Genetic and biological characterization of the anorexic anx/anx mouse

Abstract

Pathological deviations in body weight is a major health problem. It is currently unclear how normal body weight and food intake is controlled. There is even less understanding of the mechanisms behind abnormal eating behavior as in anorexia and obesity. Major support for the concept of genetic control of body weight and food intake has emerged from different animal models as well as twin- and family- studies in humans. A number of genes have been found which have an effect on food intake and body weight in animals. Some of these genes have similar effects in humans.

The mouse anorexia (anx) mutation arose spontaneously in 1976. It causes a lethal phenotype with a pronounced eating disturbance. Mutant mice (anx/anx) are characterized by poor appetite, reduced body weight, emaciated appearance and abnormal behavior including body tremors, head weaving, hyperactivity and uncoordinated gait. The anx gene was previously linked to the nonagouti (a) locus on mouse Chromosome 2. We have mapped the anx mutation to a 0.2 cM interval on Chromosome 2 by simple sequence polymorphism marker genotyping of 2422 F2 progeny from two different intercrosses. Markers from this interval were used to construct a physical map, consisting of YAC, BAC, PAC and PI clones, spanning the interval. The clones were shotgun sequenced and candidate genes identified.

Several histochemical alterations of peptides regulating food intake in the arcuate nucleus in the hypothalamus of anx/anx mice were found, using in situ hybridization and immunohistochemistry. Levels of neuropeptide Y- (NPY) and agouti gene-related protein- (AGRP) like immunoreactivities were increased in cell bodies and decreased in terminals in anx/anx mice, whereas no changes were observed in the respective mRNA levels. In contrast, markers of pro- opiomelanocortin (POMC) neurons, such as POMC, cocaine- and amphetamine-regulated transcript (CART) and the NPY receptors Y1 and Y5, had decreased mRNA levels. Taken together these results indicate that the phenotype of the anx/anx mouse may be related to the arcuate neurons themselves rather than to a particular neuropeptide.

Striatal dopamine and its metabolites were significantly lower in anx/anx mice. In addition, the activity of Na+, K+-ATPase in striatal medium spiny neurons was significantly increased in anx/anx mice compared to normal littermates. Furthermore, addition of dopamine in vitro, which normally inhibits the NA+, K+-ATPase, failed to suppress the increased activity in anx/anx neurons. This suggests an abnormal dopaminergic transmission in the striatum of anx/anx mice.

The identification of the anx gene and its product(s) may improve the understanding of food intake regulation. The fact that the anx mutation affects many of the known hypothalamic feeding regulatory molecules makes the anx gene an interesting target for development of new tools and/or pharmaceuticals for the treatment of eating disorders.