The formation of neuromuscular synapses

Src, Fyn, and Yes are not required for neuromuscular synapse formation but are necessary for stabilization of agrin-induced clusters of acetylcholine receptors

Mice deficient in src and fyn or src and yes move and breathe poorly and die perinatally, consistent with defects in neuromuscular function. Src and Fyn are associated with acetylcholine receptors (AChRs) in muscle cells, and Src and Yes can act downstream of ErbB2, suggesting roles for Src family kinases in signaling pathways regulating neuromuscular synapse formation. We studied neuromuscular synapses in src(-/-); fyn(-/-) and src(-/-); yes(-/-) mutant mice and found that muscle development, motor axon pathfinding, clustering of postsynaptic proteins, and synapse-specific transcription are normal in these double mutants, showing that these pairs of kinases are not required for early steps in synapse formation. We generated muscle cell lines lacking src and fyn and found that neural agrin and laminin-1 induced normal clustering of AChRs and that agrin induced normal tyrosine phosphorylation of the AChR beta subunit in the absence of Src and Fyn. Another Src family member, most likely Yes, was associated with AChRs and phosphorylated by agrin in myotubes lacking Src and Fyn, indicating that Yes may compensate for the loss of Src and Fyn. Nevertheless, PP1 and PP2, inhibitors of Src-class kinases, did not inhibit agrin signaling, suggesting that Src class kinase activity is dispensable for agrin-induced clustering and tyrosine phosphorylation of AChRs. AChR clusters, however, were less stable in myotubes lacking Src and Fyn but not in PP1- or PP2-treated wild-type cells. These data show that the stabilization of agrin-induced AChR clusters requires Src and Fyn and suggest that the adaptor activities, rather than the kinase activities, of these kinases are essential for this stabilization.

Localization of disulfide bonds in the frizzled module of Ror1 receptor tyrosine kinase

The frizzled (FRZ) module is a novel module type that was first identified in G-protein-coupled receptors of the frizzled and smoothened families and has since been shown to be present in several secreted frizzled-related proteins, in some modular proteases, in collagen XVIII, and in various receptor tyrosine kinases of the Ror family. The FRZ modules constitute the extracellular ligand-binding region of frizzled receptors and are known to mediate signals of WNT family members through these receptors. With an eye toward defining the structure of this important module family, we have expressed the FRZ domain of rat Ror1 receptor tyrosine kinase in Pichia pastoris. By proteolytic digestion and amino acid sequencing the disulfide bonds were found to connect the 10 conserved cysteines in a 1-5, 2-4, 3-8, 6-10, and 7-9 pattern. Circular dichroism and differential scanning calorimetry studies on the recombinant protein indicate that the disulfide-bonded FRZ module corresponds to a single, compact, and remarkably stable folding domain possessing both alpha-helices and beta-strands.

The ERBB2/HER2 receptor differentially interacts with ERBIN and PICK1 PSD-95/DLG/ZO-1 domain proteins

Identification of protein complexes associated with the ERBB2/HER2 receptor may help unravel the mechanisms of its activation and regulation in normal and pathological situations. Interactions between ERBB2/HER2 and Src homology 2 or phosphotyrosine binding domain signaling proteins have been extensively studied. We have identified ERBIN and PICK1 as new binding partners for ERBB2/HER2 that associate with its carboxyl-terminal sequence through a PDZ (PSD-95/DLG/ZO-1) domain. This peptide sequence acts as a dominant retention or targeting basolateral signal for receptors in epithelial cells. ERBIN belongs to the newly described LAP (LRR and PDZ) protein family, whose function is crucial in non vertebrates for epithelial homeostasis. Whereas ERBIN appears to locate ERBB2/HER2 to the basolateral epithelium, PICK1 is thought to be involved in the clustering of receptors. We show here that ERBIN and PICK1 bind to ERBB2/HER2 with different mechanisms, and we propose that these interactions are regulated in cells. Since ERBIN and PICK1 tend to oligomerize, further complexity of protein networks may participate in ERBB2/HER2 functions and specificity.

Patterning of muscle acetylcholine receptor gene expression in the absence of motor innervation

The patterning of skeletal muscle is thought to depend upon signals provided by motor neurons. We show that AChR gene expression and AChR clusters are concentrated in the central region of embryonic skeletal muscle in the absence of innervation. Neurally derived Agrin is dispensable for this early phase of AChR expression, but MuSK, a receptor tyrosine kinase activated by Agrin, is required to establish this AChR prepattern. The zone of AChR expression in muscle lacking motor axons is wider than normal, indicating that neural signals refine this muscle-autonomous prepattern. Neuronal Neuregulin-1, however, is not involved in this refinement process, nor indeed in synapse-specific AChR gene expression. Our results demonstrate that AChR expression is patterned in the absence of innervation, raising the possibility that similarly prepatterned muscle-derived cues restrict axon growth and initiate synapse formation.

Distinct roles of nerve and muscle in postsynaptic differentiation of the neuromuscular synapse

The development of chemical synapses is regulated by interactions between pre- and postsynaptic cells. At the vertebrate skeletal neuromuscular junction, the organization of an acetylcholine receptor (AChR)-rich postsynaptic apparatus has been well studied. Much evidence suggests that the nerve-derived protein agrin activates muscle-specific kinase (MuSK) to cluster AChRs through the synapse-specific cytoplasmic protein rapsyn. But how postsynaptic differentiation is initiated, or why most synapses are restricted to an 'end-plate band' in the middle of the muscle remains unknown. Here we have used genetic methods to address these issues. We report that the initial steps in postsynaptic differentiation and formation of an end-plate band require MuSK and rapsyn, but are not dependent on agrin or the presence of motor axons. In contrast, the subsequent stages of synaptic growth and maintenance require nerve-derived agrin, and a second nerve-derived signal that disperses ectopic postsynaptic apparatus.

Agrin-induced phosphorylation of the acetylcholine receptor regulates cytoskeletal anchoring and clustering

At the developing neuromuscular junction, a motoneuron-derived factor called agrin signals through the muscle-specific kinase receptor to induce postsynaptic aggregation of the acetylcholine receptor (AChR). The agrin signaling pathway involves tyrosine phosphorylation of the AChR beta subunit, and we have tested its role in receptor localization by expressing tagged, tyrosine-minus forms of the beta subunit in mouse Sol8 myotubes. We find that agrin-induced phosphorylation of the beta subunit occurs only on cell surface AChR, and that AChR-containing tyrosine-minus beta subunit is targeted normally to the plasma membrane. Surface AChR that is tyrosine phosphorylated is less detergent extractable than nonphosphorylated AChR, indicating that it is preferentially linked to the cytoskeleton. Consistent with this, we find that agrin treatment reduces the detergent extractability of AChR that contains tagged wild-type beta subunit but not tyrosine-minus beta subunit. In addition, agrin-induced clustering of AChR containing tyrosine-minus beta subunit is reduced in comparison to wild-type receptor. Thus, we find that agrin-induced phosphorylation of AChR beta subunit regulates cytoskeletal anchoring and contributes to the clustering of the AChR, and this is likely to play an important role in the postsynaptic localization of the receptor at the developing synapse.

Localization and modulation of ErbB receptor tyrosine kinases

The ErbB family of receptor tyrosine kinases serves as a model for understanding the propagation of growth factor signals across the plasma membrane and the interpretation of those signals into a cellular response. Recent studies point to a critical role for the accumulation of ErbBs at specific cell-surface locations in the fidelity of ErbB signaling. The past year has witnessed significant advances in our understanding of the molecular mechanisms of ErbB localization and the role of PDZ-domain-containing proteins and cell-surface glycoproteins in directly modulating signaling through ErbBs.

[Development of the central nervous system in mammals]

In humans, the nervous system is induced during the third gestational week by molecular signals coming from the mesoderm, which modulate the temporal and spatial expression of specific genes in the cells of the dorsal ectoderm. The induced neural plate closes to form the neural tube where the cells actively proliferate in the germinal zone. The neuroblasts which have completed their last division migrate along the fibers of the radial glia to which they adhere, and this movement is essential to establish the normal cerebral organization. The regional identity of the developing brain is governed by the expression of homeobox genes, and the main central structures are clearly delineated by the end of the fifth week. The cerebral cortex begins to form on the seventh week, and the early specification of the cortical areas, which is under genetic control, would be modulated later on by environmental influences. When the neurons have reached their final position, they extend an axon, using surface molecules or diffusible molecules present along its pathway as cues to reach the appropriate target and form a synapse, and this process is a critical step for the establishment of neuronal relationships. The maturation and stabilization of neural networks is characterized by the apoptotic death of roughly 50% of the neurons, due to insufficient neurotrophic support, and by the remodeling of the initial synaptic connections in the surviving neurons. These regressive events occur late in development and depend on both the interactions with the environment and the resulting neuronal activity.

Cellular and molecular insights into presynaptic assembly

Little is known about the development of presynaptic specializations. Recent studies that visualize tagged synaptic components in cultured cells and in vivo have identified molecular participants and reveal common features in cellular processes of presynaptic assembly.

Neuregulin, a factor with many functions in the life of a schwann cell

The signalling system comprising the ligand Neuregulin-1, and its receptors, ErbB2 and ErbB3, plays multiple and important roles in glial development. These include functions in early development of neural crest cells, in expansion of the Schwann cell precursor pool and in myelination. Neuregulin is one of the crucial axon-derived signals that influence development of Schwann cells. These are specialized cells that ensheath peripheral axons and provide electrical insulation. Schwann cells have also long been implicated in providing more than a simple ensheathing function. Compelling evidence for this has emerged from the analysis of mice lacking these cells, resulting from a non-functional or compromised Neuregulin signalling system. They serve as a model to study glia-nerve interactions in vivo and indicate that Schwann cells provide important neurotrophic signals, and also cues that regulate perineurium development and nerve fasciculation.

Regulation of agrin expression in hippocampal neurons by cell contact and electrical activity

Most synapses contain high concentrations of neurotransmitter receptors in the postsynaptic plasma membrane. Agrin (Ag) is an extracellular matrix protein necessary for the localization of acetylcholine receptors at the neuromuscular junction and for the differentiation of synapses in hippocampal neurons in vitro. The temporal pattern of agrin expression during the development of the central nervous system (CNS) is consistent with the notion that agrin expression is regulated during synaptogenesis. To identify the processes underlying this regulation, we have analyzed levels and alternative splicing of agrin mRNA in primary hippocampal neurons. Our results indicate that in the initial phases of synapse formation, contact-mediated processes and action potential-dependent neurotransmission regulate agrin mRNA expression, while secreted factors from glial cells, but not from hippocampal neurons, influence the alternative splicing of agrin mRNA. Previous studies have shown that specific agrin isoforms are able to induce the activation of a transcription factor and that secreted agrin associates with cellular surfaces. Therefore, we have tested whether agrin isoforms contribute to the contact-mediated induction of agrin expression in hippocampal neurons. None of the agrin isoforms tested in this study revealed this activity. Finally, we show that the role of evoked neural transmission in controlling agrin transcription changes during differentiation in vitro.

Cholinergic receptors: dual roles in transduction and plasticity

The regional distributions and possible functions of nicotinic acetylcholine receptors (nAChRs) in the developing and adult auditory rat brain are reviewed. The predominant nAChR in the auditory brainstem is the alpha7 homomeric receptor. alpha7 mRNA and protein are expressed in selected regions of the cochlear nucleus (CN), inferior colliculus (IC), medial superior olive, lateral superior olive, ventral nucleus of the lateral lemniscus and superior paraolivary nucleus. Peak expression of mRNA and protein occurs by the second postnatal week in most auditory brainstem areas. In contrast, the alpha3 and beta4 nicotinic subunits are expressed in the embryo and early in postnatal development in the CN and IC, but not other brainstem nuclei. Of particular interest is the octopus cell region of the posteroventral cochlear nucleus (PVCN). alpha3 and beta4 are down-regulated in the octopus cell region about postnatal day 10, which is the age that alpha7 is at peak expression. NAChRs play important roles in transduction and in regulating intracellular calcium. The ability of the alpha7 receptor to synchronize synaptic activity and stabilize synapses makes it a prime candidate as a mechanism underlying homeostatic plasticity in the auditory system.

Agrin-induced acetylcholine receptor clustering is mediated by the small guanosine triphosphatases Rac and Cdc42

During neuromuscular junction formation, agrin secreted from motor neurons causes muscle cell surface acetylcholine receptors (AChRs) to cluster at synaptic sites by mechanisms that are insufficiently understood. The Rho family of small guanosine triphosphatases (GTPases), including Rac and Cdc42, can mediate focal reorganization of the cell periphery in response to extracellular signals. Here, we investigated the role of Rac and Cdc42 in coupling agrin signaling to AChR clustering. We found that agrin causes marked muscle-specific activation of Rac and Cdc42 in differentiated myotubes, as detected by biochemical measurements. Moreover, this activation is crucial for AChR clustering, since the expression of dominant interfering mutants of either Rac or Cdc42 in myotubes blocks agrin-induced AChR clustering. In contrast, constitutively active Rac and Cdc42 mutants cause AChR to aggregate in the absence of agrin. By indicating that agrin-dependent activation of Rac and Cdc42 constitutes a critical step in the signaling pathway leading to AChR clustering, these findings suggest a novel role for these Rho-GTPases: the coupling of neuronal signaling to a key step in neuromuscular synaptogenesis.

Molecular mechanisms underlying the activity-linked alterations in acetylcholinesterase mRNAs in developing versus adult rat skeletal muscles

The molecular mechanisms underlying the activity-linked plasticity of acetylcholinesterase (AChE) mRNA levels in mammalian skeletal muscle have yet to be established. Here, we demonstrate that denervation of adult muscle induces a dramatic (up to 90%) and rapid (within 24 h) decrease in the abundance of AChE mRNAs. By contrast, denervation of 14-day-old rats leads to a significantly less pronounced reduction (50% of control) in the expression of AChE mRNAs. Assessment of the transcriptional activity of the AChE gene reveals that it remains essentially unchanged in adult denervated muscles, whereas it displays an approximately two- to three-fold increase (p < 0.05) in denervated muscles from 2- to 14-day-old rats. In addition, we observed a higher rate of degradation of in vitro transcribed AChE mRNAs upon incubation with protein extracts from denervated muscles. Finally, UV-crosslinking experiments reveal that denervation increases the abundance of RNA-protein interactions in the 3' untranslated region of AChE transcripts. Taken together, these data suggest that the abundance of AChE transcripts in mature muscles is controlled primarily via posttranscriptional regulatory mechanisms, whereas in neo- and postnatal muscles, both transcriptional and posttranscriptional regulation appears critical in dictating AChE mRNA levels. Accordingly, the activity-linked transcriptional regulation of the AChE gene appears to demonstrate a high level of plasticity during muscle development when maturation of the neuromuscular junctions is still occurring.

Regulated expression of the proteoglycan SPOCK in the neuromuscular system

SPOCK is prevalent in developing synaptic fields of the central nervous system (Charbonnier et al., 2000. Mech. Dev. 90, 317-321). The expression of SPOCK during neuromuscular junction (NMJ) formation was compared to agrin and acetylcholine receptor (AChR) distribution. SPOCK is detected within the myogenic masses during the early steps of embryonic development, and distributed in the cytoplasm of myotubes before coclustering with AChRs. In the adult, SPOCK is present in axons and is highly expressed by Schwann cells. SPOCK altered expression pattern after nerve lesioning, or cholinergic transmission blockade, strongly indicate that its cellular distribution at the NMJ depends on innervation.

The in vitro and in vivo phosphotyrosine map of activated MuSK

The muscle-specific receptor tyrosine kinase MuSK plays a crucial role in neuromuscular synapse formation. Activation of MuSK is induced by agrin leading to clustering of several proteins, including acetylcholine receptors, at synaptic sites. In a first step to elucidate the signal transduction cascade following MuSK activation and leading to clustering of synaptic proteins, we sought to identify the tyrosine residues that are phosphorylated in activated MuSK. We mapped the tyrosine residues that are phosphorylated in vitro and in vivo using methods that provide high sensitivity and do not require radioactive tracers. We expressed MuSK in insect cells by using a baculovirus expression vector and mapped the tyrosines that are phosphorylated in MuSK in an in vitro kinase assay using matrix-assisted laser desorption ionization MS to sequence tryptic peptides fractionated by HPLC. In addition, we isolated MuSK from Torpedo electric organ and used nanoelectrospray tandem mass spectrometry and parent ion scanning to identify the tyrosine residues that are phosphorylated in activated, endogenous MuSK in vivo. We found that six of the nineteen intracellular tyrosine residues in MuSK are phosphorylated in activated MuSK: the juxtamembrane tyrosine (Y553), the tyrosines within the activation loop (Y750, Y754, and Y755), a tyrosine near the beginning of the kinase domain (Y576), and a tyrosine (Y812) within the C-terminal lobe of the kinase domain. Our biochemical data are consistent with results from functional experiments and establish a good correlation between tyrosine residues that are phosphorylated in activated MuSK and tyrosines that are required for MuSK signaling.

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

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

The torpedo electrocyte: a model system to study membrane-cytoskeleton interactions at the postsynaptic membrane

Many aspects of the organization of the electromotor synapse of electric fish resemble the nerve-muscle junction. In particular, the postsynaptic membrane in both systems share most of their proteins. As a remarquable source of cholinergic synapses, the Torpedo electrocyte model has served to identify the most important components involved in synaptic transmission such as the nicotinic acetylcholine receptor and the enzyme acetylcholinesterase, as well as proteins associated with the subsynaptic cytoskeleton and the extracellular matrix involved in the assembly of the postsynaptic membrane, namely the 43-kDa protein-rapsyn, the dystrophin/utrophin complex, agrin, and others. This review encompasses some representative experiments that helped to clarify essential aspects of the supramolecular organization and assembly of the postsynaptic apparatus of cholinergic synapses.

The neuregulin receptor ErbB-4 interacts with PDZ-containing proteins at neuronal synapses

Neuregulins regulate the expression of ligand- and voltage-gated channels in neurons and skeletal muscle by the activation of their cognate tyrosine kinase receptors, ErbB 1-4. The subcellular distribution and mechanisms that regulate the localization of ErbB receptors are unknown. We have found that ErbB receptors are present in brain subcellular fractions enriched for postsynaptic densities (PSD). The ErbB-4 receptor is unique among the ErbB proteins because its C-terminal tail (T-V-V) conforms to a sequence that binds to a protein motif known as the PDZ domain. Using the yeast two-hybrid system, we found that the C-terminal region of ErbB-4 interacts with the three related membrane-associated guanylate kinases (MAGUKs) PSD-95/SAP90, PSD-93/chapsyn-110, and SAP 102, which harbor three PDZ domains, as well as with beta(2)-syntrophin, which has a single PDZ domain. As with N-methyl-D-aspartate (NMDA) receptors, ErbB4 interacts with the first two PDZ domains of PSD-95. Using coimmunoprecipitation assays, we confirmed the direct interactions between ErbB-4 and PSD-95 in transfected heterologous cells, as well as in vivo, where both proteins are coimmunoprecipitated from brain lysates. Moreover, evidence for colocalization of these proteins was also observed by immunofluorescence in cultured hippocampal neurons. ErbB-4 colocalizes with PSD-95 and NMDA receptors at a subset of excitatory synapses apposed to synaptophysin-positive presynaptic terminals. The capacity of ErbB receptors to interact with PDZ-domain proteins at cell junctions is conserved from invertebrates to mammals. As discussed, the interactions found between receptor tyrosine kinases and MAGUKs at neuronal synapses may have important implications for activity-dependent plasticity.

The neuregulin receptor ErbB-4 interacts with PDZ-containing proteins at neuronal synapses

Neuregulins regulate the expression of ligand- and voltage-gated channels in neurons and skeletal muscle by the activation of their cognate tyrosine kinase receptors, ErbB 1-4. The subcellular distribution and mechanisms that regulate the localization of ErbB receptors are unknown. We have found that ErbB receptors are present in brain subcellular fractions enriched for postsynaptic densities (PSD). The ErbB-4 receptor is unique among the ErbB proteins because its C-terminal tail (T-V-V) conforms to a sequence that binds to a protein motif known as the PDZ domain. Using the yeast two-hybrid system, we found that the C-terminal region of ErbB-4 interacts with the three related membrane-associated guanylate kinases (MAGUKs) PSD-95/SAP90, PSD-93/chapsyn-110, and SAP 102, which harbor three PDZ domains, as well as with beta(2)-syntrophin, which has a single PDZ domain. As with N-methyl-D-aspartate (NMDA) receptors, ErbB4 interacts with the first two PDZ domains of PSD-95. Using coimmunoprecipitation assays, we confirmed the direct interactions between ErbB-4 and PSD-95 in transfected heterologous cells, as well as in vivo, where both proteins are coimmunoprecipitated from brain lysates. Moreover, evidence for colocalization of these proteins was also observed by immunofluorescence in cultured hippocampal neurons. ErbB-4 colocalizes with PSD-95 and NMDA receptors at a subset of excitatory synapses apposed to synaptophysin-positive presynaptic terminals. The capacity of ErbB receptors to interact with PDZ-domain proteins at cell junctions is conserved from invertebrates to mammals. As discussed, the interactions found between receptor tyrosine kinases and MAGUKs at neuronal synapses may have important implications for activity-dependent plasticity.

Axonal remodeling and synaptic differentiation in the cerebellum is regulated by WNT-7a signaling

Synapse formation requires changes in cell morphology and the upregulation and localization of synaptic proteins. In the cerebellum, mossy fibers undergo extensive remodeling as they contact several granule cells and form complex, multisynaptic glomerular rosettes. Here we show that granule cells secrete factors that induce axon and growth cone remodeling in mossy fibers. This effect is blocked by the WNT antagonist, sFRP-1, and mimicked by WNT-7a, which is expressed by granule cells. WNT-7a also induces synapsin I clustering at remodeled areas of mossy fibers, a preliminary step in synaptogenesis. Wnt-7a mutant mice show a delay in the morphological maturation of glomerular rosettes and in the accumulation of synapsin I. We propose that WNT-7a can function as a synaptogenic factor.

Eye muscle sparing by the muscular dystrophies: lessons to be learned?

The devastating consequences of the various muscular dystrophies are even more obvious when a muscle or muscle group is spared. The study of the exceptional cell or tissue responses may prove to be of considerable value in the analysis of disease mechanisms. The small muscles responsible for eye movements, the extraocular muscles, have functional and morphological characteristics that set them aside from other skeletal muscles. Notably, these muscles are clinically unaffected in Duchenne/Becker, limb-girdle, and congenital muscular dystrophies, pathologies due to a broken mechanical or signaling linkage between the cytoskeleton and the extracellular matrix. Uncovering the strategies used by the extraocular muscles to "naturally" protect themselves in these diseases should contribute to knowledge of both pathogenesis and treatment. We propose that careful investigation of the cellular determinants of extraocular muscle-specific properties may provide insights into how these muscles avoid or adapt to the cascade of events leading to myofiber degeneration in the muscular dystrophies.

Aberrant development of motor axons and neuromuscular synapses in erbB2-deficient mice

Receptor tyrosine kinase erbB2, which is activated by neuregulin, is expressed in Schwann and muscle cells in the developing neuromuscular junction (NMJ). In vitro studies have shown that neuregulin promotes the survival and migration of Schwann cells and stimulates acetylcholine receptor gene transcription in cultured muscle cells. These findings suggest an important role for erbB2 in the development of the NMJ. Here we examine erbB2-deficient mice to determine whether erbB2 is required for NMJ development in vivo. Our analysis shows that there are pre- and postsynaptic defects of developing NMJ in erbB2-deficient embryos. The presynaptic defects include defasciculation and degeneration of the motor nerves, and an absence of Schwann cells. The postsynaptic defect features an impairment of junctional folds at the neuromuscular synapse in the mutants. These results demonstrate that erbB2 is essential for in vivo development of the NMJ.

Axonal signals in the assembly of neural circuitry

Recent work in Drosophila and rodents has revealed that proteins transported along axons and delivered to pathway and target cell populations play important roles in the construction of neural circuitry. Interestingly, the parallels between these systems may extend to the identities of some of the molecules involved.

Maturation and maintenance of the neuromuscular synapse: genetic evidence for roles of the dystrophin--glycoprotein complex

The dystrophin-glycoprotein complex (DGC) links the cytoskeleton of muscle fibers to their extracellular matrix. Using knockout mice, we show that a cytoplasmic DGC component, alpha-dystrobrevin (alpha-DB), is dispensable for formation of the neuromuscular junction (NMJ) but required for maturation of its postsynaptic apparatus. We also analyzed double and triple mutants lacking other cytoskeletal DGC components (utrophin and dystrophin) and myotubes lacking a alpha-DB or a transmembrane DGC component (dystroglycan). Our results suggest that alpha-DB acts via its linkage to the DGC to enhance the stability of postsynaptic specializations following their DGC-independent formation; dystroglycan may play additional roles in assembling synaptic basal lamina. Together, these results demonstrate involvement of distinct protein complexes in the formation and maintenance of the synapse and implicate the DGC in the latter process.

LIM kinase 1 accumulates in presynaptic terminals during synapse maturation

LIM kinase 1 (LIMK1) is a cytoplasmic protein kinase that is highly expressed in neurons. In transfected cells, LIMK1 binds to the cytoplasmic tail of neuregulins and regulates the breakdown of actin filaments. To identify potential functions of LIMK1 in vivo, we have determined the subcellular distribution of LIMK1 protein within neurons of the rat by using immunomicroscopy. At neuromuscular synapses in the adult hindlimb, LIMK1 was concentrated in the presynaptic terminal. However, little LIMK1 immunoreactivity was detected at neuromuscular synapses before the 2nd week after birth, and most motoneuron terminals were not strongly LIMK1 immunoreactive until the 3rd week after birth. Thus, LIMK1 accumulation at neuromuscular synapses coincided with their maturation. In contrast, SV2, like many other presynaptic terminal proteins, can be readily detected at neuromuscular synapses in the embryo. Similar to its late accumulation at developing synapses, LIMK1 accumulation at regenerating neuromuscular synapses occurred long after these synapses first formed. In the adult ventral spinal cord, LIMK1 was concentrated in a subset of presynaptic terminals. LIMK1 gradually accumulated at spinal cord synapses postnatally, reaching adult levels only after P14. This study is the first to implicate LIMK1 in the function of presynaptic terminals. The concentration of LIMK1 in adult, but not nascent, presynaptic terminals suggests a role for this kinase in regulating the structural or functional characteristics of mature synapses.

The organization of the Golgi complex and microtubules in skeletal muscle is fiber type-dependent

Skeletal muscle has a nonconventional Golgi complex (GC), the organization of which has been a subject of controversy in the past. We have now examined the distribution of the GC by immunofluorescence and immunogold electron microscopy in whole fibers from different rat muscles, both innervated and experimentally denervated. The total number of GC elements, small polarized stacks of cisternae, is quite similar in all fibers, but their intracellular distribution is fiber type-dependent. Thus, in slow-twitch, type I fibers, approximately 75% of all GC elements are located within 1 micrometer from the plasma membrane, and each nucleus is surrounded by a belt of GC elements. In contrast, in the fast-twitch type IIB fibers, most GC elements are in the fiber core, and most nuclei only have GC elements at their poles. Intermediate, type IIA fibers also have an intermediate distribution of GC elements. Interestingly, the distribution of microtubules, with which GC elements colocalize, is fiber type-dependent as well. At the neuromuscular junction, the distribution of GC elements and microtubules is independent of fiber type, and junctional nuclei are surrounded by GC elements in all fibers. After denervation of the hindlimb muscles, GC elements as well as microtubules converge toward a common pattern, that of the slow-twitch fibers, in all fibers. Our data suggest that innervation regulates the distribution of microtubules, which in turn organize the Golgi complex according to muscle fiber type.

Myotubes originating from single fast and slow satellite cells display similar patterns of AChE expression

Slow- and fast-contracting skeletal muscles of both rats and mice display significant differences in their patterns of acetylcholinesterase (AChE) expression. Although neural influences are known to account for a large proportion of these differences, intrinsic variations between fast and slow myogenic precursor cells have been implicated. In the present study, we have capitalized on the use of Immorto transgenic mice to obtain single myogenic precursor cells isolated from either slow or fast muscle fibers and determined whether these cells generated myotubes that produced distinct patterns of AChE expression as observed in vivo between slow and fast muscles. These two myotube populations displayed similar cell-associated and secreted AChE enzyme activity as well as comparable levels of AChE transcripts. Both myotube populations also expressed nearly identical molecular form profiles. By contrast, AChE activity and transcript levels were approximately two- and fivefold greater in fast skeletal muscles compared with slow ones. Together, these findings indicate that differences in AChE expression between fast and slow muscles are not due to inherent differences in myogenic precursor cells, thereby suggesting that other factors, such as innervation, play a predominant role in establishing the distinct patterns of AChE expression in these muscle types.

Chimaeric mice deficient in dystroglycans develop muscular dystrophy and have disrupted myoneural synapses

Mutations in the dystrophin gene (DMD) and in genes encoding several dystrophin-associated proteins result in Duchenne and other forms of muscular dystrophy. alpha-Dystroglycan (Dg) binds to laminins in the basement membrane surrounding each myofibre and docks with beta-Dg, a transmembrane protein, which in turn interacts with dystrophin or utrophin in the subplasmalemmal cytoskeleton. alpha- and beta-Dgs are thought to form the functional core of a larger complex of proteins extending from the basement membrane to the intracellular cytoskeleton, which serves as a superstructure necessary for sarcolemmal integrity. Dgs have also been implicated in the formation of synaptic densities of acetylcholine receptors (AChRs) on skeletal muscle. Here we report that chimaeric mice generated with ES cells targeted for both Dg alleles have skeletal muscles essentially devoid of Dgs and develop a progressive muscle pathology with changes emblematic of muscular dystrophies in humans. In addition, many neuromuscular junctions are disrupted in these mice. The ultrastructure of basement membranes and the deposition of laminin within them, however, appears unaffected in Dg-deficient muscles. We conclude that Dgs are necessary for myofibre survival and synapse differentiation or stability, but not for the formation of the muscle basement membrane, and that Dgs may have more than a purely structural function in maintaining muscle integrity.

Induction of presynaptic differentiation in cultured neurons by extracellular matrix components

Motoneurons reinnervating skeletal muscles form nerve terminals at sites of contact with a specialized basal lamina. To analyse the molecules and mechanisms that underly these responses, we introduce two systems in which basal lamina-derived components induce presynaptic differentiation of cultured neurons from chick ciliary ganglia in the absence of a postsynaptic cell. In one, ciliary neurites that contact substrates coated with a recombinant laminin beta2 fragment form varicosities that are rich in synaptic vesicle proteins, depleted of neurofilaments, and capable of depolarization-dependent exocytosis and endocytosis. Thus, a single molecule can trigger a complex, coordinated program of presynaptic differentiation. In a second system, neurites growing on cryostat sections of adult kidney form vesicle-rich, neurofilament-poor arbors on glomeruli. Glomerular basal lamina, like synaptic basal lamina, is rich in laminin beta2 and collagen (alpha3-5) IV. However, glomeruli from mutant mice lacking these proteins were capable of inducing differentiation, suggesting the glomerulus as a source of novel presynaptic organizing molecules.

Distinct domains of MuSK mediate its abilities to induce and to associate with postsynaptic specializations

Agrin released from motor nerve terminals activates a muscle-specific receptor tyrosine kinase (MuSK) in muscle cells to trigger formation of the skeletal neuromuscular junction. A key step in synaptogenesis is the aggregation of acetylcholine receptors (AChRs) in the postsynaptic membrane, a process that requires the AChR-associated protein, rapsyn. Here, we mapped domains on MuSK necessary for its interactions with agrin and rapsyn. Myotubes from MuSK(-/)- mutant mice form no AChR clusters in response to agrin, but agrin-responsiveness is restored by the introduction of rat MuSK or a Torpedo orthologue. Thus, MuSK(-/)- myotubes provide an assay system for the structure-function analysis of MuSK. Using this system, we found that sequences in or near the first of four extracellular immunoglobulin-like domains in MuSK are required for agrin responsiveness, whereas sequences in or near the fourth immunoglobulin-like domain are required for interaction with rapsyn. Analysis of the cytoplasmic domain revealed that a recognition site for the phosphotyrosine binding domain-containing proteins is essential for MuSK activity, whereas consensus binding sites for the PSD-95/Dlg/ZO-1-like domain-containing proteins and phosphatidylinositol-3-kinase are dispensable. Together, our results indicate that the ectodomain of MuSK mediates both agrin- dependent activation of a complex signal transduction pathway and agrin-independent association of the kinase with other postsynaptic components. These interactions allow MuSK not only to induce a multimolecular AChR-containing complex, but also to localize that complex to a primary scaffold in the postsynaptic membrane.

Expression of the utrophin gene during myogenic differentiation

The process of myogenic differentiation is known to be accompanied by large increases ( approximately 10-fold) in the expression of genes encoding cytoskeletal and membrane proteins including dystrophin and the acetylcholine receptor (AChR) subunits, via the effects of transcription factors belonging to the MyoD family. Since in skeletal muscle (i) utrophin is a synaptic homolog to dystrophin, and (ii) the utrophin promoter contains an E-box, we examined, in the present study, expression of the utrophin gene during myogenic differentiation using the mouse C2 muscle cell line. We observed that in comparison to myoblasts, the levels of utrophin and its transcript were approximately 2-fold higher in differentiated myotubes. In order to address whether a greater rate of transcription contributed to the elevated levels of utrophin transcripts, we performed nuclear run-on assays. In these studies we determined that the rate of transcription of the utrophin gene was approximately 2-fold greater in myotubes as compared to myoblasts. Finally, we examined the stability of utrophin mRNAs in muscle cultures by two separate methods: following transcription blockade with actinomycin D and by pulse-chase experiments. Under these conditions, we determined that the half-life of utrophin mRNAs in myoblasts was approximately 20 h and that it remained largely unaffected during myogenic differentiation. Altogether, these results show that in comparison to other synaptic proteins and to dystrophin, expression of the utrophin gene is only moderately increased during myogenic differentiation.

Non-myelin-forming perisynaptic schwann cells express protein zero and myelin-associated glycoprotein

Perisynaptic Schwann cells (PSCs) envelop axonal terminals and are physiologically distinct from the nearby myelinating Schwann cells (MSCs), which surround the same innervating motor axons. PSCs have special functions at the neuromuscular synapse, where they detect and can modulate neurotransmitter release. Although PSCs are similar to non-myelinating Schwann cells in that they do not form multiple myelin wrappings around nerve terminals, they do wrap around single nerve terminals. These differences, as well as others, lead us to question whether PSCs are truly of Schwann cell origin. We thus characterized the expression of molecules, classically associated with myelin and Schwann cells, in PSCs at the frog neuromuscular junction. We wondered whether PSCs express the Schwann cell marker protein zero (P(0)) and whether their lack of myelination was related to an absence of myelin-associated glycoprotein (MAG), a protein found in myelinating cells that is considered important in myelination. Instead, we found that PSCs express both P(0) and MAG, and other myelinating glial markers such as galactocerebroside and 2',3'-cyclic nucleotide 3'-phosphodiesterase. In denervated preparations, P(0) and MAG expression persisted, including at newly formed PSC extensions. Because PSCs do not myelinate, it is clear that expression of these proteins alone is not sufficient for myelin formation. It is possible that factors present at synapses may prevent myelination, while P(0) and MAG may mediate adhesion between nerve terminals and the surrounding PSCs. The results indicate that PSCs are of Schwann cell origin.

Changes in the distribution of peanut agglutinin (PNA) binding molecules during muscle reinnervation following nerve crush injury

Peanut agglutinin (PNA) staining during muscle reinnervation following a crushing injury of the sciatic nerve was performed in reference to the neural profiles immunolabeled with the PGP 9.5 antibody. PNA staining in the normal controls exhibited dots, granules, or lines along the length of the nerve fibers in the nerve trunk, but was faint or absent in the motor endplate. At seven days post-crush, PNA staining was detected around the vacuolated neural structures in the disorganized nerve trunk, but was still faint or absent in the motor endplate. At twenty-one days post-crush, when PGP 9.5-positive regenerating axons appeared in most of the motor endplates, PNA staining, either faint or strong, followed the pathway of the nerve fibers delineated by PGP 9.5-like immunoreactivity. During reinnervation to the motor endplates, PNA staining displayed signs of remodeling in the nerve trunk, such as marked variations in density and profile in the nerve fiber-associated dots or patches; it increased in intensity in the connective tissue covering the area of the motor endplate, as well as in the junctional myofiber surface. The structures recognizable by PNA coincided with components of the connective tissue such as collagen fibers and capillaries. Results suggest that: 1) the expression of PNA-binding molecules is dependent on the state of innervation, and 2) the spatiotemporal relationship between neural profiles and PNA staining provides sequences of axonal extension and subsequent nerve terminal maturation during regeneration in the motor endplate.

Rescue of the cardiac defect in ErbB2 mutant mice reveals essential roles of ErbB2 in peripheral nervous system development

ErbB2 receptor tyrosine kinase plays a role in neuregulin signaling and is expressed in the developing nervous system. We genetically rescued the cardiac defect of erbB2 null mutant embryos, which otherwise died at E11. These rescued erbB2 mutant mice die at birth and display a severe loss of both motor and sensory neurons. Motor and sensory axons are severely defasciculated and aberrantly projected within their final target tissues. Schwann cells are completely absent in the peripheral nerves. Schwann cell precursors are present within the DRG and proliferate normally, but their ability to migrate is decreased. Acetylcholine receptors cluster within the central band of the mutant diaphragm muscle. However, these clusters are dispersed and morphologically different from those in control muscle. Our results reveal an important role for erbB2 during normal peripheral nervous system development.

An intronic enhancer containing an N-box motif is required for synapse- and tissue-specific expression of the acetylcholinesterase gene in skeletal muscle fibers

mRNAs encoding acetylcholinesterase (AChE; EC 3.1.1.7) are highly concentrated within the postsynaptic sarcoplasm of adult skeletal muscle fibers, where their expression is markedly influenced by nerve-evoked electrical activity and trophic factors. To determine whether transcriptional regulatory mechanisms account for the synaptic accumulation of AChE transcripts at the mammalian neuromuscular synapse, we cloned a 5.3-kb DNA fragment that contained the 5' regulatory region of the rat AChE gene and generated several constructs in which AChE promoter fragments were placed upstream of the reporter gene lacZ and a nuclear localization signal (nls). Using a recently described transient expression assay system in intact skeletal muscle, we show that this AChE promoter fragment directs the synapse-specific expression of the reporter gene. Deletion analysis revealed that a 499-bp fragment located in the first intron of the AChE gene is essential for expression in muscle fibers. Further analysis showed that sequences contained within this intronic fragment were (i) functionally independent of position and orientation and (ii) inactive in hematopoietic cells. Disruption of an N-box motif located within this DNA fragment reduced by more than 80% the expression of the reporter gene in muscle fibers. In contrast, mutation of an adjacent CArG element had no effect on nlsLacZ expression. Taken together, these results indicate that a muscle-specific enhancer is present within the first intron of the AChE gene and that an intronic N-box is essential for the regulation of AChE along skeletal muscle fibers.

Watching a synapse grow: noninvasive confocal imaging of synaptic growth in Drosophila

The glutamatergic neuromuscular junction (NMJ) in Drosophila adds new boutons and branches during larval development. We generated transgenic fruit flies that express a novel green fluorescent membrane protein at the postsynaptic specialization, allowing for repeated noninvasive confocal imaging of synapses in live, developing larvae. As synapses grow, existing synaptic boutons stretch apart and new boutons insert between them; in addition, new boutons are added at the ends of existing strings of boutons. Some boutons are added de novo, while others bud from existing boutons. New branches form as multiple boutons bud from existing boutons. Nascent boutons contain active zones, T bars, and synaptic vesicles; we observe no specialized growth structures. Some new boutons exhibit a lower level of Fasciclin II, suggesting that the levels of this synaptic cell adhesion molecule vary locally during synaptic growth.

Induction of utrophin gene expression by heregulin in skeletal muscle cells: role of the N-box motif and GA binding protein

The modulation of utrophin gene expression in muscle by the nerve-derived factor agrin plausibly involves the trophic factor ARIA/heregulin. Here we show that heregulin treatment of mouse and human cultured myotubes caused a approximately 2.5-fold increase in utrophin mRNA levels. Transient transfection experiments with utrophin promoter-reporter gene constructs showed that this increase resulted from an enhanced transcription of the utrophin gene. In the case of the nicotinic acetylcholine receptor delta and epsilon subunit genes, heregulin was previously reported to stimulate transcription via a conserved promoter element, the N-box, which binds the multimeric Ets-related transcription factor GA binding protein (GABP). Accordingly, site-directed mutagenesis of a single N-box motif in the utrophin gene promoter abolished the transcriptional response to heregulin. In addition, overexpression of heregulin, or of the two GABP subunits in cultured myotubes, caused an N-box-dependent increase of the utrophin promoter activity. In vivo, direct gene transfer into muscle confirmed that heregulin regulates utrophin gene expression. Finally, electrophoretic mobility shift assays and supershift experiments performed with muscle extracts revealed that the N-box of the utrophin promoter binds GABP. These findings suggest that the subsynaptic activation of transcription by heregulin via the N-box motif and GABP are conserved among genes expressed at the neuromuscular junction. Because utrophin can functionally compensate for the lack of dystrophin, the elucidation of the molecular mechanisms regulating utrophin gene transcription may ultimately lead to therapies based on utrophin expression throughout the muscle fibers of Duchenne muscular dystrophy patients.

Seeking muscle stem cells

Skeletal muscle development requires the formation of myoblasts that can fuse with each other to form multinucleate myofibers. Distinct primary and secondary, slow and fast, populations of myofibers form by the time of birth. At embryonic, fetal, and perinatal stages of development, temporally distinct lineages of myogenic cells arise and contribute to the formation of these multiple types of myofibers. In addition, spatially distinct lineages of myogenic cells arise and form the anterior head muscles, limb (hypaxial) muscles, and dorsal (epaxial) muscles. There is strong evidence that myoblasts are produced from muscle stem cells, which are self-renewing cells that do not themselves terminally differentiate but produce progeny that are capable of becoming myoblasts and myofibers. Muscle stem cells, which may be multipotent, appear to be distinguishable from myoblasts by a number of indirect and direct criteria. Muscle stem cells arise either in unsegmented paraxial mesoderm (anterior head muscle progenitors) or in segmented mesoderm of the somites (epaxial and hypaxial muscle progenitors). These initial stages of myogenesis are regulated by positive and negative signals, including Wnt, BMP, and Shh family members, from nearby notochord, neural tube, ectoderm, and lateral mesoderm tissues. The formation of skeletal muscles, therefore, depends on the generation of spatially and temporally distinct lineages of myogenic cells. Myogenic cell lineages begin with muscle stem cells which produce the myoblasts that fuse to form myofibers.

Developmental regulation of tyrosine phosphorylation of the nicotinic acetylcholine receptor in Torpedo electrocyte

Tyrosine phosphorylation is thought to play a critical role in the clustering of acetylcholine receptors (AChR) at the developing neuromuscular junction. Yet, in vitro approaches have led to conflicting conclusions regarding the function of tyrosine phosphorylation of AChR beta subunit in AChR clustering. In this work, we followed in situ the time course of tyrosine phosphorylation of AChR in developing Torpedo electrocyte. We observed that tyrosine phosphorylation of the AChR beta and delta subunits occurs at a late stage of embryonic development after the accumulation of AChRs and rapsyn in the membrane and the onset of innervation. Interestingly, in the mature postsynaptic membrane, we observed two populations of AChR differing both in their phosphotyrosine content and distribution. Our data are consistent with the notion that tyrosine phosphorylation of the AChR is related to downstream events in the pathway regulating AChR accumulation rather than to initial clustering events.

Specific agrin isoforms induce cAMP response element binding protein phosphorylation in hippocampal neurons

The synaptic basal lamina protein agrin is essential for the formation of neuromuscular junctions. Agrin mediates the postsynaptic clustering of acetylcholine receptors and regulates transcription in muscles. Agrin expression is not restricted to motor neurons but can be demonstrated throughout the CNS. The functional significance of agrin expression in neurons other than motor neurons is unknown. To test whether agrin triggers responses in neurons that lead to the activation of transcription factors, we have analyzed phosphorylation of the transcriptional regulatory site serine 133 of the transcription factor CREB (cAMP response element binding protein) in primary hippocampal neurons. Our results indicate that the neuronal (Ag4,8), but not the non-neuronal (Ag0,0), isoform of agrin induces CREB phosphorylation in hippocampal neurons. The kinetics of agrin- and BDNF-induced CREB phosphorylation are similar: peak levels are reached in minutes and are strongly reduced 2 hr later. Neuronal responses to agrin require extracellular calcium, and, in contrast to tyrosine kinase inhibitors, the specific inhibition of protein kinase A (PKA) does not affect agrin-evoked CREB phosphorylation. Our results show that hippocampal neurons specifically respond to neuronal agrin in a Ca2+-dependent manner and via the activation of tyrosine kinases.

Kakapo, a novel cytoskeletal-associated protein is essential for the restricted localization of the neuregulin-like factor, vein, at the muscle-tendon junction site

In the Drosophila embryo, the correct association of muscles with their specific tendon cells is achieved through reciprocal interactions between these two distinct cell types. Tendon cell differentiation is initiated by activation of the EGF-receptor signaling pathway within these cells by Vein, a neuregulin-like factor secreted by the approaching myotube. Here, we describe the cloning and the molecular and genetic analyses of kakapo, a Drosophila gene, expressed in the tendons, that is essential for muscle-dependent tendon cell differentiation. Kakapo is a large intracellular protein and contains structural domains also found in cytoskeletal-related vertebrate proteins (including plakin, dystrophin, and Gas2 family members). kakapo mutant embryos exhibit abnormal muscle-dependent tendon cell differentiation. A major defect in the kakapo mutant tendon cells is the failure of Vein to be localized at the muscle-tendon junctional site; instead, Vein is dispersed and its levels are reduced. This may lead to aberrant differentiation of tendon cells and consequently to the kakapo mutant deranged somatic muscle phenotype.

A retinal axon fascicle uses spitz, an EGF receptor ligand, to construct a synaptic cartridge in the brain of Drosophila

Photoreceptor axons arriving in the Drosophila brain organize their postsynaptic target field into a precise array of five neuron "cartridge" ensembles. Here we show that Hedgehog, an initial inductive signal transported along retinal axons from the developing eye, induces postsynaptic precursor cells to express the Drosophila homolog of the epidermal growth factor receptor (EGFR). The EGFR ligand Spitz, a signal for ommatidial assembly in the compound eye, is transported to retinal axon termini in the brain where it acts as a local cue for the recruitment of five cells into a cartridge ensemble. Hedgehog and Spitz thus bring about the concerted assembly of ommatidial and synaptic cartridge units, imposing the "neurocrystalline" order of the compound eye on the postsynaptic target field.

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.

Transmembrane neuregulins interact with LIM kinase 1, a cytoplasmic protein kinase implicated in development of visuospatial cognition

The neuregulins are receptor tyrosine kinase ligands that play a critical role in the development of the heart, nervous system, and breast. Unlike many extracellular signaling molecules, such as the neurotrophins, most neuregulins are synthesized as transmembrane proteins. To determine the functions of the highly conserved neuregulin cytoplasmic tail, a yeast two-hybrid screen was performed to identify proteins that interact with the 157-amino acid sequence common to the cytoplasmic tails of all transmembrane neuregulin isoforms. This screen revealed that the neuregulin cytoplasmic tail interacts with the LIM domain region of the nonreceptor protein kinase LIM kinase 1 (LIMK1). Interaction between the neuregulin cytoplasmic tail and full-length LIMK1 was demonstrated by in vitro binding and co-immunoprecipitation assays. Transmembrane neuregulins with each of the three known neuregulin cytoplasmic tail isoforms interacted with LIMK1. In contrast, the cytoplasmic tail of TGF-alpha did not interact with LIMK1. In vivo, neuregulin and LIMK1 are co-localized at the neuromuscular synapse, suggesting that LIMK1, like neuregulin, may play a role in synapse formation and maintenance. To our knowledge, LIMK1 is the first identified protein shown to interact with the cytoplasmic tail of a receptor tyrosine kinase ligand.