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PubMed2000

Dystroglycan overexpression in vivo alters acetylcholine receptor aggregation at the neuromuscular junction.

Dystroglycan is a member of the transmembrane dystrophin glycoprotein complex in muscle that binds to the synapse-organizing molecule agrin. Dystroglycan binding and AChR aggregation are mediated by two separate domains of agrin. To test whether dystroglycan plays a role in receptor aggregation at the neuromuscular junction, we overexpressed it by injecting rabbit dystroglycan RNA into one- or two-celled Xenopus embryos. We measured AChR aggregation in myotomes by labeling them with rhodamine-alpha-bungarotoxin followed by confocal microscopy and image analysis. Dystroglycan overexpression decreased AChR aggregation at the neuromuscular junction. This result is consistent with dystroglycan competition for agrin without signaling AChR aggregation. It also supports the hypothesis that dystroglycan is not the myotube-associated specificity component, (MASC) a putative coreceptor needed for agrin to activate muscle-specific kinase (MuSK) and signal AChR aggregation. Dystroglycan was distributed along the surface of muscle membranes, but was concentrated at the ends of myotomes, where AChRs normally aggregate at synapses. Overexpressed dystroglycan altered AChR aggregation in a rostral-caudal gradient, consistent with the sequential development of neuromuscular synapses along the embryo. Increasing concentrations of dystroglycan RNA did not further decrease AChR aggregation, but decreased embryo survival. Development often stopped during gastrulation, suggesting an essential, nonsynaptic role of dystroglycan during this early period of development.

Animals↗
PubMedDate not supplied

Aggregates of acetylcholinesterase induced by acetylcholine receptor-aggregating factor.

Basal lamina-rich extracts of Torpedo californica electric organ contain a factor that causes acetylcholine receptors (AChRs) on cultured myotubes to aggregate into patches. Our previous studies have indicated that the active component of these extracts is similar to the molecules in the basal lamina which direct the aggregation of AChRs in the muscle fibre plasma membrane at regenerating neuromuscular junctions in vivo. Because it can be obtained in large amounts and assayed in controlled conditions in cell culture, the AChR-aggregating factor from electric organ may be especially useful for examining in detail how the postsynaptic apparatus of regenerating muscle is assembled. Here we demonstrate that the electric organ factor causes not only the formation of AChR aggregates on cultured myotubes, but also the formation of patches of acetylcholinesterase (AChE). This finding, together with the observation that basal lamina directs the formation of both AChR and AChE aggregates at regenerating neuromuscular junctions in vivo, leads us to hypothesize that a single component of the synaptic basal lamina causes the formation of both these synaptic specializations on regenerating myofibres.

Acetylcholinesterase↗
PubMed1997

Antisense agrin cDNA transfection blocks neuroblastoma cell-induced acetylcholine receptor aggregation when co-cultured with myotubes.

A neuroblastoma x glioma hybrid cell line, NG108-15, was able to induce the aggregation of AChRs when co-cultured with myotubes. NG108-15 cells in culture expressed agrin, producing a protein of approximately 220 kDa and a transcript of approximately 8.0 kb. The mRNA encoding the agrin isoform having no amino acid insertion at either the Y or the Z site, namely agrin0.0, was the only transcript detected in NG108-15 cells when they were cultured alone or co-cultured with myotubes. NG108-15 cells could be induced to differentiate by chemical treatment, and the chemical-induced differentiation of NG108-15 cells increased the level of agrin mRNA expression approximately fourfold while the expression of a housekeeping gene remained relatively unchanged. The increase in agrin expression of differentiated NG108-15 cells paralleled the increase in AChR-aggregating activity of differentiated NG108-15 cells, indicating that the agrin derived from NG108-15 cells could be the receptor-aggregating factor. In addition, we created a stable clonal NG108-15 cell line that was transfected with antisense agrin cDNA and its expression of agrin was abolished, while its AChR-aggregating activity was completely lost when co-cultured with myotubes. This is the first direct demonstration that NG108-15 cell-induced AChR aggregation on cultured myotubes is mediated by neuron-derived agrin.

Agrin↗
PubMed1992

Mechanism of agrin-induced acetylcholine receptor aggregation.

Agrin induces the formation of specializations on chick myotubes in culture at which several components of the postsynaptic apparatus accumulate, including acetylcholine receptors (AChRs). Agrin also induces AChR phosphorylation. Several lines of evidence suggest that agrin-induced phosphorylation of tyrosine residues in the beta subunit of the AChR is an early step in receptor aggregation: agrin-induced phosphorylation and aggregation have the same dose dependence; treatments that prevent aggregation block phosphorylation; phosphorylation begins before any detectable change in receptor distribution, reaches a maximum hours before aggregation is complete, and declines slowly together with the disappearance of aggregates after agrin is withdrawn; agrin slows the rate at which receptors are solubilized from intact myotubes by detergent extraction; and the change in receptor extractability parallels the change in phosphorylation. A model for agrin-induced AChR aggregation is presented in which phosphorylation of AChRs by an agrin-activated protein tyrosine kinase causes receptors to become attached to the cytoskeleton, which reduces their mobility and detergent extractability, and leads to the accumulation of receptors in the vicinity of the activated kinase, forming an aggregate.

Agrin↗
PubMed1995

The kinetics of bivalent ligand-bivalent receptor aggregation: ring formation and the breakdown of the equivalent site approximation.

When bivalent ligands capable of bridging binding sites on two different receptors interact with bivalent receptors, aggregates form. The aggregates can be of two types: chains (open structures containing n receptors, n-1 doubly bound ligands and 0, 1, or 2 singly bound ligands) and rings (closed structures containing n receptors and n doubly bound ligands). Both types of aggregates have been detected experimentally. In general, to determine the time dependence of the concentration of any particular aggregate requires solving an infinite set of coupled ordinary differential equations (ODEs). Perelson and DeLisi [19] showed that great simplification results if all receptor binding sites are equivalent, i.e., the binding properties of a site on a receptor are independent of the size of the aggregate the receptor is in. If only chains form, the problem reduces to solving two coupled ODEs for the concentrations of singly and doubly bound ligands. From the solutions to these ODEs, the time dependence of the entire aggregate size distribution can be determined. We show that the equivalent site approximation as formulated by Perelson and DeLisi [19] is incompatible with ring formation. We then present a modified equivalent site approximation that is useful if chains of any size can form but rings above a certain size (k) cannot. We show how to reduce the resulting infinite set of coupled ODEs to a closed system of at most 4k + 2 ODEs for the ligand concentrations, the ring concentrations, and the concentrations of all chains up to size k. Although we can only predict the kinetics of aggregate formation for aggregates of size k or less, at equilibrium the modified equivalent site approximation yields the complete aggregate size distribution.

Animals↗
PubMed1996

Quantitative and realtime correlation between receptor aggregation and intracellular calcium signal transduction.

Quantitative correlation between intracellular calcium signals and hapten density or molecular size of antigens was studied for two cell lines; hapten-specific murine B cells (TP67.21) and rat basophilic leukemia cells (RBL-2H3) with hapten-specific IgE. Magnitude of the induced calcium signal in both cells exhibited the same dependence on hapten density of antigen molecules and there existed an optimal hapten density which induced the maximum amount of calcium signal for both cells. However, they responded differently to antigens of various molecular size. In contrast to TP67.21 cells which showed larger response to larger antigen molecules, RBL-2H3 cells showed the largest response to the smallest antigen. This may possibly suggest that there exists an optimal structure of receptor aggregates for each cell. Calcium signal induced in each cell by multivalent antigen was rapidly abrogated by addition of excess hapten and this abrogation occurred both in transmembrane influx and the release from intracellular stores. We directly observed the mobilization of receptor molecules during this calcium signal abrogation at single cell level by using two fluorescent calcium probes, whose fluorescence wavelength ranges have least overlap, and confocal microscopy. During this abrogation, large clusters of receptor molecules were not affected by hapten molecules. We, therefore, conclude that these large clusters are inactive in the induction of calcium signal and smaller clusters of receptor molecules are necessary for calcium signal induction.

Animals↗
PubMed1993

Inhibition of epidermal growth factor receptor aggregation by an antibody directed against the epidermal growth factor receptor extracellular domain.

We have examined the perturbation of epidermal growth factor (EGF) receptor-receptor interactions by a monoclonal antibody (13A9) that binds to the receptor extracellular domain. While 13A9 did not inhibit EGF binding, it inhibited energy transfer between fluorescent-labeled EGF molecules bound to receptors in membranes from human A431 cells by 70-100%. This antibody also inhibited EGF-stimulated receptor dimerization in membranes as assessed by chemical cross-linking and Fab fragments of the antibody strongly inhibited the EGF-stimulated dimerization of solubilized receptors when assessed by velocity sedimentation. However, under conditions where 13A9 inhibited receptor-receptor interactions within the plasma membranes, the antibody had no effect on EGF-stimulated receptor autophosphorylation or tyrosine kinase activity toward an exogenous substrate. Moreover, although the antibody significantly inhibited receptor dimerization in A431 cells, it had no effect on EGF-stimulated changes in cytosolic free [Ca2+] or 125I-EGF uptake in these cells, or on EGF-stimulated DNA synthesis in Swiss 3T3 cells. We conclude that the dimerization of the EGF receptors in a membrane environment is not required for full activation of tyrosine kinase activity and that inhibition of the dimerization of a large fraction of EGF receptors in cells does not necessarily inhibit several EGF-mediated cellular responses.

3T3 Cells↗
PubMed1996

Antigen-mediated IGE receptor aggregation and signaling: a window on cell surface structure and dynamics.

The high-affinity receptor for immunoglobulin E, Fc epsilon RI, serves as an archtype for multisubunit immunoreceptors that mediate cell activation in response to foreign antigens. Antigen-mediated aggregation of this receptor at the surface of mast cells and basophils initiates a biochemical cascade that uses nonreceptor tyrosine kinases as key participants in the earliest steps of this signal transduction process. Cross-linking of Fc epsilon RI with ligands of well-defined structure and valency has revealed detailed information about the fundamental requirements for functionally active receptor aggregates. Cross-linking-dependent changes in the interaction of these receptors with other cellular components have been characterized with biochemical and biophysical methods to develop a more complete view of signal initiation. Recent evidence suggests that this process involves the interaction of aggregated Fc epsilon RI with specialized plasma membrane domains that may localize important signaling molecules in the vicinity of aggregated receptors. Although these various studies were aimed toward understanding the operation of one cell surface receptor, they provide new insights into plasma membrane structure and dynamics that are generally relevant to the function of most nucleated mammalian cells.

Amino Acid Sequence↗
PubMed1998

Tyrosine phosphorylation of the muscle-specific kinase is exclusively induced by acetylcholine receptor-aggregating agrin fragments.

During formation of the neuromuscular junction, the basal membrane protein agrin initiates the aggregation of acetylcholine receptors (AChR) on the surface of myotubes. A muscle-specific kinase (MuSK) becomes phosphorylated upon incubation with agrin, although it does not bind to agrin on its own. Utilizing MuSK-specific antibodies, we demonstrate that the ability of different splicing variants and truncation fragments of agrin to trigger MuSK phosphorylation and AChR aggregation are correlated. Only agrin forms which are potent inducers of AChR-clustering are able to trigger the phosphorylation of MuSK. Picomolar concentrations of agrin are already sufficient to induce MuSK phosphorylation. Similar amounts are necessary for the aggregation of AChRs as well as their phosphorylation on a tyrosine residue. The complete overlap of specificities for MuSK phosphorylation and AChR aggregation suggests that only binding of agrin to a MuSK-containing receptor complex is responsible for the initiation of AChR aggregation. In contrast, interactions of agrin with binding proteins on the muscle surface harbouring different specificities such as alpha-dystroglycan do not seem to be necessary for this process.

Agrin↗
PubMed1989

Reorganization and stabilization of acetylcholine receptor aggregates on rat myotubes.

Aggregation of acetylcholine receptors (AChRs) is an important early feature of the postsynaptic development of the vertebrae neuromuscular junction. At later stages of differentiation, aggregates are remodeled and stabilized. Aggregation of AChRs can be induced on rat myotubes in culture within 4 hr by treatment with embryonic pig brain extract (EBX). In this study, further sequential changes in the distribution of AChRs were followed by video-intensified fluorescence microscopy. These studies have revealed that groups of AChR aggregates that have formed after 4 hr in EBX are reorganized during the exposure to EBX for 20 additional hr to form a smaller number of larger, oval-shaped aggregates. We have named these two types of aggregates "4-hr aggregates" and "24-hr aggregates". This reorganization occurs by the expansion and merging of individual aggregates within a group, and by the incorporation of newly inserted AChRs. The 24-hr aggregates are an average of 15 times greater in area than 4-hr aggregates, and contain regions with an apparent AChR site density (fluorescence intensity) that is more than twice that of 4-hr aggregates. Electron microscopy of mapped 24-hr aggregates revealed that folded plasma membrane is associated with these regions, probably accounting for the elevated fluorescence. The 24-hr aggregates are more stable than 4-hr aggregates, as determined by their significantly slower disassembly after removal of EBX, elevation of temperature (38 degrees C), reduction of extracellular calcium levels (0.1 mM), or the addition of sodium azide (7 mM). This was determined by following disassembly both statistically (using fixed cultures) and by direct observations of living myotubes. These findings were confirmed by measuring the sequential changes in relative AChR site density over time in individual living myotubes. Thus, 24-hr aggregates form by the reorganization of 4-hr aggregates; exhibit a more regular, compact shape; and are more stable than 4-hr aggregates. These changes in AChR organization and aggregate stability resemble the changes occurring after the initial formation of junctional AChR aggregates during embryonic development, demonstrating additional similarities between this model system and the developing neuromuscular junction.

Animals↗
PubMed1998

Nitrocellulose particles adsorbed to immunoglobulins are a new and effective approach to induce cell activation dependent on receptor aggregation.

Nitrocellulose (NC) has proved to be a versatile tool for the isolation and characterization of various biomolecules. In this report we extend its scope by using antibody-coated NC particles to cross-link molecules on the surface of living cells. Ligation of receptors in Jurkat cells with NC-bound specific antibodies induced protein tyrosine phosphorylation patterns of cellular proteins comparable to conventional antibody cross-linking. In addition, the present study shows that application of NC particles coated with human IgA significantly activated monocytic cells via the Fc alpha receptor (Fc alphaR), whereas cross-linking of receptor-ligand complexes with isotype-specific antibody was less efficient. Subsequent immunoprecipitation and immunoblot analysis of aggregated Fc receptors (FcRs) complexed to Ig-adsorbed particles permits fast identification of molecules involved in the transmission of signals. Therefore, ligand-coated NC particles can be used to examine receptor-mediated cell activation events dependent upon extensive receptor aggregation.

Adsorption↗
PubMed1990

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.

Actinin↗
PubMed1990

Concanavalin A-induced receptor aggregation stimulates the tyrosine kinase activity of the insulin receptor in intact cells.

Concanavalin A (ConA) stimulated the phosphorylation of the beta-subunit of the insulin receptor and an Mr-185,000 protein on serine and tyrosine residues in intact H-35 rat hepatoma cells. This Mr-185,000 protein whose phosphorylation was stimulated by ConA was identical to pp185, a protein reported previously to be a putative endogenous substrate for the insulin receptor tyrosine kinase in rat hepatoma cells. In Chinese hamster ovary (CHO) cells transfected with cDNA of the human insulin receptor, tyrosine-phosphorylation of pp185 was strongly enhanced by ConA compared with the controls, suggesting that the induction of tyrosine-phosphorylation of pp185 was due to stimulation of the insulin receptor kinase by ConA. Moreover, monovalent ConA only slightly induced the tyrosine-phosphorylation of pp185, which was enhanced by the addition of anti-ConA IgG, suggesting that ConA stimulated the insulin receptor kinase mainly by the receptor cross-linking or aggregation in intact cells. These data suggest that the insulin-mimetic action of ConA is related to the autophosphorylation and activation of the insulin receptor tyrosine kinase, as well as the subsequent phosphorylation of pp185 in intact cells.

Animals↗
PubMed1992

Characterization of a recombinant extracellular domain of the type 1 tumor necrosis factor receptor: evidence for tumor necrosis factor-alpha induced receptor aggregation.

An expression plasmid encoding the extracellular portion of the human tumor necrosis factor (TNF) type 1 receptor (TNF-R1) was constructed and used to generate a stable cell line secreting soluble TNF-R1 (sTNF-R1). The sTNF-R1 was purified, and its biochemical properties and its interactions with human TNF-alpha were examined. SDS-PAGE resolved the purified sTNF-R1 into three bands of approximate Mr 24,200, 28,200, and 32,800. Sedimentation equilibrium analysis gave a molecular weight of 25,000 for sTNF-R1 whereas the molecular weight obtained by gel filtration chromatography was approximately 55,000-60,000. Scatchard analysis of [125I]TNF-alpha binding to sTNF-R1 revealed high-affinity binding (Kd = 93 pM), comparable to that observed for the intact receptor on whole cells. Competitive binding experiments showed that sTNF-R1 has a 50-60-fold higher affinity for TNF-alpha than for TNF-beta, in contrast to the equal affinities of TNF-alpha and TNF-beta for the full-length TNF-R1 transiently expressed in mammalian cells. The sTNF-R1 was found to block the cytotoxicity of TNF-alpha and TNF-beta on a murine L-M cell assay. The sizes of the sTNF-R1.TNF-alpha complex determined by gel filtration chromatography and sedimentation equilibrium were approximately 141 and 115 kDa, respectively. The stoichiometry of the complex was examined by Scatchard analysis, size-exclusion chromatography, HPLC separation, amino acid composition, sequence analysis, and sedimentation equilibrium. The data from these studies suggest that at least two molecules of sTNF-R1 can bind to a single TNF-alpha trimer.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1998

A minigene of neural agrin encoding the laminin-binding and acetylcholine receptor-aggregating domains is sufficient to induce postsynaptic differentiation in muscle fibres.

The extracellular matrix molecule agrin is both necessary and sufficient for inducing the formation of postsynaptic specializations at the neuromuscular junction (NMJ). At the mature NMJ, agrin is stably incorporated in synaptic basal lamina. The postsynapse-inducing activity of chick agrin, as assayed by its capability of causing aggregation of acetylcholine receptors (AChRs) on cultured muscle cells, maps to a 21 kDa, C-terminal domain. Binding of chick agrin to muscle basal lamina is mediated by the laminins and maps to a 25 kDa, N-terminal fragment of agrin. Here we show that an expression construct encoding a 'mini'-agrin, in which the laminin-binding fragment was fused to the AChR-clustering domain, is sufficient to induce postsynaptic differentiation in vivo when injected into non-synaptic sites of rat soleus muscle. As shown for ectopic postsynaptic differentiation induced by full-length neural agrin, myonuclei underneath the ectopic sites expressed the gene for the AChR epsilon-subunit. Altogether, our data show that a 'mini'-agrin construct encoding only a small fraction of the entire agrin protein is sufficient to induce postsynapse-like structures that are reminiscent of those induced by full-length neural agrin or innervation by motor neurons.

Agrin↗
PubMed2005

Receptor aggregation by intermembrane interactions: a Monte Carlo study.

The lateral organization of receptors on cell surfaces is critically important to their function; many receptors transmit transmembrane signals when redistributed into clusters, while the response of others is potentiated by their aggregation. Cell-cell contact can play a crucial role in receptor aggregation, even when the bonds between receptors on one cell and ligands on the other are monovalent. Monte Carlo simulations on a two-membrane model were carried out to determine whether weak enthalpic interactions among receptors in one membrane, and among ligands in another, can work synergistically to give large-scale clustering when the two membranes are brought into contact. The simulations give support to such a clustering mechanism. In addition, because clustering is a cooperative process akin to a phase separation, individual receptors and ligands may undergo repeated binding and unbinding while in a clustered "phase," and a single ligand could interact with multiple different receptor partners. The results suggest a resolution of the dichotomy between serial triggering and aggregation models of T cell activation.

Cell Membrane↗
PubMed2005

Platelet endothelial aggregation receptor 1 (PEAR1), a novel epidermal growth factor repeat-containing transmembrane receptor, participates in platelet contact-induced activation.

The present study was designed to identify novel membrane proteins that signal during platelet aggregation. Because one putative mechanism for signaling by a membrane protein involves phosphorylation, we used oligonucleotide-based microarray analyses and mass spectrometric proteomics techniques to specifically discover membrane proteins and also identify those proteins that become phosphorylated on tyrosine, threonine, or serine residues upon platelet aggregation. Surprisingly, both techniques converged to identify a novel membrane protein we have termed PEAR1 (platelet endothelial aggregation receptor 1). Sequence analysis of PEAR1 predicts a type-1 membrane protein, 15 extracellular epidermal growth factor-like repeats, and multiple cytoplasmic tyrosines. Analysis of the tissue distribution of PEAR1 showed that it was most highly expressed in platelets and endothelial cells. Upon platelet aggregation induced by physiological agonists, PEAR1 became phosphorylated on tyrosine (Tyr-925), and serine (Ser-953 and Ser-1029) residues. PEAR1 tyrosine phosphorylation was blocked by eptifibatide, an alpha(IIb)beta(3) antagonist, which inhibits platelet aggregation. Immune clustering of PEAR1 resulted in PEAR1 phosphorylation. Aggregation-induced PEAR1 tyrosine phosphorylation lead to the subsequent association with the ShcB adaptor protein. Platelet proximity induced by centrifugation also induced PEAR1 tyrosine phosphorylation, a reaction not inhibited by eptifibatide. These data suggest that PEAR1 is a novel platelet receptor that signals secondary to alpha(IIb)beta(3)-mediated platelet-platelet contacts.

Amino Acid Sequence↗
PubMed1988

Control of the aggregation factor-aggregation receptor interaction in sponges by protein kinase C.

By means of immunobiochemical and immunocytological techniques it was found that the aggregation factor (AF) from the sponge Geodia cydonium is stored in vesicles of spherulous cells. During the reaggregation process of dissociated cells, the AF which is present extracellularly was determined to be bound to the cell-surface-associated aggregation receptor (AR) only during the initial phase (0-5 h after addition of the AF to the single cell suspension). At later stages (20 h), the AF colocalized with extracellular structures, e.g., collagen and glycoconjugates. Immobilized to nitrocellulose, the AR, a molecule with Mr of 43.5 kDa, displayed its binding affinity to the AF only if it was isolated from early aggregates (5 h). The transition of the AF-susceptible to the AF-deficient state of the plasma membrane was mimicked in vitro by incubation of plasma membranes from early aggregates with purified protein kinase C. This conversion to the AF-deficient state could be prevented by the protein kinase C inhibitor staurosporine. Together with earlier findings, which revealed that the AR is phosphorylated by protein kinase C, we propose that in the sponge system this enzyme controls intercellular processes involved in morphogenesis.

Animals↗