Expression of RAPsyn (43K protein) and nicotinic acetylcholine receptor genes is not coordinately regulated in mouse muscle

VAChT overexpression increases acetylcholine at the synaptic cleft and accelerates aging of neuromuscular junctions

BACKGROUND

Cholinergic dysfunction occurs during aging and in a variety of diseases, including amyotrophic lateral sclerosis (ALS). However, it remains unknown whether changes in cholinergic transmission contributes to age- and disease-related degeneration of the motor system. Here we investigated the effect of moderately increasing levels of synaptic acetylcholine (ACh) on the neuromuscular junction (NMJ), muscle fibers, and motor neurons during development and aging and in a mouse model for amyotrophic lateral sclerosis (ALS).

METHODS

Chat-ChR2-EYFP (VAChTHyp) mice containing multiple copies of the vesicular acetylcholine transporter (VAChT), mutant superoxide dismutase 1 (SOD1G93A), and Chat-IRES-Cre and tdTomato transgenic mice were used in this study. NMJs, muscle fibers, and α-motor neurons' somata and their axons were examined using a light microscope. Transcripts for select genes in muscles and spinal cords were assessed using real-time quantitative PCR. Motor function tests were carried out using an inverted wire mesh and a rotarod. Electrophysiological recordings were collected to examine miniature endplate potentials (MEPP) in muscles.

RESULTS

We show that VAChT is elevated in the spinal cord and at NMJs of VAChTHyp mice. We also show that the amplitude of MEPPs is significantly higher in VAChTHyp muscles, indicating that more ACh is loaded into synaptic vesicles and released into the synaptic cleft at NMJs of VAChTHyp mice compared to control mice. While the development of NMJs was not affected in VAChTHyp mice, NMJs prematurely acquired age-related structural alterations in adult VAChTHyp mice. These structural changes at NMJs were accompanied by motor deficits in VAChTHyp mice. However, cellular features of muscle fibers and levels of molecules with critical functions at the NMJ and in muscle fibers were largely unchanged in VAChTHyp mice. In the SOD1G93A mouse model for ALS, increasing synaptic ACh accelerated degeneration of NMJs caused motor deficits and resulted in premature death specifically in male mice.

CONCLUSIONS

The data presented in this manuscript demonstrate that increasing levels of ACh at the synaptic cleft promote degeneration of adult NMJs, contributing to age- and disease-related motor deficits. We thus propose that maintaining normal cholinergic signaling in muscles will slow degeneration of NMJs and attenuate loss of motor function caused by aging and neuromuscular diseases.

The mechanism of acetylcholine receptor in binding MuSK in myasthenia gravis and the role of HSP90 molecular chaperone

As an autoimmune disease, myasthenia gravis is caused by the dysfunction of neural transmission. Acetylcholine is known to exert its function after entering into synaptic cleft through binding onto postsynaptic membrane. The role of acetylcholine in binding MuSK in myasthenia gravis, however, remains unknown. A total of 38 myasthenia gravis patients and 27 healthy controls were included in this study for the detection of the expression of MuSK using immunofluorescent method. Expression of both MuSK and interleukin-6 (IL-6) were measured by Western blot, followed by the correlation analysis between heat shock protein 90 (HSP90) and IL-6 which were measured by enzyme-linked immunosorbent assay (ELISA). In myasthenia gravis patients, MuSK was co-localized with acetylcholine at the postsynaptic membrane. Such accumulation of MuSK, however, did not occur in normal people. Meanwhile we also observed elevated expression of IL-6 in myasthenia gravis patients (p<0.05). ELISA assay showed higher expression of HSP90 in patients. Further signaling pathway screening revealed the activation of IL-6-mediated pathways including STAT3 and SPH2. In conclusion, MuSK was co-localized with acetylcholine in myasthenia gravis patients, with elevated expression. HSP90 in disease people can activate IL-6 mediated signaling pathways.

Neuromuscular junction immaturity and muscle atrophy are hallmarks of the ColQ-deficient mouse, a model of congenital myasthenic syndrome with acetylcholinesterase deficiency

The collagen ColQ anchors acetylcholinesterase (AChE) in the synaptic cleft of the neuromuscular junction (NMJ). It also binds MuSK and perlecan/dystroglycan, 2 signaling platforms of the postsynaptic domain. Mutations in ColQ cause a congenital myasthenic syndrome (CMS) with AChE deficiency. Because the absence of AChE does not fully explain the complexity of the syndrome and there is no curative treatment for the disease, we explored additional potential targets of ColQ by conducting a large genetic screening of ColQ-deficient mice, a model for CMS with AChE deficiency, and analyzed their NMJ and muscle phenotypes. We demonstrated that ColQ controls the development and the maturation of the postsynaptic domain by regulating synaptic gene expression. Notably, ColQ deficiency leads to an up-regulation of the 5 subunits of the nicotinic acetylcholine receptor (AChR), leading to mixed mature and immature AChRs at the NMJ of adult mice. ColQ also regulates the expression of extracellular matrix (ECM) components. However, whereas the ECM mRNAs were down-regulated in vitro, compensation seemed to occur in vivo to maintain normal levels of these mRNAs. Finally, ColQ deficiency leads to a general atrophic phenotype and hypoplasia that affect fast muscles. This study points to new specific hallmarks for this CMS.-Sigoillot, S. M., Bourgeois, F., Karmouch, J., Molgó, J., Dobbertin, A., Chevalier, C., Houlgatte, R., Léger, J., Legay, C. Neuromuscular junction immaturity and muscle atrophy are hallmarks of the ColQ-deficient mouse, a model of congenital myasthenic syndrome with acetylcholinesterase deficiency.

Failure of lysosome clustering and positioning in the juxtanuclear region in cells deficient in rapsyn

Rapsyn, a scaffold protein, is required for the clustering of acetylcholine receptors (AChRs) at contacts between motor neurons and differentiating muscle cells. Rapsyn is also expressed in cells that do not express AChRs. However, its function in these cells remains unknown. Here, we show that rapsyn plays an AChR-independent role in organizing the distribution and mobility of lysosomes. In cells devoid of AChRs, rapsyn selectively induces the clustering of lysosomes at high density in the juxtanuclear region without affecting the distribution of other intracellular organelles. However, when the same cells overexpress AChRs, rapsyn is recruited away from lysosomes to colocalize with AChR clusters on the cell surface. In rapsyn-deficient (Rapsn(-/-)) myoblasts or cells overexpressing rapsyn mutants, lysosomes are scattered within the cell and highly dynamic. The increased mobility of lysosomes in Rapsn(-/-) cells is associated with a significant increase in lysosomal exocytosis, as evidenced by increased release of lysosomal enzymes and plasma membrane damage when cells were challenged with the bacterial pore-forming toxin streptolysin-O. These findings uncover a new link between rapsyn, lysosome positioning, exocytosis and plasma membrane integrity.

Acetylcholine receptor organization in membrane domains in muscle cells: evidence for rapsyn-independent and rapsyn-dependent mechanisms

Nicotinic acetylcholine receptors (nAChR) in muscle fibers are densely packed in the postsynaptic region at the neuromuscular junction. Rapsyn plays a central role in directing and clustering nAChR during cellular differentiation and neuromuscular junction formation; however, it has not been demonstrated whether rapsyn is the only cause of receptor immobilization. Here, we used single-molecule tracking methods to investigate nAChR mobility in plasma membranes of myoblast cells during their differentiation to myotubes in the presence and absence of rapsyn. We found that in myoblasts the majority of nAChR were immobile and that ∼20% of the receptors showed restricted diffusion in small domains of ∼50 nm. In myoblasts devoid of rapsyn, the fraction of mobile nAChR was considerably increased, accompanied by a 3-fold decrease in the immobile population of nAChR with respect to rapsyn-expressing cells. Half of the mobile receptors were confined to domains of ∼120 nm. Measurements performed in heterologously transfected HEK cells confirmed the direct immobilization of nAChR by rapsyn. However, irrespective of the presence of rapsyn, about one-third of nAChR were confined in 300-nm domains. Our results show (i) that rapsyn efficiently immobilizes nAChR independently of other postsynaptic scaffold components; (ii) nAChR is constrained in confined membrane domains independently of rapsyn; and (iii) in the presence of rapsyn, the size of these domains is strongly reduced.

ColQ controls postsynaptic differentiation at the neuromuscular junction

CollagenQ (ColQ) plays an important structural role at vertebrate neuromuscular junctions (NMJs) by anchoring and accumulating acetylcholinesterase (AChE) in the extracellular matrix (ECM). Moreover, ColQ interacts with perlecan/dystroglycan and the muscle-specific receptor tyrosine kinase (MuSK), key molecules in the NMJ formation. MuSK promotes acetylcholine receptor (AChR) clustering in a process mediated by rapsyn, a cytoplasmic protein that stimulates AChR packing in clusters and regulates synaptic gene transcription. Here, we investigated a regulatory role for ColQ by comparing the clustering and expression of synaptic proteins in wild type and ColQ-deficient muscle cells in culture and at NMJ. We show first that AChR clusters are smaller and more densely packed in the absence of ColQ both in vitro and in vivo. Second, we find that like AChRs and rapsyn, MuSK mRNA levels are increased in cultured cells but not in muscles lacking ColQ. However, membrane-bound MuSK is decreased both in vitro and in vivo suggesting that ColQ controls MuSK sorting or stabilization in the muscle membrane. In line with this, our data show that activation of the MuSK signaling pathway is altered in the absence of ColQ leading to (1) perturbation of AChR clustering and/or beta-AChR subunit phosphorylation and (2) modifications of AChR mRNA level due to the lack of ColQ-MuSK interaction. Together, our results demonstrate that ColQ, in addition to its structural role, has important regulatory functions at the synapse by controlling AChR clustering and synaptic gene expression through its interaction with MuSK.

Control of rapsyn stability by the CUL-3-containing E3 ligase complex

Rapsyn is a postsynaptic protein required for clustering of nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction. Here we report the mechanism for posttranslational control of rapsyn protein stability. We confirmed that C18H9.7-encoded RPY-1 is a rapsyn homolog in Caenorhabditis elegans by showing that human rapsyn rescued rpy-1 mutant phenotypes in nematodes, as determined by levamisole assays and micropost array behavioral assays. We found that RPY-1 was degraded in the absence of functional UNC-29, a non-alpha subunit of the receptor, in an allele-specific manner, but not in the absence of other receptor subunits. The cytoplasmic loop of UNC-29 was found to be critical for RPY-1 stability. Through RNA interference screening, we found that UBC-1, UBC-12, NEDD-8, and RBX-1 were required for degradation of RPY-1. We identified cullin (CUL)-3 as a component of E3 ligase and KEL-8 as the substrate adaptor of RPY-1. Mammalian rapsyn was ubiquitinated by the CUL3/KLHL8-containing E3 ligase in vitro, and the knockdown of KLHL-8, a mammalian KEL-8 homolog, inhibited rapsyn ubiquitination in vivo, implying evolutionary conservation of the rapsyn stability control machinery. kel-8 suppression and rpy-1 overexpression in C. elegans produced a phenotype similar to that of a loss-of-function mutation of rpy-1, suggesting that control of rapsyn abundance is important for proper function of the receptor. Our results suggest a link between the control of rapsyn abundance and congenital myasthenic syndromes.

HSP90 beta regulates rapsyn turnover and subsequent AChR cluster formation and maintenance

Rapsyn, an acetylcholine receptor (AChR)-interacting protein, is essential for synapse formation at the neuromuscular junction (NMJ). Like many synaptic proteins, rapsyn turns over rapidly at synapses. However, little is known about molecular mechanisms that govern rapsyn stability. Using a differential mass-spectrometry approach, we identified heat-shock protein 90beta (HSP90beta) as a component in surface AChR clusters. The HSP90beta-AChR interaction required rapsyn and was stimulated by agrin. Inhibition of HSP90beta activity or expression, or disruption of its interaction with rapsyn attenuated agrin-induced formation of AChR clusters in vitro and impaired the development and maintenance of the NMJ in vivo. Finally, we showed that HSP90beta was necessary for rapsyn stabilization and regulated its proteasome-dependent degradation. Together, these results indicate a role of HSP90beta in NMJ development by regulating rapsyn turnover and subsequent AChR cluster formation and maintenance.

Neural agrin increases postsynaptic ACh receptor packing by elevating rapsyn protein at the mouse neuromuscular synapse

Fluorescence resonance energy transfer (FRET) experiments at neuromuscular junctions in the mouse tibialis anterior muscle show that postsynaptic acetylcholine receptors (AChRs) become more tightly packed during the first month of postnatal development. Here, we report that the packing of AChRs into postsynaptic aggregates was reduced in 4-week postnatal mice that had reduced amounts of the AChR-associated protein, rapsyn, in the postsynaptic membrane (rapsyn(+/-) mice). We hypothesize that nerve-derived agrin increases postsynaptic expression and targeting of rapsyn, which then drives the developmental increase in AChR packing. Neural agrin treatment elevated the expression of rapsyn in C2 myotubes by a mechanism that involved slowing of rapsyn protein degradation. Similarly, exposure of synapses in postnatal muscle to exogenous agrin increased rapsyn protein levels and elevated the intensity of anti-rapsyn immunofluorescence, relative to AChR, in the postsynaptic membrane. This increase in the rapsyn-to-AChR immunofluorescence ratio was associated with tighter postsynaptic AChR packing and slowed AChR turnover. Acute blockade of synaptic AChRs with alpha-bungarotoxin lowered the rapsyn-to-AChR immunofluorescence ratio, suggesting that AChR signaling also helps regulate the assembly of extra rapsyn in the postsynaptic membrane. The results suggest that at the postnatal neuromuscular synapse agrin signaling elevates the expression and targeting of rapsyn to the postsynaptic membrane, thereby packing more AChRs into stable, functionally-important AChR aggregates.

The dynamics of the rapsyn scaffolding protein at individual acetylcholine receptor clusters

Rapsyn, a cytoplasmic receptor-associated protein, is required for the clustering of acetylcholine receptors (AChRs). Although AChR dynamics have been extensively studied, little is known about the dynamics of rapsyn. Here, we used a rapsyn-green fluorescent protein (GFP) fusion protein and quantitative fluorescent imaging to study the dynamics of rapsyn in transfected C2C12 myotubes. First, we found that rapsyn-GFP expression at clusters did not alter AChR aggregation, function, or turnover. Quantification of rapsyn immunofluorescence indicated that the expression of rapsyn-GFP proteins at clusters does not increase the overall rapsyn density compared with untransfected myotube clusters. Using time lapse imaging and fluorescence recovery after photobleaching, we demonstrated that the recovery of rapsyn-GFP fluorescence at clusters was very fast, with a halftime of about approximately 1.5 h (approximately 3 times faster than AChRs). Inhibition of protein kinase C significantly altered receptor insertion, but it had no effect on rapsyn insertion. When cells were treated with the broad spectrum kinase inhibitor staurosporine, receptor insertion was decreased even further. However, inhibition of protein kinase A had no effect on insertion of either rapsyn or receptors. Finally, when cells were treated with neural agrin, rapsyn and AChRs were both directed away from preexisting clusters and accumulated together in new small clusters. These results demonstrate the remarkable dynamism of rapsyn, which may underlie the stability and maintenance of the postsynaptic scaffold and suggest that the insertion of different postsynaptic proteins may be operating independently.

Thrombin reduces MuSK and acetylcholine receptor expression along with neuromuscular contact size in vitro

In the course of studies on thrombin and its inhibitor(s) in synaptic plasticity, we addressed the question of their roles in the formation of neuromuscular junctions (NMJ) and used a model of rat neuron-myotube cocultures. We report that the size of acetylcholinesterase (AChE) patches used as a marker of neuromuscular contacts was decreased in the presence of either thrombin or SFLLRN, the agonist peptide of the thrombin receptor PAR-1, whereas it was increased with hirudin, a specific thrombin inhibitor. In an attempt to relate these neuromuscular contact size variations to molecular changes, we studied muscle-specific tyrosine kinase receptor (MuSK), acetylcholine receptor (AChR) and rapsyn expression in the presence of thrombin. We showed that thrombin did not change rapsyn gene and protein expression. However, the expression of MuSK and surface AChR proteins was diminished in both myotube cultures and neuron-myotube cocultures. These reductions in protein expression were associated with a decrease in MuSK and AChR alpha-subunit gene expression in myotube cultures but not in neuron-myotube cocultures. Moreover, the expression of the AChR epsilon-subunit gene, specifically enhanced by neuron-released factors, was not modified by thrombin in neuron-myotube cocultures. This suggests that thrombin did not affect the expression of synaptic AChRs enhanced by neuron-released factors but rather reduced the level of extrasynaptic AChRs. Taken together, these results indicate that thrombin in balance with its inhibitor(s) could modulate the formation of neuromuscular contacts in vitro by affecting the expression of two essential molecules in NMJ postsynaptic differentiation, MuSK and AChR.

Agrin regulates rapsyn interaction with surface acetylcholine receptors, and this underlies cytoskeletal anchoring and clustering

The acetylcholine receptor (AChR)-associated protein rapsyn is essential for neuromuscular synapse formation and clustering of AChRs, but its mode of action remains unclear. We have investigated whether agrin, a key nerve-derived synaptogenic factor, influences rapsyn-AChR interactions and how this affects clustering and cytoskeletal linkage of AChRs. By precipitating AChRs and probing for associated rapsyn, we found that in denervated diaphragm rapsyn associates with synaptic as well as with extrasynaptic AChRs showing that rapsyn interacts with unclustered AChRs in vivo. Interestingly, synaptic AChRs are associated with more rapsyn suggesting that clustering of AChRs may require increased interaction with rapsyn. In similar experiments in cultured myotubes, rapsyn interacted with intracellular AChRs and with unclustered AChRs at the cell surface, although surface interactions are much more prominent. Remarkably, agrin induces recruitment of additional rapsyn to surface AChRs and clustering of AChRs independently of the secretory pathway. This agrin-induced increase in rapsyn-AChR interaction strongly correlates with clustering, because staurosporine and herbimycin blocked both the increase and clustering. Conversely, laminin and calcium induced both increased rapsyn-AChR interaction and AChR clustering. Finally, time course experiments revealed that the agrin-induced increase occurs with AChRs that become cytoskeletally linked, and that this precedes receptor clustering. Thus, we propose that neural agrin controls postsynaptic aggregation of the AChR by enhancing rapsyn interaction with surface AChRs and inducing cytoskeletal anchoring and that this is an important precursor step for AChR clustering.

Metabolic stabilization of muscle nicotinic acetylcholine receptor by rapsyn

Although the metabolic half-life of muscle endplate acetylcholine receptor (AChR) changes during development and after denervation in the adult, little is known about the molecular mechanisms that influence receptor stability. We have investigated the effect on AChR turnover of its interaction with rapsyn, a 43 kDa peripheral membrane protein that is closely associated with the AChR in muscle cells and is required for its clustering at endplates. Both in transfected COS cells and in cultured myotubes from rapsyn-negative and rapsyn-positive mice, we have found that the presence of rapsyn slows the turnover of AChRs by as much as twofold. The effect was similar for both embryonic (alpha2betadeltagamma) and adult (alpha2betadeltaepsilon) AChRs and for AChRs whose beta subunit lacked a putative tyrosine phosphorylation site. Neither colchicine nor cytochalasin D altered AChR turnover or prevented the rapsyn effect. Mutant rapsyn proteins whose N-terminal myristoylation signal was eliminated, or whose C terminus or zinc-finger domains were deleted, failed to change the rate of receptor turnover. Each of these mutations affects the association of the AChR with rapsyn, suggesting that AChR stability is altered by interaction between the two proteins. Our results suggest that, in addition to its role in AChR clustering, rapsyn also functions to metabolically stabilize the AChR.

Both myoblast lineage and innervation determine fiber type and are required for expression of the slow myosin heavy chain 2 gene

Skeletal muscle fibers express members of the myosin heavy chain (MyHC) gene family in a fiber-type-specific manner. In avian skeletal muscle it is the expression of the slow MyHC isoforms that most clearly distinguishes slow- from fast-contracting fiber types. Two hypotheses have been proposed to explain fiber-type-specific expression of distinct MyHC genes during development-an intrinsic mechanism based on the formation of different myogenic lineage(s) and an extrinsic, innervation-dependent mechanism. We developed a cell culture model system in which both mechanisms were evaluated during fetal muscle development. Myoblasts isolated from prospective fast (pectoralis major) or slow (medial adductor) fetal chick muscles formed muscle fibers in cell culture, none of which expressed slow MyHC genes. By contrast, when muscle fibers formed from myoblasts derived from the slow muscle were cocultured with neural tube, the muscle fibers expressed a slow MyHC gene, while muscle fibers formed from myoblasts of fast muscle origin continued to express only fast MyHC. Motor endplates formed on the fibers derived from myoblasts of both fast and slow muscle origin in cocultures, and slow MyHC gene expression did not occur when neuromuscular transmission or depolarization was blocked. We have cloned the slow MyHC gene that is expressed in response to innervation and identified it as the slow MyHC 2 gene, the predominant adult slow isoform. cDNAs encoding portions of the three slow myosin heavy chain genes (MyHC1, slow MyHC 2, and slow MyHC 3) were isolated. Only slow MyHC 2 mRNA was demonstrated to be abundant in the cocultures of neural tube and muscle fibers derived from myoblasts of slow muscle origin. Thus, expression of the slow MyHC 2 gene in this in vitro system indicates that formation of slow muscle fiber types is dependent on both myoblast lineage (intrinsic mechanisms) and innervation (extrinsic mechanisms), and suggests neither mechanism alone is sufficient to explain formation of muscle fibers of different types during fetal development.

Effects of nicotine on spontaneous activity and underlying ionic currents in rabbit sinoatrial nodal cells

1. Effects of nicotine on the spontaneous action potentials and the underlying ionic currents in rabbit sinoatrial (SA) nodal cells were investigated using current-clamp and whole-cell voltage-clamp modes. 2. Nicotine (30 microM to 1 mM) produced a negative chronotropic effect in a concentration-dependent manner (at 1 mM by 10.6 +/- 2.8%, n = 9, p < 0.01). Nicotine at 300 microM significantly decreased the maximum rate of depolarization by 9.8 +/- 1.3% (n = 9, p < 0.05). Other action potential parameters were not affected to any significant extent. 3. Pretreatment with atropine (1 microM) and hexamethonium (1 mM) did not modify the nicotine-induced effects. After washout, these responses were reversible. 4. Isoprenaline decreased the responses induced by nicotine, but ACh increased them. 5. Nicotine at 100 microM did not affect the L-type Ca2+ current (ICa), but at 300 microM inhibited it at + 10 mV by 21.6 +/- 2.9% (n = 6, p < 0.05). The fast time constant (tau f) of the inactivation phase for ICa was not affected, but the slow one (tau s) significantly increased from 36.8 +/- 1.9 ms to 41.2 +/- 2.8 ms (n = 6) at 300 microM nicotine. The activation and inactivation kinetics (d infinity and f infinity) for ICa were not modified. 6. Nicotine also did not affect the delayed rectifier K+ current (IK) and its activation kinetic (P infinity). 7. These results suggest that nicotine depresses the action potentials and causes a negative chronotropic effect due to inhibitions of the ionic currents in the SA nodal cells.

Expression of utrophin and its mRNA in denervated mdx mouse muscle

Utrophin is a large cytoskeletal protein which shows high homology to dystrophin. In contrast to the sarcolemmal distribution of dystrophin, utrophin accumulates at the postsynaptic membrane of the neuromuscular junction. Because of its localization within this compartment of muscle fibers, expression of utrophin may be significantly influenced by the presence of the motor nerve. We tested this hypothesis by denervating muscles of mdx mouse and monitoring levels of utrophin and its mRNA by immunofluorescence, immunoblotting and RT-PCR. A significant increase in the number of utrophin positive fibers was observed by immunofluorescence 3 to 21 days after sectioning of the sciatic nerve. Quantitative analyses of utrophin and its transcripts in hindlimb muscles denervated for two weeks showed only a moderate increase in the levels of both utrophin (approximately 2-fold) and its transcript (approximately 60 to 90%). The present data suggest that although utrophin is a component of the postsynaptic membrane, its neural regulation is distinct from that of the acetylcholine receptor.

A novel synapse-associated noncoding RNA

Synaptic nuclei of innervated muscle transcribe acetylcholine receptor (AChR) genes at a much higher level than extrasynaptic nuclei. To isolate candidate synaptic regulatory molecules responsible for the unique transcriptional potential of synaptic nuclei, we have taken a subtractive hybridization approach. Here, we report the cloning and characterization of a novel synapse-associated RNA, 7H4. 7H4 is expressed selectively in the endplate zone of skeletal muscle and is upregulated during early postnatal development and after denervation. Interestingly, the 7H4 gene has no introns, and yet two different-size RNAs with identical polyadenylated 3' ends are generated. Most intriguingly, the nucleotide sequence does not contain any significant open reading frames, suggesting that 7H4 may function as a noncoding RNA.

A novel synapse-associated noncoding RNA

Synaptic nuclei of innervated muscle transcribe acetylcholine receptor (AChR) genes at a much higher level than extrasynaptic nuclei. To isolate candidate synaptic regulatory molecules responsible for the unique transcriptional potential of synaptic nuclei, we have taken a subtractive hybridization approach. Here, we report the cloning and characterization of a novel synapse-associated RNA, 7H4. 7H4 is expressed selectively in the endplate zone of skeletal muscle and is upregulated during early postnatal development and after denervation. Interestingly, the 7H4 gene has no introns, and yet two different-size RNAs with identical polyadenylated 3' ends are generated. Most intriguingly, the nucleotide sequence does not contain any significant open reading frames, suggesting that 7H4 may function as a noncoding RNA.

MyoD or Myf-5 is required for the formation of skeletal muscle

Mice carrying null mutations in the myogenic regulatory factors Myf-5 or MyoD have apparently normal skeletal muscle. To address whether these two factors functionally substitute for one another in myogenesis, mice carrying mutant Myf-5 and MyoD genes were interbred. While mice lacking both MyoD and Myf-5 were born alive, they were immobile and died soon after birth. Northern blot and S1 nuclease analyses indicated that Myf-5(-1-);MyoD(-1-) mice expressed no detectable skeletal muscle-specific mRNAs. Histological examination of these mice revealed a complete absence of skeletal muscle. Immunohistochemical analysis indicated an absence of desmin-expressing myoblast-like cells. These observations suggest that either Myf-5 or MyoD is required for the determination of skeletal myoblasts, their propagation, or both during embryonic development and indicate that these factors play, at least in part, functionally redundant roles in myogenesis.

Clustering and immobilization of acetylcholine receptors by the 43-kD protein: a possible role for dystrophin-related protein

Recombinant acetylcholine receptors (AChRs) expressed on the surface of cultured fibroblasts become organized into discrete membrane domains when the 43-kD postsynaptic protein (43k) is co-expressed in the same cells (Froehner, S.C., C. W. Luetje, P. B. Scotland, and J. Patrick, 1990. Neuron. 5:403-410; Phillips, W. D., M. C. Kopta, P. Blount, P. D. Gardner, J. H. Steinbach, and J. P. Merlie. 1991. Science (Wash. DC). 251:568-570). Here we show that AChRs present on the fibroblast cell surface prior to transfection of 43k are recruited into 43k-rich membrane domains. Aggregated AChRs show increased resistance to extraction with Triton X-100, suggesting a 43k-dependent linkage to the cytoskeleton. Myotubes of the mouse cell line C2 spontaneously display occasional AChR/43k-rich membrane domains that ranged in diameter up to 15 microns, but expressed many more when 43k was overexpressed following transfection of 43k cDNA. However, the membrane domains induced by recombinant 43k were predominantly small (< or = 2 microns). We were then interested in whether the cytoskeletal component, dystrophin related protein (DRP; Tinsley, J. M., D. J. Blake, A. Roche, U. Fairbrother, J. Riss, B. C. Byth, A. E. Knight, J. Kendrick-Jones, G. K. Suthers, D. R. Love, Y. H. Edwards, and K. E. Davis, 1992. Nature (Lond.). 360:591-593) contributed to the development of AChR clusters. Immunofluorescent anti-DRP staining was present at the earliest stages of AChR clustering at the neuromuscular synapse in mouse embryos and was also concentrated at the large AChR-rich domains on nontransfected C2 myotubes. Surprisingly, anti-DRP staining was concentrated mainly at the large, but not the small AChR clusters on C2 myotubes suggesting that DRP may be principally involved in permitting the growth of AChR clusters.

Cell type- and differentiation-dependent expression from the mouse acetylcholine receptor epsilon-subunit promoter

The nicotinic acetylcholine receptor (AChR) in adult skeletal muscle is composed of alpha-, beta-, epsilon-, and delta-subunits and is localized at the neuromuscular junction; in contrast, the more diffusely distributed fetal form is composed of alpha-, beta-, gamma-, and delta-subunits. To define sequences necessary for the transcriptional regulation of the mouse epsilon-subunit gene, we sequenced and analyzed 1036 bp upstream of the transcription start site. Using deletion analysis of the 5'-flanking region linked to the bacterial chloramphenicol acetyltransferase (CAT) gene and transfection of the resulting constructs into established cell lines, we demonstrate that a 151 bp fragment exhibits cell type- and differentiation-specific promoter activity. This activity was independent of a myogenic factor putative binding site (E-box). However, transactivation experiments with recombinant myoD, myogenin, or MRF4 showed that the E-box was functional and that MRF4 preferentially transactivates the epsilon-promoter. Thus, like other AChR promoters, the proximal region of the epsilon-promoter contains information for cell type-specific and developmental regulation of CAT and can be transactivated by myogenic factors in cultured cell lines. Unlike the other AChR promoters characterized to date, epsilon-promoter function can be partially independent of myogenic factors of the helix-loop-helix class.

Cytochalasin modulation of nicotinic cholinergic receptor expression and muscarinic receptor function in human TE671/RD cells: a possible functional role of the cytoskeleton

Previous studies have shown that cells of the TE671/RD human clonal line express muscle-type nicotinic acetylcholine receptors (nAChR) and m3-type muscarinic acetylcholine receptors (mAChR) whose numbers and function are regulated by agonist treatment and second messenger modulation. Here we show that cytochalasin treatment, which causes disruption of actin networks, induces marked changes in the numbers and distribution of nAChR, but not mAChR. Moreover, whereas cytochalasin treatment fails to alter nAChR function significantly, it acutely potentiates mAChR-mediated phosphoinositide hydrolysis. Treatment of TE671/RD cells with different cytochalasin analogues (rank order efficacy at 5 micrograms/ml is H > J = B = C = D > A = E) produces a two- to fourfold increase in numbers of membrane-bound nAChR (Bmax in units of specific 125I-labeled alpha-bungarotoxin binding per milligram of membrane protein). nAChR up-regulation is evident after 1-2 days of cytochalasin B exposure, is maximal after 3-6 days of drug treatment, and is dominated by an approximately 10-fold increase (per cell) in an intracellular nAChR pool. Cytochalasin-induced nAChR up-regulation is similar in magnitude to, but not additive with, up-regulation of nAChR following chronic exposure to nicotine or phorbol ester. Northern blot analysis shows a four- to five-fold coordinate increase in levels of mRNA that encode nAChR alpha, beta, gamma, or delta subunits in cytochalasin-treated cells, suggesting that nAChR up-regulation has a possible transcriptional basis. Studies done using a 86Rb+ efflux assay indicate that cytochalasin treatment has no significant effect on nAChR function. By contrast, cytochalasin treatment has no effect on the numbers of mAChR as assessed by binding studies with the radioantagonist 3H-labeled quinuclidinyl benzilate, but it induces marked enhancement of carbachol-stimulated, but not basal, phosphoinositide hydrolysis. These studies suggest that presumed modulation by cytochalasin treatment of cytoskeletal microfilament integrity can differentially influence expression and function of mAChR (a prototype of the metabotropic receptor superfamily) and nAChR (a prototype of the ligand-gated ion-channel superfamily). The results also suggest possible new roles for the cytoskeleton in regulation of membrane receptor expression, function, and cross talk.

Identification of a neuron-specific promoter of human aromatic L-amino acid decarboxylase gene

We have cloned the 5' region of human aromatic L-amino acid decarboxylase (AADC) gene in a cosmid and an overlapping lambda clone, and sequenced the first five exons. A 61 base pair (bp) non-coding, first exon containing for the 5' end of a human pheochromocytoma AADC cDNA was localized 16 kb upstream of exon 2, in which translation is initiated. The transcription start site was localized by RNAse mapping, primer extension and reverse transcription-PCR. The non-conventional cap site was preceded by a modified TATA box at position -29. A strong promoter was characterized in the 560 bp region upstream of the cap site by linkage to the reporter gene LacZ, and transfection in human neuroblastoma SK-N-BE and SK-N-BE-K2 cells. Using a series of constructs bearing a varying length of 5' flanking region, three positive regulatory elements have been localized in the -560 to -394, -244 to -200 and -147 to -1 regions. Negative regulatory elements were localized in the -9000 to -560 and -394 to -316 regions. Surprisingly, constructs comprising all or the major part of intron 1 were inactive, suggesting the presence of a silencer in the first intron, or incorrect splicing events. The construct containing 560 bp of 5' flanking sequence did not express in human cholinergic neuroepithelioma cells MC-I-XC, and in three non-neuronal cell lines which displayed high AADC activities: human pancreatic carcinoma cells AsPC-1, rat insulinoma cells RINm5F and mouse anterior pituitary cells AtT20. These data suggest that we have identified a neuron-specific AADC promoter. An extensive search for a second promoter responsible for AADC gene expression in non-neuronal cells only gave negative results.

Development of the neuromuscular synapse

Major advances have occurred in our understanding of the signaling events involved in neuromuscular synapse formation. In particular, it has recently been shown that agrin is necessary for synapse formation, that acetylcholine receptor genes are specifically transcribed by synaptic nuclei in response to signals from the synaptic basal lamina, and that synaptic competition between motor neurons can occur by a Hebbian mechanism in cell culture.

Differential expression of human nicotinic acetylcholine receptor alpha subunit variants in muscle and non-muscle tissues

The nicotinic acetylcholine receptor (nAChR) is an oligomeric transmembrane glycoprotein consisting of four homologous subunits in stoichiometry of alpha 2, beta (gamma or epsilon). Recently the presence of a novel exon (P3A) in human alpha AChR gene has been reported. Two variants of the human alpha subunit arise from alternate RNA splicing, one with and one without the P3A exon. However, the evolutionary origin of the P3A exon and the regulation of the expression of the two variants in human muscle and non-human tissues is currently unknown. Examination of genomic DNA from various species shows that the P3A exon sequence is present only in hominoids, old world and new world primates species and is absent in the muscle cDNA or genomic DNA from rat, mouse or dog, indicating that P3A exon is evolutionary conserved for at least 50 million years. The P3A+ variant of alpha subunit was found to be constitutively expressed in skeletal muscle, brain, heart, kidney, liver, lung and thymus, while P3A-variant was differentially expressed only in skeletal muscle. Thus it appears that the P3A+ variant is generated by 'default' selection by the splicing machinery, while expression of the P3A- variant is regulated by tissue-specific factors in the skeletal muscle. Mechanisms regulating differential expression of the alpha subunit variants may be pertinent to the pathophysiology of myasthenia gravis.

Recombinant neuromuscular synapses

The developing neuromuscular junction has provided an important paradigm for studying synapse formation. An outstanding feature of neuromuscular differentiation is the aggregation of acetylcholine receptors (AChRs) at high density in the postsynaptic membrane. While AChR aggregation is generally believed to be induced by the nerve, the mechanisms underlying aggregation remain to be clarified. A 43-kD protein (43k) normally associated with the cytoplasmic aspect of AChR clusters has long been suspected of immobilizing AChRs by linking them to the cytoskeleton. In recent studies, the AChR clustering activity of 43k has, at last, been demonstrated by expressing recombinant AChR and 43k in non-muscle cells. Mutagenesis of 43k has revealed distinct domains within the primary structure which may be responsible for plasma membrane targeting and AChR binding. Other lines of study have provided clues as to how nerve-derived (extracellular) AChR-cluster inducing factors such as agrin might activate 43k-driven postsynaptic membrane specialization.

The 5'-flanking region of the mouse muscle nicotinic acetylcholine receptor beta subunit gene promotes expression in cultured muscle cells and is activated by MRF4, myogenin and myoD

The expression of the nicotinic acetylcholine receptor (AChR) in vertebrate striated muscle is regulated both during development by nerve-evoked muscle activity and by local factors released or associated with the nerve ending. The expression pattern of AChR is achieved by coordinate regulation of four embryonic subunit mRNAs, alpha, beta, gamma and delta. We have taken the approach of identifying the similarities and differences among cis-acting regulatory elements of AChR genes to gain a better understanding of these mechanisms. Thus, to begin to define DNA sequences necessary for the transcriptional regulation of the mouse beta AChR gene, we have analyzed its 5'-flanking region. Primer extension and RNAase protection analyses showed that transcription initiates at one major and two minor sites, all of which are close to the translational initiation site. Using plasmids in which segments of the 5'-flanking region were linked to the bacterial chloramphenicol acetyltransferase (CAT) gene, we have demonstrated that 150 bp of the 5'-flanking region is active in C2 myotubes but not C2 myoblasts or NIH3T3 fibroblasts. This region contains a putative binding site for myoD, and when linked to CAT was transactivated by the muscle regulatory factors myoD, myogenin, and MRF4. Thus, a 150 bp sequence of the beta-subunit gene contains information necessary for developmental specificity and responsiveness to myogenic factors.

Nicotinic receptor-associated 43K protein and progressive stabilization of the postsynaptic membrane

An extrinsic membrane protein of apparent molecular mass 43 kDa is specifically localized in postsynaptic membranes closely associated with the nicotinic acetylcholine receptor (AChR). Since its discovery in 1977, biochemical and morphological studies have combined to provide relatively clear pictures of 43K protein structure and subcellular compartmentalization. Nevertheless, despite these advances, the precise function of this synapse-specific protein remains unclear. Data gathered in recent years indicate that the postsynaptic apparatus develops through the incremental agglomeration of receptor microaggregates; evidence derived from a number of sources points to a role for 43K protein in certain underlying reactions. In this paper, I review 43K protein structural and anatomical data and analyze evidence for its role in the organization and maintenance of the postsynaptic membrane. Finally, I offer a model presenting a view of the role of 43K protein in the ontogeny of the motor endplate.

Mutagenesis of the 43-kD postsynaptic protein defines domains involved in plasma membrane targeting and AChR clustering

The postsynaptic membrane of the neuromuscular junction contains a myristoylated 43-kD protein (43k) that is closely associated with the cytoplasmic face of the nicotinic acetylcholine receptor (AChR)-rich plasma membrane. Previously, we described fibroblast cell lines expressing recombinant AChRs. Transfection of these cell lines with 43k was necessary and sufficient for reorganization of AChR into discrete 43k-rich plasma membrane domains (Phillips, W. D., C. Kopta, P. Blount, P. D. Gardner, J. H. Steinbach, and J. P. Merlie. 1991. Science (Wash. DC). 251:568-570). Here we demonstrate the utility of this expression system for the study of 43k function by site-directed mutagenesis. Substitution of a termination codon for Asp254 produced a truncated (28-kD) protein that associated poorly with the cell membrane. The conversion of Gly2 to Ala2, to preclude NH2-terminal myristoylation, reduced the frequency with which 43k formed plasma membrane domains by threefold, but did not eliminate the aggregation of AChRs at these domains. Since both NH2 and COOH-termini seemed important for association of 43k with the plasma membrane, a deletion mutant was constructed in which the codon Gln15 was fused in-frame to Ile255 to create a 19-kD protein. This mutated protein formed 43k-rich plasma membrane domains at wild-type frequency, but the domains failed to aggregate AChRs, suggesting that the central part of the 43k polypeptide may be involved in AChR aggregation. Our results suggest that membrane association and AChR interactions are separable functions of the 43k molecule.

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.

Acetylcholine receptor-inducing activity stimulates expression of the epsilon-subunit gene of the muscle acetylcholine receptor

Motor neurons regulate the transcription of acetylcholine receptor subunit genes in postsynaptic muscle fibers both through muscle electrical activity produced by motor neuron acetylcholine release and by mechanisms independent of such transmitter release. Factors secreted by the motor neuron may mediate activity-independent regulation, including the postnatal switch from alpha 2 beta gamma delta (embryonic type) to alpha 2 beta epsilon delta (adult type) receptors. We have investigated the effect of putative trophic factors, agents affecting second-messenger systems, and muscle activity on the levels of acetylcholine receptor subunit mRNAs in primary mouse muscle cultures. We found that ARIA (acetylcholine receptor-inducing activity), a 42-kDa glycoprotein purified on the basis of its ability to increase the synthesis of acetylcholine receptors in chick myotubes, increases epsilon-subunit mRNA levels up to 10-fold. In addition, ARIA stimulated alpha-, gamma-, and delta-subunit mRNA levels 2-fold but had no effect on the expression of the beta-subunit gene. These effects of ARIA were independent of muscle activity, and they were not mimicked by calcitonin gene-related peptide nor by thyroxine, forskolin, phorbol 12-myristate 13-acetate, the calcium ionophore A23187, basic fibroblast growth factor, or transforming growth factor beta. Based on these data, we suggest that ARIA may act at the mammalian neuromuscular junction to induce adult-type acetylcholine receptors.

Distribution of Na+ channels and ankyrin in neuromuscular junctions is complementary to that of acetylcholine receptors and the 43 kd protein

We have used immunogold electron microscopy to study the organization of the acetylcholine receptor, 43 kd protein, voltage-sensitive Na+ channel, and ankyrin in the postsynaptic membrane of the rat neuromuscular junction. The acetylcholine receptor and the 43 kd protein are concentrated at the crests of the postsynaptic folds, coextensive with the subsynaptic density. In contrast, Na+ channels and ankyrin are concentrated in the membranes of the troughs and in perijunctional membranes, both characterized by discontinuous submembrane electron-dense plaques. This configuration of interspersed postsynaptic membrane domains enriched in either Na+ channels or acetylcholine receptors may facilitate the initiation of the muscle action potential. Furthermore, the results support the involvement of ankyrin in immobilizing Na+ channels in specific membrane domains, analogous to the proposed involvement of the 43 kd protein in acetylcholine receptor immobilization.

Expression of RNA transcripts for the postsynaptic 43 kDa protein in innervated and denervated rat skeletal muscle

A cDNA clone encoding the mouse muscle postsynaptic 43 kDa protein was isolated and sequenced. The amino acid sequence of this protein, which is closely associated with nicotinic acetylcholine receptors at Torpedo electrocyte and vertebrate skeletal muscle synapses, is very similar in different species. A cysteine-rich region homologous to part of the regulatory domain of protein kinase C may be important in interactions of this protein with the lipid bilayer. RNA transcripts for the 43 kDa protein increase only 2-3 fold after denervation of rat skeletal muscle, in sharp contrast to the alpha-subunit of the muscle nicotinic receptor which increases more than 30-fold. Thus, the expression of these two proteins is regulated by different mechanisms.

The mammalian 43-kD acetylcholine receptor-associated protein (RAPsyn) is expressed in some nonmuscle cells

Torpedo electric organ and vertebrate neuromuscular junctions contain the receptor-associated protein of the synapse (RAPsyn) (previously referred to as the 43K protein), a nonactin, 43,000-Mr peripheral membrane protein associated with the cytoplasmic face of postsynaptic membranes at areas of high nicotinic acetylcholine receptor (AChR) density. Although not directly demonstrated, several lines of evidence suggest that RAPsyn is involved in the synthesis and/or maintenance of such AChR clusters. Microscopic and biochemical studies had previously indicated that RAPsyn expression is restricted to differentiated, AChR-synthesizing cells. Our recent finding that RAPsyn is also produced in undifferentiated myocytes (Frail, D.E., L.S. Musil, a. Bonanno, and J.P. Merlie, 1989. Neuron. 2:1077-1086) led to to examine whether RAPsyn is synthesized in cell types that never express AChR (i.e., cells of other than skeletal muscle origin). Various primary and established rodent cell lines were metabolically labeled with [35S]methionine, and extracts were immunoprecipitated with a monospecific anti-RAPsyn serum. Analysis of these immunoprecipitates by SDS-PAGE revealed detectable RAPsyn synthesis in some (notably fibroblast and Leydig tumor cell lines and primary cardiac cells) but not all (hepatocyte- and lymphocyte-derived) cell types. These results were further substantiated by peptide mapping studies of RAPsyn immunoprecipitated from different cells and quantitation of RAPsyn-encoding mRNA levels in mouse tissues. RAPsyn synthesized in both muscle and nonmuscle cells was shown to be tightly associated with membranes. These findings demonstrate that RAPsyn is not specific to skeletal muscle-derived cells and imply that it may function in a capacity either in addition to or instead of AChR clustering.

Neural regulation of gene expression by an acetylcholine receptor promoter in muscle of transgenic mice

Motor neurons regulate the quantity and distribution of acetylcholine receptors (AChR) in the muscles they innervate. Here, we report that an AChR alpha subunit gene fragment contains cis-acting regulatory sequences that confer neural regulation as well as tissue-specific regulation of transcription. An 850 bp fragment from the 5' end of the chicken AChR alpha gene fused to the reporter gene, chloramphenicol acetyltransferase (CAT), has been introduced into the genomes of several lines of transgenic mice. Expression of CAT enzyme activity in these mice is tissue-specific; the onset of expression in embryonic muscle correlates well with that of many other muscle-specific proteins. Most importantly, CAT enzyme is down-regulated 100-fold soon after birth, an effect that can be completely reversed by denervation.