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.