LAST July, when biologists announced that grotesquely overweight mice shed fat when they were injected with a protein called leptin, a cure for obesity seemed to lie just around the corner. But some scientists were healthily sceptical, wanting to find out how leptin worked and whether fat people would respond in the same way.
Louis Tartaglia and his colleagues at Millennium Pharmaceuticals in Cambridge, Massachusetts, are beginning to provide the answers. They have identified a receptor on the surface of cells that binds to leptin, which in turn should help other scientists understand how leptin regulates body weight and aid the search for drugs to treat obesity.
Leptin is produced by a gene called ob, and is thought to suppress appetite and increase metabolic rate (This Week, 5 August 1995). To identify the leptin receptor, Tartaglia鈥檚 team attached molecules of leptin to another protein called alkaline phosphatase, which catalyses a reaction that produces a purple product. The researchers added the modified leptin to a variety of mouse tissues. After unbound leptin was washed away, the deepest purple stain appeared in a section of the brain called the choroid plexus. This tissue lines the brain鈥檚 fluid-filled cavities, or ventricles and secretes the cerebrospinal fluid that bathes the entire central nervous system.
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Next, Tartaglia and his colleagues took choroid plexus cells and isolated their mRNA, the molecules which are the intermediate step between genes and the proteins they code for. Using an enzyme called reverse transcriptase, they reversed this process of MRNA production to identify the corresponding genes. 鈥淲e then had a library of millions of samples of DNA,鈥� says Tartaglia. By injecting the samples in batches into cultured cells that originally lacked the leptin receptor, the researchers were then able to use their modified leptin to identify cells which had started to produce the receptor.
From these cells, the researchers were able to pinpoint the receptor gene, which resides on the mouse chromosome 4. It codes for a receptor protein that straddles a cell鈥檚 outer membrane (Cell, vol 83, p 1263). Tartaglia suspects that there may be other leptin receptors still waiting to be discovered. 鈥淏ut if we can demonstrate that leptin is useless at regulating body weight without our receptor, then we will know we have cloned the essential one.鈥�
Once leptin binds to the portion of the receptor that protrudes from a cell鈥檚 surface, other proteins presumably transmit a chemical message to the cell鈥檚 interior. 鈥淥besity may be caused by a mutation in the receptor gene, or it may be due to defective processing inside the cell which occurs after the protein binds to the receptor,鈥� says Tartaglia.
One big puzzle remains, however. Obese strains of mice typically produce little leptin, which is why scientists injected them with it in the first place. But obese humans seem to have high levels of leptin. Tartaglia believes that the underlying cause of severe human obesity may be similar to that behind adult-onset diabetes, where patients become unable to respond to insulin, even though it is present in large quantities. 鈥淸Obese patients] appear to be resistant to leptin,鈥� he says. 鈥淚f the resistance is only partial, we may be able to overcome the problem by designing drugs which can overstimulate the receptor.鈥�
Screening for drugs that target the receptor can begin straight away. But if the problem lies in the subsequent signalling pathway, researchers will first have to identify the signalling proteins inside the cell, and then target drugs against them. 鈥淚dentifying the receptor is an important further step in understanding how the ob gene regulates body weight,鈥� says Colin Dayan, an endocrinologist at the University of Bristol. 鈥淏ut it doesn鈥檛 immediately link all the elements together.鈥�