Aspergillus fumigatus rasA and rasB regulate the timing and morphology of asexual development

Old and new strategies in therapy and diagnosis against fungal infections

Fungal infections represent a serious global health threat. The new emerging pathogens and the spread of different forms of resistance are now hardly challenging the tools available in therapy and diagnostics. With the commonly used diagnoses, fungal identification is often slow and inaccurate, and, on the other hand, some drugs currently used as treatments are significantly affected by the decrease in susceptibility. Herein, the antifungal arsenal is critically summarized. Besides describing the old approaches and their mechanisms, advantages, and limitations, the focus is dedicated to innovative strategies which are designed, identified, and developed to take advantage of the discrepancies between fungal and host cells. Relevant pathways and their role in survival and virulence are discussed as their suitability as sources of antifungal targets. In a similar way, molecules with antifungal activity are reported as potential agents/precursors of the next generation of antimycotics. Particular attention was devoted to biotechnological entities, to their novelty and reliability, to drug repurposing and restoration, and to combinatorial applications yielding significant improvements in efficacy. KEY POINTS: • New antifungal agents and targets are needed to limit fungal morbidity and mortality. • Therapeutics and diagnostics suffer of delays in innovation and lack of targets. • Biologics, drug repurposing and combinations are the future of antifungal treatments.

LOX1- and PLP1-dependent transcriptional reprogramming is essential for injury-induced conidiophore development in a filamentous fungus

IMPORTANCE

In addition to being considered a biocontrol agent, the fungus Trichoderma atroviride is a relevant model for studying mechanisms of response to injury conserved in plants and animals that opens a new landscape in relation to regeneration and cell differentiation mechanisms. Here, we reveal the co-functionality of a lipoxygenase and a patatin-like phospholipase co-expressed in response to wounding in fungi. This pair of enzymes produces oxidized lipids that can function as signaling molecules or oxidative stress signals that, in ascomycetes, induce asexual development. Furthermore, we determined that both genes participate in the regulation of the synthesis of 13-HODE and the establishment of the physiological responses necessary for the formation of reproductive aerial mycelium ultimately leading to asexual development. Our results suggest an injury-induced pathway to produce oxylipins and uncovered physiological mechanisms regulated by LOX1 and PLP1 to induce conidiation, opening new hypotheses for the novo regeneration mechanisms of filamentous fungi.

Fungal spore swelling and germination are restricted by the macrophage phagolysosome

Many species of medically important fungi are prolific in the formation of asexual spores. Spores undergo a process of active swelling and cell wall remodelling before a germ tube is formed and filamentous growth ensues. Highly elongated germ tubes are known to be difficult to phagocytose and pose particular challenges for immune phagocytes. However, the significance of the earliest stages of spore germination during immune cell interactions has not been investigated and yet this is likely to be important for defence against sporogenous fungal pathogens. We show here that macrophages restrict the early phases of the spore germination process of Aspergillus fumigatus and Mucor circinelloides including the initial phase of spore swelling, spore germination and early polarised growth. Macrophages are therefore adept at retarding germination as well as subsequent vegetative growth which is likely to be critical for immune surveillance and protection against sporulating fungi.

Dysfunction of Ras-GAP protein AfgapA contributes to hypoxia fitness in Aspergillus fumigatus

The filamentous fungus Aspergillus fumigatus is the most important pathogenic fungus among Aspergillus species associated with aspergillosis. A. fumigatus must adapt to hypoxic microenvironments to survive and thrive in human lungs. To gain further insights into hypoxic adaptation, we generated a laboratory-evolved strain (Afs35-G20) harboring hypoxia fitness, and identified a nonsense mutation in AfgapA encoding a Ras-GAP protein, which could result in the deletion of 22 amino acids at the C-terminus. We investigated the role of AfgapA in hypoxia fitness by constructing Afs35-G20-AfgapA^WT, and ∆AfgapA. Indeed, the hypoxia fitness of Afs35-G20 was reversed by introducing AfgapA^WT. ∆AfgapA exhibited greater hypoxia fitness and hypervirulence in the silkworm infection model, indicating that AfgapA is responsible for hypoxia fitness, particularly in liquid cultures. Taken together, the AfgapA dysfunction may lead to the downregulation of its Ras substrate(s), reflecting several phenotypes such as increased hypoxia fitness, hypervirulence, poor conidiation, and conidial pigmentation. Here, we report the function of a Ras-GAP protein AfgapA in A. fumigatus for the first time.

Inter- and intra-species heterogeneity in germination of Aspergillus conidia

Aspergilli are among the most abundant fungi worldwide. They degrade organic material and can be pathogens of plants and animals. Aspergilli spread by forming high numbers of conidia. Germination of these stress resistant asexual spores is characterized by a swelling and a germ tube stage. Here, we show that conidia of Aspergillusniger,Aspergillusoryzae,Aspergillusclavatus, Aspergillusnidulans and Aspergillusterreus show different swelling and germ tube formation dynamics in pure water or in water supplemented with (in)organic nutrients. Apart from inter-species heterogeneity, intra-species heterogeneity was observed within spore populations of the aspergilli except for A.terreus. Sub-populations of conidia differing in size and/or contrast showed different swelling and germ tube formation dynamics. Together, data imply that aspergilli differ in their competitive potential depending on the substrate. Moreover, results suggest that intra-species heterogeneity provides a bet hedging mechanism to optimize survival of aspergilli.

Deubiquitinase Ubp3 regulates ribophagy and deubiquitinates Smo1 for appressorium-mediated infection by Magnaporthe oryzae

The Ubp family of deubiquitinating enzymes has been found to play important roles in plant-pathogenic fungi, but their regulatory mechanisms are still largely unknown. In this study, we revealed the regulatory mechanism of the deubiquitinating enzyme Ubp3 during the infection process of Magnaporthe oryzae. AUBP3 deletion mutant was severely defective in appressorium turgor accumulation, leading to the impairment of appressorial penetration. During appressorium formation, the mutant was also defective in glycogen and lipid metabolism. Interestingly, we found that nitrogen starvation and rapamycin treatment induced the ribophagy process in M. oryzae, which is closely dependent on Ubp3. In the ∆ubp3 mutant, the ribosome proteins and rRNAs were not well degraded on nitrogen starvation and rapamycin treatment. We also found that Ubp3 interacted with the GTPase-activating protein Smo1 and regulated its de-ubiquitination. Ubp3-dependent de-ubiquitination of Smo1 may be required for Smo1 to coordinate Ras signalling. Taken together, our results showed at least two roles of Ubp3 in M. oryzae: it regulates the ribophagy process and it regulates de-ubiquitination of GTPase-activating protein Smo1 for appressorium-mediated infection.

Nitrogen, Iron and Zinc Acquisition: Key Nutrients to Aspergillus fumigatus Virulence

Aspergillus fumigatus is a ubiquitous soil decomposer and an opportunistic pathogen that is characterized by its large metabolic machinery for acquiring nutrients from media. Lately, an ever-increasing number of genes involved in fungal nutrition has been associated with its virulence. Of these, nitrogen, iron, and zinc metabolism-related genes are particularly noteworthy, since 78% of them have a direct implication in virulence. In this review, we describe the sensing, uptake and regulation process of the acquisition of these nutrients, the connections between pathways and the virulence-implicated genes. Nevertheless, only 40% of the genes mentioned in this review have been assayed for roles in virulence, leaving a wide field of knowledge that remains uncertain and might offer new therapeutic and diagnostic targets.

Ras subfamily GTPases regulate development, aflatoxin biosynthesis and pathogenicity in the fungus Aspergillus flavus

Ras subfamily proteins are molecular switches in signal transduction pathways of many eukaryotes that regulate a variety of cellular processes. Here, the Ras subfamily, encoded by six genes, was identified in Aspergillus flavus: rasA, rasB, rasC, rab-33, rheb and rsr1. The rsr1 deletion mutant (∆rsr1), rheb deletion mutant (∆rheb) and double deletion mutant (∆rheb/rsr1) displayed significantly decreased growth and sporulation. Sclerotia formation was significantly decreased for ∆rheb or ∆rheb/rsr1 but increased for ∆rsr1. Aflatoxin production was significantly increased in ∆rheb but decreased in ∆rsr1 and ∆rheb/rsr1. We found that rsr1 and rheb are crucial for the pathogenicity of A. flavus. Quantitative proteomics identified 520 differentially expressed proteins (DEPs) for the ∆rsr1 mutant and 133 DEPs for the ∆rheb mutant. These DEPs were annotated in multiple biological processes and KEGG pathways in A. flavus. Importantly, we identified the cytokinesis protein SepA in the protein-protein interaction network of rsr1, and deletion mutants showed that SepA has pleiotropic effects on growth and AF biosynthesis, which may depend on Rsr1 for regulation in A. flavus. Our results indicated that these Ras subfamily proteins exhibited functional redundancy with each other but there were also differences in A. flavus.

Minimal nutrient requirements for induction of germination of Aspergillus niger conidia

Aspergillus niger reproduces asexually by forming conidia. Here, the minimal nutrient requirements were studied that activate germination of A. niger conidia. To this end, germination was monitored in time using an oCelloScope imager. Data was used as input in an asymmetric model to describe the process of swelling and germ tube formation. The maximum number of spores (P_max) that were activated to swell and to form germ tubes was 32.54% and 20.51%, respectively, in minimal medium with 50 mM glucose. In contrast, P_max of swelling and germ tube formation was <1% in water or 50 mM glucose. Combining 50 mM glucose with either NaNO₃, KH₂PO4, or MgSO4 increased P_max of swelling and germination up to 15.25% and 5.4%, respectively, while combining glucose with two of these inorganic components further increased these P_max values up to 25.85% and 10.99%. Next, 10 mM amino acid was combined with a phosphate buffer and MgSO₄. High (e.g. proline), intermediate and low (e.g. cysteine) inducing amino acids were distinguished. Together, a combination of an inducing carbon source with either inorganic phosphate, inorganic nitrogen or magnesium sulphate is the minimum requirement for A. niger conidia to germinate.

Overexpression of the Aspergillus fumigatus Small GTPase, RsrA, Promotes Polarity Establishment during Germination

Cell polarization comprises highly controlled processes and occurs in most eukaryotic organisms. In yeast, the processes of budding, mating and filamentation require coordinated mechanisms leading to polarized growth. Filamentous fungi, such as Aspergillus fumigatus, are an extreme example of cell polarization, essential for both vegetative and pathogenic growth. A major regulator of polarized growth in yeast is the small GTPase Rsr1, which is essential for bud-site selection. Here, we show that deletion of the putative A. fumigatus ortholog, rsrA, causes only a modest reduction of growth rate and delay in germ tube emergence. In contrast, overexpression of rsrA results in a morphogenesis defect, characterized by a significant delay in polarity establishment followed by the establishment of multiple growth axes. This aberrant phenotype is reversed when rsrA expression levels are decreased, suggesting that correct regulation of RsrA activity is crucial for accurate patterning of polarity establishment. Despite this finding, deletion or overexpression of rsrA resulted in no changes of A. fumigatus virulence attributes in a mouse model of invasive aspergillosis. Additional mutational analyses revealed that RsrA cooperates genetically with the small GTPase, RasA, to support A. fumigatus viability.

Novel Antifungal Agents and Their Activity against Aspergillus Species

There is a need for new antifungal agents, mainly due to increased incidence of invasive fungal infections (IFI), high frequency of associated morbidity and mortality and limitations of the current antifungal agents (e.g., toxicity, drug-drug interactions, and resistance). The clinically available antifungals for IFI are restricted to four main classes: polyenes, flucytosine, triazoles, and echinocandins. Several antifungals are hampered by multiple resistance mechanisms being present in fungi. Consequently, novel antifungal agents with new targets and modified chemical structures are required to combat fungal infections. This review will describe novel antifungals, with a focus on the Aspergillus species.

The Cell Wall Integrity Pathway Contributes to the Early Stages of Aspergillus fumigatus Asexual Development

Aspergillus fumigatus is a major cause of human disease. The survival of this fungus is dependent on the cell wall organization and function of its components. The cell wall integrity pathway (CWIP) is the primary signaling cascade that controls de novo synthesis of the cell wall in fungi. Abundant conidiation is a hallmark in A. fumigatus, and uptake of conidia by a susceptible host is usually the initial event in infection. The formation of conidia is mediated by the development of fungus-specific specialized structures, conidiophores, which are accompanied by cell wall remodeling. The molecular regulation of these changes in cell wall composition required for the rise of conidiophore from the solid surface and to disperse the conidia into the air is currently unknown. Here, we investigated the role of CWIP in conidiation. We show that CWIP pkcA ^G579R, ΔmpkA, and ΔrlmA mutants displayed reduced conidiation during synchronized asexual differentiation. The transcription factor RlmA directly regulated the expression of regulators of conidiation, including flbB, flbC, brlA, abaA, and rasB, as well as genes involved in cell wall synthesis and remodeling, and this affected the chitin content in aerial hyphae. Phosphorylation of RlmA and MpkA was increased during asexual differentiation. We also observed that MpkA physically associated with the proteins FlbB, FlbC, BrlA, and RasB during this process, suggesting another level of cross talk between the CWIP and asexual development pathways. In summary, our results support the conclusion that one function of the CWIP is the regulation of asexual development in filamentous fungi.IMPORTANCE A remarkable feature of the human pathogen Aspergillus fumigatus is its ability to produce impressive amounts of infectious propagules known as conidia. These particles reach immunocompromised patients and may initiate a life-threatening mycosis. The conidiation process in Aspergillus is governed by a sequence of proteins that coordinate the development of conidiophores. This process requires the remodeling of the cell wall so that the conidiophores can rise and withstand the chains of conidia. The events regulating cell wall remodeling during conidiation are currently unknown. Here, we show that the cell wall integrity pathway (CWIP) components RlmA and MpkA directly contribute to the activation of the conidiation cascade by enabling transcription or phosphorylation of critical proteins involved in asexual development. This study points to an essential role for the CWIP during conidiation and provides further insights into the complex regulation of asexual development in filamentous fungi.

Molecular Mechanisms of Conidial Germination in Aspergillus spp

Aspergilli produce conidia for reproduction or to survive hostile conditions, and they are highly effective in the distribution of conidia through the environment. In immunocompromised individuals, inhaled conidia can germinate inside the respiratory tract, which may result in invasive pulmonary aspergillosis. The management of invasive aspergillosis has become more complex, with new risk groups being identified and the emergence of antifungal resistance. Patient survival is threatened by these developments, stressing the need for alternative therapeutic strategies. As germination is crucial for infection, prevention of this process might be a feasible approach. A broader understanding of conidial germination is important to identify novel antigermination targets. In this review, we describe conidial resistance against various stresses, transition from dormant conidia to hyphal growth, the underlying molecular mechanisms involved in germination of the most common Aspergillus species, and promising antigermination targets. Germination of Aspergillus is characterized by three morphotypes: dormancy, isotropic growth, and polarized growth. Intra- and extracellular proteins play an important role in the protection against unfavorable environmental conditions. Isotropically expanding conidia remodel the cell wall, and biosynthetic machineries are needed for cellular growth. These biosynthetic machineries are also important during polarized growth, together with tip formation and the cell cycle machinery. Genes involved in isotropic and polarized growth could be effective antigermination targets. Transcriptomic and proteomic studies on specific Aspergillus morphotypes will improve our understanding of the germination process and allow discovery of novel antigermination targets and biomarkers for early diagnosis and therapy.

The farnesyltransferase β-subunit RAM1 regulates localization of RAS proteins and appressorium-mediated infection in Magnaporthe oryzae

Post-translational farnesylation can regulate subcellular localization and protein-protein interaction in eukaryotes. The function of farnesylation is not well identified in plant pathogenic fungi, particularly during the process of fungal infection. Here, through functional analyses of the farnesyltransferase β-subunit gene, RAM1, we examine the importance of protein farnesylation in the rice blast fungus Magnaporthe oryzae. Targeted disruption of RAM1 resulted in the reduction of hyphal growth and sporulation, and an increase in the sensitivity to various stresses. Importantly, loss of RAM1 also led to the attenuation of virulence on the plant host, characterized by decreased appressorium formation and invasive growth. Interestingly, the defect in appressoria formation of the Δram1 mutant can be recovered by adding exogenous cAMP and IBMX, suggesting that RAM1 functions upstream of the cAMP signalling pathway. We found that two Ras GTPases, RAS1 and RAS2, can interact with Ram1, and their plasma membrane localization was regulated by Ram1 through their C-terminal farnesylation sites. Adding a farnesyltransferase inhibitor Tipifarnib can result in similar defects as in Δram1 mutant, including decreased appressorium formation and invasive growth, as well as mislocalized RAS proteins. Our findings indicate that protein farnesylation regulates the RAS protein-mediated signaling pathways required for appressorium formation and host infection, and suggest that abolishing farnesyltransferase could be an effective strategy for disease control.

The Farnesyltransferase β-Subunit Ram1 Regulates Sporisorium scitamineum Mating, Pathogenicity and Cell Wall Integrity

The basidiomycetous fungus Sporisorium scitamineum causes a serious sugarcane smut disease in major sugarcane growing areas. Sexual mating is essential for infection to the host; however, its underlying molecular mechanism has not been fully studied. In this study, we identified a conserved farnesyltransferase (FTase) β subunit Ram1 in S. scitamineum. The ram1Δ mutant displayed significantly reduced mating/filamentation, thus of weak pathogenicity to the host cane. The ram1Δ mutant sporidia showed more tolerant toward cell wall stressor Congo red compared to that of the wild-type. Transcriptional profiling showed that Congo red treatment resulted in notable up-regulation of the core genes involving in cell wall integrity pathway in ram1Δ sporidia compared with that of WT, indicating that Ram1 may be involved in cell wall integrity regulation. In yeast the heterodimeric FTase is responsible for post-translational modification of Ras (small G protein) and a-factor (pheromone). We also identified and characterized two conserved Ras proteins, Ras1 and Ras2, respectively, and a MAT-1 pheromone precursor Mfa1. The ras1Δ, ras2Δ and mfa1Δ mutants all displayed reduced mating/filamentation similar as the ram1Δ mutant. However, both ras1Δ and ras2Δ mutants were hypersensitive to Congo red while the mfa1Δ mutant was the same as wild-type. Overall our study displayed that RAM1 plays an essential role in S. scitamineum mating/filamentation, pathogenicity, and cell wall stability.

SH3-class Ras guanine nucleotide exchange factors are essential for Aspergillus fumigatus invasive growth

Proper hyphal morphogenesis is essential for the establishment and progression of invasive disease caused by filamentous fungi. In the human pathogen Aspergillus fumigatus, signalling cascades driven by Ras and Ras‐like proteins orchestrate a wide variety of cellular processes required for hyphal growth. For activation, these proteins require interactions with Ras‐subfamily‐specific guanine nucleotide exchange factors (RasGEFs). Although Ras‐protein networks are essential for virulence in all pathogenic fungi, the importance of RasGEF proteins is largely unexplored. A. fumigatus encodes four putative RasGEFs that represent three separate classes of RasGEF proteins (SH3‐, Ras guanyl nucleotide‐releasing protein [RasGRP]–, and LTE‐class), each with fungus‐specific attributes. Here, we show that the SH3‐class and RasGRP‐class RasGEFs are required for properly timed polarity establishment during early growth and branch emergence as well as for cell wall stability. Further, we show that SH3‐class RasGEF activity is essential for polarity establishment and maintenance, a phenotype that is, at least, partially independent of the major A. fumigatus Ras proteins, RasA and RasB. Finally, loss of both SH3‐class RasGEFs resulted in avirulence in multiple models of invasive aspergillosis. Together, our findings suggest that RasGEF activity is essential for the integration of multiple signalling networks to drive invasive growth in A. fumigatus.

FIBCD1 Binds Aspergillus fumigatus and Regulates Lung Epithelial Response to Cell Wall Components

Aspergillus fumigatus (A. fumigatus) is a ubiquitous fungus of clinical importance associated with development of various pulmonary diseases and allergic hypersensitivity reactions. It is protected against environmental stress by a cell wall that contains polysaccharides such as chitin. We previously demonstrated that fibrinogen C domain-containing protein 1 (FIBCD1) is a membrane-bound protein that binds chitin through a conserved S1 binding site and is expressed in intestinal epithelium and salivary glands. Here, we further localized FIBCD1 protein expression at the surface of bronchial and alveolar human lung epithelium, observed recognition of A. fumigatus cell wall with S1 site-independent recognition. We observed FIBCD1-mediated suppression of IL-8 secretion, mucin production, and transcription of genes associated with airway inflammation and homeostasis in FIBCD1-transfected lung epithelial cells. These modulations were generally enforced by stimulation with A. fumigatus cell wall polysaccharides. In parallel, we demonstrated a FIBCD1-mediated modulation of IL-8 secretion induced by TLR2,−4, and −5. Collectively, our findings support FIBCD1 as a human lung epithelial pattern recognition receptor that recognizes the complex A. fumigatus cell wall polysaccharides and modulates the lung epithelial inflammatory response by suppressing inflammatory mediators and mucins.

ram1 gene, encoding a subunit of farnesyltransferase, contributes to growth, antifungal susceptibility to amphotericin B of Aspergillus fumigatus

Farnesylation, which is catalyzed by farnesyltransferase, is an important posttranslational process. The function of farnesyltransferase has been previously explored in Cryptococcus neoformans and Candida albicans. Aspergillus fumigatus is an important human opportunistic fungal pathogen in immunocompromised patients. Here we discover the role of the ram1 gene, encoding the β-subunit of farnesyltransferase in A. fumigatus, in the fungal growth and antifungal susceptibility. In this study the ram1 gene was disrupted using A. tumefaciens-mediated transformation. The morphology and radial growth of Δram1 were observed. Assays of disk diffusion and broth microdilution were used to determine the susceptibility of Δram1 mutant to commonly clinical used antifungals and the farnesyltransferase inhibitor manumycin A. Deletion of ram1 resulted in a reduced radial growth of A. fumigatus but did not affect the microscopic morphology. Δram1 showed increased susceptibility to the antifungal amphotericin B; however, its susceptibility to azoles and caspofungin was the same to that to the parental strain. Our data indicate that farnesyltransferase is a potential target for design new antifungal agents.

The pathogenicity ofAspergillus fumigatus, drug resistance, and nanoparticle delivery

The genus Aspergillus includes fungal species that cause major health issues of significant economic importance. These microorganisms are also the culprit for production of carcinogenic aflatoxins in grain storages, contaminating crops, and economically straining the production process. Aspergillus fumigatus is a very important pathogenic species, being responsible for high human morbidity and mortality on a global basis. The prevalence of these infections in immunosuppressed individuals is on the rise, and physicians struggle with the diagnosis of these deadly pathogens. Several virulence determinants facilitate fungal invasion and evasion of the host immune response. Metabolic functions are also important for virulence and drug resistance, since they allow fungi to obtain nutrients for their own survival and growth. Following a positive diagnostic identification, mortality rates remain high due, in part, to emerging resistance to frequently used antifungal drugs. In this review, we discuss the role of the main virulence, drug target, and drug resistance determinants. We conclude with the review of new technologies being developed to treat aspergillosis. In particular, microsphere and nanoparticle delivery systems are discussed in the context of improving drug bioavailability. Aspergillus will likely continue to cause problematic infections in immunocompromised patients, so it is imperative to improve treatment options.

C-terminus Proteolysis and Palmitoylation Cooperate for Optimal Plasma Membrane Localization of RasA in Aspergillus fumigatus

RasA is a major regulator of fungal morphogenesis and virulence in Aspergillus fumigatus. The proper localization of RasA to the plasma membrane is essential for the formation of invasive hyphae during infection. In yeast, the localization of Ras2p to the plasma membrane is orchestrated by several post-translational modifications (PTM) at the C-terminal CAAX box that are thought to occur in sequential order. These PTMs include: (1) CAAX motif farnesylation by the farnesyltransferase complex composed of Ram1p and Ram2p; (2) proteolysis of the -AAX residues by Rce1p or Ste24p; (3) methylation of the remaining prenylated cysteine residue by Ste14p, and; (4) palmitoylation at a single conserved cysteine residue mediated by the Erf2p/Erf4p palmitoyltransferase. We previously reported that homologs of each RasA PTM enzyme are conserved in A. fumigatus. Additionally, we delineated a major role for protein farnesylation in A. fumigatus growth and virulence. In this work, we characterize the post-prenylation processing enzymes of RasA in A. fumigatus. The genes encoding the RasA post-prenylation enzymes were first deleted and examined for their roles in growth and regulation of RasA. Only when strains lacked cppB, the A. fumigatus homologue of yeast RCE1, there was a significant reduction in fungal growth and conidial germination. In addition, cppB-deletion mutants displayed hypersensitivity to the cell wall-perturbing agents Calcofluor White and Congo Red and the cell wall biosynthesis inhibitor Caspofungin. In contrast to the previously published data in yeast, the deletion of post-prenylation modifying enzymes did not alter the plasma membrane localization or activation of RasA. To delineate the molecular mechanisms underlying these differences, we investigated the interplay between dual-palmitoylation of the RasA hypervariable region and CAAX proteolysis for stabilization of RasA at the plasma membrane. Our data indicate that, in the absence of proper CAAX proteolysis, RasA accumulation at the plasma membrane is stabilized by dual palmitoyl groups on the dual cysteine residues. Therefore, we conclude CAAX proteolysis and dual-palmitoylation of the hypervariable region is important for maintaining a stable attachment association of RasA with the plasma membrane to support optimal fungal growth and development.

The Aspergillus fumigatus farnesyltransferase β-subunit, RamA, mediates growth, virulence, and antifungal susceptibility

Post-translational prenylation mechanisms, including farnesylation and geranylgeranylation, mediate both subcellular localization and protein-protein interaction in eukaryotes. The prenyltransferase complex is an αβ heterodimer in which the essential α-subunit is common to both the farnesyltransferase and the geranylgeranyltransferase type-I enzymes. The β-subunit is unique to each enzyme. Farnesyltransferase activity is an important mediator of protein localization and subsequent signaling for multiple proteins, including Ras GTPases. Here, we examined the importance of protein farnesylation in the opportunistic fungal pathogen Aspergillus fumigatus through generation of a mutant lacking the farnesyltransferase β-subunit, ramA. Although farnesyltransferase activity was found to be non-essential in A. fumigatus, diminished hyphal outgrowth, delayed polarization kinetics, decreased conidial viability, and irregular distribution of nuclei during polarized growth were noted upon ramA deletion (ΔramA). Although predicted to be a target of the farnesyltransferase enzyme complex, we found that localization of the major A. fumigatus Ras GTPase protein, RasA, was only partially regulated by farnesyltransferase activity. Furthermore, the farnesyltransferase-deficient mutant exhibited attenuated virulence in a murine model of invasive aspergillosis, characterized by decreased tissue invasion and development of large, swollen hyphae in vivo. However, loss of ramA also led to a Cyp51A/B-independent increase in resistance to triazole antifungal drugs. Our findings indicate that protein farnesylation underpins multiple cellular processes in A. fumigatus, likely due to the large body of proteins affected by ramA deletion.

A Fungus-Specific Protein Domain Is Essential for RasA-Mediated Morphogenetic Signaling in Aspergillus fumigatus

Ras proteins function as conserved regulators of eukaryotic growth and differentiation and are essential signaling proteins orchestrating virulence in pathogenic fungi. Here, we report the identification of a novel N-terminal domain of the RasA protein in the filamentous fungus Aspergillus fumigatus. Whereas this domain is absent in Ras homologs of higher eukaryotes, the N-terminal extension is conserved among fungi and is characterized by a short string of two to eight amino acids terminating in an invariant arginine. For this reason, we have termed the RasA N-terminal domain the invariant arginine domain (IRD). Through mutational analyses, the IRD was found to be essential for polarized morphogenesis and asexual development, with the invariant arginine residue being most essential. Although IRD truncation resulted in a nonfunctional Ras phenotype, IRD mutation was not associated with mislocalization of the RasA protein or significant changes in steady-state RasA activity levels. Mutation of the RasA IRD diminished protein kinase A (PKA) activation and resulted in decreased interaction with the Rho-type GTPase, Cdc42. Taken together, our findings reveal novel, fungus-specific mechanisms for Ras protein function and signal transduction. IMPORTANCEAspergillus fumigatus is an important fungal pathogen against which limited treatments exist. During invasive disease, A. fumigatus hyphae grow in a highly polarized fashion, forming filaments that invade blood vessels and disseminate to distant sites. Once invasion and dissemination occur, mortality rates are high. We have previously shown that the Ras signaling pathway is an important regulator of the hyphal growth machinery supporting virulence in A. fumigatus. Here, we show that functional Ras signaling in A. fumigatus requires a novel, fungus-specific domain within the Ras protein. This domain is highly conserved among fungi, yet absent in higher eukaryotes, suggesting a potentially crucial difference in the regulation of Ras pathway activity between the human host and the fungal pathogen. Exploration of the mechanisms through which this domain regulates signaling could lead to novel antifungal therapies specifically targeting fungal Ras pathways.

Clg2p interacts with Clf and ClUrase to regulate appressorium formation, pathogenicity and conidial morphology in Curvularia lunata

Ras is a small GTPase that regulates numerous processes in the cellular development and morphogenesis of many organisms. In this study, we identified and functionally characterized the Clg2p gene of Curvularia lunata, which is homologous with the Ras protein. The Clg2p deletion mutant (ΔClg2p) had altered appressorium formation and conidial morphology and produced fewer, smaller lesions compared with the wild-type strain. When a dominant Clg2p allele was introduced into the mutant, all of these defective phenotypes were completely restored. To further understand the regulation of Clg2p in appressorium formation and conidial morphology, and its role in pathogenicity, seven Clg2p-interacting proteins were screened using a yeast two-hybrid assay. Two of these proteins, Clf, a homologue of Mst11, which corresponds to MAP kinase kinase kinase in Magnaporthe oryzae, and urate oxidase (designated ClUrase) were functionally characterized. Clg2p specifically interacted with Clf through its RA domain to regulate appressorium formation and pathogenicity, whereas the Clg2p-ClUrase interaction regulated conidial morphology without affecting fungal pathogenicity. This report is the first to elucidate the regulatory mechanism of the key Ras protein Clg2p in C. lunata.

A novel Ras GTPase (Ras3) regulates conidiation, multi-stress tolerance and virulence by acting upstream of Hog1 signaling pathway in Beauveria bassiana

Two Ras ATPases (Ras1 and Ras2) are well known to regulate antagonistically or cooperatively various cellular events in many fungi. Here we show the significance of a novel Ras homolog (Ras3) for Beauveria bassiana. Ras3 possesses five domains and two GTP/GDP switches typical for Ras family and was proven to localize to plasma membrane despite the position change of a membrane-targeting cysteine in C-terminal CAAX motif. Deletion of ras3 altered temporal transcription pattern of ras1 instead of ras2. Compared with wild-type, Δras3 grew significantly faster in a rich medium but slower in some minimal media, and produced far fewer conidia with impaired quality, which was evident with slower germination, attenuated virulence, reduced thermotolerance and decreased UV-B resistance. Moreover, Δras3 was much more sensitive to the oxidative stress of menadione than of H2O2 and to the stress of high osmolarity than of cell wall perturbation during growth. The high sensitivity of Δras3 to menadione was concurrent with reductions in both gene transcripts and total activity of superoxide dismutases. Intriguingly, the high osmosensitivity was concurrent with not only reduced transcripts of a critical transcription factor (Msn2) and most signaling proteins in the high-osmolarity-glycerol pathway of Δras3 but nearly undetectable phosphorylation signal of Hog1 hallmarking the pathway. All the changes were restored by ras3 complementation. Taken together, Ras3 is involved in the Hog1 pathway required for osmoregulation and hence can positively regulate conidiation, germination, multi-stress tolerance and virulence linked to the biological control potential of the filamentous insect pathogen.

Orchestration of Morphogenesis in Filamentous Fungi: Conserved Roles for Ras Signaling Networks

Filamentous fungi undergo complex developmental programs including conidial germination, polarized morphogenesis, and differentiation of sexual and asexual structures. For many fungi, the coordinated completion of development is required for pathogenicity, as specialized morphological structures must be produced by the invading fungus. Ras proteins are highly conserved GTPase signal transducers and function as major regulators of growth and development in eukaryotes. Filamentous fungi typically express two Ras homologues, comprising distinct groups of Ras1-like and Ras2-like proteins based on sequence homology. Recent evidence suggests shared roles for both Ras1 and Ras2 homologues, but also supports the existence of unique functions in the areas of stress response and virulence. This review focuses on the roles played by both Ras protein groups during growth, development, and pathogenicity of a diverse array of filamentous fungi.

Exploration of Aspergillus fumigatus Ras pathways for novel antifungal drug targets

Ras pathway signaling is a critical virulence determinant for pathogenic fungi. Localization of Ras to the plasma membrane (PM) is required for Ras network interactions supporting fungal growth and virulence. For example, loss of Aspergillus fumigatus RasA signaling at the PM via inhibition of palmitoylation leads to decreased growth, altered hyphal morphogenesis, decreased cell wall integrity and loss of virulence. In order to be properly localized and activated, Ras proteins must transit a series of post-translational modification (PTM) steps. These steps include farnesylation, proteolytic cleavage of terminal amino acids, carboxymethylation, and palmitoylation. Because Ras activation drives tumor development, Ras pathways have been extensively studied in mammalian cells as a potential target for anti-cancer therapy. Inhibitors of mammalian Ras interactions and PTM components have been, or are actively being, developed. This review will focus on the potential for building upon existing scaffolds to exploit fungal Ras proteins for therapy, synthesizing data from studies employing both mammalian and fungal systems.

Bypassing both surface attachment and surface recognition requirements for appressorium formation by overactive ras signaling in Magnaporthe oryzae

Magnaporthe oryzae forms a highly specialized infection structure called an appressorium for plant penetration. In M. oryzae and many other plant-pathogenic fungi, surface attachment and surface recognition are two essential requirements for appressorium formation. Development of appressoria in the air has not been reported. In this study, we found that expression of a dominant active MoRAS2(G18V) allele in M. oryzae resulted in the formation of morphologically abnormal appressoria on nonconducive surfaces, in liquid suspensions, and on aerial hyphae without attachment to hard surfaces. Both the Pmk1 mitogen-activated protein kinase cascade and cAMP signaling pathways that regulate surface recognition and appressorium morphogenesis in M. oryzae were overactivated in the MoRAS2(G18V) transformant. In mutants deleted of PMK1 or CPKA, expression of MoRAS2(G18V) had no significant effects on appressorium morphogenesis. Furthermore, expression of dominant MoRAS2 in Colletotrichum graminicola and C. gloeosporioides also caused the formation of appressorium-like structures in aerial hyphae. Overall, our data indicate that MoRas2 functions upstream from both the cAMP-PKA and Pmk1 pathways and overactive Ras signaling leads to improper activation of these two pathways and appressorium formation without surface attachment in appressorium-forming pathogens.

Control of Ras-mediated signaling in Aspergillus fumigatus

Pathogenic fungi employ numerous mechanisms to flourish in the stressful environment encountered within their mammalian hosts. Central to this arsenal for filamentous fungi is invasive growth within the host microenvironment, mediated by establishment and maintenance of polarized hyphal morphogenesis. In Aspergillus fumigatus, the RasA signal transduction pathway has emerged as a significant regulator of hyphal morphogenesis and virulence, among other processes. The factors contributing to the regulation of RasA itself are not as thoroughly understood, although proper temporal activation of RasA and spatial localization of RasA to the plasma membrane are known to play major roles. Interference with RasA palmitoylation or prenylation results in mislocalization of RasA and is associated with severe growth deficits. In addition, dysregulation of RasA activation results in severe morphologic aberrancies and growth deficits. This review highlights the relationship between RasA signaling, hyphal morphogenesis, and virulence in A. fumigatus and focuses on potential determinants of spatial and temporal RasA regulation.

Targeting fungal genes by diced siRNAs: a rapid tool to decipher gene function in Aspergillus nidulans

Background

Gene silencing triggered by chemically synthesized small interfering RNAs (siRNAs) has become a powerful tool for deciphering gene function in many eukaryotes. However, prediction and validation of a single siRNA duplex specific to a target gene is often ineffective. RNA interference (RNAi) with synthetic siRNA suffers from lower silencing efficacy, off-target effects and is cost-intensive, especially for functional genomic studies. With the explosion of fungal genomic information, there is an increasing need to analyze gene function in a rapid manner. Therefore, studies were performed in order to investigate the efficacy of gene silencing induced by RNase III-diced-siRNAs (d-siRNA) in model filamentous fungus, Aspergillus nidulans.

Methodology/Principal Findings

Stable expression of heterologous reporter gene in A. nidulans eases the examination of a new RNAi-induction route. Hence, we have optimized Agrobacterium tumefaciens-mediated transformation (AMT) of A. nidulans for stable expression of sGFP gene. This study demonstrates that the reporter GFP gene stably introduced into A. nidulans can be effectively silenced by treatment of GFP-d-siRNAs. We have shown the down-regulation of two endogenous genes, AnrasA and AnrasB of A. nidulans by d-siRNAs. We have also elucidated the function of an uncharacterized Ras homolog, rasB gene, which was found to be involved in hyphal growth and development. Further, silencing potency of d-siRNA was higher as compared to synthetic siRNA duplex, targeting AnrasA. Silencing was shown to be sequence-specific, since expression profiles of other closely related Ras family genes in d-siRNA treated AnrasA and AnrasB silenced lines exhibited no change in gene expression.

Conclusions/Significance

We have developed and applied a fast, specific and efficient gene silencing approach for elucidating gene function in A. nidulans using d-siRNAs. We have also optimized an efficient AMT in A. nidulans, which is useful for stable integration of transgenes.

Reproductive competence: a recurrent logic module in eukaryotic development

Developmental competence is the ability to differentiate in response to an appropriate stimulus, as first elaborated by Waddington in relation to organs and tissues. Competence thresholds operate at all levels of biological systems from the molecular (e.g. the cell cycle) to the ontological (e.g. metamorphosis and reproduction). Reproductive competence, an organismal process, is well studied in mammals (sexual maturity) and plants (vegetative phase change), though far less than later stages of terminal differentiation. The phenomenon has also been documented in multiple species of multicellular fungi, mostly in early, disparate literature, providing a clear example of physiological differentiation in the absence of morphological change. This review brings together data on reproductive competence in Ascomycete fungi, particularly the model filamentous fungus Aspergillus nidulans, contrasting mechanisms within Unikonts and plants. We posit reproductive competence is an elementary logic module necessary for coordinated development of multicellular organisms or functional units. This includes unitary multicellular life as well as colonial species both unicellular and multicellular (e.g. social insects such as ants). We discuss adaptive hypotheses for developmental and reproductive competence systems and suggest experimental work to address the evolutionary origins, generality and genetic basis of competence in the fungal kingdom.

A central role for Ras1 in morphogenesis of the basidiomycete Schizophyllum commune

Fungi have been used as model systems to define general processes in eukaryotes, for example, the one gene-one enzyme hypothesis, as well as to study polar growth or pathogenesis. Here, we show a central role for the regulator protein Ras in a mushroom-forming, filamentous basidiomycete linking growth, pheromone signaling, sexual development, and meiosis to different signal transduction pathways. ras1 and Ras-specific gap1 mutants were generated and used to modify the intracellular activation state of the Ras module. Transformants containing constitutive ras1 alleles (ras1(G12V) and ras1(Q61L)), as well as their compatible mating interactions, did show strong phenotypes for growth (associated with Cdc42 signaling) and mating (associated with mitogen-activated protein kinase signaling). Normal fruiting bodies with abnormal spores exhibiting a reduced germination rate were produced by outcrossing of these mutant strains. Homozygous Δgap1 primordia, expected to experience increased Ras signaling, showed overlapping phenotypes with a block in basidium development and meiosis. Investigation of cyclic AMP (cAMP)-dependent protein kinase A indicated that constitutively active ras1, as well as Δgap1 mutant strains, exhibit a strong increase in Tpk activity. Ras1-dependent, cAMP-mediated signal transduction is, in addition to the known signaling pathways, involved in fruiting body formation in Schizophyllum commune. To integrate these analyses of Ras signaling, microarray studies were performed. Mutant strains containing constitutively active Ras1, deletion of RasGap1, or constitutively active Cdc42 were characterized and compared. At the transcriptome level, specific regulation highlighting the phenotypic differences of the mutants is clearly visible.

Disparate proteome responses of pathogenic and nonpathogenic aspergilli to human serum measured by activity-based protein profiling (ABPP)

Aspergillus fumigatus is the primary pathogen causing the devastating pulmonary disease Invasive Aspergillosis in immunocompromised individuals. There is high genomic synteny between A. fumigatus and closely related rarely pathogenic Neosartorya fischeri and Aspergillus clavatus genomes. We applied activity-based protein profiling to compare unique or overexpressed activity-based probe-reactive proteins of all three fungi over time in minimal media growth and in response to human serum. We found 360 probe-reactive proteins exclusive to A. fumigatus, including known virulence associated proteins, and 13 proteins associated with stress response exclusive to A. fumigatus culture in serum. Though the fungi are highly orthologous, A. fumigatus has a significantly greater number of ABP-reactive proteins across varied biological process. Only 50% of expected orthologs of measured A. fumigatus reactive proteins were observed in N. fischeri and A. clavatus. Activity-based protein profiling identified a number of processes that were induced by human serum in A. fumigatus relative to N. fischeri and A. clavatus. These included actin organization and assembly, transport, and fatty acid, cell membrane, and cell wall synthesis. Additionally, signaling proteins regulating vegetative growth, conidiation, and cell wall integrity, required for appropriate cellular response to external stimuli, had higher activity-based probe-protein reaction over time in A. fumigatus and N. fisheri, but not in A. clavatus. Together, we show that measured proteins and physiological processes identified solely or significantly over-represented in A. fumigatus reveal a unique adaptive response to human protein not found in closely related, but rarely pathogenic aspergilli. These unique activity-based probe-protein responses to culture condition may reveal how A. fumigatus initiates pulmonary invasion leading to Invasive Aspergillosis.

Differentiated functions of Ras1 and Ras2 proteins in regulating the germination, growth, conidiation, multi-stress tolerance and virulence of Beauveria bassiana

Ras1 and Ras2 are two distinct Ras GTPases in Beauveria bassiana, an entomopathogenic fungus whose biocontrol potential against insect pests depends largely on virulence and multi-stress tolerance. The functions of both proteins were characterized for the first time by constructing dominant-active (GTP-bound) Ras1(G19V) and dominant-negative (GDP-bound) Ras1(D126A) and integrating them and normal Ras1 into wild type and ΔRas2 for a series of phenotypic and transcriptional analyses. The resultant mutants showed gradient changes of multiple phenotypes but little difference in conidial thermotolerance. Expression of Ras1(D126A) caused vigorous hyphal growth, severely defective conidiation, and increased tolerances to oxidation, cell wall disturbance, fungicide and UV-A/UV-B irradiations, but affected slightly germination, osmosensitivity and virulence. These phenotypes were antagonistically altered by mRas1(G19V) expressed in either wild type or ΔRas2, which was severely defective in conidial germination and hyphal growth and displayed intermediate changes in other mentioned phenotypes between paired mutants expressing Ras1(G19V) or Ras1(D126A) in wild type and ΔRas2. Their growth, UV tolerance or virulence was significantly correlated with cellular response to oxidation or cell wall disturbance. Transcriptional changes of 35 downstream effector genes involved in conidiation and multi-stress responses also related to most of the phenotypic changes among the mutants. Our findings highlight that Ras1 and Ras2 regulate differentially or antagonistically the germination, growth, conidiation, multi-stress tolerance and virulence of B. bassiana, thereby exerting profound effects on the fungal biocontrol potential.

Ras and Rho small G proteins: insights from the Schizophyllum commune genome sequence and comparisons to other fungi

Unlike in animal cells and yeasts, the Ras and Rho small G proteins and their regulators have not received extensive research attention in the case of the filamentous fungi. In an effort to begin to rectify this deficiency, the genome sequence of the basidiomycete mushroom Schizophyllum commune was searched for all known components of the Ras and Rho signalling pathways. The results of this study should provide an impetus for further detailed investigations into their role in polarized hyphal growth, sexual reproduction and fruiting body development. These processes have long been the targets for genetic and cell biological research in this fungus.

Plasma membrane localization is required for RasA-mediated polarized morphogenesis and virulence of Aspergillus fumigatus

Ras is a highly conserved GTPase protein that is essential for proper polarized morphogenesis of filamentous fungi. Localization of Ras proteins to the plasma membrane and endomembranes through posttranslational addition of farnesyl and palmitoyl residues is an important mechanism through which cells provide specificity to Ras signal output. Although the Aspergillus fumigatus RasA protein is known to be a major regulator of growth and development, the membrane distribution of RasA during polarized morphogenesis and the role of properly localized Ras signaling in virulence of a pathogenic mold remain unknown. Here we demonstrate that Aspergillus fumigatus RasA localizes primarily to the plasma membrane of actively growing hyphae. We show that treatment with the palmitoylation inhibitor 2-bromopalmitate disrupts normal RasA plasma membrane association and decreases hyphal growth. Targeted mutations of the highly conserved RasA palmitoylation motif also mislocalized RasA from the plasma membrane and led to severe hyphal abnormalities, cell wall structural changes, and reduced virulence in murine invasive aspergillosis. Finally, we provide evidence that proper RasA localization is independent of the Ras palmitoyltransferase homolog, encoded by erfB, but requires the palmitoyltransferase complex subunit, encoded by erfD. Our results demonstrate that plasma membrane-associated RasA is critical for polarized morphogenesis, cell wall stability, and virulence in A. fumigatus.

StRas2 regulates morphogenesis, conidiation and appressorium development in Setosphaeria turcica

The proteins of Ras family are a large group of monomeric GTPases and act as molecular switches transducing extracellular signals into the cell in higher eukaryotes. However, little is known about roles of Ras family in the foliar pathogens. In this research, we cloned the gene named StRas2 encoding Ras in Setosphaeria turcica and investigated its function by RNA interference technology. We found that the growth rate of RNAi transformants named as R1, R2, R3, R4, R5 and R6, in which the StRas2 silencing efficiency fell in turn. With the highest silencing efficiency, the transformant R1 showed anomalistic hyphae morphology, indicating its growth was significantly affected. The transformants with a middle-silencing efficiency, such as R3, R4, displayed a delay when forming appressoria and invasive hyphae. R1 could not form conidia and appressoria. However, the conidial formation in R5 and R6 was significantly reduced, and these two transformants could form appressoria and penetrate the artificial cellophane, only that its invasive hyphae were fascicular and rarely branched. The HT-toxin biological activity of all transformants showed no difference. All results suggested that StRas2 is involved in the morphogenesis, conidiation, and appressorium development and is not related to the biosynthesis of HT-toxin.

Ras-Mediated Signal Transduction and Virulence in Human Pathogenic Fungi

Signal transduction pathways regulating growth and stress responses are areas of significant study in the effort to delineate pathogenic mechanisms of fungi. In-depth knowledge of signal transduction events deepens our understanding of how a fungal pathogen is able to sense changes in the environment and respond accordingly by modulation of gene expression and re-organization of cellular activities to optimize fitness. Members of the Ras protein family are important regulators of growth and differentiation in eukaryotic organisms, and have been the focus of numerous studies exploring fungal pathogenesis. Here, the current data regarding Ras signal transduction are reviewed for three major pathogenic fungi: Cryptococcus neoformans, Candida albicans and Aspergillus fumigatus. Particular emphasis is placed on Ras-protein interactions during control of morphogenesis, stress response and virulence.

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

Regulatable Ras activity is critical for proper establishment and maintenance of polarity in Aspergillus fumigatus

Here we show that expression of a constitutively activated RasA allele, as the sole source of Ras activity, revealed novel Ras-induced phenotypes, including excessive vacuolar expansion and spontaneous lysis of hyphal compartments. These findings highlight the requirement for balanced Ras activity in the establishment and maintenance of polarized growth in filamentous fungi.

Heterotrimeric Gα protein Pga1 from Penicillium chrysogenum triggers germination in response to carbon sources and affects negatively resistance to different stress conditions

Heterotrimeric Gα protein Pga1 of Penicillium chrysogenum controls vegetative growth, conidiation and secondary metabolite production. In this work we studied the role of Pga1 in spore germination and resistance to different stress conditions. Strains G203R-T (expressing the dominant inactivating pga1(G203R) allele) and Δpga1 (deleted pga1) showed a delayed and asynchronic germination pattern, and a decrease in the percentage of germination, which occurred in only 70-80% of the total conidia. In contrast, in strains expressing the dominant activating pga1(G42R) allele, germination occurred at earlier times and in 100% of conidia. In addition, strains with the pga1(G42R) allele were able to bypass the carbon source (glucose or sucrose) requirement for germination in about 64% of conidia. Thus Pga1 plays an important, but not essential, role in germination, mediating carbon source sensing. Regulation of germination by Pga1 is probably mediated by cAMP, as intracellular levels of this secondary messenger undergo a peak before the onset of germination only in strains with an active Pga1. Pga1 activity is also a determinant factor in the resistance to different stress conditions. Absence or inactivation of Pga1 allow growth on SDS-containing minimal medium, increase resistance of conidia to thermal and oxidative stress, and increase resistance of vegetative mycelium to thermal and osmotic stress. In contrast, constitutive activation of Pga1 causes a decrease in the resistance of conidia to thermal stress and of vegetative mycelium to thermal and osmotic stress. Together with our previously reported results, we show in this work that Pga1 plays a central role in the regulation of the whole growth-developmental program of this biotechnologically important fungus.

Mmc, a gene involved in microcycle conidiation of the entomopathogenic fungus Metarhizium anisopliae

Microcycle conidiation is a survival mechanism for some fungi encountering unfavorable conditions, in which asexual spores germinate secondary spores directly without formation of mycelium. Here, we isolated a microcycle conidiation associated gene, Mmc, from Metarhizium anisopliae and obtained its full length of cDNA and DNA sequence. To clarify its roles in conidiation, we constructed an Mmc RNA interference (RNAi) vector with dual promoter system to knockdown Mmc transcript level, and then analyzed RNAi mutant phenotypes. On microcycle conidiation medium, the RNAi mutant performed normal conidiation instead of microcycle conidiation with significantly reduced growth speed and conidia yield of 5.29-fold and 3.18-fold lower, respectively, than that of the wild-type. Meanwhile, on normal conidiation medium, no significant difference was found in conidiation speed and total yield between the wild-type and RNAi mutant. These data demonstrated that the Mmc gene regulated microcycle conidiation but did not affect normal conidiation. In addition, results of heat treatment, UV-B radiation and bioassays of RNAi mutant indicated that Mmc was also involved in heat resistance but irrelevant to UV-B tolerance and virulence of M. anisopliae. This study helped understanding the regulation of sporulation of the entomopathogenic fungus M. anisopliae.

Heptahelical receptors GprC and GprD of Aspergillus fumigatus Are essential regulators of colony growth, hyphal morphogenesis, and virulence

The filamentous fungus Aspergillus fumigatus normally grows on compost or hay but is also able to colonize environments such as the human lung. In order to survive, this organism needs to react to a multitude of external stimuli. Although extensive work has been carried out to investigate intracellular signal transduction in A. fumigatus, little is known about the specific stimuli and the corresponding receptors activating these signaling cascades. Here, two putative G-protein-coupled receptors, GprC and GprD, were characterized with respect to their cellular functions. Deletion of the corresponding genes resulted in drastic growth defects as hyphal extension was reduced, germination was retarded, and hyphae showed elevated levels of branching. The growth defect was found to be temperature dependent. The higher the temperature the more pronounced was the growth defect. Furthermore, compared with the wild type, the sensitivity of the mutant strains toward environmental stress caused by reactive oxygen intermediates was increased and the mutants displayed an attenuation of virulence in a murine infection model. Both mutants, especially the DeltagprC strain, exhibited increased tolerance toward cyclosporine, an inhibitor of the calcineurin signal transduction pathway. Transcriptome analyses indicated that in both the gprC and gprD deletion mutants, transcripts of primary metabolism genes were less abundant, whereas transcription of several secondary metabolism gene clusters was upregulated. Taken together, our data suggest the receptors are involved in integrating and processing stress signals via modulation of the calcineurin pathway.

Deletion of the protein kinase A regulatory subunit leads to deregulation of mitochondrial activation and nuclear duplication in Aspergillus fumigatus

Proper regulation of the cyclic AMP-dependent protein kinase (PKA) pathway is required for normal growth and development in many fungi. We have reported that deletion of the PKA regulatory subunit gene, pkaR, in Aspergillus fumigatus leads to defects in germination and a hypersensitivity of conidia to oxidative stress. In this study, we further analyzed the defects of DeltapkaR conidia and found that a large proportion were abnormally larger than wild type. Because swelling and increased susceptibility to oxidative stress are characteristic of germinating conidia, we analyzed the metabolic activity of the conidia by mitochondrial staining. Whereas it required 4 h in rich medium for wild-type mitochondria to become active, DeltapkaR conidia harbored active mitochondria in the absence of a germinant. Furthermore, conidia of the mutant showed a dramatic loss in viability upon short-term storage in water, indicating starvation-induced death. Taken together, our data suggest that PKA activity regulates metabolic activation of resting conidia. Additionally, the DeltapkaR mutant displayed an abnormal abundance of hyphal nuclei and had increased transcript levels of several cell cycle regulatory genes. These data indicate an important role for PKA in the nuclear duplication cycle of A. fumigatus.

Transcriptional profiling identifies a role for BrlA in the response to nitrogen depletion and for StuA in the regulation of secondary metabolite clusters in Aspergillus fumigatus

Conidiation (asexual sporulation) is a key developmental process in filamentous fungi. We examined the gene regulatory roles of the Aspergillus fumigatus developmental transcription factors StuAp and BrlAp during conidiation. Conidiation was completely abrogated in an A. fumigatus DeltabrlA mutant and was severely impaired in a DeltastuA mutant. We determined the full genome conidiation transcriptomes of wild-type and DeltabrlA and DeltastuA mutant A. fumigatus and found that BrlAp and StuAp governed overlapping but distinct transcriptional programs. Six secondary metabolite biosynthetic clusters were found to be regulated by StuAp, while only one cluster exhibited BrlAp-dependent expression. The DeltabrlA mutant, but not the DeltastuA mutant, had impaired downregulation of genes encoding ribosomal proteins under nitrogen-limiting, but not carbon-limiting, conditions. Interestingly, inhibition of the target of rapamycin (TOR) pathway also caused downregulation of ribosomal protein genes in both the wild-type strain and the DeltabrlA mutant. Downregulation of these genes by TOR inhibition was associated with conidiation in the wild-type strain but not in the DeltabrlA mutant. Therefore, BrlAp-mediated repression of ribosomal protein gene expression is not downstream of the TOR pathway. Furthermore, inhibition of ribosomal protein gene expression is not sufficient to induce conidiation in the absence of BrlAp.