Identification and Deletion of Tft1, a Predicted Glycosyltransferase Necessary for Cell Wall β-1,3;1,4-Glucan Synthesis in Aspergillus fumigatus

Aspergillus fumigatus is an environmental mold that causes severe, often fatal invasive infections in immunocompromised patients. The search for new antifungal drug targets is critical, and the synthesis of the cell wall represents a potential area to find such a target. Embedded within the main β-1,3-glucan core of the A. fumigatus cell wall is a mixed linkage, β-D-(1,3;1,4)-glucan. The role of this molecule or how it is synthesized is unknown, though it comprises 10% of the glucans within the wall. While this is not a well-studied molecule in fungi, it has been studied in plants. Using the sequences of two plant mixed linkage glucan synthases, a single ortholog was identified in A. fumigatus (Tft1). A strain lacking this enzyme (tft1Δ) was generated along with revertant strains containing the native gene under the control of either the native or a strongly expressing promoter. Immunofluorescence staining with an antibody against β-(1,3;1,4)-glucan and biochemical quantification of this polysaccharide in the tft1Δ strain demonstrated complete loss of this molecule. Reintroduction of the gene into the knockout strain yielded reappearance in amounts that correlated with expected expression of the gene. The loss of Tft1 and mixed linkage glucan yielded no in vitro growth phenotype. However, there was a modest increase in virulence for the tft1Δ strain in a wax worm model. While the precise roles for β-(1,3;1,4)-glucan within A. fumigatus cell wall are still uncertain, it is clear that Tft1 plays a pivotal role in the biosynthesis of this cell wall polysaccharide.     

Determining the Subcellular Location of Synthesis and Assembly of the Cell Wall Polysaccharide (1,3; 1,4)-β-d-Glucan in Grasses

The current dogma for cell wall polysaccharide biosynthesis is that cellulose (and callose) is synthesized at the plasma membrane (PM), whereas matrix phase polysaccharides are assembled in the Golgi apparatus. We provide evidence that (1,3;1,4)-β-d-glucan (mixed-linkage glucan [MLG]) does not conform to this paradigm. We show in various grass (Poaceae) species that MLG-specific antibody labeling is present in the wall but absent over Golgi, suggesting it is assembled at the PM. Antibodies to the MLG synthases, cellulose synthase-like F6 (CSLF6) and CSLH1, located CSLF6 to the endoplasmic reticulum, Golgi, secretory vesicles, and the PM and CSLH1 to the same locations apart from the PM. This pattern was recreated upon expression of VENUS-tagged barley (Hordeum vulgare) CSLF6 and CSLH1 in Nicotiana benthamiana leaves and, consistent with our biochemical analyses of native grass tissues, shown to be catalytically active with CSLF6 and CSLH1 in PM-enriched and PM-depleted membrane fractions, respectively. These data support a PM location for the synthesis of MLG by CSLF6, the predominant enzymatically active isoform. A model is proposed to guide future experimental approaches to dissect the molecular mechanism(s) of MLG assembly.     

A Mutational Analysis of Killer Toxin Resistance in Saccharomyces Cerevisiae Identifies New Genes Involved in Cell Wall (1 -> 6)-β-Glucan Synthesis

Recessive mutations leading to killer resistance identify the KRE9, KRE10 and KRE11 genes. Mutations in both the KRE9 and KRE11 genes lead to reduced levels of (1 -> 6)-β-glucan in the yeast cell wall. The KRE11 gene encodes a putative 63-kD cytoplasmic protein, and disruption of the KRE11 locus leads to a 50% reduced level of cell wall (1 -> 6)-glucan. Structural analysis of the (1 -> 6)-β-glucan remaining in a kre11 mutant indicates a polymer smaller in size than wild type, but containing a similar proportion of (1 -> 6)- and (1 -> 3)-linkages. Genetic interactions among cells harboring mutations at the KRE11, KRE6 and KRE1 loci indicate lethality of kre11 kre6 double mutants and that kre11 is epistatic to kre1, with both gene products required to produce the mature glucan polymer at wild-type levels. Analysis of these KRE genes should extend knowledge of the β-glucan biosynthetic pathway, and of cell wall synthesis in yeast.     

A Lichen Lectin Specifically Binds to the α-1,4-Polygalactoside Moiety of Urease Located in the Cell Wall of Homologous Algae

A lectin from the lichen Evernia prunastri developing arginase activity (EC. 3.5.3.1) binds to the homologous algae that contain polygalactosilated urease (EC. 3.5.1.5) in their cell walls acting as a lectin ligand. The enzyme bound to its ligand shows to be inactive to hydrolyze of arginine. Hydrolysis of the galactoside moiety of urease in intact algae with α-1,4-galactosidase (EC. 3.2.1.22) releases high amount of D-galactose and impedes the binding of the lectin to the algal cell wall. However, the use of β-,4-galactosidase (EC.3.2.1.23) releases low amounts of D-galactose from the algal cell wall and does not change the pattern of binding of the lectin to its ligand. The production of glycosilated urease is restricted to the season in which algal cells divide and this assures the recognition of new phycobiont produced after cell division by its fungal partner.Key Words: arginase, cell wall, evernia prunastri, lectin ligand, phycobiont, urease     

In Vivo Expression of UDP-N-Acetylglucosamine: α-3-D-Mannoside β-1,2-N-Acetylglucosaminyltransferase I (GnT-1) in Aspergillus oryzae and Effects on the Sugar Chain of α-Amylase

UDP-N-Acetylglucosamine: α-3-D-mannoside β-1,2-N-acetylglucosaminyltransferase I (GnT-I) is an essential enzyme in the conversion of high mannose type oligosaccharide to the hybrid or complex type. The full length of the rat GnT-I gene was expressed in the filamentous fungus Aspergillus oryzae. A microsomal preparation from a recombinant fungus (strain NG) showed GnT-I activity that transferred N-acetylglucosamine residue to acceptor heptaose, Man₅GlcNAc₂. The N-linked sugar chain of α-amylase secreted by the strain showed a peak of novel retention on high performance liquid chromatography that was same as a reaction product of in vitro GnT-1 assay. The peak of oligosaccharide disappeared on HPLC after β-N-acetylglucosaminidase treatment. Mass analysis supported the presence of GlcNAcMan₅GlcNAc₂ as a sugar chain of α-amylase from strain NG. Chimera of GnT-I with green fluorescent protein (GFP) showed a dotted pattern of fluorescence in the mycelia, suggesting localization at Golgi vesicles. We concluded that GnT-1 was functionally expressed in A. oryzae cells and that N-acetylglucosamine residue was transferred to N-glycan of α-amylase in vivo. A. oryzae is expected to be a potential host for the production of glycoprotein with a genetically altered sugar chain.     

Drosophila α-1,4-glycosyltransferase (α-4GT1) inhibits reaper-mediated apoptosis in the eye

In a genetic screen, α-4GT1 has been identified as a potential enhancer of Hairless-mediated cell death in the eye of Drosophila. α-4GT1 encodes an α-1,4-glycosyltransferase, known to catalyze the fifth step in a series of ceramide glycosylation events. As reported for other enzymes involved in the glycosylation of ceramide, α-4GT1 is strongly expressed during oogenesis and is deposited maternally in the egg. Moreover, the protein is enriched at cell membranes. Unexpectedly, overexpression of α-4GT1 does not enhance Hairless-mediated cell death; instead, Hairless enhancement is caused by an allele of Scutoid ¹ present on the α-4GT1 chromosome. Interestingly, the downregulation of α-4GT1 during eye development amplifies cell death induction by the pro-apoptotic gene reaper. Accordingly, overexpression of α-4GT1 represses reaper-induced cell death. Thus, α-4GT1 appears to be an inhibitor of apoptosis, as has previously been observed for other ceramide glycosylating enzymes, suggesting that it likewise contributes to ceramide anchoring in the membrane. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. This work was supported by a grant from the German Science Foundation (DFG) to A.C.N. (NA427/1—2). The authors declare that they have no conflict of interest.     

Root cap specific expression of an endo-β-1,4--glucanase (cellulase): a new marker to study root development in Arabidopsis

The sloughing of root cap cells from the root tip is important because it assists the growing root in penetrating the soil. Using a promoter–reporter (GUS) and RT-PCR analysis, we identified an endo--1,4-glucanase (AtCel5) of Arabidopsis thaliana that is expressed exclusively in root cap cells of both primary and secondary roots. Expression is inhibited by high concentrations of IAA, both exogenous and internal, as well as by ABA. AtCel5 expression begins once the mature tissue pattern is established and continues for 3weeks. GUS staining is observed in both root cap cells that are still attached and cells that have already been shed. Using AtCel5-GUS as a marker, we observed that the root cap cells begin to separate at the sides of the tip while the cells of the central region of the tip separate last. Separation involves sequential tiers of intact cells that separate from the periphery of the root tip. A homozygous T-DNA insertion mutant that does not express AtCel5 forms the root cap and sheds root cap cells but sloughing is less efficient compared to wild type. The reduction in sloughing in the mutant does not affect the overall growth performance of the plant in loose media. The modest effect of abolishing AtCel5 expression suggests that there are multiple redundant genes regulating the process of sloughing of the root cap, including AtCel3/At1g71380, the paralog of the AtCel5 gene that is also expressed in the root cap cells. Thus, these two endo-1,4--d-glucanases may have a role in the sloughing of border cells from the root tip. We propose that AtCel5, provides a new molecular marker to further analyze the process of root cap cell separation and a root cap specific promoter for targeting to the environment genes with beneficial properties for plant growth.     

(1,3)-β-D-Glucan Synthase Activity in Mycelial and Cell Wall-less Phenotypes of the fz,sg, os-1 ("Slime") Mutant Strain of Neurospora crassa

Polizeli, M. L. T. M., Noventa-Jordāo, M. A., Marques da Silva, M., Jorge, J. A., and Terenzi, H. F. 1995. (1,3)-β-D-Glucan synthase activity in mycelial and cell wall-less phenotypes of the fz, sg, os-1 ("slime") mutant strain of Neurospora crassa. Experimental Mycology 19, 35-47. The cell wall-less fz, sg, os-1 ("slime") triple mutant of Neurospora crassa lacks (1,3)-βD-glucan synthase activity. fz, sg, os-1 segregants from slime × wild-type crosses initially germinate as a plasmodium (slime-like), but develop hyphae in a few hours and acquire a stable mycelial phenotype (mycelial intermediate). The cell wall-less phenotype (stable slime) can be reisolated from mycelial intermediates by filtration-enrichment selection in medium of high osmolarity. Pairs of mycelial intermediate and stable slime obtained from a single slime-like segregant were comparatively studied. Mycelial intermediate strains synthesize a cell wall with normal amounts of (1,3)-β-glucan, chitin, and other polysaccharides and possess (1,3)-β-glucan synthase activity with apparently normal properties (i.e., association with membranes, stability, K_m app, V_max, stimulation by GTP). The enzyme was dissociated by treatment with Tergitol NP-40 and NaCl into a membrane-bound catalytic center and a soluble factor which activates the enzyme in the presence of GTP. Heterologous reconstitution assays demonstrated that stable slime spheroplasts had normal activity of the soluble activating factor, but were severely deficient in membrane-bound activity. The genetic composition of the viable progeny of stable slime or mycelial intermediate × wild-type crosses failed to show differences between the two extreme phenotypes of slime. However, the analysis of heterokaryons demonstrated that the stable slime homokaryotic progeny of stable slime/wild-type heterokaryons were not viable. In contrast, the behavior of mycelial intermediate/wild-type heterokaryons was normal. Apparently, stable slime strains differed from the original mycelial intermediate in a mutation(s) which arose spontaneously during the filtration-enrichment selection applied to mycelial intermediates in order to obtain the cell wall-less phenotype. This new trait impaired conidial germination and might be the actual cause of the loss of (1,3)-β-glucan synthase activity and cell wall.     

Cloning and expression of the Bacillus circulans endo-β-1,3-1,4-glucanase gene (bglBC1) in Escherichia coli

A gene coding for the endo--1,3-1,4-glucanase of B. circulans ATCC21367 was cloned into Escherichia coli. The cloned enzyme hydrolyzed lichenan or barley -glucan to produce 3-O--cellobiosyl-d-glucose as a main product but was inactive with carboxymethyl cellulose, laminarin and xylan. The enzyme, M _r=28 kDa, remained within the cytoplasm of E. coli. A 771 bp open reading frame was in the 2 kb PstI fragment of the recombinant plasmid pLL200K. The deduced protein sequence consists of 257 amino acids and has a putative signal peptide of 26 amino acids. The amino acid sequence of the endo--1,3-1,4-glucanase showed 68 and 51% homology to previously reported endo--1,3-1,4-glucanases from Bacillus strain N-137 and B. brevis, respectively.     

Upregulated Expression of Integrin α1 in Mesangial Cells and Integrin α3 and Vimentin in Podocytes of Col4a3-Null (Alport) Mice

Alport disease in humans, which usually results in proteinuria and kidney failure, is caused by mutations to the COL4A3, COL4A4, or COL4A5 genes, and absence of collagen α3α4α5(IV) networks found in mature kidney glomerular basement membrane (GBM). The Alport mouse harbors a deletion of the Col4a3 gene, which also results in the lack of GBM collagen α3α4α5(IV). This animal model shares many features with human Alport patients, including the retention of collagen α1α2α1(IV) in GBMs, effacement of podocyte foot processes, gradual loss of glomerular barrier properties, and progression to renal failure. To learn more about the pathogenesis of Alport disease, we undertook a discovery proteomics approach to identify proteins that were differentially expressed in glomeruli purified from Alport and wild-type mouse kidneys. Pairs of cy3- and cy5-labeled extracts from 5-week old Alport and wild-type glomeruli, respectively, underwent 2-dimensional difference gel electrophoresis. Differentially expressed proteins were digested with trypsin and prepared for mass spectrometry, peptide ion mapping/fingerprinting, and protein identification through database searching. The intermediate filament protein, vimentin, was upregulated ∼2.5 fold in Alport glomeruli compared to wild-type. Upregulation was confirmed by quantitative real time RT-PCR of isolated Alport glomeruli (5.4 fold over wild-type), and quantitative confocal immunofluorescence microscopy localized over-expressed vimentin specifically to Alport podocytes. We next hypothesized that increases in vimentin abundance might affect the basement membrane protein receptors, integrins, and screened Alport and wild-type glomeruli for expression of integrins likely to be the main receptors for GBM type IV collagen and laminin. Quantitative immunofluorescence showed an increase in integrin α1 expression in Alport mesangial cells and an increase in integrin α3 in Alport podocytes. We conclude that overexpression of mesangial integrin α1 and podocyte vimentin and integrin α3 may be important features of glomerular Alport disease, possibly affecting cell-signaling, cell shape and cellular adhesion to the GBM.     

Two Novel Techniques for Determination of Polysaccharide Cross-Links Show that Crh1p and Crh2p Attach Chitin to both β(1-6)- and β(1-3)Glucan in the Saccharomyces cerevisiae Cell Wall

Previous work, using solubilization of yeast cell walls by carboxymethylation, before or after digestion with β(1-3)- or β(1-6)glucanase, followed by size chromatography, showed that the transglycosylases Crh1p and Crh2p/Utr2p were redundantly required for the attachment of chitin to β(1-6)glucan. With this technique, crh1Δ crh2Δ mutants still appeared to contain a substantial percentage of chitin linked to β(1-3)glucan. Two novel procedures have now been developed for the analysis of polysaccharide cross-links in the cell wall. One is based on the affinity of curdlan, a β(1-3)glucan, for β(1-3)glucan chains in carboxymethylated cell walls. The other consists of in situ deacetylation of cell wall chitin, generating chitosan, which can be extracted with acetic acid, either directly (free chitosan) or after digestion with different glucanases (bound chitosan). Both methodologies indicated that all of the chitin in crh1Δ crh2Δ strains is free. Reexamination of the previously used procedure revealed that the β(1-3)glucanase preparation used (zymolyase) is contaminated with a small amount of endochitinase, which caused erroneous results with the double mutant. After removing the chitinase from the zymolyase, all three procedures gave coincident results. Therefore, Crh1p and Crh2p catalyze the transfer of chitin to both β(1-3)- and β(1-6)glucan, and the biosynthetic mechanism for all chitin cross-links in the cell wall has been established.The fungal cell wall protects the cell against internal turgor pressure and external mechanical injury. To fulfill these functions, it must be endowed with a resilient structure. Presumably, the cell wall strength is largely due to the cross-links that bind together its components, mainly polysaccharides, giving rise to a tightly knit mesh (6, 11-13). Interestingly, the cross-links must be created outside the plasma membrane, because most of the polysaccharides are extruded as they are synthesized at the membrane; therefore, they do not exist inside the cell. This posits a thermodynamic problem, because there are no obvious sources of energy in the periplasmic space. About 20 years ago we proposed that the free energy may come from existing bonds in the polysaccharide chains and that the new cross-links may be originated by transglycosylation, thus creating a new linkage for each one that is broken (5).Ascertaining the mechanism of cross-link formation seemed a worthwhile endeavor, both because of the theoretical implications and because the cell wall is a proven target for antifungal compounds; therefore, more knowledge about its synthesis can be of practical interest. For this type of investigation to proceed, it was necessary to devise some method for the quantitative analysis of cell wall cross-links. We developed such a procedure for the evaluation of the proportion of cell wall chitin that is free or bound to β(1-3)- or β(1-6)glucan (4). In this methodology, chitin was specifically labeled in vivo with [¹⁴C]glucosamine; cell walls were isolated, and their proteins were eliminated by alkali treatment. The insoluble residue was solubilized by carboxymethylation and analyzed by size fractionation chromatography. By treating the cell walls with different glucanases before carboxymethylation and comparing the chromatographic profiles, we were able to determine the amount of chitin bound to the different glucans, as well as the fraction that was free (4). Armed with this procedure, we could now analyze the cell wall of different mutants that appeared to be candidates for cross-links defects. In this way we found that the two putative transglycosylases Crh1p and Crh2p were redundantly required for the formation of the chitin-β(1-6)glucan linkage. A double mutant crh1Δ crh2Δ had no chitin attached to β(1-6)glucan, although it still contained apparently normal amounts of chitin-β(1-3)glucan complex (7). Further work supported the notion that Crh1p and Crh2p function as transglycosylases, transferring portions of chitin chains to glucan (8). This confirmed our earlier hypothesis.With the initial intention of finding easier and faster methods, I devised two novel procedures for cell wall analysis. One is based on the affinity between β(1-3)glucan chains, the other on the conversion of chitin in situ into its deacetylated product, chitosan, followed by extraction of the chitosan with acetic acid before or after treatment with specific glucanases. With a wild-type strain, both procedures gave similar results to those of the carboxymethylation-chromatography technique. However, in the double mutant crh1Δ crh2Δ all of the chitin appeared to be free with both new methods. Further investigation showed that the older procedure led to erroneous results for the double mutant, because of the presence of a small amount of chitinase in the β(1-3)glucanase preparation used. After reconciling the results, I conclude that Crh1p and Crh2p are necessary for the formation of cross-links between chitin and either β(1-6) or β(1-3)glucan.     

Functional characterization of barley betaglucanless mutants demonstrates a unique role for CslF6 in (1,3;1,4)-β-D-glucan biosynthesis

(1,3;1,4)-β-D-glucans (mixed-linkage glucans) are found in tissues of members of the Poaceae (grasses), and are particularly high in barley (Hordeum vulgare) grains. The present study describes the isolation of three independent (1,3;1,4)-β-D-glucanless (betaglucanless; bgl) mutants of barley which completely lack (1,3;1,4)-β-D-glucan in all the tissues tested. The bgl phenotype cosegregates with the cellulose synthase like HvCslF6 gene on chromosome arm 7HL. Each of the bgl mutants has a single nucleotide substitution in the coding region of the HvCslF6 gene resulting in a change of a highly conserved amino acid residue of the HvCslF6 protein. Microsomal membranes isolated from developing endosperm of the bgl mutants lack detectable (1,3;1,4)-β-D-glucan synthase activity indicating that the HvCslF6 protein is inactive. This was confirmed by transient expression of the HvCslF6 cDNAs in Nicotiana benthamiana leaves. The wild-type HvCslF6 gene directed the synthesis of high levels of (1,3;1,4)-β-D-glucans, whereas the mutant HvCslF6 proteins completely lack the ability to synthesize (1,3;1,4)-β-D-glucans. The fine structure of the (1,3;1,4)-β-D-glucan produced in the tobacco leaf was also very different from that found in cereals having an extremely low DP3/DP4 ratio. These results demonstrate that, among the seven CslF and one CslH genes present in the barley genome, HvCslF6 has a unique role and is the key determinant controlling the biosynthesis of (1,3;1,4)-β-D-glucans. Natural allelic variation in the HvCslF6 gene was found predominantly within introns among 29 barley accessions studied. Genetic manipulation of the HvCslF6 gene could enable control of (1,3;1,4)-β-D-glucans in accordance with the purposes of use.     

Increased Dopaminergic Neuron Sensitivity to 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine (MPTP) in Transgenic Mice Expressing Mutant A53T α-Synuclein

Familial Parkinson’s disease (PD) has been linked to point mutations and duplication of the α-synuclein gene and mutant α-synuclein expression increases the vulnerability of neurons to exogenous insults. In this study, we analyzed the levels of dopamine and its metabolites in the olfactory bulb (OB), and nigrostriatal regions of transgenic mice expressing human, mutant A53T α-synuclein (α-syn tg) and their non-transgenic (ntg) littermates using a sub-toxic, moderate dose of MPTP to determine if mutant human α-synuclein sensitizes the central dopaminergic systems to oxidative stress. We observed that after a single, sub-lethal MPTP injection, dopamine levels were reduced in striatum and SN in both the α-syn tg and ntg mice. In the olfactory bulb, a region usually resistant to MPTP toxicity, levels were reduced only in the α-syn tg mice. In addition, we identified a significant increase in dopamine metabolism in the α-syn transgenic, but not ntg mice. Finally, MPTP treatment of α-syn tg mice was associated with a marked elevation in the oxidative product, 3-nitrotyrosine that co-migrated with α-synuclein. Cumulatively, the data support the hypothesis that mutant α-synuclein sensitizes dopaminergic neurons to neurotoxic insults and is associated with greater oxidative stress. The α-syn tg line is therefore useful to study the genetic and environmental inter-relationship in PD.     

Mutant PrP suppresses glutamatergic neurotransmission in cerebellar granule neurons by impairing membrane delivery of VGCC α(2)δ-1 Subunit

How mutant prion protein (PrP) leads to neurological dysfunction in genetic prion diseases is unknown. Tg(PG14) mice synthesize a misfolded mutant PrP which is partially retained in the neuronal endoplasmic reticulum (ER). As these mice age, they develop ataxia and massive degeneration of cerebellar granule neurons (CGNs). Here, we report that motor behavioral deficits in Tg(PG14) mice emerge before neurodegeneration and are associated with defective glutamate exocytosis from granule neurons due to impaired calcium dynamics. We found that mutant PrP interacts with the voltage-gated calcium channel α(2)δ-1 subunit, which promotes the anterograde trafficking of the channel. Owing to ER retention of mutant PrP, α(2)δ-1 accumulates intracellularly, impairing delivery of the channel complex to the cell surface. Thus, mutant PrP disrupts cerebellar glutamatergic neurotransmission by reducing the number of functional channels in CGNs. These results link intracellular PrP retention to synaptic dysfunction, indicating new modalities of neurotoxicity and potential therapeutic strategies.     

Crystal structure of α-1,3-galactosyltransferase (α3GT) in a complex with p-nitrophenyl-β-galactoside (pNPβGal)

The specificities of glycosyltransferases make them useful for the synthesis of biologically active oligosaccharides, but also restrict their range of products. In substrate engineering, substrate promiscuity is enhanced by attaching removable interactive groups to weak substrates. Thus, the attachment of β p-nitrophenyl converts galactose from a poor into a good substrate of α-1,3-galactosyltransferase. The crystallographic structure of a complex of α3GT containing p-nitrophenyl-β-galactoside shows that the p-nitrophenyl binds similarly to the N-acetylglucosamine of the substrate, N-acetyllactosamine, interacting with the indole of Trp249. p-Nitrophenyl, unlike N-acetylglucosamine, makes no H-bonds but has more non-polar interactions, making it an effective monosaccharide mimetic.