Chronic High Fat Diet Disrupts Angiotensin‐(1‐7) Mas Receptor Localization in the Arcuate Nucleus of the Hypothalamus

Abstract

Introduction The control of energy balance involves communication of peripheral hormones with brain regions controlling food intake and energy expenditure such as the arcuate nucleus of the hypothalamus (ARC). Within the ARC, two primary neuronal subpopulations control energy balance: proopiomelanocortin (POMC) neurons, which reduce food intake and increase energy expenditure; and agouti-related protein (AgRP) neurons, which inhibit POMC neurons and conversely increase food intake and suppress energy expenditure. These circuits are typically disrupted by high fat diet (HFD) leading to a chronic state of energy imbalance and obesity. Accumulating evidence suggests that HFD-induced obesity is associated with deficiency of angiotensin (Ang)-(1-7), a protective renin-angiotensin system hormone. Our recent data show that systemically administered Ang-(1-7) induces adipose thermogenesis to enhance energy expenditure and promote weight loss. We propose that effects of Ang-(1-7) on energy balance involve activation of ARC neurocircuits, but this has not been tested. Additionally, the localization and neuronal subpopulations expressing Ang-(1-7) mas receptors (MasR) in the ARC is unknown. In this study, we hypothesized that: Ang-(1-7) activates ARC neurons; MasR are expressed in the ARC and are primarily colocalized with POMC neurons; and the ability of Ang-(1-7) to activate ARC neurons as well as co-localization of MasR with POMC neurons is disrupted following chronic HFD. Methods Male C57Bl/6J mice were fed a 60% HFD or matched control diet ad libitum for 12 weeks. Mice then received subcutaneous injection of Ang-(1-7) [2 mg/kg] to induce neuronal activation in the ARC, as measured by c-fos gene expression (n=4-6/group). In a second cohort of mice, RNAscope in situ hybridization was performed on coronal ARC sections to determine co-localization of MasR mRNA within POMC versus AgRP neurons (n=5/group). Results We found that Ang-(1-7) increases the number of c-fos positive cells in the ARC (39±6 vs. 19±3 saline; p=0.022) in control diet mice. Ang-(1-7)-mediated activation of ARC neurons was attenuated in HFD mice (34±3 vs. 23±4 saline; p=0.185). The rostral-medial-caudal distribution of ARC MasR was similar between control diet and HFD mice, with no difference in percentage of MasR positive neurons between groups (18±1 and 15±5%, respectively; p=0.733). MasR were more highly co-localized to POMC versus AgRP neurons, with HFD tending to reduce these co-localizations (MasR/POMC: 49±10 control vs. 33±5% HFD, p=0.199; MasR/AgRP: 36±11 control vs.16±7% HFD, p=0.209). Conclusions These findings suggest that chronic HFD reduces the ability of Ang-(1-7) to acutely activate neurons in the ARC. Further, HFD disrupts co-localization of MasR with POMC and AgRP neurons in the ARC indicating disconnect in the endogenous neurocircuitry controlling energy balance. Further studies are needed to explore the importance of MasR in these neuronal subpopulations for energy balance, to determine the potential for targeting of Ang-(1-7) as an innovative pharmacological strategy for obesity treatment.