A redox-mediated conformational change in NQO1 controls binding to microtubules and α-tubulin acetylation

The localization of NQO1 near acetylated microtubules has led to the hypothesis that NQO1 may work in concert with the NAD⁺-dependent deacetylase SIRT2 to regulate acetyl α-tubulin (K⁴⁰) levels on microtubules. NQO1 catalyzes the oxidation of NADH to NAD⁺ and may supplement levels of NAD⁺ near microtubules to aid SIRT2 deacetylase activity. While HDAC6 has been shown to regulate the majority of microtubule acetylation at K⁴⁰, SIRT2 is also known to modulate microtubule acetylation (K⁴⁰) in the perinuclear region. In this study we examined the potential roles NQO1 may play in modulating acetyl α-tubulin levels. Knock-out or knock-down of NQO1 or SIRT2 did not change the levels of acetyl α-tubulin in 16HBE human bronchial epithelial cells and 3T3-L1 fibroblasts; however, treatment with a mechanism-based inhibitor of NQO1 (MI2321) led to a short-lived temporal increase in acetyl α-tubulin levels in both cell lines without impacting the intracellular pools of NADH or NAD⁺. Inactivation of NQO1 by MI2321 resulted in lower levels of NQO1 immunostaining on microtubules, consistent with redox-dependent changes in NQO1 conformation as evidenced by the use of redox-specific, anti-NQO1 antibodies in immunoprecipitation studies. Given the highly dynamic nature of acetylation-deacetylation reactions at α-tubulin K⁴⁰ and the crowded protein environment surrounding this site, disruption in the binding of NQO1 to microtubules may temporally disturb the physical interactions of enzymes responsible for maintaining the microtubule acetylome.

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NQO1which produces NAD and Sirt2 which uses NAD are located in the perinuclear region.

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Depleting cellular NAD⁺ led to increased levels of acetyl α-tubulin.

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Knockout or knockdown of NQO1 did not change perinuclear acetyl α-tubulin levels.

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Pharmacological inhibition of NQO1 by MI2321 increased α-tubulin acetylation.

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Redox changes in NQO1 conformation and binding modulate microtubule acetyltubulin.

NAD(P)-Driven Redox Status Contributes to Desiccation Tolerance in Acer seeds

Desiccation tolerance is a developmental program enabling seed survival in a dry state and is common in seeds categorized as orthodox. We focused on NAD and its phosphorylated form (NADP) because their continual switching between reduced (NAD(P)H) and oxidized (NAD(P)+) forms is involved in the modulation of redox signaling and the determination of the reducing power and further antioxidant responses. Norway maple and sycamore seeds representing the orthodox and recalcitrant categories, respectively, were used as models in a comparison of responses to water loss. The process of desiccation up to 10% water content (WC) was monitored in Norway maple seeds, while dehydration up to 30% WC was monitored in desiccation-sensitive sycamore seeds. Norway maple and sycamore seeds, particularly their embryonic axes, exhibited a distinct redox status during dehydration and desiccation. High NADPH levels, NAD+ accumulation, low and stable NAD(P)H/NAD(P)+ ratios expressed as reducing power and high NADPH-dependent enzyme activity were reported in Norway maple seeds and were considered attributes of orthodox-type seeds. The contrasting results of sycamore seeds contributed to their low antioxidant capacity and high sensitivity to desiccation. NADPH deficiency, low NADPH-dependent enzyme activity and lack of NAD+ accumulation were primary features of sycamore seeds, with implications for their NAD(P)H/NAD(P)+ ratios and reducing power and with effects on many seed traits. Thus, we propose that the distinct levels of pyridine nucleotides and their redox status contribute to orthodox and recalcitrant phenotype differentiation in seeds by affecting cellular redox signaling, metabolism and the antioxidant system.

Measurement of Pyridine Nucleotides in Biological Samples Using LC-MS/MS

Pyridine nucleotides which include NAD⁺, NADH, NADP, and NADPH play vital roles in many different biological processes. These metabolites can be accurately quantified in a wide variety of biological samples using LC-MS/MS. The quality and precision of these measurements was enhanced using heavy isotope-labeled internal standards and carefully crafted protocols for sample processing.

Zwitterionic HILIC tandem mass spectrometry with isotope dilution for rapid, sensitive and robust quantification of pyridine nucleotides in biological extracts

The pyridine nucleotides nicotineamide adenine dinucleotide (NAD) and nicotineamide adenine dinucleotide phosphate (NADP) are conserved coenzymes across all domains of life, and are involved in more than 200 different hydride transfer reactions supporting essential catabolic and anabolic functions. The intracellular levels of these metabolites, and the ratio of their oxidized to reduced forms regulate an extensive network of reactions ranging beyond metabolism. Hence, monitoring their intracellular levels provides information about, but not limited to, the metabolic state of a cell or tissue. Interconversion between oxidized and reduced forms, varying pH liability and varying intracellular concentrations of the different species leaves absolute quantification of the pyridine nucleotides analytically challenging. These polar metabolites are poorly retained on conventional reverseed-phase stationary phases without ion-pair reagents that contaminates the LC-system. Herein we demonstrate that zwitterionic HILIC-tandem mass spectroemtry can be applied to successfully resolve the pyridine nucleotides in biological extracts in a fast, robust and highly sensitive way. The presented method applies isotope dilution to compensate potential loss of these labile metabolites and is validated for low, medium and high biomass samples of two popular biological model systems; Escherichia coli and the human cell line JJN-3. High stability and rapid sample preparation without solvent removal allows for long sequence runs, making this method ideal for high-throughput analysis of biological extracts.

More to NAD+ than meets the eye: A regulator of metabolic pools and gene expression in Arabidopsis

Since its discovery more than a century ago, nicotinamide adenine dinucleotide (NAD⁺) is recognised as a fascinating cornerstone of cellular metabolism. This ubiquitous energy cofactor plays vital roles in metabolic pathways and regulatory processes, a fact emphasised by the essentiality of a balanced NAD⁺ metabolism for normal plant growth and development. Research on the role of NAD in plants has been predominantly carried out in the model plant Arabidopsis thaliana (Arabidopsis) with emphasis on the redox properties and cellular signalling functions of the metabolite. This review examines the current state of knowledge concerning how NAD can regulate both metabolic pools and gene expression in Arabidopsis. Particular focus is placed on recent studies highlighting the complexity of metabolic regulations involving NAD, more particularly in the mitochondrial compartment, and of signalling roles with respect to interactions with environmental fluctuations most specifically those involving plant immunity.

Redox signaling in acute oxygen sensing

Acute oxygen (O₂) sensing is essential for individuals to survive under hypoxic conditions. The carotid body (CB) is the main peripheral chemoreceptor, which contains excitable and O₂-sensitive glomus cells with O₂-regulated ion channels. Upon exposure to acute hypoxia, inhibition of K⁺ channels is the signal that triggers cell depolarization, transmitter release and activation of sensory fibers that stimulate the brainstem respiratory center to produce hyperventilation. The molecular mechanisms underlying O₂ sensing by glomus cells have, however, remained elusive. Here we discuss recent data demonstrating that ablation of mitochondrial Ndufs2 gene selectively abolishes sensitivity of glomus cells to hypoxia, maintaining responsiveness to hypercapnia or hypoglycemia. These data suggest that reactive oxygen species and NADH generated in mitochondrial complex I during hypoxia are signaling molecules that modulate membrane K⁺ channels. We propose that the structural substrates for acute O₂ sensing in CB glomus cells are “O₂-sensing microdomains” formed by mitochondria and neighboring K⁺ channels in the plasma membrane.

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Acute O₂ sensing by peripheral chemoreceptors depends on K⁺ channels.

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Mitochondrial complex I function is required for acute O₂ sensing.

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Reactive oxygen species inhibits background K⁺ channels during acute hypoxia.

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Pyridine nucleotides may signal voltage-gated K⁺ channels during acute hypoxia.

Physiological role of AOX1a in photosynthesis and maintenance of cellular redox homeostasis under high light in Arabidopsis thaliana

As plants are sessile, they often face high light (HL) stress that causes damage of the photosynthetic machinery leading to decreased photosynthesis. The importance of alternative oxidase (AOX) in optimizing photosynthesis is well documented. In the present study, the role of AOX in sustaining photosynthesis under HL was studied using AOX1a knockout mutants (aox1a) of Arabidopsis thaliana. Under growth light (GL; 50 μmol photons m(-2) s(-1)) conditions, aox1a plants did not show any changes in photosynthetic parameters, NAD(P)/H redox ratios, or respiratory O2 uptake when compared to wild-type (WT). Upon exposure to HL (700 μmol photons m(-2) s(-1)), respiratory rates did not vary between WT and aox1a. But, photosynthetic parameters related to photosystem II (PSII) and NaHCO3 dependent O2 evolution decreased, while the P700 reduction state increased in aox1a compared to WT. Further, under HL, the redox state of cellular NAD(P)/H pools increased with concomitant rise in reactive oxygen species (ROS) and malondialdehyde (MDA) content in aox1a compared to WT. In presence of HL, the transcript levels of several genes related to antioxidant, malate-oxaloacetate (malate-OAA) shuttle, photorespiratory and respiratory enzymes was higher in aox1a compared to WT. Taken together, these results demonstrate that under HL, in spite of significant increase in transcript levels of several genes mentioned above to maintain cellular redox homeostasis and minimize ROS production, Arabidopsis plants deficient in AOX1a were unable to sustain photosynthesis as is the case in WT plants.

Large changes in NAD levels associated with CD38 expression during HL-60 cell differentiation

NAD is an important cofactor involved in multiple metabolic reactions and as a substrate for several NAD-dependent signalling enzymes. One such enzyme is CD38 which, alongside synthesising Ca(2+)-releasing second messengers and acting as a cell surface receptor, has also been suggested to play a key role in NAD(+) homeostasis. CD38 is well known as a negative prognostic marker in B-CLL but the role of its enzymatic activity has not been studied in depth to date. We have exploited the HL-60 cell line as a model of inducible CD38 expression, to investigate CD38-mediated regulation intracellular NAD(+) levels and the consequences of changes in NAD(+) levels on cell physiology. Intracellular NAD(+) levels fell with increasing CD38 expression and this was reversed with the CD38 inhibitor, kuromanin confirming the key role of CD38 in NAD(+) homeostasis. We also measured the consequences of CD38 expression during the differentiation on a number of functions linked to NAD(+) and we show that some but not all NAD(+)-dependent processes are significantly affected by the lowered NAD(+) levels. These data suggest that both functional roles of CD38 might be important in the pathogenesis of B-CLL.

Activation of human biliverdin-IXα reductase by urea: generation of kinetically distinct forms during the unfolding pathway

Activation of enzymes by low concentrations of denaturants has been reported for a limited number of enzymes including lipocalin-type prostaglandin D synthase (L-PGDS) and adenylate kinase. During unfolding studies on human biliverdin-IXα reductase it was discovered that the enzyme is activated at low concentrations of urea. Under standard assay conditions the native enzyme displays pronounced substrate inhibition with biliverdin as variable substrate; however in the presence of 3M urea, the substrate inhibition is abolished and the enzyme exhibits Michaelian kinetics. When the initial rate kinetics with NADPH as variable substrate are conducted in 3M urea, the Vmax is increased 11-fold to 1.8μmol/min/mg and the apparent Km for biliverdin increases from 1 to 3μM. We report the existence of two kinetically distinct folded intermediates between the native and unfolded forms. When the period of incubation with urea was varied prior to measuring enzyme activity, the apparent Vmax was shown to decay to half that seen at zero time with a half life of 5.8minutes, while the apparent Km for NADPH remains constant at approximately 5μM. With NADH as cofactor the half life of the activated (A) form was 2.9minutes, and this form decays in 3M urea to a less active (LA) form. The apparent Km for NADH increases from 0.33mM to 2mM for the A and LA forms. These kinetically distinct species are reminiscent of the activity-enhanced and inactive forms of L-PGDS observed in the presence of urea and guanidine hydrochloride.