Measurement of H2O2 within living Drosophila during aging using a ratiometric mass spectrometry probe targeted to the mitochondrial matrix

Abstract

Hydrogen peroxide (H(2)O(2)) is central to mitochondrial oxidative damage and redox signaling, but its roles are poorly understood due to the difficulty of measuring mitochondrial H(2)O(2) in vivo. Here we report a ratiometric mass spectrometry probe approach to assess mitochondrial matrix H(2)O(2) levels in vivo. The probe, MitoB, comprises a triphenylphosphonium (TPP) cation driving its accumulation within mitochondria, conjugated to an arylboronic acid that reacts with H(2)O(2) to form a phenol, MitoP. Quantifying the MitoP/MitoB ratio by liquid chromatography-tandem mass spectrometry enabled measurement of a weighted average of mitochondrial H(2)O(2) that predominantly reports on thoracic muscle mitochondria within living flies. There was an increase in mitochondrial H(2)O(2) with age in flies, which was not coordinately altered by interventions that modulated life span. Our findings provide approaches to investigate mitochondrial ROS in vivo and suggest that while an increase in overall mitochondrial H(2)O(2) correlates with aging, it may not be causative.

Figure 1

Rationale for the Development of a Mitochondrial H₂O₂ Probe (A) Structure of mitochondria-targeted boronic acid, MitoB, and its phenol product, MitoP, formed by reaction with HOO^- (the conjugate base of H₂O₂). (B) MitoB uptake into mitochondria within tissues. MitoB is first taken up into cells driven by the plasma membrane potential (Δψ_p), and then accumulates inside the mitochondrial matrix driven by the mitochondrial membrane potential (Δψ_m), resulting in a several-hundred-fold accumulation, as predicted by the Nernst equation. Within mitochondria, MitoB is converted to MitoP in response to H₂O₂. The reaction with H₂O₂ is further enhanced by the higher pH of the matrix relative to the cytoplasm, thus the rate of MitoB to MitoP conversion is an indicator of the mitochondrial H₂O₂ concentration. To measure the MitoP/MitoB ratio, the tissue is spiked with deuterated ISs, extracted, and then quantified by LC-MS/MS.

Figure 2

Reaction of MitoB with H₂O₂ to Form MitoP (A) Oxidation of MitoB to MitoP assessed by RP-HPLC. MitoB (100 μM) was incubated at 37°C in KCl medium (pH 8.0) with no additions or with 100 μM H₂O₂ and then analyzed by RP-HPLC. A mixture of MitoB and MitoP standards (50 μM each) was also analyzed. (B) Absorbance spectra of MitoB and MitoP (100 μM) in KCl medium, showing a large difference in absorption at 285 nm due to the phenol moiety. (C) Progress curves for the reaction of MitoB with H₂O₂. The conversion of MitoB (100 μM) to MitoP by reaction with H₂O₂ is measured at 285 nm in KCl medium at 37°C (pH 8.0, unless otherwise indicated). (D) Reaction of MitoB with various peroxides. The conversion rate of MitoB (100 μM) to MitoP by reaction with 500 μM of either H₂O₂, tert-butylhydroperoxide (tBHP) or cumene hydroperoxide (cumeneHP) was measured at 285 nm in KCl medium at 37°C, pH 8.0. Data are means ± SD, n = 3.

Figure 3

LC-MS/MS Analysis of MitoB and MitoP (A) Typical chromatograms showing the m/z transitions measured simultaneously for 20 pmol standards of d₁₅-MitoB, MitoB, d₁₅-MitoP, or MitoP. Each trace is normalized to the highest total ion count peak within that trace, hence baseline noise is magnified in background relative to sample traces. (B and C) Example standard curves for MitoB and MitoP detection by LC-MS/MS. In order to quantify the LC-MS/MS data, standard curves were prepared for each experiment by spiking the appropriate biological material with a range of MitoB or MitoP concentrations and the deuterated ISs, and extracting in parallel with the samples. The results presented are from C2C12 cell pellets. Each point is the mean ± range of duplicate measurements.

Figure 4

Uptake of MitoB into Energized Mitochondria and Cells, and Detection of Oxidative Stress (A) Uptake of MitoB by energized mitochondria. An electrode selective for the TPP cation was inserted in a stirred 3 ml chamber containing KCl medium (pH 7.2) at 30°C supplemented with rotenone (4 μg/ml) and nigericin (100 nM), and calibrated by five sequential additions of 1 μM MitoB (arrowheads). Mitochondria (2 mg protein/ml) were then added, followed by the respiratory substrate succinate (10 mM) and finally FCCP (500 nM). (B) Uptake of MitoB into cells. Jurkat cells (3 × 10⁶/ml) were incubated with 5 μM MitoB, and the amount of MitoB in the cell pellets was determined by LC-MS/MS. Data are means ± range of two independent experiments. (Inset) Jurkat cells (3 × 10⁶/ml) were incubated in PBS supplemented with 1 mM pyruvate and 5 μM MitoB ± 500 nM FCCP for 1 hr. Samples (950 μl) of the cell suspension were then pelleted and the MitoB content determined by LC-MS/MS. Data are means ± SD of three replicates. ^∗∗p < 0.01 by a two-tailed Student's t test. (C) Oxidation of MitoB to MitoP within mitochondria exposed to oxidative stress. (Left) Rat liver and heart mitochondria (1 mg protein/ml) were incubated for 10 min in KCl medium (pH 7.2) at 37°C with 50 U/ml catalase and the indicated concentration of menadione. The MitoP/MitoB ratio in the mitochondrial pellet was determined by LC-MS/MS. (Right) Liver mitochondria were incubated as described above with 500 μM menadione in the presence of nigericin (100 nM). Data are means ± range of duplicate samples. (D) MitoP/MitoB ratio in the supernatant of cells exposed to oxidative stress. C2C12 cells were grown in 6-well plates seeded at 2 × 10⁵/well and incubated overnight. The cells were then supplied with fresh medium containing 5 μM MitoB, 50 U/ml catalase, and the indicated concentration of menadione and incubated for 6 hr. Controls without cells (medium only) and cells with the uncoupler FCCP (5 μM) were performed in parallel. Data are means ± SEM of three replicates (except for the medium only control, which are means ± range), and were corrected for the t = 0 value.

Figure 5

Detection of Mitochondrial H₂O₂ within Living Flies (A) Clearance of MitoB and MitoP from flies following injection. Groups of ten females were each injected with either MitoB or MitoP (∼75 pmol/fly). At the indicated times after injection, groups of flies were snap frozen and stored at -80°C until processing for LC-MS/MS analysis. Data are means ± SD of three replicates. The inset shows a plot of the natural logarithm of the compounds concentrations (as a percent of the amount at t = 0) over time to determine the first order rates of clearance for MitoB and MitoP. The slope of the linear trend line was -0.3556 hr⁻¹ (R² = 0.967). (B) Ratio of MitoP/MitoB in flies over time. Flies were injected with MitoB as in (A) and then maintained under normal aerobic conditions for the indicated time before measuring the MitoP/MitoB ratios. Data are means ± range of duplicate samples, and were corrected for the t = 0 value. (C) Effect of anoxia on the MitoP/MitoB ratio in flies. Flies that had been injected with MitoB as in (A) were incubated under anoxia (100% argon) or normoxia (21% O₂) for 3 hr, and then the MitoP/MitoB ratio was determined. Data are means ± SEM of five replicates. (D) Effect of O₂ concentration on the MitoP/MitoB ratio in flies. Flies that had been injected with MitoB as in (A) were incubated under normoxia (21% O₂) or hyperoxia (40 or 65% O₂) for 6 hr. Then the MitoP/MitoB ratio was determined. Data are means ± SEM of three to five replicates. (E) Effect of l-NAME on the MitoP/MitoB ratio within flies. Wild-type flies were pretreated with food containing water, 100 mM l-NAME, or 100 mM d-NAME for 48 hr prior to MitoB injection, and then maintained on that food for a further 3 hr. Data are means ± SD of four to five replicates. (F) Effect of paraquat on the MitoP/MitoB ratio within flies. (Left) Wild-type flies were pretreated with food ± 20 mM paraquat for 6 hr prior to MitoB injection, then maintained on that food for a further 6 hr. Data are means ± SEM of four to five replicates. (Right) Ubiquitous catalase overexpression protects against PQ toxicity. Control (UAS-cat/+) and transgenic flies overexpressing catalase (UAS-cat > daGAL4) were pretreated with food ± 20 mM paraquat for 6 hr prior to MitoB injection, then maintained on that food for a further 3 hr. Data are means ± SEM of four to five replicates. Statistical significance was determined by a two-tailed Student's t test: ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001.

Figure 6

Using MitoB to Assess the Role of Mitochondrial ROS In Vivo in Drosophila (A) Typical life span curves for wild-type Drosophila under standard conditions (n = 200 per sex). (B) Effect of age on the MitoP/MitoB ratio in wild-type flies. Flies injected with MitoB were incubated for 6 hr (females) or 3 hr (males), then extracted and analyzed by LC-MS/MS. Data are means ± SEM of three to five samples. Note that due to the different incubation times in these experiments (done for technical reasons, because males are smaller and therefore injected with proportionately less MitoB), the ratio values should not be compared directly between the sexes. (C) DR affects fecundity and life span in female Drosophila. Data for wild-type Dahomey flies were adapted from (Grandison et al., 2009). DR was achieved by diluting the concentration of yeast provided in the food, at 0.5-, 1.0-, and 2.0-fold the standard amount. The fecundity index is a measure of cumulative egg-laying per female throughout life, which decreases under DR conditions, whereas median life span increases. (D) Effect of DR on the MitoP/MitoB ratio in wild-type female flies. Flies were maintained on the indicated diets from 2 days. Flies were injected with MitoB, incubated for 6 hr, then extracted and analyzed by LC-MS/MS. Data are means ± SEM of four to five samples. (E) Effect of activity on the MitoP/MitoB ratio in wild-type female Drosophila. Flies were maintained under standard “active” conditions, which allows flying and jumping, or under “inactive” conditions from day 2, where movement is spatially restricted and only walking is possible. Flies injected with MitoB were incubated for 3 hr, then extracted and analyzed by LC-MS/MS. Data are means ± SEM of four to five samples. Statistical significance was determined by a two-tailed Student's t test: n/s p > 0.05, ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001.