Separate pathways for synapse-specific and electrical activity-dependent gene expression in skeletal muscle

The ER structural protein Rtn4A stabilizes and enhances signaling through the receptor tyrosine kinase ErbB3

ErbB3 and ErbB4 are receptor tyrosine kinases that are activated by the neuregulin (NRG) family of growth factors. These receptors govern various developmental processes, and their dysregulation contributes to several human disease states. The abundance of ErbB3 and ErbB4, and thus signaling through these receptors, is limited by the E3 ubiquitin ligase Nrdp1, which targets ErbB3 and ErbB4 for degradation. Reticulons are proteins that influence the morphology of the endoplasmic reticulum (ER) by promoting the formation of tubules, a response of cells to some stressors. We found that the ER structural protein reticulon 4A (Rtn4A, also known as Nogo-A) increased ErbB3 abundance and proliferative signaling by suppressing Nrdp1 function. Rtn4A interacted with Nrdp1 and stabilized ErbB3 in an Nrdp1-dependent manner. Rtn4A overexpression induced the redistribution of Nrdp1 from a cytosolic or perinuclear localization to ER tubules. Rtn4A knockdown in human breast tumor cells decreased ErbB3 abundance, NRG-stimulated signaling, and cellular proliferation and migration. Because proteins destined for the plasma membrane are primarily synthesized in the sheet portions of the ER, our observations suggest that Rtn4A counteracts the Nrdp1-mediated degradation of ErbB3 by sequestering the ubiquitin ligase into ER tubules. The involvement of a reticulon suggests a molecular link between ER structure and the sensitivity of cells to receptor tyrosine kinase-mediated survival signals at the cell surface.

hnRNP H enhances skipping of a nonfunctional exon P3A in CHRNA1 and a mutation disrupting its binding causes congenital myasthenic syndrome

In humans and great apes, CHRNA1 encoding the muscle nicotinic acetylcholine receptor alpha subunit carries an inframe exon P3A, the inclusion of which yields a nonfunctional alpha subunit. In muscle, the P3A(-) and P3A(+) transcripts are generated in a 1:1 ratio but the functional significance and regulation of the alternative splicing remain elusive. An intronic mutation (IVS3-8G>A), identified in a patient with congenital myasthenic syndrome, disrupts an intronic splicing silencer (ISS) and results in exclusive inclusion of the downstream P3A exon. We found that the ISS-binding splicing trans-factor was heterogeneous nuclear ribonucleoprotein (hnRNP) H and the mutation attenuated the affinity of hnRNP for the ISS approximately 100-fold. We next showed that direct placement of hnRNP H to the 3' end of intron 3 silences, and siRNA-mediated downregulation of hnRNP H enhances recognition of exon P3A. Analysis of the human genome suggested that the hnRNPH-binding UGGG motif is overrepresented close to the 3' ends of introns. Pursuing this clue, we showed that alternative exons of GRIP1, FAS, VPS13C and NRCAM are downregulated by hnRNP H. Our findings imply that the presence of the hnRNP H-binding motif close to the 3' end of an intron is an essential but underestimated splicing regulator of the downstream exon.

Activity-dependent neurotransmitter-receptor matching at the neuromuscular junction

Signaling in the nervous system requires matching of neurotransmitter receptors with cognate neurotransmitters at synapses. The vertebrate neuromuscular junction is the best studied cholinergic synapse, but the mechanisms by which acetylcholine is matched with acetylcholine receptors are not fully understood. Because alterations in neuronal calcium spike activity alter transmitter specification in embryonic spinal neurons, we hypothesized that receptor expression in postsynaptic cells follows changes in transmitter expression to achieve this specific match. We find that embryonic vertebrate striated muscle cells normally express receptors for glutamate, GABA, and glycine as well as for acetylcholine. As maturation progresses, acetylcholine receptor expression prevails. Receptor selection is altered when early neuronal calcium-dependent activity is perturbed, and remaining receptor populations parallel changes in transmitter phenotype. In these cases, glutamatergic, GABAergic, and glycinergic synaptic currents are recorded from muscle cells, demonstrating that activity regulates matching of transmitters and their receptors in the assembly of functional synapses.

Activity-dependent gene regulation in skeletal muscle is mediated by a histone deacetylase (HDAC)-Dach2-myogenin signal transduction cascade

Muscle activity contributes to muscle development and function largely by means of regulated gene expression. Many genes crucial to neuromuscular synapse formation, such as MuSK and nAChRs, are induced before muscle innervation or after muscle denervation, and this induction requires expression of the E-box binding, basic helix-loop-helix muscle-specific transcription factor, myogenin (Mgn). The mechanism by which muscle activity is coupled to gene expression is poorly defined. Here we report that inhibition of histone deacetylase (HDAC) activity attenuates the induction of activity-regulated genes in aneural myotubes and adult denervated muscle. The effect of HDAC inhibitors requires new protein synthesis, suggesting HDACs may regulate the expression of a Mgn transcriptional repressor. We identified Dach2 as a Mgn transcriptional repressor whose expression is dramatically reduced in an HDAC-dependent manner in developing aneural myotubes or adult denervated muscle. Dach2 overexpression in denervated muscle suppressed Mgn, nAChR, and MuSK gene induction, whereas Dach2 knockdown induced Mgn gene expression in innervated muscle and relieved Mgn promoter inhibition by HDAC inhibitors. Thus, a HDAC-Dach2-myogenin signaling pathway has been identified to decode nerve activity and control muscle gene expression in developing and adult skeletal muscle.

Characterization of a muscle-specific enhancer in human MuSK promoter reveals the essential role of myogenin in controlling activity-dependent gene regulation

Neuromuscular synaptogenesis is initiated by the release of agrin from motor neurons and the activation of the receptor tyrosine kinase, MuSK, in the postsynaptic membrane. MuSK gene expression is regulated by nerve-derived agrin and muscle activity. Agrin stimulates synapse-specific MuSK gene expression by activating GABP(alphabeta) transcription factors in endplate-associated myonuclei. In contrast, the mechanism by which muscle activity regulates MuSK gene expression is not known. We report on a 60-bp MuSK enhancer that confers promoter regulation by muscle differentiation, changes in intracellular calcium, and muscle activity. Within this enhancer, we identified a single E-box that is essential for this regulation. This E-box binds myogenin, and we showed that myogenin is necessary for not only MuSK but also nAChR gene regulation by muscle activity. Surprisingly, the same E-box functions in vivo to mediate muscle-specific and differentiation-dependent gene induction in zebrafish, suggesting an evolutionary conserved mechanism of regulation of synaptic protein gene expression.

A 1.3 kb promoter fragment confers spatial and temporal expression of utrophin A mRNA in mouse skeletal muscle fibers

Upregulation of utrophin in muscle is currently being examined as a potential therapy for Duchenne muscular dystrophy patients. In this context, we generated transgenic mice harboring a 1.3 kb human utrophin A promoter fragment driving expression of the lacZ gene. Characterization of reporter expression during postnatal muscle development revealed that the levels and localization of beta-galactosidase parallel expression of utrophin A transcripts. Moreover, we noted that the utrophin A promoter is more active in slow soleus muscles. Additionally, expression of the reporter gene was regulated during muscle regeneration in a manner similar to utrophin A transcripts. Together, these results show that the utrophin A promoter-lacZ construct mirrors expression of utrophin A mRNAs indicating that this utrophin A promoter fragment confers temporal and spatial patterns of expression in skeletal muscle. This transgenic mouse will be valuable as an in vivo model for developing and testing molecules aimed at increasing utrophin A expression.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Neuregulin-induced expression of the acetylcholine receptor requires endocytosis of ErbB receptors

Neuregulin-induced expression of the acetylcholine receptor (AChR) contributes to high concentration of the receptor at the neuromuscular junction (NMJ). Neuregulin-1 activates ErbB tyrosine kinases and subsequently intracellular kinases including Erk that is required for induced AChR expression. Recent studies demonstrate that ligand-induced internalization may regulate signaling of various receptor tyrosine kinases. However, the role of induced ErbB endocytosis in regulating AChR expression was unclear. Here we provide evidence that ErbB tyrosine kinases became rapidly internalized in response to neuregulin. The internalization required the kinase activity of ErbB proteins and involved a clathrin-dependent endocytic pathway. Moreover, neuregulin-induced Erk activation and AChR expression were attenuated when ErbB endocytosis was blocked. These results indicate that ErbB proteins undergo endocytosis in response to neuregulin, and this process is required for neuregulin signaling and induced AChR expression.

Histone deacetylase 9 couples neuronal activity to muscle chromatin acetylation and gene expression

Electrical activity arising from motor innervation influences skeletal muscle physiology by controlling the expression of many muscle genes, including those encoding acetylcholine receptor (AChR) subunits. How electrical activity is converted into a transcriptional response remains largely unknown. We show that motor innervation controls chromatin acetylation in skeletal muscle and that histone deacetylase 9 (HDAC9) is a signal-responsive transcriptional repressor which is downregulated upon denervation, with consequent upregulation of chromatin acetylation and AChR expression. Forced expression of Hdac9 in denervated muscle prevents upregulation of activity-dependent genes and chromatin acetylation by linking myocyte enhancer factor 2 (MEF2) and class I HDACs. By contrast, Hdac9-null mice are supersensitive to denervation-induced changes in gene expression and show chromatin hyperacetylation and delayed perinatal downregulation of myogenin, an activator of AChR genes. These findings show a molecular mechanism to account for the control of chromatin acetylation by presynaptic neurons and the activity-dependent regulation of skeletal muscle genes by motor innervation.

MUSK, a new target for mutations causing congenital myasthenic syndrome

We report the first case of a human neuromuscular transmission dysfunction due to mutations in the gene encoding the muscle-specific receptor tyrosine kinase (MuSK). Gene analysis identified two heteroallelic mutations, a frameshift mutation (c.220insC) and a missense mutation (V790M). The muscle biopsy showed dramatic pre- and postsynaptic structural abnormalities of the neuromuscular junction and severe decrease in acetylcholine receptor (AChR) epsilon-subunit and MuSK expression. In vitro and in vivo expression experiments were performed using mutant MuSK reproducing the human mutations. The frameshift mutation led to the absence of MuSK expression. The missense mutation did not affect MuSK catalytic kinase activity but diminished expression and stability of MuSK leading to decreased agrin-dependent AChR aggregation, a critical step in the formation of the neuromuscular junction. In electroporated mouse muscle, overexpression of the missense mutation induced, within a week, a phenotype similar to the patient muscle biopsy: a severe decrease in synaptic AChR and an aberrant axonal outgrowth. These results strongly suggest that the missense mutation, in the presence of a null mutation on the other allele, is responsible for the dramatic synaptic changes observed in the patient.

Lipid rafts in neuregulin signaling at synapses

Neuregulins are a family of EGF domain-containing factors that play an important role in development. In the nervous system, they promote glial differentiation, induce neurotransmitter receptor expression, and regulate synaptic plasticity. Recent studies indicate that ErbB protein tyrosine kinases, neuregulin receptors, translocate to lipid raft microdomains in the plasma membrane in response to neuregulin. Localization of ErbB proteins in lipid rafts appeared to be necessary for neuregulin signaling and regulation of synaptic plasticity. We will review recent studies of lipid rafts and neuregulin function and discuss possible roles of lipid rafts in compartmentalized neuregulin signaling and translocation of ErbB proteins to synapses.

Neuregulin-1 induces expression of Egr-1 and activates acetylcholine receptor transcription through an Egr-1-binding site

Localization of acetylcholine receptors (AChRs) to neuromuscular synapses is mediated, in part, through selective transcription of AChR genes in myofiber synaptic nuclei. Neuregulin-1 (NRG-1) and its receptors, ErbBs, are concentrated at synaptic sites, and NRG-1 activates AChR synthesis in cultured muscle cells, suggesting that NRG-1-ErbB signaling functions to activate synapse-specific transcription. Previous studies have demonstrated that NRG-1-induced transcription is conferred by cis-acting elements located within 100 bp of 5' flanking DNA from the AChR epsilon subunit gene, and that it requires a GABP binding site within this region. To determine whether additional regulatory elements have a role in NRG-1 responsiveness, we used transcriptional reporter assays in a muscle cell line, and we identified an element that is required for NRG-1-induced transcription (neuregulin response element, NRE). Proteins from myotube extracts bind the NRE and NRG-1 treatment of the cells stimulates this binding. The ability of NRG-1 to stimulate formation of a protein-DNA complex with the NRE requires induction of protein expression. The complex contains early growth response-1 (Egr-1), a member of the Egr family of transcription factors, because proteins in the complex bind specifically to an Egr consensus site, and formation of the complex is inhibited by antibodies to Egr-1. NRG-1 induces expression of Egr-1 in myotubes, which presumably is responsible for the ability of NRG-1 to stimulate protein binding to the NRE. These results suggest that NRG-1 signaling in myotubes involves induction of Egr-1 expression, which in turn serves to activate transcription of the AChR epsilon subunit gene.

Androgen receptor immunoreactivity in skeletal muscle: enrichment at the neuromuscular junction

Potential cellular targets of androgen action within skeletal muscle of the rat were determined by comparing the cellular distribution of androgen receptor (AR)-positive nuclei in the highly androgen-responsive levator ani (LA) muscle with that of the relatively androgen-unresponsive extensor digitorum longus (EDL) muscle. We found that androgen responsiveness correlates with AR expression in muscle fibers and not in fibroblasts. Results indicate that a much higher percentage of myonuclei in the LA are AR(+) than in the EDL (74% vs. 7%), correlating with differences in androgen responsiveness. Both muscles contain an equivalent proportion of AR(+) fibroblasts (approximately 62%). AR(+) nuclei were not observed in terminal Schwann cells in either muscle. These results suggest that ARs within LA muscle fibers mediate the androgen-dependent survival and growth of the LA muscle and its motoneurons. We also observed an unexpected enrichment of AR(+) myonuclei and fibroblasts proximate to neuromuscular junctions, suggesting that ARs at muscle synapses may selectively regulate synapse-specific genes important for the survival and growth of motoneurons. Although castration reduced the proportion of AR(+) fibroblasts in both muscles, the proportion of AR(+) myonuclei was reduced only in the LA. As expected, testosterone treatment prevented these effects of castration but, unexpectedly, increased the proportion of AR(+) myonuclei in the EDL to above normal. These results suggest that how AR expression in skeletal muscle is influenced by androgens depends not only on the particular muscle but on the particular cell type within that muscle.

Accelerated response of the myogenin gene to denervation in mutant mice lacking phosphorylation of myogenin at threonine 87

Gene expression in skeletal muscle is regulated by a family of myogenic basic helix-loop-helix (bHLH) proteins. The binding of these bHLH proteins, notably MyoD and myogenin, to E-boxes in their own regulatory regions is blocked by protein kinase C (PKC)-mediated phosphorylation of a single threonine residue in their basic region. Because electrical stimulation increases PKC activity in skeletal muscle, these data have led to an attractive model suggesting that electrical activity suppresses gene expression by stimulating phosphorylation of this critical threonine residue in myogenic bHLH proteins. We show that electrical activity stimulates phosphorylation of myogenin at threonine 87 (T87) in vivo and that calmodulin-dependent kinase II (CaMKII), as well as PKC, catalyzes this reaction in vitro. We find that phosphorylation of myogenin at T87 is dispensable for skeletal muscle development. We show, however, that the decrease in myogenin (myg) expression following innervation is delayed and that the increase in expression following denervation is accelerated in mutant mice lacking phosphorylation of myogenin at T87. These data indicate that two distinct innervation-dependent mechanisms restrain myogenin activity: an inactivation mechanism mediated by phosphorylation of myogenin at T87, and a second, novel regulatory mechanism that regulates myg gene activity independently of T87 phosphorylation.

Methyl-CpG binding proteins identify novel sites of epigenetic inactivation in human cancer

Methyl-CpG binding proteins (MBDs) mediate histone deacetylase-dependent transcriptional silencing at methylated CpG islands. Using chromatin immunoprecitation (ChIP) we have found that gene-specific profiles of MBDs exist for hypermethylated promoters of breast cancer cells, whilst a common pattern of histone modifications is shared. This unique distribution of MBDs is also characterized in chromosomes by comparative genomic hybridization of immunoprecipitated DNA and immunolocalization. Most importantly, we demonstrate that MBD association to methylated DNA serves to identify novel targets of epigenetic inactivation in human cancer. We combined the ChIP assay of MBDs with a CpG island microarray (ChIP on chip). The scenario revealed shows that, while many genes are regulated by multiple MBDs, others are associated with a single MBD. These target genes displayed methylation- associated transcriptional silencing in breast cancer cells and primary tumours. The candidates include the homeobox gene PAX6, the prolactin hormone receptor, and dipeptidylpeptidase IV among others. Our results support an essential role for MBDs in gene silencing and, when combined with genomic strategies, their potential to 'catch' new hypermethylated genes in cancer.

Synapse-specific gene expression at the neuromuscular junction

Agrin is the key neural factor that controls muscle postsynaptic differentiation, including the induction of synapse-specific transcription via neuregulins. In 1995, the promoter element responsible for the targeting of AChR delta and epsilon gene transcription to the skeletal muscle subsynaptic area was identified. This element, named N-box, recruits the Ets-related transcription factor GABP to AChR delta and epsilon promoters, and both the N-box and GABP are required to obtain transcriptional stimulation by neuregulins. The physiological importance of the N-box has been definitively established with the discovery of myasthenic families carrying single-point mutations in the N-box of the AChR epsilon gene promoter and showing reduced levels of AChR epsilon subunit expression. The control of synapse-specific transcription by agrin and neuregulins through the N-box and GABP is not restricted to the case of AChR genes. The same regulation holds true for the ACh esterase and utrophin genes, thus showing that nerve-induced transcriptional activation of several synapse-specific genes is triggered by a common mechanism involving agrin, neuregulins, and ultimately the N-box and Ets-related transcription factors.

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.

Compartmentalized NRG signaling and PDZ domain-containing proteins in synapse structure and function

The synapse-specific synthesis of the acetylcholine receptor (AChR) is mediated by multiple mechanisms including compartmentalized signaling induced by neuregulin (NRG). This paper presents evidence that NRG receptors--ErbB receptor tyrosine kinases interact with distinct PDZ domain-containing proteins that are localized at the neuromuscular junction (NMJ). ErbB4 associates with the PSD-95 (also known as SAP90)-family members including PSD-95, SAP97, and SAP102 whereas ErbB2 interacts with Erbin and PICK1. Although, ErbB kinases are concentrated at the NMJ, they are not colocalized with the AChR in cultured muscle cells even in the presence of agrin. Co-expression of PSD-95 causes ErbB4 to form clusters in COS cells. We propose that PDZ domain-containing proteins play a role in anchoring ErbB proteins at the neuromuscular junction, and/or mediating downstream signaling pathways. Such mechanisms could be important for the maintenance and function of the synapse.

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.

CaM kinase II-dependent suppression of nicotinic acetylcholine receptor delta-subunit promoter activity

Nerve-induced muscle activity suppresses nicotinic acetylcholine receptor (nAChR) gene expression by increasing intracellular calcium levels. This suppression is mediated by nAChR promoter sequences harboring at least 1 E-box (CANNTG) that bind myogenic helix-loop-helix transcription factors. How muscle depolarization or increased calcium mediates changes in nAChR promoter activity is not well understood. In chick muscle, protein kinase C (PKC) activation is necessary for activity-dependent nAChR gene suppression. Similar effects of PKC activation have not been found in mammalian skeletal muscle. Therefore, we used rat primary muscle cultures to screen for other calcium-regulated enzymatic activities that may mediate the effects of muscle activity and calcium on nAChR promoter activity. We report here that calcium/calmodulin-dependent protein kinase II (CaM kinase II) can specifically suppress nAChR promoter activity in mammalian muscle. This regulation was mediated by a single E-box sequence residing in the previously characterized nAChR delta-subunit genes 47-base pair activity-dependent enhancer. In vitro protein/DNA interaction studies suggest that CaM kinase II inhibits binding of the myogenic factor, myogenin, to the delta-promoter 47-base pair activity-dependent enhancer. CaM kinase activity is increased in active muscle and inhibition of this enzymatic activity results in increased nAChR delta-promoter activity. Therefore, CaM kinase II may represent a previously unappreciated activity that participates in coupling muscle depolarization to nAChR gene expression.

Denervation induces a rapid nuclear accumulation of MRF4 in mature myofibers

Muscle regulatory factor 4 (MRF4) is a member of the family of myogenic transcription factors, including MyoD, myogenin, and myf-5, that are necessary for the commitment and differentiation of mesoderm to skeletal muscle. Although the function of these transcription factors during embryonic development has been demonstrated, their role in adult muscle has remained elusive. Regulation of the MRF4 gene differs from the genes encoding the other myogenic factors in that its transcripts accumulate in neonatal muscle during maturation and continue to be expressed at relatively high levels in the adult. On the basis of its mRNA expression pattern, MRF4 has been suggested to regulate genes encoding adult contractile proteins and acetylcholine receptor subunits. To test this hypothesis, a specific antiserum was developed to study MRF4 protein expression in adult innervated and denervated muscle, because MRF4 mRNA levels increase by approximately threefold 1 day after nerve resection. By using three different immunohistochemical methods that vary widely in sensitivity, we were unable to detect MRF4 immunoreactivity in adult innervated muscles. The same results were obtained with another MRF4 antiserum generated independently. In contrast, any of these three immunologic techniques readily detected MRF4 immunoreactivity in myofiber and satellite cell nuclei of muscles denervated for 24 hours. The highest proportion of immunopositive nuclei (80%) was found 2-3 days after denervation. Immunoreactivity was no longer detectable by 14 days. There was no differential accumulation of MRF4 protein in the nuclei of satellite cells nor in sole plate (synaptic) nuclei at any time after denervation. No differences were found in the temporal accumulation of MRF4 in nuclei of type I and type II denervated myofibers, consistent with the similar distribution of MRF4 mRNAs in slow- and fast-twitch muscles. Our results are consistent with the lack of phenotype observed in the adult muscles of MRF4-null mutant mice observed by others and suggest that MRF4 may have important roles in the gene programs activated after denervation and during muscle regeneration.

Congenital myasthenic syndrome caused by a mutation in the Ets-binding site of the promoter region of the acetylcholine receptor epsilon subunit gene

Forty-two missense, truncation, or splice-site mutations of the acetylcholine receptor (AChR) subunit genes have been reported to date in patients with congenital myasthenic syndromes. Here we report a homozygous mutation, epsilon-155G --> A, in the promoter region of the AChR epsilon subunit gene that converts the Ets-binding site of the promoter region from CGGAA to CAGAA. The asymptomatic parents and brother are heterozygous and an affected sister is homozygous for epislon-155G --> A. The Ets-binding site mediates synapse specific expression of the AChR epsilon subunit gene. An identical G-to-A mutation in the mouse Ets-binding site was previously shown to decrease the binding affinity of the Ets-binding site for the GA binding protein, a transactivating factor for the Ets-binding site, and to reduce the synapse specific expression of the epsilon subunit. The decreased synaptic expression of the epsilon subunit readily accounts for the congenital myasthenic phenotype.

Development of the vertebrate neuromuscular junction

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

Synapse-specific and neuregulin-induced transcription require an ets site that binds GABPalpha/GABPbeta

Localization of acetylcholine receptors (AChRs) to neuromuscular synapses is mediated by multiple pathways. Agrin, which is the signal for one pathway, stimulates a redistribution of previously unlocalized AChRs to synaptic sites. The signal for a second pathway is not known, but this signal stimulates selective transcription of AChR genes in myofiber nuclei located near the synaptic site. Neuregulin (NRG) is a good candidate for the extracellular signal that induces synapse-specific gene expression, since NRG is concentrated at synaptic sites and activates AChR gene expression in cultured muscle cells. Previous studies have demonstrated that 181 bp of 5' flanking DNA from the AChR delta-subunit gene are sufficient to confer synapse-specific transcription in transgenic mice and NRG responsiveness in cultured muscle cells, but the critical sequences within this cis-acting regulatory region have not been identified. We transfected AChR delta-subunit-hGH gene fusions into a muscle cell line, and we show that a potential binding site for Ets proteins is required for NRG-induced gene expression. Furthermore, we produced transgenic mice carrying AChR delta-subunit-hGH gene fusions with a mutation in this NRG-response element (NRE), and we show that this NRE is necessary for synapse-specific transcription in mice. The NRE binds proteins in myotube nuclear extracts, and nucleotides that are important for NRG responsiveness are likewise critical for formation of the protein-DNA complex. This complex contains GABPalpha, an Ets protein, and GABPbeta, a protein that lacks an Ets domain but dimerizes with GABPalpha, because formation of the protein-DNA complex is inhibited by antibodies to either GABPalpha or GABPbeta. These results demonstrate that synapse-specific and NRG-induced gene expression require an Ets-binding site and suggest that GABPalpha/GABPbeta mediates the transcriptional response of the AChR delta-subunit gene to synaptic signals, including NRG.

Proximal sequences of the aldolase A fast muscle-specific promoter direct nerve- and activity-dependent expression in transgenic mice

Muscle activity is known to modulate the muscle fiber phenotype. Changes in muscle activity (normal or experimentally induced) lead to modifications of the expression status of several muscle-specific genes. However, the transcription regulatory elements involved in the adaptative response are mainly unknown. The aldolase A muscle-specific promoter, pM, is expressed in adult fast twitch muscle with a preferential expression in fast glycolytic-2B fibers. Its activity is induced during postnatal muscle maturation, suggesting a role of nerve and/or muscle activity. Indeed, denervation of gastrocnemius in newborn mice prevented the activation of the promoter in this muscle, despite the nerve-independent formation of 2B fibers. Although the nerve was necessary for pM onset during development, denervating the gastrocnemius in adults had only mild effects on pM activity. By contrast, a transgene including the pM proximal regulatory sequences that are sufficient to reproduce the 2B fiber-specific expression of the endogenous promoter was shown to be highly sensitive to both neonatal and adult denervation. Transgenes containing muscle-specific pM proximal promoter elements were used to delineate the regulatory elements involved in this response to innervation and changes in the contractile activity pattern. Nerve- and activity-dependent elements could be localized in the 130-base pair-long proximal promoter region of the human aldolase A gene.

Transcriptional pathways for synapse-specific, neuregulin-induced and electrical activity-dependent transcription

Innervation-dependent expression of acetylcholine receptor (AChR) genes in skeletal muscle is mediated by multiple transcriptional pathways. One pathway leads to activation of AChR genes selectively in synaptic nuclei and requires an Ets binding site that binds GABP. A second pathway leads to repression of AChR transcription in nuclei throughout the myofiber and requires inactivation of E-box-binding proteins, including myogenic bHLH proteins. Taken together, these studies indicate that separate pathways regulate innervation-dependent transcription.

Nonmyogenic factors bind nicotinic acetylcholine receptor promoter elements required for response to denervation

Nicotinic acetylcholine receptors (AChRs) belong to a class of muscle proteins whose expression is regulated by muscle electrical activity. In innervated muscle fiber, AChR genes are transcriptionally repressed outside of the synapse, while after denervation they become reexpressed throughout the fiber. The myogenic determination factors (MDFs) of the MyoD family have been shown to play a central role in this innervation-dependent regulation. In the chicken AChR alpha-subunit gene promoter, two E-boxes that bind MDFs are necessary to achieve the enhancement of transcription following muscle denervation. However, the deletion of promoter sequences located upstream to these E-boxes greatly impairs the response to denervation (Bessereau, J. L., Stratford- Perricaudet, L. D., Piette, J., Le Poupon, C. and Changeux, J. P. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 1304-1308). Here we identified two additional cis-regulatory elements of the alpha-subunit gene promoter that cooperate with the E-boxes in the denervation response. One region binds the Sp1 and Sp3 zinc finger transcription factors. The second region binds at least three distinct factors, among which we identified an upstream stimulatory factor, a b-ZIP-HLH transcription factor. We propose that among MDF-responsive muscle promoters, a specific combination between myogenic and nonmyogenic factors specify innervation-dependent versus innervation-independent promoters.

Regulation by ARIA/neuregulin of acetylcholine receptor gene transcription at the neuromuscular junction

Transcription of genes encoding nicotinic acetylcholine receptor (AChR) subunits (alpha, beta, gamma or epsilon, and delta) is highest in nuclei localized to the synaptic region of the muscle which contributes to maintain a high density of AChRs at the postjunctional membrane. ARIA (AChR inducing activity) is believed to be the trophic factor utilized by motor neurons to stimulate AChR synthesis in the subsynaptic area. ARIA stimulates tyrosine phosphorylation of the erbB proteins and activates the MAP kinase pathway which is required for the ARIA-mediated induction of AChR genes.

Nuclear clustering in myotubes: a proposed role in acetylcholine receptor mRNA expression

We investigated the functional relationship between nuclear topology, as expressed by degree and type of nuclear aggregation, and appearance of acetylcholine receptor (AChR) subunit mRNAs. Embryonic chick muscle cell cultures treated with the muscle activity blocking agents decamethonium (DCM), d-tubocurare (TBC), and tetrodotoxin (TTX) or co-cultured with cholinergic neurons were examined for the influence of muscle activity on nuclear aggregation and its effects on AChR alpha-, gamma-, and delta-subunit message expression. mRNA was measured by in situ hybridization and nuclei were visualized by bis-benzimide DNA staining. DCM and TBC treatments, as well as neuronal co-culture, resulted in increased nuclear clustering within myotubes and a per nucleus upregulation in mRNA expression relative to control for each subunit. The pattern of nuclear aggregation was treatment dependent, with more and larger aggregates found when myotubes were co-cultured with neurons. Moreover, as nuclear aggregates became larger: (1) nearly all nuclei within active aggregates expressed mRNA and (2) local accumulation (mRNA per unit area) was elevated relative to single nuclei, while per nucleus mRNA production decreased. To determine whether mRNA expression was transient and did not result in steady-state upregulation of AChR receptor protein, we performed a double labeling of surface AChRs with 125I-alpha-bungarotoxin (125I-alpha-BTX) concomitant to the in situ hybridization for mRNA quantification on TTX treated muscle cells. Surface receptor expression tracked mRNA expression forall types of nuclear topology observed, indicating that message levels are in fact reliable indicators of receptor population on the plasma membrane surface in myotubes. We propose that nuclear clustering is an organelle-level, accessory mechanism whereby cells concentrate relatively large amounts of AChR mRNA/protein in specific myotube regions.

Multiple nuclear proteins bind a novel cis-acting element that regulates the muscle-specific expression of the mouse nicotinic acetylcholine receptor alpha-subunit gene

Expression of the nicotinic acetylcholine receptor (AChR) is transcriptionally regulated during the development of vertebrate striated muscle. To better define regulatory elements involved in this process, site-directed mutations were made in the gene's 86 bp muscle specific enhancer. Transient expression assays in skeletal muscle C2C12 cells indicated that all three E-boxes, plus a novel sequence outside the E-boxes, are necessary for full activity of the AChR gene in myotubes. Gel mobility shift assays demonstrated that mutations in the non-E-box sequence disrupted the formation of two DNA/protein complexes while not affecting myoD binding. Methylation interference footprinting confirmed that the complexes form at nucleotides within the mutated region, and also include part of the central E-box. UV crosslinking of nuclear proteins to a DNA probe identified five proteins of 125, 81, 55, 42, and 35 kDa that bind to this region; with the 125 kDa protein being differentially bound in U.V. crosslink assays during the transition from myoblasts to myotubes. These data suggest that interactions between this DNA element and the five proteins contribute to the transcriptional control of the AChR alpha-subunit gene expression during the differentiation of skeletal muscle.

Genetic manipulations of cholinergic communication reveal trans-acting control mechanisms over acetylcholine receptors

Several approaches have been developed for genetic modulations of receptor expression. These initiated with gene cloning and heterologous expression in microinjected Xenopus oocytes, and proceeded through transgenic expression and genomic disruption of receptor genes in mice. In addition, antisense treatments have reduced receptor levels in a transient, reversible manner. Integration of foreign DNA with host genomic sequences yields both cis- and trans-acting responses. These may depend on the DNA integration site, host cells condition and most importantly, the affected signal transduction circuit. For example, acetylcholinesterase (AChE) overexpression in microinjected Xenopus tadpoles has been shown to upregulate alpha-bungarotoxin binding levels, indicating trans-acting control conferring overproduction of muscle nicotinic acetylcholine receptors. In transgenic mice expressing human AChE, the hypothermic response to oxotremorine was suppressed, reflecting modified levels of brain muscarinic receptors. To dissociate the feedback processes occurring in transfected cells from responses related to DNA integration, we examined the endogenous expression of the alpha 7 neuronal nicotinic acetylcholine receptor in PC12 cells transfected with DNA vectors carrying alternative splicing variants of human AChE mRNA. Our findings demonstrate suppression of alpha 7 receptor levels associated with the accumulation of foreign DNA in the transfected cells. Acetylcholine receptor levels thus depend on multiple elements, each of which should be considered when genetic interventions are employed.

Identification of an element crucial for the sub-synaptic expression of the acetylcholine receptor epsilon-subunit gene

The adult neuromuscular junction displays an accumulation of both the acetylcholine receptor (AChR) protein in the subneural domain of the post-synaptic membrane and the mRNAs coding for all its subunits at the level of the subjunctional "fundamental nuclei." In the course of end plate development, the epsilon-subunit, at variance with other subunits, becomes exclusively expressed at the level of the fundamental nuclei, yet at a rather late stage (around birth). To analyze the promoter region of the epsilon-subunit gene which directs its specific expression at the synapse, we used a quantitative transient in vivo expression assay in intact muscle tissue using constructs of the epsilon-subunit promoter placed upstream of the beta-galactosidase reporter gene. One crucial element for synapse-specific expression was detected between the -11 and -6 positions. Disruption of this element, either by a scanning mutation or single base mutations, greatly diminishes, or even completely inhibits, preferential expression of the transgene at the end plate. Gel shift experiments reveal the presence of a complex in nuclear muscle extracts that bind the core sequence of this element. The identification of such a site opens the possibility to identify regulatory factors responsible for compartmentalized expression at the neuromuscular junction.

Cloning and characterization of a novel transcriptional repressor of the nicotinic acetylcholine receptor delta-subunit gene

We have identified a negative cis-acting regulatory element in the nicotinic acetylcholine receptor delta-subunit gene's promoter. This element resides within a previously identified 47-base pair activity-dependent enhancer. Proteins that bind this region of DNA were cloned from a lambdagt11 innervated muscle expression library. Two cDNAs (MY1 and MY1a) were isolated that encode members of the Y-box family of transcription factors. MY1/1a RNAs are expressed at relatively high levels in heart, skeletal muscle, testis, glia, and specific regions of the central nervous system. MY1/1a are nuclear proteins that bind specifically to the coding strand of the 47-base pair enhancer and suppress delta-promoter activity in a sequence-specific manner. These results suggest a novel mechanism of repression by MY1/1a, which may contribute to the low level expression of the delta-subunit gene in innervated muscle. Finally, the gene encoding MY1/1a, Yb2, maps to the mid-distal region of mouse chromosome 6.

Transcriptional control of muscle plasticity: differential regulation of troponin I genes by electrical activity

Plasticity of the skeletal muscle phenotype can result from the selective repression and activation of gene expression in response to innervation patterns. Motoneurons, eliciting different patterns of depolarization, regulate the contractile properties of the myofibers they innervate by selectively activating expression of genes encoding fiber-type-specific (fast vs. slow) contractile proteins. We have analyzed the regulation of the troponin I slow (TnIs) and fast (TnIf) genes as a model to study the molecular mechanisms regulating fiber-type plasticity. We found that expression of the two TnI isoforms is downregulated by denervation. Moreover, TnI expression is upregulated by specific patterns of electrical activity [10 Hz vs. 100 Hz] used to depolarize muscle. We previously isolated the rat TnIs gene and demonstrated that regulatory sequences reside in its upstream region and second intron [Banerjee-Basu S, Buonanno A (1993), Mol Cell Biol 12:5024-5032]. Using transgenic mice, we show that the upstream region of the TnIs gene extending from -949 to +50 is sufficient to confer transcription specifically in slowtwitch muscles. Serial deletions of the TnIs upstream and intronic regions were generated in a CAT reporter vector to delineate transcriptional regulatory elements in transiently transfected Sol8 myotubes. Sequences necessary to confer the highest levels of TnIs transcription mapped to the upstream region between -0.95 and -0.72 kb, and to a 56 bp sequence located in the second intron. Comparison of the at sequence between -0.95 and -0.72 to the human TnIs gene identified a highly homologous region of 128 bp that we named the TnI SURE (slow upstream regulatory element). Alignment of these two SURE sequences with the quail TnI fast intronic regulatory element identified common motifs, namely, two A/T-rich sequences (A/T1 and A/T2) with homology to homeotic protein and MEF2 binding sites, a CACC box, an E box, and a novel motif (GCAGGCA) that we denoted the CAGG box. Mutation of either the A/T2 site, E box, or CAGG box practically abolish the SURE function in transfected myotubes; mutation of the A/T1 and CACC sites has a lesser effect. Using competitive electrophoretic mobility shift assays with nuclear extracts derived from Sol8 myotubes, we demonstrate specific binding to these motifs. The A/T1 and A/T2 sites are shown to form different complexes. The A/T2 site, which bears extensive homology to a MEF2 site, forms complexes that are super shifted by MEF2A antisera and that are competed by a consensus MEF2 site present in the MCK enhancer. Our results demonstrate that the linear arrangement of DNA sequence motifs is conserved in the regulatory elements of the TnI slow and fast genes and suggest that the interaction of multiple protein-DNA complexes are necessary for enhancer function.

Neuregulins and their receptors

The recent identification of an activator for the ErbB2/Neu receptor has uncovered a new family of polypeptide growth factors that undoubtedly play a major role in the regulation of neuronal growth and differentiation. These factors, called the neuregulins, are expressed in neural and mesenchymal tissues, and activate members of the epidermal growth factor family of receptor tyrosine kinases. The identification and characterization of the neuregulins and their receptors will facilitate the dissection of the biochemical pathways regulating nervous system development.

The depolarization response element in acetylcholine receptor genes is a dual-function E box

All acetylcholine receptor subunit genes contain E boxes and are blocked by membrane depolarization. We have used transfected C2C12 myogenic cells to investigate the response, to electrical stimulation and KCl, of wildtype and mutant regulatory regions of the chick acetylcholine receptor alpha, gamma and delta subunit, and the mouse MLC genes. Point mutations revealed that E boxes function as activating elements targeted by the depolarization signal. These experiments suggest, and insertion of a depolarization response element into an unrelated promoter confirms, that plasma membrane depolarization switches the depolarization response element from an activating to a repressive mode.

Regulation of the acetylcholine receptor epsilon subunit gene by recombinant ARIA: an in vitro model for transynaptic gene regulation

Structural specialization of the postsynaptic skeletal muscle membrane is in part mediated by the motor neuron-induced transcriptional regulation of synaptic muscle nuclei. ARIA, a factor that stimulates production of acetylcholine receptors (AChRs), is a candidate signaling molecule for such regulation. Here we examine the transynaptic inducing potential of this polypeptide factor. ARIA immunoreactivity is detectable at synaptic sites in vivo. In vitro, recombinant heregulin beta 1 (rHRG beta 1), the human homolog of ARIA, induces expression of the AChR epsilon gene, the subunit most sensitive to synaptic input. The inducing property of rHRG beta 1 is demonstrated most dramatically in primary muscle cultures from transgenic mice bearing an epsilon promoter-nuclear lacZ reporter transgene. Transient transfection experiments using the Sol 8 muscle cell line indicate that sequences that confer responsiveness to ARIA are located within a 150 bp epsilon subunit promoter region and are E box-independent. These results suggest that ARIA performs a vital role by directing spatially restricted gene expression at the neuromuscular junction.

Neuregulins are concentrated at nerve-muscle synapses and activate ACh-receptor gene expression

Two different signalling pathways mediate the localization of acetylcholine receptors (AChRs) to synaptic sites in skeletal muscle. The signal for one pathway is agrin, a protein that triggers a redistribution of previously unlocalized cell surface AChRs to synaptic sites. The signal for the other pathway is not known, but this signal stimulates transcription of AChR genes in myofibre nuclei near the synaptic site. Neuregulins, identified originally as a potential ligand for erbB2 (Neu differentiation factor, NDF), stimulate proliferation of Schwann cells (glial growth factor, GGF), increase the rate of AChR synthesis in cultured muscle cells (AChR-inducing activity) and are expressed in motor neurons. These results raise the possibility that neuregulin is the signal that activates AChR genes in synaptic nuclei. Here we show that neuregulin activates AChR gene expression in C2 muscle cells and that the neuregulin response element in the AChR delta-subunit gene is contained in the same 181 base pairs that confer synapse-specific expression in transgenic mice. We use antibodies to show that neuregulins are concentrated at synaptic sites and that, like the extracellular signal that stimulates synapse-specific expression, neuregulins remain at synaptic sites in the absence of nerve and muscle. We show that C2 muscle cells contain erbB2 and erbB3 messenger RNA but little or no erbB4 mRNA, and that neuregulin stimulates tyrosine phosphorylation of erbB2 and erbB3, indicating that neuregulin signalling in skeletal muscle may be mediated by a complex of erbB2 and erbB3.