Globular domains of agrin are functional units that collaborate to induce acetylcholine receptor clustering

Clinicopathological-genetic features of congenital myasthenic syndrome from a Chinese neuromuscular centre

Congenital myasthenic syndrome (CMS) encompasses a heterogeneous group of inherited disorders affecting nerve transmission across the neuromuscular junction. The aim of this study was to characterize the clinical, physiological, pathohistological and genetic features of nine unrelated Chinese patients with CMS from a single neuromuscular centre. A total of nine patients aged from neonates to 34 years were enrolled who exhibited initial symptoms. Physical examinations revealed that all patients exhibited muscle weakness. Muscle biopsies demonstrated multiple myopathological changes, including increased fibre size variation, myofibrillar network disarray, necrosis, myofiber grouping, regeneration, fibre atrophy and angular fibres. Genetic testing revealed six different mutated genes, including AGRN (2/9), CHRNE (1/9), GFPT1 (1/9), GMPPB (1/9), PLEC (3/9) and SCN4A (1/9). In addition, patients exhibited differential responses to pharmacological treatment. Prompt utilization of genetic testing will identify novel variants and expand our understanding of the phenotype of this rare syndrome. Our findings contribute to the clinical, pathohistological and genetic spectrum of congenital myasthenic syndrome in China.

Pathogenic effects of agrin V1727F mutation are isoform specific and decrease its expression and affinity for HSPGs and LRP4

Agrin is a large extracellular matrix protein whose isoforms differ in their tissue distribution and function. Motoneuron-derived y+z+ agrin regulates the formation of the neuromuscular junction (NMJ), while y-z- agrin is widely expressed and has diverse functions. Previously we identified a missense mutation (V1727F) in the second laminin globular (LG2) domain of agrin that causes severe congenital myasthenic syndrome. Here, we define pathogenic effects of the agrin V1727F mutation that account for the profound dysfunction of the NMJ. First, by expressing agrin variants in heterologous cells, we show that the V1727F mutation reduces the secretion of y+z+ agrin compared to wild type, whereas it has no effect on the secretion of y-z- agrin. Second, we find that the V1727F mutation significantly impairs binding of y+z+ agrin to both heparin and the low-density lipoprotein receptor-related protein 4 (LRP4) coreceptor. Third, molecular modeling of the LG2 domain suggests that the V1727F mutation primarily disrupts the y splice insert, and consistent with this we find that it partially occludes the contribution of the y splice insert to agrin binding to heparin and LRP4. Together, these findings identify several pathogenic effects of the V1727F mutation that reduce its expression and ability to bind heparan sulfate proteoglycan and LRP4 coreceptors involved in the muscle-specific kinase signaling pathway. These defects primarily impair the function of neural y+z+ agrin and combine to cause a severe CMS phenotype, whereas y-z- agrin function in other tissues appears preserved.

Influencing Early Stages of Neuromuscular Junction Formation through Glycocalyx Engineering

Achieving molecular control over the formation of synaptic contacts in the nervous system can provide important insights into their regulation and can offer means for creating well-defined in vitro systems to evaluate modes of therapeutic intervention. Agrin-induced clustering of acetylcholine receptors (AChRs) at postsynaptic sites is a hallmark of the formation of the neuromuscular junction, a synapse between motoneurons and muscle cells. In addition to the cognate agrin receptor LRP4 (low-density lipoprotein receptor related protein-4), muscle cell heparan sulfate (HS) glycosaminoglycans (GAGs) have also been proposed to contribute to AChR clustering by acting as agrin co-receptors. Here, we provide direct evidence for the role of HS GAGs in agrin recruitment to the surface of myotubes, as well as their functional contributions toward AChR clustering. We also demonstrate that engineering of the myotube glycocalyx using synthetic HS GAG polymers can replace native HS structures to gain control over agrin-mediated AChR clustering.

Epidermal Growth Factor - based adhesion substrates elicit myoblast scattering, proliferation, differentiation and promote satellite cell myogenic activation

The biochemical properties of muscle extracellular matrix are essential for stem cell adhesion, motility, proliferation and myogenic development. Recombinant elastin-like polypeptides are synthetic polypeptides that, besides maintaining some properties of the native protein, can be tailored by fusing bioactive sequences to their C-terminal. Our laboratory synthesized several Human Elastin-Like Polypeptides (HELP) derived from the sequence of human tropoelastin. Here, we developed a novel HELP family member by fusing the elastin-like backbone to the sequence of human Epidermal Growth Factor. We employed this synthetic protein, named HEGF, either alone or in combination with other proteins of the HELP family carrying RGD-integrin binding sites, as adhesion substrate for C2C12 myoblasts and satellite cells primary cultures. Adhesion of myoblasts to HEGF-based substrates induced scattering, decreased adhesion and cytoskeleton assembly; the concomitant presence of the RGD motifs potentiated all these effects. Recombinant substrates induced myoblasts proliferation, differentiation and the development of multinucleated myotubes, thus favoring myoblasts expansion and preserving their myogenic potential. The effects induced by adhesion substrates were inhibited by AG82 (Tyrphostin 25) and herbimycin A, indicating their dependence on the activation of both the EGF receptor and the tyrosine kinase c-src. Finally, HEGF increased the number of muscle stem cells (satellite cells) derived from isolated muscle fibers in culture, thus highlighting its potential as a novel substrate for skeletal muscle regeneration strategies.

Induction of Anti-agrin Antibodies Causes Myasthenia Gravis in Mice

Myasthenia gravis (MG) is an autoimmune disorder of the neuromuscular junction (NMJ). Most cases of MG are caused by autoantibodies against the acetylcholine receptor (AChR), muscle-specific kinase (MuSK) and low-density lipoprotein receptor-related protein 4 (LRP4). Recent studies have identified anti-agrin antibodies in MG patients lacking these three antibodies (i.e., triple negative MG). Agrin is a basal lamina protein that has two isoforms. Neural agrin (N-agrin) binds to LRP4 to activate MuSK to induce AChR clusters and is thus critical for NMJ formation. We demonstrate that mice immunized with N-agrin showed MG-associated symptoms including muscle weakness, fragmented and distorted NMJs. These effects were not observed in mice injected with muscle agrin (M-agrin), an isoform that is inactive in inducing AChR clusters. Treatment with anti-N-agrin, but not anti-M-agrin, antibodies reduced agrin-induced AChR clusters in muscle cells. Together, these observations suggest that agrin antibodies may be play a role in MG pathogenesis.

LRP4 in neuromuscular junction and bone development and diseases

Low-density lipoprotein receptor-related protein 4 (LRP4) is a member of the low-density lipoprotein receptor (LDLR) family. Recent studies have revealed multiple functions and complex signaling mechanisms of LRP4 in different organs and tissues. LPR4 mutation or malfunction has been implicated in neurological disorders including congenital myasthenic syndrome, myasthenia gravis, and diseases of bone or kidney. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Structural mechanisms of the agrin-LRP4-MuSK signaling pathway in neuromuscular junction differentiation

The neuromuscular junction (NMJ) is the most extensively studied model of neuronal synaptogenesis. Acetylcholine receptor (AChR) clustering on the postsynaptic membrane is a cardinal event in the differentiation of NMJs. AChR clustering and postsynaptic differentiation is orchestrated by sophisticated interactions among three proteins: the neuron-secreted proteoglycan agrin, the co-receptor LRP4, and the muscle-specific receptor tyrosine kinase MuSK. LRP4 and MuSK act as scaffolds for multiple binding partners, resulting in a complex and dynamic network of interacting proteins that is required for AChR clustering. In this review, we discuss the structural basis for NMJ postsynaptic differentiation mediated by the agrin-LRP4-MuSK signaling pathway.

Structural basis of agrin-LRP4-MuSK signaling

Synapses are the fundamental units of neural circuits that enable complex behaviors. The neuromuscular junction (NMJ), a synapse formed between a motoneuron and a muscle fiber, has contributed greatly to understanding of the general principles of synaptogenesis as well as of neuromuscular disorders. NMJ formation requires neural agrin, a motoneuron-derived protein, which interacts with LRP4 (low-density lipoprotein receptor-related protein 4) to activate the receptor tyrosine kinase MuSK (muscle-specific kinase). However, little is known of how signals are transduced from agrin to MuSK. Here, we present the first crystal structure of an agrin-LRP4 complex, consisting of two agrin-LRP4 heterodimers. Formation of the initial binary complex requires the z8 loop that is specifically present in neuronal, but not muscle, agrin and that promotes the synergistic formation of the tetramer through two additional interfaces. We show that the tetrameric complex is essential for neuronal agrin-induced acetylcholine receptor (AChR) clustering. Collectively, these results provide new insight into the agrin-LRP4-MuSK signaling cascade and NMJ formation and represent a novel mechanism for activation of receptor tyrosine kinases.

Thermodynamic and structural studies of carbohydrate binding by the agrin-G3 domain

Agrin is a key heparan sulfate proteoglycan involved in the development and maintenance of synaptic junctions between nerves and muscles. Agrin's important functions include clustering acetylcholine receptors on the postsynaptic membranes of muscles and binding to the muscle protein alpha-dystroglycan through its glycan chains. ITC and NMR were used to study the interactions of the C-terminal domain, agrin-G3, with carbohydrates implicated in agrin's functions. Sialic acid caps the glycan chains of alpha-dystroglycan and occurs as a posttranslational modification on the muscle-specific kinase component of the agrin receptor. We found that agrin-G3 binds sialic acid in a Ca2+-dependent manner. ITC data indicate that binding is exothermic and occurs with a 1:1 stoichiometry. NMR chemical shift changes map the sialic acid binding site to the loops that control the domain's acetylcholine receptor clustering activity. By contrast, the glycosaminoglycans heparin and heparan sulfate bind independently of Ca2+. Binding is endothermic, and the binding site spans about 12 saccharide units. The binding site for heparin occupies a similar location but is distinct from that for sialic acid. NMR translational diffusion experiments show that agrin-G3 binds heparin with a 2:1 stoichiometry. Comparisons between the muscle (B0) and neuronal (B8) isoforms of the agrin domain showed very similar Ca2+ and carbohydrate binding properties. Our work identifies agrin-G3 as a functional analogue of the concanavalin A-type lectins, highlights functional similarities between agrin and laminin G domains, and provides mechanistic clues about the roles of carbohydrates in agrin's functions.

Activation of Muscle-specific Receptor Tyrosine Kinase and Binding to Dystroglycan Are Regulated by Alternative mRNA Splicing of Agrin

Agrin induces the aggregation of postsynaptic proteins at the neuromuscular junction (NMJ). This activity requires the receptor-tyrosine kinase MuSK. Agrin isoforms differ in short amino acid stretches at two sites, called A and B, that are localized in the two most C-terminal laminin G (LG) domains. Importantly, agrin isoforms greatly differ in their activities of inducing MuSK phosphorylation and of binding to alpha-dystroglycan. By using site-directed mutagenesis, we characterized the amino acids important for these activities of agrin. We find that the conserved tripeptide asparagineglutamate-isoleucine in the eight-amino acid long insert at the B-site is necessary and sufficient for full MuSK phosphorylation activity. However, even if all eight amino acids were replaced by alanines, this agrin mutant still has significantly higher MuSK phosphorylation activity than the splice version lacking any insert. We also show that binding to alpha-dystroglycan requires at least two LG domains and that amino acid inserts at the A and the B splice sites negatively affect binding.

Crystal structure of the agrin-responsive immunoglobulin-like domains 1 and 2 of the receptor tyrosine kinase MuSK

Muscle-specific kinase (MuSK) is a receptor tyrosine kinase expressed exclusively in skeletal muscle, where it is required for formation of the neuromuscular junction. MuSK is activated by agrin, a neuron-derived heparan sulfate proteoglycan. Here, we report the crystal structure of the agrin-responsive first and second immunoglobulin-like domains (Ig1 and Ig2) of the MuSK ectodomain at 2.2 A resolution. The structure reveals that MuSK Ig1 and Ig2 are Ig-like domains of the I-set subfamily, which are configured in a linear, semi-rigid arrangement. In addition to the canonical internal disulfide bridge, Ig1 contains a second, solvent-exposed disulfide bridge, which our biochemical data indicate is critical for proper folding of Ig1 and processing of MuSK. Two Ig1-2 molecules form a non-crystallographic dimer that is mediated by a unique hydrophobic patch on the surface of Ig1. Biochemical analyses of MuSK mutants introduced into MuSK(-/-) myotubes demonstrate that residues in this hydrophobic patch are critical for agrin-induced MuSK activation.

The effect of agrin and laminin on acetylcholine receptor dynamics in vitro

Using optical imaging assays, we investigated the dynamics of acetylcholine receptors (AChRs) at laminin-associated clusters on cultured myotubes in the absence or presence of the nerve-derived clustering factor, agrin. Using fluorescence recovery after photobleaching (FRAP) on fluorescent bungarotoxin-labeled receptors, we found that approximately 9% of original fluorescence was recovered after 8 h as surface AChRs were recruited into clusters. By quantifying the loss of labeled receptors and the recovery of fluorescence after photobleaching, we estimated that the half-life of clustered receptors was approximately 4.5 h. Despite the rapid removal of receptors, the accumulation of new receptors at clusters was robust enough to maintain receptor density over time. We also found that the AChR half-life was not affected by agrin despite its role in inducing the aggregation of AChRs. Interestingly, when agrin was added to myotubes grown on laminin-coated substrates, most new receptors were not directed into preexisting laminin-induced clusters but instead formed numerous small aggregates on the entire muscle surface. Time-lapse imaging revealed that the agrin-induced clusters could be seen as early as 1 h, and agrin treatment resulted in the complete dissipation of laminin-associated clusters by 24 h. These results reveal that while laminin and agrin are involved in the clustering of receptors they are not critical to the regulation of receptor metabolic stability at these clusters, and further argue that agrin is able to rapidly and fully negate the laminin substrate clustering effect while inducing the rapid formation of new clusters.

Modulation of agrin function by alternative splicing and Ca2+ binding

The aggregation of acetylcholine receptors on postsynaptic membranes is a key step in neuromuscular junction development. This process depends on alternatively spliced forms of the proteoglycan agrin with "B-inserts" of 8, 11, or 19 residues in the protein's globular C-terminal domain, G3. Structures of the neural B8 and B11 forms of agrin-G3 were determined by X-ray crystallography. The structure of G3-B0, which lacks inserts, was determined by NMR. The agrin-G3 domain adopts a beta jellyroll fold. The B insert site is flanked by four loops on one edge of the beta sandwich. The loops form a surface that corresponds to a versatile interaction interface in the family of structurally related LNS proteins. NMR and X-ray data indicate that this interaction interface is flexible in agrin-G3 and that flexibility is reduced by Ca(2+) binding. The plasticity of the interaction interface could enable different splice forms of agrin to select between multiple binding partners.

Glycosaminoglycan-dependent and -independent inhibition of neurite outgrowth by agrin

Agrin is a proteoglycan that can inhibit neurite outgrowth from multiple neuronal types when present as a substrate. Agrin's neurite inhibitory activity is confined to the N-terminal segment of the protein (agrin N150), which contains heparan sulfate (HS) and chondroitin sulfate (CS) side chains. We have examined the activities of various purified recombinant agrin fragments and their glycosaminoglycan (GAG) side chains in neurite outgrowth inhibition. Inhibitory activity was tested using dissociated chick ciliary ganglion neurons or dorsal root ganglion explants growing on laminin or N-cadherin. Initial experiments demonstrated that agrin N150 lacking GAG chains inhibited neurite outgrowth. Both halves of N150, each containing HS and/or CS side chains, could also inhibit neurite growth. Experiments using agrin fragments in which the GAG acceptor residues were mutated, or using agrin fragments purified from cells deficient in GAG synthesis, demonstrated that inhibition by the N-terminal portion of N150 requires GAGs, but that inhibition from the C-terminal part of N150 does not. Thus, the core protein or other types of glycosylation are important for inhibition from the more C-terminal region. Our results suggest that there are two distinct mechanisms for neurite outgrowth inhibition by agrin, one that is GAG-dependent and one that is GAG-independent.

Calcium plays a critical role in determining the acetylcholine receptor-clustering activities of alternatively spliced isoforms of Agrin

Neural agrin, an extracellular matrix protein secreted by motor neurons, plays a key role in clustering of nicotinic acetylcholine receptors (AChR) on postsynaptic membranes of the neuromuscular junction. The action of agrin is critically dependent on an eight-amino acid insert (z8 insert) in the third of three consecutive laminin-like globular (G3) domains near the C terminus of neural agrin. Alternatively spliced agrin isoforms in non-neural tissue including muscle lack the z8 insert and are biologically inactive. Extracellular calcium has been shown to be imperative for the AChR-clustering activity of neural agrin. It is unclear, however, whether calcium preferentially interacts with the neural isoform or whether it acts solely as an intracellular messenger that mediates agrin signaling. Here, we report the G3 domain of rat neural agrin (AgG3z8) expressed in Pichia pastoris promoted AChR clustering on surface of C2C12 myotubes in a calcium-dependent manner. Direct binding of calcium to AgG3z8 was demonstrated by trypsin digestion and thermal denaturation experiments. Moreover, calcium induced a significant change in the conformation of AgG3z8, and the effect was correlated with an enhanced binding affinity of the protein to muscle receptor. Mutation of calcium-binding residues in the G3 domain diminished the conformational change of neural agrin, reduced its binding affinity to muscle membrane, and inhibited AChR-clustering activity. Conversely, the G3 domain of muscle agrin (AgG3z0) displayed little structural change in the presence of calcium, bound poorly to muscle surface, and was inactive in AChR-clustering assays. We conclude that distinct interactions of the G3 domain with calcium determine the biological activities of alternatively spliced agrin isoforms during synapse formation.

New insights into the roles of agrin

The heparan sulphate proteoglycan agrin is expressed as several isoforms in various tissues. Agrin is best known as a crucial organizer of postsynaptic differentiation at the neuromuscular junction, but it has recently also been implicated in the formation of the immunological synapse, the organization of the cytoskeleton and the amelioration of function in diseased muscle. So the activities of agrin might be of broader significance than previously anticipated.

Agrin in the CNS: a protein in search of a function?

The extracellular matrix molecule agrin mediates the motor neuron induced accumulation of acetylcholine receptors (AChR) at the neuromuscular junction. Agrin is also present in the CNS. However, while its spatiotemporal pattern of expression is consistent with a function in neuron-neuron synapse formation, it also suggests a role for agrin in other aspects of neural tissue morphogenesis. Here we review the data supporting these synaptic and non-synaptic functions of agrin in the CNS. The results of studies aimed at identifying a neuronal receptor for agrin (NRA) and its associated signal transduction pathways are examined. Possible roles for agrin in the etiology of diseases affecting the brain are also discussed.

Microcontact printing: a versatile technique for the study of synaptogenic molecules

During synaptogenesis information exchanged locally between synaptic partners results in precise alignment of morphological and molecular specializations. For example, agrin derived from motoneurons induces localized postsynaptic differentiation at the neuromuscular synapse. Similar information molecules are thought to act at other synapses; however, techniques for directly evaluating synaptogenic activities of such molecules are lacking. Here we use agrin-induced differentiation as a model system to validate a novel approach for characterizing synaptogenic molecules. Proteins are patterned with micron scale resolution on glass coverslips by covalent microcontact printing and these substrates are used for cell culture. Postsynaptic molecules accumulate specifically at sites of contact between muscle cells and patterned agrin: a response which is quantifiable. Our results demonstrate that microcontact printing is applicable to the analysis of cellular response to locally immobilized information molecules.

Modulation of agrin binding and activity by the CT and related carbohydrate antigens

Agrin is a nerve-derived signal that is essential for the proper organization of postsynaptic acetylcholine receptors (AChRs) at the vertebrate neuromuscular junction. It is likely that carbohydrates play a significant role in regulating agrin activity, as agrin binds multiple glycan structures and is itself a highly glycosylated protein. Here we provide support for this contention by showing that agrin can be modified with the CT antigen, a carbohydrate structure expressed at the neuromuscular junction, and by describing the resulting changes in agrin binding to neoglycoconjugates and cultured myotubes, as well as changes in agrin-dependent AChR clustering. Glycosylation of agrin with the CT antigen required the mucin domain and the dystroglycan/heparin-binding domain. The presence of the mucin domain lowered agrin binding to several N-acetyllactosaminyl-containing saccharides and C2 myotubes and lowered agrin activity in AChR clustering. Glycosylation of agrin with the CT antigen, by contrast, increased agrin binding to myotubes and potentiated its AChR clustering activity at subsaturating concentrations. Last, sialylated and nonsialylated variants of N-acetyllactosamine differentially modulated AChR clustering and agrin activity, and these changes correlated with the ability of MuSK, an agrin-stimulated kinase, to bind to these structures. These experiments demonstrate that the glycosylation state of agrin affects its activity and suggest a role for the CT antigen in modulating agrin function.

N-acetyllactosamine and the CT carbohydrate antigen mediate agrin-dependent activation of MuSK and acetylcholine receptor clustering in skeletal muscle

Galbeta1,3GalNAc and Galbeta1,4GIcNAc are the subterminal saccharide structures present on the CT carbohydrate antigen GalNAcbeta1,4[NeuAcalpha2,3]-Galbeta1-(3GalNAc or 4GIcNAc)-R, which is localized at the mammalian neuromuscular junction. Here we show that Galbeta1,3GalNAc, Galbeta1,4GIcNAc, and the CT carbohydrate antigen affect postsynaptic assembly in cultured muscle cells. Treatment of C2C12 myotubes with benzyl-O-alpha-GalNAc or neuraminidase increased peanut agglutinin (PNA) expression and AChR clustering. Induction of AChR clustering was blocked by PNA and by muscle agrin. Addition of Galbeta1,4GIcNAc or Galbeta1,3GalNAc increased AChR clustering in myotubes and muscle-specific kinase (MUSK) autophosphorylation in vitro, while NeuAcalpha2,3Galbeta1,4GIcNAc and Galbeta1,4GIc did not. Neural agrin activated MuSK in vitro if the lactosamine-containing mucin domain was present, and this activation was blocked in large part by Galbeta1,3GalNAc and Galbeta1,4GIcNAc. Agrin fragments and MuSK bound to these disaccharides with differing specificities. Overexpression of the CT carbohydrate antigen also increased AChR clustering and MuSK autophosphorylation in the presence of neural agrin. These data suggest a model in which different portions of the CT carbohydrate structure contribute to agrin-dependent signal transduction.