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.