246 Chapter 9 was also performed on explanted hearts from patients with arrhythmogenic cardiomyopathy (ARVC) negative for the pathogenic PLN R14del variant, ischemic cardiomyopathy (ICM), and control autopsy hearts (genetic variants are shown in Table I in the Supplement). Tissue sections (3 μm) of formalin-fixed and paraffin-embedded (FFPE) myocardium were stained for PLN and different UPR sensors using immunohistochemistry. Sections were manually stained for PLN (PLN ser-10 Bradilla, A010-10AP, 1:200), BiP (GRP78/BiP, Proteintech, 66574-1-lg, 1:200), PDI (PDI, Proteintech, 66422-1-Ig, 1:200) using antigen retrieval solution pH9 (PLN, BiP) and pH6 (Hsp70, PDI) and incubation overnight at 4 °C. For each heart three hotspot areas were selected: outer-and inner compact myocardium and trabecular myocardium. In each hotspot 250 cells were counted using 400X magnification. Positive cardiomyocytes were averaged as a mean score per patient. The PLN aggregates were examined by their characteristic features as described previously.13 Specifically, the size, shape, and localization were used to identify and characterize the aggresomes. BiP and PDI positive cells were scored according to dark red staining that was localized perinuclear or more diffuse in the cytosol. All cases were examined by two independent observers and guided by a certified pathologist. Protein expression analysis. The hiPSC-CMs were washed with cold PBS and lysed in RIPA buffer (Thermo Fisher) supplemented with 1X Protease and Phosphatase Inhibitor Cocktail (Sigma) for 30 minutes on ice. For BiX experiments, hiPSC-CMs were treated with 0.1 μM BiX or vehicle control (DMSΟ) for 72 hours prior to cell lysis. Protein quantification was performed using the BCA method according to manufacturer’s protocol (Pierce), and an equal amount of protein (10-20 μg) was loaded on a precast 4-20% polyacrylamide gel (Bio-Rad), followed by blotting onto a PVDF membrane using the Trans-blot Turbo system (Bio-Rad). Membranes were blocked in 5% BSA in TBS for 1 hour at room temperature. After blocking, the membranes were incubated with primary antibodies overnight while shaking at 4 ºC. After incubation with anti-mouse or anti-rabbit horseradish-coupled secondary antibody, the bands were visualized with FluroChem E (Protein Simple) imager. The antibodies used for the Western Blot analyses are shown in Table VI in the Supplement. Intracellular Calcium analysis. For ratiometric calcium imaging, dissociated hiPSC-CMs were seeded on Matrigel-coated 35 mm dishes with a 20 mm coverglass bottom (Matek). After 7 days, the cells were treated with BiX (0.1 μM) or DMSO. Seventy-two hours later, the cells were loaded with 5 µM Fura-2AM (Thermo Fisher Scientific) with 0.02% Pluronic F-127 (Thermo Fisher Scientific) in Tyrode’s solution for 10 min at room temperature. Following two washes in Tyrode’s solution, Ca2+ traces were acquired using the MultiCell HTS system (Ionoptix). The cells were electrically paced at 1.0 Hz at 37 °C. Calcium transient analysis was performed using the IonWizard software (Ionoptix).
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