Loss of growth polarity and mislocalization of septa in a Neurospora mutant altered in the regulatory subunit of cAMP-dependent protein kinase

In filamentous fungi, growth polarity (i.e. hyphal extension) and formation of septa require polarized deposition of new cell wall material. To explore this process, we analyzed a conditional Neurospora crassa mutant, mcb, which showed a complete loss of growth polarity when incubated at the restrictive temperature. Cloning and DNA sequence analysis of the mcb gene revealed that it encodes a regulatory subunit of cAMP-dependent protein kinase (PKA). Unexpectedly, the mcb mutant still formed septa when grown at the restrictive temperature, indicating that polarized deposition of wall material during septation is a process that is, at least in part, independent of polarized deposition during hyphal tip extension. However, septa formed in the mcb mutant growing at the restrictive temperature are mislocalized. Both polarized growth and septation are actin-dependent processes, and a concentration of actin patches is observed at growing hyphal tips and sites where septa are being formed. In the mcb mutant growing at the restrictive temperature, actin patches are uniformly distributed over the cell cortex; however, actin patches are still concentrated at sites of septation. Our results suggest that the PKA pathway regulates hyphal growth polarity, possibly through organizing actin patches at the cell cortex.     

Importance of calcium to the regulation of polymorphism in Wangiella (Exophiala) dermatitidis

Critical steps implicated in the polymorphism of Wangiella dermatitidis were found to be sensitive to calcium ion availability. When grown in a defined, synthetic medium under various pH and temperature conditions, two thresholds of calcium ion concentrations were identified: a lower concentration favouring non-polarized growth leading to multicellular form development and a higher concentration promoting polarized growth characterized by yeast budding or pseudo/true hyphal growth. The phenotypic transition of yeasts to multicellular forms or to hyphae was induced at both 25 and 37 degrees C in the wild-type strain by the addition of calcium to the synthetic medium adjusted to pH 2.5, which was otherwise not conducive to the production of either growth form. However, the calcium additions did not allow maintenance of polarized growth of yeasts or hyphae in a temperature-sensitive, cell-division-cycle mutant (wdcdc2) derived from the same strain and grown at 37 degrees C in the same medium adjusted to either pH 2.5 or 6.5. Instead these conditions allowed only the nonpolarized, multicellular form development associated with this conditional mutant cultured in rich media at the 37 degree C restrictive temperature for yeast bud formation. Results from experiments using the calcium chelator EGTA added to the synthetic medium supported these conclusions at neutral pH with both the wild type and the wdcdc2 mutant cultured at 37 degrees C. The results suggested that during infection different concentrations of calcium may be encountered by W. dermatitidis in different tissues, which might directly regulate its growth and polymorphism and indirectly its virulence depending on host conditions.     

Structure, function, regulation: experimental analysis in groups of non-excitable cells coupled via permeable junctions

This presentation reviews the functional properties of different permeable junctions (microplasmodesmata of filamentous cyanobacteria, septal pores of fungi, and gap junctions of animals). Examples of the distribution of power by ionic fluxes and electric current through permeable cell-cell junctions are considered. Based on the analysis of intercellular communication occurring during apical growth of hyphae of wild-type and mutant lines of the mycelial fungus Neurospora crassa, approaches to studies of the molecular and genetic mechanisms of self-organization in systems of cells with permeable junctions are discussed.     

Virulence and hyphal formation of Candida albicans require the Ste20p-like protein kinase CaCla4p

BACKGROUND: The pathogenic fungus Candida albicans is capable of a morphological transition from a unicellular budding yeast to a filamentous form. Extensive filamentous growth leads to the formation of mycelia displaying hyphae with branches and lateral buds. Hyphae have been observed to adhere to and invade host tissues more readily than the yeast form, suggesting that filamentous growth may contribute to the virulence of this major human pathogen. A molecular and genetic understanding of the potential role of morphological switching in the pathogenicity of C. albicans would be of significant benefit in view of the increasing incidence of candidiasis. RESULTS: The CaCLA4 gene of C. albicans was cloned by functional complementation of the growth defect of cells of the budding yeast Saccharomyces cerevisiae deleted for the STE20 gene and the CLA4 gene. CaCLA4 encodes a member of the Ste20p family of serine/threonine protein kinases and is characterized by a pleckstrin homology domain and a Cdc42p-binding domain in its amino-terminal non-catalytic region. Deletion of both alleles of CaCLA4 in C. albicans caused defects in hyphal formation in vitro, in both synthetic liquid and solid media, and in vivo in a mouse model for systemic candidiasis. The gene deletions reduced colonization of the kidneys in infected mice and suppressed C. albicans virulence in the mouse model. CONCLUSIONS: Our results demonstrate that the function of the CaCla4p protein kinase is essential for virulence and morphological switching of C. albicans in a mouse model. Thus, hyphal formation of C. albicans mediated by CaCla4p may contribute to the pathogenicity of this dimorphic fungus, suggesting that regulators of morphological switching may be useful targets for antifungal drugs.     

Disruption of PMR1, encoding a Ca2+-ATPase homolog in Yarrowia lipolytica, affects secretion and processing of homologous and heterologous proteins

The Yarrowia lipolytica PMR1 gene (YlPMR1) is a Saccharomyces cerevisiae PMR1 homolog which encodes a putative secretory pathway Ca2+-ATPase. In this study, we investigated the effects of a YlPMR1 disruption on the processing and secretion of native and foreign proteins in Y. lipolytica and found variable responses by the YlPMR1-disrupted mutant depending on the protein. The secretion of 32-kDa mature alkaline extracellular protease (AEP) was dramatically decreased, and incompletely processed precursors were observed in the YlPMR1-disrupted mutant. A 36- and a 52-kDa premature AEP were secreted, and an intracellular 52-kDa premature AEP was also detected. The acid extracellular protease activity of the YlPMR1-disrupted mutant was increased by 60% compared to that of the wild-type strain. The inhibitory effect of mutations in secretory pathway Ca2+-ATPase genes on the secretion of rice alpha-amylase was also observed in the Y. lipolytica and S. cerevisiae PMR1-disrupted mutants. Unlike rice alpha-amylase, the secretion of Trichoderma reesei endoglucanase I (EGI) was not influenced by the YlPMR1 disruption. However, the secreted EGI from the YlPMR1-disrupted mutant had different characteristics than that of the control. While wild-type cells secreted the hyperglycosylated form of EGI, hyperglycosylation was completely absent in the YlPMR1-disrupted mutant. Our results indicate that the effects of the YlPMR1 disruption as manifested by the phenotypic response depend on the characteristics of the reporter protein in the recombinant yeast strain evaluated.     

Impairment of calcineurin function in Neurospora crassa reveals its essential role in hyphal growth, morphology and maintenance of the apical Ca2+ gradient

The function of Neurospora crassa calcineurin was investigated in N. crassa strains transformed with a construct that provides for the inducible expression of antisense RNA for the catalytic subunit of calcineurin (cna-1). Induction of antisense RNA expression was associated with reduced levels of cna-1 mRNA and of immunodetectable CNA1 protein and decreased calcineurin enzyme activity, indicating that a conditional reduction of the target function had been achieved in antisense transformants with multiple construct integrations. Induction conditions caused growth arrest which indicated that the cna-1 gene is essential for growth of N. crassa. Growth arrest was preceded by an increase in hyphal branching, changes in hyphal morphology and concomitant loss of the distinctive tip-high Ca2+ gradient typical for growing wild-type hyphae. This demonstrates a novel and specific role for calcineurin in the precise regulation of apical growth, a common form of cellular proliferation. In vitro inhibition of N. crassa calcineurin by the complex of cyclosporin A (CsA) and cyclophilin20, and increased sensitivity of the induced transformants to the calcineurin-specific drugs CsA and FK506 imply that the drugs act in N. crassa, as in T-cells and Saccharomyces cerevisiae, by inactivating calcineurin. The finding that exposure of growing wild-type mycelium to these drugs leads to a phenotype very similar to that of the cna-1 antisense mutants is consistent with this idea.     

Microtubule cytoskeleton in hyphal growth. Response to nocodazole in a sensitive and a tolerant strain of the homobasidiomycete Schizophyllum commune

In the wild-type strains of the homobasidiomycete Schizophyllum commune microtubules were totally depolymerized by low concentrations of nocodazole, while high concentrations of benomyl only modified the structure of microtubule cytoskeleton. In the nocodazole-tolerant mutant strain NT30 the microtubule cytoskeleton remained partly functional at a nocodazole concentration which demolished the microtubules in the wild-type strains. The continuation of apical growth for several hours in the wild-type strain without cytoplasmic microtubules indicated that microtubules are not the major elements in hyphal extension growth. However, the irregular branching of the treated apical cells both in the nocodazole-sensitive and -tolerant strain suggested that an intact microtubule cytoskeleton is needed for maintaining the direct extension of the leading hyphae at the colony edge. In the nocodazole-sensitive strain growth in the absence of polymerized microtubules frequently led to the death of the apical cells even when the drug was removed. In the tolerant strain the nuclear divisions continued in spite of nocodazole, but the uninucleate hyphal compartments became multinucleate. This probably resulted from poor segregation of nuclei and septation of hyphae at telophase, which indicated that these processes might be dependent on proper polymerization of cytoplasmic microtubules in higher fungi. The different electrophoretic mobility of the beta-tubulin from the NT30 strain and its parental strains suggested that the tolerance of the NT30 to nocodazole could be due to a mutation in a beta-tubulin encoding gene.     

Expression and secretion of defined cutinase variants by Aspergillus awamori

Several cutinase variants derived by molecular modelling and site-directed mutagenesis of a cutinase gene from Fusarium solani pisi are poorly secreted by Saccharomyces cerevisiae. The majority of these variants are successfully produced by the filamentous fungus Aspergillus awamori. However, the L51S and T179Y mutations caused reductions in the levels of extracellular production of two cutinase variants by A. awamori. Metabolic labelling studies were performed to analyze the bottleneck in enzyme production by the fungus in detail. These studies showed that because of the single L51S substitution, rapid extracellular degradation of cutinase occurred. The T179Y substitution did not result in enhanced sensitivity towards extracellular proteases. Presumably, the delay in the extracellular accumulation of this cutinase variant is caused by the enhanced hydrophobicity of the molecule. Overexpression of the A. awamori gene encoding the chaperone BiP in the cutinase-producing A. awamori strains had no significant effect on the secretion efficiency of the cutinases. A cutinase variant with the amino acid changes G28A, A85F, V184I, A185L, and L189F that was known to aggregate in the endoplasmic reticulum of S. cerevisiae, resulting in low extracellular protein levels, was successfully produced by A. awamori. An initial bottleneck in secretion occurred before or during translocation into the endoplasmic reticulum but was rapidly overcome by the fungus.     

Transmission of the effect of an antifungal agent within a single hypha

A micro-compartment culture method was devised in which a single hypha of Rhizopus stolonifer growing on an agar section traversed an antifungal non-diffusible barrier to another agar section; thus the local environment of the distal or proximal part of the hypha could be controlled independently. The responses in terms of hyphal extension of the test fungus to local application of amphotericin B in this culture system were estimated by using an automatic analysing system. After hyphae had traversed the barrier, distal application of amphotericin B caused no appreciable effect on the proximal hyphae. In contrast, proximal application of amphotericin B caused inhibition of the extension of distal hyphae. The reversal of polarized cytoplasmic streaming also occurred during the inhibition of distal hyphal extension. The extents of inhibition of the distal hyphal extension and the cytoplasmic streaming were dependent upon the hyphal distance between the amphotericin B application site and the hyphal tip. These results show that the effect of an antifungal agent on a hypha depends on the region of the hypha exposed. Cytoplasmic streaming may play key role in the transmission of antifungal effects within a single hypha.     

Profilin is required for the normal timing of actin polymerization in response to thermal stress

We have used a fluorometric assay to determine the relative amounts of polymerized actin (F-actin) in wild-type and profilin mutant yeast cells. Our results indicate that profilin plays a role in maintaining normal F-actin levels in response to shifts to high temperature. Cells lacking profilin display a greater drop in F-actin levels upon such temperature shifts, and are slower to recover to initial F-actin levels than are wild-type cells. Interestingly, shifts to cold temperatures result in rapid increases of F-actin levels in wild-type and profilin null cells. We have further determined that shifting to high-osmolarity growth conditions causes a relatively slow decrease in F-actin levels in wild-type cells, and a small but rapid increase in the F-actin levels in profilin null cells. Profilin null cells contain normal concentrations of F-actin while growing exponentially at room temperature, indicating that profilin is not essential for maintaining F-actin concentrations during steady-state growth. Our data suggest that actin is inherently unstable in vivo at high temperatures, and that profilin helps to maintain actin in its filamentous state at these temperatures, perhaps by stimulating actin polymerization in a proper temporal and spatial fashion.     

Production of reactive oxygen species and release of L-glutamate during superoxide anion-induced cell death of cerebellar granule neurons

This study focuses on the different efficiencies of secretion of two fungal cutinases by Saccharomyces cerevisiae, a wild-type cutinase (CY000) and a hydrophobic mutant cutinase (CY028). Both cutinases are placed under control of the GAL7 promoter, by which the expression levels can be regulated. Wild-type cutinase was secreted at up to 25 mg per g (dry weight), while CY028 was secreted at a level of 2 mg per g (dry weight); this difference is nearly independent of the expression level. Pulse-chase experiments revealed that whereas CY000 cutinase is secreted, CY028 is irreversibly retained in the cell. Immunogold labelling followed by electron microscopy revealed colocalization of CY028 with immunoglobulin heavy-chain binding protein (BiP) in the endoplasmic reticulum (ER). The increase of wild-type cutinase expression did not result in higher levels of the molecular chaperone BiP, but BiP levels are raised by increased induction of the hydrophobic mutant cutinase. Immunoprecipitation studies showed that in contrast to the wild-type cutinase, the hydrophobic mutant cutinase interacts with BiP. These results indicate that the introduction of two exposed hydrophobic patches in cutinase results in a higher affinity for BiP which might cause the retention of this mutant cutinase in the ER.     

Purification of biologically active SPARC expressed in Saccharomyces cerevisiae

SPARC (secreted protein, acidic and rich in cysteine) is a secreted, Ca+2-binding glycoprotein that modulates interactions between cells and their immediate extracellular matrix. Traditional sources of SPARC have been mammalian bone, platelets, a basement membrane tumor, and cultured cells; most if not all preparations, however, contain platelet-derived growth factor and one or more serum proteins that bind specifically to purified SPARC. To avoid these contaminants, as well as the toxic lipid moiety associated with endotoxin, we expressed recombinant wild-type and a mutated murine SPARC in two strains of Saccharomyces cerevisiae: one strain was transfected with an expression vector encoding a proprietory signal peptide that directed the secretion of the recombinant protein. Recombinant SPARC was also purified from cell lysates of a different, nonreverting strain of S. cerevisiae that was optimized for large-scale fermentation runs. A mutant murine SPARC lacking the single glycosylation site was also expressed following substitution of Asn98 with Asp98 in the wild-type sequence. Purification of SPARC was achieved by copper-affinity and hydrophobic-interaction chromatography. Both the wild-type and the glycosylation-defective recombinant proteins exhibited high levels of activity in two bioassays with endothelial cells: inhibition of cell spreading/disruption of actin microfilaments and competition for the binding of nonrecombinant 125I-labeled SPARC to the cell surface. The availability of biologically active, recombinant SPARC will facilitate investigation of the structural and functional properties of this protein, which is expressed at high levels in healing wounds, atherosclerotic plaque, and several cancers and diseases of connective tissue.     

Neutralization of IL-12 demonstrates the existence of discrete organ-specific phases in the control of Leishmania donovani

Aspergillus nidulans grows by apical extension of multinucleate cells called hyphae that are subdivided by the insertion of crosswalls called septa. Apical cells vary in length and number of nuclei, whereas subapical cells are typically 40 microm long with three to four nuclei. Apical cells have active mitotic cycles, whereas subapical cells are arrested for growth and mitosis until branch formation reinitiates tip growth and nuclear divisions. This multicellular growth pattern requires coordination between localized growth, nuclear division, and septation. We searched a temperature-sensitive mutant collection for strains with conditional defects in growth patterning and identified six mutants (designated hyp for hypercellular). The identified hyp mutations are nonlethal, recessive defects in five unlinked genes (hypA-hypE). Phenotypic analyses showed that these hyp mutants have aberrant patterns of septation and show defects in polarity establishment and tip growth, but they have normal nuclear division cycles and can complete the asexual growth cycle at restrictive temperature. Temperature shift analysis revealed that hypD and hypE play general roles in hyphal morphogenesis, since inactivation of these genes resulted in a general widening of apical and subapical cells. Interestingly, loss of hypA or hypB function lead to a cessation of apical cell growth but activated isotropic growth and mitosis in subapical cells. The inferred functions of hypA and hypB suggest a mechanism for coordinating apical growth, subapical cell arrest, and mitosis in A. nidulans.     

The risk of seizures associated with tramadol

Several substances including proteases and restrictocin have been suggested as candidates for virulence determinants in invasive pulmonary aspergillosis. However, the roles of such substances are not well understood. This study compared the in vitro suppressive effects of Aspergillus fumigatus culture filtrates (ACFs), on the functions of human polymorphonuclear leukocytes (PMNLs), the principal cells in the host defence against aspergillus hyphae, from a clinically isolated wild-type and isogenic mutant strains which lack production of elastolytic alkaline protease (Alp) and/or restrictocin. ACFs were obtained by culturing conidia of each strain in Medium- 199 at 37 degrees C for 5 days. ACFs of the wild-type significantly (p<0.01) suppressed chemotaxis, superoxide anion (O2-) release and PMNL-mediated hyphal damage, compared with the control (Medium-199). ACFs of the mutant strains that lack Alp or restrictocin significantly (p<0.01) suppressed chemotaxis and O2(-)-release, but did not suppress hyphal damage, compared with the control. The wild-type significantly (p<0.01) suppressed chemotaxis of PMNLs compared with the mutant strains lacking Alp or restrictocin, whereas there were no significant differences in suppression of O2(-)-release and hyphal damage by PMNLs. ACF of a mutant strain that lacks both Alp and restrictocin had much less activity, but significantly (p<0.01) suppressed chemotaxis of PMNLs compared with the control. In conclusion, alkaline protease and restrictocin may play roles in the suppressive effect of Aspergillus fumigatus culture filtrates on the functions of human polymorphonuclear leukocytes. Other antiphagocytic substances produced by Aspergillus fumigatus remain to be identified.     

Mutational analysis of the Yersinia enterocolitica virC operon: characterization of yscE, F, G, I, J, K required for Yop secretion and yscH encoding YopR

Pathogenic yersiniae secrete the Yop anti-host proteins using a type-III secretion pathway. The components of the secretion machinery are encoded by three loci on the pYV plasmid: virA, virB, and virC. In this paper we describe the characterization of eight non-polar mutants of the virC locus, constructed by allelic exchange. The yscE, FG, I, J and K mutants were defective in Yop secretion and independent of Ca2+ (CI) for their growth at 37 degrees C. Substitution of the 12 N-terminal amino-acid residues of YscF impaired secretion of YopB and YopD only and led also to a CI phenotype. The culture supernatant of the yscH mutant contained all the Yops except the 18 kDa YopR. Complementation experiments and an immunoblot analysis confirmed that YopR is encoded by the yscH gene. The LD50 for the mouse of the yscH mutant was 10-fold higher than that of the parental strain indicating that YopR is involved in pathogenesis. The phenotype of the yscM mutant was similar to that of the wild-type strain. However, overproduction of YscM from a multicopy plasmid in wild-type Yersinia enterocolitica prevented Yop secretion and synthesis. A hybrid YopH-LacZ' protein, encoded by a gene transcribed from the lac promoter, was secreted by a strain overexpressing YscM, showing that the secretion machinery was still functional. These results indicate that YscM plays a role in the feedback inhibition of Yop synthesis when secretion is compromised by acting as a negative regulator of Yop synthesis.     

Targeted mutants of Cochliobolus carbonum lacking the two major extracellular polygalacturonases

The filamentous fungus Cochliobolus carbonum produces endo-alpha 1,4-polygalacturonase (endoPG), exo-alpha 1,4-polygalacturonase (exoPG), and pectin methylesterase when grown in culture on pectin. Residual activity in a pgn1 mutant (lacking endoPG) was due to exoPG activity, and the responsible protein has now been purified. After chemical deglycosylation, the molecular mass of the purified protein decreased from greater than 60 to 45 kDa. The gene that encodes exoPG, PGX1, was isolated with PCR primers based on peptide sequences from the protein. The product of PGX1, Pgx1p, has a predicted molecular mass of 48 kDa, 12 potential N-glycosylation sites, and 61% amino acid identity to an exoPG from the saprophytic fungus Aspergillus tubingensis. Strains of C. carbonum mutated in PGX1 were constructed by targeted gene disruption and by gene replacement. Growth of pgx1 mutant strains on pectin was reduced by ca. 20%, and they were still pathogenic on maize. A double pgn1/pgx1 mutant strain was constructed by crossing. The double mutant grew as well as the pgx1 single mutant on pectin and was still pathogenic despite having less than 1% of total wild-type PG activity. Double mutants retained a small amount of PG activity with the same cation-exchange retention time as Pgn1p and also pectin methylesterase and a PG activity associated with the mycelium. Continued growth of the pgn1/pgx1 mutant on pectin could be due to one or more of these residual activities.     

Identification, characterization, and In situ detection of a fruit-body-specific hydrophobin of Pleurotus ostreatus

Hydrophobins are small (length, about 100 +/- 25 amino acids), cysteine-rich, hydrophobic proteins that are present in large amounts in fungal cell walls, where they form part of the outermost layer (rodlet layer); sometimes, they can also be secreted into the medium. Different hydrophobins are associated with different developmental stages of a fungus, and their biological functions include protection of the hyphae against desiccation and attack by either bacterial or fungal parasites, hyphal adherence, and the lowering of surface tension of the culture medium to permit aerial growth of the hyphae. We identified and isolated a hydrophobin (fruit body hydrophobin 1 [Fbh1]) present in fruit bodies but absent in both monokaryotic and dikaryotic mycelia of the edible mushroom Pleurotus ostreatus. In order to study the temporal and spatial expression of the fbh1 gene, we determined the N-terminal amino acid sequence of Fbh1. We also synthesized and cloned the double-stranded cDNA corresponding to the full-length mRNA of Fbh1 to use it as a probe in both Northern blot and in situ hybridization experiments. Fbh1 mRNA is detectable in specific parts of the fruit body, and it is absent in other developmental stages.