Characterization of the LARGE family of putative glycosyltransferases associated with dystroglycanopathies

The underlying mechanisms of arenaviral entry through matriglycan

Matriglycan, a recently characterized linear polysaccharide, is composed of alternating xylose and glucuronic acid subunits bound to the ubiquitously expressed protein α-dystroglycan (α-DG). Pathogenic arenaviruses, like the Lassa virus (LASV), hijack this long linear polysaccharide to gain cellular entry. Until recently, it was unclear through what mechanisms LASV engages its matriglycan receptor to initiate infection. Additionally, how matriglycan is synthesized onto α-DG by the Golgi-resident glycosyltransferase LARGE1 remained enigmatic. Recent structural data for LARGE1 and for the LASV spike complex informs us about the synthesis of matriglycan as well as its usage as an entry receptor by arenaviruses. In this review, we discuss structural insights into the system of matriglycan generation and eventual recognition by pathogenic viruses. We also highlight the unique usage of matriglycan as a high-affinity host receptor compared with other polysaccharides that decorate cells.

Structural basis for matriglycan synthesis by the LARGE1 dual glycosyltransferase

LARGE1 is a bifunctional glycosyltransferase responsible for generating a long linear polysaccharide termed matriglycan that links the cytoskeleton and the extracellular matrix and is required for proper muscle function. This matriglycan polymer is made with an alternating pattern of xylose and glucuronic acid monomers. Mutations in the LARGE1 gene have been shown to cause life-threatening dystroglycanopathies through the inhibition of matriglycan synthesis. Despite its major role in muscle maintenance, the structure of the LARGE1 enzyme and how it assembles in the Golgi are unknown. Here we present the structure of LARGE1, obtained by a combination of X-ray crystallography and single-particle cryo-EM. We found that LARGE1 homo-dimerizes in a configuration that is dictated by its coiled-coil stem domain. The structure shows that this enzyme has two canonical GT-A folds within each of its catalytic domains. In the context of its dimeric structure, the two types of catalytic domains are brought into close proximity from opposing monomers to allow efficient shuttling of the substrates between the two domains. Together, with putative retention of matriglycan by electrostatic interactions, this dimeric organization offers a possible mechanism for the ability of LARGE1 to synthesize long matriglycan chains. The structural information further reveals the mechanisms in which disease-causing mutations disrupt the activity of LARGE1. Collectively, these data shed light on how matriglycan is synthesized alongside the functional significance of glycosyltransferase oligomerization.

Structure and receptor recognition by the Lassa virus spike complex

Lassa virus (LASV) is a human pathogen, causing substantial morbidity and mortality^1,2. Similar to other Arenaviridae, it presents a class-I spike complex on its surface that facilitates cell entry. The virus's cellular receptor is matriglycan, a linear carbohydrate that is present on α-dystroglycan^3,4, but the molecular mechanism that LASV uses to recognize this glycan is unknown. In addition, LASV and other arenaviruses have a unique signal peptide that forms an integral and functionally important part of the mature spike^5-8; yet the structure, function and topology of the signal peptide in the membrane remain uncertain^9-11. Here we solve the structure of a complete native LASV spike complex, finding that the signal peptide crosses the membrane once and that its amino terminus is located in the extracellular region. Together with a double-sided domain-switching mechanism, the signal peptide helps to stabilize the spike complex in its native conformation. This structure reveals that the LASV spike complex is preloaded with matriglycan, suggesting the mechanism of binding and rationalizing receptor recognition by α-dystroglycan-tropic arenaviruses. This discovery further informs us about the mechanism of viral egress and may facilitate the rational design of novel therapeutics that exploit this binding site.

Muscular dystrophy-dystroglycanopathy in a family of Labrador retrievers with a LARGE1 mutation

Alpha-dystroglycan (αDG) is a highly glycosylated cell surface protein with a significant role in cell-to-extracellular matrix interactions in muscle. αDG interaction with extracellular ligands relies on the activity of the LARGE1 glycosyltransferase that synthesizes and extends the heteropolysaccharide matriglycan. Abnormalities in αDG glycosylation and formation of matriglycan are the pathogenic mechanisms for the dystroglycanopathies, a group of congenital muscular dystrophies. Muscle biopsies were evaluated from related 6-week-old Labrador retriever puppies with poor suckling, small stature compared to normal litter mates, bow-legged stance and markedly elevated creatine kinase activities. A dystrophic phenotype with marked degeneration and regeneration, multifocal mononuclear cell infiltration and endomysial fibrosis was identified on muscle cryosections. Single nucleotide polymorphism (SNP) array genotyping data on the family members identified three regions of homozygosity in 4 cases relative to 8 controls. Analysis of whole genome sequence data from one of the cases identified a stop codon mutation in the LARGE1 gene that truncates 40% of the protein. Immunofluorescent staining and western blotting demonstrated the absence of matriglycan in skeletal muscle and heart from affected dogs. Compared to control, LARGE enzyme activity was not detected. This is the first report of a dystroglycanopathy in dogs.

High degree of conservation of the enzymes synthesizing the laminin-binding glycoepitope of α-dystroglycan

The dystroglycan (DG) complex plays a pivotal role for the stabilization of muscles in Metazoa. It is formed by two subunits, extracellular α-DG and transmembrane β-DG, originating from a unique precursor via a complex post-translational maturation process. The α-DG subunit is extensively glycosylated in sequential steps by several specific enzymes and employs such glycan scaffold to tightly bind basement membrane molecules. Mutations of several of these enzymes cause an alteration of the carbohydrate structure of α-DG, resulting in severe neuromuscular disorders collectively named dystroglycanopathies. Given the fundamental role played by DG in muscle stability, it is biochemically and clinically relevant to investigate these post-translational modifying enzymes from an evolutionary perspective. A first phylogenetic history of the thirteen enzymes involved in the fabrication of the so-called 'M3 core' laminin-binding epitope has been traced by an overall sequence comparison approach, and interesting details on the primordial enzyme set have emerged, as well as substantial conservation in Metazoa. The optimization along with the evolution of a well-conserved enzymatic set responsible for the glycosylation of α-DG indicate the importance of the glycosylation shell in modulating the connection between sarcolemma and surrounding basement membranes to increase skeletal muscle stability, and eventually support movement and locomotion.

The role of the Golgi apparatus in disease (Review)

The Golgi apparatus is known to underpin many important cellular homeostatic functions, including trafficking, sorting and modifications of proteins or lipids. These functions are dysregulated in neurodegenerative diseases, cancer, infectious diseases and cardiovascular diseases, and the number of disease-related genes associated with Golgi apparatus is on the increase. Recently, many studies have suggested that the mutations in the genes encoding Golgi resident proteins can trigger the occurrence of diseases. By summarizing the pathogenesis of these genetic diseases, it was found that most of these diseases have defects in membrane trafficking. Such defects typically result in mislocalization of proteins, impaired glycosylation of proteins, and the accumulation of undegraded proteins. In the present review, we aim to understand the patterns of mutations in the genes encoding Golgi resident proteins and decipher the interplay between Golgi resident proteins and membrane trafficking pathway in cells. Furthermore, the detection of Golgi resident protein in human serum samples has the potential to be used as a diagnostic tool for diseases, and its central role in membrane trafficking pathways provides possible targets for disease therapy. Thus, we also introduced the clinical value of Golgi apparatus in the present review.

HNK-1 sulfotransferase modulates α-dystroglycan glycosylation by 3-O-sulfation of glucuronic acid on matriglycan

Mutations in multiple genes required for proper O-mannosylation of α-dystroglycan are causal for congenital/limb-girdle muscular dystrophies and abnormal brain development in mammals. Previously, we and others further elucidated the functional O-mannose glycan structure that is terminated by matriglycan, [(-GlcA-β3-Xyl-α3-)n]. This repeating disaccharide serves as a receptor for proteins in the extracellular matrix. Here, we demonstrate in vitro that HNK-1 sulfotransferase (HNK-1ST/carbohydrate sulfotransferase) sulfates terminal glucuronyl residues of matriglycan at the 3-hydroxyl and prevents further matriglycan polymerization by the LARGE1 glycosyltransferase. While α-dystroglycan isolated from mouse heart and kidney is susceptible to exoglycosidase digestion of matriglycan, the functional, lower molecular weight α-dystroglycan detected in brain, where HNK-1ST expression is elevated, is resistant. Removal of the sulfate cap by a sulfatase facilitated dual-glycosidase digestion. Our data strongly support a tissue specific mechanism in which HNK-1ST regulates polymer length by competing with LARGE for the 3-position on the nonreducing GlcA of matriglycan.

TRAPPC11 functions in autophagy by recruiting ATG2B-WIPI4/WDR45 to preautophagosomal membranes

TRAPPC11 has been implicated in membrane traffic and lipid-linked oligosaccharide synthesis, and mutations in TRAPPC11 result in neuromuscular and developmental phenotypes. Here, we show that TRAPPC11 has a role upstream of autophagosome formation during macroautophagy. Upon TRAPPC11 depletion, LC3-positive membranes accumulate prior to, and fail to be cleared during, starvation. A proximity biotinylation assay identified ATG2B and its binding partner WIPI4/WDR45 as TRAPPC11 interactors. TRAPPC11 depletion phenocopies that of ATG2 and WIPI4 and recruitment of both proteins to membranes is defective upon reduction of TRAPPC11. We find that a portion of TRAPPC11 and other TRAPP III proteins localize to isolation membranes. Fibroblasts from a patient with TRAPPC11 mutations failed to recruit ATG2B-WIPI4, suggesting that this interaction is physiologically relevant. Since ATG2B-WIPI4 is required for isolation membrane expansion, our study suggests that TRAPPC11 plays a role in this process. We propose a model whereby the TRAPP III complex participates in the formation and expansion of the isolation membrane at several steps.

Distinct expression of functionally glycosylated alpha-dystroglycan in muscle and non-muscle tissues of FKRP mutant mice

The glycosylation of alpha-dystroglycan (α-DG) is crucial in maintaining muscle cell membrane integrity. Dystroglycanopathies are identified by the loss of this glycosylation leading to a breakdown of muscle cell membrane integrity and eventual degeneration. However, a small portion of fibers expressing functionally glycosylated α-DG (F-α-DG) (revertant fibers, RF) have been identified. These fibers are generally small in size, centrally nucleated and linked to regenerating fibers. Examination of different muscles have shown various levels of RFs but it is unclear the extent of which they are present. Here we do a body-wide examination of muscles from the FKRP-P448L mutant mouse for the prevalence of RFs. We have identified great variation in the distribution of RF in different muscles and tissues. Triceps shows a large increase in RFs and together with centrally nucleated fibers whereas the pectoralis shows a reduction in revertant but increase in centrally nucleated fibers from 6 weeks to 6 months of age. We have also identified that the sciatic nerve with near normal levels of F-α-DG in the P448Lneo- mouse with reduced levels in the P448Lneo+ and absent in LARGE^myd. The salivary gland of LARGE^myd mice expresses high levels of F-α-DG. Interestingly the same glands in the P448Lneo-and to a lesser degree in P448Lneo+ also maintain considerable amount of F-α-DG, indicating the non-proliferating epithelial cells have a molecular setting permitting glycosylation.

Recent advancements in understanding mammalian O-mannosylation

The post-translational glycosylation of select proteins by O-linked mannose (O-mannose or O-man) is a conserved modification from yeast to humans and has been shown to be necessary for proper development and growth. The most well studied O-mannosylated mammalian protein is α-dystroglycan (α-DG). Hypoglycosylation of α-DG results in varying severities of congenital muscular dystrophies, cancer progression and metastasis, and inhibited entry and infection of certain arenaviruses. Defects in the gene products responsible for post-translational modification of α-DG, primarily glycosyltransferases, are the basis for these diseases. The multitude of clinical phenotypes resulting from defective O-mannosylation highlights the biomedical significance of this unique modification. Elucidation of the various O-mannose biosynthetic pathways is imperative to understanding a broad range of human diseases and for the development of novel therapeutics. In this review, we will focus on recent discoveries delineating the various enzymes, structures and functions associated with O-mannose-initiated glycoproteins. Additionally, we discuss current gaps in our knowledge of mammalian O-mannosylation, discuss the evolution of this pathway, and illustrate the utility and limitations of model systems to study functions of O-mannosylation.

LARGE2-dependent glycosylation confers laminin-binding ability on proteoglycans

Both LARGE1 (formerly LARGE) and its paralog LARGE2 are bifunctional glycosyltransferases with xylosy- and glucuronyltransferase activities, and are capable of synthesizing polymers composed of a repeating disaccharide [-3Xylα1,3GlcAβ1-]. Post-translational modification of the O-mannosyl glycan of α-dystroglycan (α-DG) with the polysaccharide is essential for it to act as a receptor for ligands in the extracellular matrix (ECM), and both LARGE paralogs contribute to the modification in vivo. LARGE1 and LARGE2 have different tissue distribution profiles and enzymatic properties; however, the functional difference of the homologs remains to be determined, and α-DG is the only known substrate for the modification by LARGE1 or LARGE2. Here we show that LARGE2 can modify proteoglycans (PGs) with the laminin-binding glycan. We found that overexpression of LARGE2, but not LARGE1, mediates the functional modification on the surface of DG^-/-, Pomt1^-/- and Fktn^-/- embryonic stem cells. We identified a heparan sulfate-PG glypican-4 as a substrate for the LARGE2-dependent modification by affinity purification and subsequent mass spectrometric analysis. Furthermore, we showed that LARGE2 could modify several additional PGs with the laminin-binding glycan, most likely within the glycosaminoglycan (GAG)-protein linkage region. Our results indicate that LARGE2 can modify PGs with the GAG-like polysaccharide composed of xylose and glucuronic acid to confer laminin binding. Thus, LARGE2 may play a differential role in stabilizing the basement membrane and modifying its functions by augmenting the interactions between laminin globular domain-containing ECM proteins and PGs.

In-silico structural analysis of E509K mutation in LARGE and T192M mutation in Alpha Dystroglycan in the inhibition of glycosylation of Alpha Dystroglycan by LARGE

Impaired glycosylation of cellular receptor Alpha Dystroglycan (α-DG) leads to dystroglycanopathy. Glycoprotein α-DG is the receptor protein in the Dystrophin Associated Protein Complex (DAPC), a macromolecular gathering on muscle cell membrane to form a bridge between extracellular matrix (ECM) and cellular actin cytoskeleton. Proper glycosylation of α-DG is mediated by the glycosylating enzyme LARGE. Mutations either in α-DG or in LARGE lead to improper glycosylations of α-DG thereby hampering the formation of final Laminin binding form α-DG resulting in dystroglycanopathy. In our current work, we explored the structural changes associated with the presence of mutations in α-DG as well as in the enzyme LARGE. We further extended our research to understand the effect of the mutations onto protein-enzyme interactions. Moreover, since LARGE transfers the sugar moiety (glucuronic acid; GlcA) onto α-DG, we tried to analyze what effect the mutation in LARGE confers on this enzyme ligand interaction. This work for the first time addressed the molecular changes occurring in the structures α-DG, LARGE and their interactions and shed lights on the as yet poorly understood mechanism behind the dystroglycanopathy onset.

Identification of Differentially Expressed Genes through Integrated Study of Alzheimer's Disease Affected Brain Regions

Background

Alzheimer’s disease (AD) is the most common form of dementia in older adults that damages the brain and results in impaired memory, thinking and behaviour. The identification of differentially expressed genes and related pathways among affected brain regions can provide more information on the mechanisms of AD. In the past decade, several studies have reported many genes that are associated with AD. This wealth of information has become difficult to follow and interpret as most of the results are conflicting. In that case, it is worth doing an integrated study of multiple datasets that helps to increase the total number of samples and the statistical power in detecting biomarkers. In this study, we present an integrated analysis of five different brain region datasets and introduce new genes that warrant further investigation.

Methods

The aim of our study is to apply a novel combinatorial optimisation based meta-analysis approach to identify differentially expressed genes that are associated to AD across brain regions. In this study, microarray gene expression data from 161 samples (74 non-demented controls, 87 AD) from the Entorhinal Cortex (EC), Hippocampus (HIP), Middle temporal gyrus (MTG), Posterior cingulate cortex (PC), Superior frontal gyrus (SFG) and visual cortex (VCX) brain regions were integrated and analysed using our method. The results are then compared to two popular meta-analysis methods, RankProd and GeneMeta, and to what can be obtained by analysing the individual datasets.

Results

We find genes related with AD that are consistent with existing studies, and new candidate genes not previously related with AD. Our study confirms the up-regualtion of INFAR2 and PTMA along with the down regulation of GPHN, RAB2A, PSMD14 and FGF. Novel genes PSMB2, WNK1, RPL15, SEMA4C, RWDD2A and LARGE are found to be differentially expressed across all brain regions. Further investigation on these genes may provide new insights into the development of AD. In addition, we identified the presence of 23 non-coding features, including four miRNA precursors (miR-7, miR570, miR-1229 and miR-6821), dysregulated across the brain regions. Furthermore, we compared our results with two popular meta-analysis methods RankProd and GeneMeta to validate our findings and performed a sensitivity analysis by removing one dataset at a time to assess the robustness of our results. These new findings may provide new insights into the disease mechanisms and thus make a significant contribution in the near future towards understanding, prevention and cure of AD.

The glycosyltransferase LARGE2 is repressed by Snail and ZEB1 in prostate cancer

Reductions in both expression of the dystroglycan core protein and functional glycosylation of the α-dystroglycan (αDG) subunit have been reported in a number of cancers and may contribute to disease progression. In the case of prostate cancer, one mechanism that contributes to αDG hypoglycosylation is transcriptional down-regulation of LARGE2 (GYLTY1B), a glycosyltransferase that produces the functional (laminin-binding) glycan on αDG, but the mechanism(s) underlying reduction of LARGE2 mRNA remain unclear. Here, we show that αDG hypoglycosylation is associated with epithelial-to-mesenchymal transition (EMT)-like status. We examined immunoreactivity for both functionally-glycosylated αDG and E-cadherin by flow cytometry and the relative expression of ZEB1 mRNA and the αDG glycosyltransferase LARGE2 mRNA in prostate and other cancer cell lines by quantitative RT-PCR. To study the role of ZEB1 and other transcription factors in the regulation of LARGE2, we employed overexpression and knockdown approaches. Snail- or ZEB1-driven EMT caused αDG hypoglycosylation by repressing expression of the LARGE2 mRNA, with both ZEB1-dependent and -independent mechanisms contributing to Snail-mediated LARGE2 repression. To examine the direct regulation of LARGE2 by Snail and ZEB1 we employed luciferase reporter and chromatin immunoprecipitation assays. Snail and ZEB1 were found to bind directly to the LARGE2 promoter, specifically to E/Z-box clusters. Furthermore, analysis of gene expression profiles of clinical samples in The Cancer Genome Atlas reveals negative correlation of LARGE2 and ZEB1 expression in various cancers. Collectively, our results suggest that LARGE2 is negatively regulated by Snail and/or ZEB1, revealing a mechanistic basis for αDG hypoglycosylation during prostate cancer progression and metastasis.

Degree of Cajal-Retzius Cell Mislocalization Correlates with the Severity of Structural Brain Defects in Mouse Models of Dystroglycanopathy

The secondary dystroglycanopathies are characterized by the hypoglycosylation of alpha dystroglycan, and are associated with mutations in at least 18 genes that act on the glycosylation of this cell surface receptor rather than the Dag1 gene itself. At the severe end of the disease spectrum, there are substantial structural brain defects, the most striking of which is often cobblestone lissencephaly. The aim of this study was to determine the gene-specific aspects of the dystroglycanopathy brain phenotype through a detailed investigation of the structural brain defects present at birth in three mouse models of dystroglycanopathy-the FKRP(KD) , which has an 80% reduction in Fkrp transcript levels; the Pomgnt1null , which carries a deletion of exons 7-16 of the Pomgnt1 gene; and the Large(myd) mouse, which carries a deletion of exons 5-7 of the Large gene. We show a rostrocaudal and mediolateral gradient in the severity of brain lesions in FKRP(KD) , and to a lesser extent Pomgnt1null mice. Furthermore, the mislocalization of Cajal-Retzius cells is correlated with the gradient of these lesions and the severity of the brain phenotype in these models. Overall these observations implicate gene-specific differences in the pathogenesis of brain lesions in this group of disorders.

Downregulation of dystroglycan glycosyltransferases LARGE2 and ISPD associate with increased mortality in clear cell renal cell carcinoma

Background

Dystroglycan (DG) is a cell-surface laminin receptor that links the cytoskeleton to the extracellular matrix in a variety of epithelial tissues. Its function as a matrix receptor requires extensive glycosylation of its extracellular subunit αDG, which involves at least 13 distinct genes. Prior work has shown loss of αDG glycosylation in an assortment of carcinomas, including clear cell renal cell carcinoma (ccRCC) though the cause (s) and functional consequences of this loss are still unclear.

Methods

Using The Cancer Genome Atlas (TCGA) database, we analyzed the DG glycosylation pathway to identify changes in mRNA expression and correlation with clinical outcomes. We validated our findings with a cohort of 65 patients treated with radical nephrectomy by analyzing DG glycosylation via immunohistochemistry and gene expression via qRT-PCR.

Results

Analysis of TCGA database revealed frequent dysregulation of a subset of DG glycosyltransferases. Most notably, there was a frequent, significant downregulation of GYLTL1B (LARGE2) and ISPD. DG glycosylation is frequently impaired in ccRCC patient samples and most strongly associates with downregulation of GYLTL1B.

Conclusions

Reduced levels of GYLTL1B and ISPD mRNA associated with increased patient mortality and are the likely cause of αDG hypoglycosylation in ccRCC.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-015-0416-z) contains supplementary material, which is available to authorized users.

What do mouse models of muscular dystrophy tell us about the DAPC and its components?

There are over 30 mouse models with mutations or inactivations in the dystrophin-associated protein complex. This complex is thought to play a crucial role in the functioning of muscle, as both a shock absorber and signalling centre, although its role in the pathogenesis of muscular dystrophy is not fully understood. The first mouse model of muscular dystrophy to be identified with a mutation in a component of the dystrophin-associated complex (dystrophin) was the mdx mouse in 1984. Here, we evaluate the key characteristics of the mdx in comparison with other mouse mutants with inactivations in DAPC components, along with key modifiers of the disease phenotype. By discussing the differences between the individual phenotypes, we show that the functioning of the DAPC and consequently its role in the pathogenesis is more complicated than perhaps currently appreciated.

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.

Expression of the clustered NeuAcα2-3Galβ O-glycan determines the cell differentiation state of the cells

Human embryonic stem cells (hESCs) are pluripotent stem cells from early embryos, and their self-renewal capacity depends on the sustained expression of hESC-specific molecules and the suppressed expression of differentiation-associated genes. To discover novel molecules expressed on hESCs, we generated a panel of monoclonal antibodies against undifferentiated hESCs and evaluated their ability to mark cancer cells, as well as hESCs. MAb7 recognized undifferentiated hESCs and showed a diffuse band with molecular mass of >239 kDa in the lysates of hESCs. Although some amniotic epithelial cells expressed MAb7 antigen, its expression was barely detected in normal human keratinocytes, fibroblasts, or endothelial cells. The expression of MAb7 antigen was observed only in pancreatic and gastric cancer cells, and its levels were elevated in metastatic and poorly differentiated cancer cell lines. Analyses of MAb7 antigen suggested that the clustered NeuAcα2-3Galβ O-linked oligosaccharides on DMBT1 (deleted in malignant brain tumors 1) were critical for MAb7 binding in cancer cells. Although features of MAb7 epitope were similar with those of TRA-1-60, distribution of MAb7 antigen in cancer cells was different from that of TRA-1-60 antigen. Exposure of a histone deacetylase inhibitor to differentiated gastric cancer MKN74 cells evoked the expression of MAb7 antigen, whereas DMBT1 expression remained unchanged. Cell sorting followed by DNA microarray analyses identified the down-regulated genes responsible for the biosynthesis of MAb7 antigen in MKN74 cells. In addition, treatment of metastatic pancreatic cancer cells with MAb7 significantly abrogated the adhesion to endothelial cells. These results raised the possibility that MAb7 epitope is a novel marker for undifferentiated cells such as hESCs and cancer stem-like cells and plays a possible role in the undifferentiated cells.

Mammalian O-mannosylation pathway: glycan structures, enzymes, and protein substrates

The mammalian O-mannosylation pathway for protein post-translational modification is intricately involved in modulating cell–matrix interactions in the musculature and nervous system. Defects in enzymes of this biosynthetic pathway are causative for multiple forms of congenital muscular dystophy. The application of advanced genetic and biochemical technologies has resulted in remarkable progress in this field over the past few years, culminating with the publication of three landmark papers in 2013 alone. In this review, we will highlight recent progress focusing on the dramatic expansion of the set of genes known to be involved in O-mannosylation and disease processes, the concurrent acceleration of the rate of O-mannosylation pathway protein functional assignments, the tremendous increase in the number of proteins now known to be modified by O-mannosylation, and the recent progress in protein O-mannose glycan quantification and site assignment. Also, we attempt to highlight key outstanding questions raised by this abundance of new information.

Adeno-associated virus-mediated overexpression of LARGE rescues α-dystroglycan function in dystrophic mice with mutations in the fukutin-related protein

Multiple genes (e.g., POMT1, POMT2, POMGnT1, ISPD, GTDC2, B3GALNT2, FKTN, FKRP, and LARGE) are known to be involved in the glycosylation pathway of α-dystroglycan (α-DG). Mutations of these genes result in muscular dystrophies with wide phenotypic variability. Abnormal glycosylation of α-DG with decreased extracellular ligand binding activity is a common biochemical feature of these genetic diseases. While it is known that LARGE overexpression can compensate for defects in a few aforementioned genes, it is unclear whether it can also rescue defects in FKRP function. We examined adeno-associated virus (AAV)-mediated LARGE or FKRP overexpression in two dystrophic mouse models with loss-of-function mutations: (1) Large(myd) (LARGE gene) and (2) FKRP(P448L) (FKRP gene). The results agree with previous findings that overexpression of LARGE can ameliorate the dystrophic phenotypes of Large(myd) mice. In addition, LARGE overexpression in the FKRP(P448L) mice effectively generated functional glycosylation (hyperglycosylation) of α-DG and improved dystrophic pathologies in treated muscles. Conversely, FKRP transgene overexpression failed to rescue the defect in glycosylation and improve the phenotypes of the Large(myd) mice. Our findings suggest that AAV-mediated LARGE gene therapy may still be a viable therapeutic strategy for dystroglycanopathies with FKRP deficiency.

The absence of core fucose up-regulates GnT-III and Wnt target genes: a possible mechanism for an adaptive response in terms of glycan function

Glycans play key roles in a variety of protein functions under normal and pathological conditions, but several glycosyltransferase-deficient mice exhibit no or only mild phenotypes due to redundancy or compensation of glycan functions. However, we have only a limited understanding of the underlying mechanism for these observations. Our previous studies indicated that 70% of Fut8-deficient (Fut8(-/-)) mice that lack core fucose structure die within 3 days after birth, but the remainder survive for up to several weeks although they show growth retardation as well as emphysema. In this study, we show that, in mouse embryonic fibroblasts (MEFs) from Fut8(-/-) mice, another N-glycan branching structure, bisecting GlcNAc, is specifically up-regulated by enhanced gene expression of the responsible enzyme N-acetylglucosaminyltransferase III (GnT-III). As candidate target glycoproteins for bisecting GlcNAc modification, we confirmed that level of bisecting GlcNAc on β1-integrin and N-cadherin was increased in Fut8(-/-) MEFs. Moreover using mass spectrometry, glycan analysis of IgG1 in Fut8(-/-) mouse serum demonstrated that bisecting GlcNAc contents were also increased by Fut8 deficiency in vivo. As an underlying mechanism, we found that in Fut8(-/-) MEFs Wnt/β-catenin signaling is up-regulated, and an inhibitor against Wnt signaling was found to abrogate GnT-III expression, indicating that Wnt/β-catenin is involved in GnT-III up-regulation. Furthermore, various oxidative stress-related genes were also increased in Fut8(-/-) MEFs. These data suggest that Fut8(-/-) mice adapted to oxidative stress, both ex vivo and in vivo, by inducing various genes including GnT-III, which may compensate for the loss of core fucose functions.

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.

Adeno-associated viral-mediated LARGE gene therapy rescues the muscular dystrophic phenotype in mouse models of dystroglycanopathy

Dystroglycanopathies are a group of congenital muscular dystrophies (CMD) often caused by mutations in genes encoding glycosyltransferases that lead to hypoglycosylation of α-dystroglycan (α-DG) and reduce its extracellular matrix-binding activity. Overexpressing LARGE (formerly known as like-glycosyltransferase) generates an extracellular matrix-binding carbohydrate epitope in cells with CMD-causing mutations in not only LARGE but also other glycosyltransferases, including POMT1, POMGnT1, and fukutin, creating the possibilities of a one-for-all gene therapy. To determine the feasibility of LARGE gene therapy, a serotype 9 adeno-associated viral vector for overexpressing LARGE (AAV9-LARGE) was injected intracardially into newborns of two mouse models of CMD: the natural LARGE mutant Large(myd) mice and protein O-mannose N-acetylglucosaminyltransferase 1 (POMGnT1) knockout mice. AAV9-LARGE virus treatment yielded partial restoration of α-DG glycosylation and ligand-binding activity. The muscular dystrophy phenotype in skeletal muscles was ameliorated as revealed by significantly reduced fibrosis, necrosis, and numbers of centrally located nuclei with improved motor function. These results indicate that LARGE overexpression in vivo by AAV9-mediated gene therapy is effective at restoring functional glycosylation of α-DG and rescuing the muscular dystrophy phenotype in deficiency of not only LARGE but also POMGnT1, providing evidence that in vivo LARGE gene therapy may be broadly useful in dystroglycanopathies.

Increased levels of expression of dystroglycan may protect the heart

Dystroglycan is a major adhesion complex composed of two subunits, α and β, that undergoes extensive post-translational modifications. In particular, its α subunit is heavily decorated with sugars, influencing its basement membrane binding properties. An altered glycosylation of α-dystroglycan is at the molecular basis of muscular dystrophies defined as secondary dystroglycanopathies, that depend on malfunctioning of the enzymes in the glycosylation pathway. An increased level of transcription of the dystroglycan gene may be crucial for obtaining sufficient amounts of dystroglycan precursor substrate required for the production of the heavily glycosylated and fully functional α-dystroglycan molecule. Even slight differences in these transcriptional levels may exert a protective or pathogenetic effect, as discussed for the unique case of primary dystroglycanopathy so far identified (T192M), where the heart tissues are not affected by the pathology. Moreover, the N-terminal fragment of α-dystroglycan is also proposed to have a regulatory role in the glycosylation/maturation process.

Dmdmdx/Largemyd: a new mouse model of neuromuscular diseases useful for studying physiopathological mechanisms and testing therapies

Although muscular dystrophies are among the most common human genetic disorders, there are few treatment options available. Animal models have become increasingly important for testing new therapies prior to entering human clinical trials. The Dmd^mdx mouse is the most widely used animal model for Duchenne muscular dystrophy (DMD), presenting the same molecular and protein defect as seen in humans with the disease. However, this mouse is not useful for clinical trials because of its very mild phenotype. The mouse model for congenital myodystrophy type 1D, Large^myd, harbors a mutation in the glycosyltransferase Large gene and displays a severe phenotype. To help elucidate the role of the proteins dystrophin and LARGE in the organization of the dystrophin-glycoprotein complex in muscle sarcolemma, we generated double-mutant mice for the dystrophin and LARGE proteins. The new Dmd^mdx/Large^myd mouse model is viable and shows a severe phenotype that is associated with the lack of dystrophin in muscle. We tested the usefulness of our new mouse model for cell therapy by systemically injecting them with normal murine mesenchymal adipose stem cells (mASCs). We verified that the mASCs were hosted in the dystrophic muscle. The new mouse model has proven to be very useful for the study of several other therapies, because injected cells can be screened both through DNA and protein analysis. Study of its substantial muscle weakness will also be very informative in the evaluation of functional benefits of these therapies.

Mouse models of fukutin-related protein mutations show a wide range of disease phenotypes

Dystroglycanopathies are characterized by a reduction in the glycosylation of alpha-dystroglycan (α-DG). A common cause for this subset of muscular dystrophies is mutations in the gene of fukutin-related protein (FKRP). FKRP mutations have been associated with a wide spectrum of clinical severity from severe Walker-Warburg syndrome and muscle-eye-brain disease with brain and eye defects to mild limb-girdle muscular dystrophy 2I with myopathy only. To examine the affects of FKRP mutations on the severity of the disease, we have generated homozygous and compound heterozygous mouse models with human mutations in the murine FKRP gene. P448Lneo+ and E310delneo+ mutations result in severe dystrophic and embryonic lethal phenotypes, respectively. P448Lneo+/E310delneo+ compound heterozygotes exhibit brain defects and severe muscular dystrophies with near absence of α-DG glycosylation. Removal of the Neo(r) cassette from the P448Lneo+ homozygous mice eliminates overt brain and eye defects, and reduces severity of dystrophic phenotypes. Furthermore, introduction of the common L276I mutation to generate transgenic L276Ineo+ homozygous and L276Ineo+/P448Lneo+ and L276Ineo+/E310delneo+ compound heterozygotes results in mice displaying milder dystrophies with reduced α-DG glycosylation and no apparent brain defects. Limited sampling and variation in functionally glycosylated α-DG levels between and within muscles may explain the difficulties in correlating FKRP expression levels with phenotype in clinics. The nature of individual mutations, expression levels and status of muscle differentiation all contribute to the phenotypic manifestation. These mutant FKRP mice are useful models for the study of disease mechanism(s) and experimental therapies.

Neuromuscular disorders in zebrafish: state of the art and future perspectives

Neuromuscular disorders are a broad group of inherited conditions affecting the structure and function of the motor system with polymorphic clinical presentation and disease severity. Although individually rare, collectively neuromuscular diseases have an incidence of 1 in 3,000 and represent a significant cause of disability of the motor system. The past decade has witnessed the identification of a large number of human genes causing muscular disorders, yet the underlying pathogenetic mechanisms remain largely unclear, limiting the developing of targeted therapeutic strategies. To overcome this barrier, model systems that replicate the different steps of human disorders are increasingly being developed. Among these, the zebrafish (Danio rerio) has emerged as an excellent organism for studying genetic disorders of the central and peripheral motor systems. In this review, we will encounter most of the available zebrafish models for childhood neuromuscular disorders, providing a brief overview of results and the techniques, mainly transgenesis and chemical biology, used for genetic manipulation. The amount of data collected in the past few years will lead zebrafish to became a common functional tool for assessing rapidly drug efficacy and off-target effects in neuromuscular diseases and, furthermore, to shed light on new etiologies emerging from large-scale massive sequencing studies.

Loss of LARGE2 disrupts functional glycosylation of α-dystroglycan in prostate cancer

Dystroglycan (DG) is a cell surface receptor for extracellular matrix proteins and is involved in cell polarity, matrix organization, and mechanical stability of tissues. Previous studies documented loss of DG protein expression and glycosylation in a variety of cancer types, but the underlying mechanisms and the functional consequences with respect to cancer progression remain unclear. Here, we show that the level of expression of the βDG subunit as well as the glycosylation status of the αDG subunit inversely correlate with the Gleason scores of prostate cancers; furthermore, we show that the functional glycosylation of αDG is substantially reduced in prostate cancer metastases. Additionally, we demonstrate that LARGE2 (GYLTL1B), a gene not previously implicated in cancer, regulates functional αDG glycosylation in prostate cancer cell lines; knockdown of LARGE2 resulted in hypoglycosylation of αDG and loss of its ability to bind laminin-111 while overexpression restored ligand binding and diminished growth and migration of an aggressive prostate cancer cell line. Finally, our analysis of LARGE2 expression in human cancer specimens reveals that LARGE2 is significantly down-regulated in the context of prostate cancer, and that its reduction correlates with disease progression. Our results describe a novel molecular mechanism to account for the commonly observed hypoglycosylation of αDG in prostate cancer.

LARGE2 generates the same xylose- and glucuronic acid-containing glycan structures as LARGE

LARGE (like-glycosyltransferase) and LARGE2 (glycosyltransferase-like 1B (GYLTL1B)) are homologous Golgi glycosyltransferases possessing two catalytic domains with homology to members of glycosyltransferase families GT8 and GT49. Mutations in human and mouse Large result in muscular dystrophy due to underglycosylation of dystroglycan. The systemic function of LARGE2 is unknown, but at a cellular level the enzyme can substitute for LARGE in glycosylating dystroglycan. Here, we show that LARGE2 catalyzes the same glycosylation reaction as LARGE. It is a bifunctional glycosyltransferase using uridine diphosphate (UDP)-xylose (Xyl) and UDP-glucuronic acid (GlcA) as donor sugars to produce a xyloglucuronan with alternating Xyl and GlcA residues.

Xylosyl- and glucuronyltransferase functions of LARGE in α-dystroglycan modification are conserved in LARGE2

LARGE-dependent modification enables α-dystroglycan (α-DG) to bind to its extracellular matrix ligands. Mutations in the LARGE gene and several others involved in O-mannosyl glycan synthesis have been identified in congenital and limb-girdle muscular dystrophies that are characterized by perturbed glycosylation and reduced ligand-binding affinity of α-DG. LARGE is a bifunctional glycosyltransferase that alternately transfers xylose and glucuronic acid, thereby generating the heteropolysaccharides on α-DG that confer its ligand binding. Although the LARGE paralog LARGE2 (also referred to as GYLTL1B) has likewise been shown to enhance the functional modification of α-DG in cultured cells, its enzymatic activities have not been identified. Here, we report that LARGE2 is also a bifunctional glycosyltransferase and compare its properties with those of LARGE. By means of a high-performance liquid chromatography-based enzymatic assay, we demonstrate that like LARGE, LARGE2 has xylosyltransferase (Xyl-T) and glucuronyltransferase (GlcA-T) activities, as well as polymerizing activity. Notably, however, the pH optima of the Xyl-T and GlcA-T of LARGE2 are distinct from one another and also from those of LARGE. Our results suggest that LARGE and LARGE2 catalyze the same glycosylation reactions for the functional modification of α-DG, but that they have different biochemical properties.

O-Mannosylation and human disease

Glycosylation of proteins is arguably the most prevalent co- and post-translational modification. It is responsible for increased heterogeneity and functional diversity of proteins. Here we discuss the importance of one type of glycosylation, specifically O-mannosylation and its relationship to a number of human diseases. The most widely studied O-mannose modified protein is alpha-dystroglycan (α-DG). Recent studies have focused intensely on α-DG due to the severity of diseases associated with its improper glycosylation. O-mannosylation of α-DG is involved in cancer metastasis, arenavirus entry, and multiple forms of congenital muscular dystrophy [1, 2]. In this review, we discuss the structural and functional characteristics of O-mannose-initiated glycan structures on α-DG, enzymes involved in the O-mannosylation pathway, and the diseases that are a direct result of disruptions within this pathway.

"Casting" light on the role of glycosylation during embryonic development: insights from zebrafish

Zebrafish (Danio rerio) remains a versatile model organism for the investigation of early development and organogenesis, and has emerged as a valuable platform for drug discovery and toxicity evaluation [1-6]. Harnessing the genetic power and experimental accessibility of this system, three decades of research have identified key genes and pathways that control the development of multiple organ systems and tissues, including the heart, kidney, and craniofacial cartilage, as well as the hematopoietic, vascular, and central and peripheral nervous systems [7-31]. In addition to their application in large mutagenic screens, zebrafish has been used to model a variety of diseases such as diabetes, polycystic kidney disease, muscular dystrophy and cancer [32-36]. As this work continues to intersect with cellular pathways and processes such as lipid metabolism, glycosylation and vesicle trafficking, investigators are often faced with the challenge of determining the degree to which these pathways are functionally conserved in zebrafish. While they share a high degree of genetic homology with mouse and human, the manner in which cellular pathways are regulated in zebrafish during early development, and the differences in the organ physiology, warrant consideration before functional studies can be effectively interpreted and compared with other vertebrate systems. This point is particularly relevant for glycosylation since an understanding of the glycan diversity and the mechanisms that control glycan biosynthesis during zebrafish embryogenesis (as in many organisms) is still developing.Nonetheless, a growing number of studies in zebrafish have begun to cast light on the functional roles of specific classes of glycans during organ and tissue development. While many of the initial efforts involved characterizing identified mutants in a number of glycosylation pathways, the use of reverse genetic approaches to directly model glycosylation-related disorders is now increasingly popular. In this review, the glycomics of zebrafish and the developmental expression of their glycans will be briefly summarized along with recent chemical biology approaches to visualize certain classes of glycans within developing embryos. Work regarding the role of protein-bound glycans and glycosaminoglycans (GAG) in zebrafish development and organogenesis will also be highlighted. Lastly, future opportunities and challenges in the expanding field of zebrafish glycobiology are discussed.

Molecular cloning of a xylosyltransferase that transfers the second xylose to O-glucosylated epidermal growth factor repeats of notch

The extracellular domain of Notch contains epidermal growth factor (EGF) repeats that are extensively modified with different O-linked glycans. O-Fucosylation is essential for receptor function, and elongation with N-acetylglucosamine, catalyzed by members of the Fringe family, modulates Notch activity. Only recently, genes encoding enzymes involved in the O-glucosylation pathway have been cloned. In the Drosophila mutant rumi, characterized by a mutation in the protein O-glucosyltransferase, Notch signaling is impaired in a temperature-dependent manner, and a mouse knock-out leads to embryonic lethality. We have previously identified two human genes, GXYLT1 and GXYLT2, encoding glucoside xylosyltransferases responsible for the transfer of xylose to O-linked glucose. The identity of the enzyme further elongating the glycan to generate the final trisaccharide xylose-xylose-glucose, however, remained unknown. Here, we describe that the human gene C3ORF21 encodes a UDP-xylose:α-xyloside α1,3-xylosyltransferase, acting on xylose-α1,3-glucoseβ1-containing acceptor structures. We have, therefore, renamed it XXYLT1 (xyloside xylosyltransferase 1). XXYLT1 cannot act on a synthetic acceptor containing an α-linked xylose alone, but requires the presence of the underlying glucose. Activity on Notch EGF repeats was proven by in vitro xylosylation of a mouse Notch1 fragment recombinantly produced in Sf9 insect cells, a bacterially expressed EGF repeat from mouse Notch2 modified in vitro by Rumi and Gxylt2 and in vivo by co-expression of the enzyme with the Notch1 fragment. The enzyme was shown to be a typical type II membrane-bound glycosyltransferase localized in the endoplasmic reticulum.

Differential glycosylation of α-dystroglycan and proteins other than α-dystroglycan by like-glycosyltransferase

Genetic defects in like-glycosyltransferase (LARGE) cause congenital muscular dystrophy with central nervous system manifestations. The underlying molecular pathomechanism is the hypoglycosylation of α-dystroglycan (α-DG), which is evidenced by diminished immunoreactivity to IIH6C4 and VIA4-1, antibodies that recognize carbohydrate epitopes. Previous studies indicate that LARGE participates in the formation of a phosphoryl glycan branch on O-linked mannose or it modifies complex N- and mucin O-glycans. In this study, we overexpressed LARGE in neural stem cells deficient in protein O-mannosyltransferase 2 (POMT2), an enzyme required for O-mannosyl glycosylation. The results showed that overexpressing LARGE did not lead to hyperglycosylation of α-DG in POMT2 knockout (KO) cells but did generate IIH6C4 and VIA4-1 immunoreactivity and laminin-binding activity. Additionally, overexpressing LARGE in cells deficient in both POMT2 and α-DG generated laminin-binding IIH6C4 immunoreactivity. These results indicate that LARGE expression resulted in the glycosylation of proteins other than α-DG in the absence of O-mannosyl glycosylation. The IIH6C4 immunoreactivity generated in double-KO cells was largely removed by treatment either with peptide N-glycosidase F or with cold aqueous hydrofluoric acid, suggesting that LARGE expression caused phosphoryl glycosylation of N-glycans. However, the glycosylation of α-DG by LARGE is dependent on POMT2, indicating that LARGE expression only modifies O-linked mannosyl glycans of α-DG. Thus, LARGE expression mediates the phosphoryl glycosylation of not only O-mannosyl glycans including those on α-DG but also N-glycans on proteins other than α-DG.

Sarcolemma instability during mechanical activity in Largemyd cardiac myocytes with loss of dystroglycan extracellular matrix receptor function

The abnormal glycosylation and loss of extracellular matrix receptor function of the protein dystroglycan (DG) lead to the development of muscular dystrophy and cardiomyopathy. Dystroglycan is an important receptor for extracellular matrix proteins, such as laminin, in the basement membrane surrounding muscle. Large(myd) mice have a null mutation in a gene encoding the glycosyltransferase LARGE that results in abnormal glycosylation of α-DG and phenotypes similar to those in human α-DG glycosylation-deficient muscular dystrophy. Here, we show that Large(myd) hearts with the loss of DG extracellular matrix receptor function display a cardiomyopathy characterized by myocyte damage in patches of cells positive for membrane impermeant dyes. To examine the cellular mechanisms, we show that isolated adult cardiac myocytes from Large(myd) mice retain normal laminin-dependent cell adhesion, cell surface laminin deposition and basement membrane assembly. However, although isolated adult cardiac myocytes with the loss of α-DG glycosylation adhere normally to laminin substrates both passively and in the presence of mechanical activity, Large(myd) myocytes rapidly take up membrane impermeant dye following cyclical cell stretching. Therefore, while other cell surface laminin receptors are likely responsible for myocardial cell adhesion to the basement membrane, DG has a unique function of stabilizing the cardiac myocyte plasma membrane during repetitive mechanical activity by tightly binding the transmembrane dystrophin-glycoprotein complex to the extracellular matrix. This function of DG to stabilize the myocyte membrane during normal physiologic cell length changes is likely critical for the prevention of the myocardial damage and subsequent remodeling observed in α-DG glycosylation-deficient muscular dystrophies.

Large induces functional glycans in an O-mannosylation dependent manner and targets GlcNAc terminals on alpha-dystroglycan

Alpha-dystroglycan (α-DG) is a ubiquitously expressed receptor for extracellular matrix proteins and some viruses, and plays a pivotal role in a number of pathological events, including cancer progression, muscular dystrophies, and viral infection. The O-glycans on α-DG are essential for its ligand binding, but the biosynthesis of the functional O-glycans remains obscure. The fact that transient overexpression of LARGE, a putative glycosyltransferase, up-regulates the functional glycans on α-DG to mediate its ligand binding implied that overexpression of LARGE may be a novel strategy to treat disorders with hypoglycosylation of α-DG. In this study, we focus on the effects of stable overexpression of Large on α-DG glycosylation in Chinese hamster ovary (CHO) cell and its glycosylation deficient mutants. Surprisingly, stable overexpression of Large in an O-mannosylation null deficient Lec15.2 CHO cells failed to induce the functional glycans on α-DG. Introducing the wild-type DPM2 cDNA, the deficient gene in the Lec15.2 cells, fully restored the Large-induced functional glycosylation, suggesting that Large induces the functional glycans in a DPM2/O-mannosylation dependent manner. Furthermore, stable overexpression of Large can effectively induce functional glycans on N-linked glycans in the Lec8 cells and ldlD cells growing in Gal deficient media, in both of which circumstances galactosylation are deficient. In addition, supplement of Gal to the ldlD cell culture media significantly reduces the amount of functional glycans induced by Large, suggested that galactosylation suppresses Large to induce the functional glycans. Thus our results revealed a mechanism by which Large competes with galactosyltransferase to target GlcNAc terminals to induce the functional glycans on α-DG.

Glycomarkers for muscular dystrophy

During the last 10 years it has become apparent that a significant subset of inherited muscular dystrophy is caused by errors in the glycosylation of α-dystroglycan. Many of these dystrophies are also associated with abnormalities of the central nervous system. Dystroglycan has to be fully glycosylated in order bind to its ligands. To date, six genes have been shown to be essential for functional dystroglycan glycosylation and most, if not all, of these genes act in the formation of O-mannosyl glycans. Genetic heterogeneity indicates that other genes are involved in this pathway. Identification of these additional genes would increase our understanding of this specific and essential glycosylation pathway.

Transgenic overexpression of LARGE induces α-dystroglycan hyperglycosylation in skeletal and cardiac muscle

Background

LARGE is one of seven putative or demonstrated glycosyltransferase enzymes defective in a common group of muscular dystrophies with reduced glycosylation of α-dystroglycan. Overexpression of LARGE induces hyperglycosylation of α-dystroglycan in both wild type and in cells from dystroglycanopathy patients, irrespective of their primary gene defect, restoring functional glycosylation. Viral delivery of LARGE to skeletal muscle in animal models of dystroglycanopathy has identical effects in vivo, suggesting that the restoration of functional glycosylation could have therapeutic applications in these disorders. Pharmacological strategies to upregulate Large expression are also being explored.

Methodology/Principal Findings

In order to asses the safety and efficacy of long term LARGE over-expression in vivo, we have generated four mouse lines expressing a human LARGE transgene. On observation, LARGE transgenic mice were indistinguishable from the wild type littermates. Tissue analysis from young mice of all four lines showed a variable pattern of transgene expression: highest in skeletal and cardiac muscles, and lower in brain, kidney and liver. Transgene expression in striated muscles correlated with α-dystroglycan hyperglycosylation, as determined by immunoreactivity to antibody IIH6 and increased laminin binding on an overlay assay. Other components of the dystroglycan complex and extracellular matrix ligands were normally expressed, and general muscle histology was indistinguishable from wild type controls. Further detailed muscle physiological analysis demonstrated a loss of force in response to eccentric exercise in the older, but not in the younger mice, suggesting this deficit developed over time. However this remained a subclinical feature as no pathology was observed in older mice in any muscles including the diaphragm, which is sensitive to mechanical load-induced damage.

Conclusions/Significance

This work shows that potential therapies in the dystroglycanopathies based on LARGE upregulation and α-dystroglycan hyperglycosylation in muscle should be safe.

Fukutin-related protein is essential for mouse muscle, brain and eye development and mutation recapitulates the wide clinical spectrums of dystroglycanopathies

Mutations in fukutin-related protein (FKRP) cause a common subset of muscular dystrophies characterized by aberrant glycosylation of alpha-dystroglycan (α-DG), collectively known as dystroglycanopathies. The clinical variations associated with FKRP mutations range from mild limb-girdle muscular dystrophy type 2I with predominantly muscle phenotypes to severe Walker-Warburg syndrome and muscle-eye-brain disease with striking structural brain and eye defects. In the present study, we have generated animal models and demonstrated that ablation of FKRP functions is embryonic lethal and that the homozygous-null embryos die before reaching E12.5. The homozygous knock-in mouse carrying the missense P448L mutation almost completely lacks functional glycosylation of α-DG in muscles and brain, validating the essential role of FKRP in the functional glycosylation of α-DG. However, the knock-in mouse survives and develops a wide range of structural abnormalities in the central nervous system, characteristics of neuronal migration defects. The brain and eye defects are highly reminiscent of the phenotypes seen in severe dystroglycanopathy patients. In addition, skeletal muscles develop progressive muscular dystrophy. Our results confirm that post-translational modifications of α-DG are essential for normal development of the brain and eyes. In addition, both the mutation itself and the levels of FKRP expression are equally critical for the survival of the animals. The exceptionally wide clinical spectrums recapitulated in the P448L mice also suggest the involvement of other factors in the disease progression. The mutant mouse represents a valuable model to further elucidate the functions of FKRP and develop therapies for FKRP-related muscular dystrophies.

Rodent host cell/Lassa virus interactions: evolution and expression of α-Dystroglycan, LARGE-1 and LARGE-2 genes, with special emphasis on the Mastomys genus

Arenaviruses are usually rodent-borne viruses that constitute a major threat for human health. Among them, Lassa Fever Virus (LFV) occurs in Western Africa where it infects hundreds of thousands of people annually. According to the most recent surveys, LFV is hosted by one of the multimammate rats, Mastomys natalensis, but has never been detected in its sibling and sometimes sympatric species Mastomys erythroleucus. This pattern suggests that intrinsic, i.e. genetic properties underlie such a drastic epidemiological difference (M. natalensis as a reservoir vs. M. erythroleucus as a non-reservoir species). Here we investigate genomic differences between these two closely related rodent species by focusing on three genes that have recently been described as pivotal for LFV/human cell interactions: Dystroglycan (the LFV cellular receptor), LARGE-1 and LARGE-2 (two enzymes that are essential to Dystroglycan functioning). For all three genes, sequence analyses showed that amino-acid chains undergo extremely strong purifying selective pressures, and indicated that no nucleotide (therefore no tertiary structure) change can be advocated to explain species-specific differences in LFV-cellular mediation. Nevertheless, preliminary studies of kidney-specific expression profiles suggested that important species-specific differences exist between Mastomys species. Taking into account current knowledge about LFV-human cell interactions, our results may point towards a possible role for LARGE-1 and LARGE-2 enzymes at the intracellular replication level of the virus, rather than at the LFV-host cell receptor binding step.

Congenital muscular dystrophy. Part II: a review of pathogenesis and therapeutic perspectives

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. In the last number of this journal, we presented the main clinical and diagnostic data concerning the different subtypes of CMD. In this second part of the review, we analyse the main reports from the literature concerning the pathogenesis and the therapeutic perspectives of the most common subtypes of CMD: MDC1A with merosin deficiency, collagen VI related CMDs (Ullrich and Bethlem), CMDs with abnormal glycosylation of alpha-dystroglycan (Fukuyama CMD, Muscle-eye-brain disease, Walker Warburg syndrome, MDC1C, MDC1D), and rigid spine syndrome, another much rare subtype of CMDs not related with the dystrophin/glycoproteins/extracellular matrix complex.

Protein O-mannosylation in animal development and physiology: from human disorders to Drosophila phenotypes

Protein O-mannosylation has a profound effect on the development and physiology of mammalian organisms. Mutations in genes affecting O-mannosyl glycan biosynthesis result in congenital muscular dystrophies. The main pathological mechanism triggered by O-mannosylation defects is a compromised interaction of cells with the extracellular matrix due to abnormal glycosylation of alpha-dystroglycan. Hypoglycosylation of alpha-dystroglycan impairs its ligand-binding activity and results in muscle degeneration and failure of neuronal migration. Recent experiments revealed the existence of compensatory mechanisms that could ameliorate defects of O-mannosylation. However, these mechanisms remain poorly understood. O-mannosylation and dystroglycan pathway genes show remarkable evolutionary conservation in a wide range of metazoans. Mutations and downregulation of these genes in zebrafish and Drosophila result in muscle defects and degeneration, also causing neurological phenotypes, which suggests that O-mannosylation has similar functions in mammals and lower animals. Thus, future studies in genetically tractable model organisms, such as zebrafish and Drosophila, should help to reveal molecular and genetic mechanisms of mammalian O-mannosylation and its role in the regulation of dystroglycan function.

Identification, characterization, and biosynthesis of a novel N-glycan modification in the fruiting body of the basidiomycete Coprinopsis cinerea

Coprinopsis cinerea is a model organism for fruiting body development in homobasidiomycetes. Here, we focused on N-linked oligosaccharides (NLO) of cell wall proteins in the hyphae of two developmental stages, vegetative mycelium and fruiting body. High mannose-type glycans were the most commonly found structures. In addition, we observed a novel glycan, predominantly present in fruiting body. This oligosaccharide structure was of the high mannose type with at least five mannoses and a bisecting alpha1-4 N-acetylglucosamine (GlcNAc) at the beta-mannose of the N-glycan core. The transferase responsible for this modification, CcGnt1 (C. cinerea GlcNAc transferase 1), was identified and expressed in insect cells. In vitro activity of CcGnt1 was demonstrated. This novel glycosyltransferase belongs to the glycosyltransferase family 8 (GT8) and is predicted to be a type II membrane protein. Expression of the CcGnt1 locus was up-regulated in fruiting body, but down-regulation of expression by means of RNAi decreased the level of bisected NLO; however had no apparent effect on fruiting body formation.

Mutational and functional analysis of Large in a novel CHO glycosylation mutant

Inactivating mutations of Large reduce the functional glycosylation of alpha-dystroglycan (alpha-DG) and lead to muscular dystrophy in mouse and humans. The N-terminal domain of Large is most similar to UDP-glucose glucosyltransferases (UGGT), and the C-terminal domain is related to the human i blood group transferase beta1,3GlcNAcT-1. The amino acids at conserved motifs DQD+1 and DQD+3 in the UGGT domain are necessary for mammalian UGGT activity. When the corresponding residues were mutated to Ala in mouse Large, alpha-DG was not functionally glycosylated. A similar result was obtained when a DXD motif in the beta1,3GlcNAcT-1 domain was mutated to AIA. Therefore, the first putative glycosyltransferase domain of Large has properties of a UGGT and the second of a typical glycosyltransferase. Co-transfection of Large mutants affected in the different glycosyltransferase domains did not lead to complementation. While Large mutants were more localized to the endoplasmic reticulum than wild-type Large or revertants, all mutants were in the Golgi, and only very low levels of Golgi-localized Large were necessary to generate functional alpha-DG. When Large was overexpressed in ldlD.Lec1 mutant Chinese hamster ovary (CHO) cells which synthesize few, if any, mucin O-GalNAc glycans and no complex N-glycans, functional alpha-DG was produced, presumably by modifying O-mannose glycans. To investigate mucin O-GalNAc glycans as substrates of Large, a new CHO mutant Lec15.Lec1 that lacked O-mannose and complex N-glycans was isolated and characterized. Following transfection with Large, Lec15.Lec1 cells also generated functionally glycosylated alpha-DG. Thus, Large may act on the O-mannose, complex N-glycans and mucin O-GalNAc glycans of alpha-DG.

Controlled enlargement of the glycoprotein vesicle surrounding a volvox embryo requires the InvB nucleotide-sugar transporter and is required for normal morphogenesis

Here, we report our analysis of a mutant of Volvox carteri, InvB, whose embryos fail to execute inversion, the process in which each Volvox embryo normally turns itself inside-out at the end of embryogenesis, thereby achieving the adult configuration. The invB gene encodes a nucleotide-sugar transporter that exhibits GDP-mannose transport activity when expressed in yeast. In wild-type embryos, the invB transcript is maximally abundant before and during inversion. A mannoside probe (fluorescent concanavalin A) stains the glycoprotein-rich gonidial vesicle (GV) surrounding wild-type embryos much more strongly than it stains the GV surrounding InvB embryos. Direct measurements revealed that throughout embryogenesis the GV surrounding a wild-type embryo increases in size much more than the GV surrounding an InvB embryo does, and the fully cleaved InvB embryo is much more tightly packed within its GV than a wild-type embryo is. To test the hypothesis that the restraint imposed by a smaller than normal GV directly causes the inversion defect in the mutant, we released InvB embryos from their GVs microsurgically. The resulting embryos inverted normally, demonstrating that controlled enlargement of the GV, by a process in which requires the InvB nucleotide-sugar transporter, is essential to provide the embryo sufficient space to complete inversion.

Receptor binding and cell entry of Old World arenaviruses reveal novel aspects of virus-host interaction

Ten years ago, the first cellular receptor for the prototypic arenavirus lymphocytic choriomeningitis virus (LCMV) and the highly pathogenic Lassa virus (LASV) was identified as alpha-dystroglycan (alpha-DG), a versatile receptor for proteins of the extracellular matrix (ECM). Biochemical analysis of the interaction of alpha-DG with arenaviruses and ECM proteins revealed a strikingly similar mechanism of receptor recognition that critically depends on specific sugar modification on alpha-DG involving a novel class of putative glycosyltransferase, the LARGE proteins. Interestingly, recent genome-wide detection and characterization of positive selection in human populations revealed evidence for positive selection of a locus within the LARGE gene in populations from Western Africa, where LASV is endemic. While most enveloped viruses that enter the host cell in a pH-dependent manner use clathrin-mediated endocytosis, recent studies revealed that the Old World arenaviruses LCMV and LASV enter the host cell predominantly via a novel and unusual endocytotic pathway independent of clathrin, caveolin, dynamin, and actin. Upon internalization, the virus is rapidly delivered to endosomes via an unusual route of vesicular trafficking that is largely independent of the small GTPases Rab5 and Rab7. Since infection of cells with LCMV and LASV depends on DG, this unusual endocytotic pathway could be related to normal cellular trafficking of the DG complex. Alternatively, engagement of arenavirus particles may target DG for an endocytotic pathway not normally used in uninfected cells thereby inducing an entry route specifically tailored to the pathogen's needs.

Post-translational modification of {alpha}-dystroglycan is not critical for lymphocytic choriomeningitis virus receptor function in vivo

alpha-Dystroglycan (alpha-DG) is a ubiquitously expressed molecule that has been identified as a cellular receptor for lymphocytic choriomeningitis virus (LCMV) and other arenaviruses. Recently, it was demonstrated that LCMV receptor function is critically dependent on post-translational modifications, namely glycosylation. In particular, it was shown that O-mannosylation, a rare type of mammalian O-linked glycosylation, is important in determining the binding of LCMV to its cellular receptor. All studies carried out so far showed a dependence on glycosylation in LCMV receptor function in vitro. This work extended these studies to two in vivo models of alpha-DG hypoglycosylation. The results confirm earlier findings on the in vitro dependence of carbohydrate modifications in LCMV receptor function. However, experiments in animal models showed that this dependence was only very weak in vivo. It is likely that alternative receptors or alternative entry pathways may account for this attenuated in vivo phenotype.