The Structure of the N-terminal Region of Murine Skeletal Muscle α-Dystroglycan Discloses a Modular Architecture

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.

The α-dystroglycan N-terminus is a broad-spectrum antiviral agent against SARS-CoV-2 and enveloped viruses

The COVID-19 pandemic has shown the need to develop effective therapeutics in preparedness for further epidemics of virus infections that pose a significant threat to human health. As a natural compound antiviral candidate, we focused on α-dystroglycan, a highly glycosylated basement membrane protein that links the extracellular matrix to the intracellular cytoskeleton. Here we show that the N-terminal fragment of α-dystroglycan (α-DGN), as produced in E. coli in the absence of post-translational modifications, blocks infection of SARS-CoV-2 in cell culture, human primary gut organoids and the lungs of transgenic mice expressing the human receptor angiotensin I-converting enzyme 2 (hACE2). Prophylactic and therapeutic administration of α-DGN reduced SARS-CoV-2 lung titres and protected the mice from respiratory symptoms and death. Recombinant α-DGN also blocked infection of a wide range of enveloped viruses including the four Dengue virus serotypes, influenza A virus, respiratory syncytial virus, tick-borne encephalitis virus, but not human adenovirus, a non-enveloped virus in vitro. This study establishes soluble recombinant α-DGN as a broad-band, natural compound candidate therapeutic against enveloped viruses.

N-terminal domain on dystroglycan enables LARGE1 to extend matriglycan on α-dystroglycan and prevents muscular dystrophy

Dystroglycan (DG) requires extensive post-translational processing and O-glycosylation to function as a receptor for extracellular matrix (ECM) proteins containing laminin-G (LG) domains. Matriglycan is an elongated polysaccharide of alternating xylose (Xyl) and glucuronic acid (GlcA) that binds with high affinity to ECM proteins with LG domains and is uniquely synthesized on α-dystroglycan (α-DG) by like-acetylglucosaminyltransferase-1 (LARGE1). Defects in the post-translational processing or O-glycosylation of α-DG that result in a shorter form of matriglycan reduce the size of α-DG and decrease laminin binding, leading to various forms of muscular dystrophy. Previously, we demonstrated that protein O-mannose kinase (POMK) is required for LARGE1 to generate full-length matriglycan on α-DG (~150-250 kDa) (Walimbe et al., 2020). Here, we show that LARGE1 can only synthesize a short, non-elongated form of matriglycan in mouse skeletal muscle that lacks the DG N-terminus (α-DGN), resulting in an ~100-125 kDa α-DG. This smaller form of α-DG binds laminin and maintains specific force but does not prevent muscle pathophysiology, including reduced force production after eccentric contractions (ECs) or abnormalities in the neuromuscular junctions. Collectively, our study demonstrates that α-DGN, like POMK, is required for LARGE1 to extend matriglycan to its full mature length on α-DG and thus prevent muscle pathophysiology.

Identification and Modeling of a GT-A Fold in the α-Dystroglycan Glycosylating Enzyme LARGE1

The acetylglucosaminyltransferase-like protein LARGE1 is an enzyme that is responsible for the final steps of the post-translational modifications of dystroglycan (DG), a membrane receptor that links the cytoskeleton with the extracellular matrix in the skeletal muscle and in a variety of other tissues. LARGE1 acts by adding the repeating disaccharide unit [-3Xyl-α1,3GlcAβ1-] to the extracellular portion of the DG complex (α-DG); defects in the LARGE1 gene result in an aberrant glycosylation of α-DG and consequent impairment of its binding to laminin, eventually affecting the connection between the cell and the extracellular environment. In the skeletal muscle, this leads to degeneration of the muscular tissue and muscular dystrophy. So far, a few missense mutations have been identified within the LARGE1 protein and linked to congenital muscular dystrophy, and because no structural information is available on this enzyme, our understanding of the molecular mechanisms underlying these pathologies is still very limited. Here, we generated a 3D model structure of the two catalytic domains of LARGE1, combining different molecular modeling approaches. Furthermore, by using molecular dynamics simulations, we analyzed the effect on the structure and stability of the first catalytic domain of the pathological missense mutation S331F that gives rise to a severe form of muscle–eye–brain disease.

Protective role for the N-terminal domain of α-dystroglycan in Influenza A virus proliferation

α-Dystroglycan (α-DG) is a highly glycosylated basement membrane receptor that is cleaved by the proprotein convertase furin, which releases its N-terminal domain (α-DGN). Before cleavage, α-DGN interacts with the glycosyltransferase LARGE1 and initiates functional O-glycosylation of the mucin-like domain of α-DG. Notably, α-DGN has been detected in a wide variety of human bodily fluids, but the physiological significance of secreted α-DGN remains unknown. Here, we show that mice lacking α-DGN exhibit significantly higher viral titers in the lungs after Influenza A virus (IAV) infection (strain A/Puerto Rico/8/1934 H1N1), suggesting an inability to control virus load. Consistent with this, overexpression of α-DGN before infection or intranasal treatment with recombinant α-DGN prior and during infection, significantly reduced IAV titers in the lungs of wild-type mice. Hemagglutination inhibition assays using recombinant α-DGN showed in vitro neutralization of IAV. Collectively, our results support a protective role for α-DGN in IAV proliferation.

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.

Neurexins - versatile molecular platforms in the synaptic cleft

Neurexins constitute a large family of synaptic organizers. Their extracellular domains protrude into the synaptic cleft where they can form transsynaptic bridges with different partners. A unique constellation of structural elements within their ectodomains enables neurexins to create molecular platforms within the synaptic cleft that permit a large portfolio of partners to be recruited, assembled and their interactions to be dynamically regulated. Neurexins and their partners are implicated in neuropsychiatric diseases including autism spectrum disorder and schizophrenia. Detailed understanding of the mechanisms that underlie neurexin interactions may in future guide the design of tools to manipulate synaptic connections and their function, in particular those involved in the pathogenesis of neuropsychiatric disease.

The effect of the pathological V72I, D109N and T190M missense mutations on the molecular structure of α-dystroglycan

Dystroglycan (DG) is a highly glycosylated protein complex that links the cytoskeleton with the extracellular matrix, mediating fundamental physiological functions such as mechanical stability of tissues, matrix organization and cell polarity. A crucial role in the glycosylation of the DG α subunit is played by its own N-terminal region that is required by the glycosyltransferase LARGE. Alteration in this O-glycosylation deeply impairs the high affinity binding to other extracellular matrix proteins such as laminins. Recently, three missense mutations in the gene encoding DG, mapped in the α-DG N-terminal region, were found to be responsible for hypoglycosylated states, causing congenital diseases of different severity referred as primary dystroglycanopaties.To gain insight on the molecular basis of these disorders, we investigated the crystallographic and solution structures of these pathological point mutants, namely V72I, D109N and T190M. Small Angle X-ray Scattering analysis reveals that these mutations affect the structures in solution, altering the distribution between compact and more elongated conformations. These results, supported by biochemical and biophysical assays, point to an altered structural flexibility of the mutant α-DG N-terminal region that may have repercussions on its interaction with LARGE and/or other DG-modifying enzymes, eventually reducing their catalytic efficiency.

Structural flexibility of human α-dystroglycan

Dystroglycan (DG), composed of α and β subunits, belongs to the dystrophin-associated glycoprotein complex. α-DG is an extracellular matrix protein that undergoes a complex post-translational glycosylation process. The bifunctional glycosyltransferase like-acetylglucosaminyltransferase (LARGE) plays a crucial role in the maturation of α-DG, enabling its binding to laminin. We have already structurally analyzed the N-terminal region of murine α-DG (α-DG-Nt) and of a pathological single point mutant that may affect recognition of LARGE, although the structural features of the potential interaction between LARGE and DG remain elusive. We now report on the crystal structure of the wild-type human α-DG-Nt that has allowed us to assess the reliability of our murine crystallographic structure as a α-DG-Nt general model. Moreover, we address for the first time both structures in solution. Interestingly, small-angle X-ray scattering (SAXS) reveals the existence of two main protein conformations ensembles. The predominant species is reminiscent of the crystal structure, while the less populated one assumes a more extended fold. A comparative analysis of the human and murine α-DG-Nt solution structures reveals that the two proteins share a common interdomain flexibility and population distribution of the two conformers. This is confirmed by the very similar stability displayed by the two orthologs as assessed by biochemical and biophysical experiments. These results highlight the need to take into account the molecular plasticity of α-DG-Nt in solution, as it can play an important role in the functional interactions with other binding partners.

Evolution and diversity of cadherins and catenins

Cadherin genes encode a superfamily of conserved transmembrane proteins that share an adhesive ectodomain composed of tandem cadherin repeats. More than 100 human cadherin superfamily members have been identified, which can be classified into three families: major cadherins, protocadherins and cadherin-related proteins. These superfamily members are involved in diverse fundamental cellular processes including cell-cell adhesion, morphogenesis, cell recognition and signaling. Epithelial cadherin (E-cadherin) is the founding cadherin family member. Its cytoplasmic tail interacts with the armadillo catenins, p120 and β-catenin. Further, α-catenin links the cadherin/armadillo catenin complex to the actin filament network. Even genomes of ancestral metazoan species such as cnidarians and placozoans encode a limited number of distinct cadherins and catenins, emphasizing the conservation and functional importance of these gene families. Moreover, a large expansion of the cadherin and catenin families coincides with the emergence of vertebrates and reflects a major functional diversification in higher metazoans. Here, we revisit and review the functions, phylogenetic classifications and co-evolution of the cadherin and catenin protein families.

Expansion of divergent SEA domains in cell surface proteins and nucleoporin 54

SEA (sea urchin sperm protein, enterokinase, agrin) domains, many of which possess autoproteolysis activity, have been found in a number of cell surface and secreted proteins. Despite high sequence divergence, SEA domains were also proposed to be present in dystroglycan based on a conserved autoproteolysis motif and receptor-type protein phosphatase IA-2 based on structural similarity. The presence of a SEA domain adjacent to the transmembrane segment appears to be a recurring theme in quite a number of type I transmembrane proteins on the cell surface, such as MUC1, dystroglycan, IA-2, and Notch receptors. By comparative sequence and structural analyses, we identified dystroglycan-like proteins with SEA domains in Capsaspora owczarzaki of the Filasterea group, one of the closest single-cell relatives of metazoans. We also detected novel and divergent SEA domains in a variety of cell surface proteins such as EpCAM, α/ε-sarcoglycan, PTPRR, collectrin/Tmem27, amnionless, CD34, KIAA0319, fibrocystin-like protein, and a number of cadherins. While these proteins are mostly from metazoans or their single cell relatives such as choanoflagellates and Filasterea, fibrocystin-like proteins with SEA domains were found in several other eukaryotic lineages including green algae, Alveolata, Euglenozoa, and Haptophyta, suggesting an ancient evolutionary origin. In addition, the intracellular protein Nucleoporin 54 (Nup54) acquired a divergent SEA domain in choanoflagellates and metazoans.

An evaluation of the evolution of the gene structure of dystroglycan

Background

Dystroglycan (DG) is an adhesion receptor complex composed of two non-covalently associated subunits, transcribed from a single gene. The extracellular α-DG is highly and heterogeneously glycosylated and binds with high affinity to laminins, and the transmembrane β-DG binds intracellular dystrophin. Multiple cellular functions have been proposed for DG, notwithstanding that its role in skeletal muscle appears central as demonstrated by both primary and secondary severe muscular dystrophic phenotypes collectively known as dystroglycanopathies. We recently analysed the molecular phylogeny of the DG core protein and identified the α/β interface, transmembrane and cytoplasmic domains of β-DG as the most conserved region. It was also identified that the IG2_MAT_NU region has been independently duplicated in multiple lineages.

Results

To understand the evolution of dystroglycan in more depth, we investigated dystroglycan gene structure in 35 species representative of the phyla in which dystroglycan has been identified (i.e., all metazoan phyla except Ctenophora). The gene structure of three exons and two introns is remarkably conserved. However, additional lineage-specific introns were identified, which interrupt the coding sequence at distinct points, were identified in multiple metazoan groups, most prominently in ecdysozoans.

Conclusions

A coding DNA sequence (CDS) intron that interrupts the encoding of the IG1 domain is universally conserved and this intron is longer in gnathostomes (jawed vertebrates) than in other metazoans. Lineage-specific gain of additional introns has occurred notably in ecdysozoans, where multiple introns interrupt the large 3′ exon. More limited intron gain has also occurred in placozoa, cnidarians, urochordates and the DG paralogues of lamprey and teleost fish.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-016-2322-x) contains supplementary material, which is available to authorized users.

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.

The evolution of the dystroglycan complex, a major mediator of muscle integrity

Basement membrane (BM) extracellular matrices are crucial for the coordination of different tissue layers. A matrix adhesion receptor that is important for BM function and stability in many mammalian tissues is the dystroglycan (DG) complex. This comprises the non-covalently-associated extracellular α-DG, that interacts with laminin in the BM, and the transmembrane β-DG, that interacts principally with dystrophin to connect to the actin cytoskeleton. Mutations in dystrophin, DG, or several enzymes that glycosylate α-DG underlie severe forms of human muscular dystrophy. Nonwithstanding the pathophysiological importance of the DG complex and its fundamental interest as a non-integrin system of cell-ECM adhesion, the evolution of DG and its interacting proteins is not understood. We analysed the phylogenetic distribution of DG, its proximal binding partners and key processing enzymes in extant metazoan and relevant outgroups. We identify that DG originated after the divergence of ctenophores from porifera and eumetazoa. The C-terminal half of the DG core protein is highly-conserved, yet the N-terminal region, that includes the laminin-binding region, has undergone major lineage-specific divergences. Phylogenetic analysis based on the C-terminal IG2_MAT_NU region identified three distinct clades corresponding to deuterostomes, arthropods, and mollusks/early-diverging metazoans. Whereas the glycosyltransferases that modify α-DG are also present in choanoflagellates, the DG-binding proteins dystrophin and laminin originated at the base of the metazoa, and DG-associated sarcoglycan is restricted to cnidarians and bilaterians. These findings implicate extensive functional diversification of DG within invertebrate lineages and identify the laminin-DG-dystrophin axis as a conserved adhesion system that evolved subsequent to integrin-ECM adhesion, likely to enhance the functional complexity of cell-BM interactions in early metazoans.

Genetic Engineering of Dystroglycan in Animal Models of Muscular Dystrophy

In skeletal muscle, dystroglycan (DG) is the central component of the dystrophin-glycoprotein complex (DGC), a multimeric protein complex that ensures a strong mechanical link between the extracellular matrix and the cytoskeleton. Several muscular dystrophies arise from mutations hitting most of the components of the DGC. Mutations within the DG gene (DAG1) have been recently associated with two forms of muscular dystrophy, one displaying a milder and one a more severe phenotype. This review focuses specifically on the animal (murine and others) model systems that have been developed with the aim of directly engineering DAG1 in order to study the DG function in skeletal muscle as well as in other tissues. In the last years, conditional animal models overcoming the embryonic lethality of the DG knock-out in mouse have been generated and helped clarifying the crucial role of DG in skeletal muscle, while an increasing number of studies on knock-in mice are aimed at understanding the contribution of single amino acids to the stability of DG and to the possible development of muscular dystrophy.

The Structure of the T190M Mutant of Murine α-Dystroglycan at High Resolution: Insight into the Molecular Basis of a Primary Dystroglycanopathy

The severe dystroglycanopathy known as a form of limb-girdle muscular dystrophy (LGMD2P) is an autosomal recessive disease caused by the point mutation T192M in α-dystroglycan. Functional expression analysis in vitro and in vivo indicated that the mutation was responsible for a decrease in posttranslational glycosylation of dystroglycan, eventually interfering with its extracellular-matrix receptor function and laminin binding in skeletal muscle and brain. The X-ray crystal structure of the missense variant T190M of the murine N-terminal domain of α-dystroglycan (50-313) has been determined, and showed an overall topology (Ig-like domain followed by a basket-shaped domain reminiscent of the small subunit ribosomal protein S6) very similar to that of the wild-type structure. The crystallographic analysis revealed a change of the conformation assumed by the highly flexible loop encompassing residues 159-180. Moreover, a solvent shell reorganization around Met190 affects the interaction between the B1-B5 anti-parallel strands forming part of the floor of the basket-shaped domain, with likely repercussions on the folding stability of the protein domain(s) and on the overall molecular flexibility. Chemical denaturation and limited proteolysis experiments point to a decreased stability of the T190M variant with respect to its wild-type counterpart. This mutation may render the entire L-shaped protein architecture less flexible. The overall reduced flexibility and stability may affect the functional properties of α-dystroglycan via negatively influencing its binding behavior to factors needed for dystroglycan maturation, and may lay the molecular basis of the T190M-driven primary dystroglycanopathy.

α-dystroglycan is a potential target of matrix metalloproteinase MMP-2

Dystroglycan (DG) is a member of the glycoprotein complex associated to dystrophin and composed by two subunits, the β-DG, a transmembrane protein, and the α-DG, an extensively glycosylated extracellular protein. The β-DG ectodomain degradation by the matrix metallo-proteinases (i.e., MMP-2 and MMP-9) in both, pathological and physiological conditions, has been characterized in detail in previous publications. Since the amounts of α-DG and β-DG at the cell surface decrease when gelatinases are up-regulated, we investigated the degradation of α-DG subunit by MMP-2. Present data show, for the first time, that the proteolysis of α-DG indeed occurs on a native glycosylated molecule enriched from rabbit skeletal muscle. In order to characterize the α-DG portion, which is more prone to cleavage by MMP-2, we performed different degradations on tailored recombinant domains of α-DG spanning the whole subunit. The overall bulk of results casts light on a relevant susceptibility of the α-DG to MMP-2 degradation with particular reference to its C-terminal domain, thus opening a new scenario on the role of gelatinases (in particular of MMP-2) in the degradation of this glycoprotein complex, taking place in the course of pathological processes.

X-ray structure of the mature ectodomain of phogrin

Phogrin/IA-2β and ICA512/IA-2 are two paralogs receptor-type protein-tyrosine phosphatases (RPTP) that localize in secretory granules of various neuroendocrine cells. In pancreatic islet β-cells, they participate in the regulation of insulin secretion, ensuring proper granulogenesis, and β-cell proliferation. The role of their cytoplasmic tail has been partially unveiled, while that of their luminal region remains unclear. To advance the understanding of its structure-function relationship, the X-ray structure of the mature ectodomain of phogrin (ME phogrin) at pH 7.4 and 4.6 has been solved at 1.95- and 2.01-Å resolution, respectively. Similarly to the ME of ICA512, ME phogrin adopts a ferredoxin-like fold: a sheet of four antiparallel β-strands packed against two α-helices. Sequence conservation among vertebrates, plants and insects suggests that the structural similarity extends to all the receptor family. Crystallized ME phogrin is monomeric, in agreement with solution studies but in striking contrast with the behavior of homodimeric ME ICA512. The structural details that may cause the quaternary structure differences are analyzed. The results provide a basis for building models of the overall orientation and oligomerization state of the receptor in biological membranes.

Insights from molecular dynamics simulations: structural basis for the V567D mutation-induced instability of zebrafish alpha-dystroglycan and comparison with the murine model

A missense amino acid mutation of valine to aspartic acid in 567 position of alpha-dystroglycan (DG), identified in dag1-mutated zebrafish, results in a reduced transcription and a complete absence of the protein. Lacking experimental structural data for zebrafish DG domains, the detailed mechanism for the observed mutation-induced destabilization of the DG complex and membrane damage, remained unclear. With the aim to contribute to a better clarification of the structure-function relationships featuring the DG complex, three-dimensional structural models of wild-type and mutant (V567D) C-terminal domain of alpha-DG from zebrafish were constructed by a template-based modelling approach. We then ran extensive molecular dynamics (MD) simulations to reveal the structural and dynamic properties of the C-terminal domain and to evaluate the effect of the single mutation on alpha-DG stability. A comparative study has been also carried out on our previously generated model of murine alpha-DG C-terminal domain including the I591D mutation, which is topologically equivalent to the V567D mutation found in zebrafish. Trajectories from MD simulations were analyzed in detail, revealing extensive structural disorder involving multiple beta-strands in the mutated variant of the zebrafish protein whereas local effects have been detected in the murine protein. A biochemical analysis of the murine alpha-DG mutant I591D confirmed a pronounced instability of the protein. Taken together, the computational and biochemical analysis suggest that the V567D/I591D mutation, belonging to the G beta-strand, plays a key role in inducing a destabilization of the alpha-DG C-terminal Ig-like domain that could possibly affect and propagate to the entire DG complex. The structural features herein identified may be of crucial help to understand the molecular basis of primary dystroglycanopathies.

Hypoglycosylation of dystroglycan due to T192M mutation: a molecular insight behind the fact

Abnormal glycosylation of dystroglycan (DG), a transmembrane glycoprotein, results in a group of diseases known as dystroglycanopathy. A severe dystroglycanopathy known as the limb girdle disease MDDGC9 [OMIM: 613818] occurs as a result of hypoglycosylation of alpha subunit of DG. Reasons behind this has been traced back to a point mutation (T192M) in DG that leads to weakening of interactions of DG protein with laminin and subsequent loss of signal flow through the DG protein. In this work we have tried to analyze the molecular details of the interactions between DG and laminin1 in order to propose a mechanism about the onset of the disease MDDGC9. We have observed noticeable changes between the modeled structures of wild type and mutant DG proteins. We also have employed molecular docking techniques to study and compare the binding interactions between laminin1 and both the wild type and mutant DG proteins. The docking simulations have revealed that the mutant DG has weaker interactions with laminin1 as compared to the wild type DG. Till date there are no previous reports that deal with the elucidation of the interactions of DG with laminin1 from the molecular level. Our study is therefore the first of its kind which analyzes the differences in binding patterns of laminin1 with both the wild type and mutant DG proteins. Our work would therefore facilitate analysis of the molecular mechanism of the disease MDDGC9. Future work based on our results may be useful for the development of suitable drugs against this disease.

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.

Recent advances in the molecular pathogenesis of dystonia-plus syndromes and heredodegenerative dystonias

The majority of studies investigating the molecular pathogenesis and cell biology underlying dystonia have been performed in individuals with primary dystonia. This includes monogenic forms such as DYT1and DYT6 dystonia, and primary focal dystonia which is likely to be multifactorial in origin. In recent years there has been renewed interest in non-primary forms of dystonia including the dystonia-plus syndromes and heredodegenerative disorders. These are caused by a variety of genetic mutations and their study has contributed to our understanding of the neuronal dysfunction that leads to dystonia These findings have reinforced themes identified from study of primary dystonia including abnormal dopaminergic signalling, cellular trafficking and mitochondrial function. In this review we highlight recent advances in the understanding of the dystonia-plus syndromes and heredodegenerative dystonias.

New insights into the evolution of metazoan cadherins and catenins

E-Cadherin and β-catenin are the best studied representatives of the superfamilies of transmembrane cadherins and intracellular armadillo catenins, respectively. However, in over 600 million years of multicellular animal evolution, these two superfamilies have diversified remarkably both structurally and functionally. Although their basic building blocks, respectively, the cadherin repeat domain and the armadillo repeat domain, predate metazoans, the specific and complex domain compositions of the different family members and their functional roles in cell adhesion and signaling appear to be key features for the emergence of multicellular animal life. Basal animals such as placozoans and sponges have a limited number of distinct cadherins and catenins. The origin of vertebrates, in particular, coincided with a large increase in the number of cadherins and armadillo proteins, including modern "classical" cadherins, protocadherins, and plakophilins. Also, α-catenins increased. This chapter introduces the many different family members and describes the putative evolutionary relationships between them.

C. elegans dystroglycan coordinates responsiveness of follower axons to dorsal/ventral and anterior/posterior guidance cues

Neural development in metazoans is characterized by the establishment of initial process tracts by pioneer axons and the subsequent extension of follower axons along these pioneer processes. Mechanisms governing the fidelity of follower extension along pioneered routes are largely unknown. In C. elegans, formation of the right angle-shaped lumbar commissure connecting the lumbar and preanal ganglia is an example of pioneer/follower dynamics. We find that the dystroglycan ortholog DGN-1 mediates the fidelity of follower lumbar commissure axon extension along the pioneer axon route. In dgn-1 mutants, the axon of the pioneer PVQ neuron faithfully establishes the lumbar commissure, but axons of follower lumbar neurons, such as PVC, frequently bypass the lumbar commissure and extend along an oblique trajectory directly toward the preanal ganglion. In contrast, disruption of the UNC-6/netrin guidance pathway principally perturbs PVQ ventral guidance to pioneer the lumbar commissure. Loss of DGN-1 in unc-6 mutants has a quantitatively similar effect on follower axon guidance regardless of PVQ axon route, indicating that DGN-1 does not mediate follower/pioneer adhesion. Instead, DGN-1 appears to block premature responsiveness of follower axons to a preanal ganglion-directed guidance cue, which mediates ventral-to-anterior reorientation of lumbar commissure axons. Deletion analysis shows that only the most N-terminal DGN-1 domain is required for these activities. These studies suggest that dystroglycan modulation of growth cone responsiveness to conflicting guidance cues is important for restricting follower axon extension to the tracts laid down by pioneers.

Insertion of a myc-tag within α-dystroglycan domains improves its biochemical and microscopic detection

BACKGROUND

Epitope tags and fluorescent fusion proteins have become indispensable molecular tools for studies in the fields of biochemistry and cell biology. The knowledge collected on the subdomain organization of the two subunits of the adhesion complex dystroglycan (DG) enabled us to insert the 10 amino acids myc-tag at different locations along the α-subunit, in order to better visualize and investigate the DG complex in eukaryotic cells.

RESULTS

We have generated two forms of DG polypeptides via the insertion of the myc-tag 1) within a flexible loop (between a.a. 170 and 171) that separates two autonomous subdomains, and 2) within the C-terminal domain in position 500. Their analysis showed that double-tagging (the β-subunit is linked to GFP) does not significantly interfere with the correct processing of the DG precursor (pre-DG) and confirmed that the α-DG N-terminal domain is processed in the cell before α-DG reaches its plasma membrane localization. In addition, myc insertion in position 500, right before the second Ig-like domain of α-DG, proved to be an efficient tool for the detection and pulling-down of glycosylated α-DG molecules targeted at the membrane.

CONCLUSIONS

Further characterization of these and other myc-permissive site(s) will represent a valid support for the study of the maturation process of pre-DG and could result in the creation of a new class of intrinsic doubly-fluorescent DG molecules that would allow the monitoring of the two DG subunits, or of pre-DG, in cells without the need of antibodies.

DAG1, no gene for RNA regulation?

DAG1 encodes for a precursor protein that liberates the two subunits featured by the dystroglycan (DG) adhesion complex that are involved in an increasing number of cellular functions in a wide variety of cells and tissues. Aside from the proteolytic events producing the α and β subunits, especially the former undergoes extensive "post-production" modifications taking place within the ER/Golgi where its core protein is both N- and O-decorated with sugars. These post-translational events, that are mainly orchestrated by a plethora of certified, or putative, glycosyltransferases, prelude to the excocytosis-mediated trafficking and targeting of the DG complex to the plasma membrane. Extensive genetic and biochemical evidences have been accumulated so far on α-DG glycosylation, while little is know on possible regulatory events underlying the chromatine activation, transcription or post-transcription (splicing and escape from the nucleus) of DAG1 or of its mRNA. A scenario is envisaged in which cells would use a sort of preferential, and scarcely regulated, route for DAG1 activation, that would imply fast mRNA transcription, maturation and export to the cytosol, and would prelude to the multiple time-consuming enzymatic post-translational activities needed for its glycosylation. Such a provocative view might be helpful to trigger future work aiming at disclosing the complete molecular mechanisms underlying DAG1 activation and at improving our knowledge of any pre-translational step that is involved in dystroglycan regulation.

Neural maintenance roles for the matrix receptor dystroglycan and the nuclear anchorage complex in Caenorhabditis elegans

Recent studies in Caenorhabditis elegans have revealed specific neural maintenance mechanisms that protect soma and neurites against mispositioning due to displacement stresses, such as muscle contraction. We report that C. elegans dystroglycan (DG) DGN-1 functions to maintain the position of lumbar neurons during late embryonic and larval development. In the absence of DGN-1 the cell bodies of multiple lumbar neuron classes are frequently displaced anterior of their normal positions. Early but not later embryonic panneural expression of DGN-1 rescues positional maintenance, suggesting that dystroglycan is required for establishment of a critical maintenance pathway that persists throughout later developmental stages. Lumbar neural maintenance requires only a membrane-tethered N-terminal domain of DGN-1 and may involve a novel extracellular partner for dystroglycan. A genetic screen for similar lumbar maintenance mutants revealed a role for the nesprin/SYNE family protein ANC-1 as well as for the extracellular protein DIG-1, previously implicated in lumbar neuron maintenance. The involvement of ANC-1 reveals a previously unknown role for nucleus-cytoskeleton interactions in neural maintenance. Genetic analysis indicates that lumbar neuron position is maintained in late embryos by parallel DGN-1/DIG-1 and ANC-1-dependent pathways, and in larvae by separate DGN-1 and ANC-1 pathways. The effect of muscle paralysis on late embryonic- or larval-stage maintenance defects in mutants indicates that lumbar neurons are subject to both muscle contraction-dependent and contraction-independent displacement stresses, and that different maintenance pathways may protect against specific types of displacement stress.

A gain-of-glycosylation mutation associated with myoclonus-dystonia syndrome affects trafficking and processing of mouse ε-sarcoglycan in the late secretory pathway

Missense mutations in the SGCE gene encoding ε-sarcoglycan account for approximately 15% of SGCE-positive cases of myoclonus-dystonia syndrome (MDS) in humans. In this study, we show that while the majority of MDS-associated missense mutants modeled with a murine ε-sarcoglycan cDNA are substrates for endoplasmic reticulum-associated degradation, one mutant, M68T (analogous to human c.275T>C, p.M92T), located in the Ig-like domain of ε-sarcoglycan, results in a gain-of-glycosylation mutation producing a protein that is targeted to the plasma membrane, albeit at reduced levels compared to wild-type ε-sarcoglycan. Removal of the ectopic N-linked glycan failed to restore efficient plasma membrane targeting of M68T demonstrating that the substitution rather than the glycan was responsible for the trafficking defect of this mutant. M68T also colocalized with CD63-positive vesicles in the endosomal-lysosomal system and was found to be more susceptible to lysosomal proteolysis than wild-type ε-sarcoglycan. Finally, we demonstrate impaired ectodomain shedding of M68T, a process that occurs physiologically for ε-sarcoglycan resulting in the lysosomal trafficking of the intracellular C-terminal domain of the protein. Our findings show that functional analysis of rare missense mutations can provide a mechanistic insight into the pathogenesis of MDS and the physiological role of ε-sarcoglycan.

A second Ig-like domain identified in dystroglycan by molecular modelling and dynamics

Dystroglycan (DG) is a cell surface receptor which is composed of two subunits that interact noncovalently, namely α- and β-DG. In skeletal muscle, DG is the central component of the dystrophin-glycoprotein complex (DGC) that anchors the actin cytoskeleton to the extracellular matrix. To date only the three-dimensional structure of the N-terminal region of α-DG has been solved by X-ray crystallography. To expand such a structural analysis, a theoretical molecular model of the murine α-DG C-terminal region was built based on folding recognition/threading techniques. Although there is no a significant (<30%) sequence homology with the N-terminal region of α-DG, protein fold recognition methods found a significant resemblance to the α-DG N-terminal crystallographic structure. Our in silico structural prediction identified two subdomains in this region. Amino acid residues ∼ 500-600 of α-DG were predicted to adopt an immunoglobulin-like (Ig-like) β-sandwich fold. Such modeled domain includes the β-DG binding epitope of α-DG and, confirming our previous experimental results, suggests that the linear epitope (residues 550-565) assumes a β-strand conformation. The remaining segment of the α-DG C-terminal region (residues 601-653) is organized in a coil-helix-coil motif. A 20-ns molecular dynamics simulation in explicit water solvent provided support to the predicted Ig-like model structure. The identification of a second Ig-like domain in DG represents another important step towards a full structural and functional description of the α/β DG interface. Preliminary characterization of a novel recombinant peptide (505-600) encompassing this second Ig-like domain demonstrates that it is soluble and stable, further corroborating our in silico analysis.

A dystroglycan mutation associated with limb-girdle muscular dystrophy

Dystroglycan, which serves as a major extracellular matrix receptor in muscle and the central nervous system, requires extensive O-glycosylation to function. We identified a dystroglycan missense mutation (Thr192→Met) in a woman with limb-girdle muscular dystrophy and cognitive impairment. A mouse model harboring this mutation recapitulates the immunohistochemical and neuromuscular abnormalities observed in the patient. In vitro and in vivo studies showed that the mutation impairs the receptor function of dystroglycan in skeletal muscle and brain by inhibiting the post-translational modification, mediated by the glycosyltransferase LARGE, of the phosphorylated O-mannosyl glycans on α-dystroglycan that is required for high-affinity binding to laminin.

Mutagenesis at the alpha-beta interface impairs the cleavage of the dystroglycan precursor

The interaction between a-dystroglycan (alpha-DG) and beta-dystroglycan (beta-DG), the two constituent subunits of the adhesion complex dystroglycan, is crucial in maintaining the integrity of the dystrophin-glycoprotein complex. The importance of the alpha-beta interface can be seen in the skeletal muscle of humans affected by severe conditions, such as Duchenne muscular dystrophy, where the alpha-beta interaction can be secondarily weakened or completely lost, causing sarcolemmal instability and muscular necrosis. The reciprocal binding epitopes of the two subunits reside within the C-terminus of alpha-DG and the ectodomain of beta-DG. As no ultimate structural data are yet available on the alpha-beta interface, site-directed mutagenesis was used to identify which specific amino acids are involved in the interaction. A previous alanine-scanning analysis of the recombinant beta-DG ectodomain allowed the identification of two phenylalanines important for alpha-DG binding, namely F692 and F718. In this article, similar experiments performed on the alpha-DG C-terminal domain pinpointed two residues, G563 and P565, as possible binding counterparts of the two beta-DG phenylalanines. In 293-Ebna cells, the introduction of alanine residues instead of F692, F718, G563 and P565 prevented the cleavage of the DG precursor that liberates alpha- and beta-DG, generating a pre-DG of about 160 kDa. This uncleaved pre-DG tetramutant is properly targeted at the cell membrane, is partially glycosylated and still binds laminin in pull-down assays. These data reinforce the notion that DG processing and its membrane targeting are two independent processes, and shed new light on the molecular mechanism that drives the maturation of the DG precursor.

Molecular evolution of the cadherin superfamily

This review deals with the large and pleiotropic superfamily of cadherins and its molecular evolution. We compiled literature data and an in-depth phylogenetic analysis of more than 350 members of this superfamily from about 30 species, covering several but not all representative branches within metazoan evolution. We analyzed the sequence homology between either ectodomains or cytoplasmic domains, and we reviewed protein structural data and genomic architecture. Cadherins and cadherin-related molecules are defined by having an ectodomain in which at least two consecutive calcium-binding cadherin repeats are present. There are usually 5 or 6 domains, but in some cases as many as 34. Additional protein modules in the ectodomains point at adaptive evolution. Despite the occurrence of several conserved motifs in subsets of cytoplasmic domains, these domains are even more diverse than ectodomains and most likely have evolved separately from the ectodomains. By fine tuning molecular classifications, we reduced the number of solitary superfamily members. We propose a cadherin major branch, subdivided in two families and 8 subfamilies, and a cadherin-related major branch, subdivided in four families and 11 subfamilies. Accordingly, we propose a more appropriate nomenclature. Although still fragmentary, our insight into the molecular evolution of these remarkable proteins is steadily growing. Consequently, we can start to propose testable hypotheses for structure-function relationships with impact on our models of molecular evolution. An emerging concept is that the ever evolving diversity of cadherin structures is serving dual and important functions: specific cell adhesion and intricate cell signaling.

Processing and secretion of the N-terminal domain of alpha-dystroglycan in cell culture media

Alpha-dystroglycan (alpha-DG) plays a crucial role in maintaining the stability of muscle cell membrane. Although it has been shown that the N-terminal domain of alpha-DG (alpha-DG-N) is cleaved by a proprotein convertase, its physiological significance remains unclear. We show here that native alpha-DG-N is secreted by a wide variety of cultured cells into the culture media. The secreted alpha-DG-N was both N- and O-glycosylated. Finally, a small amount of alpha-DG-N was detectable in the normal human serum. These observations indicate that the cleavage of alpha-DG-N is a widespread event and suggest that the secreted alpha-DG-N might be transported via systemic circulation in vivo.

Biology of the striated muscle dystrophin-glycoprotein complex

Since its first description in 1990, the dystrophin-glycoprotein complex has emerged as a critical nexus for human muscular dystrophies arising from defects in a variety of distinct genes. Studies in mammals widely support a primary role for the dystrophin-glycoprotein complex in mechanical stabilization of the plasma membrane in striated muscle and provide hints for secondary functions in organizing molecules involved in cellular signaling. Studies in model organisms confirm the importance of the dystrophin-glycoprotein complex for muscle cell viability and have provided new leads toward a full understanding of its secondary roles in muscle biology.

Dystroglycan: a possible mediator for reducing congenital muscular dystrophy?

Alpha-dystroglycan is a highly glycosylated peripheral protein forming a complex with the membrane-spanning beta-dystroglycan and establishing a connection between the extracellular matrix and the cytoskeleton. In skeletal muscle, as part of the larger dystrophin-glycoprotein complex, dystroglycan is believed to be essential for maintaining the structural and functional stability of muscle fibers. Recent work highlights the role of abnormal dystroglycan glycosylation at the basis of glycosyltransferase-deficient congenital muscular dystrophies. Notably, modulation of glycosyltransferase activity can restore alpha-dystroglycan receptor function in these disorders. Moreover, transgenic approaches favoring the interaction between dystroglycan and the extracellular matrix molecules also represent an innovative way to restore skeletal muscle structure. These pioneering approaches might comprise an important first step towards the design of gene-transfer-based strategies for the rescue of congenital muscular dystrophies involving dystroglycan.

Duplication of the dystroglycan gene in most branches of teleost fish

Background

The dystroglycan (DG) complex is a major non-integrin cell adhesion system whose multiple biological roles involve, among others, skeletal muscle stability, embryonic development and synapse maturation. DG is composed of two subunits: α-DG, extracellular and highly glycosylated, and the transmembrane β-DG, linking the cytoskeleton to the surrounding basement membrane in a wide variety of tissues. A single copy of the DG gene (DAG1) has been identified so far in humans and other mammals, encoding for a precursor protein which is post-translationally cleaved to liberate the two DG subunits. Similarly, D. rerio (zebrafish) seems to have a single copy of DAG1, whose removal was shown to cause a severe dystrophic phenotype in adult animals, although it is known that during evolution, due to a whole genome duplication (WGD) event, many teleost fish acquired multiple copies of several genes (paralogues).

Results

Data mining of pufferfish (T. nigroviridis and T. rubripes) and other teleost fish (O. latipes and G. aculeatus) available nucleotide sequences revealed the presence of two functional paralogous DG sequences. RT-PCR analysis proved that both the DG sequences are transcribed in T. nigroviridis. One of the two DG sequences harbours an additional mini-intronic sequence, 137 bp long, interrupting the uncomplicated exon-intron-exon pattern displayed by DAG1 in mammals and D. rerio. A similar scenario emerged also in D. labrax (sea bass), from whose genome we have cloned and sequenced a new DG sequence that also harbours a shorter additional intronic sequence of 116 bp. Western blot analysis confirmed the presence of DG protein products in all the species analysed including two teleost Antarctic species (T. bernacchii and C. hamatus).

Conclusion

Our evolutionary analysis has shown that the whole-genome duplication event in the Class Actinopterygii (ray-finned fish) involved also DAG1. We unravelled new important molecular genetic details about fish orthologous DGs, which might help to increase the current knowledge on DG expression, maturation and targeting and on its physiopathological role in higher organisms.

Concerted mutation of Phe residues belonging to the ?-dystroglycan ectodomain strongly inhibits the interaction with ?-dystroglycan in�vitro

The dystroglycan adhesion complex consists of two noncovalently interacting proteins: alpha-dystroglycan, a peripheral extracellular subunit that is extensively glycosylated, and the transmembrane beta-dystroglycan, whose cytosolic tail interacts with dystrophin, thus linking the F-actin cytoskeleton to the extracellular matrix. Dystroglycan is thought to play a crucial role in the stability of the plasmalemma, and forms strong contacts between the extracellular matrix and the cytoskeleton in a wide variety of tissues. Abnormal membrane targeting of dystroglycan subunits and/or their aberrant post-translational modification are often associated with several pathologic conditions, ranging from neuromuscular disorders to carcinomas. A putative functional hotspot of dystroglycan is represented by its intersubunit surface, which is contributed by two amino acid stretches: approximately 30 amino acids of beta-dystroglycan (691-719), and approximately 15 amino acids of alpha-dystroglycan (550-565). Exploiting alanine scanning, we have produced a panel of site-directed mutants of our two consolidated recombinant peptides beta-dystroglycan (654-750), corresponding to the ectodomain of beta-dystroglycan, and alpha-dystroglycan (485-630), spanning the C-terminal domain of alpha-dystroglycan. By solid-phase binding assays and surface plasmon resonance, we have determined the binding affinities of mutated peptides in comparison to those of wild-type alpha-dystroglycan and beta-dystroglycan, and shown the crucial role of two beta-dystroglycan phenylalanines, namely Phe692 and Phe718, for the alpha-beta interaction. Substitution of the alpha-dystroglycan residues Trp551, Phe554 and Asn555 by Ala does not affect the interaction between dystroglycan subunits in vitro. As a preliminary analysis of the possible effects of the aforementioned mutations in vivo, detection through immunofluorescence and western blot of the two dystroglycan subunits was pursued in dystroglycan-transfected 293-Ebna cells.

C. elegans dystroglycan DGN-1 functions in epithelia and neurons, but not muscle, and independently of dystrophin

The C. elegans dystroglycan (DG) homolog DGN-1 is expressed in epithelia and neurons, and localizes to basement membrane (BM) surfaces. Unlike vertebrate DG, DGN-1 is not expressed in muscle or required for muscle function. dgn-1 null mutants are viable but sterile owing to severe disorganization of the somatic gonad epithelium, and show defects in vulval and excretory cell epithelia and in motoneuron axon guidance. The defects resemble those of epi-1 laminin alphaB mutants, suggesting that DGN-1 serves as a receptor for laminin. dgn-1(0)/+ animals are fertile but show gonad migration defects in addition to the defects seen in homozygotes, indicating that DGN-1 function is dosage sensitive. Phenotypic analyses show that DGN-1 and dystrophin-associated protein complex (DAPC) components have distinct and independent functions, in contrast to the situation in vertebrate muscle. The DAPC-independent functions of DGN-1 in epithelia and neurons suggest that vertebrate DG may also act independently of dystrophin/utrophin in non-muscle tissues.

The structure of Leishmania mexicana ICP provides evidence for convergent evolution of cysteine peptidase inhibitors

Clan CA, family C1 cysteine peptidases (CPs) are important virulence factors and drug targets in parasites that cause neglected diseases. Natural CP inhibitors of the I42 family, known as ICP, occur in some protozoa and bacterial pathogens but are absent from metazoa. They are active against both parasite and mammalian CPs, despite having no sequence similarity with other classes of CP inhibitor. Recent data suggest that Leishmania mexicana ICP plays an important role in host-parasite interactions. We have now solved the structure of ICP from L. mexicana by NMR and shown that it adopts a type of immunoglobulin-like fold not previously reported in lower eukaryotes or bacteria. The structure places three loops containing highly conserved residues at one end of the molecule, one loop being highly mobile. Interaction studies with CPs confirm the importance of these loops for the interaction between ICP and CPs and suggest the mechanism of inhibition. Structure-guided mutagenesis of ICP has revealed that residues in the mobile loop are critical for CP inhibition. Data-driven docking models support the importance of the loops in the ICP-CP interaction. This study provides structural evidence for the convergent evolution from an immunoglobulin fold of CP inhibitors with a cystatin-like mechanism.

O Mannosylation of alpha-dystroglycan is essential for lymphocytic choriomeningitis virus receptor function

Alpha-dystroglycan (alpha-DG) was identified as a common receptor for lymphocytic choriomeningitis virus (LCMV) and several other arenaviruses including the human pathogenic Lassa fever virus. Initial work postulated that interactions between arenavirus glycoproteins and alpha-DG are based on protein-protein interactions. We found, however, that susceptibility toward LCMV infection differed in various cell lines despite them expressing comparable levels of DG, suggesting that posttranslational modifications of alpha-DG would be involved in viral receptor function. Here, we demonstrate that glycosylation of alpha-DG, and in particular, O mannosylation, which is a rare type of O-linked glycosylation in mammals, is essential for LCMV receptor function. Cells that are defective in components of the O-mannosylation pathway showed strikingly reduced LCMV infectibility. As defective O mannosylation is associated with severe clinical symptoms in mammals such as congenital muscular dystrophies, it is likely that LCMV and potentially other arenaviruses may have selected this conserved and crucial posttranslational modification as the primary target structure for cell entry and infection.

Alpha-dystroglycan, the usual suspect?

An increasing number of congenital muscular dystrophies might originate from genetic abnormalities of glycosyltransferases genes which are believed to target the alpha subunit of the dystroglycan (DG) adhesion complex as their major enzymatic substrate. alpha-DG is highly glycosylated and peripherally associated with the sarcolemma of skeletal muscle and the plasma membrane in a wide variety of cells. Several lines of evidence indicate that alpha-DG hypoglycosylation might represent the primary molecular event characterizing congenital dystrophies, since it is likely to alter alpha-DG high-affinity binding to laminin and other extracellular molecules, thus negatively influencing the basement-membrane/cytoskeleton axis and eventually leading to sarcolemmal instability, infiltration of myofibers and congenital weakness. For this reason, congenital diseases such as Walker-Warburg Syndrome or Muscle-Eye-Brain disease, have been recently denominated 'secondary dystroglycanopathies'. However, some crucial points need to be fully addressed in order to finally assess the degree of involvement of alpha-DG in congenital muscular diseases, for example: the possibility that mutations hitting the DG gene might lead to primary dystroglycanopathies; the putative functional or pathological role of hypoglycosylated - or even hyperglycosylated - alpha-DG molecules; or also the compensatory role played by the recently identified paralogue glycosyltransferases in alpha-DG sugar decoration.

CHDL: A cadherin-like domain in Proteobacteria and Cyanobacteria

We identified a cadherin-like domain (CHDL) using computational analysis. The CHDL domain is mostly distributed in Proteobacteria and Cyanobacteria, although it is also found in some eukaryotic proteins. Prediction of three-dimensional protein folding indicated that the CHDL domain has an immunoglobulin beta-sandwich fold and belongs to the cadherin superfamily. The CHDL domain does not have LDRE and DxNDN motifs, which are conserved in the cadherin domain, but has three other motifs: PxAxxD, DxDxD and YT-V/I-S/T-D, which might contribute to forming a calcium-binding site. The identification of this cadherin-like domain indicates that the cadherin superfamily may exhibit wider sequence and structural diversity than previously appreciated. Domain architecture analysis revealed that the CHDL domain is also associated with other adhesion domains as well as enzyme domains. Based on computational analysis and previous experimental data, we predict that the CHDL domain has calcium-binding and also carbohydrate-binding activity.

The dystroglycan complex: From biology to cancer

Dystroglycan (DG), a non-integrin adhesion molecule, is a pivotal component of the dystrophin-glycoprotein complex, that is expressed in skeletal muscle and in a wide variety of tissues at the interface between the basement membrane (BM) and the cell membrane. DG has been mainly studied for its role in skeletal muscle cell stability and its alterations in muscular diseases, such as dystrophies. However, accumulating evidence have implicated DG in a variety of other biological functions, such as maturation of post-synaptic elements in the central and peripheral nervous system, early morphogenesis, and infective pathogens targeting. Moreover, DG has been reported to play a role in regulating cytoskeletal organization, cell polarization, and cell growth in epithelial cells. Recent studies also indicate that abnormalities in the expression of DG frequently occur in human cancers and may play a role in both the process of tumor progression and in the maintenance of the malignant phenotype. This paper reviews the available information on the biology of DG, the abnormalities found in human cancers, and the implications of these findings with respect to our understanding of cancer pathogenesis and to the development of novel strategies for a better management of cancer patients.