Eric L. Gustafson

Molecular identification and characterization of the platelet ADP receptor targeted by thienopyridine antithrombotic drugs

ADP plays a critical role in modulating thrombosis and hemostasis. ADP initiates platelet aggregation by simultaneous activation of two G protein-coupled receptors, P2Y1 and P2Y12. Activation of P2Y1 activates phospholipase C and triggers shape change, while P2Y12 couples to Gi to reduce adenylyl cyclase activity. P2Y12 has been shown to be the target of the thienopyridine drugs, ticlopidine and clopidogrel. Recently, we cloned a human orphan receptor, SP1999, highly expressed in brain and platelets, which responded to ADP and had a pharmacological profile similar to that of P2Y12. To determine whether SP1999 is P2Y12, we generated SP1999-null mice. These mice appear normal, but they exhibit highly prolonged bleeding times, and their platelets aggregate poorly in responses to ADP and display a reduced sensitivity to thrombin and collagen. These platelets retain normal shape change and calcium flux in response to ADP but fail to inhibit adenylyl cyclase. In addition, oral clopidogrel does not inhibit aggregation responses to ADP in these mice. These results demonstrate that SP1999 is indeed the elusive receptor, P2Y12. Identification of the target receptor of the thienopyridine drugs affords us a better understanding of platelet function and provides tools that may lead to the discovery of more effective antithrombotic therapies.

A receptor autoradiographic and in situ hybridization analysis of the distribution of the 5-ht7 receptor in rat brain.

1 Receptor autoradiography and in situ hybridization histochemistry have been used to delineate the distribution of the 5‐ht7 receptor and its mRNA in rat brain. Receptor autoradiographic studies were performed using [3H]‐5‐carboxamidotryptamine (5‐CT) as the radioligand. The binding characteristics of the masking compounds were determined in Cos‐7 cells transfected with a panel of 5‐HT receptor subtype cDNAs, including the rat 5‐ht7 cDNA. In situ hybridization studies were carried out with 35S‐labelled oligonucleotide probes to the rat 5‐ht7 mRNA. 2 Specific binding of [3H]‐5‐CT was observed in many areas of the rat brain. Following co‐incubation with 1 μm ergotamine, this binding was completely eliminated. After addition of the masking ligands, [3H]‐5‐CT binding remained in layers 1–3 of cortex, septum, globus pallidus, thalamus, hypothalamus, centromedial amygdala, substantia nigra, periaquaductal gray, and superior colliculus. Addition of the antagonist, methiothepin, to the incubation regimen eliminated most of the remaining [3H]‐5‐CT binding in the brain, with the exception of the globus pallidus and substantia nigra. 3 The 5‐ht7 mRNA was discretely localized in rat brain. The most intense hybridization signals were observed over the thalamus, the anterior hippocampal rudiment, and over the CA3 region of the hippocampus. Other regions containing hybridization signals included the septum, the hypothalamus, the centromedial amygdala and the periaquaductal gray. The regions exhibiting a modest receptor binding signal after methiothepin incubation, the globus pallidus and the substantia nigra, contained no 5‐ht7 hybridization signals, suggesting a non‐5‐ht7 subtype in these two related structures. 4 The distribution of the 5‐ht7 receptor and its mRNA is suggestive of multiple roles for this novel 5‐HT receptor, within several brain systems. The limbic system (centromedial amygdala, anterior hippocampal rudiment, hypothalamus) is particularly well‐represented, indicating a potential role for the 5‐ht7 receptor in affective processes.

ADP Is the Cognate Ligand for the Orphan G Protein-coupled Receptor SP1999

P2Y receptors are a class of G protein-coupled receptors activated primarily by ATP, UTP, and UDP. Five mammalian P2Y receptors have been cloned so far including P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11. P2Y1, P2Y2, and P2Y6 couple to the activation of phospholipase C, whereas P2Y4 and P2Y11 couple to the activation of both phospholipase C and the adenylyl cyclase pathways. Additional ADP receptors linked to Gαi have been described but have not yet been cloned. SP1999 is an orphan G protein-coupled receptor, which is highly expressed in brain, spinal cord, and blood platelets. In the present study, we demonstrate that SP1999 is a Gαi-coupled receptor that is potently activated by ADP. In an effort to identify ligands for SP1999, fractionated rat spinal cord extracts were assayed for Ca2+ mobilization activity against Chinese hamster ovary cells transiently transfected with SP1999 and chimeric Gα subunits (Gαq/i). A substance that selectively activated SP1999-transfected cells was identified and purified through a series of chromatographic steps. Mass spectral analysis of the purified material definitively identified it as ADP. ADP was subsequently shown to inhibit forskolin-stimulated adenylyl cyclase activity through selective activation of SP1999 with an EC50 of 60 nm. Other nucleotides were able to activate SP1999 with a rank order of potency 2-MeS-ATP = 2-MeS-ADP > ADP = adenosine 5′-O-2-(thio)diphosphate > 2-Cl-ATP > adenosine 5′-O-(thiotriphosphate). Thus, SP1999 is a novel, Gαi-linked receptor for ADP.

Localization of messenger RNAs encoding three GABA transporters in rat brain : an in situ hybridization study

Localization of the messenger RNAs encoding three gamma-aminobutyric acid (GABA) transporters, termed GAT-1, GAT-2, and GAT-3, has been carried out in rat brain using radiolabeled oligonucleotide probes and in situ hybridization histochemistry. Hybridization signals for GAT-1 mRNA were observed over many regions of the rat brain, including the retina, olfactory bulb, neocortex, ventral pallidum, hippocampus, and cerebellum. At the microscopic level, this signal appeared to be restricted to neuronal profiles, and the overall distribution of GAT-1 mRNA closely paralleled that seen in other studies with antibodies to GABA. Areas containing hybridization signals for GAT-3 mRNA included the retina, olfactory bulb, subfornical organ, hypothalamus, midline thalamus, and brainstem. In some regions, the hybridization signal for GAT-3 seemed to be preferentially distributed over glial cells, although hybridization signals were also observed over neurons, particularly in the retina and olfactory bulb. Notably, hybridization signal for GAT-3 mRNA was absent from the neocortex and cerebellar cortex, and was very weak in the hippocampus. In contrast to the parenchymal localization obtained for GAT-1 and GAT-3 mRNAs, hybridization signals for GAT-2 mRNA were found only over the leptomeninges (pia and arachnoid). The differential distribution of the three GABA transporters described here suggests that while each plays a role in GABA uptake, they do so via distinct cellular populations.

Targeted deletion of Gpbar1 protects mice from cholesterol gallstone formation

The Gpbar1 [G-protein-coupled BA (bile acid) receptor 1] is a recently identified cell-surface receptor that can bind and is activated by BAs, but its physiological role is unclear. Using targeted deletion of the Gpbar1 gene in mice, we show that the gene plays a critical role in the maintenance of bile lipid homoeostasis. Mice lacking Gpbar1 expression were viable, developed normally and did not show significant difference in the levels of cholesterol, BAs or any other bile constituents. However, they did not form cholesterol gallstones when fed a cholic acid-containing high-fat diet, and liver-specific gene expression indicated that Gpbar1-deficient mice have altered feedback regulation of BA synthesis. These results suggest that Gpbar1 plays a critical role in the formation of gallstones, possibly via a regulatory mechanism involving the cholesterol 7alpha-hydroxylase pathway.

Distribution of a rat galanin receptor mRNA in rat brain

In situ hybridization histochemistry has been employed to determine the distribution of the mRNA encoding a recently cloned rat galanin receptor (rGalR1). The galanin receptor mRNA has been found to be discretely localized in rat brain. The most intense hybridization signals were found over neurons in the nucleus of the lateral olfactory tract, in the ventral posterior hippocampus, and in the lateral external subdivision of the parabrachial nucleus. A number of other brain regions also contain significant hybridization signals, including the hypothalamus, brain stem and spinal cord. The localization of rGalR1mRNA indicates that this receptor may play a role in the varied functions ascribed to GAL, among them feeding, cognition and modulation of sensory information.

Identification of overexpression of orphan G protein-coupled receptor GPR49 in human colon and ovarian primary tumors.

We used gene expression profiling to probe differences in transcriptional output between 15 panels of colon tumor and matched normal colon tissues. This analysis revealed that GPR49, an orphan G Protein-Coupled Receptor (GPCR) is over-expressed in 66% (10/15) colon tumors compared with normal colon tissues. Subsequent analysis of an additional 39 sets of matched normal and tumor colon tissues by real-time quantitative reverse transcriptase confirmed the up-regulation of this receptor. The differential expression of GPR49 between normal and tumor tissue was significant (P > 0.001). GPR49 was up-regulated in 25 of 39 (64%) colon primary tumor tissues. In addition to colon tumors, GPR49 was also found to be up-regulated in 18 of 33 (53%) ovarian primary tumor tissues analyzed by RT-PCR. Moreover, the expression level of GPR49 in colon and ovarian tumors increased in more advanced tumors suggesting a role for the receptor in tumor progression. The selective over-expression of GPR49 in tumor tissues was further illustrated by specific immunohistochemical staining of colon and ovarian tumor tissues, a finding that correlates with the mRNA expression of the receptor. In addition, expression of GPR49 induced transformation in a ligand-dependent manner and Knockdown of GPR49 mRNA level induced apoptosis in colon tumor cells. These novel findings provide a foundation for further studies and suggest a potential role for GPR49 in tumorigenesis.

Distribution of the neuropeptide Y Y2 receptor mRNA in rat central nervous system

Our group has recently reported the expression cloning of the human neuropeptide Y Y2 receptor DNA and subsequently the cloning of the rat homologue. These studies have made it possible to localize the mRNA encoding this NPY receptor subtype in rat tissues. We have, thus, carried out in situ hybridization studies, using radiolabeled oligonucleotide probes to the rat Y2 receptor mRNA, to determine the distribution of Y2 mRNA in rat brain and limited peripheral ganglia. Probe specificity was confirmed by testing antisense and sense probes in transfected cells. In rat brain, hybridization signals obtained with the antisense probes were discrete and were restricted to neuronal profiles in specific subregions of the cortex, hippocampus, amygdala, thalamus, hypothalamus, mesencephalon and pons. Among the regions exhibiting the most intense labeling were the CA3 region of the hippocampus, the arcuate nucleus of the hypothalamus and layer 3 of the piriform cortex. Other regions containing labeled neurons included the medial amygdala, the centromedial thalamic nucleus, the dorsal raphe, the dorsal motor nucleus of the vagus and the trigeminal ganglion. The present results indicate that the mRNA encoding the Y2 receptor is discretely localized in the rat brain and that the distribution is generally consistent with previous radioligand-binding studies. This study should help clarify the relationship between the Y2 receptor distribution and functional studies of NPY receptor subtype classification and provides further evidence for the involvement of the Y2 receptor in multiple physiological processes.

GPR119 is required for physiological regulation of glucagon-like peptide-1 secretion but not for metabolic homeostasis

G protein-coupled receptor 119 (GPR119) is expressed in pancreatic islets and intestine, and is involved in insulin and incretin hormone release. GPR119-knockout (Gpr119(-/-)) mice were reported to have normal islet morphology and normal size, body weight (BW), and fed/fasted glucose levels. However, the physiological function of GPR119 and its role in maintaining glucose homeostasis under metabolic stress remain unknown. Here, we report the phenotypes of an independently generated line of Gpr119(-/-) mice under basal and high-fat diet (HFD)-induced obesity. Under low-fat diet feeding, Gpr119(-/-) mice show normal plasma glucose and lipids, but have lower BWs and lower post-prandial levels of active glucagon-like peptide 1 (GLP-1). Nutrient-stimulated GLP-1 release is attenuated in Gpr119(-/-) mice, suggesting that GPR119 plays a role in physiological regulation of GLP-1 secretion. Under HFD-feeding, both Gpr119(+)(/)(+) and Gpr119(-/-) mice gain weight similarly, develop hyperinsulinemia and hyperleptinemia, but not hyperglycemia or dyslipidemia. Glucose and insulin tolerance tests did not reveal a genotypic difference. These data show that GPR119 is not essential for the maintenance of glucose homeostasis. Moreover, we found that oleoylethanolamide (OEA), reported as a ligand for GPR119, was able to suppress food intake in both Gpr119(+)(/)(+) and Gpr119(-/-) mice, indicating that GPR119 is not required for the hypophagic effect of OEA. Our results demonstrate that GPR119 is important for incretin and insulin secretion, but not for appetite suppression.

Lack of FFAR1/GPR40 does not protect mice from high-fat diet-induced metabolic disease.

OBJECTIVE—FFAR1/GPR40 is a G-protein–coupled receptor expressed predominantly in pancreatic islets mediating free fatty acid–induced insulin secretion. However, the physiological role of FFAR1 remains controversial. It was previously reported that FFAR1 knockout (Ffar1−/−) mice were resistant to high-fat diet–induced hyperinuslinemia, hyperglycemia, hypertriglyceridemia, and hepatic steatosis. A more recent report suggested that although FFAR1 was necessary for fatty acid–induced insulin secretion in vivo, deletion of FFAR1 did not protect pancreatic islets against fatty acid–induced islet dysfunction. This study is designed to investigate FFAR1 function in vivo using a third line of independently generated Ffar1−/− mice in the C57BL/6 background. RESEARCH DESIGN AND METHODS—We used CL-316,243, a β3 adrenergic receptor agonist, to acutely elevate blood free fatty acids and to study its effect on insulin secretion in vivo. Ffar1+/+ (wild-type) and Ffar1−/− (knockout) mice were placed on two distinct high-fat diets to study their response to diet-induced obesity. RESULTS—Insulin secretion was reduced by ∼50% in Ffar1−/− mice, confirming that FFAR1 contributes significantly to fatty acid stimulation of insulin secretion in vivo. However, Ffar1+/+ and Ffar1−/− mice had similar weight, adiposity, and hyperinsulinemia on high-fat diets, and Ffar1−/− mice showed no improvement in glucose or insulin tolerance tests. In addition, high-fat diet induced comparable levels of lipid accumulation in livers of Ffar1+/+ and Ffar1−/− mice. CONCLUSIONS—FFAR1 is required for normal insulin secretion in response to fatty acids; however, Ffar1−/− mice are not protected from high-fat diet–induced insulin resistance or hepatic steatosis.