Substrate-bound agrin induces expression of acetylcholine receptor epsilon-subunit gene in cultured mammalian muscle cells

A link between agrin signalling and Cav3.2 at the neuromuscular junction in spinal muscular atrophy

SMN protein deficiency causes motoneuron disease spinal muscular atrophy (SMA). SMN-based therapies improve patient motor symptoms to variable degrees. An early hallmark of SMA is the perturbation of the neuromuscular junction (NMJ), a synapse between a motoneuron and muscle cell. NMJ formation depends on acetylcholine receptor (AChR) clustering triggered by agrin and its co-receptors lipoprotein receptor-related protein 4 (LRP4) and transmembrane muscle-specific kinase (MuSK) signalling pathway. We have previously shown that flunarizine improves NMJs in SMA model mice, but the mechanisms remain elusive. We show here that flunarizine promotes AChR clustering in cell-autonomous, dose- and agrin-dependent manners in C2C12 myotubes. This is associated with an increase in protein levels of LRP4, integrin-beta-1 and alpha-dystroglycan, three agrin co-receptors. Furthermore, flunarizine enhances MuSK interaction with integrin-beta-1 and phosphotyrosines. Moreover, the drug acts on the expression and splicing of Agrn and Cacna1h genes in a muscle-specific manner. We reveal that the Cacna1h encoded protein Cav3.2 closely associates in vitro with the agrin co-receptor LRP4. In vivo, it is enriched nearby NMJs during neonatal development and the drug increases this immunolabelling in SMA muscles. Thus, flunarizine modulates key players of the NMJ and identifies Ca_v3.2 as a new protein involved in the NMJ biology.

Synapse-specific Lrp4 mRNA enrichment requires Lrp4/MuSK signaling, muscle activity and Wnt non-canonical pathway

Background

The neuromuscular junction (NMJ) is a peripheral synapse critical to muscle contraction. Like acetylcholine receptors (AChRs), many essential proteins of NMJ are extremely concentrated at the postjunctional membrane. However, the mechanisms of synapse-specific concentration are not well understood; furthermore, it is unclear whether signaling molecules critical to NMJ formation and maintenance are also locally transcribed.

Results

We studied the β-gal activity encoded by a lacZ cassette driven by the promoter of the Lrp4 gene. As reported for Lrp4 mRNA, β-gal was in the central region in embryonic muscles and at the NMJ after its formation. However, β-gal was no longer in the central areas of muscle fibers in Lrp4 or MuSK mutant mice, indicating a requirement of Lrp4/MuSK signaling. This phenotype could be rescued by transgenic expression of LRP4 with a transmembrane domain but not soluble ECD in Lrp4 mutant mice. β-gal and AChR clusters were distributed in a broader region in lacZ/ECD than that of heterozygous lacZ/+ mice, indicating an important role of the transmembrane domain in Lrp4 signaling. Synaptic β-gal activity became diffused after denervation or treatment with µ-conotoxin, despite its mRNA was increased, indicating synaptic Lrp4 mRNA enrichment requires muscle activity. β-gal was also diffused in aged mice but became re-concentrated after muscle stimulation. Finally, Lrp4 mRNA was increased in C2C12 myotubes by Wnt ligands in a manner that could be inhibited by RKI-1447, an inhibitor of ROCK in Wnt non-canonical signaling. Injecting RKI-1447 into muscles of adult mice diminished Lrp4 synaptic expression.

Conclusions

This study demonstrates that synapse-specific enrichment of Lrp4 mRNA requires a coordinated interaction between Lrp4/MuSK signaling, muscle activity, and Wnt non-canonical signaling. Thus, the study provides a new mechanism for Lrp4 mRNA enrichment. It also provides a potential target for the treatment of NMJ aging and other NMJ-related diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00619-z.

Zonisamide upregulates neuregulin-1 expression and enhances acetylcholine receptor clustering at the in vitro neuromuscular junction

Decreased acetylcholine receptor (AChR) clustering compromises signal transmission at the neuromuscular junction (NMJ) in myasthenia gravis, congenital myasthenic syndromes, and motor neuron diseases. Although the enhancement of AChR clustering at the NMJ is a promising therapeutic strategy for these maladies, no drug is currently available for this enhancement. We previously reported that zonisamide (ZNS), an anti-epileptic and anti-Parkinson's disease drug, enhances neurite elongation of the primary spinal motor neurons (SMNs). As nerve sprouting occurs to compensate for the loss of AChR clusters in human diseases, we examined the effects of ZNS on AChR clustering at the NMJ. To this end, we established a simple and quick co-culture system to reproducibly make in vitro NMJs using C2C12 myotubes and NSC34 motor neurons. ZNS at 1-20 μM enhanced the formation of AChR clusters dose-dependently in co-cultured C2C12 myotubes but not in agrin-treated single cultured C2C12 myotubes. We observed that molecules that conferred responsiveness to ZNS were not secreted into the co-culture medium. We found that 10 μM ZNS upregulated the expression of neuregulin-1 (Nrg1) in co-cultured cells but not in single cultured C2C12 myotubes or single cultured NSC34 motor neurons. In accordance with this observation, inhibition of the Nrg1/ErbB signaling pathways nullified the effect of 10 μM ZNS on the enhancement of AChR clustering in in vitro NMJs. Although agrin was not induced by 10 μM ZNS in co-cultured cells, anti-agrin antibody attenuated ZNS-mediated enhancement of AChR clustering. We conclude that ZNS enhances agrin-dependent AChR-clustering by upregulating the Nrg1/ErbB signaling pathways in the presence of NMJs.

The Structure, Function, and Physiology of the Fetal and Adult Acetylcholine Receptor in Muscle

The neuromuscular junction (NMJ) is a highly developed synapse linking motor neuron activity with muscle contraction. A complex of molecular cascades together with the specialized NMJ architecture ensures that each action potential arriving at the motor nerve terminal is translated into an action potential in the muscle fiber. The muscle-type nicotinic acetylcholine receptor (AChR) is a key molecular component located at the postsynaptic muscle membrane responsible for the generation of the endplate potential (EPP), which usually exceeds the threshold potential necessary to activate voltage-gated sodium channels and triggers a muscle action potential. Two AChR isoforms are found in mammalian muscle. The fetal isoform is present in prenatal stages and is involved in the development of the neuromuscular system whereas the adult isoform prevails thereafter, except after denervation when the fetal form is re-expressed throughout the muscle. This review will summarize the structural and functional differences between the two isoforms and outline congenital and autoimmune myasthenic syndromes that involve the isoform specific AChR subunits.

Emergence of functional neuromuscular junctions in an engineered, multicellular spinal cord-muscle bioactuator

Three-dimensional (3D) biomimetic systems hold great promise for the study of biological systems in vitro as well as for the development and testing of pharmaceuticals. Here, we test the hypothesis that an intact segment of lumbar rat spinal cord will form functional neuromuscular junctions (NMJs) with engineered, 3D muscle tissue, mimicking the partial development of the peripheral nervous system (PNS). Muscle tissues are grown on a 3D-printed polyethylene glycol (PEG) skeleton where deflection of the backbone due to muscle contraction causes the displacement of the pillar-like "feet." We show that spinal cord explants extend a robust and complex arbor of motor neurons and glia in vitro. We then engineered a "spinobot" by innervating the muscle tissue with an intact segment of lumbar spinal cord that houses the hindlimb locomotor central pattern generator (CPG). Within 7 days of the spinal cord being introduced to the muscle tissue, functional neuromuscular junctions (NMJs) are formed, resulting in the development of an early PNS in vitro. The newly innervated muscles exhibit spontaneous contractions as measured by the displacement of pillars on the PEG skeleton. Upon chemical excitation, the spinal cord-muscle system initiated muscular twitches with a consistent frequency pattern. These sequences of contraction/relaxation suggest the action of a spinal CPG. Chemical inhibition with a blocker of neuronal glutamate receptors effectively blocked contractions. Overall, these data demonstrate that a rat spinal cord is capable of forming functional neuromuscular junctions ex vivo with an engineered muscle tissue at an ontogenetically similar timescale.

Extracellular signal-regulated kinases 1 and 2 regulate neuromuscular junction and myofiber phenotypes in mammalian skeletal muscle

The neuromuscular junction is the synapse between a motor neuron of the spinal cord and a skeletal muscle fiber in the periphery. Reciprocal interactions between these excitable cells, and between them and others cell types present within the muscle tissue, shape the development, homeostasis and plasticity of skeletal muscle. An important aim in the field is to understand the molecular mechanisms underlying these cellular interactions, which include identifying the nature of the signals and receptors involved but also of the downstream intracellular signaling cascades elicited by them. This review focuses on work that shows that skeletal muscle fiber-derived extracellular signal-regulated kinases 1 and 2 (ERK1/2), ubiquitous and prototypical intracellular mitogen-activated protein kinases, have modulatory roles in the maintenance of the neuromuscular synapse and in the acquisition and preservation of fiber type identity in skeletal muscle.

Nerve, Muscle, and Synaptogenesis

The vertebrate skeletal neuromuscular junction (NMJ) has long served as a model system for studying synapse structure, function, and development. Over the last several decades, a neuron-specific isoform of agrin, a heparan sulfate proteoglycan, has been identified as playing a central role in synapse formation at all vertebrate skeletal neuromuscular synapses. While agrin was initially postulated to be the inductive molecule that initiates synaptogenesis, this model has been modified in response to work showing that postsynaptic differentiation can develop in the absence of innervation, and that synapses can form in transgenic mice in which the agrin gene is ablated. In place of a unitary mechanism for neuromuscular synapse formation, studies in both mice and zebrafish have led to the proposal that two mechanisms mediate synaptogenesis, with some synapses being induced by nerve contact while others involve the incorporation of prepatterned postsynaptic structures. Moreover, the current model also proposes that agrin can serve two functions, to induce synaptogenesis and to stabilize new synapses, once these are formed. This review examines the evidence for these propositions, and concludes that it remains possible that a single molecular mechanism mediates synaptogenesis at all NMJs, and that agrin acts as a stabilizer, while its role as inducer is open to question. Moreover, if agrin does not act to initiate synaptogenesis, it follows that as yet uncharacterized molecular interactions are required to play this essential inductive role. Several alternatives to agrin for this function are suggested, including focal pericellular proteolysis and integrin signaling, but all require experimental validation.

A Cell Culture System to Investigate the Presynaptic Control of Subsynaptic Membrane Differentiation at the Neuromuscular Junction

For decades the neuromuscular junction (NMJ) has been a favorite preparation to investigate basic mechanisms of synaptic function and development. As its function is to transmit action potentials in a 1:1 ratio from motor neurons to muscle fibers, the NMJ shows little or no functional plasticity, a property that makes it poorly suited to investigate mechanisms of use-dependent adaptations of synaptic function, which are thought to underlie learning and memory formation in the brain. On the other hand, the NMJ is unique in that the differentiation of the subsynaptic membrane is regulated by one major factor secreted from motor neurons, agrin. As a consequence, myotubes grown on a laminin substrate that is focally impregnated with recombinant neural agrin closely resemble the situation in vivo, where agrin secreted from motor neurons binds to the basal lamina of the NMJ's synaptic cleft to induce and maintain the subsynaptic muscle membrane. We provide here a detailed protocol through which acetylcholine receptor clusters are induced in cultured myotubes contacting laminin-attached agrin, enabling molecular, biochemical and cell biological analyses including high resolution microscopy in 4D. This preparation is ideally suited to investigate the mechanisms involved in the assembly of the postsynaptic muscle membrane, providing distinct advantages over inducing AChR clusters using soluble agrin.

Sema3A Reduces Sprouting of Adult Rod Photoreceptors In Vitro

Purpose

Rod photoreceptor terminals respond to retinal injury with retraction and sprouting. Since the guidance cue Semaphorin3A (Sema3A) is observed in the retina after injury, we asked whether Sema3A contributes to structural plasticity in rod photoreceptors.

Methods

We used Western blots and alkaline phosphatase (AP)-tagged neuropilin-1 (NPN-1) to detect the expression of Sema3A in an organotypic model of porcine retinal detachment. We then examined Sema3A binding to cultured salamander rod photoreceptors using AP-tagged Sema3A. For functional analysis, we used a microspritzer to apply a gradient of Sema3A-Fc to isolated salamander rod photoreceptors over 24 hours.

Results

Sema3A protein was biochemically detected in porcine retinal explants in the retina 7, 24, and 72 hours after detachment. In sections, NPN-1 receptor was bound to the inner and outer retina. For isolated rod photoreceptors, Sema3A localized to synaptic terminals and to neuritic processes after 1 week in vitro. In microspritzed rod photoreceptors, process initiation occurred away from high concentrations of Sema3A. Sema3A significantly decreased the number of processes formed by rod photoreceptors although the average length of processes was not affected. The cellular orientation of rod photoreceptors relative to the microspritzer also significantly changed over time; this effect was reduced with the Sema3A inhibitor, xanthofulvin.

Conclusion

Sema3A is expressed in the retina after detachment, binds to rod photoreceptors, affects cell orientation, and reduces photoreceptor process initiation in vitro. Our results suggest that Sema3A contributes to axonal retraction in retinal injury, whereas rod neuritic sprouting and regenerative synaptogenesis may require a reduction in semaphorin signaling.

Zonisamide Enhances Neurite Elongation of Primary Motor Neurons and Facilitates Peripheral Nerve Regeneration In Vitro and in a Mouse Model

No clinically applicable drug is currently available to enhance neurite elongation after nerve injury. To identify a clinically applicable drug, we screened pre-approved drugs for neurite elongation in the motor neuron-like NSC34 cells. We found that zonisamide, an anti-epileptic and anti-Parkinson’s disease drug, promoted neurite elongation in cultured primary motor neurons and NSC34 cells in a concentration-dependent manner. The neurite-scratch assay revealed that zonisamide enhanced neurite regeneration. Zonisamide was also protective against oxidative stress-induced cell death of primary motor neurons. Zonisamide induced mRNA expression of nerve growth factors (BDNF, NGF, and neurotrophin-4/5), and their receptors (tropomyosin receptor kinase A and B). In a mouse model of sciatic nerve autograft, intragastric administration of zonisamide for 1 week increased the size of axons distal to the transected site 3.9-fold. Zonisamide also improved the sciatic function index, a marker for motor function of hindlimbs after sciatic nerve autograft, from 6 weeks after surgery. At 8 weeks after surgery, zonisamide was protective against denervation-induced muscle degeneration in tibialis anterior, and increased gene expression of Chrne, Colq, and Rapsn, which are specifically expressed at the neuromuscular junction. We propose that zonisamide is a potential therapeutic agent for peripheral nerve injuries as well as for neuropathies due to other etiologies.

CLASP2-dependent microtubule capture at the neuromuscular junction membrane requires LL5β and actin for focal delivery of acetylcholine receptor vesicles

A novel mechanism is described for the agrin-mediated focal delivery of acetylcholine receptors (AChRs) to the postsynaptic membrane of the neuromuscular junction. Microtubule capture mediated by CLASP2 and its interaction partner, LL5β, and an intact subsynaptic actin cytoskeleton are both required for focal AChR transport to the synaptic membrane.

A hallmark of the neuromuscular junction (NMJ) is the high density of acetylcholine receptors (AChRs) in the postsynaptic muscle membrane. The postsynaptic apparatus of the NMJ is organized by agrin secreted from motor neurons. The mechanisms that underlie the focal delivery of AChRs to the adult NMJ are not yet understood in detail. We previously showed that microtubule (MT) capture by the plus end–tracking protein CLASP2 regulates AChR density at agrin-induced AChR clusters in cultured myotubes via PI3 kinase acting through GSK3β. Here we show that knockdown of the CLASP2-interaction partner LL5β by RNAi and forced expression of a CLASP2 fragment blocking the CLASP2/LL5β interaction inhibit microtubule capture. The same treatments impair focal vesicle delivery to the clusters. Consistent with these findings, knockdown of LL5β at the NMJ in vivo reduces the density and insertion of AChRs into the postsynaptic membrane. MT capture and focal vesicle delivery to agrin-induced AChR clusters are also inhibited by microtubule- and actin-depolymerizing drugs, invoking both cytoskeletal systems in MT capture and in the fusion of AChR vesicles with the cluster membrane. Combined our data identify a transport system, organized by agrin through PI3 kinase, GSK3β, CLASP2, and LL5β, for precise delivery of AChR vesicles from the subsynaptic nuclei to the overlying synaptic membrane.

Acetylcholine receptor (AChR) clustering is regulated both by glycogen synthase kinase 3β (GSK3β)-dependent phosphorylation and the level of CLIP-associated protein 2 (CLASP2) mediating the capture of microtubule plus-ends

The postsynaptic apparatus of the neuromuscular junction (NMJ) traps and anchors acetylcholine receptors (AChRs) at high density at the synapse. We have previously shown that microtubule (MT) capture by CLASP2, a MT plus-end-tracking protein (+TIP), increases the size and receptor density of AChR clusters at the NMJ through the delivery of AChRs and that this is regulated by a pathway involving neuronal agrin and several postsynaptic kinases, including GSK3. Phosphorylation by GSK3 has been shown to cause CLASP2 dissociation from MT ends, and nine potential phosphorylation sites for GSK3 have been mapped on CLASP2. How CLASP2 phosphorylation regulates MT capture at the NMJ and how this controls the size of AChR clusters are not yet understood. To examine this, we used myotubes cultured on agrin patches that induce AChR clustering in a two-dimensional manner. We show that expression of a CLASP2 mutant, in which the nine GSK3 target serines are mutated to alanine (CLASP2-9XS/9XA) and are resistant to GSK3β-dependent phosphorylation, promotes MT capture at clusters and increases AChR cluster size, compared with myotubes that express similar levels of wild type CLASP2 or that are noninfected. Conversely, myotubes expressing a phosphomimetic form of CLASP2 (CLASP2-8XS/D) show enrichment of immobile mutant CLASP2 in clusters, but MT capture and AChR cluster size are reduced. Taken together, our data suggest that both GSK3β-dependent phosphorylation and the level of CLASP2 play a role in the maintenance of AChR cluster size through the regulated capture and release of MT plus-ends.

Recycling of acetylcholine receptors at ectopic postsynaptic clusters induced by exogenous agrin in living rats

During the development of the neuromuscular junction, motor axons induce the clustering of acetylcholine receptors (AChRs) and increase their metabolic stability in the muscle membrane. Here, we asked whether the synaptic organizer agrin might regulate the metabolic stability and density of AChRs by promoting the recycling of internalized AChRs, which would otherwise be destined for degradation, into synaptic sites. We show that at nerve-free AChR clusters induced by agrin in extrasynaptic membrane, internalized AChRs are driven back into the ectopic synaptic clusters where they intermingle with pre-existing and new receptors. The extent of AChR recycling depended on the strength of the agrin stimulus, but not on the development of junctional folds, another hallmark of mature postsynaptic membranes. In chronically denervated muscles, in which both AChR stability and recycling are significantly decreased by muscle inactivity, agrin maintained the amount of recycled AChRs at agrin-induced clusters at a level similar to that at denervated original endplates. In contrast, AChRs did not recycle at agrin-induced clusters in C2C12 or primary myotubes. Thus, in muscles in vivo, but not in cultured myotubes, neural agrin promotes the recycling of AChRs and thereby increases their metabolic stability.

Agrin regulates CLASP2-mediated capture of microtubules at the neuromuscular junction synaptic membrane

Agrin regulates acetylcholine receptors at the neuromuscular junction by locally stabilizing microtubules through the plus end tracking proteins CLASP2 and CLIP-170.

Agrin is the major factor mediating the neuronal regulation of postsynaptic structures at the vertebrate neuromuscular junction, but the details of how it orchestrates this unique three-dimensional structure remain unknown. Here, we show that agrin induces the formation of the dense network of microtubules in the subsynaptic cytoplasm and that this, in turn, regulates acetylcholine receptor insertion into the postsynaptic membrane. Agrin acted in part by locally activating phosphatidylinositol 3-kinase and inactivating GSK3β, which led to the local capturing of dynamic microtubules at agrin-induced acetylcholine receptor (AChR) clusters, mediated to a large extent by the microtubule plus-end tracking proteins CLASP2 and CLIP-170. Indeed, in the absence of CLASP2, microtubule plus ends at the subsynaptic muscle membrane, the density of synaptic AChRs, the size of AChR clusters, and the numbers of subsynaptic muscle nuclei with their selective gene expression programs were all reduced. Thus, the cascade linking agrin to CLASP2-mediated microtubule capturing at the synaptic membrane is essential for the maintenance of a normal neuromuscular phenotype.

Neuregulin/ErbB regulate neuromuscular junction development by phosphorylation of α-dystrobrevin

Neuregulin/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Neuregulin (NRG)/ErbB signaling is involved in numerous developmental processes in the nervous system, including synapse formation and function in the central nervous system. Although intensively investigated, its role at the neuromuscular synapse has remained elusive. Here, we demonstrate that loss of neuromuscular NRG/ErbB signaling destabilized anchoring of acetylcholine receptors (AChRs) in the postsynaptic muscle membrane and that this effect was caused by dephosphorylation of α-dystrobrevin1, a component of the postsynaptic scaffold. Specifically, in mice in which NRG signaling to muscle was genetically or pharmacologically abolished, postsynaptic AChRs moved rapidly from the synaptic to the perisynaptic membrane, and the subsynaptic scaffold that anchors the AChRs was impaired. These defects combined compromised synaptic transmission. We further show that blockade of NRG/ErbB signaling abolished tyrosine phosphorylation of α-dystrobrevin1, which reduced the stability of receptors in agrin-induced AChR clusters in cultured myotubes. Our data indicate that NRG/ErbB signaling maintains high efficacy of synaptic transmission by stabilizing the postsynaptic apparatus via phosphorylation of α-dystrobrevin1.

Soluble miniagrin enhances contractile function of engineered skeletal muscle

Neural agrin plays a pleiotropic role in skeletal muscle innervation and maturation, but its specific effects on the contractile function of aneural engineered muscle remain unknown. In this study, neonatal rat skeletal myoblasts cultured within 3-dimensional engineered muscle tissue constructs were treated with 10 nM soluble recombinant miniagrin and assessed using histological, biochemical, and functional assays. Depending on the treatment duration and onset time relative to the stage of myogenic differentiation, miniagrin was found to induce up to 1.7-fold increase in twitch and tetanus force amplitude. This effect was associated with the 2.3-fold up-regulation of dystrophin gene expression at 6 d after agrin removal and enhanced ACh receptor (AChR) cluster formation, but no change in cell number, expression of muscle myosin, or important aspects of intracellular Ca(2+) handling. In muscle constructs with endogenous ACh levels suppressed by the application of α-NETA, miniagrin increased AChR clustering and twitch force amplitude but failed to improve intracellular Ca(2+) handling and increase tetanus-to-twitch ratio. Overall, our studies suggest that besides its synaptogenic function that could promote integration of engineered muscle constructs in vivo, neural agrin can directly promote the contractile function of aneural engineered muscle via mechanisms distinct from those involving endogenous ACh.

Matrix metalloproteinase-3 in myasthenia gravis compared to other neurological disorders and healthy controls

MMP-3 is capable of degrading a variety of proteins, including agrin, which plays a critical role in neuromuscular signaling by controlling acetylcholine receptor clustering. High MMP-3 levels in a proportion of myasthenia gravis (MG) patients have been reported. A pathogenic role of MMP-3 in other neurological disorders has been suggested but not proven. We have therefore examined the levels of MMP-3 in 124 MG patients and compared them to 59 multiple sclerosis (MS) patients, 74 epilepsy patients, 33 acute stroke patients, and 90 healthy controls. 15.3% of the patients in the MG group were MMP-3-positive (defined as higher than cutoff value 48 ng/mL) with very high mean MMP-3 concentration (79.9 ng/mL), whereas the proportion of MMP-3 positive patients in the MS (3.4%), epilepsy (6.7%), stroke (0%), and the control group (4.4%) was significantly lower. Mean MMP-3 concentration in the total MG group (25.5 ng/mL) was significantly higher than in the MS (16.6 ng/mL) and stroke (11.7 ng/mL) groups, but did not differ significantly from the epilepsy (19.4 ng/mL) and the control group (23.4 ng/mL). MMP-3 may have a specific pathogenic effect in MG in addition to being associated with autoimmune diseases in general.

Neural agrin controls maturation of the excitation-contraction coupling mechanism in human myotubes developing in vitro

The aim of this study was to elucidate the mechanisms responsible for the effects of innervation on the maturation of excitation-contraction coupling apparatus in human skeletal muscle. For this purpose, we compared the establishment of the excitation-contraction coupling mechanism in myotubes differentiated in four different experimental paradigms: 1) aneurally cultured, 2) cocultured with fetal rat spinal cord explants, 3) aneurally cultured in medium conditioned by cocultures, and 4) aneurally cultured in medium supplemented with purified recombinant chick neural agrin. Ca2+ imaging indicated that coculturing human muscle cells with rat spinal cord explants increased the fraction of cells showing a functional excitation-contraction coupling mechanism. The effect of spinal cord explants was mimicked by treatment with medium conditioned by cocultures or by addition of 1 nM of recombinant neural agrin to the medium. The treatment with neural agrin increased the number of human muscle cells in which functional ryanodine receptors (RyRs) and dihydropyridine-sensitive L-type Ca2+ channels were detectable. Our data are consistent with the hypothesis that agrin, released from neurons, controls the maturation of the excitation-contraction coupling mechanism and that this effect is due to modulation of both RyRs and L-type Ca2+ channels. Thus, a novel role for neural agrin in skeletal muscle maturation is proposed.

The proteome survey of an electricity-generating organ (Torpedo californica electric organ)

Torpedo californica is a species in class Chondrichthyes. Electric rays have evolved the electric organ, which is similar to the mammalian neuromuscular junction (NMJ). Here, we took a combined cDNA sequencing and proteomic approach to define the molecular constituents of the T. californica electric organ. For soluble proteins, 2-DE was used and 224 protein spots were mapped. Plasma membrane fractions were analyzed using the shotgun approach (LC-MS/MS). A Torpedo cDNA library was constructed and 607 cDNA clones were sequenced. Identification of electric organ proteins was done using cross-species comparisons, and a custom database was constructed from cDNA translations. We unambiguously identified 121 proteins and transcripts, 103 of which were novel additions to the existing databases of Torpedo fish. Fifteen proteins of known function, but not previously associated with either the electroplaque or NMJ, were present at high abundance. These included the heat shock and oxidative stress proteins, annexin V (calelectrin), and plectin 1. Most interesting were the unambiguous matches to 11 human ORFs of unknown function, including four potential RNA splicing proteins, a vacuolar sorting protein, and a tetraspanin containing protein. This analysis identified proteins that may play a role in the higher vertebrate neuromuscular junction or other electrical synapses.

A nanofabricated planar aperture as a mimic of the nerve-muscle contact during synaptogenesis

The release of synaptogenic factors by the nerve terminal plays a central role in the aggregation of neurotransmitter receptors at the postsynaptic membrane, a precisely timed and localized process that is essential for the correct formation and functioning of the synapse. This process has been difficult to re-capitulate in cell culture because present cell stimulation methods do not have sufficient spatiotemporal control of the delivery of soluble signals. We cultured myotubes atop nanofabricated planar apertures (2-8 microm diameter) to focally stimulate the muscle cell membrane with neural agrin, a synaptogenic factor released by motor neurons during development. Focal agrin delivery through the apertures after myotube fusion results in local aggregation of acetylcholine receptors (AChRs) in the vicinity of the apertures, a process reminiscent of AChR clustering at innervation sites. Since the apertures are spatially organized in microarrays, multiple experiments can be run in parallel on one device. The technique has wide applicability in cell-cell communication studies and cell-based bioassays.

Local induction of acetylcholine receptor clustering in myotube cultures using microfluidic application of agrin

During neuromuscular synaptogenesis, the exchange of spatially localized signals between nerve and muscle initiates the coordinated focal accumulation of the acetylcholine (ACh) release machinery and the ACh receptors (AChRs). One of the key first steps is the release of the proteoglycan agrin focalized at the axon tip, which induces the clustering of AChRs on the postsynaptic membrane at the neuromuscular junction. The lack of a suitable method for focal application of agrin in myotube cultures has limited the majority of in vitro studies to the application of agrin baths. We used a microfluidic device and surface microengineering to focally stimulate muscle cells with agrin at a small portion of their membrane and at a time and position chosen by the user. The device is used to verify the hypothesis that focal application of agrin to the muscle cell membrane induces local aggregation of AChRs in differentiated C2C12 myotubes.

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.

Kinase- and rapsyn-independent activities of the muscle-specific kinase (MuSK)

The muscle-specific receptor tyrosine kinase (MuSK) is co-localized with nicotinic acetylcholine receptors (AChRs) in the postsynaptic membrane of the skeletal neuromuscular junction, and is required for all known aspects of postsynaptic differentiation. Studies in vitro have shown that Z(+)-agrin, a nerve-derived proteoglycan, activates MuSK's kinase activity to promote clustering of AChRs and MuSK itself with a cytoplasmic, receptor-associated protein, rapsyn. These studies, however, have used soluble forms of agrin, whereas agrin is cell- or matrix-attached in vivo. We show here that immobilized (particle- or cell-attached) agrin but not soluble agrin is able to aggregate MuSK in the absence of rapsyn and that this aggregation does not require MuSK's kinase activity but does require MuSK's cytoplasmic domain. Moreover, immobilized agrin can promote clustering of AChRs by a mechanism that requires MuSK and rapsyn but does not require MuSK's kinase activity. These results imply that rapsyn and signaling components activated by MuSK kinase may be dispensable for some early aspects of postsynaptic differentiation.

Regulation of MuSK expression by a novel signaling pathway

MuSK is a receptor tyrosine kinase essential for neuromuscular junction formation. Expression of the MuSK gene is tightly regulated during development and at the neuromuscular junction. However, little is known about molecular mechanisms regulating its gene expression. Here we report a characterization of the promoter of the mouse MuSK gene. The transcription of MuSK starts at multiple sites with a major site 51 nt upstream of the translation start site. We have identified an E-box-like cis-element that is both required and sufficient for differentiation-dependent transcription. Interestingly, the promoter activity of the MuSK gene did not respond to neuregulin, a factor believed to mediate the synapse-specific transcription of acetylcholine receptor subunit genes. Rather, MuSK expression is increased in muscle cells stimulated with Wnt or at conditions when the Wnt signaling was activated. These results suggest a novel mechanism for the MuSK synapse-specific expression.

Erbb2 regulates neuromuscular synapse formation and is essential for muscle spindle development

Neuregulins and their Erbb receptors have been implicated in neuromuscular synapse formation by regulating gene expression in subsynaptic nuclei. To analyze the function of Erbb2 in this process, we have inactivated the Erbb2 gene in developing muscle fibers by Cre/Lox-mediated gene ablation. Neuromuscular synapses form in the mutant mice, but the synapses are less efficient and contain reduced levels of acetylcholine receptors. Surprisingly, the mutant mice also show proprioceptive defects caused by abnormal muscle spindle development. Sensory Ia afferent neurons establish initial contact with Erbb2-deficient myotubes. However, functional spindles never develop. Taken together, our data suggest that Erbb2 signaling regulates the formation of both neuromuscular synapses and muscle spindles.

A novel pathway for MuSK to induce key genes in neuromuscular synapse formation

At the developing neuromuscular junction the Agrin receptor MuSK is the central organizer of subsynaptic differentiation induced by Agrin from the nerve. The expression of musk itself is also regulated by the nerve, but the mechanisms involved are not known. Here, we analyzed the activation of a musk promoter reporter construct in muscle fibers in vivo and in cultured myotubes, using transfection of multiple combinations of expression vectors for potential signaling components. We show that neuronal Agrin by activating MuSK regulates the expression of musk via two pathways: the Agrin-induced assembly of muscle-derived neuregulin (NRG)-1/ErbB, the pathway thought to regulate acetylcholine receptor (AChR) expression at the synapse, and via a direct shunt involving Agrin-induced activation of Rac. Both pathways converge onto the same regulatory element in the musk promoter that is also thought to confer synapse-specific expression to AChR subunit genes. In this way, a positive feedback signaling loop is established that maintains musk expression at the synapse when impulse transmission becomes functional. The same pathways are used to regulate synaptic expression of AChR epsilon. We propose that the novel pathway stabilizes the synapse early in development, whereas the NRG/ErbB pathway supports maintenance of the mature synapse.

MAPK signal transduction pathway mediates agrin effects on neurite elongation in cultured hippocampal neurons

We have previously shown that agrin regulates the rates of axonal and dendritic elongation by modulating the expression of microtubule-associated proteins in cultured hippocampal neurons. However, the mechanisms by which agrin-induced signals are propagated to the nucleus where they can lead to the phosphorylation, and hence the activation, of transcription factors, are not known. In the present study, we identified downstream elements that play essential roles in the agrin-signaling pathway in developing central neurons. Our results indicate that agrin induces the combined activation of the extracellular signal-regulated kinases (ERK1/ERK2) and p38 in central neurons. In addition, they showed that PD98059 and SB202190, synthetic inhibitors of ERK1/ERK2 and p38 respectively, prevented the changes in the rate of neurite elongation induced by agrin in cultured hippocampal neurons. Collectively, these results suggest that agrin might modulate the expression of neuron-specific genes involved in neurite elongation by inducing CREB phosphorylation through the activation of the MAPK signal transduction pathway in cultured hippocampal neurons.

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.

Clustering of nicotinic acetylcholine receptors: from the neuromuscular junction to interneuronal synapses

Fast and accurate synaptic transmission requires high-density accumulation of neurotransmitter receptors in the postsynaptic membrane. During development of the neuromuscular junction, clustering of acetylcholine receptors (AChR) is one of the first signs of postsynaptic specialization and is induced by nerve-released agrin. Recent studies have revealed that different mechanisms regulate assembly vs stabilization of AChR clusters and of the postsynaptic apparatus. MuSK, a receptor tyrosine kinase and component of the agrin receptor, and rapsyn, an AChR-associated anchoring protein, play crucial roles in the postsynaptic assembly. Once formed, AChR clusters and the postsynaptic membrane are stabilized by components of the dystrophin/utrophin glycoprotein complex, some of which also direct aspects of synaptic maturation such as formation of postjunctional folds. Nicotinic receptors are also expressed across the peripheral and central nervous system (PNS/CNS). These receptors are localized not only at the pre- but also at the postsynaptic sites where they carry out major synaptic transmission. In neurons, they are found as clusters at synaptic or extrasynaptic sites, suggesting that different mechanisms might underlie this specific localization of nicotinic receptors. This review summarizes the current knowledge about formation and stabilization of the postsynaptic apparatus at the neuromuscular junction and extends this to explore the synaptic structures of interneuronal cholinergic synapses.

Agrin regulates neuronal responses to excitatory neurotransmitters in vitro and in vivo

Agrin mediates motor neuron-induced differentiation of the postsynaptic apparatus of the neuromuscular junction but its function in brain remains unknown. Here we report that expression of c-fos, induced by activation of nicotinic or glutamatergic receptors, was significantly lower in cortical neurons cultured from agrin-deficient mutant mouse embryos compared to wildtype. Agrin-deficient neurons also exhibited increased resistance to excitotoxic injury. Treatment with recombinant agrin restored glutamate-induced c-fos expression and excitotoxicity of the agrin-deficient neurons to near wild-type levels, confirming the agrin dependence of the phenotype. The observation that c-fos induction by activation of voltage-gated Ca2+ channels is also reduced in agrin-deficient neurons raises the possibility that agrin may play a wider role by regulating responses to Ca(2+)-mediated signals. Consistent with the decline in response of cultured mutant neurons to glutamate, decreases in kainic acid-induced seizure and mortality were observed in adult agrin heterozygous mice. Together, these data demonstrate that agrin plays an important role in defining neuronal responses to excitatory neurotransmitters both in vitro and in vivo.

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.

Expression of ciliary neurotrophic factor receptor in myasthenia gravis

In myasthenia gravis, high anti-nicotinic receptor (AChR) antibody titers are not always associated with severity of the disease, suggesting the involvement of other pathological effectors. We showed that ciliary neurotrophic factor receptor (CNTFR) gene expression was higher in muscle biopsy tissue from severely affected MG patients regardless of anti-nAChR antibody titer. This increase was also triggered in vitro by a seric factor from MG patients. CNTFR protein expression was decreased in muscles from seropositive MG patients only.Altogether, our data indicate that the alteration of CNTFR expression in some MG patients could contribute to the severity of the disease in a subgroup of patients.

Agrin differentially regulates the rates of axonal and dendritic elongation in cultured hippocampal neurons

In the present study, we examined the role of agrin in axonal and dendritic elongation in central neurons. Dissociated hippocampal neurons were grown in the presence of either recombinant agrin or antisense oligonucleotides designed to block agrin expression. Our results indicate that agrin differentially regulates axonal and dendritic growth. Recombinant agrin decreased the rate of elongation of main axons but induced the formation of axonal branches. On the other hand, agrin induced both dendritic elongation and dendritic branching. Conversely, cultured hippocampal neurons depleted of agrin extended longer, nonbranched axons and shorter dendrites when compared with controls. These changes in the rates of neurite elongation and branching were paralleled by changes in the composition of the cytoskeleton. In the presence of agrin, there was an upregulation of the expression of microtubule-associated proteins MAP1B, MAP2, and tau. In contrast, a downregulation of the expression of these MAPs was detected in agrin-depleted cells. Taken collectively, these results suggest an important role for agrin as a trigger of the transcription of neuro-specific genes involved in neurite elongation and branching in central neurons.

Agrin differentially regulates the rates of axonal and dendritic elongation in cultured hippocampal neurons

In the present study, we examined the role of agrin in axonal and dendritic elongation in central neurons. Dissociated hippocampal neurons were grown in the presence of either recombinant agrin or antisense oligonucleotides designed to block agrin expression. Our results indicate that agrin differentially regulates axonal and dendritic growth. Recombinant agrin decreased the rate of elongation of main axons but induced the formation of axonal branches. On the other hand, agrin induced both dendritic elongation and dendritic branching. Conversely, cultured hippocampal neurons depleted of agrin extended longer, nonbranched axons and shorter dendrites when compared with controls. These changes in the rates of neurite elongation and branching were paralleled by changes in the composition of the cytoskeleton. In the presence of agrin, there was an upregulation of the expression of microtubule-associated proteins MAP1B, MAP2, and tau. In contrast, a downregulation of the expression of these MAPs was detected in agrin-depleted cells. Taken collectively, these results suggest an important role for agrin as a trigger of the transcription of neuro-specific genes involved in neurite elongation and branching in central neurons.

Targeting transcription to the neuromuscular synapse

Concomitant with innervation, genes coding for components of the neuromuscular junction become exclusively expressed in subsynaptic nuclei. A six-base pair element, the N box, can confer synapse-specific transcription to the acetylcholine nicotinic receptor delta and epsilon subunit, utrophin, and acetylcholine esterase genes. N box-dependent synaptic expression is stimulated by the nerve-derived signal agrin and the trophic factor neuregulin, which triggers the MAPK and JNK signaling pathways, to ultimately allow activation by the N box binding Ets transcription factor GABP.

Effects of purified recombinant neural and muscle agrin on skeletal muscle fibers in vivo

Aggregation of acetylcholine receptors (AChRs) in muscle fibers by nerve-derived agrin plays a key role in the formation of neuromuscular junctions. So far, the effects of agrin on muscle fibers have been studied in culture systems, transgenic animals, and in animals injected with agrin--cDNA constructs. We have applied purified recombinant chick neural and muscle agrin to rat soleus muscle in vivo and obtained the following results. Both neural and muscle agrin bind uniformly to the surface of innervated and denervated muscle fibers along their entire length. Neural agrin causes a dose-dependent appearance of AChR aggregates, which persist > or = 7 wk after a single application. Muscle agrin does not cluster AChRs and at 10 times the concentration of neural agrin does not reduce binding or AChR-aggregating activity of neural agrin. Electrical muscle activity affects the stability of agrin binding and the number, size, and spatial distribution of the neural agrin--induced AChR aggregates. Injected agrin is recovered from the muscles together with laminin and both proteins coimmunoprecipitate, indicating that agrin binds to laminin in vivo. Thus, the present approach provides a novel, simple, and efficient method for studying the effects of agrin on muscle under controlled conditions in vivo.

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.

Muscle activity-resistant acetylcholine receptor accumulation is induced in places of former motor endplates in ectopically innervated regenerating rat muscles

Expression of acetylcholine receptors (AChRs) in the extrajunctional muscle regions, but not in the neuromuscular junctions, is repressed by propagated electric activity in muscle fibers. During regeneration, subsynaptic-like specializations accumulating AChRs are induced in new myotubes by agrin attached to the synaptic basal lamina at the places of former motor endplates even in the absence of innervation. We examined whether AChRs still accumulated at these places when the regenerating muscles were ectopically innervated and the former synaptic places became extrajunctional. Rat soleus muscles were injured by bupivacaine and ischemia to produce complete myofiber degeneration. The soleus muscle nerve was permanently severed and the muscle was ectopically innervated by the peroneal nerve a few millimeters away from the former junctional region. After 4 weeks of regeneration, the muscles contracted upon nerve stimulation, showed little atrophy and the cross-section areas of their fibers were completely above the range in non-innervated regenerating muscles, indicating successful innervation. Subsynaptic-like specializations in the former junctional region still accumulated AChRs (and acetylcholinesterase) although no motor nerve endings were observed in their vicinity and the cross-section area of their fibers clearly demonstrated that they were ectopically innervated. We conclude that the expression of AChRs at the places of the former neuromuscular junctions in the ectopically innervated regenerated soleus muscles is activity-independent.

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.

Electrical activity and postsynapse formation in adult muscle: gamma-AChRs are not required

Skeletal muscle fibers will not accept hyperinnervation by foreign motor axons unless they are paralyzed, suggesting that paralysis makes them receptive to innervation, e.g., by upregulating extrasynaptic expression of gamma-AChRs and/or of the agrin receptor MuSK. To examine the involvement of these parameters in paralysis-mediated synapse induction, ectopic expression of agrin, a factor from motor neurons controlling neuromuscular synapse formation, was made dependent on the administration of doxycycline in innervated adult muscle fibers. In response to doxycycline-induced agrin secretion, adult fibers did form ectopic postsynaptic specializations, even when they were electrically active, lacked fetal AChRs, and expressed normal low levels of MuSK. These data demonstrate that paralysis and changes associated with it are not required for agrin-induced postsynapse formation. They suggest that paralyzed muscle induces synapse formation via the release of factors that make motor neurites contact muscle fibers and secrete agrin.

The Ets transcription factor GABP is required for postsynaptic differentiation in vivo

At chemical synapses, neurotransmitter receptors are concentrated in the postsynaptic membrane. During the development of the neuromuscular junction, motor neurons induce aggregation of acetylcholine receptors (AChRs) underneath the nerve terminal by the redistribution of existing AChRs and preferential transcription of the AChR subunit genes in subsynaptic myonuclei. Neural agrin, when expressed in nonsynaptic regions of muscle fibers in vivo, activates both mechanisms resulting in the assembly of a fully functional postsynaptic apparatus. Several lines of evidence indicate that synaptic transcription of AChR genes is primarily dependent on a promoter element called N-box. The Ets-related transcription factor growth-associated binding protein (GABP) binds to this motif and has thus been suggested to regulate synaptic gene expression. Here, we assessed the role of GABP in synaptic gene expression and in the formation of postsynaptic specializations in vivo by perturbing its function during postsynaptic differentiation induced by neural agrin. We find that neural agrin-mediated activation of the AChR epsilon subunit promoter is abolished by the inhibition of GABP function. Importantly, the number of AChR aggregates formed in response to neural agrin was strongly reduced. Moreover, aggregates of acetylcholine esterase and utrophin, two additional components of the postsynaptic apparatus, were also reduced. Together, these results are the first direct in vivo evidence that GABP regulates synapse-specific gene expression at the neuromuscular junction and that GABP is required for the formation of a functional postsynaptic apparatus.

The Ets transcription factor GABP is required for postsynaptic differentiation in vivo

At chemical synapses, neurotransmitter receptors are concentrated in the postsynaptic membrane. During the development of the neuromuscular junction, motor neurons induce aggregation of acetylcholine receptors (AChRs) underneath the nerve terminal by the redistribution of existing AChRs and preferential transcription of the AChR subunit genes in subsynaptic myonuclei. Neural agrin, when expressed in nonsynaptic regions of muscle fibers in vivo, activates both mechanisms resulting in the assembly of a fully functional postsynaptic apparatus. Several lines of evidence indicate that synaptic transcription of AChR genes is primarily dependent on a promoter element called N-box. The Ets-related transcription factor growth-associated binding protein (GABP) binds to this motif and has thus been suggested to regulate synaptic gene expression. Here, we assessed the role of GABP in synaptic gene expression and in the formation of postsynaptic specializations in vivo by perturbing its function during postsynaptic differentiation induced by neural agrin. We find that neural agrin-mediated activation of the AChR epsilon subunit promoter is abolished by the inhibition of GABP function. Importantly, the number of AChR aggregates formed in response to neural agrin was strongly reduced. Moreover, aggregates of acetylcholine esterase and utrophin, two additional components of the postsynaptic apparatus, were also reduced. Together, these results are the first direct in vivo evidence that GABP regulates synapse-specific gene expression at the neuromuscular junction and that GABP is required for the formation of a functional postsynaptic apparatus.

Regulation and functional significance of utrophin expression at the mammalian neuromuscular synapse

Duchenne muscular dystrophy (DMD) is caused by the absence of full-length dystrophin molecules in skeletal muscle fibers. In normal muscle, dystrophin is found along the length of the sarcolemma where it links the intracellular actin cytoskeleton to the extracellular matrix, via the dystrophin-associated protein (DAP) complex. Several years ago, an autosomal homologue to dystrophin, termed utrophin, was identified and shown to be expressed in a variety of tissues, including skeletal muscle. However, in contrast to the localization of dystrophin in extrajunctional regions of muscle fibers, utrophin preferentially accumulates at the postsynaptic membrane of the neuromuscular junction in both normal and DMD adult muscle fibers. Since it has recently been suggested that the upregulation of utrophin might functionally compensate for the lack of dystrophin in DMD, considerable interest is now directed toward the elucidation of the various regulatory mechanisms presiding over expression of utrophin in normal and dystrophic skeletal muscle fibers. In this review, we discuss some of the most recent data relevant to our understanding of the impact of myogenic differentiation and innervation on the expression and localization of utrophin in skeletal muscle fibers.

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.

Synapse formation and agrin expression in stratospheroid cultures from embryonic chick retina

Stratospheroids are three-dimensional cellular spheres which develop in vitro through the proliferation and differentiation of retinal neuroepithelial precursor cells. We investigated synapse formation in stratospheroids by analyzing the development of aggregates of synapse-associated molecules and of electron microscopically identifiable synaptic specializations. Our results show that the first aggregates of the GABA(A) receptor, the glycine receptor, and gephyrin appear in the inner plexiform layer after 8 days in culture simultaneously with the development of the first active zones and postsynaptic densities. In contrast, presynaptic molecules including synaptophysin could be detected in the inner plexiform layer before synaptogenesis, suggesting functions for these molecules in addition to neurotransmitter exocytosis at mature synapses. Similar to the retina in vivo, synapses were not found in the nuclear layers of stratospheroids. We also analyzed the isoform pattern, expression, and distribution of the extracellular matrix molecule agrin, a key regulator during formation, maintenance, and regeneration of the neuromuscular junction. In stratospheroids, several agrin isoforms were expressed as highly glycosylated proteins with an apparent molecular weight of approximately 400 kDa, similar to the molecular weight of agrin in the retina in vivo. The expression specifically of the neuronal isoforms of agrin was concurrent with the onset of synaptogenesis. Moreover, the neuronal agrin isoforms were exclusively found in the synapse-containing inner plexiform layer, whereas other agrin isoforms were associated also with the inner limiting membrane and with Müller glial cells. These results show that synapse formation is very similar in stratospheroids and in the retina in vivo, and they suggest an important role for agrin during CNS development.

Formation of the neuromuscular junction. Agrin and its unusual receptors

Synapses are essential relay stations for the transmission of information between neurones and other cells. An ordered and tightly regulated formation of these structures is crucial for the functioning of the nervous system. The induction of the intensively studied synapse between nerve and muscle is initiated by the binding of neurone-specific isoforms of the basal membrane protein agrin to receptors on the surface of myotubes. Agrin activates a receptor complex that includes the muscle-specific kinase and most likely additional, yet to be identified, components. Receptor activation leads to the aggregation of acetylcholine receptors (AChR) and other proteins of the postsynaptic apparatus. This activation process has unique features which distinguish it from other receptor tyrosine kinases. In particular, the autophosphorylation of the kinase domain, which usually induces the recruitment of adaptor and signalling molecules, is not sufficient for AChR aggregation. Apparently, interactions of the extracellular domain with unknown components are also required for this process. Agrin binds to a second protein complex on the muscle surface known as the dystrophin-associated glycoprotein complex. This binding forms one end of a molecular link between the extracellular matrix and the cytoskeleton. While many components of the machinery triggering postsynaptic differentiation have now been identified, our picture of the molecular pathway causing the redistribution of synaptic proteins is still incomplete.

Evidence of an agrin receptor in cortical neurons

Agrin plays a key role in directing the differentiation of the vertebrate neuromuscular junction. Understanding agrin function at the neuromuscular junction has come via molecular genetic analyses of agrin as well as identification of its receptor and associated signal transduction pathways. Agrin is also expressed by many populations of neurons in brain, but its role remains unknown. Here we show, in cultured cortical neurons, that agrin induces expression of the immediate early gene c-fos in a concentration-dependent and saturable manner, as expected for a signal transduction pathway activated by a cell surface receptor. Agrin is active in cortical neurons at picomolar concentrations, is Ca(2+) dependent, and is inhibited by heparin and staurosporine. Despite marked differences in acetylcholine receptor (AChR)-clustering activity, all alternatively spliced forms of agrin are equally potent inducers of c-fos in cortical neurons. A similar, isoform-independent response to agrin was also observed in cultures prepared from the hippocampus and cerebellum. Only agrin with high AChR-clustering activity was effective in cultured muscle, whereas non-neuronal cells were agrin insensitive. Although consistent with a receptor tyrosine kinase model similar to the muscle-specific kinase-myotube-associated specificity component complex in muscle, our data suggest that CNS neurons express a unique agrin receptor. Evidence that neuronal signal transduction is mediated via an increase in intracellular Ca(2+) means that agrin is well situated to influence important Ca(2+)-dependent functions in brain, including neuronal growth, differentiation, and adaptive changes in gene expression associated with synaptic remodeling.

Evidence of an agrin receptor in cortical neurons

Agrin plays a key role in directing the differentiation of the vertebrate neuromuscular junction. Understanding agrin function at the neuromuscular junction has come via molecular genetic analyses of agrin as well as identification of its receptor and associated signal transduction pathways. Agrin is also expressed by many populations of neurons in brain, but its role remains unknown. Here we show, in cultured cortical neurons, that agrin induces expression of the immediate early gene c-fos in a concentration-dependent and saturable manner, as expected for a signal transduction pathway activated by a cell surface receptor. Agrin is active in cortical neurons at picomolar concentrations, is Ca(2+) dependent, and is inhibited by heparin and staurosporine. Despite marked differences in acetylcholine receptor (AChR)-clustering activity, all alternatively spliced forms of agrin are equally potent inducers of c-fos in cortical neurons. A similar, isoform-independent response to agrin was also observed in cultures prepared from the hippocampus and cerebellum. Only agrin with high AChR-clustering activity was effective in cultured muscle, whereas non-neuronal cells were agrin insensitive. Although consistent with a receptor tyrosine kinase model similar to the muscle-specific kinase-myotube-associated specificity component complex in muscle, our data suggest that CNS neurons express a unique agrin receptor. Evidence that neuronal signal transduction is mediated via an increase in intracellular Ca(2+) means that agrin is well situated to influence important Ca(2+)-dependent functions in brain, including neuronal growth, differentiation, and adaptive changes in gene expression associated with synaptic remodeling.