Role of the cytoskeleton in the formation, stabilization, and removal of acetylcholine receptor clusters in cultured muscle cells

HDAC6 regulates microtubule stability and clustering of AChRs at neuromuscular junctions

Microtubules (MTs) are known to be post-translationally modified at the neuromuscular junction (NMJ), hence increasing their stability. To date however, the function(s) of the dynamic MT network and its relative stability in the formation and maintenance of NMJs remain poorly described. Stabilization of the MT is dependent in part on its acetylation status, and HDAC6 is capable of reversing this post-translational modification. Here, we report that HDAC6 preferentially accumulates at NMJs and that it contributes to the organization and the stability of NMJs. Indeed, pharmacological inhibition of HDAC6 protects against MT disorganization and reduces the size of acetylcholine receptor (AChR) clusters. Moreover, the endogenous HDAC6 inhibitor paxillin interacts with HDAC6 in skeletal muscle cells, colocalizes with AChR aggregates, and regulates the formation of AChR. Our findings indicate that the focal insertion of AChRs into the postsynaptic membrane is regulated by stable MTs and highlight how an MT/HDAC6/paxillin axis participates in the regulation of AChR insertion and removal to control the structure of NMJs.

Cullin E3 Ligase Activity Is Required for Myoblast Differentiation

The role of cullin E3-ubiquitin ligases for muscle homeostasis is best known during muscle atrophy, as the cullin-1 substrate adaptor atrogin-1 is among the most well-characterized muscle atrogins. We investigated whether cullin activity was also crucial during terminal myoblast differentiation and aggregation of acetylcholine receptors for the establishment of neuromuscular junctions in vitro. The activity of cullin E3-ligases is modulated through post-translational modification with the small ubiquitin-like modifier nedd8. Using either the Nae1 inhibitor MLN4924 (Pevonedistat) or siRNA against nedd8 in early or late stages of differentiation on C2C12 myoblasts, and primary satellite cells from mouse and human, we show that cullin E3-ligase activity is necessary for each step of the muscle cell differentiation program in vitro. We further investigate known transcriptional repressors for terminal muscle differentiation, namely ZBTB38, Bhlhe41, and Id1. Due to their identified roles for terminal muscle differentiation, we hypothesize that the accumulation of these potential cullin E3-ligase substrates may be partially responsible for the observed phenotype. MLN4924 is currently undergoing clinical trials in cancer patients, and our experiments highlight concerns on the homeostasis and regenerative capacity of muscles in these patients who often experience cachexia.

Cytoskeleton and Adhesion in Myogenesis

The function of muscle is to contract, which means to exert force on a substrate. The adaptations required for skeletal muscle differentiation, from a prototypic cell, involve specialization of housekeeping cytoskeletal contracting and supporting systems into crystalline arrays of proteins. Here I discuss the changes that all three cytoskeletal systems (microfilaments, intermediate filaments, and microtubules) undergo through myogenesis. I also discuss their interaction, through the membrane, to extracellular matrix and to other cells, where force will be exerted during contraction. The three cytoskeletal systems are necessary for the muscle cell and must exert complementary roles in the cell. Muscle is a responsive system, where structure and function are integrated: the structural adaptations it undergoes depend on force production. In this way, the muscle cytoskeleton is a portrait of its physiology. I review the cytoskeletal proteins and structures involved in muscle function and focus particularly on their role in myogenesis, the process by which this incredible muscle machine is made. Although the focus is on skeletal muscle, some of the discussion is applicable to cardiac and smooth muscle.

Transmembrane mechanisms in the assembly of the postsynaptic apparatus at the neuromuscular junction

The vertebrate neuromuscular junction (NMJ) is marked by molecular specializations that include postsynaptic clusters of acetylcholine receptor (AChR) and acetylcholinesterase (AChE). Whereas AChRs are aggregated in the postsynaptic muscle membrane to a density of 10,000/mum(2), AChE is concentrated, also to a high density, in the synaptic basement membrane (BM). In recent years considerable progress has been made in understanding the cellular and molecular mechanisms of AChR clustering. It is known that during the early stages of motoneuron-muscle interaction, the nerve-secreted proteoglycan agrin activates the muscle-specific kinase MuSK, which leads to the formation of a postsynaptic cytoskeletal scaffold that immobilizes and concentrates AChRs through a process generally accepted to involve diffusion-mediated trapping of the receptors. We have recently tested this diffusion-trap model at the single molecule level for the first time by using quantum-dot labeling to track individual AChRs during NMJ development. Our results showed that single AChRs exhibit Brownian-type movement, with diffusion coefficients of 10(-11) to 10(-9)cm(2)/s, until they become immobilized at "traps" assembled in response to synaptogenic stimuli. Thus, free diffusion of AChRs is an integral part of their clustering mechanism. What is the mechanism for AChE clustering? We previously showed that the A(12) asymmetric form of AChE binds to perlecan, a heparan-sulfate proteoglycan which in turn interacts with the transmembrane dystroglycan complex. Through this linkage AChE becomes bound to the muscle membrane and, like AChRs, may exhibit lateral mobility along the membrane. Consistent with this idea, pre-existent AChE at the cell surface becomes clustered together with AChRs following synaptogenic stimulation. Future studies testing diffusion-mediated trapping of AChE should provide insights into the synaptic localization of BM-bound molecules at the NMJ.

The role of the cytoskeleton in neuromuscular junction formation

The cytoskeleton plays a vital role in neuromuscular junction (NMJ) formation. It is responsible for shaping synaptic membrane into folds opposed to presynaptic active zones and anchoring acetylcholine receptors (AChRs) to the crest of the junctional folds. Acetylcholine receptors (AChRs) associate with the actin cytoskeleton, the disruption of which affects spontaneous and agrin-induced AChR clusters (Prives et al., 1982; Connolly, 1984; Peng and Phelan, 1984; Bloch, 1986; Dai et al., 2000). How AChRs are tethered to the actin cytoskeleton remains unclear.

Generation of noncentrosomal microtubule arrays

In most proliferating and migrating animal cells, the centrosome is the main site for microtubule (MT) nucleation and anchoring, leading to the formation of radial MT arrays in which MT minus ends are anchored at the centrosomes and plus ends extend to the cell periphery. By contrast, in most differentiated animal cell types, including muscle, epithelial and neuronal cells, as well as most fungi and vascular plant cells, MTs are arranged in noncentrosomal arrays that are non-radial. Recent studies suggest that these noncentrosomal MT arrays are generated by a three step process. The initial step involves formation of noncentrosomal MTs by distinct mechanisms depending on cell type: release from the centrosome, catalyzed nucleation at noncentrosomal sites or breakage of pre-existing MTs. The second step involves transport by MT motor proteins or treadmilling to sites of assembly. In the final step, the noncentrosomal MTs are rearranged into cell-type-specific arrays by bundling and/or capture at cortical sites, during which MTs acquire stability. Despite their relative stability, the final noncentrosomal MT arrays may still exhibit dynamic properties and in many cases can be remodeled.

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.

Lithium inhibits a late step in agrin-induced AChR aggregation

Agrin activates an intracellular signaling pathway to induce the formation of postsynaptic specializations on muscle fibers. In myotubes in culture, this pathway has been shown to include autophosphorylation of the muscle-specific kinase MuSK, activation of Src-family kinases, tyrosine phosphorylation of the acetylcholine receptor (AChR) beta subunit, a decrease in receptor detergent extractability, and the accumulation of AChRs into high-density aggregates. Here we report that treating chick myotubes with lithium prevented any detectable agrin-induced change in AChR distribution without affecting the number of AChRs or the agrin-induced change in AChR tyrosine phosphorylation and detergent extractability. Lithium treatment also increased the rate at which AChR aggregates disappeared when agrin was removed. The effects of lithium developed slowly over the course of approximately 12 h. Thus, sensitivity to lithium identifies a late step in the agrin signaling pathway, after agrin-induced MuSK and AChR phosphorylation, that is necessary for the recruitment of AChRs into visible aggregates.

Expression of Eph receptors in skeletal muscle and their localization at the neuromuscular junction

The participation of ephrins and Eph receptors in guiding motor axons during muscle innervation has been well documented, but little is known about their expression and functional significance in muscle at later developmental stages. Our present study investigates the expression and localization of Eph receptors and ephrins in skeletal muscle. Prominent expression of EphA4, EphA7, and ephrin-A ligands was detected in muscle during embryonic development. More importantly, both EphA4 and EphA7, as well as ephrin-A2, were localized at the neuromuscular junction (NMJ) of adult muscle. Despite their relative abundance, they were not localized at the synapses during embryonic stages. The concentration of EphA4, EphA7, and ephrin-A2 at the NMJ was observed at postnatal stages and the synaptic localization became prominent at later developmental stages. In addition, expression of Eph receptors was increased by neuregulin and after nerve injury. Furthermore, we demonstrated that overexpression of EphA4 led to tyrosine phosphorylation of the actin-binding protein cortactin and that EphA4 was coimmunoprecipitated with cortactin in muscle. Taken together, our findings indicate that EphA4 is associated with the actin cytoskeleton. Since actin cytoskeleton is critical to the formation and stability of NMJ, the present findings raise the intriguing possibility that Eph receptors may have a novel role in NMJ formation and/or maintenance.

The actin-driven movement and formation of acetylcholine receptor clusters

A new method was devised to visualize actin polymerization induced by postsynaptic differentiation signals in cultured muscle cells. This entails masking myofibrillar filamentous (F)-actin with jasplakinolide, a cell-permeant F-actin-binding toxin, before synaptogenic stimulation, and then probing new actin assembly with fluorescent phalloidin. With this procedure, actin polymerization associated with newly induced acetylcholine receptor (AChR) clustering by heparin-binding growth-associated molecule-coated beads and by agrin was observed. The beads induced local F-actin assembly that colocalized with AChR clusters at bead-muscle contacts, whereas both the actin cytoskeleton and AChR clusters induced by bath agrin application were diffuse. By expressing a green fluorescent protein-coupled version of cortactin, a protein that binds to active F-actin, the dynamic nature of the actin cytoskeleton associated with new AChR clusters was revealed. In fact, the motive force generated by actin polymerization propelled the entire bead-induced AChR cluster with its attached bead to move in the plane of the membrane. In addition, actin polymerization is also necessary for the formation of both bead and agrin-induced AChR clusters as well as phosphotyrosine accumulation, as shown by their blockage by latrunculin A, a toxin that sequesters globular (G)-actin and prevents F-actin assembly. These results show that actin polymerization induced by synaptogenic signals is necessary for the movement and formation of AChR clusters and implicate a role of F-actin as a postsynaptic scaffold for the assembly of structural and signaling molecules in neuromuscular junction formation.

Evidence for phosphorylation-dependent internalization of recombinant human rho1 GABAC receptors

1. Recombinant wild-type or mutant human rho1 GABA receptors were expressed in human embryonic kidney (HEK) 293 or monkey COS-7 cells and studied using the patch clamp technique. 2. Standard whole-cell recordings with 4 mM Mg-ATP in the patch pipette induced a time-dependent decrease in the GABA-activated current (IGABA) amplitude that was not the result of a decrease in GABA sensitivity. In contrast, IGABA remained stable when recordings were obtained using the perforated patch configuration or with standard whole-cell recording and no Mg-ATP in the patch pipette. 3. The inhibitors of serine/threonine protein kinases KN-62 (20 microM) or staurosporine (20 nM) prevented the time-dependent decrease in the amplitude of IGABA seen in the presence of ATP. Alkaline phosphatase (220 U ml-1), when added to the patch pipette in the absence of ATP, induced a transient potentiation of IGABA. Although the protein kinase C (PKC) activator 4beta-phorbol 12-myristate, 13-acetate (PMA) did not reduce the amplitude of IGABA, inclusion of the catalytic domain of PKC in the recording pipette accelerated the time-dependent decrease in current amplitude. These data suggest that phosphorylation is involved in the regulation of the amplitude of IGABA. 4. Mutation of the three PKC consensus sequences of the rho1 receptor had no significant effect on the decline in IGABA, indicating that direct phosphorylation of these putative sites on the rho1 receptor does not underlie the time-dependent decrease in amplitude. 5. In COS-7 cells transfected with wild-type rho1 receptors, the amplitude of IGABA had completely recovered to the original value when the same cells were repatched after 30-40 min, indicating that the decline in IGABA was a reversible process. 6. The inhibitor of actin filament formation cytochalasin B, when added to the patch pipette in the absence of ATP, induced a time-dependent inactivation suggesting that the actin cytoskeleton may play a role in the regulation of the amplitude. 7. Coincident with the decrease in the amplitude of IGABA, the cell capacitance significantly decreased in the presence of ATP in the patch pipette. This decrease in capacitance was not observed in the absence of Mg-ATP. The decrease in the membrane surface area suggests that receptor internalization could be a potential mechanism for the observed inactivation. 8. At 32 C, compared with 22 C, the rate and magnitude of the decline was increased dramatically. In contrast, at 16 C, no significant change in IGABA was observed over the 20 min recording time. This marked temperature sensitivity is consistent with receptor internalization as a mechanism for the time-dependent decline in IGABA. 9. The specificity of the decrease in IGABA was assessed by coexpressing the voltage-dependent potassium channel Kv1.4 along with the rho1 receptor in HEK293 cells. The amplitude of the potassium current (IKv1.4) exhibited very little decrement in comparison to IGABA suggesting that the putative GABA receptor internalization was not the consequence of a non-specific membrane retrieval.

Metabolic stabilization of muscle nicotinic acetylcholine receptor by rapsyn

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

Metabolic stabilization of muscle nicotinic acetylcholine receptor by rapsyn

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

Role of actin in anchoring postsynaptic receptors in cultured hippocampal neurons: differential attachment of NMDA versus AMPA receptors

We used actin-perturbing agents and detergent extraction of primary hippocampal cultures to test directly the role of the actin cytoskeleton in localizing GABAA receptors, AMPA- and NMDA-type glutamate receptors, and potential anchoring proteins at postsynaptic sites. Excitatory postsynaptic sites on dendritic spines contained a high concentration of F-actin that was resistant to cytochalasin D but could be depolymerized using the novel compound latrunculin A. Depolymerization of F-actin led to a 40% decrease in both the number of synaptic NMDA receptor (NMDAR1) clusters and the number of AMPA receptor (GluR1)-labeled spines. The nonsynaptic NMDA receptors appeared to remain clustered and to coalesce in cell bodies. alpha-Actinin-2, which binds both actin and NMDA receptors, dissociated from the receptor clusters, but PSD-95 remained associated with both the synaptic and nonsynaptic receptor clusters, consistent with a proposed cross-linking function. AMPA receptors behaved differently; on GABAergic neurons, the clusters redistributed to nonsynaptic sites, whereas on pyramidal neurons, many of the clusters appeared to disperse. Furthermore, in control neurons, AMPA receptors were detergent extractable from pyramidal cell spines, whereas AMPA receptors on GABAergic neurons and NMDA receptors were unextractable. GABAA receptors were not dependent on F-actin for the maintenance or synaptic localization of clusters. These results indicate fundamental differences in the mechanisms of receptor anchoring at postsynaptic sites, both regarding the anchoring of a single receptor (the AMPA receptor) in pyramidal cells versus GABAergic interneurons and regarding the anchoring of different receptors (AMPA vs NMDA receptors) at a single class of postsynaptic sites on pyramidal cell dendritic spines.

Role of actin in anchoring postsynaptic receptors in cultured hippocampal neurons: differential attachment of NMDA versus AMPA receptors

We used actin-perturbing agents and detergent extraction of primary hippocampal cultures to test directly the role of the actin cytoskeleton in localizing GABAA receptors, AMPA- and NMDA-type glutamate receptors, and potential anchoring proteins at postsynaptic sites. Excitatory postsynaptic sites on dendritic spines contained a high concentration of F-actin that was resistant to cytochalasin D but could be depolymerized using the novel compound latrunculin A. Depolymerization of F-actin led to a 40% decrease in both the number of synaptic NMDA receptor (NMDAR1) clusters and the number of AMPA receptor (GluR1)-labeled spines. The nonsynaptic NMDA receptors appeared to remain clustered and to coalesce in cell bodies. alpha-Actinin-2, which binds both actin and NMDA receptors, dissociated from the receptor clusters, but PSD-95 remained associated with both the synaptic and nonsynaptic receptor clusters, consistent with a proposed cross-linking function. AMPA receptors behaved differently; on GABAergic neurons, the clusters redistributed to nonsynaptic sites, whereas on pyramidal neurons, many of the clusters appeared to disperse. Furthermore, in control neurons, AMPA receptors were detergent extractable from pyramidal cell spines, whereas AMPA receptors on GABAergic neurons and NMDA receptors were unextractable. GABAA receptors were not dependent on F-actin for the maintenance or synaptic localization of clusters. These results indicate fundamental differences in the mechanisms of receptor anchoring at postsynaptic sites, both regarding the anchoring of a single receptor (the AMPA receptor) in pyramidal cells versus GABAergic interneurons and regarding the anchoring of different receptors (AMPA vs NMDA receptors) at a single class of postsynaptic sites on pyramidal cell dendritic spines.

Intracellular calcium regulates agrin-induced acetylcholine receptor clustering

Agrin is an extracellular matrix protein that directs neuromuscular junction formation. Early signal transduction events in agrin-mediated postsynaptic differentiation include activation of a receptor tyrosine kinase and phosphorylation of acetylcholine receptors (AChRs), but later steps in this pathway are unknown. Here, we have investigated the role of intracellular calcium in agrin-induced AChR clustering on cultured myotubes. Clamping intracellular calcium levels by loading with the fast chelator BAPTA inhibited agrin-induced AChR aggregation. In addition, preexisting AChR aggregates dispersed under these conditions, indicating that the maintenance of AChR clusters is similarly dependent on intracellular calcium fluxes. The decrease in AChR clusters in BAPTA-loaded cells was dose-dependent and reversible, and no change in the number or mobility of AChRs was observed. Clamping intracellular calcium did not block agrin-induced tyrosine phosphorylation of the AChR beta-subunit, indicating that intracellular calcium fluxes are likely to act downstream from or parallel to AChR phosphorylation. Finally, the targets of the intracellular calcium are likely to be close to the calcium source, since agrin-induced AChR clustering was unaffected in cells loaded with EGTA, a slower-binding calcium chelator. These findings distinguish a novel step in the signal transduction mechanism of agrin and raise the possibility that the pathways mediating agrin- and activity-driven changes in synaptic architecture could intersect at the level of intracellular calcium fluxes.

Intracellular calcium regulates agrin-induced acetylcholine receptor clustering

Agrin is an extracellular matrix protein that directs neuromuscular junction formation. Early signal transduction events in agrin-mediated postsynaptic differentiation include activation of a receptor tyrosine kinase and phosphorylation of acetylcholine receptors (AChRs), but later steps in this pathway are unknown. Here, we have investigated the role of intracellular calcium in agrin-induced AChR clustering on cultured myotubes. Clamping intracellular calcium levels by loading with the fast chelator BAPTA inhibited agrin-induced AChR aggregation. In addition, preexisting AChR aggregates dispersed under these conditions, indicating that the maintenance of AChR clusters is similarly dependent on intracellular calcium fluxes. The decrease in AChR clusters in BAPTA-loaded cells was dose-dependent and reversible, and no change in the number or mobility of AChRs was observed. Clamping intracellular calcium did not block agrin-induced tyrosine phosphorylation of the AChR beta-subunit, indicating that intracellular calcium fluxes are likely to act downstream from or parallel to AChR phosphorylation. Finally, the targets of the intracellular calcium are likely to be close to the calcium source, since agrin-induced AChR clustering was unaffected in cells loaded with EGTA, a slower-binding calcium chelator. These findings distinguish a novel step in the signal transduction mechanism of agrin and raise the possibility that the pathways mediating agrin- and activity-driven changes in synaptic architecture could intersect at the level of intracellular calcium fluxes.

Heterologous retinal cultured neurons and cell adhesion molecules induce clustering of acetylcholine receptors and polynucleation in mouse muscle BC3H-1 clonal cell line

Several features of the clonal cell line BC3H-1 resemble those of embryonic muscle cells at their early stage of development (Patrick et al.: J Biol Chem 252:2143-2153, 1977). Under normal culture conditions, fully differentiated BC3H-1 cells possess a spindle-shaped, muscle-like morphology with a single nucleus. Like embryonic muscle cell counterparts, they neither express the epsilon subunit nor exhibit the clustered organization of the nicotinic acetylcholine receptor (AChR) characteristic of mature myocytes. Instead, AChRs are evenly distributed upon the sarcolemma upon differentiation. Here we report that BC3H-1 cells can be induced to express AChR clusters by co-culturing the cell line with heterologous retinal neurons, which establish contacts with former cells. Clustering was also triggered by pretreating the culture substratum with several cell adhesion molecules. Polynucleation, a phenomenon observed in mature myotubes in vivo, was also observed in BC3H-1 cells under the two sets of experimental conditions. AChR clustering and polynucleation, both characteristic of the mature postsynaptic region, can thus be induced in BC3H-1 cells by at least two quite distinct pathways: neuronal cell contact and/or cell adhesion molecules. It is suggested that heterologous retinal neurons can elicit only an incomplete differentiation of BC3H-1 cells, failing to concentrate the clusters beneath the neuronal endings, and failing to induce expression of the epsilon subunit, characteristic of the mature AChR.

Requirement of a colchicine-sensitive component of the cytoskeleton for acetylcholine receptor recovery

1. The effect of colchicine treatment on acetylcholine receptor function was examined in potassium depolarized, voltage-clamped snake twitch fibre endplates. Receptor function was assessed by analysis of miniature endplate currents (m.e.p.c.) as well as acetylcholine (ACh)-induced single channel currents. 2. Pretreatment of snake muscle fibres with colchicine (10 microM to 100 microM) for 16-18 h had no effect on m.e.p.c. amplitude or decay rates. At higher concentrations (1 mM), there was a slight decrease in the average m.e.p.c. amplitude. 3. Colchicine produced a concentration-dependent decrease in the extent of m.e.p.c. amplitude recovery following a 10 min exposure to 540 microM carbachol. Exposure of 100 microM colchicine-treated preparations to 0.5 microM staurosporine further reduced the extent of m.e.p.c. amplitude recovery following carbachol exposure. 4. The decrease in m.e.p.c. amplitude following carbachol exposure was not due to a shift in the m.e.p.c. reversal potential. In addition, the distribution of m.e.p.c. amplitudes remained unimodal in both control and colchicine (100 microM)-treated preparations following carbachol exposure. 5. In addition to the normal, large conductance (approximately 48 pS) ACh-activated channels, a population of small conductance (approximately 29 pS) channels was observed in colchicine-treated preparations following exposure to carbachol. In preparations treated with both colchicine and staurosporine and then exposed to carbachol, the conductance of these small channels was identical to that of colchicine or staurosporine alone. 6. We suggest that prolonged exposure of snake twitch fibre endplates to agonist results in the activation and desensitization of ACh receptors. Furthermore, we propose that for a subpopulation of the inactivated receptors, restoration of function requires both the integrity of a subsynaptic cytoskeletal component and phosphorylation by a staurosporine-sensitive protein kinase. One plausible mechanism is that some receptors become destabilized in the membrane and phosphorylation of a cytoskeletal component, whose distribution may depend on an intact microtubular system, is required to re-anchor these receptors. If this anchoring process is inhibited either by disruption of the cytoskeleton with colchicine, or inhibition of the kinase by staurosporine, these receptors remain activatable, but have a reduced conductance.

Co-localization of 1,4-dihydropyridine receptor alpha 2/delta subunit and N-CAM during early myogenesis in vitro

The surface distribution of the alpha 2/delta subunit of the 1,4-dihydropyridine receptor and its topographical relationship with the neural cell adhesion molecule (N-CAM) were investigated during early myogenesis in vitro, by double immunocytochemical labeling with the monoclonal antibody 3007 and an anti-N-CAM polyclonal antiserum. The monoclonal antibody 3007 has been previously shown to immunoprecipitate dihydropyridine receptor from skeletal muscle T-tubules. In further immunoprecipitation experiments on such preparations and muscle cell cultures, it was demonstrated here that the monoclonal antibody 3007 exclusively recognizes the alpha 2/delta subunit of the 1,4-dihydropyridine receptor. In rabbit muscle cell cultures, the labeling for both alpha 2/delta and N-CAM was first detected on myoblasts, in the form of spots on the membrane and perinuclear patches. Spots of various sizes organized in aggregates were then found on the membrane of myotubes. At fusion (T0), aggregates of N-CAM spots alone were found at the junction between fusing cells. At T6 and later stages, all alpha 2/delta aggregates present on myotubes co-localized with N-CAM, while less than 3% of N-CAM aggregates did not co-localize with alpha 2/delta. A uniform N-CAM staining also made its appearance. At T12, when myotubes showed prominent contractility, alpha 2/delta-N-CAM aggregates diminished in size. Dispersed alpha 2/delta spots of a small regular size spread over the whole surface of the myotubes and alignments of these spots became visible. Corresponding N-CAM spots were now occasionally seen, and uniform N-CAM staining was prominent. These results show that alpha 2/delta and N-CAM are co-localized and that their distributions undergo concomitant changes during early myogenesis until the T-tubule network starts to be organized. This suggest that these two proteins might jointly participate in morphogenetic events preceding the formation of T-tubules.

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.

Agrin induces alpha-actinin, filamin, and vinculin to co-localize with AChR clusters on cultured chick myotubes

Agrin induces discrete high-density patches of acetylcholine receptors (AChRs) and other synaptic components on cultured myotubes in a manner that resembles synaptic differentiation. Furthermore, agrin-like molecules are present at developing neuromuscular junctions in vivo. This provides us with a unique opportunity to manipulate AChR patching in order to examine the role of cytoskeletal components. Cultured chick myotubes were fixed and labeled to visualize the distributions of actin, alpha-actinin, filamin, tropomyosin, and vinculin. Overnight exposure to agrin caused a small amount of alpha-actinin, filamin, and vinculin to reorganize into discrete clusters. Double-labeling studies revealed that 78% of the AChR clusters were associated with detectable concentrations of filamin, 70% with alpha-actinin, and 58% with vinculin. Filamin even showed congruence to AChRs within clustered regions. By contrast, actin (visualized with fluorescein-phalloidin) and tropomyosin did not show specific associations with agrin-induced AChR clusters. The accumulation of cytoskeletal components at AChRs clusters raised the possibility that cytoskeletal rearrangements direct AChR clustering. However, a time course of agrin-induced clustering that focused on filamin revealed that most of the early AChR clusters (3-6 h) were not associated with detectable amounts of cytoskeletal material. The accumulation of cytoskeletal material at later times (12-18 h) may imply a role in maintenance and stabilization, but it appears unlikely that these cytoskeletal elements initiate AChR clustering on myotubes.

Rotational diffusion of acetylcholine receptors on cultured rat myotubes

The rotational mobility of acetylcholine receptors (AChR) in the plasma membrane of living rat myotubes in culture is measured in this study by polarized fluorescence recovery after photobleaching (PFRAP). These AChR are known to exist in two distinct classes, evident by labeling with rhodamine alpha-bungarotoxin; clustered AChR that are aggregated in a pattern of highly concentrated speckles and streaks, with each cluster occupying an area of approximately 1,000 microns 2; and nonclustered AChR that appear as diffuse labeling. PFRAP results reported here show that: (a) most clustered AChR (approximately 86%) are rotationally immobile within a time scale of at least several seconds; and (b) most nonclustered AChR (approximately 76%) are rotationally mobile with characteristic times ranging from less than 50 ms to 0.1 s. External cross-linking with the tetravalent lectin concanavalin A immobilizes many nonclustered AChR. PFRAP experiments in the presence of carbachol or cytochalasin D show that the restraints to rotational motion in clusters are remarkably immune to treatments that disperse clusters or disrupt cytoplasmic actin. The experiments also demonstrate the feasibility of using PFRAP to measure rotational diffusion on selected microscopic areas of living nondeoxygenated cells labeled with standard fluorescence probes over a very wide range of time scales, and they also indicate what technical improvements would make PFRAP even more practicable.

Association of cytoskeletal proteins with newly formed acetylcholine receptor aggregates induced by embryonic brain extract

Aggregates of acetylcholine receptors (AChR) in muscle cell membranes are associated with accumulations of certain cytoskeletal and peripheral membrane proteins. We treated cultured rat myotubes briefly with embryonic brain extract (EBX) to promote AChR aggregation and determined the distribution of several of these proteins at early stages of aggregation. EBX-treated and control cultures were stained with tetramethylrhodamine-alpha-bungarotoxin to identify AChR aggregates and were then frozen and sectioned on a cryostat. These sections were stained with primary antibodies and fluoresceinated secondary antibodies to localize cytoskeletal proteins. The distributions of AChRs and cytoskeletal proteins was examined qualitatively and analyzed by a semiquantitative assay. Qualitatively, the 43K protein had a distribution that was virtually identical to that of AChR in both control and EBX-treated cultures, and it always colocalized with early AChR aggregates. The 58K protein similarly colocalized with early AChR aggregates, but it was also in aggregate-free areas of muscle membrane. The association of vinculin with the aggregates was quantitatively similar to that of the 43K and 58K proteins, but, qualitatively, its distribution did not follow that of the AChR as closely. Like the 58K protein and vinculin, alpha-actinin, filamin, and actin were concentrated in AChR aggregates and were also enriched elsewhere. However, they were less closely associated with the aggregates, both quantitatively and qualitatively. These results show that AChR aggregates induced by EBX tend to be enriched in the same cytoskeletal proteins that are present at the neuromuscular junction in vivo and at AChR clusters formed at sites of cell-substrate adhesion in vitro. Semiquantitative analysis also revealed that the fractional area of the cell surface associated with vinculin, alpha-actinin, and the 58K protein was the same in controls and EBX-treated myotubes, although the area enriched in AChR and the 43K protein increased about three-fold upon EBX treatment. These results suggest that AChR aggregates may form preferentially in membrane regions that are already enriched in these proteins.

cAMP delays disappearance of gap junctions between pairs of rat hepatocytes in primary culture

Freshly isolated adult rat hepatocytes were found to be coupled through gap junctions, but coupling decreased abruptly 5-8 h after plating the cells on plastic culture dishes in physiological saline containing insulin and fetal calf serum. Loss of intercellular coupling was associated with disappearance of 27-kDa gap junction protein and of gap junctions seen by electron microscopy or immunocytochemistry. This disappearance of coupling was delayed approximately 8 h by treatment of the cultures with membrane permeant adenosine 3',5'-cyclic monophosphate (cAMP) [but not guanosine 3',5'-cyclic monophosphate (cGMP)] derivatives. Levels of gap junction protein and anatomically identified junctions were also maintained by 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP). Level of mRNA encoding the gap junction protein was maintained longer in cells treated with 8-BrcAMP than in untreated cells, but 8-BrcAMP did not detectably increase the transcription rate. Thus prolongation of coupling must result at least partially from extension of the lifetime of gap junction mRNA, allowing translation of message and assembly of channels for a longer period after plating. Treatment of cells with mRNA or protein synthesis inhibitors (alpha-amanitin and cycloheximide) prolonged coupling to a similar extent as did treatment with 8-BrcAMP. alpha-Amanitin blocked transcription of gap junction mRNA, but levels of cytoplasmic mRNA encoding the 27-kDa gap junction protein were maintained, presumably by block of transcription of an mRNA degrading factor. The factor is probably a protein, since a similar effect on mRNA level was produced in cycloheximide-treated cells. Cells cultured in control medium were also observed to flatten as they became uncoupled, whereas cells cultured for as long as 16 h in elevated 8-BrcAMP remained round and well coupled. The correlation between shape and coupling strength was not obtained after treatment with the microtubule stabilizing agent, taxol, which maintained the spherical shape of the cells but did not delay the disappearance of dye coupling. Nocodazole, which blocks microtubule formation, also maintained the spherical shape of the cells but delayed the disappearance of dye coupling. In addition to gating by covalent modification or other mechanisms, hormones and drugs that alter the intracellular cAMP concentration may affect intercellular communication by changing the lifetime of the mRNA encoding the main gap junction protein, thereby decreasing or increasing its synthesis. In addition, cAMP may act by decreasing removal of junctions from appositional membranes.

Cytoskeletal architecture of neuromuscular junction: localization of vinculin

Cytoskeletons underneath the postsynaptic membrane of neuromuscular junctions were studied by using a quick-freeze deep-etch method and immunoelectron microscopy of ultrathin frozen sections. In a quick-freeze deep-etched replica of fresh, unfixed muscles, 8.9 +/- 1.5-nm particles were present on the true postsynaptic membrane surface. Underneath this receptor-rich postsynaptic membrane, networks of fine filaments were observed. These cytoskeletal networks were more clearly observed in extracted samples. In these samples, diameters of the filaments which formed networks were measured. In the platinum replica, three kinds of filament were recognized--12 nm, 9 nm, and 7 nm in diameter. The 12-nm filament seemed to correspond to the intermediate filament. The other two filaments formed meshworks between intermediate filaments and plasma membrane. In ultrathin frozen sections vinculin label was localized just beneath the plasma membrane. Thirty-six percent of the label was within 18 nm from the cytoplasmic side of the plasma membrane and 50% was within 30 nm. Taking the size of the vinculin molecule into account, it was concluded that vinculin is localized just beneath the plasma membrane and might play some role in anchoring filaments which formed meshworks underneath the plasma membrane.

An unusual beta-spectrin associated with clustered acetylcholine receptors

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane is an early event in the formation of the neuromuscular junction. The mechanism of clustering is still unknown, but is generally believed to be mediated by the postsynaptic cytoskeleton. We have identified an unusual isoform of beta-spectrin which colocalizes with AChR in AChR clusters isolated from rat myotubes in vitro. A related antigen is present postsynaptically at the neuromuscular junction of the rat. Immunoprecipitation, peptide mapping and immunofluorescence show that the beta-spectrin in AChR clusters resembles but is distinct from the beta-spectrin of human erythrocytes. alpha-Spectrin appears to be absent from AChR clusters. Semiquantitative immunofluorescence techniques indicate that there are from two to seven beta-spectrin molecules present for every clustered AChR, the higher values being obtained from rapidly prepared clusters, the lower values from clusters that require several minutes or more for isolation. Upon incubation of isolated AChR clusters for 1 h at room temperature, beta-spectrin is slowly depleted and the AChR redistribute into microaggregates. The beta-spectrin that remains associated with the myotube membrane is concentrated at these microaggregates. beta-Spectrin is quantitatively lost from clusters upon digestion with chymotrypsin, which causes AChR to redistribute in the plane of the membrane. These results suggest that AChR in clusters is closely linked to an unusual isoform of beta-spectrin.

Fine structure of synaptogenesis in the vertebrate central nervous system

This article reviews studies of the formation of synaptic junctions in the vertebrate central nervous system. It is focused on electron microscopic investigations of synaptogenesis, although insights from other disciplines are interwoven where appropriate, as are findings from developing peripheral and invertebrate nervous systems. The first part of the review is concerned with the morphological maturation of synapses as described from both qualitative and quantitative perspectives. Next, epigenetic influences on synaptogenesis are examined, and later in the article the concept of epigenesis is integrated with that of hierarchy. It is suggested that the formation of synaptic junctions may take place as an ordered progression of epigenetically modulated events wherein each level of cellular affinity becomes subordinate to the one that follows. The ultimate determination of whether a synapse is maintained, modified or dissolved would be made by the changing molecular fabric of its junctional membranes. In closing, a hypothetical model of synaptogenesis is proposed, and an hierarchial order of events is associated with a speculative synaptogenic sequence. Key elements of this hypothesis are 1) epigenetic factors that facilitate generally appropriate interactions between neurites; 2) independent expression of surface specializations that contain sufficient information for establishing threshold recognition between interacting neurites; 3) exchange of molecular information that biases the course of subsequent junctional differentiation and ultimately results in 4) the stabilization of synaptic junctions into functional connectivity patterns.

Acetylcholine receptors and concanavalin A-binding sites on cultured Xenopus muscle cells: electrophoresis, diffusion, and aggregation

Using digitally analyzed fluorescence videomicroscopy, we have examined the behavior of acetylcholine receptors and concanavalin A binding sites in response to externally applied electric fields. The distributions of these molecules on cultured Xenopus myoballs were used to test a simple model which assumes that electrophoresis and diffusion are the only important processes involved. The model describes the distribution of concanavalin A sites quite well over a fourfold range of electric field strengths; the results suggest an average diffusion constant of approximately 2.3 X 10(-9) cm2/s. At higher electric field strengths, the asymmetry seen is substantially less than that predicted by the model. Acetylcholine receptors subjected to electric fields show distributions substantially different from those predicted on the basis of simple electrophoresis and diffusion, and evidence a marked tendency to aggregate. Our results suggest that this aggregation is due to lateral migration of surface acetylcholine receptors, and is dependent on surface interactions, rather than the rearrangement of microfilaments or microtubules. The data are consistent with a diffusion-trap mechanism of receptor aggregation, and suggest that the event triggering receptor localization is a local increase in the concentration of acetylcholine receptors, or the electrophoretic concentration of some other molecular species. These observations suggest that, whatever mechanism(s) trigger initial clustering events in vivo, the accumulation of acetylcholine receptors can be substantially enhanced by passive, diffusion-mediated aggregation.

Regulation of agrin-induced acetylcholine receptor aggregation by Ca++ and phorbol ester

Agrin, a protein extracted from the electric organ of Torpedo californica, induces the formation of specializations on cultured chick myotubes that resemble the postsynaptic apparatus at the neuromuscular junction. The aim of the studies reported here was to characterize the effects of agrin on the distribution of acetylcholine receptors (AChRs) and cholinesterase as a step toward determining agrin's mechanism of action. When agrin was added to the medium bathing chick myotubes small (less than 4 micron 2) aggregates of AChRs began to appear within 2 h and increased rapidly in number until 4 h. Over the next 12-20 h the number of aggregates per myotube decreased as the mean size of each aggregate increased to approximately 15 micron 2. The accumulation of AChRs into agrin-induced aggregates occurred primarily by lateral migration of AChRs already in the myotube plasma membrane at the time agrin was added to the cultures. Aggregates of AChRs and cholinesterase remained as long as agrin was present in the medium; if agrin was removed the number of aggregates declined slowly. The formation and maintenance of agrin-induced AChR aggregates required Ca++, Co++ and Mn++ inhibited agrin-induced AChR aggregation and increased the rate of aggregate dispersal. Mg++ and Sr++ could not substitute for Ca++. Agrin-induced receptor aggregation also was inhibited by phorbol 12-myristate 13-acetate, an activator of protein kinase C, and by inhibitors of energy metabolism. The similarities between agrin's effects on cultured myotubes and events that occur during formation of neuromuscular junctions support the hypothesis that axon terminals release molecules similar to agrin that induce the differentiation of the postsynaptic apparatus.

A myasthenic syndrome with congenital paucity of secondary synaptic clefts: CPSC syndrome

Two cases of a newly recognized myasthenic syndrome were investigated (CPSC syndrome). The course of the disease was observed for periods of 6 and 3 years. In infancy, exacerbations of the symptoms occurred during febrile illness, but thereafter the clinical course was stable and the children appeared to be only slightly handicapped. Biopsies were taken from the intercostal muscle in both patients. Microelectrode studies revealed small Mepp amplitudes. Light microscopy demonstrated predominance of type I fibers and focal type-grouping. There was a lowered cholinesterase activity and frequent branching of preterminal axons. Electron microscopy revealed that there were few, if any, folds of the postsynaptic membrane and that there were no signs of degeneration. Methods for localization of acetylcholine receptors (AChR) revealed a deficiency and altered distribution of AChRs at these postsynaptic membranes and the occurrence of extrajunctional AChRs in some muscle fibers. It is concluded that the syndrome is a clinicopathological entity, characterized morphologically by a congenital paucity of secondary synaptic clefts (CPSC syndrome).

The disappearance of a cyclin-like protein and the appearance of statin is correlated with the onset of differentiation during myogenesis in vitro

We have used monoclonal antibodies to statin (S-44) and a cyclin-like protein (S-132) to examine the distribution of these two antigens in proliferating and in nonproliferating populations of cells. We have found that this cyclin-like protein is present in proliferating fibroblasts, whereas statin is absent from these same cell populations; in contrast, in senescent populations of fibroblasts the cyclin-like antigen disappears and statin labeling of nuclei appears. During myogenesis in rat muscle cell cultures, S-132 labeling is present in proliferating myoblasts and disappears after cells fuse and differentiate as multinucleated myotubes. In contrast, statin is absent from proliferating myoblasts, but appears when these cells become postmitotic and begin to differentiate. Similar results were seen during chick myogenesis. We have also found similar results during serum-starvation-induced differentiation in neuroblastoma cells. These results indicate that the cyclin-like protein disappears and statin appears upon commitment to differentiation in vitro, and the presence or the absence of these proteins appears to provide cellular markers for the transition from the proliferative to the nonproliferative state during differentiation.

Development of the electromotor system of Torpedo marmorata: distribution of extracellular matrix and cytoskeletal components during acetylcholine receptor focalization

A combination of direct fluorescence and indirect immunofluorescence microscopy has been used to compare the distribution of the acetylcholine receptor with the distribution of major cytoskeletal and extracellular matrix components during electrocyte differentiation in the electric organs of Torpedo marmorata. Laminin, fibronectin and extracellular matrix proteoglycan are always more extensively distributed around the differentiating cell than the acetylcholine receptor-rich patch that forms on the ventral surface of the cell. The distribution of acetylcholinesterase within the ventral surface of the differentiating electrocyte closely resembles the distribution of the acetylcholine receptor. Areas of apparently high acetylcholine receptor density within the ventrally forming acetylcholine receptor-rich patch are always areas of apparently high extracellular matrix proteoglycan density but are not always areas of high laminin or fibronectin density. Desmin levels appear to increase at the onset of differentiation and desmin initially accumulates in the ventral pole of each myotube as it begins to form an electrocyte. During differentiation F-actin-positive filament bundles are observed that extend from the nuclei down to the ventrally forming acetylcholine receptor-rich patch. Most filament bundles terminate in the acetylcholine receptor-rich region of the cell membrane. Electron-microscopic autoradiography suggests that the filament bundles attach to the membrane at sites where small acetylcholine receptor clusters are found. The results of this study suggest that, out of the four extracellular matrix components studied, only the distribution of acetylcholinesterase (which may be both matrix- and membrane-bound at this stage) closely parallels that of the acetylcholine receptor, and that F-actin filament bundles terminate in a region of the cell that is becoming an area of high acetylcholine receptor density.

Acetylcholine receptor clustering and nuclear movement in muscle fibers in culture

We have studied the formation of acetylcholine receptor (AChR) clusters and the behavior of myonuclei in rat and chick skeletal muscle cells grown in cell culture. These cells were treated with a factor derived from Torpedo electric extracellular matrix, which causes a large increase in their number of AChR clusters. We found that these clusters were located preferentially in membrane regions above myonuclei. This cluster-nucleus colocalization is explained by our finding that most of the nuclei near clusters remain relatively stationary, while most of those away from clusters are able to translocate throughout the myotube. In some cases, clusters clearly formed first, then nuclei migrated underneath and became immobilized. If clustered AChRs later dispersed, their associated nuclei resumed moving. These results suggest that AChR clustering initiates an extensive cytoskeletal rearrangement that causes the subcluster localization of organelles, potentially providing a stable source of newly synthesized AChRs for insertion into the cluster.

Platelet-derived growth factor stimulation of [3H]-glucosamine incorporation in density-arrested BALB/c-3T3 cells

G0/G1 traverse in density-arrested BALB/c-3T3 cells is controlled by multiple serum-derived growth factors. Platelet-derived growth factor (PDGF) initiates a proliferative response, whereas factors present in plasma facilitate progression through G0/G1. In the absence of competence formation, progression factors are unable to stimulate cell cycle traverse. We have identified the stimulation of a biochemical process specific to competence formation in BALB/c-3T3 cells. PDGF treated BALB/c-3T3 cells incorporated 5-10-fold more [3H]-glucosamine (GlcN) into acid-insoluble material as compared to platelet-poor plasma (PPP) treated cultures. Increased GlcN incorporation occurred in density-arrested BALB/c-3T3 cells in response to treatment with other competence factors, fibroblast growth factor, and Ca3 (PO4)2 and was not due to cell-cycle traverse. Stimulation of [3H]-GlcN incorporation by PDGF was time dependent, and increased incorporation of [3H]-GlcN into protein required de novo protein synthesis. Several mechanisms through which PDGF could increase GlcN incorporation into cellular material were examined. Results of these studies suggest an increase in the cellular capacity to glycosylate proteins is a response to or a part of competence formation.

Thymic B lymphocyte clones from patients with myasthenia gravis secrete monoclonal striational autoantibodies reacting with myosin, alpha actinin, or actin

Striational autoantibodies (StrAb), which react with elements of skeletal muscle cross-striations, occur frequently in patients with thymoma associated with myasthenia gravis (MG). Dissociated thymic lymphocytes from 22 of 72 MG patients secreted StrAb when cultured with PWM. A high yield of EBV-transformed B cell lines was established from thymus, thymoma, and peripheral blood of seven patients with MG, but clones secreting StrAb arose only from the three patients who had StrAb in their sera. The monoclonal StrAb bound to A bands or I bands in skeletal muscle of human, rat, and frog. One bound to mitochondria in addition to myofibrillar I bands. None bound to nuclei, smooth muscle, or gastric mucosal cells. In immunoblot analyses and ELISAs the monoclonal StrAb bound to muscle and nonmuscle isotypes of myosin, alpha actinin, and/or actin. All bound to contractile proteins common to thymus and muscle, and one selectively immunostained epithelial cells of the thymic medulla. From these antigenic specificities we suggest that StrAb might arise as an immune response directed against the cytoskeletal anchoring proteins associated with nicotinic acetylcholine receptors in thymic epithelial cells undergoing neoplastic transformation to thymoma.

Early stages in the formation and stabilization of acetylcholine receptor aggregates on cultured myotubes: sensitivity to temperature and azide

We have studied the effects of temperature and sodium azide on the formation and stability of embryonic brain extract (EBX)2-induced acetylcholine receptor (AChR) aggregates on myotubes. Sequential changes in AChR distribution were studied on living myotubes in culture by video-intensified fluorescence microscopy. Aggregate formation was temperature dependent, increasing sharply from 24-36 degrees, maximal at 36-37 degrees, and virtually blocked at 38-40 degrees. Whereas aggregate size increased rapidly with time (up to 4 hr) at 36 degrees, at 18-24 degrees small (less than or equal to 1 micron) "microaggregates" formed and accumulated for up to 10 hr. Aggregates formed within 1.5 hr at the sites of microaggregates (formed after 4 hr at 23 degrees) if the temperature was raised to 36 degrees. However, if EBX was removed, the microaggregates on 50% of myotubes disassembled within 1.5 hr. The formation of microaggregates at 23 degrees and aggregates at 36 degrees was reversibly inhibited by sodium azide. These results show that clusters of microaggregates are the precursors of aggregates, and suggest that microaggregate clouds represent a discrete, labile, ATP-dependent stage in aggregate formation. Aggregates that had formed after 4 hr in the presence of EBX disassembled slowly (within 12-14 hr) following removal of EBX at 36 degrees, and even more slowly at 23-30 degrees. However, a temperature shift to 38 degrees, or the addition of azide, resulted in a rapid but reversible disassembly of aggregates (within 4 hr). Thus, newly formed aggregates appear to be relatively stable structures, while microaggregate clouds are labile, tending to disassemble or evolve into aggregates.

Organization of filaments underneath the plasma membrane of developing chicken skeletal muscle cells in vitro revealed by the freeze-dry and rotary replica method

Cytoskeletal organization and its association with plasma membranes in embryonic chick skeletal muscle cells in vitro was studied by the freeze-drying and rotary-shadowing method of physically ruptured cells. The cytoskeletal filaments underlying the plasma membranes were sparse in myogenic cells at the stage when cells exhibited great lipid fluidity in plasma membranes (fusion competent mononucleated myoblasts and recently fused young myotubes). Myotubes at more advanced stages of development possessed a highly interconnected dense filamentous network just underneath the cell membrane. This subsarcolemmal network was composed predominantly of 8-10 nm filaments; they were identified as actin filaments because of their decoration with myosin subfragment-1. Fine fibrils having a diameter of 3-5 nm were found on the protoplasmic surface of the plasmalemma at both the early and advanced stages of development. They were associated with the subsarcolemmal cytoskeletal filaments. Short 2-5 nm cross-linking filaments were occasionally seen between filaments in the subsarcolemmal network. We conclude that, although the subsarcolemmal cytoskeletal network contains many actin filaments, this domain appears to play some role in preserving the cell shape in the form of the membrane skeleton rather than membrane mobility.

Demonstration of the association of the cell-surface enzyme, 5'-nucleotidase, with microvillar microfilaments by phalloidin shift on velocity sedimentation gradients

The cell-surface enzyme 5'-nucleotidase in microvilli from 13762 rat mammary adenocarcinoma cells remains largely associated with microfilament-containing high-speed pellets from Triton X-100 extracts of the microvilli. The fraction remaining with the insoluble portion is higher under ionic conditions which enhance microfilament stability. To minimize trapping and cosedimentation we have analyzed the distribution of microfilaments and 5'-nucleotidase activity on velocity sedimentation sucrose gradients of the microvillar extracts. A large fraction of the total enzyme activity is found in the filament fractions in the middle of the gradient. When phalloidin is included in the extraction buffer to stabilize the microfilaments, both the microfilaments and the bulk of the nucleotidase activity are shifted further into the gradients. Both the position of the filament fraction and the percentage of the total nucleotidase activity remaining with the filament fraction varies with extraction buffer composition and conditions. Nonetheless, under all conditions tested, a large percentage of the activity was shifted, along with the microfilaments, in the presence of phalloidin. These results are consistent with a specific association of 5'-nucleotidase with microfilaments in the ascites tumor cell microvilli.

Differential responses of L5 and rat primary muscle cells to factors in rat brain extract

Crude brain extract (100,000 g supernate from newborn or fetal rat brain homogenate) was studied for its effects on the number and distribution of acetylcholine receptors (AChRs) on myotubes of the L5 cloned myogenic cell line and compared to that of rat primary cultures. Gamma counting, light autoradiography and scanning electron microscopic autoradiography were used. We found that the L5 cells responded to the brain extract with an increase in the average AChR site density (2-5-fold) and with an increase in AChR clustering. Clustering was manifested by both an increase in the number of AChR clusters and in the ratio of receptor site density within clusters relative to that between clusters. The increase in average AChR site density was shown to be due to an increase in the rate of AChR insertion into the surface membrane with little change in the rate of receptor degradation. As also previously reported, the rat myotubes had a similar clustering response but only a very slight (approximately 1.2-fold) increase in average AChR site density. The surface area of myotubes was also increased slightly (approximately 1.2-1.3-fold) by the brain extract. Autoradiography viewed by scanning EM was found to be very useful in illustrating the shape and distribution of the receptor clusters. After the brain extract was fractionated on Sephadex G-200, the fractions with greatest clustering activity could be separated from those causing predominantly an increase in receptor site density. Increased receptor site density was primarily produced by the low molecular weight fractions (less than 12 kD), whereas the strongest (but not exclusive) effect on clustering was produced by the high molecular weight fractions (greater than 140 kD). Furthermore, the two cell types assayed had different sensitivity to the different factors. L5 cells responded to both the high and low molecular weight factors while rat primary cells are sensitive primarily to the high molecular weight factors.