In vitro excitation of purified membrane fragments by cholinergic agonists : III. Comparison of the dose-response curves to decamethonium with the corresponding binding curves of decamethonium to the cholinergic receptor

Spatial and temporal signal processing and decision making by MAPK pathways

Mitogen-activated protein kinase (MAPK) pathways are conserved from yeast to man and regulate a variety of cellular processes, including proliferation and differentiation. Recent developments show how MAPK pathways perform exquisite spatial and temporal signal processing and underscores the importance of studying the dynamics of signaling pathways to understand their physiological response. The importance of dynamic mechanisms that process input signals into graded downstream responses has been demonstrated in the pheromone-induced and osmotic stress-induced MAPK pathways in yeast and in the mammalian extracellular signal-regulated kinase MAPK pathway. Particularly, recent studies in the yeast pheromone response have shown how positive feedback generates switches, negative feedback enables gradient detection, and coherent feedforward regulation underlies cellular memory. More generally, a new wave of quantitative single-cell studies has begun to elucidate how signaling dynamics determine cell physiology and represents a paradigm shift from descriptive to predictive biology.

Dose-response aligned circuits in signaling systems

Cells use biological signal transduction pathways to respond to environmental stimuli and the behavior of many cell types depends on precise sensing and transmission of external information. A notable property of signal transduction that was characterized in the Saccharomyces cerevisiae yeast cell and many mammalian cells is the alignment of dose-response curves. It was found that the dose response of the receptor matches closely the dose responses of the downstream. This dose-response alignment (DoRA) renders equal sensitivities and concordant responses in different parts of signaling system and guarantees a faithful information transmission. The experimental observations raise interesting questions about the nature of the information transmission through DoRA signaling networks and design principles of signaling systems with this function. Here, we performed an exhaustive computational analysis on network architectures that underlie the DoRA function in simple regulatory networks composed of two and three enzymes. The minimal circuits capable of DoRA were examined with Michaelis-Menten kinetics. Several motifs that are essential for the dynamical function of DoRA were identified. Systematic analysis of the topology space of robust DoRA circuits revealed that, rather than fine-tuning the network's parameters, the function is primarily realized by enzymatic regulations on the controlled node that are constrained in limiting regions of saturation or linearity.

Cell signaling: what is the signal and what information does it carry?

This paper reviews key findings from quantitative study of the yeast pheromone response system. Most come from single cell experiments that quantify molecular events the system uses to operate. After induction, signal propagation is relatively slow; peak activity takes minutes to reach the nucleus. At each measurement point along the transmission chain, signal rises, overshoots, peaks, and declines toward steady state. At at least one measurement point, this decline depends on negative feedback. The system senses and relays percent receptor occupancy, and one effect of the feedback is to maximize precision of this transmitted information. Over time, the system constantly adjusts quantitative behaviors to convey extracellular ligand concentration faithfully. These behaviors and mechanisms that control them are likely to be general for metazoan signaling systems.

Negative feedback that improves information transmission in yeast signalling

Haploid Saccharomyces cerevisiae yeast cells use a prototypic cell signalling system to transmit information about the extracellular concentration of mating pheromone secreted by potential mating partners. The ability of cells to respond distinguishably to different pheromone concentrations depends on how much information about pheromone concentration the system can transmit. Here we show that the mitogen-activated protein kinase Fus3 mediates fast-acting negative feedback that adjusts the dose response of the downstream system response to match the dose response of receptor-ligand binding. This 'dose-response alignment', defined by a linear relationship between receptor occupancy and downstream response, can improve the fidelity of information transmission by making downstream responses corresponding to different receptor occupancies more distinguishable and reducing amplification of stochastic noise during signal transmission. We also show that one target of the feedback is a previously uncharacterized signal-promoting function of the regulator of G-protein signalling protein Sst2. Our work suggests that negative feedback is a general mechanism used in signalling systems to align dose responses and thereby increase the fidelity of information transmission.

Interactions of the nicotinic acetylcholine receptor transmembrane segments with the lipid bilayer in native receptor-rich membranes

Proper ion channel function of the nicotinic acetylcholine receptor (nAChR) requires the interaction of the protein with distinct lipid species present in the receptor's membrane microenvironment. Two classes of lipid binding sites present at the protein-membrane interface have been postulated: annular binding sites primarily occupied by phospholipids and non-annular binding sites mainly occupied by cholesterol [Jones & McNamee (1988) Biochemistry 27, 2364-2374]. We investigated the binding of these lipids to the nAChR and potential dynamics of these interactions during events associated with signal transduction by electron spin resonance spectroscopy (ESR) using spin-labeled analogues of phospholipids, androstane, and stearic acid. Protein-lipid interactions were characterized in receptor-rich membranes prepared from Torpedo californica electric tissue preserving the native lipid environment of the nAChR. We found a strong preference of the receptor for the phosphatidylserine (PS) analogue as compared to the other probes. Up to 57% of PS were perturbed by the membrane protein, while the fraction of motionally restricted lipid for the other analogues was on the order of 30%. After removal of the extramembrane portions of the membrane-bound receptor, we observed a loss of binding sites for the spin-labeled analogue of androstane and for stearic acid, but not for phospholipids and sphingomyelin analogues. Our results demonstrate the existence of topologically distinct lipid binding sites for different lipid species. In the case of cholesterol, extramembrane portions of the receptor are involved, whereas the transmembrane segments meet the requirements for the binding of phospholipids. Tyrosine phosphorylation of the nAChR did not affect protein-lipid interactions in samples of intact nAChR. Similarly, no significant changes were observed in the presence of carbamoylcholine at concentrations that caused rapid and quantitative desensitization of the nAChR.

Muscarinic cholinoceptors in the rabbit's myometrium: a study of the relationship between binding and response

The relationship between [3H](-)-quinuclinidyl benzilate [( 3H](-)QNB) binding and muscarinic cholinoceptors in the myometrium of the oestrogen treated rabbit was studied. [3H](-)QNB binding was specific, saturable and reversible. The ability of muscarinic agonists and antagonists and some of their stereoisomers to inhibit [3H](-)QNB binding and to stimulate or inhibit contraction of the myometrium were almost identical suggesting that [3H](-)QNB binds specifically to the muscarinic cholinoceptor in this tissue. There appeared to be no receptor reserve in this tissue and whilst structural and stereospecific requirements for binding were almost identical to those for the muscarinic cholinoceptor in the guinea pig ileum requirements for activation were not. Competition binding experiments with pirenzepine suggested that antagonist binding sites were heterogeneous whilst those with 4-diphenylacetoxy-N-methyl-piperidine MeBr did not. Results were discussed with regard to simple and complex models of muscarinic cholinoceptor interactions.

Tetrodotoxin receptors in membrane fragments: purification from Electrophorus electricus electroplax and binding properties

A tetrodotoxin receptor-rich preparation of membrane fragments from the electric organ of Electrophorus electricus is described. The specific binding of neurotoxins and freeze-fracture electron microscopy are used as tools to identify and to characterize membrane fractions. Freeze-fracture electron micrographs of the electric organ demonstrate a high density of membrane particles in the extrasynaptic regions. Density gradient fractions show a broad distribution of [3H]tetrodotoxin, [3H]saxitoxin and 125I-labelled bungarotoxin binding in the range of 1.04--1.15 g/ml sucrose densities, with specific neurotoxin binding up to approx. 5 pmol/mg protein. Carrier-free column electrophoresis of density gradient fractions yields a subfraction with tetrodotoxin and alpha-neurotoxin binding up to 30 pmol/mg protein. The major part of the membrane fragments forms vesicles, which are separated by lectin chromatography into an outside-out and inside-out population. The latter represents at least 50% of the material of a density gradient fraction. For the association of tetrodotoxin, a bimolecular kinetic constant kf greater than or equal to 3.10(5) M-1.s-1 is determined. The dissociation constant is k'b = 2.5.10(-2)s-1. These data are in agreement with a thermodynamic dissociation constant of Kd = 20 nM as determined earlier for E. electricus membrane fragments by equilibrium methods (Grünhagen, H.H., Rack, M., Stämpfli, R., Fasold, H. and Reiter, P. (1981) Arch. Biochem. Biophys. 206, in the press). However, these association kinetics of tetrodotoxin binding in vitro are significantly different from kinetics determined electrophysiologically in Rana (Wagner, H.H. and Ulbricht, W. (1975) Pflügers Arch. 359, 297--315) or Xenopus (Schwarz, J.R., Ulbricht, W. and Wagner, H.H. (1973) J. Physiol. 233, 167--194).

Acetylcholine receptor-controlled ion flux in electroplax membrane vesicles: identification and characterization of membrane properties that affect ion flux measurements

Several intrinsic properties of acetylcholine receptor-rich membrane vesicles prepared from Electrophorus electricus, which need to be considered in measurements of receptor-mediated ion flux, have been identified. One of these properties is a slow exchange of inorganic ions in the vesicles. The slow exchange of ions is not related to the receptor-mediated flux of ions and accounts for 30-35% of the efflux observed. A method to separate this process from the receptor-controlled flux has been developed. It has also been shown, using a light-scattering method, that aggregation-disaggregation of the vesicles can interfere with the efflux measurements, and a method to overcome this problem has been developed. The difference in the amplitude of effluxes induced by saturating amounts of carbamylcholine and gramicidin has been investigated and has been shown not to be due to a receptor-controlled process; therefore, the amplitude difference does not need to be considered in understanding the receptor-controlled process.

Inactivation (desensitization) of the acetylcholine receptor in Electrophorus electricus membrane vesicles by carbamylcholine: comparison between ion flux and alpha-bungarotoxin binding

The inactivation (desensitization) of the acetylcholine receptor by carbamylcholine, a stable analogue of acetylcholine, has been investigated in eel Ringer's solution, pH 7.0, 0 degrees C, by measurements of (i) ion flux and (ii) the kinetics of the reaction of [125I]-alpha-bungarotoxin with the receptor. The effect of preincubation with carbamylcholine is significantly different in the two types of measurement. In both the receptor-controlled flux of inorganic ions and the toxin-binding kinetics a biphasic process has been observed (Hess, G.P., Lipkowitz, S., Struve, G.E., 1978, Proc. Nat. Acad. Sci. USA 75:1703; Hess, G.P. et al., 1975, Biochem. Biophys. Res. Commun. 64:1018; Bulger, J.E. et al., 1977, Biochemistry 16:684), only the initial fast phase of which is inhibited and the subsequent slow phase persists. However, preincubation with carbamylcholine per se has no effect on the toxin reaction. The results obtained are consistent with the proposal of Katz and Thesleff (Katz, B., Thesleff, S., 1957, J. Physiol. (London) 138:65) that the active form of the receptor is converted to an inactive form in the presence of acetylcholine receptor ligands, and with our previous experiments (Hess et al., 1978) which indicated that one receptor form is responsible for the initial fast phase of both the receptor-controlled ion flux and the toxin binding reaction, and that its conversion to the other form results in the slow phases in these two measurements.

Ca2+ control of electrolyte permeability in plasma membrane vesicles from cat pancreas

The influence of Ca2+ and other cations on electrolyte permeability has been studied in isolated membrane vesicles from cat pancreas. Ca2+ in the micromolar to millimolar concentration range, as well as Mg2+, Sr2+, Mn2+ and La3+ at a tested concentration 10(-4) M, increased Na+ permeability when applied at the vesicle inside. When added to the vesicle outside, however, they decreased Na+ permeability. Ba2+ was effective from the outside but not from the vesicle inside. When Ca2+ was present at both sides of the membrane, Na+ efflux was not affected as compared to that in the absence of Ca2+. Monovalent cations such as Rb+, Cs+, K+, Tris+ and choline+ decreased Na+ permeability when present at the vesicle outside at a concentration range of 10 to 100 mM. Increasing Na+ concentrations from 10 to 100 mM at the vesicle inside increased Na+ permeability. The temperature dependence of Na+ efflux revealed that the activation energy increased in the lower temperature range (0 to 10 degrees C) when Ca2+ was present at the outside or at both sides, but not when present at the vesicle inside only or in the absence of Ca2+. The results suggest that the Ca2+ outside effect is due to binding of calcium to negatively charged phospholipids with a consequent reduction of both fluidity and Na+ permeability of the membrane. The Ca2+-inside effect most likely involves interaction with proteins with consequent increase in Na+ permeability. The data are consistent with current hypotheses on secretagogue-induced fluid secretion in acinar cells of the pancreas according to which secretagogues elicit NaCl and fluid secretion by liberating Ca2+ from cellular membranes and by stimulating Ca2+ influx into the cell. The increased intracellular Ca2+ concentration in turn increases the contraluminal Na+ permeability which leads to NaCl influx. The luminal sodium pump finally transports Na+ ions into the lumen.

Cholinomimetic teratogens: studies with chicken embryos

The cholinomimetic compounds carbachol, decamethonium, neostigmine, succinylcholine, trimethylphenylammonium, and others were tested for their interference with normal chick development. All these compounds led to abnormalities of the cervical vertebrae; at higher dosage interference with normal morphogenesis involved the whole vertebral column. Hypoplasia of the leg muscles occurred with lower incidence. Responses, tested with carbachol, rose from 24 to 72 and 96 h, then declined to 120 h of incubation. Two of the cholinometic compounds used in combined treatment produced a high degree of synergism. Gallamine, benzoquinomium, butyrylcholine, and bethanechol had protective effects. Acetylcholine, at high dosage, caused defects different from the above. It is suggested that the cholinomimetic teratogens interfere with normal development by displacing acetylcholine from its receptors or by forming complexes with it.

Acetylcholine receptors

The idea that certain drugs and neurotransmitters produce their effects by combining with specific receptors was first clearly expressed by Langley (1905) on the basis of the selective and localized effect of nicotine on striated muscle fibres. In 1914, Langley published a paper in which the antagonism between ‘curari’ and nicotine was analysed and measured as the ratio by which the nicotine concentration had to be increased in order to produce a standard response in the presence of tubocurarine. It was clear that Langley had in mind the idea of competition between nicotine and curare for the receptor sites and it was surprising that he did not formulatethe theory quantitatively, particularly since Hill, working in Langley's laboratory in 1909, published a mathematical analysis of the action of nicotine on frog muscle giving kinetic and equilibrium equations based on the law of mass action, which could easily have been extended to give an account of competitive antagonism. Barger & Dale (1910) did not favour the idea of specific receptors for sympathomimetic amines.

Changes in extrinsic fluorescence intensity of the electroplax membrane during electrical excitation

The fluorescent dye 1-anilinonapthalene-8-sulfonic acid (ANS) has been used as a probe of changes in membrane conformation accompanying excitation of the electroplax ofElectrophorus electricus. ANS binds reversibly to the excitable membrane at rest. During generation of an action potential, an increase in ANS-fluorescence intensity is observed which resembles but does not strictly follow the membrane potential. Experiments using the current-clamp technique have demonstrated a linear relationship between the change in membrane potential and the change in ANS fluorescence intensity. The change in fluorescence intensity is not a consequence of binding to membrane sites of increased affinity nor of an electrophoretic concentration of ANS molecules at the membrane surface.It is not known whether the change in fluorescence intensity is due to a change in quantum yield of bound ANS or to an increase in the amount of bound ANS. In either case, the change in fluorescence intensity may be interpreted as a change in membrane conformation.