Body weight is regulated by a physiological process called energy homeostasis, whereby alterations in body fat stores trigger compensatory changes in appetite and energy expenditure that resist weight loss. In this process, fluctuations in nutrients and adiposity-associated hormones (e.g., leptin and insulin) communicate the status of peripheral energy stores to the brain. The neuronal pathways that mediate the effects of these peripheral "adiposity signals" are rapidly being elucidated in animal models. Information regarding the molecular mediators of energy homeostasis in the human brain, however, is limited because the relevant molecules have not yet been measured in living subjects.
The goal of this proposal is to measure one of these mediators—long-chain fatty acyl-CoA (FACoA) lipids—in the human brain using state-of-the-art, non-invasive, voxel-localized proton magnetic resonance spectroscopy (1H-MRS). Recent data suggest that acute elevations of hypothalamic FACoA (H-FACoA) concentrations exert anorexic effects similar to those of leptin and insulin, and H-FACoA are implicated as possible intracellular mediators of these hormones' actions.
This project will entail the following specific aims:
1. Refine an 1H-MRS technique to measure H-FACoA concentration in rats. We will first optimize MRS acquisition parameters in rats, using the insulin-induced 6-8 fold elevation of H-FACoA content to help identify the H-FACoA spectral signature. MRS data will be validated against the gold-standard measurement of H-FACoA content in tissue samples by negative-ion chemical ionization gas chromatography mass spectrometry (NCI-GC/MS). We will then determine if MRS can detect physiologic changes in H-FACoA induced by fasting, again validating our results with NCI-GC/MS.
2. Utilize 1H-MRSmethods that we have refined and validated in rats to measure the concentration and regulation of H-FACoA in humans. Once our MRS method has been optimized and validated in rats, we will apply this technique to humans. Paralleling our rat studies, initial human experiments will involve pharmacologic insulin administration to facilitate identification of the H-FACoA spectrum. We will then use MRS to measure H-FACoA levels in the fasting vs. fed state, to determine if fasting decreases H-FACoA in humans, as predicted by the model of these molecules as anorexic mediators in energy homeostasis. Based on rat studies using NCI-GC/MS, we anticipate that fasting will decrease H-FACoA content. To examine the individual roles for insulin and glucose fluctuations in this response, we will determine the impact on H-FACoA of fasting during a hyperinsulinemic clamp, under either euglycemic or hyperglycemic conditions. To investigate potential relationships between H-FACoA and energy-regulatory hormones and nutrients, circulating levels of insulin, leptin, ghrelin, glucose, triglycerides, and free-fatty acids will be concomitantly measured in these experiments.
With these studies, we hope not only to shed light on the role of H-FACoA in human energy homeostasis, but also to develop, for the first time, a non-invasive method to measure molecules involved in body-weight regulation in the human brain. In future studies, we plan to apply this technique to quantify other molecules that are hypothesized to participate in energy homeostasis and can potentially be detected by MRS. These include malonyl-CoA, gamma amino butyric acid, glucose, and N-acetylglucosamine. We also hope to expand our future studies to examine brain areas outside the hypothalamus that regulate energy balance.