Functional analysis of the Campylobacter jejuni N-linked protein glycosylation pathway

We describe in this report the characterization of the recently discovered N-linked glycosylation locus of the human bacterial pathogen Campylobacter jejuni, the first such system found in a species from the domain Bacteria. We exploited the ability of this locus to function in Escherichia coli to demonstrate through mutational and structural analyses that variant glycan structures can be transferred onto protein indicating the relaxed specificity of the putative oligosaccharyltransferase PglB. Structural data derived from these variant glycans allowed us to infer the role of five individual glycosyltransferases in the biosynthesis of the N-linked heptasaccharide. Furthermore, we show that C. jejuni- and E. coli-derived pathways can interact in the biosynthesis of N-linked glycoproteins. In particular, the E. coli encoded WecA protein, a UDP-GlcNAc: undecaprenylphosphate GlcNAc-1-phosphate transferase involved in glycolipid biosynthesis, provides for an alternative N-linked heptasaccharide biosynthetic pathway bypassing the requirement for the C. jejuni-derived glycosyltransferase PglC. This is the first experimental evidence that biosynthesis of the N-linked glycan occurs on a lipid-linked precursor prior to transfer onto protein. These findings provide a framework for understanding the process of N-linked protein glycosylation in Bacteria and for devising strategies to exploit this system for glycoengineering.

ALG9 mannosyltransferase is involved in two different steps of lipid-linked oligosaccharide biosynthesis

N-linked protein glycosylation follows a conserved pathway in eukaryotic cells. The assembly of the lipid-linked core oligosaccharide Glc3Man9GlcNAc2, the substrate for the oligosaccharyltransferase (OST), is catalyzed by different glycosyltransferases located at the membrane of the endoplasmic reticulum (ER). The substrate specificity of the different glycosyltransferase guarantees the ordered assembly of the branched oligosaccharide and ensures that only completely assembled oligosaccharide is transferred to protein. The glycosyltransferases involved in this pathway are highly specific, catalyzing the addition of one single hexose unit to the lipid-linked oligosaccharide (LLO). Here, we show that the dolichylphosphomannose-dependent ALG9 mannosyltransferase is the exception from this rule and is required for the addition of two different alpha-1,2-linked mannose residues to the LLO. This report completes the list of lumen-oriented glycosyltransferases required for the assembly of the LLO.

Promoter analysis of cgl2, a galectin encoding gene transcribed during fruiting body formation in Coprinopsis cinerea (Coprinus cinereus)

In the homobasidiomycete Coprinopsis cinerea, expression of the two fruiting body-specific galectins, CGL1 and CGL2, is controlled by nutrients, light and darkness and the A mating type genes. In this study, we analyzed the promoter of the cgl2 gene by measuring transcript levels by quantitative real-time PCR and show that regulation of CGL2 expression occurs at the transcriptional level. A minimal promoter sufficient to confer regulated expression of a heterologous reporter gene and comprising 627 base pairs from the start codon was defined. On the minimal promoter we identified a 120 bp sequence mediating induction of the cgl2 gene in constant darkness. Along with direct repeats (TGGAAG/TGGAAG/GGAA), the sequence contains a CRE consensus site (cAMP-responsive element, TGCGTCA) suggesting the involvement of cAMP signaling in cgl2 activation. No specific elements responsible for light repression and mating type regulation were found in the promoter.

Ligand interactions of the Coprinopsis cinerea galectins

The basidiomycete Coprinopsis cinerea (Coprinus cinereus) expresses two fruiting body-specific isolectins (CGL1 and CGL2) that belong to the family of galectins. Understanding the role of these beta-galactoside binding lectins is still in the beginning. Even though the prerequisites for substrate binding are well understood, it is not known how discrimination between potential substrates is achieved and what kind of influence this has on the function in a distinct cellular context. Precise knowledge of the expression of galectins and their ligands will aid in elucidating their function. In Coprinopsis, the developmentally regulated ligands for galectins co-localise with galectin expression in the veil surrounding the developing primordium and the outer cells of the young stipe. In addition, galectin ligands are observed in the hymenium. The subcellular localisation of the galectin ligands suggests these to be present in cellular compartments distinct from galectin transport. The sensitivity of the in situ interactions with exogenous galectin towards detergents and organic solvents infers that these ligands are lipid-borne. Accordingly, lipid fractions from primordia are shown to contain galectin-binding compounds. Based on these results and the determined binding specificity towards substituted beta-galactosides we hypothesise that beta-galactoside-containing lipids (basidiolipids) found in mushrooms are physiological ligands for the galectins in C. cinerea.

Engineering N-linked protein glycosylation with diverse O antigen lipopolysaccharide structures in Escherichia coli

Campylobacter jejuni has a general N-linked protein glycosylation system that can be functionally transferred to Escherichia coli. In this study, we engineered E. coli cells in a way that two different pathways, protein N-glycosylation and lipopolysaccharide (LPS) biosynthesis, converge at the step in which PglB, the key enzyme of the C. jejuni N-glycosylation system, transfers O polysaccharide from a lipid carrier (undecaprenyl pyrophosphate) to an acceptor protein. PglB was the only protein of the bacterial N-glycosylation machinery both necessary and sufficient for the transfer. The relaxed specificity of the PglB oligosaccharyltransferase toward the glycan structure was exploited to create novel N-glycan structures containing two distinct E. coli or Pseudomonas aeruginosa O antigens. PglB-mediated transfer of polysaccharides might be valuable for in vivo production of O polysaccharides-protein conjugates for use as antibacterial vaccines.

Congenital disorder of glycosylation (CDG) Ig: report on a patient and review of the literature

We report a new patient with CDG Ig and review the five other known patients. From the data on this small number of patients, it seems that the association of psychomotor retardation, male hypogenitalism and decreased serum IgG in a patient with a type 1 pattern of serum sialotransferrins might be a clue to the diagnosis of CDG Ig.

Role of peg formation in clamp cell fusion of homobasidiomycete fungi

In most filamentous basidiomycetes, clamp cells are found at the septa of dikaryotic mycelia. Clamp cell formation starts at hyphal tip cells with the development of a lateral bulge at a position slightly apical to the future septum. Relative to the growth direction of the hypha, the protrusion expands backwards into a hook-like structure. Next, the two genetically different haploid nuclei within the hyphal tip cell divide. A septum appears between clamp cell and hyphal tip cell, thereby trapping one nucleus within the clamp cell. Another septum is laid within the hypha, separating a nucleus of the other type in the newly generated subapical hyphal cell from the two different nuclei kept together in the new apical hyphal cell. Through fusion of clamp and subapical cell, the two solitary nuclei become united within the subapical hyphal compartment. In 1933, Buller described subapical formation of a peg to which the clamp cell fuses as an additional, subsequently neglected step in this series of events. In this study, we represent evidence for subapical peg formation and its role in clamp cell fusion. Our observations potentially indicate a B mating type regulated extracellular communication between clamp and subapical hyphal cell.

The N-X-S/T consensus sequence is required but not sufficient for bacterial N-linked protein glycosylation

In the Gram-negative bacterium Campylobacter jejuni there is a pgl (protein glycosylation) locus-dependent general N-glycosylation system of proteins. One of the proteins encoded by pgl locus, PglB, a homolog of the eukaryotic oligosaccharyltransferase component Stt3p, is proposed to function as an oligosaccharyltransferase in this prokaryotic system. The sequence requirements of the acceptor polypeptide for N-glycosylation were analyzed by reverse genetics using the reconstituted glycosylation of the model protein AcrA in Escherichia coli. As in eukaryotes, the N-X-S/T sequon is an essential but not a sufficient determinant for N-linked protein glycosylation. This conclusion was supported by the analysis of a novel C. jejuni glycoprotein, HisJ. Export of the polypeptide to the periplasm was required for glycosylation. Our data support the hypothesis that eukaryotic and bacterial N-linked protein glycosylation are homologous processes.

Structure and functional analysis of the fungal galectin CGL2

Recognition of and discrimination between potential glyco-substrates is central to the function of galectins. Here we dissect the fundamental parameters responsible for such selectivity by the fungal representative, CGL2. The 2.1 A crystal structure of CGL2 and five substrate complexes reveal that this prototype galectin achieves increased substrate specificity by accommodating substituted oligosaccharides of the mammalian blood group A/B type in an extended binding cleft. Kinetic studies on wild-type and mutant CGL2 proteins demonstrate that the tetrameric organization is essential for functionality. The geometric constraints due to the orthogonal orientation of the four binding sites have important consequences on substrate binding and selectivity.

Roles of N-linked glycans in the endoplasmic reticulum

From a process involved in cell wall synthesis in archaea and some bacteria, N-linked glycosylation has evolved into the most common covalent protein modification in eukaryotic cells. The sugars are added to nascent proteins as a core oligosaccharide unit, which is then extensively modified by removal and addition of sugar residues in the endoplasmic reticulum (ER) and the Golgi complex. It has become evident that the modifications that take place in the ER reflect a spectrum of functions related to glycoprotein folding, quality control, sorting, degradation, and secretion. The glycans not only promote folding directly by stabilizing polypeptide structures but also indirectly by serving as recognition "tags" that allow glycoproteins to interact with a variety of lectins, glycosidases, and glycosyltranferases. Some of these (such as glucosidases I and II, calnexin, and calreticulin) have a central role in folding and retention, while others (such as alpha-mannosidases and EDEM) target unsalvageable glycoproteins for ER-associated degradation. Each residue in the core oligosaccharide and each step in the modification program have significance for the fate of newly synthesized glycoproteins.

Identification and functional analysis of a defect in the human ALG9 gene: definition of congenital disorder of glycosylation type IL

Defects of lipid-linked oligosaccharide assembly lead to alterations of N-linked glycosylation known as "type I congenital disorders of glycosylation" (CDG). Dysfunctions along this stepwise assembly pathway are characterized by intracellular accumulation of intermediate lipid-linked oligosaccharides, the detection of which contributes to the identification of underlying enzymatic defects. Using this approach, we have found, in a patient with CDG, a deficiency of the ALG9 alpha 1,2 mannosyltransferase enzyme, which causes an accumulation of lipid-linked-GlcNAc(2)Man(6) and -GlcNAc(2)Man(8) structures, which was paralleled by the transfer of incomplete oligosaccharides precursors to protein. A homozygous point-mutation 1567G-->A (amino acid substitution E523K) was detected in the ALG9 gene. The functional homology between the human ALG9 and Saccharomyces cerevisiae ALG9, as well as the deleterious effect of the E523K mutation detected in the patient with CDG, were confirmed by a yeast complementation assay lacking the ALG9 gene. The ALG9 defect found in the patient with CDG--who presented with developmental delay, hypotonia, seizures, and hepatomegaly--shows that efficient lipid-linked oligosaccharide synthesis is required for proper human development and physiology. The ALG9 defect presented here defines a novel form of CDG named "CDG-IL."

Deficiency of the first mannosylation step in the N-glycosylation pathway causes congenital disorder of glycosylation type Ik

Defects of N-linked glycosylation represent diseases with multiple organ involvements that are classified as congenital disorders of glycosylation (CDG). In recent years, several CDG types have been attributed to defects of dolichol-linked oligosaccharide assembly in the endoplasmic reticulum. The profiling of [3H]mannose-labeled lipid-linked oligosaccharides was instrumental in identifying most of these glycosylation disorders. However, this method is poorly suited for the identification of short lipid-linked oligosaccharide biosynthesis defects. To adequately resolve deficiencies affecting the first steps of lipid-linked oligosaccharide formation, we have used a non-radioactive procedure employing the fluorescence detection of 2-aminobenzamide-coupled oligosaccharides after HPLC separation. By applying this method, we have detected the accumulation of dolichylpyrophosphate-GlcNAc2 in a previously untyped CDG patient. The accumulation pattern suggested a deficiency of the ALG1 beta1,4 mannosyltransferase, which adds the first mannose residue to lipid-linked oligosaccharides. This was supported by the finding that this CDG patient was compound heterozygous for three mutations in the ALG1 gene, leading to the amino acid substitutions S150R and D429E on one allele and S258L on the other. The detrimental effect of these mutations on ALG1 protein function was demonstrated in a complementation assay using alg1 Saccharomyces cerevisiae yeast mutants. The ALG1 mannosyltransferase defect described here represents a novel type of CDG, which should be referred to as CDG-Ik.

Multiple endoplasmic reticulum-associated pathways degrade mutant yeast carboxypeptidase Y in mammalian cells

The degradation of misfolded and unassembled proteins by the endoplasmic reticulum (ER)-associated degradation (ERAD) has been shown to occur mainly through the ubiquitin-proteasome pathway after transport of the protein to the cytosol. Recent work has revealed a role for N-linked glycans in targeting aberrant glycoproteins to ERAD. To further characterize the molecular basis of substrate recognition and sorting during ERAD in mammalian cells, we expressed a mutant yeast carboxypeptidase Y (CPY*) in CHO cells. CPY* was retained in the ER in un-aggregated form, and degraded after a 45-min lag period. Degradation was predominantly by a proteasome-independent, non-lysosomal pathway. The inhibitor of ER mannosidase I, kifunensine, blocked the degradation by the alternate pathway but did not affect the proteasomal fraction of degradation. Upon inhibition of glucose trimming, the initial lag period was eliminated and degradation thus accelerated. Our results indicated that, although the proteasome is a major player in ERAD, alternative routes are present in mammalian cells and can play an important role in the disposal of both glycoproteins and non-glycoproteins.

Congenital scoliosis

Congenital scoliosis is the most frequent congenital deformity of the spine. Congenital curvatures are due to anomalous development of the vertebrae (failure of formation and/or segmentation). Congenital scoliosis is believed to be related to an insult to the fetus during spine embryological development, and associated malformations (heart, spinal cord, kidney.) are frequently observed. A perfect understanding of the natural history of the deformity and the treatment principles will allow best management of these complex spine deformities. New imaging techniques like three-dimensional computed tomography (CT) and magnetic resonance imaging (MRI) are important tools for analyzing the underlying deformity and understanding the evolution of the complex deformities. The mainstay of treatment is either observation or, in case of curve progression (>10 degrees /year), surgery. Different surgeries are described with two main principles: (1) prophylactic surgeries like hemiepiphysiodesis or in situ fusions that will prevent worsening or allow progressive correction over time, and (2) corrective surgeries, with spinal fusion with or without spinal resection. Exceptional procedures (e.g. spinal column resection or halo distraction) can be attempted in cases of very severe deformity. Congenital curves must be carefully observed to choose the least invasive procedure at the right time and to minimize spinal cord risks.

Regulation of hyphal growth and sporulation of the insect pathogenic fungus Entomophthora thripidum in vitro

Entomophthora thripidum is an obligate biotrophic insect pathogenic fungus that grows as protoplasts within the hemocoel of thrips. Prior to penetration through the insect cuticle and spore formation at the insect surface the protoplasts switch to hyphal growth. In vitro, the differentiation to hyphal growth was a prerequisite for the subsequent formation of infectious spores and was detected 10-20 days after inoculation. E. thripidum secreted a factor that autoinduced the differentiation to hyphal growth. The discovery of this activity inducing hyphal growth made possible the reliable production of spores, the infection of host insects and the consecutive re-isolation of the fungus from the infected insects.

The validity of clinical examination in the diagnosis of loosening of components in total hip arthroplasty

We analysed follow-up data from 18,486 primary total hip arthroplasties performed between 1967 and 2001 to assess the validity of clinical procedures in diagnosing loosening of prosthetic components. Sensitivity, specificity and predictive values were estimated with the radiological definition of loose or not loose as the 'gold standard'. The prevalence of acetabular loosening increased from 0.6% to 13.9% during the period of the study and that of femoral loosening from 0.9% to 12.1%. Sensitivities and positive predictive values were low, suggesting that clinical procedures could not replace radiological assessment in the identification of loose prostheses. Specificities and negative predictive values were constantly above 0.86. The possibility of there being a prosthesis which is not loose in asymptomatic patients was consequently very high, particularly during the first five to six years after operation. The necessity of periodic clinical and radiological follow-up examinations of asymptomatic patients during the first five to six years after operation remains questionable. Symptomatic patients, however, require radiological assessment.

N-linked glycosylation in Campylobacter jejuni and its functional transfer into E. coli

N-linked protein glycosylation is the most abundant posttranslation modification of secretory proteins in eukaryotes. A wide range of functions are attributed to glycan structures covalently linked to asparagine residues within the asparagine-X-serine/threonine consensus sequence (Asn-Xaa-Ser/Thr). We found an N-linked glycosylation system in the bacterium Campylobacter jejuni and demonstrate that a functional N-linked glycosylation pathway could be transferred into Escherichia coli. Although the bacterial N-glycan differs structurally from its eukaryotic counterparts, the cloning of a universal N-linked glycosylation cassette in E. coli opens up the possibility of engineering permutations of recombinant glycan structures for research and industrial applications.

Influence of activated A and B mating-type pathways on developmental processes in the basidiomycete Coprinus cinereus

The A and B mating type pathways in Coprinus cinereus monokaryons can be activated by transformation with cloned genes from strains of compatible mating types. The presence of heterologous A mating-type genes (Aon) induces production of submerged chlamydospores, hyphal knots and sclerotia in cultures kept in the dark. Upon illumination of transformants of certain strains (218), fruiting body primordia may develop that arrest before karyogamy. Furthermore, formation of aerial spores (oidia) is repressed by the action of A mating type genes in the dark, but light overrides this repression. Heterologous B mating type genes enhance the effects of the A genes on developmental processes, and partially repress the negative action of light on A-mediated regulation of development. Most notably, A-induced fruiting occurs more efficiently and earlier when the B mating type pathway is also active (Bon). However, activation of the B pathway alone is not sufficient to induce fruiting. Unlike A-activated transformants, A+ B-activated transformants of monokaryon 218 form mature fruiting bodies. Therefore, the B genes control fruiting body maturation at the stage of karyogamy. Basidia within the fruiting bodies that were analysed contained four spores in a typical post-meiotic arrangement. In the absence of an activated A mating type pathway, B mating type genes cause deformation and hyperbranching of vegetative hyphae, a reduction in aerial mycelium, and invasion of the agar substrate - a phenotype resembling the "flat" phenotype known from B-activated Schizophyllum commune strains. B-activated transformants usually show enhanced production of chlamydospores and hyphal knots, but maturation of sclerotia is variably efficient. Activation of the B mating type pathway in monokaryons blocked acceptance of nuclei, but not activation of the A mating type pathway.

A biological approach to treating disc degeneration: not for today, but maybe for tomorrow

The intervertebral disc unites the vertebrae in the spine, providing the flexibility required for bending and twisting and resisting the compression inflicted by gravity when in an upright posture. The discs have a complex structure, with the outer annulus fibrosus having lamellae of organized collagen fibrils and the inner nucleus pulposus having a more random collagen organization and an abundance of aggregating proteoglycans. This composite nature endows the disc with both the tension-resisting properties of a ligament and the compression-resisting properties of articular cartilage. Unfortunately, disc structure and function does not remain optimal throughout life, but undergoes progressive degeneration, commencing in the young adult, and is particularly evident in the nucleus pulposus. With time, disc degeneration may result in clinical symptoms, such as low back pain, and require medical intervention. Such treatment may involve removal of the offending disc by surgery rather than its repair, which would be the preferred course of action. In the near future, current bioengineering techniques may offer the possibility of repairing the damaged disc, if an engineered tissue with the appropriate functional properties can be generated to augment the ailing disc. In this report, we summarized our recent results, in which disc cells were implanted into a scaffold of collagen and hyaluronan, or entrapped into a chitosan gel, and growth factors were used to modulate matrix synthesis in an attempt to produce a tissue with a similar molecular composition to native nucleus pulposus tissue.