SLC25A51 is a mammalian mitochondrial NAD+ transporter

MDH2 produced OAA is a metabolic switch rewiring the fuelling of respiratory chain and TCA cycle

The mitochondrial respiratory chain (RC) enables many metabolic processes by regenerating both mitochondrial and cytosolic NAD⁺ and ATP. The oxidation by the RC of the NADH metabolically produced in the cytosol involves redox shuttles as the malate-aspartate shuttle (MAS) and is of paramount importance for cell fate. However, the specific metabolic regulations allowing mitochondrial respiration to prioritize NADH oxidation in response to high NADH/NAD⁺ redox stress have not been elucidated. The recent discovery that complex I (NADH dehydrogenase), and not complex II (Succinate dehydrogenase), can assemble with other respiratory chain complexes to form functional entities called respirasomes, led to the assumption that this supramolecular organization would favour NADH oxidation. Unexpectedly, characterization of heart and liver mitochondria demonstrates that the RC systematically favours electrons provided by the 'respirasome free' complex II. Our results demonstrate that the preferential succinate driven respiration is tightly controlled by OAA levels, and that OAA feedback inhibition of complex II rewires RC fuelling increasing NADH oxidation capacity. This new regulatory mechanism synergistically increases RC's NADH oxidative capacity and rewires MDH2 driven anaplerosis of the TCA, preventing malate production from succinate to favour oxidation of cytosolic malate. This regulatory mechanism synergistically adjusts RC and TCA fuelling in response to extramitochondrial malate produced by the MAS.

Overexpression of SLC25A51 promotes hepatocellular carcinoma progression by driving aerobic glycolysis through activation of SIRT5

Solute carrier family 25 member 20 (SLC25A51) is a newly identified mammalian mitochondrial NAD⁺ transporter. However, the clinicopathological and biological significance of SLC25A51 in human cancers, including hepatocellular carcinoma (HCC), remains unclear. The aim of this study was to define the role of SLC25A51 in HCC progression. Here we demonstrate that SLC25A51 is significantly overexpressed in human HCC specimens and cell lines, caused by, at least in partial, the decrease of miR-212-3p. SLC25A51 overexpression is positively correlated with the clinicopathological characteristics of vascular invasion and tumor diameter, as well as poor survival in patients with HCC. Knockdown of SLC25A51 attenuated, while overexpression of SLC25A51 enhanced the growth and metastasis of HCC cells both in vitro and in vivo. Mechanistically, glucose metabolism reprogramming from oxidative phosphorylation to glycolysis by activation of mitochondrial sirtuin 5 (SIRT5) was found to contribute to the promotion of growth and metastasis by SLC25A51 in HCC cells. Together, these findings reveal important roles of SLC25A51 in HCC tumorigenesis and suggest SLC25A51 as a promising prognostic marker and therapeutic target for treating HCC.

The hallmarks of cancer metabolism: Still emerging

Metabolism of cancer cells is geared toward biomass production and proliferation. Since the metabolic resources within the local tissue are finite, this can lead to nutrient depletion and accumulation of metabolic waste. To maintain growth in these conditions, cancer cells employ a variety of metabolic adaptations, the nature of which is collectively determined by the physiology of their cell of origin, the identity of transforming lesions, and the tissue in which cancer cells reside. Furthermore, select metabolites not only serve as substrates for energy and biomass generation, but can also regulate gene and protein expression and influence the behavior of non-transformed cells in the tumor vicinity. As they grow and metastasize, tumors can also affect and be affected by the nutrient distribution within the body. In this hallmark update, recent advances are incorporated into a conceptual framework that may help guide further research efforts in exploring cancer cell metabolism.

Weighing in on Adipogenesis

Obesity is a growing health concern worldwide because of its contribution to metabolic syndrome, type II diabetes, insulin resistance (IR), and numerous cancers. In obesity, white adipose tissue (WAT) expands through two mechanisms: increase in adipocyte cell number by precursor cell differentiation through the process of adipogenesis (hyperplasia) and increase in existing mature adipocyte cell size (hypertrophy). While hypertrophy is associated with the negative effects of obesity on metabolic health, such as inflammation and lipotoxicity, adipogenesis prevents obesity-mediated metabolic decline. Moreover, in metabolically healthy obesity adipogenesis is increased. Thus, it is vital to understand the mechanistic basis for adipose expansion to inform novel therapeutic approaches to mitigate the dysfunction of this tissue and associated diseases. In this mini-review, we summarize recent studies on the regulation of adipogenesis and provide a perspective on targeting adipogenesis as a potential therapeutic avenue for metabolic disorders.

The assembly, regulation and function of the mitochondrial respiratory chain

The mitochondrial oxidative phosphorylation system is central to cellular metabolism. It comprises five enzymatic complexes and two mobile electron carriers that work in a mitochondrial respiratory chain. By coupling the oxidation of reducing equivalents coming into mitochondria to the generation and subsequent dissipation of a proton gradient across the inner mitochondrial membrane, this electron transport chain drives the production of ATP, which is then used as a primary energy carrier in virtually all cellular processes. Minimal perturbations of the respiratory chain activity are linked to diseases; therefore, it is necessary to understand how these complexes are assembled and regulated and how they function. In this Review, we outline the latest assembly models for each individual complex, and we also highlight the recent discoveries indicating that the formation of larger assemblies, known as respiratory supercomplexes, originates from the association of the intermediates of individual complexes. We then discuss how recent cryo-electron microscopy structures have been key to answering open questions on the function of the electron transport chain in mitochondrial respiration and how supercomplexes and other factors, including metabolites, can regulate the activity of the single complexes. When relevant, we discuss how these mechanisms contribute to physiology and outline their deregulation in human diseases.

Meeting Report: Aging Research and Drug Discovery

Aging is the single largest risk factor for most chronic diseases, and thus possesses large socioeconomic interest to continuously aging societies. Consequently, the field of aging research is expanding alongside a growing focus from the industry and investors in aging research. This year’s 8th Annual Aging Research and Drug Discovery (ARDD) meeting was organized as a hybrid meeting from August 30th to September 3rd 2021 with more than 130 attendees participating on-site at the Ceremonial Hall at University of Copenhagen, Denmark, and 1800 engaging online. The conference comprised of presentations from 75 speakers focusing on new research in topics including mechanisms of aging and how these can be modulated as well as the use of AI and new standards of practices within aging research. This year, a longevity workshop was included to build stronger connections with the clinical community.

AGC1 Deficiency: Pathology and Molecular and Cellular Mechanisms of the Disease

AGC1/Aralar/Slc25a12 is the mitochondrial carrier of aspartate-glutamate, the regulatory component of the NADH malate-aspartate shuttle (MAS) that transfers cytosolic redox power to neuronal mitochondria. The deficiency in AGC1/Aralar leads to the human rare disease named "early infantile epileptic encephalopathy 39" (EIEE 39, OMIM # 612949) characterized by epilepsy, hypotonia, arrested psychomotor neurodevelopment, hypo myelination and a drastic drop in brain aspartate (Asp) and N-acetylaspartate (NAA). Current evidence suggest that neurons are the main brain cell type expressing Aralar. However, paradoxically, glial functions such as myelin and Glutamine (Gln) synthesis are markedly impaired in AGC1 deficiency. Herein, we discuss the role of the AGC1/Aralar-MAS pathway in neuronal functions such as Asp and NAA synthesis, lactate use, respiration on glucose, glutamate (Glu) oxidation and other neurometabolic aspects. The possible mechanism triggering the pathophysiological findings in AGC1 deficiency, such as epilepsy and postnatal hypomyelination observed in humans and mice, are also included. Many of these mechanisms arise from findings in the aralar-KO mice model that extensively recapitulate the human disease including the astroglial failure to synthesize Gln and the dopamine (DA) mishandling in the nigrostriatal system. Epilepsy and DA mishandling are a direct consequence of the metabolic defect in neurons due to AGC1/Aralar deficiency. However, the deficits in myelin and Gln synthesis may be a consequence of neuronal affectation or a direct effect of AGC1/Aralar deficiency in glial cells. Further research is needed to clarify this question and delineate the transcellular metabolic fluxes that control brain functions. Finally, we discuss therapeutic approaches successfully used in AGC1-deficient patients and mice.

Circadian REV-ERBs repress E4bp4 to activate NAMPT-dependent NAD+ biosynthesis and sustain cardiac function

The heart is a highly metabolic organ that uses multiple energy sources to meet its demand for ATP production. Diurnal feeding-fasting cycles result in substrate availability fluctuations which, together with increased energetic demand during the active period, impose a need for rhythmic cardiac metabolism. The nuclear receptors REV-ERBα and β are essential repressive components of the molecular circadian clock and major regulators of metabolism. To investigate their role in the heart, here we generated mice with cardiomyocyte (CM)-specific deletion of both Rev-erbs, which died prematurely due to dilated cardiomyopathy. Loss of Rev-erbs markedly downregulated fatty acid oxidation genes prior to overt pathology, which was mediated by induction of the transcriptional repressor E4BP4, a direct target of cardiac REV-ERBs. E4BP4 directly controls circadian expression of Nampt and its biosynthetic product NAD+ via distal cis-regulatory elements. Thus, REV-ERB-mediated E4BP4 repression is required for Nampt expression and NAD+ production by the salvage pathway. Together, these results highlight the indispensable role of circadian REV-ERBs in cardiac gene expression, metabolic homeostasis and function.

Cancer treatment-induced NAD+ depletion in premature senescence and late cardiovascular complications

Numerous studies have revealed the critical role of premature senescence induced by various cancer treatment modalities in the pathogenesis of aging-related diseases. Senescence-associated secretory phenotype (SASP) can be induced by telomere dysfunction. Telomeric DNA damage response induced by some cancer treatments can persist for months, possibly accounting for long-term sequelae of cancer treatments. Telomeric DNA damage-induced mitochondrial dysfunction and increased reactive oxygen species production are hallmarks of premature senescence. Recently, we reported that the nucleus-mitochondria positive feedback loop formed by p90 ribosomal S6 kinase (p90RSK) and phosphorylation of S496 on ERK5 (a unique member of the mitogen-activated protein kinase family that is not only a kinase but also a transcriptional co-activator) were vital signaling events that played crucial roles in linking mitochondrial dysfunction, nuclear telomere dysfunction, persistent SASP induction, and atherosclerosis. In this review, we will discuss the role of NAD⁺ depletion in instigating SASP and its downstream signaling and regulatory mechanisms that lead to the premature onset of atherosclerotic cardiovascular diseases in cancer survivors.

Advances of Glycometabolism Engineering in Chinese Hamster Ovary Cells

As the most widely used mammalian cell line, Chinese hamster ovary (CHO) cells can express various recombinant proteins with a post translational modification pattern similar to that of the proteins from human cells. During industrial production, cells need large amounts of ATP to support growth and protein expression, and since glycometabolism is the main source of ATP for cells, protein production partly depends on the efficiency of glycometabolism. And efficient glycometabolism allows less glucose uptake by cells, reducing production costs, and providing a better mammalian production platform for recombinant protein expression. In the present study, a series of progresses on the comprehensive optimization in CHO cells by glycometabolism strategy were reviewed, including carbohydrate intake, pyruvate metabolism and mitochondrial metabolism. We analyzed the effects of gene regulation in the upstream and downstream of the glucose metabolism pathway on cell’s growth and protein expression. And we also pointed out the latest metabolic studies that are potentially applicable on CHO cells. In the end, we elaborated the application of metabolic models in the study of CHO cell metabolism.

The role of protein acetylation in regulating mitochondrial fusion and fission

The dynamic processes of mitochondrial fusion and fission determine the shape of mitochondria, which can range from individual fragments to a hyperfused network, and influence mitochondrial function. Changes in mitochondrial shape can occur rapidly, allowing mitochondria to adapt to specific cues and changing cellular demands. Here, we will review what is known about how key proteins required for mitochondrial fusion and fission are regulated by their acetylation status, with acetylation promoting fission and deacetylation enhancing fusion. In particular, we will examine the roles of NAD+ dependant sirtuin deacetylases, which mediate mitochondrial acetylation, and how this post-translational modification provides an exquisite regulatory mechanism to co-ordinate mitochondrial function with metabolic demands of the cell.

Nutritional reprogramming of mouse liver proteome is dampened by metformin, resveratrol, and rapamycin

Nutrient sensing pathways influence metabolic health and aging, offering the possibility that diet might be used therapeutically, alone or with drugs targeting these pathways. We used the Geometric Framework for Nutrition to study interactive and comparative effects of diet and drugs on the hepatic proteome in mice across 40 dietary treatments differing in macronutrient ratios, energy density, and drug treatment (metformin, rapamycin, resveratrol). There was a strong negative correlation between dietary energy and the spliceosome and a strong positive correlation between dietary protein and mitochondria, generating oxidative stress at high protein intake. Metformin, rapamycin, and resveratrol had lesser effects than and dampened responses to diet. Rapamycin and metformin reduced mitochondrial responses to dietary protein while the effects of carbohydrates and fat were downregulated by resveratrol. Dietary composition has a powerful impact on the hepatic proteome, not just on metabolic pathways but fundamental processes such as mitochondrial function and RNA splicing.

Pharmacology and Potential Implications of Nicotinamide Adenine Dinucleotide Precursors

Coenzyme I (nicotinamide adenine dinucleotide, NAD+/NADH) and coenzyme II (nicotinamide adenine dinucleotide phosphate, NADP+/NADPH) are involved in various biological processes in mammalian cells. NAD+ is synthesised through the de novo and salvage pathways, whereas coenzyme II cannot be synthesised de novo. NAD+ is a precursor of coenzyme II. Although NAD+ is synthesised in sufficient amounts under normal conditions, shortage in its supply due to over consumption and its decreased synthesis has been observed with increasing age and under certain disease conditions. Several studies have proved that in a wide range of tissues, such as liver, skin, muscle, pancreas, and fat, the level of NAD+ decreases with age. However, in the brain tissue, the level of NADH gradually increases and that of NAD+ decreases in aged people. The ratio of NAD+/NADH indicates the cellular redox state. A decrease in this ratio affects the cellular anaerobic glycolysis and oxidative phosphorylation functions, which reduces the ability of cells to produce ATP. Therefore, increasing the exogenous NAD+ supply under certain disease conditions or in elderly people may be beneficial. Precursors of NAD+ have been extensively explored and have been reported to effectively increase NAD+ levels and possess a broad range of functions. In this review article, we discuss the pharmacokinetics and pharmacodynamics of NAD+ precursors.

NAD+ Metabolism in Cardiac Health, Aging, and Disease

Nicotinamide adenine dinucleotide (NAD⁺) is a central metabolite involved in energy and redox homeostasis as well as in DNA repair and protein deacetylation reactions. Pharmacological or genetic inhibition of NAD⁺-degrading enzymes, external supplementation of NAD⁺ precursors, and transgenic overexpression of NAD⁺-generating enzymes have wide positive effects on metabolic health and age-associated diseases. NAD⁺ pools tend to decline with normal aging, obesity, and hypertension, which are all major risk factors for cardiovascular disease, and NAD⁺ replenishment extends healthspan, avoids metabolic syndrome, and reduces blood pressure in preclinical models. In addition, experimental elevation of NAD⁺ improves atherosclerosis, ischemic, diabetic, arrhythmogenic, hypertrophic, or dilated cardiomyopathies, as well as different modalities of heart failure. Here, we critically discuss cardiomyocyte-specific circuitries of NAD⁺ metabolism, comparatively evaluate distinct NAD⁺ precursors for their preclinical efficacy, and raise outstanding questions on the optimal design of clinical trials in which NAD⁺ replenishment or supraphysiological NAD⁺ elevations are assessed for the prevention or treatment of major cardiac diseases. We surmise that patients with hitherto intractable cardiac diseases such as heart failure with preserved ejection fraction may profit from the administration of NAD⁺ precursors. The development of such NAD⁺-centered treatments will rely on technological and conceptual progress on the fine regulation of NAD⁺ metabolism.

Interplay between ADP-ribosyltransferases and essential cell signaling pathways controls cellular responses

Signaling cascades provide integrative and interactive frameworks that allow the cell to respond to signals from its environment and/or from within the cell itself. The dynamic regulation of mammalian cell signaling pathways is often modulated by cascades of protein post-translational modifications (PTMs). ADP-ribosylation is a PTM that is catalyzed by ADP-ribosyltransferases and manifests as mono- (MARylation) or poly- (PARylation) ADP-ribosylation depending on the addition of one or multiple ADP-ribose units to protein substrates. ADP-ribosylation has recently emerged as an important cell regulator that impacts a plethora of cellular processes, including many intracellular signaling events. Here, we provide an overview of the interplay between the intracellular diphtheria toxin-like ADP-ribosyltransferase (ARTD) family members and five selected signaling pathways (including NF-κB, JAK/STAT, Wnt-β-catenin, MAPK, PI3K/AKT), which are frequently described to control or to be controlled by ADP-ribosyltransferases and how these interactions impact the cellular responses.

First we eat, then we do everything else: The dynamic metabolic regulation of efferocytosis

Clearance of apoptotic cells, or "efferocytosis," is essential for diverse processes including embryonic development, tissue turnover, organ regeneration, and immune cell development. The human body is estimated to remove approximately 1% of its body mass via apoptotic cell clearance daily. This poses several intriguing cell metabolism problems. For instance, phagocytes such as macrophages must induce or suppress metabolic pathways to find, engulf, and digest apoptotic cells. Then, phagocytes must manage the potentially burdensome biomass of the engulfed apoptotic cell. Finally, phagocytes reside in complex tissue architectures that vary in nutrient availability, the types of dying cells or debris that require clearance, and the neighboring cells they interact with. Here, we review advances in our understanding of these three key areas of phagocyte metabolism. We end by proposing a model of efferocytosis that integrates recent findings and establishes a new paradigm for testing how efferocytosis prevents chronic inflammatory disease and autoimmunity.

Ablation of mitochondrial DNA results in widespread remodeling of the mitochondrial complexome

So-called ρ0 cells lack mitochondrial DNA and are therefore incapable of aerobic ATP synthesis. How cells adapt to survive ablation of oxidative phosphorylation remains poorly understood. Complexome profiling analysis of ρ0 cells covered 1,002 mitochondrial proteins and revealed changes in abundance and organization of numerous multiprotein complexes including previously not described assemblies. Beyond multiple subassemblies of complexes that would normally contain components encoded by mitochondrial DNA, we observed widespread reorganization of the complexome. This included distinct changes in the expression pattern of adenine nucleotide carrier isoforms, other mitochondrial transporters, and components of the protein import machinery. Remarkably, ablation of mitochondrial DNA hardly affected the complexes organizing cristae junctions indicating that the altered cristae morphology in ρ0 mitochondria predominantly resulted from the loss of complex V dimers required to impose narrow curvatures to the inner membrane. Our data provide a comprehensive resource for in-depth analysis of remodeling of the mitochondrial complexome in response to respiratory deficiency.

Learning from Yeast about Mitochondrial Carriers

Mitochondria are organelles that play an important role in both energetic and synthetic metabolism of eukaryotic cells. The flow of metabolites between the cytosol and mitochondrial matrix is controlled by a set of highly selective carrier proteins localised in the inner mitochondrial membrane. As defects in the transport of these molecules may affect cell metabolism, mutations in genes encoding for mitochondrial carriers are involved in numerous human diseases. Yeast Saccharomyces cerevisiae is a traditional model organism with unprecedented impact on our understanding of many fundamental processes in eukaryotic cells. As such, the yeast is also exceptionally well suited for investigation of mitochondrial carriers. This article reviews the advantages of using yeast to study mitochondrial carriers with the focus on addressing the involvement of these carriers in human diseases.

Aryl hydrocarbon receptor (AHR) functions in infectious and sterile inflammation and NAD+-dependent metabolic adaptation

Aryl hydrocarbon receptor (AHR) research has shifted from exploring dioxin toxicity to elucidation of various physiologic AHR functions. Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to exert cellular stress-mediated sterile inflammatory responses in exposed human tissues but may be lethal in sensitive species. Inflammation can be thought of as the extreme end of a spectrum ranging from homeostasis to stress responses (sterile inflammation) and to defense against infection (infectious inflammation). Defense against bacterial infection by generation of reactive oxygen species has to be strictly controlled and may use up a considerable amount of energy. NAD⁺-mediated energy metabolism adapts to various inflammatory responses. As examples, the present commentary tries to integrate responses of AHR and NAD⁺-consuming enzymes (PARP7/TiPARP, CD38 and sirtuins) into infectious and stress-induced inflammatory responses, the latter exemplified by nonalcoholic fatty liver disease (NAFLD). TCDD toxicity models in sensitive species provide hints to molecular AHR targets of energy metabolism including gluconeogenesis and glycolysis. AHR research remains challenging and promising.

NAD+ flux is maintained in aged mice despite lower tissue concentrations

NAD⁺ is an essential coenzyme for all living cells. NAD⁺ concentrations decline with age, but whether this reflects impaired production or accelerated consumption remains unclear. We employed isotope tracing and mass spectrometry to probe age-related changes in NAD⁺ metabolism across tissues. In aged mice, we observed modest tissue NAD⁺ depletion (median decrease ∼30%). Circulating NAD⁺ precursors were not significantly changed, and isotope tracing showed the unimpaired synthesis of nicotinamide from tryptophan. In most tissues of aged mice, turnover of the smaller tissue NAD⁺ pool was modestly faster such that absolute NAD⁺ biosynthetic flux was maintained, consistent with more active NAD⁺-consuming enzymes. Calorie restriction partially mitigated age-associated NAD⁺ decline by decreasing consumption. Acute inflammatory stress induced by LPS decreased NAD⁺ by impairing synthesis in both young and aged mice. Thus, the decline in NAD⁺ with normal aging is relatively subtle and occurs despite maintained NAD⁺ production, likely due to increased consumption.

Preclinical and clinical evidence of NAD+ precursors in health, disease, and ageing

NAD⁺ is a fundamental molecule in human life and health as it participates in energy metabolism, cell signalling, mitochondrial homeostasis, and in dictating cell survival or death. Emerging evidence from preclinical and human studies indicates an age-dependent reduction of cellular NAD⁺, possibly due to reduced synthesis and increased consumption. In preclinical models, NAD⁺ repletion extends healthspan and / or lifespan and mitigates several conditions, such as premature ageing diseases and neurodegenerative diseases. These findings suggest that NAD⁺ replenishment through NAD⁺ precursors has great potential as a therapeutic target for ageing and age-predisposed diseases, such as Alzheimer's disease. Here, we provide an updated review on the biological activity, safety, and possible side effects of NAD⁺ precursors in preclinical and clinical studies. Major NAD⁺ precursors focused on by this review are nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), and the new discovered dihydronicotinamide riboside (NRH). In summary, NAD⁺ precursors have an exciting therapeutic potential for ageing, metabolic and neurodegenerative diseases.

NAD+ Degrading Enzymes, Evidence for Roles During Infection

Declines in cellular nicotinamide adenine dinucleotide (NAD) contribute to metabolic dysfunction, increase susceptibility to disease, and occur as a result of pathogenic infection. The enzymatic cleavage of NAD⁺ transfers ADP-ribose (ADPr) to substrate proteins generating mono-ADP-ribose (MAR), poly-ADP-ribose (PAR) or O-acetyl-ADP-ribose (OAADPr). These important post-translational modifications have roles in both immune response activation and the advancement of infection. In particular, emergent data show viral infection stimulates activation of poly (ADP-ribose) polymerase (PARP) mediated NAD⁺ depletion and stimulates hydrolysis of existing ADP-ribosylation modifications. These studies are important for us to better understand the value of NAD⁺ maintenance upon the biology of infection. This review focuses specifically upon the NAD⁺ utilising enzymes, discusses existing knowledge surrounding their roles in infection, their NAD⁺ depletion capability and their influence within pathogenic infection.

NAD+ in Alzheimer's Disease: Molecular Mechanisms and Systematic Therapeutic Evidence Obtained in vivo

Mitochondria in neurons generate adenosine triphosphate (ATP) to provide the necessary energy required for constant activity. Nicotinamide adenine dinucleotide (NAD⁺) is a vital intermediate metabolite involved in cellular bioenergetics, ATP production, mitochondrial homeostasis, and adaptive stress responses. Exploration of the biological functions of NAD⁺ has been gaining momentum, providing many crucial insights into the pathophysiology of age-associated functional decline and diseases, such as Alzheimer’s disease (AD). Here, we systematically review the key roles of NAD⁺ precursors and related metabolites in AD models and show how NAD⁺ affects the pathological hallmarks of AD and the potential mechanisms of action. Advances in understanding the molecular roles of NAD⁺-based neuronal resilience will result in novel approaches for the treatment of AD and set the stage for determining whether the results of exciting preclinical trials can be translated into the clinic to improve AD patients’ phenotypes.

Pseudogene Transcripts in Head and Neck Cancer: Literature Review and In Silico Analysis

Once considered nonfunctional, pseudogene transcripts are now known to provide valuable information for cancer susceptibility, including head and neck cancer (HNC), a serious health problem worldwide, with about 50% unimproved overall survival over the last decades. The present review focuses on the role of pseudogene transcripts involved in HNC risk and prognosis. We combined current literature and in silico analyses from The Cancer Genome Atlas (TCGA) database to identify the most deregulated pseudogene transcripts in HNC and their genetic variations. We then built a co-expression network and performed gene ontology enrichment analysis to better understand the pseudogenes’ interactions and pathways in HNC. In the literature, few pseudogenes have been studied in HNC. Our in silico analysis identified 370 pseudogene transcripts associated with HNC, where SPATA31D5P, HERC2P3, SPATA31C2, MAGEB6P1, SLC25A51P1, BAGE2, DNM1P47, SPATA31C1, ZNF733P and OR2W5 were found to be the most deregulated and presented several genetic alterations. NBPF25P, HSP90AB2P, ZNF658B and DPY19L2P3 pseudogenes were predicted to interact with 12 genes known to participate in HNC, DNM1P47 was predicted to interact with the TP53 gene, and HLA-H pseudogene was predicted to interact with HLA-A and HLA-B genes. The identified pseudogenes were associated with cancer biology pathways involving cell communication, response to stress, cell death, regulation of the immune system, regulation of gene expression, and Wnt signaling. Finally, we assessed the prognostic values of the pseudogenes with the Kaplan–Meier Plotter database, and found that expression of SPATA31D5P, SPATA31C2, BAGE2, SPATA31C1, ZNF733P and OR2W5 pseudogenes were associated with patients’ survival. Due to pseudogene transcripts’ potential for cancer diagnosis, progression, and as therapeutic targets, our study can guide new research to HNC understanding and development of new target therapies.

Transcriptomic Analysis of Long Noncoding RNA and mRNA Expression Profiles in the Amygdala of Rats with Bone Cancer Pain-Depression Comorbidity

Bone cancer pain (BCP)–depression comorbidity has become a complex clinical problem during cancer treatment; however, its underlying molecular mechanisms have not been clarified. Several long noncoding RNAs (lncRNAs) have been demonstrated to be promising therapeutic targets in depression, but research on the role of lncRNAs in BCP–depression comorbidity has been limited. Therefore, high-throughput RNA sequencing was performed to detect differentially expressed profiles in the amygdala of a BCP–depression rat model in this study. We detected 330 differentially expressed mRNAs (DEmRNAs) and 78 differentially expressed lncRNAs (DElncRNAs) in the BCP–depression comorbidity model and then verified the expression of six DEmRNAs and six DElncRNAs with the greatest degrees of difference by RT-qPCR. Furthermore, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that differentially expressed genes were strongly enriched in inflammatory and immunologic systemic responses. Then the nuclear factor kappa B (NF-κB) signaling pathway and the Th17 differentiation pathway showed significant differences, as determined by Western blot analysis. Finally, we constructed a protein–protein interaction (PPI) network to explore the potential regulatory mechanism of DEmRNAs. In conclusion, our study reveals a new resource for the understanding of dysregulated lncRNAs and mRNAs in BCP–depression comorbidity and provides novel potential therapeutic targets for further approaches.

Mitochondria in Neuronal Health: From Energy Metabolism to Parkinson's Disease

Mitochondria are the main suppliers of neuronal adenosine triphosphate and play a critical role in brain energy metabolism. Mitochondria also serve as Ca²⁺ sinks and anabolic factories and are therefore essential for neuronal function and survival. Dysregulation of neuronal bioenergetics is increasingly implicated in neurodegenerative disorders, particularly Parkinson's disease. This review describes the role of mitochondria in energy metabolism under resting conditions and during synaptic transmission, and presents evidence for the contribution of neuronal mitochondrial dysfunction to Parkinson's disease.

3D Adipose Tissue Culture Links the Organotypic Microenvironment to Improved Adipogenesis

Obesity and type 2 diabetes are strongly associated with adipose tissue dysfunction and impaired adipogenesis. Understanding the molecular underpinnings that control adipogenesis is thus of fundamental importance for the development of novel therapeutics against metabolic disorders. However, translational approaches are hampered as current models do not accurately recapitulate adipogenesis. Here, a scaffold-free versatile 3D adipocyte culture platform with chemically defined conditions is presented in which primary human preadipocytes accurately recapitulate adipogenesis. Following differentiation, multi-omics profiling and functional tests demonstrate that 3D adipocyte cultures feature mature molecular and cellular phenotypes similar to freshly isolated mature adipocytes. Spheroids exhibit physiologically relevant gene expression signatures with 4704 differentially expressed genes compared to conventional 2D cultures (false discovery rate < 0.05), including the concerted expression of factors shaping the adipogenic niche. Furthermore, lipid profiles of >1000 lipid species closely resemble patterns of the corresponding isogenic mature adipocytes in vivo (R2 = 0.97). Integration of multi-omics signatures with analyses of the activity profiles of 503 transcription factors using global promoter motif inference reveals a complex signaling network, involving YAP, Hedgehog, and TGFβ signaling, that links the organotypic microenvironment in 3D culture to the activation and reinforcement of PPARγ and CEBP activity resulting in improved adipogenesis.

Disrupting Mitochondrial Electron Transfer Chain Complex I Decreases Immune Checkpoints in Murine and Human Acute Myeloid Leukemic Cells

Oxidative metabolism is crucial for leukemic stem cell (LSC) function and drug resistance in acute myeloid leukemia (AML). Mitochondrial metabolism also affects the immune system and therefore the anti-tumor response. The modulation of oxidative phosphorylation (OxPHOS) has emerged as a promising approach to improve the therapy outcome for AML patients. However, the effect of mitochondrial inhibitors on the immune compartment in the context of AML is yet to be explored. Immune checkpoints such as ectonucleotidase CD39 and programmed dead ligand 1 (PD-L1) have been reported to be expressed in AML and linked to chemo-resistance and a poor prognosis. In the present study, we first demonstrated that a novel selective electron transfer chain complex (ETC) I inhibitor, EVT-701, decreased the OxPHOS metabolism of murine and human cytarabine (AraC)-resistant leukemic cell lines. Furthermore, we showed that while AraC induced an immune response regulation by increasing CD39 expression and by reinforcing the interferon-γ/PD-L1 axis, EVT-701 reduced CD39 and PD-L1 expression in vitro in a panel of both murine and human AML cell lines, especially upon AraC treatment. Altogether, this work uncovers a non-canonical function of ETCI in controlling CD39 and PD-L1 immune checkpoints, thereby improving the anti-tumor response in AML.

NAD+ homeostasis in human health and disease

Depletion of nicotinamide adenine dinucleotide (NAD+ ), a central redox cofactor and the substrate of key metabolic enzymes, is the causative factor of a number of inherited and acquired diseases in humans. Primary deficiencies of NAD+ homeostasis are the result of impaired biosynthesis, while secondary deficiencies can arise due to other factors affecting NAD+ homeostasis, such as increased NAD+ consumption or dietary deficiency of its vitamin B3 precursors. NAD+ depletion can manifest in a wide variety of pathological phenotypes, ranging from rare inherited defects, characterized by congenital malformations, retinal degeneration, and/or encephalopathy, to more common multifactorial, often age-related, diseases. Here, we discuss NAD+ biochemistry and metabolism and provide an overview of the etiology and pathological consequences of alterations of the NAD+ metabolism in humans. Finally, we discuss the state of the art of the potential therapeutic implications of NAD+ repletion for boosting health as well as treating rare and common diseases, and the possibilities to achieve this by means of the different NAD+ -enhancing agents.

Welcome to the Family: Identification of the NAD+ Transporter of Animal Mitochondria as Member of the Solute Carrier Family SLC25

Subcellular compartmentation is a fundamental property of eukaryotic cells. Communication and metabolic and regulatory interconnectivity between organelles require that solutes can be transported across their surrounding membranes. Indeed, in mammals, there are hundreds of genes encoding solute carriers (SLCs) which mediate the selective transport of molecules such as nucleotides, amino acids, and sugars across biological membranes. Research over many years has identified the localization and preferred substrates of a large variety of SLCs. Of particular interest has been the SLC25 family, which includes carriers embedded in the inner membrane of mitochondria to secure the supply of these organelles with major metabolic intermediates and coenzymes. The substrate specificity of many of these carriers has been established in the past. However, the route by which animal mitochondria are supplied with NAD⁺ had long remained obscure. Only just recently, the existence of a human mitochondrial NAD⁺ carrier was firmly established. With the realization that SLC25A51 (or MCART1) represents the major mitochondrial NAD⁺ carrier in mammals, a long-standing mystery in NAD⁺ biology has been resolved. Here, we summarize the functional importance and structural features of this carrier as well as the key observations leading to its discovery.

SIRT1-dependent restoration of NAD+ homeostasis after increased extracellular NAD+ exposure

In the last several years, NAD+ supplementation has emerged as an innovative and safe therapeutic strategy for a wide spectrum of disorders, including diabetes and neuropathy. However, critical questions remain as to how NAD+ and its precursors are taken up by cells, as well as the effects of long-lasting intracellular NAD+ (iNAD+) increases. Here, we investigated the kinetics of iNAD+ levels in different cell types challenged with prolonged exposure to extracellular NAD+ (eNAD+). Surprisingly, we found that after the initial increase, iNAD+ contents decreased back to control levels (iNAD+ resetting). Focusing our attention on HeLa cells, we found that oxygen and ATP consumption occurred with similar temporal kinetics after eNAD+ exposure. Using [³H]NAD+ and [¹⁴C]NAD+, we determined that NAD+ resetting was not due to increased dinucleotide extrusion but rather due to reduced uptake of cleaved NAD+ products. Indeed, eNAD+ exposure reduced the expression of the ecto-5′-nucleotidase CD73, the nicotinamide adenine mononucleotide transporter solute carrier family 12 member 8, and the nicotinamide riboside kinase. Interestingly, silencing the NAD+-sensor enzyme sirtuin 1 prevented eNAD+-dependent transcriptional repression of ecto-5′-nucleotidase, solute carrier family 12 member 8, and nicotinamide riboside kinase, as well as iNAD+ resetting. Our findings provide the first evidence for a sirtuin 1–mediated homeostatic response aimed at maintaining physiological iNAD+ levels in conditions of excess eNAD+ availability. These data may be of relevance for therapies designed to support the NAD+ metabolome via extracellular supplementation of the dinucleotide or its precursors.

Neurotransmitters responsible for purinergic motor neurotransmission and regulation of GI motility

Classical concepts of peripheral neurotransmission were insufficient to explain enteric inhibitory neurotransmission. Geoffrey Burnstock and colleagues developed the idea that ATP or a related purine satisfies the criteria for a neurotransmitter and serves as an enteric inhibitory neurotransmitter in GI muscles. Cloning of purinergic receptors and development of specific drugs and transgenic mice have shown that enteric inhibitory responses depend upon P2Y₁ receptors in post-junctional cells. The post-junctional cells that transduce purinergic neurotransmitters in the GI tract are PDGFRα⁺ cells and not smooth muscle cells (SMCs). PDGFRα⁺ cells express P2Y₁ receptors, are activated by enteric inhibitory nerve stimulation and generate Ca²⁺ oscillations, express small-conductance Ca²⁺-activated K⁺ channels (SK3), and generate outward currents when exposed to P2Y₁ agonists. These properties are consistent with post-junctional purinergic responses, and similar responses and effectors are not functional in SMCs. Refinements in methodologies to measure purines in tissue superfusates, such as high-performance liquid chromatography (HPLC) coupled with etheno-derivatization of purines and fluorescence detection, revealed that multiple purines are released during stimulation of intrinsic nerves. β-NAD⁺ and other purines, better satisfy criteria for the purinergic neurotransmitter than ATP. HPLC has also allowed better detection of purine metabolites, and coupled with isolation of specific types of post-junctional cells, has provided new concepts about deactivation of purine neurotransmitters. In spite of steady progress, many unknowns about purinergic neurotransmission remain and require additional investigation to understand this important regulatory mechanism in GI motility.

Evolving concepts in NAD+ metabolism

NAD(H) and NADP(H) have traditionally been viewed as co-factors (or co-enzymes) involved in a myriad of oxidation-reduction reactions including the electron transport in the mitochondria. However, NAD pathway metabolites have many other important functions, including roles in signaling pathways, post-translational modifications, epigenetic changes, and regulation of RNA stability and function via NAD-capping of RNA. Non-oxidative reactions ultimately lead to the net catabolism of these nucleotides, indicating that NAD metabolism is an extremely dynamic process. In fact, recent studies have clearly demonstrated that NAD has a half-life in the order of minutes in some tissues. Several evolving concepts on the metabolism, transport, and roles of these NAD pathway metabolites in disease states such as cancer, neurodegeneration, and aging have emerged in just the last few years. In this perspective, we discuss key recent discoveries and changing concepts in NAD metabolism and biology that are reshaping the field. In addition, we will pose some open questions in NAD biology, including why NAD metabolism is so fast and dynamic in some tissues, how NAD and its precursors are transported to cells and organelles, and how NAD metabolism is integrated with inflammation and senescence. Resolving these questions will lead to significant advancements in the field.

Metabolism and Chronic Inflammation: The Links Between Chronic Heart Failure and Comorbidities

Heart failure (HF) patients often suffer from multiple comorbidities, such as diabetes, atrial fibrillation, depression, chronic obstructive pulmonary disease, and chronic kidney disease. The coexistance of comorbidities usually leads to multi morbidity and poor prognosis. Treatments for HF patients with multi morbidity are still an unmet clinical need, and finding an effective therapy strategy is of great value. HF can lead to comorbidity, and in return, comorbidity may promote the progression of HF, creating a vicious cycle. This reciprocal correlation indicates there may be some common causes and biological mechanisms. Metabolism remodeling and chronic inflammation play a vital role in the pathophysiological processes of HF and comorbidities, indicating metabolism and inflammation may be the links between HF and comorbidities. In this review, we comprehensively discuss the major underlying mechanisms and therapeutic implications for comorbidities of HF. We first summarize the potential role of metabolism and inflammation in HF. Then, we give an overview of the linkage between common comorbidities and HF, from the perspective of epidemiological evidence to the underlying metabolism and inflammation mechanisms. Moreover, with the help of bioinformatics, we summarize the shared risk factors, signal pathways, and therapeutic targets between HF and comorbidities. Metabolic syndrome, aging, deleterious lifestyles (sedentary behavior, poor dietary patterns, smoking, etc.), and other risk factors common to HF and comorbidities are all associated with common mechanisms. Impaired mitochondrial biogenesis, autophagy, insulin resistance, and oxidative stress, are among the major mechanisms of both HF and comorbidities. Gene enrichment analysis showed the PI3K/AKT pathway may probably play a central role in multi morbidity. Additionally, drug targets common to HF and several common comorbidities were found by network analysis. Such analysis has already been instrumental in drug repurposing to treat HF and comorbidity. And the result suggests sodium-glucose transporter-2 (SGLT-2) inhibitors, IL-1β inhibitors, and metformin may be promising drugs for repurposing to treat multi morbidity. We propose that targeting the metabolic and inflammatory pathways that are common to HF and comorbidities may provide a promising therapeutic strategy.

NAD+ and NAFLD - caution, causality and careful optimism

The prevalence of non-alcoholic fatty liver disease (NAFLD) is increasing worldwide, and new treatments are sorely needed. Nicotinamide adenine dinucleotide (NAD⁺ ) has been proposed as a potential target to prevent and reverse NAFLD. NAD⁺ is an important redox factor for energy metabolism and is used as a substrate by a range of enzymes, including sirtuins (SIRT), which regulates histone acetylation, transcription factor activity and mitochondrial function. NAD⁺ is also a precursor for reduced nicotinamide adenine dinucleotide phosphate (NADPH), which is an important component of the antioxidant defense system. NAD⁺ precursors such as nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN) are available as over-the-counter dietary supplements, and oral supplementation with these precursors increases hepatic NAD⁺ levels and prevents hepatic lipid accumulation in pre-clinical models of NAFLD. NAD⁺ precursors have also been found to improve hepatic mitochondrial function and decrease oxidative stress in pre-clinical NAFLD models. NAD⁺ repletion also prevents NAFLD progression to non-alcoholic steatohepatitis (NASH), as NAD⁺ precursor supplementation is associated with decreased hepatic stellate cell activation, and decreased fibrosis. However, initial clinical trials have only shown modest effects when NAD⁺ precursors were administrated to people with obesity. We review the available pre-clinical investigations of NAD⁺ supplementation for targeting NAFLD, and discuss how data from the first clinical trials can be reconciled with observations from preclinical research.

Protective Effects of Nicotinamide Adenine Dinucleotide and Related Precursors in Alzheimer's Disease: A Systematic Review of Preclinical Studies

Data from preclinical studies propose nicotinamide adenine dinucleotide (NAD⁺) as a neuroprotective and bioenergetics stimulant agent to treat Alzheimer's disease (AD); however, there seems to be inconsistency between behavioral and molecular outcomes. We performed this systematic review to provide a better understanding of the effects of NAD⁺ in rodent AD models and to summarize the literature.Studies were identified by searching PubMed, EMBASE, Scopus, Google Scholar, and the reference lists of relevant review articles published through December 2020. The search strategy was restricted to articles about NAD⁺, its derivatives, and their association with cognitive function in AD rodent models. The initial search yielded 320 articles, of which 11 publications were included in our systematic review.Based on the primary outcomes, it was revealed that NAD⁺ improves learning and memory. The secondary endpoints also showed neuroprotective effects of NAD⁺ on different AD models. The proposed neuroprotective mechanisms included, but were not limited to, the attenuation of the oxidative stress, inflammation, and apoptosis, while enhancing the mitochondrial function.The current systematic review summarizes the preclinical studies on NAD⁺ precursors and provides evidence favoring the pro-cognitive effects of such components in rodent models of AD.

Computational studies of the mitochondrial carrier family SLC25. Present status and future perspectives

The members of the mitochondrial carrier family, also known as solute carrier family 25 (SLC25), are transmembrane proteins involved in the translocation of a plethora of small molecules between the mitochondrial intermembrane space and the matrix. These transporters are characterized by three homologous domains structure and a transport mechanism that involves the transition between different conformations. Mutations in regions critical for these transporters’ function often cause several diseases, given the crucial role of these proteins in the mitochondrial homeostasis. Experimental studies can be problematic in the case of membrane proteins, in particular concerning the characterization of the structure–function relationships. For this reason, computational methods are often applied in order to develop new hypotheses or to support/explain experimental evidence. Here the computational analyses carried out on the SLC25 members are reviewed, describing the main techniques used and the outcome in terms of improved knowledge of the transport mechanism. Potential future applications on this protein family of more recent and advanced in silico methods are also suggested.

Biomimetic Molecular Clamp Nanopores for Simultaneous Quantifications of NAD+ and NADH

NADH/NAD⁺ is pivotal to fundamental biochemistry research and molecular diagnosis, but recognition and detection for them are a big challenge at the single-molecule level. Inspired by the biological system, here, we designed and synthesized a biomimetic NAD⁺/NADH molecular clamp (MC), octakis-(6-amino-6-deoxy)-γ-cyclomaltooctaose, and harbored in the engineered α-HL(M113R)₇ nanopore, forming a novel single-molecule biosensor. The single-molecule measurement possesses high selectivity and a high signal-to-noise ratio, allowing to simultaneously recognize and detect for sensing NADH/NAD⁺ and their transformations.

Nicotinamide adenine dinucleotide metabolism in the immune response, autoimmunity and inflammageing

Metabolism is dynamically regulated to accompany immune cell function, and altered immunometabolism can result in impaired immune responses. Concomitantly, the pharmacological manipulation of metabolic processes offers an opportunity for therapeutic intervention in inflammatory disorders. The nicotinamide adenine dinucleotide (NAD⁺) is a critical metabolic intermediate that serves as enzyme cofactor in redox reactions, and is also used as a co‐substrate by many enzymes such as sirtuins, adenosine diphosphate ribose transferases and synthases. Through these activities, NAD⁺ metabolism regulates a broad spectrum of cellular functions such as energy metabolism, DNA repair, regulation of the epigenetic landscape and inflammation. Thus, the manipulation of NAD⁺ availability using pharmacological compounds such as NAD⁺ precursors can have immune‐modulatory properties in inflammation. Here, we discuss how the NAD⁺ metabolism contributes to the immune response and inflammatory conditions, with a special focus on multiple sclerosis, inflammatory bowel diseases and inflammageing.

Uncovering the Invisible: Mono-ADP-ribosylation Moved into the Spotlight

Adenosine diphosphate (ADP)-ribosylation is a nicotinamide adenine dinucleotide (NAD+)-dependent post-translational modification that is found on proteins as well as on nucleic acids. While ARTD1/PARP1-mediated poly-ADP-ribosylation has extensively been studied in the past 60 years, comparably little is known about the physiological function of mono-ADP-ribosylation and the enzymes involved in its turnover. Promising technological advances have enabled the development of innovative tools to detect NAD+ and NAD+/NADH (H for hydrogen) ratios as well as ADP-ribosylation. These tools have significantly enhanced our current understanding of how intracellular NAD dynamics contribute to the regulation of ADP-ribosylation as well as to how mono-ADP-ribosylation integrates into various cellular processes. Here, we discuss the recent technological advances, as well as associated new biological findings and concepts.

Maestro of the SereNADe: SLC25A51 Orchestrates Mitochondrial NAD. Trends Biochem Sci 2021;46:348-350. [PMID: 33618948 PMCID: PMC8344040 DOI: 10.1016/j.tibs.2021.02.001]

Recently, three groups, Girardi et al., Kory et al., and Luongo et al., independently identified solute carrier (SLC) 25A51 as the long-sought, major mitochondrial NAD⁺ transporter in mammalian cells. These studies not only deorphan an uncharacterized transporter of the SLC25A family, but also shed light on other aspects of NAD⁺ biology.

Transporters at the Interface between Cytosolic and Mitochondrial Amino Acid Metabolism

Mitochondria are central organelles that coordinate a vast array of metabolic and biologic functions important for cellular health. Amino acids are intricately linked to the bioenergetic, biosynthetic, and homeostatic function of the mitochondrion and require specific transporters to facilitate their import, export, and exchange across the inner mitochondrial membrane. Here we review key cellular metabolic outputs of eukaryotic mitochondrial amino acid metabolism and discuss both known and unknown transporters involved. Furthermore, we discuss how utilization of compartmentalized amino acid metabolism functions in disease and physiological contexts. We examine how improved methods to study mitochondrial metabolism, define organelle metabolite composition, and visualize cellular gradients allow for a more comprehensive understanding of how transporters facilitate compartmentalized metabolism.

Mitochondrial NAD+ Controls Nuclear ARTD1-Induced ADP-Ribosylation

In addition to its role as an electron transporter, mitochondrial nicotinamide adenine dinucleotide (NAD⁺) is an important co-factor for enzymatic reactions, including ADP-ribosylation. Although mitochondria harbor the most intra-cellular NAD⁺, mitochondrial ADP-ribosylation remains poorly understood. Here we provide evidence for mitochondrial ADP-ribosylation, which was identified using various methodologies including immunofluorescence, western blot, and mass spectrometry. We show that mitochondrial ADP-ribosylation reversibly increases in response to respiratory chain inhibition. Conversely, H₂O₂-induced oxidative stress reciprocally induces nuclear and reduces mitochondrial ADP-ribosylation. Elevated mitochondrial ADP-ribosylation, in turn, dampens H₂O₂-triggered nuclear ADP-ribosylation and increases MMS-induced ARTD1 chromatin retention. Interestingly, co-treatment of cells with the mitochondrial uncoupler FCCP decreases PARP inhibitor efficacy. Together, our results suggest that mitochondrial ADP-ribosylation is a dynamic cellular process that impacts nuclear ADP-ribosylation and provide evidence for a NAD⁺-mediated mitochondrial-nuclear crosstalk.

Polyphosphate-dependent nicotinamide adenine dinucleotide (NAD) kinase: A novel missing link in human mitochondria

Polyphosphate [poly(P)] is described as a homopolymer of inorganic phosphates. Nicotinamide adenine dinucleotide kinase (NAD kinase) catalyzes the phosphorylation of NAD⁺ to NADP⁺ in the presence of ATP (ATP-NAD kinase). Novel NAD kinase that explicitly phosphorylates NAD⁺ to NADP⁺ using poly(P), besides ATP [ATP/poly(P)-NAD kinase], was found in bacteria, in particular, Gram-positive bacteria, and the gene encoding ATP/poly(P)-NAD kinase was also newly identified in Mycobacterium tuberculosis H37Rv. Both NAD kinases required multi-homopolymeric structures for activity expression. The enzymatic and genetic results, combined with their primary and tertiary structures, have led to the discovery of a long-awaited human mitochondrial NAD kinase. This discovery showed that the NAD kinase is a bacterial type of ATP/poly(P)-NAD kinase. These pioneering findings, i.e., ATP/poly(P)-NAD kinase, NAD kinase gene, and human mitochondrial NAD kinase, have significantly enhanced research on the biochemistry, molecular biology, and evolutionary biology of NAD kinase, mitochondria, and poly(P), including some biotechnological knowledge applicable to NADP⁺ production.

Epistasis-driven identification of SLC25A51 as a regulator of human mitochondrial NAD import

About a thousand genes in the human genome encode for membrane transporters. Among these, several solute carrier proteins (SLCs), representing the largest group of transporters, are still orphan and lack functional characterization. We reasoned that assessing genetic interactions among SLCs may be an efficient way to obtain functional information allowing their deorphanization. Here we describe a network of strong genetic interactions indicating a contribution to mitochondrial respiration and redox metabolism for SLC25A51/MCART1, an uncharacterized member of the SLC25 family of transporters. Through a combination of metabolomics, genomics and genetics approaches, we demonstrate a role for SLC25A51 as enabler of mitochondrial import of NAD, showcasing the potential of genetic interaction-driven functional gene deorphanization.

Lactate Limits T Cell Proliferation via the NAD(H) Redox State

Immune cell function is influenced by metabolic conditions. Low-glucose, high-lactate environments, such as the placenta, gastrointestinal tract, and the tumor microenvironment, are immunosuppressive, especially for glycolysis-dependent effector T cells. We report that nicotinamide adenine dinucleotide (NAD⁺), which is reduced to NADH by lactate dehydrogenase in lactate-rich conditions, is a key point of metabolic control in T cells. Reduced NADH is not available for NAD⁺-dependent enzymatic reactions involving glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and 3-phosphoglycerate dehydrogenase (PGDH). We show that increased lactate leads to a block at GAPDH and PGDH, leading to the depletion of post-GAPDH glycolytic intermediates, as well as the 3-phosphoglycerate derivative serine that is known to be important for T cell proliferation. Supplementing serine rescues the ability of T cells to proliferate in the presence of lactate-induced reductive stress. Directly targeting the redox state may be a useful approach for developing novel immunotherapies in cancer and therapeutic immunosuppression.

Quinn et al. report that lactate has an acidity-independent suppressive effect on effector T cell proliferation mediated through a shift from NAD⁺ to NADH (lactate-induced reductive stress). This impairs glycolysis and glucose-derived serine production, which is required for effector T cell proliferation.