Cytoskeletal and Ca2+ regulation of hyphal tip growth and initiation

Recent advances in the development of Aspergillus for protein production

Aspergillus had been widely used in the industrial production of recombinant proteins. In addition to the safety and broad substrate utilization spectrum, its efficient post-translational modification and strong protein secretion capacity have significant advantages for developing an excellent protein-producing cell factory in industrial production. However, the difficulties in genetic manipulation of Aspergillus and varying expression levels of different heterologous proteins hampered its further development and application. Recently, the development of CRISPR genome editing and high-throughput screening platforms has facilitated the Aspergillus development of a wide range of modifications and applications. Meanwhile, multi-omics analysis and multiplexed genetic engineering have promoted effective knowledge mining. This paper provides a comprehensive and updated review of these advances, including high-throughput screening, genome editing, protein expression modules, and fermentation optimization. It also highlights and discusses the latest significant progress, aiming to provide a practical guide for implementing Aspergillus as an efficient protein-producing cell factory.

SEM and qRT-PCR revealed quercetin inhibits morphogenesis of Aspergillus flavus conidia via modulating calcineurin-Crz1 signalling pathway

ASPERGILLUS FLAVUS

exploits diverse mechanisms to survive during exposure to antifungal agents including morphogenesis. Germination of dormant conidia involves cascades of reactions integrated into the signalling pathway. This study documents the effect of phytochemical-quercetin on A. flavus during germination of conidia using scanning electron microscopy (SEM). Significant inhibition of conidial swelling of A. flavus in comparison to control was observed at 4 and 7 h Quantitative real-time PCR for genes from calcium signalling pathway and heat-shock proteins family showed up-regulation of heat shock (Hsp70 and Hsp90) and calcium signalling pathway genes (calcium-transporting ATPase and calmodulin) in response to quercetin at initial 4 h in comparison to control sample whereas up-regulation of Hsp70, calcineurin and transcription factor Crz1, were observed in both the treated samples. Gene encoding for calcium-kinase, cAMP, Rho-gdp, Plc and Pkc showed a constitutively higher level of expression in quercetin-treated sample in comparison to control at both time points. These data showed a clear response from genes encoding calcineurin-Crz1 signalling pathways and may find its application in the screening of antifungal agents.

ABBREVIATIONS

Hsp: Hear shock protein; MIC: Minimum Inhibitory Concentration; SEM: Scanning Electron Microscopy; qRT-PCR: Quantitative Real-Time Polymerase Chain Reaction.

Comparative proteomic analysis reveals the regulatory network of the veA gene during asexual and sexual spore development of Aspergillus cristatus

Aspergillus cristatus is the predominant fungal population during fermentation of Chinese Fuzhuan brick tea, and belongs to the homothallic fungal group that undergoes a sexual stage without asexual conidiation under hypotonic conditions, while hypertonic medium induces initiation of the asexual stage and completely blocks sexual development. However, the veA deletion mutant only produces conidia in hypotonic medium after a 24-h culture, but both asexual and sexual spores are observed after 72 h. The veA gene is one of the key genes that positively regulates sexual and negatively regulates asexual development in A. cristatus. To elucidate the molecular mechanism of how VeA regulates asexual and sexual spore development in A. cristatus, 2D electrophoresis (2-DE) combined with MALDI-tandem ToF MS analysis were applied to identify 173 differentially expressed proteins (DEPs) by comparing the agamotype (24 h) and teleomorph (72 h) with wild-type (WT) A. cristatus strains. Further analysis revealed that the changed expression pattern of Pmk1-MAPK and Ser/Thr phosphatase signaling, heat shock protein (Hsp) 90 (HSP90), protein degradation associated, sulphur-containing amino acid biosynthesis associated, valine, leucine, isoleucine, and arginine biosynthesis involved, CYP450 and cytoskeletal formation associated proteins were involved in the production of conidia in agamotype of A. cristatus. Furthermore, the deletion of veA in A. cristatus resulted in disturbed process of transcription, translation, protein folding, amino acid metabolism, and secondary metabolism. The carbohydrate and energy metabolism were also greatly changed, which lied in the suppression of anabolism through pentose phosphate pathway (PPP) but promotion of catabolism through glycolysis and tricarboxylic acid (TCA) cycle. The energy compounds produced in the agamotype were mainly ATP and NADH, whereas they were NADPH and FAD in the teleomorph. These results will contribute to the existing knowledge on the complex role of VeA in the regulation of spore development in Aspergillus and provide a framework for functional investigations on the identified proteins.

SERCA directs cell migration and branching across species and germ layers

Branching morphogenesis underlies organogenesis in vertebrates and invertebrates, yet is incompletely understood. Here, we show that the sarco-endoplasmic reticulum Ca2+ reuptake pump (SERCA) directs budding across germ layers and species. Clonal knockdown demonstrated a cell-autonomous role for SERCA in Drosophila air sac budding. Live imaging of Drosophila tracheogenesis revealed elevated Ca2+ levels in migratory tip cells as they form branches. SERCA blockade abolished this Ca2+ differential, aborting both cell migration and new branching. Activating protein kinase C (PKC) rescued Ca2+ in tip cells and restored cell migration and branching. Likewise, inhibiting SERCA abolished mammalian epithelial budding, PKC activation rescued budding, while morphogens did not. Mesoderm (zebrafish angiogenesis) and ectoderm (Drosophila nervous system) behaved similarly, suggesting a conserved requirement for cell-autonomous Ca2+ signaling, established by SERCA, in iterative budding.

The phenotype of a phospholipase C (plc-1) mutant in a filamentous fungus, Neurospora crassa

In the filamentous fungus Neurospora crassa, phospholipase C may play a role in hyphal extension at the growing tips as part of a growth-sensing mechanism that activates calcium release from internal stores to mediate continued expansion of the hyphal tip. One candidate for a tip-localized phospholipase C is PLC-1. We characterized morphology and growth characteristics of a knockout mutant (KO plc-1) and a RIP mutated strain (RIP plc-1) (missense mutations and a nonsense mutation render the gene product non-functional). Growth and hyphal cytology of wildtype and KO plc-1 were similar, but the RIP plc-1 mutant grew slower and exhibited abnormal membrane structures at the hyphal tip, imaged using the fluorescence dye FM4-64. To test for causes of the slower growth of the RIP plc-1 mutant, we examined its physiological poise compared to wildtype and the KO plc-1 mutant. The electrical properties of all three strains and the electrogenic contribution of the plasma membrane H(+)-ATPase (identified by cyanide inhibition) were the same. Responses to high osmolarity were also similar. However, the RIP plc-1 mutant had a significantly lower turgor, a possible cause of its slower growth. While growth of all three strains was inhibited by the phospholipase C inhibitor 3-nitrocoumarin, the RIP plc-1 mutant did not exhibit hyphal bursting after addition of the inhibitor, observed in both wildtype and the KO plc-1 mutant. Although the plc-1 gene is not obligatory for tip growth, the phenotype of the RIP plc-1 mutant - abnormal tip cytology, lower turgor and resistance to inhibitor-induced hyphal bursting - suggest it does play a role in tip growth. The expression of a dysfunctional plc-1 gene may cause a shift to alternative mechanism(s) of growth sensing in hyphal extension.

Structural and functional organization of growing tips of Neurospora crassa Hyphae

Data are presented on a variety of intracellular structures of the vegetative hyphae of the filamentous fungus Neurospora crassa and the involvement of these structures in the tip growth of the hyphae. Current ideas on the molecular and genetic mechanisms of tip growth and regulation of this process are considered. On the basis of comparison of data on behaviors of mitochondria and microtubules and data on the electrical heterogeneity of the hyphal apex, a hypothesis is proposed about a possible supervisory role of the longitudinal electric field in the structural and functional organization of growing tips of the N. crassa hyphae.

Role of the zinc finger transcription factor SltA in morphogenesis and sterigmatocystin biosynthesis in the fungus Aspergillus nidulans

Potassium, a widely accepted macronutrient, is vital for many physiological processes such as regulation of cell volume, maintenance of intracellular pH, synthesis of proteins and activation of enzymes in filamentous fungi. Another cation, calcium, plays an essential role in many signaling processes from lower to higher eukaryotes. Imbalance in the intracellular ionic levels of potassium or calcium causes adverse effects on cell growth, morphology and development, and eventually death. Previous studies on the adaptation of Aspergillus nidulans to salt and osmotic stress conditions have revealed the role of SltA, a C₂H₂ zinc finger transcription factor in cation homeostasis. SltA is highly conserved in the Ascomycota phylum with no identifiable homolog in S. cerevisiae and other yeast-like fungi, and prevents toxicity by the cations Na⁺, K⁺, Li⁺, Cs⁺ and Mg²⁺, but not by Ca²⁺. However its role in morphology and biosynthesis of natural products such as mycotoxins remained unknown. This study shows the first characterization of the role of calcium and SltA fungal homologs in morphogenesis using the model system A. nidulans. Addition of potassium to sltA deletion mutants resulted in decreased levels of sterigmatocystin production. A similar phenotype was observed for both types of mutants in veA1 and veA⁺ genetic background. Expression of the sterigmatocystin genes aflR and stcU was strongly reduced in sltA deletion mutant when K⁺ was added. Additionally, increased concentrations of K⁺ drastically reduced sexual and asexual development, as well as radial growth in deletion sltA colonies. This reduction was accompanied by lower expression of the morphology related genes nsdD, steA and brlA. Interestingly, addition of calcium was able to stimulate asexual and sexual development and remediate the deletion sltA phenotype, including defects in morphology and toxin production.

The social network: deciphering fungal language

It has been estimated that up to one quarter of the world's biomass is of fungal origin, comprising approximately 1.5 million species. In order to interact with one another and respond to environmental cues, fungi communicate with their own chemical languages using a sophisticated series of extracellular signals and cellular responses. A new appreciation for the linkage between these chemical languages and developmental processes in fungi has renewed interest in these signalling molecules, which can now be studied using post-genomic resources. In this Review, we focus on the molecules that are secreted by the largest phylum of fungi, the Ascomycota, and the quest to understand their biological function.

Role of four calcium transport proteins, encoded by nca-1, nca-2, nca-3, and cax, in maintaining intracellular calcium levels in Neurospora crassa

We have examined the distribution of calcium in Neurospora crassa and investigated the role of four predicted calcium transport proteins. The results of cell fractionation experiments showed 4% of cellular calcium in mitochondria, approximately 11% in a dense vacuolar fraction, 40% in an insoluble form that copurifies with microsomes, and 40% in a high-speed supernatant, presumably from large vacuoles that had broken. Strains lacking NCA-1, a SERCA-type Ca(2+)-ATPase, or NCA-3, a PMC-type Ca(2+)-ATPase, had no obvious defects in growth or distribution of calcium. A strain lacking NCA-2, which is also a PMC-type Ca(2+)-ATPase, grew slowly in normal medium and was unable to grow in high concentrations of calcium tolerated by the wild type. Furthermore, when grown in normal concentrations of calcium (0.68 mM), this strain accumulated 4- to 10-fold more calcium than other strains, elevated in all cell fractions. The data suggest that NCA-2 functions in the plasma membrane to pump calcium out of the cell. In this way, it resembles the PMC-type enzymes of animal cells, not the Pmc1p enzyme in Saccharomyces cerevisiae that resides in the vacuole. Strains lacking the cax gene, which encodes a Ca(2+)/H(+) exchange protein in vacuolar membranes, accumulate very little calcium in the dense vacuolar fraction but have normal levels of calcium in other fractions. The cax knockout strain has no other observable phenotypes. These data suggest that "the vacuole" is heterogeneous and that the dense vacuolar fraction contains an organelle that is dependent upon the CAX transporter for accumulation of calcium, while other components of the vacuolar system have multiple calcium transporters.

Genes encoding a striatin-like protein (ham-3) and a forkhead associated protein (ham-4) are required for hyphal fusion in Neurospora crassa

Cell-cell fusion during fertilization and between somatic cells is an integral process in eukaryotic development. In Neurospora crassa, the hyphal anastomosis mutant, ham-2, fails to undergo somatic fusion. In both humans and Saccharomyces cerevisiae, homologs of ham-2 are found in protein complexes that include homologs to a striatin-like protein and a forkhead-associated (FHA) protein. We identified a striatin (ham-3) gene and a FHA domain (ham-4) gene in N. crassa; strains containing mutations in ham-3 and ham-4 show severe somatic fusion defects. However, ham-3 and ham-4 mutants undergo mating-cell fusion, indicating functional differences in somatic versus sexual fusion events. The ham-2 and ham-3 mutants are female sterile, while ham-4 mutants are fertile. Homozygous crosses of ham-2, ham-3 and ham-4 mutants show aberrant meiosis and abnormally shaped ascospores. These data indicate that, similar to humans, the HAM proteins may form different signaling complexes that are important during both vegetative and sexual development in N. crassa.

Deletion of Mid1, a putative stretch-activated calcium channel in Claviceps purpurea, affects vegetative growth, cell wall synthesis and virulence

The putative Claviceps purpurea homologue of the Saccharomyces cerevisiae stretch-activated calcium ion channel Mid1 was investigated for its role in vegetative growth, differentiation and pathogenicity on rye (Secale cereale). Gene replacement mutants of Cl. purpurea mid1 were not affected in polar growth and branching in axenic culture but showed a significantly reduced growth rate. The growth defect could not be complemented by Ca2+ supplementation, in contrast to mid1 mutants in yeast, but the altered sensitivity of the mutants to changes in external and internal Ca2+ concentrations indicates some role of Mid1 in Ca2+ homeostasis. The major effect of mid1 deletion, however, was the complete loss of virulence: infected rye plants showed no disease symptoms at all. Detailed analyses of in vitro-infected rye ovaries demonstrated that the Δmid1 mutants had multiple apical branches and were unable to infect the host tissue, suggesting that Mid1 is essential for generating the necessary mechanical force for penetration. This is believed to be the first report of an essential role for a Mid1 homologue in the virulence of a plant-pathogenic fungus.

Structure and distribution of organelles and cellular location of calcium transporters in Neurospora crassa

We wanted to examine the cellular locations of four Neurospora crassa proteins that transport calcium. However, the structure and distribution of organelles in live hyphae of N. crassa have not been comprehensively described. Therefore, we made recombinant genes that generate translational fusions of putative organellar marker proteins with green or red fluorescent protein. We observed putative endoplasmic reticulum proteins, encoded by grp-78 and dpm, in the nuclear envelope and associated membranes. Proteins of the vacuolar membrane, encoded by vam-3 and vma-1, were in an interconnected network of small tubules and vesicles near the hyphal tip, while in more distal regions they were in large and small spherical vacuoles. Mitochondria, visualized with tagged ARG-4, were abundant in all regions of the hyphae. Similarly, we tagged the four N. crassa proteins that transport calcium with green or red fluorescent protein to examine their cellular locations. NCA-1 protein, a homolog of the SERCA-type Ca(2+)-ATPase of animal cells, colocalized with the endoplasmic reticulum markers. The NCA-2 and NCA-3 proteins are homologs of Ca(2+)-ATPases in the vacuolar membrane in yeast or in the plasma membrane in animal cells. They colocalized with markers in the vacuolar membrane, and they also occurred in the plasma membrane in regions of the hyphae more than 1 mm from the tip. The cax gene encodes a Ca(2+)/H(+) exchange protein found in vacuoles. As expected, the CAX protein localized to the vacuolar compartment. We observed, approximately 50 to 100 mum from the tip, a few spherical organelles that had high amounts of tagged CAX protein and tagged subunits of the vacuolar ATPase (VMA-1 and VMA-5). We suggest that this organelle, not described previously in N. crassa, may have a role in sequestering calcium.

Reconstruction of signaling networks regulating fungal morphogenesis by transcriptomics

Coordinated control of hyphal elongation and branching is essential for sustaining mycelial growth of filamentous fungi. In order to study the molecular machinery ensuring polarity control in the industrial fungus Aspergillus niger, we took advantage of the temperature-sensitive (ts) apical-branching ramosa-1 mutant. We show here that this strain serves as an excellent model system to study critical steps of polar growth control during mycelial development and report for the first time a transcriptomic fingerprint of apical branching for a filamentous fungus. This fingerprint indicates that several signal transduction pathways, including TORC2, phospholipid, calcium, and cell wall integrity signaling, concertedly act to control apical branching. We furthermore identified the genetic locus affected in the ramosa-1 mutant by complementation of the ts phenotype. Sequence analyses demonstrated that a single amino acid exchange in the RmsA protein is responsible for induced apical branching of the ramosa-1 mutant. Deletion experiments showed that the corresponding rmsA gene is essential for the growth of A. niger, and complementation analyses with Saccharomyces cerevisiae evidenced that RmsA serves as a functional equivalent of the TORC2 component Avo1p. TORC2 signaling is required for actin polarization and cell wall integrity in S. cerevisiae. Congruently, our microscopic investigations showed that polarized actin organization and chitin deposition are disturbed in the ramosa-1 mutant. The integration of the transcriptomic, genetic, and phenotypic data obtained in this study allowed us to reconstruct a model for cellular events involved in apical branching.

The polarisome component SpaA localises to hyphal tips of Aspergillus niger and is important for polar growth

Hyphal tip growth is a key feature of filamentous fungi, however, the molecular mechanism(s) that regulate cell polarity are poorly understood. On the other hand, much more is known about polarised growth in the yeast Saccharomyces cerevisiae. Here, the proteins Spa2p, Bni1p, Bud6p and Pea2p form a protein complex named the polarisome known to be important for the assurance of polar growth. We searched the genome of Aspergillus niger and identified homologues for Spa2p, Bni1p, Bud6p but not for Pea2p. We characterised the function of the Spa2p homologue SpaA by determining its cellular localisation and by constructing deletion and overexpressing mutant strains. SpaA was found to be localised exclusively at the hyphal tip, suggesting that SpaA can be used as marker for polarisation. Deletion and overexpression of spaA resulted in reduced growth rate, increased hyphal diameter and polarity defects, indicating that one of the functions of SpaA is to ensure polarity maintenance. In addition, we could show that SpaA is able to complement the defective haploid invasive growth phenotype of a S. cerevisiae SPA2 null mutant. We suggest that the function of SpaA is to ensure maximal polar growth rate in A. niger.

Cytoskeletal elements and intracellular transport

Recent advances in the understanding of the functions of various components of the cytoskeleton indicate that, besides serving a structural role, the cytoskeletal elements may regulate the transport of several proteins in the cell. Studies reveal that there are co-operative interactions between the actin and microtubule cytoskeletons including functional overlap in the transport influenced by different motor families. Multiple motors are probably involved in the control of the dynamics of many proteins and intriguing hints about how these motors are co-ordinated are appearing. It has been shown that some of the intermediate elements also participate in selected intracellular transport mechanisms. In view of the author's preoccupation with the steroid receptor systems, special attention has been given to the role of the cytoskeletal elements, particularly actin, in the intracellular transport of steroid receptors and receptor-related proteins.

Aspergillus nidulans class V and VI chitin synthases CsmA and CsmB, each with a myosin motor-like domain, perform compensatory functions that are essential for hyphal tip growth

The polarized synthesis of cell wall components such as chitin is essential for the hyphal tip growth of filamentous fungi. The actin cytoskeleton is known to play important roles in the determination of hyphal polarity in Aspergillus nidulans. Previously, we suggested that CsmA, a chitin synthase with a myosin motor-like domain (MMD), was involved in polarized chitin synthesis in a manner dependent on the interaction between the MMD and the actin cytoskeleton. The genome database indicates that A. nidulans possesses another gene encoding another chitin synthase with an MMD. In this study, we characterized this gene, which we designated csmB. The csmB null mutants examined were viable, although they exhibited defective phenotypes, including the formation of balloons and intrahyphal hyphae and the lysis of subapical regions, which were similar to those obtained with csmA null mutants. Moreover, csmA csmB double null mutants were not viable. Mutants in which csmB was deleted and the expression of csmA was under the control of the alcA promoter were viable but severely impaired in terms of hyphal growth under alcA-repressing conditions. We revealed that CsmB with three copies of a FLAG epitope tag localized at the hyphal tips and forming septa, and that the MMD of CsmB was able to bind to actin filaments in vitro. These results suggest that CsmA and CsmB perform compensatory functions that are essential for hyphal tip growth.

The role of tip-localized mitochondria in hyphal growth

Hyphal tip-growing organisms have a high density of tip-localized mitochondria which maintain a membrane potential based on Rhodamine 123 fluorescence, but do not produce ATP based on the absence of significant oxygen consumption. Two possible roles of these mitochondria in tip growth were examined: Calcium sequestration and biogenesis, because tip-high cytoplasmic calcium gradients are a common feature of tip-growing organisms, and the volume expansion as the tip extends would require a continuous supply of additional mitochondria. Co-localization of calcium-sensitive fluorescent dye and mitochondria-specific fluorescent dyes showed that the tip-localized mitochondria do contain calcium, and therefore, may function in calcium clearance from the cytoplasm. Short-term inhibition of DNA synthesis or mitochondrial protein synthesis did not affect either tip growth, or mitochondrial shape or distribution. Therefore, mitochondrial biogenesis may not occur from the tip-localized mitochondria in hyphal organisms.

Molecules into cells: specifying spatial architecture

A living cell is not an aggregate of molecules but an organized pattern, structured in space and in time. This article addresses some conceptual issues in the genesis of spatial architecture, including how molecules find their proper location in cell space, the origins of supramolecular order, the role of the genes, cell morphology, the continuity of cells, and the inheritance of order. The discussion is framed around a hierarchy of physiological processes that bridge the gap between nanometer-sized molecules and cells three to six orders of magnitude larger. Stepping stones include molecular self-organization, directional physiology, spatial markers, gradients, fields, and physical forces. The knowledge at hand leads to an unconventional interpretation of biological order. I have come to think of cells as self-organized systems composed of genetically specified elements plus heritable structures. The smallest self that can be fairly said to organize itself is the whole cell. If structure, form, and function are ever to be computed from data at a lower level, the starting point will be not the genome, but a spatially organized system of molecules. This conclusion invites us to reconsider our understanding of what genes do, what organisms are, and how living systems could have arisen on the early Earth.

Surface ultrastructure and elasticity in growing tips and mature regions of Aspergillus hyphae describe wall maturation

This study reports the first direct, high-resolution physical and structural evidence of wall changes during hyphal tip growth, visualized by atomic force microscopy (AFM) inAspergillus nidulans. Images from AFM and cryo-scanning electron microscopy provided comparable information, but AFM was also able to image and physically probe living cells. AFM images showed changes in the surface ultrastructure ofA. nidulanshyphae, from newly deposited walls at hyphal tips to fully mature walls, as well as additional changes at young branches arising from mature walls. Surface architecture during wall maturation correlated with changes in the relative viscoelasticity (compliance per unit applied force) of walls measured by force spectroscopy (FS) in growingA. nidulanshyphae. Growing tips showed greater viscoelasticity than mature walls, despite equal support from turgor. Branch tips had comparable viscoelasticity to hyphal tips, unlike the mature wall from which they grew. FS also revealed differences in surface hydrophilicity between newly deposited and mature walls, with the tips being more hydrophilic. The hydrophilicity of young branch tips was similar to that of hyphal tips, and different from that of mature walls. Taken together, AFM images and FS data suggest that theA. nidulanswall matures following deposition at the hyphal tip.

Calcineurin-binding protein Cbp1 directs the specificity of calcineurin-dependent hyphal elongation during mating in Cryptococcus neoformans

Mating and virulence of the human fungal pathogen Cryptococcus neoformans are controlled by calcineurin, a serine-threonine-specific calcium-activated phosphatase that is the target of the immunosuppressive drugs cyclosporine A and FK506. In previous studies, a calcineurin binding protein (Cbp1, Rcn1, Dscr1/Csp1-3/MCIP1-3) that is conserved from yeasts to humans has been identified, but whether this protein functions to regulate calcineurin activity or facilitate calcineurin function as a signaling effector has been unclear. Here we show that, like calcineurin, Cbp1 is required for mating in C. neoformans. By contrast, Cbp1 plays no role in promoting calcineurin-dependent growth at 37 degrees C and is not essential for haploid fruiting. Site-directed mutagenesis studies provide evidence that tandem phosphorylation and dephosphorylation of two serine residues in the conserved SP repeat motif are critical for Cbp1 function. Epistasis analysis supports models in which Cbp1 functions coordinately with calcineurin to direct hyphal elongation during mating. Taken together, these findings provide insights into the roles of Cbp1 as an accessory subunit or effector of calcineurin-specific signaling pathways, which may be features conserved among the calcipressins to govern calcineurin signaling in immune cells, cardiomyocytes, and neurons of multicellular eukaryotes.

The dynamic behaviour of microtubules and their contributions to hyphal tip growth in Aspergillus nidulans

Creating and maintaining cell polarity are complex processes that are not fully understood. Fungal hyphal tip growth is a highly polarized and dynamic process involving both F-actin and microtubules (MTs), but the behaviour and roles of the latter are unclear. To address this issue, MT dynamics and subunit distribution were analysed in a strain of Aspergillus nidulans expressing GFP-alpha-tubulin. Apical MTs are the most dynamic, the bulk of which move tipwards from multiple subapical spindle pole bodies, the only clear region of microtubule nucleation detected. MTs populate the apex predominantly by elongation at rates about three times faster than tip extension. This polymerization was facilitated by the tipward migration of MT subunits, which generated a tip-high gradient. Subapical regions of apical cells showed variable tubulin subunit distributions, without tipward flow, while subapical cells showed even tubulin subunit distribution and low MT dynamics. Short MTs, of a similar size to those reported in axons, also occasionally slid into the apex. During mitosis in apical cells, MT populations at the tip varied. Cells with less distance between the tip and the first nucleus were more likely to loose normal MT populations and dynamics. Reduced MTs in the tip, during mitosis or after exposure to the MT inhibitor carbendazim (MBC), generally correlated with reduced, but continuing growth and near-normal tip morphology. In contrast, the actin-disrupting agent latrunculin B reduced growth rates much more severely and dramatically distorted tip morphology. These results suggest substantial independence between MTs and hyphal tip growth and a more essential role for F-actin. Among MT-dependent processes possibly contributing to tip growth is the transportation of vesicles. However, preliminary ultrastructural data indicated a lack of direct MT-organelle interactions. It is suggested that the population of dynamic apical MTs enhance migration of the 'cytomatrix', thus ensuring that organelles and proteins maintain proximity to the constantly elongating tip.

Cell cycle arrest during S or M phase generates polarized growth via distinct signals in Candida albicans

Treatments that perturb DNA synthesis or mitosis will activate checkpoints that prevent cell cycle progression and cell proliferation. In yeast-form cells of the fungal pathogen Candida albicans, exposure to hydroxyurea (HU) or shutting off expression of the polo-like kinase CaCDC5 blocked nuclear division and spindle elongation, but activated a highly polarized growth mode. We have used transcription profiling both to characterize the initiation and progression of this polar growth pattern and to determine how cell elongation may be linked to the cell cycle in C. albicans. Different gene expression patterns during early stages of cell elongation support the concept that CaCdc5p-depleted and HU-exposed cells were blocked at different stages of the cell cycle, and suggest that different signals may generate the common polarized growth phenotype. Consistent with this, BUB2 expression was modulated in CaCdc5p-depleted cells, and absence of BUB2 prevented the maintenance of cell polarization, resulting in multibudded, pseudohyphal cells with constrictions. In contrast, HU-induced filaments did not modulate or require BUB2, but were dependent on the GTPase Ras1p. However, at later stages of cell elongation, transcription profiles were more similar, and comparisons with serum-induced hyphae revealed that the cell cycle-arrested filaments expressed several targets of the hyphal signalling pathways. Thus, arresting the yeast cell cycle in S or M phase generates a polarized growth pattern through different mechanisms in C. albicans, and maintenance of the polar growth mode can ultimately lead to the expression of hyphal-associated cell wall and virulence-related factors, in the absence of any external stimuli.

Hyperbranching induced by cold-shock orsnow-flake mutation inNeurospora crassa is prevented by addition of exogenous calcium

Hyphal tip growth is a highly polarized process of cell extension, which may be affected by chemical and physical stress. Neurospora crassa exposed to cold-shock lost its polarized growth and dichotomous branches were detected. These effects were not observed in the presence of 500 mM Ca2+. We compared here the morphological pattern of a snow-flake mutant (sn) and the wild-type (wt) exposed to 4 degrees C. Hyphal morphology, nuclei, actin and microtubule distribution were analyzed. No effects on sn hyphal morphology were detected at 4 degrees C. Exogenous Ca2+ converted sn to an essentially wt appearance. The results presented here suggest that sn mutation and cold-shock treatment have affected Ca2+ influx since addition of this cation to sn (30 degrees C) and to wt (4 degrees C) maintained polarized growth and normal nuclear and microtubules distribution.

CsmA, a class V chitin synthase with a myosin motor-like domain, is localized through direct interaction with the actin cytoskeleton in Aspergillus nidulans

One of the essential features of fungal morphogenesis is the polarized synthesis of cell wall components such as chitin. The actin cytoskeleton provides the structural basis for cell polarity in Aspergillus nidulans, as well as in most other eukaryotes. A class V chitin synthase, CsmA, which contains a myosin motor-like domain (MMD), is conserved among most filamentous fungi. The DeltacsmA null mutant showed remarkable abnormalities with respect to cell wall integrity and the establishment of polarity. In this study, we demonstrated that CsmA tagged with 9x HA epitopes localized near actin structures at the hyphal tips and septation sites and that its MMD was able to bind to actin. Characterization of mutants bearing a point mutation or deletion in the MMD suggests that the interaction between the MMD and actin is not only necessary for the proper localization of CsmA, but also for CsmA function. Thus, the finding of a direct interaction between the chitin synthase and the actin cytoskeleton provides new insight into the mechanisms of polarized cell wall synthesis and fungal morphogenesis.

Identification and complementation of abnormal hyphal branch mutants ahbA1 and ahbB1 in Aspergillus nidulans

Branching generates new axes of polar growth in filamentous fungi and is critical for development, reproduction, and pathogenicity. To investigate branching we screened an Aspergillus nidulans temperature-sensitive mutant collection for abnormal hyphal branch (ahb) mutants. We identified two mutants, ahbA1, which showed reduced branching relative to wild type at restrictive temperature, and ahbB1, which showed increased branching relative to wild type at restrictive temperature. Both mutants also showed abnormal conidiophore development at restrictive temperature. The ahbA1 hypobranching mutant showed defects in nuclear division and hydroxyurea resistance. Complementation and sequencing showed that ahbA1 is a previously identified allele of the cell cycle regulator nimX. The ahbB1 hyperbranching mutant had an increased number of nuclei, was osmotically remedial and Calcofluor resistant. The ahbB gene is predicted to encode a novel protein that has homologues exclusively in filamentous fungi. The C-terminal domain of the predicted AhbB protein showed homology with the heme-binding domain of a cytochrome P450 protein and sequencing of the ahbB1 mutant allele showed that the lesion lies just before this putative heme-binding domain. The ahbB1 mutant showed increased sensitivity to the ergosterol biosynthesis inhibitor imidazole. Our results suggest a link between nuclear division and branching and a possible role for membrane synthesis in branching.

Possible role of ionic gradients in the apical growth of Neurospora crassa

The effects of the Ca2+/H+ exchanger A23187 and the K+/H+ exchanger nigericin on the growth of Neurospora crassa were analyzed. Both ionophores had the same effects on the fungus. They both inhibited growth in liquid media, apical extension being more affected than protein synthesis. A sudden challenge to either ionophore on solid media rapidly stopped hyphal extension. Additionally, both ionophores induced profuse mycelium branching and upward hyphal growth. Hyphae growing on nigericin-containing media also burst at the apex. Both ionophores caused a rapid inhibition in the apically-occurring synthesis of structural wall polysaccharides, but they did not affect mitochondrial energy conservation. With the use of DiBAC, a membrane-potential sensitive fluorophore, it was excluded that their effects were due to depletion of the plasma membrane potential. Considering that both ionophores exchange H+ for different metallic ions, we concluded that their effect was due to dissipation of a proton gradient, which is directly or indirectly involved in the apical growth of the fungus.

Calcium gradient dependence of Neurospora crassa hyphal growth

A tip-high cytoplasmic calcium gradient has been identified as a requirement for hyphal growth in the fungus Neurospora crassa. The Ca2+ gradient is less steep compared to wall vesicle, wall incorporation and vesicular Ca2+ gradients, but this can be explained by Ca2+ diffusion. Analysis of the relation between the rate of hyphal growth and the spatial distribution of tip-localized calcium indicates that hyphal growth rates depend upon the tip-localized calcium concentration. It is not the steepness of the calcium gradient, but tip-localized calcium and the difference in tip-localized calcium versus subapical calcium concentration which correlate closely with hyphal growth rate. A minimal concentration difference between the apex and subapical region of 30 nM is required for growth to occur. The calcium concentration dependence of growth may relate directly to biochemical functions of calcium in hyphal extension, such as vesicle fusion and enzyme activation during cellular expansion. Initiation of tip growth may rely upon random Ca2+ motions causing localized regions of elevated calcium. Continued hyphal expansion may activate a stretch-activated phospholipase C which would increase tip-localized inositol 1,4,5-trisphosphate (IP3). Hyphal expansion, induced by mild hypoosmotic treatment, does increase diacylglycerol, the other product of phospholipase C activity. This is consistent with evidence that IP3-activated Ca2+ channels generate and maintain the tip-high calcium gradient.

Depletion of a polo-like kinase in Candida albicans activates cyclase-dependent hyphal-like growth

Morphogenesis in the fungal pathogen Candida albicans is an important virulence-determining factor, as a dimorphic switch between yeast and hyphal growth forms can increase pathogenesis. We identified CaCDC5, a cell cycle regulatory polo-like kinase (PLK) in C. albicans and demonstrate that shutting off its expression induced cell cycle defects and dramatic changes in morphology. Cells lacking CaCdc5p were blocked early in nuclear division with very short spindles and unseparated chromatin. GFP-tagged CaCdc5p localized to unseparated spindle pole bodies, the spindle, and chromatin, consistent with a role in spindle elongation at an earlier point in the cell cycle than that described for the homologue Cdc5p in yeast. Strikingly, the cell cycle defects were accompanied by the formation of hyphal-like filaments under yeast growth conditions. Filament growth was determinate, as the filaments started to die after 24 h. The filaments resembled serum-induced hyphae with respect to morphology, organization of cytoplasmic microtubules, localization of nuclei, and expression of hyphal-specific components. Filament formation required CaCDC35, but not EFG1 or CPH1. Similar defects in spindle elongation and a corresponding induction of filaments occurred when yeast cells were exposed to hydroxyurea. Because CaCdc5p does not appear to act as a direct repressor of hyphal growth, the data suggest that a target of CaCdc5p function is associated with hyphal-like development. Thus, an internal, cell cycle-related cue can activate hyphal regulatory networks in Candida.

Spatial and cellular localization of calcium-dependent protease (CDP II) in Allomyces arbuscula

Immunogold labeling of calcium-dependent neutral protease II (CDPII) with specific antibodies in near median longitudinal ultrathin sections of Allomyces arbuscula showed that the enzyme is predominantly localized in the growing hyphal and rhizoidal apices. The tips in both cell type had more enzyme than the distal regions and showed a gradient distribution. Labeling of the ultrathin sections and western blot analysis of purified subcellular fractions showed that CDPII is mainly cytosolic. Catalytic activity of the enzyme measured with synthetic substrate (Bz-Arg-pNA) showed that 90% of its activity is present in the soluble fraction, although a small amount is associated with the nuclei (0.2%), plasma membranes (0.7%) and microsomes (3.9%). This association is discussed in the context of the functional role of the enzyme and its possible localized activation. Western blot analysis of the crude extract and indirect immunofluorescence of the fixed permeabilized hypahe after treatment with CDPII showed that the alpha-tubulin is a specific target of the enzyme.

Class V chitin synthase determines pathogenesis in the vascular wilt fungus Fusarium oxysporum and mediates resistance to plant defence compounds

Chitin, a beta-1,4-linked polysaccharide of N-acetylglucosamine, is a major structural component of fungal cell walls. Fungi have multiple classes of chitin synthases that catalyse N-acetylglucosamine polymerization. Here, we demonstrate the requirement for a class V chitin synthase during host infection by the vascular wilt pathogen Fusarium oxysporum. The chsV gene was identified in an insertional mutagenesis screen for pathogenicity mutants. ChsV has a putative myosin motor and a chitin synthase domain characteristic of class V chitin synthases. The chsV insertional mutant and a gene replacement mutant of F. oxysporum display morphological abnormalities such as hyphal swellings that are indicative of alterations in cell wall structure and can be partially restored by osmotic stabilizer. The mutants are unable to infect and colonize tomato plants or to grow invasively on tomato fruit tissue. They are also hypersensitive to plant antimicrobial defence compounds such as the tomato phytoanticipin alpha-tomatine or H2O2. Reintroduction of a functional chsV copy into the mutant restored the growth phenotype of the wild-type strain. These data suggest that F. oxysporum requires a specific class V chitin synthase for pathogenesis, most probably to protect itself against plant defence mechanisms.

Force and compliance: rethinking morphogenesis in walled cells

In the turgid cells of plants, protists, fungi, and bacteria, walls resist swelling; they also confer shape on the cell. These two functions are not unrelated: cell physiologists have generally agreed that morphogenesis turns on the deformation of existing wall and the deposition of new wall, while turgor pressure produces the work of expansion. In 1990, I summed up consensus in a phrase: "localized compliance with the global force of turgor pressure." My purpose here is to survey the impact of recent discoveries on the traditional conceptual framework. Topics include the recognition of a cytoskeleton in bacteria; the tide of information and insight about budding in yeast; the role of the Spitzenkörper in hyphal extension; calcium ions and actin dynamics in shaping a tip; and the interplay of protons, expansins and cellulose fibrils in cells of higher plants.

Good fungi gone bad: the corruption of calcineurin

Calcineurin is a Ca(2+)/calmodulin-activated protein phosphatase that is conserved in eukaryotes, from yeast to humans, and is the conserved target of the immunosuppressive drugs cyclosporin A (CsA) and FK506. Genetic studies in yeast and fungi established the molecular basis of calcineurin inhibition by the cyclophilin A-CsA and FKBP12-FK506 complexes. Calcineurin also functions in fungi to control a myriad of physiological processes including cell cycle progression, cation homeostasis, and morphogenesis. Recent investigations into the molecular mechanisms of pathogenesis in Candida albicans and Cryptococcus neoformans, two fungi that cause life-threatening infections in humans, have revealed an essential role for calcineurin in morphogenesis, virulence, and antifungal drug action. Novel non-immunosuppressive analogs of the calcineurin inhibitors CsA and FK506 that retain antifungal activity have been identified and hold promise as candidate antifungal drugs. In addition, comparisons of calcineurin function in both fungi and humans may identify fungal-specific components of calcineurin-signaling pathways that could be targeted for therapy, as well as conserved elements of calcium signaling events.

Ca(2+) shuttling in vesicles during tip growth in Neurospora crassa

Tip-growing organisms maintain an apparently essential tip-high gradient of cytoplasmic Ca(2+). In the oomycete Saprolegnia ferax, in pollen tubes and root hairs, the gradient is produced by a tip-localized Ca(2+) influx from the external medium. Such a gradient is normally dispensable for Neurospora crassa hyphae, which may maintain their Ca(2+) gradient by some form of internal recycling. We localized Ca(2+) in N. crassa hyphae at the ultrastructural level using two techniques (a) electron spectroscopic imaging of freeze-dried hyphae and (b) pyroantimoniate precipitation. The results of both methods support the presence of Ca(2+) in the wall vesicles and Golgi body equivalents, providing a plausible mechanism for the generation and maintenance of the gradient by Ca(2+) shuttling in vesicles to the apex, without exogenous Ca(2+) influx. Ca(2+) sequestration into the vesicles seems to be dependent on Ca(2+)-ATPases since cyclopiazonic acid, a specific inhibitor of Ca(2+) pumps, eliminated all Ca(2+) deposits from the vesicles of N. crassa.

Regulation of the tip-high [Ca2+] gradient in growing hyphae of the fungus Neurospora crassa

Previous work has shown that hyphal elongation in the fungus Neurospora crassa requires a tip-high cytosolic Ca2+ gradient. The source of the Ca2+ appears to be intracellular stores as there is no net transplasma membrane Ca2+ flux at the elongating hyphal tip and modification of ion fluxes across the plasma membrane using voltage clamp is without effect on tip growth. To decode the internal mechanisms which generate and maintain the tip-high Ca2+ gradient we first identified calcium regulators which affect hyphal growth and morphology, then determined how they modify cytosolic [Ca2+] and the actin cytoskeleton using fluorescent dyes and confocal microscopy. Cyclopiazonic acid (a known inhibitor of the endoplasmic reticulum calcium ATPase) inhibits growth and increases cytoplasmic [Ca2+] in the basal region 10-25 microm behind the hyphal tip. 2-APB (2-aminoethoxydiphenyl borate, an inhibitor of IP3-induced Ca2+ release) inhibits hyphal elongation and dissipates the tip-high Ca2 gradient 0-10 microm from the tip. Microinjections of the IP3 receptor agonists adenophostin A and IP3 (but not control microinjections of the biologically inactive L-IP3) transiently inhibited growth and induced subapical branches. IP3 microinjections, but not L-IP3, lowered tip-localized [Ca2+] and increased basal [Ca2+]. Even though their effect on [Ca2+] gradients was different, both cyclopiazonic acid and 2-APB disrupted similarly the normal actin pattern at the hyphal apex. Conversely, disruption of actin with latrunculin B dissipated tip-localized Ca2+. We conclude that the tip-high Ca2+ gradient is generated internally by Ca2+ sequestration into endoplasmic reticulum behind the tip and Ca2+ release via an IP3 receptor from tip-localized vesicles whose location is maintained by the actin cytoskeleton.