Probing the role of point mutations in the cyp51A gene from Aspergillus fumigatus in the model yeast Saccharomyces cerevisiae

Azole-resistant strains of Aspergillus fumigatus have been detected and the underlying molecular mechanisms of resistance characterized. Point mutations in the cyp51A gene have been proved to be related to azole resistance in A. fumigatus clinical strains and with different resistance profiles depending on the amino acid change (G54E, G54V, G54R, G54W, M220V, M220K, M220T, M220I). The aim of this work was to express A. fumigatus cyp51A genes in the yeast Saccharomyces cerevisiae in order to better assess the contribution of each independent amino acid substitution to resistance. A tetracycline regulatable system allowing repression of the endogenous essential ERG11 gene was used. The expression of Aspergillus cyp51A alleles could efficiently restore the absence of ERG11 in S. cerevisiae. In general, S. cerevisiae clones expressing. A. fumigatus cyp51A alleles from azole-resistant isolates showed higher MICs to all azoles tested than those expressing alleles from susceptible isolates. The azole susceptibility profiles obtained in S. cerevisiae upon expression of specific cyp51A alleles recapitulated susceptibility profiles observed from their A. fumigatus origins. In conclusion this work supports the concept that characteristics of specific A. fumigatus cyp51A alleles could be investigated in the heterologous host S. cerevisia

Incidence of zygomycosis in transplant recipients

AbstractRecently, a remarkable increase in the incidence of zygomycosis has been reported from institutions in the USA and Europe. The use of voriconazole for the treatment of aspergillosis and, less frequently, the use of echinocandins as empirical treatment for invasive fungal infections are thought to be responsible for the increase. In addition, an increased incidence of this infection has been observed in transplant recipients, including both haematopoietic stem cell transplant (HSCT) and solid organ transplant (SOT) patients. There are no global surveys on the prevalence or incidence of zygomycosis, but data from individual institutions and countries show that zygomycosis is an emerging infection. The increased incidence of zygomycosis most probably reflects a greater frequency of predisposing factors, such as higher numbers of patients undergoing HSCT and immunosuppressive chemotherapy. In addition, the emergence of rare pathogens as a result of the rise in the use of antifungal therapy against common species can be postulated. Further, the availability of antifungal agents with activity profiles that are more specific for selected fungi increases the necessity of identifying pathogenic fungi; the frequency of Zygomycetes infections may have been underestimated until now because therapeutic decisions did not depend on the precise identification of pathogenic fungi

High-resolution melting assay for genotyping variants of the CYP2C19 enzyme and predicting voriconazole effectiveness

Voriconazole is a triazole antifungal agent recommended as primary treatment for invasive aspergillosis, as well as some other mold infections. However, it presents some pharmacokinetic singularities that lead to a great variability intra- and interindividually, nonlinear pharmacokinetics, and a narrow therapeutic range. Most experts have recommended tracing the levels of voriconazole in patients when receiving treatment. This azole is metabolized through the hepatic enzyme complex cytochrome P450 (CYPP450), with the isoenzyme CYP2C19 being principally involved. Allelic variations (polymorphisms) of the gene that encodes this enzyme are known to contribute to variability in voriconazole exposure. Three different allelic variants, CYP2C19*17, CYP2C19*2, and CYP2C19*3, could explain most of the phenotypes related to the voriconazole metabolism and some of its pharmacokinetic singularities. We designed a rapid molecular method based on high-resolution melting to characterize these polymorphisms in a total of 142 samples, avoiding sequencing. Three PCRs were designed with similar cycling conditions to run simultaneously. The results showed that our method represents a fast, accurate, and inexpensive means to study these variants related to voriconazole metabolism. In clinical practice, this could offer a useful tool to individually optimize therapy and reduce expenses in patients with fungal infections.National Institute of Health Carlos III (AES13PI13/01817Research Project MPY 1367/13). L.B.-M. has a contract supported by theMinisterio de Ciencia e Innovación, Instituto de Salud Carlos III, cofinanced by the EuropeanDevelopment Regional Fund (EDRF) “A Way to Achieve Europe” and the SpanishNetwork for the Research in Infectious Diseases (REIPI; RD12/0015/0015). B.M.-R. is astudent in the Master’s Program entitled “Microbiología Aplicada a la Salud Pública eInvestigación en Enfermedades Infecciosas,” Alcalá de Henares University, Madrid,Spain. A.C. and C.C. were supported by the Northern Portugal Regional OperationalProgram (NORTE 2020) under the Portugal 2020 Partnership Agreement through theEuropean Regional Development Fund (FEDER; NORTE-01-0145-FEDER-000013) and theFundação Para a Ciência e Tecnologia (FCT; IF/00735/2014 [A.C.] and SFRH/BPD/96176/2013 [C.C.])

Candida tropicalis antifungal cross-resistance is related to different azole target (Erg11p) modifications

ABSTARCT: Candida tropicalis ranks between third and fourth among Candida species most commonly isolated from clinical specimens. Invasive candidiasis and candidemia are treated with amphotericin B or echinocandins as first-line therapy, with extended-spectrum triazoles as acceptable alternatives. Candida tropicalis is usually susceptible to all antifungal agents, although several azole drug-resistant clinical isolates are being reported. However, C. tropicalis resistant to amphotericin B is uncommon, and only a few strains have reliably demonstrated a high level of resistance to this agent. The resistance mechanisms operating in C. tropicalis strains isolated from clinical samples showing resistance to azole drugs alone or with amphotericin B cross-resistance were elucidated. Antifungal drug resistance was related to mutations of the azole target (Erg11p) with or without alterations of the ergosterol biosynthesis pathway. The antifungal drug resistance shown in vitro correlated very well with the results obtained in vivo using the model host Galleria mellonella. Using this panel of strains, the G. mellonella model system was validated as a simple, nonmammalian minihost model that can be used to study in vitro-in vivo correlation of antifungals in C. tropicalis. The development in C. tropicalis of antifungal drug resistance with different mechanisms during antifungal treatment has potential clinical impact and deserves specific prospective studies

A Sterol-Regulatory Element Binding Protein Is Required for Cell Polarity, Hypoxia Adaptation, Azole Drug Resistance, and Virulence in Aspergillus fumigatus

At the site of microbial infections, the significant influx of immune effector cells and the necrosis of tissue by the invading pathogen generate hypoxic microenvironments in which both the pathogen and host cells must survive. Currently, whether hypoxia adaptation is an important virulence attribute of opportunistic pathogenic molds is unknown. Here we report the characterization of a sterol-regulatory element binding protein, SrbA, in the opportunistic pathogenic mold, Aspergillus fumigatus. Loss of SrbA results in a mutant strain of the fungus that is incapable of growth in a hypoxic environment and consequently incapable of causing disease in two distinct murine models of invasive pulmonary aspergillosis (IPA). Transcriptional profiling revealed 87 genes that are affected by loss of SrbA function. Annotation of these genes implicated SrbA in maintaining sterol biosynthesis and hyphal morphology. Further examination of the SrbA null mutant consequently revealed that SrbA plays a critical role in ergosterol biosynthesis, resistance to the azole class of antifungal drugs, and in maintenance of cell polarity in A. fumigatus. Significantly, the SrbA null mutant was highly susceptible to fluconazole and voriconazole. Thus, these findings present a new function of SREBP proteins in filamentous fungi, and demonstrate for the first time that hypoxia adaptation is likely an important virulence attribute of pathogenic molds

A proteomic approach to investigating gene cluster expression and secondary metabolite functionality in Aspergillus fumigatus.

A combined proteomics and metabolomics approach was utilised to advance the identification and characterisation of secondary metabolites in Aspergillus fumigatus. Here, implementation of a shotgun proteomic strategy led to the identification of non-redundant mycelial proteins (n = 414) from A. fumigatus including proteins typically under-represented in 2-D proteome maps: proteins with multiple transmembrane regions, hydrophobic proteins and proteins with extremes of molecular mass and pI. Indirect identification of secondary metabolite cluster expression was also achieved, with proteins (n = 18) from LaeA-regulated clusters detected, including GliT encoded within the gliotoxin biosynthetic cluster. Biochemical analysis then revealed that gliotoxin significantly attenuates H2O2-induced oxidative stress in A. fumigatus (p>0.0001), confirming observations from proteomics data. A complementary 2-D/LC-MS/MS approach further elucidated significantly increased abundance (p<0.05) of proliferating cell nuclear antigen (PCNA), NADH-quinone oxidoreductase and the gliotoxin oxidoreductase GliT, along with significantly attenuated abundance (p<0.05) of a heat shock protein, an oxidative stress protein and an autolysis-associated chitinase, when gliotoxin and H2O2 were present, compared to H2O2 alone. Moreover, gliotoxin exposure significantly reduced the abundance of selected proteins (p<0.05) involved in de novo purine biosynthesis. Significantly elevated abundance (p<0.05) of a key enzyme, xanthine-guanine phosphoribosyl transferase Xpt1, utilised in purine salvage, was observed in the presence of H2O2 and gliotoxin. This work provides new insights into the A. fumigatus proteome and experimental strategies, plus mechanistic data pertaining to gliotoxin functionality in the organism

HacA-Independent Functions of the ER Stress Sensor IreA Synergize with the Canonical UPR to Influence Virulence Traits in Aspergillus fumigatus

Endoplasmic reticulum (ER) stress is a condition in which the protein folding capacity of the ER becomes overwhelmed by an increased demand for secretion or by exposure to compounds that disrupt ER homeostasis. In yeast and other fungi, the accumulation of unfolded proteins is detected by the ER-transmembrane sensor IreA/Ire1, which responds by cleaving an intron from the downstream cytoplasmic mRNA HacA/Hac1, allowing for the translation of a transcription factor that coordinates a series of adaptive responses that are collectively known as the unfolded protein response (UPR). Here, we examined the contribution of IreA to growth and virulence in the human fungal pathogen Aspergillus fumigatus. Gene expression profiling revealed that A. fumigatus IreA signals predominantly through the canonical IreA-HacA pathway under conditions of severe ER stress. However, in the absence of ER stress IreA controls dual signaling circuits that are both HacA-dependent and HacA-independent. We found that a ΔireA mutant was avirulent in a mouse model of invasive aspergillosis, which contrasts the partial virulence of a ΔhacA mutant, suggesting that IreA contributes to pathogenesis independently of HacA. In support of this conclusion, we found that the ΔireA mutant had more severe defects in the expression of multiple virulence-related traits relative to ΔhacA, including reduced thermotolerance, decreased nutritional versatility, impaired growth under hypoxia, altered cell wall and membrane composition, and increased susceptibility to azole antifungals. In addition, full or partial virulence could be restored to the ΔireA mutant by complementation with either the induced form of the hacA mRNA, hacAi, or an ireA deletion mutant that was incapable of processing the hacA mRNA, ireAΔ10. Together, these findings demonstrate that IreA has both HacA-dependent and HacA-independent functions that contribute to the expression of traits that are essential for virulence in A. fumigatus

SREBP Coordinates Iron and Ergosterol Homeostasis to Mediate Triazole Drug and Hypoxia Responses in the Human Fungal Pathogen Aspergillus fumigatus

Sterol regulatory element binding proteins (SREBPs) are a class of basic helix-loop-helix transcription factors that regulate diverse cellular responses in eukaryotes. Adding to the recognized importance of SREBPs in human health, SREBPs in the human fungal pathogens Cryptococcus neoformans and Aspergillus fumigatus are required for fungal virulence and susceptibility to triazole antifungal drugs. To date, the exact mechanism(s) behind the role of SREBP in these observed phenotypes is not clear. Here, we report that A. fumigatus SREBP, SrbA, mediates regulation of iron acquisition in response to hypoxia and low iron conditions. To further define SrbA's role in iron acquisition in relation to previously studied fungal regulators of iron metabolism, SreA and HapX, a series of mutants were generated in the ΔsrbA background. These data suggest that SrbA is activated independently of SreA and HapX in response to iron limitation, but that HapX mRNA induction is partially dependent on SrbA. Intriguingly, exogenous addition of high iron or genetic deletion of sreA in the ΔsrbA background was able to partially rescue the hypoxia growth, triazole drug susceptibility, and decrease in ergosterol content phenotypes of ΔsrbA. Thus, we conclude that the fungal SREBP, SrbA, is critical for coordinating genes involved in iron acquisition and ergosterol biosynthesis under hypoxia and low iron conditions found at sites of human fungal infections. These results support a role for SREBP–mediated iron regulation in fungal virulence, and they lay a foundation for further exploration of SREBP's role in iron homeostasis in other eukaryotes

Fungal chitinases: diversity, mechanistic properties and biotechnological potential

Chitin derivatives, chitosan and substituted chito-oligosaccharides have a wide spectrum of applications ranging from medicine to cosmetics and dietary supplements. With advancing knowledge about the substrate-binding properties of chitinases, enzyme-based production of these biotechnologically relevant sugars from biological resources is becoming increasingly interesting. Fungi have high numbers of glycoside hydrolase family 18 chitinases with different substrate-binding site architectures. As presented in this review, the large diversity of fungal chitinases is an interesting starting point for protein engineering. In this review, recent data about the architecture of the substrate-binding clefts of fungal chitinases, in connection with their hydrolytic and transglycolytic abilities, and the development of chitinase inhibitors are summarized. Furthermore, the biological functions of chitinases, chitin and chitosan utilization by fungi, and the effects of these aspects on biotechnological applications, including protein overexpression and autolysis during industrial processes, are discussed in this review