Protein O-mannosyltransferase activities in lymphoblasts from patients with α-dystroglycanopathies

Protein O-mannosylation across kingdoms and related diseases: From glycobiology to glycopathology

The post-translational glycosylation of proteins by O-linked α-mannose is conserved from bacteria to humans. Due to advances in high-throughput mass spectrometry-based approaches, a variety of glycoproteins are identified to be O-mannosylated. Various proteins with O-mannosylation are involved in biological processes, providing essential necessity for proper growth and development. In this review, we summarize the process and regulation of O-mannosylation. The multi-step O-mannosylation procedures are quite dynamic and complex, especially when considering the structural and functional inspection of the involved enzymes. The widely studied O-mannosylated proteins in human include α-Dystroglycan (α-DG), cadherins, protocadherins, and plexin, and their aberrant O-mannosylation are associated with many diseases. In addition, O-mannosylation also contributes to diverse functions in lower eukaryotes and prokaryotes. Finally, we present the relationship between O-mannosylation and gut microbiota (GM), and elucidate that O-mannosylation in microbiome is of great importance in the dynamic balance of GM. Our study provides an overview of the processes of O-mannosylation in mammalian cells and other organisms, and also associated regulated enzymes and biological functions, which could contribute to the understanding of newly discovered O-mannosylated glycoproteins.

Structure of the eukaryotic protein O-mannosyltransferase Pmt1-Pmt2 complex

In eukaryotes, a nascent peptide entering the endoplasmic reticulum (ER) is scanned by two Sec61-translocon-associated large membrane machines for protein N-glycosylation and protein O-mannosylation, respectively. While the structure of the eight-protein oligosaccharyltransferase complex has been determined recently, the structures of mannosyltransferases of the PMT family, which are an integral part of ER protein homeostasis, are still unknown. Here we report cryo-EM structures of the S. cerevisiae Pmt1–Pmt2 complex bound to a donor and an acceptor peptide at 3.2-Å resolution, showing that each subunit contains 11 transmembrane helices and a lumenal β-trefoil fold termed the MIR domain. The structures reveal the substrate recognition model and confirm an inverting mannosyl-transferring reaction mechanism by the enzyme complex. Furthermore, we found that the transmembrane domains of Pmt1 and Pmt2 share a structural fold with the catalytic subunits of oligosaccharyltransferases, confirming a previously proposed evolutionary relationship between protein O-mannosylation and protein N-glycosylation.

Molecular genetics of the POMT1-related muscular dystrophy-dystroglycanopathies

Protein O-mannosyltransferase 1 (POMT1) is a critical enzyme participating in the first step of protein O-mannosylation. Mutations in the coding gene, POMT1, have been described to be related to a series of autosomal recessive disorders associated with defective alpha-dystroglycan glycosylation, later termed muscular dystrophy-dystroglycanopathies (MDDGs). MDDGs are characterized by a broad phenotypic spectrum of congenital muscular dystrophy or later-onset limb-girdle muscular dystrophy, accompanied by variable degrees of intellectual disability, brain defects, and ocular abnormalities. To date, at least 76 disease-associated mutations in the POMT1 gene, including missense, nonsense, splicing, deletion, insertion/duplication, and insertion-deletion mutations, have been reported in the literature. In this review, we highlight the present knowledge of the identified disease-associated POMT1 gene mutations and genetic animal models related to the POMT1 gene. This review may help further normative classification of phenotypes, assist in definite clinical and genetic diagnoses, and genetic counseling, and may comprehensively improve our understanding of the basis of complex phenotypes and possible pathogenic mechanisms involved.

Glycosylation with ribitol-phosphate in mammals: New insights into the O-mannosyl glycan

BACKGROUND

O-mannosyl glycans have been found in a limited number of glycoproteins of the brain, nerves, and skeletal muscles, particularly in α-dystroglycan (α-DG). Defects in O-mannosyl glycan on α-DG are the primary cause of a group of congenital muscular dystrophies, which are collectively termed α-dystroglycanopathy. Recent studies have revealed various O-mannosyl glycan structures, which can be classified as core M1, core M2, and core M3 glycans. Although many dystroglycanopathy genes are involved in core M3 processing, the structure and biosynthesis of core M3 glycan remains only partially understood.

SCOPE OF REVIEW

This review presents recent findings about the structure, biosynthesis, and pathology of O-mannosyl glycans.

MAJOR CONCLUSIONS

Recent studies have revealed that the entire structure of core M3 glycan, including ribitol-5-phosphate, is a novel structure in mammals; its unique biosynthetic pathway has been elucidated by the identification of new causative genes for α-dystroglycanopathies and their functions.

GENERAL SIGNIFICANCE

O-mannosyl glycan has a novel, unique structure that is important for the maintenance of brain and muscle functions. These findings have opened up a new field in glycoscience. These studies will further contribute to the understanding of the pathomechanism of α-dystroglycanopathy and the development of glycotherapeutics. This article is part of a Special Issue entitled Neuro-glycoscience, edited by Kenji Kadomatsu and Hiroshi Kitagawa.

Analysis of phenotype, enzyme activity and genotype of Chinese patients with POMT1 mutation

Protein O-mannosyltransferase 1 (POMT1) is a glycosyltransferase involved in α-dystroglycan glycosylation. POMT1 mutations cause a wide spectrum of clinical conditions from Walker-Warburg syndrome (WWS), which involves muscle, eye and brain abnormalities, to mild forms of limb-girdle muscular dystrophy with mental retardation. We aimed to elucidate the impact of different POMT1 mutations on the clinical phenotype. We report five Chinese patients with POMT1 mutations: one had a typical clinical manifestation of WWS, and the other four were diagnosed with congenital muscular dystrophy with mental retardation of varying severity. We analyzed the influence of the POMT1 mutations on POMT activity by assaying the patients' muscles and cultured skin fibroblasts. We demonstrated different levels of decreased POMT activity that correlated highly with decreased α-dystroglycan glycosylation. Our results suggest that POMT activity is inversely proportional to clinical severity, and demonstrate that skin fibroblasts can be used for differential diagnosis of patients with α-dystroglycanopathies. We have provided clinical, histological, enzymatic and genetic evidence of POMT1 involvement in five unrelated Chinese patients.

Glycobiology of α-dystroglycan and muscular dystrophy

Most proteins are modified by glycans, which can modulate the biological properties and functions of glycoproteins. The major glycans can be classified into N-glycans and O-glycans according to their glycan-peptide linkage. This review will provide an overview of the O-mannosyl glycans, one subtype of O-glycans. Originally, O-mannosyl glycan was only known to be present on a limited number of glycoproteins, especially α-dystroglycan (α-DG). However, once a clear relationship was established between O-mannosyl glycan and the pathological mechanisms of some congenital muscular dystrophies in humans, research on the biochemistry and pathology of O-mannosyl glycans has been expanding. Because α-DG glycosylation is defective in congenital muscular dystrophies, which also feature abnormal neuronal migration, these disorders are collectively called α-dystroglycanopathies. In this article, I will describe the structure, biosynthesis and pathology of O-mannosyl glycans.

Endogenous glucuronyltransferase activity of LARGE or LARGE2 required for functional modification of α-dystroglycan in cells and tissues

Mutations in the LARGE gene have been identified in congenital muscular dystrophy (CMD) patients with brain abnormalities. Both LARGE and its paralog, LARGE2 (also referred to as GYLTL1B) are bifunctional glycosyltransferases with xylosyltransferase (Xyl-T) and glucuronyltransferase (GlcA-T) activities, and are capable of forming polymers consisting of [-3Xyl-α1,3GlcAβ1-] repeats. LARGE-dependent modification of α-dystroglycan (α-DG) with these polysaccharides is essential for the ability of α-DG to act as a receptor for ligands in the extracellular matrix. Here we report on the endogenous enzymatic activities of LARGE and LARGE2 in mice and humans, using a newly developed assay for GlcA-T activity. We show that normal mouse and human cultured cells have endogenous LARGE GlcA-T, and that this activity is absent in cells from the Large(myd) (Large-deficient) mouse model of muscular dystrophy, as well as in cells from CMD patients with mutations in the LARGE gene. We also demonstrate that GlcA-T activity is significant in the brain, heart, and skeletal muscle of wild-type and Large2(-/-) mice, but negligible in the corresponding tissues of the Large(myd) mice. Notably, GlcA-T activity is substantial, though reduced, in the kidneys of both the Large(myd) and Large2(-/-) mice, consistent with the observation of α-DG/laminin binding in these contexts. This study is the first to test LARGE activity in samples as small as cryosections and, moreover, provides the first direct evidence that not only LARGE, but also LARGE2, is vital to effective functional modification of α-DG in vivo.

Diagnostic approach to the congenital muscular dystrophies

Congenital muscular dystrophies (CMDs) are early onset disorders of muscle with histological features suggesting a dystrophic process. The congenital muscular dystrophies as a group encompass great clinical and genetic heterogeneity so that achieving an accurate genetic diagnosis has become increasingly challenging, even in the age of next generation sequencing. In this document we review the diagnostic features, differential diagnostic considerations and available diagnostic tools for the various CMD subtypes and provide a systematic guide to the use of these resources for achieving an accurate molecular diagnosis. An International Committee on the Standard of Care for Congenital Muscular Dystrophies composed of experts on various aspects relevant to the CMDs performed a review of the available literature as well as of the unpublished expertise represented by the members of the committee and their contacts. This process was refined by two rounds of online surveys and followed by a three-day meeting at which the conclusions were presented and further refined. The combined consensus summarized in this document allows the physician to recognize the presence of a CMD in a child with weakness based on history, clinical examination, muscle biopsy results, and imaging. It will be helpful in suspecting a specific CMD subtype in order to prioritize testing to arrive at a final genetic diagnosis.

Protein O-mannosylation in metazoan organisms

Protein O-mannosylation is a special type of glycosylation that plays prominent roles in metazoans, affecting development and physiology of the nervous system and muscles. A major biological effect of O-mannosylation involves the regulation of α-dystroglycan, a membrane glycoprotein mediating cell-extracellular matrix interactions. Genetic defects of O-mannosylation result in the loss of ligand-binding activity of α-dystroglycan and cause congenital muscular dystrophies termed dystroglycanopathies. Recent progress in mass spectrometry and in vitro analyses has shed new light on the mechanism of α-dystroglycan glycosylation; however, this mechanism is underlain by complex genetic and molecular elements that remain poorly understood. Protein O-mannosylation is evolutionarily conserved in metazoans, yet the pathway is simplified and more amenable to genetic analyses in invertebrate organisms, indicating that genetically tractable in vivo models could facilitate research in this area. This unit describes recent methodological strategies for studying protein O-mannosylation using in vitro and in vivo approaches.

Cardiomyopathy in patients with POMT1-related congenital and limb-girdle muscular dystrophy

Protein-o-mannosyl transferase 1 (POMT1) is a glycosyltransferase involved in α-dystroglycan (α-DG) glycosylation. Clinical phenotype in POMT1-mutated patients ranges from congenital muscular dystrophy (CMD) with structural brain abnormalities, to limb-girdle muscular dystrophy (LGMD) with microcephaly and mental retardation, to mild LGMD. No cardiac involvement has until now been reported in POMT1-mutated patients. We report three patients who harbored compound heterozygous POMT1 mutations and showed left ventricular (LV) dilation and/or decrease in myocardial contractile force: two had a LGMD phenotype with a normal or close-to-normal cognitive profile and one had CMD with mental retardation and normal brain MRI. Reduced or absent α-DG immunolabeling in muscle biopsies were identified in all three patients. Bioinformatic tools were used to study the potential effect of POMT1-detected mutations. All the detected POMT1 mutations were predicted in silico to interfere with protein folding and/or glycosyltransferase function. The report on the patients described here has widened the clinical spectrum associated with POMT1 mutations to include cardiomyopathy. The functional impact of known and novel POMT1 mutations was predicted with a bioinformatics approach, and results were compared with previous in vitro studies of protein-o-mannosylase function.

Absence of post-phosphoryl modification in dystroglycanopathy mouse models and wild-type tissues expressing non-laminin binding form of α-dystroglycan

α-Dystroglycan (α-DG) is a membrane-associated glycoprotein that interacts with several extracellular matrix proteins, including laminin and agrin. Aberrant glycosylation of α-DG disrupts its interaction with ligands and causes a certain type of muscular dystrophy commonly referred to as dystroglycanopathy. It has been reported that a unique O-mannosyl tetrasaccharide (Neu5Ac-α2,3-Gal-β1,4-GlcNAc-β1,2-Man) and a phosphodiester-linked modification on O-mannose play important roles in the laminin binding activity of α-DG. In this study, we use several dystroglycanopathy mouse models to demonstrate that, in addition to fukutin and LARGE, FKRP (fukutin-related protein) is also involved in the post-phosphoryl modification of O-mannose on α-DG. Furthermore, we have found that the glycosylation status of α-DG in lung and testis is minimally affected by defects in fukutin, LARGE, or FKRP. α-DG prepared from wild-type lung- or testis-derived cells lacks the post-phosphoryl moiety and shows little laminin-binding activity. These results show that FKRP is involved in post-phosphoryl modification rather than in O-mannosyl tetrasaccharide synthesis. Our data also demonstrate that post-phosphoryl modification not only plays critical roles in the pathogenesis of dystroglycanopathy but also is a key determinant of α-DG functional expression as a laminin receptor in normal tissues and cells.

Different roles of the two components of human protein O-mannosyltransferase, POMT1 and POMT2

Protein O-mannosyltransferase 1 (POMT1) and its homolog, POMT2, are responsible for the catalysis of the first step in O-mannosyl glycan synthesis. Mutations in their genes are associated with a type of congenital muscular dystrophy called Walker-Warburg syndrome. Arg(64), Glu(78) and Arg(138) in the N-terminus region of ScPmt1p, a POMT homolog in Saccharomyces cerevisiae, are important for transferase activity. Arg(138) is also essential for complex formation with ScPmt2p. Here we examined the effects of replacing the corresponding residues in human POMT1 and POMT2 with Ala on complex formation and enzymatic activity. The human POMT1 mutants lost almost all transferase activity while the POMT2 mutants retained enzymatic activity. Neither mutant lost its ability to form complexes with the native counter component. These results indicate that ScPmtps and human POMTs have different mechanisms of complex formation. They also suggest that human POMT1 and POMT2 have discrete functions since the effect of amino acid substitutions on enzymatic activity are different.

Congenital muscular dystrophy type 1D (MDC1D) due to a large intragenic insertion/deletion, involving intron 10 of the LARGE gene

Mutation of the LARGE gene is the rarest of the six known genetic causes of α-dystroglycanopathy. We report further a family with MDC1D due to a complex genomic rearrangement that was not apparent on standard sequencing of LARGE. Two sisters in a consanguineous family had moderate mental retardation and cerebellar malformations, together with dystrophic changes and markedly reduced α-dystroglycan glycosylation staining on muscle biopsy. There was homozygous linkage to the LARGE locus but sequencing of LARGE coding regions was normal. Analysis of LARGE cDNA showed an abnormal sequence inserted between exons 10 and 11, in most of the transcripts, predicted to introduce a premature stop codon. The abnormal sequence mapped to a spliced EST (DA935254) of unknown function, normally located at 100 kb centromeric of LARGE on chromosome 22q12.3. Quantitative PCR analysis of the EST and adjacent regions showed twice the normal copy number in patients' genomic DNA samples, consistent with a large intra-chromosomal duplication inserted into intron 10 of LARGE in a homozygous state. This insertion was associated with deletion of a central region of intron 10, but the exact break points of the deletion/duplication were not found, suggesting that an even more complex rearrangement may have occurred. The exact function of LARGE, a golgi protein, remains uncertain. POMT and POMGnT enzyme activities were normal in patients' lymphoblast cells, suggesting that defects in LARGE do not affect the initiation of O-mannosyl glycans.

POMGnT1, POMT1, and POMT2 mutations in congenital muscular dystrophies

Alpha-dystroglycanopathies are a group of rare inherited neuromuscular disorders characterized by reduced glycosylation of alpha-dystroglycan (alpha-DG). Mutations in six genes (POMT1, POMT2, POMGNT1, FKTN, FKRP, and LARGE) have been identified in patients with alpha-dystroglycanopathies. Due to an extremely broad clinical spectrum and relatively poor phenotype-genotype correlation, diagnosis of alpha-dystroglycanopathies is difficult and requires searching for mutations gene by gene. At present, of the six proteins involved on alpha-dystroglycanopathies, the function of the gene products is only known for POMT1, POMT2, and POMGnT1, all responsible for the O-mannosylglycan biosynthesis. This chapter describes the assay protocols to diagnose patients with alpha-dystroglycanopathy by measuring glycosyltransferase activity.

Role of N-glycans in maintaining the activity of protein O-mannosyltransferases POMT1 and POMT2

The complex of protein O-mannosyltransferase 1 (POMT1) and POMT2 catalyzes the initial step of O-mannosyl glycan biosynthesis. The mutations in either POMT1 or POMT2 can lead to Walker-Warburg syndrome, a congenital muscular dystrophy with abnormal neuronal migration. Here, we used three algorithms for predicting transmembrane helices to construct the secondary structural models of human POMT1 and POMT2. In these models, POMT1 and POMT2 have seven- and nine-transmembrane helices and contain four and five potential N-glycosylation sites, respectively. To determine whether these sites are actually glycosylated, we prepared mutant proteins that were defective in each site by site-directed mutagenesis. Three of the POMT1 sites and all of the POMT2 sites were found to be N-glycosylated, suggesting that these sites face the luminal side of the endoplasmic reticulum. Mutation of any single site did not significantly affect POMT activity, but mutations of all N-glycosylation sites of either POMT1 or POMT2 caused a loss of POMT activity. The loss of activity appeared to be due to the decreased hydrophilicity. These results suggest that the N-glycosylation of POMT1 and POMT2 is required for maintaining the conformation as well as the activity of the POMT1-POMT2 complex.

Congenital muscular dystrophy. Part I: a review of phenotypical and diagnostic aspects

The congenital muscular dystrophies (CMDs) are a group of genetically and clinically heterogeneous hereditary myopathies with preferentially autosomal recessive inheritance, that are characterized by congenital hypotonia, delayed motor development and early onset of progressive muscle weakness associated with dystrophic pattern on muscle biopsy. The clinical course is broadly variable and can comprise the involvement of the brain and eyes. From 1994, a great development in the knowledge of the molecular basis has occurred and the classification of CMDs has to be continuously up dated. We initially present the main clinical and diagnostic data concerning the CMDs related to changes in the complex dystrophin-associated glycoproteins-extracellular matrix: CMD with merosin deficiency (CMD1A), collagen VI related CMDs (Ullrich CMD and Bethlem myopathy), CMDs with abnormal glycosylation of alpha-dystroglycan (Fukuyama CMD, Muscle-eye-brain disease, Walker-Warburg syndrome, CMD1C, CMD1D), and the much rarer CMD with integrin deficiency. Finally, we present other forms of CMDs not related with the dystrophin/glycoproteins/extracellular matrix complex (rigid spine syndrome, CMD1B, CMD with lamin A/C deficiency), and some apparently specific clinical forms not yet associated with a known molecular mechanism. The second part of this review concerning the pathogenesis and therapeutic perspectives of the different subtypes of CMD will be described in a next number.

POMT2 intragenic deletions and splicing abnormalities causing congenital muscular dystrophy with mental retardation

BACKGROUND

Alpha-dystroglycanopathies are a group of congenital muscular dystrophies (CMDs) with autosomal recessive inheritance characterized by abnormal glycosylation of alpha-dystroglycan. Although six genetic causes have been identified (FKTN, POMGNT1, POMT1, POMT2, FKRP, and LARGE) many alpha-dystroglycanopathy patients remain without a genetic diagnosis after standard exon sequencing. To date POMT2 mutations have been identified in CMD cases with a wide range of clinical severities from Walker-Warburg syndrome to limb girdle muscular dystrophy without structural brain or ocular involvement.

METHODS

We analyzed POMT2 in six CMD patients, who had severe diffuse muscle weakness, generalized joint contractures, microcephaly, severe mental retardation and elevated CK levels. Eye involvement was absent or limited to myopia or strabismus. We sequenced the coding regions of POMT2 using genomic DNA and cDNA generated from blood lymphocytes or B lymphoblastoid cell lines. Quantitative PCR analysis of genomic DNA was used to identify and determine the breakpoints of large deletions.

RESULTS

We report five novel mutations in POMT2, four of which were outside of coding exons, two large genomic deletions and two intronic single base substitutions that induced aberrant mRNA splicing.

CONCLUSIONS

Large scale DNA rearrangements (such as large deletions) and cryptic splice mutations, that can be missed on standard sequencing of genomic DNA, may be relatively common in POMT2. Additional techniques, such as sequencing of cDNA are needed to identify all mutations. These results also confirm that POMT2 mutations are an important cause of the less severe alpha-dystroglycanopathy phenotypes.

Residual laminin-binding activity and enhanced dystroglycan glycosylation by LARGE in novel model mice to dystroglycanopathy

Hypoglycosylation and reduced laminin-binding activity of α-dystroglycan are common characteristics of dystroglycanopathy, which is a group of congenital and limb-girdle muscular dystrophies. Fukuyama-type congenital muscular dystrophy (FCMD), caused by a mutation in the fukutin gene, is a severe form of dystroglycanopathy. A retrotransposal insertion in fukutin is seen in almost all cases of FCMD. To better understand the molecular pathogenesis of dystroglycanopathies and to explore therapeutic strategies, we generated knock-in mice carrying the retrotransposal insertion in the mouse fukutin ortholog. Knock-in mice exhibited hypoglycosylated α-dystroglycan; however, no signs of muscular dystrophy were observed. More sensitive methods detected minor levels of intact α-dystroglycan, and solid-phase assays determined laminin binding levels to be ∼50% of normal. In contrast, intact α-dystroglycan is undetectable in the dystrophic Large^myd mouse, and laminin-binding activity is markedly reduced. These data indicate that a small amount of intact α-dystroglycan is sufficient to maintain muscle cell integrity in knock-in mice, suggesting that the treatment of dystroglycanopathies might not require the full recovery of glycosylation. To examine whether glycosylation defects can be restored in vivo, we performed mouse gene transfer experiments. Transfer of fukutin into knock-in mice restored glycosylation of α-dystroglycan. In addition, transfer of LARGE produced laminin-binding forms of α-dystroglycan in both knock-in mice and the POMGnT1 mutant mouse, which is another model of dystroglycanopathy. Overall, these data suggest that even partial restoration of α-dystroglycan glycosylation and laminin-binding activity by replacing or augmenting glycosylation-related genes might effectively deter dystroglycanopathy progression and thus provide therapeutic benefits.