Circadian tracking of nicotinamide cofactor levels in an immortalized suprachiasmatic nucleus cell line

Emerging models for the molecular basis of mammalian circadian timing

Mammalian circadian timekeeping arises from a transcription-based feedback loop driven by a set of dedicated clock proteins. At its core, the heterodimeric transcription factor CLOCK:BMAL1 activates expression of Period, Cryptochrome, and Rev-Erb genes, which feed back to repress transcription and create oscillations in gene expression that confer circadian timing cues to cellular processes. The formation of different clock protein complexes throughout this transcriptional cycle helps to establish the intrinsic ∼24 h periodicity of the clock; however, current models of circadian timekeeping lack the explanatory power to fully describe this process. Recent studies confirm the presence of at least three distinct regulatory complexes: a transcriptionally active state comprising the CLOCK:BMAL1 heterodimer with its coactivator CBP/p300, an early repressive state containing PER:CRY complexes, and a late repressive state marked by a poised but inactive, DNA-bound CLOCK:BMAL1:CRY1 complex. In this review, we analyze high-resolution structures of core circadian transcriptional regulators and integrate biochemical data to suggest how remodeling of clock protein complexes may be achieved throughout the 24 h cycle. Defining these detailed mechanisms will provide a foundation for understanding the molecular basis of circadian timing and help to establish new platforms for the discovery of therapeutics to manipulate the clock.

Circadian rhythm of metabolic oscillation in suprachiasmatic nucleus depends on the mitochondrial oxidation state, reflected by cytochrome C oxidase and lactate dehydrogenase

Metabolic activity in the suprachiasmatic nucleus (SCN), a center of biological rhythm, is higher during the daytime than at night. The rhythmic oscillation in the SCN is feedback controlled by the Clock/Bmal1 heterodimer binding to the E-box in target genes (e.g., Arg-vasopressin). Similar transcriptional regulation by Npas2/Bmal1 heterodimer formation operates in the brain, which is dependent on the redox state (i.e., NAD/NADH). To clarify the metabolic function of SCN in relation to the redox state and glycolysis levels, we measured glucose, lactate dehydrogenase (LDH), LDH mRNA, and cytochrome C oxidase, energy-producing biochemical materials from mitochondria/cytosol, in rats kept under a light-dark cycle. Mitochondrial cytochrome C oxidase activity, measured by the changes in absorption at 550 nm, was higher during the light period than during the dark period. Glucose concentration was higher during the light period. In contrast, LDH and its coding mRNA were higher during the dark period. Mitochondrial aggregation, which is reflected by mitochondrial membrane potential, indexed by JC-1 fluorescence, was higher during the light period. The results indicate that the glycolysis energy pathway in the SCN, which exhits higher metabolic activity during the day than at night, might be involved in the generation of circadian rhythm.

Circadian rhythm of enolase in suprachiasmatic nucleus depends on mitochondrial function

Metabolic activity in the suprachiasmatic nucleus (SCN), a center of biological rhythm, is higher during the daytime than at night. The rhythmic oscillation in the SCN is feedback controlled by the CLOCK/BMAL1 heterodimer binding to the E-box in target genes (e.g., Arg- vasopressin). Similar transcriptional regulation by NPAS2/BMAL1 heterodimer formation operates in the brain, which depends on the redox state (i.e., NAD/NADH). To clarify the metabolic function of SCN in relation to the redox state, two-dimensional electrophoresis was carried out on the mitochondrial fraction of SCN, obtained from rats kept under a light:dark cycle and constant under dim light. The electrophoretic pattern with TOF-mass spectrometry analysis revealed that enolase catalyzes the interconversion of 2-phosphoglycerate and phosphoenolpyruvate. The enolase activity, coupled with lactate dehydrogenase, was higher during the light period than that in the dark. However, enolase mRNA, analyzed by RT-PCR, showed higher levels during the dark period than in the light. The clock gene products Per2, Bmal1, Rev-erbα, and AVP mRNA in the mitochondrial fraction of SCN developed a circadian rhythm showing almost the same peak time as that in whole SCN. These mRNA rhythms ran free except for that of Rev-erbα mRNA. The results indicate that, in the glycolysis-related energy pathway, enolase might be involved in higher metabolic activity during the day than at night, at least in part.

Interactions between light, mealtime and calorie restriction to control daily timing in mammals

Daily variations in behaviour and physiology are controlled by a circadian timing system consisting of a network of oscillatory structures. In mammals, a master clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, adjusts timing of other self-sustained oscillators in the brain and peripheral organs. Synchronisation to external cues is mainly achieved by ambient light, which resets the SCN clock. Other environmental factors, in particular food availability and time of feeding, also influence internal timing. Timed feeding can reset the phase of the peripheral oscillators whilst having almost no effect in shifting the phase of the SCN clockwork when animals are exposed (synchronised) to a light-dark cycle. Food deprivation and calorie restriction lead not only to loss of body mass (>15%) and increased motor activity, but also affect the timing of daily activity, nocturnal animals becoming partially diurnal (i.e. they are active during their usual sleep period). This change in behavioural timing is due in part to the fact that metabolic cues associated with calorie restriction affect the SCN clock and its synchronisation to light.

Subcellular and tissue localization of NAD kinases from Arabidopsis: compartmentalization of de novo NADP biosynthesis

The de novo biosynthesis of the triphosphopyridine NADP is catalyzed solely by the ubiquitous NAD kinase family. The Arabidopsis (Arabidopsis thaliana) genome contains two genes encoding NAD+ kinases (NADKs), annotated as NADK1, NADK2, and one gene encoding a NADH kinase, NADK3, the latter isoform preferring NADH as a substrate. Here, we examined the tissue-specific and developmental expression patterns of the three NADKs using transgenic plants stably transformed with NADK promoter::glucuronidase (GUS) reporter gene constructs. We observed distinct spatial and temporal patterns of GUS activity among the NADK::GUS plants. All three NADK::GUS transgenes were expressed in reproductive tissue, whereas NADK1::GUS activity was found mainly in the roots, NADK2::GUS in leaves, and NADK3::GUS was restricted primarily to leaf vasculature and lateral root primordia. We also examined the subcellular distribution of the three NADK isoforms using NADK-green fluorescent protein (GFP) fusion proteins expressed transiently in Arabidopsis suspension-cultured cells. NADK1 and NADK2 were found to be localized to the cytosol and plastid stroma, respectively, consistent with previous work, whereas NADK3 localized to the peroxisomal matrix via a novel type 1 peroxisomal targeting signal. The specific subcellular and tissue distribution profiles among the three NADK isoforms and their possible non-overlapping roles in NADP(H) biosynthesis in plant cells are discussed.

Neurogenetics of food anticipation

Circadian clocks enable the organisms to anticipate predictable cycling events in the environment. The mechanisms of the main circadian clock, localized in the suprachiasmatic nuclei of the hypothalamus, involve intracellular autoregulatory transcriptional loops of specific genes, called clock genes. In the suprachiasmatic clock, circadian oscillations of clock genes are primarily reset by light, thus allowing the organisms to be in phase with the light-dark cycle. Another circadian timing system is dedicated to preparing the organisms for the ongoing meal or food availability: the so-called food-entrainable system, characterized by food-anticipatory processes depending on a circadian clock whose location in the brain is not yet identified with certainty. Here we review the current knowledge on food anticipation in mice lacking clock genes or feeding-related genes. The food-entrainable clockwork in the brain is currently thought to be made of transcriptional loops partly divergent from those described in the light-entrainable suprachiasmatic nuclei. Possible confounding effects associated with behavioral screening of meal anticipation in mutant mice are also discussed.

Hypothalamic cell lines to investigate neuroendocrine control mechanisms

The hypothalamus is the control center for most physiological processes; yet has been difficult to study due to the inherent heterogeneity of this brain region. For this reason, researchers have turned towards cell models. Primary hypothalamic cultures are difficult to maintain, are heterogeneous neuronal and glial cell populations and often contain a minimal number of viable peptide-secreting neurons. In contrast, immortalized, clonal cell lines represent an unlimited, homogeneous population of neurons that can be manipulated using a number of elegant molecular techniques. Cell line studies and in vivo experimentation are complementary and together provide a powerful tool to drive scientific discovery. This review focuses on three key neuroendocrine systems: energy homeostasis, reproduction, and circadian rhythms; and the use of hypothalamic cell lines to dissect the complex pathways utilized by individual neurons in these systems.

Separation of flavins and nicotinamide cofactors in Chinese hamster ovary cells by capillary electrophoresis

Simultaneous extraction, separation and quantitation of reduced nicotinamide adenine dinucleotide (NADH), reduced nicotinamide adenine dinucleotide phosphate (NADPH), flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) in Chinese Hamster Ovary (CHO) cells were investigated. The separation of flavins and nicotinamide cofactors was performed by capillary electrophoresis with laser-induced fluorescence detection at the excitation wavelength of 325 nm. The separation protocol was established by investigating the excitation wavelength, high voltage and effects of buffer nature, pH and concentration. All endogenous fluorophores riboflavin, FAD, FMN, NADH and NADPH show wide linear range of quantitation. The limits of detection for the five compounds ranged from 4.5 to 23 nM. Extraction conditions were optimized for high-efficiency recovery of all endogenous fluorophores from CHO cells. To account for the complex matrix of cell extracts, a standard addition method was used to quantify FAD, FMN, NADH and NADPH in CHO cells. The quantitative results should be useful to reveal the metabolic status of cells. The protocols for extraction, separation and quantitation are readily adaptable to normal and cancer cell lines for the analysis of endogenous fluorophores.

Combination of NADPH and copper ions generates proteinase K-resistant aggregates from recombinant prion protein

Recent studies have demonstrated that the octapeptide repeats of the N-terminal region of prion protein may be responsible for de novo generation of infectious prions in the absence of template. Here we demonstrate that PrP-(23-98), an N-terminal portion of PrP, is converted to aggregates upon incubation with NADPH and copper ions. Other pyridine nucleotides possessing a phosphate group on the adenine-linked ribose moiety (the reduced form of nicotinamide adenine dinucleotide 3'-phosphate, nicotinic acid adenine dinucleotide phosphate, and NADP) were also effective in promoting aggregation, but NADH and NAD had no effect. The aggregation was attenuated by the metal chelator EDTA or by modification of histidyl residues with diethyl pyrocarbonate. The aggregates are amyloid-like as judged by the binding of thioflavin T, a fluorescent probe for amyloid, but do not exhibit fibrillar structures according to electron micrography. Interestingly the aggregates were resistant to proteinase K digestion. Likewise NADPH and zinc ions caused aggregation of PrP-(23-98), but the resulting aggregates were susceptible to degradation by proteinase K. Upon incubation with NADPH and copper ions, the full-length molecule PrP-(23-231) also formed proteinase K-resistant amyloid-like aggregates. Because it is possible that PrP, NADPH, and copper ions could associate in certain tissues, the aggregation observed in this study may be involved in prion initiation especially in the nonfamilial types of prion diseases.

Quantitation of nicotinamide and serotonin derivatives and detection of flavins in neuronal extracts using capillary electrophoresis with multiphoton-excited fluorescence

Capillary electrophoresis (CE) with multiphoton-excited fluorescence detection (CE-MPE) allows low-background analysis of spectrally distinct fluorophores using a single long-wavelength laser. Extracts were prepared from immortalized rat raphe nuclei neurons, and were analyzed by CE-MPE. Native fluorescence was detected from reduced nicotinamide adenine dinucleotide (NADH) and its phosphorylated form (NADPH), flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN), riboflavin, serotonin, and 5-hydroxytryptophan (5HTrp). Quantitation of exogenous serotonin (taken up by cells) and endogenous NADH and 5HTrp was possible using internal standards or standard addition. This system should be useful to study monamine oxidase inhibitors (MAOIs) and selective serotonin reuptake inhibitors (SSRIs).