Studying ischemia and reperfusion in isolated neonatal rat ventricular myocytes using coverslip hypoxia

Monitoring mitochondrial calcium in cardiomyocytes during coverslip hypoxia using a fluorescent lifetime indicator

An increase in mitochondrial calcium via the mitochondrial calcium uniporter (MCU) has been implicated in initiating cell death in the heart during ischemia-reperfusion (I/R) injury. Measurement of calcium during I/R has been challenging due to the pH sensitivity of indicators coupled with the fall in pH during I/R. The development of a pH-insensitive indicator, mitochondrial localized Turquoise Calcium fluorescence Lifetime Sensor (mito-TqFLITS), allows for quantifying mitochondrial calcium during I/R via fluorescent lifetime imaging. Mitochondrial calcium was monitored using mito-TqFLITS, in neonatal mouse ventricular myocytes (NMVM) isolated from germline MCU-KO mice and MCUfl/fl treated with CRE-recombinase to acutely knockout MCU. To simulate ischemia, a coverslip was placed on a monolayer of NMVMs to prevent access to oxygen and nutrients. Reperfusion was induced by removing the coverslip. Mitochondrial calcium increases threefold during coverslip hypoxia in MCU-WT. There is a significant increase in mitochondrial calcium during coverslip hypoxia in germline MCU-KO, but it is significantly lower than in MCU-WT. We also found that compared to WT, acute MCU-KO resulted in no difference in mitochondrial calcium during coverslip hypoxia and reoxygenation. To determine the role of mitochondrial calcium uptake via MCU in initiating cell death, we used propidium iodide to measure cell death. We found a significant increase in cell death in both the germline MCU-KO and acute MCU-KO, but this was similar to their respective WTs. These data demonstrate the utility of mito-TqFLITS to monitor mitochondrial calcium during simulated I/R and further show that germline loss of MCU attenuates the rise in mitochondrial calcium during ischemia but does not reduce cell death.

Oxidative stress in the mitochondrial matrix underlies ischemia/reperfusion-induced mitochondrial instability

Ischemia and reperfusion affect multiple elements of cardiomyocyte electrophysiology, especially within the mitochondria. We previously showed that in cardiac monolayers, upon reperfusion after coverslip-induced ischemia, mitochondrial inner membrane potential (ΔΨ) unstably oscillates between polarized and depolarized states, and ΔΨ instability corresponds with arrhythmias. Here, through confocal microscopy of compartment-specific molecular probes, we investigate the mechanisms underlying the postischemic ΔΨ oscillations, focusing on the role of Ca²⁺ and oxidative stress. During reperfusion, transient ΔΨ depolarizations occurred concurrently with periods of increased mitochondrial oxidative stress (5.07 ± 1.71 oscillations/15 min, N = 100). Supplementing the antioxidant system with GSH monoethyl ester suppressed ΔΨ oscillations (1.84 ± 1.07 oscillations/15 min, N = 119, t test p = 0.027) with 37% of mitochondrial clusters showing no ΔΨ oscillations (versus 4% in control, odds ratio = 14.08, Fisher's exact test p < 0.001). We found that limiting the production of reactive oxygen species using cyanide inhibited postischemic ΔΨ oscillations (N = 15, t test p < 10^-5). Furthermore, ΔΨ oscillations were not associated with any discernable pattern in cell-wide oxidative stress or with the changes in cytosolic or mitochondrial Ca²⁺. Sustained ΔΨ depolarization followed cytosolic and mitochondrial Ca²⁺ increase and was associated with increased cell-wide oxidative stress. Collectively, these findings suggest that transient bouts of increased mitochondrial oxidative stress underlie postischemic ΔΨ oscillations, regardless of Ca²⁺ dynamics.

Low-cost, open-source cell culture chamber for regulating physiologic oxygen levels

The physiological oxygen levels for several mammalian cell types in vivo are considered to be hypoxic (low oxygen tension), but the vast majority of in vitro mammalian cell culture is conducted at atmospheric oxygen levels of around 21%. In order to understand the impact of low oxygen environments on cells, oxygen levels need to be regulated during in vitro culture. Two common methods for simulating a hypoxic environment are through the regulation of gas composition or chemical induction. Chemically mimicking hypoxia can have adverse effects such as reducing cell viability, making oxygen regulation in cell culture chambers crucial for long-term culture. However, oxygen-regulating cell culture incubators and commercial hypoxia chambers may not always be a viable option due to cost and limited customization. Other low-cost chambers have been developed, but they tend to lack control systems or are fairly small scale. Thus, the objective of this project was to design and develop a low-cost, open-source, controllable, and reproducible hypoxia chamber that can fit inside a standard cell culture incubator. This design allows for the control of O₂ between 1 and 21%, while maintaining CO₂ levels at 5%, as well as monitoring of temperature, pressure, and relative humidity. Testing showed our hypoxia chamber was able to maintain CO₂ levels at 5% and hypoxic O₂ levels at 1% and 5% for long-term cell culture. This simple and easy-to-manufacture design uses off the shelf components, and the total material cost was $832.47 (USD).

Modulation of cardiac Na+,K+-ATPase cell surface abundance by simulated ischemia-reperfusion and ouabain preconditioning

Na(+),K(+)-ATPase and cell survival were investigated in a cellular model of ischemia-reperfusion (I/R)-induced injury and protection by ouabain-induced preconditioning (OPC). Rat neonatal cardiac myocytes were subjected to 30 min of substrate and coverslip-induced ischemia followed by 30 min of simulated reperfusion. This significantly compromised cell viability as documented by lactate dehydrogenase release and Annexin V/propidium iodide staining. Total Na(+),K(+)-ATPase α(1)- and α(3)-polypeptide expression remained unchanged, but cell surface biotinylation and immunostaining studies revealed that α(1)-cell surface abundance was significantly decreased. Na(+),K(+)-ATPase-activity in crude homogenates and (86)Rb(+) transport in live cells were both significantly decreased by about 30% after I/R. OPC, induced by a 4-min exposure to 10 μM ouabain that ended 8 min before the beginning of ischemia, increased cell viability in a PKCε-dependent manner. This was comparable with the protective effect of OPC previously reported in intact heart preparations. OPC prevented I/R-induced decrease of Na(+),K(+)-ATPase activity and surface expression. This model also revealed that Na(+),K(+)-ATPase-mediated (86)Rb(+) uptake was not restored to control levels in the OPC group, suggesting that the increased viability was not conferred by an increased Na(+),K(+)-ATPase-mediated ion transport capacity at the cell membrane. Consistent with this observation, transient expression of an internalization-resistant mutant form of Na(+),K(+)-ATPase α(1) known to have increased surface abundance without increased ion transport activity successfully reduced I/R-induced cell death. These results suggest that maintenance of Na(+),K(+)-ATPase cell surface abundance is critical to myocyte survival after an ischemic attack and plays a role in OPC-induced protection. They further suggest that the protection conferred by increased surface expression of Na(+),K(+)-ATPase may be independent of ion transport.

Early long-term L-T3 replacement rescues mitochondria and prevents ischemic cardiac remodelling in rats

3,5,3′-Levo-triiodothyronine (L-T3) is essential for DNA transcription, mitochondrial biogenesis and respiration, but its circulating levels rapidly decrease after myocardial infarction (MI). The main aim of our study was to test whether an early and sustained normalization of L-T3 serum levels after MI exerts myocardial protective effects through a mitochondrial preservation. Seventy-two hours after MI induced by anterior interventricular artery ligation, rats were infused with synthetic L-T3 (1.2 μg/kg/day) or saline over 4 weeks. Compared to saline, L-T3 infusion restored FT3 serum levels at euthyroid state (3.0 ± 0.2 versus 4.2 ± 0.3 pg/ml), improved left ventricular (LV) ejection fraction (39.5 ± 2.5 versus 65.5 ± 6.9%), preserved LV end-systolic wall thickening in the peri-infarct zone (6.34 ± 3.1 versus 33.7 ± 6.21%) and reduced LV infarct-scar size by approximately 50% (all P < 0.05). Moreover, L-T3 significantly increased angiogenesis and cell survival and enhanced the expression of nuclear-encoded transcription factors involved in these processes. Finally, L-T3 significantly increased the expression of factors involved in mitochondrial DNA transcription and biogenesis, such as hypoxic inducible factor-1α, mitochondrial transcription factor A and peroxisome proliferator activated receptor γ coactivator-1α, in the LV peri-infarct zone. To further explore mechanisms of L-T3 protective effects, we exposed isolated neonatal cardiomyocytes to H₂O₂ and found that L-T3 rescued mitochondrial biogenesis and function and protected against cell death via a mitoKATP dependent pathway. Early and sustained physiological restoration of circulating L-T3 levels after MI halves infarct scar size and prevents the progression towards heart failure. This beneficial effect is likely due to enhanced capillary formation and mitochondrial protection.