A dystroglycan mutation (p.Cys667Phe) associated to muscle-eye-brain disease with multicystic leucodystrophy results in ER-retention of the mutant protein

Live cell optical super-resolution microscopy of dystroglycan mutants as a model for dystroglycanopathies in multiple cell lines

Introduction

Dystroglycan (DG) is an adhesion complex comprising two subunits, α-DG and β-DG, which interact non-covalently at the plasma membrane. As a component of the dystrophin-glycoprotein complex DGC, DG plays a crucial role in linking the cytoskeleton to the surrounding basement membranes. Rare primary point mutations in the DAG1 gene have been identified in patients with various forms of neuromuscular dystrophy, ranging in phenotype from mild to severe.

Methods

To gain a deeper understanding of the molecular mechanisms underlying these pathologies, we have designed a series of chimeric GFP-tagged full-length α/β-DG constructs and expressed them in three different cell lines (U-2OS, HEK-293T and C2C12). Wild-type DG constructs were compared to their counterparts carrying pathologic missense mutations previously described in patients, namely, L84F, T190M and C667F and with the mutant I591D, i.e., the topological equivalent of V567D identified in zebrafish.

Results

Live super-resolution fluorescence microscopy showed that the C667F mutant is retained within the ER/Golgi while the T190M and wild-type proteins are correctly localized to the plasma membrane in all 3 cell lines. The L84F mutant exhibits a delay in trafficking to the plasma membrane in two of the cell lines, while localizing strongly at the plasma membrane in the high-expression HEK-293T cells. Similarly, the I591D mutant accumulated at the plasma membrane in the HEK-293T cells, in contrast to the clear retention in the endoplasmic reticulum/Golgi apparatus observed in U-2OS and C2C12 cells.

Discussion

Our data demonstrate the importance of using a range of different cell lines for a comprehensive study of DG mutants or variants by live cell optical super-resolution microscopy.

Molecular basis of proteolytic cleavage regulation by the extracellular matrix receptor dystroglycan

The dystrophin-glycoprotein-complex (DGC), anchored by the transmembrane protein dystroglycan, functions to mechanically link the extracellular matrix and actin cytoskeleton. Breaking this connection is associated with diseases such as muscular dystrophy, yet cleavage of dystroglycan by matrix-metalloproteinases (MMPs) remains an understudied mechanism to disrupt the DGC. We determined the crystal structure of the membrane-adjacent domain (amino acids 491-722) of E. coli expressed human dystroglycan to understand MMP cleavage regulation. The structural model includes tandem immunoglobulin-like (IGL) and sperm/enterokinase/agrin-like (SEAL) domains, which support proteolysis in diverse receptors to facilitate mechanotransduction, membrane protection, and viral entry. The structure reveals a C-terminal extension that buries the MMP site by packing into a hydrophobic pocket, a unique mechanism of MMP cleavage regulation. We further demonstrate structure-guided and disease-associated mutations disrupt proteolytic regulation using a cell-surface proteolysis assay. Thus disrupted proteolysis is a potentially relevant mechanism for "breaking" the DGC link to contribute to disease pathogenesis.

The missense mutation C667F in murine β-dystroglycan causes embryonic lethality, myopathy and blood-brain barrier destabilization

Dystroglycan (DG) is an extracellular matrix receptor consisting of an α- and a β-DG subunit encoded by the DAG1 gene. The homozygous mutation (c.2006G>T, p.Cys669Phe) in β-DG causes muscle-eye-brain disease with multicystic leukodystrophy in humans. In a mouse model of this primary dystroglycanopathy, approximately two-thirds of homozygous embryos fail to develop to term. Mutant mice that are born undergo a normal postnatal development but show a late-onset myopathy with partially penetrant histopathological changes and an impaired performance on an activity wheel. Their brains and eyes are structurally normal, but the localization of mutant β-DG is altered in the glial perivascular end-feet, resulting in a perturbed protein composition of the blood-brain and blood-retina barrier. In addition, α- and β-DG protein levels are significantly reduced in muscle and brain of mutant mice. Owing to the partially penetrant developmental phenotype of the C669F β-DG mice, they represent a novel and highly valuable mouse model with which to study the molecular effects of β-DG functional alterations both during embryogenesis and in mature muscle, brain and eye, and to gain insight into the pathogenesis of primary dystroglycanopathies.

Analysis of the GFP-labelled β-dystroglycan interactome in HEK-293 transfected cells reveals novel intracellular networks

Dystroglycan (DG) is a cell adhesion complex that is widely expressed in tissues. It is composed by two subunits, α-DG, a highly glycosylated protein that interacts with several extracellular matrix proteins, and transmembrane β-DG whose, cytodomain binds to the actin cytoskeleton. Glycosylation of α-DG is crucial for functioning as a receptor for its multiple extracellular binding partners. Perturbation of α-DG glycosylation is the central event in the pathogenesis of severe pathologies such as muscular dystrophy and cancer. β-DG acts as a scaffold for several cytoskeletal and nuclear proteins and very little is known about the fine regulation of some of these intracellular interactions and how they are perturbed in diseases. To start filling this gap by identifying uncharacterized intracellular networks preferentially associated with β-DG, HEK-293 cells were transiently transfected with a plasmid carrying the β-DG subunit with GFP fused at its C-terminus. With this strategy, we aimed at forcing β-DG to occupy multiple intracellular locations instead of sitting tightly at its canonical plasma membrane milieu, where it is commonly found in association with α-DG. Immunoprecipitation by anti-GFP antibodies followed by shotgun proteomic analysis led to the identification of an interactome formed by 313 exclusive protein matches for β-DG binding. A series of already known β-DG interactors have been found, including ezrin and emerin, whilst significant new matches, which include potential novel β-DG interactors and their related networks, were identified in diverse subcellular compartments, such as cytoskeleton, endoplasmic reticulum/Golgi, mitochondria, nuclear membrane and the nucleus itself. Of particular interest amongst the novel identified matches, Lamina-Associated Polypeptide-1B (LAP1B), an inner nuclear membrane protein, whose mutations are known to cause nuclear envelopathies characterized by muscular dystrophy, was found to interact with β-DG in HEK-293 cells. This evidence was confirmed by immunoprecipitation, Western blotting and immunofluorescence experiments. We also found by immunofluorescence experiments that LAP1B looses its nuclear envelope localization in C2C12 DG-knock-out cells, suggesting that LAP1B requires β-DG for a proper nuclear localization. These results expand the role of β-DG as a nuclear scaffolding protein and provide novel evidence of a possible link between dystroglycanopathies and nuclear envelopathies displaying with muscular dystrophy.

The Role of β-Dystroglycan in Nuclear Dynamics

Dystroglycan is a ubiquitously expressed heterodimeric cell-surface laminin receptor with roles in cell adhesion, signalling, and membrane stabilisation. More recently, the transmembrane β-subunit of dystroglycan has been shown to localise to both the nuclear envelope and the nucleoplasm. This has led to the hypothesis that dystroglycan may have a structural role at the nuclear envelope analogous to its role at the plasma membrane. The biochemical fraction of myoblast cells clearly supports the presence of dystroglycan in the nucleus. Deletion of the dystroglycan protein by disruption of the DAG1 locus using CRISPR/Cas9 leads to changes in nuclear size but not overall morphology; moreover, the Young's modulus of dystroglycan-deleted nuclei, as determined by atomic force microscopy, is unaltered. Dystroglycan-disrupted myoblasts are also no more susceptible to nuclear stresses including chemical and mechanical, than normal myoblasts. Re-expression of dystroglycan in DAG1-disrupted myoblasts restores nuclear size without affecting other nuclear parameters.

Exocyst-mediated membrane trafficking of the lissencephaly-associated ECM receptor dystroglycan is required for proper brain compartmentalization

To assemble a brain, differentiating neurons must make proper connections and establish specialized brain compartments. Abnormal levels of cell adhesion molecules disrupt these processes. Dystroglycan (Dg) is a major non-integrin cell adhesion receptor, deregulation of which is associated with dramatic neuroanatomical defects such as lissencephaly type II or cobblestone brain. The previously established Drosophila model for cobblestone lissencephaly was used to understand how Dg is regulated in the brain. During development, Dg has a spatiotemporally dynamic expression pattern, fine-tuning of which is crucial for accurate brain assembly. In addition, mass spectrometry analyses identified numerous components associated with Dg in neurons, including several proteins of the exocyst complex. Data show that exocyst-based membrane trafficking of Dg allows its distinct expression pattern, essential for proper brain morphogenesis. Further studies of the Dg neuronal interactome will allow identification of new factors involved in the development of dystroglycanopathies and advance disease diagnostics in humans.

A molecular overview of the primary dystroglycanopathies

Dystroglycan is a major non-integrin adhesion complex that connects the cytoskeleton to the surrounding basement membranes, thus providing stability to skeletal muscle. In Vertebrates, hypoglycosylation of α-dystroglycan has been strongly linked to muscular dystrophy phenotypes, some of which also show variable degrees of cognitive impairments, collectively termed dystroglycanopathies. Only a small number of mutations in the dystroglycan gene, leading to the so called primary dystroglycanopathies, has been described so far, as opposed to the ever-growing number of identified secondary or tertiary dystroglycanopathies (caused by genetic abnormalities in glycosyltransferases or in enzymes involved in the synthesis of the carbohydrate building blocks). The few mutations found within the autonomous N-terminal domain of α-dystroglycan seem to destabilise it to different degrees, without influencing the overall folding and targeting of the dystroglycan complex. On the contrary other mutations, some located at the α/β interface of the dystroglycan complex, seem to be able to interfere with its maturation, thus compromising its stability and eventually leading to the intracellular engulfment and/or partial or even total degradation of the dystroglycan uncleaved precursor.

The roles of dystroglycan in the nervous system: insights from animal models of muscular dystrophy

Dystroglycan is a cell membrane protein that binds to the extracellular matrix in a variety of mammalian tissues. The α-subunit of dystroglycan (αDG) is heavily glycosylated, including a special O-mannosyl glycoepitope, relying upon this unique glycosylation to bind its matrix ligands. A distinct group of muscular dystrophies results from specific hypoglycosylation of αDG, and they are frequently associated with central nervous system involvement, ranging from profound brain malformation to intellectual disability without evident morphological defects. There is an expanding literature addressing the function of αDG in the nervous system, with recent reports demonstrating important roles in brain development and in the maintenance of neuronal synapses. Much of these data are derived from an increasingly rich array of experimental animal models. This Review aims to synthesize the information from such diverse models, formulating an up-to-date understanding about the various functions of αDG in neurons and glia of the central and peripheral nervous systems. Where possible, we integrate these data with our knowledge of the human disorders to promote translation from basic mechanistic findings to clinical therapies that take the neural phenotypes into account.

Summary: Dystroglycan is a ubiquitous matrix receptor linked to brain and muscle disease. Unraveling the functions of this protein will inform basic and translational research on neural development and muscular dystrophies.

Network-based association analysis to infer new disease-gene relationships using large-scale protein interactions

Protein-protein interactions integrated with disease-gene associations represent important information for revealing protein functions under disease conditions to improve the prevention, diagnosis, and treatment of complex diseases. Although several studies have attempted to identify disease-gene associations, the number of possible disease-gene associations is very small. High-throughput technologies have been established experimentally to identify the association between genes and diseases. However, these techniques are still quite expensive, time consuming, and even difficult to perform. Thus, based on currently available data and knowledge, computational methods have served as alternatives to provide more possible associations to increase our understanding of disease mechanisms. Here, a new network-based algorithm, namely, Disease-Gene Association (DGA), was developed to calculate the association score of a query gene to a new possible set of diseases. First, a large-scale protein interaction network was constructed, and the relationship between two interacting proteins was calculated with regard to the disease relationship. Novel plausible disease-gene pairs were identified and statistically scored by our algorithm using neighboring protein information. The results yielded high performance for disease-gene prediction, with an F-measure of 0.78 and an AUC of 0.86. To identify promising candidates of disease-gene associations, the association coverage of genes and diseases were calculated and used with the association score to perform gene and disease selection. Based on gene selection, we identified promising pairs that exhibited evidence related to several important diseases, e.g., inflammation, lipid metabolism, inborn errors, xanthomatosis, cerebellar ataxia, cognitive deterioration, malignant neoplasms of the skin and malignant tumors of the cervix. Focusing on disease selection, we identified target genes that were important to blistering skin diseases and muscular dystrophy. In summary, our developed algorithm is simple, efficiently identifies disease–gene associations in the protein-protein interaction network and provides additional knowledge regarding disease-gene associations. This method can be generalized to other association studies to further advance biomedical science.