No abstract

How genes with newly characterized functions originate remains a fundamental question. PMCHL1 and PMCHL2 , two chimeric genes derived from the melanin-concentrating hormone ( MCH ) gene, offer an opportunity to examine such an issue in the human lineage. Detailed structural, expression, and phylogenetic analysis showed that the PMCHL1 gene was created near 25 million years ago (Ma) by a complex mechanism of exon shuffling through retrotransposition of an antisense MCH messenger RNA coupled to de novo creation of splice sites. PMCHL2 arose 5 to 10 Ma by an event of duplication involving a large chromosomal region encompassing the PMCHL1 locus. The RNA expression patterns of those chimeric genes suggest that they have been submitted to strong regulatory constraints during primate evolution.

Melanin-concentrating hormone (MCH) is a cyclic nona- Melanin-concentrating hormone (MCH)1 has been initially described in fish as a heptadecapeptide (Asp-Thr-Met-Arg-CysMet-Val-Gly-Arg-Val-Tyr-Arg-Pro-Cys-Trp-Glu-Val (1)). Its structure was relatively conserved throughout evolution, although in mammals the sequence of MCH is a nonadecapeptide with differences mainly in the N terminus (Asp-Phe-Asp-MetLeu-Arg-Cys-Met-Leu-Gly-Arg-Val-Tyr-Arg-Pro-Cys-Trp-GlnVal (2)). In rodents, there are now several lines of evidence for the involvement of MCH in the central regulation of feeding behavior as reviewed by Tritos and Maratos-Flier (3). The MCH peptide and its receptor are expressed in the hypothalamus, a region involved in energy balance and food intake (4 -7). In this particular brain area, MCH mRNA is overexpressed and up-regulated during fasting in ob/ob mice as well as in rats (8, 9). Intra-cerebroventricular injections of MCH promote feeding in mice and rats (9 -12). Finally, transgenic mice lacking the MCH gene are lean and hypophagic (13). Interestingly, in peripheral tissues, MCH also stimulates the release of leptin from isolated rat adipocytes (14). The lack of suitable binding conditions, mainly due to the hydrophobic and sticky nature of MCH itself or derivatives (15, 16), was probably a limitation for expression cloning of the receptor. The MCH receptor was nevertheless recently identified by several groups using reverse pharmacology (17-21). The MCH function was assigned to the previously described orphan receptor SLC-1 (22, 23), using inhibition of forskolin-stimulated cAMP production and induction of calcium rise.Receptor cloning and association of functional tests open the way to the search for pharmacological tools, especially receptor antagonists that are needed to study receptor functions. One of the possible strategies to this goal is the chemical modification of the natural peptide including peptide shortening, amino acid substitution, and conformation restriction with the help of structure-activity relationships and modeling studies toward optimized nonpeptide ligands. Such a strategy has been successful for the design of subtype-specific antagonists of neuropeptide Y receptors (24 -27).In the case of MCH, only two sets of data have been published on the pharmacological action and binding affinity of MCH analogues in vitro. A first series of experiments with fish MCH on fish, frog, or other batrachian skin assays were reported (28 -32), showing that fish MCH could be shortened at * This work was supported by a a Convention CIFRE between the Association Nationale de la Recherche Technique, the Institut de Recherches SERVIER and the Centre National de la Recherche Scientifique (to T. S.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.*