Fasting, but Not Aging, Dramatically Alters the Redox Status of Cysteine Residues on Proteins in Drosophila melanogaster

Abstract

Altering the redox state of cysteine residues on protein surfaces is an important response to environmental challenges. Although aging and fasting alter many redox processes, the role of cysteine residues is uncertain. To address this, we used a redox proteomic technique, oxidative isotope-coded affinity tags (OxICAT), to assess cysteine-residue redox changes in Drosophila melanogaster during aging and fasting. This approach enabled us to simultaneously identify and quantify the redox state of several hundred cysteine residues in vivo. Cysteine residues within young flies had a bimodal distribution with peaks at ∼10% and ∼85% reversibly oxidized. Surprisingly, these cysteine residues did not become more oxidized with age. In contrast, 24 hr of fasting dramatically oxidized cysteine residues that were reduced under fed conditions while also reducing cysteine residues that were initially oxidized. We conclude that fasting, but not aging, dramatically alters cysteine-residue redox status in D. melanogaster.

Graphical abstract

Figure 1

Assessment of Protein Cysteine-Residue Redox State in Flies (A) Schematic showing how exposed cysteine residue can be reversibly oxidized and reduced by GSH/glutaredoxin (Grx) and Trx. (B) OxICAT methodology. Flies are rapidly frozen, and the heads and thoraces are homogenized in 100% TCA to separate solubilized protein from the exoskeleton and then diluted to 20% TCA to precipitate proteins. The protein homogenate is then reacted with the Light ICAT reagent (L-ICAT, red) to label reduced cysteine residues (Pr-SH). After reduction of reversibly oxidized cysteine residues (Pr-SX), these thiols are reacted with the heavy ICAT reagent (H-ICAT, blue). After tryptic digestion and enrichment of labeled peptides, the biotin tags are cleaved off before separation by liquid chromatography and analysis by mass spectrometry, enabling the peptide sequence and the ratio of heavy and light labeled cysteine-containing peptides to be determined simultaneously. (C) A typical chromatogram from control flies (UAS-cat/+). A cysteine peptide oxidized and reduced pair (retention time = 39 min) is highlighted. (D) Chromatograms for the heavy and light labeled peptide eluting at 39 min are shown. The percentage oxidation of that cysteine residue was determined (bar chart). (E) The peptide eluting at 39 min was identified by mass spectrometry as a component of thioredoxin reductase-1 (TrxR1). This gene encodes both a mitochondrial and a shorter cytoplasmic splice variant. The peptide could arise from either isoform but has been numbered as Cys142 from the mitochondrial isoform. See also Figure S1.

Figure 2

OxICAT Analysis of Control Young Female D. melanogaster (A) Ion count for peptides plotted against percentage oxidation of the cysteine residue. The ion count is the log₁₀ intensity of the sum of the heavy and light peptides. Data are the averages over three to five biological replicates. Red cross is cysteine residue 142 from TrxR1 (Figure 1C). (B) Distribution of total cysteine residue oxidation levels. Plotted are the means of the proportion of the total number of peptides containing unique cysteine residues in each 5% quantile of percentage oxidation across five biological replicates (mean ± SEM). Total unique peptides = 491. (C) Plasma membrane Na⁺/K⁺ ATPase. D. melanogaster Na⁺/K⁺ ATPase contains an α subunit and a β subunit with multiple isoforms. The monomeric structure from S. acanthias containing subunit α (green) and subunit β1 (yellow) is 77% and 25% homologous to the α and β2 subunits of D. melanogaster, which were detected by OxICAT. Cysteine residues on the S. acanthias structure present in homologous positions in the D. melanogaster α and β2 subunits are numbered. Cysteines observed by OxICAT are shown in red, and those not detected are blue. Disulfide cysteine partners are also labeled. The table shows the oxidation state of each cysteine in young control untreated flies. (D) Oxidation state of protein cysteine residues in mitochondria. Peptides from Figure 2B that are mitochondrial are plotted as the mean of the proportion of the total number of peptides in each 5% quantile of percentage oxidation across five biological replicates (mean ± SEM). Of 214 proteins identified in Figure 2A, 87 are mitochondrial, corresponding to 214 unique cysteine residues. (E) Comparison of peptides detected by OxICAT with transcript abundance. Whole-fly transcript intensity data were annotated to the head and thorax OxICAT dataset to characterize fly cysteines observable by mass spectrometry. Transcripts are subdivided by abundance into blocks that are √2 of the upper and lower bounds of the block immediately to the left. The percentage of both the observed OxICAT cysteine population (mean transcript abundance = 1,119; n = 849) and the total cysteine population (mean transcript abundance = 134; n = ∼135,000) falling within each transcript abundance block are on the y axis. See also Figure S2.

Figure 3

Reversible Oxidation Levels of Cysteine Residues in Aging D. melanogaster (A) Lifespan of control female flies. Cohorts of flies were taken to analyze protein cysteine residue redox state of young (7 days), middle-aged (28 days), and old (56 days) flies. (B) Distribution of cysteine peptides plotted against their redox states for 7-, 28-, and 56-day-old control flies. Data are means ± SEM. The red curve is for 7-day-old control flies. (C) Oxidation state of cysteine residues present in 56-day-old flies plotted against 7-day-old flies. The dotted line slope = 1, while the continuous line is the least-squares best-fit line to the data. Data from 263 unique peptides identified at least three times under both conditions are plotted. Red symbols (n = 6) indicate low-stringency significance with p < 0.05 assessed by a non-paired, two-tailed Student’s t test. See also Figure S3.

Figure 4

Effect of Exogenous Oxidants on Protein Cysteine-Residue Redox State and Fly Survival (A) Survival of young (7 days) control and catalase-overexpressing flies after exposure to H₂O₂. Arrow indicates when cohorts are collected (24-hr treatment). (B) Distribution of cysteine peptides plotted against redox states of the cysteine residues for control flies after exposure to H₂O₂. Data show the mean of five biological samples where each cysteine residue identified is sorted into corresponding 5% quantiles, and the resulting distributions are averaged (mean ± SEM). Dashed line indicates the untreated control (cf. Figure 2B). (C) Oxidation state of cysteine residues present in at least three biological replicates exposed to H₂O₂ plotted against the same cysteine residues present in at least three biological replicates of controls. Dotted line slope = 1, whereas the continuous line is the best fit to the data. Red symbols (n = 12) indicate cysteine residues significantly different following a non-paired, two-tailed Student’s t test (p < 0.05). Total unique peptides = 452. (D) Survival of young (7 days) control and catalase overexpressing flies after exposure to PQ. Arrow indicates where cohorts are sampled (24-hr treatment). (E) Distribution of cysteine-containing peptides plotted against redox states of the cysteine residues for control flies after exposure to PQ. Means are across five biological replicates of the relative number of cysteine residues within each 5% quantile. The dashed line is the untreated control cohort (cf. Figure 2B). (F) Oxidation state of cysteine residues in control flies exposed to PQ plotted against untreated flies. Dotted line slope = 1, while the continuous line is the line of best fit. Each symbol represents a cysteine residue identified in at least three biological replicates of the untreated as well as the PQ-treated cohort. Red symbols identify cysteine residues (n = 68; p < 0.05; non-paired, two-tailed Student’s t test with low-stringency significance). The blue symbols (n = 33) indicate a high-stringency significance (Benjamini-Hochberg test). Total unique peptides = 452. See also Figure S4.

Figure 5

Effect of Fasting on Protein Cysteine-Residue Oxidation and Survival (A) Survival of young (7 days) control and catalase-overexpressing flies during fasting. Arrow indicates where cohorts are sampled (24-hr treatment). (B) Distribution of cysteine peptides plotted against redox states of the cysteine residues for control flies after 24-hr fasting. Shown is the mean for the relative numbers of cysteine residues in each 5% quantile of the five biological replicates. The dashed line is the curve for the untreated control (cf. Figure 2B) cohort. (C) Oxidation state of cysteine residues present in control flies upon 24-hr fasting compared to untreated cohorts. Dotted line slope = 1, while the continuous line is the best fit to the data. Each symbol represents a cysteine residue identified in at least three biological replicates of both the control untreated as well as the fasted cohort. Red symbols identify cysteine residues (n = 252) with p < 0.05 (non-paired, two-tailed Student’s t test), while blue symbols (n = 200) indicate a high-stringency significance (Benjamini-Hochberg test). Total unique peptides = 387. (D) Distribution of cysteine peptides plotted against redox states of the cysteine residues for catalase-overexpressing flies after 24-hr fasting. Shown is the mean for the relative numbers of cysteine residues in each 5% quantile of the five biological replicates. Dashed line is the distribution for untreated catalase-overexpressing flies on control food. (E) Oxidation state of cysteine residues in catalase-overexpressing flies upon 24 hr fasting against untreated cohorts. Dotted line slope = 1, while the continuous line is the best fit to the data. Each symbol represents a cysteine residue identified in at least three biological replicates of both the untreated and fasted cohorts. Red symbols identify cysteine residues (n = 96) with p < 0.05 (non-paired, two-tailed Student’s t test). Blue symbols (n = 51) indicate high-stringency significance assessed (Benjamini-Hochberg test). Total unique peptides = 440. (F) Oxidation state of cysteine residues present upon 24-hr fasting in catalase-overexpressing flies plotted against control flies. Dotted line slope = 1, while the continuous line is best fit to the data. Each symbol represents a cysteine residue that was identified in at least three biological replicates of both the fasted control and catalase-overexpressing flies. Red symbols identify cysteine residues (n = 13) p < 0.05 (non-paired, two-tailed Student’s t test). Total unique peptides = 601. See also Figures S5–S8.