The Leptin Receptor*

Of the many genetic obesity syndromes, none have been as intensively studied as ob/ob and db/db mice. These two mutant mice were originally identified over 30 years ago (1, 2) and shown to be a result of two distinct single gene mutations residing on mouse chromosomes 6 (ob) and 4 (db). The phenotypes and pathophysiologies of these two mice have been studied for decades and described in well over 1000 publications. However, the nature of the lesions or primary defects was not revealed until very recently. Perhaps the most informative early studies on the nature of the ob and db primary defects were the parabiosis experiments (partial connection of the circulatory systems of animals through grafting) performed throughout the 1970s (3) (reviewed in Ref. 4). Parabiosis of an ob/obmouse and a lean control resulted in partial normalization of body weight in the ob/obmutant mouse. This led to the proposal that ob/obmice were deficient in a circulating factor that could be restored through the blood of the lean animal. However, db/db mice that underwent parabiosis with lean controls did not exhibit body weight normalization. This suggested that db/db mice may be defective in their ability to respond to the putative satiety factor, perhaps because they were defective in the receptor for this molecule. The Obese (ob) Gene and Its Product, Leptin Despite intensive interest in the nature of the putative satiety factor missing in ob/ob mice, biochemical strategies failed to identify it. It was not until a genetic/positional cloning strategy was employed that the gene corresponding to the ob locus and its gene product were ultimately identified (5). The wild type ob gene encodes a protein of about 16 kDa that is preceded by a secretory hydrophobic signal peptide. It is expressed in adipose tissue in multiple mammalian species including mice and humans. The development of antibody reagents confirmed that this factor (leptin) is found at high levels in blood, consistent with the previous parabiosis studies (6). Since the cloning of the ob gene numerous studies have described the regulation of the leptin mRNA and protein. Although the purpose of this review is not to comprehensively examine the growing literature on the regulation of the leptin ligand, it is important to briefly summarize a few aspects of leptin expression and regulation that are key in interpreting the biology of the leptin receptor. The leptin transcript appears to be expressed fairly specifically in adipose tissue (5), although it is also detectable in human placenta on poly(A) Northern blots. Steady state levels of the leptin mRNA and protein are elevated in a variety of rodent obesity models (6–9). These observations have led to the proposal that leptin serves as an “adipostat,” informing the body of the status of energy storage in the adipose tissue so that appropriate changes in appetite, metabolism, and nutrient partitioning can be signaled via the leptin receptor. Dramatic regulation of the leptin transcript and protein has also been observed in response to short term alterations in food intake (7, 9–11); fasting results in dramatic down-regulation, and excessive caloric intake results in up-regulation. It is therefore plausible that an important role for leptin is mediating the response to starvation (12). Acute effects on leptin mRNA and protein have also been observed in response to a variety of stimuli including glucocorticoids, cytokines, and insulin (10, 13, 14). There has now also been considerable analysis of leptin regulation in humans. The leptin mRNA is regulated in humans by both changes in the percentage of body fat as well as changes in food intake (6, 15–18). However, the degree of mRNA regulation in humans is less impressive than that seen in rodents. Importantly, at the protein level human leptin is dramatically regulated, with changes approaching those seen in the rodent obesities and fasting (6, 15). The protein is much higher in individuals with an increased percentage of body fat and is down-regulated during body weight loss. This parallel regulation in mice and humans may imply that leptin is functioning in humans as it is in rodents, although further studies are required to directly address this. An obvious and important question has been whether a significant portion of human obesity can be due to mutations in the ob gene. The ob coding region has been sequenced from hundreds of human individuals, but mutations have not been found (16, 19, 20). Although mutations affecting mRNA levels can reside outside of the coding region, individuals with severely reduced leptin mRNA levels have also not yet been described (15–18), suggesting that the number of such individuals will not be high. On the other hand, genotyping of microsatellite markers that span the ob gene region has suggested linkage of this region with extreme human obesity (21, 22). Considerable excitement has been generated by the observations that administration of recombinant leptin to rodents results in food intake reduction and weight loss (23–26). Although the potency of leptin is highest in mice that are completely deficient in this protein (ob/ob), significant effects can be seen at higher doses in normal mice and mice with diet-induced obesity. Such studies have brought hope that leptin may be an effective treatment even in some obesities that are not due to leptin deficiencies. Of particular interest are studies that have investigated the effects of centrally administered leptin. These studies showed that leptin injection into the lateral or third brain ventricle produced reduction in food intake and weight loss, strongly implying that leptin could act directly on receptors within the central nervous system (23, 26). Weight loss in rodents following leptin administration appears to be due to not only decreases in food intake but also increases in energy expenditure (24, 25). Although the mechanisms of increased energy utilization are likely to be complex, one important component may involve the activation of brown adipose tissue (27). The observation that leptin deficiencies are not common in human obesity and, in fact, that leptin levels appear to be up-regulated as the percentage of body fat increases has suggested that resistance to normal or elevated levels of leptin may be more important than inadequate leptin production in human obesity (15). This line of thought has been further strengthened by the parallel situation of type II diabetics, many of whom exhibit severe insulin resistance while producing elevated levels of insulin. These observations further stimulated interest in the identification of the receptor for leptin and the analysis of leptin signal reception. They also heightened interest in the db/dbmouse, a model of total leptin resistance (23–26) and elevated leptin levels (8). Cloning of the Leptin Receptor (OB-R) The identification of the receptor for the leptin protein was realized through an expression cloning strategy (28). Tagged versions of leptin were generated either through a traditional iodination strategy or by generating recombinant fusion proteins between leptin and secreted placental alkaline phosphatase. These tagged reagents were then used to identify a tissue source expressing a cell surface leptin binding activity (28, 29). Significant and specific leptin binding was detected in the mouse choroid plexus. To clone this leptin binding activity a murine choroid plexus cDNA library was constructed, and cells transfected with this library were screened with a leptin-alkaline phosphatase fusion protein. From this screen, cDNAs were identified that encoded a cell surface * This minireview will be reprinted in the 1997 Minireview Compendium, which will be available in December, 1997. ‡ To whom correspondence should be addressed: Millennium Pharmaceuticals, 640 Memorial Dr., Cambridge, MA 02139. Tel. : 617-679-7110; Fax: 617-679-7370; E-mail: tartaglia@mpi.com. 1 X. Weng and L. A. Tartaglia, unpublished observations. Minireview THE JOURNAL OF BIOLOGICAL CHEMISTRY Vol. 272, No. 10, Issue of March 7, pp. 6093–6096, 1997 © 1997 by The American Society for Biochemistry and Molecular Biology, Inc. Printed in U.S.A.