NAD(P)HX repair deficiency causes central metabolic perturbations in yeast and human cells

Pericyte-Specific Secretome Profiling in Hypoxia Using TurboID in a Multicellular In Vitro Spheroid-Model

Cellular communication within the brain is imperative for maintaining homeostasis and mounting effective responses to pathological triggers like hypoxia. However, a comprehensive understanding of the precise composition and dynamic release of secreted molecules has remained elusive, confined primarily to investigations using isolated monocultures. To overcome these limitations, we utilized the potential of TurboID, a non-toxic biotin ligation enzyme, to capture and enrich secreted proteins specifically originating from human brain pericytes in spheroid co-cultures with human endothelial cells and astrocytes. This approach allowed us to characterize the pericyte secretome within a more physiologically relevant multi-cellular setting encompassing the constituents of the blood-brain barrier (BBB). Through a combination of mass spectrometry and multiplex immunoassays, we identified a wide spectrum of different secreted proteins by pericytes. Our findings demonstrate that the pericytes secretome is profoundly shaped by their inter-cellular communication with other BBB-residing cells. Moreover, we identified substantial differences in the secretory profiles between hypoxic and normoxic pericytes. Mass spectrometry analysis showed that hypoxic pericytes in co-culture increase their release of signals related to protein secretion, mTOR signalling and the complement system, while hypoxic pericytes in monocultures showed an upregulation in proliferative pathways including G2M checkpoints, E2F- and Myc-targets. In addition, hypoxic pericytes show an upregulation of proangiogenic proteins such as VEGFA but display downregulation of canonical proinflammatory cytokines such as CXCL1, MCP-1, and CXCL6. Understanding the specific composition of secreted proteins in the multi-cellular brain microvasculature is crucial for advancing our knowledge of brain homeostasis and the mechanisms underlying pathology. This study has implications for the identification of targeted therapeutic strategies aimed at modulating microvascular signalling in brain pathologies associated with hypoxia.

A Systematic Review on Quiescent State Research Approaches in S. cerevisiae

Quiescence, the temporary and reversible arrest of cell growth, is a fundamental biological process. However, the lack of standardization in terms of reporting the experimental details of quiescent cells and populations can cause confusion and hinder knowledge transfer. We employ the systematic review methodology to comprehensively analyze the diversity of approaches used to study the quiescent state, focusing on all published research addressing the budding yeast Saccharomyces cerevisiae. We group research articles into those that consider all cells comprising the stationary-phase (SP) population as quiescent and those that recognize heterogeneity within the SP by distinguishing phenotypically distinct subpopulations. Furthermore, we investigate the chronological age of the quiescent populations under study and the methods used to induce the quiescent state, such as gradual starvation or abrupt environmental change. We also assess whether the strains used in research are prototrophic or auxotrophic. By combining the above features, we identify 48 possible experimental setups that can be used to study quiescence, which can be misleading when drawing general conclusions. We therefore summarize our review by proposing guidelines and recommendations pertaining to the information included in research articles. We believe that more rigorous reporting on the features of quiescent populations will facilitate knowledge transfer within and between disciplines, thereby stimulating valuable scientific discussion.

Severe NAD(P)HX Dehydratase (NAXD) Neurometabolic Syndrome May Present in Adulthood after Mild Head Trauma

We have previously reported that pathogenic variants in a key metabolite repair enzyme NAXD cause a lethal neurodegenerative condition triggered by episodes of fever in young children. However, the clinical and genetic spectrum of NAXD deficiency is broadening as our understanding of the disease expands and as more cases are identified. Here, we report the oldest known individual succumbing to NAXD-related neurometabolic crisis, at 32 years of age. The clinical deterioration and demise of this individual were likely triggered by mild head trauma. This patient had a novel homozygous NAXD variant [NM_001242882.1:c.441+3A>G:p.?] that induces the mis-splicing of the majority of NAXD transcripts, leaving only trace levels of canonically spliced NAXD mRNA, and protein levels below the detection threshold by proteomic analysis. Accumulation of damaged NADH, the substrate of NAXD, could be detected in the fibroblasts of the patient. In agreement with prior anecdotal reports in paediatric patients, niacin-based treatment also partly alleviated some clinical symptoms in this adult patient. The present study extends our understanding of NAXD deficiency by uncovering shared mitochondrial proteomic signatures between the adult and our previously reported paediatric NAXD cases, with reduced levels of respiratory complexes I and IV as well as the mitoribosome, and the upregulation of mitochondrial apoptotic pathways. Importantly, we highlight that head trauma in adults, in addition to paediatric fever or illness, may precipitate neurometabolic crises associated with pathogenic NAXD variants.

Clinical and biochemical distinctions for a metabolite repair disorder caused by NAXD or NAXE deficiency

The central cofactors NAD(P)H are prone to damage by hydration, resulting in formation of redox-inactive derivatives designated NAD(P)HX. The highly conserved enzymes NAD(P)HX dehydratase (NAXD) and NAD(P)HX epimerase (NAXE) function to repair intracellular NAD(P)HX. Recently, pathogenic variants in both the NAXD and NAXE genes were associated with rapid deterioration and death after an otherwise trivial fever, infection, or illness in young patients. As more patients are identified, distinct clinical features are emerging depending on the location of the pathogenic variant. In this review, we carefully catalogued the clinical features of all published NAXD deficiency patients and found distinct patterns in clinical presentations depending on which subcellular compartment is affected by the enzymatic deficiency. Exon 1 of NAXD contains a mitochondrial propeptide, and a unique cytosolic isoform is initiated from an alternative start codon in exon 2. NAXD deficiency patients with variants that affect both the cytosolic and mitochondrial isoforms present with neurological defects, seizures and skin lesions. Interestingly, patients with NAXD variants exclusively affecting the mitochondrial isoform present with myopathy, moderate neuropathy and a cardiac presentation, without the characteristic skin lesions, seizures or neurological degeneration. This suggests that cytosolic NAD(P)HX repair may protect from neurological damage, whereas muscle fibres may be more sensitive to mitochondrial NAD(P)HX damage. A deeper understanding of the clinical phenotype may facilitate rapid identification of new cases and allow earlier therapeutic intervention. Niacin-based therapies are promising, but advances in disease modelling for both NAXD and NAXE deficiency may identify more specific compounds as targeted treatments. In this review, we found distinct patterns in the clinical presentations of NAXD deficiency patients based on the location of the pathogenic variant, which determines the subcellular compartment that is affected by the enzymatic deficiency.

Metabolite Damage and Damage Control in a Minimal Genome

Analysis of the genes retained in the minimized Mycoplasma JCVI-Syn3A genome established that systems that repair or preempt metabolite damage are essential to life. Several genes known to have such functions were identified and experimentally validated, including 5-formyltetrahydrofolate cycloligase, coenzyme A (CoA) disulfide reductase, and certain hydrolases. Furthermore, we discovered that an enigmatic YqeK hydrolase domain fused to NadD has a novel proofreading function in NAD synthesis and could double as a MutT-like sanitizing enzyme for the nucleotide pool. Finally, we combined metabolomics and cheminformatics approaches to extend the core metabolic map of JCVI-Syn3A to include promiscuous enzymatic reactions and spontaneous side reactions. This extension revealed that several key metabolite damage control systems remain to be identified in JCVI-Syn3A, such as that for methylglyoxal.

Functional and ligand binding studies of NAD(P)H hydrate dehydratase enzyme from vancomycin-resistant Staphylococcus aureus by NMR spectroscopic approach, including saturation transfer difference (STD-NMR) spectroscopy

NADH and NADPH are labile coenzymes that undergo hydration by enzymatic reaction or by heat at 5,6 double bond, and convert into non-functional hydrates, NADHX and NADPHX, respectively. The NAD(P)H hydrate dehydratase enzyme catalyzes the dehydration of S-NADHX/S-NADPHX at the expense of ATP, and thus contributes in the nicotinamide nucleotide repair process. This enzyme is also known as "metabolite-proofreading enzyme". Herein, we report the molecular cloning and expression of this highly conserved enzyme of vancomycin-resistant Staphylococcus aureus (VRSA). Its functional and inhibition studies were performed for the first time by NMR spectroscopy. NMR studies showed the dehydration of S epimer of NADHX, in the presence of R-NADHX and cyc-NADHX, by NAD(P)H hydrate dehydratase. In addition, by employing the STD-NMR approach, a library of drugs and natural products (total 79) were evaluated for their binding interactions with the NAD(P)H hydrate dehydratase enzyme. Among them, seven compounds showed ligand-like interactions with the enzyme, and thus functional activity of the enzyme was again checked in the presence of each ligand. Compound 2 (Thiamine HCl) was found to fully inhibit the enzyme's function, and recognized as a potential inhibitor. Current study demonstrates that this enzyme deserves further studies as a potential drug target, as its inhibition can disrupt the normal metabolism of pathogenic VRSA.

NAXE deficiency: A neurometabolic disorder of NAD(P)HX repair amenable for metabolic correction

The NAD(P)HX repair system is a metabolite damage repair mechanism responsible for restoration of NADH and NADPH after their inactivation by hydration. Deficiency in either of its two enzymes, NAD(P)HX dehydratase (NAXD) or NAD(P)HX epimerase (NAXE), causes a fatal neurometabolic disorder characterized by decompensations precipitated by inflammatory stress. Clinical findings include rapidly progressive muscle weakness, ataxia, ophthalmoplegia, and motor and cognitive regression, while neuroimaging abnormalities are subtle or nonspecific, making a clinical diagnosis challenging. During stress, nonenzymatic conversion of NAD(P)H to NAD(P)HX increases, and in the absence of repair, NAD(P)H is depleted, and NAD(P)HX accumulates, leading to decompensation; however, the contribution of each to the metabolic derangement is not established. Herein, we summarize the clinical knowledge of NAXE deficiency from 30 cases and lessons learned about disease pathogenesis from cell cultures and model organisms and describe a metabolomics signature obtained by untargeted metabolomics analysis in one case at the time of crisis and after initiation of treatment. Overall, biochemical findings support a model of acute depletion of NAD⁺, signs of mitochondrial dysfunction, and altered lipidomics. These findings are further substantiated by untargeted metabolomics six months post-crisis showing that niacin supplementation reverses primary metabolomic abnormalities concurrent with improved clinical status.

Approaches for completing metabolic networks through metabolite damage and repair discovery

Metabolites are prone to damage, either via enzymatic side reactions, which collectively form the underground metabolism, or via spontaneous chemical reactions. The resulting non-canonical metabolites that can be toxic, are mended by dedicated "metabolite repair enzymes." Deficiencies in the latter can cause severe disease in humans, whereas inclusion of repair enzymes in metabolically engineered systems can improve the production yield of value-added chemicals. The metabolite damage and repair loops are typically not yet included in metabolic reconstructions and it is likely that many remain to be discovered. Here, we review strategies and associated challenges for unveiling non-canonical metabolites and metabolite repair enzymes, including systematic approaches based on high-resolution mass spectrometry, metabolome-wide side-activity prediction, as well as high-throughput substrate and phenotypic screens.

Clinical and biochemical footprints of inherited metabolic diseases. VI. Metabolic dermatoses

Cutaneous signs and symptoms may facilitate the diagnosis or can help in identifying complications or side effects of overtreatment of inherited metabolic diseases. The principal manifestations can be grouped into vascular lesions, ichthyosis, papular and nodular skin lesions, abnormal pigmentation, photosensitivity, skin laxity, hair shaft involvement, and nail abnormalities. We have summarized associations of these cutaneous signs and symptoms in 252 inherited metabolic diseases. This represents the sixth of a series of articles attempting to create and maintain a comprehensive list of clinical and metabolic differential diagnoses according to system involvement.

Chemical and Biochemical Reactivity of the Reduced Forms of Nicotinamide Riboside

All life forms require nicotinamide adenine dinucleotide, NAD⁺, and its reduced form NADH. They are redox partners in hundreds of cellular enzymatic reactions. Changes in the intracellular levels of total NAD (NAD⁺ + NADH) and the (NAD⁺/NADH) ratio can cause cellular dysfunction. When not present in protein complexes, NADH and its phosphorylated form NADPH degrade through intricate mechanisms. Replenishment of a declining total NAD pool can be achieved with biosynthetic precursors that include one of the reduced forms of nicotinamide riboside (NR⁺), NRH. NRH, like NADH and NADPH, is prone to degradation via oxidation, hydration, and isomerization and, as such, is an excellent model compound to rationalize the nonenzymatic metabolism of NAD(P)H in a biological context. Here, we report on the stability of NRH and its propensity to isomerize and irreversibly degrade. We also report the preparation of two of its naturally occurring isomers, their chemical stability, their reactivity toward NRH-processing enzymes, and their cell-specific cytotoxicity. Furthermore, we identify a mechanism by which NRH degradation causes covalent peptide modifications, a process that could expose a novel type of NADH-protein modifications and correlate NADH accumulation with "protein aging." This work highlights the current limitations in detecting NADH's endogenous catabolites and in establishing the capacity for inducing cellular dysfunction.

Systemic metabolite profiling reveals sexual dimorphism of AIBP control of metabolism in mice

Emerging studies indicate that APOA-I binding protein (AIBP) is a secreted protein and functions extracellularly to promote cellular cholesterol efflux, thereby disrupting lipid rafts on the plasma membrane. AIBP is also present in the mitochondria and acts as an epimerase, facilitating the repair of dysfunctional hydrated NAD(P)H, known as NAD(P)H(X). Importantly, AIBP deficiency contributes to lethal neurometabolic disorder, reminiscent of the Leigh syndrome in humans. Whereas cyclic NADPHX production is proposed to be the underlying cause, we hypothesize that an unbiased metabolic profiling may: 1) reveal new clues for the lethality, e.g., changes of mitochondrial metabolites., and 2) identify metabolites associated with new AIBP functions. To this end, we performed unbiased and profound metabolic studies of plasma obtained from adult AIBP knockout mice and control littermates of both genders. Our systemic metabolite profiling, encompassing 9 super pathways, identified a total of 640 compounds. Our studies demonstrate a surprising sexual dimorphism of metabolites affected by AIBP deletion, with more statistically significant changes in the AIBP knockout female vs male when compared with the corresponding controls. AIBP knockout trends to reduce cholesterol but increase the bile acid precursor 7-HOCA in female but not male. Complex lipids, phospholipids, sphingomyelin and plasmalogens were reduced, while monoacylglycerol, fatty acids and the lipid soluble vitamins E and carotene diol were elevated in AIBP knockout female but not male. NAD metabolites were not significantly different in AIBP knockout vs control mice but differed for male vs female mice. Metabolites associated with glycolysis and the Krebs cycle were unchanged by AIBP knockout. Importantly, polyamine spermidine, critical for many cellular functions including cerebral cortex synapses, was reduced in male but not female AIBP knockout. This is the first report of a systemic metabolite profile of plasma samples from AIBP knockout mice, and provides a metabolic basis for future studies of AIBP regulation of cellular metabolism and the pathophysiological presentation of AIBP deficiency in patients.

Detecting early myocardial ischemia in rat heart by MALDI imaging mass spectrometry

Diagnostics of myocardial infarction in human post-mortem hearts can be achieved only if ischemia persisted for at least 6–12 h when certain morphological changes appear in myocardium. The initial 4 h of ischemia is difficult to diagnose due to lack of a standardized method. Developing a panel of molecular tissue markers is a promising approach and can be accelerated by characterization of molecular changes. This study is the first untargeted metabolomic profiling of ischemic myocardium during the initial 4 h directly from tissue section. Ischemic hearts from an ex-vivo Langendorff model were analysed using matrix assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) at 15 min, 30 min, 1 h, 2 h, and 4 h. Region-specific molecular changes were identified even in absence of evident histological lesions and were segregated by unsupervised cluster analysis. Significantly differentially expressed features were detected by multivariate analysis starting at 15 min while their number increased with prolonged ischemia. The biggest significant increase at 15 min was observed for m/z 682.1294 (likely corresponding to S-NADHX—a damage product of nicotinamide adenine dinucleotide (NADH)). Based on the previously reported role of NAD⁺/NADH ratio in regulating localization of the sodium channel (Na_v1.5) at the plasma membrane, Na_v1.5 was evaluated by immunofluorescence. As expected, a fainter signal was observed at the plasma membrane in the predicted ischemic region starting 30 min of ischemia and the change became the most pronounced by 4 h. Metabolomic changes occur early during ischemia, can assist in identifying markers for post-mortem diagnostics and improve understanding of molecular mechanisms.

AIBP, Angiogenesis, Hematopoiesis, and Atherogenesis

PURPOSE OF REVIEW

The goal of this manuscript is to summarize the current understanding of the secreted APOA1 binding protein (AIBP), encoded by NAXE, in angiogenesis, hematopoiesis, and inflammation. The studies on AIBP illustrate a critical connection between lipid metabolism and the aforementioned endothelial and immune cell biology.

RECENT FINDINGS

AIBP dictates both developmental processes such as angiogenesis and hematopoiesis, and pathological events such as inflammation, tumorigenesis, and atherosclerosis. Although cholesterol efflux dictates AIBP-mediated lipid raft disruption in many of the cell types, recent studies document cholesterol efflux-independent mechanism involving Cdc42-mediated cytoskeleton remodeling in macrophages. AIBP disrupts lipid rafts and impairs raft-associated VEGFR2 but facilitates non-raft-associated NOTCH1 signaling. Furthermore, AIBP can induce cholesterol biosynthesis gene SREBP2 activation, which in turn transactivates NOTCH1 and supports specification of hematopoietic stem and progenitor cells (HSPCs). In addition, AIBP also binds TLR4 and represses TLR4-mediated inflammation. In this review, we summarize the latest research on AIBP, focusing on its role in cholesterol metabolism and the attendant effects on lipid raft-regulated VEGFR2 and non-raft-associated NOTCH1 activation in angiogenesis, SREBP2-upregulated NOTCH1 signaling in hematopoiesis, and TLR4 signaling in inflammation and atherogenesis. We will discuss its potential therapeutic applications in angiogenesis and inflammation due to selective targeting of activated cells.

Cutaneous manifestations of NAXD deficiency - A case report

Metabolism is a tightly regulated sequence of events, supported by key reactions between enzymes and enzyme-specific substrates. These reactions have the potential to produce metabolic side products that can have deleterious effects to further key metabolic reactions. The nicotinamide repair system consists of two partner enzymes, NAD(P)HX epimerase (NAXE) and NAD(P)HX dehydratase (NAXD). These enzymes regulate the levels of metabolic side products. Here we present a case of an 11-month old child who presented to our paediatric department with pyrexia, lethargy and multiple cutaneous lesions on the background of NAXD deficiency, a lethal neurometabolic disorder of early childhood. Despite early intervention with intravenous antibiotics, the patient failed to improve and subsequently passed away. The skin lesions were thought to be a consequence of systemic disease rather than a propagator of infection. Clinicians should be aware of this incredibly rare metabolic disease, its potential to cause widespread systemic dysfunction and the developing avenues for management.

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Physical stress, infection or pyrexia have the potential to produce NADPHX.

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NADPHX is a metabolite with the ability to inhibit biosynthesis pathways.

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NAXE and NAXD are two cofactor enzymes that break down this toxic metabolite.

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NAXD deficiency is a rare metabolic disorder with potentially fatal outcomes

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Here we present a rare case of NAXD deficiency and its cutaneous manifestations.

Pyridoxamine-phosphate oxidases and pyridoxamine-phosphate oxidase-related proteins catalyze the oxidation of 6-NAD(P)H to NAD(P)

6-NADH and 6-NADPH are strong inhibitors of several dehydrogenases that may form spontaneously from NAD(P)H. They are known to be oxidized to NAD(P)+ by mammalian renalase, an FAD-linked enzyme mainly present in heart and kidney, and by related bacterial enzymes. We partially purified an enzyme oxidizing 6-NADPH from rat liver, and, surprisingly, identified it as pyridoxamine-phosphate oxidase (PNPO). This was confirmed by the finding that recombinant mouse PNPO oxidized 6-NADH and 6-NADPH with catalytic efficiencies comparable to those observed with pyridoxine- and pyridoxamine-5'-phosphate. PNPOs from Escherichia coli, Saccharomyces cerevisiae and Arabidopsis thaliana also displayed 6-NAD(P)H oxidase activity, indicating that this 'side-activity' is conserved. Remarkably, 'pyridoxamine-phosphate oxidase-related proteins' (PNPO-RP) from Nostoc punctiforme, A. thaliana and the yeast S. cerevisiae (Ygr017w) were not detectably active on pyridox(am)ine-5'-P, but oxidized 6-NADH, 6-NADPH and 2-NADH suggesting that this may be their main catalytic function. Their specificity profiles were therefore similar to that of renalase. Inactivation of renalase and of PNPO in mammalian cells and of Ygr017w in yeasts led to the accumulation of a reduced form of 6-NADH, tentatively identified as 4,5,6-NADH3, which can also be produced in vitro by reduction of 6-NADH by glyceraldehyde-3-phosphate dehydrogenase or glucose-6-phosphate dehydrogenase. As 4,5,6-NADH3 is not a substrate for renalase, PNPO or PNPO-RP, its accumulation presumably reflects the block in the oxidation of 6-NADH. These findings indicate that two different classes of enzymes using either FAD (renalase) or FMN (PNPOs and PNPO-RPs) as a cofactor play an as yet unsuspected role in removing damaged forms of NAD(P).

Unexpected NADPH Hydratase Activity in the Nitrile Reductase QueF from Escherichia coli

The nitrile reductase QueF catalyzes NADPH-dependent reduction of the nitrile group of preQ₀ (7-cyano-7-deazaguanine) into the primary amine of preQ₁ (7-aminomethyl-7-deazaguanine), a biologically unique reaction important in bacterial nucleoside biosynthesis. Here we have discovered that the QueF from Escherichia coli-its D197A and E89L variants in particular (apparent k_cat ≈10^-2  min^-1 )-also catalyze the slow hydration of the C5=C6 double bond of the dihydronicotinamide moiety of NADPH. The enzymatically C6-hydrated NADPH is a 3.5:1 mixture of R and S forms and rearranges spontaneously through anomeric epimerization (β→α) and cyclization at the tetrahydronicotinamide C6 and the ribosyl O2. NADH and 1-methyl- or 1-benzyl-1,4-dihydronicotinamide are not substrates of the enzymatic hydration. Mutagenesis results support a QueF hydratase mechanism, in which Cys190-the essential catalytic nucleophile for nitrile reduction-acts as the general acid for protonation at the dihydronicotinamide C5 of NADPH. Thus, the NADPH hydration in the presence of QueF bears mechanistic resemblance to the C=C double bond hydration in natural hydratases.

How evolution shapes enzyme selectivity - lessons from aminoacyl-tRNA synthetases and other amino acid utilizing enzymes

Aminoacyl-tRNA synthetases (AARSs) charge tRNA with their cognate amino acids. Many other enzymes use amino acids as substrates, yet discrimination against noncognate amino acids that threaten the accuracy of protein translation is a hallmark of AARSs. Comparing AARSs to these other enzymes allowed us to recognize patterns in molecular recognition and strategies used by evolution for exercising selectivity. Overall, AARSs are 2-3 orders of magnitude more selective than most other amino acid utilizing enzymes. AARSs also reveal the physicochemical limits of molecular discrimination. For example, amino acids smaller by a single methyl moiety present a discrimination ceiling of ~200, while larger ones can be discriminated by up to 10⁵ -fold. In contrast, substrates larger by a hydroxyl group challenge AARS selectivity, due to promiscuous H-bonding with polar active site groups. This 'hydroxyl paradox' is resolved by editing. Indeed, when the physicochemical discrimination limits are reached, post-transfer editing - hydrolysis of tRNAs charged with noncognate amino acids, evolved. The editing site often selectively recognizes the edited noncognate substrate using the very same feature that the synthetic site could not efficiently discriminate against. Finally, the comparison to other enzymes also reveals that the selectivity of AARSs is an explicitly evolved trait, showing some clear examples of how selection acted not only to optimize catalytic efficiency with the target substrate, but also to abolish activity with noncognate threat substrates ('negative selection').

Inborn errors of metabolite repair

It is traditionally assumed that enzymes of intermediary metabolism are extremely specific and that this is sufficient to prevent the production of useless and/or toxic side-products. Recent work indicates that this statement is not entirely correct. In reality, enzymes are not strictly specific, they often display weak side activities on intracellular metabolites (substrate promiscuity) that resemble their physiological substrate or slowly catalyse abnormal reactions on their physiological substrate (catalytic promiscuity). They thereby produce non-classical metabolites that are not efficiently metabolised by conventional enzymes. In an increasing number of cases, metabolite repair enzymes are being discovered that serve to eliminate these non-classical metabolites and prevent their accumulation. Metabolite repair enzymes also eliminate non-classical metabolites that are formed through spontaneous (ie, not enzyme-catalysed) reactions. Importantly, genetic deficiencies in several metabolite repair enzymes lead to 'inborn errors of metabolite repair', such as L-2-hydroxyglutaric aciduria, D-2-hydroxyglutaric aciduria, 'ubiquitous glucose-6-phosphatase' (G6PC3) deficiency, the neutropenia present in Glycogen Storage Disease type Ib or defects in the enzymes that repair the hydrated forms of NADH or NADPH. Metabolite repair defects may be difficult to identify as such, because the mutated enzymes are non-classical enzymes that act on non-classical metabolites, which in some cases accumulate only inside the cells, and at rather low, yet toxic, concentrations. It is therefore likely that many additional metabolite repair enzymes remain to be discovered and that many diseases of metabolite repair still await elucidation.

Novel NAXE variants as a cause for neurometabolic disorder: implications for treatment

Neurometabolic disorders are often inherited and complex disorders that result from abnormalities of enzymes important for development and function of the nervous system. Recently, biallelic mutations in NAXE (APOA1BP) were found in patients with an infantile, lethal, neurometabolic disease. Here, exome sequencing was performed in two affected sisters and their healthy parents. The best candidate, NAXE, was tested for replication in exome sequencing data from 4351 patients with neurodevelopmental disorders. Quantitative RT-PCR, western blot and form factor analysis were performed to assess NAXE expression, protein levels and to analyze mitochondrial morphology in fibroblasts. Vitamin B3 was administered to one patient. Compound heterozygous missense (c.757G>A: p.Gly253Ser) and splicing (c.665-1G>A) variants in NAXE were identified in both affected sisters. In contrast to the previously reported patients with biallelic NAXE variants, our patients showed a milder phenotype with disease onset in early adulthood with psychosis, cognitive impairment, seizures, cerebellar ataxia and spasticity. The symptoms fluctuated. Additional screening of NAXE identified three novel homozygous missense variants (p.Lys245Gln, p.Asp218Asn, p.Ile214Val) in three patients with overlapping phenotype (fluctuating disease course, respiratory insufficiency, movement disorder). Lastly, patients with the c.665-1G>A splicing variant showed a significant reduction of NAXE expression compared to control fibroblasts and undetectable NAXE protein levels compared to control fibroblasts. Based on the metabolic pathway, vitamin B3 and coenzyme Q treatment was introduced in one patient in addition to antiepileptic treatment. This combination and avoidance of triggers was associated with continuous motor and cognitive improvement. The NAXE variants identified in this study suggest a loss-of-function mechanism leading to an insufficient NAD(P)HX repair system. Importantly, symptoms of patients with NAXE variants may improve with vitamin B3/coenzyme Q administration.

Metabolite Repair Enzymes Control Metabolic Damage in Glycolysis

Hundreds of metabolic enzymes work together smoothly in a cell. These enzymes are highly specific. Nevertheless, under physiological conditions, many perform side-reactions at low rates, producing potentially toxic side-products. An increasing number of metabolite repair enzymes are being discovered that serve to eliminate these noncanonical metabolites. Some of these enzymes are extraordinarily conserved, and their deficiency can lead to diseases in humans or embryonic lethality in mice, indicating their central role in cellular metabolism. We discuss how metabolite repair enzymes eliminate glycolytic side-products and prevent negative interference within and beyond this core metabolic pathway. Extrapolating from the number of metabolite repair enzymes involved in glycolysis, hundreds more likely remain to be discovered that protect a wide range of metabolic pathways.

NAD(P)HX dehydratase (NAXD) deficiency: a novel neurodegenerative disorder exacerbated by febrile illnesses

Physical stress, including high temperatures, may damage the central metabolic nicotinamide nucleotide cofactors [NAD(P)H], generating toxic derivatives [NAD(P)HX]. The highly conserved enzyme NAD(P)HX dehydratase (NAXD) is essential for intracellular repair of NAD(P)HX. Here we present a series of infants and children who suffered episodes of febrile illness-induced neurodegeneration or cardiac failure and early death. Whole-exome or whole-genome sequencing identified recessive NAXD variants in each case. Variants were predicted to be potentially deleterious through in silico analysis. Reverse-transcription PCR confirmed altered splicing in one case. Subject fibroblasts showed highly elevated concentrations of the damaged cofactors S-NADHX, R-NADHX and cyclic NADHX. NADHX accumulation was abrogated by lentiviral transduction of subject cells with wild-type NAXD. Subject fibroblasts and muscle biopsies showed impaired mitochondrial function, higher sensitivity to metabolic stress in media containing galactose and azide, but not glucose, and decreased mitochondrial reactive oxygen species production. Recombinant NAXD protein harbouring two missense variants leading to the amino acid changes p.(Gly63Ser) and p.(Arg608Cys) were thermolabile and showed a decrease in Vmax and increase in KM for the ATP-dependent NADHX dehydratase activity. This is the first study to identify pathogenic variants in NAXD and to link deficient NADHX repair with mitochondrial dysfunction. The results show that NAXD deficiency can be classified as a metabolite repair disorder in which accumulation of damaged metabolites likely triggers devastating effects in tissues such as the brain and the heart, eventually leading to early childhood death.

3-Phosphoglycerate Transhydrogenation Instead of Dehydrogenation Alleviates the Redox State Dependency of Yeast de Novo l-Serine Synthesis

The enzymatic mechanism of 3-phosphoglycerate to 3-phosphohydroxypyruvate oxidation, which forms the first step of the main conserved de novo serine synthesis pathway, has been revisited recently in certain microorganisms. While this step is classically considered to be catalyzed by an NAD-dependent dehydrogenase (e.g., PHGDH in mammals), evidence has shown that in Pseudomonas, Escherichia coli, and Saccharomyces cerevisiae, the PHGDH homologues act as transhydrogenases. As such, they use α-ketoglutarate, rather than NAD⁺, as the final electron acceptor, thereby producing D-2-hydroxyglutarate in addition to 3-phosphohydroxypyruvate during 3-phosphoglycerate oxidation. Here, we provide a detailed biochemical and sequence-structure relationship characterization of the yeast PHGDH homologues, encoded by the paralogous SER3 and SER33 genes, in comparison to the human and other PHGDH enzymes. Using in vitro assays with purified recombinant enzymes as well as in vivo growth phenotyping and metabolome analyses of yeast strains engineered to depend on either Ser3, Ser33, or human PHGDH for serine synthesis, we confirmed that both yeast enzymes act as transhydrogenases, while the human enzyme is a dehydrogenase. In addition, we show that the yeast paralogs differ from the human enzyme in their sensitivity to inhibition by serine as well as hydrated NADH derivatives. Importantly, our in vivo data support the idea that a 3PGA transhydrogenase instead of dehydrogenase activity confers a growth advantage under conditions where the NAD⁺:NADH ratio is low. The results will help to elucidate why different species evolved different reaction mechanisms to carry out a widely conserved metabolic step in central carbon metabolism.

The chemistry of the vitamin B3 metabolome

The functional cofactors derived from vitamin B3 are nicotinamide adenine dinucleotide (NAD⁺), its phosphorylated form, nicotinamide adenine dinucleotide phosphate (NADP⁺) and their reduced forms (NAD(P)H). These cofactors, together referred as the NAD(P)(H) pool, are intimately implicated in all essential bioenergetics, anabolic and catabolic pathways in all forms of life. This pool also contributes to post-translational protein modifications and second messenger generation. Since NAD⁺ seats at the cross-road between cell metabolism and cell signaling, manipulation of NAD⁺ bioavailability through vitamin B3 supplementation has become a valuable nutritional and therapeutic avenue. Yet, much remains unexplored regarding vitamin B3 metabolism. The present review highlights the chemical diversity of the vitamin B3-derived anabolites and catabolites of NAD⁺ and offers a chemical perspective on the approaches adopted to identify, modulate and measure the contribution of various precursors to the NAD(P)(H) pool.