Visualization of Nicotine Adenine Dinucleotide Redox Homeostasis with Genetically Encoded Fluorescent Sensors

RNAi screens identify HES4 as a regulator of redox balance supporting pyrimidine synthesis and tumor growth

NADH/NAD+ redox balance is pivotal for cellular metabolism. Systematic identification of NAD(H) redox regulators, although currently lacking, would help uncover unknown effectors critically implicated in the coordination of growth metabolism. In this study, we performed a genome-scale RNA interference (RNAi) screen to globally survey the genes involved in redox modulation and identified the HES family bHLH transcription factor HES4 as a negative regulator of NADH/NAD+ ratio. Functionally, HES4 is shown to be crucial for maintaining mitochondrial electron transport chain (ETC) activity and pyrimidine synthesis. More specifically, HES4 directly represses transcription of SLC44A2 and SDS, thereby inhibiting mitochondrial choline oxidation and cytosolic serine deamination, respectively, which, in turn, ensures coenzyme Q reduction capacity for DHODH-mediated UMP synthesis and serine-derived dTMP production. Accordingly, inhibition of choline oxidation preserves mitochondrial serine catabolism and ETC-coupled redox balance. Furthermore, HES4 protein stability is enhanced under EGFR activation, and increased HES4 levels facilitate EGFR-driven tumor growth and predict poor prognosis of lung adenocarcinoma. These findings illustrate an unidentified mechanism, underlying pyrimidine biosynthesis in the intersection between serine and choline catabolism, and underscore the physiological importance of HES4 in tumor metabolism.

Transient inhibition of sodium-glucose cotransporter 2 after ischemia/reperfusion injury ameliorates chronic kidney disease

Sodium-glucose cotransporter 2 (SGLT2) inhibitor, dapagliflozin (Dapa), exhibited nephroprotective effects in patients with chronic kidney disease (CKD). We assessed the efficacy of short-term Dapa administration following acute kidney injury (AKI) in preventing CKD. Male Wistar rats were randomly assigned to Sham surgery, bilateral ischemia for 30 minutes (abbreviated as IR), and IR + Dapa groups. Daily treatment with Dapa was initiated just 24 hours after IR and maintained for only 10 days. Initially, rats were euthanized at this point to study early renal repair. After severe AKI, Dapa promptly restored creatinine clearance (CrCl) and significantly reduced renal vascular resistance compared with the IR group. Furthermore, Dapa effectively reversed the mitochondrial abnormalities, including increased fission, altered mitophagy, metabolic dysfunction, and proapoptotic signaling. To study this earlier, another set of rats was studied just 5 days after AKI. Despite persistent renal dysfunction, our data reveal a degree of mitochondrial protection. Remarkably, a 10-day treatment with Dapa demonstrated effectiveness in preventing CKD transition in an independent cohort monitored for 5 months after AKI. This was evidenced by improvements in proteinuria, CrCl, glomerulosclerosis, and fibrosis. Our findings underscore the potential of Dapa in preventing maladaptive repair following AKI, emphasizing the crucial role of early intervention in mitigating AKI long-term consequences.

Roles of NAD+ in Health and Aging

NAD+, the essential metabolite involved in multiple reactions such as the regulation of cellular metabolism, energy production, DNA repair, mitophagy and autophagy, inflammation, and neuronal function, has been the subject of intense research in the field of aging and disease over the last decade. NAD+ levels decline with aging and in some age-related diseases, and reduction in NAD+ affects all the hallmarks of aging. Here, we present an overview of the discovery of NAD+, the cellular pathways of producing and consuming NAD+, and discuss how imbalances in the production rate and cellular request of NAD+ likely contribute to aging and age-related diseases including neurodegeneration. Preclinical studies have revealed great potential for NAD+ precursors in promotion of healthy aging and improvement of neurodegeneration. This has led to the initiation of several clinical trials with NAD+ precursors to treat accelerated aging, age-associated dysfunctions, and diseases including Alzheimer's and Parkinson's. NAD supplementation has great future potential clinically, and these studies will also provide insight into the mechanisms of aging.

Genetically encoded ATP and NAD(P)H biosensors: potential tools in metabolic engineering

To date, many metabolic engineering tools and strategies have been developed, including tools for cofactor engineering, which is a common strategy for bioproduct synthesis. Cofactor engineering is used for the regulation of pyridine nucleotides, including NADH/NAD+ and NADPH/NADP+, and adenosine triphosphate/adenosine diphosphate (ATP/ADP), which is crucial for maintaining redox and energy balance. However, the intracellular levels of NADH/NAD+, NADPH/NADP+, and ATP/ADP cannot be monitored in real time using traditional methods. Recently, many biosensors for detecting, monitoring, and regulating the intracellular levels of NADH/NAD+, NADPH/NADP+, and ATP/ADP have been developed. Although cofactor biosensors have been mainly developed for use in mammalian cells, the potential application of cofactor biosensors in metabolic engineering in bacterial and yeast cells has received recent attention. Coupling cofactor biosensors with genetic circuits is a promising strategy in metabolic engineering for optimizing the production of biochemicals. In this review, we focus on the development of biosensors for NADH/NAD+, NADPH/NADP+, and ATP/ADP and the potential application of these biosensors in metabolic engineering. We also provide critical perspectives, identify current research challenges, and provide guidance for future research in this promising field.

PDK4-dependent hypercatabolism and lactate production of senescent cells promotes cancer malignancy

Senescent cells remain metabolically active, but their metabolic landscape and resulting implications remain underexplored. Here, we report upregulation of pyruvate dehydrogenase kinase 4 (PDK4) upon senescence, particularly in some stromal cell lines. Senescent cells display a PDK4-dependent increase in aerobic glycolysis and enhanced lactate production but maintain mitochondrial respiration and redox activity, thus adopting a special form of metabolic reprogramming. Medium from PDK4+ stromal cells promotes the malignancy of recipient cancer cells in vitro, whereas inhibition of PDK4 causes tumor regression in vivo. We find that lactate promotes reactive oxygen species production via NOX1 to drive the senescence-associated secretory phenotype, whereas PDK4 suppression reduces DNA damage severity and restrains the senescence-associated secretory phenotype. In preclinical trials, PDK4 inhibition alleviates physical dysfunction and prevents age-associated frailty. Together, our study confirms the hypercatabolic nature of senescent cells and reveals a metabolic link between cellular senescence, lactate production, and possibly, age-related pathologies, including but not limited to cancer.

Pelargonium sidoides extract mediates nephrotoxicity through mitochondrial malfunction and cytoskeleton destabilization

We investigated the cytotoxic effect of Pelargonium sidoides extract on Madin-Darby canine kidney (MDCK) cells. P. sidoides extract decreased the cell viability in a dose dependent manner (> 0.2%). The extract of P. sidoides decreased the mitochondrial action potential, increased the number of reactive oxygen species (ROS) inside the cell, and caused nicotinamide adenine dinucleotide hydride (NADH) to be released, all of which are signs of mitochondrial dysfunction. The results of unbiased mRNA sequencing showed that 0.3% P. sidoides extract upregulates the apoptosis-related gene (BBC3). This finding was supported by immunoblot analysis of apoptosis signal pathways, which included Bcl-2, Bax, cytochrome C (CytC), cleaved caspase 3 (CC3), cleaved caspase 7 (CC7), cleaved caspase 9 (CC9) and cleaved PARP (CP). It is interesting to note that the elevated levels of Bax, CytC, CC3, CC7, and CC9, as well as CP, were suppressed by N-acetyl-L-cysteine (NAC) pretreatment, which points to ROS-mediated apoptosis. The small GTPases, RhoA, and Rac1/cdc42-GTP-bound active form were all lowered when P. sidoides extract was used. Also, RhoA-related cytoskeleton signals (ROCK, p-LIMK1/2, p-cofilin) and Rac1/cdc42-related signals (N-WASP, WAVE-2) were inhibited by P. sidoides extract. NAC or RhoA/Rac1/cdc42 activator pretreatment reduced P. sidoides extract-induced actin destabilization. In this work, P. sidoides extract promotes apoptosis by causing mitochondrial dysfunction and cytoskeleton disassembly.

Exercise-induced changes to the fiber type-specific redox state in human skeletal muscle are associated with aerobic capacity

The benefits of exercise involve skeletal muscle redox state alterations of nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). We determined the fiber-specific effects of acute exercise on the skeletal muscle redox state in healthy adults. Muscle biopsies were obtained from 19 participants (11 M, 8 F; 26 ± 4 yr) at baseline (fasted) and 30 min and 3 h after treadmill exercise at 80% maximal oxygen consumption (V̇o2max). Muscle samples were probed for autofluorescence of NADH (excitation at 340-360 nm) and oxidized flavoproteins (Fp; excitation at 440-470 nm) and subsequently, fiber typed to quantify the redox signatures of individual muscle fibers. Redox state was calculated as the oxidation-to-reduction redox ratio: Fp/(Fp + NADH). At baseline, pair-wise comparisons revealed that the redox ratio of myosin heavy chain (MHC) I fibers was 7.2% higher than MHC IIa (P = 0.023, 95% CI: 5.2, 9.2%) and the redox ratio of MHC IIa was 8.0% higher than MHC IIx (P = 0.035, 95% CI: 6.8, 9.2%). MHC I fibers also displayed greater NADH intensity than MHC IIx (P = 0.007) and greater Fp intensity than both MHC IIa (P = 0.019) and MHC IIx (P < 0.0001). Fp intensities increased in all fiber types (main effect, P = 0.039) but redox ratios did not change (main effect, P = 0.483) 30 min after exercise. The change in redox ratio was positively correlated with capillary density in MHC I (rho = 0.762, P = 0.037), MHC IIa fibers (rho = 0.881, P = 0.007), and modestly in MHC IIx fibers (rho = 0. 771, P = 0.103). These findings support the use of redox autofluorescence to interrogate skeletal muscle metabolism.NEW & NOTEWORTHY This study is the first to use autofluorescent imaging to describe differential redox states within human skeletal muscle fiber types with exercise. Our findings highlight an easy and efficacious technique for assessing skeletal muscle redox in humans.

The autophagy-NAD axis in longevity and disease

Autophagy is an intracellular degradation pathway that recycles subcellular components to maintain metabolic homeostasis. NAD is an essential metabolite that participates in energy metabolism and serves as a substrate for a series of NAD+-consuming enzymes (NADases), including PARPs and SIRTs. Declining levels of autophagic activity and NAD represent features of cellular ageing, and consequently enhancing either significantly extends health/lifespan in animals and normalises metabolic activity in cells. Mechanistically, it has been shown that NADases can directly regulate autophagy and mitochondrial quality control. Conversely, autophagy has been shown to preserve NAD levels by modulating cellular stress. In this review we highlight the mechanisms underlying this bidirectional relationship between NAD and autophagy, and the potential therapeutic targets it provides for combatting age-related disease and promoting longevity.

Lighting the light reactions of photosynthesis by means of redox-responsive genetically encoded biosensors for photosynthetic intermediates

Oxygenic photosynthesis involves light and dark phases. In the light phase, photosynthetic electron transport provides reducing power and energy to support the carbon assimilation process. It also contributes signals to defensive, repair, and metabolic pathways critical for plant growth and survival. The redox state of components of the photosynthetic machinery and associated routes determines the extent and direction of plant responses to environmental and developmental stimuli, and therefore, their space- and time-resolved detection in planta becomes critical to understand and engineer plant metabolism. Until recently, studies in living systems have been hampered by the inadequacy of disruptive analytical methods. Genetically encoded indicators based on fluorescent proteins provide new opportunities to illuminate these important issues. We summarize here information about available biosensors designed to monitor the levels and redox state of various components of the light reactions, including NADP(H), glutathione, thioredoxin, and reactive oxygen species. Comparatively few probes have been used in plants, and their application to chloroplasts poses still additional challenges. We discuss advantages and limitations of biosensors based on different principles and propose rationales for the design of novel probes to estimate the NADP(H) and ferredoxin/flavodoxin redox poise, as examples of the exciting questions that could be addressed by further development of these tools. Genetically encoded fluorescent biosensors are remarkable tools to monitor the levels and/or redox state of components of the photosynthetic light reactions and accessory pathways. Reducing equivalents generated at the photosynthetic electron transport chain in the form of NADPH and reduced ferredoxin (FD) are used in central metabolism, regulation, and detoxification of reactive oxygen species (ROS). Redox components of these pathways whose levels and/or redox status have been imaged in plants using biosensors are highlighted in green (NADPH, glutathione, H2O2, thioredoxins). Analytes with available biosensors not tried in plants are shown in pink (NADP+). Finally, redox shuttles with no existing biosensors are circled in light blue. APX, ASC peroxidase; ASC, ascorbate; DHA, dehydroascorbate; DHAR, DHA reductase; FNR, FD-NADP+ reductase; FTR, FD-TRX reductase; GPX, glutathione peroxidase; GR, glutathione reductase; GSH, reduced glutathione; GSSG, oxidized glutathione; MDA, monodehydroascorbate; MDAR, MDA reductase; NTRC, NADPH-TRX reductase C; OAA, oxaloacetate; PRX, peroxiredoxin; PSI, photosystem I; PSII: photosystem II; SOD, superoxide dismutase; TRX, thioredoxin.

Ultrasensitive sensors reveal the spatiotemporal landscape of lactate metabolism in physiology and disease

Despite its central importance in cellular metabolism, many details remain to be determined regarding subcellular lactate metabolism and its regulation in physiology and disease, as there is sensitive spatiotemporal resolution of lactate distribution, and dynamics remains a technical challenge. Here, we develop and characterize an ultrasensitive, highly responsive, ratiometric lactate sensor, named FiLa, enabling the monitoring of subtle lactate fluctuations in living cells and animals. Utilizing FiLa, we demonstrate that lactate is highly enriched in mammalian mitochondria and compile an atlas of subcellular lactate metabolism that reveals lactate as a key hub sensing various metabolic activities. In addition, FiLa sensors also enable direct imaging of elevated lactate levels in diabetic mice and facilitate the establishment of a simple, rapid, and sensitive lactate assay for point-of-care clinical screening. Thus, FiLa sensors provide powerful, broadly applicable tools for defining the spatiotemporal landscape of lactate metabolism in health and disease.

Assessment of the Peroxisomal Redox State in Living Cells Using NADPH- and NAD+/NADH-Specific Fluorescent Protein Sensors

The pyridine nucleotides NAD(H) and NADP(H) are key molecules in cellular metabolism, and measuring their levels and oxidation states with spatiotemporal precision is of great value in biomedical research. Traditional methods to assess the redox state of these metabolites are intrusive and prohibit live-cell quantifications. This obstacle was surpassed by the development of genetically encoded fluorescent biosensors enabling dynamic measurements with subcellular resolution in living cells. Here, we provide step-by-step protocols to monitor the intraperoxisomal NADPH levels and NAD⁺/NADH redox state in cellulo by using targeted variants of iNAP1 and SoNar, respectively.

Genetically encoded biosensors for evaluating NAD+/NADH ratio in cytosolic and mitochondrial compartments

The ratio of oxidized to reduced NAD (NAD+/NADH) sets intracellular redox balance and antioxidant capacity. Intracellular NAD is compartmentalized and the mitochondrial NAD+/NADH ratio is intricately linked to cellular function. Here, we report the monitoring of the NAD+/NADH ratio in mitochondrial and cytosolic compartments in live cells by using a modified genetic biosensor (SoNar). The fluorescence signal of SoNar targeted to mitochondria (mt-SoNar) or cytosol (ct-SoNar) responded linearly to physiological NAD+/NADH ratios in situ. NAD+/NADH ratios in cytosol versus mitochondria responded rapidly, but differently, to acute metabolic perturbations, indicating distinct NAD pools. Subcellular NAD redox balance regained homeostasis via communications through malate-aspartate shuttle. Mitochondrial and cytosolic NAD+/NADH ratios are influenced by NAD+ precursor levels and are distinctly regulated under pathophysiological conditions. Compartment-targeted biosensors and real-time imaging allow assessment of subcellular NAD+/NADH redox signaling in live cells, enabling future mechanistic research of NAD redox in cell biology and disease development.

Shining a light on NAD- and NADP-based metabolism in plants

The pyridine nucleotides nicotinamide adenine dinucleotide [NAD(H)] and nicotinamide adenine dinucleotide phosphate [NADP(H)] simultaneously act as energy transducers, signalling molecules, and redox couples. Recent research into photosynthetic optimisation, photorespiration, immunity, hypoxia/oxygen signalling, development, and post-harvest metabolism have all identified pyridine nucleotides as key metabolites. Further understanding will require accurate description of NAD(P)(H) metabolism, and genetically encoded fluorescent biosensors have recently become available for this purpose. Although these biosensors have begun to provide novel biological insights, their limitations must be considered and the information they provide appropriately interpreted. We provide a framework for understanding NAD(P)(H) metabolism and explore what fluorescent biosensors can, and cannot, tell us about plant biology, looking ahead to the pressing questions that could be answered with further development of these tools.

Genetically Encoded Fluorescent Redox Indicators for Unveiling Redox Signaling and Oxidative Toxicity

Redox-active molecules play essential roles in cell homeostasis, signaling, and other biological processes. Dysregulation of redox signaling can lead to toxic effects and subsequently cause diseases. Therefore, real-time tracking of specific redox-signaling molecules in live cells would be critical for deciphering their functional roles in pathophysiology. Fluorescent protein (FP)-based genetically encoded redox indicators (GERIs) have emerged as valuable tools for monitoring the redox states of various redox-active molecules from subcellular compartments to live organisms. In the first section of this review, we overview the background, focusing on the sensing mechanisms of various GERIs. Next, we review a list of selected GERIs according to their analytical targets and discuss their key biophysical and biochemical properties. In the third section, we provide several examples which applied GERIs to understanding redox signaling and oxidative toxicology in pathophysiological processes. Lastly, a summary and outlook section is included.

Imaging Biomarkers for Monitoring the Inflammatory Redox Landscape in the Brain

Inflammation is one key process in driving cellular redox homeostasis toward oxidative stress, which perpetuates inflammation. In the brain, this interplay results in a vicious cycle of cell death, the loss of neurons, and leakage of the blood–brain barrier. Hence, the neuroinflammatory response fuels the development of acute and chronic inflammatory diseases. Interrogation of the interplay between inflammation, oxidative stress, and cell death in neurological tissue in vivo is very challenging. The complexity of the underlying biological process and the fragility of the brain limit our understanding of the cause and the adequate diagnostics of neuroinflammatory diseases. In recent years, advancements in the development of molecular imaging agents addressed this limitation and enabled imaging of biomarkers of neuroinflammation in the brain. Notable redox biomarkers for imaging with positron emission tomography (PET) tracers are the 18 kDa translocator protein (TSPO) and monoamine oxygenase B (MAO–B). These findings and achievements offer the opportunity for novel diagnostic applications and therapeutic strategies. This review summarizes experimental as well as established pharmaceutical and biotechnological tools for imaging the inflammatory redox landscape in the brain, and provides a glimpse into future applications.

NAMPT and NAPRT: Two Metabolic Enzymes With Key Roles in Inflammation

Nicotinamide phosphoribosyltransferase (NAMPT) and nicotinate phosphoribosyltransferase (NAPRT) are two intracellular enzymes that catalyze the first step in the biosynthesis of NAD from nicotinamide and nicotinic acid, respectively. By fine tuning intracellular NAD levels, they are involved in the regulation/reprogramming of cellular metabolism and in the control of the activity of NAD-dependent enzymes, including sirtuins, PARPs, and NADases. However, during evolution they both acquired novel functions as extracellular endogenous mediators of inflammation. It is well-known that cellular stress and/or damage induce release in the extracellular milieu of endogenous molecules, called alarmins or damage-associated molecular patterns (DAMPs), which modulate immune functions through binding pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), and activate inflammatory responses. Increasing evidence suggests that extracellular (e)NAMPT and eNAPRT are novel soluble factors with cytokine/adipokine/DAMP-like actions. Elevated eNAMPT were reported in several metabolic and inflammatory disorders, including obesity, diabetes, and cancer, while eNAPRT is emerging as a biomarker of sepsis and septic shock. This review will discuss available data concerning the dual role of this unique family of enzymes.

Illuminating NAD+ Metabolism in Live Cells and In Vivo Using a Genetically Encoded Fluorescent Sensor

Understanding of NAD⁺ metabolism provides many critical insights into health and diseases, yet highly sensitive and specific detection of NAD⁺ metabolism in live cells and in vivo remains difficult. Here, we present ratiometric, highly responsive genetically encoded fluorescent indicators, FiNad, for monitoring NAD⁺ dynamics in living cells and animals. FiNad sensors cover physiologically relevant NAD⁺ concentrations and sensitively respond to increases and decreases in NAD⁺. Utilizing FiNad, we performed a head-to-head comparison study of common NAD⁺ precursors in various organisms and mapped their biochemical roles in enhancing NAD⁺ levels. Moreover, we showed that increased NAD⁺ synthesis controls morphofunctional changes of activated macrophages, and directly imaged NAD⁺ declines during aging in situ. The broad utility of the FiNad sensors will expand our mechanistic understanding of numerous NAD⁺-associated physiological and pathological processes and facilitate screening for drug or gene candidates that affect uptake, efflux, and metabolism of this important cofactor.

Subcellular Characterization of Nicotinamide Adenine Dinucleotide Biosynthesis in Metastatic Melanoma by Using Organelle-Specific Biosensors

Aim: Nicotinamide adenine dinucleotide (NAD⁺) plays central roles in a wide array of normal and pathological conditions. Inhibition of NAD⁺ biosynthesis can be exploited therapeutically in cancer, including melanoma. To obtain quantitation of NAD⁺ levels in live cells and to address the issue of the compartmentalization of NAD⁺ biosynthesis, we exploited a recently described genetically encoded NAD⁺ biosensor (LigA-circularly permutated Venus), which was targeted to the cytosol, mitochondria, and nuclei of BRAF-V600E A375 melanoma cells, a model of metastatic melanoma (MM). Results: FK866, a specific inhibitor of nicotinamide phosphoribosyltransferase (NAMPT), the main NAD⁺-producing enzyme in MM cells, was used to monitor NAD⁺ depletion kinetics at the subcellular level in biosensor-transduced A375 cells. In addition, we treated FK866-blocked A375 cells with NAD⁺ precursors, including nicotinamide, nicotinic acid, nicotinamide riboside, and quinolinic acid, highlighting an organelle-specific capacity of each substrate to rescue from NAMPT block. Expression of NAD⁺ biosynthetic enzymes was then biochemically studied in isolated organelles, revealing the presence of NAMPT in all three cellular compartments, whereas nicotinate phosphoribosyltransferase was predominantly cytosolic and mitochondrial, and nicotinamide riboside kinase mitochondrial and nuclear. In keeping with biosensor data, quinolinate phosphoribosyltransferase was expressed at extremely low levels. Innovation and Conclusions: Throughout this work, we validated the use of genetically encoded NAD⁺ biosensors to characterize subcellular distribution of NAD⁺ production routes in MM. The chance of real-time monitoring of NAD⁺ fluctuations after chemical perturbations, together with a deeper comprehension of the cofactor biosynthesis compartmentalization, strengthens the foundation for a targeted strategy of NAD⁺ pool manipulation in cancer and metabolic diseases.

Analysis of redox landscapes and dynamics in living cells and in vivo using genetically encoded fluorescent sensors

Cellular oxidation-reduction reactions are mainly regulated by pyridine nucleotides (NADPH/NADP⁺ and NADH/NAD⁺), thiols, and reactive oxygen species (ROS) and play central roles in cell metabolism, cellular signaling, and cell-fate decisions. A comprehensive evaluation or multiplex analysis of redox landscapes and dynamics in intact living cells is important for interrogating cell functions in both healthy and disease states; however, until recently, this goal has been limited by the lack of a complete set of redox sensors. We recently reported the development of a series of highly responsive, genetically encoded fluorescent sensors for NADPH that substantially strengthen the existing toolset of genetically encoded sensors for thiols, H₂O₂, and NADH redox states. By combining sensors with unique spectral properties and specific subcellular targeting domains, our approach allows simultaneous imaging of up to four different sensors. In this protocol, we first describe strategies for multiplex fluorescence imaging of these sensors in single cells; then we demonstrate how to apply these sensors to study changes in redox landscapes during the cell cycle, after macrophage activation, and in living zebrafish. This approach can be adapted to different genetically encoded fluorescent sensors and various analytical platforms such as fluorescence microscopy, high-content imaging systems, flow cytometry, and microplate readers. A typical preparation of cells or zebrafish expressing different sensors takes 2-3 d; microscopy imaging or flow-cytometry analysis can be performed within 5-60 min.

Nicotine Adenine Dinucleotides: The Redox Currency of the Cell

There has been tremendous and rapidly growing interest in understanding intermediary metabolism as a key aspect of both normal cellular function and as a participant in the molecular pathogenesis of many different complex diseases. This area of research naturally intersects at virtually every level with the substantial and expanding body of knowledge regarding mechanisms of cellular redox balance. In this Forum, the contributing authors address specifically the union of intermediary metabolism and redox biology through detailed consideration of the biochemistry and biology of nicotine adenine dinucleotides, the cell's "redox currency." From technical considerations of how to measure nicotine adenine dinucleotides all the way to detailed treatments of their potential roles in specific disease states, this Forum provides a thorough introduction to a topic that is positioned to be at the heart of the next wave of research in metabolism and redox biology. Antioxid. Redox Signal. 28, 165-166.