Unraveling the novel structure and biosynthetic pathway of O-linked glycans in the Golgi apparatus of the human pathogenic yeast Cryptococcus neoformans

Silencing of MNT1 and PMT2 Shows the Importance of O-Linked Glycosylation During the Sporothrix schenckii-Host Interaction

Sporothrix schenckii is a pathogenic fungus of worldwide distribution and one of the etiological agents of sporotrichosis. The cell wall is the first point of contact with host cells; therefore, its composition has been widely studied. It has a cell wall composed of chitin, β-glucans, and glycoproteins modified with N-linked and O-linked glycans. Protein O-linked glycosylation is mediated by two gene families, PMT and MNT. Therefore, we evaluated the relevance of protein O-linked glycosylation during the interaction of S. schenckii with the host. Independent silencing of the MNT1 and PMT2 was accomplished by interference RNA. Morphological analyses revealed defects in cell morphology in both yeast and mycelial cells; however, these defects differed between MNT1 and PMT2 silencing. Subsequently, the cell wall was characterized, and the silencing of these genes markedly changed cell wall organization. When the silenced strains interacted with human peripheral blood mononuclear cells, a reduced ability to stimulate the proinflammatory cytokines IL-6 and TNFα was found. However, the PMT2-silenced mutants also stimulated higher levels of IL-10 and IL-1β. Interaction with macrophages and neutrophils was also altered, with increased phagocytosis and decreased extracellular trap formation in both sets of silenced strains. Survival assays in Galleria mellonella larvae showed that silencing of any of these genes reduced the ability of S. schenckii to kill the host. In addition, the mutant strains showed defects in the adhesion to extracellular matrix proteins. These data indicate that MNT1 and PMT2 are relevant for cell wall synthesis and interaction with the host.

Effects of altered N-glycan structures of Cryptococcus neoformans mannoproteins, MP98 (Cda2) and MP84 (Cda3), on interaction with host cells

Cryptococcus neoformans is an opportunistic human fungal pathogen causing lethal meningoencephalitis. It has several cell wall mannoproteins (MPs) identified as immunoreactive antigens. To investigate the structure and function of N-glycans assembled on cryptococcal cell wall MPs in host cell interactions, we purified MP98 (Cda2) and MP84 (Cda3) expressed in wild-type (WT) and N-glycosylation-defective alg3 mutant (alg3Δ) strains. HPLC and MALDI-TOF analysis of the MP proteins from the WT revealed protein-specific glycan structures with different extents of hypermannosylation and xylose/xylose phosphate addition. In alg3Δ, MP98 and MP84 had truncated core N-glycans, containing mostly five and seven mannoses (M5 and M7 forms), respectively. In vitro adhesion and uptake assays indicated that the altered core N-glycans did not affect adhesion affinities to host cells although the capacity to induce the immune response of bone-marrow derived dendritic cells (BMDCs) decreased. Intriguingly, the removal of all N-glycosylation sites on MP84 increased adhesion to host cells and enhanced the induction of cytokine secretion from BMDCs compared with that on MP84 carrying WT N-glycans. Therefore, the structure-dependent effects of N-glycans suggested their complex roles in modulating the interaction of MPs with host cells to avoid nonspecific adherence to host cells and host immune response hyperactivation.

Identification of an α-(16)-Mannosyltransferase Contributing To Biosynthesis of the Fungal-Type Galactomannan α-Core-Mannan Structure in Aspergillus fumigatus

Fungal-type galactomannan, a cell wall component of Aspergillus fumigatus, is composed of α-(1→2)-/α-(1→6)-linked mannan and β-(1→5)-/β-(1→6)-linked galactofuran side chains. Recently, CmsA and CmsB were identified as the α-(1→2)-mannosyltransferases involved in the biosynthesis of the α-core-mannan. However, the α-(1→6)-mannosyltransferase involved in the biosynthesis of the α-core-mannan has not been identified yet. In this study, we analyzed 9 putative α-(1→6)-mannosyltransferase gene disruption strains of A. fumigatus. The ΔanpA strain resulted in decreased mycelial elongation and reduced conidia formation. Proton nuclear magnetic resonance analysis revealed that the ΔanpA strain failed to produce the α-core-mannan of fungal-type galactomannan. We also found that recombinant AnpA exhibited much stronger α-(1→6)-mannosyltransferase activity toward α-(1→2)-mannobiose than α-(1→6)-mannobiose in vitro. Molecular simulations corroborated the fact that AnpA has a structure that can recognize the donor and acceptor substrates suitable for α-(1→6)-mannoside bond formation and that its catalytic activity would be specific for the elongation of the α-core-mannan structure in vivo. The identified AnpA is similar to Anp1p, which is involved in the elongation of the N-glycan outer chain in budding yeast, but the building sugar chain structure is different. The difference was attributed to the difference in substrate recognition of AnpA, which was clarified by simulations based on protein conformation. Thus, even proteins that seem to be functionally identical due to amino acid sequence similarity may be glycosyltransferase enzymes that make different glycans upon detailed analysis. This study describes an example of such a case. IMPORTANCE Fungal-type galactomannan is a polysaccharide incorporated into the cell wall of filamentous fungi belonging to the subphylum Pezizomycotina. Biosynthetic enzymes of fungal-type galactomannan are potential targets for antifungal drugs and agrochemicals. In this study, we identified an α-(1→6)-mannosyltransferase responsible for the biosynthesis of the α-core-mannan of fungal-type galactomannan, which has not been known for a long time. The findings of this study shed light on processes that shape this cellular structure while identifying a key enzyme essential for the biosynthesis of fungal-type galactomannan.

Extension of O-Linked Mannosylation in the Golgi Apparatus Is Critical for Cell Wall Integrity Signaling and Interaction with Host Cells in Cryptococcus neoformans Pathogenesis

The human-pathogenic yeast Cryptococcus neoformans assembles two types of O-linked glycans on its proteins. In this study, we identified and functionally characterized the C. neoformans CAP6 gene, encoding an α1,3-mannosyltransferase responsible for the second mannose addition to minor O-glycans containing xylose in the Golgi apparatus. Two cell surface sensor proteins, Wml1 (WSC/Mid2-like) and Wml2, were found to be independent substrates of Cap6-mediated minor or Ktr3-mediated major O-mannosylation, respectively. The double deletion of KTR3 and CAP6 (ktr3Δ cap6Δ) completely blocked the mannose addition at the second position of O-glycans, resulting in the accumulation of proteins with O-glycans carrying only a single mannose. Tunicamycin (TM)-induced phosphorylation of the Mpk1 mitogen-activated protein kinase (MAPK) was greatly decreased in both ktr3Δ cap6Δ and wml1Δ wml2Δ strains. Transcriptome profiling of the ktr3Δ cap6Δ strain upon TM treatment revealed decreased expression of genes involved in the Mpk1-dependent cell wall integrity (CWI) pathway. Consistent with its defective growth under several stress conditions, the ktr3Δ cap6Δ strain was avirulent in a mouse model of cryptococcosis. Associated with this virulence defect, the ktr3Δ cap6Δ strain showed decreased adhesion to lung epithelial cells, decreased proliferation within macrophages, and reduced transcytosis of the blood-brain barrier (BBB). Notably, the ktr3Δ cap6Δ strain showed reduced induction of the host immune response and defective trafficking of ergosterol, an immunoreactive fungal molecule. In conclusion, O-glycan extension in the Golgi apparatus plays critical roles in various pathobiological processes, such as CWI signaling and stress resistance and interaction with host cells in C. neoformans. IMPORTANCE Cryptococcus neoformans assembles two types of O-linked glycans on its surface proteins, the more abundant major O-glycans that do not contain xylose residues and minor O-glycans containing xylose. Here, we demonstrate the role of the Cap6 α1,3-mannosyltransferase in the synthesis of minor O-glycans. Previously proposed to be involved in capsule biosynthesis, Cap6 works with the related Ktr3 α1,2-mannosyltransferase to synthesize O-glycans on their target proteins. We also identified two novel C. neoformans stress sensors that require Ktr3- and Cap6-mediated posttranslational modification for full function. Accordingly, the ktr3Δ cap6Δ double O-glycan mutant strain displays defects in stress signaling pathways, CWI, and ergosterol trafficking. Furthermore, the ktr3Δ cap6Δ strain is completely avirulent in a mouse infection model. Together, these results demonstrate critical roles for O-glycosylation in fungal pathogenesis. As there are no human homologs for Cap6 or Ktr3, these fungus-specific mannosyltransferases are novel targets for antifungal therapy.

Cryptococcus neoformans can utilize ferritin as an iron source

Ferritin, a major iron storage protein in vertebrates, supplies iron upon iron deficiency. Ferritin is also found extracellularly, and acts as an iron carrier and a contributor to the immune response to invading microbes. Some microbial pathogens take advantage of ferritin as an iron source upon infection. However, no information is currently available on whether the human fungal pathogen Cryptococcus neoformans can acquire iron from ferritin. Here, we found that C. neoformans grew well in the presence of ferritin as a sole iron source. We showed that the binding of ferritin to the surface of C. neoformans is necessary and that acidification may contribute to ferritin-iron utilization by the fungus. Our data also revealed that the high-affinity reductive iron uptake system in C. neoformans is required for ferritin-iron acquisition. Furthermore, phagocytosis of C. neoformans by macrophages led to increased intracellular ferritin levels, suggesting that iron is sequestered by ferritin in infected macrophages. The increase in intracellular ferritin levels was reversed upon infection with a C. neoformans mutant deficient in the high-affinity reductive iron uptake system, indicating that this system plays a major role in iron acquisition in the phagocytosed C. neoformans in macrophages.

LAY SUMMARY

Cryptococcus neoformans is an opportunistic fungal pathogen causing life-threatening pulmonary disease and cryptococcal meningitis, mainly in immunocompromised patients. In this study, we found that C. neoformans can use ferritin, a major iron storage protein in vertebrates, as a sole iron source.

Role of Protein Glycosylation in Interactions of Medically Relevant Fungi with the Host

Protein glycosylation is a highly conserved post-translational modification among organisms. It plays fundamental roles in many biological processes, ranging from protein trafficking and cell adhesion to host–pathogen interactions. According to the amino acid side chain atoms to which glycans are linked, protein glycosylation can be divided into two major categories: N-glycosylation and O-glycosylation. However, there are other types of modifications such as the addition of GPI to the C-terminal end of the protein. Besides the importance of glycoproteins in biological functions, they are a major component of the fungal cell wall and plasma membrane and contribute to pathogenicity, virulence, and recognition by the host immunity. Given that this structure is absent in host mammalian cells, it stands as an attractive target for developing selective compounds for the treatment of fungal infections. This review focuses on describing the relationship between protein glycosylation and the host–immune interaction in medically relevant fungal species.

Cryptococcus extracellular vesicles properties and their use as vaccine platforms

Whereas extracellular vesicle (EV) research has become commonplace in different biomedical fields, this field of research is still in its infancy in mycology. Here we provide a robust set of data regarding the structural and compositional aspects of EVs isolated from the fungal pathogenic species Cryptococcus neoformans, C. deneoformans and C. deuterogattii. Using cutting-edge methodological approaches including cryogenic electron microscopy and cryogenic electron tomography, proteomics, and flow cytometry, we revisited cryptococcal EV features and suggest a new EV structural model, in which the vesicular lipid bilayer is covered by mannoprotein-based fibrillar decoration, bearing the capsule polysaccharide as its outer layer. About 10% of the EV population is devoid of fibrillar decoration, adding another aspect to EV diversity. By analysing EV protein cargo from the three species, we characterized the typical Cryptococcus EV proteome. It contains several membrane-bound protein families, including some Tsh proteins bearing a SUR7/PalI motif. The presence of known protective antigens on the surface of Cryptococcus EVs, resembling the morphology of encapsulated virus structures, suggested their potential as a vaccine. Indeed, mice immunized with EVs obtained from an acapsular C. neoformans mutant strain rendered a strong antibody response in mice and significantly prolonged their survival upon C. neoformans infection.

Ultrastructural Study of Cryptococcus neoformans Surface During Budding Events

Cryptococcus neoformans is a fungal pathogen that causes life-threatening infections in immunocompromised individuals. It is surrounded by three concentric structures that separate the cell from the extracellular space: the plasma membrane, the cell wall and the polysaccharide (PS) capsule. Although several studies have revealed the chemical composition of these structures, little is known about their ultrastructural organization and remodeling during C. neoformans budding events. Here, by combining the latest and most accurate light and electron microscopy techniques, we describe the morphological remodeling that occurs among the capsule, cell wall and plasma membrane during budding in C. neoformans. Our results show that the cell wall deforms to generate a specialized region at one of the cell’s poles. This region subsequently begins to break into layers that are slightly separated from each other and with thick tips. We also observe a reorganization of the capsular PS around the specialized regions. While daughter cells present their PS fibers aligned in the direction of budding, mother cells show a similar pattern but in the opposite direction. Also, daughter cells form multilamellar membrane structures covering the continuous opening between both cells. Together, our findings provide compelling ultrastructural evidence for C. neoformans surface remodeling during budding, which may have important implications for future studies exploring these remodeled specialized regions as drug-targets against cryptococcosis.

Glycomics, Glycoproteomics, and Glycogenomics: An Inter-Taxa Evolutionary Perspective

Glycosylation is a highly diverse set of co- and posttranslational modifications of proteins. For mammalian glycoproteins, glycosylation is often site-, tissue-, and species-specific and diversified by microheterogeneity. Multitudinous biochemical, cellular, physiological, and organismic effects of their glycans have been revealed, either intrinsic to the carrier proteins or mediated by endogenous reader proteins with carbohydrate recognition domains. Furthermore, glycans frequently form the first line of access by or defense from foreign invaders, and new roles for nucleocytoplasmic glycosylation are blossoming. We now know enough to conclude that the same general principles apply in invertebrate animals and unicellular eukaryotes-different branches of which spawned the plants or fungi and animals. The two major driving forces for exploring the glycomes of invertebrates and protists are (i) to understand the biochemical basis of glycan-driven biology in these organisms, especially of pathogens, and (ii) to uncover the evolutionary relationships between glycans, their biosynthetic enzyme genes, and biological functions for new glycobiological insights. With an emphasis on emerging areas of protist glycobiology, here we offer an overview of glycan diversity and evolution, to promote future access to this treasure trove of glycobiological processes.

Genome-wide functional analysis of phosphatases in the pathogenic fungus Cryptococcus neoformans

Phosphatases, together with kinases and transcription factors, are key components in cellular signalling networks. Here, we present a systematic functional analysis of the phosphatases in Cryptococcus neoformans, a fungal pathogen that causes life-threatening fungal meningoencephalitis. We analyse 230 signature-tagged mutant strains for 114 putative phosphatases under 30 distinct in vitro growth conditions, revealing at least one function for 60 of these proteins. Large-scale virulence and infectivity assays using insect and mouse models indicate roles in pathogenicity for 31 phosphatases involved in various processes such as thermotolerance, melanin and capsule production, stress responses, O-mannosylation, or retromer function. Notably, phosphatases Xpp1, Ssu72, Siw14, and Sit4 promote blood-brain barrier adhesion and crossing by C. neoformans. Together with our previous systematic studies of transcription factors and kinases, our results provide comprehensive insight into the pathobiological signalling circuitry of C. neoformans.

Phosphatases are key components in cellular signalling networks. Here, the authors present a systematic functional analysis of phosphatases of the fungal pathogen Cryptococcus neoformans, revealing roles in virulence, stress responses, O-mannosylation, retromer function and other processes.

Cryptococcus neoformans Evades Pulmonary Immunity by Modulating Xylose Precursor Transport

Cryptococcus neoformans is a fungal pathogen that kills almost 200,000 people each year and is distinguished by abundant and unique surface glycan structures that are rich in xylose. A mutant strain of C. neoformans that cannot transport xylose precursors into the secretory compartment is severely attenuated in virulence in mice yet surprisingly is not cleared. We found that this strain failed to induce the nonprotective T helper cell type 2 (Th2) responses characteristic of wild-type infection, instead promoting sustained interleukin 12p40 (IL-12p40) induction and increased IL-17A (IL-17) production. It also stimulated dendritic cells to release high levels of proinflammatory cytokines, a behavior we linked to xylose expression. We further discovered that inducible bronchus-associated lymphoid tissue (iBALT) forms in response to infection with either wild-type cryptococci or the mutant strain with reduced surface xylose; although iBALT formation is slowed in the latter case, the tissue is better organized. Finally, our temporal studies suggest that lymphoid structures in the lung restrict the spread of mutant fungi for at least 18 weeks after infection, which is in contrast to ineffective control of the pathogen after infection with wild-type cells. These studies demonstrate the role of xylose in modulation of host response to a fungal pathogen and show that cryptococcal infection triggers iBALT formation.

Core N-Glycan Structures Are Critical for the Pathogenicity of Cryptococcus neoformans by Modulating Host Cell Death

Cryptococcus neoformans is a human-pathogenic fungal pathogen that causes life-threatening meningoencephalitis in immunocompromised individuals. To investigate the roles of N-glycan core structure in cryptococcal pathogenicity, we constructed mutant strains of C. neoformans with defects in the assembly of lipid-linked N-glycans in the luminal side of the endoplasmic reticulum (ER). Deletion of ALG3 (alg3Δ), which encodes dolichyl-phosphate-mannose (Dol-P-Man)-dependent α-1,3-mannosyltransferase, resulted in the production of truncated neutral N-glycans carrying five mannose residues as a major species. Despite moderate or nondetectable defects in virulence-associated phenotypes in vitro, the alg3Δ mutant was avirulent in a mouse model of systemic cryptococcosis. Notably, the mutant did not show defects in early stages of host cell interaction during infection, including attachment to lung epithelial cells, opsonic/nonopsonic phagocytosis, and manipulation of phagosome acidification. However, the ability to drive macrophage cell death was greatly decreased in this mutant, without loss of cell wall remodeling capacity. Furthermore, deletion of ALG9 and ALG12, encoding Dol-P-Man-dependent α-1,2-mannosyltransferases and α-1,6-mannosyltransferases, generating truncated core N-glycans with six and seven mannose residues, respectively, also displayed remarkably reduced macrophage cell death and in vivo virulence. However, secretion levels of interleukin-1β (IL-1β) were not reduced in the bone marrow-derived dendritic cells obtained from Asc- and Gsdmd-deficient mice infected with the alg3Δ mutant strain, excluding the possibility that pyroptosis is a main host cell death pathway dependent on intact core N-glycans. Our results demonstrated N-glycan structures as a critical feature in modulating death of host cells, which is exploited by as a strategy for host cell escape for dissemination of C. neoformansIMPORTANCE We previously reported that the outer mannose chains of N-glycans are dispensable for the virulence of C. neoformans, which is in stark contrast to findings for the other human-pathogenic yeast, Candida albicans Here, we present evidence that an intact core N-glycan structure is required for C. neoformans pathogenicity by systematically analyzing alg3Δ, alg9Δ, and alg12Δ strains that have defects in lipid-linked N-glycan assembly and in in vivo virulence. The alg null mutants producing truncated core N-glycans were defective in inducing host cell death after phagocytosis, which is triggered as a mechanism of pulmonary escape and dissemination of C. neoformans, thus becoming inactive in causing fatal infection. The results clearly demonstrated the critical features of the N-glycan structure in mediating the interaction with host cells during fungal infection. The delineation of the roles of protein glycosylation in fungal pathogenesis not only provides insight into the glycan-based fungal infection mechanism but also will aid in the development of novel antifungal agents.

Unraveling synthesis of the cryptococcal cell wall and capsule

Fungal pathogens cause devastating infections in millions of individuals each year, representing a huge but underappreciated burden on human health. One of these, the opportunistic fungus Cryptococcus neoformans, kills hundreds of thousands of patients annually, disproportionately affecting people in resource-limited areas. This yeast is distinguished from other pathogenic fungi by a polysaccharide capsule that is displayed on the cell surface. The capsule consists of two complex polysaccharide polymers: a mannan substituted with xylose and glucuronic acid, and a galactan with galactomannan side chains that bear variable amounts of glucuronic acid and xylose. The cell wall, with which the capsule is associated, is a matrix of alpha and beta glucans, chitin, chitosan, and mannoproteins. In this review, we focus on synthesis of the wall and capsule, both of which are critical for the ability of this microbe to cause disease and are distinct from structures found in either model yeasts or the mammals afflicted by this infection. Significant research effort over the last few decades has been applied to defining the synthetic machinery of these two structures, including nucleotide sugar metabolism and transport, glycosyltransferase activities, polysaccharide export, and assembly and association of structural elements. Discoveries in this area have elucidated fundamental biology and may lead to novel targets for antifungal therapy. In this review, we summarize the progress made in this challenging and fascinating area, and outline future research questions.

Functional characterization of the Sporothrix schenckii Ktr4 and Ktr5, mannosyltransferases involved in the N-linked glycosylation pathway

Sporothrix schenckii is one of the causative agents of the deep-seated mycosis sporotrichosis, a fungal infection with worldwide distribution. Fungus-specific molecules and biosynthetic pathways are potential targets for the development of new antifungal drugs. The MNT1/KRE2 gene family is a group of genes that encode fungus-specific Golgi-resident mannosyltransferases that participate in the synthesis of O-linked and N-linked glycans. While this family is composed of five and nine members in Candida albicans and Saccharomyces cerevisiae, respectively, the S. schenckii genome contains only three putative members. MNT1 has been previously characterized as an enzyme that participates in the synthesis of both N-linked and O-linked glycans. Here, we aimed to establish the functional role of the two remaining family members, KTR4 and KTR5, in the protein glycosylation pathways by using heterologous complementation in C. albicans mutants lacking genes of the MNT1/KRE2 family. The two S. schenckii genes restored defects in the elaboration of N-linked glycans, but no complementation of mutants that synthesize truncated O-linked glycans was observed. Therefore, our results suggest that MNT1 is the sole member with a role in O-linked glycan elaboration, whereas the three family members have redundant activity in the S. schenckii N-linked glycan synthesis.

Xylose donor transport is critical for fungal virulence

Cryptococcus neoformans, an AIDS-defining opportunistic pathogen, is the leading cause of fungal meningitis worldwide and is responsible for hundreds of thousands of deaths annually. Cryptococcal glycans are required for fungal survival in the host and for pathogenesis. Most glycans are made in the secretory pathway, although the activated precursors for their synthesis, nucleotide sugars, are made primarily in the cytosol. Nucleotide sugar transporters are membrane proteins that solve this topological problem, by exchanging nucleotide sugars for the corresponding nucleoside phosphates. The major virulence factor of C. neoformans is an anti-phagocytic polysaccharide capsule that is displayed on the cell surface; capsule polysaccharides are also shed from the cell and impede the host immune response. Xylose, a neutral monosaccharide that is absent from model yeast, is a significant capsule component. Here we show that Uxt1 and Uxt2 are both transporters specific for the xylose donor, UDP-xylose, although they exhibit distinct subcellular localization, expression patterns, and kinetic parameters. Both proteins also transport the galactofuranose donor, UDP-galactofuranose. We further show that Uxt1 and Uxt2 are required for xylose incorporation into capsule and protein; they are also necessary for C. neoformans to cause disease in mice, although surprisingly not for fungal viability in the context of infection. These findings provide a starting point for deciphering the substrate specificity of an important class of transporters, elucidate a synthetic pathway that may be productively targeted for therapy, and contribute to our understanding of fundamental glycobiology.

Tracing Genetic Exchange and Biogeography of Cryptococcus neoformans var. grubii at the Global Population Level

Cryptococcus neoformans var. grubii is the causative agent of cryptococcal meningitis, a significant source of mortality in immunocompromised individuals, typically human immunodeficiency virus/AIDS patients from developing countries. Despite the worldwide emergence of this ubiquitous infection, little is known about the global molecular epidemiology of this fungal pathogen. Here we sequence the genomes of 188 diverse isolates and characterize the major subdivisions, their relative diversity, and the level of genetic exchange between them. While most isolates of C. neoformans var. grubii belong to one of three major lineages (VNI, VNII, and VNB), some haploid isolates show hybrid ancestry including some that appear to have recently interbred, based on the detection of large blocks of each ancestry across each chromosome. Many isolates display evidence of aneuploidy, which was detected for all chromosomes. In diploid isolates of C. neoformans var. grubii (serotype AA) and of hybrids with C. neoformans var. neoformans (serotype AD) such aneuploidies have resulted in loss of heterozygosity, where a chromosomal region is represented by the genotype of only one parental isolate. Phylogenetic and population genomic analyses of isolates from Brazil reveal that the previously "African" VNB lineage occurs naturally in the South American environment. This suggests migration of the VNB lineage between Africa and South America prior to its diversification, supported by finding ancestral recombination events between isolates from different lineages and regions. The results provide evidence of substantial population structure, with all lineages showing multi-continental distributions; demonstrating the highly dispersive nature of this pathogen.

Genetic alteration of UDP-rhamnose metabolism in Botrytis cinerea leads to the accumulation of UDP-KDG that adversely affects development and pathogenicity

Botrytis cinerea is a model plant-pathogenic fungus that causes grey mould and rot diseases in a wide range of agriculturally important crops. A previous study has identified two enzymes and corresponding genes (bcdh, bcer) that are involved in the biochemical transformation of uridine diphosphate (UDP)-glucose, the major fungal wall nucleotide sugar precursor, to UDP-rhamnose. We report here that deletion of bcdh, the first biosynthetic gene in the metabolic pathway, or of bcer, the second gene in the pathway, abolishes the production of rhamnose-containing glycans in these mutant strains. Deletion of bcdh or double deletion of both bcdh and bcer has no apparent effect on fungal development or pathogenicity. Interestingly, deletion of the bcer gene alone adversely affects fungal development, giving rise to altered hyphal growth and morphology, as well as reduced sporulation, sclerotia production and virulence. Treatments with wall stressors suggest the alteration of cell wall integrity. Analysis of nucleotide sugars reveals the accumulation of the UDP-rhamnose pathway intermediate UDP-4-keto-6-deoxy-glucose (UDP-KDG) in hyphae of the Δbcer strain. UDP-KDG could not be detected in hyphae of the wild-type strain, indicating fast conversion to UDP-rhamnose by the BcEr enzyme. The correlation between high UDP-KDG and modified cell wall and developmental defects raises the possibility that high levels of UDP-KDG result in deleterious effects on cell wall composition, and hence on virulence. This is the first report demonstrating that the accumulation of a minor nucleotide sugar intermediate has such a profound and adverse effect on a fungus. The ability to identify molecules that inhibit Er (also known as NRS/ER) enzymes or mimic UDP-KDG may lead to the development of new antifungal drugs.

Yeasts for Global Happiness: report of the 14th International Congress on Yeasts (ICY14) held in Awaji Island

The 14th International Congress on Yeasts (ICY14) was held at Awaji Yumebutai International Conference Center (Awaji, Hyogo) in Japan from 11 to 15 September 2016. The main slogan of ICY14 was 'Yeasts for Global Happiness', which enabled us to acknowledge the high-potential usefulness of yeasts contributing to the global happiness in terms of food/beverage, health/medicine and energy/environment industries, as well as to basic biosciences. In addition, two more concepts were introduced: 'from Japan to the world' and 'from senior to junior'. As it was the first ICY meeting held in Japan or other Asian countries, ICY14 provided a good opportunity to widely spread the great achievements by Japanese and Asian yeast researchers, such as those by the 2016 Nobel Laureate Dr. Yoshinori Ohsumi, and also, to convey the fun and importance of yeasts to the next generation of researchers from Asia and all over the world. As a result, a total of 426 yeast lovers from 42 countries (225 overseas and 201 domestic participants) with different generations attended ICY14 to share the latest knowledge of a wide range of yeast research fields and to join active and constructive scientific discussions.

Essential gene discovery in the basidiomycete Cryptococcus neoformans for antifungal drug target prioritization

Fungal diseases represent a major burden to health care globally. As with other pathogenic microbes, there is a limited number of agents suitable for use in treating fungal diseases, and resistance to these agents can develop rapidly. Cryptococcus neoformans is a basidiomycete fungus that causes cryptococcosis worldwide in both immunocompromised and healthy individuals. As a basidiomycete, it diverged from other common pathogenic or model ascomycete fungi more than 500 million years ago. Here, we report C. neoformans genes that are essential for viability as identified through forward and reverse genetic approaches, using an engineered diploid strain and genetic segregation after meiosis. The forward genetic approach generated random insertional mutants in the diploid strain, the induction of meiosis and sporulation, and selection for haploid cells with counterselection of the insertion event. More than 2,500 mutants were analyzed, and transfer DNA (T-DNA) insertions in several genes required for viability were identified. The genes include those encoding the thioredoxin reductase (Trr1), a ribosome assembly factor (Rsa4), an mRNA-capping component (Cet1), and others. For targeted gene replacement, the C. neoformans homologs of 35 genes required for viability in ascomycete fungi were disrupted, meiosis and sporulation were induced, and haploid progeny were evaluated for their ability to grow on selective media. Twenty-one (60%) were found to be required for viability in C. neoformans. These genes are involved in mitochondrial translation, ergosterol biosynthesis, and RNA-related functions. The heterozygous diploid mutants were evaluated for haploinsufficiency on a number of perturbing agents and drugs, revealing phenotypes due to the loss of one copy of an essential gene in C. neoformans. This study expands the knowledge of the essential genes in fungi using a basidiomycete as a model organism. Genes that have no mammalian homologs and are essential in both Cryptococcus and ascomycete human pathogens would be ideal for the development of antifungal drugs with broad-spectrum activity.

IMPORTANCE

Fungal infections are very common in humans but may be neglected due to misdiagnosis and inattention. Cryptococcus neoformans is a yeast that infects mainly immunocompromised people, causing high mortality rates in developing countries. The fungus infects the lungs, crosses the blood-brain barrier, and invades the cerebrospinal fluid, causing fatal meningitis. C. neoformans infections are treated with amphotericin B, flucytosine, and azoles, all developed decades ago. However, problems with antifungal agents highlight the urgent need for more-effective drugs to treat C. neoformans and other invasive fungal infections. These issues include the negative side effects of amphotericin B, the spontaneous resistance of C. neoformans to azoles, and the inefficacy of the echinocandin antifungals. In this study, we report the identification of C. neoformans essential genes as targets for the development of novel antifungals. Because of the level of evolutionary divergence between C. neoformans and the ascomycetes, a subset of these genes is likely essential in all fungi. Genes identified in this study represent an excellent starting point for the future development of new antifungals by pharmaceutical companies.