Tracking variations in nicotinamide cofactors extracted from cultured cells using capillary electrophoresis with multiphoton excitation of fluorescence

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

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

Quantification of riboflavin, flavin mononucleotide, and flavin adenine dinucleotide in mammalian model cells by CE with LED-induced fluorescence detection

Cultured mammalian cells essential are model systems in basic biology research, production platforms of proteins for medical use, and testbeds in synthetic biology. Flavin cofactors, in particular flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), are critical for cellular redox reactions and sense light in naturally occurring photoreceptors and optogenetic tools. Here, we quantified flavin contents of commonly used mammalian cell lines. We first compared three procedures for extraction of free and noncovalently protein-bound flavins and verified extraction using fluorescence spectroscopy. For separation, two CE methods with different BGEs were established, and detection was performed by LED-induced fluorescence with limit of detections (LODs 0.5-3.8 nM). We found that riboflavin (RF), FMN, and FAD contents varied significantly between cell lines. RF (3.1-14 amol/cell) and FAD (2.2-17.0 amol/cell) were the predominant flavins, while FMN (0.46-3.4 amol/cell) was found at markedly lower levels. Observed flavin contents agree with those previously extracted from mammalian tissues, yet reduced forms of RF were detected that were not described previously. Quantification of flavins in mammalian cell lines will allow a better understanding of cellular redox reactions and optogenetic tools.

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.

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).

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

Nicotinamide adenine dinucleotides can exhibit a daily rhythm in plants and regulate the activity of mammalian clock-like transcription factors in vitro. Because one such redox-sensitive transcription factor is present in the master circadian clock of the brain (the suprachiasmatic nuclei, SCN) and the SCN exhibits a characteristic daily rhythm in glucose usage, nicotinamide cofactors might be expected to influence, exhibit, and/or reflect biological rhythms in SCN cells. Therefore, cofactors were extracted from a model SCN cell line at 3 h intervals over 1-2 day periods and samples were analyzed by capillary electrophoresis with multiphoton excitation of fluorescence. Natively fluorescent reduced cofactors (nicotinamide adenine dinucleotide, NADH, and its phosphorylated form, NADPH) were assayed directly, and nonfluorescent oxidized cofactors (nicotinamide adenine dinucleotide, NAD, and its phosphorylated form, NADP) were enzymatically reduced to their fluorescent counterparts before analysis. In the first day after a synchronizing pulse of fetal bovine serum, a dramatic upregulation in cellular NADH content was observed, consistent with a response to serum insulin; this was accompanied by a smaller decrease in NADPH redox state, which may indicate scavenging of reactive oxygen species generated by increased cellular metabolism. However, when cells were investigated after these early phenomena had recovered or stabilized, no circadian NAD(P)(H) rhythms were observed. During these studies, the NADH/NAD(H) concentration ratio in SCN2.2 cells (0.13+/-0.03) was not high enough to activate clock-like transcription factors. Although the NADPH/NADP(H) concentration ratio was more appropriate (0.8+/-0.1), the intracellular NADPH concentration was < or = 0.7 mM, far too low for half-maximal DNA binding of clock-like transcription factors in vitro. Moreover, these concentration and ratio values represent cellular averages, and free cofactors should be much lower in the cell nucleus. Our data show that SCN2.2 cells maintain nearly constant circadian NAD(P)(H) levels, and that the previously reported in vitro relationship between clock-like transcription factors and NAD(P)(H) does not appear to be biologically relevant.