Methods: We utilised skeletal muscle myotube models to induce NAD+ depletion, repletion and excess and conducted metabolic tracing to provide comprehensive and detailed analysis of the consequences of altered NAD+ metabolism on central carbon metabolic pathways. We used stable isotope tracers, [1,2-13C] D-glucose and [U-13C] glutamine, and conducted combined 2D-1H,13C-heteronuclear single quantum coherence (HSQC) NMR spectroscopy and GC-MS analysis.
Results: NAD+ excess driven by nicotinamide riboside (NR) supplementation within skeletal muscle cells results in enhanced nicotinamide clearance, but had no effect on energy homeostasis or central carbon metabolism. Nicotinamide phosphoribosyltransferase (NAMPT) inhibition induced NAD+ depletion and resulted in equilibration of metabolites upstream of glyceraldehyde phosphate dehydrogenase (GAPDH). Aspartate production through glycolysis and TCA cycle activity is increased in response to low NAD+, which is rapidly reversed with repletion of the NAD+ pool using NR. NAD+ depletion reversibly inhibits cytosolic GAPDH activity, but retains mitochondrial oxidative metabolism, suggesting differential effects of this treatment on sub-cellular pyridine pools. When supplemented, NR efficiently reverses these metabolic consequences. However, the functional relevance of increased aspartate levels after NAD+ depletion remains unclear, and requires further investigation.
Conclusions: These data highlight the need to consider carbon metabolism and clearance pathways when investigating NAD+ precursor usage in models of skeletal muscle physiology.
Bibliographical noteOakey LA, Fletcher RS, Elhassan YS et al. Metabolic tracing reveals novel adaptations to skeletal muscle cell energy production pathways in response to NAD+ depletion [version 1; referees: 2 approved]. Wellcome Open Res 2018, 3:147 (https://doi.org/10.12688/wellcomeopenres.14898.1)
- skeletal muscle
- isotopic tracing