1512-P: Brown Adipose NMNAT1 Is Necessary for Maintaining NAD+ Levels but Dispensable for Systemic Energy Metabolism

Brown adipose tissue (BAT) is rich in mitochondria and expresses high levels of uncoupling protein 1 (UCP1), enabling it to regulate thermogenesis and system energy expenditure. NAD+ (nicotinamide adenine dinucleotide) is a key cofactor for maintaining mitochondrial function by serving as a redox carrier and signaling molecule. To investigate the regulation and function of NAD+ in BAT, we measured the expression of genes involved in NAD+ synthesis and catabolism in mice under room temperature, cold challenge, and thermoneutral conditions. We found that the expression of nicotinamide nucleotide adenylyltransferase 1 (NMNAT1), the final enzyme for NAD+ synthesis, was suppressed by thermoneutrality and increased by cold challenge in BAT. We, therefore, generated brown adipocyte-specific NMNAT1 knockout mice (NMNAT1-bKO) by crossing the NMNAT1 flox/flox mice to the UCP1-Cre mice. NMNAT1 deletion reduced NAD+ levels by approximately 50%, accompanied by the accumulated precursors, including NMN and NaMN in BAT. Despite the significant reduction of BAT NAD+, NMNAT1-bKO mice showed comparable body weight, normal glucose tolerance and insulin sensitivity, and unchanged systemic energy metabolism, including O2 consumption, CO2 production, and RER in the metabolic cage studies. Further cold challenges and high-fat diet feeding did not induce any differences in the metabolic phenotype. RNA-seq analysis of BAT from NMNAT1-bKO and control mice under cold challenge revealed enrichment of genes involved in mitochondrial function and lipid metabolic process. These results show that BAT NMNAT1 is necessary for maintaining NAD+ levels but dispensable for systemic energy metabolism, possibly due to compensatory upregulation of mitochondrial genes.