Expression and functional characterisation of a synthetic version of the human D4 dopamine receptor in a stable human cell line

Therapeutic potential of dopamine agonists in the treatment of type 2 diabetes mellitus

Diabetes is a global health concern that has affected almost 415 million people globally. Bromocriptine is a dopamine D2 agonist, which is a Food and Drug Administration (FDA)-approved drug to treat type 2 diabetes mellitus (T2DM) patients. However, it is considered that a novel treatment therapy is required which can be used in the treatment of diabetes with or without other antidiabetic agents. Dopamine agonists are usually used in neurological disorders like Parkinson's disease (PD), restless leg syndrome, and hyperprolactinemia. However, dopamine agonists including bromocriptine and cabergoline are also effective in reducing the glycemic level in T2DM patients. Bromocriptine was formerly used for the treatment of PD, hyperprolactinemia, and restless leg syndrome, but now it is used for improving glycemic levels as well as reducing free fatty acids and triglycerides. In addition, cabergoline has been found to be effective in glycemic control, but this drug is yet to be approved by the FDA due to its limitations and lack of study. Findings of the clinical trials of bromocriptine have suggested that it reduces almost 0.4-0.8% glycated hemoglobin and cardiovascular risk by 40% in insulin-resistant patients. Moreover, the safe use of bromocriptine in obese T2DM patients makes it a more attractive option as it causes weight loss. Indeed, bromocriptine is a novel therapy for T2DM patients, as its mechanism of action is unique in T2DM patients with minimal adverse effects. This review summarizes the potential of dopamine agonists in the treatment of T2DM.

Selective reconstitution of human D4 dopamine receptor variants with Gi alpha subtypes

G protein-coupled receptors (GPCRs) are seven-transmembrane (TM) helical proteins that bind extracellular molecules and transduce signals by coupling to heterotrimeric G proteins in the cytoplasm. The human D4 dopamine receptor is a particularly interesting GPCR because the polypeptide loop linking TM helices 5 and 6 (loop i3) may contain from 2 to 10 similar direct hexadecapeptide repeats. The precise role of loop i3 in D4 receptor function is not known. To clarify the role of loop i3 in G protein coupling, we constructed synthetic genes for the three main D4 receptor variants. D4-2, D4-4, and D4-7 receptors contain 2, 4, and 7 imperfect hexadecapeptide repeats in loop i3, respectively. We expressed and characterized the synthetic genes and found no significant effect of the D4 receptor polymorphisms on antagonist or agonist binding. We developed a cell-based assay where activated D4 receptors coupled to a Pertussis toxin-sensitive pathway to increase intracellular calcium concentration. Studies using receptor mutants showed that the regions of loop i3 near TM helices 5 and 6 were required for G protein coupling. The hexadecapeptide repeats were not required for G protein-mediated calcium flux. Cell membranes containing expressed D4 receptors and receptor mutants were reconstituted with purified recombinant G protein alpha subunits. The results show that each D4 receptor variant is capable of coupling to several G(i)alpha subtypes. Furthermore, there is no evidence of any quantitative difference in G protein coupling related to the number of hexadecapeptide repeats in loop i3. Thus, loop i3 is required for D4 receptors to activate G proteins. However, the polymorphic region of the loop does not appear to affect the specificity or efficiency of G(i)alpha coupling.

Functional characterization of corticotropin-releasing factor type 1 receptor endogenously expressed in human embryonic kidney 293 cells

The endogenous expression in human embryonic kidney 293 (HEK293) cells of corticotropin-releasing factor (CRF) receptors was detected. High-affinity binding sites for human CRF (K(i)=3.6 nM), ovine CRF (K(i)=4.6 nM), rat urocortin (K(i)=2.2 nM), sauvagine (K(i)=2.4 nM) and astressin (K(i)=4.3 nM) with the pharmacological characteristics for CRF type 1 (CRF(1)) receptors and B(max) values of approximately 30 fmol/mg protein were determined. The four CRF receptor agonists nonselectively stimulated cAMP production in HEK293 cells at low agonist concentrations, whereas the antagonist astressin shifted the dose-response curve for ovine CRF significantly rightward. Transfection of the pcDNA3 vector into HEK293 cells strongly reduced the expression of the endogenous CRF receptor. Northern blot analysis revealed the expression of a CRF(1) transcript in human neuronal tissues, HEK293, human NTera-2 (NT2) carcinoma, Y-79 retinoblastoma and African green monkey kidney (COS-7) cells. Neither by Northern blot analysis nor by reverse transcriptase PCR (RT-PCR), the expression of CRF(2) could be detected. In cAMP stimulation experiments, functional CRF receptors were detected in these cell lines. These data show that HEK293 and other cell lines endogenously express CRF(1) receptors.

Pharmacological characterization of extracellular acidification rate responses in human D2(long), D3 and D4.4 receptors expressed in Chinese hamster ovary cells

This study characterized pharmacologically the functional responses to agonists at human dopamine D2(long) (hD2), D3 (hD3) and D4.4 (hD4) receptors separately expressed in cloned cells using the cytosensor microphysiometer. Dopaminergic receptor agonists caused increases in extracellular acidification rate in adherent Chinese hamster ovary (CHO) clones expressing hD2, hD3 or hD4 receptors. Acidification rate responses to agonists in other cell lines expressing these receptors were smaller than those in adherent CHO cells. The time courses and maximum increases in acidification rate of the agonist responses in adherent CHO cells were different between the three dopamine receptor clones. Responses were blocked by pretreatment of cells with pertussis toxin or amiloride analogues. Most agonists had full intrinsic activity at each of the dopamine receptor subtypes, as compared to quinpirole, however both enantiomers of UH-232 and (-)3-PPP were partial agonists in this assay system. The functional potency of full agonists at each of the three receptors expressed in CHO cells was either higher than, or similar to, the apparent inhibition constants (Ki) determined in [125I]-iodosulpride competition binding studies. Functional selectivities of the agonists were less than radioligand binding selectivities. The rank orders of agonist potencies and selectivities were similar, but not identical, to the rank orders of radioligand binding affinities and selectivities. The dopamine receptor antagonists, iodosulpride and clozapine, had no effect on basal acidification rates but inhibited acidification responses in CHO cells to quinpirole in an apparently competitive manner. Antagonist potencies closely matched their radioligand binding affinities in these cells.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Effects of dopamine on L-type Ca2+ current in single atrial and ventricular myocytes of the rat

1. The effects of dopamine on the L-type Ca2+ current (ICa,L) of both atrial and ventricular single myocytes and on the force of contraction of atrial trabeculae in rat heart were investigated. 2. Dopamine increased atrial ICa,L at concentrations higher than 1 microM, but had little or no effect on ICa,L at lower concentrations. The increase in ICa,L at high concentrations was reversed by propranolol and acetylcholine, but not by phentolamine. Activation and inactivation kinetics of ICa,L were not altered by dopamine. 3. In rat ventricular myocytes in which the D4 receptor mRNA does not express, dopamine (20-100 microM) also increased the ICa,L amplitude and propranolol reversed this effect. 4. Clozapine, a potent D4 receptor antagonist, blocked the augmenting effect of dopamine on ICa,L. However, this effect could be explained by beta-antagonism, since clozapine also inhibited the isoprenaline effect. 5. In the atrial trabeculae, the increase in contraction by dopamine (1 to 30 microM) was reversed by 1 microM propranolol, but not by 2 microM phentolamine. Low doses of dopamine (0.01 to 0.3 microM) did not affect the contraction in the controls or during a modest stimulation of the beta-adrenoceptor with 0.01 microM isoprenaline. 6. These results indicate that the positive inotropic action of dopamine is mediated through direct stimulation of the beta-adrenoceptor in both atrial and ventricular myocytes. Involvement of D4 receptor appears unlikely in the regulation of the atrial contraction.

The rat D4 dopamine receptor couples to cone transducin (Galphat2) to inhibit forskolin-stimulated cAMP accumulation

Based on its expression pattern and pharmacology, the D4 dopamine receptor may play a role in schizophrenia. Thus it is of interest to know what signaling pathways are utilized by this receptor. Previously, we showed that activation of D4 receptors in a mouse mesencephalic neuronal cell line (MN9D) inhibited forskolin-stimulated cAMP accumulation in a pertussis toxin-sensitive (Ptx-sensitive) fashion. Of the known Ptx-sensitive G-protein alpha subunits, MN9D-expressed Galphai2, GalphaoA, and GalphaoB; however, none of these coupled to the D4 receptor. Using a low stringency polymerase chain reaction cloning method, we found an additional Ptx-sensitive G-protein cone transducin (Galphat2) expressed in the MN9D cells. We also found that Galphat2 mRNA is highly expressed in rat mesencephalic tissue. To test the hypothesis that the D4 receptor couples to Galphat2, we cotransfected MN9D cells with the D4 receptor and a mutagenized Ptx-resistant Galphat2 subunit (mGalphat2). Application of the dopaminergic agonist quinpirole to cotransfected cells inhibited forskolin-stimulated cAMP accumulation in the presence or absence of Ptx. To our knowledge, this is the first report demonstrating that the D4 dopamine receptor functionally couples to a specific G-protein and that a non-opsin-like receptor can couple with a transducin subunit.

Molecular attributes of dopamine receptors: new potential for antipsychotic drug development

The neurotransmitter dopamine (DA) plays a central role in the control of motor function, emotional states, and endocrine physiology. The discovery that schizophrenic symptoms can be alleviated by neuroleptic drugs and the finding that these drugs interact at dopamine receptors has indicated involvement of the dopamine system in schizophrenia. The dopamine system has recently been shown to rely on the activation of five distinct subtypes of DA receptors (D1-D5) identified by molecular cloning, and pharmacological studies have specifically implicated the D2-like receptors (D2R, D3R and D4R) in antipsychotic action. In addition, the localization of D3R and D4R expression in the mesolimbic/mesocortical DA pathways is consistent with their proposed involvement in affective behaviour, and suggests that drugs developed specifically for these receptor subtypes might have potent antipsychotic activity with a lower propensity for extrapyramidal, endocrine, and cognitive side-effects.