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Decreased nicotinamide adenine dinucleotide (NAD + ) levels have been shown to contribute to metabolic dysfunction during aging. NAD + decline can be partially prevented by knockout of the enzyme CD38. However, it is not known how CD38 is regulated during aging, and how its ecto-enzymatic activity impacts NAD + homeostasis. Here we show that increases in CD38 in white adipose tissue (WAT) and liver during aging is mediated by accumulation of CD38 + immune cells. Inflammation increases CD38 and decreases NAD + . In addition, senescent cells and their secreted signals promote accumulation of CD38 + cells in WAT, and ablation of senescent cells or their secretory phenotype decrease CD38, partially reversing NAD + decline. Finally, blocking the ecto-enzymatic activity of CD38 can increase NAD + through a nicotinamide mononucleotide (NMN)-dependent process. Our findings demonstrate that senescence-induced inflammation promotes accumulation of CD38 in immune cells that through its ecto-enzymatic activity decreases levels of NMN and NAD + .

Summary Mitochondria require nicotinamide adenine dinucleotide (NAD + ) in order to carry out the fundamental processes that fuel respiration and mediate cellular energy transduction. Mitochondrial NAD + transporters have been identified in yeast and plants 1 , 2 but their very existence is controversial in mammals 3 – 5 . Here we demonstrate that mammalian mitochondria are capable of taking up intact NAD + and identify SLC25A51 (an essential 6 , 7 mitochondrial protein of previously unknown function, also known as MCART1) as a mammalian mitochondrial NAD + transporter. Loss of SLC25A51 decreases mitochondrial but not whole-cell NAD + content, impairs mitochondrial respiration, and blocks the uptake of NAD + into isolated mitochondria. Conversely, overexpression of SLC25A51 or a nearly identical paralog, SLC25A52, increases mitochondrial NAD + levels and restores NAD + uptake into yeast mitochondria lacking endogenous NAD + transporters. Together, these findings identify SLC25A51 as the first transporter capable of importing NAD + into mammalian mitochondria.

NADH provides electrons for aerobic ATP production. In cells deprived of oxygen or with impaired electron transport chain activity, NADH accumulation can be toxic. To minimize such toxicity, elevated NADH inhibits the classical NADH producing pathways: glucose, glutamine, and fat oxidation. Here, through deuterium tracing studies in cultured cells and mice, we show that folatedependent serine catabolism also produces substantial NADH. Strikingly, when respiration is impaired, serine catabolism through methylene tetrahydrofolate dehydrogenase (MTHFD2) becomes a major NADH source. In cells whose respiration is slowed by hypoxia, metformin, or genetic lesions, mitochondrial serine catabolism inhibition partially normalizes NADH levels and facilitates cell growth. In mice with engineered mitochondrial complex I deficiency (NDUSF4-/-), serine's contribution to NADH is elevated and progression of spasticity is modestly slowed by pharmacological blockade of serine degradation. Thus, when respiration is impaired, serine catabolism contributes to toxic NADH accumulation.

A primary goal of metabolomics is to identify all biologically important metabolites. One powerful approach is liquid chromatography-high resolution mass spectrometry (LC-MS), yet most LC-MS peaks remain unidentified. Here, we present a global network optimization approach, NetID, to annotate untargeted LC-MS metabolomics data. We consider all experimentally observed ion peaks together, and assign annotations to all of them simultaneously so as to maximize a score that considers properties of peaks (known masses, retention times, MS/MS fragmentation patterns) as well network constraints that arise based on mass difference between peaks. Global optimization results in accurate peak assignment and trackable peak-peak relationships. Applying this approach to yeast and mouse data, we identify a half-dozen novel metabolites, including thiamine and taurine derivatives. Isotope tracer studies indicate active flux through these metabolites. Thus, NetID applies existing metabolomic knowledge and global optimization to annotate untargeted metabolomics data, revealing novel metabolites.

Small cell size preserves the function of hematopoietic stem cells (HSCs); HSC enlargement during aging causes their dysfunction.

Controllable rotational manipulation enables multi-dimensional imaging and rapid screening of single cells and model organisms. Current approaches to rotationally maneuver small objects depend on optical, magnetic, or electrical properties of the sample under investigation. This dependence renders the existing methods sample-specific which limits their applicability. Here we present a new rotational manipulation method based on oscillating sidewall sharp-edge microstructures and thin glass slides in a microchannel. This method is independent of the intrinsic properties of sample under investigation and can be effectively applied to particles, cells, and multicellular organisms.

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