Figure 6.

Flow cytometry assay to quantify selective autophagy. (A) Diagram of the bicistronic reporter construct. Ribosomes are recruited to both the 5′ end of the bicistronic construct to translate the test fusion protein fused to GFP and at the IRES to translate the stable, nonaggregating reference protein chFP. The test protein is either htt exon 1 with a nonpathogenic poly-Gln repeat htt(Q25) fused to GFP or htt with an expanded poly-Gln tract, htt(Q47)-GFP, which promotes aggregation. (B) The addition of dox for 96 h decreased autophagy in the bicistronic stably expressing cell lines similar to the parental line, m5-7, as measured by the decreased levels of phosphatidylethanolamine-modified LC3 (LC3-II). (C) Western blot (WB) analysis of the accumulation of htt(Q)-GFP after autophagy shutoff. At steady-state (t = 0 h), the levels of both htt(Q25)-GFP and htt(Q47)-GFP were virtually undetectable. After autophagy shutoff, there was a robust increase in the level of htt(Q47)-GFP, as detected by GFP Western blot, whereas there was very little or no detectable increase in htt(Q25)-GFP, GAPDH, LDH, or chFP. (D) Quantitative flow cytometry time course analysis measuring selective autophagy in live cells. The ASI is the ratio of the relative increase in green and red fluorescence, as assessed by two-color flow cytometry, in response to dox. In control experiments, the ASI of two soluble, nonaggregating proteins, htt(Q25)-GFP and chFP, was 1. In contrast, the ASI of the aggregation-prone protein, htt(Q47)-GFP, compared with chFP after autophagy shutoff (+dox; 100 h) was 2. Data represent three independent experiments performed on different days. **, P ≤ 0.01; ***, P ≤ 0.005. (E) Flow cytometry time course analysis of cell death at different time points after autophagy shutoff (+dox). The degree of cell death correlated directly with increased htt(Q47)-GFP levels after autophagy shutoff and was significantly enhanced in cells expressing htt(Q47)-GFP compared with htt(Q25)-GFP (***, P ≤ 0.005). Data represent three independent experiments performed on different days. (F) AQUA using heavy isotope–labeled GFP standard in addition to the routine Ub standards after P2UBA pull-down of soluble and insoluble lysates from htt(Q)-GFP-IRES-chFP bicistronic expressing cells before (−dox) and after autophagy shutoff (+dox). The experimental GFP peptide was only detected in two of the four experiments, and the data shown are representative of these. In the other two experiments, the experimental GFP peptide was below the threshold of detection. (D–F) Error bars indicate SEM. *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.005.