roGFP2-based biosensor analyses

The roGFP2-based biosensors roGFP2-Orp1 and Grx1-roGFP2 were used in living iPSC-CMs to assess the level of ROS (Figure 20, 21 and 22). Since iPSC-CMs are hard to transfect via lipofection with vectors as big as the biosensors, a lentiviral approach was used to generate biosensor expressing iPSC-CMs. For this purpose, HEK-293T cells were transfected with Addgenes second generation lentiviral system including the envelope and packaging plasmids pMD2.G and psPAX2. The virus-containing cell supernatant was used for transduction of iPSC-CMs. Observed visually, a sufficient amount of iPSC-CMs usually expressed roGFP2-Orp1 or Grx1-roGFP2 72 hours after transduction start (Figure 20 A, B). Also, biosensor-expressing iPSC-CMs still showed beating activity and no differences compared to non-transduced cells were observed. However, when the cells were cultured for about one week after transduction, usually more transduced cells died than non-transduced. To avoid this, the cells were used for measurements about five days after start of transduction. When excited at 405 and 485 nm and measured at 515 nm, the biosensor containing cells exhibited stronger fluorescence than non-transduced cells (Figure 20 C, D). As expected, addition of the oxidant diamide during measurement resulted in a decline of the signal at 485 nm excitation and a rise at 405 nm. Addition of the reductant DTT caused the opposite effect. These observations confirm the functionality of roGFP2-Orp1 and Grx1-roGFP2 in iPSC-CMs (Figure 20 C, D).

roGFP2-based biosensor measurements may be used for relative comparisons but do not give absolute values. To enable comparability of different iPSC-CM differentiation experiments, the degree of roGFP2 oxidation (OxD) had to be determined for every measurement. For this reason, conditions of diamide and DTT treatment had to be determined where roGFP2 would be completely oxidized (OxD = 1) or reduced (OxD = 0). The 405/485nm ratio of both roGFP2-Orp1 and Grx1-roGFP2 increased after treatment with 1, 2.5, 5 and 10 µM diamide, but did not further increase with higher concentrations, indicating that a plateau was reached (Figure 21 A, B). Treatment of roGFP2-Orp1-expressing iPSC-CMs with 0.05, 0.5, 5, 50 and 90 mM DTT only caused a mild decrease of the 405/485nm ratio (Figure 21 C). However, pretreatment of Grx1-roGFP2 expressing iPSC-CMs with H2O2

revealed the potential of DTT to reduce the 405/485nm ratio: Adding 10 mM DTT to iPSC-CMs pretreated with up to 20 µM H2O2 caused the ratio to drop to the level of untreated cells (Figure 21 D). For these reasons, the concentrations of 500 µM diamide and 10 mM DTT have been chosen to induce complete oxidation and complete reduction of roGFP2 in following experiments. Using the fluorescence intensities of fully oxidized and fully reduced conditions, OxD of roGFP2 was calculated with equation 2 (see chapter 3.2.8).

Figure 20: roGFP2-based biosensors roGFP2-Orp1 and Grx1-roGFP2 in iPSC-CMs. A, B: roGFP2-Orp1 expression in iPSC-CMs (representative for Grx1-roGFP2). C, D: Detection of Grx-roGFP2 fluorescence intensity at 510 nm upon excitation with 405 and 485 nm. Oxidation (diamide) and reduction (DTT) causes respective dynamic changes in fluorescence intensity. Empty: non-transduced iPSC-CMs.

Depicted measurements are representative for roGFP2-Orp1.

roGFP2-Orp1 expressing Ctrl- and ACT-iPSC-CMs showed a comparable level of roGFP2 oxidation without DOX treatment (Figure 22 A). DOX treatment caused a dose-dependent increase in oxidation in both groups of up to 2.5-fold, which was significant for DOX concentrations of 0.75, 1 and 5 µM (Figure 22 A, B).

Figure 21: Determination of diamide and DTT concentrations for the induction of complete

oxidation and reduction of roGFP2-Orp1 and Grx1-roGFP2 in iPSC-CMs. A: roGFP2-Orp1 oxidation in iPSC-CMs upon diamide application. B: Grx1-roGFP2 oxidation in iPSC-CMs upon diamide application.

C: roGFP2-Orp1 reduction in iPSC-CMs upon DTT application. D: Grx1-roGFP2 reduction in iPSC-CMs pretreated with H2O2 upon DTT application.

Analyses of Grx1-roGFP2 redox state revealed a higher oxidation in ACT-iPSC-CMs compared to the Ctrl-iPSC-CMs without DOX as well as after treatment with all tested DOX concentrations (Figure 22 C). Upon treatment with 0.5 µM DOX, OxD of Grx1-roGFP2 was significantly higher in ACT-iPSC-CMs, compared to Ctrl-iPSC-CMs. A mild decrease of oxidation was observed in the control group after treatment with low DOX concentrations of 0.25 and 0.5 µM. Overall, no clear DOX dose-dependency was observed in iPSC-CMs of both groups. Relative changes induced by DOX were comparable between Ctrl- and ACT-iPSC-CMs.

5 µM DOX treatment caused a 1.4-fold increase in both groups (Figure 22 D). EGSH was calculated and is shown in Table 4. Treatment with 5 µM DOX caused an oxidation of about 6 mV. Taken together, oxidation of Grx1-roGFP2 was greater in iPSC-CMs of ACT patients compared to controls.

Figure 22: Analysis of ROS in iPSC-CMs using genetically encoded biosensors roGFP2-Orp1 and Grx1-roGFP2. A: H2O2 production in iPSC-CMs of both groups represented by roGFP2-Orp1 OxD.

B: Relative DOX-induced change in roGFP2-Orp1 OxD. Sample number: 5 Ctrl-iPSC-CM differentiations, 8 ACT-iPSC-CM differentiations. C: Grx1-roGFP2 oxidation in iPSC-CMs of both groups. D: Relative DOX-induced change in Grx1-roGFP2 OxD. Sample number: 5 Ctrl-iPSC-CM differentiations, 6 ACT-iPSC-CM differentiations. Mean + SEM. * p < 0.05, ** p < 0.01, *** p <

0.001.

differentiations, 6 ACT-iPSC-CM differentiations.

Ca²⁺ belongs to the most important second messengers in CMs since it is the basis for contraction and relaxation, determines electrophysiological properties of the cell and is involved in gene transcription (see chapter 1.1.1.3). Cytosolic Ca²⁺ transients were visualized by loading iPSC-CMs with the Ca²⁺ sensitive dye Fluo-4 and the use of confocal laser scanning microscopy. Thereby, the Ca²⁺ transient rise time, the relative amplitude and the decay time could be assessed (Figure 23). Isoprenaline, which activates β-adrenergic signaling, was used to additionally stimulate the iPSC-CMs.

The Ca²⁺ transient rise time was comparable at basal conditions between Ctrl- and ACT-iPSC-CMs (Figure 24 A). It was reduced by 100 nM isoprenaline in Ctrl- and ACT-iPSC-ACT-iPSC-CMs.

Treatment with 0.25 µM DOX caused a significant decrease in both groups, whereas no Figure 23: Ca²⁺ transient visualization. Ca²⁺ transients were analyzed using Fluo-4 live cell staining and confocal laser scanning microscopy. A suitable region in the cytoplasm of iPSC-CMs was measured for 20 sec with the line scanning mode and relative fluorescence intensities were plotted. The Ca²⁺

transient rise time, amplitude and decay time were analyzed.