Glucose-limited chemostat culture of Chinese hamster ovary cells producing recombinant human interferon-gamma

A Chinese hamster ovary (CHO) cell line expressing recombinant human interferon-gamma (IFN-gamma) was grown under glucose limitation in a chemostate at a constant dilution rate of 0.015 h(-1) with glucose feed concentrations of 2.75 mM and 4.25 mM. The changes in cell concentration that accompanied changes in the glucose feed concentration indicated that the cells were glucose-limited. The cell yield on glucose remained constant, but there was a decline in residual glucose concentration and a reduced lactate yield from glucose in the latter stages of the culture. The consumption rates for many of the essential amino acids were increased later in the culture. The volumetric rate of interferon-gamma production was maintained throughout the course of this culture, indicating that IFN-gamma expression was stable under these conditions. However, the specific rate of IFN-gamma production was significantly lower at the higher glucose feed concentration. Under glucose limitation, the proportion of fully glycosylated IFN-gamma produced by these cells was less than that produced in the early stages of batch cultures. The proportion of fully glycosylated IFN-gamma increased during transient periods of glucose excess, suggesting that the culture environment influences the glycosylation of IFN-gamma.

Modelling the interdependence between the stoichiometry of receptor oligomerization and ligand binding for a coexisting dimer/tetramer receptor system

Many G protein-coupled receptors have been shown to exist as oligomers, but the oligomerization state and the effects of this on receptor function are unclear. For some G protein-coupled receptors, in ligand binding assays, different radioligands provide different maximal binding capacities. Here we have developed mathematical models for co-expressed dimeric and tetrameric species of receptors. We have considered models where the dimers and tetramers are in equilibrium and where they do not interconvert and we have also considered the potential influence of the ligands on the degree of oligomerization. By analogy with agonist efficacy, we have considered ligands that promote, inhibit or have no effect on oligomerization. Cell surface receptor expression and the intrinsic capacity of receptors to oligomerize are quantitative parameters of the equations. The models can account for differences in the maximal binding capacities of radioligands in different preparations of receptors and provide a conceptual framework for simulation and data fitting in complex oligomeric receptor situations.

Mechanisms underlying agonist efficacy

Agonist efficacy is a measure of how well an agonist can stimulate a response system linked to a receptor. Efficacy can be assessed in functional assays and various parameters (E(max), K(A)/EC(50), E(max).K(A)/EC(50)) determined. The E(max).K(A)/EC(50) parameter provides a good estimate of efficacy across the full range of efficacy. A convenient assay for the efficacy of agonists for some receptors is provided by the [(35)S]GTP[S] (guanosine 5'-[gamma-[(35)S]thio]triphosphate)-binding assay. In this assay, the normal GTP-binding event in GPCR (G-protein-coupled receptor) activation is replaced by the binding of the non-hydrolysable analogue [(35)S]GTP[S]. This assay may be used to profile ligands for their efficacy, and an example here is the D(2) dopamine receptor where an efficacy scale has been set up using this assay. The mechanisms underlying the assay have been probed. The time course of [(35)S]GTP[S] binding follows a pseudo-first-order reaction with [(35)S]GTP[S] binding reaching equilibrium after approx. 3 h. The [(35)S]GTP[S]-binding event is the rate-determining step in the assay. Agonists regulate the maximal level of [(35)S]GTP[S] bound, rather than the rate constant for binding. The [(35)S]GTP[S]-binding assay therefore determines agonist efficacy on the basis of the amount of [(35)S]GTP[S] bound rather than the rate of binding.

Analysis of second messenger pathways stimulated by different chemokines acting at the chemokine receptor CCR5

The chemokine receptor, CCR5, responds to several chemokines leading to changes in activity in several signalling pathways. Here, we investigated the ability of different chemokines to provide differential activation of pathways. The effects of five CC chemokines acting at CCR5 were investigated for their ability to inhibit forskolin-stimulated 3'-5'-cyclic adenosine monophosphate (cAMP) accumulation and to stimulate Ca(2+) mobilisation in Chinese hamster ovary (CHO) cells expressing CCR5. Macrophage inflammatory protein 1alpha (D26A) (MIP-1alpha (D26A), CCL3 (D26A)), regulated on activation, normal T-cell expressed and secreted (RANTES, CCL5), MIP-1beta (CCL4) and monocyte chemoattractant protein 2 (MCP-2, CCL8) were able to inhibit forskolin-stimulated cAMP accumulation, whilst MCP-4 (CCL13) could not elicit a response. CCL3 (D26A), CCL4, CCL5, CCL8 and CCL13 were able to stimulate Ca(2+) mobilisation through CCR5, although CCL3 (D26A) and CCL5 exhibited biphasic concentration-response curves. The Ca(2+) responses induced by CCL4, CCL5, CCL8 and CCL13 were abolished by pertussis toxin, whereas the response to CCL3 (D26A) was only partially inhibited by pertussis toxin, indicating G(i/o)-independent signalling induced by this chemokine. Although the rank order of potency of chemokines was similar between the two assays, certain chemokines displayed different pharmacological profiles in cAMP inhibition and Ca(2+) mobilisation assays. For instance, whilst CCL13 could not inhibit forskolin-stimulated cAMP accumulation, this chemokine was able to induce Ca(2+) mobilisation via CCR5. It is concluded that different chemokines acting at CCR5 can induce different pharmacological responses, which may account for the broad spectrum of chemokines that can act at CCR5.

Mechanisms of G protein activation via the D2 dopamine receptor: evidence for persistent receptor/G protein interaction after agonist stimulation

BACKGROUND AND PURPOSE

The aim of this report is to study mechanisms of G protein activation by agonists.

EXPERIMENTAL APPROACH

The association and dissociation of guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding at G proteins in membranes of CHO cells stably transfected with the human dopamine D(2short) receptor was studied in the presence of a range of agonists.

KEY RESULTS

Binding of [(35)S]GTPgammaS was dissociable in the absence of agonist and dissociation was accelerated both in rate and extent by dopamine, an effect which was blocked by the dopamine D(2) receptor antagonist raclopride and by suramin, which inhibits receptor/G protein interaction. A range of agonists of varying efficacy increased the rate of dissociation of [(35)S]GTPgammaS binding, with the more efficacious agonists resulting in faster dissociation. Agonists were able to dissociate about 70% of the pre-bound [(35)S]GTPgammaS, leaving a component which may not be accessible to the agonist-bound receptor. The dissociable component of the [(35)S]GTPgammaS binding was reduced with longer association times and increased [(35)S]GTPgammaS concentrations.

CONCLUSIONS AND IMPLICATIONS

These data are consistent with [(35)S]GTPgammaS binding being initially to receptor-linked G proteins and then to G proteins which have separated from the agonist bound receptor. Under the conditions used typically for [(35)S]GTPgammaS binding assays, therefore, much of the agonist-receptor complex remains in proximity to G proteins after they have been activated by agonist.

Assays for enhanced activity of low efficacy partial agonists at the D(2) dopamine receptor

BACKGROUND AND PURPOSE

Low efficacy partial agonists at the D2 dopamine receptor may be useful for treating schizophrenia. In this report we describe a method for assessing the efficacy of these compounds based on stimulation of [35S]GTPgammaS binding.

EXPERIMENTAL APPROACH

Agonist efficacy was assessed from [(35)S]GTPgammaS binding to membranes of CHO cells expressing D2 dopamine receptors in buffers with and without Na+. Effects of Na+ on receptor/G protein coupling were assessed using agonist/[3H]spiperone competition binding assays.

KEY RESULTS

When [35S]GTPgammaS binding assays were performed in buffers containing Na+, some agonists (aripiprazole, AJ-76, UH-232) exhibited very low efficacy whereas other agonists exhibited measurable efficacy. When Na+ was substituted by N-methyl D-glucamine, the efficacy of all agonists increased (relative to that of dopamine) but particularly for aripiprazole, aplindore, AJ-76, (-)-3-PPP and UH-232. In ligand binding assays, substitution of Na+ by N-methyl D-glucamine increased receptor/G protein coupling for some agonists -. aplindore, dopamine and (-)-3-PPP - but for aripiprazole, AJ-76 and UH-232 there was little effect on receptor/G protein coupling.

CONCLUSIONS AND IMPLICATIONS

Substitution of Na+ by NMDG increases sensitivity in [(35)S]GTPgammaS binding assays so that very low efficacy agonists were detected clearly. For some agonists the effect seems to be mediated via enhanced receptor/G protein coupling whereas for others the effect is mediated at another point in the G protein activation cycle. AJ-76, aripiprazole and UH-232 seem particularly sensitive to this change in assay conditions. This work provides a new method to discover these very low efficacy agonists.

Agonist regulation of D(2) dopamine receptor/G protein interaction. Evidence for agonist selection of G protein subtype

The D(2) dopamine receptor has been expressed in Sf21 insect cells together with the G proteins G(o) and G(i2), using the baculovirus system. Expression levels of receptor and G protein (alpha, beta, and gamma subunits) in the two preparations were similar as shown by binding of [(3)H]spiperone and quantitative Western blot, respectively. For several agonists, binding data were fitted best by a two-binding site model in either preparation, showing interaction of expressed receptor and G protein. For some agonists, binding to the higher affinity site was of higher affinity in D(2)/G(o) than in the D(2)/G(i2) preparation. Some agonists exhibited binding data that were best fitted by a two-binding site model in D(2)/G(o) and a one-binding site model in D(2)/G(i2). Therefore, receptor/G protein interaction seemed to be stronger in the D(2)/G(o) preparation. Agonist stimulation of [(35)S]GTP gamma S (guanosine 5'-3-O-(thio)triphosphate) binding in the two preparations also gave evidence for higher affinity D(2)/G(o) interaction. In the D(2)/G(o) preparation, agonist stimulation of [(35)S]GTP gamma S binding occurred at higher potency for several agonists, and a higher stimulation (relative to dopamine) was achieved in D(2)/G(o) compared with D(2)/G(i2). Some agonists were able to stimulate [(35)S]GTP gamma S binding in the D(2)/G(o) preparation but not in D(2)/G(i2). The extent of D(2) receptor selectivity for G(o) over G(i2) is therefore dependent on the agonist used, and thus agonists may stabilize different conformations of the receptor with different abilities to couple to and activate G proteins.

Dopamine D2 receptor dimer formation: evidence from ligand binding

We have examined the binding of two radioligands ([(3)H]spiperone and [(3)H]raclopride) to D(2) dopamine receptors expressed in Chinese hamster ovary cells. In saturation binding experiments in the presence of sodium ions, both radioligands labeled a similar number of sites, whereas in the absence of sodium ions [(3)H]raclopride labeled about half the number of sites labeled by [(3)H]spiperone. In competition experiments in the absence of sodium ions, however, raclopride was able to inhibit [(3)H]spiperone binding fully. In saturation analyses with [(3)H]spiperone in the absence of sodium ions raclopride exerted noncompetitive effects, decreasing the number of sites labeled by the radioligand. These data are interpreted in terms of a model where the receptor exists as a dimer, and in the absence of sodium ions, raclopride exerts negative cooperativity across the dimer both for its own binding and the binding of spiperone. A model of the receptor has been produced that provides a good description of the experimental phenomena described here.

Mechanisms of inverse agonism of antipsychotic drugs at the D(2) dopamine receptor: use of a mutant D(2) dopamine receptor that adopts the activated conformation

The antipsychotic drugs have been shown to be inverse agonists at the D(2) dopamine receptor. We have examined the mechanism of this inverse agonism by making mutations in residue T343 in the base of the sixth transmembrane spanning region of the receptor. T343R, T343S and T343K mutant D(2) dopamine receptors were made and the T343R mutant characterized in detail. The T343R mutant D(2) dopamine receptor exhibits properties of a receptor that resides more in the activated state, namely increased agonist binding affinity (independent of G-protein coupling and dependent on agonist efficacy), increased agonist potency in functional tests (adenylyl cyclase inhibition) and increased inverse agonist effects. The binding of agonists to the mutant receptor also shows sensitivity to sodium ions, unlike the native receptor, so that isomerization of the receptor to its inactive state may be driven by sodium ions. The binding of inverse agonists to the receptor is, however, unaffected by the mutation. We conclude that inverse agonism at this receptor is not achieved by the inverse agonist binding preferentially to the non-activated state of the receptor over the activated state. Rather the inverse agonist appears to bind to all forms of the receptor but then renders the receptor inactive.

Antipsychotic drugs: importance of dopamine receptors for mechanisms of therapeutic actions and side effects

Interaction of the antipsychotic drugs with dopamine receptors of the D2, D3, or D4 subclasses is thought to be important for their mechanisms of action. Consideration of carefully defined affinities of the drugs for these three receptors suggests that occupancy of the D4 subclass is not mandatory for achieving antipsychotic effects, but actions at D2 or D3 receptors may be important. A major difference between typical and atypical antipsychotic drugs is in the production of extrapyramidal side effects by the typical drugs. Production of extrapyramidal side effects by typical drugs seems to be due to the use of the drugs at doses where striatal D2 receptor occupancy exceeds approximately 80%. Use of these drugs at doses that do not produce this level of receptor blockade enables them to be used therapeutically without producing these side effects. The antipsychotic drugs have been shown to act as inverse agonists at D2 and D3 dopamine receptors, and this property may be important for the antipsychotic effects of the drugs. It is suggested that the property of inverse agonism leads to a receptor up-regulation upon prolonged treatment, and this alters the properties of dopamine synapses. Several variants of the dopamine receptors exist with different DNA sequences and in some cases different amino acid sequences. These variants may have different properties that alter the effects of dopamine and the antipsychotic drugs. The determination of such variants in patients may help in the prediction of drug responsiveness.

Site-directed mutations in the third intracellular loop of the serotonin 5-HT(1A) receptor alter G protein coupling from G(i) to G(s) in a ligand-dependent manner

The effect of mutations (V344E and T343A/V344E) in the third intracellular loop of the serotonin 5-HT(1A) receptor expressed transiently in human embryonic kidney 293 cells have been examined in terms of receptor/G protein interaction and signaling. Serotonin, (R)-8-hydroxy-2-dipropylaminotetralin [(R)-8-OH-DPAT], and buspirone inhibited cyclic AMP production in cells expressing native and mutant 5-HT(1A) receptors. Serotonin, however, produced inverse bell-shaped cyclic AMP concentration-response curves at native and mutant 5-HT(1A) receptors, indicating coupling not only to G(i)/G(o), but also to G(s). (R)-8-OH-DPAT, however, induced stimulation of cyclic AMP production only after inactivation of G(i)/G(o) proteins by pertussis toxin and only at the mutant receptors. The partial agonist buspirone was unable to induce coupling to G(s) at any of the receptors, even after pertussis toxin treatment. The basal activities of native and mutant 5-HT(1A) receptors in suppressing cyclic AMP levels were not found to be significantly different. The receptor binding characteristics of the native and mutant receptors were investigated using the novel 5-HT(1A) receptor antagonist [(3)H]NAD-299. For other receptors, analogous mutations have produced constitutive activation. This does not occur for the 5-HT(1A) receptor, and for this receptor the mutations seem to alter receptor/G protein coupling, allowing ligand-dependent coupling of receptor to G(s) in addition to G(i)/G(o) proteins.

Regulation of human D(1), d(2(long)), d(2(short)), D(3) and D(4) dopamine receptors by amiloride and amiloride analogues

1. The modulatory effects of the allosteric effectors methylisobutylamiloride (MIA), benzamil and amiloride have been examined at human D(1), D(2), D(3) and D(4) dopamine receptors. The subtype selectivity and the mechanism of action of this allosteric regulation was examined. 2. In radioligand dissociation experiments each modulator accelerated dissociation from all four receptor subtypes indicating allosteric regulation. MIA displayed selectivity for the D(3) subtype for acceleration of radioligand dissociation. 3. In equilibrium binding (pseudo-competition) experiments the three compounds inhibited radioligand binding at the four receptor subtypes. Inhibition curves for D(1), D(2(short)), D(2(long)) and D(3) receptors were described by Hill coefficients exceeding unity and data were fitted best by a model that assumes binding of modulator to both the primary and allosteric binding sites of the receptor (the allosteric/competitive model). 4. At the D(4) subtype, Hill coefficients of unity described the binding data for amiloride and benzamil, consistent with competitive inhibition. The Hill coefficient for MIA at the D(4) subtype was less than unity and data could be fitted well by the allosteric/competitive model, but it was not possible to define unambiguously the modulatory mechanism. For this effect a better definition of the mechanism could be obtained by simultaneous analysis of data obtained in the presence of a range of concentrations of a purely competitive ligand. 5. MIA reduced the potency with which dopamine stimulated [(35)S]-GTPgammaS binding at the D(2) receptor. The effects of MIA could be described by the allosteric/competitive model with effects of MIA to inhibit the binding of dopamine but not its ability to induce a response.

Effect of multiple serine/alanine mutations in the transmembrane spanning region V of the D2 dopamine receptor on ligand binding

Three conserved serine residues (Ser193, Ser194, and Ser197) in transmembrane spanning region (TM) V of the D2 dopamine receptor have been mutated to alanine, individually and in combination, to explore their role in ligand binding and G protein coupling. The multiple Ser -->Ala mutations had no effect on the binding of most antagonists tested, including [3H]spiperone, suggesting that the multiple mutations did not affect the overall conformation of the receptor protein. Double or triple mutants containing an Ala197 mutation showed a decrease in affinity for domperidone, whereas Ala193 mutants showed an increased affinity for a substituted benzamide, remoxipride. However, dopamine showed large decreases in affinity (>20-fold) for each multiple mutant receptor containing the Ser193Ala mutation, and the high-affinity (coupled) state of the receptor (in the absence of GTP) could not be detected for any of the multiple mutants. A series of monohydroxylated phenylethylamines and aminotetralins was tested for their binding to the native and multiple mutant D2 dopamine receptors. The results obtained suggest that Ser193 interacts with the hydroxyl of S-5-hydroxy-2-dipropylaminotetralin (OH-DPAT) and Ser197 with the hydroxyl of R-5-OH-DPAT. We predict that Ser193 interacts with the hydroxyl of R-7-OH-DPAT and the 3-hydroxyl (m-hydroxyl) of dopamine. Therefore, the conserved serine residues in TMV of the D2 dopamine receptor are involved in hydrogen bonding interactions with selected antagonists and most agonists tested and also enable agonists to stabilise receptor-G protein coupling.

Mechanisms of agonism and inverse agonism at serotonin 5-HT1A receptors

Mechanisms of agonist and inverse agonist action at the serotonin 5-HT1A receptor have been studied using the modulation of guanosine 5'-O-(3-[35S]thiotriphosphate) ([35S]GTPgammaS) binding in membranes of Chinese hamster ovary (CHO) cells expressing the receptor (CHO-5-HTA1A cells). A range of agonists increased [35S]GTPgammaS binding with different potencies and to different maximal extents, whereas two compounds, methiothepin and spiperone, inhibited both agonist-stimulated and basal [5S]GTPgammaS binding, thus exhibiting inverse agonism. Potencies of agonists to stimulate [35S]GTPgammaS binding in membranes from CHO-5-HT1A cells were reduced by adding increasing concentrations of GDP to assays, whereas changes in sodium ion concentration did not affect agonist potency. The maximal effect of the agonists was increased by increasing sodium ion concentrations. The affinities of agonists in ligand binding assays were unaffected by changes in sodium ion concentration. Increasing GDP in the assays of the inverse agonists increased potency for spiperone to inhibit [35S]GTPgammaS binding and had no effect for methiothepin, in agreement with the sensitivity of these compounds to guanine nucleotides in ligand binding assays. Potencies for these inverse agonists were unaffected by changes in sodium ion concentration. These data were simulated using the extended ternary complex model. These simulations showed that the data obtained with agonists were consistent with these compounds achieving agonism by stabilising the ternary complex. For inverse agonists, the simulations showed that the mechanism for spiperone may be to stabilise forms of the receptor uncoupled from G proteins. Methiothepin, however, probably does not alter the equilibrium distribution of different receptor species; rather, this inverse agonist may stabilise an inactive form of the receptor that can still couple to G protein.

G-protein coupled receptors: conformations and states

Activation of G-protein coupled receptors by agonists is thought to involve the stabilisation of a ternary complex of agonist/receptor/G-protein, leading to effector activation, but this mechanism may be an oversimplification, as follows: (a) Agonist binding to the free receptor (uncoupled from G-proteins) is not a neutral event, but includes a component of the activation process and may be described in terms of the stabilisation of a partly activated form of the receptor (R*) that is able to couple to the G-protein. Stabilisation of R*, therefore, may contribute to agonist efficacy. Also, determinations of agonist affinity even in the absence of G-protein coupling do not necessarily describe the affinities of agonists for the ground state of the receptor. (b) R* is a partly activated intermediate between the ground state of the receptor (R) and the activated form coupled to G-protein (R*G). There is some indication that different agonists may stabilise different conformational states of the receptor, i.e. different R* species. (c) Agonists also stabilise the activated, coupled form of the receptor (AR*G), and for some agonists acting at a single receptor, the activated states may be similar, although there is evidence for other agonists that different activated states with different activities may be stabilised. (d) Two or more efficacy-generating steps are involved in the activation of G-protein coupled receptors by agonists: the stabilisation of R*, the stabilisation of R*G, and possibly the modulation of the activity of the activated state (AR*G). (e) The experimentally observed excess of G-proteins over receptors in membranes is inconsistent with data obtained from ligand-binding assays on these receptors. Receptors and G-proteins, therefore, may exist in some form of higher order array with cooperative interactions.

Pathology and drug action in schizophrenia: insights from molecular biology

Schizophrenia is a severe disorder of personality which has a genetic basis. Schizophrenia arises from a change in brain development. There is no strong evidence that disturbances in neurotransmitter systems are a primary cause. Anti-psychotic drugs act primarily through D2 and D3 dopamine receptors. The atypical drug clozapine may act through a number of different receptors, including D2, D3 and D4 dopamine receptors. Anti-psychotic drugs are inverse agonists at D2 dopamine receptors.

Comparison of the ability of dopamine receptor agonists to inhibit forskolin-stimulated adenosine 3'5'-cyclic monophosphate (cAMP) accumulation via D2L (long isoform) and D3 receptors expressed in Chinese hamster ovary (CHO) cells

The pharmacological properties of the human D2L (long isoform) and rat D3 dopamine receptors in functional assays were examined. A range of dopamine agonists were assessed for their ability to inhibit adenosine 3'5'-cyclic monophosphate (cAMP) accumulation via the two receptors expressed stably in Chinese hamster ovary cells. Dopamine caused a significantly greater maximal inhibition (P < 0.05) of cAMP accumulation via the D2L receptor (approximately 70%) as compared to the D3 receptor (approximately 50%). The pattern of agonist effects was different at the two receptors. The absolute and relative potencies for inhibition of cAMP accumulation were different for a range of agonists acting at the two receptors. Similarly, the maximal inhibitions achieved by a range of agonists were different for the two receptors.

Constraints on the transport and glycosylation of recombinant IFN-gamma in Chinese hamster ovary and insect cells

In this study we compare intracellular transport and processing of a recombinant glycoprotein in mammalian and insect cells. Detailed analysis of the N-glycosylation of recombinant human IFN-gamma by matrix-assisted laser-desorption mass spectrometry showed that the protein secreted by Chinese hamster ovary and baculovirus-infected insect Sf9 cells was associated with complex sialylated or truncated tri-mannosyl core glycans, respectively. However, the intracellular proteins were predominantly associated with high-mannose type oligosaccharides (Man-6 to Man-9) in both cases, indicating that endoplasmic reticulum to cis-Golgi transport is a predominant rate-limiting step in both expression systems. In CHO cells, although there was a minor intracellular subpopulation of sialylated IFN-gamma glycoforms identical to the secreted product (therefore associated with late-Golgi compartments or secretory vesicles), no other intermediates were evident. Therefore, anterograde transport processes in the Golgi stack do not limit secretion. In Sf9 insect cells, there was no direct evidence of post-ER glycan-processing events other than core fucosylation and de-mannosylation, both of which were glycosylation site-specific. To investigate the influence of nucleotide-sugar availability on cell-specific glycosylation, the cellular content of nucleotide-sugar substrates in both mammalian and insect cells was quantitatively determined by anion-exchange HPLC. In both host cell types, UDP-hexose and UDP-N-acetylhexosamine were in greater abundance relative to other substrates. However, unlike CHO cells, sialyltransferase activity and CMP-NeuAc substrate were not present in uninfected or baculovirus-infected Sf9 cells. Similar data were obtained for other insect cell hosts, Sf21 and Ea4. We conclude that although the limitations on intracellular transport and secretion of recombinant proteins in mammalian and insect cells are similar, N-glycan processing in Sf insect cells is limited, and that genetic modification of N-glycan processing in these insect cell lines will be constrained by substrate availability to terminal galactosylation.

Role of conserved serine residues in the interaction of agonists with D3 dopamine receptors

To understand the role of conserved serine residues in the fifth transmembrane domain (Ser192, Ser193, and Ser196) of the D3 dopamine receptor, these have been mutated individually to alanine, and the ligand binding properties of the mutant receptors have been evaluated. The mutations had little or no effect on the binding of the antagonist spiperone and the agonist quinpirole, indicating that the overall conformation of the receptor was unaffected. The binding of dopamine and 7-hydroxydipropylaminotetralin, agonists containing hydroxyl groups, was, however, of lower affinity for the Ser192 mutation but unaffected by the other mutations (Ser193 and Ser196). Therefore, for the agonists tested, the hydroxyl groups interact exclusively with Ser192.

Agonism and inverse agonism at dopamine D2-like receptors

1. The processes that follow the binding of ligands to receptors are critical for their physiological functions. In the present paper I intend to review our own work and the work of other laboratories attempting to understand these processes for the dopamine D2-like receptors (D2, D3, D4) and how they contribute to the mechanisms of drug action. It is thought that the key event in agonist action for these receptors is the stabilization, by the agonist, of the agonist-receptor-G-protein ternary complex. The majority of the work I shall describe has been performed using recombinant receptors expressed in cell lines and the mechanisms of receptor action have been probed using ligand binding (competition vs [3H]-spiperone), the stimulation of [35S]-GTP gamma S binding and inhibition of adenylyl cyclase. 2. Measures of the ability of agonists to stabilize the agonist-receptor-G-protein ternary complex may be obtained in ligand-binding studies using the ratio of dissociation constants for the higher and lower affinity states (KI/KH ratio). The stimulation of [35S]-GTP gamma S binding provides a very convenient assay for agonist action and allows the determination of agonist potency and maximal response. Estimates of these quantities may also be obtained from the inhibition of adenylyl cyclase. For a range of agonists at the D2 receptor, there is a tendency for high values of KI/KH to predict high maximal activity and vice versa, but there is no general correlation. This suggests that the simple scheme of agonist action depending on the stabilization of the ternary complex is an over-simplification and further efficacy determining steps need to be included. For a number of receptors, including the D2 and D3 receptors, it has now been shown that there is activity in the absence of agonist (so-called constitutive activity). This agonist-independent activity can be inhibited by compounds previously considered to be antagonists (e.g. the antipsychotic drugs). Therefore, these compounds are inverse agonists rather than antagonists. The mechanism of this inverse agonist effect is unclear and we are examining this using a variety of biochemical approaches, including the use of constitutively active mutants. 3. The mechanisms of agonism and inverse agonism may be probed using biochemical assays and these studies are of great relevance to the understanding of drug action.

Pharmacological characterisation of the D2 dopamine receptor expressed in the yeast Schizosaccharomyces pombe

The rat D2(long) dopamine receptor has been expressed in the fission yeast Schizosaccharomyces pombe at levels of about 1 pmol/mg of protein. The recombinant receptor, analysed in ligand binding experiments, exhibits properties typical of a D2 dopamine receptor and the affinities of antagonists agree with values obtained for the receptor expressed in mammalian systems although the affinities of some antagonists are lower. Substituted benzamide antagonists show lower affinities in the absence of sodium ions whereas clozapine and classical antagonists mostly show higher affinities. Agonist binding is insensitive to the effects of GTP indicating lack of a stable interaction with G-proteins.

Agonist action at D2(long) dopamine receptors: ligand binding and functional assays

1. The activities of a range of agonists at D2(long) dopamine receptors expressed in CHO cells have been determined in ligand binding and in a functional assay, the stimulation of [35S]-GTPgammaS binding. 2. For several agonists (apomorphine, dopamine, pergolide, quinpirole, NPA, ropinirole, talipexole) binding in the absence of added guanine nucleotides was best described in terms of interaction at higher and lower affinity states, whereas for other agonists (bromocriptine, DHEC, lisuride, 3-PPP) a one binding site model was a good description of the data. In the presence of GTP (100 microM) all agonist binding data were best described by a one site model. 3. All of the agonists tested increased [35S]-GTPgammaS binding above the basal level and the maximal effects and potencies of the agonists in this test were different. There was no clear relation between the ability of an agonist to stabilize the formation of the ternary complex of agonist/receptor/G-protein and the maximal activity of the agonist or the amplification factor (ratio of dissociation constant for binding to receptor to EC50 in functional assay). 4. A comparison was made between the profiles of the D2(short) and D2(long) receptor isoforms in these assays.

Activation of microtubule-associated protein kinase (Erk) and p70 S6 kinase by D2 dopamine receptors

The ability of human and rat D2(short) and D2(long) dopamine receptors to activate microtubule-associated protein (MAP) kinase (Erk1/2) and p70 S6 kinase has been investigated in recombinant cells expressing these receptors. In cells expressing the D2(short) receptor, dopamine activated both enzymes in a transient manner but with very different time courses, with activation of Erk being much quicker. Activation of both enzymes by dopamine was dose-dependent and could be prevented by a range of selective dopamine antagonists. Excellent correlations were observed between the potencies of the antagonists for blocking enzyme activation and their affinities for the D2 dopamine receptor. Activation of Erk and of p70 S6 kinase via the D2 dopamine receptors was prevented by pretreatment of the cells with pertussis toxin, indicating the involvement of G proteins of the Gi or Go family. Inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase) were found to block substantially, but not completely, activation of p70 S6 kinase by dopamine, suggesting the involvement of PI 3-kinase-dependent and -independent signalling pathways in its control by dopamine. p70 S6 kinase activation was completely blocked by rapamycin. In the case of Erk, activation was partially blocked by wortmannin or LY294002, indicating a possible link with PI 3-kinase.

Expression of cytoskeletal proteins differentiates between progressors and non-progressors in treated idiopathic membranous nephropathy

Myofibroblasts play an important role in wound healing in a variety of tissue injuries. They have also been implicated in tissue fibrosis including renal scarring. This study was aimed at defining their role in one of the commonest forms of nephrotic syndrome in adults, namely membranous nephropathy. We have studied 21 patients with biopsy proven idiopathic membranous nephropathy who were treated with glucocorticoids, attempting to define the role of myofibroblasts (alpha-smooth muscle actin-positive as well as vimentin-positive cells) in the progression of this form of nephropathy. There were 13 non-progressors (NP) and 8 progressors (P). The clinical, histological, and immunohistochemical characteristics of both groups were compared. Immunohistochemical staining for myofibroblasts cytoplasmic markers a-smooth muscle actin (alpha-SMA) and vimentin relied on an avidin-biotin-peroxidase method. The level of blood pressure, degree of proteinuria, severity of interstitial infiltrate and interstitial fibrosis did not differentiate P from NP. However, vascular sclerosis was more severe in P compared to NP (p < 0.016) and its severity predicted the subsequent functional outcome (slope of the 1/serum creatinine against time; r2 = 0.618, p < 0.01). Mesangial alpha-SMA was significantly higher in P (31 +/- 18.6%) than in NP (14.5 +/- 9.8%), p < 0.015. Interstitial alpha-SMA immunostain was also higher in P but did not reach statistical significance. However, the number of interstitial myofibroblasts (alpha-SMA positive cells) closely predicted the subsequent rate of the progression of chronic renal failure (r2 = 0.919, p < 0.0001). Mesangial vimentin expression was not different between both groups. By contrast, interstitial vimentin immunostain was higher in P (19.1 +/- 8.8%) compared to NP (7.9+/-5.6 %), p < 0.002. These data suggest that the expression of mesangial and interstitial cytoskeletal proteins (alpha-SMA and vimentin) may have useful prognostic implications as they appear to differentiate between patients with membranous nephropathy who respond to immunosuppression and those who continue to progress.

Agonist action at D2(short) dopamine receptors determined in ligand binding and functional assays

Mechanisms of agonist action at the G protein-coupled D2(short) dopamine receptor expressed in Chinese hamster ovary cells have been investigated. Agonist binding was assayed in the presence and absence of GTP (100 microM). Data in the absence of GTP were fitted best by a two-site model (apomorphine, dopamine, 10,11-dihydroxy-N-n-propylnorapomorphine hydrochloride, and quinpirole) or a one-site model [bromocriptine, dihydroergocristine, and (-)-3-(3-hydroxyphenyl)-N-propylpiperidine hydrochloride], whereas in the presence of GTP a one-site model was the best fit for all compounds. Agonist binding parameters were used to provide a measure of the ability of the agonist to stabilise the ternary complex of agonist/receptor/G protein. Agonist stimulation of [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTPgammaS) binding for a range of agonist concentrations was measured and the EC50 and maximal effects determined. The initial rates of [35S]GTPgammaS binding induced by maximally stimulating agonist concentrations were also recorded. Simultaneous inhibition of agonist-stimulated [35S]GTPgammaS binding and receptor occupancy by spiperone was determined. Agonist inhibition of forskolin-stimulated cyclic AMP accumulation was determined for a range of agonist concentrations and the EC50 and maximal inhibition recorded. The data on the maximal agonist responses showed that it was possible to detect a spectrum of agonist efficacy (partial and full agonism) in both functional assays. The data on the apparent potencies of agonists to elicit the functional responses showed that different extents of amplification of response were seen for different agonists in both assays. The maximal activity data have been compared with the stabilisation of the agonist/receptor/G protein ternary complex as measured in binding assays.

Evidence that antipsychotic drugs are inverse agonists at D2 dopamine receptors

1. The effects of a number of D2-like dopamine receptor antagonists have been determined on forskolin-stimulated cyclic AMP accumulation in Chinese hamster ovary (CHO) cells expressing the human D2short dopamine receptor (CHO-D2S cells). 2. Dopamine inhibited the effect of forskolin (as expected for a D2 receptor). However, all of the antagonists tested, apart from UH232 and (-)-butaclamol, were able to increase cyclic AMP accumulation above the forskolin control level. (+)-Butaclamol elicited a similar stimulation of forskolin-stimulated cyclic AMP accumulation in a CHO cell line expressing human D2long dopamine receptors whereas it exhibited no stimulating effect on forskolin-stimulated cyclic AMP accumulation in untransfected CHO-K1 cells. 3. There was a strong correlation between the EC50 values of these compounds for potentiation of cyclic AMP accumulation and their Ki values from radioligand binding experiments in CHO-D2S cells. 4. The effects of both (+)-butaclamol and dopamine in CHO-D2S cells were inhibited by pre-treatment with pertussis toxin indicating a role for Gi/Go proteins. 5. UH232 did not significantly affect forskolin-stimulated cyclic AMP accumulation but this substance was able to inhibit the effects of both dopamine and (+)-butaclamol in a concentration-dependent manner. Thus the effects of (+)-butaclamol on forskolin-stimulated cyclic AMP accumulation are mediated directly via the D2 receptor rather than by reversal of the effects of an endogenous agonist. 6. These data suggest that the D2 dopamine receptor antagonists tested here, many of which are used clinically as antipsychotic drugs, are in fact inverse agonists at human D2 dopamine receptors.

D2-like dopamine receptors are not detectable on human peripheral blood lymphocytes

The binding of [3H]-nemonapride to human peripheral blood lymphocytes (PBL) was studied using various competing ligands specific for D2-like dopamine receptors. There is no detectable stereoselectivity for the stereoisomers of butaclamol, and competitions with haloperidol and sulpiride also show no evidence of specific binding to D2-like dopamine receptors. RT-PCR of RNA from human lymphocytes showed that there is no detectable D2 mRNA (even with nested PCR). D3 mRNA was, however, detectable by RT-PCR, but only at low levels that could not be detected by Northern blots of PBL total RNA.

Regulation of D2 dopamine receptors by amiloride and amiloride analogs

Allosteric regulation of rat D2 dopamine receptors by amiloride and amiloride analogs has been studied by investigating their ability to accelerate the dissociation of [3H]spiperone from the receptors expressed in Ltk- cells. The amiloride analogs were more potent and produced a greater maximal effect on the rate of [3H]spiperone dissociation than did amiloride. [3H]Spiperone dissociation was biphasic and could be resolved into contributions from fast and slow rates in the absence and presence of the modulators. Methylisobutylamiloride accelerated both the fast and slow rates of dissociation and modulated the proportions of the two rates. The association of [3H]spiperone in the absence of modulators was also biphasic. The combination of the two sets of association and dissociation rate constants gave very similar equilibrium dissociation constants, and this was confirmed in equilibrium binding experiments that were consistent with a single binding site. It is proposed that there are two binding states for [3H]spiperone that can be distinguished kinetically but not in equilibrium binding experiments. The proportions of these states are differentially modulated with the use of sodium ions and magnesium ions, whereas GTP has no significant effect. Allosteric regulation of [3H]spiperone binding by methylisobutylamiloride could also be observed in saturation and inhibition binding experiments. These effects can be accounted for in a model in which the modulator binds to the competitive site of the receptor and to an allosteric site on the receptor from which it exerts negatively cooperative effects on [3H]spiperone binding to the competitive site and positively cooperative effects on the binding of the modulator to the competitive site.

The energetics of ligand binding at catecholamine receptors

One of the key events in the actions of agonists and antagonists is their binding to receptors. Understanding this event is of interest in terms of understanding receptor function but it also has immense practical relevance for the design of drugs. If the ligand-binding process could be understood in detail, including the nature of the interactions made between ligand and receptor, then this could help in the design of more-selective drugs. The interaction of a ligand with its receptor is clearly of importance in determining the specificity of ligand action but ligand-receptor interaction also initiates the processes of signalling that are exhibited in the efficacy of ligand action. Here Philip Strange considers these events for catecholamine receptors, concentrating mostly on dopamine receptors; where necessary the discussion is widened to include other receptor systems.

Pharmacological analysis of dopamine stimulation of [35S]-GTP gamma S binding via human D2short and D2long dopamine receptors expressed in recombinant cells

1. The activation of G-proteins by agonist-occupied D2 or D3 dopamine receptors in membranes from recombinant cells expressing the cloned receptors has been analysed by a [35S]-guanosine 5'-[gamma-thio] triphosphate ([35S]-GTP gamma S) binding assay. 2. The rate of [35S]-GTP gamma S binding was increased by dopamine in a dose-dependent manner in membranes from CHO cells stably expressing either the D2short or D2long dopamine receptor. 3. The dopamine-induced stimulation of [35S]-GTP gamma S binding could be inhibited by a range of antagonists. Affinities for antagonists derived from the inhibition of the dopamine stimulation of [35S]-GTP gamma S binding correlated very well with affinities derived from radioligand binding studies. 4. When the maximum [35S]-GTP gamma S binding responses stimulated by dopamine acting at different receptor subtypes were compared, there was a tendency for the stimulation via the D2short receptor to be greater than via the D2long receptor and for the stimulation via the D3 dopamine receptor to be less than for either D2 receptor. These differences in maximal response were also seen when the inhibitory effects of dopamine on adenylyl cyclase via the three receptor subtypes were compared. 5. The stimulation of [35S]-GTP gamma S binding by dopamine in membranes from recombinant cells therefore provides an excellent system for studying the molecular nature of agonism and the receptor/G-protein interactions for these receptors.

Investigation of the role of conserved serine residues in the long form of the rat D2 dopamine receptor using site-directed mutagenesis

Three serine residues (Ser193, Ser194, Ser197) in the fifth transmembrane-spanning region of the D2 dopamine receptor have been mutated separately to alanine and the effects of the mutations determined in ligand-binding experiments with [3H] spiperone. For many antagonists the mutations had little effect, showing that the overall conformation of the mutant receptors was similar to that of the native, although there were effects on the binding of certain antagonists. The effect of the mutations on agonist binding to the free receptor (uncoupled from G proteins) was determined in the presence of GTP (100 microM). This showed that there was no single mode of binding of catecholamine agonists to the receptor and that all three serine residues can participate in the binding of some agonists, possibly through hydrogen bonds to the catechol hydroxyl groups. Coupling of the mutant receptors to G proteins was assessed from agonist-binding curves in the absence of GTP, when higher and lower affinity agonist-binding sites were seen. Receptor/G protein coupling was generally unaffected by the Ala193 and Ala194 mutations, but the Ala197 mutation eliminated receptor/G protein coupling for some agonists. These data show that the interactions of agonists with the free and coupled forms of the receptor are different.

Characterisation of recombinant serotonin 5-HT1A receptors expressed in Chinese hamster ovary cells: the agonist [3H]lisuride labels free receptor and receptor coupled to G protein

Serotonin 5-HT1A receptors expressed stably in recombinant Chinese hamster ovary cells have been studied using radioligand binding with the radiolabelled agonist [3H]lisuride. Competition studies with a range of antagonists versus [3H]lisuride confirmed that all of the specific [3H]lisuride binding was to 5-HT1A receptors on the cells. Competition studies with the antagonist spiperone and several agonists gave data that fitted best to two-binding-site models. The affinities of these competing ligands at the two classes of sites were generally in agreement with their corresponding affinities determined in previous work with either 8-[3H]hydroxydipropylaminotetralin ([3H]8-OH-DPAT; labels receptor coupled to G protein) or [3H]spiperone (labels free receptor). Saturation analyses with [3H]lisuride showed that this radioligand labels a single class of binding sites, but the level of radioligand binding was approximately twice that seen when either [3H]8-OH-DPAT or [3H]spiperone was used. [3H]Lisuride binding was partially inhibited by addition of guanine nucleotides, and the extent of inhibition decreased as the [3H]lisuride concentration was increased. This inhibition was due to the effect of guanine nucleotide to decrease slightly the affinity of [3H]lisuride for binding to the 5-HT1A receptors on the cells. It is concluded that [3H]lisuride can label both the free receptor and the receptor coupled to G proteins but with slightly different affinities and that these two states of the receptor exist in roughly equal amounts in the cells. Agonists generally have a higher affinity for the receptor coupled to G protein, whereas antagonists, with the exception of spiperone (which has a higher affinity for the free receptor), have roughly equal affinities for the free receptor and the receptor coupled to G proteins.

pH dependence of ligand binding to D2 dopamine receptors

The binding of a range of ligands to D2 dopamine receptors in bovine caudate nucleus and recombinant CHO cells expressing the receptor has been determined at different pH values between 4.5 and 8.5. The maximum number of D2 dopamine receptor binding sites in each tissue was not affected by the change in pH, but the affinity of ligands for binding to the receptors was decreased as the pH was decreased. For classical dopamine antagonists, e.g. spiperone and haloperidol, the data on pH dependence of the dissociation constant for receptor binding indicated that the protonation of a single ionizing group on the receptor (pKa approximately 6) influenced the binding process. For antagonists of the substituted benzamide class, the data indicated that the protonation of two ionizing groups (pKa between 6 and 7) influenced the ligand binding process. These ionizing residues may correspond to Asp 114 for the classical antagonists and Asp 114 and Asp 80 for the substituted benzamide antagonists. Further evidence for the participation of carboxyl residues in the ligand binding process was obtained from the inhibition by N,N'-dicyclohexylcarbodiimide of the binding of [3H]spiperone and [3H]YM 09151-2 to D2 receptors in the recombinant CHO cells.

[3H]nemonapride and [3H]spiperone label equivalent numbers of D2 and D3 dopamine receptors in a range of tissues and under different conditions

[3H]Nemonapride and [3H]spiperone are very widely used to study dopaminergic systems in vitro and in vivo, but it has been reported that [3H]nemonapride and [3H]spiperone give markedly different Bmax values for preparations of D2 dopamine receptors from recombinant cell lines or animal tissues. We have used the two radioligands in parallel to study a range of dopamine receptors [D2(short), D2(long), and D3] in different buffers. Bmax values derived using either radioligand differ by an average of < 20%, independent of receptor type or buffer conditions. All competition experiments show that the two ligands compete at a single site. It seems that [3H]spiperone and [3H]nemonapride do not differentiate between different forms or populations of D2-like receptors.