Experimental measurement of endocytosis in fungal hyphae

Material and mechanical behavior of bracket fungi context as a mechanically versatile structural layer

Bracket fungi sporocarps present promising environmentally friendly alternatives to harmful and wasteful structural applications with their high strength-to-weight ratio mechanical properties. Kingdom Fungi is estimated to have over three million species, yet only 4% of the species have been described by mycologists, and their mechanical behavior has been under-explored. This work aims to characterize the material behavior and mechanical properties of bracket fungi as a whole through micro-mechanical tensile testing combined with microstructural imaging and analysis of two representative species. The context layer from three distinctive fresh bracket sporocarps is used in this study. At the microstructure level, the bracket fungi have a preferred alignment in the hyphal network, which correlates to the radial direction. The bracket fungi exhibit an anisotropic mechanical behavior with higher ultimate tensile strength and elastic modulus in the radial direction, while the strain to failure is higher in the transverse direction. However, the bracket fungi exhibit an isotropic energy absorption, or toughness, behavior, with no statistically significant difference between the radial and transverse directions. The characterization of anisotropic mechanical properties and isotropic energy absorption will inspire the exploration of bracket fungi as a viable alternative to applications in various industries, such as aerospace and agriculture.

Cdc42 mobility and membrane flows regulate fission yeast cell shape and survival

Polarized exocytosis induced by local Cdc42 GTPase activity results in membrane flows that deplete low-mobility membrane-associated proteins. A reaction-diffusion particle model comprising Cdc42 positive feedback activation, hydrolysis by GTPase-activating proteins (GAPs), and flow-induced displacement by exo/endocytosis shows that flow-induced depletion of low mobility GAPs promotes polarization. We modified Cdc42 mobility in Schizosaccharomyces pombe by replacing its prenylation site with 1, 2 or 3 repeats of the Rit C-terminal membrane-binding domain (ritC), yielding alleles with progressively lower mobility and increased flow-coupling. While Cdc42-1ritC cells are viable and polarized, Cdc42-2ritC polarize poorly and Cdc42-3ritC are inviable, in agreement with model's predictions. Deletion of Cdc42 GAPs restores viability to Cdc42-3ritC cells, verifying the model's prediction that GAP deletion increases Cdc42 activity at the expense of polarization. Our work demonstrates how membrane flows are an integral part of Cdc42-driven pattern formation and require Cdc42-GTP to turn over faster than the surface on which it forms.

Biochemical Characterization of Recombinant Enterococcus faecalis EntV Peptide to Elucidate Its Antihyphal and Antifungal Mechanisms against Candida albicans

Candida albicans is a common opportunistic fungus in humans, whose morphological switch between yeast and hyphae forms represents a key virulence trait. Developing strategies to inhibit C. albicans hyphal growth may provide insights into designs of novel antivirulent therapeutics. Importantly, the gut commensal bacterium, Enterococcus faecalis, secretes a bacteriocin EntV which has potent antivirulent and antifungal effects against C. albicans in infection models; however, hampered by the challenges to access large quantities of bioactive EntV, the detailed understanding of its mechanisms on C. albicans has remained elusive. In this work, we biochemically reconstituted the proteolytic cleavage reaction to obtain recombinant EntV88-His6 on a large preparative scale, providing facile access to the C-terminal EntV construct. Under in vitro C. albicans hyphal assay with specific inducers, we demonstrated that EntV88-His6 exhibits potent bioactivity against GlcNAc-triggered hyphal growth. Moreover, with fluorescent FITC-EntV88-His6, we revealed that EntV88-His6 enters C. albicans via endocytosis and perturbs the proper localization of the polarisome scaffolding Spa2 protein. Our findings provide important clues on EntV's mechanism of action. Surprisingly, we showed that EntV88-His6 does not affect C. albicans yeast cell growth but potently exerts cytotoxicity against C. albicans under hyphal-inducing conditions in vitro. The combination of EntV88-His6 and GlcNAc displays rapid killing of C. albicans, rendering it a promising antivirulent and antifungal agent.

Natural Product Aloesin Significantly Inhibits Spore Germination and Appressorium Formation in Magnaporthe oryzae

This study aims to determine the effects of the natural product aloesin against Magnaporthe oryzae. The results exposed that aloesin had a high inhibitory effect on appressorium formation (the EC50 value was 175.26 μg/mL). Microscopic examination revealed that 92.30 ± 4.26% of M. oryzae spores could be broken down by 625.00 μg/mL of aloesin, and the formation rate of appressoria was 4.74 ± 1.00% after 12 h. M. oryzae mycelial growth was weaker than that on the control. The enzyme activity analysis results indicated that aloesin inhibited the activities of polyketolase (PKS), laccase (LAC), and chain-shortening catalytic enzyme (Aayg1), which are key enzymes in melanin synthesis. The inhibition rate by aloesin of PKS, LAC, and Aayg1 activity was 32.51%, 33.04%, and 43.38%, respectively. The proteomic analysis showed that actin expression was downregulated at 175.62 μg/mL of aloesin, which could reduce actin bundle formation and prevent the polar growth of hyphae in M. oryzae. This is the first report showing that aloesin effectively inhibits conidia morphology and appressorium formation in M. oryzae.

Using fimbrin to quantify the endocytic subapical collar during polarized growth in three filamentous fungi

Filamentous fungi produce specialized cells called hyphae. These cells grow by polarized extension at their apex, which is maintained by the balance of endocytosis and exocytosis at the apex. Although endocytosis has been well characterized in other organisms, the details of endocytosis and its role in maintaining polarity during hyphal growth in filamentous fungi is comparatively sparsely studied. In recent years, a concentrated region of protein activity that trails the growing apex of hyphal cells has been discovered. This region, dubbed the "endocytic collar" (EC), is a dynamic 3-dimensional region of concentrated endocytic activity, the disruption of which results in the loss of hyphal polarity. Here, fluorescent protein-tagged fimbrin was used as a marker to map the collar during growth of hyphae in three fungi: Aspergillus nidulans, Colletotrichum graminicola, and Neurospora crassa. Advanced microscopy techniques and novel quantification strategies were then utilized to quantify the spatiotemporal localization and recovery rates of fimbrin in the EC during hyphal growth. Correlating these variables with hyphal growth rate revealed that the strongest observed relationship with hyphal growth is the distance by which the EC trails the apex, and that measured endocytic rate does not correlate strongly with hyphal growth rate. This supports the hypothesis that endocytic influence on hyphal growth rate is better explained by spatiotemporal regulation of the EC than by the raw rate of endocytosis.

Going with the membrane flow: the impact of polarized secretion on bulk plasma membrane flows

Even the simplest cells show a remarkable degree of intracellular patterning. Like developing multicellular organisms, single cells break symmetry to establish polarity axes, pattern their cortex and interior, and undergo morphogenesis to acquire sometimes complex shapes. Symmetry-breaking and molecular patterns can be established through coupling of negative and positive feedback reactions in biochemical reaction-diffusion systems. Physical forces, perhaps best studied in the contraction of the metazoan acto-myosin cortex, which induces cortical and cytoplasmic flows, also serve to pattern-associated components. A less investigated physical perturbation is the in-plane flow of plasma membrane material caused by membrane trafficking. In this review, we discuss how bulk membrane flows can be generated at sites of active polarized secretion and growth, how they affect the distribution of membrane-associated proteins, and how they may be harnessed for patterning and directional movement in cells across the tree of life.

Chemotropism and Cell-Cell Fusion in Fungi

Fungi exhibit an enormous variety of morphologies, including yeast colonies, hyphal mycelia, and elaborate fruiting bodies. This diversity arises through a combination of polar growth, cell division, and cell fusion. Because fungal cells are nonmotile and surrounded by a protective cell wall that is essential for cell integrity, potential fusion partners must grow toward each other until they touch and then degrade the intervening cell walls without impacting cell integrity. Here, we review recent progress on understanding how fungi overcome these challenges. Extracellular chemoattractants, including small peptide pheromones, mediate communication between potential fusion partners, promoting the local activation of core cell polarity regulators to orient polar growth and cell wall degradation. However, in crowded environments, pheromone gradients can be complex and potentially confusing, raising the question of how cells can effectively find their partners. Recent findings suggest that the cell polarity circuit exhibits searching behavior that can respond to pheromone cues through a remarkably flexible and effective strategy called exploratory polarization.

F-actin dynamics following mechanical injury of Trichoderma atroviride and Neurospora crassa hyphae

We investigated hyphae regeneration in Trichoderma atroviride and Neurospora crassa, with particular focus on determining the role of the actin cytoskeleton after mechanical injury. Filamentous actin (F-actin) dynamics were observed by live-cell confocal microscopy in both T. atroviride and N. crassa strains expressing Lifeact-GFP. In growing hyphae of both fungi, F-actin localized in three different structural forms: patches, cables and actomyosin rings. Most patches were conspicuously arranged in a collar in the hyphal subapex. A strong F-actin signal, likely actin filaments, colocalized with the core of the Spitzenkörper. Filaments and cables of F-actin we observed along the cortex throughout hyphae. Following mechanical damage at the margin of growing mycelia of T. atroviride and N. crassa, the severed hyphae lost their cytoplasmic contents, but plugging of the septal pore by a Woronin body, the rest of the hyphal tube remained whole. In both fungi, patches of F-actin began accumulating next to the plugged septum. Regeneration was attained by the emergence of a new hyphal tube as an extension of the plugged septum wall. The septum wall was gradually remodeled into the apical wall of the emerging hypha. Whereas in T. atroviride the re-initiation of polarized growth took about ∼1 h, in N. crassa, actin patch accumulation began almost immediately, and new growing hyphae were observed ∼30 min after injury. By confocal microscopy, we found that chitin synthase 1 (CHS-1), a microvesicle (chitosome) component, accumulated next to the plugged septum in regenerating hyphae of N. crassa. We concluded that the actin cytoskeleton plays a key role in hyphal regeneration by supporting membrane remodeling, helping to facilitate transport of vesicles responsible for new wall growth and organization of the new tip-growth apparatus.

Twinfilin regulates actin assembly and Hexagonal peroxisome 1 (Hex1) localization in the pathogenesis of rice blast fungus Magnaporthe oryzae

Actin assembly at the hyphal tip is key for polar growth and pathogenesis of the rice blast fungus Magnaporthe oryzae. The mechanism of its precise assemblies and biological functions is not understood. Here, we characterized the role of M. oryzae Twinfilin (MoTwf) in M. oryzae infection through organizing the actin cables that connect to Spitzenkörper (Spk) at the hyphal tip. MoTwf could bind and bundle the actin filaments. It formed a complex with Myosin2 (MoMyo2) and the Woronin body protein Hexagonal peroxisome 1 (MoHex1). Enrichment of MoMyo2 and MoHex1 in the hyphal apical region was disrupted in a ΔMotwf loss-of-function mutant, which also showed a decrease in the number and width of actin cables. These findings indicate that MoTwf participates in the virulence of M. oryzae by organizing Spk-connected actin filaments and regulating MoHex1 distribution at the hyphal tip.

RCO-3 and COL-26 form an external-to-internal module that regulates the dual-affinity glucose transport system in Neurospora crassa

BACKGROUND

Low- and high-affinity glucose transport system is a conserved strategy of microorganism to cope with environmental glucose fluctuation for their growth and competitiveness. In Neurospora crassa, the dual-affinity glucose transport system consists of a low-affinity glucose transporter GLT-1 and two high-affinity glucose transporters HGT-1/HGT-2, which play diverse roles in glucose transport, carbon metabolism, and cellulase expression regulation. However, the regulation of this dual-transporter system in response to environmental glucose fluctuation is not yet clear.

RESULTS

In this study, we report that a regulation module consisting of a downstream transcription factor COL-26 and an upstream non-transporting glucose sensor RCO-3 regulates the dual-affinity glucose transport system in N. crassa. COL-26 directly binds to the promoter regions of glt-1, hgt-1, and hgt-2, whereas RCO-3 is an upstream factor of the module whose deletion mutant resembles the Δcol-26 mutant phenotypically. Transcriptional profiling analysis revealed that Δcol-26 and Δrco-3 mutants had similar transcriptional profiles, and both mutants had impaired response to a glucose gradient. We also showed that the AMP-activated protein kinase (AMPK) complex is involved in regulation of the glucose transporters. AMPK is required for repression of glt-1 expression in starvation conditions by inhibiting the activity of RCO-3.

CONCLUSIONS

RCO-3 and COL-26 form an external-to-internal module that regulates the glucose dual-affinity transport system. Transcription factor COL-26 was identified as the key regulator. AMPK was also involved in the regulation of the dual-transporter system. Our findings provide novel insight into the molecular basis of glucose uptake and signaling in filamentous fungi, which may aid in the rational design of fungal strains for industrial purposes.

Improvement of gibberellin production by a newly isolated Fusarium fujikuroi mutant

AIMS

To obtain and investigate the potential mechanism for GA₃ production in Fusarium fujikuroi GA-251, a high GA₃ producer.

METHODS AND RESULTS

Fusarium fujikuroi IMI 58289 was bred with Cobalt-60 (⁶⁰ Co) radiation and lithium chloride treatment. The best mutant strain GA-251 was obtained for the subsequent optimization of fermentation conditions. The yield of GA₃ by GA-251 was 2100 mg l^-1 , while the wild-type strain was 100 mg l^-1 , which is a 21-fold increase in the yield. To elucidate the mechanism of high GA₃ yield of GA-251, the genome was sequenced and compared with wild-type strain IMI 58289. The results showed 2295 single nucleotide polymorphisms, 1242 small indels and 30 structural variants. These mutations were analysed and enriched in the MAPK signalling pathway, the mRNA surveillance pathway and endocytosis. The potential reasons for the improved GA₃ biosynthesis were investigated.

CONCLUSIONS

The potential mechanism of high GA₃ yield was attributed to endocytosis pathway and histone modification proteins family.

SIGNIFICANCE AND IMPACT OF THE STUDY

A mutant strain GA-251 in this work that could potentially be utilized in the industrial yield of GA₃ . The comparative genome analysis would shed light onto the mechanism of yield improvement and be a theoretical guide for further metabolic engineering.

Localization of NPFxD motif-containing proteins in Aspergillus nidulans

During growth, filamentous fungi produce polarized cells called hyphae. It is generally presumed that polarization of hyphae is dependent upon secretion through the Spitzenkörper, as well as a mechanism called apical recycling, which maintains a balance between the tightly coupled processes of endocytosis and exocytosis. Endocytosis predominates in an annular domain called the sub-apical endocytic collar, which is located in the region of plasma membrane 1-5 μm distal to the Spitzenkörper. It has previously been proposed that one function of the sub-apical endocytic collar is to maintain the apical localization of polarization proteins. These proteins mark areas of polarization at the apices of hyphae. However, as hyphae grow, these proteins are displaced along the membrane and some must then be removed at the sub-apical endocytic collar in order to maintain the hyphoid shape. While endocytosis is fairly well characterized in yeast, comparatively little is known about the process in filamentous fungi. Here, a bioinformatics approach was utilized to identify 39 Aspergillus nidulans proteins that are predicted to be cargo of endocytosis based on the presence of an NPFxD peptide motif. This motif is a necessary endocytic signal sequence first established in Saccharomyces cerevisiae, where it marks proteins for endocytosis through an interaction with the adapter protein Sla1p. It is hypothesized that some proteins that contain this NPFxD peptide sequence in A. nidulans will be potential targets for endocytosis, and therefore will localize either to the endocytic collar or to more proximal polarized regions of the cell, e.g. the apical dome or the Spitzenkörper. To test this, a subset of the motif-containing proteins in A. nidulans was tagged with GFP and the dynamic localization was evaluated. The documented localization patterns support the hypothesis that the motif marks proteins for localization to the polarized cell apex in growing hyphae.

Current views on endocytosis in filamentous fungi

Filamentous fungi grow by adding cell wall and membrane exclusively at the apex of tubular structures called hyphae. Growth was previously believed to occur only through exocytosis at the Spitzenkörper, an organised body of secretory macro- and microvesicles found only in growing hyphae. More recent work has indicated that an area deemed the sub-apical collar is enriched for endocytosis and is also required for hyphal growth. It is now generally believed that polarity of filamentous fungi is achieved through the balancing of the processes of endocytosis and exocytosis at these two areas. This review is an update on the current progress and understanding surrounding the occurrence of endocytosis and its spatial regulation as they pertain to growth and pathogenicity in filamentous fungi.

Magnaporthe oryzae fimbrin organizes actin networks in the hyphal tip during polar growth and pathogenesis

Magnaporthe oryzae causes rice blast disease, but little is known about the dynamic restructuring of the actin cytoskeleton during its polarized tip growth and pathogenesis. Here, we used super-resolution live-cell imaging to investigate the dynamic organization of the actin cytoskeleton in M. oryzae during hyphal tip growth and pathogenesis. We observed a dense actin network at the apical region of the hyphae and actin filaments originating from the Spitzenkörper (Spk, the organizing center for hyphal growth and development) that formed branched actin bundles radiating to the cell membrane. The actin cross-linking protein Fimbrin (MoFim1) helps organize this actin distribution. MoFim1 localizes to the actin at the subapical collar, the actin bundles, and actin at the Spk. Knockout of MoFim1 resulted in impaired Spk maintenance and reduced actin bundle formation, preventing polar growth, vesicle transport, and the expansion of hyphae in plant cells. Finally, transgenic rice (Oryza sativa) expressing RNA hairpins targeting MoFim1 exhibited improved resistance to M. oryzae infection, indicating that MoFim1 represents an excellent candidate for M. oryzae control. These results reveal the dynamics of actin assembly in M. oryzae during hyphal tip development and pathogenesis, and they suggest a mechanism in which MoFim1 organizes such actin networks.

Recent advances in understanding Candida albicans hyphal growth

Morphological changes are critical for the virulence of a range of plant and human fungal pathogens. Candida albicans is a major human fungal pathogen whose ability to switch between different morphological states is associated with its adaptability and pathogenicity. In particular, C. albicans can switch from an oval yeast form to a filamentous hyphal form, which is characteristic of filamentous fungi. What mechanisms underlie hyphal growth and how are they affected by environmental stimuli from the host or resident microbiota? These questions are the focus of intensive research, as understanding C. albicans hyphal growth has broad implications for cell biological and medical research.

Regulation of Candida albicans Hyphal Morphogenesis by Endogenous Signals

Candida albicans is a human commensal fungus that is able to assume several morphologies, including yeast, hyphal, and pseudohyphal. Under a range of conditions, C. albicans performs a regulated switch to the filamentous morphology, characterized by the emergence of a germ tube from the yeast cell, followed by a mold-like growth of branching hyphae. This transition from yeast to hyphal growth has attracted particular attention, as it has been linked to the virulence of C. albicans as an opportunistic human pathogen. Signal transduction pathways that mediate the induction of the hyphal transcription program upon the imposition of external stimuli have been extensively investigated. However, the hyphal morphogenesis transcription program can also be induced by internal cellular signals, such as inhibition of cell cycle progression, and conversely, the inhibition of hyphal extension can repress hyphal-specific gene expression, suggesting that endogenous cellular signals are able to modulate hyphal gene expression as well. Here we review recent developments in the regulation of the hyphal morphogenesis of C. albicans, with emphasis on endogenous morphogenetic signals.