(A) Schematic showing the sensor design, with a cpFP (PDB: 3evp) interposed between the two T-Rex subunits (blue and orange), and a change of fluorescence upon binding of NADH (black). (B) Excitation and emission spectra in the control condition (solid black), after addition of 100 μM NAD⁺ (dash purple), or 100 μM NAD⁺ and 0.2 μM NADH (solid green), normalized to the peak intensity in the control condition. For excitation spectra, emission was measured at 510 ± 5 nm; for emission spectra, excitation was at 400 ± 2.5 nm. (C) Green to red fluorescence ratios at the indicated [NAD⁺] at pH 7.2 plotted against [NADH]. (D) Fluorescence ratios at the indicated [NAD⁺] and pH plotted against R′. (E) Fluorescence ratios at the indicated [NAD⁺] and pH plotted against [NADH] × [H⁺]/[NAD⁺]. Fluorescence ratios (mean ± SEM, n = 3) were normalized to the control condition in the absence of pyridine nucleotides at pH 7.2 at 25°C.

(A) Green to red fluorescence ratios at the indicated pH, plotted against R′ or R (above the plot), with 80 μM NAD⁺ at 25°C. (B) Fluorescence ratios at the indicated pH, temperature, and ATP:ADP ratios (low, 0.3; high, 3.6; with total adenine nucleotides of 4.6 mM), plotted against R′ or R (above the plot), with 80 μM NAD⁺. Fluorescence ratios (mean ± SEM, n = 3) were normalized to the control condition in the absence of pyridine nucleotides at pH 7.2. (C) Kinetics of fluorescence signal upon addition of pyruvate and LDH to Peredox pre-equilibrated with saturating NADH at 25°C or 35°C, normalized to initial and final values.

(A) Left: Confocal green and red fluorescence images of two cultured mouse neuroblastoma Neuro-2a cells expressing Peredox supplied with 10 mM lactate, 10 mM lactate and 0.2 mM pyruvate, or 10 mM pyruvate. Scale bar 20 μm. Middle: Pseudocolored pixel by pixel green to red ratio images. The slight edge effect seen is likely due to the optical z-shift of 1.5 μm between the green and red confocal images. Right: Widefield differential interference contrast (DIC), nuclear staining, and the overlay image. (B) Time course of fluorescence ratios of four Neuro-2a cells perfused with the indicated lactate:pyruvate ratios. (C) Steady state fluorescence responses of Neuro-2a cells plotted against extracellular lactate:pyruvate ratios, with lactate of 10 mM or 20 mM (mean ± SEM, n = 12–15 cells from two independent experiments). (D) Steady state fluorescence responses in (D) plotted against the predicted R′, by assuming the LDH reaction was at equilibrium and a pH of 7.4. Data of purified Peredox proteins were with 80 μM NAD⁺ and 4.6 mM total adenine nucleotides at 35°C. Line fitted with a logistic function using a Hill coefficient of 1.8.

(A) Steady state fluorescence responses of Neuro-2a cells plotted against concentrations of extracellular glucose (mean ± SEM, n = 19 cells from two independent experiments). (B) Steady state fluorescence responses of Neuro-2a cells plotted against total concentrations of extracellular lactate and pyruvate, with a constant lactate:pyruvate ratio of 10, and glucose of 10 mM or 0 mM (mean ± SEM, n = 7 cells from three independent experiments). For the alternate y axis, the predicted NAD⁺:NADH ratio was calculated from purified protein measurements. p < 0.001 (paired t-test) for all conditions in 10 mM vs. 0 mM glucose.

Time course of fluorescence ratios of primary mouse cortical astrocytes or neurons expressing Peredox-NLS and perfused with solutions as indicated (mean ± SEM, n = 4–5 cells from four independent experiments). For the alternate y axis, the predicted NAD⁺:NADH ratio was calculated from purified protein measurements. p < 0.01 (paired t-test) for astrocytes vs. neurons prior to the 10 mM lactate condition.

(A) Heat maps of normalized fluorescence ratios of MCF-10A cells stably expressing Peredox-NLS plotted against time. After 38 minutes of baseline, cells were treated with 1 μM NVP-BEZ235 (left) or DMSO (right). As a control, 20 mM lactate, 20 mM lactate and 1 mM pyruvate, and 20 mM pyruvate and 0.4 mM iodoacetate were applied at red, blue, and black arrows, respectively. For each group, ~700 cells from six fields in two experiments were collected. While the dynamic range was less than the usual 2.5-fold, this was likely due to inadequate control of extracellular lactate and pyruvate concentrations, as these cells were imaged in the 24-well plate formats and solutions were changed without rinsing, as opposed to imaging in a chamber under continuous perfusion of fresh solutions. Fluorescence ratios binned in increments of 0.012. (B) Histograms of normalized fluorescence ratios after one-hour treatment with DMSO (black) or NVP-BEZ235 (blue), binned in increments of 0.027. (C) Histograms of normalized fluorescence ratios from the indicated groups after calibration with 20 mM lactate (L), 20 mM lactate and 1 mM pyruvate (L+P), and 20 mM pyruvate and 0.4 mM iodoacetate (P+I).