Book Description

Obesity is a rapidly growing worldwide health crisis estimated to affect more than one-third of adults in the United States alone. Comorbidities associated with obesity include an increased risk of hypertension, insulin resistance, diabetes mellitus, and cardiovascular disease. Despite a well-established clinical association, the underlying mechanisms and optimal treatment approaches for obesity remain poorly understood. Lifestyle modifications, such as diet and exercise, are only modestly effective for achieving long-term weight loss. Additionally, several anti-obesity drugs have been withdrawn from the market due to limited efficacy and adverse cardiovascular and other off-target effects. This illustrates the critical need to identify new mechanisms that can be targeted to promote positive metabolic changes in obesity without adversely impacting blood pressure and cardiovascular functions. In this regard, recent research has underscored the importance of understanding how peripheral hormones interact with the brain to influence metabolic outcomes in obesity. In particular, the arcuate nucleus of the hypothalamus (ARC) has emerged as a critical brain region that receives input from peripheral hormones and can modulate autonomic nervous system pathways controlling energy balance, glucose homeostasis, and cardiovascular function. Identifying hormonal targets that interact with ARC circuits to promote positive metabolic phenotypes in the absence of adverse cardiovascular effects could be a major advance for obesity treatment. Emerging evidence from our laboratory and others suggests the renin-angiotensin system (RAS) may provide this ideal hormonal target. Overactivation of the hormone angiotensin (Ang) II is often observed in obese patients and closely correlates with insulin resistance and hypertension. Obesity is also associated with deficiency of Ang-(1-7), a protective hormone of the RAS that acts at mas receptors (MasR) to mitigate the deleterious actions of Ang II. Accumulating evidence shows that Ang-(1-7) lowers blood pressure and cardiovascular sympathetic tone as well as has direct positive metabolic effects including reducing body weight by enhancing energy expenditure (EE) and reversing whole-body glucose intolerance and insulin resistance in rodent models of obesity and metabolic syndrome. The finding that Ang-(1-7) increases EE may suggest effects on neural circuits originating within brain regions controlling energy balance, such as the ARC. Despite this, potential neural mechanisms by which Ang-(1-7) improves energy balance and glucose homeostasis are still poorly understood. The overarching hypothesis of this dissertation is that Ang-(1-7) MasR within the ARC are protective for metabolic function. More specifically, we determined in mouse models if: (1) MasR are localized to ARC neurons; and (2) MasR in the ARC are important for regulation of energy balance and glucose homeostasis under normal conditions and during the development of obesity. We also assessed the importance of ARC MasR to blood pressure regulation, to provide insight into the importance of this circuit to integrated cardiometabolic function in obesity. To test this hypothesis, this research employed cutting edge molecular methods (e.g., RNAscope in situ hybridization) to assess ARC MasR localization in a well-established mouse model of obesity that closely mimics the human condition. Additionally, we employed physiological methods (e.g., body composition analysis, insulin and glucose tolerance tests, radiotelemetry) in a novel transgenic mouse model that allowed for examination of the impact of MasR deletion from ARC neurons on in vivo integrated metabolic and cardiovascular outcomes. There are several important findings that emerged from these studies. First, we found that Ang-(1-7) MasR are widely distributed throughout the rostral to caudal extent of the ARC, with more MasR positive neurons in the ARC of females compared with males, and high fat diet (HFD) tending to upregulate this expression in both sexes. Second, MasR in the ARC protect against the development of HFD-induced insulin resistance in both sexes, particularly in females, without effects on glucose tolerance. Third, in contrast to our hypothesis, ARC MasR do not appear to play a major role in the control of energy balance as measured by body mass and composition. Finally, ARC MasR appear protective for cardiovascular regulation, with deletion of these receptors elevating blood pressure under control diet conditions. These findings support divergence of MasR pathways within the ARC controlling metabolic versus cardiovascular functions. Overall, an improved understanding of how neural circuits controlling energy balance are influenced by interactions with the RAS brings new insight to the mechanistic basis of obesity, to potentially help identify new pharmacological targets. The findings presented in this dissertation highlight the importance of interactions of the RAS with ARC neurocircuits controlling metabolic and cardiovascular functions under normal conditions and in the context of diet-induced obesity. Additionally, the finding that females have more MasR positive neurons in the ARC compared to males in our rodent model may provide a mechanism for their observed protection against obesity-related insulin resistance. While further studies are needed to explore the role of ARC MasR in Ang-(1-7) effects on metabolic and cardiovascular regulation, and the precise neuronal subpopulations within the ARC and downstream neural and signaling pathways involved, these findings provide support for targeting Ang-(1-7) pathways as an innovative strategy for treatment of obesity and related metabolic and cardiovascular complications. Potential approaches to target Ang-(1-7) chronically are currently in development and include stable analogues, oral formulations, MasR agonists, and ACE2 activators.