Congenital disorders of glycosylation: genetic model systems lead the way.

N-linked glycosylation is the most frequent modification of secretory proteins in eukaryotic cells. The highly conserved glycosylation process is initiated in the endoplasmic reticulum (ER), where the Glc(3)Man(9)GlcNAc(2) oligosaccharide is assembled on the lipid carrier dolichylpyrophosphate and then transferred to selected asparagine residues of polypeptide chains. In recent years, several inherited human diseases, congenital disorders of glycosylation (CDG), have been associated with deficiencies in this pathway. The ER-associated glycosylation pathway has been studied in the budding yeast Saccharomyces cerevisiae, and this model system has been invaluable in elucidating the molecular basis of novel types of CDG.

A new case of ALG8 deficiency (CDG Ih).

UNLABELLED: Congenital disorders of glycosylation (CDG) represent an expanding group of inherited diseases. One of them, ALG8 deficiency (CDG Ih), leads to protein N-glycosylation defects caused by malfunction of glucosyltransferase 2 (Dol-P-Glc:Glc1-Man(9)-GlcNAc(2)-P-P-Dol glucosyltransferase) resulting in inefficient addition of the second glucose residue onto lipid-linked oligosaccharides. So far, only five patients have been described with ALG8 deficiency. We present a new patient with neonatal onset. The girl was born at the 29th week of gestation complicated by oligohydramnios. Although the early postnatal adaptation was uneventful (Apgar score 8 and 9 at 5 and 10 min), generalized oedema, multifocal myoclonic seizures, and bleeding due to combined coagulopathy were present from the first day. Diarrhoea progressing to protein-losing enteropathy with ascites and pericardial effusion developed in the third week of life. Pharmacoresistant seizures and cortical, cerebellar and optic nerve atrophy indicated neurological involvement. No symptoms of liver disease except coagulopathy were observed; however, steatofibrosis with cholestasis was found at autopsy. The girl died at the age of 2 months owing to the progressive general oedema, bleeding and cardio-respiratory insufficiency. Molecular analysis revealed two heterozygous mutations in the ALG8 gene: c.139A>C (p.T47P) and the novel mutation c.1090C>T (p.R364X). CONCLUSION: The prognosis of patients with ALG8 deficiency is unfavourable. The majority of affected children have early onset of the disease with heterogeneous symptoms including multiple organ dysfunction, coagulopathy and protein-losing enteropathy. Neurological impairment is not a general clinical symptom, but it has to be taken into consideration when thinking about ALG8 deficiency.

RFT1-CDG: deafness as a novel feature of congenital disorders of glycosylation.

Congenital disorders of glycosylation (CDG) are genetic diseases due to defects in the synthesis of glycans and in the attachment of glycans to lipids and proteins. Actually, some 42 CDG are known including defects in protein N-glycosylation, in protein O-glycosylation, in lipid glycosylation, and in multiple and other glycosylation pathways. Most CDG are multisystem diseases and a large number of signs and symptoms have already been reported in CDG. An exception to this is deafness. This symptom has not been observed as a consistent feature in CDG. In 2008, a novel defect was identified in protein N-glycosylation, namely in RFT1. This is a defect in the assembly of N-glycans. RFT1 is involved in the transfer of Man(5)GlcNAc(2)-PP-Dol from the cytoplasmic to the luminal side of the endoplasmic reticulum. According to the novel nomenclature (non-italicized gene symbol followed by -CDG) this defect is named RFT1-CDG. Recently, three other patients with RFT1-CDG have been reported and here we report two novel patients. Remarkably, all six patients with RFT1-CDG show sensorineural deafness as part of a severe neurological syndrome. We conclude that RFT1-CDG is the first 'deafness-CDG'. CDG should be included in the work-up of congenital, particularly syndromic, hearing loss.

On the nomenclature of congenital disorders of glycosylation (CDG).

A new nomenclature of CDG is proposed because the current one is too complex for clinicians and provides no added value.

Deficiency of dolichol-phosphate-mannose synthase-1 causes congenital disorder of glycosylation type Ie.

Congenital disorders of glycosylation (CDG), formerly known as carbohydrate-deficient glycoprotein syndromes, lead to diseases with variable clinical pictures. We report the delineation of a novel type of CDG identified in 2 children presenting with severe developmental delay, seizures, and dysmorphic features. We detected hypoglycosylation on serum transferrin and cerebrospinal fluid beta-trace protein. Lipid-linked oligosaccharides in the endoplasmic reticulum of patient fibroblasts showed an accumulation of the dolichyl pyrophosphate Man(5)GlcNAc(2) structure, compatible with the reduced dolichol-phosphate-mannose synthase (DolP-Man synthase) activity detected in these patients. Accordingly, 2 mutant alleles of the DolP-Man synthase DPM1 gene, 1 with a 274C>G transversion, the other with a 628delC deletion, were detected in both siblings. Complementation analysis using DPM1-null murine Thy1-deficient cells confirmed the detrimental effect of both mutations on the enzymatic activity. Furthermore, mannose supplementation failed to improve the glycosylation status of DPM1-deficient fibroblast cells, thus precluding a possible therapeutic application of mannose in the patients. Because DPM1 deficiency, like other subtypes of CDG-I, impairs the assembly of N-glycans, this novel glycosylation defect was named CDG-Ie.

MPDU1 mutations underlie a novel human congenital disorder of glycosylation, designated type If.

Deficiencies in the pathway of N-glycan biosynthesis lead to severe multisystem diseases, known as congenital disorders of glycosylation (CDG). The clinical appearance of CDG is variable, and different types can be distinguished according to the gene that is altered. In this report, we describe the molecular basis of a novel type of the disease in three unrelated patients diagnosed with CDG-I. Serum transferrin was hypoglycosylated and patients' fibroblasts accumulated incomplete lipid-linked oligosaccharide precursors for N-linked protein glycosylation. Transfer of incomplete oligosaccharides to protein was detected. Sequence analysis of the Lec35/MPDU1 gene, known to be involved in the use of dolichylphosphomannose and dolichylphosphoglucose, revealed mutations in all three patients. Retroviral-based expression of the normal Lec35 cDNA in primary fibroblasts of patients restored normal lipid-linked oligosaccharide biosynthesis. We concluded that mutations in the Lec35/MPDU1 gene cause CDG. This novel type was termed CDG-If.

Expression of BCL-2 protein enhances the survival of mouse fibrosarcoid cells in tumor necrosis factor-mediated cytotoxicity.

Tumor necrosis factor (TNF) kills some types of tumor cells in vitro and participates in tumor elimination in vivo. TNF has been shown to kill cells by altering their mitochondria structurally and functionally. The oncogene BCL-2 codes for a protein located in the inner membrane of mitochondria which is able to inhibit the commitment to cell death in various cell types. We have therefore investigated whether TNF-mediated killing of the cell line L929 could be modulated by expression of the protein BCL-2. We report here that L929 cells transfected with a BCL-2 expression vector have an increased survival compared to wild type cells after TNF challenge. The protective effect is greatest at moderate TNF concentrations and is still significant at concentrations that killed 100% of wild type cells. The action of BCL-2 is selective inasmuch as cells are not protected against other cytotoxic agents blocking various mitochondrial functions. We show that cells expressing BCL-2 have a higher mitochondrial membrane potential (delta psi) than wild type cells. The increase in delta psi could be linked with the enhanced survival of cells after TNF challenge. Indeed, we found that treatment of wild type L929 cells with the ionophore nigericin, which increases delta psi, protects them even at high TNF concentrations.

The remodeling of glycoconjugates in mice.

A role for glycoconjugates in mediating cellular interactions is well established. To further understand the formation, function and regulation of various glycoconjugates in vivo, gene targeting approaches have been applied to glycosyltransferase and glycosidase enzymes involved in different biosynthetic pathways. The growing number of gene targeted mice generated have brought confirmations of the importance of both core and terminal glycosylation enzymes in normal development and physiology. Of particular interest has been the degree of cell and tissue specificity of phenotypes generated by systemic null mutations as well as the number of enzymes belonging to multigene families having overlapping activities.

TNF alpha alters mitochondrial membrane potential in L929 but not in TNF alpha-resistant L929.12 cells: relationship with the expression of stress proteins, annexin 1 and superoxide dismutase activity.

Tumour necrosis factor alpha (TNF alpha) cytotoxicity is mediated, at least in part, by oxidative stress and phospholipase A2 activation. The first post-receptor events to be observed in TNF alpha-sensitive lines are the generation of superoxide anion (O2-) within the mitochondria and the activation of phospholipase A2. Using the lipophilic dye JC-1 to determine mitochondrial membrane potential, we showed that TNF alpha induces time-dependent alterations in mitochondrial membrane potential in L929 cells but not in the TNF alpha-resistant L929. 12 subclone. Heat shock (HS) proteins (HSP) and superoxide dismutase (SOD) have been shown to protect cells from TNF alpha cytotoxicity, while glucose regulated proteins (GRP) and annexins might also be involved in cellular protection. We thus compared the expression of HSP, grp78 and annexin 1 as well as SOD activity in TNF alpha sensitive and resistant lines. We found no difference in the expression of HSP, grp78 or annexin 1, but an increase in the constitutive activity of SOD in the L929.12 cells as compared to L929. Furthermore, SOD was inducible by TNF alpha in L929 cells, but not in L929.12 cells. These data suggest that in TNF alpha-resistant lines, mitochondrial damage by TNF alpha is prevented by an increase in SOD rather than in overexpression of stress proteins or annexins.

[Transgenic mice in basic research]. / Transgene Mäuse in der Grundlagenforschung.

What are transgenic mice and what do we learn from them? In this review, we focus on the generation of "classical" transgenic and "knock-out" mice. The establishment of transgenic and gene-targeted mice provides an unique tool to study the function(s) of a given gene in the context of a whole organism. Based on selected examples, we demonstrate the potential of this transgenic technology to understand the interactions between cells, organs and organ systems in genetically engineered mice.

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.

The galactosyltransferase family.

Galactose is transferred via several linkages to acceptor structures by galactosyltransferase enzymes. In prokaryotes, galactose is mainly found on lipopolysaccharides and capsular polysaccharides. In eukaryotes, galactosyltransferases, which are localized in the Golgi apparatus, are involved in the formation of several classes of glycoconjugates and in lactose biosynthesis. Although they sometimes catalyze identical reactions, prokaryotic and eukaryotic galactosyltransferases share only little structural similarities. In mammals, 19 distinct galactosyltransferase enzymes have been characterized to date. These enzymes catalyze the transfer of galactose via beta1-4, beta1-3, alpha1-3 and alpha1-4 linkages. The present review focuses on the description of these mammalian galactosyltransferases.

Alterations in antioxidant defences in lung and liver of mice infected with influenza A virus.

We investigated the possible involvement of oxidative mechanisms in the pathogenesis of influenza A/PR8/34 virus infection in mice. As a biochemical marker of oxidative stress, we determined the endogenous concentrations of the antioxidants glutathione and vitamins C and E in their reduced and oxidized forms in the lungs, liver and blood plasma of control and infected animals. Following intranasal infection with 8 to 10 LD50, influenza virus was detected in the lungs, but not in the plasma, liver or other organs. Infection resulted in a decrease in the total concentration of glutathione and vitamins C and E, whereas no relevant change in the ratio of oxidized to total concentration of antioxidants was observed. Changes in the concentration of hepatic antioxidants were significant in the early stages of the infection. The results suggest that hepatic alterations may be caused indirectly by mechanisms related to the host response to virus infection. The observed general decrease in the antioxidant buffering capacity may reduce the ability of tissues to protect against potential oxidative stress. Such stress can occur during bacterial superinfections, which are common in influenza, thereby rendering the host more susceptible to the pathogenic effects of such agents. In addition, reactive oxygen species produced in the lung may inactivate protease inhibitors, resulting in increased protease activity. Using an in vitro system consisting of alpha 1-antiprotease, trypsin and HOCl as the oxidant, we have shown that the infectivity of influenza viruses can be increased up to 10,000-fold by proteolytic cleavage of haemagglutinin, leading to activation of the fusogenic properties of this protein.

Tumour necrosis factor-alpha induces superoxide anion generation in mitochondria of L929 cells.

Within a few minutes after addition to L929 cells, tumour necrosis factor-alpha (TNF alpha) induced an increase in lucigenin-enhanced chemiluminescence that could be inhibited by superoxide dismutase. The generation of superoxide anion (O2.-) was sensitive to treatment with rotenone, antimycin A and cyanide, indicating that the signal originated from mitochondria. The mechanism of production of O2.- was shown to be independent of ATP synthesis, as uncoupling of this event from mitochondrial electron transport did not alter the generation of O2.- induced by TNF alpha. Chemiluminescence was further dependent on the presence of extracellular calcium, suggesting a role for this cation as a second messenger. This hypothesis was supported by the finding that inhibition of mitochondrial calcium uptake by Ruthenium Red exerted a protective effect on TNF alpha-treated L929 cells. Increased O2.- generation was followed by a marked decrease in mitochondrial dehydrogenase activity and cellular ATP levels, while cell membrane permeability was moderately increased. A role for mitochondrial O2.- generation in TNF alpha cytotoxicity was further supported by the finding that resistant L929 cells had decreased ability to produce O2.- in response to TNF alpha. In addition, we detected a decreased activity of the mitochondrial enzyme succinate dehydrogenase in these cells, suggesting that this component of the respiratory chain might be an important contributor to the TNF alpha-induced generation of O2.-.

Lipopolysaccharide synergizes with tumour necrosis factor-alpha in cytotoxicity assays.

Lipopolysaccharide (LPS) from Escherichia coli was found to synergize with human recombinant tumour necrosis factor-alpha (TNF-alpha) in the lysis of L929 and WEHI 164 (clone 13) murine fibroblasts, two cell lines classically used in TNF-alpha bioassays. The effect was noted with TNF-alpha at low (sublytic or lightly lytic) concentrations and was significant for LPS concentrations in the ng range. The LPS effect could be inhibited by polymyxin B, and was not observed when the TNF-alpha assay was performed in the absence of actinomycin D. Enhancement of TNF-alpha lysis by LPS occurred in several assays for determining TNF-alpha, including MTT cleavage, crystal violet staining and lactate dehydrogenase release. Synergism was obtained only when LPS and TNF-alpha were added to cells simultaneously, but not when applied in sequence. The reported synergism may be relevant for TNF-alpha determinations by bioassay, and for the understanding of pathophysiology of Gram-negative sepsis.

A kinetic study of immune mediators in the lungs of mice infected with influenza A virus.

We investigated a broad spectrum of immunoactive mediators in a mouse model of influenza. ICR mice (4-5 wk old) that were infected with a 10 LD50 dose of influenza A/PR8/34 virus died after 6 days without evidence of bacterial superinfection. Maximal virus titers were reached by day 2 postinfection, whereas the multifocal pneumonia with mononuclear cell infiltration reached its maximum at the end of infection. We measured the cytokines IL-1 alpha, IL-1 beta, IL-2, IL-3, IL-4, IL-6, IFN-gamma, TNF-alpha, granulocyte (G)/macrophage (M)-CSF, G-CSF, M-CSF, and the lipid mediators leukotriene B4 and platelet-activating factor in the cellfree bronchoalveolar lavage fluid of mice during infection. We found an early increase of IL-1 alpha, IL-1 beta, IL-6, TNF-alpha, GM-CSF, IFN-gamma, and leukotriene B4. Levels of these factors peaked between 36 h and day 3 postinfection, with the exception of IL-6 that remained at elevated levels throughout infection. G-CSF and M-CSF increased slowly and reached a maximum by day 5 postinfection. We were unable to detect IL-2, IL-3, or IL-4. PAF remained at the same level throughout infection. Our results suggest that lung-resident cells, and possibly the alveolar macrophages, participate actively in the onset of the inflammatory response against the invading virus. The inability to detect the T cell products IL-2, IL-3, and IL-4 was unexpected considering the role of T cells in the elimination of the virus in infected mice. Our observation confirms thus earlier findings about the inability of specific T cell clones to elicit an unspecific antiviral effect.

Secretion and purification of recombinant beta1-4 galactosyltransferase from insect cells using pFmel-protA, a novel transposition-based baculovirus transfer vector.

The palette of transfer vectors available for generation of recombinant baculoviruses based on transposition-mediated recombination has been enlarged by constructing the pFmel-protA vector. The pFmel-protA plasmid includes the honeybee melittin secretion signal and a Staphylococcus aureus protein A fusion protein tag, which allows the secretion and purification of recombinant proteins. Using this system, the human beta1-4 galactosyltransferase-I protein was expressed in Sf9 insect cells at a level ranging from 22 to 28 U (4.8 to 6.0 mg)/L. The protein A tag enabled a simple monitoring of recombinant protein expression by enzyme-linked immunosorbent assay and Western blotting. Single step purification was achieved by immunoglobulin G affinity chromatography achieving a recovery yield of 28% and a specific activity of 1.9 U per mg of recombinant protein.

Molecular cloning of a human UDP-galactose:GlcNAcbeta1,3GalNAc beta1, 3 galactosyltransferase gene encoding an O-linked core3-elongation enzyme.

Using the full-length amino-acid sequences of the human beta1,3 galactosyltransferase (beta3GalT)-I, -II and III enzymes as query, we have identified an additional member of the beta3GalT gene family within a sequenced region of the human chromosome 21 as found in GenBank. The novel human beta3GalT-V gene included an open reading frame of 933 bp encoding a protein of 310 amino acids with a short N-terminal cytoplasmic tail, a single predicted transmembrane domain and a large lumenal catalytic domain. The human beta3GalT-V protein showed 34%, 27%, 31% and 23% sequence identity with the human beta3GalT-I, -II, -III and -IV enzymes, respectively. The expression of beta3GalT-V as a recombinant protein in Sf9 insect cells confirmed the galactosyltransferase activity catalyzed by this enzyme. Similarly to beta3GalT-I, -II and -III, the beta3GalT-V enzyme used beta-linked GlcNAc as an acceptor, but unlike the former enzymes beta3GalT-V exhibited a marked preference for the O-linked core3 GlcNAcbeta1,3GalNAc substrate. The beta3GalT-V gene was mainly expressed in human small intestine and to a lesser extent in pancreas and testis. Although beta3GalT-V transcripts were not detected in normal colon tissue, based on Northern analysis, beta3GalT-V mRNA was found in the adenocarcinoma cell line Colo 205.

T-cell-specific deletion of a polypeptide N-acetylgalactosaminyl-transferase gene by site-directed recombination.

UDP-N-acetylgalactosamine (GalNAc): polypeptide N-acetylgalactosaminyltransferase (polypeptide GalNAc-T) catalyzes transfer of the monosaccharide GalNAc to serine and threonine residues, thereby initiating O-linked oligosaccharide biosynthesis. Previous studies have suggested the possibility of multiple polypeptide GalNAc-Ts, although attachment of saccharide units to polypeptide or lipid in generating oligosaccharide structures in vertebrates has been dependent upon the activity of single gene products. To address this issue and to determine the relevance of Oglycosylation variation in T-cell ontogeny, we have directed Cre/loxP mutagenic recombination to the polypeptide GalNAc-T locus in gene-targeted mice. Resulting deletion in the catalytic region of polypeptide GalNAc-T occurred to completion on both alleles in thymocytes and was found in peripheral T cells, but not among other cell types. Thymocyte O-linked oligosaccharide formation persisted in the absence of a functional targeted polypeptide GalNAc-T allele as determined by O-glycan-specific lectin binding. T-cell development and colonization of secondary lymphoid organs were also normal. These results indicate a complexity in vertebrate O-glycan biosynthesis that involves multiple polypeptide GalNAc-Ts. We infer the potential for protein-specific O-glycan formation governed by distinct polypeptide GalNAc-Ts.