Hypothalamic control of body weight: role of tri-iodothyronine (T3)

Understanding the ways that the brain controls energy intake and expenditure has benefits both for improved animal production and for identification of strategies and therapeutics to counteract obesity in man. Studying these processes in the Siberian hamster provides new insights because it undergoes an annual cycle of body weight gain due to fat deposition in spring and summer, but then survives winter by reducing its appetite and by burning its fat reserves. These opposing states of weight gain and loss can be induced in hamsters simply by changing the photoperiod in which the hamsters live. Long days induce the summer anabolic state whereas short days induce the winter catabolic state. The overall aim of this project is to understand how the mammalian brain can establish this catabolic state which results in long-term body weight loss. This project builds upon our previous studies which compared fat and lean hamsters and found changes in gene expression that were restricted to two discrete regions within the hypothalamus, the region of the brain known to be important in controlling food intake and energy metabolism. The most important finding is that the gene encoding the enzyme which converts active thyroid hormone (T3) into its inactive form is expressed at far higher levels as the period of decreased appetite and body weight loss begins. The enzyme is only found in cells lining the ventricular system in the hypothalamus, and the consequence of the change is that there is a massive reduction of the availability of thyroid hormone in the brain in the winter catabolic state. We already know that the availability of thyroid hormone in the brain is functionally important because we have conducted pilot experiments where we have replaced small amounts of active thyroid hormone into the hypothalamus in hamsters exposed to short photoperiods when levels naturally fall. This treatment was able to block all the winter body weight loss that usually occurs in short days. The objectives of this project are to understand how thyroid hormone acts within the hypothalamus, and to determine exactly how this change in thyroid hormone availability is brought about. The project will characterise exactly what replacement of thyroid hormone does, by measuring food intake, body weight and metabolic rate as assessed by oxygen consumption and carbon dioxide production using an automated monitoring system. It will then find out which other genes are affected by thyroid hormone replacement by measuring their expression in sections of brain taken from hamsters at the end of the experiment. Thyroid hormone is important for the initial development of the brain, where stem cells divide to form neurons and supporting cells, so we will pay particular attention to a group of genes which are involved in the generation of new neurons. We will find out if the development of the winter catabolic state represents a 'plastic' change in the adult brain. We will also look at a group of genes that are involved in communication within the brain by retinoic acid, a vitamin A derivative because we have already detected changes in this signalling pathway in fat and lean hamsters. Once we have identified genes that are affected by thyroid hormone, we will test their function by altering their expression in experimental hamsters, and then determining whether they can still show the normal metabolic changes when exposed to short days. We will use the technique of gene therapy whereby viral particles are modified to carry DNA into the hamster's brain, and this will then be incorporated and expressed. This is a safe technique because the viruses are modified so that they cannot replicate to infect other tissue. Our preliminary studies show that virally-mediated gene therapy can produce long-term changes in hypothalamic gene expression, so this is an effective way to discover the function of those genes which are regulated by thyroid hormone.

The Siberian hamster displays profound annual changes in energy balance which are regulated by photoperiod. In short days its appetite decreases and it metabolises fat reserves thus loses body weight. Our overall aim is to understand how the hypothalamus can generate this chronic catabolic state. We have previously identified a striking upregulation of the type III de-iodinase gene (DIO-3) in ependymal cells in hamsters exposed to short days. This encodes the enzyme responsible for converting tri-iodothyronine (T3) into its inactive form (T2). These cells are the only hypothalamic site of thyroid hormone transport and conversion, so the upregulation of DIO-3 causes a major decline in T3 in the hypothalamus at the time when hamsters enter the catabolic state. We have conducted a preliminary study in which replacement of T3 via intrahypothalamic implants prevented the weight loss that normally occurs in short days. The objective of this project is to understand how T3 acts within the hypothalamus. We will characterise the physiological actions of T3 replacement on food intake, locomotor activity, metabolic rate, body temperature and torpor bouts, and to use in situ hybridization to determine which of the known 'seasonal' genes are regulated by T3 treatment. The focus will be on genes involved in neurogenesis as thyroid hormone is important in this process and we already have evidence of seasonal changes in other markers of neurogenesis and on those involved in retinoic acid signalling, as thyroid hormone signalling interacts with this pathway and we have previously detected seasonal changes in expression of both retinol/retinoic acid transporter genes and in RAR/RXR subunit genes. Once we have identified candidate genes that are regulated by T3, we will test their function by inducing overexpression using a recombinant adeno-associated viral gene therapy strategy. We have already successfully conducted pilot studies with this approach.