Melanin concentrating hormone, or MCH, is a cyclic 19 amino acid neuropeptide that functions as a regulator of food intake and energy balance. In many vertebrate species, including humans, MCH is produced in the hypothalamus, which is associated with behaviors such as eating, drinking, aggression and sexual behavior. MCH is also produced at various peripheral sites, including the gastrointestinal tract and testis.
The postulated role of MCH in feeding behavior and body weight is confirmed by the finding that i.c.v. injection of MCH into the lateral ventrical of the hypothalamus increases caloric consumption in rats over similarly treated control animals. Furthermore, rats having the ob/ob genotype exhibit a 50–80% increase in MCH mRNA expression as compared to leaner ob/+ genotype mice. MCH knockout mice are leaner than mice that produce MCH, but are otherwise genetically identical, due to hypophagia and an increased metabolic rate.
MCH activity is mediated via binding to specific receptors. Like other G protein-coupled receptors (e.g., neuropeptide Y (NPY) and beta-adrenergic receptors), MCH receptors are membrane-spanning proteins that consist of a single contiguous amino acid chain comprising an extracellular N-terminal domain, seven membrane-spanning alpha helical domains (connected by three intracellular loop domains alternating with three extracellular loop domains), and an intracellular C-terminal domain. Signal transduction is initiated by the binding of MCH to the receptor. This elicits conformational changes in the extracellular domains. When the receptor is functioning properly, these conformational changes propagate through the transmembrane domains and result in a coordinated change in the intracellular portions of the receptor. This precise alteration in the intracellular domains acts to trigger the associated G-protein complex to modulate intracellular signaling.
MCH1R is a 353 amino acid, 7-transmembrane, alpha-helical, G protein-coupled receptor, initially reported as orphan receptor SCL-1 by Kolakowski et al. (1996) FEBS Lett. 398:253–58 and Lakaye et al. (1998) Biochim. Biophys. Acta 1401:216–220. Chambers et al. (1999) Nature 400:261–65 and Saito et al. (1999) Nature 400:265–69 subsequently showed that SLC-1 was an MCH receptor. Immunohistochemistry studies of rat brain sections indicate that the MCH1R is widely expressed in brain. MCH1R expression is found in olfactory tubercle, cerebral cortex, substantia nigra, basal forebrain CA1, CA2, and CA3 field of the hippocampus, amygdala, and in nuclei of the hypothalamus, thalamus, midbrain and hindbrain. Strong signals are observed in the ventromedial and dorsomedial nuclei of the hypothalamus, two areas of the brain involved in feeding behavior. Upon binding MCH, MCH1R expressed in HEK 293 cells mediate a dose dependent release of intracellular calcium. Cells expressing MCH1R also exhibit a pertussis toxin sensitive dose-dependent inhibition of forskolin-elevated cyclic AMP, indicating that the receptor couples to a GI/O G-protein alpha subunit.
Recently, a second MCH receptor (designated MCH2R) was identified (WO 01/70975; WO 01/07606; WO 00/49046; An et al., Proc. Natl. Acad. Sci. USA (2001) 98:7576–7581; Sailer et al., Proc. Natl. Acad. Sci. USA (2001) 98:7564–7569; Hill et al., J. Biol. Chem. (2001) 276:20125–20129; Mori et al., Biochem. Biophys. Res. Commun. (2001) 283:1013–1018). The human sequence (Hill et al.) has GenBank Accession Number AF347063. MCH2R has an overall amino acid identity of more than 30% with MCH1R, and is detected specifically in the same regions of the brain as MCH1R.
Agents capable of modulating MCH receptor activity are highly desirable for the treatment of obesity, eating disorders (e.g., bulimia and anorexia), sexual disorders (e.g., anorgasmic or psychogenic impotence) and metabolic disorders, such as diabetes. Isolated MCH receptors (e.g., as components of membrane preparations), cells expressing such receptors and cloned MCH receptor genes are needed to facilitate the discovery of such agents.
Accordingly, there is a need in the art for additional MCH receptor sequences. The present invention fulfills this need, and provides further related advantages.