The stearoyl-coenzyme A desaturase 1 is essential for virulence and membrane stress in Candida parapsilosis through unsaturated fatty acid production

The Heat Shock Transcription Factor HsfA Plays a Role in Membrane Lipids Biosynthesis Connecting Thermotolerance and Unsaturated Fatty Acid Metabolism in Aspergillus fumigatus

Thermotolerance is a remarkable virulence attribute of Aspergillus fumigatus, but the consequences of heat shock (HS) to the cell membrane of this fungus are unknown, although this structure is one of the first to detect changes in ambient temperature that imposes on the cell a prompt adaptative response. Under high-temperature stress, fungi trigger the HS response controlled by heat shock transcription factors, such as HsfA, which regulates the expression of heat shock proteins. In yeast, smaller amounts of phospholipids with unsaturated fatty acid (FA) chains are synthesized in response to HS, directly affecting plasma membrane composition. The addition of double bonds in saturated FA is catalyzed by Δ9-fatty acid desaturases, whose expression is temperature-modulated. However, the relationship between HS and saturated/unsaturated FA balance in membrane lipids of A. fumigatus in response to HS has not been investigated. Here, we found that HsfA responds to plasma membrane stress and has a role in sphingolipid and phospholipid unsaturated biosynthesis. In addition, we studied the A. fumigatus Δ9-fatty acid desaturase sdeA and discovered that this gene is essential and required for unsaturated FA biosynthesis, although it did not directly affect the total levels of phospholipids and sphingolipids. sdeA depletion significantly sensitizes mature A. fumigatus biofilms to caspofungin. Also, we demonstrate that hsfA controls sdeA expression, while SdeA and Hsp90 physically interact. Our results suggest that HsfA is required for the adaptation of the fungal plasma membrane to HS and point out a sharp relationship between thermotolerance and FA metabolism in A. fumigatus. IMPORTANCE Aspergillus fumigatus causes invasive pulmonary aspergillosis, a life-threatening infection accounting for high mortality rates in immunocompromised patients. The ability of this organism to grow at elevated temperatures is long recognized as an essential attribute for this mold to cause disease. A. fumigatus responds to heat stress by activating heat shock transcription factors and chaperones to orchestrate cellular responses that protect the fungus against damage caused by heat. Concomitantly, the cell membrane must adapt to heat and maintain physical and chemical properties such as the balance between saturated/unsaturated fatty acids. However, how A. fumigatus connects these two physiological responses is unclear. Here, we explain that HsfA affects the synthesis of complex membrane lipids such as phospholipids and sphingolipids and controls the enzyme SdeA, which produces monounsaturated fatty acids, raw material for membrane lipids. These findings suggest that forced dysregulation of saturated/unsaturated fatty acid balance might represent novel strategies for antifungal therapy.

Integrative functional analysis uncovers metabolic differences between Candida species

Candida species are a dominant constituent of the human mycobiome and associated with the development of several diseases. Understanding the Candida species metabolism could provide key insights into their ability to cause pathogenesis. Here, we have developed the BioFung database, providing an efficient annotation of protein-encoding genes. Along, with BioFung, using carbohydrate-active enzyme (CAZymes) analysis, we have uncovered core and accessory features across Candida species demonstrating plasticity, adaption to the environment and acquired features. We show a greater importance of amino acid metabolism, as functional analysis revealed that all Candida species can employ amino acid metabolism. However, metabolomics revealed that only a specific cluster of species (AGAu species—C. albicans, C. glabrata and C. auris) utilised amino acid metabolism including arginine, cysteine, and methionine metabolism potentially improving their competitive fitness in pathogenesis. We further identified critical metabolic pathways in the AGAu cluster with biomarkers and anti-fungal target potential in the CAZyme profile, polyamine, choline and fatty acid biosynthesis pathways. This study, combining genomic analysis, and validation with gene expression and metabolomics, highlights the metabolic diversity with AGAu species that underlies their remarkable ability to dominate they mycobiome and cause disease.

Metabolic differences between Candida species are uncovered using the BioFung database alongside genomic and metabolic analysis.

The role of lipid droplets in microbial pathogenesis

The nonpolar lipids present in cells are mainly triacylglycerols and steryl esters. When cells are provided with an abundance of nutrients, these storage lipids accumulate. As large quantities of nonpolar lipids cannot be integrated into membranes, they are isolated from the cytosolic environment in lipid droplets. As specialized, inducible cytoplasmic organelles, lipid droplets have functions beyond the regulation of lipid metabolism, in cell signalling and activation, membrane trafficking and control of inflammatory mediator synthesis and secretion. Pathogens, including fungi, viruses, parasites, or intracellular bacteria can induce and may benefit from lipid droplets in infected cells. Here we review biogenesis of lipid droplets as well as the role of lipid droplets in the pathogenesis of selected viruses, bacteria, protists and yeasts.

HapX, an Indispensable bZIP Transcription Factor for Iron Acquisition, Regulates Infection Initiation by Orchestrating Conidial Oleic Acid Homeostasis and Cytomembrane Functionality in Mycopathogen Beauveria bassiana

Conidial maturation and germination are highly coupled physiological processes in filamentous fungi that are critical for the pathogenicity of mycopathogens. Compared to the mechanisms involved in conidial germination, those of conidial reserves during maturation are less understood. The insect-pathogenic fungus Beauveria bassiana, as a representative species of filamentous fungi, is important for applied and fundamental research. In addition to its conserved roles in fungal adaptation to iron status, the bZIP transcription factor HapX acts as a master regulator involved in conidial virulence and regulates fatty acid/lipid metabolism. Further investigation revealed that the Δ9-fatty acid desaturase gene (Ole1) is a direct downstream target of HapX. This study reveals the HapX-Ole1 pathway involved in the fatty acid/lipid accumulation associated with conidial maturation and provides new insights into the startup mechanism of infection caused by spores from pathogenic fungi.

In pathogenic filamentous fungi, conidial germination not only is fundamental for propagation in the environment but is also a critical step of infection. In the insect mycopathogen Beauveria bassiana, we genetically characterized the role of the basic leucine zipper (bZIP) transcription factor HapX (BbHapX) in conidial nutrient reserves and pathogen-host interaction. Ablation of BbHapX resulted in an almost complete loss of virulence in the topical inoculation and intrahemocoel injection assays. Comparative transcriptomic analysis revealed that BbHapX is required for fatty acid (FA)/lipid metabolism, and biochemical analyses indicated that BbHapX loss caused a significant reduction in conidial FA contents. Exogenous oleic acid could partially or completely restore the impaired phenotypes of the ΔBbHapX mutant, including germination rate, membrane integrity, vegetative growth, and virulence. BbHapX mediates fungal iron acquisition which is not required for desaturation of stearic acid. Additionally, inactivation of the Δ9-fatty acid desaturase gene (BbOle1) generated defects similar to those of the ΔBbHapX mutant; oleic acid also had significant restorative effects on the defective phenotypes of the ΔBbOle1 mutant. A gel retarding assay revealed that BbHapX directly regulated the expression of BbOle1. Lipidomic analyses indicated that both BbHapX and BbOle1 contributed to the homeostasis of phospholipids with nonpolar tails derived from oleic acid; therefore, exogenous phospholipids could significantly restore membrane integrity. These data reveal that the HapX-Ole1 pathway contributes to conidial fatty acid/lipid reserves and that there are important links between the lipid biology and membrane functionality involved in the early stages of infection caused by B.bassiana.

IMPORTANCE Conidial maturation and germination are highly coupled physiological processes in filamentous fungi that are critical for the pathogenicity of mycopathogens. Compared to the mechanisms involved in conidial germination, those of conidial reserves during maturation are less understood. The insect-pathogenic fungus Beauveria bassiana, as a representative species of filamentous fungi, is important for applied and fundamental research. In addition to its conserved roles in fungal adaptation to iron status, the bZIP transcription factor HapX acts as a master regulator involved in conidial virulence and regulates fatty acid/lipid metabolism. Further investigation revealed that the Δ9-fatty acid desaturase gene (Ole1) is a direct downstream target of HapX. This study reveals the HapX-Ole1 pathway involved in the fatty acid/lipid accumulation associated with conidial maturation and provides new insights into the startup mechanism of infection caused by spores from pathogenic fungi.

Candida parapsilosis: from Genes to the Bedside

Patients with suppressed immunity are at the highest risk for hospital-acquired infections. Among these, invasive candidiasis is the most prevalent systemic fungal nosocomial infection. Over recent decades, the combined prevalence of non-albicans Candida species outranked Candida albicans infections in several geographical regions worldwide, highlighting the need to understand their pathobiology in order to develop effective treatment and to prevent future outbreaks. Candida parapsilosis is the second or third most frequently isolated Candida species from patients. Besides being highly prevalent, its biology differs markedly from that of C. albicans, which may be associated with C. parapsilosis' increased incidence. Differences in virulence, regulatory and antifungal drug resistance mechanisms, and the patient groups at risk indicate that conclusions drawn from C. albicans pathobiology cannot be simply extrapolated to C. parapsilosis Such species-specific characteristics may also influence their recognition and elimination by the host and the efficacy of antifungal drugs. Due to the availability of high-throughput, state-of-the-art experimental tools and molecular genetic methods adapted to C. parapsilosis, genome and transcriptome studies are now available that greatly contribute to our understanding of what makes this species a threat. In this review, we summarize 10 years of findings on C. parapsilosis pathogenesis, including the species' genetic properties, transcriptome studies, host responses, and molecular mechanisms of virulence. Antifungal susceptibility studies and clinician perspectives are discussed. We also present regional incidence reports in order to provide an updated worldwide epidemiology summary.

Biotechnological potential of insect fatty acid-modifying enzymes

There are more than one million described insect species. This species richness is reflected in the diversity of insect metabolic processes. In particular, biosynthesis of secondary metabolites, such as defensive compounds and chemical signals, encompasses an extraordinarily wide range of chemicals that are generally unparalleled among natural products from other organisms. Insect genomes, transcriptomes and proteomes thus offer a valuable resource for discovery of novel enzymes with potential for biotechnological applications. Here, we focus on fatty acid (FA) metabolism-related enzymes, notably the fatty acyl desaturases and fatty acyl reductases involved in the biosynthesis of FA-derived pheromones. Research on insect pheromone-biosynthetic enzymes, which exhibit diverse enzymatic properties, has the potential to broaden the understanding of enzyme specificity determinants and contribute to engineering of enzymes with desired properties for biotechnological production of FA derivatives. Additionally, the application of such pheromone-biosynthetic enzymes represents an environmentally friendly and economic alternative to the chemical synthesis of pheromones that are used in insect pest management strategies.

Analysis of Piscirickettsia salmonis Metabolism Using Genome-Scale Reconstruction, Modeling, and Testing

Piscirickettsia salmonis is an intracellular bacterial fish pathogen that causes piscirickettsiosis, a disease with highly adverse impact in the Chilean salmon farming industry. The development of effective treatment and control methods for piscireckttsiosis is still a challenge. To meet it the number of studies on P. salmonis has grown in the last couple of years but many aspects of the pathogen's biology are still poorly understood. Studies on its metabolism are scarce and only recently a metabolic model for reference strain LF-89 was developed. We present a new genome-scale model for P. salmonis LF-89 with more than twice as many genes as in the previous model and incorporating specific elements of the fish pathogen metabolism. Comparative analysis with models of different bacterial pathogens revealed a lower flexibility in P. salmonis metabolic network. Through constraint-based analysis, we determined essential metabolites required for its growth and showed that it can benefit from different carbon sources tested experimentally in new defined media. We also built an additional model for strain A1-15972, and together with an analysis of P. salmonis pangenome, we identified metabolic features that differentiate two main species clades. Both models constitute a knowledge-base for P. salmonis metabolism and can be used to guide the efficient culture of the pathogen and the identification of specific drug targets.

Three, two, one yeast fatty acid desaturases: regulation and function

Fatty acid composition of biological membranes functionally adapts to environmental conditions by changing its composition through the activity of lipid biosynthetic enzymes, including the fatty acid desaturases. Three major desaturases are present in yeasts, responsible for the generation of double bonds in position C9-C10, C12-C13 and C15-C16 of the carbon backbone. In this review, we will report data addressed to define the functional role of basidiomycete and ascomycete yeast desaturase enzymes in response to various external signals and the regulation of the expression of their corresponding genes. Many yeast species have the complete set of three desaturases; however, only the Δ9 desaturase seems to be necessary and sufficient to ensure yeast viability. The evolutionary issue of this observation will be discussed.

Role of Protein Glycosylation in Candida parapsilosis Cell Wall Integrity and Host Interaction

Candida parapsilosis is an important, emerging opportunistic fungal pathogen. Highly mannosylated fungal cell wall proteins are initial contact points with host immune systems. In Candida albicans, Och1 is a Golgi α1,6-mannosyltransferase that plays a key role in the elaboration of the N-linked mannan outer chain. Here, we disrupted C. parapsilosis OCH1 to gain insights into the contribution of N-linked mannosylation to cell fitness and to interactions with immune cells. Loss of Och1 in C. parapsilosis resulted in cellular aggregation, failure of morphogenesis, enhanced susceptibility to cell wall perturbing agents and defects in wall composition. We removed the cell wall O-linked mannans by β-elimination, and assessed the relevance of mannans during interaction with human monocytes. Results indicated that O-linked mannans are important for IL-1β stimulation in a dectin-1 and TLR4-dependent pathway; whereas both, N- and O-linked mannans are equally important ligands for TNFα and IL-6 stimulation, but neither is involved in IL-10 production. Furthermore, mice infected with C. parapsilosis och1Δ null mutant cells had significantly lower fungal burdens compared to wild-type (WT)-challenged counterparts. Therefore, our data are the first to demonstrate that C. parapsilosis N- and O-linked mannans have different roles in host interactions than those reported for C. albicans.

Candida parapsilosis produces prostaglandins from exogenous arachidonic acid and OLE2 is not required for their synthesis

Prostaglandins are C20 fatty acid metabolites with diverse biological functions. In mammalian cells, prostaglandins are produced from arachidonic acid (AA) via cyclooxygenases (COX1 and COX2). Although fungi do not possess cyclooxygenase homologues, several pathogenic species are able to produce prostaglandins from host-derived arachidonic acid. In this study, we characterized the prostaglandin profile of the emerging human pathogen Candida parapsilosis with HPLC-MS and compared it to that of C. albicans. We found that both species synthesized prostaglandins (mainly PGD2 and PGE2) from exogenous AA. Furthermore, as OLE2 has been associated with prostaglandin synthesis in C. albicans, we generated homozygous OLE2 deletion mutants in C. parapsilosis and examined their PGE2 production. However, the PGE2 production of the OLE2 KO strain was similar to that of wild type (WT), indicating that OLE2 is not required for prostaglandin synthesis in C. parapsilosis. Interestingly, analyses of the fatty acid composition of WT and OLE2 KO cells by gas chromatography (GC) highlighted the accumulation of palmitoleic and oleic acid in the OLE2 deletion mutant. The OLE2 KO cells were killed more efficiently by human monocytes-derived macrophages (MDMs) as well as induced higher interleukin-10 (IL-10) secretion, indicating that OLE2 affects the virulence of C. parapsilosis. Taken together, these results contribute to the better understanding of fatty acid biosynthesis pathways in C. parapsilosis.

Influence of glucose concentration on the structure and quantity of biofilms formed by Candida parapsilosis

Candida parapsilosis is nowadays an emerging opportunistic pathogen and its increasing incidence is part related to the capacity to produce biofilm. In addition, one of the most important C. parapsilosis pathogenic risk factors includes the organisms' selective growth capabilities in hyperalimentation solutions. Thus, in this study, we investigated the role of glucose in C. parapsilosis biofilm modulation, by studying biofilm formation, matrix composition, and structure. Moreover, the expression of biofilm-related genes (BCR1, FKS1 and OLE1) was analysed in the presence of different glucose percentages. The results demonstrated the importance of glucose in the modulation of C. parapsilosis biofilm. The concentration of glucose had direct implications on the C. parapsilosis transition of yeast cells to pseudohyphae. Additionally, it was demonstrated that biofilm-related genes BCR1, FKS1, and OLE1 are involved in biofilm modulation as a result of glucose. The mechanism by which glucose enhances biofilm formation is not fully understood; however, with this study we were able to demonstrate that C. parapsilosis responds to stress conditions caused by elevated levels of glucose by upregulating genes related to biofilm formation (BCR1, FKS1 and OLE1).

Membrane fluidity and temperature sensing are coupled via circuitry comprised of Ole1, Rsp5, and Hsf1 in Candida albicans

Temperature is a ubiquitous environmental variable which can profoundly influence the physiology of living cells as it changes over time and space. When yeast cells are exposed to a sublethal heat shock, normal metabolic functions become repressed and the heat shock transcription factor Hsf1 is activated, inducing heat shock proteins (HSPs). Candida albicans, the most prevalent human fungal pathogen, is an opportunistic pathogen that has evolved as a relatively harmless commensal of healthy individuals. Even though C. albicans occupies thermally buffered niches, it has retained the classic heat shock response, activating Hsf1 during slow thermal transitions such as the increases in temperature suffered by febrile patients. However, the mechanism of temperature sensing in fungal pathogens remains enigmatic. A few studies with Saccharomyces cerevisiae suggest that thermal stress is transduced into a cellular signal at the level of the membrane. In this study, we manipulated the fluidity of C. albicans membrane to dissect mechanisms of temperature sensing. We determined that in response to elevated temperature, levels of OLE1, encoding a fatty acid desaturase, decrease. Subsequently, loss of OLE1 triggers expression of FAS2, encoding a fatty acid synthase. Furthermore, depletion of OLE1 prevents full activation of Hsf1, thereby reducing HSP expression in response to heat shock. This reduction in Hsf1 activation is attributable to the E3 ubiquitin ligase Rsp5, which regulates OLE1 expression. To our knowledge, this is the first study to define a molecular link between fatty acid synthesis and the heat shock response in the fungal kingdom.

Δ12-Fatty acid desaturase from Candida parapsilosis is a multifunctional desaturase producing a range of polyunsaturated and hydroxylated fatty acids

Numerous Δ12-, Δ15- and multifunctional membrane fatty acid desaturases (FADs) have been identified in fungi, revealing great variability in the enzymatic specificities of FADs involved in biosynthesis of polyunsaturated fatty acids (PUFAs). Here, we report gene isolation and characterization of novel Δ12/Δ15- and Δ15-FADs named CpFad2 and CpFad3, respectively, from the opportunistic pathogenic yeast Candida parapsilosis. Overexpression of CpFad3 in Saccharomyces cerevisiae strains supplemented with linoleic acid (Δ9,Δ12-18:2) and hexadecadienoic acid (Δ9,Δ12-16:2) leads to accumulation of Δ15-PUFAs, i.e., α-linolenic acid (Δ9,Δ12,Δ15-18:3) and hexadecatrienoic acid with an unusual terminal double bond (Δ9,Δ12,Δ15-16:3). CpFad2 produces a range of Δ12- and Δ15-PUFAs. The major products of CpFad2 are linoleic and hexadecadienoic acid (Δ9,Δ12-16:2), accompanied by α-linolenic acid and hexadecatrienoic acid (Δ9,Δ12,Δ15-16:3). Using GC/MS analysis of trimethylsilyl derivatives, we identified ricinoleic acid (12-hydroxy-9-octadecenoic acid) as an additional product of CpFad2. These results demonstrate that CpFAD2 is a multifunctional FAD and indicate that detailed analysis of fatty acid derivatives might uncover a range of enzymatic selectivities in other Δ12-FADs from budding yeasts (Ascomycota: Saccharomycotina).

The natural diyne-furan fatty acid EV-086 is an inhibitor of fungal delta-9 fatty acid desaturation with efficacy in a model of skin dermatophytosis

Human fungal infections represent a therapeutic challenge. Although effective strategies for treatment are available, resistance is spreading, and many therapies have unacceptable side effects. A clear need for novel antifungal targets and molecules is thus emerging. Here, we present the identification and characterization of the plant-derived diyne-furan fatty acid EV-086 as a novel antifungal compound. EV-086 has potent and broad-spectrum activity in vitro against Candida, Aspergillus, and Trichophyton spp., whereas activities against bacteria and human cell lines are very low. Chemical-genetic profiling of Saccharomyces cerevisiae deletion mutants identified lipid metabolic processes and organelle organization and biogenesis as targets of EV-086. Pathway modeling suggested that EV-086 inhibits delta-9 fatty acid desaturation, an essential process in S. cerevisiae, depending on the delta-9 fatty acid desaturase OLE1. Delta-9 unsaturated fatty acids-but not saturated fatty acids-antagonized the EV-086-mediated growth inhibition, and transcription of the OLE1 gene was strongly upregulated in the presence of EV-086. EV-086 increased the ratio of saturated to unsaturated free fatty acids and phosphatidylethanolamine fatty acyl chains, respectively. Furthermore, EV-086 was rapidly taken up into the lipid fraction of the cell and incorporated into phospholipids. Together, these findings demonstrate that EV-086 is an inhibitor of delta-9 fatty acid desaturation and that the mechanism of inhibition might involve an EV-086-phospholipid. Finally, EV-086 showed efficacy in a guinea pig skin dermatophytosis model of topical Trichophyton infection, which demonstrates that delta-9 fatty acid desaturation is a valid antifungal target, at least for dermatophytoses.

Candida parapsilosis and the neonate: epidemiology, virulence and host defense in a unique patient setting

Invasive candidiasis is a common problem in premature infants that leads to high morbidity and mortality. Although Candida albicans has historically been the most prominent species involved in these infections and has therefore been the subject of the most study, Candida parapsilosis is increasing in frequency, and neonates are disproportionately affected. This article reviews unique aspects of the epidemiology of this organism as well as strategies for prophylaxis against invasive candidiasis in general. Additionally, important differences between C. parapsilosis and C. albicans are coming to light related to virulence determinants and interactions with components of host immunity. These developments are reviewed while highlighting the significant gaps in our understanding that remain to be elucidated.

A potent plant-derived antifungal acetylenic acid mediates its activity by interfering with fatty acid homeostasis

6-Nonadecynoic acid (6-NDA), a plant-derived acetylenic acid, exhibits strong inhibitory activity against the human fungal pathogens Candida albicans, Aspergillus fumigatus, and Trichophyton mentagrophytes. In the present study, transcriptional profiling coupled with mutant and biochemical analyses were conducted using the model yeast Saccharomyces cerevisiae to investigate its mechanism of action. 6-NDA elicited a transcriptome response indicative of fatty acid stress, altering the expression of genes that are required for yeast growth in the presence of oleate. Mutants of S. cerevisiae lacking transcription factors that regulate fatty acid β-oxidation showed increased sensitivity to 6-NDA. Fatty acid profile analysis indicated that 6-NDA inhibited the formation of fatty acids longer than 14 carbons in length. In addition, the growth inhibitory effect of 6-NDA was rescued in the presence of exogenously supplied oleate. To investigate the response of a pathogenic fungal species to 6-NDA, transcriptional profiling and biochemical analyses were also conducted in C. albicans. The transcriptional response and fatty acid profile of C. albicans were comparable to those obtained in S. cerevisiae, and the rescue of growth inhibition with exogenous oleate was also observed in C. albicans. In a fluconazole-resistant clinical isolate of C. albicans, a fungicidal effect was produced when fluconazole was combined with 6-NDA. In hyphal growth assays, 6-NDA inhibited the formation of long hyphal filaments in C. albicans. Collectively, our results indicate that the antifungal activity of 6-NDA is mediated by a disruption in fatty acid homeostasis and that 6-NDA has potential utility in the treatment of superficial Candida infections.