Coverslip hypoxia: a novel method for studying cardiac myocyte hypoxia and ischemia in vitro

Differential effects of coverslip-induced hypoxia and cobalt chloride mimetic hypoxia on cellular stress, metabolism, and nuclear structure

Hypoxia has profound effects on cell physiology, both in normal or pathological settings like cancer. In this study, we asked whether a variant of coverslip-induced hypoxia that recapitulates the conditions found in the tumor microenvironment would elicit similar cellular responses compared to the well established model of cobalt chloride-induced hypoxia. Comparable levels of nuclear HIF-1α were observed after 24 h of coverslip-induced hypoxia or cobalt chloride treatment in CAL-27 oral squamous carcinoma cells. However, cellular stress levels assessed by reactive oxygen species production and lipid droplet accumulation were markedly increased in coverslip-induced hypoxia compared to cobalt chloride treatment. Conversely, mitochondrial ATP production sharply decreased after coverslip-induced hypoxia but was preserved in the presence of cobalt chloride. Coverslip-induced hypoxia also had profound effects in nuclear organization, assessed by changes in nuclear dry mass distribution, whereas these effects were much less marked after cobalt chloride treatment. Taken together, our results show that coverslip-induced hypoxia effects on cell physiology and structure are more pronounced than mimetic hypoxia induced by cobalt chloride treatment. Considering also the simplicity of coverslip-induced hypoxia, our results therefore underscore the usefulness of this method to recapitulate in vitro the effects of hypoxic microenvironments encountered by cells in vivo.

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.

Mitochondrial membrane potential instability on reperfusion after ischemia does not depend on mitochondrial Ca2+ uptake

Physiologic Ca2+ entry via the Mitochondrial Calcium Uniporter (MCU) participates in energetic adaption to workload but may also contribute to cell death during ischemia/reperfusion (I/R) injury. The MCU has been identified as the primary mode of Ca2+ import into mitochondria. Several groups have tested the hypothesis that Ca2+ import via MCU is detrimental during I/R injury using genetically-engineered mouse models, yet the results from these studies are inconclusive. Furthermore, mitochondria exhibit unstable or oscillatory membrane potentials (ΔΨm) when subjected to stress, such as during I/R, but it is unclear if the primary trigger is an excess influx of mitochondrial Ca2+ (mCa2+), reactive oxygen species (ROS) accumulation, or other factors. Here, we critically examine whether MCU-mediated mitochondrial Ca2+ uptake during I/R is involved in ΔΨm instability, or sustained mitochondrial depolarization, during reperfusion by acutely knocking out MCU in neonatal mouse ventricular myocyte (NMVM) monolayers subjected to simulated I/R. Unexpectedly, we find that MCU knockout does not significantly alter mCa2+ import during I/R, nor does it affect ΔΨm recovery during reperfusion. In contrast, blocking the mitochondrial sodium-calcium exchanger (mNCE) suppressed the mCa2+ increase during Ischemia but did not affect ΔΨm recovery or the frequency of ΔΨm oscillations during reperfusion, indicating that mitochondrial ΔΨm instability on reperfusion is not triggered by mCa2+. Interestingly, inhibition of mitochondrial electron transport or supplementation with antioxidants stabilized I/R-induced ΔΨm oscillations. The findings are consistent with mCa2+ overload being mediated by reverse-mode mNCE activity and supporting ROS-induced ROS release as the primary trigger of ΔΨm instability during reperfusion injury.

Generation of myocyte agonal Ca2+ waves and contraction bands in perfused rat hearts following irreversible membrane permeabilisation

Although irreversible cardiomyocyte injury provokes intracellular Ca²⁺ ([Ca²⁺]_i) overload, the underlying dynamics of this response and its effects on cellular morphology remain unknown. We therefore visualised rapid-scanning confocal fluo4-[Ca²⁺]_i dynamics and morphology of cardiomyocytes in Langendorff-perfused rat hearts following saponin-membrane permeabilisation. Our data demonstrate that 0.4% saponin-treated myocytes immediately exhibited high-frequency Ca²⁺ waves (131.3 waves/min/cell) with asynchronous, oscillatory contractions having a mean propagation velocity of 117.8 μm/s. These waves slowly decreased in frequency, developed a prolonged decay phase, and disappeared in 10 min resulting in high-static, fluo4-fluorescence intensity. The myocytes showing these waves displayed contraction bands, i.e., band-like actin-fibre aggregates with disruption of sarcomeric α-actinin. The contraction bands were not attenuated by the abolition of Ca²⁺ waves under pretreatment with ryanodine plus thapsigargin, but were partially attenuated by the calpain inhibitor MDL28170, while mechanical arrest of the myocytes by 2,3-butanedione monoxime completely attenuated contraction-band formation. The depletion of adenosine 5'-triphosphate by the mitochondrial electron uncoupler carbonyl cyanide 4-trifluoromethoxy phenylhydrazone also attenuated Ca²⁺ waves and contraction bands. Overall, saponin-induced myocyte [Ca²⁺]_i overload provokes agonal Ca²⁺ waves and contraction bands. Contraction bands are not the direct consequence of the waves but are caused by cross-bridge interactions of the myocytes under calpain-mediated proteolysis.

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.

Acute pH alterations do not impact cardiac mitochondrial respiration in naked mole-rats or mice

Energetically demanding conditions such as hypoxia and exercise favour anaerobic metabolism (glycolysis), which leads to acidification of the cellular milieu from ATP hydrolysis and accumulation of the anaerobic end-product, lactate. Cellular acidification may damage mitochondrial proteins and/or alter the H⁺ gradient across the mitochondrial inner membrane, which may in turn impact mitochondrial respiration and thus aerobic ATP production. Naked mole-rats are among the most hypoxia-tolerant mammals, and putatively experience intermittent environmental and systemic hypoxia while resting and exercising in their underground burrows. Previous studies in naked mole-rat brain, heart, and skeletal muscle mitochondria have demonstrated adaptations that favour improved efficiency in hypoxic conditions; however, the impact of cellular acidification on mitochondrial function has not been explored. We hypothesized that, relative to hypoxia-intolerant mice, naked mole-rat cardiac mitochondrial respiration is less sensitive to cellular pH changes. To test this, we used high-resolution respirometry to measure mitochondrial respiration by permeabilized cardiac muscle fibres from naked mole-rats and mice exposed in vitro to a pH range from 6.6 to 7.6. Surprisingly, we found that acute pH changes do not impact cardiac mitochondrial respiration or compromise mitochondrial integrity in either species. Our results suggest that acute alterations of cellular pH have minimal impact on cardiac mitochondrial respiration.

MitoWave: Spatiotemporal analysis of mitochondrial membrane potential fluctuations during I/R

Mitochondria exhibit unstable inner membrane potentials (ΔΨ_m) when subjected to stress, such as during ischemia/reperfusion (I/R). Understanding the mechanism of ΔΨ_m instability involves characterizing and quantifying this phenomenon in an unbiased and reproducible manner. Here, we describe a simple analytical workflow called "MitoWave" that combines wavelet transform methods and image segmentation to unravel dynamic ΔΨ_m changes in the cardiac mitochondrial network during I/R. In vitro ischemia was affected by placing a glass coverslip on a monolayer of neonatal mouse ventricular myocytes for 1 h and removing the coverslip to allow for reperfusion, revealing complex oscillatory ΔΨ_m. MitoWave analysis was then used to identify individual mitochondrial clusters within the cells and track their intrinsic oscillation frequencies over the course of reperfusion. Responses segregated into five typical behaviors were quantified by MitoWave that were corroborated by visual inspection of the time series. Statistical analysis of the distribution of oscillating mitochondrial clusters during reperfusion showed significant differences between the five different outcomes. Features such as the time point of ΔΨ_m depolarization during I/R, area of mitochondrial clusters, and time-resolved frequency components during reperfusion were determined per cell and per mitochondrial cluster. Mitochondria from neonatal mouse ventricular myocytes subjected to I/R oscillate in the frequency range of 8.6-45 mHz, with a mean of 8.73 ± 4.35 mHz. Oscillating clusters had smaller areas ranging from 49.8 ± 1.2 μm², whereas nonoscillating clusters had larger areas 66 ± 1.5 μm². A negative correlation between frequency and mitochondrial cluster area was observed. We also observed that late ΔΨ_m loss during ischemia correlated with early ΔΨ_m stabilization after oscillation on reperfusion. Thus, MitoWave analysis provides a semiautomated method to quantify complex time-resolved mitochondrial behavior in an easy-to-follow workflow, enabling unbiased, reproducible quantitation of complex nonstationary cellular phenomena.

Fenofibrate Protects Cardiomyocytes from Hypoxia/Reperfusion- and High Glucose-Induced Detrimental Effects

Lesions caused by high glucose (HG), hypoxia/reperfusion (H/R), and the coexistence of both conditions in cardiomyocytes are linked to an overproduction of reactive oxygen species (ROS), causing irreversible damage to macromolecules in the cardiomyocyte as well as its ultrastructure. Fenofibrate, a peroxisome proliferator-activated receptor alpha (PPARα) agonist, promotes beneficial activities counteracting cardiac injury. Therefore, the objective of this work was to determine the potential protective effect of fenofibrate in cardiomyocytes exposed to HG, H/R, and HG+H/R. Cardiomyocyte cultures were divided into four main groups: (1) control (CT), (2) HG (25 mM), (3) H/R, and (4) HG+H/R. Our results indicate that cell viability decreases in cardiomyocytes undergoing HG, H/R, and both conditions, while fenofibrate improves cell viability in every case. Fenofibrate also decreases ROS production as well as nicotinamide adenine dinucleotide phosphate oxidase (NADPH) subunit expression. Regarding the antioxidant defense, superoxide dismutase (SOD Cu²⁺/Zn²⁺ and SOD Mn²⁺), catalase, and the antioxidant capacity were decreased in HG, H/R, and HG+H/R-exposed cardiomyocytes, while fenofibrate increased those parameters. The expression of nuclear factor erythroid 2-related factor 2 (Nrf2) increased significantly in treated cells, while pathologies increased the expression of its inhibitor Keap1. Oxidative stress-induced mitochondrial damage was lower in fenofibrate-exposed cardiomyocytes. Endothelial nitric oxide synthase was also favored in cardiomyocytes treated with fenofibrate. Our results suggest that fenofibrate preserves the antioxidant status and the ultrastructure in cardiomyocytes undergoing HG, H/R, and HG+H/R preventing damage to essential macromolecules involved in the proper functioning of the cardiomyocyte.

A distinctive distribution of hypoxia-inducible factor-1α in cultured renal tubular cells with hypoperfusion simulated by coverslip placement

Chronic hypoxia in the renal tubulointerstitium plays a key role in the progression of chronic kidney disease (CKD). It is therefore important to investigate tubular hypoxia and the activity of hypoxia-inducible factor (HIF)-1α in response to hypoxia. Rarefaction of the peritubular capillary causes hypoperfusion in CKD; however, the effect of hypoperfusion on HIFs has rarely been investigated. We induced hypoperfusion caused by coverslip placement in human kidney-2 cells, and observed an oxygen gradient under the coverslip. Immunocytochemistry of HIF-1α showed a doughnut-shaped formation on the edge of a pimonidazole-positive area, which we named the "HIF-ring". The oxygen tension of the HIF-ring was estimated to be between approximately 4 mmHg and 20 mmHg. This result was not compatible with those of past research showing HIF-1α accumulation in the anoxic range with homogeneous oxygen tension. We further observed the presence of a pH gradient under a coverslip, as well as a shift of the HIF ring due to changes in the pH of the culture medium, suggesting that the HIF ring was formed by suppression of HIF-1α related to low pH. This research demonstrated that HIF-1α activation mimics the physiological state in cultured cells with hypoperfusion.

Paradoxical Onset of Arrhythmic Waves from Depolarized Areas in Cardiac Tissue Due to Curvature-Dependent Instability

The generation of abnormal excitations in pathological regions of the heart is a main trigger for lethal cardiac arrhythmias. Such abnormal excitations, also called ectopic activity, often arise from areas with local tissue heterogeneity or damage accompanied by localized depolarization. Finding the conditions that lead to ectopy is important to understand the basic biophysical principles underlying arrhythmia initiation and might further refine clinical procedures. In this study, we are the first to address the question of how geometry of the abnormal region affects the onset of ectopy using a combination of experimental, in silico, and theoretical approaches. We paradoxically find that, for any studied geometry of the depolarized region in optogenetically modified monolayers of cardiac cells, primary ectopic excitation originates at areas of maximal curvature of the boundary, where the stimulating electrotonic currents are minimal. It contradicts the standard critical nucleation theory applied to nonlinear waves in reaction-diffusion systems, where a higher stimulus is expected to produce excitation more easily. Our in silico studies reveal that the nonconventional ectopic activity is caused by an oscillatory instability at the boundary of the damaged region, the occurrence of which depends on the curvature of that boundary. The onset of this instability is confirmed using the Schrödinger equation methodology proposed by Rinzel and Keener [SIAM J. Appl. Math. 43, 907 (1983)]. Overall, we show distinctively novel insight into how the geometry of a heterogeneous cardiac region determines ectopic activity, which can be used in the future to predict the conditions that can trigger cardiac arrhythmias.

Restricted exchange microenvironments for cell culture

Metabolite diffusion in tissues produces gradients and heterogeneous microenvironments that are not captured in standard 2D cell culture models. Here we describe restricted exchange environment chambers (REECs) in which diffusive gradients are formed and manipulated on length scales approximating those found in vivo. In REECs, cells are grown in 2D in an asymmetric chamber (<50 μL) formed between a coverglass and a glass bottom cell culture dish separated by a thin (~100 μm) gasket. Diffusive metabolite exchange between the chamber and bulk media occurs through one or more openings micromachined into the coverglass. Cell-generated concentration gradients form radially in REECs with a single round opening (~200 μm diameter). At steady state only cells within several hundred micrometers of the opening experience metabolite concentrations that permit survival which is analogous to diffusive exchange near a capillary in tissue. The chamber dimensions, the openings' shape, size, and number, and the cellular density and metabolic activity define the gradient structure. For example, two parallel slots above confluent cells produce the 1D equivalent of a spheroid. Using REECs, we found that fibroblasts align along the axis of diffusion while MDCK cells do not. MDCK cells do, however, exhibit significant morphological variations along the diffusive gradient.

Engineering prokaryotic channels for control of mammalian tissue excitability

The ability to directly enhance electrical excitability of human cells is hampered by the lack of methods to efficiently overexpress large mammalian voltage-gated sodium channels (VGSC). Here we describe the use of small prokaryotic sodium channels (BacNa_v) to create de novo excitable human tissues and augment impaired action potential conduction in vitro. Lentiviral co-expression of specific BacNa_v orthologues, an inward-rectifying potassium channel, and connexin-43 in primary human fibroblasts from the heart, skin or brain yields actively conducting cells with customizable electrophysiological phenotypes. Engineered fibroblasts ('E-Fibs') retain stable functional properties following extensive subculture or differentiation into myofibroblasts and rescue conduction slowing in an in vitro model of cardiac interstitial fibrosis. Co-expression of engineered BacNa_v with endogenous mammalian VGSCs enhances action potential conduction and prevents conduction failure during depolarization by elevated extracellular K⁺, decoupling or ischaemia. These studies establish the utility of engineered BacNa_v channels for induction, control and recovery of mammalian tissue excitability.

Glycolytic Enzymes Localize to Synapses under Energy Stress to Support Synaptic Function

Changes in neuronal activity create local and transient changes in energy demands at synapses. Here we discover a metabolic compartment that forms in vivo near synapses to meet local energy demands and support synaptic function in Caenorhabditis elegans neurons. Under conditions of energy stress, glycolytic enzymes redistribute from a diffuse localization in the cytoplasm to a punctate localization adjacent to synapses. Glycolytic enzymes colocalize, suggesting the ad hoc formation of a glycolysis compartment, or a "glycolytic metabolon," that can maintain local levels of ATP. Local formation of the glycolytic metabolon is dependent on presynaptic scaffolding proteins, and disruption of the glycolytic metabolon blocks the synaptic vesicle cycle, impairs synaptic recovery, and affects locomotion. Our studies indicate that under energy stress conditions, energy demands in C. elegans synapses are met locally through the assembly of a glycolytic metabolon to sustain synaptic function and behavior. VIDEO ABSTRACT.

Glucose-insulin-potassium solution protects ventricular myocytes of neonatal rat in an in vitro coverslip ischemia/reperfusion model

Background and Objectives

The benefit of high glucose-insulin-potassium (GIK) solution in clinical applications is controversial. We established a neonatal rat ventricular myocyte (NRVM) in vitro coverslip ischemia/reperfusion (I/R) model and investigated the effects of GIK solution on suppressing reactive oxygen species (ROS) and upregulating O-GlcNacylation, which protects cells from ischemic injury.

Materials and Methods

NRVMs were isolated from postnatal day 3-4 Sprague-Dawley rat pups and grown in Dulbecco's modified Eagle's medium containing high glucose (4.5 g/L), fetal bovine serum, and penicillin/streptomycin. The effects of the GIK solution on ROS production, apoptosis, and expression of O-GlcNAc and O-GlcNAc transferase (OGT) were investigated in the coverslip I/R model.

Results

Covering the 24-well culture plates for 3 hr with 12 mm diameter coverslips resulted in the appropriate ischemic shock. Glucose and insulin synergistically reduced ROS production, protected NRVM dose-dependently from apoptosis, and altered O-GlcNAc and OGT expression.

Conclusion

The high GIK solution protected NRVM from I/R injury in vitro by reducing ROS and altering O-GlcNacylation.

Mitochondrial instability during regional ischemia-reperfusion underlies arrhythmias in monolayers of cardiomyocytes

Regional depolarization of the mitochondrial network can alter cellular electrical excitability and increase the propensity for reentry, in part, through the opening of sarcolemmal KATP channels. Mitochondrial inner membrane potential (ΔΨm) instability or oscillation can be induced in myocytes by exposure to reactive oxygen species (ROS), laser excitation, or glutathione depletion, and is thought to be a major factor in arrhythmogenesis during ischemia-reperfusion. Nevertheless, the correlation between ΔΨm recovery kinetics and reperfusion-induced arrhythmias has been difficult to demonstrate experimentally. Here, we investigate the relationship between subcellular changes in ΔΨm, cellular glutathione redox potential, electrical excitability, and wave propagation during coverslip-induced ischemia-reperfusion (IR) in neonatal rat ventricular myocyte (NRVM) monolayers. Ischemia led to decreased action potential amplitude and duration followed by electrical inexcitability after ~15min of ischemia. ΔΨm depolarization occurred in two phases during ischemia: in phase 1 (<30min ischemia), mitochondrial clusters within individual NRVMs depolarized, while phase 2 ΔΨm depolarization (30-60min) was characterized by global functional collapse of the mitochondrial network across the whole ischemic region of the monolayer, typically involving a propagating metabolic wave. Oxidation of the glutathione (GSSG:GSH) redox potential occurred during ischemia, followed by recovery upon reperfusion (i.e., lifting the coverslip). ΔΨm recovered in the mitochondria of individual myocytes quite rapidly upon reperfusion (<5min), but was highly unstable, characterized by subcellular oscillations or flickering of clusters of mitochondria in NRVMs across the reperfused region. Electrical excitability also recovered in a heterogeneous manner, providing an arrhythmogenic substrate which led to formation of sustained reentry. Treatment with 4'-chlorodiazepam, a peripheral benzodiazepine receptor ligand, prevented ΔΨm oscillation, improved GSH recovery rate, and prevented reentry during reperfusion, indicating that stabilization of mitochondrial network dynamics is important for preventing post-ischemic arrhythmias. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

Pacing-induced non-uniform ca(2+) dynamics in rat atria revealed by rapid-scanning confocal microscopy

Intracellular Ca(2+) ([Ca(2+)]i) dynamics in isolated myocytes differ between the atria and ventricles due to the distinct t-tubular distributions. Although cellular aspects of ventricular [Ca(2+)]i dynamics in the heart have been extensively studied, little is known about those of atrial myocytes in situ. Here we visualized precise [Ca(2+)]i dynamics of atrial myocytes in Langendorff-perfused rat hearts by rapid-scanning confocal microscopy. Of 16 fluo-4-loaded hearts imaged during pacing up to 4-Hz, five hearts showed spatially uniform Ca(2+) transients on systole among individual cells, whereas no discernible [Ca(2+)]i elevation developed during diastole. In contrast, the remaining hearts showed non-uniform [Ca(2+)]i dynamics within and among the cells especially under high-frequency (4 Hz) excitation, where subcellular cluster-like [Ca(2+)]i rises or wave-like [Ca(2+)]i propagation occurred on excitation. Such [Ca(2+)]i inhomogeneity was more pronounced at high-frequency pacing, showing beat-to-beat Ca(2+) transient alternans. Despite such non-uniform dynamics, cessation of burst pacing of the atria was not followed by emergence of spontaneous Ca(2+) waves, indicating minor Ca(2+)-releasing potentials of the sarcoplasmic reticulum (SR). In summary, rat atria display a propensity to show non-uniform [Ca(2+)]i dynamics on systole due to impaired Ca(2+)-release from the SR and paucity of t-tubules. Our results provide an important basis for understanding atrial pathophysiology.

Pre-conditioning with CDP-choline attenuates oxidative stress-induced cardiac myocyte death in a hypoxia/reperfusion model

Background. CDP-choline is a key intermediate in the biosynthesis of phosphatidylcholine, which is an essential component of cellular membranes, and a cell signalling mediator. CDP-choline has been used for the treatment of cerebral ischaemia, showing beneficial effects. However, its potential benefit for the treatment of myocardial ischaemia has not been explored yet. Aim. In the present work, we aimed to evaluate the potential use of CDP-choline as a cardioprotector in an in vitro model of ischaemia/reperfusion injury. Methods. Neonatal rat cardiac myocytes were isolated and subjected to hypoxia/reperfusion using the coverslip hypoxia model. To evaluate the effect of CDP-choline on oxidative stress-induced reperfusion injury, the cells were incubated with H₂O₂ during reperfusion. The effect of CDP-choline pre- and postconditioning was evaluated using the cell viability MTT assay, and the proportion of apoptotic and necrotic cells was analyzed using the Annexin V determination by flow cytometry. Results. Pre- and postconditioning with 50 mg/mL of CDP-choline induced a significant reduction of cells undergoing apoptosis after hypoxia/reperfusion. Preconditioning with CDP-choline attenuated postreperfusion cell death induced by oxidative stress. Conclusion. CDP-choline administration reduces cell apoptosis induced by oxidative stress after hypoxia/reperfusion of cardiac myocytes. Thus, it has a potential as cardioprotector in ischaemia/reperfusion-injured cardiomyocytes.

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.

Optical imaging of arrhythmias in the cardiomyocyte monolayer

In recent years, cultured cardiac cell monolayers have become a contemporary experimental preparation for the study of fundamental mechanisms that underlie normal and pathologic electrophysiology at the tissue level. Ion channels and gap junctions in the cardiomyocyte monolayer may be modulated using drugs that suppress or enhance certain channels/junctions, or by genetic silencing or overexpression. The cardiomyocyte monolayer is particularly well suited for studies of functional electrophysiologic properties of mixtures of cardiac and noncardiac cells (eg, myofibroblasts), which otherwise would be difficult to investigate. Optical mapping of monolayers has provided insight into mechanisms that can set the stage for arrhythmias, such as unidirectional conduction block, gap junction uncoupling, ischemia, alternans, and anisotropy, and continues to enhance our understanding of basic electrophysiologic mechanisms.

Development of an in vitro model of myotube ischemia

Critical limb ischemia causes severe damage to the skeletal muscle. This study develops a reproducible model of myotube ischemia by simulating, in vitro, the critical parameters that occur in skeletal muscle ischemia. Monolayers of C2C12 myoblasts were differentiated into mature myotubes and exposed to nutrition depletion, hypoxia and hypercapnia for variable time periods. A range of culture media and gas mixture combinations were used to obtain an optimum ischemic environment. Nuclear staining, cleaved caspase-3 and lactate dehydrogenase (LDH) release assay were used to assess apoptosis and myotube survival. HIF-1α concentration of cell lysates, pH of conditioned media as well as partial pressures of oxygen (PO₂) and carbon dioxide (PCO₂) in the media were used to confirm ischemic simulation. Culturing myotubes in depleted media, in a gas mixture containing 20% CO+80% N₂ for 6-12 h increased the PCO₂ and decreased the pH and PO₂ of culture media. This attempts to mimic the in vivo ischemic state of skeletal muscle. These conditions were used to study the potential tissue-protective effects of erythropoietin (EPO) in C2C12 myotubes exposed to ischemia. EPO (60 ng/ml) suppressed LDH release, decreased cleaved caspase-3 and reduced the number of apoptotic nuclei, suggesting significantly decreased ischemia-induced apoptosis in myotubes (P<0.01) and a potential role in tissue protection. Additional therapeutic agents designed for tissue protection can also be evaluated using this model.

Engineered heart tissue enables study of residual undifferentiated embryonic stem cell activity in a cardiac environment

Embryonic stem cell (ESC) derivatives are a promising cell source for cardiac cell therapy. Mechanistic studies upon cell injection in conventional animal models are limited by inefficient delivery and poor cell survival. As an alternative, we have used an engineered heart tissue (EHT) based on neonatal rat cardiomyocytes (CMs) cultivated with electrical field stimulation as an in vitro model to study cell injection. We injected (0.001, 0.01, and 0.1 million) and tracked (by qPCR and histology) undifferentiated yellow-fluorescent protein transgenic mouse ESCs and Flk1 + /PDGFRα+ cardiac progenitor (CPs) cells, to investigate the effect of the cardiac environment on cell differentiation, as well as to test whether our in vitro model system could recapitulate the formation of teratoma-like structures commonly observed upon in vivo ESC injection. By 8 days post-injection, ESCs were spatially segregated from the cardiac cell population; however, ESC injection increased survival of CMs. The presence of ESCs blocked electrical conduction through the tissue, resulting in a 46% increase in the excitation threshold. Expression of mouse cardiac troponin I, was markedly increased in CP injected constructs compared to ESC injected constructs at all time points and cell doses tested. As early as 2 weeks, epithelial and ganglion-like structures were observed in ESC injected constructs. By 4 weeks of ESC injection, teratoma-like structures containing neural, epithelial, and connective tissue were observed in the constructs. Non-cardiac structures were observed in the CP injected constructs only after extended culture (4 weeks) and only at high cell doses, suggesting that these cells require further enrichment or differentiation prior to transplantation. Our data indicate that the cardiac environment of host tissue and electrical field stimulation did not preferentially guide the differentiation of ESCs towards the cardiac lineage. In the same environment, injection of CP resulted in a more robust cardiac differentiation than injection of ESC. Our data demonstrate that the model-system developed herein can be used to study the functional effects of candidate stem cells on the host myocardium, as well as to measure the residual activity of undifferentiated cells present in the mixture.

Modulation of Na(+)-K(+)-ATPase cell surface abundance through structural determinants on the α1-subunit

Through their ion-pumping and non-ion-pumping functions, Na(+)-K(+)-ATPase protein complexes at the plasma membrane are critical to intracellular homeostasis and to the physiological and pharmacological actions of cardiotonic steroids. Alteration of the abundance of Na(+)-K(+)-ATPase units at the cell surface is one of the mechanisms for Na(+)-K(+)-ATPase regulation in health and diseases that has been closely examined over the past few decades. We here summarize these findings, with emphasis on studies that explicitly tested the involvement of defined regions or residues on the Na(+)-K(+)-ATPase α1 polypeptide. We also report new findings on the effect of manipulating Na(+)-K(+)-ATPase membrane abundance by targeting one of these defined regions: a dileucine motif of the form [D/E]XXXL[L/I]. In this study, opossum kidney cells stably expressing rat α1 Na(+)-K(+)-ATPase or a mutant where the motif was disrupted (α1-L499V) were exposed to 30 min of substrate/coverslip-induced-ischemia followed by reperfusion (I-R). Biotinylation studies suggested that I-R itself acted as an inducer of Na(+)-K(+)-ATPase internalization and that surface expression of the mutant was higher than the native Na(+)-K(+)-ATPase before and after ischemia. Annexin V/propidium iodide staining and lactate dehydrogenase release suggested that I-R injury was reduced in α1-L499V-expressing cells compared with α1-expressing cells. Hence, modulation of Na(+)-K(+)-ATPase cell surface abundance through structural determinants on the α-subunit is an important mechanism of regulation of cellular Na(+)-K(+)-ATPase in various physiological and pathophysiological conditions, with a significant impact on cell survival in face of an ischemic stress.

Assessment of fetal canine cardiac myocyte survival in a dual-side flow bioreactor with modeled hypoxia/reperfusion injury

The present study introduces a new experimental model of hypoxia/reperfusion injury using a newly developed bioreactor system. The injury is introduced and kept localized via fluid dynamic manipulation. Using low Reynolds number fluid flow, regions of the culture can be injured while maintaining physiological conditions in the remaining culture. This approach enables both normal and injured cells within the same monolayer to be investigated side-by-side. The current study evaluated the ability of the model to induce localized reperfusion injury in a monolayer of fetal canine cardiomyocytes (FCCs). Significant apoptosis was found in the hypoxia/reperfusion-injured but not normal-flow regions of the myocyte cultures. The model holds the potential to help elucidate the fundamental mechanisms of hypoxic/reperfusion insults in myocardium.

Electrophysiological consequences of acute regional ischemia/reperfusion in neonatal rat ventricular myocyte monolayers

BACKGROUND

Electrophysiological changes promoting arrhythmias during acute regional ischemia/reperfusion are challenging to study in intact cardiac tissue because of complex 3-dimensional myocardial and vascular geometry. We characterized electrophysiological alterations and arrhythmias during regional ischemia/reperfusion in a simpler 2-dimensional geometry of cultured neonatal rat ventricular myocyte monolayers.

METHODS AND RESULTS

Optical mapping of intracellular Ca (Ca(i)) and voltage was performed with the use of Rhod 2-AM and Rh-237, respectively. Regional ischemia was mimicked by covering the central portion of monolayer with a glass coverslip, and reperfusion was mimicked by removing the coverslip. Monolayers were stained with fluorescent antibodies to detect total and dephosphorylated connexin-43 at various time points. During coverslip ischemia, action potential duration shortened, Ca(i) transient duration was prolonged, and local conduction velocity (CV) slowed progressively, with loss of excitability after 10.6 +/- 3.6 minutes. CV slowing was accompanied by connexin-43 dephosphorylation. During ischemia, spontaneous reentry occurred in 5 of 11 monolayers, initiated by extrasystoles arising from the border zone or unidirectional conduction block of paced beats. On reperfusion, excitability recovered within 1.0 +/- 0.8 minutes, but CV remained depressed for 9.0 +/- 3.0 minutes, promoting reentry in the reperfused zone. As connexin-43 phosphorylation recovered in the reperfused zone, CV normalized, and arrhythmias resolved.

CONCLUSIONS

Acute regional ischemia/reperfusion in neonatal rat ventricular myocyte monolayers recapitulates electrophysiological alterations and arrhythmias similar to those observed during acute coronary occlusion/reperfusion in intact hearts. During early reperfusion, slow recovery from connexin-43 dephosphorylation leads to persistent CV slowing, creating a highly arrhythmogenic substrate.

Region of slowed conduction acts as core for spiral wave reentry in cardiac cell monolayers

Pathophysiological heterogeneity in cardiac tissue is related to the occurrence of arrhythmias. Of importance are regions of slowed conduction, which have been implicated in the formation of conduction block and reentry. Experimentally, it has been a challenge to produce local heterogeneity in a manner that is both reversible and well controlled. Consequently, we developed a dual-zone superfusion chamber that can dynamically create a small (5 mm) central island of heterogeneity in cultured cardiac cell monolayers. Three different conditions were studied to explore the effect of regionally slowed conduction on wave propagation and reentry: depolarization by elevated extracellular potassium, sodium channel inhibition with lidocaine, and cell-cell decoupling with palmitoleic acid. Using optical mapping of transmembrane voltage, we found that the central region of slowed conduction always served as the core region around which a spiral wave formed and then revolved following a period of rapid pacing. Because of the localized slowing in the core region, we observed experimentally for the first time an S shape of the spiral wave front near its tip. These results indicate that a small region of slowed conduction can play a crucial role in the formation, anchoring, and modulation of reentrant spiral waves.

Differentially regulated functional gene clusters identified during ischemia and reperfusion in isolated cardiac myocytes using coverslip hypoxia

INTRODUCTION

Coverslip hypoxia (CSH) is a recently described method for producing rapid and severe ischemia derived from the metabolic activity of synchronously contracting isolated neonatal rat ventricular myocytes (NRVMs). While the effect of acute ischemia produced by CSH is documented, the contribution of reperfusion to cell viability has not been fully studied.

METHODS

We therefore used fluorescence microscopy and expression profiling by microarray to determine the morphological and genetic effects in NRVMs of both the ischemic and reperfusion events of CSH.

RESULTS

Fluorescence microscopy studies in coverslipped NRVMs showed cell death at 1 h as previously reported. Matched samples coverslipped for up to 2 h and then reperfused 18 h showed myocyte recovery prior to but not beyond 1 h upon post-staining, suggesting a limited window of recovery. Expression profiling of more than 30,000 genes using total RNA collected from NRVMs subjected to varying periods of ischemia and reperfusion revealed 103 genes regulated at least 2-fold at p<10(-7). These genes fall into discrete functional groups including apoptosis, metabolism, and hypoxia/acidosis. The regulation of a subset of genes from these groups was confirmed by RT-PCR. Interestingly, the hypoxia/acidosis gene BNip3 (a Bcl-2 family member implicated in hypoxia/acidosis-associated cell death) was upregulated early during ischemia and persisted throughout reperfusion. In addition, other hypoxia/acidosis genes such as heme oxygenase 1, pyruvate dehydrogenase kinase 1, prolyl hydroxylases, and hypoxia-inducible protein 2 were upregulated.

DISCUSSION

These data suggest that the ischemic and reperfusion events created by CSH induce gene regulation within distinct functional groups related to in vivo ischemia.

Experimental strategies to improve in vitro models of renal ischemia

Ischemia has elicited a great deal of interest among the scientific community due to its role in life-threatening pathologies such as cancer, stroke, acute renal failure, and myocardial infarction. Oxygen deprivation (hypoxia) associated with ischemia has recently become a subject of intense scrutiny. New investigators may find it challenging to induce hypoxic injury in vitro. Researchers may not always be aware of the experimental barriers that contribute to this phenomenon. Furthermore, ischemia is associated with other major insults, such as excess carbon dioxide (hypercapnia), nutrient deprivation, and accumulation of cellular wastes. Ideally, these conditions should also be incorporated into in vitro models. Therefore, the motivation behind this review is to: i. delineate major in vivo ischemic insults; ii. identify and explain critical in vitro parameters that need to be considered when simulating ischemic pathologies; iii. provide recommendations to improve experiments; and as a result, iv. enhance the validity of in vitro results for understanding clinical ischemic pathologies. Undoubtedly, it is not possible to completely replicate the in vivo environment in an ex vivo model system. In fact, the primary goal of many in vitro studies is to elucidate the role of specific stimuli during in vivo pathological events. This review will present methodologies that may be implemented to improve the applicability of in vitro models for understanding the complex pathological mechanisms of ischemia. Finally, although these topics will be discussed within the context of renal ischemia, many are pertinent for cellular models of other organ systems and pathologies.

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

In vitro experimental models designed to study the effects of hypoxia and ischemia typically employ oxygen-depleted media and/or hypoxic chambers. These approaches, however, allow for metabolites to diffuse away into a large volume and may not replicate the local buildup of metabolic byproducts that occur in ischemic myocardium in vivo. Coverslip hypoxia (CSH) is a recently described method for studying hypoxia and ischemia derived from the byproducts and metabolites of contractile ventricular myocytes. Hence, this method is dependent on the purity and contractile activity of the isolated myocytes. We describe herein methods for isolating neonatal rat ventricular myocytes with these characteristics, as well as means for performing CSH, identifying viable and compromised myocytes after coverslipping, and tracking pH changes during CSH.

Three-dimensional mitochondrial arrangement in ventricular myocytes: from chaos to order

We have developed a novel method to quantitatively analyze mitochondrial positioning in three dimensions. Using this method, we compared the relative positioning of mitochondria in adult rat and rainbow trout (Oncorhynchus mykiss) ventricular myocytes. Energetic data suggest that trout, in contrast to the rat, have two subpopulations of mitochondria in their cardiomyocytes. Therefore, we speculated whether trout cardiomyocytes exhibit two types of mitochondrial patterns. Stacks of confocal images of mitochondria were acquired in live cardiomyocytes. The images were processed and mitochondrial centers were detected automatically. The mitochondrial arrangement was analyzed by calculating the three-dimensional probability density and distribution functions describing the distances between neighboring mitochondrial centers. In the rat (8 cells with a total of 7,546 mitochondrial centers), intermyofibrillar mitochondria are highly ordered and arranged in parallel strands. These strands are separated by approximately 1.8 mum and can be found in any transversal direction relative to each other. Neighboring strands exhibit the same mitochondrial periodicity. In contrast to the rat, trout ventricular myocytes (22 cells; 5,528 mitochondrial centers) exhibit a relatively chaotic mitochondrial pattern. Neighboring mitochondria can be found in any direction relative to each other. Thus, two potential subpopulations of mitochondria in trout are not distinguishable by their pattern. The developed method required minor interaction in the filtering of the mitochondrial centers. It is therefore a practical approach to describe intracellular organization and may also be used for analysis of time-dependent organizational changes. The obtained quantitative description of mitochondrial organization is a requisite for accurate mathematical analysis of mitochondrial systems biology.