Real-time redox measurements during endoplasmic reticulum stress reveal interlinked protein folding functions

Abstract

Disruption of protein folding in the endoplasmic reticulum (ER) causes unfolded proteins to accumulate, triggering the unfolded protein response (UPR). UPR outputs in turn decrease ER unfolded proteins to close a negative feedback loop. However, because it is infeasible to directly measure the concentration of unfolded proteins in vivo, cells are generically described as experiencing "ER stress" whenever the UPR is active. Because ER redox potential is optimized for oxidative protein folding, we reasoned that measureable redox changes should accompany unfolded protein accumulation. To test this concept, we employed fluorescent protein reporters to dynamically measure ER redox status and UPR activity in single cells. Using these tools, we show that diverse stressors, both experimental and physiological, compromise ER protein oxidation when UPR-imposed homeostatic control is lost. Using genetic analysis we uncovered redox heterogeneities in isogenic cell populations, and revealed functional interlinks between ER protein folding, modification, and quality control systems.

Figure 1. Characterization of an ER-localized redox-sensitive GFP (eroGFP)

(A) Fluorescence excitation spectra of recombinant fully reduced (red triangles) and oxidized (blue circles) eroGFP. Emission was measured at 520 nm. (B) Excitation spectra of recombinant eroGFP through titration of reduced to oxidized lipoic acid (ratios in inset—total 10 mM). (C) Fraction-reduced eroGFP—data from (B)—as a function of ratios of reduced to oxidized lipoic acid expressed as redox potential values. (D) Representative confocal image of cells expressing eroGFP and Sec61-mCherry. (E) eroGFP ratio (defined as the ratio of fluorescence from excitation at 488 nm vs. 405 nm expressed in log₂ space and normalized to the untreated case—UT) in cells treated with the indicated concentration of DTT or H₂O₂. Data represent means +/− SD from three independent experiments. (F) Northern blot for HAC1 mRNA in cells expressing vector or eroGFP, treated with or without tunicamycin, Tm, (1µg/ml) for 1 hr. HAC1^u denotes unspliced and HAC1ⁱ spliced mRNA respectively. An asterisk signifies the HAC1 5’ exon.

Figure 2. Dynamic monitoring of ER redox status and UPR activity during pharmacologically-induced ER stress

(A) Schematic of composite reporter gene. (B) Schematic showing configuration of flow cytometer laser lines and filters. A syringe-pump periodically (~10 minutes) injected 25µl of sample from cultures growing in bioreactors into a flow cytometer. (C–F) Time courses of eroGFP ratio or UPR-RFP metric histograms in wild-type cells during treatment with DTT (2mM) or Tm (1µg/ml). The eroGFP ratio and UPR-RFP metrics are normalized to wild-type unstressed cells. Color represents percentage of cells at a given metric value and time point. Dashed gray line signifies time of DTT or Tm addition. (G) Non-reducing SDS-PAGE and immunoblot (anti-GFP) of AMS-treated protein extracts from untreated cells (lane 1), cells treated with 10mM DTT for 30 min (lane 2), or 2µg/ml Tm for 3 hrs (lane 3).

Figure 3. Dynamic monitoring of ER redox status and UPR activity in ER oxidative folding, quality control, and UPR signaling mutants during pharmacologically-induced ER stress

(A–L) Median eroGFP ratio (green circles) and UPR-RFP metric (red x-marks) during treatment with DTT (2mM) or Tm (1µg/ml) for wild-type and indicated mutants. Values are normalized to wild-type unstressed cells. Dashed gray line signifies time at which stressor was added.

Figure 4. Dynamic monitoring of ER redox status and UPR activity during inositol starvation

(A–F) Time courses of eroGFP ratio or UPR-RFP metric histograms in wild-type, ire1Δ, or hac1Δ cells starved for inositol at time, t=0. (G) Growth curves for the time courses of (A–F). (H–J) Snapshots of hac1Δ cell populations for eroGFP ratio vs. Cy5 signal at three time points. Mothers (m) have a high Cy5 signal (log₂ Cy5 > 10.5) while daughters (d) have a low Cy5 signal (log₂ Cy5 < 10.5). Time courses of eroGFP histograms for hac1Δ mothers (K) and daughters (L). The three time points of (H–J) are indicated in (L). Color represents percentage of cells at a given metric value and time point.

Figure 5. Dynamic monitoring of ER redox status and UPR activity during overexpression of secretory proteins

(A–R) Time courses of eroGFP ratio or UPR-RFP metric histograms in wild-type (WT) or hrd1Δ cells expressing CPY, CPY*, CPY*-0000, CPY*-1cys, or CPY*-Cysless under control of the copper-inducible CUP1 promoter. Color represents percentage of cells at a given metric value and time point. The dashed gray line signifies time of CuSO₄ (200µM or 500µM) addition.

Figure 6. Analysis of ER oxidative folding, quality control, and UPR signaling mutants under stress

(A, B) Median trajectories of wild-type or mutant cells (treated with DTT or Tm) through UPR-RFP metric and eroGFP ratio two-dimensional space. The trajectories begin at the last data point before stress addition, and conclude when the time course ends (signified by the arrow). Consecutive white circles within each trajectory line are separated by 30 minutes. Color denotes each mutant. (C, D) Growth curves for wild-type cells treated with DTT (2mM) or Tm (1µg/ml). (E, F) The first derivative of median UPR-RFP with respect to time, d(UPR-RFP)/dt, vs. time for wild-type and mutants treated with DTT or Tm. (G) Integrated eroGFP ratios over the time courses for each mutant normalized to wild-type and represented as heat maps ranging from blue (negative values) to yellow (positive values). Data for (A–G) are from Figure 3. (H) Histogram of ire1Δ cells expressing eroGFP, reconstituted with a 1NM-PP1-sensitized IRE1 allele, and treated with DMSO (black line), 30µM 1NM-PP1 (green line), DMSO and 1µg/ml Tm (red line), and 30µM 1NM-PP1 and 1µg/ml Tm (yellow line). Inset shows median eroGFP ratio change for each treatment. Error bars are SD of three independent experiments.

Figure 7. Conceptual model of ER stress and UPR-mediated compensation

ER oxidative folding stress, as measured by increased eroGFP ratios, can be provoked by various perturbations to ER protein folding, modification, and quality control systems. In addition, increased protein secretion and nutrient (inositol) deprivation cause ER oxidative folding stress in some cells. UPR activity, as measured by UPR-RFP, provides information on the strength of compensatory mechanisms. Triangles indicate that each variable is a continuum. Four cell states are depicted at extremes of each continuum, with positions of mutants indicated across the two variables. See text for further details.