Over the past several decades a role for adaptive thermogenesis in brown adipose tissue (BAT) and beiging of white adipose tissue (WAT) has emerged as a potential therapeutic avenue to combat human obesity. However, the brain circuitry that controls BAT and WAT has not been fully mapped. Determining the neural pathway that controls both BAT and WAT beiging is the first step toward safely manipulating these tissues to combat obesity. As obesity has a higher prevalence in females, it is one goal of this study to examine the neural mechanism underlying BAT thermogenesis and WAT beiging in females. In females only, bone morphogenic protein 8B (BMP8B) injection in the ventromedial hypothalamus (VMH) increased orexin (OX) expression in downstream lateral hypothalamic (LH) neurons, BAT thermogenesis and WAT beiging. This effect of BMP8B was blocked by LH OX receptor antagonists, was dependent on circulating ovarian estrogens and required inhibition of the cellular fuel sensor AMP-activated protein kinase (AMPK) in the VMH. We have already shown that estrogen blunts activation of the ventrolateral VMH (vlVMH) glucose inhibited (GI) neurons in low glucose by inhibiting AMPK. Interestingly, estrogen receptors are densely clustered in the vlVMH. Further, GI neurons make up ~60% of the neurons in the vlVMH. There is a high correlation between VMH neurons that produce nitric oxide (NO) in response to low glucose and GI neurons. Moreover, VMH GI neurons require neuronal nitric oxide synthase (nNOS) to sense glucose. These data lead us to hypothesize that vlVMH nNOS-GI neurons exert a tonic inhibition of the downstream LH OX neurons which activate BAT thermogenesis and WAT beiging. Further, that BMP8B, like estrogen, increases the inhibitory effect of glucose on vlVMH GI neurons allowing BAT thermogenesis and WAT beiging to increase. To test this hypothesis, the applicant will first use a combination of electrophysiological recordings, optogenetics and virally assisted circuit mapping in mice expressing cre-recombinase in nNOS neurons to examine the neural connection between the vlVMH nNOS-GI and LH OX neurons in vitro. Based on the literature she will determine whether inhibitors of group III metabotropic glutamate receptors (mGluR) and/or GABA receptors mediate the inhibitory effect of vlVMH nNOS-GI neurons on LH OX neurons. She will then use chemogenetics in nNOS cre mice to determine how VMH nNOS neurons regulate BAT thermogenesis and WAT beiging in vivo. Finally, she will determine whether BMP8B directly regulates vlVMH nNOS-GI neurons which project to the LH. This project will uncover the neural mechanisms by which estrogen and BMP8B increase BAT thermogenesis and WAT beiging in females. These data are important for developing therapies to counter the increased incidence of obesity and metabolic disorders in post-menopausal women.
Brown adipose tissue (BAT) thermogenesis and white adipose tissue (WAT) beiging have been proposed to be possible therapeutic avenues for combatting diet induced obesity, but are modulated differently in males and females. The ventromedial hypothalamus (VMH), which has estrogen sensitive glucose inhibited (GI) neurons in its ventrolateral section, is part of the neuronal pathway that controls BAT thermogenesis and WAT beiging. This proposal will investigate whether estrogen sensitive GI neurons of the ventrolateral VMH control the female specific initiation of BAT thermogenesis and WAT beiging.
