Quantification of cardiomyocyte contraction based on image correlation analysis

Assessing Cardiac Contractility From Single Molecules to Whole Hearts

Fundamentally, the heart needs to generate sufficient force and power output to dynamically meet the needs of the body. Cardiomyocytes contain specialized structures referred to as sarcomeres that power and regulate contraction. Disruption of sarcomeric function or regulation impairs contractility and leads to cardiomyopathies and heart failure. Basic, translational, and clinical studies have adapted numerous methods to assess cardiac contraction in a variety of pathophysiological contexts. These tools measure aspects of cardiac contraction at different scales ranging from single molecules to whole organisms. Moreover, these studies have revealed new pathogenic mechanisms of heart disease leading to the development of novel therapies targeting contractility. In this review, the authors explore the breadth of tools available for studying cardiac contractile function across scales, discuss their strengths and limitations, highlight new insights into cardiac physiology and pathophysiology, and describe how these insights can be harnessed for therapeutic candidate development and translational.

Unlocking cardiac motion: assessing software and machine learning for single-cell and cardioid kinematic insights

The heart coordinates its functional parameters for optimal beat-to-beat mechanical activity. Reliable detection and quantification of these parameters still represent a hot topic in cardiovascular research. Nowadays, computer vision allows the development of open-source algorithms to measure cellular kinematics. However, the analysis software can vary based on analyzed specimens. In this study, we compared different software performances in in-silico model, in-vitro mouse adult ventricular cardiomyocytes and cardioids. We acquired in-vitro high-resolution videos during suprathreshold stimulation at 0.5-1-2 Hz, adapting the protocol for the cardioids. Moreover, we exposed the samples to inotropic and depolarizing substances. We analyzed in-silico and in-vitro videos by (i) MUSCLEMOTION, the gold standard among open-source software; (ii) CONTRACTIONWAVE, a recently developed tracking software; and (iii) ViKiE, an in-house customized video kinematic evaluation software. We enriched the study with three machine-learning algorithms to test the robustness of the motion-tracking approaches. Our results revealed that all software produced comparable estimations of cardiac mechanical parameters. For instance, in cardioids, beat duration measurements at 0.5 Hz were 1053.58 ms (MUSCLEMOTION), 1043.59 ms (CONTRACTIONWAVE), and 937.11 ms (ViKiE). ViKiE exhibited higher sensitivity in exposed samples due to its localized kinematic analysis, while MUSCLEMOTION and CONTRACTIONWAVE offered temporal correlation, combining global assessment with time-efficient analysis. Finally, machine learning reveals greater accuracy when trained with MUSCLEMOTION dataset in comparison with the other software (accuracy > 83%). In conclusion, our findings provide valuable insights for the accurate selection and integration of software tools into the kinematic analysis pipeline, tailored to the experimental protocol.

Stretch Harmonizes Sarcomere Strain Across the Cardiomyocyte

BACKGROUND

Increasing cardiomyocyte contraction during myocardial stretch serves as the basis for the Frank-Starling mechanism in the heart. However, it remains unclear how this phenomenon occurs regionally within cardiomyocytes, at the level of individual sarcomeres. We investigated sarcomere contractile synchrony and how intersarcomere dynamics contribute to increasing contractility during cell lengthening.

METHODS

Sarcomere strain and Ca2+ were simultaneously recorded in isolated left ventricular cardiomyocytes during 1 Hz field stimulation at 37 °C, at resting length and following stepwise stretch.

RESULTS

We observed that in unstretched rat cardiomyocytes, differential sarcomere deformation occurred during each beat. Specifically, while most sarcomeres shortened during the stimulus, ≈10% to 20% of sarcomeres were stretched or remained stationary. This nonuniform strain was not traced to regional Ca2+ disparities but rather shorter resting lengths and lower force production in systolically stretched sarcomeres. Lengthening of the cell recruited additional shortening sarcomeres, which increased contractile efficiency as less negative, wasted work was performed by stretched sarcomeres. Given the known role of titin in setting sarcomere dimensions, we next hypothesized that modulating titin expression would alter intersarcomere dynamics. Indeed, in cardiomyocytes from mice with titin haploinsufficiency, we observed greater variability in resting sarcomere length, lower recruitment of shortening sarcomeres, and impaired work performance during cell lengthening.

CONCLUSIONS

Graded sarcomere recruitment directs cardiomyocyte work performance, and harmonization of sarcomere strain increases contractility during cell stretch. By setting sarcomere dimensions, titin controls sarcomere recruitment, and its lowered expression in haploinsufficiency mutations impairs cardiomyocyte contractility.

Crack Sensing of Cardiomyocyte Contractility with High Sensitivity and Stability

Measuring myocardial contractility is of great value in exploring cardiac pathogenesis and quantifying drug efficacy. Among the biosensing platforms developed for detecting the weak contractility of a single layer of cardiomyocytes (CMs), thin brittle metal membrane sensors with microcracks are highly sensitive. However, their poor stability limits the application in long-term measurement. Here, we report a high stability crack sensor fabricated by deposition of a 105 nm thick Ag/Cr with microcracks onto a carbon nanotubes-polydimethylsiloxane (CNT-PDMS) layer. This brittle-tough bilayer crack sensor achieved high sensitivity (gauge factor: 108 241.7), a wide working range (0.01-44%), and high stability (stable period >2 000 000 cycles under the strain caused by a monolayer of CMs). During 14-day continuously monitoring CMs culturing and drug treatment testings, the device demonstrated high sensitivity and stability to record the dynamic change caused by contractility of the CMs.

Changes in extracellular matrix in failing human non-ischemic and ischemic hearts with mechanical unloading

Ischemic and non-ischemic cardiomyopathies have distinct etiologies and underlying disease mechanisms, which require in-depth investigation for improved therapeutic interventions. The goal of this study was to use clinically obtained myocardium from healthy and heart failure patients, and characterize the changes in extracellular matrix (ECM) in ischemic and non-ischemic failing hearts, with and without mechanical unloading. Using tissue engineering methodologies, we also investigated how diseased human ECM, in the absence of systemic factors, can influence cardiomyocyte function. Heart tissues from heart failure patients with ischemic and non-ischemic cardiomyopathy were compared to explore differential disease phenotypes and reverse remodeling potential of left ventricular assisted device (LVAD) support at transcriptomic, proteomic and structural levels. The collected data demonstrated that the differential ECM compositions recapitulated the disease microenvironment and induced cardiomyocytes to undergo disease-like functional alterations. In addition, our study also revealed molecular profiles of non-ischemic and ischemic heart failure patients and explored the underlying mechanisms of etiology-specific impact on clinical outcome of LVAD support and tendency towards reverse remodeling.

Image entropy-based label-free functional characterization of human induced pluripotent stem cell-derived 3D cardiac spheroids

Human induced pluripotent stem cell-derived cardiac spheroids (iPSC-CSs) in 3D possess tremendous potential for treating heart diseases and screening drugs for their cardiac effect. The beating pattern (including beating frequency and amplitude) of iPSC-CSs is a direct indicator of their health and function. However, detecting the beating pattern of 3D cardiac spheroid is not well studied and the probes commonly used for labeling cardiomyocytes for their beating pattern detection is toxic during long-term culture. Here, we reveal that the beating pattern of 3D iPSC-CSs can be conveniently detected/quantified by calculating the relative change of entropy in all the frames/images of non-fluorescent optical signal without labeling any cells. The entropy rate superpixel segmentation method is used for image segmentation in frames containing multiple or aggregated iPSC-CSs to identify individual iPSC-CSs, enabling rapid detection/quantification of the beating pattern of each iPSC-CS. Moreover, the responses of iPSC-CSs to both anticancer and cardiac drugs can be reliably detected with the image entropy-based label-free method in terms of their beating patterns. This novel label-free approach may be valuable for convenient and efficient functional evaluation of 3D and 2D cardiac constructs, which is important not only for drug screening but also the advancement of manufacturing functional cardiac constructs to treat heart diseases.

A Numerical Model of a Perforated Microcantilever Covered with Cardiomyocytes to Improve the Performance of the Microcantilever Sensor

A few simple polymeric microsystems, such as microcantilever sensors, have recently been developed for the preliminary screening of cardiac toxicity. The microcantilever deflection produced by a change in the cardiomyocyte (CM) contraction force is important for understanding the mechanism of heart failure. In this study, a new numerical model is proposed to analyze the contractile behavior of CMs cultured on a perforated microcantilever surface for improving the performance of the microcantilever sensor. First, the surface traction model is used to investigate the bending displacement of the plain microcantilever. In order to improve the bending effect, a new numerical model is developed to analyze the bending behavior of the perforated microcantilever covered with CMs. Compared with the designed molds, the latter yields better results. Finally, a simulation analysis is proposed based on a finite element method to verify the presence of a preformed mold. Moreover, the effects of various factors on the bending displacement, including microcantilever size, Young's modulus, and porosity factor, are investigated. Both the simulation and numerical results have good consistency, and the maximum error between the numerical and simulation results is not more than 3.4%, even though the porosity factor reaches 0.147. The results show that the developed mold opens new avenues for CM microcantilever sensors to detect cardiac toxicity.

Improvement of the signal-to-noise ratio in a low power self-mixing interferometer using a coupled interferometric effect

We present experimental results of a low-emission self-mixing interferometer that uses a coupled interferometric effect to improve the signal produced by a vibrating target. This method is intended to be useful in applications where the target is prone to be damaged by high-intensity laser sources. The beam of a Fabry-Perot laser diode is split and ∼21% of the original emission is used to measure the harmonic micro-displacements of the target using the self-mixing effect. A portion of the residual beam, which also carries the interferometric information related to the target displacement, is reinjected back into the laser cavity by means of a fixed reflector, causing a second interferometric phenomenon that improves the signal-to-noise ratio of the measurement by up to ∼13 dB. A theoretical description of the phenomena is also proposed. Further, we apply this technique to the two most common self-mixing sensing schemes: internal photodiode and junction voltage. The reported results show good agreement with theory and prove the capability of the method to enhance the SNR in SMI schemes.

Creation of a contractile biomaterial from a decellularized spinach leaf without ECM protein coating: An in vitro study

Myocardial infarction (MI) results in the death of cardiac tissue, decreases regional contraction, and can lead to heart failure. Tissue engineered cardiac patches containing human induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs) can restore contractile function. However, cells within thick patches require vasculature for blood flow. Recently, we demonstrated fibronectin coated decellularized leaves provide a suitable scaffold for hiPS-CMs. Yet, the necessity of this additional coating step is unclear. Therefore, we compared hiPS-CM behavior on decellularized leaves coated with collagen IV or fibronectin extracellular matrix (ECM) proteins to noncoated leaves for up to 21 days. Successful coating was verified by immunofluorescence. Similar numbers of hiPS-CMs adhered to coated and noncoated decellularized leaves for 21 days. At Day 14, collagen IV coated leaves contracted more than noncoated leaves (3.25 ± 0.39% vs. 1.54 ± 0.60%; p < .05). However, no differences in contraction were found between coated leaves, coated tissue culture plastic (TCP), noncoated leaves, or noncoated TCP at other time points. No significant differences were observed in hiPS-CM spreading or sarcomere lengths on leaves with or without coating. This study demonstrates that cardiac scaffolds can be created from decellularized leaves without ECM coatings. Noncoated decellularized leaf surfaces facilitate robust cell attachment for an engineered tissue patch.

Muscular Thin Films for Label-Free Mapping of Excitation Propagation in Cardiac Tissue

Muscular thin films (MTFs), have already found a variety of applications in cardiac tissue engineering and in building of lab-on-a-chip systems. Here we present a novel approach to label-free mapping of excitation waves in the cardiomyocyte cell cultures with the use of MTFs. Neonatal rat ventricular cardiomyocytes were cultured on polydimethylsiloxane (PDMS) thin films and observed by means of off-axis illumination. Inflexions of the membrane created by the contraction of cardiomyocytes led to formation of patterns of bright and dark areas on the surface of the membrane. These patterns were recorded and analyzed for the monitoring of the contraction propagation. The method was compared with a standard optical mapping technique based on the use of a Ca²⁺-sensitive fluorescent dye. A good consistency of the results obtained by these two methods was demonstrated. The proposed method is non-toxic and might be of particular interest for the purpose of continuous monitoring in test systems based on human induced pluripotent stem cells.

Diffraction-based technology for the monitoring of contraction dynamics in 3D and 2D tissue models

We present a novel optical device developed for the monitoring of dynamic behavior in extended 3D-tissue models in various culture environments based on variations in their speckle patterns. The results presented point out the benefit of the technology in terms of detection, accuracy, sensitivity and a reasonable read-out speed as well as reproducibility for the measurements and monitoring of cardiac contractions. We show that the optical read-out technology is suitable for long time monitoring and for drug screening. The method is discussed and compared to other techniques, in particular calcium imaging. The device is flexible and easily adaptable to 2D and 3D-tissue model screenings using different culture environments. The technology can be parallelized for automated read-out of different multi-well-plate formats.

Spatiotemporal Characterizations of Spontaneously Beating Cardiomyocytes with Adaptive Reference Digital Image Correlation

We developed an Adaptive Reference-Digital Image Correlation (AR-DIC) method that enables unbiased and accurate mechanics measurements of moving biological tissue samples. We applied the AR-DIC analysis to a spontaneously beating cardiomyocyte (CM) tissue, and could provide correct quantifications of tissue displacement and strain for the beating CMs utilizing physiologically-relevant, sarcomere displacement length-based contraction criteria. The data were further synthesized into novel spatiotemporal parameters of CM contraction to account for the CM beating homogeneity, synchronicity, and propagation as holistic measures of functional myocardial tissue development. Our AR-DIC analyses may thus provide advanced non-invasive characterization tools for assessing the development of spontaneously contracting CMs, suggesting an applicability in myocardial regenerative medicine.

Clonal Isolation of Human Pluripotent Stem Cells on Nanofibrous Substrates Reveals an Advanced Subclone for Cardiomyocyte Differentiation

Human pluripotent stem cells (hPSCs) have been widely used for various applications including disease modeling and regenerative medicine, among others. Recently, an increasing number of studies has focused on heterogeneity among hPSCs, which could affect cell quality and subsequent applications. In this study, a nanofibrous platform is developed for single human induced pluripotent stem cell isolation and culture. One type of single cell-derived subclone is established and found to have a distinct morphology compared to other subclones. When used for differentiation toward cardiomyocytes, this type of subclone demonstrates higher differentiation efficiency, increased maturation, and stronger beating compared to those derived from the other subclones. The findings provide a convenient method for single-cell isolation and culture, and demonstrate that variations in differentiation tendencies exist among subclones from the same cell line. This substrate adhesion-based selection process could be used to obtain cell lines with improved differentiation efficiency toward cardiomyocytes and other cell types, which would be advantageous for studies in various fields.

Multi-parametric cell profiling with a CMOS quad-modality cellular interfacing array for label-free fully automated drug screening

Cells are complex systems with concurrent multi-physical responses, and cell physiological signals are often encoded with spatiotemporal dynamics and further coupled with multiple cellular activities. However, most existing electronic sensors are only single-modality and cannot capture multi-parametric cellular responses. In this paper, a 1024-pixel CMOS quad-modality cellular interfacing array that enables multi-parametric cell profiling for drug development is presented. The quad-modality CMOS array features cellular impedance characterization, optical detection, extracellular potential recording, and biphasic current stimulation. The fibroblast transparency and surface adhesion are jointly monitored by cellular impedance and optical sensing modalities for comprehensive cell growth evaluation. Simultaneous current stimulation and opto-mechanical monitoring based on cardiomyocytes are demonstrated without any stimulation/sensing dead-zone. Furthermore, drug dose-dependent multi-parametric feature extractions in cardiomyocytes from their extracellular potentials and opto-mechanical signals are presented. The CMOS array demonstrates great potential for fully automated drug screening and drug safety assessments, which may substantially reduce the drug screening time and cost in future new drug development.

Functional video-based analysis of 3D cardiac structures generated from human embryonic stem cells

Human embryonic stem cells (hESCs) differentiated into cardiomyocytes (CM) often develop into complex 3D structures that are composed of various cardiac cell types. Conventional methods to study the electrophysiology of cardiac cells are patch clamp and microelectrode array (MEAs) analyses. However, these methods are not suitable to investigate the contractile features of 3D cardiac clusters that detach from the surface of the culture dishes during differentiation. To overcome this problem, we developed a video-based motion detection software relying on the optical flow by Farnebäck that we call cBRA (cardiac beat rate analyzer). The beating characteristics of the differentiated cardiac clusters were calculated based on the local displacement between two subsequent images. Two differentiation protocols, which profoundly differ in the morphology of cardiac clusters generated and in the expression of cardiac markers, were used and the resulting CM were characterized. Despite these differences, beat rates and beating variabilities could be reliably determined using cBRA. Likewise, stimulation of β-adrenoreceptors by isoproterenol could easily be identified in the hESC-derived CM. Since even subtle changes in the beating features are detectable, this method is suitable for high throughput cardiotoxicity screenings.

Phenotypic assays for analyses of pluripotent stem cell-derived cardiomyocytes

Stem cell-derived cardiomyocytes (CMs) hold great hopes for myocardium regeneration because of their ability to produce functional cardiac cells in large quantities. They also hold promise in dissecting the molecular principles involved in heart diseases and also in drug development, owing to their ability to model the diseases using patient-specific human pluripotent stem cell (hPSC)-derived CMs. The CM properties essential for the desired applications are frequently evaluated through morphologic and genotypic screenings. Even though these characterizations are necessary, they cannot in principle guarantee the CM functionality and their drug response. The CM functional characteristics can be quantified by phenotype assays, including electrophysiological, optical, and/or mechanical approaches implemented in the past decades, especially when used to investigate responses of the CMs to known stimuli (eg, adrenergic stimulation). Such methods can be used to indirectly determine the electrochemomechanics of the cardiac excitation-contraction coupling, which determines important functional properties of the hPSC-derived CMs, such as their differentiation efficacy, their maturation level, and their functionality. In this work, we aim to systematically review the techniques and methodologies implemented in the phenotype characterization of hPSC-derived CMs. Further, we introduce a novel approach combining atomic force microscopy, fluorescent microscopy, and external electrophysiology through microelectrode arrays. We demonstrate that this novel method can be used to gain unique information on the complex excitation-contraction coupling dynamics of the hPSC-derived CMs.

Direct Mechanical Stimulation of Stem Cells: A Beating Electromechanically Active Scaffold for Cardiac Tissue Engineering

The combination of stem cell therapy with a supportive scaffold is a promising approach to improving cardiac tissue engineering. Stem cell therapy can be used to repair nonfunctioning heart tissue and achieve myocardial regeneration, and scaffold materials can be utilized in order to successfully deliver and support stem cells in vivo. Current research describes passive scaffold materials; here an electroactive scaffold that provides electrical, mechanical, and topographical cues to induced human pluripotent stem cells (iPS) is presented. The poly(lactic-co-glycolic acid) fiber scaffold coated with conductive polymer polypyrrole (PPy) is capable of delivering direct electrical and mechanical stimulation to the iPS. The electroactive scaffolds demonstrate no cytotoxic effects on the iPS as well as an increased expression of cardiac markers for both stimulated and unstimulated protocols. This study demonstrates the first application of PPy as a supportive electroactive material for iPS and the first development of a fiber scaffold capable of dynamic mechanical actuation.

Autonomous beating rate adaptation in human stem cell-derived cardiomyocytes

The therapeutic success of human stem cell-derived cardiomyocytes critically depends on their ability to respond to and integrate with the surrounding electromechanical environment. Currently, the immaturity of human cardiomyocytes derived from stem cells limits their utility for regenerative medicine and biological research. We hypothesize that biomimetic electrical signals regulate the intrinsic beating properties of cardiomyocytes. Here we show that electrical conditioning of human stem cell-derived cardiomyocytes in three-dimensional culture promotes cardiomyocyte maturation, alters their automaticity and enhances connexin expression. Cardiomyocytes adapt their autonomous beating rate to the frequency at which they were stimulated, an effect mediated by the emergence of a rapidly depolarizing cell population, and the expression of hERG. This rate-adaptive behaviour is long lasting and transferable to the surrounding cardiomyocytes. Thus, electrical conditioning may be used to promote cardiomyocyte maturation and establish their automaticity, with implications for cell-based reduction of arrhythmia during heart regeneration.

Inferring intra-cellular mechanics using geometric metamorphosis: A preliminary study

Mechanotransduction plays an important role in sub-cellular processes and is an active area of research. Determining the forces/strains that the intra-cellular structures experience is vital for developing quantitative models of cellular behavior. Established techniques such as traction force microscopy, digital image correlation etc. track surface forces and kinematics of intra-cellular structures. However, difficulties arise when cells cannot be seeded on micro-patterned substrates or the intra-cellular structures vary (unstable landmarks). Here, we applied geometric metamorphosis, a global image registration method, to determine the kinematic profile of a cell during cell division. The method does not require stable landmarks, the registration is non-local in nature and constraints such as volume conservation can be enforced. The cell wall was tracked over time and a sequence of transformations relating the cell wall at the start of cytokinesis to the configuration prior to the daughters completely separate was determined. These transformations are associated with a scalar metric and a statistical atlas describing the wall kinematics from multiple tracking's of the wall shape is constructed. Using these transformations, the cellular kinematics can be described using a Lagrangian frame of reference and the evolution of a material point property can be easily modeled. To demonstrate this, we use the kinematic data derived from the atlas along with a model of stress-fiber (de)formation dynamics to simulate the stress-fiber configuration as the cell domain deforms.

Methods for in vitro functional analysis of iPSC derived cardiomyocytes - Special focus on analyzing the mechanical beating behavior

A rapidly increasing number of papers describing novel iPSC models for cardiac diseases are being published. To be able to understand the disease mechanisms in more detail, we should also take the full advantage of the various methods for analyzing these cell models. The traditionally and commonly used electrophysiological analysis methods have been recently accompanied by novel approaches for analyzing the mechanical beatingbehavior of the cardiomyocytes. In this review, we provide first a concise overview on the methodology for cardiomyocyte functional analysis and then concentrate on the video microscopy, which provides a promise for a new faster yet reliable method for cardiomyocyte functional analysis. We also show how analysis conditions may affect the results. Development of the methodology not only serves the basic research on the disease models, but could also provide the much needed efficient early phase screening method for cardiac safety toxicology. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Centrifugal microfluidic platform for single-cell level cardiomyocyte-based drug profiling and screening

Drug screening and profiling is an important phase in drug discovery, development, and marketing. However, some profiling tests are not routinely done because of the needed additional technical skills and costly maintenance, which leads to cases of unexpected side effects or adverse drug reactions (ADRs). This study presents the design and operation of a microfluidic chip for single-cell level drug screening and profiling as an alternative platform for this purpose. Centrifugation was utilized to trap isolated single and groups of primary cultured neonatal rat cardiomyocytes in the same chip. In the off-spin operation of the chip, the cells can be observed under a microscope and movies of the beat motion can be recorded. The beat profiles of the cells were generated by image correlation analysis of the recorded video to study the contractile characteristics (beating rate, beating strength, and inter-beat duration). By utilizing this non-invasive tool, long term continuous monitoring, right after trapping, was made possible and cell growth and dynamics were successfully observed in the chip. Media and liquid replacement does not require further centrifugation but instead utilizes capillary flow only. The effect of carbachol (100 μM) and isoproterenol (4 μg mL(-1)) on single cells and groups of cells was demonstrated and the feature for immunostaining (β-actin) applicability of the chip was revealed. Furthermore, these findings can be helpful for the headway of non-invasive profiling of cardiomyocytes and for future chip design and operation of high-throughput lab-on-a-chip devices.

Automated multi-parameter measurement of cardiomyocytes dynamics with digital holographic microscopy

Compounds tested during drug development may have adverse effects on the heart; therefore all new chemical entities have to undergo extensive preclinical assessment for cardiac liability. Conventional intensity-based imaging techniques are not robust enough to provide detailed information for cell structure and the captured images result in low-contrast, especially to cell with semi-transparent or transparent feature, which would affect the cell analysis. In this paper we show, for the first time, that digital holographic microscopy (DHM) integrated with information processing algorithms automatically provide dynamic quantitative phase profiles of beating cardiomyocytes. We experimentally demonstrate that relevant parameters of cardiomyocytes can be obtained by our automated algorithm based on DHM phase signal analysis and used to characterize the physiological state of resting cardiomyocytes. Our study opens the possibility of automated quantitative analysis of cardiomyocyte dynamics suitable for further drug safety testing and compounds selection as a new paradigm in drug toxicity screens.

Lipid emulsion rapidly restores contractility in stunned mouse cardiomyocytes: a comparison with therapeutic hypothermia

OBJECTIVES

Cooling following cardiac arrest can improve survival significantly. However, delays in achieving target temperature may decrease the overall benefits of cooling. Here, we test whether lipid emulsion, a clinically approved drug reported to exert cardioprotection, can rescue heart contractility in the setting of delayed cooling in stunned mouse cardiomyocytes.

DESIGN

Cell culture study.

SETTING

Academic research laboratory.

SUBJECTS

Cardiomyocytes isolated from 1- to 2-day-old C57BL6 mice.

INTERVENTIONS

Cardiomyocytes were exposed to 30 minutes of ischemia followed by 90 minutes of reperfusion and 10 minutes of isoproterenol with nine interventions: 1) no additional treatment; 2) intraischemic cooling at 32 °C initiated 10 minutes prior to reperfusion; 3) delayed cooling started 20 minutes after reperfusion; 4) lipid emulsion + delayed cooling; 5) lipid emulsion (0.25%) administered at reperfusion; 6) lipid emulsion + intraischemic cooling; 7) delayed lipid emulsion; 8) lipid emulsion + delayed cooling + Akt inhibitor (API-2, 10 µM); and 9) lipid emulsion + delayed cooling + Erk inhibitor (U0126, 10 µM). Inhibitors were given to cells 1 hour prior to ischemia.

MEASUREMENTS AND MAIN RESULTS

Contractility was recorded by real-time phase-contrast imaging and analyzed with pulse image velocimetry in MATLAB (Mathworks, Natick, MA). Ischemia diminished cell contraction. The cardioprotective effect of cooling was diminished when delayed but was rescued by lipid emulsion. Further, lipid emulsion on its own improved recovery of the contractility to a greater extent as intraischemic cooling. However, cotreatment of lipid emulsion and intraischemic cooling did not further improve the recovery compared to either treatment alone. Furthermore, Akt and Erk inhibitors blocked lipid emulsion-induced protection.

CONCLUSIONS

Lipid emulsion improved contractility and rescued contractility in the context of delayed cooling. This protective effect required Akt and Erk signaling. Lipid emulsion might serve as a treatment or adjunct to cooling in ameliorating myocardial ischemia/reperfusion injury.

Video image-based analysis of single human induced pluripotent stem cell derived cardiomyocyte beating dynamics using digital image correlation

Background

The functionality of a cardiomyocyte is primarily measured by analyzing the electrophysiological properties of the cell. The analysis of the beating behavior of single cardiomyocytes, especially ones derived from stem cells, is challenging but well warranted. In this study, a video-based method that is non-invasive and label-free is introduced and applied for the study of single human cardiomyocytes derived from induced pluripotent stem cells.

Methods

The beating of dissociated stem cell-derived cardiomyocytes was visualized with a microscope and the motion was video-recorded. Minimum quadratic difference, a digital image correlation method, was used for beating analysis with geometrical sectorial cell division and radial/tangential directions. The time series of the temporal displacement vector fields of a single cardiomyocyte was computed from video data. The vector field data was processed to obtain cell-specific, contraction-relaxation dynamics signals. Simulated cardiomyocyte beating was used as a reference and the current clamp of real cardiomyocytes was used to analyze the electrical functionality of the beating cardiomyocytes.

Results

Our results demonstrate that our sectorized image correlation method is capable of extracting single cell beating characteristics from the video data of induced pluripotent stem cell-derived cardiomyocytes that have no clear movement axis, and that the method can accurately identify beating phases and time parameters.

Conclusion

Our video analysis of the beating motion of single human cardiomyocytes provides a robust, non-invasive and label-free method to analyze the mechanobiological functionality of cardiomyocytes derived from induced pluripotent stem cells. Thus, our method has potential for the high-throughput analysis of cardiomyocyte functions.

Microdomain heterogeneity in 3D affects the mechanics of neonatal cardiac myocyte contraction

Cardiac muscle cells are known to adapt to their physical surroundings, optimizing intracellular organization and contractile function for a given culture environment. A previously developed in vitro model system has shown that the inclusion of discrete microscale domains (or microrods) in three dimensions (3D) can alter long-term growth responses of neonatal ventricular myocytes. The aim of this work was to understand how cellular contact with such a domain affects various mechanical changes involved in cardiac muscle cell remodeling. Myocytes were maintained in 3D gels over 5 days in the presence or absence of 100-μm-long microrods, and the effect of this local heterogeneity on cell behavior was analyzed via several imaging techniques. Microrod abutment resulted in approximately twofold increases in the maximum displacement of spontaneously beating myocytes, as based on confocal microscopy scans of the gel xy-plane or the myocyte long axis. In addition, microrods caused significant increases in the proportion of aligned myofibrils (≤20° deviation from long axis) in fixed myocytes. Microrod-related differences in axial contraction could be abrogated by long-term interruption of certain signals of the RhoA-/Rho-associated kinase (ROCK) or protein kinase C (PKC) pathway. Furthermore, microrod-induced increases in myocyte size and protein content were prevented by ROCK inhibition. In all, the data suggest that microdomain heterogeneity in 3D appears to promote the development of axially aligned contractile machinery in muscle cells, an observation that may have relevance to a number of cardiac tissue engineering interventions.

Cardiac myocytes' dynamic contractile behavior differs depending on heart segment

Cardiac myocytes originating from different parts of the heart exhibit varying morphology and ultrastructure. However, the difference in their dynamic behavior is unclear. We examined the contraction of cardiac myocytes originating from the apex, ventricle, and atrium, and found that their dynamic behavior, such as amplitude and frequency of contraction, differs depending on the heart segment of origin. Using video microscopy and high-precision image correlation, we found that: (1) apex myocytes exhibited the highest contraction rate (∼17 beats/min); (2) ventricular myocytes exhibited the highest contraction amplitude (∼5.2 micron); and (3) as myocyte contraction synchronized, their frequency did not change significantly, but the amplitude of contraction increased in apex and ventricular myocytes. In addition, as myocyte cultures mature they formed contractile filaments, further emphasizing the difference in myocyte dynamics is persistent. These results suggest that the dynamic behavior (in addition to static properties) of myocytes is dependent on their segment of origin.

Orthotropic active strain models for the numerical simulation of cardiac biomechanics

A model for the active deformation of cardiac tissue considering orthotropic constitutive laws is introduced and studied. In particular, the passive mechanical properties of the myocardium are described by the Holzapfel-Ogden relation, whereas the activation model is based on the concept of active strain. There, an incompatible intermediate configuration is considered, which entails a multiplicative decomposition between active and passive deformation gradients. The underlying Euler-Lagrange equations for minimizing the total energy are written in terms of these deformation factors, where the active part is assumed to depend, at the cell level, on the electrodynamics and on the specific orientation of the cardiomyocytes. The active strain formulation is compared with the classical active stress model from both numerical and modeling perspectives. The well-posedness of the linear system derived from a generic Newton iteration of the original problem is analyzed, and different mechanical activation functions are considered. Taylor-Hood and MINI finite elements are used in the discretization of the overall mechanical problem. The results of several numerical experiments show that the proposed formulation is mathematically consistent and is able to represent the main features of the phenomenon, while allowing savings in computational costs.

In situ mechanical analysis of cardiomyocytes at nano scales

Nanomechanical behaviors of single living cardiomyocytes are quantitatively observed using calculated torsions and deflections of an AFM cantilever. The lateral contractions are related to the calcium intensity within rather than the vertical beating power of the cardiomyocytes. Drug-induced nanomechanical changes of cardiomyocytes were further investigated by measuring lateral contractions in real time.

Micromechanical regulation in cardiac myocytes and fibroblasts: implications for tissue remodeling

Cells of the myocardium are at home in one of the most mechanically dynamic environments in the body. At the cellular level, pulsatile stimuli of chamber filling and emptying are experienced as cyclic strains (relative deformation) and stresses (force per unit area). The intrinsic characteristics of tension-generating myocytes and fibroblasts thus have a continuous mechanical interplay with their extrinsic surroundings. This review explores the ways that the micromechanics at the scale of single cardiac myocytes and fibroblasts have been measured, modeled, and recapitulated in vitro in the context of adaptation. Both types of cardiac cells respond to externally applied strain, and many of the intracellular mechanosensing pathways have been identified with the careful manipulation of experimental variables. In addition to strain, the extent of loading in myocytes and fibroblasts is also regulated by cues from the microenvironment such as substrate surface chemistry, stiffness, and topography. Combinations of these structural cues in three dimensions are needed to mimic the micromechanical complexity derived from the extracellular matrix of the developing, healthy, or pathophysiologic heart. An understanding of cardiac cell micromechanics can therefore inform the design and composition of tissue engineering scaffolds or stem cell niches for future applications in regenerative medicine.

Introduction A: recent advances in cytometry instrumentation, probes, and methods--review

Cytometric techniques are continually being improved, refined, and adapted to new applications. This chapter briefly outlines recent advances in the field of cytometry with the main focus on new instrumentations in flow and image cytometry as well as new probes suitable for multiparametric analyses. There is a remarkable trend for miniaturizing cytometers, developing label-free and fluorescence-free analytical approaches, and designing "intelligent" probes. Furthermore, new methods for analyzing complex data for extracting relevant information are reviewed.

Whole-cell-analysis of live cardiomyocytes using wide-field interferometric phase microscopy

We apply wide-field interferometric microscopy techniques to acquire quantitative phase profiles of ventricular cardiomyocytes in vitro during their rapid contraction with high temporal and spatial resolution. The whole-cell phase profiles are analyzed to yield valuable quantitative parameters characterizing the cell dynamics, without the need to decouple thickness from refractive index differences. Our experimental results verify that these new parameters can be used with wide field interferometric microscopy to discriminate the modulation of cardiomyocyte contraction dynamics due to temperature variation. To demonstrate the necessity of the proposed numerical analysis for cardiomyocytes, we present confocal dual-fluorescence-channel microscopy results which show that the rapid motion of the cell organelles during contraction preclude assuming a homogenous refractive index over the entire cell contents, or using multiple-exposure or scanning microscopy.

An integrated formulation of anisotropic force-calcium relations driving spatio-temporal contractions of cardiac myocytes

Isolated cardiac myocytes exhibit spontaneous patterns of rhythmic contraction, driven by intracellular calcium waves. In order to study the coupling between spatio-temporal calcium dynamics and cell contraction in large deformation regimes, a new strain-energy function, describing the influence of sarcomere length on the calcium-dependent generation of active intracellular stresses, is proposed. This strain-energy function includes anisotropic passive and active contributions that were first validated separately from experimental stress-strain curves and stress-sarcomere length curves, respectively. An extended validation of this formulation was then conducted by considering this strain-energy function as the core of an integrated mechano-chemical three-dimensional model of cardiac myocyte contraction, where autocatalytic intracellular calcium dynamics were described by a representative two-variable model able to generate realistic intracellular calcium waves similar to those observed experimentally. Finite-element simulations of the three-dimensional cell model, conducted for different intracellular locations of triggering calcium sparks, explained very satisfactorily, both qualitatively and quantitatively, the contraction patterns of cardiac myocytes observed by time-lapse videomicroscopy. This integrative approach of the mechano-chemical couplings driving cardiac myocyte contraction provides a comprehensive framework for analysing active stress regulation and associated mechano-transduction processes that contribute to the efficiency of cardiac cell contractility in both physiological and pathological contexts.

Clinical applications of slide-based cytometry--an update

Slide-based cytometric approaches open the possibility to obtain quantitative and objective data from specimens that so far have not been accessible to this kind of analysis. In this review, we will highlight the specific advantages of slide-based cytometry (SBC) and show the applications that have been established for clinical samples. Focuses are cytomic analyses of oncological and hematological samples where the slide-based concept turned out to open new dimensions in understanding underlying cellular networks. We review the recent literature and point out future applications.