Heterogeneity of Receptors and Signal Transduction Pathways in Smooth Muscle Cells of the Gut
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Abstract It is well-established that polypeptide hormones elicit their biological effects by binding to receptors expressed on the surface of responsive cells. The characterization of cell surface receptors has proceeded in two distinct phases. Initially, the majority of receptors were described on the basis of their ability to bind radiolabelled derivatives of a particular hormone. These studies allowed the specificity of receptors to be determined and the equilibrium and kinetic characteristics of binding to be investigated. In some cases, through the additional use of chemical cross-linking reagents, an estimate of the apparent molecular weight of receptors was also possible. In general, however, the low abundance of receptors and their integral membrane location hindered their detailed biochemical characterization. Only with the advent and application of appropriate molecular genetic techniques has the predicted primary structure of many receptors been determined. The aim of the following review is to summarize the important structural features that unite receptors of a given family and to highlight themes common to the function of receptors from different families.
Class C GPCR
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Rhodopsin-like receptors
Immune receptor
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PELP-1
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Radioligand
Radioligand Assay
Receptor expression
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Abstract The binding properties, solubilization, and sensitivity to enzymatic treatment of the Fc receptors of guinea pig peritoneal macrophages were studied. The receptors on the cell surface were saturable with soluble immune complexes, and the binding showed linearity in the Scatchard plot. The observed linearity probably reflected preferential binding of the immune complexes of relatively large sizes. When receptors on the cells were operationally saturated, the number of antibody molecules in the cell-bound complexes was calculated to be 6.5 × 105/cell. By inhibition test on the binding of 125I-labeled soluble immune complexes to macrophages, receptor activity was found in the 20,000 × G supernatant fraction of sonically disrupted macrophages, which was precipitable at 100,000 × G. Treatment of the 20,000 × G supernatant or intact cells with Nonidet P-40 solubilized Fc receptors. In the presence of detergent, these receptors existed as molecules with a Stokes radius somewhat larger than that of IgG but they reaggregated when the detergent was removed. Treatment of the sonic lysate of macrophages with phospholipase C revealed that Fc receptor activity on 100,000 × G-percipitable cell fragments was lost as indicated by the binding inhibition test. The loss of activity was shown to be not due to release of the receptors from the cell fragment. By using surface-labeled macrophages it was demonstrated that the activity of the receptors was restored by treating the phospholipase C-inactivated cell fragments with Nonidet P-40 as indicated by direct binding of solubilized receptors to insoluble immune complexes. This restoration of activity was probably brought about by binding of detergent to the receptors. These results suggest the possibility that Fc receptors on cell membranes require the structural support of phospholipids for their activity.
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Soluble cytokine receptors naturally arise from genes encoding membrane-bound receptors or are direct derivatives of the receptors themselves. There is mounting evidence that soluble receptors play important roles in human disease states. In many cases, soluble receptors appear to play an integral part in the dynamic interaction between ligands and their membrane-bound receptors in maintaining and restoring health after a pathological insult but, in some instances, dysregulated expression of soluble receptors can contribute to disease pathology. Nonetheless, an appreciation of the biological actions of soluble receptors, particularly as cytokine inhibitors, has led to their therapeutic use in human diseases. Although early clinical trails of soluble receptors have had unexpected toxicities, their application in medicine continues to advance and it is likely that soluble receptors will join hormones, cytokines, and growth factors as established biological therapies.
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Rhodopsin-like receptors
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ZAJAC, IHOR(Hahnemann Medical College, Philadelphia, Pa.), AND RICHARDL. CROWELL.Location andregeneration ofenterovirus receptors ofHeLacells. J.Bacteriol. 89:1097-1100. 1965.-Treatment ofliveHeLacells withchymotrypsin ortrypsin completely inactivated theviral receptors forcoxsackievirus B3andpoliovirus Ti, respectively. Enzyme-treated cells regained their enterovirus receptor activity after incubation inculture medium.Theexistence ofintracellular receptors forvirus could notbedemonstrated whenliveHeLacells were treated withenzyme prior todisruptionandfractionation. Theseresults suggested thatreceptors forenteroviruses are limited tothecell surface. Itwasreported recently (Zajac andCrowell, 1965) thatproteolytic enzymes willselectively inactivate enteroviral receptors on liveHeLa cells without affecting cell viability. Thisfinding hasmadeitpossible toreinvestigate theintracellular distribution of enterovirus receptors (Holland andMcLaren, 1961;Philipson and Bengtsson, 1962) andtostudy receptor regeneration. Thiscommunication provides evidence that enteroviral receptors arelimited to thecell surface, and thatenteroviral receptors are regenerated inamannersimilar tothatfound for receptors formyxoviruses (Fazekas deSt.Groth, 1948;Stone, 1948a, b;Baluda, 1958; Marcus, 1959) andforEMC virus (Kodza andJungeblut, 1958).
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Mauthner cell
Biological neural network
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Nerve net
Neocortex
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