CART in the regulation of appetite and energy homeostasis

Abstract

CART is a key neuropeptide with predominant expression in the hypothalamus central to appetite control and energy homeostasis. However, the underlying mechanisms and sites of action of CART remain unclear, leading to discrepant implications for CART function. This is mainly due to the lack of specific pharmacological tools as the CART receptor(s) are yet to be identified. Utilizing a novel CART-cre knock-in mouse line, this work aims to determine the impact of CART action on energy balance, by means of transgenic mouse models, CART introduction or neuronal activation, and changes in ambient temperature. To study the potential contribution of CART to thermoregulation and diet-induced obesity, CART-deficient CARTcre/cre mice were subjected to thermoneutrality (30 ˚C) and high fat diet (HFD). Whilst CART KO mice displayed higher adiposity and lower lean mass across temperatures, especially under HFD, the elevated weight gain and thermogenesis under standard housing (22 ˚C) was reversed at thermoneutrality. These effects were accompanied by distinctive neurochemical profiles in CART KO mice. In addition to temperature- and diet-dependence, CART function further exhibited brain region-specificity, as evident in the differential effects of chronic CART introduction via the FLEX switch design into the Arc or LHA, two hypothalamic areas critical for energy balance. Intriguingly, while intra-LHA CART induced obesogenic traits in CARTcre/cre mice, the opposite was observed for the CART-positive CARTcre/+ mice. In contrast, intra-Arc CART displayed anorexigenic effects, particularly in CARTcre/+ mice. The distinctions across genotypes also highlighted in the disparate responses upon CART neuron activation in the Arc or LHA enabled by the DREADD technology. Substantially, both energy expenditure and physical activity were decreased and increased upon CART neuron stimulation in the Arc and LHA, respectively, particularly in CARTcre/+ mice. The discrete phenotypes may attribute to the interaction of endogenous CART with other neuromodulators such as NPY. Accordingly, selective deletion of NPY-Y1 receptor on CART neurons achieved in our unique CARTcre/+;Y1lox/lox mouse line suppressed adipose accretion and weight gain, endorsing the direct inhibition of CART action by Y1R signaling. In sum, this study demonstrates that CART activity exhibits pivotal, divergent and site-specific effects on food intake and energy balance.