Anesthetic preconditioning protects cardiomyocytes from oxidative stress-induced injury, but it is ineffective in patients with diabetes mellitus. To address the role of hyperglycemia in the inability of diabetic individuals to be preconditioned, we used human cardiomyocytes differentiated from induced pluripotent stem cells generated from patients with or without type 2 diabetes mellitus (DM-iPSC- and N-iPSC-CMs, respectively) to investigate the efficacy of preconditioning in varying glucose conditions (5, 11, and 25 mM).
Induced pluripotent stem cells were induced to generate cardiomyocytes by directed differentiation. For subsequent studies, cardiomyocytes were identified by genetic labeling with enhanced green fluorescent protein driven by a cardiac-specific promoter. Cell viability was analyzed by lactate dehydrogenase assay. Confocal microscopy was utilized to measure opening of the mitochondrial permeability transition pore and the mitochondrial adenosine 5'-triphosphate-sensitive potassium channels.
Isoflurane (0.5 mM) preconditioning protected N-iPSC- and DM-iPSC-CMs from oxidative stress-induced lactate dehydrogenase release and mitochondrial permeability transition pore opening in 5 mM and 11 mM glucose. Isoflurane triggered mitochondrial adenosine-5'-triphosphate-sensitive potassium channel opening in N-iPSC-CMs in 5 mM and 11 mM glucose and in DM-iPSC-CMs in 5 mM glucose; 25 mM glucose disrupted anesthetic preconditioning-mediated protection in DM-iPSC- and N-iPSC-CMs.
The opening of mitochondrial adenosine 5'-triphosphate-sensitive potassium channels are disrupted in DM-iPSC-CMs in 11 mM and 25 mM glucose and in N-iPSC-CMs in 25 mM glucose. Cardiomyocytes derived from healthy donors and patients with a specific disease, such as diabetes in this study, open possibilities in studying genotype- and phenotype-related pathologies in a human-relevant model.
What We Already Know about This Topic
Previous studies have demonstrated that anesthetic preconditioning protects cardiomyocytes from oxidative stress-induced injury, but is ineffective in patients with diabetes mellitus.
Using human cardiomyocytes induced from pluripotent stem cells generated from patients with and without diabetes mellitus, the present study investigated the effect of anesthetic preconditioning in the presence of varying glucose concentrations.
What This Article Tells Us That Is New
High glucose had detrimental effects on anesthetic preconditioning in cardiomyocytes derived from human pluripotent stem cells. Adenosine 5′-triphosphate-sensitive potassium channels are differentially effected by anesthetic preconditioning in human cardiomyocytes induced from pluripotent stem cells generated from patients with and without diabetes mellitus.
THE World Health Organization estimates that diabetes mellitus affects nearly 200 million people worldwide, with 90% of them having type 2. The risk of cardiovascular disease increases dramatically for those diagnosed with diabetes.1Anesthetic preconditioning (APC) can reduce myocardial injury following an ischemia and reperfusion injury.2,–,4However, diabetes or hyperglycemia in the nondiabetic myocardium have been shown to abolish APC in animal models.5–6
Tanaka et al. discovered that streptozotocin-induced diabetic canines could not be efficiently protected from myocardial infarction with isoflurane preconditioning.5They also observed that infarct size was directly related to blood glucose levels. Kehl et al. observed that moderate hyperglycemia attenuated the protective effects of 0.5 minimum alveolar concentration (MAC) isoflurane, but not 1.0 MAC isoflurane in canines. However, severe hyperglycemia attenuated the protective effects of 0.5 and 1.0 MAC isoflurane.6These animal studies provided great insight into APC in diabetes mellitus; however, they could not address a genetic component of this phenomenon because of the nature of inducing diabetes in this experimental model. Moreover, it is unclear whether we could extend these mechanisms delineated in animal models to human myocardium.
Induced pluripotent stem cells (iPSCs) used in this study were generated from human skin fibroblasts by introducing pluripotency factors. Reprogramming the cells allows them to obtain characteristics of pluripotent stem cells.7As such, iPSCs have the capability to differentiate into cardiomyocytes8–9while maintaining their genotype. Our laboratory has previously shown that cardiomyocytes derived from human embryonic stem cells (hESC-CMs) exhibit competent responses to APC compared with various animal models and human myocardium.10The generation of iPSCs from patient-specific-derived fibroblasts enables us to study human disease mechanisms that were nearly impossible to be investigated in the past. In this study, iPSCs derived from nondiabetic and type 2 diabetic individuals were used to generate cardiomyocytes (DM-iPSC-CMs and N-iPSC-CMs, respectively). Others have shown that differentiated cardiomyocytes behave very similar to adult cardiomyocytes on multiple levels, including exhibition of spontaneous and rhythmic contractions, and expression of cardiac specific markers.9,–,11Thus, DM-iPSC-CMs and N-iPSC-CMs provide us with a unique in vitro model to investigate environmental factors, such as high glucose, that affect APC in diabetic myocardium. Because of the lack of experimentally available human myocardium, reliable type 2 diabetic animal models, and the disconnect between animal and human studies, our model has the unique ability to investigate a human disease and its ability to attenuate APC-induced cardioprotection for the first time. Our study investigates the detrimental effects of high glucose on APC-mediated cardiac protection in cardiomyocytes derived from a nondiabetic and a type 2 diabetic individual.
Materials and Methods
Human-induced Pluripotent Stem Cell Culture
Dermal fibroblasts were isolated from one type 2 diabetic and one nondiabetic patient, and these patient-specific fibroblasts were reprogrammed into iPSCs12–13(DM-iPSCs and N-iPSCs, respectively). Both lines were gifts from Dr. George Daley, M.D., Ph.D. (Professor, Director, Stem Cell Transplantation Program, Howard Hughes Medical Institute and Children's Hospital, Boston, MA) and Dr. Stephen Duncan, Ph.D. (Professor, Vice-Chairman Cell Biology, Neurobiology, and Anatomy, Director, MCW Program in Regenerative Medicine and Stem Cell Biology, Medical College of Wisconsin, Milwaukee, WI), respectively, and iPSC lines were cultured according to our previously described protocol.10Mouse embryonic fibroblasts were inactivated with mitomycin C (Sigma-Aldrich, St. Louis, MO) and plated in Dulbecco Modified Eagle Medium (Millipore Bioscience Research Reagents, Temecula, CA) complemented with 10% fetal bovine serum (Invitrogen, Carlsbad, CA) and 1% nonessential amino acids (Millipore Bioscience Research Reagents). iPSCs were maintained on inactivated mouse embryonic fibroblasts in hypoxic conditions (4% O2, 5% CO2) with iPSC culture medium consisting of Dulbecco Modified Eagle Medium/Ham F12 medium (Invitrogen) supplemented with 20% knockout serum (Invitrogen), 1% nonessential amino acids (Invitrogen), 1% penicillin-streptomycin (Invitrogen), L-glutamine (Millipore), β-mercaptoethanol (Sigma–Aldrich), and 4 ng/ml human recombinant basic fibroblast growth factor (Invitrogen). iPSC colonies were passaged every 6–8 days, and iPSC passages 45–65 were used in the subsequent study.
Cardiac Differentiation of N-iPSCs and DM-iPSCs
Colonies of iPSCs were mechanically dissociated into small aggregates and plated onto dishes precoated with reduced growth factor matrigel (BD Biosciences, San Jose, CA), and cultured under hypoxic (4% O2, 5% CO2) conditions. Cells were supplemented with iPSC medium conditioned by inactivated mouse embryonic fibroblasts for 7 days. Next, cells were grown in Roswell Park Memorial Institute (RPMI)/B27 medium (Invitrogen) supplemented with growth factors activin A (50 ng/ml; R&D Systems, Minneapolis, MN) and bone morphogenetic protein-4 (10 ng/ml; R&D) for 5 days. Finally, cells were plated in normoxic conditions (20% O2) with RPMI/B27 and matured for 3 months before use in subsequent experiments.
Microdissection and Single Cell Dissociation
Three months following differentiation, cell clusters were mechanically dissociated under a dissecting microscope (SMZ1000; Nikon, Tokyo, Japan) using a 27-gauge needle and enzymatically dispersed in 0.05% trypsin-EDTA (Invitrogen) for 5 min. Trypsin was inactivated by Dulbecco Modified Eagle Medium containing 10% fetal bovine serum and individual cells were plated onto matrigel-coated coverslips at low density.
Cells cultured on matrigel-coated glass coverslips were fixed with 1% paraformaldehyde for 30 min and then were washed three times with phosphate-buffered saline. Cells were permeabilized with 0.5% Triton X-100 (Sigma-Aldrich) and blocked with 10% donkey serum for 30 min. Cells were incubated with primary antibodies for sarcomeric α-actinin (1:100 dilution; Sigma-Aldrich) or anticardiac-specific troponin T (1:100; Thermo Fisher Scientific, Waltham, MA) for 1 h at 37°C. After three washes, cells were incubated with secondary antibody Alexa Fluor 594 (1:1,000, Invitrogen) for 1 h at room temperature. Nuclei were stained with TOPRO-3 (1:1,000; Invitrogen). Coverslips were then mounted onto slides and images were acquired with a laser-scanning confocal microscope (Nikon Eclipse TE2000-U).
Genetic Marking of iPSC-derived Cardiomyocytes with a Lentiviral Vector
Genetic marking of iPSC-CMs was similarly conducted in our previous study.10In brief, 3 days after dissociation, cardiomyocytes were transduced with a lentiviral vector encoding human myosin light chain-2v (MLC-2v)-driven enhanced green fluorescent protein (eGFP: multiplicity of infection, 2.2 × 104). The MLC-2v promoter is specific for ventricular myocytes.11,14–15The MLC-2v-eGFP cassette (provided by Lior Gepstein, M.D., Ph.D., Professor, The Bruce Rappaport Institute in the Medical Sciences, Faculty of Medicine, Technion-Israel Institute of Technology, Haifa, Israel16) was subcloned into lentiviral transfer plasmid pHR(+)c.Ub.MCSoligo.R(−)W(+). Lentiviral vector assembly and titering were performed as previously described.17–18
Lactate Dehydrogenase Assay
Lactate dehydrogenase (LDH) activity was assessed for determination of cell viability. LDH was measured using a colorimetric cytotoxicity assay kit according to the manufacturer's directions as an indicator of cell membrane damage (Roche Diagnostics Corporation, Indianapolis, IN). iPSC-CMs were plated at 1 × 104cells/well in 96-well plates and cultured in 100 μl of RPMI/B27 medium. The medium was exchanged every 2 days for 1 week. Cardiomyocytes were treated with either a control (RPMI/B27) medium or preconditioned with 0.5 mM isoflurane for 10 min and then replaced with control medium. Both control and experimental groups had a 2 h exposure of 10 mM H2O2(Calbiochem, LA Jolla, CA) to induce oxidative stress. Experiments were conducted in RPMI/B27 media containing either 5 mM glucose with 20 mM mannitol, an osmotic control, 11 mM glucose with 14 mM mannitol, or 25 mM glucose throughout the experiment. Absorbance data were read in the assay plate spectrophotometrically at 490 nm (reference wavelength, 600 nm) with a microplate reader (BioTek Instruments, Inc., Winooski, VT). Only control and experimental groups that had an oxidative stress-induced LDH release were analyzed. After subtracting background absorbance of cell-free medium, total LDH released into medium was calculated.
Laser-scanning Confocal Microscopy for Mitochondrial Membrane Potential and Mitochondrial Permeability Transition Pore (mPTP) Opening Assays
Five days following lentiviral vector transduction, imaging was performed using a confocal microscope (Eclipse TE2000-U; Nikon) and data were analyzed with Image J software (National Institutes of Health, Bethesda, MD). Living iPSC-CMs were identified by detecting green fluorescent cells, indicating MLC-2v-driven eGFP expression. Experiments were conducted in either 5 mM glucose, 11 mM glucose, or 25 mM glucose. Tetramethylrhodamine ethyl ester (TMRE; 30 nM, Invitrogen) was used to detect mitochondrial membrane potential (ΔΨm) in iPSC-CMs for the purpose of detecting opening of mitochondrial adenosine 5′-triphosphate-sensitive potassium (mitoKATP) channels. Opening of mitoKATPchannels induces a depolarization, observed as a loss of TMRE fluorescence. iPSC-CMs were exposed to either isoflurane (0.5 mM), diazoxide (100 mM; Sigma-Aldrich), a mitoKATPactivator, or isoflurane plus 5-hydroxydecanoate (5-HD; 500 μM; Sigma-Aldrich), a mitoKATPinhibitor, to determine the activity of mitoKATPchannels. Data are normalized to baseline (100%). Baseline fluorescence intensity was recorded for five frames (1 frame/min) and then treatment was added and fluorescence intensity was recorded for 10 frames (1 frame/min). For statistical analysis, the average values of the baseline measurements were compared with the average values of the last five frames following treatment to determine changes in TMRE fluorescence from baseline. Rarely, cardiomyocytes would shift during the experimental protocol and were not included in the analysis. Opening of the mPTP was assessed as described previously in our laboratory,19a method based on mPTP induction by photoexcitation-generated oxidative stress.20,–,23The mPTP opening was detected by rapid dissipation of ΔΨm, observed as a loss of TMRE fluorescence, which is sensitive to mPTP opening inhibition.19
Application of Anesthetic Preconditioning
Appropriate volumes of isoflurane stock solution (Baxter, Deerfield, IL) were sonicated into media to achieve 0.5 mM (approximately 1 MAC) concentration, as done in previous studies.10,24Cardiomyocytes were anesthetically preconditioned with culture media containing 0.5 mM isoflurane for 10 min followed by a washout with culture media in LDH and mPTP experiments. Culture medium containing 0.5 mM isoflurane was placed onto cardiomyocytes to trigger mitoKATPopening. At the end of each experiment, isoflurane concentration was analyzed by gas chromatography.
Data are presented as mean ± SD. Each experimental group consisted of iPSC-derived CMs from at least three separate differentiations; n indicates the number of independent experiments. For the statistical analyses, SigmaStat 3.0 software (Systat Software, Inc., San Jose, CA) was used. Statistical comparisons were performed using one-way anova with Holm–Sidak correction for multiple testing over all comparisons within each cell line or unpaired Student t test where appropriate. P < 0.05 was considered significant.
Differentiation and Characterization of iPSC-derived Cardiomyocytes
Cardiomyocytes differentiated from both N-iPSCs and DM-iPSCs were observed contracting in culture dishes approximately 14 days following treatment with activin-A and bone morphogenetic protein-4 (fig. 1). Contracting N-iPSC-CMs and DM-iPSC-CMs are shown in Supplemental Digital Content 1 and 2, respectively (see video, Supplemental Digital Content 1, http://links.lww.com/ALN/A867, and see video, Supplemental Digital Content 2, http://links.lww.com/ALN/A868). Immunostaining verified that dissociated cardiomyocytes from both N-iPSC and DM-iPSC populations had striation-like patterns confirmed by both cardiac-specific troponin T (figs. 2A, 2B) and sarcomeric α-actinin staining (figs. 2C, 2D).
iPSC-derived Cardiomyocytes Express eGFP Under the Control of Cardiac-specific Promoter MLC-2v
To properly identify living iPSC-CMs, determine differentiation efficiency, and ensure that subsequent confocal microscopy experiments were conducted on cardiomyocytes, we genetically labeled cells with a lentiviral vector that expressed eGFP under the transcriptional control of cardiac-specific promoter, MLC-2v (fig. 3). The differentiation efficiency was determined to be high in N-iPSC- and DM-iPSC-CMs, 67 ± 4%, 71 ± 4%, respectively (figs. 3B–D; n = 6/group). eGFP-positive cardiomyocytes were selected for subsequent confocal microscopy experiments.
APC Protects Both N-iPSC- and DM-iPSC-derived Cardiomyocytes in 5 mM and 11 mM but Not in 25 mM Glucose against Oxidative Stress
To determine whether APC could protect cardiomyocytes derived from both N-iPSCs and DM-iPSCs from oxidative stress-induced damage, we exposed control and anesthetic preconditioned cardiomyocytes to 10 mM H2O2for 2 h in 5 mM, 11 mM, or 25 mM glucose environment. N-iPSC-CMs that were preconditioned with isoflurane before H2O2exposure had a significantly lower LDH release in 5 mM glucose compared with stress control, 128 ± 4%versus 152 ± 4%, respectively (fig. 4A; n = 7/group) and in 11 mM glucose compared with stress control, 131 ± 6%versus 151 ± 6%, respectively (fig. 4A; n = 8/group). APC did not reduce LDH release in N-iPSC-CMs in 25 mM glucose compared with the stress control (151 ± 9%vs. 150 ± 7%, respectively; fig. 4A; n = 7/group). DM-iPSC-CMs also had a reduction in LDH release when preconditioned with isoflurane compared with the stress control in 5 mM glucose (119 ± 3%vs. 148 ± 3%, respectively; fig. 4B; n = 7/group) and in 11 mM glucose (124 ± 4%vs. 146 ± 3%, respectively; fig. 4B; n = 8/group). However, the APC-induced reduction in LDH release in DM-iPSC-CMs was lost in the presence of 25 mM glucose (fig. 4B; (n = 7/group).
Preconditioning with Isoflurane Delays Opening of the mPTP in Both N-iPSC- and DM-iPSC-derived Cardiomyocytes in 5 mM and 11 mM but Not in 25 mM Glucose
APC delays opening of the mPTP and thus reduces cell death in ischemia-reperfusion injury.10,19,25Opening of the mPTP causes a rapid collapse in ΔΨm, which can be detected following rapid loss of TMRE fluorescence intensity in iPSC-CMs (fig. 5A). APC increased the mPTP opening time in N-iPSC-CMs in 5 mM glucose compared with control, 139 ± 6%versus 100 ± 5% (fig. 5B; n = 22 cells/group). APC also increased mPTP opening time in DM-iPSC-CMs in 5 mM glucose compared with control, 128 ± 4%versus 100 ± 5% (fig. 5C; n = 25 cells/group). In addition, APC increased mPTP opening time in N-iPSC-CMs in 11 mM glucose, 131 ± 7%versus 100 ± 5% (fig. 5B; n = 32), and in DM-iPSC-CMs in 11 mM glucose, 122 ± 9%versus 100 ± 5%, respectively (figs. 5B and 5C; n = 30 cells/group). The APC-induced delay in mPTP opening was lost in N-iPSC-CMS in 25 mM glucose, 100 ± 6%versus 100 ± 5% (fig. 5B; n = 25 cells/group), and in DM-iPSC-CMs when the cells were exposed to 25 mM glucose, 102 ± 9%versus 100 ± 6% (fig. 5C; n = 24 cells/group).
Isoflurane Triggers Opening of the mitoKATPChannels and Depolarizes Mitochondria in iPSC-derived Cardiomyocytes
Isoflurane induced opening of mitoKATPchannels, causing a mild mitochondrial depolarization, which is observed as a partial loss of TMRE fluorescence intensity; the addition of 5-HD, a mitoKATPinhibitor, attenuated a portion of this depolarization (fig. 6A). Diazoxide, a mitoKATPopener, triggered opening of the mitoKATPchannels and a subsequent loss of ΔΨmin N-iPSC-CMs in 5 mM (n = 56 cells/group), 11 mM (n = 60 cells/group), and 25 mM glucose (n = 63 cells/group) compared with baseline (fig. 6B). Diazoxide also opened mitoKATPchannels in DM-iPSC-CMs in 5 mM (n = 62 cells/group), 11 mM (n = 65 cells/group), and 25 mM glucose (n = 68/cells group) compared with baseline (fig. 6C). The addition of isoflurane (0.5 mM) decreased TMRE fluorescence, indicating a mitochondrial depolarization in N-iPSC-CMs in 5 mM glucose (decrease by 34 ± 5%), which was attenuated in the presence of 500 mM 5-HD (decrease by 18 ± 5%) (fig. 6B; n = 56 cells/group). Isoflurane induced a mitochondrial depolarization in DM-iPSC-CMs in 5 mM glucose (decrease by 21 ± 4%), which was attenuated in the presence of 500 mM 5-HD (decrease by 9 ± 4%) (fig. 6C; n = 62 cells/group). This indicated opening of mitoKATPchannels by isoflurane. However, N-iPSC-CMs in 25 mM glucose had the same extent of decrease in TMRE fluorescence between isoflurane and isoflurane + 5-HD treatment, indicating the inability of isoflurane to open mitoKATPchannels (fig. 6B; n = 63 cells/group). Isoflurane did not open mitoKATPchannels to a greater extent than when isoflurane + 5-HD was present in DM-iPSC-CMs in 11 mM and 25 mM glucose (fig. 6C; n = 68 cells/group). The ability of isoflurane-induced mitochondrial depolarization in the presence of mitoKATPchannel blocker suggests that isoflurane decreases ΔΨmin part by other mechanisms.26
In the present study, we investigated the effects of APC on iPSC-derived cardiomyocytes from a healthy individual and a type 2 diabetic patient in varying glucose conditions. Our major findings are summarized as follows. 1) iPSCs reprogrammed from nondiabetic and type 2 diabetic human fibroblasts could be differentiated into cardiomyocytes with directed differentiation, as supported by visualization of contracting cell regions; expression of cardiac-specific, highly organized contractile myofilaments; and positive genetic labeling with cardiac-specific marker (MLC 2v-eGFP). 2) Preconditioning with isoflurane reduced LDH release and delayed mPTP opening in both N-iPSC-CMs and unexpectedly in DM-iPSC-CMs in 5 mM and 11 mM, but not 25 mM glucose. 3) Isoflurane triggered the opening of mitoKATPchannels in N-iPSC-CMs in 5 mM and 11 mM glucose and in DM-iPSC-CMs, but only in 5 mM glucose. In summary, both N-iPSC- and DM-iPSC-CMs could be protected in 5 mM glucose, whereas 11 mM glucose attenuated isoflurane-induced mitoKATPopening in DM-iPSC-CMs, and 25 mM glucose abrogated all cardioprotective effects provided by APC in both DM-iPSC- and N-iPSC-CMs, indicating that 25 mM glucose was the predominant mechanism in the attenuation of APC in diabetic individuals.
We have shown that preconditioning with isoflurane attenuates cell damage and triggers cardioprotective mechanisms in iPSC-CMs against oxidative stress in 5 mM and 11 mM glucose. We recorded isoflurane-induced mitoKATPopening and a delay in oxidative stress-induced mPTP opening in N-iPSC-CMs in 5 mM and 11 mM glucose. iPSC-CMs responded similarly to APC as in studies of adult cardiomyocytes27and hESC-CMs,10indicating that patient-specific iPSC-derived cardiomyocytes can be a vital tool in understanding human diseases not only at the genetic level, but can also test how environmental conditions play a role in a disease phenotype.
We achieved a high rate of differentiation efficiency; approximately 70% of cells expressed eGFP driven by the cardiomyocyte-specific promoter MLC-2v (fig. 3). N-iPSC-CMs and DM-iPSC-CMs both had striation-like patterns (fig. 2), indicating structural integrity similar to adult human cardiomyocytes and hESC-CMs.28We utilized growth factors to differentiate our iPSCs into cardiomyocytes based on the presence of these factors in regulating heart development in the embryo.28A previously published cardiac differentiation protocol for the induction of human embryonic stem cells and differentiation into cardiomyocytes was used for iPSC differentiation.10We are confident that cardiomyocytes derived from iPSCs can serve as a powerful tool to understand different human diseases and how environment plays a role in attenuating the cardioprotection provided by APC. Cardiomyocytes derived from iPSCs share many characteristics of human adult cardiomyocytes: contractions, cardio-specific immunostaining that illustrates the highly organized contractile sarcomeres, atrial and ventricular-like action potentials, and the exhibition of functioning APC mediators.
iPSC-derived cardiomyocytes have many unique advantages over models that are currently available to study human diseases. The first shortcoming of many disease animal models is that the model itself does not replicate the complexity of the disease in its entirety. Specifically, the streptozotocin-induced diabetic models do not alter the genome of the model. Many of the type 2 diabetic murine models,29although more relevant, do not exhibit all the components of type 2 diabetes in humans. In addition, there are many shortcomings of using an animal model compared with a human disease.30As stem cell biology advances, pluripotent stem cell-derived cell types are being utilized in drug discovery and toxicity screening.31–32The use of pluripotent stem cells to investigate disease models have become more prevalent as well.33,–,35Clearly, a human-type model will have many advantages over animal models as we continue to study human diseases.
Our laboratory has extensively investigated APC in animal models,3,19,25human myocardium,27and in hESC-CMs.10It was unclear whether iPSC-CMs could be protected from oxidative stress and what role glucose had in disabling the cardioprotection provided by preconditioning with isoflurane. In this study we showed that cardiomyocytes derived from both N-iPSCs and DM-iPSCs could be preconditioned with isoflurane; however, the presence of 25 mM glucose obliterated the APC-induced cardioprotective effects in each respective cell line. To further investigate the underlying mechanisms of the protection provided by APC in iPSC-CMs, we tested mitoKATPchannel activity and mPTP opening. We showed that isoflurane partly depolarized mitochondria in a 5-HD-sensitive manner to the greatest extent in N-iPSC-CMs in 5 mM and 11 mM glucose and in DM-iPSC-CMs in 5 mM glucose. This isoflurane-induced depolarization suggests that opening of the mitoKATPchannels plays a role in cardioprotection.3,36N-iPSC-CMs in 25 mM glucose and DM-iPSC-CMs in 11 mM and 25 mM glucose did not show a significant difference in isoflurane-induced mitoKATPopening compared with groups treated with the mitoKATPchannel blocker, 5-HD. This is not surprising, as others have indicated that both diabetes and hyperglycemia have shown to attenuate isoflurane-induced mitoKATPopening.37–38
It is well known that isoflurane acts on the electron transport chain, specifically complex I,26and the isoflurane-induced inhibition of this complex could explain the mitochondrial depolarization that we observed in the 5-HD groups. However, APC protected DM-iPSC-CMs in 11 mM glucose from oxidative stress and caused an APC-induced delay in mPTP opening, suggesting that isoflurane may elicit cell protection via parallel pathways that do not fully depend on the opening of mitoKATPchannels. In addition, isoflurane may induce mitoKATPchannel opening in a few channels or at levels that we could not accurately record with confocal microscopy. Further studies are needed to understand the role of mitoKATPchannels in cardioprotective strategies in iPSC-CMs, especially at moderately high glucose levels where APC provided protection without the measureable aid of mitoKATPchannels opening.
In addition, we investigated the effects of preconditioning on the delay of the mPTP opening. Typically, the mPTP opens during an ischemia-reperfusion injury resulting in an increase in the permeability of large solutes in the inner membrane of the mitochondria. mPTP opening dissipates ΔΨm, initiating cell death pathways.39Preconditioning cardiomyocytes with volatile anesthetics has been shown to delay the opening of the mPTP.40–41Delaying the mPTP opening preserves the mitochondrial bioenergetics and overall improves the cellular and mitochondrial function. Our results indicate that APC delays opening of the mPTP in N-iPSC- and DM-iPSC-CMs in 5 mM and 11 mM glucose, but not in 25 mM glucose conditions, which is in line with LDH experiments suggesting that iPSC-CMs derived from type 2 diabetic patients can be preconditioned and high glucose conditions may be playing a key role in attenuating cardioprotective effects provided by APC in these patients. The presence of 25 mM glucose alone attenuated APC-mediated delay in mPTP opening. Studies in rat cardiomyocytes have shown that pyruvate, a product of glycolysis, obliterated APC-induced delay in mPTP opening in ΔΨm-dependent manner.24The delay in mPTP opening in N-iPSC-CMs in 5 mM and 11 mM glucose was similar to observations made in rat cardiomyocytes,19human adult cardiomyoyctes,27and hESC-CMs.10
A limitation of our study is the use of in vitro generated cardiomyocytes. Our iPSC-CMs may lack some of the characteristics of adult cardiomyocytes. However, with the parameters that we investigated, N-iPSC-CMs responded to APC similarly as other rat, rabbit, guinea pig, canine, human, and hESC-CM models that our department has previously investigated. To induce an injury, we generated oxidative stress with H2O2or photoexcitation, which does not fully represent the conditions of an ischemia and reperfusion event. Specifically, the use of 10 mM H2O2to induce an oxidative stress environment in our iPSC-CMs was excessively higher than H2O2levels used in animal studies. We have observed that our stem cell-derived cardiomyocytes are more resistant to oxidative stress-induced injury. Further studies are needed to investigate the mechanisms involved in the resistance of iPSC-CMs to oxidative stress. Nevertheless, H2O2-induced injury is widely accepted for studying reperfusion injury as it mimics the most important type of stress during this injury: oxidative stress. Finally, we solely investigated the effects of APC on cardiomyocytes. APC not only protects cardiomyocytes directly, but also indirectly through its action on other cell types in the heart, such as endothelial cells,42or by modulation of inflammatory response.43
In conclusion, for the first time our study shows that preconditioning with isoflurane protects cardiomyocytes derived from nondiabetic iPSCs from oxidative stress in 5 mM and 11 mM glucose. 25 mM glucose had detrimental effects on cardioprotection provided by APC in both nondiabetic and type 2 diabetic patient-derived cardiac lines. Unexpectedly, type 2 diabetic patient-specific iPSCs could be anesthetically preconditioned in 5 mM and 11 mM glucose. Additional studies and cell lines are needed to further understand the role of genetics in the disease phenotype, and identify potential components that may be contributing to the inability of human diabetic patients to be preconditioned.
The similarity between our study conducted on iPSC-CMs to our previous work in animal models, human cardiomyocytes, and hESC-CMs proves that cardiomyocytes derived from patient-specific iPSCs are a suitable model to further understand preconditioning in a human model and the underlying mechanisms by which disease phenotypes alter the protection provided by APC. Development of iPSC-derived ventricular cardiomyocytes from healthy individuals and patients with various diseases in conjunction with modulating external factors, such as varying glucose levels, provides a powerful tool for translational research.
The authors thank Katelyn Revak, B.S. (Research Technician, Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, Wisconsin), for technical assistance and Aniko Szabo, Ph.D. (Associate Professor, Institute for Health and Society, Division of Biostatistics, Medical College of Wisconsin) for help with the statistical analysis.