Fukutin is prerequisite to ameliorate muscular dystrophic phenotype by myofiber-selective LARGE expression

Laminin α5_CD239_Spectrin is a candidate association that compensates the linkage between the basement membrane and cytoskeleton in skeletal muscle fibers

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Laminin α5_CD239_spectrin complex is a candidate linkage in sarcolemma.

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The linkage molecules are expressed in embryonic and regenerative muscle fibers.

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CD239 expression is upregulated by steroid therapy for muscular dystrophy.

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The compensatory linkage may be a therapeutic target for muscular dystrophy.

The linkage between the basement membrane (BM) and cytoskeleton is crucial for muscle fiber stability and signal transduction. Mutations in the linkage molecules can cause various types of muscular dystrophies. The different severities and times of onset suggest that compensatory linkages occur at the sarcolemma. Cluster of differentiation 239 (CD239) binds to the α5 subunit of laminin-511 extracellularly and is connected to spectrin intracellularly, resulting in a linkage between the BM and cytoskeleton. In this study, we explored the linkage of laminin α5_CD239_spectrin in skeletal muscles. Although laminin α5, CD239, and spectrin were present in embryonic skeletal muscles, they disappeared in adult skeletal muscle tissues, except for the soleus and diaphragm. Laminin α5_CD239_spectrin was localized in the skeletal muscle tissues of Duchenne muscular dystrophy and congenital muscular dystrophy mouse models. The experimental regeneration of skeletal muscle increased the CD239-mediated linkage, indicating that it responds to regeneration, but not to genetic influence. Furthermore, in silico analysis showed that laminin α5_CD239_spectrin was upregulated by steroid therapy for muscular dystrophy. Therefore, CD239-mediated linkage may serve as a therapeutic target to prevent the progression of muscular dystrophy.

CDP-ribitol prodrug treatment ameliorates ISPD-deficient muscular dystrophy mouse model

Ribitol-phosphate modification is crucial for the functional maturation of α-dystroglycan. Its dysfunction is associated with muscular dystrophy, cardiomyopathy, and central nervous system abnormalities; however, no effective treatments are currently available for diseases caused by ribitol-phosphate defects. In this study, we demonstrate that prodrug treatments can ameliorate muscular dystrophy caused by defects in isoprenoid synthase domain containing (ISPD), which encodes an enzyme that synthesizes CDP-ribitol, a donor substrate for ribitol-phosphate modification. We generated skeletal muscle-selective Ispd conditional knockout mice, leading to a pathogenic reduction in CDP-ribitol levels, abnormal glycosylation of α-dystroglycan, and severe muscular dystrophy. Adeno-associated virus-mediated gene replacement experiments suggested that the recovery of CDP-ribitol levels rescues the ISPD-deficient pathology. As a prodrug treatment strategy, we developed a series of membrane-permeable CDP-ribitol derivatives, among which tetraacetylated CDP-ribitol ameliorated the dystrophic pathology. In addition, the prodrug successfully rescued abnormal α-dystroglycan glycosylation in patient fibroblasts. Consequently, our findings provide proof-of-concept for supplementation therapy with CDP-ribitol and could accelerate the development of therapeutic agents for muscular dystrophy and other diseases caused by glycosylation defects.

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.

[A Japanese family with POMT2-related limb girdle muscular dystrophy]

Mutations in the gene encoding the protein O-mannosyl-transferase 2 (POMT2) are known to cause autosomal recessive limb girdle muscular dystrophy type 14 (LGMDR14). No Japanese patient with LGMDR14 has been reported previously. Here, we report three patients with LGMDR14 in one family. The first and second patients harbored a novel homozygous mutation of c.1568A>G, while the third harbored a compound heterozygous mutation of c.1568A>G and c.869C>T. The novel c.1568A>G mutation is classified as likely pathogenic by the guideline of the American College of Medical Genetics and Genomics. Similar to previous cases, all three patients presented difficulty walking and cognitive impairment, and the hamstring muscles were severely affected. Although eye abnormality has only been reported in one previous case, two our patients showed eye abnormalities. As POMT2 enzymatic activity has been demonstrated in the mammalian retina, an eye abnormality may represent a phenotype associated with POMT2 mutation.

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, Largemyd/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.

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.

Elimination of fukutin reveals cellular and molecular pathomechanisms in muscular dystrophy-associated heart failure

Heart failure is the major cause of death for muscular dystrophy patients, however, the molecular pathomechanism remains unknown. Here, we show the detailed molecular pathogenesis of muscular dystrophy-associated cardiomyopathy in mice lacking the fukutin gene (Fktn), the causative gene for Fukuyama muscular dystrophy. Although cardiac Fktn elimination markedly reduced α-dystroglycan glycosylation and dystrophin-glycoprotein complex proteins in sarcolemma at all developmental stages, cardiac dysfunction was observed only in later adulthood, suggesting that membrane fragility is not the sole etiology of cardiac dysfunction. During young adulthood, Fktn-deficient mice were vulnerable to pathological hypertrophic stress with downregulation of Akt and the MEF2-histone deacetylase axis. Acute Fktn elimination caused severe cardiac dysfunction and accelerated mortality with myocyte contractile dysfunction and disordered Golgi-microtubule networks, which were ameliorated with colchicine treatment. These data reveal fukutin is crucial for maintaining myocyte physiology to prevent heart failure, and thus, the results may lead to strategies for therapeutic intervention.

Mutations in Ftkn cause Fukuyama muscular dystrophy, and heart failure is the main cause of death in thes patients. Here the authors show that acute elimination of Fktn in adult mice causes early mortality, and this is associated with myocyte dysfunction, with disorganised Golg-microtubule networks, and that the pathology can be ameliorated with colchicine treatment.

Temporal requirement of dystroglycan glycosylation during brain development and rescue of severe cortical dysplasia via gene delivery in the fetal stage

Congenital muscular dystrophies (CMDs) are characterized by progressive weakness and degeneration of skeletal muscle. In several forms of CMD, abnormal glycosylation of α-dystroglycan (α-DG) results in conditions collectively known as dystroglycanopathies, which are associated with central nervous system involvement. We recently demonstrated that fukutin, the gene responsible for Fukuyama congenital muscular dystrophy, encodes the ribitol-phosphate transferase essential for dystroglycan function. Brain pathology in patients with dystroglycanopathy typically includes cobblestone lissencephaly, mental retardation, and refractory epilepsy; however, some patients exhibit average intelligence, with few or almost no structural defects. Currently, there is no effective treatment for dystroglycanopathy, and the mechanisms underlying the generation of this broad clinical spectrum remain unknown. Here, we analysed four distinct mouse models of dystroglycanopathy: two brain-selective fukutin conditional knockout strains (neuronal stem cell-selective Nestin-fukutin-cKO and forebrain-selective Emx1-fukutin-cKO), a Fukutin^Hp strain with the founder retrotransposal insertion in the fukutin gene, and a spontaneous Large-mutant Large^myd strain. These models exhibit variations in the severity of brain pathology, replicating the clinical heterogeneity of dystroglycanopathy. Immunofluorescence analysis of the developing cortex suggested that residual glycosylation of α-DG at embryonic day 13.5 (E13.5), when cortical dysplasia is not yet apparent, may contribute to subsequent phenotypic heterogeneity. Surprisingly, delivery of fukutin or Large into the brains of mice at E12.5 prevented severe brain malformation in Emx1-fukutin-cKO and Large^myd/myd mice, respectively. These findings indicate that spatiotemporal persistence of functionally glycosylated α-DG may be crucial for brain development and modulation of glycosylation during the fetal stage could be a potential therapeutic strategy for dystroglycanopathy.

Keeping an eye on congenital disorders of O-glycosylation: A systematic literature review

Congenital disorders of glycosylation (CDG) are a rapidly growing family comprising >100 genetic diseases. Some 25 CDG are pure O-glycosylation defects. Even among this CDG subgroup, phenotypic diversity is broad, ranging from mild to severe poly-organ/system dysfunction. Ophthalmic manifestations are present in 60% of these CDG. The ophthalmic manifestations in N-glycosylation-deficient patients have been described elsewhere. The present review documents the spectrum and incidence of eye disorders in patients with pure O-glycosylation defects with the aim of assisting diagnosis and management and promoting research.

LARGE expression in different types of muscular dystrophies other than dystroglycanopathy

BACKGROUND

Alpha-dystroglycan (αDG) is an extracellular peripheral glycoprotein that acts as a receptor for both extracellular matrix proteins containing laminin globular domains and certain arenaviruses. An important enzyme, known as Like-acetylglucosaminyltransferase (LARGE), has been shown to transfer repeating units of -glucuronic acid-β1,3-xylose-α1,3- (matriglycan) to αDG that is required for functional receptor as an extracellular matrix protein scaffold. The reduction in the amount of LARGE-dependent matriglycan result in heterogeneous forms of dystroglycanopathy that is associated with hypoglycosylation of αDG and a consequent lack of ligand-binding activity. Our aim was to investigate whether LARGE expression showed correlation with glycosylation of αDG and histopathological parameters in different types of muscular dystrophies, except for dystroglycanopathies.

METHODS

The expression level of LARGE and glycosylation status of αDG were examined in skeletal muscle biopsies from 26 patients with various forms of muscular dystrophy [Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), sarcoglycanopathy, dysferlinopathy, calpainopathy, and merosin and collagen VI deficient congenital muscular dystrophies (CMDs)] and correlation of results with different histopathological features was investigated.

RESULTS

Despite the fact that these diseases are not caused by defects of glycosyltransferases, decreased expression of LARGE was detected in many patient samples, partly correlating with the type of muscular dystrophy. Although immunolabelling of fully glycosylated αDG with VIA4-1 was reduced in dystrophinopathy patients, no significant relationship between reduction of LARGE expression and αDG hypoglycosylation was detected. Also, Merosin deficient CMD patients showed normal immunostaining with αDG despite severe reduction of LARGE expression.

CONCLUSIONS

Our data shows that it is not always possible to correlate LARGE expression and αDG glycosylation in different types of muscular dystrophies and suggests that there might be differences in αDG processing by LARGE which could be regulated under different pathological conditions.

Gonorazky H, Cohn RD, Campbell C. Treating pediatric neuromuscular disorders: The future is now

Pediatric neuromuscular diseases encompass all disorders with onset in childhood and where the primary area of pathology is in the peripheral nervous system. These conditions are largely genetic in etiology, and only those with a genetic underpinning will be presented in this review. This includes disorders of the anterior horn cell (e.g., spinal muscular atrophy), peripheral nerve (e.g., Charcot-Marie-Tooth disease), the neuromuscular junction (e.g., congenital myasthenic syndrome), and the muscle (myopathies and muscular dystrophies). Historically, pediatric neuromuscular disorders have uniformly been considered to be without treatment possibilities and to have dire prognoses. This perception has gradually changed, starting in part with the discovery and widespread application of corticosteroids for Duchenne muscular dystrophy. At present, several exciting therapeutic avenues are under investigation for a range of conditions, offering the potential for significant improvements in patient morbidities and mortality and, in some cases, curative intervention. In this review, we will present the current state of treatment for the most common pediatric neuromuscular conditions, and detail the treatment strategies with the greatest potential for helping with these devastating diseases.

Cystic kidneys in fetal Walker-Warburg syndrome with POMT2 mutation: Intrafamilial phenotypic variability in four siblings and review of literature

Walker-Warburg syndrome (WWS) is a severe form of congenital muscular dystrophy secondary to α-dystroglycanopathy with muscle, brain, and eye abnormalities often leading to death in the first weeks of life. It is transmitted in an autosomal recessive pattern, and has been linked to at least 15 different genes; including protein O-mannosyltransferase 1 (POMT1), protein O-mannosyltransferase 2 (POMT2), protein O-mannose beta-1,2-N acetylglucosaminyltransferase (POMGNT1), fukutin (FKTN), isoprenoid synthase domain-containing protein (ISPD), and other genes. We report on a consanguineous family with four consecutive siblings affected by this condition with lethal outcome in three (still birth), and termination of the fourth pregnancy based on antenatal MRI identification of brain and kidney anomalies that heralded proper and deep clinical phenotyping. The diagnosis of WWS was suggested based on the unique collective phenotype comprising brain anomalies in the form of lissencephaly, subcortical/subependymal heterotopia, and cerebellar hypoplasia shared by all four siblings; microphthalmia in one sibling; and large cystic kidneys in the fetus and another sibling. Other unshared neurological abnormalities included hydrocephalus and Dandy-Walker malformation. Whole exome sequencing of the fetus revealed a highly conserved missense mutation in POMT2 that is known to cause WWS with brain and eye anomalies.In conclusion, the heterogeneous clinical presentation in the four affected conceptions with POMT2 mutation expands the current clinical spectrum of POMT2-associated WWS to include large cystic kidneys; and confirms intra-familial variability in terms of brain, kidney, and eye anomalies.

Postnatal Gene Therapy Improves Spatial Learning Despite the Presence of Neuronal Ectopia in a Model of Neuronal Migration Disorder

Patients with type II lissencephaly, a neuronal migration disorder with ectopic neurons, suffer from severe mental retardation, including learning deficits. There is no effective therapy to prevent or correct the formation of neuronal ectopia, which is presumed to cause cognitive deficits. We hypothesized that learning deficits were not solely caused by neuronal ectopia and that postnatal gene therapy could improve learning without correcting the neuronal ectopia formed during fetal development. To test this hypothesis, we evaluated spatial learning of cerebral cortex-specific protein O-mannosyltransferase 2 (POMT2, an enzyme required for O-mannosyl glycosylation) knockout mice and compared to the knockout mice that were injected with an adeno-associated viral vector (AAV) encoding POMT2 into the postnatal brains with Barnes maze. The data showed that the knockout mice exhibited reduced glycosylation in the cerebral cortex, reduced dendritic spine density on CA1 neurons, and increased latency to the target hole in the Barnes maze, indicating learning deficits. Postnatal gene therapy restored functional glycosylation, rescued dendritic spine defects, and improved performance on the Barnes maze by the knockout mice even though neuronal ectopia was not corrected. These results indicate that postnatal gene therapy improves spatial learning despite the presence of neuronal ectopia.

Glycan Engineering for Cell and Developmental Biology

Cell-surface glycans are a diverse class of macromolecules that participate in many key biological processes, including cell-cell communication, development, and disease progression. Thus, the ability to modulate the structures of glycans on cell surfaces provides a powerful means not only to understand fundamental processes but also to direct activity and elicit desired cellular responses. Here, we describe methods to sculpt glycans on cell surfaces and highlight recent successes in which artificially engineered glycans have been employed to control biological outcomes such as the immune response and stem cell fate.

The Muscular Dystrophy Gene TMEM5 Encodes a Ribitol β1,4-Xylosyltransferase Required for the Functional Glycosylation of Dystroglycan

A defect in O-mannosyl glycan is the cause of α-dystroglycanopathy, a group of congenital muscular dystrophies caused by aberrant α-dystroglycan (α-DG) glycosylation. Recently, the entire structure of O-mannosyl glycan, [3GlcAβ1-3Xylα1]_n-3GlcAβ1-4Xyl-Rbo5P-1Rbo5P-3GalNAcβ1-3GlcNAcβ1-4 (phospho-6)Manα1-, which is required for the binding of α-DG to extracellular matrix ligands, has been proposed. However, the linkage of the first Xyl residue to ribitol 5-phosphate (Rbo5P) is not clear. TMEM5 is a gene product responsible for α-dystroglycanopathy and was reported as a potential enzyme involved in this linkage formation, although the experimental evidence is still incomplete. Here, we report that TMEM5 is a xylosyltransferase that forms the Xylβ1-4Rbo5P linkage on O-mannosyl glycan. The anomeric configuration and linkage position of the product (β1,4 linkage) was determined by NMR analysis. The introduction of two missense mutations in TMEM5 found in α-dystroglycanopathy patients impaired xylosyltransferase activity. Furthermore, the disruption of the TMEM5 gene by CRISPR/Cas9 abrogated the elongation of the (-3GlcAβ1-3Xylα1-) unit on O-mannosyl glycan. Based on these results, we concluded that TMEM5 acts as a UDP-d-xylose:ribitol-5-phosphate β1,4-xylosyltransferase in the biosynthetic pathway of O-mannosyl glycan.

Carbohydrate-binding domain of the POMGnT1 stem region modulates O-mannosylation sites of α-dystroglycan

The dystrophin glycoprotein complex, which connects the cell membrane to the basement membrane, is essential for a variety of biological events, including maintenance of muscle integrity. An O-mannose-type GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man structure of α-dystroglycan (α-DG), a subunit of the complex that is anchored to the cell membrane, interacts directly with laminin in the basement membrane. Reduced glycosylation of α-DG is linked to some types of inherited muscular dystrophy; consistent with this relationship, many disease-related mutations have been detected in genes involved in O-mannosyl glycan synthesis. Defects in protein O-linked mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGnT1), a glycosyltransferase that participates in the formation of GlcNAc-β1,2-Man glycan, are causally related to muscle-eye-brain disease (MEB), a congenital muscular dystrophy, although the role of POMGnT1 in postphosphoryl modification of GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man glycan remains elusive. Our crystal structures of POMGnT1 agreed with our previous results showing that the catalytic domain recognizes substrate O-mannosylated proteins via hydrophobic interactions with little sequence specificity. Unexpectedly, we found that the stem domain recognizes the β-linked GlcNAc of O-mannosyl glycan, an enzymatic product of POMGnT1. This interaction may recruit POMGnT1 to a specific site of α-DG to promote GlcNAc-β1,2-Man clustering and also may recruit other enzymes that interact with POMGnT1, e.g., fukutin, which is required for further modification of the GalNAc-β1,3-GlcNAc-β1,4-(phosphate-6)-Man glycan. On the basis of our findings, we propose a mechanism for the deficiency in postphosphoryl modification of the glycan observed in POMGnT1-KO mice and MEB patients.

ISPD gene homozygous deletion identified by SNP array confirms prenatal manifestation of Walker-Warburg syndrome

Walker-Warburg syndrome (WWS) is a rare form of autosomal recessive, congenital muscular dystrophy that is associated with brain and eye anomalies. Several genes encoding proteins involved in abnormal α-dystroglycan glycosylation have been implicated in the aetiology of WWS, most recently the ISPD gene. Typical WWS brain anomalies, such as cobblestone lissencephaly, hydrocephalus and cerebellar malformations, can be prenatally detected through routine ultrasound examinations. Here, we report two karyotypically normal foetuses with multiple brain anomalies that corresponded to WWS symptoms. Using a SNP-array examination on the amniotic fluid DNA, a homozygous microdeletion was identified at 7p21.2p21.1 within the ISPD gene. Published data and our findings led us to the conclusion that a homozygous segmental intragenic deletion of the ISPD gene causes the most severe phenotype of Walker-Warburg syndrome. Our results also clearly supports the use of chromosomal microarray analysis as a first-line diagnostic test in patients with a foetus with one or more major structural abnormalities identified on ultrasonographic examination.