Abnormal metabolism of mannose in families with carbohydrate-deficient glycoprotein syndrome type 1

Mannose: a potential saccharide candidate in disease management

There are a plethora of antibiotic resistance cases and humans are marching towards another big survival test of evolution along with drastic climate change and infectious diseases. Ever since the first antibiotic [penicillin], and the myriad of vaccines, we were privileged to escape many infectious disease threats. The survival technique of pathogens seems rapidly changing and sometimes mimicking our own systems in such a perfect manner that we are left unarmed against them. Apart from searching for natural alternatives, repurposing existing drugs more effectively is becoming a familiar approach to new therapeutic opportunities. The ingenious use of revolutionary artificial intelligence-enabled drug discovery techniques is coping with the speed of such alterations. D-Mannose is a great hope as a nutraceutical in drug discovery, against CDG, diabetes, obesity, lung disease, and autoimmune diseases and recent findings of anti-tumor activity make it interesting along with its role in drug delivery enhancing techniques. A very unique work done in the present investigation is the collection of data from the ChEMBL database and presenting the targetable proteins on pathogens as well as on humans. It shows Mannose has 50 targets and the majority of them are on human beings. The structure and conformation of certain monosaccharides have a decisive role in receptor pathogen interactions and here we attempt to review the multifaceted roles of Mannose sugar, its targets associated with different diseases, as a natural molecule having many success stories as a drug and future hope for disease management.

Graphical abstract

Glucose-responsive nanoparticles designed via a molecular-docking-driven method for insulin delivery

Nocturnal blood glucose regulation was one of the key challenges in diabetic treatments. However, development of the smart insulin complexes with mild and glucose-responsive delivering performances was mostly relied on experience of the senior researchers and numerous confirmation experiments. In this work, a series of bioinspired fatty-acid-modified glucose-responsive insulin-delivering polymeric nanoparticles were designed. The molecular docking technique was utilized to efficiently screen the fatty-acid-derived functional groups. The results provided the basis for polymer functionalization and simplified the optimization experiments. For the optimized formulation (C10MS), insulin-loaded C10MS successfully fulfilled the nocturnal-glycemic-controlling requirement of the diabetic rats with lower occurrence of hypoglycemia than the conventional insulin injection schemes. Such formulation also possessed good biocompatibility with the moderate elimination kinetics in vivo, which matched the demand of bio-safety in the daily treatments. Overall, this work opened up a new path for efficient design of functional polymeric materials.

Treatment Options in Congenital Disorders of Glycosylation

Despite advances in the identification and diagnosis of congenital disorders of glycosylation (CDG), treatment options remain limited and are often constrained to symptomatic management of disease manifestations. However, recent years have seen significant advances in treatment and novel therapies aimed both at the causative defect and secondary disease manifestations have been transferred from bench to bedside. In this review, we aim to give a detailed overview of the available therapies and rising concepts to treat these ultra-rare diseases.

Dietary mannose supplementation in phosphomannomutase 2 deficiency (PMM2-CDG)

Background

PMM2-CDG (CDG-Ia) is the most frequent N-glycosylation disorder. While supplying mannose to PMM2-deficient fibroblasts corrects the altered N-glycosylation in vitro, short term therapeutic approaches with mannose supplementation in PMM2-CDG patients have been unsuccessful. Mannose found no further mention in the design of a potential therapy for PMM2-CDG in the past years, as it applies to be ineffective. This retrospective study analyzes the first long term mannose supplementation in 20 PMM2-CDG patients. Mannose was given at a total of 1–2 g mannose/kg b.w./d divided into 5 single doses over a mean time of 57,75 ± 25,85 months. Protein glycosylation, blood mannose concentration and clinical presentation were monitored in everyday clinical practice.

Results

After a mean time period of more than 1 year the majority of patients showed significant improvements in protein glycosylation.

Conclusion

Dietary mannose supplementation shows biological effects in PMM2-CDG patients improving glycosylation in the majority of patients. A double-blind randomized study is needed to examine the role of mannose in the design of a therapy for children with PMM2-CDG in more detail.

Unsuccessful intravenous D-mannose treatment in PMM2-CDG

Background

PMM2-CDG (Phosphomannomutase 2 - Congenital disorder of glycosylation-Ia; CDG-Ia) is the most common glycosylation defect, often presenting as a severe multisystem disorder that can be fatal within the first years of life. While mannose treatment has been shown to correct glycosylation in vitro and in vivo in mice, no convincing effects have been observed in short-term treatment trials in single patients so far.

Results

We report on a boy with a severe PMM2-CDG who received a continuous intravenous mannose infusion over a period of 5 months during the first year of life in a dose of 0.8 g/kg/day. N-glycosylation of serum glycoproteins and mannose concentrations in serum were studied regularly. Unfortunately, no biochemical or clinical improvement was observed, and the therapy was terminated at age 9 months.

Conclusion

Postnatal intravenous D-mannose treatment seems to be ineffective in PMM2-CDG.

CDG Therapies: From Bench to Bedside

Congenital disorders of glycosylation (CDG) are a group of genetic disorders that affect protein and lipid glycosylation and glycosylphosphatidylinositol synthesis. More than 100 different disorders have been reported and the number is rapidly increasing. Since glycosylation is an essential post-translational process, patients present a large range of symptoms and variable phenotypes, from very mild to extremely severe. Only for few CDG, potentially curative therapies are being used, including dietary supplementation (e.g., galactose for PGM1-CDG, fucose for SLC35C1-CDG, Mn²⁺ for TMEM165-CDG or mannose for MPI-CDG) and organ transplantation (e.g., liver for MPI-CDG and heart for DOLK-CDG). However, for the majority of patients, only symptomatic and preventive treatments are in use. This constitutes a burden for patients, care-givers and ultimately the healthcare system. Innovative diagnostic approaches, in vitro and in vivo models and novel biomarkers have been developed that can lead to novel therapeutic avenues aiming to ameliorate the patients’ symptoms and lives. This review summarizes the advances in therapeutic approaches for CDG.

Mannose metabolism: more than meets the eye

Mannose is a simple sugar with a complex life. It is a welcome therapy for genetic and acquired human diseases, but it kills honeybees and blinds baby mice. It could cause diabetic complications. Mannose chemistry, metabolism, and metabolomics in cells, tissues and mammals can help explain these multiple systemic effects. Mannose has good, bad or ugly outcomes depending on its steady state levels and metabolic flux. This review describes the role of mannose at cellular level and its impact on organisms.

Mannose efflux from the cells: a potential source of mannose in blood

All mammals have 50-100 μM mannose in their blood. However, the source of the dynamic pool of mannose in blood is unknown. Most of it is thought to be derived from glucose in the cells. We studied mannose uptake and release by various cell types. Interestingly, our results show that mannose taken up by the cells through transporters is handled differently from the mannose released within the cells due to glycan processing of protein-bound oligosaccharides. Although more than 95% of incoming mannose is catabolized, most of the mannose released by intracellular processing is expelled from the cells as free mannose predominantly via a nocodazole-sensitive sugar transporter. Under physiological conditions, incoming mannose is more accessible to hexokinase, whereas mannose released within the cells is protected from HK and therefore has a different fate. Our data also suggest that generation of free mannose due to the processing of glycoconjugates composed of glucose-derived mannose and its efflux from the cells can account for most of the mannose found in blood and its steady state maintenance.

Simultaneous determination of serum mannose and glucose concentrations in dog serum using high performance liquid chromatography

Serum mannose and glucose concentrations in dogs before and after eating a meal were determined simultaneously with a recently established HPLC method combined with a UV and fluorescence detection system of p-aminobenzoic acid ethyl ester (ABEE)-derivatized monosaccharides. In this newly established HPLC method, detection limits were 0.09 micromol/L for mannose and 0.04 mmol/L for glucose. Linearity of peak areas vs. amounts of mannose and glucose in the range of 0.27-320 micromol/L and 0.13-64 mmol/L were observed, respectively. The value of the glucose content measured by the HPLC method was in good agreement with that of the commonly used enzymatic method (control). Serum glucose concentrations in dogs 90 min after the meal were almost the same as those before the meal, whereas serum mannose concentrations decreased significantly after the meal. This HPLC method may be useful for determination of monosaccharides in animal blood.

Free sugar and sugar alcohol concentrations in human breast milk

OBJECTIVES

The goal was to determine the free sugars and polyols in human breast milk, both term and preterm, for comparison with formula milks.

METHODS

Methodology was developed for the measurement of the free sugars and polyols in breast milk. There were 16 samples collected from women who delivered at term and 17 samples from women delivering preterm. For purposes of comparison, samples were also collected from four commercial formulas for term infants and eight from commercial formulas for preterm infants as well as one sample of cows' milk. All samples were frozen immediately and analyzed by high-performance liquid chromatography techniques.

RESULTS

Except for lactose concentrations, no significant differences were detected for all other sugars and polyols between term and preterm breast milk samples. Within breast milk samples, two patterns emerged, with one group containing additional elution peaks for compounds not yet identified. A second group did not contain these compounds. There were a number of significant differences between breast milk and formulas, particularly for inositol, glycerol, glucose, and galactose. All milks contained significant concentrations of mannose.

CONCLUSIONS

There are no significant differences among breast milk samples for free sugar and polyol concentrations except lactose. However, some milk contains additional peaks that could be of dietary or genetic origin. Formula milk has relatively high concentrations of glucose and galactose compared with breast milk, suggesting some lactose hydrolysis.

Hepatic glycogen breakdown is implicated in the maintenance of plasma mannose concentration

D-mannose is an essential monosaccharide constituent of glycoproteins and glycolipids. However, it is unknown how plasma mannose is supplied. The aim of this study was to explore the source of plasma mannose. Oral administration of glucose resulted in a significant decrease of plasma mannose concentration after 20 min in fasted normal rats. However, in fasted type 2 diabetes model rats, plasma mannose concentrations that were higher compared with normal rats did not change after the administration of glucose. When insulin was administered intravenously to fed rats, it took longer for plasma mannose concentrations to decrease significantly in diabetic rats than in normal rats (20 and 5 min, respectively). Intravenous administration of epinephrine to fed normal rats increased the plasma mannose concentration, but this effect was negated by fasting or by administration of a glycogen phosphorylase inhibitor. Epinephrine increased mannose output from the perfused liver of fed rats, but this effect was negated in the presence of a glucose-6-phosphatase inhibitor. Epinephrine also increased the hepatic levels of hexose 6-phosphates, including mannose 6-phosphate. When either lactate alone or lactate plus alanine were administered as gluconeogenic substrates to fasted rats, the concentration of plasma mannose did not increase. When lactate was used to perfuse the liver of fasted rats, a decrease, rather than an increase, in mannose output was observed. These findings indicate that hepatic glycogen is a source of plasma mannose.

Defectos congénitos de la glucosilación: últimos avances y experiencia española

Congenital disorders of glycosylation (CDG) are a group of inherited disorders caused by defects in the synthesis and processing of the linked glycans of glycoproteins and other molecules. The first patients with CDG were described in 1980. Fifteen years later, phosphomannomutase was found to be the basis of the most frequent type: CDG-Ia. Over the last years, several novel types have been identified related to the N-glycosylation pathway, affecting enzymes or transporters of the cytosol, endoplasmic reticulum or the Golgi compartment. CDGs are multisystemic disorders, mainly affecting the central nervous system. Yet CDG-Ib and Ih are mainly hepato-intestinal diseases. Recently, several defects involving the O-glycosylation pathways have been described, indicating that some congenital muscular dystrophies and neuronal migration disorders are caused by congenital disorders of glycosylation.

Congenital disorders of glycosylation: a review

Congenital disorders of glycosylation (CDGs) are a rapidly growing group of inherited disorders caused by defects in the synthesis and processing of the asparagine(ASN)-linked oligosaccharides of glycoproteins. The first CDG patients were described in 1980. Fifteen years later, a phosphomannomutase deficiency was found as the basis of the most frequent type, CDG-Ia. In recent years several novel types have been identified. The N-glycosylation pathway is highly conserved from yeast to human, and the rapid progress in this field can largely be attributed to the systematic application of the knowledge of yeast mutants. Up to now, eight diseases have been characterized, resulting from enzyme or transport defects in the cytosol, endoplasmic reticulum, or Golgi compartment. CDGs affect all organs and particularly the CNS, except for CDG-Ib, which is mainly a hepatic-intestinal disease.

Functional significance of PMM2 mutations in mildly affected patients with congenital disorders of glycosylation Ia

PURPOSE

Congenital disorders of glycosylation (CDG) result from mutations in N-glycan biosynthesis. Mutations in phosphomannomutase (PMM2) cause CDG-Ia. Here, we report four clinically mild patients and their mutations in PMM2.

METHODS

Analysis of the PMM2 cDNA and gene revealed the mutations affecting the glycosylation efficiency.

RESULTS

The patients have 30% to 50% normal PMM activity in fibroblasts due to different mutations in PMM2, and we studied the effect of each mutation on the PMM activity in a Saccharomyces cerevisiae expression system.

CONCLUSIONS

Each patient carried a severe mutation that decreased the PMM activity to less than 10% as well as a relatively mild mutation. A new mutation, deletion of base 24, changed the reading frame. The C9Y, C241S, and L32R mutations showed 27% to 45% activity when expressed in the eukaryotic expression system, and the more severe D148N was shown to be thermolabile.

Functional analysis of novel mutations in a congenital disorder of glycosylation Ia patient with mixed Asian ancestry

Congenital disorders of glycosylation (CDG) are caused by autosomal recessive mutations in genes affecting N-glycan biosynthesis. Mutations in the PMM2 gene, which encodes the enzyme phosphomannomutase (mannose 6-phosphate <--> mannose 1-phosphate), give rise to the most common form: CDG-Ia. These patients typically present with dysmorphic features and neurological abnormalities, cerebellar hypoplasia, ataxia, hypotonia, and coagulopathy, in addition to feeding problems. However, the clinical symptoms vary greatly. The great majority of known CDG-Ia patients are of European descent where the most common mutant alleles originated. This ethnic bias can also be explained by lack of global awareness of the disorder. Here we report an Asian patient with prominent systemic features that we diagnosed with CDG-Ia resulting from two new mutations in the PMM2 gene (310C --> G resulting in L104V and an intronic mutation IVS1-1G --> A). The latter mutation seems to result in lower mRNA levels, and the L104V has been functionally analyzed in a yeast expression system together with known mutations. The Filipino and Cambodian origins of the parents show that CDG-Ia mutations occur in these ethnic groups as well as in Caucasians.

Analysis of multiple mutations in the hALG6 gene in a patient with congenital disorder of glycosylation Ic

Congenital disorder of glycosylation Ic is caused by mutations in the hALG6 gene that encodes an alpha-1,3 glucosyltransferase. This enzyme is required for the addition of the first glucose residue to the lipid-linked oligosaccharide precursor for N-linked glycosylation. Here we describe the biochemical and molecular analysis of a patient with three mutations in the hALG6 gene. The maternal allele has an intronic G --> A mutation resulting in skipping of exon3 (IVS3 + 5G > A). This produces a nonfunctional enzyme as shown by its inability to restore normal glycosylation in a Saccharomyces cerevisiae strain lacking a functional ALG6. The paternal allele has two mutations. One is a deletion of three bases (895-897delATA) leading to an in-frame deletion of isoleucine 299 (delI299) located in a transmembrane domain. The second mutation on the same allele 911T > C causes a F304S change. When expressed in the ALG6 deficient yeast strain, this allele restores glycosylation but the mRNA is unstable or inefficiently transcribed, contributing to the impaired glycosylation in the patient.

Dolichol phosphate mannose synthase (DPM1) mutations define congenital disorder of glycosylation Ie (CDG-Ie)

Congenital disorders of glycosylation (CDGs) are metabolic deficiencies in glycoprotein biosynthesis that usually cause severe mental and psychomotor retardation. Different forms of CDGs can be recognized by altered isoelectric focusing (IEF) patterns of serum transferrin (Tf). Two patients with these symptoms and similar abnormal Tf IEF patterns were analyzed by metabolic labeling of fibroblasts with ¿2-(3)Hmannose. The patients produced a truncated dolichol-linked precursor oligosaccharide with 5 mannose residues, instead of the normal precursor with 9 mannose residues. Addition of 250 microM mannose to the culture medium corrected the size of the truncated oligosaccharide. Microsomes from fibroblasts of these patients were approximately 95% deficient in dolichol-phosphate-mannose (Dol-P-Man) synthase activity, with an apparent K(m) for GDP-Man approximately 6-fold higher than normal. DPM1, the gene coding for the catalytic subunit of Dol-P-Man synthase, was altered in both patients. One patient had a point mutation, C(274)G, causing an R(92)G change in the coding sequence. The other patient also had the C(274)G mutation and a 13-bp deletion that presumably resulted in an unstable transcript. Defects in DPM1 define a new glycosylation disorder, CDG-Ie.

Carbohydrate-deficient glycoprotein syndrome type IA (phosphomannomutase-deficiency)

The carbohydrate-deficient glycoprotein or CDG syndromes (OMIM 212065) are a recently delineated group of genetic, multisystem diseases with variable dysmorphic features. The known CDG syndromes are characterized by a partial deficiency of the N-linked glycans of secretory glycoproteins, lysosomal enzymes, and probably also membranous glycoproteins. Due to the deficiency of terminal N-acetylneuraminic acid or sialic acid, the glycan changes can be observed in serum transferrin or other glycoproteins using isoelectrofocusing with immunofixation as the most widely used diagnostic technique. Most patients show a serum sialotransferrin pattern characterized by increased di- and asialotransferrin bands (type I pattern). The majority of patients with type I are phosphomannomutase deficient (type IA), while in a few other patients, deficiencies of phosphomannose isomerase (type IB) or endoplasmic reticulum glucosyltransferase (type IC) have been demonstrated. This review is an update on CDG syndrome type IA.

Protein glycosylation in development and disease

N- and O-linked glycan structures of cell surface and secreted glycoproteins serve a variety of functions related to cell-cell communication in systems affecting development and disease. The more sophisticated N-glycan biosynthesis pathway of metazoans diverges from that of yeast with the appearance of the medial-Golgi beta-N-acetylglucosaminyltransferases (GlcNAc-Ts). Tissue-specific regulation of medial- and trans-Golgi glycosyltransferases contribute structural diversity to glycoproteins in metazoans, and this can affect their molecular properties including localization, half-life, and biological activity. Null mutations in glycosyltransferase genes positioned later in the biosynthetic pathway disrupt expression of smaller subsets of glycan structures and are progressively milder in phenotype. In this review, we examine data on targeted mutations affecting glycosylation in mice and congenital mutations in man, with a view to understanding the molecular functions of glycan structures as modulators of glycoprotein activity. Finally, pathology associated with the expression of GlcNAc-Ts in cancer and diabetes-induced cardiac hypertrophy suggest that inhibitors of these enzymes may have therapeutic value.

Alteration of mannose transport in fibroblasts from type I carbohydrate deficient glycoprotein syndrome patients

The aim of the present study was to explore how mannose enters fibroblasts derived from a panel of children suffering from different subtypes of type I carbohydrate deficient glycoprotein syndrome: seven carbohydrate deficient glycoprotein syndrome subtype Ia (phosphomannomutase deficiency), two carbohydrate deficient glycoprotein syndrome subtype Ib (phosphomannose isomerase deficiency) and two carbohydrate deficient glycoprotein syndrome subtype Ix (not identified deficiency). We showed that a specific mannose transport system exists in all the cells tested but has different characteristics with respect to carbohydrate deficient glycoprotein syndrome subtypes. Subtype Ia fibroblasts presented a mannose uptake equivalent or higher (maximum 1.6-fold) than control cells with a D-[2-3H]-mannose incorporation in nascent N-glycoproteins decreased up to 7-fold. Compared to control cells, the mannose uptake was greatly stimulated in subtype Ib (4.0-fold), due to lower Kuptake and higher Vmax values. Subtype Ib cells showed an increased incorporation of D-[2-3H]-mannose into nascent N-glycoproteins. Subtype Ix fibroblasts presented an intermediary status with mannose uptake equivalent to the control but with an increased incorporation of D-[2-3H]-mannose in nascent N-glycoproteins. All together, our results demonstrate quantitative and/or qualitative modifications in mannose transport of all carbohydrate deficient glycoprotein syndrome fibroblasts in comparison to control cells, with a relative homogeneity within a considered subtype of carbohydrate deficient glycoprotein syndrome. These results are consistent with the possible use of mannose as a therapeutic agent in carbohydrate deficient glycoprotein syndrome Ib and Ix.

Zonal rotor purification and characterization of "mannosomes": a tubular membrane system in gastropod mollusc digestive gland

Mannitol oxidase (manox) is an H2O2-generating oxidase apparently unique to molluscs and especially abundant in alimentary tissues. In the digestive gland it is localized to an organelle ("mannosome") that forms an unusual tubular membrane system. We have developed a novel centrifugation procedure for >100-fold purification of these membranes in 20% yield from approximately 30 g of digestive gland of the slug Arion ater. Mannosomes from several other gastropod species are also substantially purified by the procedure. Four successive density gradient separations are employed which minimize structural damage by exploiting near isosmotic conditions early on and by completely avoiding traumatic pelleting and resuspension. Plasma membrane contamination is reduced by digitonin-induced density perturbation. The purified preparation is characterized by a predominant 68-kDa integral membrane protein and retains the in situ appearance of hexagonally arranged tubules with an enveloping outer membrane.