Agrin-related molecules are concentrated at acetylcholine receptor clusters in normal and aneural developing muscle

Effects of physical exercise on neuromuscular junction degeneration during ageing: A systematic review

The neuromuscular junction (NMJ) is a specialized chemical synapse that converts neural impulses into muscle action. Age-associated NMJ degeneration, which involves nerve terminal and postsynaptic decline, denervation, and loss of motor units, significantly contributes to muscle weakness and dysfunction. Although physical training has been shown to make substantial modifications in NMJ of both young and aged animals, the results are often influenced by methodological variables in existing studies. Moreover, there is still lack of strong consensus on the specific effects of exercise on improving the morphology and function of the ageing NMJ. Consequently, the purpose of this study was to conduct a systematic review to elucidate the effects of exercise training on NMJ compartments in the elderly. We conducted a systematic review using PubMed, Embase, and Web of Science databases, employing relevant keywords. Two independent reviewers selected studies that detailed NMJ changes during exercise in ageing, written in English, and available in full text. In total, 20 papers were included. We examined the altered adaptation of the NMJ to exercise, focusing on presynaptic and postsynaptic structures and myofibers in older animals or humans. Our findings indicated that aged NMJs exhibited different adaptive responses to physical exercise compared to younger counterparts. Endurance training, compared with resistance and voluntary exercise regimens, was found to have a more pronounced effect on NMJ structural remodeling, particularly in fast twitch muscle fibers. Physical exercise was observed to promote the formation and maintenance of acetylcholine receptor (AChR) clusters by increasing the recombinant docking protein 7 (Dok7) expression and stabilizing Agrin and lipoprotein receptor-related protein 4 (LRP4). These insights suggest that research on exercise-related therapies could potentially attenuate the progression of neuromuscular degeneration. Translational potential of this article: This systematic review provides a detailed overview of the effects of different types of physical exercise on improving NMJ in the elderly, providing scientific support for the timely intervention of muscle degeneration in the elderly by physical exercise, and providing help for the development of new therapeutic interventions in the future.

Sensorimotor nerve lesion of upper airway in patients with obstructive sleep apnea

The pathogenesis of obstructive sleep apnea (OSA) remains controversial. The role of anatomic stenosis is indisputable, and neural regulation of the upper airway remains to be elucidated. The upper airway maintains patency through the upper airway reflex. Lesions in any link of the reflex can increase the collapsibility of the upper airway. In this study, we investigated sensorimotor nerve lesions and their possible relationship with OSA. Tissue samples were obtained from the pharyngopalatine arch in 47 patients with OSA and 45 control participants to examine changes in the expression levels of myelin basic protein (MBP) and agrin through immunohistochemistry and western blotting. Downregulation of MBP in the mucosa reflects myelinated degeneration of mucosal sensory nerve axons, whereas upregulation of agrin in the neuromuscular junction reflects synaptic regeneration following denervation. The two neural factors correlate significantly with polysomnographic parameters, such as the apnea hypopnea index and lowest oxygen saturation. Our findings suggest that sensorimotor nerve damage in the upper airway of patients with OSA may be associated closely with the mechanism of OSA.

Autoantibodies to agrin in myasthenia gravis patients

To determine if patients with myasthenia gravis (MG) have antibodies to agrin, a proteoglycan released by motor neurons and is critical for neuromuscular junction (NMJ) formation, we collected serum samples from 93 patients with MG with known status of antibodies to acetylcholine receptor (AChR), muscle specific kinase (MuSK) and lipoprotein-related 4 (LRP4) and samples from control subjects (healthy individuals and individuals with other diseases). Sera were assayed for antibodies to agrin. We found antibodies to agrin in 7 serum samples of MG patients. None of the 25 healthy controls and none of the 55 control neurological patients had agrin antibodies. Two of the four triple negative MG patients (i.e., no detectable AChR, MuSK or LRP4 antibodies, AChR-/MuSK-/LRP4-) had antibodies against agrin. In addition, agrin antibodies were detected in 5 out of 83 AChR+/MuSK-/LRP4- patients but were not found in the 6 patients with MuSK antibodies (AChR-/MuSK+/LRP4-). Sera from MG patients with agrin antibodies were able to recognize recombinant agrin in conditioned media and in transfected HEK293 cells. These sera also inhibited the agrin-induced MuSK phosphorylation and AChR clustering in muscle cells. Together, these observations indicate that agrin is another autoantigen in patients with MG and agrin autoantibodies may be pathogenic through inhibition of agrin/LRP4/MuSK signaling at the NMJ.

Wnt signaling in skeletal muscle dynamics: myogenesis, neuromuscular synapse and fibrosis

The signaling pathways activated by Wnt ligands are related to a wide range of critical cell functions, such as cell division, migration, and synaptogenesis. Here, we summarize compelling evidence on the role of Wnt signaling on several features of skeletal muscle physiology. We briefly review the role of Wnt pathways on the formation of muscle fibers during prenatal and postnatal myogenesis, highlighting its role on the activation of stem cells of the adult muscles. We also discuss how Wnt signaling regulates the precise formation of neuromuscular synapses, by modulating the differentiation of presynaptic and postsynaptic components, particularly regarding the clustering of acetylcholine receptors on the muscle membrane. In addition, based on previous evidence showing that Wnt pathways are linked to several diseases, such as Alzheimer's and cancer, we address recent studies indicating that Wnt signaling plays a key role in skeletal muscle fibrosis, a disease characterized by an increase in the extracellular matrix components leading to failure in muscle regeneration, tissue disorganization and loss of muscle activity. In this context, we also discuss the possible cross-talk between the Wnt/β-catenin pathway with two other critical profibrotic pathways, transforming growth factor β and connective tissue growth factor, which are potent stimulators of the accumulation of connective tissue, an effect characteristic of the fibrotic condition. As it has emerged in other pathological conditions, we suggests that muscle fibrosis may be a consequence of alterations of Wnt signaling activity.

Dual roles for Wnt signalling during the formation of the vertebrate neuromuscular junction

Wnt proteins play prominent roles in different aspects of neuronal development culminating with the formation of complex neuronal circuits. Here, we discuss new studies addressing the function of Wnt signalling at the peripheral neuromuscular junction (NMJ). In both, invertebrate and vertebrate organisms, Wnt signalling promotes and also inhibits the assembly of the neuromuscular synapse. Here, we focus our attention on recent studies at the vertebrate NMJ that demonstrate that some Wnt proteins collaborate with the Agrin-MuSK signalling to induce post-synaptic differentiation. In contrast, Wnts that activate the Wnt/β-catenin signalling inhibit post-synaptic differentiation. The dual function of different Wnts might finely modulate the proper apposition of the pre- and post-synaptic terminals during NMJ formation and growth.

The Wnt and BMP families of signaling morphogens at the vertebrate neuromuscular junction

The neuromuscular junction has been extensively employed in order to identify crucial determinants of synaptogenesis. At the vertebrate neuromuscular synapse, extracellular matrix and signaling proteins play stimulatory and inhibitory roles on the assembly of functional synapses. Studies in invertebrate species have revealed crucial functions of early morphogens during the assembly and maturation of the neuromuscular junction. Here, we discuss growing evidence addressing the function of Wnt and Bone morphogenetic protein (BMP) signaling pathways at the vertebrate neuromuscular synapse. We focus on the emerging role of Wnt proteins as positive and negative regulators of postsynaptic differentiation. We also address the possible involvement of BMP pathways on motor neuron behavior for the assembly and/or regeneration of the neuromuscular junction.

T-shaped arrangement of the recombinant agrin G3-IgG Fc protein

Agrin is a large heparin sulphate proteoglycan with multiple domains, which is located in the extracellular matrix. The C-terminal G3 domain of agrin is functionally one of the most important domains. It harbors an α-dystroglycan binding site and carries out acetylcholine receptor clustering activities. In the present study, we have fused the G3 domain of agrin to an IgG Fc domain to produce a G3-Fc fusion protein that we intend to use as a tool to investigate new binding partners of agrin. As a first step of the study, we have characterized the recombinant fusion protein using a multidisciplinary approach using dynamic light scattering, analytical ultracentrifugation and small angle X-ray scattering (SAXS). Interestingly, our SAXS analysis using the high-resolution structures of G3 and Fc domain as models indicates that the G3-Fc protein forms a T-shaped molecule with the G3 domains extruding perpendicularly from the Fc scaffold. To validate our models, we have used the program HYDROPRO to calculate the hydrodynamic properties of the solution models. The calculated values are in excellent agreement with those determined experimentally.

A synaptic nidogen: developmental regulation and role of nidogen-2 at the neuromuscular junction

Background

The skeletal neuromuscular junction is a useful model for elucidating mechanisms that regulate synaptogenesis. Developmentally important intercellular interactions at the neuromuscular junction are mediated by the synaptic portion of a basal lamina that completely ensheaths each muscle fiber. Basal laminas in general are composed of four main types of glycosylated proteins: laminins, collagens IV, heparan sulfate proteoglycans and nidogens (entactins). The portion of the muscle fiber basal lamina that passes between the motor nerve terminal and postsynaptic membrane has been shown to bear distinct isoforms of the first three of these. For laminins and collagens IV, the proteins are deposited by the muscle; a synaptic proteoglycan, z-agrin, is deposited by the nerve. In each case, the synaptic isoform plays key roles in organizing the neuromuscular junction. Here, we analyze the fourth family, composed of nidogen-1 and -2.

Results

In adult muscle, nidogen-1 is present throughout muscle fiber basal lamina, while nidogen-2 is concentrated at synapses. Nidogen-2 is initially present throughout muscle basal lamina, but is lost from extrasynaptic regions during the first three postnatal weeks. Neuromuscular junctions in mutant mice lacking nidogen-2 appear normal at birth, but become topologically abnormal as they mature. Synaptic laminins, collagens IV and heparan sulfate proteoglycans persist in the absence of nidogen-2, suggesting the phenotype is not secondary to a general defect in the integrity of synaptic basal lamina. Further genetic studies suggest that synaptic localization of each of the four families of synaptic basal lamina components is independent of the other three.

Conclusion

All four core components of the basal lamina have synaptically enriched isoforms. Together, they form a highly specialized synaptic cleft material. Individually, they play distinct roles in the formation, maturation and maintenance of the neuromuscular junction.

Seeking long-term relationship: axon and target communicate to organize synaptic differentiation

Synapses form after growing axons recognize their appropriate targets. The subsequent assembly of aligned pre and postsynaptic specializations is critical for synaptic function. This highly precise apposition of presynaptic elements (i.e. active zones) to postsynaptic specializations (i.e. neurotransmitter receptor clusters) strongly suggests that communication between the axon and target is required for synaptic differentiation. What trans-synaptic factors drive such differentiation at vertebrate synapses? First insights into the answers to this question came from studies at the neuromuscular junction (NMJ), where axon-derived agrin and muscle-derived laminin beta2 induce post and presynaptic differentiation, respectively. Recent work has suggested that axon- and target-derived factors similarly drive synaptic differentiation at central synapses. Specifically, WNT-7a, neuroligin, synaptic cell adhesion molecule (SynCAM) and fibroblast growth factor-22 (FGF-22) have all been identified as target-derived presynaptic organizers, whereas axon-derived neuronal activity regulated pentraxin (Narp), ephrinB and neurexin reciprocally co-ordinate postsynaptic differentiation. In addition to these axon- and target-derived inducers of synaptic differentiation, factors released from glial cells have also been implicated in regulating synapse assembly. Together, these recent findings have profoundly advanced our understanding of how precise appositions are established during vertebrate nervous system development.

Agrin becomes concentrated at neuroeffector junctions in developing rodent urinary bladder

The formation of somatic neuromuscular junctions in skeletal muscle is regulated by an extracellular matrix protein called agrin. Here, we have examined the expression and localization of agrin during development of the rodent urinary bladder, as a first step to examining its possible role at autonomic neuroeffector junctions in smooth muscle. We have found that agrin is expressed on the surface of developing smooth muscle cells and in the basement membrane underlying the urothelium. More importantly, agrin is progressively concentrated at parasympathetic varicosities during postnatal development and is present at virtually all junctions in mature muscle. Reverse transcription/polymerase chain reaction analysis has shown that pelvic ganglion neurons that innervate the bladder express LN/z8 agrin, whereas bladder smooth muscle expresses LN/z- agrin. Together, these results demonstrate that nerve and/or muscle agrin becomes localized at cholinergic parasympathetic varicosities in smooth muscle, where it could play a role in the maturation of the neuroeffector junction.

Synergistic effects of neuregulin and agrin on muscle acetylcholine receptor expression

The proper function of neuromuscular junctions requires an extremely high density of acetylcholine receptors (AChRs) that may be achieved from neuron-derived factors including agrin and neuregulin. Here, we show that neuregulin-1 and agrin co-localize at neuromuscular junctions in vivo and form complexes when co-transfected into COS-7 cells. When these COS-7 cells are cultured with myotubes, synergistic effects are observed for AChR clustering, membrane insertion of new AChRs, and induction of AChR mRNA. Even a muscle form of agrin that lacks intrinsic clustering activities by itself, significantly enhances neuregulin-induced clustering and insertion of AChRs. While the heparin-binding (A) domain of agrin is required for agrin localization in the extracellular matrix adjacent to AChR clusters, the heparan sulfate-containing domain of agrin is needed for the synergistic effects and co-localization with neuregulin-1. These results suggest that matrix interactions between exogenously supplied agrin and neuregulin-1 on the muscle surface provide a localized source of signaling factors needed to produce high densities of AChRs at neuromuscular junctions.

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.

Neuregulin expression at neuromuscular synapses is modulated by synaptic activity and neurotrophic factors

The proper formation of neuromuscular synapses requires ongoing synaptic activity that is translated into complex structural changes to produce functional synapses. One mechanism by which activity could be converted into these structural changes is through the regulated expression of specific synaptic regulatory factors. Here we demonstrate that blocking synaptic activity with curare reduces synaptic neuregulin expression in a dose-dependent manner yet has little effect on synaptic agrin or a muscle-derived heparan sulfate proteoglycan. These changes are associated with a fourfold increase in number and a twofold reduction in average size of synaptic acetylcholine receptor clusters that appears to be caused by excessive axonal sprouting with the formation of new, smaller acetylcholine receptor clusters. Activity blockade also leads to threefold reductions in brain-derived neurotrophic factor and neurotrophin 3 expression in muscle without appreciably changing the expression of these same factors in spinal cord. Adding back these or other neurotrophic factors restores synaptic neuregulin expression and maintains normal end plate band architecture in the presence of activity blockade. The expression of neuregulin protein at synapses is independent of spinal cord and muscle neuregulin mRNA levels, suggesting that neuregulin accumulation at synapses is independent of transcription. These findings suggest a local, positive feedback loop between synaptic regulatory factors that translates activity into structural changes at neuromuscular synapses.

Neuregulin expression at neuromuscular synapses is modulated by synaptic activity and neurotrophic factors

The proper formation of neuromuscular synapses requires ongoing synaptic activity that is translated into complex structural changes to produce functional synapses. One mechanism by which activity could be converted into these structural changes is through the regulated expression of specific synaptic regulatory factors. Here we demonstrate that blocking synaptic activity with curare reduces synaptic neuregulin expression in a dose-dependent manner yet has little effect on synaptic agrin or a muscle-derived heparan sulfate proteoglycan. These changes are associated with a fourfold increase in number and a twofold reduction in average size of synaptic acetylcholine receptor clusters that appears to be caused by excessive axonal sprouting with the formation of new, smaller acetylcholine receptor clusters. Activity blockade also leads to threefold reductions in brain-derived neurotrophic factor and neurotrophin 3 expression in muscle without appreciably changing the expression of these same factors in spinal cord. Adding back these or other neurotrophic factors restores synaptic neuregulin expression and maintains normal end plate band architecture in the presence of activity blockade. The expression of neuregulin protein at synapses is independent of spinal cord and muscle neuregulin mRNA levels, suggesting that neuregulin accumulation at synapses is independent of transcription. These findings suggest a local, positive feedback loop between synaptic regulatory factors that translates activity into structural changes at neuromuscular synapses.

The agrin/muscle-specific kinase pathway: new targets for autoimmune and genetic disorders at the neuromuscular junction

The increasing understanding of the structural complexity of the neuromuscular junction (NMJ), and the processes that are important in its development, suggests many possible new disease targets. Here, we summarize briefly the genetic and autoimmune disorders that affect neuromuscular transmission, and the identified targets, including new evidence that antibodies to muscle-specific receptor tyrosine kinase (MuSK) are involved in the pathogenesis of acetylcholine receptor (AChR) antibody-negative myasthenia gravis. We then review the development of the NMJ, focusing on the important roles of nerve-derived agrin and MuSK in clustering of AChRs and other essential components of the NMJ.

Sodium channel distribution on uninnervated and innervated embryonic skeletal myotubes

Acetylcholine receptor (AChR) and sodium (Na(+)) channel distributions within the membrane of mature vertebrate skeletal muscle fibers maximize the probability of successful neuromuscular transmission and subsequent action potential propagation. AChRs have been studied intensively as a model for understanding the development and regulation of ion channel distribution within the postsynaptic membrane. Na(+) channel distributions have received less attention, although there is evidence that the temporal accumulation of Na(+) channels at developing neuromuscular junctions (NMJs) may differ between species. Even less is known about the development of extrajunctional Na(+) channel distributions. To further our understanding of Na(+) channel distributions within junctional and extrajunctional membranes, we used a novel voltage-clamp method and fluorescent probes to map Na(+) channels on embryonic chick muscle fibers as they developed in vitro and in vivo. Na(+) current densities on uninnervated myotubes were approximately one-tenth the density found within extrajunctional regions of mature fibers, and showed several-fold variations that could not be explained by a random scattering of single channels. Regions of high current density were not correlated with cellular landmarks such as AChR clusters or myonuclei. Under coculture conditions, AChRs rapidly concentrated at developing synapses, while Na(+) channels did not show a significant increase over the 7 day coculture period. In vivo investigations supported a significant temporal separation between Na(+) channel and AChR aggregation at the developing NMJ. These data suggest that extrajunctional Na(+) channels cluster together in a neuronally independent manner and concentrate at the developing avian NMJ much later than AChRs.

Src, Fyn, and Yes are not required for neuromuscular synapse formation but are necessary for stabilization of agrin-induced clusters of acetylcholine receptors

Mice deficient in src and fyn or src and yes move and breathe poorly and die perinatally, consistent with defects in neuromuscular function. Src and Fyn are associated with acetylcholine receptors (AChRs) in muscle cells, and Src and Yes can act downstream of ErbB2, suggesting roles for Src family kinases in signaling pathways regulating neuromuscular synapse formation. We studied neuromuscular synapses in src(-/-); fyn(-/-) and src(-/-); yes(-/-) mutant mice and found that muscle development, motor axon pathfinding, clustering of postsynaptic proteins, and synapse-specific transcription are normal in these double mutants, showing that these pairs of kinases are not required for early steps in synapse formation. We generated muscle cell lines lacking src and fyn and found that neural agrin and laminin-1 induced normal clustering of AChRs and that agrin induced normal tyrosine phosphorylation of the AChR beta subunit in the absence of Src and Fyn. Another Src family member, most likely Yes, was associated with AChRs and phosphorylated by agrin in myotubes lacking Src and Fyn, indicating that Yes may compensate for the loss of Src and Fyn. Nevertheless, PP1 and PP2, inhibitors of Src-class kinases, did not inhibit agrin signaling, suggesting that Src class kinase activity is dispensable for agrin-induced clustering and tyrosine phosphorylation of AChRs. AChR clusters, however, were less stable in myotubes lacking Src and Fyn but not in PP1- or PP2-treated wild-type cells. These data show that the stabilization of agrin-induced AChR clusters requires Src and Fyn and suggest that the adaptor activities, rather than the kinase activities, of these kinases are essential for this stabilization.

Src, Fyn, and Yes are not required for neuromuscular synapse formation but are necessary for stabilization of agrin-induced clusters of acetylcholine receptors

Mice deficient in src and fyn or src and yes move and breathe poorly and die perinatally, consistent with defects in neuromuscular function. Src and Fyn are associated with acetylcholine receptors (AChRs) in muscle cells, and Src and Yes can act downstream of ErbB2, suggesting roles for Src family kinases in signaling pathways regulating neuromuscular synapse formation. We studied neuromuscular synapses in src(-/-); fyn(-/-) and src(-/-); yes(-/-) mutant mice and found that muscle development, motor axon pathfinding, clustering of postsynaptic proteins, and synapse-specific transcription are normal in these double mutants, showing that these pairs of kinases are not required for early steps in synapse formation. We generated muscle cell lines lacking src and fyn and found that neural agrin and laminin-1 induced normal clustering of AChRs and that agrin induced normal tyrosine phosphorylation of the AChR beta subunit in the absence of Src and Fyn. Another Src family member, most likely Yes, was associated with AChRs and phosphorylated by agrin in myotubes lacking Src and Fyn, indicating that Yes may compensate for the loss of Src and Fyn. Nevertheless, PP1 and PP2, inhibitors of Src-class kinases, did not inhibit agrin signaling, suggesting that Src class kinase activity is dispensable for agrin-induced clustering and tyrosine phosphorylation of AChRs. AChR clusters, however, were less stable in myotubes lacking Src and Fyn but not in PP1- or PP2-treated wild-type cells. These data show that the stabilization of agrin-induced AChR clusters requires Src and Fyn and suggest that the adaptor activities, rather than the kinase activities, of these kinases are essential for this stabilization.

Distinct roles of nerve and muscle in postsynaptic differentiation of the neuromuscular synapse

The development of chemical synapses is regulated by interactions between pre- and postsynaptic cells. At the vertebrate skeletal neuromuscular junction, the organization of an acetylcholine receptor (AChR)-rich postsynaptic apparatus has been well studied. Much evidence suggests that the nerve-derived protein agrin activates muscle-specific kinase (MuSK) to cluster AChRs through the synapse-specific cytoplasmic protein rapsyn. But how postsynaptic differentiation is initiated, or why most synapses are restricted to an 'end-plate band' in the middle of the muscle remains unknown. Here we have used genetic methods to address these issues. We report that the initial steps in postsynaptic differentiation and formation of an end-plate band require MuSK and rapsyn, but are not dependent on agrin or the presence of motor axons. In contrast, the subsequent stages of synaptic growth and maintenance require nerve-derived agrin, and a second nerve-derived signal that disperses ectopic postsynaptic apparatus.

The Agrin/MuSK signaling pathway is spatially segregated from the neuregulin/ErbB receptor signaling pathway at the neuromuscular junction

The neuregulin/erbB receptor and agrin/MuSK pathways are critical for communication between the nerve, muscle, and Schwann cell that establishes the precise topological arrangement at the vertebrate neuromuscular junction (NMJ). ErbB2, erbB3, and erbB4 as well as neuregulin, agrin, and MuSK are known to be concentrated at the NMJ. Here we have examined NMJs from gastrocnemius muscle of adult rat using immunofluorescence confocal microscopy to characterize in detail the distribution of these proteins relative to the distribution of acetylcholine receptors (AChRs). We have determined that erbB2 and erbB4 are enriched in the depths of the secondary junctional folds on the postsynaptic muscle membrane. In contrast, erbB3 at the NMJ was concentrated at presynaptic terminal Schwann cells. This distribution strongly argues that erbB2/erbB4 heterodimers are the functional postsynaptic neuregulin receptors of the NMJ. Neuregulin was localized to the axon terminal, secondary folds, and terminal Schwann cells, where it was in a position to signal through erbB receptors. MuSK was concentrated in the postsynaptic primary gutter region where it was codistributed with AChRs. Agrin was present at the axon terminal and in the basal lamina associated with the primary gutter region, but not in the secondary junctional folds. The differential distributions of the neuregulin and agrin signaling pathways argue against neuregulin and erbB receptors being localized to the NMJ via direct interactions with either agrin or MuSK.

The Agrin/MuSK signaling pathway is spatially segregated from the neuregulin/ErbB receptor signaling pathway at the neuromuscular junction

The neuregulin/erbB receptor and agrin/MuSK pathways are critical for communication between the nerve, muscle, and Schwann cell that establishes the precise topological arrangement at the vertebrate neuromuscular junction (NMJ). ErbB2, erbB3, and erbB4 as well as neuregulin, agrin, and MuSK are known to be concentrated at the NMJ. Here we have examined NMJs from gastrocnemius muscle of adult rat using immunofluorescence confocal microscopy to characterize in detail the distribution of these proteins relative to the distribution of acetylcholine receptors (AChRs). We have determined that erbB2 and erbB4 are enriched in the depths of the secondary junctional folds on the postsynaptic muscle membrane. In contrast, erbB3 at the NMJ was concentrated at presynaptic terminal Schwann cells. This distribution strongly argues that erbB2/erbB4 heterodimers are the functional postsynaptic neuregulin receptors of the NMJ. Neuregulin was localized to the axon terminal, secondary folds, and terminal Schwann cells, where it was in a position to signal through erbB receptors. MuSK was concentrated in the postsynaptic primary gutter region where it was codistributed with AChRs. Agrin was present at the axon terminal and in the basal lamina associated with the primary gutter region, but not in the secondary junctional folds. The differential distributions of the neuregulin and agrin signaling pathways argue against neuregulin and erbB receptors being localized to the NMJ via direct interactions with either agrin or MuSK.

Neural regulation of alpha-dystroglycan biosynthesis and glycosylation in skeletal muscle

Alpha-dystroglycan (alpha-DG) is part of a complex of cell surface proteins linked to dystrophin or utrophin, which is distributed over the myofiber surface and is concentrated at neuromuscular junctions. In laminin overlays of muscle extracts from developing chick hindlimb muscle, alpha-DG first appears at embryonic day (E) 10 with an apparent molecular mass of 120 kDa. By E15 it is replaced by smaller (approximately 100 kDa) and larger (approximately 150 kDa) isoforms. The larger form increases in amount and in molecular mass (>200 kDa) as the muscle is innervated and the postsynaptic membrane differentiates (E10-E20), and then decreases dramatically in amount after hatching. In myoblasts differentiating in culture the molecular mass of alpha-DG is not significantly increased by their replication, fusion, or differentiation into myotubes. Monoclonal antibody IIH6, which recognizes a carbohydrate epitope on alpha-DG, preferentially binds to the larger forms, suggesting that the core protein is differentially glycosylated beginning at E16. Consistent with prior observations implicating the IIH6 epitope in laminin binding, the smaller forms of alpha-DG bind more weakly to laminin affinity columns than the larger ones. In blots of adult rat skeletal muscle probed with radiolabeled laminin or monoclonal antibody IIH6, alpha-DG appears as a >200-kDa band that decreases in molecular mass but increases in intensity following denervation. Northern blots reveal a single mRNA transcript, indicating that the reduction in molecular mass of alpha-DG after denervation is not obviously a result of alternative splicing but is likely due to posttranslational modification of newly synthesized molecules. The regulation of alpha-DG by the nerve and its increased affinity for laminin suggest that glycosylation of this protein may be important in myofiber-basement membrane interactions during development and after denervation.

Expression, distribution and ultrastructural localization of the synapse-organizing molecule agrin in the mature avian retina

At the vertebrate neuromuscular junction the extracellular matrix molecule agrin is responsible for the formation, maintenance and regeneration of most if not all postsynaptic specializations. Several agrin isoforms are generated by alternative splicing which differ in their function and which are all expressed in the CNS. To analyse the role of agrin in the CNS, we investigated the expression and ultrastructural localization of agrin in the posthatched chick retina. In situ hybridization revealed the presence of agrin mRNA in all cellular layers of the mature retina, indicating that most if not all major retinal cell types synthesize agrin. Pan-specific as well as isoform-specific antiagrin antisera stained the optic fibre layer and the outer plexiform layer. However, only the pan-specific antiserum additionally stained the inner limiting membrane. Immunoelectron microscopy showed that in the optic fibre layer agrin was associated with ganglion cell axons and that at least part of this agrin corresponds to a neuronal isoform of agrin. In the outer plexiform layer, agrin was localized in the cleft between the photoreceptor terminals and the invaginating horizontal and bipolar cell dendrites. In the synapse-containing inner plexiform layer both antisera revealed punctate immunoreactivity. This staining corresponded to agrin concentrated in the synaptic cleft of conventional synapses as determined by preembedding immunoelectron microscopy. Agrin is thus concentrated at mature interneuronal synapses as it is at the neuromuscular junction, consistent with a role of agrin during formation and/or maintenance of synapses in the CNS.

Formation of functional synaptic connections between cultured cortical neurons from agrin-deficient mice

Numerous studies suggest that the extracellular matrix protein agrin directs the formation of the postsynaptic apparatus at the neuromuscular junction (NMJ). Strong support for this hypothesis comes from the observation that the high density of acetylcholine receptors (AChR) normally present at the neuromuscular junction fails to form in muscle of embryonic agrin mutant mice. Agrin is expressed by many populations of neurons in the central nervous system (CNS), suggesting that this molecule may also play a role in neuron-neuron synapse formation. To test this hypothesis, we examined synapse formation between cultured cortical neurons isolated from agrin-deficient mouse embryos. Our data show that glutamate receptors accumulate at synaptic sites on agrin-deficient neurons. Moreover, electrophysiological analysis demonstrates that functional glutamatergic and gamma-aminobutyric acid (GABA)ergic synapses form between mutant neurons. The frequency and amplitude of miniature postsynaptic glutamatergic and GABAergic currents are similar in mutant and age-matched wild-type neurons during the first 3 weeks in culture. These results demonstrate that neuron-specific agrin is not required for formation and early development of functional synaptic contacts between CNS neurons, and suggest that mechanisms of interneuronal synaptogenesis are distinct from those regulating synapse formation at the neuromuscular junction.

Alternatively spliced isoforms of nerve- and muscle-derived agrin: their roles at the neuromuscular junction

Agrin induces synaptic differentiation at the skeletal neuromuscular junction (NMJ); both pre- and postsynaptic differentiation are drastically impaired in its absence. Multiple alternatively spliced forms of agrin that differ in binding characteristics and bioactivity are synthesized by nerve and muscle cells. We used surgical chimeras, isoform-specific mutant mice, and nerve-muscle cocultures to determine the origins and nature of the agrin required for synaptogenesis. We show that agrin containing Z exons (Z+) is a critical nerve-derived inducer of postsynaptic differentiation, whereas neural isoforms containing a heparin binding site (Y+) and all muscle-derived isoforms are dispensable for major steps in synaptogenesis. Our results also suggest that the requirement of agrin for presynaptic differentiation is mediated indirectly by its ability to promote postsynaptic production or localization of appropriate retrograde signals.

Development of the vertebrate neuromuscular junction

We describe the formation, maturation, elimination, maintenance, and regeneration of vertebrate neuromuscular junctions (NMJs), the best studied of all synapses. The NMJ forms in a series of steps that involve the exchange of signals among its three cellular components--nerve terminal, muscle fiber, and Schwann cell. Although essentially any motor axon can form NMJs with any muscle fiber, an additional set of cues biases synapse formation in favor of appropriate partners. The NMJ is functional at birth but undergoes numerous alterations postnatally. One step in maturation is the elimination of excess inputs, a competitive process in which the muscle is an intermediary. Once elimination is complete, the NMJ is maintained stably in a dynamic equilibrium that can be perturbed to initiate remodeling. NMJs regenerate following damage to nerve or muscle, but this process differs in fundamental ways from embryonic synaptogenesis. Finally, we consider the extent to which the NMJ is a suitable model for development of neuron-neuron synapses.

BEN/SC1/DM-GRASP expression during neuromuscular development: a cell adhesion molecule regulated by innervation

BEN/SC1/DM-GRASP is a cell adhesion molecule belonging to the Ig superfamily that is transiently expressed during avian embryogenesis in a variety of cell types, including the motoneurons of the spinal cord. We have investigated the pattern of BEN expression during neuromuscular development of the chick. We show that both motoneurons and their target myoblasts express BEN during early embryonic development and that the protein becomes restricted at neuromuscular contacts as soon as postsynaptic acetylcholine receptor clusters are observed in muscle fibers. Muscle cells grown in vitro express and maintain BEN expression even when they fuse and give rise to mature myotubes. When embryos are deprived of innervation by neural tube ablation, BEN expression is observed in muscle fibers, whereas, in control, the protein is already restricted at neuromuscular synaptic sites. These results demonstrate that all myogenic cells intrinsically express BEN and maintain the protein in the absence of innervation. Conversely, when neurons are added to myogenic cultures, BEN is rapidly downregulated in muscle cells, demonstrating that innervation controls the restricted pattern of BEN expression seen in innervated muscles. After nerve section in postnatal muscles, BEN protein becomes again widely spread over muscle fibers. When denervated muscles are allowed to be reinnervated, the protein is reexpressed in regenerating motor axons, and reinnervation of synaptic sites leads to the concentration of BEN at neuromuscular junctions. Our results suggest that BEN cell adhesion molecule acts both in the formation of neuromuscular contacts during development and in the events leading to muscle reinnervation.

Expression patterns of transmembrane and released forms of neuregulin during spinal cord and neuromuscular synapse development

We mapped the distribution of neuregulin and its transmembrane precursor in developing, embryonic chick and mouse spinal cord. Neuregulin mRNA and protein were expressed in motor and sensory neurons shortly after their birth and levels steadily increased during development. Expression of the neuregulin precursor was highest in motor and sensory neuron cell bodies and axons, while soluble, released neuregulin accumulated along early motor and sensory axons, radial glia, spinal axonal tracts and neuroepithelial cells through associations with heparan sulfate proteoglycans. Neuregulin accumulation in the synaptic basal lamina of neuromuscular junctions occurred significantly later, coincident with a reorganization of muscle extracellular matrix resulting in a relative concentration of heparan sulfate proteoglycans at endplates. These results demonstrate an early axonal presence of neuregulin and its transmembrane precursor at developing synapses and a role for heparan sulfate proteoglycans in regulating the temporal and spatial sites of soluble neuregulin accumulation during development.

BEN/SC1/DM-GRASP expression during neuromuscular development: a cell adhesion molecule regulated by innervation

BEN/SC1/DM-GRASP is a cell adhesion molecule belonging to the Ig superfamily that is transiently expressed during avian embryogenesis in a variety of cell types, including the motoneurons of the spinal cord. We have investigated the pattern of BEN expression during neuromuscular development of the chick. We show that both motoneurons and their target myoblasts express BEN during early embryonic development and that the protein becomes restricted at neuromuscular contacts as soon as postsynaptic acetylcholine receptor clusters are observed in muscle fibers. Muscle cells grown in vitro express and maintain BEN expression even when they fuse and give rise to mature myotubes. When embryos are deprived of innervation by neural tube ablation, BEN expression is observed in muscle fibers, whereas, in control, the protein is already restricted at neuromuscular synaptic sites. These results demonstrate that all myogenic cells intrinsically express BEN and maintain the protein in the absence of innervation. Conversely, when neurons are added to myogenic cultures, BEN is rapidly downregulated in muscle cells, demonstrating that innervation controls the restricted pattern of BEN expression seen in innervated muscles. After nerve section in postnatal muscles, BEN protein becomes again widely spread over muscle fibers. When denervated muscles are allowed to be reinnervated, the protein is reexpressed in regenerating motor axons, and reinnervation of synaptic sites leads to the concentration of BEN at neuromuscular junctions. Our results suggest that BEN cell adhesion molecule acts both in the formation of neuromuscular contacts during development and in the events leading to muscle reinnervation.

Neurotrophins regulate agrin-induced postsynaptic differentiation

The precise orchestration of synaptic differentiation is critical for efficient information exchange in the nervous system. The nerve-muscle synapse forms in response to agrin, which is secreted from the motor nerve terminal and induces the clustering of acetylcholine receptors (AChRs) and other elements of the postsynaptic apparatus on the subjacent muscle cell surface. In view of the highly restricted spatial localization and the plasticity of neuromuscular junctions, it seems likely that synapse formation and maintenance are regulated by additional, as-yet-unidentified factors. Here, we tested whether neurotrophins modulate the agrin-induced differentiation of postsynaptic specializations. We show that both brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4) inhibit agrin-induced AChR clustering on cultured myotubes. Nerve growth factor and NT-3 are without effect. Muscle cells express full-length TrkB, the cognate receptor for BDNF and NT-4. Direct activation of this receptor by anti-TrkB antibodies mimicked the BDNF/NT-4 inhibition of agrin-induced AChR clustering. This BDNF/NT-4 inhibition is likely to be an intrinsic mechanism for regulating AChR clustering, because neutralization of endogenous TrkB ligands resulted in elevated levels of AChR clustering even in the absence of added agrin. Finally, high concentrations of agrin can occlude the BDNF/NT-4 inhibition of AChR clustering. These results indicate that an interplay between agrin and neurotrophins can regulate the formation of postsynaptic specializations. They also suggest a mechanism for the suppression of postsynaptic specializations at nonjunctional regions.

Muscle-specific agrin isoforms reduce phosphorylation of AChR gamma and delta subunits in cultured muscle cells

The accumulation of nicotinic acetylcholine receptors (AChRs) at neuromuscular synapses is triggered by agrin, a protein that is synthesized by both nerve and muscle. Nerve-derived agrin, which contains an amino acid insert at a conserved splice site in the carboxy-terminal part of the protein, induces AChR aggregation and causes tyrosine phosphorylation of the AChR beta subunit. In contrast, agrin isoforms synthesized by muscle cells lack such an insert and have no effect on AChR distribution. In order to identify possible functional roles of muscle-derived agrin we have analyzed further the effect of various fragments of recombinant agrin on AChR phosphorylation. A carboxy-terminal fragment of muscle agrin, c95A0B0, reduced AChR gamma and delta subunit phosphorylation when added to C2C12 myotubes in culture. Although c95A0B0 had no effect on AChR beta subunit phosphorylation when added alone, it inhibited AChR beta subunit phosphorylation and AChR aggregation by the nerve-specific agrin isoform c95A4B8. We conclude that muscle-derived agrin can influence, both directly and indirectly, AChR phosphorylation. Such changes may play a role in the formation, maintenance, or function of the neuromuscular junction.

Agrin orchestrates synaptic differentiation at the vertebrate neuromuscular junction

The synapse is a key structure that is involved in perception, learning and memory. Understanding the sequence of steps that is involved in establishing synapses during development might also help to understand mechanisms that cause changes in synapses during learning and memory. For practical reasons, most of our current knowledge of synapse development is derived from studies of the vertebrate neuromuscular junction (NMJ). Several lines of evidence strongly suggest that motor axons release the molecule agrin to induce the formation of the postsynaptic apparatus in muscle fibers. Recent advances implicate proteins such as dystroglycan, MuSK, and rapsyn in the transduction of agrin signals. Recently, additional functions of agrin have been discovered, including the upregulation of gene transcription in myonuclei and the control of presynaptic differentiation. Agrin therefore appears to play a unique role in controlling synaptic differentiation on both sides of the NMJ.

Antisense agrin cDNA transfection blocks neuroblastoma cell-induced acetylcholine receptor aggregation when co-cultured with myotubes

A neuroblastoma x glioma hybrid cell line, NG108-15, was able to induce the aggregation of AChRs when co-cultured with myotubes. NG108-15 cells in culture expressed agrin, producing a protein of approximately 220 kDa and a transcript of approximately 8.0 kb. The mRNA encoding the agrin isoform having no amino acid insertion at either the Y or the Z site, namely agrin0.0, was the only transcript detected in NG108-15 cells when they were cultured alone or co-cultured with myotubes. NG108-15 cells could be induced to differentiate by chemical treatment, and the chemical-induced differentiation of NG108-15 cells increased the level of agrin mRNA expression approximately fourfold while the expression of a housekeeping gene remained relatively unchanged. The increase in agrin expression of differentiated NG108-15 cells paralleled the increase in AChR-aggregating activity of differentiated NG108-15 cells, indicating that the agrin derived from NG108-15 cells could be the receptor-aggregating factor. In addition, we created a stable clonal NG108-15 cell line that was transfected with antisense agrin cDNA and its expression of agrin was abolished, while its AChR-aggregating activity was completely lost when co-cultured with myotubes. This is the first direct demonstration that NG108-15 cell-induced AChR aggregation on cultured myotubes is mediated by neuron-derived agrin.

Evidence that agrin directly influences presynaptic differentiation at neuromuscular junctions in vitro

The synaptic protein agrin is required for aspects of both pre- and postsynaptic differentiation at neuromuscular junctions. Although a direct effect of agrin on postsynaptic differentiation, presumably through the MuSK receptor, is established, it is not clear whether agrin directly affects the presynaptic nerve. To provide evidence on this point, we used anti-agrin IgG to disrupt agrin function in chick ciliary ganglion (CG) neuron/myotube cocultures. In cocultures grown in the presence of 200 microg/ml anti-agrin IgG, clustering of acetylcholine receptors (AChRs), extracellular matrix proteins, and the synaptic vesicle protein synaptotagmin (syt) at nerve-muscle contacts was inhibited. Syt clustering was still inhibited in the presence of 100 microg/ml blocking antibody, while the postsynaptic clustering of AChRs, heparan sulphate proteoglycan, and s-laminin was retained. Additionally, in CG neurons cultured with COS cells expressing agrin A0B0, which lacks the ability to signal postsynaptic differentiation, syt clustering was induced and this clustering was also blocked by anti-agrin IgG. Our results demonstrate that agrin function is acutely required for pre- and postsynaptic differentiation in vitro, and strongly suggest that agrin is directly involved in the induction of presynaptic differentiation.

Laminin-induced acetylcholine receptor clustering: an alternative pathway

The induction of acetylcholine receptor (AChR) clustering by neurally released agrin is a critical, early step in the formation of the neuromuscular junction. Laminin, a component of the muscle fiber basal lamina, also induces AChR clustering. We find that induction of AChR clustering in C2 myotubes is specific for laminin-1; neither laminin-2 (merosin) nor laminin-11 (a synapse-specific isoform) are active. Moreover, laminin-1 induces AChR clustering by a pathway that is independent of that used by neural agrin. The effects of laminin-1 and agrin are strictly additive and occur with different time courses. Most importantly, laminin- 1-induced clustering does not require MuSK, a receptor tyrosine kinase that is part of the receptor complex for agrin. Laminin-1 does not cause tyrosine phosphorylation of MuSK in C2 myotubes and induces AChR clustering in myotubes from MuSK-/- mice that do not respond to agrin. In contrast to agrin, laminin-1 also does not induce tyrosine phosphorylation of the AChR, demonstrating that AChR tyrosine phosphorylation is not required for clustering in myotubes. Laminin-1 thus acts by a mechanism that is independent of that used by agrin and may provide a supplemental pathway for AChR clustering during synaptogenesis.

Neural activity affects distribution of glutamate receptors during neuromuscular junction formation in Drosophila embryos

Changes in the distribution and density of transmitter receptors in the postsynaptic cell are required steps for functional synapse formation. We raised antibodies against Drosophila glutamate receptors (DGluR-II) and visualized the distribution of receptors during neuromuscular junction formation in embryos. In wild-type embryos, embryonic development is complete within 22 hr after egg lying (AEL) and neuromuscular junction (NMJ) formation begins at 13 hr AEL. At the time of initial synapse formation, DGluR-IIs appeared as clusters closely associated with some muscle nuclei. Subsequently, these nonjunctional clusters dispersed while DGluR-IIs accumulated at the junctional region. In a paralytic temperature-sensitive mutant, para(ts1), neural activity decreases drastically at restrictive temperatures. When neural activity was blocked throughout synaptogenesis by rearing embryos at a restrictive temperature prior to the beginning of synaptogenesis, 12 hr AEL, the dispersal of extrajunctional clusters was significantly suppressed and no accumulation of receptors at the junction was observed at 22 hr AEL. However, when neural activity was blocked later, by rearing embryos at a restrictive temperature from 13 hr AEL, DGluR-IIs did not accumulate at the NMJ, although extrajunctional clusters dispersed normally. These findings suggest that the neural activity differentially regulates dissipation of receptor clusters in the nonjunctional region and accumulation of receptors at the junctional region.

Non-neural agrin codistributes with acetylcholine receptors during early differentiation of Torpedo electrocytes

Agrin, an extracellular matrix protein synthesized by nerves and muscles is known to promote the clustering of acetylcholine receptors and other synaptic proteins in cultured myotubes. This observation suggests that agrin may provide at least part of the signal for synaptic specialization in vivo. The extracellular matrix components agrin, laminin and merosin bind to alpha-dystroglycan, a heavily glycosylated peripheral protein part of the dystrophin-glycoprotein complex, previously characterized in the sarcolemma of skeletal and cardiac muscles and at the neuromuscular junction. In order to understand further the function of agrin and alpha DG in the genesis of the acetylcholine receptor-rich membrane domain, the settling of components of the dystrophin-glycoprotein complex and agrin was followed by immunofluorescence localization in developing Torpedo marmorata electrocytes. In 40–45 mm Torpedo embryos, a stage of development at which the electrocytes exhibit a definite structural polarity, dystrophin, alpha/beta-dystroglycan and agrin accumulated concomitantly with acetylcholine receptors at the ventral pole of the cells. Among these components, agrin appeared as the most intensely concentrated and sharply localized. The scarcity of the nerve-electrocyte synaptic contacts at this stage of development, monitored by antibodies against synaptic vesicles, further indicates that before innervation, the machinery for acetylcholine receptor clustering is provided by electrocyte-derived agrin rather than by neural agrin. These observations suggest a two-step process of acetylcholine receptor clustering involving: (i) an instructive role of electrocyte-derived agrin in the formation of a dystrophin-based membrane scaffold upon which acetylcholine receptor molecules would accumulate according to a diffusion trap model; and (ii) a maturation and/or stabilization step controlled by neural agrin. In the light of these data, the existence of more than one agrin receptor is postulated to account for the action of agrin variants at different stages of the differentiation of the postsynaptic membrane in Torpedo electrocytes.

Defective neuromuscular synaptogenesis in agrin-deficient mutant mice

During neuromuscular synapse formation, motor axons induce clustering of acetylcholine receptors (AChRs) in the muscle fiber membrane. The protein agrin, originally isolated from the basal lamina of the synaptic cleft, is synthesized and secreted by motoneurons and triggers formation of AChR clusters on cultured myotubes. We show here postsynaptic AChR aggregates are markedly reduced in number, size, and density in muscles of agrin-deficient mutant mice. These results support the hypothesis that agrin is a critical organizer of postsynaptic differentiation does occur in the mutant, suggesting the existence of a second-nerve-derived synaptic organizing signal. In addition, we show that intramuscular nerve branching and presynaptic differentiation are abnormal in the mutant, phenotypes which may reflect either a distinct effect of agrin or impaired retrograde signaling from a defective postsynaptic apparatus.

Alternative splicing of agrin alters its binding to heparin, dystroglycan, and the putative agrin receptor

Agrin is a heparan sulfate proteoglycan that induces aggregation of acetylcholine receptors (AChRs) at the neuromuscular synapse. This aggregating activity is modulated by alternative splicing. Here, we compared binding of agrin isoforms to heparin, alpha-dystroglycan, and cultured myotubes. We find that the alternatively spliced 4 amino acids insert (KSRK) is required for heparin binding. The binding affinity of agrin isoforms to alpha-dystroglycan correlates neither with binding to heparin nor with their AChR-aggregating activities. Moreover, the minimal fragment sufficient to induce AChR aggregation does not bind to alpha-dystroglycan. Nevertheless, this fragment still binds to cultured muscle cells. Its binding is completed only by agrin isoforms that are active in AChR aggregation, and therefore this binding site is likely to represent the receptor that initiates AChR clustering.

Biochemical and functional characterization of basal lamina-bound agrin in the chick central nervous system

Agrin is a high-molecular weight extracellular matrix molecule, initially purified from the electric organ of the marine ray Torpedo californica, which induces on the surface of cultured myotubes the formation of postsynaptic specializations similar to those found at the neuromuscular junction. Agrin immunoreactivity is highly concentrated in the basal lamina of the synaptic cleft but is also found in a number of other tissues where its function is not known. We characterized agrin associated with two basal laminae from the central nervous system, the inner limiting membrane of the retina and the mesencephalic external limiting membrane. A major broad band with an apparent molecular weight of > 300 kDa was identified in immunoblots of isolated basal laminae from retina, mesencephalon, kidney and muscle, showing that basal lamina-bound agrin from the central nervous system and that from non-neural tissues have similar molecular sizes. Agrin is stably but not covalently bound to the inner limiting membrane and could be completely removed only with strong detergents. Agrin could be partially extracted with buffers that are also able to partially release acetylcholine receptor aggregation activity from the neuromuscular junction or from the electric organ. Despite these immunological and biochemical similarities, agrin from both central nervous system-derived basal laminae was not able to induce acetylcholine receptor aggregation on cultured myotubes. This shows that functionally different agrin isoforms are associated with basal laminae in the central nervous system compared to the neuromuscular junction or the electric organ.

An amino-terminal extension is required for the secretion of chick agrin and its binding to extracellular matrix

Agrin is an extracellular matrix (ECM) protein with a calculated relative molecular mass of more than 200 kD that induces the aggregation of acetylcholine receptors (AChRs) at the neuromuscular junction. This activity has been mapped to its COOH terminus. In an attempt to identify the functions of the NH2-terminal end, we have now characterized full-length chick agrin. We show that chick agrin encoded by a previously described cDNA is not secreted from transfected cells. Secretion is achieved with a construct that includes an additional 350 bp derived from the 5' end of chick agrin mRNA. Recombinant agrin is a heparan sulfate proteoglycan (HSPG) of more than 400 kD with glycosaminoglycan side chains attached only to the NH2-terminal half. Endogenous agrin in tissue homogenates also has an apparent molecular mass of > 400 kD. While the amino acid sequence encoded by the 350-bp extension has no homology to published rat agrin, it includes a stretch of 15 amino acids that is 80% identical to a previously identified bovine HSPG. The extension is required for binding of agrin to ECM. AChR aggregates induced by recombinant agrin that includes the extension are considerably smaller than those induced by agrin fragments, suggesting that binding of agrin to ECM modulates the size of receptor clusters. In addition, we found a site encoding seven amino acids at the NH2-terminal end of agrin that is alternatively spliced. While motor neurons express the splice variant with the seven amino acid long insert, muscle cells mainly synthesize isoforms that lack this insert. In conclusion, the cDNAs described here code for chick agrin that has all the characteristics previously allocated to endogenous agrin.

Agrin is a differentiation-inducing "stop signal" for motoneurons in vitro

Proteins of the synaptic basal lamina are important in directing the differentiation of motor nerve terminals. One synaptic basal lamina protein, agrin, which influences postsynaptic muscle differentiation, has been suggested to influence nerve terminals as well. To test this hypothesis, we cocultured chick ciliary ganglion neurons with agrin-expressing CHO cells. Ciliary ganglion neurons, but not sensory neurons, adhered five times as well to agrin-expressing cells as to untransfected cells. Further, ciliary ganglion neurites were growth inhibited upon contact with agrin-expressing cells. Finally, the synaptic vesicle protein synaptotagmin became concentrated at contacts between ciliary ganglion neurites and agrin-expressing cells. These activities were shared by neuronal and muscle-derived agrin isoforms, consistent with the hypothesis that muscle agrin may influence the presynaptic axon. Our results suggest that agrin influences the growth and differentiation of motoneurons in vivo.

Acetylcholine receptor clustering in C2 muscle cells requires chondroitin sulfate

Proteoglycans have been implicated in the clustering of acetylcholine receptors (AChRs) on cultured myotubes and at the neuromuscular junction. We report that the presence of chondroitin sulfate is associated with the ability of cultured myotubes to form spontaneous clusters of AChRs. Three experimental manipulations of wild type C2 cells in culture were found to affect both glycosaminoglycans (GAGs) and AChR clustering in concert. Chlorate was found to have dose-dependent negative effects both on GAG sulfation and on the frequency of AChR clusters. When extracellular calcium was raised from 1.8 to 6.8 mM in cultures of wild-type C2 myotubes, increases were observed both in the level of cell layer-associated chondroitin sulfate and in the frequency of AChR clusters. Culture of wild-type C2 myotubes in the presence of chondroitinase ABC eliminated cell layer-associated chondroitin sulfate while leaving heparan sulfate intact and simultaneously prevented the formation of AChR clusters. Treatment with either chlorate or chondroitinase inhibited AChR clustering only if begun prior to the spontaneous formation of clusters. We propose that chondroitin sulfate plays an essential role in the initiation of AChR clustering and in the early events of synapse formation on muscle.

Expression of agrin mRNA is altered following seizures in adult rat brain

Agrin mRNA is broadly distributed throughout the adult rat brain, consistent with its proposed role in synaptogenesis and the organization of synaptic proteins in the central nervous system. The present study examined the effect of neuronal activity on agrin mRNA expression in adult rat forebrain using the hilus lesion paradigm for seizure induction and in situ hybridization and polymerase chain reaction techniques for quantification and characterization of agrin mRNA content. Seizures induced rapid, prolonged, and region-specific changes in agrin mRNA expression with the most prominent alterations occurring in hippocampal and cortical neurons. However, there were no detectable perturbations in the relative abundance of alternatively spliced agrin transcripts in affected brain regions. Activity-dependent changes in agrin expression suggest a role for this protein in modifications of synaptic structure associated with functional synaptic plasticity.

ARIA is concentrated in the synaptic basal lamina of the developing chick neuromuscular junction

ARIA is a member of a family of polypeptide growth and differentiation factors that also includes glial growth factor (GGF), neu differentiation factor, and heregulin. ARIA mRNA is expressed in all cholinergic neurons of the central nervous systems of rats and chicks, including spinal cord motor neurons. In vitro, ARIA elevates the rate of acetylcholine receptor incorporation into the plasma membrane of primary cultures of chick myotubes. To study whether ARIA may regulate the synthesis of junctional synaptic acetylcholine receptors in chick embryos, we have developed riboprobes and polyclonal antibody reagents that recognize isoforms of ARIA that include an amino-terminal immunoglobulin C2 domain and examined the expression and distribution of ARIA in motor neurons and at the neuromuscular junction. We detected significant ARIA mRNA expression in motor neurons as early as embryonic day 5, around the time that motor axons are making initial synaptic contacts with their target muscle cells. In older embryos and postnatal animals, we found ARIA protein concentrated in the synaptic cleft at neuromuscular junctions, consistent with transport down motor axons and release at nerve terminals. At high resolution using immunoelectron microscopy, we detected ARIA immunoreactivity exclusively in the synaptic basal lamina in a pattern consistent with binding to synapse specific components on the presynaptic side of the basal lamina. These results support a role for ARIA as a trophic factor released by motor neuron terminals that may regulate the formation of mature neuromuscular synapses.

Possible strategies for treatment of SMA patients: a neurobiologist's view

This paper discusses possible strategies that might prevent or alleviate muscle weakness of SMA patients and hence improve their condition. The strategies discussed are as follows. (1) Prevention of motoneurone death. To achieve this two main approaches have been applied. Firstly, trophic factors have been used to prevent motoneurone death after nerve injury and clinically in diseases such as motoneurone disease. The results of these attempts will be described. Secondly, the possibility that injured motoneurones die as a result of the excitotoxic effects of the excitatory transmitter glutamate will be explored. Evidence will be presented which indicates that blocking glutamate receptors can rescue injured motoneurones from death. (2) Replacement of lost motoneurones by embryonic grafts. Motoneurones from grafts of embryonic spinal cord have been shown to survive in the adult spinal cord and are able to reinnervate skeletal muscles. The potential and practical problems of this approach will be discussed. (3) Expansion or motor unit territory of surviving motoneurones. Such an expansion of the territory occupied by individual motor units can be achieved by encouraging sprouting and ensuring that the newly formed connections between the motoneurone and muscle fibres are maintained, so that individual motor units are capable of developing more force. Strategies to achieve such an expansion of motor unit territory will be described. Finally, combinations of some of these approaches are considered.

Agrin: toward a molecular understanding of synapse regeneration

One of the foremost challenges to repairing damage after stroke, trauma, or disease is the regeneration of synaptic connections between neurons. Here, we consider recent strides in our understanding of the molecular basis of synapse formation and regeneration. We will focus on the protein agrin, a key player in synaptogenesis at neuromuscular junctions and perhaps at central nervous system synapses as well. Insights into agrin and its receptor could guide the development of rational therapies to combat neuronal degeneration. We will also consider recent surprising and provocative data linking the mechanisms of synapse formation and the cellular pathology in Duchenne muscular dystrophy.