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

Structure-guided discovery of protein and glycan components in native mastigonemes

Mastigonemes, the hair-like lateral appendages lining cilia or flagella, participate in mechanosensation and cellular motion, but their constituents and structure have remained unclear. Here, we report the cryo-EM structure of native mastigonemes isolated from Chlamydomonas at 3.0 Å resolution. The long stem assembles as a super spiral, with each helical turn comprising four pairs of anti-parallel mastigoneme-like protein 1 (Mst1). A large array of arabinoglycans, which represents a common class of glycosylation in plants and algae, is resolved surrounding the type II poly-hydroxyproline (Hyp) helix in Mst1. The EM map unveils a mastigoneme axial protein (Mstax) that is rich in heavily glycosylated Hyp and contains a PKD2-like transmembrane domain (TMD). Mstax, with nearly 8,000 residues spanning from the intracellular region to the distal end of the mastigoneme, provides the framework for Mst1 assembly. Our study provides insights into the complexity of protein and glycan interactions in native bio-architectures.

Endogenous reductase activities for the generation of ribitol-phosphate, a CDP-ribitol precursor, in mammals

The core M3 O-mannosyl glycan on α-dystroglycan serves as the binding epitope for extracellular matrix molecules. Defects in core M3 glycans cause congenital muscular dystrophies that are collectively known as dystroglycanopathies. The core M3 glycan contains a tandem D-ribitol-5-phosphate (Rbo5P) structure, which is synthesized by the Rbo5P-transferases fukutin and fukutin-related protein using CDP-ribitol (CDP-Rbo) as a donor substrate. CDP-Rbo is synthesized from CTP and Rbo5P by CDP-Rbo pyrophosphorylase A. However, the Rbo5P biosynthesis pathway has yet to be elucidated in mammals. Here, we investigated the reductase activities toward four substrates, including ribose, ribulose, ribose-phosphate and ribulose-phosphate, to identify the intracellular Rbo5P production pathway and elucidated the role of the aldo-keto reductases AKR1A1, AKR1B1 and AKR1C1 in those pathways. It was shown that the ribose reduction pathway is the endogenous pathway that contributes most to Rbo5P production in HEK293T cells and that AKR1B1 is the major reductase in this pathway.

Glycan Modifications as Regulators of Stem Cell Fate

Glycosylation is a process where proteins or lipids are modified with glycans. The presence of glycans determines the structure, stability, and localization of glycoproteins, thereby impacting various biological processes, including embryogenesis, intercellular communication, and disease progression. Glycans can influence stem cell behavior by modulating signaling molecules that govern the critical aspects of self-renewal and differentiation. Furthermore, being located at the cell surface, glycans are utilized as markers for stem cell pluripotency and differentiation state determination. This review aims to provide a comprehensive overview of the current literature, focusing on the effect of glycans on stem cells with a reflection on the application of synthetic glycans in directing stem cell differentiation. Additionally, this review will serve as a primer for researchers seeking a deeper understanding of how synthetic glycans can be used to control stem cell differentiation, which may help establish new approaches to guide stem cell differentiation into specific lineages. Ultimately, this knowledge can facilitate the identification of efficient strategies for advancing stem cell-based therapeutic interventions.

Misfolding of fukutin-related protein (FKRP) variants in congenital and limb girdle muscular dystrophies

Fukutin-related protein (FKRP, MIM ID 606596) variants cause a range of muscular dystrophies associated with hypo-glycosylation of the matrix receptor, α-dystroglycan. These disorders are almost exclusively caused by homozygous or compound heterozygous missense variants in the FKRP gene that encodes a ribitol phosphotransferase. To understand how seemingly diverse FKRP missense mutations may contribute to disease, we examined the synthesis, intracellular dynamics, and structural consequences of a panel of missense mutations that encompass the disease spectrum. Under non-reducing electrophoresis conditions, wild type FKRP appears to be monomeric whereas disease-causing FKRP mutants migrate as high molecular weight, disulfide-bonded aggregates. These results were recapitulated using cysteine-scanning mutagenesis suggesting that abnormal disulfide bonding may perturb FKRP folding. Using fluorescence recovery after photobleaching, we found that the intracellular mobility of most FKRP mutants in ATP-depleted cells is dramatically reduced but can, in most cases, be rescued with reducing agents. Mass spectrometry showed that wild type and mutant FKRP differentially associate with several endoplasmic reticulum (ER)-resident chaperones. Finally, structural modelling revealed that disease-associated FKRP missense variants affected the local environment of the protein in small but significant ways. These data demonstrate that protein misfolding contributes to the molecular pathophysiology of FKRP-deficient muscular dystrophies and suggest that molecules that rescue this folding defect could be used to treat these disorders.

Deletion of POMT2 in Zebrafish Causes Degeneration of Photoreceptors

Mutations in the extracellular matrix protein eyes shut homolog (EYS) are a common cause of retinitis pigmentosa, a blinding disease characterized by photoreceptor degeneration. EYS binds to matriglycan, a carbohydrate modification on O-mannosyl glycan substitutions of the cell-surface glycoprotein α-dystroglycan. Patients with mutations in enzymes required for the biosynthesis of matriglycan exhibit syndromic retinal atrophy, along with brain malformations and congenital muscular dystrophy. Protein O-mannosyltransferase 2 (POMT2) is an enzyme required for the synthesis of O-mannosyl glycans. To evaluate the roles of O-mannosyl glycans in photoreceptor health, we generated protein O-mannosyltransferase 2 (pomt2) mutant zebrafish by CRISPR. pomt2 mutation resulted in a loss of matriglycan and abolished binding of EYS protein to α-dystroglycan. Mutant zebrafish presented with hydrocephalus and hypoplasia of the cerebellum, as well as muscular dystrophy. EYS protein was enriched near photoreceptor connecting cilia in the wild-type, but its presence and proper localization was significantly reduced in mutant animals. The mutant retina exhibited mis-localization of opsins and increased apoptosis in both rod and cone photoreceptors. Immunofluorescence intensity of G protein subunit alpha transducin 2 (GNAT2) antibody (a general cone marker) and 1D4 antibody (a long double cone marker) in mutant retinas did not differ from wild-type retinas at 1-month post fertilization, but was reduced at 6 months post fertilization, indicating significant cone degeneration. These data suggest that POMT2-mediated O-mannosyl glycosylation is required for EYS protein localization to the connecting cilium region and photoreceptor survival.

Compound Heterozygous FKTN Variants in a Patient with Dilated Cardiomyopathy Led to an Aberrant α-Dystroglycan Pattern

Fukutin encoded by FKTN is a ribitol 5-phosphate transferase involved in glycosylation of α-dystroglycan. It is known that mutations in FKTN affect the glycosylation of α-dystroglycan, leading to a dystroglycanopathy. Dystroglycanopathies are a group of syndromes with a broad clinical spectrum including dilated cardiomyopathy and muscular dystrophy. In this study, we reported the case of a patient with muscular dystrophy, early onset dilated cardiomyopathy, and elevated creatine kinase levels who was a carrier of the compound heterozygous variants p.Ser299Arg and p.Asn442Ser in FKTN. Our work showed that compound heterozygous mutations in FKTN lead to a loss of fully glycosylated α-dystroglycan and result in cardiomyopathy and end-stage heart failure at a young age.

Cancer Malignancy Is Correlated with Upregulation of PCYT2-Mediated Glycerol Phosphate Modification of α-Dystroglycan

The dystrophin–glycoprotein complex connects the cytoskeleton with base membrane components such as laminin through unique O-glycans displayed on α-dystroglycan (α-DG). Genetic impairment of elongation of these glycans causes congenital muscular dystrophies. We previously identified that glycerol phosphate (GroP) can cap the core part of the α-DG O-glycans and terminate their further elongation. This study examined the possible roles of the GroP modification in cancer malignancy, focusing on colorectal cancer. We found that the GroP modification critically depends on PCYT2, which serves as cytidine 5′-diphosphate-glycerol (CDP-Gro) synthase. Furthermore, we identified a significant positive correlation between cancer progression and GroP modification, which also correlated positively with PCYT2 expression. Moreover, we demonstrate that GroP modification promotes the migration of cancer cells. Based on these findings, we propose that the GroP modification by PCYT2 disrupts the glycan-mediated cell adhesion to the extracellular matrix and thereby enhances cancer metastasis. Thus, the present study suggests the possibility of novel approaches for cancer treatment by targeting the PCYT2-mediated GroP modification.

Chemical and Chemo-Enzymatic Syntheses of Glycans Containing Ribitol Phosphate Scaffolding of Matriglycan

Ribitol phosphate modifications to the core M3 O-mannosyl glycan are important for the functional maturation of α-dystroglycan. Three sequentially extended partial structures of the core M3 O-mannosyl glycan including a tandem ribitol phosphate were regio- and stereo-selectively synthesized: Rbo5P-3GalNAcβ, Rbo5P-1Rbo5P-3GalNAcβ, and Xylβ1-4Rbo5P-1Rbo5P-3GalNAcβ (Rbo5P, d-ribitol-5-phosphate; GalNAc, N-acetyl-d-galactosamine; Xyl, d-xylose). Rbo5P-3GalNAcβ with p-nitrophenyl at the aglycon part served as a substrate for ribitol phosphate transferase (FKRP, fukutin-related protein), and its product was glycosylated by the actions of a series of glycosyltransferases, namely, ribitol xylosyltransferase 1 (RXYLT1), β1,4-glucuronyltransferase 1 (B4GAT1), and like-acetyl-glucosaminyltransferase (LARGE). Rbo5P-3GalNAcβ equipped with an alkyne-type aglycon was also active for FKRP. The molecular information obtained on FKRP suggests that Rbo5P-3GalNAcβ derivatives are the minimal units required as the acceptor glycan for Rbo5P transfer and may serve as a precursor for the elongation of the core M3 O-mannosyl glycan.

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.

Synthesis of the matriglycan hexasaccharide, -3Xylα1-3GlcAβ1-trimer and its interaction with laminin

Matriglycan hexasaccharide (-3Xylα1-3GlcAβ1)3-O(C2H4O)3CH2CCH and the biotin conjugate was synthesized. The hexasaccharide showed good interaction with laminin-G-like domains 4 and 5 of laminin-α2 using saturation transfer difference-NMR and bio-layer interferometry.

Biosynthetic Mechanisms and Biological Significance of Glycerol Phosphate-Containing Glycan in Mammals

Bacteria contain glycerol phosphate (GroP)-containing glycans, which are important constituents of cell-surface glycopolymers such as the teichoic acids of Gram-positive bacterial cell walls. These glycopolymers comprising GroP play crucial roles in bacterial physiology and virulence. Recently, the first identification of a GroP-containing glycan in mammals was reported as a variant form of O-mannosyl glycan on α-dystroglycan (α-DG). However, the biological significance of such GroP modification remains largely unknown. In this review, we provide an overview of this new discovery of GroP-containing glycan in mammals and then outline the recent progress in elucidating the biosynthetic mechanisms of GroP-containing glycans on α-DG. In addition, we discuss the potential biological role of GroP modification along with the challenges and prospects for further research. The progress in this newly identified glycan modification will provide insights into the phylogenetic implications of glycan.

31 P-MRS of healthy human brain: Measurement of guanosine diphosphate mannose at 7 T

Guanosine diphosphate mannose (GDP-Man) is the donor substrate required for mannosylation in the synthesis of glycoproteins, glycolipids and the newly discovered glycoRNA. Normal GDP-Man biosynthesis plays a crucial role in support of a variety of cellular functions, including cell recognition, cell communication and immune responses against viruses. Here, we report the detection of GDP-Man in human brain for the first time, using ³¹ P MRS at 7 T. The presence of GDP-Man is evidenced by the detection of a weak ³¹ P doublet at -10.7 ppm that can be assigned to the phosphomannosyl group (Pβ) of the GDP-Man molecule. This weak but well-resolved signal lies 0.9 ppm upfield of UDP(G) Pβ-multiplet from a mixture of UDP-Glc, UDP-Gal, UDP-GlcNAc and UDP-GalNAc. In reference to ATP (2.8 mM), the concentration of GDP-Man in human brain was estimated to be 0.02 ± 0.01 mM, about 15-fold lower than the total concentration of UDP(G) (0.30 ± 0.04, N = 17) and consistent with previous reports of UDP-Man in cells and brain tissue extracts measured by high-performance liquid chromatography. The reproducibility of the measured GDP-Man between test and 2-week retest was 21% ± 15% compared with 5% ± 4% for UDP(G) (N = 7). The measured concentrations of GDP-Man and UDP(G) are linearly correlated ([UDP(G)] = 4.3 [GDP-Man] + 0.02, with R = 0.66 and p = 0.0043), likely reflecting the effect of shared sugar precursors, which may vary among individuals in response to variation in nutritional intake and consumption. Given that GDP-Man has another set of doublet (Pα) at -8.3 ppm that overlaps with NAD(H) and UDP(G)-Pα signals, the amount of GDP-Man could potentially interfere with the deconvolution of these mixed signals in composition analysis. Importantly, this new finding may be useful in advancing our understanding of glycosylation and its role in the development of cancer, as well as infectious and neurodegenerative diseases.

Neuronal Dystroglycan regulates postnatal development of CCK/cannabinoid receptor-1 interneurons

Background

The development of functional neural circuits requires the precise formation of synaptic connections between diverse neuronal populations. The molecular pathways that allow GABAergic interneuron subtypes in the mammalian brain to initially recognize their postsynaptic partners remain largely unknown. The transmembrane glycoprotein Dystroglycan is localized to inhibitory synapses in pyramidal neurons, where it is required for the proper function of CCK+ interneurons. However, the precise temporal requirement for Dystroglycan during inhibitory synapse development has not been examined.

Methods

In this study, we use NEX^Cre or Camk2a^CreERT2 to conditionally delete Dystroglycan from newly-born or adult pyramidal neurons, respectively. We then analyze forebrain development from postnatal day 3 through adulthood, with a particular focus on CCK+ interneurons.

Results

In the absence of postsynaptic Dystroglycan in developing pyramidal neurons, presynaptic CCK+ interneurons fail to elaborate their axons and largely disappear from the cortex, hippocampus, amygdala, and olfactory bulb during the first two postnatal weeks. Other interneuron subtypes are unaffected, indicating that CCK+ interneurons are unique in their requirement for postsynaptic Dystroglycan. Dystroglycan does not appear to be required in adult pyramidal neurons to maintain CCK+ interneurons. Bax deletion did not rescue CCK+ interneurons in Dystroglycan mutants during development, suggesting that they are not eliminated by canonical apoptosis. Rather, we observed increased innervation of the striatum, suggesting that the few remaining CCK+ interneurons re-directed their axons to neighboring areas where Dystroglycan expression remained intact.

Conclusion

Together these findings show that Dystroglycan functions as part of a synaptic partner recognition complex that is required early for CCK+ interneuron development in the forebrain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13064-021-00153-1.

PCYT2 synthesizes CDP-glycerol in mammals and reduced PCYT2 enhances the expression of functionally glycosylated α-dystroglycan

α-Dystroglycan (α-DG) is a highly glycosylated cell-surface protein. Defective O-mannosyl glycan on α-DG is associated with muscular dystrophies and cancer. In the biosynthetic pathway of the O-mannosyl glycan, fukutin (FKTN) and fukutin-related protein (FKRP) transfer ribitol phosphate (RboP). Previously, we reported that FKTN and FKRP can also transfer glycerol phosphate (GroP) from CDP-glycerol (CDP-Gro) and showed the inhibitory effects of CDP-Gro on functional glycan synthesis by preventing glycan elongation in vitro. However, whether mammalian cells have CDP-Gro or associated synthetic machinery has not been elucidated. Therefore, the function of CDP-Gro in mammals is largely unknown. Here, we reveal that cultured human cells and mouse tissues contain CDP-Gro using liquid chromatography tandem-mass spectrometry (LC-MS/MS). By performing the enzyme activity assay of candidate recombinant proteins, we found that ethanolamine-phosphate cytidylyltransferase (PCYT2), the key enzyme in de novo phosphatidylethanolamine biosynthesis, has CDP-Gro synthetic activity from glycerol-3-phosphate (Gro3P) and CTP. In addition, knockdown of PCYT2 dramatically reduced cellular CDP-Gro. These results indicate that PCYT2 is a CDP-Gro synthase in mammals. Furthermore, we found that the expression of functionally glycosylated α-DG is increased by reducing PCYT2 expression. Our results suggest an important role for CDP-Gro in the regulation of α-DG function in mammals.

The structure of POMGNT2 provides new insights into the mechanism to determine the functional O-mannosylation site on α-dystroglycan

Defects in the O‐mannosyl glycan of α‐dystroglycan (α‐DG) are associated with α‐dystroglycanopathy, a group of congenital muscular dystrophies. While α‐DG has many O‐mannosylation sites, only the specific positions can be modified with the functional O‐mannosyl glycan, namely, core M3‐type glycan. POMGNT2 is a glycosyltransferase which adds β1,4‐linked GlcNAc to the O‐mannose (Man) residue to acquire core M3‐type glycan. Although it is assumed that POMGNT2 extends the specific O‐Man residues around particular amino acid sequences, the details are not well understood. Here, we determined a series of crystal structures of POMGNT2 with and without the acceptor O‐mannosyl peptides and identified the critical interactions between POMGNT2 and the acceptor peptide. POMGNT2 has an N‐terminal catalytic domain and a C‐terminal fibronectin type III (FnIII) domain and forms a dimer. The acceptor peptide is sandwiched between the two protomers. The catalytic domain of one protomer recognizes the O‐mannosylation site (TPT motif), and the FnIII domain of the other protomer recognizes the C‐terminal region of the peptide. Structure‐based mutational studies confirmed that amino acid residues of the catalytic domain interacting with mannose or the TPT motif are essential for POMGNT2 enzymatic activity. In addition, the FnIII domain is also essential for the activity and it interacts with the peptide mainly by hydrophobic interaction. Our study provides the first atomic‐resolution insights into specific acceptor recognition by the FnIII domain of POMGNT2. The catalytic mechanism of POMGNT2 is proposed based on the structure.

The Role of APP O-Glycosylation in Alzheimer's Disease

The number of people with dementia is increasing rapidly due to the increase in the aging population. Alzheimer’s disease (AD) is a type of neurodegenerative dementia caused by the accumulation of abnormal proteins. Genetic mutations, smoking, and several other factors have been reported as causes of AD, but alterations in glycans have recently been demonstrated to play a role in AD. Amyloid-β (Aβ), a cleaved fragment of APP, is the source of senile plaque, a pathological feature of AD. APP has been reported to undergo N- and O-glycosylation, and several Polypeptide N-acetylgalactosaminyltransferases (ppGalNAc-Ts) have been shown to have catalytic activity for the transfer of GalNAc to APP. Since O-glycosylation in the proximity of a cleavage site in many proteins has been reported to be involved in protein processing, O-glycans may affect the cleavage of APP during the Aβ production process. In this report, we describe new findings on the O-glycosylation of APP and Aβ production.

Stereo- and Regioselective Synthesis of O-Mannosyl Glycan Containing Matriglycan and a Part of Tandem Ribitol Phosphate

We herein successfully synthesized two pivotal structures of O-mannosyl glycan: (1) the matriglycan-repeating tetrasaccharide Xylα1-3GlcAβ1-3Xylα1-3GlcAβ and (2) the link between matriglycan and a part of tandem ribitol phosphate, Xylα1-3GlcAβ1-4Xylβ1-4Rbo, in a regio- and stereocontrolled manner. The disaccharide unit with the α-linkage of xylose was obtained by adopting the conformational fixation of the xylopyranoside ring and a specific solvation system of diastereoselective solubility.

Congenital hearing impairment associated with peripheral cochlear nerve dysmyelination in glycosylation-deficient muscular dystrophy

Hearing loss (HL) is one of the most common sensory impairments and etiologically and genetically heterogeneous disorders in humans. Muscular dystrophies (MDs) are neuromuscular disorders characterized by progressive degeneration of skeletal muscle accompanied by non-muscular symptoms. Aberrant glycosylation of α-dystroglycan causes at least eighteen subtypes of MD, now categorized as MD-dystroglycanopathy (MD-DG), with a wide spectrum of non-muscular symptoms. Despite a growing number of MD-DG subtypes and increasing evidence regarding their molecular pathogeneses, no comprehensive study has investigated sensorineural HL (SNHL) in MD-DG. Here, we found that two mouse models of MD-DG, Large^myd/myd and POMGnT1-KO mice, exhibited congenital, non-progressive, and mild-to-moderate SNHL in auditory brainstem response (ABR) accompanied by extended latency of wave I. Profoundly abnormal myelination was found at the peripheral segment of the cochlear nerve, which is rich in the glycosylated α-dystroglycan–laminin complex and demarcated by “the glial dome.” In addition, patients with Fukuyama congenital MD, a type of MD-DG, also had latent SNHL with extended latency of wave I in ABR. Collectively, these findings indicate that hearing impairment associated with impaired Schwann cell-mediated myelination at the peripheral segment of the cochlear nerve is a notable symptom of MD-DG.

Hearing loss (HL) is one of the most common sensory impairments and heterogeneous disorders in humans. Up to 60% of HL cases are caused by genetic factors, and approximately 30% of genetic HL cases are syndromic. Although 400–700 genetic syndromes are associated with sensorineural HL (SNHL), caused due to problems in the nerve pathways from the cochlea to the brain, only about 45 genes are known to be associated with syndromic HL. Muscular dystrophies (MDs) are neuromuscular disorders characterized by progressive degeneration of skeletal muscle accompanied by non-muscular symptoms. MD-dystroglycanopathy (MD-DG), caused by aberrant glycosylation of α-dystroglycan, is an MD subtype with a wide spectrum of non-muscular symptoms. Despite a growing number of MD-DG subtypes (at least 18), no comprehensive study has investigated SNHL in MD-DG. Here, we found that hearing impairment was associated with abnormal myelination of the peripheral segment of the cochlear nerve caused by impaired dystrophin–dystroglycan complex in two mouse models (type 3 and 6) of MD-DG and in patients (type 4) with MD-DG. This is the first comprehensive study investigating SNHL in MD-DG. Our findings may provide new insights into understanding the pathogenic characteristics and mechanisms underlying inherited syndromic hearing impairment.

Eyes shut homolog (EYS) interacts with matriglycan of O-mannosyl glycans whose deficiency results in EYS mislocalization and degeneration of photoreceptors

Mutations in eyes shut homolog (EYS), a secreted extracellular matrix protein containing multiple laminin globular (LG) domains, and in protein O-mannose β1, 2-N-acetylglucosaminyl transferase 1 (POMGnT1), an enzyme involved in O-mannosyl glycosylation, cause retinitis pigmentosa (RP), RP25 and RP76, respectively. How EYS and POMGnT1 regulate photoreceptor survival is poorly understood. Since some LG domain-containing proteins function by binding to the matriglycan moiety of O-mannosyl glycans, we hypothesized that EYS interacted with matriglycans as well. To test this hypothesis, we performed EYS Far-Western blotting assay and generated pomgnt1 mutant zebrafish. The results showed that EYS bound to matriglycans. Pomgnt1 mutation in zebrafish resulted in a loss of matriglycan, retention of synaptotagmin-1-positive EYS secretory vesicles within the outer nuclear layer, and diminished EYS protein near the connecting cilia. Photoreceptor density in 2-month old pomgnt1 mutant retina was similar to the wild-type animals but was significantly reduced at 6-months. These results indicate that EYS protein localization to the connecting cilia requires interaction with the matriglycan and that O-mannosyl glycosylation is required for photoreceptor survival in zebrafish. This study identified a novel interaction between EYS and matriglycan demonstrating that RP25 and RP76 are mechanistically linked in that O-mannosyl glycosylation controls targeting of EYS protein.

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.

Crystal structures of fukutin-related protein (FKRP), a ribitol-phosphate transferase related to muscular dystrophy

α-Dystroglycan (α-DG) is a highly-glycosylated surface membrane protein. Defects in the O-mannosyl glycan of α-DG cause dystroglycanopathy, a group of congenital muscular dystrophies. The core M3 O-mannosyl glycan contains tandem ribitol-phosphate (RboP), a characteristic feature first found in mammals. Fukutin and fukutin-related protein (FKRP), whose mutated genes underlie dystroglycanopathy, sequentially transfer RboP from cytidine diphosphate-ribitol (CDP-Rbo) to form a tandem RboP unit in the core M3 glycan. Here, we report a series of crystal structures of FKRP with and without donor (CDP-Rbo) and/or acceptor [RboP-(phospho-)core M3 peptide] substrates. FKRP has N-terminal stem and C-terminal catalytic domains, and forms a tetramer both in crystal and in solution. In the acceptor complex, the phosphate group of RboP is recognized by the catalytic domain of one subunit, and a phosphate group on O-mannose is recognized by the stem domain of another subunit. Structure-based functional studies confirmed that the dimeric structure is essential for FKRP enzymatic activity.

Fukutin-related protein (FKRP) catalyses the addition of ribitol-phosphate (RboP) to the O-mannosyl glycan of α-dystroglycan and mutations in FKRP cause dystroglycanopathy. Here the authors provide insights into its oligomerization and recognition of the substrates, CDP-Rbo and the RboP-(phospho-)core M3 glycan, by determining the crystal structures of human FKRP.

FAM3B/PANDER-Like Carbohydrate-Binding Domain in a Glycosyltransferase, POMGNT1

Protein O-mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGNT1) is one of the gene products responsible for α-dystroglycanopathy, which is a type of congenital muscular dystrophy caused by O-mannosyl glycan defects. The originally identified function of POMGNT1 was as a glycosyltransferase that catalyzes the formation of the GlcNAcβ1-2Man linkage of O-mannosyl glycan, but the enzyme function is not essential for α-dystroglycanopathy pathogenesis. Our recent study revealed that the stem domain of POMGNT1 has a carbohydrate-binding ability, which recognizes the GalNAcβ1-3GlcNAc structure. This carbohydrate-binding activity is required for the formation of the ribitol phosphate (RboP)-3GalNAcβ1-3GlcNAc structure by fukutin. This protocol describes methods to assess the carbohydrate-binding activity of the POMGNT1 stem domain.

Muscular, Ocular and Brain Involvement Associated with a De Novo 11q13.2q14.1 Duplication: Contribution to the Differential Diagnosis of Muscle-Eye-Brain Congenital Muscular Dystrophy

Muscular weakness and hypotonia may be associated with multisystem involvement giving rise to complex phenotypes, many of which are uncharacterized. We report a patient presenting with congenital hypotonia and severe ocular and brain abnormalities, evoking a Muscle Eye Brain disease (MEB). She had global muscular weakness, hypotonia and amyotrophy, joint hyperlaxity, kyphoscoliosis, respiratory insufficiency, dysmorphic features and severe intellectual disability. Brain MRI showed cortical atrophy and hypoplasia of the corpus callosum. Normal CK levels, non-progressive course and absence of dystrophic features or α-dystroglycan abnormalities on the muscle biopsy were not typical of MEB. CGH array identified a large de novo duplication in chromosome 11, including regions partially duplicated in three other patients with common clinical features. This report adds to the differential diagnosis of complex phenotypes characterized by muscular, ocular and CNS involvement and highlights the potential contribution of still unrecognized chromosomal abnormalities to these phenotypes.

Structural biology of laminins

Laminins are large cell-adhesive glycoproteins that are required for the formation and function of basement membranes in all animals. Structural studies by electron microscopy in the early 1980s revealed a cross-shaped molecule, which subsequently was shown to consist of three distinct polypeptide chains. Crystallographic studies since the mid-1990s have added atomic detail to all parts of the laminin heterotrimer. The three short arms of the cross are made up of continuous arrays of disulphide-rich domains. The globular domains at the tips of the short arms mediate laminin polymerization; the surface regions involved in this process have been identified by structure-based mutagenesis. The long arm of the cross is an α-helical coiled coil of all three chains, terminating in a cell-adhesive globular region. The molecular basis of cell adhesion to laminins has been revealed by recent structures of heterotrimeric integrin-binding fragments and of a laminin fragment bound to the carbohydrate modification of dystroglycan. The structural characterization of the laminin molecule is essentially complete, but we still have to find ways of imaging native laminin polymers at molecular resolution.

Muscular Dystrophy with Ribitol-Phosphate Deficiency: A Novel Post-Translational Mechanism in Dystroglycanopathy

Muscular dystrophy is a group of genetic disorders characterized by progressive muscle weakness. In the early 2000s, a new classification of muscular dystrophy, dystroglycanopathy, was established. Dystroglycanopathy often associates with abnormalities in the central nervous system. Currently, at least eighteen genes have been identified that are responsible for dystroglycanopathy, and despite its genetic heterogeneity, its common biochemical feature is abnormal glycosylation of alpha-dystroglycan. Abnormal glycosylation of alpha-dystroglycan reduces its binding activities to ligand proteins, including laminins. In just the last few years, remarkable progress has been made in determining the sugar chain structures and gene functions associated with dystroglycanopathy. The normal sugar chain contains tandem structures of ribitol-phosphate, a pentose alcohol that was previously unknown in humans. The dystroglycanopathy genes fukutin, fukutin-related protein (FKRP), and isoprenoid synthase domain-containing protein (ISPD) encode essential enzymes for the synthesis of this structure: fukutin and FKRP transfer ribitol-phosphate onto sugar chains of alpha-dystroglycan, and ISPD synthesizes CDP-ribitol, a donor substrate for fukutin and FKRP. These findings resolved long-standing questions and established a disease subgroup that is ribitol-phosphate deficient, which describes a large population of dystroglycanopathy patients. Here, we review the history of dystroglycanopathy, the properties of the sugar chain structure of alpha-dystroglycan, dystroglycanopathy gene functions, and therapeutic strategies.

Glycosylation in health and disease

The glycome describes the complete repertoire of glycoconjugates composed of carbohydrate chains, or glycans, that are covalently linked to lipid or protein molecules. Glycoconjugates are formed through a process called glycosylation and can differ in their glycan sequences, the connections between them and their length. Glycoconjugate synthesis is a dynamic process that depends on the local milieu of enzymes, sugar precursors and organelle structures as well as the cell types involved and cellular signals. Studies of rare genetic disorders that affect glycosylation first highlighted the biological importance of the glycome, and technological advances have improved our understanding of its heterogeneity and complexity. Researchers can now routinely assess how the secreted and cell-surface glycomes reflect overall cellular status in health and disease. In fact, changes in glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis; the composition of the glycome also affects kidney function in health and disease. New insights into the structure and function of the glycome can now be applied to therapy development and could improve our ability to fine-tune immunological responses and inflammation, optimize the performance of therapeutic antibodies and boost immune responses to cancer. These examples illustrate the potential of the emerging field of 'glycomedicine'.

Analysis of α-Dystroglycan/LG Domain Binding Modes: Investigating Protein Motifs That Regulate the Affinity of Isolated LG Domains

Dystroglycan (DG) is an adhesion complex that links the cytoskeleton to the surrounding extracellular matrix in skeletal muscle and a wide variety of other tissues. It is composed of a highly glycosylated extracellular α-DG associated noncovalently with a transmembrane β-DG whose cytodomain interacts with dystrophin and its isoforms. Alpha-dystroglycan (α-DG) binds tightly and in a calcium-dependent fashion to multiple extracellular proteins and proteoglycans, each of which harbors at least one, or, more frequently, tandem arrays of laminin-globular (LG) domains. Considerable biochemical and structural work has accumulated on the α-DG-binding LG domains, highlighting a significant heterogeneity in ligand-binding properties of domains from different proteins as well as between single and multiple LG domains within the same protein. Here we review biochemical, structural, and functional information on the LG domains reported to bind α-dystroglycan. In addition, we have incorporated bioinformatics and modeling to explore whether specific motifs responsible for α-dystroglycan recognition can be identified within isolated LG domains. In particular, we analyzed the LG domains of slits and agrin as well as those of paradigmatic α-DG non-binders such as laminin-α3. While some stretches of basic residues may be important, no universally conserved motifs could be identified. However, the data confirm that the coordinated calcium atom within the LG domain is needed to establish an interaction with the sugars of α-DG, although it appears that this alone is insufficient to mediate significant α-DG binding. We develop a scenario involving different binding modes of a single LG domain unit, or tandemly repeated units, with α-DG. A variability of binding modes might be important to generate a range of affinities to allow physiological regulation of this interaction, reflecting its crucial biological importance.

Glycan-based biomarkers for diagnosis of cancers and other diseases: Past, present, and future

Glycans are essential biomolecules in regulating human physiology and pathology ranging from signal transduction to microbial infections. Developing complex human diseases, such as cancer, diabetes, and cardiovascular diseases, are a combination of genetic and environmental factors. Genetics dominates embryonic development and the passing of genes to the next generation whereas the information in glycans reflects the impact of internal and external environmental factors, such as diseases, lifestyle, and social factors, on a person's health and disease. The reason behind this is that glycans are not directly encoded in a genetic template. Instead, they are assembled dynamically by hundreds of enzymes organized in more than 10 complex biosynthetic pathways. Any environmental changes affecting enzymatic activities or the availability of high-energy monosaccharide donors in a specific location will disturb the final structure of glycans. The glycan structure-dependent biological activities subsequently enable or disable gene expressions, which partially explain that it is difficult to pinpoint specific genetic defects to aging-associated diseases. Glycan-based biomarkers are currently used for diagnosis of diabetes, cancers, and other complex diseases. We will recapitulate the discovery of glucose, glycated proteins, glycan-, and glycoprotein-based biomarkers followed by summarizing clinically used glycan/glycoprotein-based biomarkers. The potential serum/plasma-derived N- and O-linked glycans as biomarkers will also be discussed.

Laminin G-like domains: dystroglycan-specific lectins

A unique O-mannose-linked glycan on the transmembrane protein dystroglycan binds a number of extracellular matrix proteins containing laminin G-like (LG) domains. The dystroglycan-matrix interaction is essential for muscle function: disrupted biosynthesis of the matrix-binding modification causes several forms of muscular dystrophy. The complete chemical structure of this modification has been deciphered in the past few years. We now know that LG domains bind to a glycosaminoglycan-like polysaccharide of [-3GlcAβ1,3Xylα1-] units, termed matriglycan, that is attached to a highly unusual heptasaccharide linker. X-ray crystallography has revealed the principles of Ca²⁺-dependent matriglycan binding by LG domains. In this review, the new structural insights are applied to the growing number of LG domain-containing proteins that bind dystroglycan. It is proposed that LG domains be recognised as 'D-type' lectins to indicate their conserved function in dystroglycan binding.

Retinal ganglion cell axon sorting at the optic chiasm requires dystroglycan

In the developing visual system, retinal ganglion cell (RGC) axons project from the retina to several distal retinorecipient regions in the brain. Several molecules have been implicated in guiding RGC axons in vivo, but the role of extracellular matrix molecules in this process remains poorly understood. Dystroglycan is a laminin-binding transmembrane protein important for formation and maintenance of the extracellular matrix and basement membranes and has previously been implicated in axon guidance in the developing spinal cord. Using two genetic models of functional dystroglycan loss, we show that dystroglycan is necessary for correct sorting of contralateral and ipsilateral RGC axons at the optic chiasm. Mis-sorted axons still target retinorecipient brain regions and persist in adult mice, even after axon pruning is complete. Our results highlight the importance of the extracellular matrix for axon sorting at an intermediate choice point in the developing visual circuit.

A new case expanding the mutation and phenotype spectrum of TMEM5-related alpha-dystroglycanopathy

Dystroglycanopathies are a diverse group of neuromuscular disorders caused by aberrant glycosylation of alpha-dystroglycan. TMEM5 is one of many glycosyltransferases recently described to be associated with alpha-dystroglycanopathies. We report the case of a 15-year-old boy suffering from a congenital muscular dystrophy with elevated serum creatine kinase levels and an almost complete absence of alpha-dystroglycan in muscle biopsy. The clinical course was milder than any previously reported case and did not include brain or eye defects. Standard next-generation sequencing analysis revealed a homozygous mutation in the donor splice site region of exon 5 in TMEM5 (c.914+6 T>G). Available in-silico prediction tools anticipated a reduced efficiency of the splice site. Subsequent cDNA sequencing confirmed the expression of a truncated transcript of TMEM5 lacking exon 5, hence leading to an in-frame deletion in the exostosin domain of the protein. This report expands the clinical and mutation spectrum of alpha-dystroglycanopathies.

CDP-glycerol inhibits the synthesis of the functional O-mannosyl glycan of α-dystroglycan

α-Dystroglycan (α-DG) is a highly glycosylated cell-surface laminin receptor. Defects in the O-mannosyl glycan of an α-DG with laminin-binding activity can cause α-dystroglycanopathy, a group of congenital muscular dystrophies. In the biosynthetic pathway of functional O-mannosyl glycan, fukutin (FKTN) and fukutin-related protein (FKRP), whose mutated genes underlie α-dystroglycanopathy, sequentially transfer ribitol phosphate (RboP) from CDP-Rbo to form a tandem RboP unit (RboP-RboP) required for the synthesis of the laminin-binding epitope on O-mannosyl glycan. Both RboP- and glycerol phosphate (GroP)-substituted glycoforms have recently been detected in recombinant α-DG. However, it is unclear how GroP is transferred to the O-mannosyl glycan or whether GroP substitution affects the synthesis of the O-mannosyl glycan. Here, we report that, in addition to having RboP transfer activity, FKTN and FKRP can transfer GroP to O-mannosyl glycans by using CDP-glycerol (CDP-Gro) as a donor substrate. Kinetic experiments indicated that CDP-Gro is a less efficient donor substrate for FKTN than is CDP-Rbo. We also show that the GroP-substituted glycoform synthesized by FKTN does not serve as an acceptor substrate for FKRP and that therefore further elongation of the outer glycan chain cannot occur with this glycoform. Finally, CDP-Gro inhibited the RboP transfer activities of both FKTN and FKRP. These results suggest that CDP-Gro inhibits the synthesis of the functional O-mannosyl glycan of α-DG by preventing further elongation of the glycan chain. This is the first report of GroP transferases in mammals.

Impairment of photoreceptor ribbon synapses in a novel Pomt1 conditional knockout mouse model of dystroglycanopathy

Hypoglycosylation of α-dystroglycan (α-DG) resulting from deficiency of protein O-mannosyltransferase 1 (POMT1) may cause severe neuromuscular dystrophies with brain and eye anomalies, named dystroglycanopathies. The retinal involvement of these disorders motivated us to generate a conditional knockout (cKO) mouse experiencing a Pomt1 intragenic deletion (exons 3-4) during the development of photoreceptors, mediated by the Cre recombinase expressed from the cone-rod homeobox (Crx) gene promoter. In this mouse, retinal α-DG was unglycosylated and incapable of binding laminin. Retinal POMT1 deficiency caused significant impairments in both electroretinographic recordings and optokinetic reflex in Pomt1 cKO mice, and immunohistochemical analyses revealed the absence of β-DG and of the α-DG-interacting protein, pikachurin, in the outer plexiform layer (OPL). At the ultrastructural level, noticeable alterations were observed in the ribbon synapses established between photoreceptors and bipolar cells. Therefore, O-mannosylation of α-DG in the retina carried out by POMT1 is crucial for the establishment of proper synapses at the OPL and transmission of visual information from cones and rods to their postsynaptic neurons.

Uniparental disomy unveils a novel recessive mutation in POMT2

Mutations in POMT2 are most commonly associated with Walker-Warburg syndrome and Muscle-Eye-Brain disease, but can also cause limb girdle muscular dystrophy (LGMD2N). We report a case of LGMD due to a novel mutation in POMT2 unmasked by uniparental isodisomy. The patient experienced proximal muscle weakness from three years of age with minimal progression. She developed progressive contractures and underwent unilateral Achilles tenotomy. By age 11, she had borderline low left ventricular ejection fraction and mild restrictive lung disease. Muscle biopsy showed mild dystrophic changes with selective reduction in α-dystroglycan immunostaining. Sequencing of POMT2 showed a novel homozygous c.1502A>C variant that was predicted to be probably pathogenic. Fibroblast complementation studies showed lack of functional glycosylation rescued by wild-type POMT2 expression. Chromosomal microarray showed a single 15 Mb copy number neutral loss of heterozygosity on chromosome 14 encompassing POMT2. RNAseq verified homozygosity at this locus. Together, our findings indicate maternal uniparental isodisomy causing LGMD2N.

β-dystroglycan is regulated by a balance between WWP1-mediated degradation and protection from WWP1 by dystrophin and utrophin

Dystroglycan is a ubiquitous membrane protein that functions as a mechanical connection between the extracellular matrix and cytoskeleton. In skeletal muscle, dystroglycan plays an indispensable role in regulating muscle regeneration; a malfunction in dystroglycan is associated with muscular dystrophy. The regulation of dystroglycan stability is poorly understood. Here, we report that WWP1, a member of NEDD4 E3 ubiquitin ligase family, promotes ubiquitination and subsequent degradation of β-dystroglycan. Our results indicate that dystrophin and utrophin protect β-dystroglycan from WWP1-mediated degradation by competing with WWP1 for the shared binding site at the cytosolic tail of β-dystroglycan. In addition, we show that a missense mutation (arginine 440 to glutamine) in WWP1-which is known to cause muscular dystrophy in chickens-increases the ubiquitin ligase-mediated ubiquitination of both β-dystroglycan and WWP1. The R440Q missense mutation in WWP1 decreases HECT domain-mediated intramolecular interactions to relieve autoinhibition of the enzyme. Our results provide new insight into the regulation of β-dystroglycan degradation by WWP1 and other Nedd4 family members and improves our understanding of dystroglycan-related disorders.

Perspectives on Glycosylation and Its Congenital Disorders

Congenital disorders of glycosylation (CDG) are a rapidly expanding group of metabolic disorders that result from abnormal protein or lipid glycosylation. They are often difficult to clinically diagnose because they broadly affect many organs and functions and lack clinical uniformity. However, recent technological advances in next-generation sequencing have revealed a treasure trove of new genetic disorders, expanded the knowledge of known disorders, and showed a critical role in infectious diseases. More comprehensive genetic tools specifically tailored for mammalian cell-based models have revealed a critical role for glycosylation in pathogen-host interactions, while also identifying new CDG susceptibility genes. We highlight recent advancements that have resulted in a better understanding of human glycosylation disorders, perspectives for potential future therapies, and mysteries for which we continue to seek new insights and solutions.