Catalytic Reaction Mechanism of Bacterial GH92 α-1,2-Mannosidase: A QM/MM Metadynamics Study.

The catalytic mechanism of a C a + 2 ${C{a}^{+2}}$ -dependent family 92 α ${{\rm \alpha }}$ -mannosidase, which is abundantly present in human gut flora and malfunctions leading to the lysosomal storage disease α-mannosidosis, has been investigated using quantum mechanics/molecular mechanics and metadynamics methods. Computational efforts show that the enzyme follows a conformational itinerary of and the C a + 2 ${C{a}^{+2}}$ ion serves a dual purpose, as it not only distorts the sugar ring but also plays a crucial role in orchestrating the arrangement of catalytic residues. This orchestration, in turn, contributes to the facilitation of O S 2 ${{{\rm \ }}^{{\rm O}}{{\rm S}}_{2}}$ conformers for the ensuing reaction. This mechanistic insight is well-aligned with the experimental predictions of the catalytic pathway, and the computed energies are of the same order of magnitude as the experimental estimations. Hence, our results extend the mechanistic understanding of glycosidases.

Pharmacological Chaperones for the Treatment of α-Mannosidosis.

α-Mannosidosis (AM) results from deficient lysosomal α-mannosidase (LAMAN) activity and subsequent substrate accumulation in the lysosome, leading to severe pathology. Many of the AM-causative mutations compromise enzyme folding and could be rescued with purpose-designed pharmacological chaperones (PCs). We found that PCs combining a LAMAN glycone-binding motif based on the 5 N,6 O-oxomethylidenemannojirimycin (OMJ) glycomimetic core and different aglycones, in either mono- or multivalent displays, elicit binding modes involving glycone and nonglycone enzyme regions that reinforce the protein folding and stabilization potential. Multivalent derivatives exhibited potent enzyme inhibition that generally prevailed over the chaperone effect. On the contrary, monovalent OMJ derivatives with LAMAN aglycone binding area-fitting substituents proved effective as activity enhancers for several mutant LAMAN forms in AM patient fibroblasts and/or transfected MAN2 B1-KO cells. This translated into a significant improvement in endosomal/lysosomal function, reverting not only the primary LAMAN substrate accumulation but also the additional downstream consequences such as cholesterol accumulation.

Alpha-Mannosidosis: Therapeutic Strategies.

Alpha-mannosidosis (α-mannosidosis) is a rare lysosomal storage disorder with an autosomal recessive inheritance caused by mutations in the gene encoding for the lysosomal α-d-mannosidase. So far, 155 variants from 191 patients have been identified and in part characterized at the biochemical level. Similarly to other lysosomal storage diseases, there is no relationship between genotype and phenotype in alpha-mannosidosis. Enzyme replacement therapy is at the moment the most effective therapy for lysosomal storage disease, including alpha-mannosidosis. In this review, the genetic of alpha-mannosidosis has been described together with the results so far obtained by two different therapeutic strategies: bone marrow transplantation and enzyme replacement therapy. The primary indication to offer hematopoietic stem cell transplantation in patients affected by alpha-mannosidosis is preservation of neurocognitive function and prevention of early death. The results obtained from a Phase I⁻II study and a Phase III study provide evidence of the positive clinical effect of the recombinant enzyme on patients with alpha-mannosidosis.

Intestinal Epithelial Cell-specific Deletion of α-Mannosidase II Ameliorates Experimental Colitis.

Inflammatory bowel disease (IBD) is a refractory disease of the gastrointestinal tract that is believed to develop in genetically susceptible individuals. Glycosylation, a type of post-translational modification, is involved in the development of a wide range of diseases, including IBD, by modulating the function of various glycoproteins. To identify novel genes contributing to the development of IBD, we analyzed single nucleotide polymorphisms (SNPs) of glycosylation-related genes in IBD patients and identified MAN2A1, encoding alpha-mannosidase II (α-MII), as a candidate gene. α-MII plays a crucial, but not exclusive, role in the maturation of N-glycans. We also observed that intestinal epithelial cells (IECs), which establish the first-line barrier and regulate gut immunity, selectively expressed α-MII with minimal expression of its isozyme, alpha-mannosidase IIx (α-MIIx). This led us to hypothesize that IEC-intrinsic α-MII is implicated in the pathogenesis of IBD. To test this hypothesis, we generated IEC-specific α-MII-deficient (α-MIIΔIEC) mice. Although α-MII deficiency has been shown to have a minimal effect on N-glycan maturation in most cell types due to the compensation by α-MIIx, ablation of α-MII impaired the maturation of N-glycans in IECs. α-MIIΔIEC mice were less susceptible to dextran sulfate sodium-induced colitis compared with control littermates. In accordance with this, neutrophil infiltration in the colonic mucosa was attenuated in α-MIIΔIEC mice. Furthermore, gene expression levels of neutrophil-attracting chemokines were downregulated in the colonic tissue. These results suggest that IEC-intrinsic α-MII promotes intestinal inflammation by facilitating chemokine expression. We propose SNPs in MAN2A1 as a novel genetic factor for IBD.Key words: inflammatory bowel disease, alpha-mannosidase II, intestinal epithelial cell, N-glycosylation.

Diffusion Tensor Imaging for Assessing Brain Gray and White Matter Abnormalities in a Feline Model of α-Mannosidosis.

α-Mannosidosis (AMD) is an autosomal recessively inherited lysosomal storage disorder affecting brain function and structure. We performed ex vivo and in vivo diffusion tensor imaging (DTI) on the brains of AMD-affected cats to assess gray and white matter abnormalities. A multi-atlas approach was used to generate a brain template to process the ex vivo DTI data. The probabilistic label method was used to measure fractional anisotropy (FA), mean diffusivity, axial diffusivity, and radial diffusivity values from gray and white matter regions from ex vivo DTI. Regional analysis from various regions of the gray matter (frontal cortex, cingulate gyrus, caudate nucleus, hippocampus, thalamus, and occipital cortex), and white matter (corpus callosum, corticospinal tract, cerebral peduncle, external and internal capsule) was also performed on both ex vivo and in vivo DTI. Ex vivo DTI revealed significantly reduced FA from both gray and white matter regions in AMD-affected cats compared to controls. Significantly reduced FA was also observed from in vivo DTI of AMD-affected cats compared to controls, with lower FA values observed in all white matter regions. We also observed significantly increased axial and radial diffusivity values in various gray and white matter regions in AMD cats from both ex vivo and in vivo DTI data. Imaging findings were correlated with histopathologic analyses suggesting that DTI studies can further aid in the characterization of AMD by assessing the microstructural abnormalities in both white and gray matter.

Alpha-mannosidosis: correlation between phenotype, genotype and mutant MAN2B1 subcellular localisation.

BACKGROUND: Alpha-mannosidosis is caused by mutations in MAN2B1, leading to loss of lysosomal alpha-mannosidase activity. Symptoms include intellectual disabilities, hearing impairment, motor function disturbances, facial coarsening and musculoskeletal abnormalities. METHODS: To study the genotype-phenotype relationship for alpha-mannosidosis 66 patients were included. Based on the predicted effect of the mutations and the subcellular localisation of mutant MAN2B1 in cultured cells, the patients were divided into three subgroups. Clinical and biochemical data were collected. Correlation analyses between each of the three subgroups of genotype/subcellular localisation and the clinical and biochemical data were done to investigate the potential relationship between genotype and phenotype in alpha-mannosidosis. Statistical analyses were performed using the SPSS software. Analyses of covariance were performed to describe the genotype-phenotype correlations. The phenotype parameters were modelled by the mutation group and age as a covariate. P values of <0.05 were considered as statistically significant. RESULTS: Complete MAN2B1 genotypes were established for all patients. We found significantly higher scores in the Leiter-R test, lower concentrations of CSF-oligosaccharides, higher point scores in the Bruininks-Oseretsky Test of Motor Proficiency subtests (BOT-2); Upper limb coordination and Balance, and a higher FVC% in patients in subgroup 3, harbouring at least one variant that allows localisation of the mutant MAN2B1 protein to the lysosomes compared to subgrou 2 and/or subgroup 1 with no lysosomal localization of the mutant MAN2B1 protein. CONCLUSION: Our results indicate a correlation between the MAN2B1 genotypes and the cognitive function, upper limb coordination, balance, FVC% and the storage of oligosaccharides in CSF. This correlation depends on the subcellular localisation of the mutant MAN2B1 protein.

amamutdb.no: A relational database for MAN2B1 allelic variants that compiles genotypes, clinical phenotypes, and biochemical and structural data of mutant MAN2B1 in α-mannosidosis.

α-Mannosidosis is an autosomal recessive lysosomal storage disorder caused by mutations in the MAN2B1 gene, encoding lysosomal α-mannosidase. The disorder is characterized by a range of clinical phenotypes of which the major manifestations are mental impairment, hearing impairment, skeletal changes, and immunodeficiency. Here, we report an α-mannosidosis mutation database, amamutdb.no, which has been constructed as a publicly accessible online resource for recording and analyzing MAN2B1 variants (http://amamutdb.no). Our aim has been to offer structured and relational information on MAN2B1 mutations and genotypes along with associated clinical phenotypes. Classifying missense mutations, as pathogenic or benign, is a challenge. Therefore, they have been given special attention as we have compiled all available data that relate to their biochemical, functional, and structural properties. The α-mannosidosis mutation database is comprehensive and relational in the sense that information can be retrieved and compiled across datasets; hence, it will facilitate diagnostics and increase our understanding of the clinical and molecular aspects of α-mannosidosis. We believe that the amamutdb.no structure and architecture will be applicable for the development of databases for any monogenic disorder.

The N-glycan glycoprotein deglycosylation complex (Gpd) from Capnocytophaga canimorsus deglycosylates human IgG.

C. canimorsus 5 has the capacity to grow at the expenses of glycan moieties from host cells N-glycoproteins. Here, we show that C. canimorsus 5 also has the capacity to deglycosylate human IgG and we analyze the deglycosylation mechanism. We show that deglycosylation is achieved by a large complex spanning the outer membrane and consisting of the Gpd proteins and sialidase SiaC. GpdD, -G, -E and -F are surface-exposed outer membrane lipoproteins. GpdDEF could contribute to the binding of glycoproteins at the bacterial surface while GpdG is a endo-ß-N-acetylglucosaminidase cleaving the N-linked oligosaccharide after the first N-linked GlcNAc residue. GpdC, resembling a TonB-dependent OM transporter is presumed to import the oligosaccharide into the periplasm after its cleavage from the glycoprotein. The terminal sialic acid residue of the oligosaccharide is then removed by SiaC, a periplasm-exposed lipoprotein in direct contact with GpdC. Finally, most likely degradation of the oligosaccharide proceeds sequentially from the desialylated non reducing end by the action of periplasmic exoglycosidases, including ß-galactosidases, ß-N-Acetylhexosaminidases and α-mannosidases.

Impaired lysosomal trimming of N-linked oligosaccharides leads to hyperglycosylation of native lysosomal proteins in mice with alpha-mannosidosis.

Alpha-mannosidosis is caused by the genetic defect of the lysosomal alpha-d-mannosidase (LAMAN), which is involved in the breakdown of free alpha-linked mannose-containing oligosaccharides originating from glycoproteins with N-linked glycans, and thus manifests itself in an extensive storage of mannose-containing oligosaccharides. Here we demonstrate in a model of mice with alpha-mannosidosis that native lysosomal proteins exhibit elongated N-linked oligosaccharides as shown by two-dimensional difference gel electrophoresis, deglycosylation assays, and mass spectrometry. The analysis of cathepsin B-derived oligosaccharides revealed a hypermannosylation of glycoproteins in mice with alpha-mannosidosis as indicated by the predominance of extended Man3GlcNAc2 oligosaccharides. Treatment with recombinant human alpha-mannosidase partially corrected the hyperglycosylation of lysosomal proteins in vivo and in vitro. These data clearly demonstrate that LAMAN is involved not only in the lysosomal catabolism of free oligosaccharides but also in the trimming of asparagine-linked oligosaccharides on native lysosomal proteins.

Reversal of peripheral and central neural storage and ataxia after recombinant enzyme replacement therapy in alpha-mannosidosis mice.

Despite the progress in the treatment of lysosomal storage disorders (LSDs) mainly by enzyme replacement therapy, only limited success was reported in targeting the appropriate lysosomal enzyme into the brain. This prevents efficient clearance of neuronal storage, which is present in many of these disorders including alpha-mannosidosis. Here we show that the neuropathology of a mouse model for alpha-mannosidosis can be efficiently treated using recombinant human alpha-mannosidase (rhLAMAN). After intravenous administration of different doses (25-500 U/kg), rhLAMAN was widely distributed among tissues, and immunohistochemistry revealed lysosomal delivery of the injected enzyme. Whereas low doses (25 U/kg) led to a significant clearance (<70%) in visceral tissues, higher doses were needed for a clear effect in central and peripheral nervous tissues. A distinct reduction (<50%) of brain storage required repeated high-dose injections (500 U/kg), whereas lower doses (250 U/kg) were sufficient for clearance of stored substrates in peripheral neurons of the trigeminal ganglion. Successful transfer across the blood-brain barrier was evident as the injected enzyme was found in hippocampal neurons, leading to a nearly complete disappearance of storage vacuoles. Importantly, the decrease in neuronal storage in the brain correlated with an improvement of the neuromotor disabilities found in untreated alpha-mannosidosis mice. Uptake of rhLAMAN seems to be independent of mannose-6-phosphate receptors, which is consistent with the low phosphorylation profile of the enzyme. These data suggest that high-dose injections of low phosphorylated enzymes might be an interesting option to efficiently treat LSDs with CNS involvement.

Oligosaccharide transport: pumping waste from the ER into lysosomes.

N-glycans play important roles during the folding and secretion of glycoproteins. Surprisingly, during the N-glycosylation of glycoproteins, considerable amounts of unconjugated polymannose-type oligosaccharides ('free OS') are generated. Although free oligosaccharides have no known function in mammalian cells, a sophisticated cellular machinery enables them to be cleared from the endoplasmic reticulum (ER) into the cytosol and then re-enter the endomembrane system at the level of the lysosome. One possible function of this pathway is to stop free OS from interfering with the carbohydrate-dependent aspects of glycoprotein folding and transport along the secretory pathway.

Biochemical and morphological expression of early prenatal caprine beta-mannosidosis.

Lysosomal storage diseases associated with early-onset pathological changes may require prenatal therapy to avert the profound effects of the metabolic error on organs, especially the central nervous system. The present investigation determined the extent of expression of beta-mannosidase deficiency in the caprine fetus at 62 days of gestation, near the end of the period of immunotolerance when donor cells can engraft in various organs without immune rejection and supply missing enzyme. Three pairs of obligate carrier goats from the beta-mannosidosis colony were mated. Out of six fetuses delivered at 62 days of gestation, one (V385) was identified by measurement of beta-mannosidase activity as the only fetus affected with beta-mannosidosis. Thin-layer chromatography and quantitation of oligosaccharides revealed the presence of tri- and disaccharides, typical of beta-mannosidosis, only in V385. Morphological analysis revealed cytoplasmic vacuolation typical of beta-mannosidosis in V385; in thyroid, spinal cord, and kidney, the pattern of vacuolation was similar to, but less severe than, that observed previously in newborn affected goats. On the basis of these results, it will be possible to determine the effects of prenatal cell transplantation therapeutic strategies performed during the period of immunotolerance by monitoring phenotypic characteristics after treatment.

Lectin activities of cytokines and growth factors: function and implications for pathology.

The discovery that certain cytokines have carbohydrate-binding (lectin) properties opens new concepts in the understanding of their mechanism of action. The carbohydrate-recognition domain, which is localized opposite to the receptor-binding domain, makes these molecules bi-functional. The expression of the biological activity of the cytokine relies on its carbohydrate-binding activity which allows the association of the cytokine receptor with molecular complexes comprising the specific kinase involved in receptor phosphorylation and in specific signal transduction. It is expected that blood accumulation of free or membrane-bound glucan ligands of cytokines may dramatically perturb their endogenous function inducing specific immunodeficiencies.

mRNA levels for central nervous system myelin proteins in myelin deficiency of caprine beta-mannosidosis.

Caprine beta-mannosidosis is an inherited lysosomal storage disease that leads to a deficiency of oligodendrocytes and hypomyelination. Our previous results demonstrated that low levels of myelin-associated glycoprotein (MAG), 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) and proteolipid protein (PLP) found in CNS samples correlated with decreased yields of myelin. However, there was a relative preservation of myelin basic protein (MBP) in the spinal cord samples of affected goats. This report shows that the amounts of myelin protein mRNAs in the spinal cords of affected goats relative to control goats are also decreased. The levels of mRNA for MAG, MBP and PLP in affected goat spinal cords compared with those of controls were equally decreased to approximately 50% for the three myelin proteins. This suggests that the relative preservation of MBP protein in the spinal cords is not due to a higher MBP mRNA level, but might be due to a difference in post-transcriptional processes.

Myelin-associated glycoprotein (MAG) in myelin deficiency of caprine beta-mannosidosis.

Caprine beta-mannosidosis is an inherited lysosomal storage disorder due to a deficiency of beta-mannosidase which cleaves beta-linked mannose residues from the ends of N-asparagine linked oligosaccharides of glycoproteins. Histological and chemical examination has revealed a deficiency of compact myelin in the brains and spinal cords of affected goats. Since myelin-associated glycoprotein (MAG) is glycosylated and its metabolism could be directly affected in this disease, we investigated the possibility of a differential treatment of MAG in caprine beta-mannosidosis in comparison to non-glycosylated myelin proteins. MAG, myelin basic protein (MBP), 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP), proteolipid protein (PLP) and glial fibrillary acidic protein (GFAP) were quantified by western blot analysis in whole homogenates of spinal cords and hemispheres from affected goats at 1, 3 and 6 days of age and from normal controls. The yields of isolated myelin from the spinal cords of affected goats varied from 37 to 63% of normal and were 7% or less of normal from the hemispheres. In mutant spinal cords, the deficits of MAG, CNP and PLP measured in whole homogenates corresponded reasonably well with the decreased myelin yields, but the levels of MBP were consistently much closer to control levels than those of the other myelin proteins. A greater deficiency of PLP than MBP was also apparent in the myelin fractions purified from the affected spinal cords. In homogenates of mutant hemispheres, MAG, MBP, PLP and CNP were undetectable or at trace levels in comparison to controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical and immunohistochemical studies on overgrown gingival tissues associated with mannosidosis.

The gingival tissues of a male patient suffering from mannosidosis and presenting with gingival overgrowth have been studied. Routine histological assessment highlighted the presence of highly enlarged and vacuolated lymphocytes. The morphology of the connective tissues, fibroblasts and epithelium appeared normal. Immunohistochemical staining of the tissues for chondroitin sulfate proteoglycan demonstrated a normal distribution of this component throughout the connective tissues and intense staining associated with the vacuolated lymphocytes. In vitro studies indicated that fibroblasts isolated from the overgrown tissue did not differ from age and sex matched control fibroblasts with respect to proliferation, protein and proteoglycan synthesis. Taken together, these findings imply that the gingvial overgrowth noted in this patient was not due to a defect in the resident fibroblasts but rather reflected a secondary response to the tissues to impaired host defence mechanisms.

Mammalian beta-D-mannosidase and beta-mannosidosis.

Lysosomal beta-D-mannosidase is the last exoglycosidase involved in the sequential degradation of the N-glycosylproteins glycans. Research on this enzyme was restricted before the discovery of its hereditary deficiency, first in goat (1981) and later in man (1986). We describe the biochemical aspects of these beta-mannosidosis and the properties of the beta-mannosidases of mammalian origin. Our own results concerning human enzyme (from kidney and urine, seminal plasma and blood cells) suggest that, apart from the case of the inherited disease, beta-mannosidase may become a useful tool in other pathologies.

Oligosaccharides accumulated in the bovine beta-mannosidosis kidney.

The phenotype of bovine beta-mannosidosis (beta-mannosidase deficiency), recently identified in Salers cattle, is similar to the caprine form of the disease (Abbitt et al., 1991). This investigation was designed to characterize accumulated kidney oligosaccharides in bovine beta-mannosidosis. Oligosaccharides were extracted from the kidney of an affected Salers calf and purified by chromatographic techniques. The amount of accumulating oligosaccharides in 1 g of wet tissue was about 21 mumol. Structures of derivatized oligosaccharides were characterized by high-performance liquid chromatography, mass spectrometry, methylation analysis and sequential exoglycosidase digestions. The major accumulating oligosaccharides were Man beta 1-4GlcNAc and Man beta 1-4GlcNAc beta 1-4GlcNAc. Oligosaccharides accumulating in minor amounts were Man beta 1-4GlcNAc beta 1-4Man beta 1-4GlcNAc, Man alpha 1-6Man beta 1-4GlcNAc beta 1-4GlcNAc and Man beta 1-4GlcNAc beta 1-4Man beta 1-4GlcNAc beta 1-4GlcNAc. As in caprine beta-mannosidosis, oligosaccharides with terminal beta-mannose residues and cleaved as well as uncleaved chitobiose linkages were identified in bovine beta-mannosidosis kidney. The accumulating oligosaccharides in tissue were thus identical in bovine and caprine beta-mannosidosis; however, the source of the novel oligosaccharides remains to be determined.