Investigation of the role of thyroid hormone receptor beta in the ventromedial nucleus of the hyptholamus in the mediation of energy homeostasis

The hypothalamus is a region of the central nervous system that is important in the regulation energy homeostasis. Thyroid hormone action within the hypothalamus has been shown to influence food intake and body weight and these effects may be mediated through the nuclear transcription factor thyroid hormone receptor beta (TRβ), which is highly expressed within the ventromedial nucleus (VMN) of the hypothalamus. Recombinant adeno-associated virus (rAAV) expressing Cre recombinase (rAAV-Cre) was synthesised. This was injected bilaterally into the VMN of adult mice which were bred for locus of crossover of phage P1 (LoxP) sites within the THRB gene TRβflox/flox) which encodes TRβ . Cre recombination in these mice introduces a frame shift, so that only truncated products are encoded. This results in TRβ knock down in the VMN. Recombinant AAV encoding green fluorescent protein (GFP) (rAAV-GFP) was injected bilaterally into the VMN of control mice. Cre mediated tissue specific knockdown of TRβ in the VMN resulted in a highly significant increase in body weight over sixty three days. The cumulative food intake over the course of the study was significantly greater in mice with TRβ knock down in the VMN compared with controls. Furthermore, mice with intra-VMN TRβ knock down mice failed to become obese when pair-fed to the food intake of control animals, but when allowed to ad libitum feed once again, gained significantly more weight and ate significantly more than controls. The importance of hyperphagia to the obese phenotype was further underscored by the level of uncoupling protein-1 mRNA in brown adipose tissue which was unaltered in intra-VMN TRβ knock down mice. This work is the first to show the essential function of TRβ in the VMN in the regulation of food intake and body weight by demonstrating that TRβ knock down in the VMN results in hyperphagia and obesity