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

Two Pathways Regulate Differential Expression of nAChRs Between the Orbicularis Oris and Gastrocnemius

BACKGROUND

We previously demonstrated differential expression of nicotinic acetylcholine receptors (nAChRs) in the facial nerve-innervated orbicularis oris and somatic nerve-innervated gastrocnemius, which contribute to different sensitivities to muscle relaxants. Furthermore, the orbicularis oris exhibits less sensitivity to muscle relaxants after facial nerve injury, which is also related to upregulation of nAChRs. Here, we explored the regulatory mechanism for the different expression patterns. Because the agrin/Lrp4/MuSK/rapsyn and neuregulin1/ErbB signaling pathways are indispensable for maintaining the expression of nAChRs, we examined the activity of these two signaling pathways in gastrocnemius and orbicularis oris innervated by normal or injured facial nerves.

MATERIALS AND METHODS

A quantitative analysis of these two signaling pathways was realized by immunofluorescence, and immunoprecipitation was applied to detect the level of phosphorylated MuSK in the gastrocnemius and orbicularis oris innervated by normal or injured facial nerves in adult rats.

RESULTS

ErbB and the phosphorylated MuSK were expressed more in orbicularis oris than in gastrocnemius (P < 0.05). No significant difference was found in the expression of agrin/Lrp4/MuSK/rapsyn. After facial nerve injury, the level of agrin and the percentage of phosphorylated MuSK decreased significantly, although the expression levels of MuSK, rapsyn, and neuregulin1/ErbB were highly upregulated.

CONCLUSIONS

Differential expression of the neuregulin1/ErbB signaling pathway may account for the different expression patterns of nAChRs at the neuromuscular junctions of the orbicularis oris and gastrocnemius. Overexpression of MuSK and rapsyn may contribute to upregulation of nAChRs after facial nerve injury.

Structural Similarities between Neuregulin 1-3 Isoforms Determine Their Subcellular Distribution and Signaling Mode in Central Neurons

The Neuregulin (NRG) family of ErbB ligands is comprised of numerous variants originating from the use of different genes, alternative promoters, and splice variants. NRGs have generally been thought to be transported to axons and presynaptic terminals where they signal via ErbB3/4 receptors in paracrine or juxtacrine mode. However, we recently demonstrated that unprocessed pro-NRG2 accumulates on cell bodies and proximal dendrites, and that NMDAR activity is required for shedding of its ectodomain by metalloproteinases. Here we systematically investigated the subcellular distribution and processing of major NRG isoforms in rat hippocampal neurons. We show that NRG1 isotypes I and II, which like NRG2 are single-pass transmembrane proteins with an Ig-like domain, share the same subcellular distribution and ectodomain shedding properties. We furthermore show that NRG3, like CRD-NRG1, is a dual-pass transmembrane protein that harbors a second transmembrane domain near its amino terminus. Both NRG3 and CRD-NRG1 cluster on axons through juxtacrine interactions with ErbB4 present on GABAergic interneurons. Interestingly, although single-pass NRGs accumulate as unprocessed proforms, axonal puncta of CRD-NRG1 and NRG3 are comprised of processed protein. Mutations of CRD-NRG1 and NRG3 that render them resistant to BACE cleavage, as well as BACE inhibition, result in the loss of axonal puncta and in the accumulation of unprocessed proforms in neuronal soma. Together, these results define two groups of NRGs with distinct membrane topologies and fundamentally different targeting and processing properties in central neurons. The implications of this functional diversity for the regulation of neuronal processes by the NRG/ErbB pathway are discussed.SIGNIFICANCE STATEMENT Numerous Neuregulins (NRGs) are generated through the use of different genes, promoters, and alternative splicing, but the functional significance of this evolutionary conserved diversity remains poorly understood. Here we show that NRGs can be categorized by their membrane topologies. Single-pass NRGs, such as NRG1 Types I/II and NRG2, accumulate as unprocessed proforms on cell bodies, and their ectodomains are shed by metalloproteinases in response to NMDA receptor activation. By contrast, dual-pass CRD-NRG1 and NRG3 are constitutively processed by BACE and accumulate on axons where they interact with ErbB4 in juxtacrine mode. These findings reveal a previously unknown functional relationship between membrane topology, protein processing, and subcellular distribution, and suggest that single- and dual-pass NRGs regulate neuronal functions in fundamentally different ways.

Detection of an interaction between prion protein and neuregulin I-β1 by fluorescence resonance energy transfer analysis

Cellular prion protein (PrP) copurifies with neuregulin type I-β1 (NRG I-β1), but no interaction has been detected by a general immunoprecipitation study. We speculate that PrP interacts with NRG I-β1. Here, the interaction of PrP with NRG I-β1 was detected by measuring fluorescence resonance energy transfer (FRET) between enhanced blue (EBFP) and enhanced green (EGFP) fluorescent protein-fusion proteins. Full-length PrP interacted with EGFP in addition to NRG I-β1. From this result, we deduced that PrP interacts with EGFP through its unstructured N-terminal domain. We therefore detected FRET between PrP deleting the N-terminal domain and NRG I-β1. In contrast, the C-terminal domain of PrP interacted with NRG I-β1 and the proteins dissociated completely in the presence of sodium chloride. This interaction occurs at the nanomolar level, which is important for the reaction to be functional in organisms. We concluded that PrP interacted with NRG I-β1 through its C-terminal domain.

Role of extracellular matrix proteins and their receptors in the development of the vertebrate neuromuscular junction

The vertebrate neuromuscular junction (NMJ) remains the best-studied model for understanding the mechanisms involved in synaptogenesis, due to its relatively large size, its simplicity of patterning, and its unparalleled experimental accessibility. During neuromuscular development, each skeletal myofiber secretes and deposits around its extracellular surface an assemblage of extracellular matrix (ECM) proteins that ultimately form a basal lamina. This is also the case at the NMJ, where the motor nerve contributes additional factors. Before most of the current molecular components were known, it was clear that the synaptic ECM of adult skeletal muscles was unique in composition and contained factors sufficient to induce the differentiation of both pre- and postsynaptic membranes. Biochemical, genetic, and microscopy studies have confirmed that agrin, laminin (221, 421, and 521), collagen IV (α3-α6), collagen XIII, perlecan, and the ColQ-bound form of acetylcholinesterase are all synaptic ECM proteins with important roles in neuromuscular development. The roles of their many potential receptors and/or binding proteins have been more difficult to assess at the genetic level due to the complexity of membrane interactions with these large proteins, but roles for MuSK-LRP4 in agrin signaling and for integrins, dystroglycan, and voltage-gated calcium channels in laminin-dependent phenotypes have been identified. Synaptic ECM proteins and their receptors are involved in almost all aspects of synaptic development, including synaptic initiation, topography, ultrastructure, maturation, stability, and transmission.

Bone marrow-derived mesenchymal stem cells up-regulate acetylcholine receptor delta subunit through NRG/ErbB3-mediated mitogen-activated protein kinase pathway

To investigate the effect of bone marrow-derived mesenchymal stem cells (BMSCs) on the expression of acetylcholine receptor delta subunit (AChRd), the murine skeletal muscle cell line Sol8 were grown in DMEM with 20% fetal bovine serum added with (conditional medium group) or without (control group) conditional medium of BMSC cells for 48 hours. RT-PCR and Western blot were performed to access the mRNA and protein levels of AChRd in Sol8 cells, respectively. Western blot was used to detect total and phosphorylated protein levels of Ras, Raf-1, Mek1/2, and Erk1/2, respectively. NRG-1 antibody added in conditional medium of BMSCs, si-ErbB3, and four Ras/Raf/MEK/ERK pathway inhibitors (FTS, Sulindac, U0126, and PD98059) were using to investigate the effect of AChRd levels. Our studies indicated that expression of AChRd was significantly enhanced in the conditional medium group when compared with those in control group and phosphorylation of Ras, Raf, Erk1/2 in Sol8 cells was also increased. Although gene silencing for ErbB3 gene, adding of NRG-1 antibody in conditional medium of BMSCs or treatment of Ras/Raf/MEK/ERK pathway inhibitors can down-regulate expression of AChRd and phosphorylation, which suggesting that the Ras/Raf/MEK/ERK pathway may be involved in BMSCs-induced expression of AChRd.

Neuregulin-1 potentiates agrin-induced acetylcholine receptor clustering through muscle-specific kinase phosphorylation

At neuromuscular synapses, neural agrin (n-agrin) stabilizes embryonic postsynaptic acetylcholine receptor (AChR) clusters by signalling through the muscle-specific kinase (MuSK) complex. Live imaging of cultured myotubes showed that the formation and disassembly of primitive AChR clusters is a dynamic and reversible process favoured by n-agrin, and possibly other synaptic signals. Neuregulin-1 is a growth factor that can act through muscle ErbB receptor kinases to enhance synaptic gene transcription. Recent studies suggest that neuregulin-1-ErbB signalling can modulate n-agrin-induced AChR clustering independently of its effects on transcription. Here we report that neuregulin-1 increased the size of developing AChR clusters when injected into muscles of embryonic mice. We investigated this phenomenon using cultured myotubes, and found that in the ongoing presence of n-agrin, neuregulin-1 potentiates AChR clustering by increasing the tyrosine phosphorylation of MuSK. This potentiation could be blocked by inhibiting Shp2, a postsynaptic tyrosine phosphatase known to modulate the activity of MuSK. Our results provide new evidence that neuregulin-1 modulates the signaling activity of MuSK and hence might function as a second-order regulator of postsynaptic AChR clustering at the neuromuscular synapse. Thus two classic synaptic signalling systems (neuregulin-1 and n-agrin) converge upon MuSK to regulate postsynaptic differentiation.

Chromatin modifications that support acetylcholine receptor gene activation are established during muscle cell determination and differentiation

Localization of acetylcholine receptors (AChRs) to the postsynaptic region of muscle is mediated in part by transcriptional mechanisms. An important way of regulating transcription is through targeting histone modifications on chromatin to distinct gene loci. Using chromatin immunoprecipitation, we examined the developmental regulation of certain histone modifications at the AChR epsilon subunit locus, including methylations at lysine residues K4 and K27 and acetylations at K9 and K14. We modeled various stages of muscle development in cell culture, including pre-determined cells, committed but undifferentiated myoblasts, and differentiated myotubes, and modeled synaptic myotube nuclei by stimulating myotubes with neuregulin (NRG) 1. We found that a pattern of histone modifications associated with transcriptional activation is targeted to the AChR epsilon subunit locus in myotubes prior to stimulation with NRG1 and does not change upon addition of NRG1. Instead, we found that during muscle cell determination and differentiation, specific histone modifications are targeted to the AChR epsilon subunit locus. Within the gene, at K4, dimethylation is induced during muscle cell determination, while trimethylation is induced during differentiation. At K27, loss of trimethylation and appearance of monomethylation occurs during determination and differentiation. In addition, in a region upstream of the gene, K4 di- and trimethylation, and K9/14 acetylation are induced in a distinct developmental pattern, which may reflect a functional regulatory element. These results suggest synaptic signaling does not directly target histone modifications but rather the histone modification pattern necessary for transcriptional activation is previously established in a series of steps during muscle development.

[Synapse formation and regeneration]

Synapse formation is probably the key process in neural development allowing signal transmission between nerve cells. As an interesting model of synapse maturation, we considered first the neuromuscular junction (NMJ), whose development is particularly dependent on intercellular interactions between the motor nerve and the skeletal muscle. Nerve and muscle have distinct roles in synaptic compartment differentiation. The initial steps of this differentiation and motor endplate formation require several postsynaptic molecular agents including agrin, the tyrosine kinase receptor MuSK and rapsyn. The agrin or motoneuron dependence of this process continues to be debated while the following steps of axonal growth and postsynaptic apparatus maintenance essentially depend on neuronal agrin and a neuron-specific signal dispersing ectopic AChR aggregate remainders, possibly mediated by acetylcholine itself. Neuregulin is essentially involved in Schwann's cell survival and guidance for axonal growth. In this paper, we will discuss the similarities between Central Nervous System (CNS) synaptic formation and Motor innervation. The limited ability of the CNS to create new synapses after nervous system injury will be then discussed with a final consideration of some new strategies elaborated to circumvent the limitations of lesion extension processes.

The neuregulin-1 receptor erbB4 controls glutamatergic synapse maturation and plasticity

Neuregulin-1 (NRG1) signaling participates in numerous neurodevelopmental processes. Through linkage analysis, nrg1 has been associated with schizophrenia, although its pathophysiological role is not understood. The prevailing models of schizophrenia invoke hypofunction of the glutamatergic synapse and defects in early development of hippocampal-cortical circuitry. Here, we show that the erbB4 receptor, as a postsynaptic target of NRG1, plays a key role in activity-dependent maturation and plasticity of excitatory synaptic structure and function. Synaptic activity leads to the activation and recruitment of erbB4 into the synapse. Overexpressed erbB4 selectively enhances AMPA synaptic currents and increases dendritic spine size. Preventing NRG1/erbB4 signaling destabilizes synaptic AMPA receptors and leads to loss of synaptic NMDA currents and spines. Our results indicate that normal activity-driven glutamatergic synapse development is impaired by genetic deficits in NRG1/erbB4 signaling leading to glutamatergic hypofunction. These findings link proposed effectors in schizophrenia: NRG1/erbB4 signaling perturbation, neurodevelopmental deficit, and glutamatergic hypofunction.

Neuregulin 1 is a pronociceptive cytokine that is regulated by progesterone in the spinal cord: implications for sex specific pain modulation

Sex differences in the magnitude of response to thermal and tactile stimuli have been demonstrated in both clinical and animal studies. Females typically display lower threshold responses to painful stimuli as compared to males. We have previously observed sexually dimorphic expression of the growth factor, neuregulin 1 (NRG1) following L5 nerve root ligation (LR) in male and female rats. In the present study, we sought to determine which gonadal hormones were involved in regulating NRG1 expression following L5 nerve root ligation. We observed that expression of NRG1 mRNA and the neuregulin receptors, ErbB2 and ErbB4 in the lumbar spinal cord was facilitated by the presence of progesterone in female rats following L5 nerve root ligation. An increase in NRG1 protein and NRG1 immunoreactivity was also observed in the ipsilateral spinal cord of progesterone treated female rats as compared to ovariectomized female rats and male rats at day 14 following LR. NRG1 immunoreactivity was equally colocalized with either the astrocytic marker, GFAP, and with NeuN labeled neurons 14days following L5 nerve root ligation. Intrathecal administration of recombinant NRG1-beta1 protein significantly decreased the hindpaw tactile withdrawal threshold in male rats, ovariectomized female rats, and progesterone treated female rats. These results demonstrate a role for progesterone-dependent regulation of glial and/or neuronal neuregulin 1 in female rats in mediating sex differences in nociception. Furthermore, our results suggest that NRG1 may be involved in central sensitization during the maintenance phase, but not in the initiation of persistent pain in female rats.

Neuromuscular synapse formation in mice lacking motor neuron- and skeletal muscle-derived Neuregulin-1

The localization of acetylcholine receptors (AChRs) to the vertebrate neuromuscular junction is mediated, in part, through selective transcription of AChR subunit genes in myofiber subsynaptic nuclei. Agrin and the muscle-specific receptor tyrosine kinase, MuSK, have critical roles in synapse-specific transcription, because AChR genes are expressed uniformly in mice lacking either agrin or MuSK. Several lines of evidence suggest that agrin and MuSK stimulate synapse-specific transcription indirectly by regulating the distribution of other cell surface ligands, which stimulate a pathway for synapse-specific gene expression. This putative secondary signal for directing AChR gene expression to synapses is not known, but Neuregulin-1 (Nrg-1), primarily based on its presence at synapses and its ability to induce AChR gene expression in vitro, has been considered a good candidate. To study the role of Nrg-1 at neuromuscular synapses, we inactivated nrg-1 in motor neurons, skeletal muscle, or both cell types, using mice that express Cre recombinase selectively in developing motor neurons or in developing skeletal myofibers. We find that AChRs are clustered at synapses and that synapse-specific transcription is normal in mice lacking Nrg-1 in motor neurons, myofibers, or both cell types. These data indicate that Nrg-1 is dispensable for clustering AChRs and activating AChR genes in subsynaptic nuclei during development and suggest that these aspects of postsynaptic differentiation are dependent on Agrin/MuSK signaling without a requirement for a secondary signal.

Differential localization of ErbB receptor ensembles influences their signaling in hippocampal neurons

Our studies indicate that ErbB complexes participate in both survival and synaptic plasticity signals of hippocampal neurons but in a manner that depends on the subcellular localization of the receptor ensembles. Using dissociated hippocampal cultures, we found that neurons, rather than glial cells, are the primary targets of ErbB receptor ligands such as epidermal growth factor and heregulin. Further investigation demonstrated that ErbB receptors distribute differentially in hippocampal neurons with the epidermal growth factor receptor confined to neural cell bodies and the p185(c-neu) and ErbB4 receptors distributed to both neural soma and neurites. Activation of ErbB receptor and downstream signaling molecules were observed in neurites only after heregulin stimulation. The receptor complex which mediated neurite located signals was the p185(c-neu)/ErbB4 heterodimer. Colocalization of p185(c-neu), but not epidermal growth factor receptor, with postsynaptic density protein 95 suggests that the heregulin signaling contributes to synapse specific activities. However, the epidermal growth factor receptor complex mediates physiological survival signals, as neuronal survival was enhanced by epidermal growth factor, rather than heregulin. Collectively, these studies indicate that different ErbB ensembles localize to different locations on the neuron to mediate distinct signals and functions.

Assembly of the postsynaptic membrane at the neuromuscular junction: paradigm lost

Studies of the vertebrate skeletal neuromuscular junction led to an influential model of how neurotransmitter receptors accumulate in the postsynaptic membrane. In this model, motor axons organize postsynaptic development by secreting neuregulin to induce acetylcholine receptor gene transcription in specialized subsynaptic nuclei, agrin to cluster diffuse receptors in the postsynaptic membrane, and acetylcholine to evoke electrical activity that promotes synaptic maturation. However, new studies in this area have first, demonstrated that axons sometimes innervate pre-existing receptor clusters; second, recast the roles of agrin and neuregulin; third, revealed early effects of neurotransmission; fourth, questioned the role of subsynaptic myonuclei; fifth, shown that elaborately-branched postsynaptic structures can form aneurally; and sixth, raised the possibility that neurotransmitter affects receptor type as well as distribution. These recent studies challenge the widely-held paradigms, although not the results that led to them, and suggest a new model for neuromuscular synaptogenesis.

Schwann cell-derived neuregulin-2α can function as a cell-attached activator of muscle acetylcholine receptor expression

Here we show that neuregulin-2 (Nrg-2) alpha- and beta-isoforms can activate acetylcholine receptor (AChR) transcription as surface-attached ligands. More importantly, we demonstrate that Schwann cells that express Nrg-2alpha on their cell surface, the same Nrg-2 isoform expressed by terminal Schwann cells at the neuromuscular junction, can induce AChR expression if brought into cell-to-cell contact with myotubes specifically expressing ErbB4. These Schwann cells, the D6P2T cell line, induce AChR expression apparently as well as 293T cells transfected with Nrg-2beta, the isoform with the highest AChR-inducing activity when presented in a soluble form. These results provide a potential role for the previously reported, paradoxical perisynaptic accumulation of Nrg-2alpha, the isoform with the least AChR-inducing activity when presented in a soluble form. They also raise the possibility that Schwann cell-derived Nrg-2 could activate ErbB receptors on the synaptic sarcolemma and that this could account, at least in part, for the Nrg-mediated regulation of AChR expression.

Stimulation of acetylcholine receptor transcription by neuregulin-2 requires an N-box response element and is regulated by alternative splicing

The neuregulin (Nrg) family of growth/differentiation factors is encoded by at least four genes in the mammalian genome: nrg-1, nrg-2, nrg-3 and nrg-4. Nrg-1 and Nrg-2 share the highest homology within the family, and the primary RNA transcripts from their encoding genes are subjected to extensive alternative splicing. Although little is known about the biological function of Nrg-2-4, their structural similarity with Nrg-1 suggests that they could account for some of the activities presently attributed to Nrg-1. Thus, at the neuromuscular junction Nrg-1 has been a favored candidate for the signal that activates selective acetylcholine receptor (AChR) transcription in synaptic myonuclei. However, we have recently shown that like Nrg-1, Nrg-2 can also activate AChR transcription in cultured myotubes and accumulates at the synaptic site. Synapse-specific and Nrg-1-induced AChR transcription require an enhancer sequence, the N-box, which is also mutated in some patients with congenital myasthenia gravis. Here, we show that Nrg-2-induced AChR transcription requires an N-box motif and is regulated by alternative splicing. We also show that unique Nrg-2 isoforms are differentially distributed between spinal cord and skeletal muscle, the tissues that harbor the cellular components of the neuromuscular synapse.

Synapses form in skeletal muscles lacking neuregulin receptors

The formation of the neuromuscular junction (NMJ) is directed by reciprocal interactions between motor neurons and muscle fibers. Neuregulin (NRG) and Agrin from motor nerve terminals are both implicated. Here, we demonstrate that NMJs can form in the absence of the NRG receptors ErbB2 and ErbB4 in mouse muscle. Postsynaptic differentiation is, however, induced by Agrin. We therefore conclude that NRG signaling to muscle is not required for NMJ formation. The effects of NRG signaling to muscle may be mediated indirectly through Schwann cells.

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.

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 induces the expression of transcription factors and myosin heavy chains typical of muscle spindles in cultured human muscle

Neuregulin (NRG) (also known as ARIA, GGF, and other names) is a heparin sulfate proteoglycan secreted into the neuromuscular junction by innervating motor and sensory neurons. An integral part of synapse formation, we have analyzed NRG-induced changes in gene expression over 48 h in primary human myotubes. We show that in addition to increasing the expression of acetylcholine receptors on the myotube surface, NRG treatment results in a transient increase of several members of the early growth response (Egr) family of transcription factors. Three Egrs, Egr1, -2, and -3, are induced within the first hour of NRG treatment, with Egr1 and -3 RNA levels showing the most significant increases of approximately 9- and 16-fold, respectively. Also noted was a corresponding increase in protein levels for both of these transcription factors. Previous literature indicates that Egr3 expression is required for the formation of muscle spindle fibers, sensory organs that are distinct from skeletal muscle contractile fibers. At the molecular level, muscle spindle fibers express a unique subset of myosin heavy chains. Two isoforms of the myosin heavy chain, the slow development and neonatal, were found to be increased in our myotube cultures after 48 h of treatment with NRG. Taken together, these results indicate that not only can NRG induce the expression of a transcription factor key to spindle fiber development (Egr3), but that a portion of this developmental process can be replicated in vitro.

Neuregulin Inhibits Acetylcholine Receptor Aggregation in Myotubes

The high local concentration of acetylcholine receptors (AChRs) at the vertebrate neuromuscular junction results from their aggregation by the agrin/MuSK signaling pathway and their synthetic up-regulation by the neuregulin/ErbB pathway. Here, we show a novel role for the neuregulin/ErbB pathway, the inhibition of AChR aggregation on the muscle surface. Treatment of C2C12 myotubes with the neuregulin epidermal growth factor domain decreased the number of both spontaneous and agrin-induced AChR clusters, in part by increasing the rate of cluster disassembly. Upon cluster disassembly, AChRs were internalized into caveolae (as identified by caveolin-3). Time-lapse microscopy revealed that individual AChR clusters fragmented into puncta, and application of neuregulin accelerated the rate at which AChR clusters decreased in area without affecting the density of AChRs remaining in individual clusters (as measured by the fluorescence intensity/unit area). We propose that this novel action of neuregulin regulates synaptic competition at the developing neuromuscular junction.

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.

Autocrine activation of ErbB2/ErbB3 receptor complex by NRG-1 in non-small cell lung cancer cell lines

Our prior studies identified co-expression of the human epidermal growth factor-like receptors 2 (ErbB2) and 3 (ErbB3), as well as the growth factor neuregulin-1 (NRG-1) in normal lung epithelium and lung cancers. As ErbB2 and ErbB3 dimerize to produce a high affinity receptor for NRG-1, we postulated that an autocrine growth loop was present in transformed and non-transformed pulmonary epithelial cells. To test this hypothesis, we examined four cell lines derived from human non-small cell carcinomas for: (1) ErbB2 and ErbB3 expression and endogenous activation; (2) NRG-1 expression and secretion/shedding; and (3) the effect of receptor blockade on autocrine receptor activation. Our studies found that ErbB2 and ErbB3 were expressed by each of these cell lines. In addition, the NRG-1 gene was also expressed with both major isoforms of NRG-1 (NRG-1alpha and NRG-1beta) found intracellularly. Only the NRG-1alpha isoform, however, was found secreted/shed into the culture medium. The secreted/shed NRG-1alpha was capable of activating the ErbB2/ErbB3 receptor complex expressed on the breast adenocarcinoma cell line MCF-7. Basal ErbB2 phosphorylation was identified in all lung cancer cell lines and was inhibited with an antibody that blocked the NRG-1 binding site on ErbB3. Taken together, these data show that secreted NRG-1alpha can activate the ErbB2/ErbB3 heterodimer in an autocrine fashion. The identification of a NRG-1alpha/ErbB2/ErbB3 autocrine loop raises the possibility that interruption of this loop may have therapeutic potential in lung cancer.

Neuregulin-2 is developmentally regulated and targeted to dendrites of central neurons

Neuregulin-1 (NRG-1) regulates numerous aspects of neural development and synaptic plasticity; the functions of NRG-2 and NRG-3 are presently unknown. As a first step toward understanding how NRGs contribute to distinct aspects of neural development and function, we characterized their regional and subcellular expression patterns in developing brain. The expression of NRG-1-3 mRNAs was compared postnatally (P0, P7, adult) by using in situ hybridization. NRG-1 expression is highest at birth, whereas NRG-2 mRNA levels increase with development; expression of both genes is restricted to distinct brain regions. In contrast, NRG-3 transcripts are abundant in most brain regions throughout development. NRG-2 antibodies were generated to analyze protein processing, expression, and subcellular distribution. As with NRG-1, the transmembrane NRG-2 proprotein is proteolytically processed in transfected HEK 293 cells and in neural tissues, and its ectodomain is exposed and accumulates on the neuron surface. Despite the structural similarities between NRG-1 and NRG-2, we unexpectedly found that NRG-2 colocalizes with MAP2 in proximal primary dendrites of hippocampal neurons in culture and in vivo, although it is not detectable in axons or in axon terminals. These findings were confirmed with NRG-2 ectodomain antisera and epitope-tagged recombinant protein. In cerebellum, NRG-2 colocalizes with calbindin in proximal dendrites and soma of Purkinje cells. In contrast, NRG-1 is highly expressed in axons of dissociated hippocampal neurons, as well as in somas and dendrites. The distinct temporal, regional, and subcellular expression of NRG-2 suggests its unique and nonredundant role in neural function.

Phosphorylation-elicited quaternary changes of GA binding protein in transcriptional activation

Enrichment of nicotinic acetylcholine receptors (nAChR) on the tip of the subjunctional folds of the postsynaptic membrane is a central event in the development of the vertebrate neuromuscular junction. This is attained, in part, through a selective transcription in the subsynaptic nuclei, and it has recently been shown that the GA binding protein (GABP) plays an important role in this compartmentalized expression. The neural factor heregulin (HRG) activates nAChR transcription in cultured cells by stimulating a signaling cascade of protein kinases. Hence, it is speculated that GABP becomes activated by phosphorylation, but the mechanism has remained elusive. To fully understand the consequences of GABP phosphorylation, we examined the effect of heregulin-elicited GABP phosphorylation on cellular localization, DNA binding, transcription, and mobility. We demonstrate that HRG-elicited phosphorylation dramatically changes the transcriptional activity and mobility of GABP. While phosphorylation of GABPbeta seems to be dispensable for these changes, phosphorylation of GABPalpha is crucial. Using fluorescence resonance energy transfer, we furthermore showed that phosphorylation of threonine 280 in GABPalpha triggers reorganizations of the quaternary structure of GABP. Taken together, these results support a model in which phosphorylation-elicited structural changes of GABP enable engagement in certain interactions leading to transcriptional activation.

Cyclic AMP-dependent protein kinase A and CREB are involved in neuregulin-induced synapse-specific expression of acetylcholine receptor gene

Neuregulin is reported to stimulate synapse-specific transcription of acetylcholine receptor (AChR) genes in the skeletal muscle fiber by multiple signaling pathways such as ERK, PI3K, and JNK. The co-localization of PKA mRNA with AChR and ErbBs, receptors for neuregulin, at the confined region of synapse implicates the putative role of PKA in neuregulin-induced AChR gene expression. In the present study, we found that mRNA and protein of a regulatory subunit of PKA (PKARIalpha) were concentrated at synaptic sites of the rat sternomastoid muscle fiber, while those of ERK and PI3K were uniformly distributed throughout the muscle fiber. Neuregulin (100 ng/ml) increased both PKA activity in the nucleus and AChRdelta subunit gene transcription in cultured Sol8 myotubes. These increases were significantly blocked by a specific PKA inhibitor H-89 (100 nM) and an adenylcyclase inhibitor SQ 22536 (200 microM) (72.5% and 60.1%, respectively). Furthermore, neuregulin phosphorylated CREB, a well-known down-stream transcription factor of PKA. While H-89 inhibited CREB phosphorylation, H-89 and PD098059 (50 microM), a specific MEK1/2 inhibitor, did not inhibit the phosphorylation of ERK and CREB, respectively, suggesting no cross-talk between PKA and ERK pathways. In conclusion, neuregulin increases AChRdelta subunit gene transcription, in part, by the activation of PKA/CREB, an alternative route to the previously reported ERK signaling pathway.

Neuregulin 1 and susceptibility to schizophrenia

The cause of schizophrenia is unknown, but it has a significant genetic component. Pharmacologic studies, studies of gene expression in man, and studies of mouse mutants suggest involvement of glutamate and dopamine neurotransmitter systems. However, so far, strong association has not been found between schizophrenia and variants of the genes encoding components of these systems. Here, we report the results of a genomewide scan of schizophrenia families in Iceland; these results support previous work, done in five populations, showing that schizophrenia maps to chromosome 8p. Extensive fine-mapping of the 8p locus and haplotype-association analysis, supplemented by a transmission/disequilibrium test, identifies neuregulin 1 (NRG1) as a candidate gene for schizophrenia. NRG1 is expressed at central nervous system synapses and has a clear role in the expression and activation of neurotransmitter receptors, including glutamate receptors. Mutant mice heterozygous for either NRG1 or its receptor, ErbB4, show a behavioral phenotype that overlaps with mouse models for schizophrenia. Furthermore, NRG1 hypomorphs have fewer functional NMDA receptors than wild-type mice. We also demonstrate that the behavioral phenotypes of the NRG1 hypomorphs are partially reversible with clozapine, an atypical antipsychotic drug used to treat schizophrenia.

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.

Sodium channel mRNAs at the neuromuscular junction: distinct patterns of accumulation and effects of muscle activity

Voltage-gated sodium channels (VGSCs) are highly concentrated at the neuromuscular junction (NMJ) in mammalian skeletal muscle. Here we test the hypothesis that local upregulation of mRNA contributes to this accumulation. We designed radiolabeled antisense RNA probes, specific for the "adult" Na(V)1.4 and "fetal" Na(V)1.5 isoforms of VGSC in mammalian skeletal muscle, and used them in in situ hybridization studies of rat soleus muscles. Na(V)1.4 mRNA is present throughout normal adult muscles but is highly concentrated at the NMJ, in which the amount per myonucleus is more than eightfold greater than away from the NMJ. Na(V)1.5 mRNA is undetectable in innervated muscles but is dramatically upregulated by denervation. In muscles denervated for 1 week, both Na(V)1.4 and Na(V)1.5 mRNAs are present throughout the muscle, and both are concentrated at the NMJ. No Na(V)1.5 mRNA was detectable in denervated muscles stimulated electrically for 1 week in vivo. Neither denervation nor stimulation had any significant effect on the level or distribution of Na(V)1.4 mRNA. We conclude that factors, probably derived from the nerve, lead to the increased concentration of VGSC mRNAs at the NMJ. In addition, the expression of Na(V)1.5 mRNA is downregulated by muscle activity, both at the NMJ and away from it.

A novel role of neuregulin in skeletal muscle. Neuregulin stimulates glucose uptake, glucose transporter translocation, and transporter expression in muscle cells

Neuregulins regulate the expression of acetylcholine receptor genes and induce development of the neuromuscular junction in muscle. In studying whether neuregulins regulate glucose uptake in muscle, we analyzed the effect of a recombinant neuregulin, (r)heregulin-beta1-(177-244) (HRG), on L6E9 muscle cells, which express the neuregulin receptors ErbB2 and ErbB3. L6E9 responded acutely to HRG by a time- and concentration-dependent stimulation of 2-deoxyglucose uptake. HRG-induced stimulation of glucose transport was additive to the effect of insulin. The acute stimulation of the glucose transport induced by HRG was a consequence of the translocation of GLUT4, GLUT1, and GLUT3 glucose carriers to the cell surface. The effect of HRG on glucose transport was dependent on phosphatidylinositol 3-kinase activity. HRG also stimulated glucose transport in the incubated soleus muscle and was additive to the effect of insulin. Chronic exposure of L6E9 cells to HRG potentiated myogenic differentiation, and under these conditions, glucose transport was also stimulated. The activation of glucose transport after chronic HRG exposure was due to enhanced cell content of GLUT1 and GLUT3 and to increased abundance of these carriers at the plasma membrane. However, under these conditions, GLUT4 expression was markedly down-regulated. Muscle denervation is associated with GLUT1 induction and GLUT4 repression. In this connection, muscle denervation caused a marked increase in the content of ErbB2 and ErbB3 receptors, which occurred in the absence of alterations in neuregulin mRNA levels. This fact suggests that neuregulins regulate glucose transporter expression in denervated muscle. We conclude that neuregulins regulate glucose uptake in L6E9 muscle cells by mechanisms involving the recruitment of glucose transporters to the cell surface and modulation of their expression. Neuregulins may also participate in the adaptations in glucose transport that take place in the muscle fiber after denervation.

ErbB transmembrane tyrosine kinase receptors are differentially expressed throughout the adult rat central nervous system

The neuregulin (NRG) family of growth and differentiation factors and their erbB receptors contribute importantly to the development of the nervous system, but their distribution and function in the adult brain are poorly understood. The present study showed that erbB2, erbB3, and erbB4 transcripts and protein are distributed throughout all areas of adult rat brain. These three receptors were differentially expressed in neurons and glia. Some neurons expressed only a subset of erbB kinases, whereas other neurons expressed all three erbB receptors but sequestered each of these polypeptides into distinct cellular compartments. In synapse-rich regions, erbB immunoreactivity appeared as punctate-, axon-, and/or dendrite-associated staining, suggesting that NRGs are involved in the formation and maintenance of synapses in adult brain. ErbB labeling also was present in neuronal soma, indicating that NRGs act at sites in addition to the synapse. Glia in adult brain also differentially expressed erbB3 and erbB4. Approximately half of the erbB3 labeling in white matter was associated with S100beta+/glial fibrillary acidic protein negative macroglia (i.e., oligodendrocytes or glial fibrillary acidic protein negative astrocytes). In contrast, macroglia in gray matter did not express erbB3. The remaining erbB3 immunoreactivity in white matter and erbB4 glial staining seemed to be associated with microglia. These results showed that erbB receptors are expressed widely in adult rat brain and that each erbB receptor subtype has a distinct distribution. The differential distributions of erbB receptors in neurons and glia and the known functional differences between these kinases suggest that NRGs have distinct effects on these cells. The continued expression of NRGs and their erbB receptors in mature brain also implies that these molecules perform important functions in the brain throughout life.

Cdk5 is involved in neuregulin-induced AChR expression at the neuromuscular junction

Here we describe an important involvement of Cdk5/p35 in regulating the gene expression of acetylcholine receptor (AChR) at the neuromuscular synapse. Cdk5 and p35 were prominently expressed in embryonic muscle, and concentrated at the neuromuscular junction in adulthood. Neuregulin increased the p35-associated Cdk5 kinase activity in the membrane fraction of cultured C2C12 myotubes. Co-immunoprecipitation studies revealed the association between Cdk5, p35 and ErbB receptors in muscle and cultured myotubes. Inhibition of Cdk5 activity not only blocked the NRG-induced AChR transcription, but also attenuated ErbB activation in cultured myotubes. In light of our finding that overexpression of p35 alone led to an increase in AChR promoter activity in muscle, Cdk5 activation is sufficient to mediate the up-regulation of AChR gene expression. Taken together, these results reveal the unexpected involvement of Cdk5/p35 in neuregulin signaling at the neuromuscular synapse.

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

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

Syne-1, a dystrophin- and Klarsicht-related protein associated with synaptic nuclei at the neuromuscular junction

We describe a novel protein, Syne-1, that is associated with nuclear envelopes in skeletal, cardiac, and smooth muscle cells. Syne-1 contains multiple spectrin repeats similar to those found in dystrophin and utrophin, as well as a domain homologous to the carboxyl-terminal of Klarsicht, a protein associated with nuclei and required for a subset of nuclear migrations in Drosophila. In adult skeletal muscle fibers, levels of Syne-1 are highest in the nuclei that lie beneath the postsynaptic membrane at the neuromuscular junction. These nuclei are transcriptionally specialized, expressing genes for synaptic components at higher levels than extrasynaptic nuclei in the same cytoplasm. Syne-1 is the first protein found to be selectively associated with synaptic nuclei. Syne-1 becomes concentrated in synaptic nuclei postnatally. It remains synaptically enriched following denervation or degeneration/regeneration, and is also present at high levels in the central nuclei of dystrophic myotubes. The location and structure of Syne-1 suggest that it may participate in the migration of myonuclei in myotubes and/or their anchoring at the postsynaptic apparatus. Finally, we identify a homologous gene, syne-2, that is expressed in an overlapping but distinct pattern.

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

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

Comparison of neuregulin-1 expression in olfactory ensheathing cells, Schwann cells and astrocytes

Recently we demonstrated that a member of the neuregulin-1 (NRG-1) family of growth factors is a mitogen and survival factor for olfactory ensheathing cells (OECs). OECs are specialized glial cells within the olfactory system that are believed to play a role in the continual nerve re-growth of this tissue. OECs share properties with both astrocytes and Schwann cells but are likely to be a distinct glial cell type. NRG-1s have been found to be important regulators of Schwann cells in vivo, but the role of NRG-1 for OECs is less clear. The nrg-1 gene produces at least 12 different isoforms, that are likely to have different functions, due to alternative splicing of its mRNA. In this study, the expression of NRG-1 mRNAs in OECs was compared with other glial cells and their corresponding tissue sources. Cultured glial cells, unlike their tissue sources, expressed NRG-1 mRNAs containing the alpha EGF-like domain and expressed only the type 1beta isoform that lacks the glycosylated spacer domain. This correlated with expression of these isoforms during olfactory nerve degeneration in vivo. Although OECs expressed mRNA for all NRG-1 isoforms, the protein could not be detected in concentrated supernatant, or on the cell surface by immunofluorescence, but was detected in the nucleus or cytoplasm (depending on the isoform). These data support the hypothesis that NRG-1s play a functional role in OEC biology.

Neuregulin-increased expression of acetylcholine receptor epsilon-subunit gene requires ErbB interaction with Shc

Selective transcription of acetylcholine receptor (AChR) subunit genes by neuregulin is one of the mechanisms involved in the synaptic localization of AChRs to the neuromuscular junction. Neuregulin stimulates ErbB receptor tyrosine kinases and subsequently activates the Ras/ERK pathway, which is required for neuregulin-mediated induction of AChR subunit genes in muscle cells and synapse-specific expression in vivo. Here we investigated the neuregulin transduction mechanism that leads to ERK activation after ErbB receptor tyrosine phosphorylation. Neuregulin increases the association of the adaptor proteins Grb2 and Shc with both ErbB2 and ErbB3 in C2C12 muscle cells. Dephosphorylation of the tyrosine-phosphorylated ErbB proteins abolished their association with both Grb2 and Shc, suggesting a tyrosine phosphorylation-dependent interaction. The interaction of Shc with the ErbB receptors is mediated by Shc's phosphotyrosine-binding domain. In addition, neuregulin increased tyrosine phosphorylation of Shc. Mutagenesis approaches demonstrated that tyrosine phosphorylation of Shc is required for neuregulin induction of AChR subunit gene expression. Taken together, these data indicate that the interaction of ErbB receptors with Grb2 alone is insufficient for neuregulin-activated transcription, but that ErbB receptor signaling via Shc is necessary and important.

Regulation of neuregulin-mediated acetylcholine receptor synthesis by protein tyrosine phosphatase SHP2

Synapse-specific expression of the nicotinic acetylcholine receptor (AChR) is believed to be mediated by neuregulin, an epidermal growth factor-like trophic factor released by somatic motoneurons at the neuromuscular junction (NMJ). Neuregulin stimulates ErbB2, ErbB3, and ErbB4, members of the ErbB family of receptor tyrosine kinases. SHP2 is a cytoplasmic protein tyrosine phosphatase containing two Src homology 2 domains near its N terminus, and has been shown to be a positive mediator of mitogenic responses to various growth factors. We found that SHP2 interacted with ErbB2 and ErbB3 after neuregulin stimulation of muscle cells. Expression of SHP2 in C2C12 mouse muscle cells attenuated the neuregulin-induced expression of an AChR epsilon-promoter reporter gene, whereas a catalytically inactive SHP2 mutant or a mutant lacking the N-terminal Src homology 2 (SH2) domain enhanced reporter expression, suggesting that SHP2 negatively regulates the neuregulin signaling pathway. In fibroblast cells that express a mutant SHP2 with a targeted deletion of the N-terminal SH2 domain, neuregulin-mediated activation of the Ras/Raf/extracellular signal-regulated kinase cascade was enhanced. Furthermore, we found that SHP2 immunoreactivity colocalized with the staining of alpha-bungarotoxin, a marker of the NMJ. These results demonstrate a negative role of SHP2 in the neuregulin signal that leads to AChR gene expression at the NMJ.

Essential roles of c-JUN and c-JUN N-terminal kinase (JNK) in neuregulin-increased expression of the acetylcholine receptor epsilon-subunit

Neuregulin is a neural factor implicated in upregulation of acetylcholine receptor (AChR) synthesis at the neuromuscular junction. Previous studies have demonstrated that the extracellular signal-regulated kinase (ERK) subgroup of MAP kinases is required for neuregulin-induced AChR gene expression. We report here that the neuregulin-mediated increase in AChR epsilon-subunit mRNA was a delayed response in C2C12 muscle cells. Neuregulin induced expression of immediate early genes c-jun and c-fos, which followed and depended on the ERK activation. Treatment of muscle cells with cycloheximide to inhibit c-JUN synthesis at the protein level and suppression of c-JUN function by a dominant-negative mutant blocked neuregulin-induced expression of the epsilon-subunit gene, indicating an essential role of c-JUN in neuregulin signaling. Furthermore, neuregulin activated c-JUN N-terminal kinase (JNK) in C2C12 muscle cells. Blockade of JNK activation by overexpressing dominant-negative MKK4 inhibited epsilon-promoter activation. Moreover, overexpression of the JNK dominant-negative mutant inhibited neuregulin-mediated expression of the epsilon-transgene and endogenous epsilon-mRNA. Taken together, our results demonstrate important roles of c-JUN and JNK in neuregulin-mediated expression of the AChR epsilon-subunit gene and suggest that neuregulin activates multiple signaling cascades that converge to regulate AChR epsilon-subunit gene expression.

Distinct phenotypes of mutant mice lacking agrin, MuSK, or rapsyn

Differentiation of the postsynaptic membrane at the neuromuscular junction requires agrin, a nerve-derived signal; MuSK, a critical component of the agrin receptor in muscle; and rapsyn, a protein that interacts with acetylcholine receptors (AChRs). We showed previously that nerve-induced AChR aggregation is dramatically impaired in knockout mice lacking agrin, MuSK, or rapsyn. However, the phenotypes of these mutants differed in several respects, suggesting that the pathway from agrin to MuSK to rapsyn is complex. Here, we compared the effects of these mutations on two aspects of synaptic differentiation: AChR clustering and transcriptional specialization of synapse-associated myonuclei. First, we show that a plant lectin, VVA-B4, previously shown to act downstream of agrin, can induce AChR clusters on MuSK-deficient but not rapsyn-deficient myotubes in culture. Thus, although both MuSK and rapsyn are required for AChR clustering in vivo, only rapsyn is essential for cluster formation per se. Second, we show that neuregulin, a nerve-derived inducer of AChR gene expression, activates AChR gene expression in cultured agrin- and MuSK-deficient myotubes, even though synapse-specific transcriptional specialization is disrupted in agrin and MuSK mutants in vivo. We propose that agrin works through MuSK to determine a synaptogenic region within which synaptic differentiation occurs.

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.

ERK MAP kinase activation is required for acetylcholine receptor inducing activity-induced increase in all five acetylcholine receptor subunit mRNAs as well as synapse-specific expression of acetylcholine receptor epsilon-transgene

The AChR is a pentamer of four different subunits in a stoichiometry of alpha2betagammadelta in embryonic and alpha2betaepsilondelta in adult animals. Transcription of AChR subunit genes is most active in synaptic nuclei in adult skeletal muscle cells, and is regulated by neural factors such as ARIA. We report here that ARIA up-regulated specifically the expression of all five AChR subunits in C2C12 cells. The mRNA level of erbB2, erbB3, rapsyn, MuSK, SHP-2 and beta-actin remained unchanged in response to ARIA stimulation in C2C12 cells. The ARIA-induced increase in AChR subunit expression in C2C12 cells was inhibited by the erbB kinase inhibitor tyrphostin AG1478 and the MEK inhibitor PD98059, but not by the PI3 kinase inhibitor wortmannin, suggesting an important role of the erbB protein tyrosine kinases and MAP kinase in the regulation of the expression of the five different AChR subunits. To determine the signaling pathways in vivo, we studied the expression of reporter genes driven by the epsilon-promoter in injected muscles. The in vivo expression of the epsilon-transgene was inhibited by co-expression of dominant negative mutants of key components in the MAP kinase pathway including ras, raf and MEK, but not the dominant negative mutant of PI3 kinase. These results suggest that ERK MAP kinase activation is required for ARIA-induced increase in all five AChR subunit mRNAs as well as synapse-specific expression of AChR epsilon-transgene.

Neu differentiation factor stimulates phosphorylation and activation of the Sp1 transcription factor

Neu differentiation factors (NDFs), or neuregulins, are epidermal growth factor-like growth factors which bind to two tyrosine kinase receptors, ErbB-3 and ErbB-4. The transcription of several genes is regulated by neuregulins, including genes encoding specific subunits of the acetylcholine receptor at the neuromuscular junction. Here, we have examined the promoter of the acetylcholine receptor epsilon subunit and delineated a minimal CA-rich sequence which mediates transcriptional activation by NDF (NDF-response element [NRE]). Using gel mobility shift analysis with an NRE oligonucleotide, we detected two complexes that are induced by treatment with neuregulin and other growth factors and identified Sp1, a constitutively expressed zinc finger phosphoprotein, as a component of one of these complexes. Phosphatase treatment, two-dimensional gel electrophoresis, and an in-gel kinase assay indicated that Sp1 is phosphorylated by a 60-kDa kinase in response to NDF-induced signals. Moreover, Sp1 seems to act downstream of all members of the ErbB family and thus may funnel the signaling of the ErbB network into the nucleus.

Processing of ARIA and release from isolated nerve terminals

The neuromuscular junction is a specialized synapse in that every action potential in the presynaptic nerve terminal results in an action potential in the postsynaptic membrane, unlike most interneuronal synapses where a single presynaptic input makes only a small contribution to the population postsynaptic response. The postsynaptic membrane at the neuromuscular junction contains a high density of neurotransmitter (acetylcholine) receptors and a high density of voltage-gated Na+ channels. Thus, the large acetylcholine activated current occurs at the same site where the threshold for action potential generation is low. Acetylcholine receptor inducing activity (ARIA), a 42 kD protein, that stimulates synthesis of acetylcholine receptors and voltage-gated Na+ channels in cultured myotubes, probably plays the same roles at developing and mature motor endplates in vivo. ARIA is synthesized as part of a larger, transmembrane, precursor protein called proARIA. Delivery of ARIA from motor neuron cell bodies in the spinal cord to the target endplates involves several steps, including proteolytic cleavage of proARIA. ARIA is also expressed in the central nervous system and it is abundant in the molecular layer of the cerebellum. In this paper we describe our first experiments on the processing and release of ARIA from subcellular fractions containing synaptosomes from the chick cerebellum as a model system.

Synapse formation molecules in muscle and autonomic ganglia: the dual constraint hypothesis

In 1970 it was thought that if the motor-nerve supply to a muscle was interrupted and then allowed to regenerate into the muscle, motor-synaptic terminals most often formed presynaptic specializations at random positions over the surface of the constituent muscle fibres, so that the original spatial pattern of synapses was not restored. However, in the early 1970s a systematic series of experiments were carried out showing that if injury to muscles was avoided then either reinnervation or cross-reinnervation reconstituted the pattern of synapses on the muscle fibres according to an analysis using the combined techniques of electrophysiology, electronmicroscopy and histology on the muscles. It was thus shown that motor-synaptic terminals are uniquely restored to their original synaptic positions. This led to the concept of the synaptic site, defined as that region on a muscle fibre that contains molecules for triggering synaptic terminal formation. However, nerves in developing muscles were found to form connections at random positions on the surface of the very short muscle cells, indicating that these molecules are not generated by the muscle but imprinted by the nerves themselves; growth in length of the cells on either side of the imprint creates the mature synaptic site in the approximate middle of the muscle fibres. This process is accompanied at first by the differentiation of an excess number of terminals at the synaptic site, and then the elimination of all but one of the terminals. In the succeeding 25 years, identification of the synaptic site molecules has been a major task of molecular neurobiology. This review presents an historical account of the developments this century of the idea that synaptic-site formation molecules exist in muscle. The properties that these molecules must possess if they are to guide the differentiation and elimination of synaptic terminals is considered in the context of a quantitative model of this process termed the dual-constraint hypothesis. It is suggested that the molecules agrin, ARIA, MuSK and S-laminin have suitable properties according to the dual-constraint hypothesis to subserve this purpose. The extent to which there is evidence for similar molecules at neuronal synapses such as those in autonomic ganglia is also considered.

Regulation of ion channel expression in neural cells by hormones and growth factors

Voltage-and ligand-gated ion channels are key players in synaptic transmission and neuron-glia communication in the nervous system. Expression of these proteins can be regulated at several levels (transcriptional, translational, or posttranslational) and by multiple extracellular factors in the developing and mature nervous system. A wide variety of hormones and growth factors have been identified as important in neural cell differentiation, which is a complex process involving the acquisition of cell-type-specific ion channel phenotypes. Much literature has already accumulated describing the structural and functional characteristics of ion channels, but relatively little is known about the factors that influence their synthesis and cell surface expression, although this area has generated considerable interest in the context of neural cell development. This article reviews several examples of regulated expression of these channels by cellular factors, namely peptide growth factors and steroid hormones, and discusses, where applicable, current understanding of molecular mechanisms underlying such regulation of voltage-and neurotransmitter-gated ion channels.