Deletion of the sec4 homolog srgA from Aspergillus fumigatus is associated with an impaired stress response, attenuated virulence and phenotypic heterogeneity

Small GTPase FoSec4-Mediated Protein Secretion Is Important for Polarized Growth, Reproduction and Pathogenicity in the Banana Fusarium Wilt Fungus Fusarium odoratissimum

Apical secretion at hyphal tips is important for the growth and development of filamentous fungi. In this study, we analyzed the role of the Rab GTPases FoSec4 involved in the secretion of the banana wilt fungal pathogen Fusarium odoratissimum. We found that the deletion of FoSEC4 affects the activity of extracellular hydrolases and protein secretion, indicating that FoSec4 plays an important role in the regulation of protein secretion in F. odoratissimum. As a typical Rab GTPase, Sec4 participates in the Rab cycle through the conversion between the active GTP-bound state and the inactive GDP-bound state, which is regulated by guanine nucleate exchange factors (GEFs) and GTPase-activating proteins (GAPs). We further found that FoSec2 can interact with dominant-negative FoSec4 (GDP-bound and nucleotide-free form, FoSec4DN), and that FoGyp5 can interact with dominant active FoSec4 (GTP-bound and constitutively active form, FoSec4CA). We evaluated the biofunctions of FoSec4, FoSec2 and FoGyp5, and found that FoSec4 is involved in the regulation of vegetative growth, reproduction, pathogenicity and the environmental stress response of F. odoratissimum, and that FocSec2 and FoGyp5 perform biofunctions consistent with FoSec4, indicating that FoSec2 and FoGyp5 may work as the GEF and the GAP, respectively, of FoSec4 in F. odoratissimum. We further found that the amino-terminal region and Sec2 domain are essential for the biological functions of FoSec2, while the carboxyl-terminal region and Tre-2/Bub2/Cdc16 (TBC) domain are essential for the biological functions of FoGyp5. In addition, FoSec4 mainly accumulated at the hyphal tips and partially colocalized with Spitzenkörper; however, FoGyp5 accumulated at the periphery of Spitzenkörper, suggesting that FoGyp5 may recognize and inactivate FoSec4 at a specific location in hyphal tips.

Identifying Conserved Generic Aspergillus spp. Co-Expressed Gene Modules Associated with Germination Using Cross-Platform and Cross-Species Transcriptomics

Aspergillus spp. is an opportunistic human pathogen that may cause a spectrum of pulmonary diseases. In order to establish infection, inhaled conidia must germinate, whereby they break dormancy, start to swell, and initiate a highly polarized growth process. To identify critical biological processes during germination, we performed a cross-platform, cross-species comparative analysis of germinating A. fumigatus and A. niger conidia using transcriptional data from published RNA-Seq and Affymetrix studies. A consensus co-expression network analysis identified four gene modules associated with stages of germination. These modules showed numerous shared biological processes between A. niger and A. fumigatus during conidial germination. Specifically, the turquoise module was enriched with secondary metabolism, the black module was highly enriched with protein synthesis, the darkgreen module was enriched with protein fate, and the blue module was highly enriched with polarized growth. More specifically, enriched functional categories identified in the blue module were vesicle formation, vesicular transport, tubulin dependent transport, actin-dependent transport, exocytosis, and endocytosis. Genes important for these biological processes showed similar expression patterns in A. fumigatus and A. niger, therefore, they could be potential antifungal targets. Through cross-platform, cross-species comparative analysis, we were able to identify biologically meaningful modules shared by A. fumigatus and A. niger, which underscores the potential of this approach.

Galleria mellonella as a screening tool to study virulence factors of Aspergillus fumigatus

The invertebrate Galleria mellonella has increasingly and widely been used in the last few years to study complex host–microbe interactions. Aspergillus fumigatus is one of the most pathogenic fungi causing life-threatening diseases in humans and animals. Galleria mellonella larvae has been proven as a reliable model for the analysis of pathogenesis and virulence factors, enable to screen a large number of A. fumigatus strains. This review describes the different uses of G. mellonella to study A. fumigatus and provides a comparison of the different protocols to trace fungal pathogenicity. The review also includes a summary of the diverse mutants tested in G. mellonella, and their respective contribution to A. fumigatus virulence. Previous investigations indicated that G. mellonella should be considered as an interesting tool even though a mammalian model may be required to complete and verify initial data.

A Heterogeneously Expressed Gene Family Modulates the Biofilm Architecture and Hypoxic Growth of Aspergillus fumigatus

The genus Aspergillus encompasses human pathogens such as Aspergillus fumigatus and industrial powerhouses such as Aspergillus niger In both cases, Aspergillus biofilms have consequences for infection outcomes and yields of economically important products. However, the molecular components influencing filamentous fungal biofilm development, structure, and function remain ill defined. Macroscopic colony morphology is an indicator of underlying biofilm architecture and fungal physiology. A hypoxia-locked colony morphotype of A. fumigatus has abundant colony furrows that coincide with a reduction in vertically oriented hyphae within biofilms and increased low oxygen growth and virulence. Investigation of this morphotype has led to the identification of the causative gene, biofilm architecture factor A (bafA), a small cryptic open reading frame within a subtelomeric gene cluster. BafA is sufficient to induce the hypoxia-locked colony morphology and biofilm architecture in A. fumigatus Analysis across a large population of A. fumigatus isolates identified a larger family of baf genes, all of which have the capacity to modulate hyphal architecture, biofilm development, and hypoxic growth. Furthermore, introduction of A. fumigatusbafA into A. niger is sufficient to generate the hypoxia-locked colony morphology, biofilm architecture, and increased hypoxic growth. Together, these data indicate the potential broad impacts of this previously uncharacterized family of small genes to modulate biofilm architecture and function in clinical and industrial settings.IMPORTANCE The manipulation of microbial biofilms in industrial and clinical applications remains a difficult task. The problem is particularly acute with regard to filamentous fungal biofilms for which molecular mechanisms of biofilm formation, maintenance, and function are only just being elucidated. Here, we describe a family of small genes heterogeneously expressed across Aspergillus fumigatus strains that are capable of modifying colony biofilm morphology and microscopic hyphal architecture. Specifically, these genes are implicated in the formation of a hypoxia-locked colony morphotype that is associated with increased virulence of A. fumigatus Synthetic introduction of these gene family members, here referred to as biofilm architecture factors, in both A. fumigatus and A. niger additionally modulates low oxygen growth and surface adherence. Thus, these genes are candidates for genetic manipulation of biofilm development in aspergilli.

The class V myosin interactome of the human pathogen Aspergillus fumigatus reveals novel interactions with COPII vesicle transport proteins

The human fungal pathogen Aspergillus fumigatus causes life-threatening invasive aspergillosis in immunocompromised individuals. Adaptation to the host environment is integral to survival of A. fumigatus and requires the coordination of short- and long-distance vesicular transport to move essential components throughout the fungus. We previously reported the importance of MyoE, the only class V myosin, for hyphal growth and virulence of A. fumigatus. Class V myosins are actin-based, cargo-carrying motor proteins that contain unique binding sites for specific cargo. Specific cargo carried by myosin V has not been identified in any fungus, and previous studies have only identified single components that interact with class V myosins. Here we utilized a mass spectrometry-based whole proteomic approach to identify MyoE interacting proteins in A. fumigatus for the first time. Several proteins previously shown to interact with myosin V through physical and genetic approaches were confirmed, validating our proteomic analysis. Importantly, we identified novel MyoE-interacting proteins, including members of the cytoskeleton network, cell wall synthesis, calcium signaling and a group of coat protein complex II (COPII) proteins involved in the endoplasmic reticulum (ER) to Golgi transport. Furthermore, we analyzed the localization patterns of the COPII proteins, UsoA (Uso1), SrgE (Sec31), and SrgF (Sec23), which suggested a potential role for MyoE in ER to Golgi trafficking.

Benzothiourea Derivatives Target the Secretory Pathway of the Human Fungal Pathogen Cryptococcus neoformans

Cryptococcus neoformans is one of the most important human fungal pathogens and causes life-threatening meningoencephalitis in immunocompromised patients. The current gold standard therapy for C. neoformans meningoencephalitis is based on medications that are over 50 years old and is not readily available in regions with high disease burden. Here, we report the mycologic, mechanistic, and pharmacologic characterization of a set of benzothioureas with highly selective fungicidal activity against C. neoformans. In addition, to direct antifungal activity, benzothioureas inhibit C. neoformans virulence traits. On the basis of a set of phenotypic, biochemical, and biophysical assays, the benzothioureas (BTUs) inhibit the late secretory pathway (post-Golgi), possibly through a direct interaction with Sav1, an orthologue of the Sec4-class small GTPase. Importantly, pharmacological characterization of the BTUs indicates it readily penetrates the blood-brain barrier. Together, our data support the further development of this scaffold as an antifungal agent with a novel mechanism of action against C. neoformans.

A Rab GTPase protein FvSec4 is necessary for fumonisin B1 biosynthesis and virulence in Fusarium verticillioides

Rab GTPases are responsible for a variety of membrane trafficking and vesicular transportation in fungi. But the role of Rab GTPases in Fusarium verticillioides, one of the key corn pathogens worldwide, remains elusive. These Small GTPases in fungi, particularly those homologous to Saccharomyces cerevisiae Sec4, are known to be associated with protein secretion, vesicular trafficking, secondary metabolism and pathogenicity. In this study, our aim was to investigate the molecular functions of FvSec4 in F. verticillioides associated with physiology and virulence. Interestingly, the FvSec4 null mutation did not impair the expression of key conidiation-related genes. Also, the mutant did not show any defect in sexual development, including perithecia production. Meanwhile, GFP-FvSec4 localized to growing hyphal tips and raised the possibility that FvSec4 is involved in protein trafficking and endocytosis. The mutant exhibited defect in corn stalk rot virulence and also significant alteration of fumonisin B1 production. The mutation led to higher sensitivity to oxidative and cell wall stress agents, and defects in carbon utilization. Gene complementation fully restored the defects in the mutant demonstrating that FvSec4 plays important roles in these functions. Taken together, our data indicate that FvSec4 is critical in F. verticillioides hyphal development, virulence, mycotoxin production and stress responses.

Off the wall: The rhyme and reason of Neurospora crassa hyphal morphogenesis

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Chitin and β-1,3-glucan synthases are transported separately in chitosomes and macrovesicles.

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Chitin synthases occupy the core of the SPK; β-1,3-glucan synthases the outer layer.

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CHS-4 arrival to the SPK and septa is CSE-7 dependent.

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Rabs YPT-1 and YPT-31 localization at the SPK mimics that of chitosomes and macrovesicles.

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The exocyst acts as a tether between the SPK outer layer vesicles and the apical PM.

The fungal cell wall building processes are the ultimate determinants of hyphal shape. In Neurospora crassa the main cell wall components, β-1,3-glucan and chitin, are synthesized by enzymes conveyed by specialized vesicles to the hyphal tip. These vesicles follow different secretory routes, which are delicately coordinated by cargo-specific Rab GTPases until their accumulation at the Spitzenkörper. From there, the exocyst mediates the docking of secretory vesicles to the plasma membrane, where they ultimately get fused. Although significant progress has been done on the cellular mechanisms that carry cell wall synthesizing enzymes from the endoplasmic reticulum to hyphal tips, a lot of information is still missing. Here, the current knowledge on N. crassa cell wall composition and biosynthesis is presented with an emphasis on the underlying molecular and cellular secretory processes.

FgSec2A, a guanine nucleotide exchange factor of FgRab8, is important for polarized growth, pathogenicity and deoxynivalenol production in Fusarium graminearum

Sec4/Rab8 is one of the well-studied members of the Rab GTPase family, previous studies have shown that Sec4/Rab8 crucially promotes the pathogenesis of phytopathogens, but the upstream regulators of Rab8 are still unknown. Here, we have identified two Sec2 homologues FgSec2A and FgSec2B in devastating fungal pathogen Fusarium graminearum and investigated their functions and interactions with FgRab8 by live-cell imaging, genetic and functional analyses. Yeast two-hybrid assay shows that FgSec2A specifically interacts with FgRab8DN(N123I) and itself. Importantly, FgSec2A is required for growth, conidiation, DON production and virulence of F. graminearum. Live-cell imaging shows that FgSec2A and FgSec2B are both localized to the tip region of hyphae and conidia. Both N-terminal region and Sec2 domain of FgSec2A are essential for its function, but not for localization, whereas the C-terminal region is important for its polarized localization. Furthermore, constitutively active FgRab8CA(Q69L) partially rescues the defects of ΔFgsec2A. Consistently, FgSec2A is required for the polarized localization of FgRab8. Finally, FgSec2A and FgSec2B show partial functions, but FgSec2A does not interact and co-localize with FgSec2B. Taken together, these results indicate that FgSec2A acts as a FgRab8 guanine nucleotide exchange factor and is necessary for polarized growth, DON production and pathogenicity in F. graminearum.

Two Rab GTPases play different roles in conidiation, trap formation, stress resistance, and virulence in the nematode-trapping fungus Arthrobotrys oligospora

Rab GTPases are the largest group of the small GTPases family, which play a pivotal role in the secretion of proteins. Arthrobotrys oligospora is a representative nematode-trapping fungus that can produce adhesive networks to capture nematodes. In this study, the roles of two Rab GTPases AoRab-7A and AoRab-2 were characterized by gene knockout in the fungus A. oligospora. The disruption of AoRab-7A hindered the mycelial growth in different media, the conidiation of ΔAoRab-7A transformants was almost abolished, and the transcription of four sporulation-related genes (AbaA, FluG, Hyp1, and VosA) was downregulated compared to the wild-type strain (WT). Furthermore, the tolerance of the ΔAoRab-7A mutants to sodium dodecyl sulfate (SDS) and H₂O₂ was also significantly reduced compared to the WT, and the transcription of several genes related to environmental resistance, such as genes for catalase and trehalose synthase, was downregulated. Similarly, the extracellular proteolytic activity was decreased. Importantly, the ΔAoRab-7A mutants were unable to produce traps and capture nematodes. However, the disruption of gene AoRab-2 only affected the conidiation slightly but non-significantly, while other phenotypic traits were unaffected. Moreover, the gene AoRab-7A was also involved in the autophagy induced by nitrogen deprivation in A. oligospora. Our results revealed for the first time that the Rab GTPases are involved in the regulation of mycelial growth, conidiation, trap formation, stress resistance, and pathogenicity in the nematode-trapping fungus A. oligospora.

Effect of secretory pathway gene overexpression on secretion of a fluorescent reporter protein in Aspergillus nidulans

Background

The considerable capacity of filamentous fungi for the secretion of proteins is the basis for multi-billion dollar industries producing enzymes and proteins with therapeutic value. The stepwise pathway from translation to secretion is therefore well studied, and genes playing major roles in the process have been identified through transcriptomics. The assignment of function to these genes has been enabled in combination with gene deletion studies. In this work, 14 genes known to play a role in protein secretion in filamentous fungi were overexpressed in Aspergillus nidulans. The background strain was a fluorescent reporter secreting mRFP. The overall effect of the overexpressions could thus be easily monitored through fluorescence measurements, while the effects on physiology were determined in batch cultivations and surface growth studies.

Results

Fourteen protein secretion pathway related genes were overexpressed with a tet-ON promoter in the RFP-secreting reporter strain and macromorphology, physiology and protein secretion were monitored when the secretory genes were induced. Overexpression of several of the chosen genes was shown to cause anomalies on growth, micro- and macro-morphology and protein secretion levels. While several constructs exhibited decreased secretion of the model protein, the overexpression of the Rab GTPase RabD resulted in a 40 % increase in secretion in controlled bioreactor cultivations. Fluorescence microscopy revealed alterations of protein localization in some of the constructed strains, giving further insight into potential roles of the investigated genes.

Conclusions

This study demonstrates the possibility of significantly increasing cellular recombinant protein secretion by targeted overexpression of secretion pathway genes. Some gene targets investigated here, including genes from different compartments of the secretory pathway resulted in no significant change in protein secretion, or in significantly lowered protein titres. As the 14 genes selected in this study were previously shown to be upregulated during protein secretion, our results indicate that increased expression may be a way for the cell to slow down secretion in order to cope with the increased protein load. By constructing a secretion reporter strain, the study demonstrates a robust way to study the secretion pathway in filamentous fungi.

Electronic supplementary material

The online version of this article (doi:10.1186/s40694-016-0021-y) contains supplementary material, which is available to authorized users.

The Small GTPase MoSec4 Is Involved in Vegetative Development and Pathogenicity by Regulating the Extracellular Protein Secretion in Magnaporthe oryzae

The Rab GTPase proteins play important roles in the membrane trafficking, and consequently protein secretion and development of eukaryotic organisms. However, little is known about the function of Rab GTPases in Magnaporthe oryzae. To further explore the function of Rab GTPases, we deleted the ortholog of the yeast Sec4p protein in M. oryzae, namely MoSEC4. The ΔMosec4 mutant is defective in polarized growth and conidiation, and it displays decreased appressorium turgor pressure and attenuated pathogenicity. Notably, the biotrophic invasive hyphae produced in rice cells are more bulbous and compressed in the ΔMosec4 mutant. Further studies showed that deletion of the MoSEC4 gene resulted in decreased secretion of extracellular enzymes and mislocalization of the cytoplasmic effector PWL2-mCherry-NLS. In accordance with a role in secretion, the GFP-MoSec4 fusion protein mainly accumulates at tips of growing vegetative hyphae. Our results suggest that the MoSec4 protein plays important roles in the secretion of extracellular proteins and consequently hyphal development and pathogenicity in the rice blast fungus.

The exocyst complex: delivery hub for morphogenesis and pathogenesis in filamentous fungi

Regulated by several small GTPases, the octameric exocyst complex directs the docking and tethering of exocytic vesicles to the destined plasma membrane sites, providing the precise spatiotemporal control of exocytosis. Although the exocyst components are well conserved among various fungal species, the mechanisms for the regulation of its assembly and activity are diverse. Exocytosis is crucial for the generation of cell polarity as well as the delivery of effector proteins in filamentous fungi, and thus plays an important role for fungal morphogenesis and pathogenicity on plant hosts. This review focuses on current findings about the roles of the exocyst complex in the morphogenesis and pathogenesis of filamentous fungi.

Rab GTPases are essential for membrane trafficking-dependent growth and pathogenicity in Fusarium graminearum

Rab GTPases represent the largest subfamily of Ras-related small GTPases and regulate membrane trafficking. Vesicular transport is a general mechanism that governs intracellular membrane trafficking along the endocytic and exocytic pathways in all eukaryotic cells. Fusarium graminearum is a filamentous fungus and causes the devastating and economically important head blight of wheat and related species. The mechanism of vesicular transport is not well understood, and little is known about Rab GTPases in F. graminearum. In this study, we systematically characterized all eleven FgRabs by live cell imaging and genetic analysis. We find that FgRab51 and FgRab52 are important for the endocytosis, FgRab7 localizes to the vacuolar membrane and regulates the fusion of vacuoles and autophagosomes, and FgRab8 and FgRab11 are important for polarized growth and/or exocytosis. Furthermore, both endocytic and exocytic FgRabs are required for vegetative growth, conidiogenesis, sexual reproduction, as well as pathogenesis and deoxynivalenol metabolism in F. graminearum. Thus, we conclude that Rab GTPases are essential for membrane trafficking-dependent growth and pathogenicity in F. graminearum.