The mitochondrial NAD+ transporter SLC25A51 is a fasting-induced gene affecting SIRT3 functions

Abstract

Introduction: Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism and energy production. NAD⁺-dependent deacetylase sirtuin 3 (SIRT3) regulates the acetylation levels of mitochondrial proteins that are involved in mitochondrial homeostasis. Fasting up-regulates hepatic SIRT3 activity, which requires mitochondrial NAD⁺. What is the mechanism, then, to transport more NAD⁺ into mitochondria to sustain enhanced SIRT3 activity during fasting?

Objective: SLC25A51 is a recently discovered mitochondrial NAD⁺ transporter. We tested the hypothesis that, during fasting, increased expression of SLC25A51 is needed for enhanced mitochondrial NAD⁺ uptake to sustain SIRT3 activity. Because the fasting-fed cycle and circadian rhythm are closely linked, we further tested the hypothesis that SLC25A51 is a circadian regulated gene.

Methods: We examined Slc25a51 expression in the liver of fasted mice, and examined its circadian rhythm in wild-type mice and those with liver-specific deletion of the clock gene BMAL1 (LKO). We suppressed Slc25a51 expression in hepatocytes and the mouse liver using shRNA-mediated knockdown, and then examined mitochondrial NAD⁺ levels, SIRT3 activities, and acetylation levels of SIRT3 target proteins (IDH2 and ACADL). We measured mitochondrial oxygen consumption rate using Seahorse analysis in hepatocytes with reduced Slc25a51 expression.

Results: We found that fasting induced the hepatic expression of Slc25a51, and its expression showed a circadian rhythm-like pattern that was disrupted in LKO mice. Reduced expression of Slc25a51 in hepatocytes decreased mitochondrial NAD⁺ levels and SIRT3 activity, reflected by increased acetylation of SIRT3 targets. Slc25a51 knockdown reduced the oxygen consumption rate in intact hepatocytes. Mice with reduced Slc25a51 expression in the liver manifested reduced hepatic mitochondrial NAD⁺ levels, hepatic steatosis and hypertriglyceridemia.

Conclusions: Slc25a51 is a fasting-induced gene that is needed for hepatic SIRT3 functions.

Keywords: Mitochondria; NAD; SIRT3; SLC25A51.