Karen J. Moore

Identification and Expression Cloning of a Leptin Receptor, OB-R

The ob gene product, leptin, is an important circulating signal for the regulation of body weight. To identify high affinity leptin-binding sites, we generated a series of leptin-alkaline phosphatase (AP) fusion proteins as well as [125I]leptin. After a binding survey of cell lines and tissues, we identified leptin-binding sites in the mouse choroid plexus. A cDNA expression library was prepared from mouse choroid plexus and screened with a leptin-AP fusion protein to identify a leptin receptor (OB-R). OB-R is a single membrane-spanning receptor most related to the gp130 signal-transducing component of the IL-6 receptor, the G-CSF receptor, and the LIF receptor. OB-R mRNA is expressed not only in choroid plexus, but also in several other tissues, including hypothalamus. Genetic mapping of the gene encoding OB-R shows that it is within the 5.1 cM interval of mouse chromosome 4 that contains the db locus.

Identification and Characterization of the Mouse Obesity Gene tubby: A Member of a Novel Gene Family

The mutated gene responsible for the tubby obesity phenotype has been identified by positional cloning. A single base change within a splice donor site results in the incorrect retention of a single intron in the mature tub mRNA transcript. The consequence of this mutation is the substitution of the carboxy-terminal 44 amino acids with 24 intron-encoded amino acids. The normal transcript appears to be abundantly expressed in the hypothalamus, a region of the brain involved in body weight regulation. Variation in the relative abundance of alternative splice products is observed between inbred mouse strains and appears to correlate with an intron length polymorphism. This allele of tub is a candidate for a previously reported diet-induced obesity quantitative trait locus on mouse chromosome 7.

Insight into the microphthalmia gene

The murine microphthalmia gene (mi) is one of the last multi-allelic, classic coat-colour genes to be cloned in the mouse and, similar to many of these genes, encodes an exciting molecule that is is involved in multiple developmental processes. The existence of the numerous alleles has allowed the molecular dissection of the function of the MI bHLH-Zip transcription factor in vivo and offers a unique opportunity to understand the function of a multimeric transcription factor throughout development and in many tissues. It is also the gene mutated in some patients with the human deafness syndrome, Waardenburgs syndrome type II, and hence helps to understand this syndrome.

The mahogany protein is a receptor involved in suppression of obesity

Genetic studies have shown that mutations within the mahogany locus suppress the pleiotropic phenotypes, including obesity, of the agouti-lethal-yellow mutant,. Here we identify the mahogany gene and its product; this study, to our knowledge, represents the first positional cloning of a suppressor gene in the mouse. Expression of the mahogany gene is broad; however, in situ hybridization analysis emphasizes the importance of its expression in the ventromedial hypothalamic nucleus, a region that is intimately involved in the regulation of body weight and feeding. We present new genetic studies that indicate that the mahogany locus does not suppress the obese phenotype of the melanocortin-4-receptor null allele or those of the monogenic obese models (Lepdb, tub and Cpefat). However, mahogany can suppress diet-induced obesity, the mechanism of which is likely to have implications for therapeutic intervention in common human obesity. The amino-acid sequence of the mahogany protein suggests that it is a large, single-transmembrane-domain receptor-like molecule, with a short cytoplasmic tail containing a site that is conserved between Caenorhabditis elegans and mammals. We propose two potential, alternative modes of action for mahogany: one draws parallels with the mechanism of action of low-affinity proteoglycan receptors such as fibroblast growth factor and transforming growth factor-β, and the other suggests that mahogany itself is a signalling receptor.

Genetic Background Determines the Extent of Atherosclerosis in ApoE-Deficient Mice

Two strains of ApoE-deficient mice were found to have markedly different plasma lipoprotein profiles and susceptibility to atherosclerosis when fed either a low-fat chow or a high-fat Western-type diet. FVB/NJ ApoE-deficient (FVB E0) mice had higher total cholesterol, HDL cholesterol, ApoA1, and ApoA2 levels when compared with C57BL/6J ApoE-deficient (C57 E0) mice. At 16 weeks of age, mean aortic root atherosclerotic lesion area was 7- to 9-fold higher in chow diet-fed C57 E0 mice and 3.5-fold higher in Western diet-fed C57 E0 mice compared with FVB E0 mice fed similar diets. Lesion area in chow diet-fed first-generation mice from a strain intercross was intermediate in size compared with parental values. The distribution of the lesion area in 150 chow diet-fed second-generation progeny spanned the range of the lesion area in both parental strains. There were no correlations between total cholesterol, non-HDL cholesterol, HDL cholesterol, ApoA1, ApoA2, ApoJ, or anti-cardiolipin antibodies and lesion area in the second-generation progeny. Thus, a genomic approach may succeed in identifying the genes responsible for the variation in atherosclerosis susceptibility in these 2 strains of ApoE-deficient mice, which could not be explained by measured plasma parameters.

Mouse chromosome 6

The 1999 consensus map for Chr 6 is presented in Fig. 1. There are now 1215 loci, including 266 named genes, on the chromosome. These genes include the gene complexes for Igk, Tcrb, Hoxa, Ly49 and Ly55. The map has been constructed by first integrating data from the multilocus crosses. Additional data from smaller crosses and RI strains were then added. The map has been refined using physical data from YAC, BAC and P1 contigs, where available, and from the RH maps. A number of new loci have been added, including 51 new named or sequenced genes and 28 new unidentified ESTs, most of the latter mapped in the Jackson BSS backcross. These ESTs appear to be clustered, perhaps coinciding with regions of high gene density. The resources available at MGI (http://www.informatics.jax.org/) have been invaluable. The mouse Unigene database (http://www.ncbi.nlm.nih.gov/UniGene/ Mm.Home.html) now has a collection of ESTs for known mapped genes and identifies more than 115 genes on mouse Chr 6. The positions of several loci have been moved. D6Wsu163e has been moved from position 49, where it was erroneously placed, to 60. Other genes, such as tc and Tgfa, have been moved small distances based on recent mapping results. Positions of several microsatellite markers have also been moved, based on YAC contigs and RH data.