Masazumi Kamohara

Molecular identification of nicotinic acid receptor.

Nicotinic acid and its derivative, Acipimox, have been widely used in the treatment of hyperlipidemia. Pharmacological studies have demonstrated that they exert the beneficial effect through the activation of a Gi-protein-coupled receptor on adipocyte, which has remained elusive to date. Here we show that a novel GPCR, designated HM74b because of its high similarity to HM74, is a receptor for nicotinic acid. HM74b mRNA is found in human, murine, and rat adipose tissues. Nicotinic acid and Acipimox inhibit forskolin-stimulated intracellular cAMP accumulation in human HM74b-expressing cells and activate GTP gamma S binding in a dose-dependent manner. [3H]Nicotinic acid specifically binds to HM74b-expressing membrane and its binding is replaced by Acipimox. This finding will open a new phase of research on the physiological role of nicotinic acid and will be a clue to develop novel antihyperlipidemic drugs.

Molecular cloning and characterization of prokineticin receptors.

Recent studies have identified two novel biofunctional proteins, termed prokineticin 1/EG-VEGF and prokineticin 2, which were mammalian homologues of mamba MIT1 and frog Bv8. Prokineticins have been demonstrated to exert their physiological functions through G-protein coupled receptors (GPCRs). In this study, we report the molecular identification of two endogenous prokineticin receptors, designated PK-R1 and PK-R2, through a search of the human genomic DNA database. PK-R1, locating in chromosome 2, and PK-R2, locating in chromosome 20p13, shared 87% homology, which was an extremely high value among known GPCRs. In functional assays, mammalian cells expressing PK-Rs responded to prokineticins in a concentration-dependent manner. Tissue distribution analysis revealed that expression of PK-R1 was observed in the testis, medulla oblongata, skeletal muscle and skin, while that of PK-R2 showed preferential expression in the central nervous system. The tissue distribution of PK-Rs reported in this paper suggests that the prokineticins play multifunctional roles in vivo.

The novel G-protein coupled receptor SALPR shares sequence similarity with somatostatin and angiotensin receptors.

A cDNA encoding a novel G-protein coupled receptor (GPCR) was isolated from a human cerebral cortex cDNA library by low stringency hybridization screening. This putative seven-transmembrane domain receptor of 469 amino acids was designated SALPR (Somatostatin- and Angiotensin- Like Peptide Receptor). SALPR shares the highest amount of amino acid similarity with the somatostatin (35% with SSTR5) and angiotensin receptors (31% with AT1). Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that the SALPR mRNA is predominantly expressed in human brain regions, particularly the substantia nigra and pituitary, although the mRNA can also be detected in the peripheral tissues, albeit at low levels. Chromosomal mapping by radiation hybrid analysis localized the human SALPR gene to the chromosome 5p15.1-5p14. Transient expression of SALPR in COS-1 cells did not produce any binding sites for somatostatin or angiotensin II, indicating the necessity for further study to discover its ligand and physiological significance.

The evolutionarily conserved G protein-coupled receptor SREB2/GPR85 influences brain size, behavior, and vulnerability to schizophrenia

The G protein-coupled receptor (GPCR) family is highly diversified and involved in many forms of information processing. SREB2 (GPR85) is the most conserved GPCR throughout vertebrate evolution and is expressed abundantly in brain structures exhibiting high levels of plasticity, e.g., the hippocampal dentate gyrus. Here, we show that SREB2 is involved in determining brain size, modulating diverse behaviors, and potentially in vulnerability to schizophrenia. Mild overexpression of SREB2 caused significant brain weight reduction and ventricular enlargement in transgenic (Tg) mice as well as behavioral abnormalities mirroring psychiatric disorders, e.g., decreased social interaction, abnormal sensorimotor gating, and impaired memory. SREB2 KO mice showed a reciprocal phenotype, a significant increase in brain weight accompanying a trend toward enhanced memory without apparent other behavioral abnormalities. In both Tg and KO mice, no gross malformation of brain structures was observed. Because of phenotypic overlap between SREB2 Tg mice and schizophrenia, we sought a possible link between the two. Minor alleles of two SREB2 SNPs, located in intron 2 and in the 3′ UTR, were overtransmitted to schizophrenia patients in a family-based sample and showed an allele load association with reduced hippocampal gray matter volume in patients. Our data implicate SREB2 as a potential risk factor for psychiatric disorders and its pathway as a target for psychiatric therapy.

cDNA cloning and characterization of porcine histamine H4 receptor.

The cDNA encoding histamine H4 receptor was cloned from the porcine spleen cDNA library. Porcine H4 receptor, which shares 72% homology with its human counterpart, bound to histamine in receptor-expressing mammalian cells. Isolation of the porcine H4 receptor, which is important for understanding of the pharmacology, will aid in better interpretation of physiological role of this subtype of histamine receptor.

Both V1A and V1B vasopressin receptors deficiency result in impaired glucose tolerance

[Arg(8)]-vasopressin (AVP) is involved in the regulation of glucose homeostasis via vasopressin V(1A) and vasopressin V(1B) receptor. Our previous studies have demonstrated that vasopressin V(1A) receptor deficient (V(1A)R(-/-)) mice exhibited hyperglycemia, vasopressin V(1B) receptor deficient (V(1B)R(-/-)) mice, in contrast, exhibited hypoglycemia with hypoinsulinemia. These findings indicate that vasopressin V(1A) receptor deficiency results in decreased insulin sensitivity, whereas vasopressin V(1B) receptor deficiency results in increased insulin sensitivity. In our previous and present studies, we used the glucose tolerance test to investigate glucose tolerance in mutant mice, lacking either the vasopressin V(1A) receptor, the vasopressin V(1B) receptor, or both receptors, that were kept on a high-fat diet. Glucose and insulin levels were lower in V(1B)R(-/-) mice than in wild type (WT) mice when both groups were fed the high-fat diet, which indicates that the insulin sensitivity of the V(1B)R(-/-) mice was enhanced. V(1A)R(-/-) mice on the high-fat diet, on the other hand, exhibited overt obesity, along with an impaired glucose tolerance, while WT mice on the high-fat diet did not. Next, in order to assess the effect of vasopressin V(1B) receptor deficiency on the development of glucose intolerance caused by vasopressin V(1A) receptor deficiency, we generated mice that were deficient for both vasopressin V(1A) receptor and vasopressin V(1B) receptor (V(1AB)R(-/-)), fed them a high-fat diet, and examined their glucose tolerances using the glucose tolerance test. Glucose tolerance was impaired in V(1AB)R(-/-) mice, suggesting that the effects of vasopressin V(1B) receptor deficiency could not influence the development of hyperglycemia promoted by vasopressin V(1A) receptor deficiency, and that blockade of both receptors could lead to impaired glucose tolerance.

Molecular modeling of vasopressin receptor and in silico screening of V1b receptor antagonists

Introduction: G protein-coupled receptors (GPCRs) are integral membrane proteins which contain seven-transmembrane-spanning alpha-helices. GPCR-mediated signaling has been associated with various human diseases, positioning GPCRs as attractive targets in the drug discovery field. Recently, through advances in protein engineering and crystallography, the number of resolved GPCR structures has increased dramatically. This growing availability of GPCR structures has greatly accelerated structure-based drug design (SBDD) and in silico screening for GPCR-targeted drug discovery. Areas covered: The authors introduce the current status of X-ray crystallography of GPCRs and what has been revealed from the resolved crystal structures. They also review the recent advances in SBDD and in silico screening for GPCR-targeted drug discovery and discuss a docking study, using homology modeling, with the discovery of potent antagonists of the vasopressin 1b receptor. Expert opinion: Several innovative protein engineering techniques and crystallographic methods have greatly accelerated SBDD, not only for already-resolved GPCRs but also for those structures which remain unclear. These technological advances are expected to enable the determination of GPCR-fragment complexes, making it practical to perform fragment-based drug discovery. This paves the way for a new era of GPCR-targeted drug discovery.