Targeting NAD Metabolism to Improve Glucose Homeostasis in Obesity and Aging

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Targeting NAD Metabolism to Improve Glucose Homeostasis in Obesity and Aging We are exploring the hypothesis that nicotinamide adenine dinucleotide (NAD) metabolism can be targeted to improve physiology in aged and obese individuals. NAD is a ubiquitous molecule that is required as a redox cofactor or substrate for hundreds of enzymes within the cell. It is derived from dietary tryptophan, niacin, nicotinamide, or synthetic intermediates. Prolonged deficiency in all of these precursors leads to pellagra (characterized by dermatitis, diarrhea, and dementia) and eventually death. It recent years, it has become appreciated that NAD concentration falls in many tissues with age or during obesity, and that the pathogenesis of many diseases includes a component of NAD depletion within the target tissue. Multiple groups, including ours, have established that high doses of precursors have therapeutic effects in many rodent models of disease. However, much remains to be learned about NAD metabolism, even as nutraceutical formulations containing precursors are being marketed to the general public. Here, we propose building on our use of stable-isotope methods to understand how supplemental NAD precursors are metabolized in vivo (Aim 1). We have already shown that for the most commonly administered precursors, nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), oral dosing results in the delivery of compounds to the liver only, whereas intravenous delivery reaches many tissues. We propose expanding these methods to include other precursors, tissues, and delivery methods that are commonly used in experiments, and to address outstanding questions such as whether precursors cross the blood-brain barrier and how they influence mitochondrial NAD levels. We also apply new methodology to examine NAD turnover rates in addition to steady state concentrations. In Aim 2, we take advantage of two model systems for NAD limitation that were developed during the previous award period (liver regeneration and muscle-specific Nampt knockout) to explore the downstream mechanisms that are affected by insufficient NAD availability. Finally, in Aim 3, we explore the consequences of NAD depletion in a condition with enormous clinical significance: heart failure. It is now established that NAD depletion occurs in human heart failure and several mouse models have suggested a protective effect of NAD precursors. Here, we propose the generation of a novel mouse model to test whether NAD deficiency per se is sufficient to cause the metabolic or functional changes associated with heart failure. Based on our data suggesting that the heart benefits more from intravenous precursors that from oral dosing, we will also collaborate with Dan Kelly's group here at Penn to test the utility of intravenous NR in two human-relevant models: heart secondary to ischemic injury or treatment with a chemotherapeutic drug. Together, these studies will reveal fundamental details of how NAD metabolism influences physiology, and will help guide efforts to develop novel therapeutic approaches for the treatment or prevention of diseases associated with obesity and aging.