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PubMed1991

Comparison of agrin-like proteins from the extracellular matrix of chicken kidney and muscle with neural agrin, a synapse organizing protein.

Agrin is a synapse-organizing protein that is concentrated in embryonic motor neurons and the synaptic basal lamina of the neuromuscular junction. Agrin or closely related proteins are also associated with most other basal laminae. Here I report that the major agrin-like proteins from the nervous system and other tissues of the chicken are immunochemically and biochemically similar. Four major agrin-like proteins of approximately 60, 72, 80, and 90 kDa were identified on immunoblots of agrin preparations from both neural and non-neural tissues. Agrin-like proteins from embryonic chicken brain and adult kidney were similar in amino acid composition. Rabbit antisera against each of the kidney proteins labeled basement membranes of several tissues, as well as spinal cord motor neurons. These antibodies specifically precipitated and inhibited acetylcholine receptor (AChR)-aggregating activity from the chicken nervous system and Torpedo electric organ. Thus, the agrin-like proteins of non-neural tissues in the chicken are closely related to agrin from the nervous system. However, the AChR-aggregating activity of chicken agrin preparations differed depending on the tissue of origin. Agrin enriched by immunoaffinity chromatography from the central nervous system induced large numbers of AChR aggregates on cultured myotubes. In contrast, agrin preparations from other chicken tissues induced dramatically fewer and smaller AChR aggregates. The difference in biological activity between these agrin preparations may reflect differential inactivation or the existence of tissue- or cell-specific isoforms of agrin.

Agrin↗

PubMed1991

The putative agrin receptor binds ligand in a calcium-dependent manner and aggregates during agrin-induced acetylcholine receptor clustering.

Agrin derived from Torpedo electric organ induces the clustering of acetylcholine receptors (AChRs) on cultured myotubes. As a first step toward characterizing the plasma membrane receptor for agrin, we have examined agrin binding to cultured myotubes. Agrin binding is saturable as measured by radioimmunoassay and, like agrin-induced AChR clustering, requires extracellular calcium. Immunofluorescence shows that on myotubes incubated with agrin at 4 degrees C, agrin binds in a uniform, finely punctate pattern that correlates poorly with the distribution of AChRs. Myotubes stimulated with agrin at 37 degrees C for greater than or equal to 2 hr show a coclustering of agrin binding sites and AChRs. By contrast, if anti-AChR antibodies are used either to cluster or to internalize AChRs, the distribution and number of agrin binding sites remain unchanged. The aggregation and calcium dependence of the putative agrin receptor may represent important control points in postsynaptic differentiation.

Agrin↗

PubMed1992

Muscle-derived agrin in cultured myotubes: expression in the basal lamina and at induced acetylcholine receptor clusters.

The synaptic basal lamina (SBL) directs key aspects of the differentiation of regenerating neuromuscular junctions. A range of experiments indicate that agrin or a closely related molecule is stably associated with the SBL and participates in inducing the formation of the postsynaptic apparatus after damage to adult muscle. The selective concentration of agrin-related molecules in the SBL suggests that agrin is secreted locally by cellular components of the nerve-muscle synapse. In vivo studies on aneural embryonic muscle indicate that the muscle cell is one source of the agrin-like molecules in the SBL. Here we have used cultured chick muscle cells to study the expression of agrin-related molecules in the absence of innervation. Immunofluorescence and immunoelectron microscopy show that myogenic cells in culture express agrin-related molecules on their surfaces, and that at least a subset of these molecules are associated with the basal lamina. Moreover, in short term cultures agrin-like molecules accumulate on the surfaces of myogenic cells grown in unsupplemented basal media. We quantified the expression of agrin-like molecules on the cell surface using a solid-phase radioimmune assay. The expression of these molecules is relatively low during the first 6 days of culture and increases fourfold during the second week. The stimulation of the expression of agrin-related molecules in these long-term cultures requires the presence of chick embryo extract or fetal calf serum. We also characterized the expression of muscle-derived agrin-like molecules at clusters of AChR. These agrin-related molecules are not consistently colocalized at spontaneous AChR aggregates; however, they are selectively concentrated at greater than or equal to 90% of the AChR clusters that are induced by Torpedo agrin. These data, together with previous results from in vivo developmental experiments indicate that the agrin-like molecules in the synaptic basal lamina are derived at least in part from the muscle cell. In addition, the expression of agrin-like molecules can be regulated by soluble factors present in CEE and FBS. Finally, the selective localization of these molecules at induced AChR clusters, taken together with their localization in the basal lamina, suggests that agrin-like molecules secreted by the muscle cell play an important role in the formation and/or the stabilization of the postsynaptic apparatus.

Agrin↗

PubMed1991

Agrin induces phosphorylation of the nicotinic acetylcholine receptor.

Agrin causes acetylcholine receptors (AChRs) on chick myotubes in culture to aggregate, forming specializations that resemble the postsynaptic apparatus at the vertebrate skeletal neuromuscular junction. Here we report that treating chick myotubes with agrin caused an increase in phosphorylation of the AChR beta, gamma, and delta subunits. H-7, a potent inhibitor of several protein serine kinases, blocked agrin-induced phosphorylation of the gamma and delta subunits, but did not prevent either agrin-induced AChR aggregation or phosphorylation of the beta subunit. Experiments with anti-phosphotyrosine antibodies demonstrated that agrin caused an increase in tyrosine phosphorylation of the beta subunit that began within 30 min of adding agrin to the myotube cultures, reached a plateau by 3 hr, and was blocked by treatments known to block agrin-induced AChR aggregation. Anti-phosphotyrosine antibodies labeled agrin-induced specializations as they do the postsynaptic apparatus. These results suggest that agrin-induced tyrosine phosphorylation of the beta subunit may play a role in regulating AChR distribution.

Agrin↗

PubMed1992

Mechanism of agrin-induced acetylcholine receptor aggregation.

Agrin induces the formation of specializations on chick myotubes in culture at which several components of the postsynaptic apparatus accumulate, including acetylcholine receptors (AChRs). Agrin also induces AChR phosphorylation. Several lines of evidence suggest that agrin-induced phosphorylation of tyrosine residues in the beta subunit of the AChR is an early step in receptor aggregation: agrin-induced phosphorylation and aggregation have the same dose dependence; treatments that prevent aggregation block phosphorylation; phosphorylation begins before any detectable change in receptor distribution, reaches a maximum hours before aggregation is complete, and declines slowly together with the disappearance of aggregates after agrin is withdrawn; agrin slows the rate at which receptors are solubilized from intact myotubes by detergent extraction; and the change in receptor extractability parallels the change in phosphorylation. A model for agrin-induced AChR aggregation is presented in which phosphorylation of AChRs by an agrin-activated protein tyrosine kinase causes receptors to become attached to the cytoskeleton, which reduces their mobility and detergent extractability, and leads to the accumulation of receptors in the vicinity of the activated kinase, forming an aggregate.

Agrin↗

PubMed1992

The agrin gene codes for a family of basal lamina proteins that differ in function and distribution.

We isolated two cDNAs that encode isoforms of agrin, the basal lamina protein that mediates the motor neuron-induced aggregation of acetylcholine receptors on muscle fibers at the neuromuscular junction. Both proteins are the result of alternative splicing of the product of the agrin gene, but unlike agrin, they are inactive in standard acetylcholine receptor aggregation assays. They lack one (agrin-related protein 1) or two (agrin-related protein 2) regions in agrin that are required for its activity. Expression studies provide evidence that both proteins are present in the nervous system and muscle and that, in muscle, myofibers and Schwann cells synthesize the agrin-related proteins while the axon terminals of motor neurons are the sole source of agrin.

Agrin↗

PubMed1992

Agrin released by motor neurons induces the aggregation of acetylcholine receptors at neuromuscular junctions.

To test the hypothesis that agrin mediates motor neuron-induced aggregation of acetylcholine receptors (AChRs) in skeletal muscle fibers and to determine whether the agrin active in this process is released by motor neurons, we raised polyclonal antibodies to purified ray agrin that blocked its receptor aggregating activity. When the antibodies were applied to chick motor neuron--chick myotube cocultures, they inhibited the formation of AChR aggregates at and near neuromuscular contacts, demonstrating that agrin plays a role in the induction of the aggregates. Rat motor neurons, like chick motor neurons, induce AChR aggregates on chick myotubes. This effect was not inhibited by our antibodies, indicating that, although the antibodies inhibited the activity of chick agrin, they did not have a similar effect on rat agrin. We conclude that agrin released by rat motor neurons induced the chick myotubes to aggregate AChRs.

Agrin↗

PubMed1992

Early appearance of and neuronal contribution to agrin-like molecules at embryonic frog nerve-muscle synapses formed in culture.

Antibodies against chicken and Torpedo agrin were used for immunofluorescent staining in order to assess the spatial distribution and temporal appearance of agrin-like molecules at newly formed synaptic contacts in cultures of embryonic Xenopus nerve and muscle cells. The antibodies stained Xenopus neuromuscular junctions and removed ACh receptor (AChR)-aggregating activity from extracts of Xenopus brain. Immunofluorescence was observed at almost all nerve-induced AChR aggregates, even at microaggregates in cocultures as young as 7.5 hr and at nerve-muscle contacts less than 2 hr old. Microdeposits of immunofluorescence extended as far distally as, or farther than, the microaggregates of AChRs along young nerve-muscle contacts. They also occurred along portions of growing neurites that were not in contact with muscle. By contrast, immunofluorescence was rarely observed at the nonsynaptic aggregates of AChRs that form on noninnervated muscle cells. These results raise the possibility that neuronally derived microaggregates of agrin-like molecules may be primary sites of nerve-induced clustering of AChRs, and they indicate that these molecules are present at embryonic nerve-muscle synapses from the very onset of AChR aggregation. The cellular origin of the agrin-like molecules at synapses was examined in cross-species cocultures in which the neurons and muscle cells were obtained from embryos of Xenopus laevis and Rana pipiens. Immunofluorescent staining with anti-agrin antibodies reactive at both Rana and Xenopus neuromuscular junctions revealed immunofluorescence at AChR aggregates along nerve-muscle contacts involving both cross-species combinations. Immunofluorescent staining with an anti-agrin antibody reactive at Rana but not at Xenopus neuromuscular junctions was positive only at cross-species nerve-muscle contacts involving Rana neurons. These results provide the first demonstration that embryonic neurons supply agrin-like molecules to the synapses they form with embryonic muscle cells.

Agrin↗

PubMed1992

cDNA that encodes active agrin.

Agrin is thought to mediate the motor neuron-induced aggregation of AChRs and AChE on the surface of muscle fibers at neuromuscular junctions. We have isolated a cDNA from a chick brain library that, based on sequence homology and expression experiments, codes for active agrin. Examination of the sequence reveals considerable similarity to homologous cDNAs previously isolated from ray and rat libraries. A conspicuous difference is an insertion of 33 bp in chick agrin cDNA, which endows the encoded protein with AChR/AChE aggregating activity. Homologous transcripts having the 33 bp insertion were detected in the ray CNS, which indicates that an insertion of similar size is conserved in agrin in many, if not all, vertebrate species. Results of in situ hybridization studies and PCR experiments on mRNA isolated from motor neuron-enriched fractions of the spinal cord indicate that, consistent with the agrin hypothesis, motor neurons contain transcripts that code for active agrin.

Agrin↗

PubMed1991

Agrin induces alpha-actinin, filamin, and vinculin to co-localize with AChR clusters on cultured chick myotubes.

Agrin induces discrete high-density patches of acetylcholine receptors (AChRs) and other synaptic components on cultured myotubes in a manner that resembles synaptic differentiation. Furthermore, agrin-like molecules are present at developing neuromuscular junctions in vivo. This provides us with a unique opportunity to manipulate AChR patching in order to examine the role of cytoskeletal components. Cultured chick myotubes were fixed and labeled to visualize the distributions of actin, alpha-actinin, filamin, tropomyosin, and vinculin. Overnight exposure to agrin caused a small amount of alpha-actinin, filamin, and vinculin to reorganize into discrete clusters. Double-labeling studies revealed that 78% of the AChR clusters were associated with detectable concentrations of filamin, 70% with alpha-actinin, and 58% with vinculin. Filamin even showed congruence to AChRs within clustered regions. By contrast, actin (visualized with fluorescein-phalloidin) and tropomyosin did not show specific associations with agrin-induced AChR clusters. The accumulation of cytoskeletal components at AChRs clusters raised the possibility that cytoskeletal rearrangements direct AChR clustering. However, a time course of agrin-induced clustering that focused on filamin revealed that most of the early AChR clusters (3-6 h) were not associated with detectable amounts of cytoskeletal material. The accumulation of cytoskeletal material at later times (12-18 h) may imply a role in maintenance and stabilization, but it appears unlikely that these cytoskeletal elements initiate AChR clustering on myotubes.

Actinin↗

PubMed1992

Structure and chromosomal localization of the mammalian agrin gene.

Agrin, a component of the synaptic basal lamina, has been shown to induce clustering of ACh receptors on the surface of muscle fibers. Analysis of cDNAs isolated from a rat embryonic spinal cord library demonstrated that agrin contains domains similar to regions of protease inhibitors, laminin and epidermal growth factor. The domain structure of agrin is further revealed here in an analysis of the agrin gene. Two additional internal repeated sequences are defined: one rich in cysteine residues with no homology to other proteins, and another similar to the laminin G domain, which is involved in heparin binding. Alternative RNA splicing at two positions in the gene predicts up to eight possible forms of the agrin protein. The gene (symbol AGRN/Agrn) has been assigned to chromosome 1 region pter-p32 in human and to mouse chromosome 4.

Agrin↗

PubMed1990

Inhibition of agrin-induced acetylcholine-receptor aggregation by heparin, heparan sulfate, and other polyanions.

Heparin and heparan sulfate have been shown to block nerve-induced acetylcholine-receptor (AChR) aggregation at developing neuromuscular junctions. We found that heparin, heparan sulfate, and a wide variety of other polyanions also inhibited agrin-induced AChR aggregation. The more highly charged the polyanion, the more potent it was as an inhibitor. Inhibition of agrin-induced AChR aggregation was due, at least in part, to the formation of a complex between the polyanion and agrin that was inactive. These findings are consistent with the hypothesis that nerve-induced aggregation of AChRs is mediated by the release of agrin, or a closely related protein, from axon terminals and suggest that a polyanion, such as a sulfated proteoglycan, may be involved in the interaction of agrin with its receptor on the myotube surface.

Agrin↗

PubMed1991

Structure and expression of a rat agrin.

Agrin is a component of the basal lamina that causes the aggregation of acetylcholine receptors on cultured muscle fibers. An agrin cDNA clone isolated from electromotor neurons of a marine ray was used to characterize the corresponding cDNAs from a rat embryonic spinal cord library. Analysis of a set of clones predicts a 1940 amino acid protein containing 141 cysteine residues. The predicted protein has nine domains homologous to protease inhibitors, a region similar to domain III of laminin, and four epidermal growth factor repeats. The agrin gene is expressed in rat embryonic nervous system and muscle. The rat agrin protein is concentrated at synapses, where it may play a role in development and regeneration.

Agrin↗

PubMed1991

Agrin mediates cell contact-induced acetylcholine receptor clustering.

One of the important events in synapse formation is the accumulation of neurotransmitter receptors beneath the presynaptic nerve terminal. Agrin is a component of the synaptic basal lamina that induces the clustering of acetylcholine receptors when bath-applied to muscle fibers in culture. When a cDNA encoding a putative agrin protein is transfected into cells, the molecule is secreted and concentrated on the extracellular surface. Coculture of transfected cells with muscle fibers induces the formation of receptor patches at contact sites. These results demonstrate that expression of a single gene encoding agrin confers receptor clustering that is restricted to specific sites of cell-muscle contact.

Agrin↗

PubMed1992

RNA splicing regulates agrin-mediated acetylcholine receptor clustering activity on cultured myotubes.

Agrin is a component of the synaptic basal lamina that induces the clustering of acetylcholine receptors (AChRs) on muscle fibers. A region near the carboxyl terminus of the protein exists in four forms that are generated by alternative RNA splicing. All four alternatively spliced forms of agrin are active in inducing AChR clusters on rat primary and C2-derived muscle fibers. In contrast, only two forms of the protein, each containing an 8 amino acid insert, are capable of inducing clusters on myotubes of S27 cells, a C2 variant that has defective proteoglycans. These two forms are also most active in inducing clusters on chick myotubes. This pattern of differential activity suggests that RNA splicing of agrin transcripts and interactions with proteoglycans or other components of basal lamina have important roles in regulating the localization of neurotransmitter receptors at synaptic sites.

Agrin↗

PubMed1992

Agrin isoforms and their role in synaptogenesis.

Agrin is thought to mediate the motor neuron-induced aggregation of synaptic proteins on the surface of muscle fibers at neuromuscular junctions. Recent experiments provide direct evidence in support of this hypothesis, reveal the nature of agrin immunoreactivity at sites other than neuromuscular junctions, and have resulted in findings that are consistent with the possibility that agrin plays a role in synaptogenesis throughout the nervous system.

Agrin↗

PubMed1991

Effect of agrin on the distribution of acetylcholine receptors and sodium channels on adult skeletal muscle fibers in culture.

We used the loose patch voltage clamp technique and rhodamine-conjugated alpha-bungarotoxin to study the regulation of Na channel (NaCh) and acetylcholine receptor (AChR) distribution on dissociated adult skeletal muscle fibers in culture. The aggregate of AChRs and NaChs normally found in the postsynaptic membrane of these cells gradually fragmented and dispersed from the synaptic region after several days in culture. This dispersal was the result of the collagenase treatment used to dissociate the cells, suggesting that a factor associated with the extracellular matrix was responsible for maintaining the high concentration of AchRs and NaChs at the neuromuscular junction. We tested whether the basal lamina protein agrin, which has been shown to induce the aggregation of AChRs on embryonic myotubes, could similarly influence the distribution of NaChs. By following identified fibers, we found that agrin accelerated both the fragmentation of the endplate AChR cluster into smaller patches as well as the appearance of new AChR clusters away from the endplate. AChR patches which were fragments of the original endplate retained a high density of NaChs, but no new NaCh hotspots were found elsewhere on the fiber, including sites of newly formed AChR clusters. The results are consistent with the hypothesis that extracellular signals regulate the distribution of AChRs and NaChs on skeletal muscle fibers. While agrin probably serves this function for the AChR, it does not appear to play a role in the regulation of the NaCh distribution.

Agrin↗

PubMed1991

The mechanism of agrin-induced acetylcholine receptor aggregation.

Agrin, a protein isolated from the synapse-rich electric organ of Torpedo californica, induces the formation of specializations on myotubes in culture which resemble the post-synaptic apparatus at the vertebrate skeletal neuromuscular junction. For example, the specializations contain aggregates of acetylcholine receptors and acetylcholinesterase. This report summarizes the evidence that the formation of the post-synaptic apparatus at developing and regenerating neuromuscular junctions is triggered by the release of agrin from motor axon terminals and describes results of recent experiments which suggest that agrin-induced tyrosine phosphorylation of the beta subunit of the acetylcholine receptor may play a role in receptor aggregation.

Agrin↗