An Extracellular Pathway for Dystroglycan Function in Acetylcholine Receptor Aggregation and Laminin Deposition in Skeletal Myotubes

Agrin plays a major role in the coalescence of the aquaporin-4 clusters induced by gamma-1-containing laminin

The basement membrane that seperates the endothelial cells and astrocytic endfeet that comprise the blood-brain barrier is rich in collagen, laminin, agrin, and perlecan. Previous studies have demonstrated that the proper recruitment of the water-permeable channel aquaporin-4 (AQP4) to astrocytic endfeet is dependent on interactions between laminin and the receptor dystroglycan. In this study, we conducted a deeper investigation into how the basement membrane might further regulate the expression, localization, and function of AQP4, using primary astrocytes as a model system. We found that treating these cells with laminin causes endogenous agrin to localize to the cell surface, where it co-clusters with β-dystroglycan (β-DG). Conversely, agrin sliencing profoundly disrupts β-DG clustering. As in the case of laminin111, Matrigel™, a complete basement membrane analog, also causes the clustering of AQP4 and β-DG. This clustering, whether induced by laminin111 or Matrigel™ is inhibited when the astrocytes are first incubated with an antibody against the γ1 subunit of laminin, suggesting that the latter is crucial to the process. Finally, we showed that laminin111 appears to negatively regulate AQP4-mediated water transport in astrocytes, suppressing the cell swelling that occurs following a hypoosmotic challenge. This suppression is abolished if DG expression is silenced, again demonstrating the central role of this receptor in relaying the effects of laminin.

Conditional Inactivation of Nf1 and Pten in Schwann Cells Results in Abnormal Neuromuscular Junction Maturation

The neuromuscular junction (NMJ) consists of three components, namely presynaptic motor neurons, postsynaptic muscle fibers and perisynaptic Schwann cells (PSCs). The role of Schwann cells (SCs) in regulating NMJ structural and functional development remains unclear. In this study, mice with conditional inactivation of neurofibromin 1 (Nf1) and phosphatase and tensin homolog (Pten), specifically in SCs, resulted in delayed NMJ maturation that led to delayed muscle growth, recapitulating the muscular dystrophy condition observed in human neurofibromatosis type I syndrome (NF1) patients. Expression levels of NMJ development related molecules such as cholinergic receptor, nicotinic, alpha polypeptide 1 (Chrna1), agrin (Agrn), dystrophin, muscular dystrophy (Dmd), laminin, beta 2 (Lamb2) and dystroglycan 1 (Dag1) were also downregulated. To further explore the molecular alterations in these SCs, NF1- and PTEN-related pathways were analyzed in mutant sciatic nerves. As expected, hyperactive RAS/PI3K/AKT/mTOR signaling pathways were identified, suggesting the importance of these pathways for NMJ development, and subsequent muscle maturation.

Making muscle: skeletal myogenesis in vivo and in vitro

Skeletal muscle is the largest tissue in the body and loss of its function or its regenerative properties results in debilitating musculoskeletal disorders. Understanding the mechanisms that drive skeletal muscle formation will not only help to unravel the molecular basis of skeletal muscle diseases, but also provide a roadmap for recapitulating skeletal myogenesis in vitro from pluripotent stem cells (PSCs). PSCs have become an important tool for probing developmental questions, while differentiated cell types allow the development of novel therapeutic strategies. In this Review, we provide a comprehensive overview of skeletal myogenesis from the earliest premyogenic progenitor stage to terminally differentiated myofibers, and discuss how this knowledge has been applied to differentiate PSCs into muscle fibers and their progenitors in vitro.

Laminin is instructive and calmodulin dependent kinase II is non-permissive for the formation of complex aggregates of acetylcholine receptors on myotubes in culture

Previous work has shown that myotubes cultured on laminin-coated substrates form complex aggregates of synaptic proteins that are similar in shape and composition to neuromuscular junctions (NMJs). Here we show that laminin instructs the location of complex aggregates which form only on the lower surface when laminin is coated onto culture dishes but over the entire cell when laminin is added in solution. Silencing of myotubes by agents that block electrical activity (tetrodotoxin, verapamil) or by inhibitors of calmodulin dependent kinase (CaMKII) render the myotube permissive for the formation of complex aggregates. Treatment with laminin alone will facilitate the formation of complex aggregates hours later when myotubes are made permissive by inhibiting CaMKII. The AChR agonist carbachol disperses pre formed aggregates suggesting that non-permissiveness may involve active dispersal of AChRs. The permissive period requires ongoing protein synthesis. The latter may reflect a requirement for rapsyn, which turns over rapidly, and is necessary for aggregation. Consistent with this geldanamycin, an agent that increases rapsyn turnover disrupts complex aggregates. Agrin is well known to induce small clusters of AChRs but does not induce complex aggregates even though aggregate formation requires MuSK, a receptor tyrosine kinase activated by agrin. Dystroglycan (DG) is the major laminin receptor mediating complex aggregate formation with some contribution from β1 integrins. In addition, there is a pool of CaMKII associated with DG. We discuss how these permissive and instructive mechanisms bear on NMJ formation in vivo.

Integrins regulate centrosome integrity and astrocyte polarization following a wound

In response to a wound, astrocytes in culture extend microtubule-rich processes and polarize, orienting their centrosomes and Golgi apparatus woundside. β1 Integrin null astrocytes fail to extend processes toward the wound, and are disoriented, and often migrate away orthogonal, to the wound. The centrosome is unusually fragmented in β1 integrin null astrocytes. Expression of a β1 integrin cDNA in the null background yields cells with intact centrosomes that polarize and extend processes normally. Fragmented centrosomes rapidly assemble following integrin ligation and cell attachment. However, several experiments indicated that cell adhesion is not necessary. For example, astrocytes in suspension expressing a chimeric β1 subunit that can be activated by an antibody assemble centrosomes suggesting that β1 activation is sufficient to cause centrosome assembly in the absence of cell adhesion. siRNA knockdown of PCM1, a major centrosomal protein, inhibits cell polarization, consistent with the notion that centrosomes are necessary for polarity and that integrins regulate polarity via centrosome integrity. Screening inhibitors of molecules downstream of integrins indicate that neither FAK nor ILK is involved in regulation of centrosome integrity. In contrast, blebbistatin, a specific inhibitor of non-muscle myosin II (NMII), mimics the response of β1 integrin null astrocytes by disrupting centrosome integrity and cell polarization. Blebbistatin also inhibits integrin-mediated centrosome assembly in astrocytes attaching to fibronectin, consistent with the hypothesis that NMII functions downstream of integrins in regulating centrosome integrity.

Identification of new dystroglycan complexes in skeletal muscle

The dystroglycan complex contains the transmembrane protein β-dystroglycan and its interacting extracellular mucin-like protein α-dystroglycan. In skeletal muscle fibers, the dystroglycan complex plays an important structural role by linking the cytoskeletal protein dystrophin to laminin in the extracellular matrix. Mutations that affect any of the proteins involved in this structural axis lead to myofiber degeneration and are associated with muscular dystrophies and congenital myopathies. Because loss of dystrophin in Duchenne muscular dystrophy (DMD) leads to an almost complete loss of dystroglycan complexes at the myofiber membrane, it is generally assumed that the vast majority of dystroglycan complexes within skeletal muscle fibers interact with dystrophin. The residual dystroglycan present in dystrophin-deficient muscle is thought to be preserved by utrophin, a structural homolog of dystrophin that is up-regulated in dystrophic muscles. However, we found that dystroglycan complexes are still present at the myofiber membrane in the absence of both dystrophin and utrophin. Our data show that only a minority of dystroglycan complexes associate with dystrophin in wild type muscle. Furthermore, we provide evidence for at least three separate pools of dystroglycan complexes within myofibers that differ in composition and are differentially affected by loss of dystrophin. Our findings indicate a more complex role of dystroglycan in muscle than currently recognized and may help explain differences in disease pathology and severity among myopathies linked to mutations in DAPC members.

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.

Stress and muscular dystrophy: a genetic screen for dystroglycan and dystrophin interactors in Drosophila identifies cellular stress response components

In Drosophila, like in humans, Dystrophin Glycoprotein Complex (DGC) deficiencies cause a life span shortening disease, associated with muscle dysfunction. We performed the first in vivo genetic interaction screen in ageing dystrophic muscles and identified genes that have not been shown before to have a role in the development of muscular dystrophy and interact with dystrophin and/or dystroglycan. Mutations in many of the found interacting genes cause age-dependent morphological and heat-induced physiological defects in muscles, suggesting their importance in the tissue. Majority of them is phylogenetically conserved and implicated in human disorders, mainly tumors and myopathies. Functionally they can be divided into three main categories: proteins involved in communication between muscle and neuron, and interestingly, in mechanical and cellular stress response pathways. Our data show that stress induces muscle degeneration and accelerates age-dependent muscular dystrophy. Dystrophic muscles are already compromised; and as a consequence they are less adaptive and more sensitive to energetic stress and to changes in the ambient temperature. However, only dystroglycan, but not dystrophin deficiency causes extreme myodegeneration induced by energetic stress suggesting that dystroglycan might be a component of the low-energy pathway and act as a transducer of energetic stress in normal and dystrophic muscles.

Nuclear translocation of beta-dystroglycan reveals a distinctive trafficking pattern of autoproteolyzed mucins

Dystroglycan (DG) is an extracellular matrix receptor implicated in muscular dystrophies and cancers. DG belongs to the membrane-tethered mucin family and is composed of extracellular (alpha-DG) and transmembrane (beta-DG) subunits stably coupled at the cell surface. These two subunits are generated by autoproteolysis of a monomeric precursor within a distinctive protein motif called sea urchin-enterokinase-agrin (SEA) domain, yet the purpose of this cleavage and heterodimer creation is uncertain. In this study, we identify a functional nuclear localization signal within beta-DG and show that, in addition to associating with alpha-DG at the cell surface, the full-length and glycosylated beta-DG autonomously traffics to the cytoplasm and nucleoplasm in a process that occurs independent of alpha-DG ligand binding. The trafficking pattern of beta-DG mirrors that of MUC1-C, the transmembrane subunit of the related MUC1 oncoprotein, also a heterodimeric membrane-tethered mucin created by SEA autoproteolysis. We show that the transmembrane subunits of both MUC1 and DG transit the secretory pathway prior to nuclear targeting and that their monomeric precursors maintain the capacity for nuclear trafficking. A screen of breast carcinoma cell lines of distinct pathophysiological origins revealed considerable variability in the nuclear partitioning of beta-DG, indicating that nuclear localization of beta-DG is regulated, albeit independent of extracellular ligand binding. These findings point to novel intracellular functions for beta-DG, with possible disease implications. They also reveal an evolutionarily conserved role for SEA autoproteolysis, serving to enable independent functions of mucin transmembrane subunits, enacted by a shared and poorly understood pathway of segregated subunit trafficking.

Integrins and dystroglycan regulate astrocyte wound healing: the integrin beta1 subunit is necessary for process extension and orienting the microtubular network

Monolayers of astrocytes in culture respond to a scrape wound by orienting towards the wound and extending processes that will repair it. We show here that they also upregulate the expression of extracellular matrix (ECM) proteins, laminin, and chondroitin sulfated proteoglycan, that are deposited in astrocytic scars in vivo. We have previously shown that the major functional ECM receptors on astrocytes are dystroglycan (DG) plus integrins alpha1beta1, alpha5beta1, alpha6beta1, and alphavbeta3. Consistent with this, laminin fragments that activate alpha1beta1 integrin, alpha6beta1 integrin, and DG all contribute to attachment. During astrocyte attachment, or process extension, integrins and DG are found at the leading edge of the lammelipodium, though they change in distribution with the extent of attachment and the alpha and beta subunits of DG can be spatially uncoupled. Functionally, inhibitory antibodies to DG and integrin alpha1beta1 or the RGD peptide all inhibit process extension, showing that ligand engagement of integrins and DG contribute to process extension. Astrocytes differentiated from DG or beta1 null ES cells respond very differently to wounding. The former fail to extend process and cell polarization is disrupted partially. However, beta1 null astrocytes not only fail to extend processes perpendicular to the wound, but cell polarization is completely disrupted and cells migrate randomly into the wound. We conclude that integrins are essential for astrocyte polarity.

Muscle dystroglycan organizes the postsynapse and regulates presynaptic neurotransmitter release at the Drosophila neuromuscular junction

Background

The Dystrophin-glycoprotein complex (DGC) comprises dystrophin, dystroglycan, sarcoglycan, dystrobrevin and syntrophin subunits. In muscle fibers, it is thought to provide an essential mechanical link between the intracellular cytoskeleton and the extracellular matrix and to protect the sarcolemma during muscle contraction. Mutations affecting the DGC cause muscular dystrophies. Most members of the DGC are also concentrated at the neuromuscular junction (NMJ), where their deficiency is often associated with NMJ structural defects. Hence, synaptic dysfunction may also intervene in the pathology of dystrophic muscles. Dystroglycan is a central component of the DGC because it establishes a link between the extracellular matrix and Dystrophin. In this study, we focused on the synaptic role of Dystroglycan (Dg) in Drosophila.

Methodology/Principal Findings

We show that Dg was concentrated postsynaptically at the glutamatergic NMJ, where, like in vertebrates, it controls the concentration of synaptic Laminin and Dystrophin homologues. We also found that synaptic Dg controlled the amount of postsynaptic 4.1 protein Coracle and alpha-Spectrin, as well as the relative subunit composition of glutamate receptors. In addition, both Dystrophin and Coracle were required for normal Dg concentration at the synapse. In electrophysiological recordings, loss of postsynaptic Dg did not affect postsynaptic response, but, surprisingly, led to a decrease in glutamate release from the presynaptic site.

Conclusion/Significance

Altogether, our study illustrates a conservation of DGC composition and interactions between Drosophila and vertebrates at the synapse, highlights new proteins associated with this complex and suggests an unsuspected trans-synaptic function of Dg.

SEA domain proteolysis determines the functional composition of dystroglycan

Post-translational modifications of the extracellular matrix receptor dystroglycan (DG) determine its functional state, and defects in these modifications are linked to muscular dystrophies and cancers. A prominent feature of DG biosynthesis is a precursor cleavage that segregates the ligand-binding and transmembrane domains into the noncovalently attached alpha- and beta-subunits. We investigate here the structural determinants and functional significance of this cleavage. We show that cleavage of DG elicits a conspicuous change in its ligand-binding activity. Mutations that obstruct this cleavage result in increased capacity to bind laminin, in part, due to enhanced glycosylation of alpha-DG. Reconstitution of DG cleavage in a cell-free expression system demonstrates that cleavage takes place in the endoplasmic reticulum, providing a suitable regulatory point for later processing events. Sequence and mutational analyses reveal that the cleavage occurs within a full SEA (sea urchin, enterokinase, agrin) module with traits matching those ascribed to autoproteolysis. Thus, cleavage of DG constitutes a control point for the modulation of its ligand-binding properties, with therapeutic implications for muscular dystrophies. We provide a structural model for the cleavage domain that is validated by experimental analysis and discuss this cleavage in the context of mucin protein and SEA domain evolution.

A Putative Src Homology 3 Domain Binding Motif but Not the C-terminal Dystrophin WW Domain Binding Motif Is Required for Dystroglycan Function in Cellular Polarity in Drosophila

The conserved dystroglycan-dystrophin (Dg.Dys) complex connects the extracellular matrix to the cytoskeleton. In humans as well as Drosophila, perturbation of this complex results in muscular dystrophies and brain malformations and in some cases cellular polarity defects. However, the regulation of the Dg.Dys complex is poorly understood in any cell type. We now find that in loss-of-function and overexpression studies more than half (34 residues) of the Dg proline-rich conserved C-terminal regions can be truncated without significantly compromising its function in regulating cellular polarity in Drosophila. Notably, the truncation eliminates the WW domain binding motif at the very C terminus of the protein thought to mediate interactions with dystrophin, suggesting that a second, internal WW binding motif can also mediate this interaction. We confirm this hypothesis by using a sensitive fluorescence polarization assay to show that both WW domain binding sites of Dg bind to Dys in humans (K(d) = 7.6 and 81 microM, respectively) and Drosophila (K(d) = 16 and 46 microM, respectively). In contrast to the large deletion mentioned above, a single proline to an alanine point mutation within a predicted Src homology 3 domain (SH3) binding site abolishes Dg function in cellular polarity. This suggests that an SH3-containing protein, which has yet to be identified, functionally interacts with Dg.

Defective peripheral nerve myelination and neuromuscular junction formation in fukutin-deficient chimeric mice

Dystroglycan is a central component of the dystrophin-glycoprotein complex that links the extracellular matrix with cytoskeleton. Recently, mutations of the genes encoding putative glycosyltransferases were identified in several forms of congenital muscular dystrophies accompanied by brain anomalies and eye abnormalities, and aberrant glycosylation of alpha-dystroglycan has been implicated in their pathogeneses. These diseases are now collectively called alpha-dystroglycanopathy. In this study, we demonstrate that peripheral nerve myelination is defective in the fukutin-deficient chimeric mice, a mouse model of Fukuyama-type congenital muscular dystrophy, which is the most common alpha-dystroglycanopathy in Japan. In the peripheral nerve of these mice, the density of myelinated nerve fibers was significantly decreased and clusters of abnormally large non-myelinated axons were ensheathed by a single Schwann cell, indicating a defect of the radial sorting mechanism. The sugar chain moiety and laminin-binding activity of alpha-dystroglycan were severely reduced, while the expression of beta1-integrin was not altered in the peripheral nerve of the chimeric mice. We also show that the clustering of acetylcholine receptor is defective and neuromuscular junctions are fragmented in appearance in these mice. Expression of agrin and laminin as well as the binding activity of alpha-dystroglycan to these ligands was severely reduced at the neuromuscular junction. These results demonstrate that fukutin plays crucial roles in the myelination of peripheral nerve and formation of neuromuscular junction. They also suggest that defective glycosylation of alpha-dystroglycan may play a role in the impairment of these processes in the deficiency of fukutin.

Dystroglycan expression is reduced during prostate tumorigenesis and is regulated by androgens in prostate cancer cells

Prostate cancer, the most frequently diagnosed cancer in Western men, can display a high variability in term of clinical aggressiveness and prognosis and none of the available markers is able to accurately predict its clinical course. Dystroglycan (DG), a non-integrin adhesion molecule, is a complex formed by two subunits, alpha- and beta-DG, which bind to extracellular matrix molecules and cytoskeleton, respectively. DG expression is frequently reduced in human cancers and has been related to tumor grade and aggressiveness. This study investigated the role of DG in human prostate tumorigenesis and its suitability as a prognostic marker. The expression level of extracellular alpha-DG subunit was frequently reduced in human prostate cancer cell lines and primary tumors and the percentage of positive tumor cells was significantly further decreased in vivo following androgen ablation therapy (median = 1%) compared to pre-treatment samples (median = 28%). A significant relationship was observed between alpha-DG staining on the post-treatment samples and tumor recurrence. A dose- and time-dependent decrease of DG expression also occurred in human prostate cancer cells following treatment with the anti-androgen flutamide. Stable expression of an exogenous DG cDNA in the LNCaP human prostate carcinoma cell line resulted in a marked inhibition of both anchorage-dependent and independent growth and of the in vivo tumorigenicity. These findings confirm and extend previous evidence that disturbances in the function of the DG complex might contribute to the definition of the malignant behavior of prostate cancer cells and suggest that androgens might regulate DG expression in these cells.

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.