A role of tyrosine phosphorylation in the formation of acetylcholine receptor clusters induced by electric fields in cultured Xenopus muscle cells

Protein Tyrosine Phosphatase Receptor Type R (PTPRR) Reduces AChR Clustering by Dephosphorylating MuSK

Neuromuscular junction (NMJ) formation and maintenance depend on the proper localization and concentration of various molecules at synaptic contact sites. Acetylcholine receptor (AChR) clustering on the postsynaptic membrane is a cardinal event in NMJ formation. Muscle-specific tyrosine kinase (MuSK), which functions depending on its phosphorylation, plays an essential role in AChR clustering. In the present study, we used plasmid-based biochemical screening and determined that protein tyrosine phosphatase receptor type R (PTPRR) is responsible for dephosphorylating MuSK on tyrosine residue 754. Furthermore, we showed that PTPRR significantly reduced MuSK-dependent AChR clustering in C2C12 myotubes. Collectively, these data illustrate a negative regulation function of PTPRR in AChR clustering.

Roles of tyrosine kinases and phosphatases in the formation and dispersal of acetylcholine receptor clusters

The formation of acetylcholine receptor (AChR) clusters at the postsynaptic muscle membrane in response to motor innervation is a key event in the development of the neuromuscular junction. The synaptic AChR clustering process is initiated by motor axon-released agrin, which activates a tyrosine kinase-based signaling pathway to cause AChR aggregation. In cultured muscle cells, AChR clustering is elicited by diverse nonneural signals, and this process is also mediated by tyrosine kinases. Conversely, the formation of new AChR clusters induced by innervation or nonneural stimuli is unfailingly associated with the dispersal of pre-existing AChR clusters, and this process is mediated by tyrosine phosphatases. In this review, we address how local kinase activation leads to global phosphatase action in muscle. More specifically, we discuss the roles of Src kinase and the SH2 domain-containing tyrosine phosphatase Shp-2 in establishing a regenerative mechanism to propagate the AChR cluster dispersing signal extrasynaptically and in defining the boundary of cluster formation subsynaptically.

Nerve, Muscle, and Synaptogenesis

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

Mechanism of acetylcholine receptor cluster formation induced by DC electric field

Background

The formation of acetylcholine receptor (AChR) cluster is a key event during the development of the neuromuscular junction. It is induced through the activation of muscle-specific kinase (MuSK) by the heparan-sulfate proteoglycan agrin released from the motor axon. On the other hand, DC electric field, a non-neuronal stimulus, is also highly effective in causing AChRs to cluster along the cathode-facing edge of muscle cells.

Methodology/Principal Findings

To understand its molecular mechanism, quantum dots (QDs) were used to follow the movement of AChRs as they became clustered under the influence of electric field. From analyses of trajectories of AChR movement in the membrane, it was concluded that diffuse receptors underwent Brownian motion until they were immobilized at sites of cluster formation. This supports the diffusion-mediated trapping model in explaining AChR clustering under the influence of this stimulus. Disrupting F-actin cytoskeleton assembly and interfering with rapsyn-AChR interaction suppressed this phenomenon, suggesting that these are integral components of the trapping mechanism induced by the electric field. Consistent with the idea that signaling pathways are activated by this stimulus, the localization of tyrosine-phosphorylated forms of AChR β-subunit and Src was observed at cathodal AChR clusters. Furthermore, disrupting MuSK activity through the expression of a kinase-dead form of this enzyme abolished electric field-induced AChR clustering.

Conclusions

These results suggest that DC electric field as a physical stimulus elicits molecular reactions in muscle cells in the form of cathodal MuSK activation in a ligand-free manner to trigger a signaling pathway that leads to cytoskeletal assembly and AChR clustering.

GSK-3β is essential for physiological electric field-directed Golgi polarization and optimal electrotaxis

Endogenous electrical fields (EFs) at corneal and skin wounds send a powerful signal that directs cell migration during wound healing. This signal therefore may serve as a fundamental regulator directing cell polarization and migration. Very little is known of the intracellular and molecular mechanisms that mediate EF-induced cell polarization and migration. Here, we report that Chinese hamster ovary (CHO) cells show robust directional polarization and migration in a physiological EF (0.3–1 V/cm) in both dissociated cell culture and monolayer culture. An EF of 0.6 V/cm completely abolished cell migration into wounds in monolayer culture. An EF of higher strength (≥1 V/cm) is an overriding guidance cue for cell migration. Application of EF induced quick phosphorylation of glycogen synthase kinase 3β (GSK-3β) which reached a peak as early as 3 min in an EF. Inhibition of protein kinase C (PKC) significantly reduced EF-induced directedness of cell migration initially (in 1–2 h). Inhibition of GSK-3β completely abolished EF-induced GA polarization and significantly inhibited the directional cell migration, but at a later time (2–3 h in an EF). Those results suggest that GSK-3β is essential for physiological EF-induced Golgi apparatus (GA) polarization and optimal electrotactic cell migration.

Integrins and ion channels in cell migration: implications for neuronal development, wound healing and metastatic spread

Cells migration is necessary for proper embryonic development and adult tissue remodeling. Its mechanisms determine the physiopathology of processes such as neuronal targeting, inflammation, wound healing and metastatic spread. Crawling of cells onto solid surfaces requires a controlled sequence of cell protrusions and retractions that mainly depends on sophisticated regulation of the actin cytoskeleton, although the contribution of microtubules should not be neglected. This process is triggered and modulated by a combination of diffusible and fixed environmental signals. External cues are sensed and integrated by membrane receptors, including integrins, which transduce these signals into cellular signaling pathways, often centered on the small GTPase proteins belonging to the Rho family. These pathways regulate the coordinated cytoskeletal rearrangements necessary for proper timing of adhesion, contraction and detachement at the front and rear side of cells finding their way through the extracellular spaces. The overall process involves continuous modulation of cell motility, shape and volume, in which ion channels play major roles. In particular, Ca2+ signals have both global and local regulatory effects on cell motility, because they target the contractile proteins as well as many regulatory proteins. After reviewing the fundamental mechanisms of eukaryotic cell migration onto solid substrates, we briefly describe how integrin receptors and ion channels are involved in cell movement. We next examine a few processes in which these mechanisms have been studied in depth. We thus illustrate how integrins and K+ channels control cell volume and migration, how intracellular Ca2+ homeostasis affects the motility of neuronal growth cones and what is known about the ion channel roles in epithelial cell migration. These mechanisms are implicated in a variety of pathological processes, such as the disruption of neural circuits and wound healing. Finally, we describe the interaction between neoplastic cells and their local environment and how derangement of adhesion can lead to metastatic spread. It is likely that the cellular mechanisms controlled by integrin receptors, ion channels or both participate in the entire metastatic process. Until now, however, evidence is limited to a few steps of the metastatic cascade, such as brain tumor invasiveness.

New insights into the regulation of ion channels by integrins

By controlling cell adhesion to the extracellular matrix, integrin receptors regulate processes as diverse as cell migration, proliferation, differentiation, apoptosis, and synaptic stability. Because the underlying mechanisms are generally accompanied by changes in transmembrane ion flow, a complex interplay occurs between integrins, ion channels, and other membrane transporters. This reciprocal interaction regulates bidirectional signal transduction across the cell surface and may take place at all levels of control, from transcription to direct conformational coupling. In particular, it is becoming increasingly clear that integrin receptors form macromolecular complexes with ion channels. Besides contributing to the membrane localization of the channel protein, the integrin/channel complex can regulate a variety of downstream signaling pathways, centered on regulatory proteins like tyrosine kinases and small GTPases. In turn, the channel protein usually controls integrin activation and expression. We review some recent advances in the field, with special emphasis on hematology and neuroscience. Some oncological implications are also discussed.

A biochemical approach to wound healing through the use of modalities

Wound healing is a complex pathway that is energy dependent. Nonhealing wounds frequently require the use of physical modalities to achieve healing. There is much debate over which treatment modality to use, with varying clinical results in the literature. This review paper describes a common biochemical pathway that helps the clinician understand, at a molecular level, how the transference of energy to a wound can result in positive clinical results. The mechanisms of action for ultraviolet light, electrical stimulation, and ultrasound are reviewed along with a proposed biochemical roadmap. An emphasis on protein biochemistry is supported with an extensive review of the literature.

Molecular regulation of postsynaptic differentiation at the neuromuscular junction

The neuromuscular junction (NMJ) is a synapse that develops between a motor neuron and a muscle fiber. A defining feature of NMJ development in vertebrates is the re-distribution of muscle acetylcholine (ACh) receptors (AChRs) following innervation, which generates high-density AChR clusters at the postsynaptic membrane and disperses aneural AChR clusters formed in muscle before innervation. This process in vivo requires MuSK, a muscle-specific receptor tyrosine kinase that triggers AChR re-distribution when activated; rapsyn, a muscle protein that binds and clusters AChRs; agrin, a nerve-secreted heparan-sulfate proteoglycan that activates MuSK; and ACh, a neurotransmitter that stimulates muscle and also disperses aneural AChR clusters. Moreover, in cultured muscle cells, several additional muscle- and nerve-derived molecules induce, mediate or participate in AChR clustering and dispersal. In this review we discuss how regulation of AChR re-distribution by multiple factors ensures aggregation of AChRs exclusively at NMJs.

The Ras/Raf-1/MEK1/ERK Signaling Pathway Coupled to Integrin Expression Mediates Cholinergic Regulation of Keratinocyte Directional Migration

The physiologic mechanisms that determine directionality of lateral migration are a subject of intense research. Galvanotropism in a direct current (DC) electric field represents a natural model of cell re-orientation toward the direction of future migration. Keratinocyte migration is regulated through both the nicotinic and muscarinic classes of acetylcholine (ACh) receptors. We sought to identify the signaling pathway mediating the cholinergic regulation of chemotaxis and galvanotropism. The pharmacologic and molecular modifiers of the Ras/Raf-1/MEK1/ERK signaling pathway altered both chemotaxis toward choline and galvanotropism toward the cathode in a similar way, indicating that the same signaling steps were involved. The galvanotropism was abrogated due to inhibition of ACh production by hemicholinium-3 and restored by exogenously added carbachol. The concentration gradients of ACh and choline toward the cathode in a DC field were established by high-performance liquid chromatographic measurements. This suggested that keratinocyte galvanotaxis is, in effect, chemotaxis toward the concentration gradient of ACh, which it creates in a DC field due to its highly positive charge. A time-course immunofluorescence study of the membrane redistribution of ACh receptors in keratinocytes exposed to a DC field revealed rapid relocation to and clustering at the leading edge of alpha7 nicotinic and M(1) muscarinic receptors. Their inactivation with selective antagonists or small interfering RNAs inhibited galvanotropism, which could be prevented by transfecting the cells with constitutively active MEK1. The end-point effect of the cooperative signaling downstream from alpha7 and M(1) through the MEK1/ERK was an up-regulated expression of alpha(2) and alpha(3) integrins, as judged from the results of real-time PCR and quantitative immunoblotting. Thus, alpha7 works together with M(1) to orient a keratinocyte toward direction of its future migration. Both alpha7 and M(1) apparently engage the Ras/Raf/MEK/ERK pathway to up-regulate expression of the "sedentary" integrins required for stabilization of the lamellipodium at the keratinocyte leading edge.

Electrical stimulation for wound healing: a review of evidence from in vitro studies, animal experiments, and clinical trials

This article reviews theories linked to endogenous bioelectric currents and the role they may play in wound repair with further appraisal of in vitro and in vivo research related to the effects of clinically applicable electrical currents on protein synthesis, cell migration, and antibacterial outcomes. In addition, studies on the effects of electrical stimulation (ES) on skin grafts, donor sites, and musculocutaneous flaps in animals are evaluated, as well as assessments of numerous clinical reports that examined the effects of ES on angiogenesis, perfusion, PtcO2, and epithelialization. Finally, a plethora of clinical trials related to the responses of chronic lower extremity wounds to ES therapy are reviewed, with emphasis on wounds caused by venous insufficiency, diabetic neuropathy, and ischemia in patients with and without diabetes mellitus. A glossary that addresses ES terminology is also included.

Differential regulation of keratinocyte chemokinesis and chemotaxis through distinct nicotinic receptor subtypes

Nicotinergic agents can act as both chemokines and chemoattractants for cell migration. Epidermal keratinocytes both synthesize acetylcholine and use it as a paracrine and autocrine regulator of cell motility. To gain a mechanistic insight into nicotinergic control of keratinocyte motility, we determined types of nicotinic acetylcholine receptors and signaling pathways regulating keratinocyte chemokinesis and chemotaxis, using respective modifications of the agarose gel keratinocyte outgrowth assay. Random migration of keratinocytes was significantly (P<0.05) inhibited by hemicholinum-3, a metabolic inhibitor of acetylcholine synthesis, as well as by the alpha-conotoxins MII and AuIB, preferentially blocking alpha3-containing nicotinic acetylcholine receptors. The use of antisense oligonucleotides specific for nicotinic-acetylcholine-receptor subunits and knockout mice demonstrated pivotal role for the alpha3beta2 channel in mediating acetylcholine-dependent chemokinesis. Signaling pathways downstream of alpha3beta2 included activation of the protein-kinase-C isoform delta and RhoA-dependent events. The nicotinergic chemotaxis of keratinocytes was most pronounced towards the concentration gradient of choline, a potent agonist of alpha7 nicotinic acetylcholine receptor. The alpha7-preferring antagonist alpha-bungarotoxin significantly (P<0.05) diminished keratinocyte chemotaxis, further suggesting a central role for the alpha7 nicotinic acetylcholine receptor. This hypothesis was confirmed in experiments with anti-alpha7 antisense oligonucleotides and alpha7-knockout mice. The signaling pathway mediating alpha7-dependent keratinocyte chemotaxis included intracellular calcium, activation of calcium/calmodulin-dependent protein-kinase II, conventional isoforms of protein-kinase C, phosphatidylinositol-3-kinase and engagement of Rac/Cdc42. Redistribution of alpha7 immunoreactivity to the leading edge of keratinocytes upon exposure to a chemoattractant preceded crescent shape formation and directional migration. Application of high-resolution deconvolution microscopy demonstrated that, on the cell membrane of keratinocytes, the nicotinic acetylcholine receptor subunits localize with the integrin beta1. The obtained results demonstrate for the first time that alpha3 and alpha7 nicotinic acetylcholine receptors regulate keratinocyte chemokinesis and chemotaxis, respectively, and identify signaling pathways mediating these functions, which has clinical implications for wound healing and control of cancer metastases.

Confocal microscopic study of GABA(A) receptors in Xenopus oocytes after rat brain mRNA injection: modulation by tyrosine kinase activity

The expression of GABA(A) receptors in Xenopus oocytes injected with rat brain mRNA was studied by immunocytochemistry and evaluation of the distribution of fluorescent probes at the confocal microscope. The beta(2/3) subunit distributed exclusively on the membrane at the animal pole of the oocytes. Treatment of oocytes for 20 min with the protein tyrosine kinase inhibitor genistein, 200 microM, resulted in a lower presence of GABA(A) receptors on the membrane. The inactive genistein analogue daidzein, 200 microM, had no effect even with a 30 min treatment. Alkaline phosphatase but not a protein tyrosine phosphatase, when injected into oocytes, reduced GABA(A) receptor membrane expression. The data indicate that protein tyrosine phosphorylation modulates the expression on the plasma membrane of presynthesized GABA(A) receptors.

Epidermal growth factor receptor relocalization and kinase activity are necessary for directional migration of keratinocytes in DC electric fields

Human keratinocytes migrate towards the negative pole in DC electric fields of physiological strength. This directional migration is promoted by epidermal growth factor (EGF). To investigate how EGF and its receptor (EGFR) regulate this directionality, we first examined the effect of protein tyrosine kinase inhibitors, including PD158780, a specific inhibitor for EGFR, on this response. At low concentrations, PD158780 inhibited keratinocyte migration directionality, but not the rate of migration; at higher concentrations, it reduced the migration rate as well. The less specific inhibitors, genistein, lavendustin A and tyrphostin B46, reduced the migration rate, but did not affect migration directionality. These data suggest that inhibition of EGFR kinase activity alone reduces directed motility, and inhibition of multiple tyrosine kinases, including EGFR, reduces the cell migration rate. EGFR redistribution also correlates with directional migration. EGFR concentrated on the cathodal face of the cell as early as 5 minutes after exposure to electric fields. PD158780 abolished EGFR localization to the cathodal face. These data suggest that EGFR kinase activity and redistribution in the plasma membrane are required for the directional migration of keratinocytes in DC electric fields. This study provides the first insights into the mechanisms of directed cell migration in electric fields.

Regulation of ligand-gated ion channels by protein phosphorylation

The studies discussed in this review demonstrate that phosphorylation is an important mechanism for the regulation of ligand-gated ion channels. Structurally, ligand-gated ion channels are heteromeric proteins comprised of homologous subunits. For both the AChR and the GABA(A) receptor, each subunit has a large extracellular N-terminal domain, four transmembrane domains, a large intracellular loop between transmembrane domains M3 and M4, and an extracellular C-terminal domain (Fig. 1B). All the phosphorylation sites on these receptors have been mapped to the major intracellular loop between M3 and M4 (Table 1). In contrast, glutamate receptors appear to have a very large extracellular N-terminal domain, one membrane hairpin loop, three transmembrane domains, a large extracellular loop between transmembrane domains M3 and M4, and an intracellular C-terminal domain (Fig. 1C). Most phosphorylation sites on glutamate receptors have been shown to be on the intracellular C-terminal domain, although some have been suggested to be on the putative extracellular loop between M3 and M4 (Table 1). A variety of extracellular factors and intracellular signal transduction cascades are involved in regulating phosphorylation of these ligand-gated ion channels (Fig. 2). Once again, the AChR at the neuromuscular junction is the most fully understood system. Phosphorylation of the AChR by PKA is stimulated synaptically by the neuropeptide CGRP and in an autocrine fashion by adenosine released from the muscle in response to acetylcholine. In addition, acetylcholine, via calcium influx through the AChR, appears to activate calcium-dependent kinases including PKC to stimulate serine phosphorylation of the receptor. Presently, agrin is the only extracellular factor known to stimulate phosphorylation of the AChR on tyrosine residues. For glutamate receptors, non-NMDA receptor phosphorylation by PKA is stimulated by dopamine, while NMDA receptor phosphorylation by PKA and PKC can be induced via the activation of beta-adrenergic receptors, and metabotropic glutamate or opioid receptors, respectively. In addition, Ca2+ influx through the NMDA receptor has been shown to activate PKC. CaMKII, and calcineurin, resulting in phosphorylation of AMPA receptors (by CaMKII) and inactivation of NMDA receptors (at least in part through calcineurin). In contrast to the AChR and glutamate receptors, no information is presently available regarding the identities of the extracellular factors and intracellular signal transduction cascades that regulate phosphorylation of the GABA(A) receptor. Surely, future studies will be aimed at further clarifying the molecular mechanisms by which the central receptors are regulated. The presently understood functional effects of ligand-gated ion channel phosphorylation are diverse. At the neuromuscular junction, a regulation of the AChR desensitization rate by both serine and tyrosine phosphorylation has been demonstrated. In addition, tyrosine phosphorylation of the AChR or other synaptic components appears to play a role in AChR clustering during synaptogenesis. For the GABA(A) receptor, the data are complex. Both activation and inhibition of GABA(A) receptor currents as a result of PKA and PKC phosphorylation have been reported, while phosphorylation by PTK enhances function. The predominant effect of glutamate receptor phosphorylation by a variety of kinases is a potentiation of the peak current response. However, PKC also modulates clustering of NMDA receptors. This complexity in the regulation of ligand-gated ion channels by phosphorylation provides diverse mechanisms for mediating synaptic plasticity. In fact, accumulating evidence supports the involvement of protein phosphorylation and dephosphorylation of AMPA receptors in LTP and LTD respectively. There has been a dramatic increase in our understanding of the nature by which phosphorylation regulates ligand-gated ion channels. However, many questions remain unanswered. (AB

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.

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.

The dystrophinopathies: an alternative to the structural hypothesis

Abnormal expression of the cytoskeletal protein dystrophin has deleterious consequences for skeletal muscle, cardiac muscle, and the central nervous system. A complete failure to express the protein produces Duchenne muscular dystrophy (DMD), in which there is extensive and progressive skeletal muscle necrosis, the development of a life-threatening dilated cardiomyopathy, and mild mental retardation. Dystrophin binds the F-actin cytoskeleton and is normally expressed in a complex of transmembrane proteins (the "dystrophin protein complex") that interact with external components of the basal lamina. One pathogenic model for DMD (the "structural hypothesis") suggests that this complex forms a structural bridge between the external basal lamina and the internal cytoskeleton and that the absence of dystrophin produces a defect in membrane structural support that renders skeletal muscle susceptible to plasmalemmal ruptures (or "tears") during the course of contractile activity. This review attempts to critically evaluate the structural hypothesis for DMD and presents an opposing model (the "channel aggregation model") that highlights the role of dystrophin in organizing the membrane cytoskeleton and the role of the cytoskeleton in aggregating ion channels and neurotransmitter receptors. Since ion channel aggregation is a process that is common across organ systems, the idea that channel function can be altered when aggregated ion channels interact with a dystrophic cytoskeleton has immediate implications for the expression of the dystrophinopathies in skeletal muscle, cardiac muscle, and the central nervous system.

A role of tyrosine phosphatase in acetylcholine receptor cluster dispersal and formation

Innervation of the skeletal muscle involves local signaling, leading to acetylcholine receptor (AChR) clustering, and global signaling, manifested by the dispersal of preexisting AChR clusters (hot spots). Receptor tyrosine kinase (RTK) activation has been shown to mediate AChR clustering. In this study, the role of tyrosine phosphatase (PTPase) in the dispersal of hot spots was examined. Hot spot dispersal in cultured Xenopus muscle cells was initiated immediately upon the presentation of growth factor-coated beads that induce both AChR cluster formation and dispersal. Whereas the density of AChRs decreased with time, the fine structure of the hot spot remained relatively constant. Although AChR, rapsyn, and phosphotyrosine disappeared, a large part of the original hot spot-associated cytoskeleton remained. This suggests that the dispersal involves the removal of a key linkage between the receptor and its cytoskeletal infrastructure. The rate of hot spot dispersal is inversely related to its distance from the site of synaptic stimulation, implicating the diffusible nature of the signal. PTPase inhibitors, such as pervanadate or phenylarsine oxide, inhibited hot spot dispersal. In addition, they also affected the formation of new clusters in such a way that AChR microclusters extended beyond the boundary set by the clustering stimuli. Furthermore, by introducing a constitutively active PTPase into cultured muscle cells, hot spots were dispersed in a stimulus- independent fashion. This effect of exogenous PTPase was also blocked by pervanadate. These results implicate a role of PTPase in AChR cluster dispersal and formation. In addition to RTK activation, synaptic stimulation may also activate PTPase which acts globally to destabilize preexisting AChR hot spots and locally to facilitate AChR clustering in a spatially discrete manner by countering the action of RTKs.

Protein tyrosine phosphorylation: implications for synaptic function

The phosphorylation of proteins on tyrosine residues, initially believed to be primarily involved in cell growth and differentiation, is now recognized as having a critical role in regulating the function of mature cells. The brain exhibits one of the highest levels of tyrosine kinase activity in the adult animal and the synaptic region is particularly rich in tyrosine kinases and tyrosine phosphorylated proteins. Recent studies have described the effects of tyrosine phosphorylation on the activities of a number of proteins which are potentially involved in the regulation of synaptic function. Furthermore, it is becoming apparent that tyrosine phosphorylation is involved in the modification of synaptic activity, such as occurs during depolarization, the induction of long-term potentiation or long-term depression, and ischemia. Changes in the activities of tyrosine kinases and/or protein tyrosine phosphatases which are associated with synaptic structures may result in altered tyrosine phosphorylation of proteins located at the synapse leading to both short-term and long-lasting changes in synaptic and neuronal function.

Human corneal epithelial cells reorient and migrate cathodally in a small applied electric field

PURPOSE

To test whether human corneal epithelial cells (HCECs) respond to small applied electric fields (EFs) in a similar manner to bovine corneal epithelial cells (BCECs), the orientation and directed migration in small EFs of both primary cultures and of a human corneal epithelial cell line were quantified.

METHODS

Primary cultures of human corneal epithelial cells (PHCECs) and transformed human corneal epithelial cells (THCECs) were exposed to EFs (100 mV/mm-250 mV/mm) in different media. Cell migration was traced using an image analyser.

RESULTS

PHCECs and THCECs reoriented and migrated towards the cathode (negative pole) when cultured in small direct current (dc) EFs. Both the reorientation and directional migration were voltage- and serum-dependent, as shown previously for bovine cells. PHCECs and THCECs showed significant perpendicular orientation in EFs at 150 mV/mm in medium with serum, while at the same voltage, no significant orientation was found in serum free medium. PHCECs started to show perpendicular reorientation around 30 min after onset of EF at 150 mV/mm. They showed significant directional migration at 150 mV/mm, with directedness of 0.35 +/- 0.07 and a migration rate of 9.1 +/- 0.7 microns/h (n = 90), both significantly higher than that of cells in serum free medium. Addition of EGF-induced significant reorientation and directional migration of THCECs at 100 mV/mm. Additionally, as for BCECs, which remained viable and responsive to electric fields for at least 75 h at 150 mV/mm, THCECs also remained viable and showed responsiveness during long periods of exposure to EFs (at least 20 h).

CONCLUSIONS

Cultured human primary CECs and a human corneal epithelial cell line both responded to small EFs with perpendicular reorientation and cathodally-directed migration. Cell responses were qualitatively similar to those reported previously for bovine CECs. The endogenous EFs generated by wounded cornea may play an important role in promoting cell shape changes and directed migration of CECs during the healing process.

Intercellular communication that mediates formation of the neuromuscular junction

Reciprocal signals between the motor axon and myofiber induce structural and functional differentiation in the developing neuromuscular junction (NMJ). Elevation of presynaptic acetylcholine (ACh) release on nerve-muscle contact and the correlated increase in axonal-free calcium are triggered by unidentified membrane molecules. Restriction of axon growth to the developing NMJ and formation of active zones for ACh release in the presynaptic terminal may be induced by molecules in the synaptic basal lamina, such as S-laminin, heparin binding growth factors, and agrin. Acetylcholine receptor (AChR) synthesis by muscle cells may be increased by calcitonin gene-related peptide (CGRP), ascorbic acid, and AChR-inducing activity (ARIA)/heregulin, which is the best-established regulator. Heparin binding growth factors, proteases, adhesion molecules, and agrin all may be involved in the induction of AChR redistribution to form postsynaptic-like aggregates. However, the strongest case has been made for agrin's involvement. "Knockout" experiments have implicated agrin as a primary anterograde signal for postsynaptic differentiation and muscle-specific kinase (MuSK), as a putative agrin receptor. It is likely that both presynaptic and postsynaptic differentiation are induced by multiple molecular signals. Future research should reveal the physiological roles of different molecules, their interactions, and the identity of other molecular participants.

Common molecular mechanisms in field- and agrin-induced acetylcholine receptor clustering

1. The aggregation of acetylcholine receptors at the developing neuromuscular junction is critical to the development and function of this synapse. In vitro studies have shown that receptor aggregation can be induced by the finding of agrin to the muscle cell surface and by the electric field-induced concentration of a (nonreceptor) molecule at the cathodal cell pole. 2. We report here on the interaction between agrin binding and electric fields with respect to the distribution of receptors and agrin binding sites. 3. (a) Pretreatment of cells with agrin completely blocks the development of field-induced receptor clusters. (b) Field-induced aggregation of receptors precedes the field-induced aggregation of agrin binding sites by approximately 30 min. (c) Electric fields prevent agrin-induced receptor clustering despite the presence of agrin binding sites and freely diffusing receptors. 4. These results indicate that another membrane component-but not the agrin binding site and not the receptor-is required for agrin-induced receptor clustering. They also suggest that electric fields and agrin cause receptor clustering via common molecular mechanisms.

Functional interaction of Src family kinases with the acetylcholine receptor in C2 myotubes

Tyrosine phosphorylation of the beta subunit of the acetylcholine receptor (AChR) has been postulated to play a role in AChR clustering during development of the neuromuscular junction. We have investigated the mechanism of this phosphorylation in mammalian C2 myotubes and report that the tyrosine kinase Src binds and phosphorylates glutathione S-transferase fusion proteins containing the N-terminal half of the cytoplasmic loop of the beta subunit. No binding occurs to the related kinases Fyn or Yes or to the corresponding regions from the gamma and delta subunits. Furthermore, AChRs affinity-isolated from C2 myotubes using alpha-bungarotoxin-Sepharose were specifically associated with Src and Fyn and had tyrosine-phosphorylated beta subunits. We suggest that AChRs are initially phosphorylated by Src and subsequently bind Fyn in a phosphotyrosine-dependent manner. These interactions are likely to play an important role in construction of the specialized postsynaptic membrane during synaptogenesis.

Modulation and selection of neurotransmitter responses during synapse formation between identified leech neurons

1. Serotonin (5-HT) modulates two different responses in the pressure sensitive neurons (P) of the leech: an inhibitory, Cl- dependent synaptic response and a depolarizing extrasynaptic response. 2. Serotonergic Retzius cells (R) in vivo and in culture elicit inhibitory Cl- dependent responses in P neurons. Moreover, at discrete sites of contact between R and P cells, the excitatory response to 5-HT is gradually lost prior to synapse formation. This phenomenon is specifically mediated by R cells. 3. The extrasynaptic response is mediated by cation channels sensitive to protein kinase C (PKC). Cation channels are present at the sites of contact but they become insensitive to PKC. Moreover, cation channels from single P cells are no longer modulated by PKC if they are inserted (by cramming the patch pipette) into the cytoplasm of a P cell in contact with an R cell. 4. Blockers of tyrosine kinases prevent the uncoupling of cation channel modulation and inhibit synapse formation between the R and the P neurons. 5. We suggest that cell contact induces an intracellular, tyrosine kinase-dependent signal as part of the mechanism of neuronal recognition leading to synapse formation.

Signaling from Neural Impulses to Genes

Nerve impulses regulate expression of genes that control receptors, channels, enzymes, and structural proteins. This activity-dependent feedback allows adaptation to changing requirements and environmental conditions. The signal transduction mechanisms carrying information from the cell membrane to the nucleus are becoming well characterized, but a more dynamic view of intracellular signaling is emerging to explain cellular responses to specific patterns of neural impulses. This review analyzes this interface between electrophysiology and molecular cell biology to examine the signals, substrates, and processes that enable the nervous system to regulate its structure and function as a consequence of its own operation.

Signaling from Neural Impulses to Genes

Nerve impulses regulate expression of genes that control receptors, channels, enzymes, and structural proteins. This activity-dependent feedback allows adaptation to changing requirements and environmental conditions. The signal transduction mechanisms carrying information from the cell membrane to the nucleus are becoming well characterized, but a more dynamic view of intracellular signaling is emerging to explain cellular responses to specific patterns of neural impulses. This review analyzes this interface between electrophysiology and molecular cell biology to examine the signals, substrates, and processes that enable the nervous system to regulate its structure and function as a consequence of its own operation.

Single channel evidence for a cytoskeletal defect involving acetylcholine receptors and calcium influx in cultured dystrophic (mdx) myotubes

Single channel events that exhibited the conductance, event duration, and ion selectivity characteristics of calcium leakage activity (CLA) were recorded in association with acetylcholine receptor (AChR) activity in cultured nondystrophic myotubes. The CLA was observed in the presence or absence of acetylcholine (ACh), and at normal or elevated concentrations of calcium. In contrast to results from nondystrophic myotubes, cell-attached patches from several cultured dystrophic (mdx) myotubes exhibited 100% CLA with no AChR activity, even though ACh was present in the pipette solution. Acquisition of an inside-out patch from these membrane areas produced a profound decrease in CLA and the appearance of AChR events exhibiting typical conductance and event duration characteristics. These results suggest that CLA in dystrophic muscle is produced, in part, by unusual physical interactions between AChRs and the dystrophic cytoskeleton that are mediated by the action of intracellular modulators responsible for aggregating and stabilizing AChRs.

Orientation and directed migration of cultured corneal epithelial cells in small electric fields are serum dependent

Reorientation and migration of cultured bovine corneal epithelial cells (CECs) in an electric field were studied. Electric field application was designed to model the laterally directed, steady direct current electric fields which arise in an injured corneal epithelium. Single cells cultured in media containing 10% foetal bovine serum showed significant galvanotropism, reorienting to lie perpendicular to electric field vector with a threshold field strength of less than 100 mV/mm. Cells cultured in serum-free medium showed no reorientation until 250 mV/mm. Addition of EGF, bFGF or TGF-beta 1 singly or in combination to serum free medium significantly restored the reorientation response at low field strengths. Both the mean translocation rate and directedness of cell migration were serum dependent. Cultured in medium with serum or serum plus added EGF, single cells showed obvious cathodal migration at 100 mV/mm. Increasing electric field strength enhanced the cathodal directedness of single cell migration. Supplementing serum free medium with growth factors restored the cathodal directed migration of single cells and highest directedness was found for the combination of EGF and TGF-beta 1. Corneal epithelial sheets also migrated towards the cathode in electric fields. Serum or individual growth factors stimulated CEC motility (randomly directed). Applied fields did not further augment migration rates but added a vector to stimulated migration. Electric fields which are present in wounded cornea interact with other environmental factors and may impinge on CECs migration during wound healing. Therapies which combine the application of growth factors and electric fields may be useful clinically.

Density and diffusion limited aggregation in membranes

Aggregation of membrane molecules is a crucial phenomenon in developing organisms, a classic example being the aggregation of post-synaptic receptors during synaptogenesis. Our understanding of the molecular events involved is improving, but most models of the aggregation or concentration process do not address binding events on the molecular level. An exception is the study of diffusion limited aggregation, in which the aggregation process is simulated on a molecular level. In this analysis, however, important physical parameters such as molecular size, diffusion constant and initial density are not addressed. Thus no predictions about the rate at which such aggregates will form is possible. In the present work the model of diffusion limited aggregation is extended to incorporate these parameters and make the corresponding predictions.

Tyrosine phosphorylation and synapse formation at the neuromuscular junction

Protein tyrosine phosphorylation is prevalent throughout the nervous system. It has been implicated to play an important role in the development and maintenance of neuronal functions. In the past few years significant advances have been made in our understanding of the molecular mechanisms of synapse formation and synaptic plasticity. Protein tyrosine phosphorylation appears to be important in the neuron-induced synthesis of the nicotinic acetylcholine receptor and aggregation of synaptic proteins at the neuromuscular junction during development. In addition, protein tyrosine phosphorylation may regulate the ion channel activity of the nicotinic acetylcholine receptor.

Tyrosine phosphorylation during synapse formation between identified leech neurons

1. We have examined whether tyrosine phosphorylation is required for synapse formation between identified neurons from the central nervous system of the leech in culture. 2. Within a few hours of contact with the cell body of the serotonergic Retzius neuron (R cell), the soma of the postsynaptic pressure-sensitive neuron (P cell), but not the R cell, could be labelled intracellularly with an antibody against phosphotyrosine residues. The labelling seemed specific for P cells contacted by R cells, as it was greatly reduced in pairs of either R or P cells and in single cells. Genistein (20 microM) and lavendustin A (10 microM), selective inhibitors of tyrosine kinases, blocked the labelling of contacted P cells, whereas their ineffective analogues (genistein and lavendustin B) had no effect on labelling. 3. R cell contact also induced the loss of an extrasynaptic, depolarizing response (due to modulation of cation channels) to serotonin (5-HT) in the P cell within a few days of juxtaposing cell bodies and within an hour of contact with growth cones. Treatment of the neurons with the tyrosine kinase inhibitors (but not the ineffective analogues) prevented the loss of the depolarizing response and of single cation channel modulation by 5-HT. 4. R cells formed inhibitory, Cl(-)-dependent synapses with P cells. Synapse formation was prevented by the tyrosine kinase inhibitors but not by their ineffective analogues. These compounds had no obvious effect on neurite outgrowth or cell adhesion. We conclude that tyrosine phosphorylation is a signal during the formation of this synapse.

Phosphorylation of the nicotinic acetylcholine receptor by protein tyrosine kinases

Most neurotransmitter receptors examined to date are either regulated by phosphorylation or contain consensus sequences for phosphorylation by protein kinases. The nicotinic acetylcholine receptor (AChR), which mediates depolarization at the neuromuscular junction, has served as a model for the study of the structure, function, and regulation of ligand-gated ion channels. The AChR is phosphorylated by protein kinase A, protein kinase C, and an unidentified protein tyrosine kinase. Tyrosine phosphorylation of the AChR is correlated with a modulation of the rate of receptor desensitization and is associated with AChR clustering. We showed that agrin, a neuronally derived extracellular matrix protein, induces AChR clustering and tyrosine phosphorylation. In addition, we identified two protein tyrosine kinases, Fyn and Fyk, that appear to be involved in the regulation of synaptic transmission at the neuromuscular junction by phosphorylating the AChR. The two kinases are highly expressed in Torpedo electric organ, a tissue enriched in synaptic components including the AChR. As demonstrated by coimmunoprecipitation, Fyn and Fyk associate with the AChR. Furthermore, the AChR is phosphorylated in Fyn and Fyk immunoprecipitates. We investigated the molecular basis for the association of the AChR with Fyn and Fyk using fusion proteins derived from the kinases. The AChR bound specifically to the SH2 domain fusion proteins of Fyn and Fyk. The association of the AChR with the SH2 domains is dependent on the state of AChR tyrosine phosphorylation and is mediated by the delta subunit of the receptor. These data provide evidence that the protein tyrosine kinases Fyn and Fyk may act to phosphorylate the AChR in vivo.

Distribution of alpha-dystroglycan during embryonic nerve-muscle synaptogenesis

The distribution of alpha-dystroglycan (alpha DG) relative to acetylcholine receptors (AChRs) and neural agrin was examined by immunofluorescent staining with mAb IIH6 in cultures of nerve and muscle cells derived from Xenopus embryos. In Western blots probed with mAb IIH6, alpha DG was evident in membrane extracts of Xenopus muscle but not brain. alpha DG immunofluorescence was present at virtually all synaptic clusters of AChRs and neural agrin. Even microclusters of AChRs and agrin at synapses no older than 1-2 h (the earliest examined) had alpha DG associated with them. alpha DG was also colocalized at the submicrometer level with AChRs at nonsynaptic clusters that have little or no agrin. The number of large (> 4 microns) nonsynaptic clusters of alpha DG, like the number of large nonsynaptic clusters of AChRs, was much lower on innervated than on noninnervated cells. When mAb IIH6 was included in the culture medium, the large nonsynaptic clusters appeared fragmented and less compact, but the accumulation of agrin and AChRs along nerve-muscle contacts was not prevented. It is concluded that during nerve-muscle synaptogenesis, alpha DG undergoes the same nerve-induced changes in distribution as AChRs. We propose a diffusion trap model in which the alpha DG-transmembrane complex participates in the anchoring and recruitment of AChRs and alpha DG during the formation of synaptic as well as nonsynaptic AChR clusters.

Tyrosine kinase inhibition: an approach to drug development

Protein tyrosine kinases (PTKs) regulate cell proliferation, cell differentiation, and signaling processes in the cells of the immune system. Uncontrolled signaling from receptor tyrosine kinases and intracellular tyrosine kinases can lead to inflammatory responses and to diseases such as cancer, atherosclerosis, and psoriasis. Thus, inhibitors that block the activity of tyrosine kinases and the signaling pathways they activate may provide a useful basis for drug development. This article summarizes recent progress in the development of PTK inhibitors and demonstrates their potential use in the treatment of disease.

Signalling synapse formation between identified neurons

We have investigated the signals between identified leech neurons during the formation of specific synapses in culture. At an inhibitory serotonergic synapse between two well-studied neurons, the postsynaptic cell has an additional (extrasynaptic) excitatory response to 5-HT which may underly a form of activity-dependent modulation. Thus, the presynaptic neuron must select which 5-HT response will be activated and which will be excluded at its synapses. The selection of these responses preceded synapse formation and was specifically induced at sites of contact with the presynaptic neuron, this not being observed for other cell pairings. Aldehyde-fixed presynaptic cells were equally effective, unless pre-treated with trypsin or wheat germ agglutinin, suggesting that contact with a specific cell-surface glycoprotein induced this physiological change in 5-HT sensitivity. The mechanism underlying the selective loss of the extrasynaptic response has been examined by single channel recording. Cation channels in the postsynaptic neuron were modulated by protein kinase C (PKC) upon binding of 5-HT to a 5-HT2 receptor. However, at sites of contact with the presynaptic neuron, the channels were no longer sensitive to PKC. Furthermore, when cation channels from uncontacted neurons were inserted or 'crammed' into contacted neurons, they were rapidly rendered insensitive to PKC, demonstrating a cytoplasmic signal for the uncoupling of channel modulation. Interestingly, the cytoplasm of contacted postsynaptic neurons showed immunoreactivity for tyrosine phosphorylation: exposure of the neurons to specific inhibitors of tyrosine kinases prevented tyrosine phosphorylation, the loss of cation channel modulation and synapse formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentration of pp125 focal adhesion kinase (FAK) at the myotendinous junction

Focal adhesion kinase is a recently characterized tyrosine kinase that is concentrated at focal contacts in cultured cells. It is thought to play an important role in the regulation of the integrin-based signal transduction mechanism involved in the assembly of this membrane specialization. In this study, we examined the immunocytochemical distribution of focal adhesion kinase in Xenopus skeletal muscle and its role in the formation of two sarcolemmal specializations, the myotendinous junction and the neuromuscular junction, using a monoclonal antibody (2A7) against this protein. Immunoprecipitation of Xenopus embryonic tissues with this antibody demonstrated a single band at a relative molecular mass of 116 kDa. A distinct concentration of immunolabeling for focal adhesion kinase was observed at the myotendinous junction of muscle fibers in vivo. At this site, the labeling for this protein is correlated with an accumulation of phosphotyrosine immunolabeling. Focal adhesion kinase was not concentrated at the neuromuscular junction in muscle cells either in vivo or in vitro. However, it was localized at spontaneously formed acetylcholine receptor clusters in cultured Xenopus myotomal muscle cells, although its distribution was not exactly congruent with that of the receptors. In these cells, the accumulation focal adhesion kinase was induced by polystyrene microbeads. In addition, beads also induce the formation of acetylcholine receptor clusters and myotendinous junction-like specializations. By following the appearance of the focal adhesion kinase relative to the formation of these sarcolemmal specializations at bead-muscle contacts in cultured muscle cells, we conclude that the accumulation of this protein was in pace with the development of the myotendinous junction, but occurred well after the clustering of acetylcholine receptors. These results suggest that focal adhesion kinase may be involved in the development and/or maintenance of the myotendinous junction through an integrin-based signaling system. Although it can accumulate at acetylcholine receptor clusters formed in culture, it does not appear to be involved in the development of the neuromuscular junction.

Tyrosine and serine phosphorylation of dystrophin and the 58-kDa protein in the postsynaptic membrane of Torpedo electric organ

Dystrophin associates with a 58-kDa and an 87-kDa protein in the postsynaptic membrane of the Torpedo electric organ. We have previously shown that the 87-kDa protein is a major phosphotyrosine-containing protein in these membranes. Immunoprecipitation of the 87-kDa protein from phosphorylated postsynaptic membranes results in coimmunoprecipitation of additional phosphorproteins. These phosphorproteins are identified as dystrophin and the 58-kDa protein. Monoclonal antibodies to dystrophin and the 58-kDa protein immunoprecipitate phosphorylated forms of these proteins from postsynaptic membranes phosphorylated in vitro. Phosphoamino acid analysis reveals that dystrophin and the 58-kDa protein are phosphorylated on serine and tyrosine residues. In addition, both dystrophin and the 58-kDa protein are shown to be phosphorylated on tyrosine residues in vivo. These results suggest that the synaptic function of dystrophin and its associated proteins, the 58-kDa and 87-kDa proteins, may be modulated by tyrosine and serine protein phosphorylation.

Staurosporine inhibits agrin-induced acetylcholine receptor phosphorylation and aggregation

Agrin, a protein that mediates nerve-induced acetylcholine receptor (AChR) aggregation at developing neuromuscular junctions, has been shown to cause an increase in phosphorylation of the beta, gamma, and delta subunits of AChRs in cultured myotubes. As a step toward understanding the mechanism of agrin-induced AChR aggregation, we examined the effects of inhibitors of protein kinases on AChR aggregation and phosphorylation in chick myotubes in culture. Staurosporine, an antagonist of both protein serine and tyrosine kinases, blocked agrin-induced AChR aggregation in a dose-dependent manner; 50% inhibition occurred at approximately 2 nM. The extent of inhibition was independent of agrin concentration, suggesting an effect downstream of the interaction of agrin with its receptor. Staurosporine blocked agrin-induced phosphorylation of the AChR beta subunit, which occurs at least in part on tyrosine residues, but did not reduce phosphorylation of the gamma and delta subunits, which occurs on serine/threonine residues. Staurosporine also prevented the agrin-induced decrease in the rate at which AChRs are extracted from intact myotubes by mild detergents. H-7, an antagonist of protein serine kinases, inhibited agrin-induced phosphorylation of the gamma and delta subunits but did not block agrin-induced phosphorylation of the AChR beta subunit, AChR aggregation, or the decrease in AChR extractability. The results provide support for the hypothesis that tyrosine phosphorylation of the beta subunit plays a role in agrin-induced AChR aggregation.

Tyrosine phosphorylation and acetylcholine receptor cluster formation in cultured Xenopus muscle cells

Aggregation of the nicotinic acetylcholine receptor (AChR) at sites of nerve-muscle contact is one of the earliest events to occur during the development of the neuromuscular junction. The stimulus presented to the muscle by nerve and the mechanisms underlying postsynaptic differentiation are not known. The purpose of this study was to examine the distribution of phosphotyrosine (PY)-containing proteins in cultured Xenopus muscle cells in response to AChR clustering stimuli. Results demonstrated a distinct accumulation of PY at AChR clusters induced by several stimuli, including nerve, the culture substratum, and polystyrene microbeads. AChR microclusters formed by external cross-linking did not show PY colocalization, implying that the accumulation of PY in response to clustering stimuli was not due to the aggregation of basally phosphorylated AChRs. A semi-quantitative determination of the time course for development of PY labeling at bead contacts revealed early PY accumulation within 15 min of contact before significant AChR aggregation. At later stages (within 15 h), the AChR signal came to approximate the PY signal. We have reported the inhibition of bead-induced AChR clustering in response to beads by a tyrphostin tyrosine kinase inhibitor (RG50864) (Peng, H. B., L. P. Baker, and Q. Chen. 1991. Neuron. 6:237-246). RG50864 also inhibited PY accumulation at bead contacts, providing evidence for tyrosine kinase activation in response to the bead stimulus. These results suggest that tyrosine phosphorylation may play an important role in the generative stages of cluster formation, and may involve protein(s) other than or in addition to AChRs.