Protocols for Studying How Thyroid Hormone Receptors Trigger Adaptive Thermogenesis in Mice.

The ability of thyroid hormone T3 to stimulate energy expenditure and regulate different aspects of whole body metabolism has been recognized for over a 100 years. Indeed in 1895 Adolf Magnus Levy was already describing the influence of the thyroid on setting the basal metabolic rate. Now it has been well characterized that the level of circulating T3 is correlated with energy expenditure both in humans and in rodent models (Mullur et al., Physiol Rev 94:355-382, 2014; Silva, Thyroid 5:481-492, 1995). Hypothyroidism and hyperthyroidism are respectively associated with low and high energy expenditure leading to high and low body mass index. Moreover, T3 also reduces the level of cholesterol in the serum, which is one of the key risk factors to develop atherosclerosis (Mullur et al., Physiol Rev 94:355-382, 2014; Hak et al., Ann Intern Med 132:270-278, 2000). Given the worldwide pandemic of obesity and associated metabolic disorders in the last two decades, the metabolic activities of T3 gained a renewed interest. However, one has to keep in mind that despite its desirable actions, T3 cannot be used as a pharmacological agent since it also triggers unacceptable effects including tachycardia, lean mass loss, and osteoporosis. To solve this conundrum, a recent intense effort has been dedicated to understand how T3 fulfills its different activities, looking for the target tissues and the specific T3 receptor (TR) involved. Indeed recently it was shown that injection of T3 in the brain could efficiently trigger thermogenesis (López et al., Nat Med 16:1001-1008, 2010). This questioned the classical paradigm in which most metabolic actions were the result of local T3 action in the different metabolic tissues. To tackle this problem, many new models of transgenic mice carrying selective Knock-out and Knock-in mutation of TR in specific metabolic tissues are currently generated using conditional mutant alleles of THRA (Quignodon et al., Mol Endocrinol 21:2350-2360, 2007) and THRB (Billon et al., Endocrinology 155:2735-2745, 2014). In parallel some new compounds that allow the targeting of T3 to selective metabolic tissues (Finan et al., Cell 167:843-857, 2016) have also been obtained.In this chapter we will provide and comment two different protocols that are useful to study the adaptive thermogenesis in response to two physiological stresses: cold exposure and high fat diet feeding. They can also be used to test the thermogenic activity of the new designed compounds.