Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Receptor Aggregation”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 541 records · Page 30Linked to original sources
PubMed1992

Molecular economy and antibody function: the evolution of a Protecton.

The humoral immune response protects against a very large array of pathogens which attempt to escape immune recognition by changing the antigens they display. When looked at from the point of two competing sets of DNA (i.e., the pathogens vs. the host), there is a vastly larger pool of mutating pathogen DNA than in, say, a mouse. The stratagems that allow a tiny fraction of the mouse's genome to effectively compete with a hugely diverse array of pathogens is analyzed in terms of how antibody functions and how the immune system avoids such pitfalls as self-recognition and destruction. This review is a more general description of a lengthy series of papers which detailed the evolution of the Protecton. Starting from the obvious, that is the concentration-dependence of antibody function, it is apparent that the functional antibody repertoire must be relatively small if a sufficient concentration of specific antibody is to be produced in time to arrest the growth of pathogens and eventually eliminate them. Thus, commonly quoted estimates of antibody repertoires in the range from greater than 10(10) to "complete" (infinite?) must be seriously in error. Other well known "facts", such as D-diversity, and B cell signaling by receptor aggregation are also shown to be lacking in biological commonsense.

Animals↗
PubMed1992

Nicotinic receptor-associated 43K protein and progressive stabilization of the postsynaptic membrane.

An extrinsic membrane protein of apparent molecular mass 43 kDa is specifically localized in postsynaptic membranes closely associated with the nicotinic acetylcholine receptor (AChR). Since its discovery in 1977, biochemical and morphological studies have combined to provide relatively clear pictures of 43K protein structure and subcellular compartmentalization. Nevertheless, despite these advances, the precise function of this synapse-specific protein remains unclear. Data gathered in recent years indicate that the postsynaptic apparatus develops through the incremental agglomeration of receptor microaggregates; evidence derived from a number of sources points to a role for 43K protein in certain underlying reactions. In this paper, I review 43K protein structural and anatomical data and analyze evidence for its role in the organization and maintenance of the postsynaptic membrane. Finally, I offer a model presenting a view of the role of 43K protein in the ontogeny of the motor endplate.

Amino Acid Sequence↗
PubMed2004

Repeated apomorphine administration alters dopamine D1 and D2 receptor densities in pigeon basal telencephalon.

When pigeons are repeatedly administered a dose of apomorphine they show an increasing behavioral response, much as rodents do. In birds this expresses itself in an augmented pecking response. This sensitization is assumed to be largely due to a conditioning process. Here we present evidence that sensitization is accompanied by an alteration of the D(1) to D(2) dopamine receptor densities. An experimental group of pigeons was repeatedly injected with apomorphine, and a control group with saline. The basal forebrain tissue, known to be rich in dopamine receptors, was subjected to binding assays using tritiated specific D(1) and D(2) dopamine receptor antagonists. There was a trend towards an increase in D(1) and a significant decrease in D(2) receptor densities in apomorphine-treated birds compared to the saline-treated controls. We conclude that extended apomorphine treatment modifies the D(1) dopamine receptor density in the opposite manner to the D(2) dopamine receptor density.

Animals↗
PubMed2004

Mercury decreases the frequency of induced but not spontaneous clustering of acetylcholine receptors.

Studies with environmental levels of various metals typically focus on observable neurological symptoms in newborns and adults. Use of the C2C12 skeletal muscle cell line as a developmental model enabled us to test whether environmental insults prevented myotube formation or the assembly of the postsynaptic component of the neuromuscular synapse. Specifically, we asked whether the inorganic metal mercury interfered with the fusion of myoblasts into myotubes, acetylcholine receptor (AChR) clustering, or the agrin signaling events that precede AChR clustering. C2C12 myotubes grown in culture medium containing 10 microM mercuric chloride were morphologically indistinguishable from control myotubes at the light-microscopic level, and myoblasts fused into myotubes normally. However, myotubes pretreated with mercury demonstrated a decreased frequency of AChR clustering induced by agrin and other experimental manipulations. Furthermore, mercury pretreatment decreased the agrin-induced tyrosine phosphorylation of the AChR beta subunit, thus inhibiting the agrin signal transduction pathway. In contrast, mercury failed to decrease the frequency of spontaneous AChR clustering, suggesting that spontaneous AChR clustering differs from agrin-induced AChR clustering in some significant way.

Animals↗
PubMed2000

Sialic acid inhibits agrin signaling in C2 myotubes.

Acetylcholine receptor (AChR) clustering is an early event in neuromuscular synapse formation that is commonly studied using muscle cell culture. Motor neuron-derived agrin induces the postsynaptic tyrosine phosphorylation of both a muscle-specific kinase (MuSK) and the AChR beta-subunit. These phosphorylation events are required for AChR clustering, suggesting an agrin-driven signaling pathway. Both the phosphorylation events and AChR clustering can also be induced by neuraminidase, an enzyme that cleaves sialic acid from glycoconjugates, suggesting that neuraminidase is able to activate the agrin signaling pathway. A postulated signal for postsynaptic differentiation at sites of nerve-muscle contact during vertebrate development is the enzymatic removal of basal lamina components. We show here that bath-applied sialic acid has an effect directly opposite that of agrin or neuraminidase. Sialic acid not only decreases AChR clustering but also diminishes the tyrosine phosphorylation of MuSK and the AChR beta-subunit signal-transduction events normally driven by agrin. However, sialic acid does not prevent agrin-binding molecules from colocalizing with the decreased number of AChR clusters that do form, suggesting that sialic acid is acting to inhibit the agrin signaling pathway downstream of agrin binding to the muscle cell membrane. We propose a regulatory role for sialic acid in the signal transduction events of neuromuscular synapse formation, in which agrin or neuraminidase can overcome this sialic acid repression, resulting in the clustering of AChRs and other postsynaptic molecules.

Agrin↗
PubMed1989

Binding of plasma membrane glycoproteins to the cytoskeleton during patching and capping is consistent with an entropy-enhancement model.

Concentrations of concanavalin A that induced patching and capping of cell surface receptors on Dictyostelium discoideum also induce binding of the receptors to the cortical cytoskeleton, which was isolated by density-gradient centrifugation. The receptors were solubilized by deoxycholate, purified by affinity chromatography, and used to determine whether the receptors bound directly to the cytoskeletal protein, actin. As the concentration of actin was increased, many of the receptors became bound to purified filamentous rabbit muscle actin, even in the absence of concanavalin A. As in the ligation-induced binding of receptors to the cortical cytoskeleton in cells, concanavalin A induced much stronger binding of the purified receptors to filamentous actin. The results were consistent with a previously stated hypothesis that induction of receptor binding to the cytoskeleton during their patching and capping is driven by clustering the receptors, which reduces their translational entropy and by doing so enhances their avidity for the cytoskeleton.

Actins↗
PubMed1996

Arachidonate-induced eicosanoid synthesis in RBL-2H3 cells: stimulation with antigen or A23187 induces prolonged activation of 5-lipoxygenase.

We studied the ability of rat basophilic leukemia (RBL-2H3) cells stimulated with either IgE/antigen or calcium ionophore, A23187, to synthesize LTC4 and PGD2 after addition of exogenous arachidonic acid. RBL-2H3 cells preferentially synthesized PGD2 in response to stimulation with low concentrations of antigen or A23187 while higher concentrations also resulted in a marked synthesis of LTC4. The synthesis of LTC4 was dependent upon initial activation of 5-lipoxygenase by IgE/antigen or A23187, since arachidonic acid lone failed to induce LTC4 synthesis. Following the addition of IgE/antigen or A23187 alone, the synthesis of PGD2 and LTC4 was essentially complete by 10 min. To determine whether a limitation of substrate precluded further eicosanoid synthesis, exogenous arachidonic acid was added to washed cells 15-145 min following the initial stimulation with IgE/antigen or A23187, PGD2 and LTC4 synthesis was resumed following the addition of arachidonic acid to washed prestimulated cells, demonstrating that the termination of eicosanoid synthesis in RBL-2H3 cells was nor caused by the inactivation of cyclooxygenase and 5-lipoxygenase. DNP-lysine was added to cells previously stimulated with IgE/antigen to stop receptor aggregation and this greatly inhibited subsequent production of LTC4 following the addition of arachidonic acid, suggesting that ongoing stimulation of Fc epsilon XsRI was required for LTC4 synthesis in this setting. These results indicate that the magnitude of a physiologic stimulus (IgE/antigen) can profoundly affect the arachidonate metabolites produced by mast cells and that the synthesis of these metabolites quickly becomes limited by substrate availability rather than the activity of cyclooxygenase or 5-lipoxygenase.

Animals↗
PubMed1992

Cross-linking of surface immunoglobulin activates src-related tyrosine kinases in WEHI 231 cells.

Crosslinking of sIgM on the B cell line WEHI 231 with anti-sIgM antibody induces protein tyrosine phosphorylation, implicating protein tyrosine kinases (PTKs) in sIg-mediated signal transduction. We have analyzed this cell line for members of the src family of PTKs and have evaluated whether these PTKs might be involved in the process of sIgM-mediated signaling. Our results show that Blk, Lyn, Lck, and Hck are detectable in WEHI 231 cells. Addition of antibodies to sIgM were found to variably stimulate the activities of Blk, Lyn, Lck, and Hck as measured by immune-complex protein kinase assays. Autophosphorylation of these src PTKs, as assessed by reaction with anti-phosphotyrosine antibodies, increased over the time course of sIgM-mediated activation. Co-immunoprecipitation studies to investigate the potential physical interaction of src PTKs with the sIgM receptor complex revealed that, under digitonin and Brij 96 lysis conditions Lyn, Lck, Hck, but not Blk associated with sIgM.

Animals↗
PubMed1988

Monoclonal antibody internalization and degradation during modulation of the CD3/T-cell receptor complex.

Although it is well known that the CD3/T-cell receptor (TCR) complex modulates from the surface of T cells upon exposure to monoclonal antibodies (mAb) directed against it, the fate of bound mAb has not been yet elucidated. We therefore perform direct binding experiments of 125I-labeled mAb against CD3 or TCR to investigate their fate in Jurkat T cells. We demonstrated that all mAb were progressively internalized and degraded in Jurkat T cells and that this degradation was inhibited by chloroquine, an inhibitor of lysosomal degradation enzymes. The sequestration of anti-CD3 mAb in acid compartments was furthermore shown using cytofluorometry. All together our results show that antibodies against CD3 or against TCR follow the same endocytic pathway.

Antibodies, Monoclonal↗
PubMed1988

Differential immunomodulation by anti-CD2 monoclonal antibodies of anti-CD3-induced T cell activation: dependence upon the individual anti-CD3 monoclonal antibody used for activation.

Two monoclonal antibodies (mAb) recognizing different CD2 epitopes each inhibited anti-CD3-induced proliferation and anti-CD3-induced increase in surface CD2 expression. The magnitude of inhibition by either anti-CD2 mAb was dependent upon which anti-CD3 mAb was used as the stimulus, being more pronounced when the anti-CD3 mAb 454 was used as the stimulus than when either anti-CD3 mAb 147 or 446 was the stimulus. The effects of neuraminidase-treated sheep erythrocytes (which bind to CD2) were also more pronounced on mAb 454-induced proliferation than on mAb 147- or 446-induced proliferation. Furthermore, the effects of preincubation with anti-CD2 mAb depended upon the responder status of the donor to IgG1 anti-CD3 mAb. Preincubation of high-responder cells with anti-CD2 mAb had little effect on subsequent IgG1 anti-CD3-induced proliferation. In contrast, preincubation of low-responder cells with anti-CD2 mAb usually augmented the otherwise small proliferative response to IgG1 anti-CD3 mAb. Taken together, these observations suggest that interaction of surface CD2 with ligand alters the response of T cells to anti-CD3 mAb, but these effects depend upon the individual anti-CD3 mAb used for stimulation. These studies raise the possibility that perturbation of different parts of the CD3-T cell antigen receptor complex may lead to different sequelae, and, as a result, the T cell may respond to a given immunomodulator in different ways.

Antibodies, Monoclonal↗
PubMed1990

Lymphoma models for B cell activation and tolerance. VIII. Cross-desensitization by sIgM and sIgD and its effects on growth regulation by anti-isotype antibodies.

ECH408-1 is a murine B cell lymphoma expressing idiotypically and allotypically distinguishable transfected and endogenous IgD. Previously, we demonstrated that this cell line was not growth inhibited by antibodies directed at membrane IgD, but could be inhibited by antibodies which crosslink membrane IgM. Herein, we demonstrate that both anti-mu and anti-delta will cause calcium mobilization in this transfected cell line; this is followed by a period during which antibodies against the alternative isotype are unable to induce significant increases in intracellular calcium concentrations. This phenomenon, called "desensitization," is short-lived, lasting 20 min. We further demonstrate that acute desensitization of these cells by anti-delta has no effect on immediate growth inhibition which is elicited by anti-mu. These data confirm our earlier proposal that the rapid, initial calcium response seen in these lymphomas is not required for the negative signal for growth. Moreover, we also demonstrate that pretreatment of these lymphoma cells with phorbol myristate acetate (PMA) also renders these lymphoma cells temporarily incapable of manifesting a significant calcium signal. Nonetheless, PMA-pretreated B lymphoma cells are not altered in their subsequent sensitivity to anti-mu growth inhibition, nor are they affected in their resistance to inhibition by anti-delta. Our data confirm the proposal that neither the calcium signal nor protein kinase-C activation is involved in the modulation of B lymphoma growth.

Animals↗
PubMed1991

Stimulation of human Jurkat cells by monoclonal antibody crosslinking of transfected-Ly-6A.2 (TAP) molecules.

Monoclonal antibody crosslinking of phosphatidylinositol-anchored Ly-6A.2 molecules on the surface of murine T lymphocytes leads to cell activation and secretion of IL-2. To examine the potential activity of these molecules in human T cells we transfected the Ly-6A.2 gene into Jurkat cells. Transfection of Jurkat cells with genomic Ly-6A.2 sequences results in low levels of Ly-6A.2 on the cell surface. However, linking the Ly-6A.2 sequences to the enhancer from the human CD2 gene results in greatly increased expression of Ly-6A.2. These molecules are anchored to the membrane via a phosphatidylinositol linkage. Crosslinking of Ly-6A.2 molecules with soluble mAb stimulates the transfected Jurkat cells to produce IL-2. This stimulation is abrogated by treatment with phosphatidylinositol-specific phospholipase C. The transfected human T cells displayed the same unusual crosslinking requirements for stimulation with anti-Ly-6A.2 mAbs as previously observed for murine T cells. Crosslinking of Ly-6A.2 with soluble antibodies is stimulatory, whereas immobilized antibodies are inactive. The crosslinking requirements for antiCD3 mAb stimulation display a reciprocal pattern. These data demonstrate that the Ly-6A.2 pathway for T cell activation is conserved between human and murine T cells.

Antigens, Differentiation, T-Lymphocyte↗
PubMed1991

IgE and IgG are synergistic in antigen-mediated release of thromboxane from human lung macrophages.

The mechanism of IgE-mediated release of thromboxane A2 from human lung macrophages has been studied using a monoclonal chimeric human/mouse IgE antibody and its specific antigen. The cells could be sensitized at 37 degrees C but not at 4 degrees C by incubation with IgE, and released a significant amount of thromboxane A2 (TXA2), measured as the stable hydrolysis product TXB2, in response to an anti-chimeric IgE antibody. In contrast, stimulation of IgE-sensitized macrophages with the specific antigen produced less than 10% of this response. A similar time course for the release of TXB2 and the formation of inositol monophosphate in the presence of LiCl was observed. Cleavage of the Fc domain of the anti-chimeric IgE antibody substantially eliminated its capacity to stimulate IgE-sensitized cells. However, the weak or undetectable response to chimeric IgE plus specific antigen was substantially potentiated by an antigen-specific chimeric IgG antibody. IgG-sensitized macrophages did not respond to antigen challenge by the release of TXB2. Preincubation of the cells with a monoclonal antibody against the low affinity receptor for IgE (Fc epsilon RII/CD23) did not prevent IgE sensitization. We conclude that cell-bound IgE antibody cannot induce the release of TXB2 but has fixed antigen which then must interact with specific IgG antibody and IgG receptors to induce mediator release.

Antigen-Antibody Complex↗
PubMed1992

Surface mu heavy chain expressed on pre-B lymphomas transduces Ca2+ signals but fails to cause growth arrest of pre-B lymphomas.

Little is known about the role of signals transduced by cell surface IgM (sIgM) expressed during early B cell development. A subclone (1.6) of the late pre-B cell lymphoma 70Z/3.12 was used to study signal transduction by surface mu heavy (H) chain before and after transition to the early immature B cell stage, and the functional consequences thereof. Although kappa L chain expression can be induced on 1.6 cells by LPS or cytokines, immunoprecipitations indicated that the non-induced 1.6 cells expressed mu H chain with an alternative protein(s) which may be a surrogate light chain(s). Consistent with this, anti-mu but not anti-kappa or anti-lambda antibodies caused transient Ca2+ mobilization in noninduced 1.6 cells. The Ca2+ signal was derived from both intracellular stores and Ca2+ influx in either noninduced cells or in cells that had been preinduced to express kappa L chain. Thus, the ability of mu H chain to mobilize Ca2+ as a second messenger does not depend upon the expression of mature L chains. The immature B lymphomas, WEHI-231 and CH1, express mature forms of IgM and undergo growth arrest when stimulated by anti-mu antibody. In contrast, signals generated by mu H chain on either noninduced or preinduced 1.6 cells or in the sIgM+ pre-B cell transfectant 300-19 mu lambda 36/8 did not cause growth arrest. These results suggest that mu H chain expressed on pre-B cells is capable of mobilizing Ca2+, but that this signal alone is insufficient to induce growth arrest in the pre-B cell.

B-Lymphocytes↗
PubMed1985

Formation of acetylcholine receptor clusters at neuromuscular junction in Xenopus cultures.

The formation of acetylcholine receptor (AChR) clusters at the neuromuscular junction was investigated by observing the sequential changes in AChR cluster distribution on cultured Xenopus muscle cells. AChRs were labeled with tetramethylrhodamine-conjugated alpha-bungarotoxin (TMR-alpha BT). Before innervation AChRs were distributed over the entire surface of muscle cells with occasional spots of high density (hot spots). When the nerve contacted the muscle cell, the large existing hot spots disappeared and small AChR clusters (less than 1 micron in diameter) initially emerged from the background along the area of nerve contact. They grew in size, increased in number, and fused to form larger clusters over a period of 1 or 2 days. Receptor clusters did not migrate as a whole as observed during "cap" formation in B lymphocytes. The rate of recruitment of AChRs at the nerve-muscle junction varied from less than 50 binding sites to 1000 sites/hr for alpha BT. In this study the diffusion-trap mechanism was tested for the nerve-induced receptor accumulation. The diffusion coefficient of diffusely distributed AChRs was measured using the fluorescence photobleaching recovery method and found to be 2.45 X 10(-10) cm2/sec at 22 degrees C. There was no significant difference in these values among the muscle cells cultured without nerve, the non-nerve-contacted muscle cells in nerve-muscle cultures, and the nerve-contacted muscle cells. It was found that the diffusion of receptors in the membrane is not rate-limiting for AChR accumulation.

Animals↗
PubMed1991

Lectin-binding receptors, Na+,K(+)-ATPase, and acetylcholinesterase on immunocyte plasma membrane of Limulus polyphemus.

Plasma membrane receptors are crucial for nonself tissue recognition. Using concanavalin A (Con A), wheat germ agglutinin, peanut agglutinin, soybean agglutinin (SBA), and winged pea agglutinin, five lectin-binding receptor molecules have been recognized on the plasma membrane of the granulocyte (immunocyte) of the horseshoe crab, Limulus polyphemus. Only Con A and SBA caused capping of surface receptors. On the basis of the known functions of these lectin-binding receptor molecules in other invertebrates and vertebrates, their roles in phagocytosis, encapsulation, signaling, and possibly in complement pathway activation are postulated. In addition to lectin-binding receptors, Na+,K(+)-ATPase and acetylcholinesterase were detected on the plasma membrane. Because Limulus dates back to some 200 million years, the antiquity of these molecules is suggested. Furthermore, some of the lectin-binding surface receptors have the potential to be used as markers to separate different kinds of hemocytes in higher arthropods and to distinguish between normal and neoplastic cells in humans.

Acetylcholinesterase↗
PubMed1991

The lipid bilayer of acetylcholine receptor clusters of cultured rat myotubes is organized into morphologically distinct domains.

We have studied the composition and organization of the lipid bilayer at the large, substrate-associated clusters of acetylcholine receptors (AChR) that form in cultured rat myotubes. These clusters have a characteristic morphology consisting of alternating linear domains of AChR-rich and AChR-poor membrane, the latter involved in attaching the myotube to the substrate. We partially purified AChR clusters by extracting cultured rat myotubes with the cholesterol-specific detergent, saponin. The lipid bilayer of the cluster preparation was analyzed biochemically and the substructure of the bilayers was studied morphologically using the fluorescent probes, dansyl polymyxin B, and 3,3'-di(C12H25 and C18H37) indocarbocyanine iodide (C12- and C18-diI). Our results demonstrate that preparations of AChR clusters have a lipid composition biochemically similar to that of the surrounding plasma membrane. Morphologically, however, the lipid bilayer appears to be arranged into domains that resemble the interdigitating pattern seen for the AChR. This distinctive lipid organization is not due to the use of saponin to purify clusters, as we obtained similar results with clusters isolated by physically shearing myotube cultures. The domain-like organization of the bilayer at clusters is disrupted by treatments that disperse AChR clusters in intact myotubes or that remove peripheral membrane proteins from isolated clusters. This suggests that such proteins may contribute to the organization of the bilayer. Two additional factors may also contribute to the organization of the bilayer: physical constraints imposed by sites of substrate attachment and, to a lesser extent, "boundary" lipid associated with AChR.

Actins↗
PubMed1988

Correlated muscle and nerve development in the bullfrog cutaneous pectoris.

The development of the cutaneous pectoris muscle was studied and compared with the differentiation of its peripheral nerve in bullfrog (Rana catesbeiana) tadpoles and frogs by light and electron microscopic techniques. This muscle preparation was chosen for this study because it possesses a number of advantages for (and has become a model system for) the study of correlated nerve-muscle development. At the earliest stage examined (stage XI) the presumptive muscle did not contain any contractile or morphologically distinguishable myotubes, but was contacted by the well-defined cutaneous pectoris nerve trunk. Myotubes were present at stage XII, the same time that nerve-associated acetylcholine receptor aggregations and nerve-evoked muscle contractions were first observed. The adult number of axons was present in the cutaneous pectoris nerve at stage XII, but no axons were myelinated. Gradually thereafter, the number of muscle fibers increased and the cutaneous pectoris axons became myelinated. By stages XX and XXI, but prior to metamorphic climax (stage XXV), the adult numbers of cutaneous pectoris muscle fibers and myelinated and unmyelinated nerve fibers were present. These numbers did not change significantly between stages XX and XXI, through metamorphosis, and in the adult, even during the period of the most rapid loss of multiple innervation in the first 2 weeks after metamorphosis. These results show that the nerve was present and in contact with the cutaneous pectoris muscle from the earliest stages of development prior to muscle differentiation, at a time when the muscle was a disorganized mass of undifferentiated cells. Such early contact suggests that the nerve may have a significant influence on muscle maturation.

Animals↗