Diversification of a protein kinase cascade: IME-2 is involved in nonself recognition and programmed cell death in Neurospora crassa

Ubiquitin proteolysis of a CDK-related kinase regulates titan cell formation and virulence in the fungal pathogen Cryptococcus neoformans

Fungal pathogens often undergo morphological switches, including cell size changes, to adapt to the host environment and cause disease. The pathogenic yeast Cryptococcus neoformans forms so-called 'titan cells' during infection. Titan cells are large, polyploid, display alterations in cell wall and capsule, and are more resistant to phagocytosis and various types of stress. Titan cell formation is regulated by the cAMP/PKA signal pathway, which is stimulated by the protein Gpa1. Here, we show that Gpa1 is activated through phosphorylation by a CDK-related kinase (Crk1), which is targeted for degradation by an E3 ubiquitin ligase (Fbp1). Strains overexpressing CRK1 or an allele lacking a PEST domain exhibit increased production of titan cells similarly to the fbp1∆ mutant. Conversely, CRK1 deletion results in reduced titan cell production, indicating that Crk1 stimulates titan cell formation. Crk1 phosphorylates Gpa1, which then localizes to the plasma membrane and activates the cAMP/PKA signal pathway to induce cell enlargement. Furthermore, titan cell-overproducing strains trigger increased Th1 and Th17 cytokine production in CD4+ T cells and show attenuated virulence in a mouse model of systemic cryptococcosis. Overall, our study provides insights into the regulation of titan cell formation and fungal virulence.

Protein kinase Ime2 is associated with mycelial growth, conidiation, osmoregulation, and pathogenicity in Fusarium oxysporum

Fusarium oxysporum f.sp. niveum is one of the most serious diseases impairing watermelon yield and quality. Inducer of meiosis 2 (Ime2) is the founding member of a family of serine/threonine protein kinases and plays important roles in yeasts and other filamentous fungi. In this study, we analyzed the functions of FoIme2, the ortholog of Saccharomyces cerevisiae Ime2 in F. oxysporum f.sp. niveum. The FoIme2-deleted mutants exhibited obvious morphological abnormalities, including slower vegetative growth, more branches in the edge hyphae and a reduction in conidia production. Compared to the wild type, the mutants were hypersensitive to the osmotic stressor NaCl but were more insensitive to the membrane stressor SDS. The deletion of FoIme2 also caused a reduction in pathogenicity. Transcriptional analysis revealed that FoIme2 acts downstream of FoOpy2 which is an upstream sensor of the MAPK kinase cascade. These results indicate that FoIme2 is important in the development and pathogenicity of F. oxysporum, and provide new insight for the analysis of the pathogenic mechanism of F. oxysporum.

Fungal Cell Death: The Beginning of the End

Death is an important part of an organism's existence and also marks the end of life. On a cellular level, death involves the execution of complex processes, which can be classified into different types depending on their characteristics. Despite their "simple" lifestyle, fungi carry out highly specialized and sophisticated mechanisms to regulate the way their cells die, and the pathways underlying these mechanisms are comparable with those of plants and metazoans. This review focuses on regulated cell death in fungi and discusses the evidence for the occurrence of apoptotic-like, necroptosis-like, pyroptosis-like death, and the role of the NLR proteins in fungal cell death. We also describe recent data on meiotic drive elements involved in "spore killing" and the molecular basis of allorecognition-related cell death during cell fusion of genetically dissimilar cells. Finally, we discuss how fungal regulated cell death can be relevant in developing strategies to avoid resistance and tolerance to antifungal agents.

Sexual Differentiation Is Coordinately Regulated by Cryptococcus neoformans CRK1 and GAT1

The heterothallic basidiomycetous fungus Cryptococcus neoformans has two mating types, MATa and MATα. Morphological progression of bisexual reproduction in C. neoformans is as follows: yeast to hyphal transition, filament extension, basidium formation, meiosis, and sporulation. C. neoformans Cdk-related kinase 1 (CRK1) is a negative regulator of bisexual mating. In this study, we characterized the morphological features of mating structures in the crk1 mutant and determined the genetic interaction of CRK1 in the regulatory networks of sexual differentiation. In the bilateral crk1 mutant cross, despite shorter length of filaments than in the wild-type cross, dikaryotic filaments and other structures still remained intact during bisexual mating, but the timing of basidium formation was approximately 18 h earlier than in the cross between wild type strains. Furthermore, gene expression analyses revealed that CRK1 modulated the expression of genes involved in the progression of hyphal elongation, basidium formation, karyogamy and meiosis. Phenotypic results showed that, although deletion of C. neoformans CRK1 gene increased the efficiency of bisexual mating, filamentation in the crk1 mutant was blocked by MAT2 or ZNF2 mutation. A bioinformatics survey predicted the C. neoformans GATA transcriptional factor Gat1 as a potential substrate of Crk1 kinase. Our genetic and phenotypic findings revealed that C. neoformansGAT1 and CRK1 formed a regulatory circuit to negatively regulate MAT2 to control filamentation progression and transition during bisexual mating.

Nutrient sensing-the key to fungal p53-like transcription factors?

The mammalian tumour suppressor protein, p53, plays an important role in cell cycle control, DNA repair and apoptotic cell death. Transcription factors belonging to the "p53-like" superfamily are found exclusively in the Amorphea branch of eukaryotes, which includes animals, fungi and slime molds. Many members of the p53-like superfamily (proteins containing p53, Rel/Dorsal, T-box, STAT, Runt, Ndt80, and the CSL DNA-binding domains) are involved in development. Two families of p53-like proteins (Ndt80 and CSL) are widespread in fungi as well as animals. The Basidiomycetes and the Ascomycetes have undergone reciprocal loss of the Ndt80 and CSL classes of transcription factors, with the CSL class preserved in only one branch of Ascomycetes and the Ndt80 class found in only one branch of Basidiomycetes. Recent studies have greatly expanded the known functions of fungal Ndt80-like proteins and shown that they play important roles in sexual reproduction, cell death, N-acetylglucosamine sensing and catabolism, secondary metabolism, and production of extracellular hydrolases such as proteases, chitinases and cellulases. In the opportunistic pathogen, Candida albicans, Ndt80-like proteins are essential for hyphal growth and virulence and also play a role in antifungal resistance. These recent studies have confirmed that nutrient sensing is a common feature of fungal Ndt80-like proteins and is also found in fungal CSL-like transcription factors, which in animals is the mediator of Notch signalling. Thus, nutrient sensing may represent the ancestral role of the p53-like superfamily.

Regulated Forms of Cell Death in Fungi

Cell death occurs in all domains of life. While some cells die in an uncontrolled way due to exposure to external cues, other cells die in a regulated manner as part of a genetically encoded developmental program. Like other eukaryotic species, fungi undergo programmed cell death (PCD) in response to various triggers. For example, exposure to external stress conditions can activate PCD pathways in fungi. Calcium redistribution between the extracellular space, the cytoplasm and intracellular storage organelles appears to be pivotal for this kind of cell death. PCD is also part of the fungal life cycle, in which it occurs during sexual and asexual reproduction, aging, and as part of development associated with infection in phytopathogenic fungi. Additionally, a fungal non-self-recognition mechanism termed heterokaryon incompatibility (HI) also involves PCD. Some of the molecular players mediating PCD during HI show remarkable similarities to major constituents involved in innate immunity in metazoans and plants. In this review we discuss recent research on fungal PCD mechanisms in comparison to more characterized mechanisms in metazoans. We highlight the role of PCD in fungi in response to exogenic compounds, fungal development and non-self-recognition processes and discuss identified intracellular signaling pathways and molecules that regulate fungal PCD.

An Evolutionary Perspective on Yeast Mating-Type Switching

Cell differentiation in yeast species is controlled by a reversible, programmed DNA-rearrangement process called mating-type switching. Switching is achieved by two functionally similar but structurally distinct processes in the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe. In both species, haploid cells possess one active and two silent copies of the mating-type locus (a three-cassette structure), the active locus is cleaved, and synthesis-dependent strand annealing is used to replace it with a copy of a silent locus encoding the opposite mating-type information. Each species has its own set of components responsible for regulating these processes. In this review, we summarize knowledge about the function and evolution of mating-type switching components in these species, including mechanisms of heterochromatin formation, MAT locus cleavage, donor bias, lineage tracking, and environmental regulation of switching. We compare switching in these well-studied species to others such as Kluyveromyces lactis and the methylotrophic yeasts Ogataea polymorpha and Komagataella phaffii. We focus on some key questions: Which cells switch mating type? What molecular apparatus is required for switching? Where did it come from? And what is the evolutionary purpose of switching?

Suppressor of fusion, a Fusarium oxysporum homolog of Ndt80, is required for nutrient-dependent regulation of anastomosis

Heterokaryon formation is an essential step in asexual recombination in Fusarium oxysporum. Filamentous fungi have an elaborate nonself recognition machinery to prevent formation and proliferation of heterokaryotic cells, called heterokaryon incompatibility (HI). In F. oxysporum the regulation of this machinery is not well understood. In Neurospora crassa, Vib-1, a putative transcription factor of the p53-like Ndt80 family of transcription factors, has been identified as global regulator of HI. In this study we investigated the role of the F. oxysporum homolog of Vib-1, called Suf, in vegetative hyphal and conidial anastomosis tube (CAT) fusion and HI. We identified a novel function for an Ndt80 homolog as a nutrient-dependent regulator of anastomosis. Strains carrying the SUF deletion mutation display a hyper-fusion phenotype during vegetative growth as well as germling development. In addition, conidial paring of incompatible SUF deletion strains led to more heterokaryon formation, which is independent of suppression of HI. Our data provides further proof for the divergence in the functions of different members Ndt80 family. We propose that Ndt80 homologs mediate responses to nutrient quality and quantity, with specific responses varying between species.

Extreme Diversity in the Regulation of Ndt80-Like Transcription Factors in Fungi

The Saccharomyces cerevisiaeNdt80 protein is the founding member of a class of p53-like transcription factors that is known as the NDT80/PhoG-like DNA-binding family. The number of NDT80-like genes in different fungi is highly variable and their roles, which have been examined in only a few species, include regulation of meiosis, sexual development, biofilm formation, drug resistance, virulence, the response to nutrient stress and programmed cell death. The protein kinase Ime2 regulates the single NDT80 gene present in S. cerevisiae. In this study we used a genetic approach to investigate whether the Aspergillus nidulansIme2 homolog, ImeB, and/or protein kinases MpkC, PhoA and PhoB regulate the two NDT80-like genes (xprG and ndtA) in A. nidulans. Disruption of imeB, but not mpkC, phoA or phoB, led to increased extracellular protease activity and a defect in mycotoxin production similar to the xprG1 gain-of-function mutation. Quantitative RT-PCR showed that ImeB is a negative regulator of xprG expression and XprG is a negative regulator of xprG and ndtA expression. Thus, in contrast to Ime2, which is a positive regulator of NDT80 in S. cerevisiae, ImeB is a negative regulator as in Neurospora crassa. However, the ability of Ndt80 to autoregulate NDT80 is conserved in A. nidulans though the autoregulatory effect is negative rather than positive. Unlike N. crassa, a null mutation in imeB does not circumvent the requirement for XprG or NdtA. These results show that the regulatory activities of Ime2 and Ndt80-like proteins display an extraordinarily level of evolutionary flexibility.

Identification of Allorecognition Loci in Neurospora crassa by Genomics and Evolutionary Approaches

Understanding the genetic and molecular bases of the ability to distinguish self from nonself (allorecognition) and mechanisms underlying evolution of allorecognition systems is an important endeavor for understanding cases where it becomes dysfunctional, such as in autoimmune disorders. In filamentous fungi, allorecognition can result in vegetative or heterokaryon incompatibility, which is a type of programmed cell death that occurs following fusion of genetically different cells. Allorecognition is genetically controlled by het loci, with coexpression of any combination of incompatible alleles triggering vegetative incompatibility. Herein, we identified, characterized, and inferred the evolutionary history of candidate het loci in the filamentous fungus Neurospora crassa. As characterized het loci encode proteins carrying an HET domain, we annotated HET domain genes in 25 isolates from a natural population along with the N. crassa reference genome using resequencing data. Because allorecognition systems can be affected by frequency-dependent selection favoring rare alleles (i.e., balancing selection), we mined resequencing data for HET domain loci whose alleles displayed elevated levels of variability, excess of intermediate frequency alleles, and deep gene genealogies. From these analyses, 34 HET domain loci were identified as likely to be under balancing selection. Using transformation, incompatibility assays and genetic analyses, we determined that one of these candidates functioned as a het locus (het-e). The het-e locus has three divergent allelic groups that showed signatures of positive selection, intra- and intergroup recombination, and trans-species polymorphism. Our findings represent a compelling case of balancing selection functioning on multiple alleles across multiple loci potentially involved in allorecognition.

A protein kinase screen of Neurospora crassa mutant strains reveals that the SNF1 protein kinase promotes glycogen synthase phosphorylation

Glycogen functions as a carbohydrate reserve in a variety of organisms and its metabolism is highly regulated. The activities of glycogen synthase and glycogen phosphorylase, the rate-limiting enzymes of the synthesis and degradation processes, respectively, are regulated by allosteric modulation and reversible phosphorylation. To identify the protein kinases affecting glycogen metabolism in Neurospora crassa, we performed a screen of 84 serine/threonine kinase knockout strains. We identified multiple kinases that have already been described as controlling glycogen metabolism in different organisms, such as NcSNF1, NcPHO85, NcGSK3, NcPKA, PSK2 homologue and NcATG1. In addition, many hypothetical kinases have been implicated in the control of glycogen metabolism. Two kinases, NcIME-2 and NcNIMA, already functionally characterized but with no functions related to glycogen metabolism regulation, were also identified. Among the kinases identified, it is important to mention the role of NcSNF1. We showed in the present study that this kinase was implicated in glycogen synthase phosphorylation, as demonstrated by the higher levels of glycogen accumulated during growth, along with a higher glycogen synthase (GSN) ±glucose 6-phosphate activity ratio and a lesser set of phosphorylated GSN isoforms in strain Ncsnf1KO, when compared with the wild-type strain. The results led us to conclude that, in N. crassa, this kinase promotes phosphorylation of glycogen synthase either directly or indirectly, which is the opposite of what is described for Saccharomyces cerevisiae. The kinases also play a role in gene expression regulation, in that gdn, the gene encoding the debranching enzyme, was down-regulated by the proteins identified in the screen. Some kinases affected growth and development, suggesting a connection linking glycogen metabolism with cell growth and development.

VIB1, a link between glucose signaling and carbon catabolite repression, is essential for plant cell wall degradation by Neurospora crassa

Filamentous fungi that thrive on plant biomass are the major producers of hydrolytic enzymes used to decompose lignocellulose for biofuel production. Although induction of cellulases is regulated at the transcriptional level, how filamentous fungi sense and signal carbon-limited conditions to coordinate cell metabolism and regulate cellulolytic enzyme production is not well characterized. By screening a transcription factor deletion set in the filamentous fungus Neurospora crassa for mutants unable to grow on cellulosic materials, we identified a role for the transcription factor, VIB1, as essential for cellulose utilization. VIB1 does not directly regulate hydrolytic enzyme gene expression or function in cellulosic inducer signaling/processing, but affects the expression level of an essential regulator of hydrolytic enzyme genes, CLR2. Transcriptional profiling of a Δvib-1 mutant suggests that it has an improper expression of genes functioning in metabolism and energy and a deregulation of carbon catabolite repression (CCR). By characterizing new genes, we demonstrate that the transcription factor, COL26, is critical for intracellular glucose sensing/metabolism and plays a role in CCR by negatively regulating cre-1 expression. Deletion of the major player in CCR, cre-1, or a deletion of col-26, did not rescue the growth of Δvib-1 on cellulose. However, the synergistic effect of the Δcre-1; Δcol-26 mutations circumvented the requirement of VIB1 for cellulase gene expression, enzyme secretion and cellulose deconstruction. Our findings support a function of VIB1 in repressing both glucose signaling and CCR under carbon-limited conditions, thus enabling a proper cellular response for plant biomass deconstruction and utilization.

CZT-1 is a novel transcription factor controlling cell death and natural drug resistance in Neurospora crassa

We pinpoint CZT-1 (cell death–activated zinc cluster transcription factor) as a novel transcription factor involved in tolerance to cell death induced by the protein kinase inhibitor staurosporine in Neurospora crassa. Transcriptional profiling of staurosporine-treated wild-type cells by RNA-sequencing showed that genes encoding the machinery for protein synthesis are enriched among the genes repressed by the drug. Functional category enrichment analyses also show that genes encoding components of the mitochondrial respiratory chain are downregulated by staurosporine, whereas genes involved in endoplasmic reticulum activities are upregulated. In contrast, a staurosporine-treated Δczt-1 deletion strain is unable to repress the genes for the respiratory chain and to induce the genes related to the endoplasmic reticulum, indicating a role for CZT-1 in the regulation of activity of these organelles. The Δczt-1 mutant strain displays increased reactive oxygen species accumulation on insult with staurosporine. A genome-wide association study of a wild population of N. crassa isolates pointed out genes associated with a cell death role of CZT-1, including catalase-1 (cat-1) and apoptosis-inducing factor–homologous mitochondrion-associated inducer of death 2 (amid-2). Importantly, differences in the expression of czt-1 correlates with resistance to staurosporine among wild isolate strains. Our results reveal a novel transcription factor that regulates drug resistance and cell death in response to staurosporine in laboratory strains as well as in wild isolates of N. crassa.

Global analysis of serine/threonine and tyrosine protein phosphatase catalytic subunit genes in Neurospora crassa reveals interplay between phosphatases and the p38 mitogen-activated protein kinase

Protein phosphatases are integral components of the cellular signaling machinery in eukaryotes, regulating diverse aspects of growth and development. The genome of the filamentous fungus and model organism Neurospora crassa encodes catalytic subunits for 30 protein phosphatase genes. In this study, we have characterized 24 viable N. crassa phosphatase catalytic subunit knockout mutants for phenotypes during growth, asexual development, and sexual development. We found that 91% of the mutants had defects in at least one of these traits, whereas 29% possessed phenotypes in all three. Chemical sensitivity screens were conducted to reveal additional phenotypes for the mutants. This resulted in the identification of at least one chemical sensitivity phenotype for 17 phosphatase knockout mutants, including novel chemical sensitivities for two phosphatase mutants lacking a growth or developmental phenotype. Hence, chemical sensitivity or growth/developmental phenotype was observed for all 24 viable mutants. We investigated p38 mitogen-activated protein kinase (MAPK) phosphorylation profiles in the phosphatase mutants and identified nine potential candidates for regulators of the p38 MAPK. We demonstrated that the PP2C class phosphatase pph-8 (NCU04600) is an important regulator of female sexual development in N. crassa. In addition, we showed that the Δcsp-6 (ΔNCU08380) mutant exhibits a phenotype similar to the previously identified conidial separation mutants, Δcsp-1 and Δcsp-2, that lack transcription factors important for regulation of conidiation and the circadian clock.