance is known to arise from amino acid substitutions in β-(1,3)-d-glucan synthase encoded by the fks1 gene. Yet these clinical isolates did not contain mutations in fks1, indicating an undefined resistance mechanism. To explore this new mechanism, we used a laboratory-derived strain, RG101, with a nearly identical caspofungin (CAS) susceptibility phenotype that also does not contain fks1 mutations. Glucan synthase isolated from RG101 was fully sensitive to echinocandins. Yet exposure of RG101 to CAS during growth yielded a modified enzyme that was drug insensitive (4 log orders) in kinetic inhibition assays, and this insensitivity was also observed for enzymes isolated from clinical isolates. To understand this alteration, we analyzed whole-enzyme posttranslational modifications (PTMs) but found none linked to resistance. However, analysis of the lipid microenvironment of the enzyme with resistance induced by CAS revealed a prominent increase in the abundances of dihydrosphingosine (DhSph) and phytosphingosine (PhSph). Exogenous addition of DhSph and PhSph to the sensitive enzyme recapitulated the drug insensitivity of the CAS-derived enzyme. Further analysis demonstrated that CAS induces mitochondrion-derived reactive oxygen species (ROS) and that dampening ROS formation by antimycin A or thiourea eliminated drug-induced resistance. We conclude that CAS induces cellular stress, promoting formation of ROS and triggering an alteration in the composition of plasma membrane lipids surrounding glucan synthase, rendering it insensitive to echinocandins.

Cell wall glucans of fungi. A review

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Glucans are the most abundant compounds in the fungal cell walls.

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The most common type of glucose bonding is 1 → 3, both alpha and beta.

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Microfibrillar glucans with chitin provide rigidity to the fungal wall.

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Fungal beta glucans act as PAMPS during infection of animals and plants.

Glucans are the most abundant polysaccharides in the cell walls of fungi, and their structures are highly variable. Accordingly, their glucose moieties may be joined through either or both alpha (α) or beta (β) linkages, they are either lineal or branched, and amorphous or microfibrillar. Alpha 1,3 glucans sensu strictu (pseudonigerans) are the most abundant alpha glucans present in the cell walls of fungi, being restricted to dikarya. They exist in the form of structural microfibrils that provide resistance to the cell wall. The structure of beta glucans is more complex. They are linear or branched, and contain mostly β 1,3 and β 1,6 linkages, existing in the form of microfibrils. Together with chitin they constitute the most important structural components of fungal cell walls. They are the most abundant components of the cell walls in members of all fungal phyla, with the exception of Microsporidia, where they are absent.

Taking into consideration the importance of glucans in the structure and physiology of the fungi, in the present review we describe the following aspects of these polysaccharides: i) types and distribution of fungal glucans, ii) their structure, iii) their roles, iv) the mechanism of synthesis of the most important ones, and v) the phylogentic relationships of the enzymes involved in their synthesis.

Current perspectives on echinocandin class drugs

It has been nearly a decade since caspofungin was approved for clinical use as the first echinocandin class antifungal agent, followed by micafungin and anidulafungin. The echinocandin drugs target the fungal cell wall by inhibiting the synthesis of β-1,3-D-glucan, a critical cell wall component of many pathogenic fungi. They are fungicidal for Candida spp. and fungistatic for moulds, such as Aspergillus fumigatus, where they induce abnormal morphology and growth properties. The echinocandins have a limited antifungal spectrum but are highly active against most Candida spp., including azole-resistant strains and biofilms. As they target glucan synthase, an enzyme absent in mammalian cells, the echinocandins have a favorable safety profile. They show potent MIC and epidemiological cutoff values against susceptible Candida and Aspergillus isolates, and the frequency of resistance is low. When clinical breakthrough occurs, it is associated with high MIC values and mutations in Fks subunits of glucan synthase, which can reduce the sensitivity of the enzyme to the drug by several thousand-fold. Such strains were not adequately captured by an early clinical breakpoint for susceptibility prompting a revised lower value, which addresses the FKS resistance mechanism and new pharmacokinetic/pharmacodynamic studies. Elevated MIC values unlinked to therapeutic failure can occur and result from adaptive cell behavior, which is FKS-independent and involves the molecular chaperone Hsp90 and the calcineurin pathway. Mutations in FKS1 and/or FKS2 alter the kinetic properties of glucan synthase, which reduces the relative fitness of mutant strains causing them to be less pathogenic. The echinocandin drugs also modify the cell wall architecture exposing buried glucans, which in turn induce a variety of important host immune responses. Finally, the future for glucan synthase inhibitors looks bright with the development of new orally active compounds.

Fungal beta(1,3)-D-glucan synthesis

The polysaccharide beta(1,3)-D-glucan is a component of the cell wall of many fungi. Synthesis of the linear polymer is catalysed by UDP-glucose beta(1,3)-D-glucan beta(3)-D-glucosyltransferase. Because this enzyme has a key role in fungal cell-wall synthesis, and because many organisms that are responsible for human mycoses, including Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans, produce walls that are rich in beta(1,3)-glucan, it has been and remains the focus of intensive study. From early characterization of the enzymatic activity in Saccharomyces cerevisiae, advances have been made in purification of the enzyme, identification of essential subunits and description of regulatory circuitry that controls expression and localization of different components of the multisubunit enzyme complex. Progress in each of these areas has been enhanced dramatically by the availability of specific inhibitors of the enzymatic reaction that produces beta(1,3)-glucan. These natural product inhibitors have utility both as tools to dissect the biology of beta(1,3)-glucan synthase and as sources for development of semisynthetic derivatives with clinical utility in treatment of human fungal disease. This review will focus on the biochemistry, genetics and regulation of the enzyme.

High molecular weight precursors of glucans in Saccharomyces cerevisiae

Nascent beta-1,3 glucan synthesized by mixed membrane fractions from Saccharomyces cerevisiae was solubilized by extraction with hot SDS or urea. Nature of the material was analyzed by electrophoresis and gel filtration. As determined by gel filtration, Mr of synthesized glucans exceeded 1,500 kDa, but was below 20,000 kDa. This nascent material served as an acceptor for further glucose transfer reactions, giving rise to glucan molecules over 20,000 kDa. It is suggested that the high Mr precursor components represent protein-bound glucan molecules in transit to the cell surface.

Isolation and partial characterization of uronic acid-containing glycoproteins from Mucor rouxii

Five different fractions containing uronic acids associated with protein were isolated from the cytoplasm of the filamentous form of Mucor rouxii. A single fraction was isolated from the cell wall by hot sodium dodecyl sulfate followed by ion exchange column chromatography. Two cytoplasmic entities (peaks I and II) were not adsorbed to DEAE Bio-Gel A. The molecular mass of peaks I to V ranged from 16.5 to 210 kDa. The protein-uronic acid ratios were different for each fraction. The cell wall fraction showed a molecular mass of 16.5 kDa, similar to that of peak II but with differences in chromatographic behavior and protein-uronic acid ratio. The possible role of these molecules as acceptors of sugar residues during polyuronide chain growth is discussed.

Killer toxin secretion through the cell wall of the yeast Pichia anomala

A secreted killer toxin was detected through the cell wall of Pichia anomala cells by ultrastructural immunodetection with a specific monoclonal antibody (MAb KT4). MAb KT4 was successively detected by colloidal gold labeled streptavidin and biotinylated anti-mouse F(ab')2 antibodies. The antigenic determinants of the toxin were localized throughout the cytoplasm and the cell wall of killer yeast cells. The Lowicryl K4M-immunogold method gave very satisfactory results and showed that the killer toxin was somewhat concentrated in the yeast cell wall layers before being exported into the medium. In agreement with previous reports, the binding of MAb KT4 suggested that the P. anomala killer toxin secretion did not result from a homogeneous diffusion across the yeast cell wall.

Biosynthesis of beta-glucans in fungi

Glucans are the most abundant polysaccharides present in fungi. The present review provides updated information on the structure and synthesis of beta-glucans in fungal cells. Synthesis of these polymers made up of B1,3 chains with a variable degree of B1,6 branching involves several reactions: initiation, chain elongation and branching, of which the most studied one is the elongation step. This reaction, catalyzed by the so-called glucan synthetases, utilizes UDPG as sugar donor. Properties of glucan synthetases are extremely variable depending on the fungal species, and their developmental stage. Because of the importance of these polysaccharides it is anticipated that comprehension of their mechanism of synthesis, is important for the understanding of cell wall assembly and cell growth and morphogenesis, as well as for the design of specific antifungal drugs.

A protein-sialyl polymer complex involved in colominic acid biosynthesis. Effect of tunicamycin

A protein-NeuAc complex involved in colominic acid biosynthesis has been identified in membrane preparations of Escherichia coli K-235. This compound had an Mr (estimated by SDS/polyacrylamide-gel electrophoresis and autoradiography) of about 100,000 and played the role of an 'initiator' or 'primer' (endogenous acceptor) in the synthesis of the whole polymer. Incubations of E. coli membranes with CMP-[14C]NeuAc (CMP-N-[14C]acetylneuraminic acid) pointed to the existence of a protein fraction (primer acceptor) that linked residues of sialic acid (N-acetylneuraminic acid, NeuAc) up to a maximal size, later releasing them as low-Mr sialyl polymers (LMrS, Mr less than 10,000). In the presence of colominic acid (final acceptor) the radioactivity linked to the protein quickly decreased, appearing stoichiometrically bound to the whole polysaccharide. When membrane preparations were previously digested with Streptomyces proteinase or de-activated by heating (80 degrees C, 10 min), no incorporation of labelled NeuAc into trichloroacetic acid-insoluble material was detected. These results suggested that colominic acid molecules are synthesized while they are bound to a proteinaceous acceptor that is subsequently excised in the presence of colominic acid, generating the native protein. The antibiotic tunicamycin inhibited the biosynthesis of colominic acid, affecting the synthesis of this protein-(NeuAc)n intermediate. All these results are described here for the first time.