Mitochondrial NAD kinase: function and mechanism in metabolism

Project Summary Dysregulated energy metabolism is intrinsically linked to the development of metabolic disorders, such as non-alcoholic steatohepatitis (NASH) and type 2 diabetes mellitus (T2DM). As universal electron carriers, both nicotinamide adenine dinucleotide (NAD) and its phosphorylated form (NADP) play essential roles in energy metabolism. The NAD kinase (NADK), which phosphorylates NAD to generate NADP, is exquisitely sensitive to nutritional or stress signals. While the cytosolic NADK has been characterized, little is known about generation and maintenance of NADP from mitochondria, the central organelle responsible for metabolic process and energy production, until our recent discovery that the uncharacterized human gene C5ORF33 encodes the long- sought mitochondrial NADK, referred to as MNADK. We have shown that MNADK, as a nutritionally-regulated, liver-enriched mitochondrial NADK, functions as the only de novo mitochondrial NADP biosynthesis pathway. Recently, we accumulated strong preliminary evidence that MNADK is required to maintain energy homeostasis, redox state, and insulin sensitivity and that repression of MNADK activity by protein S-nitrosylation modification under the high-fat diet is largely responsible for hepatic steatosis and insulin resistance induced by overnutrition. MNADK functions as a key regulator of acetylation of major metabolic transcriptional regulators. These observations lead us to hypothesize that MNADK is required to maintain energy homeostasis and insulin sensitivity and that repression of MNADK activity by overnutrition critically contributes to the development of NASH and T2DM. Mechanically, MNADK not only plays major roles in maintaining mitochondrial function and anti-oxidative protection, but also functions as a key regulator of acetylation of major mitochondrial regulators or enzymes that shape metabolic adaptability following metabolic challenges. In this application, we will utilize molecular and cellular approaches, genetically-engineered mouse models, as well as bioinformatics to critically address the functions and mechanisms by which the sole mitochondrial NAD kinase, MNADK, maintains energy homeostasis and protects from metabolic disorders. Our goal will be achieved by two aims: Aim 1, to determine the functional significance of MNADK in preserving hepatic energy homeostasis and thus mitigating the risk of metabolic disorders; and Aim 2, to decipher the molecular mechanism by which MNADK maintains mitochondrial function and energy metabolism. Upon completion of this project, we will have defined the function and mechanism by which the mitochondria NAD kinase preserves energy homeostasis and thus mitigates the risk of metabolic disease. Our proposed research will not only open a new paradigm for the study on molecular basis underlying energy metabolism, but also have important implications in the prevention and treatment of metabolic disease.