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Integrated Analysis of Contractile Kinetics, Force Generation, and Electrical Activity in Single Human Stem Cell-Derived Cardiomyocytes

Summary The quantitative analysis of cardiomyocyte function is essential for stem cell-based approaches for the in vitro study of human cardiac physiology and pathophysiology. We present a method to comprehensively assess the function of single human pluripotent stem cell-derived cardiomyocyte (hPSC-CMs) through simultaneous quantitative analysis of contraction kinetics, force generation, and electrical activity. We demonstrate that statistical analysis of movies of contracting hPSC-CMs can be used to quantify changes in cellular morphology over time and compute contractile kinetics. Using a biomechanical model that incorporates substrate stiffness, we calculate cardiomyocyte force generation at single-cell resolution and validate this approach with conventional traction force microscopy. The addition of fluorescent calcium indicators or membrane potential dyes allows the simultaneous analysis of contractility and calcium handling or action potential morphology. Accordingly, our approach has the potential for broad application in the study of cardiac disease, drug discovery, and cardiotoxicity screening.


INTRODUCTION
Advanced heart failure represents a leading cause of mortality and morbidity in the developed world. The clinical syndrome results from an inability of the cardiac output to meet the metabolic demands of affected individuals. Most commonly, this results from a loss of myocardial cell viability or function (de Tombe, 1998;Narula et al., 1998). Cardiomyocytes (CMs), the basic functional units of the myocardium, produce force by shortening and thickening during each contractile cycle to generate the forward flow of blood. In vitro, myocardial function has been studied at the single-cell level or by myocardial muscle constructs as a surrogate for in vivo myocardium (Zimmermann et al., 2006). The use of adult CMs isolated from the myocardium of adult rodents and other animals for in vitro studies of cardiac physiology and pathophysiology has been an established method since the 1970s (Glick et al., 1974). As a result, most techniques used to quantify the contractility of CMs have been optimized for cells with distinct edges and highly developed sarcomeres.
Recent advances in stem cell biology have greatly increased the efficiency of cardiac differentiation of human pluripotent stem cells . Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) are now used widely for in vitro studies  and as cell sources for regenerative cardiovascular medicine (Chong et al., 2014;Zimmermann et al., 2006). However, hPSC-CMs display a relatively less mature phenotype and often lack distinct cell edges and highly developed sarcomeres, making the study of their contractility with traditional techniques difficult. This has prompted a number of laboratories to focus on the functional maturation of stem cell-derived CMs . Although progress has been made in this regard, the goal of culturing fully mature human CMs from hPSC-CMs remains elusive, highlighting the need for novel methods to functionally characterize CMs at different developmental states.
Two widely used methods to quantify the contractile kinetics of adult CMs are edge detection and sarcomere length measurements (Bub et al., 2010;Chen et al., 2014). Edge detection technology relies on automatically detecting changes in the position of the longitudinal edges of a CM over time. Accordingly, its application must be optimized for the scale, clarity, and orientation of the images being analyzed. Commercially available edge detection tools used to study CMs, for example, have been optimized to detect the outer edges of horizontally aligned isolated adult rod-like CMs that are either in suspension or attached to glass (Chen et al., 2014). These tools are therefore not ideal for the assessment of hPSC-CMs with indistinct borders. Moreover, glass is not an ideal substrate for CMs when studying their contraction kinetics because the stiffness of glass far exceeds the force generated by contracting CMs. Alternative approaches for the quantification of contractility of adult CMs include assessment of the change of sarcomere length over time. This approach requires the presence of distinct sarcomeres (Bub et al., 2010) and is therefore not very well suited for the study of hPSC-CMs. Several approaches have been described recently for analyzing motion in movies of beating hPSC-CMs, collectively referred to as optical flow analysis. These approaches include motion vector analysis after manual segmentation (Ahola et al., 2014), block-matching algorithms combined with motion vector analysis (Hayakawa et al., 2014), or the evaluation of the correlation between intensity vectors of frames within a movie (Maddah et al., 2015) to yield a unit-less or dual-peaked curve representing the beating signal. These approaches, however, do not directly allow for the quantitative assessment of fractional shortening and force generation kinetics, key features of cardiomyocyte physiology. CM force generation has been assessed previously by a number of different methods, including fluorescent microsphere-based traction force microscopy, atomic force microscopy, and micropost deformation measurements McCain et al., 2014;Rodriguez et al., 2014). These techniques are highly specialized, require advanced instrumentation, and cannot be easily combined with optical measurements of contractile kinetics, measurements of calcium cycling, or action potentials.
Here we present a methodology for the quantitative analysis of CM contractile kinetics and force generation that can be used in hPSC-CMs as well as isolated adult CMs. Our approach is not based on tracking the motion of parts of the cell but, rather, on quantifying the total amount of change in cell morphology over time. We use a previously well validated statistical tool to analyze the similarity between frames in movies of contracting human embryonic stem cell-derived cardiomyocytes (hESC-CMs) to generate a similarity matrix. This matrix represents the change in cell morphology over time and is used to compute the contractile kinetics of hESC-CMs. We combine this methodology with a biomechanical model to concurrently calculate the CM force generation as the CMs deform a flexible substrate. We validate this approach in human stem cellderived CMs as well as in murine adult myocardial cells at single-cell resolution as well as in myocardial clusters. We further demonstrate our ability to detect subtle changes in the contractile kinetics and force generation of hESC-CMs in response to pharmacological intervention with isoproterenol and verapamil. Furthermore, we show that our methodology can be applied simultaneously with other single-cell physiological assays for the quantification of calcium cycling and action potentials. Finally, we show the utility of this method in assessing the cardiotoxicity of drugs such as dofetilide.

Morphologic Similarity Measure to Produce Contraction Curves
The goal of this study is to generate a high-throughput unbiased and robust methodology for the analysis of contraction kinetics and force generation of stem cell-derived CMs that is compatible with fluorescence-based calcium cycling and action potential assessment. We therefore performed directed differentiation to generate hESC-CMs. On days 13-20 of in vitro differentiation, CMs were dissociated into single-cell suspension and plated on a polydimethylsiloxane (PDMS) flexible substrate at single-cell density. By culturing these CMs on the PDMS flexible substrate, the cells could contract against strain (McCain et al., 2014). After 14 days of culture on the flexible substrate, cells were visualized in a temperature-and CO 2 -controlled chamber by differential interference contrast (DIC) imaging as described in the Experimental Procedures. We acquired movies of the contracting cells at a temporal resolution of at least 100 frames/s (fps).
As an example to illustrate our methodology, a 300-frame movie at 100 fps of a single hESC-derived rod-like CM undergoing a single contraction cycle was analyzed (Movie S1). As shown, the CM of Movie S1 is in a pre-contractile relaxed state between frames 1 (F1) and F140 and shortens between F140 and F165 before returning to the baselinerelaxed state. We first sought to determine the change in cell morphology during these 300 frames. We therefore quantitatively assessed the similarity of F1 to every other frame in the movie by performing a pairwise similarity comparison (SC) using a well established statistical tool ( Figure 1A; Garakani et al., 2015;Drubin et al., 2006;Garakani, 2008;Hack et al., 2004;C.A. MacRae et al., 2005, In-ternational Conference on Systems Biology, conference presentation). This equation calculates the SC for all pairs of frames captured at times i and j (i s j). Each pair of frames is compared based on the color intensity of spatially corresponding pixels defined by an intensity function I x,y,t' , where x and y are integers defining the coordinates of a given pixel in a particular frame and t' is an integer corresponding to the time of capture of the frame. The intensity function values of all pixels in each frame are summated over N, where N is the total width 3 total height of each frame. The similarity function is commutative so that SC i,j = SC j,i . Furthermore, SC i,j = 1 if i = j. Therefore, SC is defined as having a value between 0 and 1 where, the more similar two frames are to each other, the closer their SC value is to 1. We plotted the pairwise SC value of F1 against every other frame, which yielded a preliminary analysis of the contraction cycle ( Figure 1B). The SC of the relaxed frame F1 versus the pre-contractile frames F2-F140 resulted in relatively high SC values (highly similar). The SC values of F1 versus the contractile frames (F141-F165) dropped dramatically (highly dissimilar). And, finally, the SC values of F1 versus the post-contractile frames returned to the high baseline value (again highly similar).
To visually display the outcome of the SC analysis, we used Fiji to calculate the digital differences between F1 and the pre-contractile frame F100. Spatially corresponding pixels that have a similar intensity result in a dark pixel, and corresponding pixels that have a less similar intensity result in a lighter pixel. This calculation resulted in an essentially black image with a faint, apparently random white-speckled background and an associated SC of 0.923 because these two frames are both acquired while the cell is in a relaxed state and very similar, although not identical. ( Figure 1C). The digital difference of F1 and the contractile frame F165 resulted in a high-contrast black-and-white image with an associated SC of 0.301, indicating a clear signal because the change in cell morphology has resulted in a change of the intensity of many pixels compared with the corresponding pixels in the other frame ( Figure 1D). Finally, the digital difference between F1 and the post-contractile frame F300 again yielded an essentially black image with a faint white-speckled background and an associated SC of 0.917 ( Figure 1E).
In contrast, pairwise analysis of contractile frame F165 to every other frame resulted in an inverted-appearing contractile cycle ( Figure 1F). The SC value of F165 versus the relaxed frames was very low (highly dissimilar), became progressively higher (more similar) for the contractile frames, and dropped again for the post-contractile frames. Likewise, the digital difference of F165 versus the pre-contractile frame F100 was a high-contrast image with an associated SC of 0.294 ( Figure 1G). The digital difference of F165 versus the contractile frame F166 was essentially black with a faint white-speckled background and an associated SC of 0.930 ( Figure 1H). And, finally, the digital difference of F165 versus the post-contractile frame F300 was again a high-contrast black-and-white image with an associated SC value of 0.302 ( Figure 1I).
To capture all of the similarity data in the movie, we performed a pairwise comparison of each frame in Movie S1 The equation to calculate SC between two frames. Each pair of frames is compared based on the intensity (grayscale value) of pixels in the same location of the two frames. The intensity is defined by the function I x,y,i , where x and y define the coordinates of a given pixel in frame i. The values of all pixels in each frame are summed over the frame area (N). The similarity function is commutative so that SC i,j = SC j,I . The SC of two identical frames is defined as 1.
(B) A plot of SCs of frame F1 versus every other frame of Movie S1. Blue, red, and green arrows refer to the SCs for F1 versus F100, F165, and F300, respectively. (C) The digital difference between the relaxed frame F1 (top) and the pre-contractile frame F100 (center) is displayed at the bottom with the associated SC value.
(D) The digital difference between F1 (top) and the contractile frame F165 (center) is displayed at the bottom with the associated SC value.
(E) The digital difference between F1 (top) and the post-contractile frame F300 (center) is displayed at the bottom with the associated SC value. Scale bar, 10 mm. See also Movie S1.
versus every other frame to produce a similarity matrix of 300 rows 3 300 columns, where each row of the matrix contains a series of SCs for a specific frame versus every other frame (Table S1). The 300 3 300 similarity matrix was then plotted as a series of similarity curves in a three-dimensional graph ( To obtain a more physiological assessment of the CM contraction cycle, we used the average of the similarity curves of pre-contractile frames ( Figure 2C) as the baseline for the maximally relaxed state. We defined the pre-contractile period as the terminal 15% of the inter-contraction interval, as determined by a rolling linear regression analysis of all frames (Figures S1A and S1B). Next we averaged these similarity curves to obtain the baseline adjusted similarity comparison (BASiC) curve. We then normalized the maximum and minimum SC values to manually measured cell length at end-diastole and peak systole (as described in detail in the Supplemental Experimental Procedures). This allowed us to generate a normalized contraction curve of CMs with percent cell length plotted against time (Figure 2D). Manually measured cell length measurements were highly similar between independent observers, with a mean difference of 0.3 mm (95% limit of agreement, À1.8-1.2 mm; Figure S1C).
We then empirically validated this quantitative analysis and demonstrated that the BASiC-generated cell length curves correlate with contraction curves generated by manual cell length measurement, automated edge detection measurement, and sarcomere length measurement with Pearson's correlation coefficients of greater than 0.98 in adult murine CMs ( Figures 3A, 3C, and 3E; Movie S2). We then repeated a similar type of analysis on rodlike hESC-CMs and demonstrated that BASiC-generated cell length curves were highly correlated with manual cell length curves, with Pearson's correlation coefficients of 0.98 ( Figures 3B, 3D, and 3E; Movie S1). Importantly, automated edge detection methods could not generate usable contraction curves for hESC-derived CMs because of the relatively low-contrast edges of these cell types, highlighting the need for an automated alternative to edge detection. Similarly, these cells do not have well developed sarcomeres.

Figure 2. Generation of the Contraction
Curve from the Similarity Matrix (A and B) Three-dimensional graph (A) displaying the similarity matrix (B) of Movie S1, where the x and y axes display frame numbers and the z axis displays the SC value. The highlighted black lines display the SC plots of F1 and F165 versus every other frame. The blue, red, and green planes map areas of the three-dimensional graph to the associated portion of the matrix. (C) Three-dimensional graph of the selected pre-contractile SC curves. (D) BASiC normalized to cell length and time for the generation of a standard contraction curve. See also Figure S1 and Table S1.
In addition, we analyzed the similarity between contraction curves generated by BASiC and manual measurements of irregularly shaped hESC-CMs cultured on PDMS (i.e., non-rod-like), hESC-CM clusters cultured on PDMS or in suspension, and hESC-CM monolayers on polystyrene (Figures S2A-S2I). We found Pearson's correlation coefficients of above 0.95 under all of these conditions, indicating the applicability of BASiC to analyze CM contractility in variable setups. Further testing revealed that differences in frame rate, light intensity, and resolution during image acquisition had a minimal effect on BASiC analysis ( Figures S2J-S2O).

Mechanical Model for Quantification of Force Generation
We then sought to quantify the force generated by individual CMs by traction force microscopy (TFM), a well validated methodology used to estimate the force generated by individual CMs (McCain et al., 2014). In traditional TFM, cells are plated on flexible substrates with fluorescent microspheres attached to the surface of the substrate. As cells shorten, they deform the substrate and, with it, move the attached fluorescent microspheres. Because the stiffness of the substrate is known, it is possible to estimate the contractile force required for its deformation from the movement of the fluorescent microspheres. Because cell shortening is not measured directly but, rather, inferred from microsphere movement, this approach relies on the assumption that microsphere movement and cell shortening are directly correlated. To test this underlying assumption, we cultured hESC-CMs on flexible PDMS substrates embedded with fluorescent microspheres. Cell shortening correlated closely with microsphere movement, validating this assumption ( Figure S3A; Movie S3). We then calculated the force generated by single contracting rod-like CMs from the microsphere movement ( Figures  4A, 4E, and 4F) with standard techniques and obtained results similar to previous reports (Kuo et al., 2012;McCain et al., 2014). An important limitation of standard TFM approaches is the use of fluorescently labeled microspheres, limiting their compatibility for use in conjunction with fluorescence-based physiological assays such as optical mapping of calcium cycling and action potential. To address this limitation and capitalize on the power of our methodology, we utilized a mechanical model based on cell dimensions, cell shortening, and growth substrate properties to calculate the force generated by single contracting myocytes without the use of fluorescent microspheres (described in detail in the Supplemental Experimental Procedures). A direct comparison of both TFM methodologies (with and without microspheres) revealed highly similar force curves with a mean Pearson's correlation coefficient of 0.96 ( Figures 4B and 4C). Likewise, the mean difference in peak force was 8.8 mN/mm 2 (95% limits of agreement, À33.8-16.2 mN/mm 2 ; Figure 4D). We then repeated these experiments with irregularly shaped  Figure S2 and Movies S1 and S2.
hESC-CMs and CM clusters on PDMS and again found highly similar force curves with average Pearson's correlation coefficients of above 0.95. The ratio of peak force calculated by TFM with BASiC over peak force calculated by TFM with microspheres was 1.02 ± 0.04 for CM clusters and 1.40 ± 0.15 for irregularly shaped CMs (Figures S3B-S3G). The correlation in force calculations between TFM with microspheres and BASiC was not affected by the application of the non-selective b-adrenergic agonist isoproterenol ( Figures S3H and S3I).

Quantification of Pharmacotherapy Effects
To evaluate the sensitivity of our methodology in detecting small changes in the contractile behavior of individual hESC-CMs, we examined the dose response of two bioac-tive agents used widely in cardiovascular research. Verapamil is an L-type calcium channel blocker that has been shown to decrease CM contractility (Dolnikov et al., 2006;Turnbull et al., 2014). Conversely, isoproterenol has been shown to increase CM contractility (Brito-Martins et al., 2008). hESC-CMs were paced with field stimulation at a rate of 1.2 Hz, and movies of single rod-like CMs contracting on PDMS gels were captured at baseline and after treatment with 10 and 100 nM of verapamil or isoproterenol. CM contraction curves were then generated as described above. Calcium cycling was also concurrently assessed via fluorescence imaging with the calcium dye Fluo-4 AM. In a control experiment, the effect of Fluo-4 AM on hESC-CM contractility was analyzed over a period of 60 min, and no significant differences in contractility  Figure S3 and Movie S3.
were found ( Figure S4A-S4G). In an additional control experiment with hESC-CMs, the calcium dye intensity remained stable over a period of 60 min ( Figure S4H). Treatment with verapamil resulted in decreased contractility and calcium transients, as assessed by fluorescent signal, fractional shortening, and force curves ( Figure 5A; Movie S4). Quantitative analysis of the contraction kinetics and calcium cycling revealed negative inotropic and lusitropic effects on CM contractility by the administration of verapamil, with a significant reduction in maximum shortening velocity, peak contractile force, maximum relengthening velocity, and calcium transient amplitude ( Figures 5B-5G). In contrast, treatment with isoproterenol resulted in progressive positive inotropic and lusitropic effects with a significant increase in maximum shortening velocity, peak contractile force, and maximum relengthening velocity. We also observed significant decreases in time to 50% peak shortening and time to 90% relengthening ( Figure 6). We analyzed the responses to these pharmacological interventions as the average fold change of multiple cells. As expected, because of the heterogeneous nature of hESC-CMs, there was some degree of variability in the functional characteristics of these myocytes at baseline and in response to verapamil ( Figure S4I-S4N) and isoproterenol ( Figures S5A-S5F). Subsequently, isolated adult mouse CMs field-stimulated at 1 Hz were treated with 100 nM isoproterenol, contraction curves were generated with BASiC, and a significant positive inotropic effect was again observed (Figure S5G-S5M).
Fluorescent transmembrane voltage reporters have been used previously to study toxic electrophysiological changes after treatment with anti-arrhythmic drugs such as dofetilide, a blocker of the rapid component of the delayed rectifier current (IKr). Prior work has demonstrated that dofetilide prolongs the cardiac action potential and can be proarrhythmic at clinically prescribed doses (Leyton-Mange et al., 2014;Roden, 2004). Most studies to date, however, have examined the effect of such drugs exclusively on action potential characteristics (Braam et Figure S4 and Movie S4. that, by concurrently examining CM contractility and action potential characteristics, we would be able to better assess the cardiotoxicity of such commonly used drugs. Movies of single rod-like CMs contracting on PDMS gels were captured at baseline and after treatment with 10 nM dofetilide. Action potential morphology was concurrently assessed via fluorescence imaging with the transmembrane voltage reporter FluoVolt (Miller et al., 2012). At baseline, hESC-CMs exhibited regular intervals between action potentials and contractions, with consistent peak amplitudes even in the absence of field stimulation ( Figure 7A). Upon treatment with the arrhythmogenic agent dofetilide, however, cells exhibited multiple toxic responses, including early afterdepolarizations ( Figure 7B; Movie S5), irregular interbeat intervals (IBIs; Figure 7C), and irregular contraction peak amplitudes (PAs; Figure 7D). We then quantified the coefficient of variation (COV) for both the IBI and PA for the cells before and after dofetilide treatment and compared the measurements for the action potential profile and the contractility profile. Although some of the toxic effects were identified in both the action potential and contraction profiles, others were only apparent in the contraction curve, highlighting the need to examine excitation-contraction coupling after treatment with potentially cardiotoxic agents.

DISCUSSION
The advent of human induced pluripotent stem cell technology coupled to advances in directed cardiac differentiation protocols has opened new avenues for the study of human myocardial physiology and pathophysiology. Recent studies have focused on disease-specific human induced PSC-CMs' electrophysiology and calcium cycling properties for cardiac disease modeling or, alternatively, on myocardial contractility (Itzhaki et al., 2011;Lan et al., 2013). To date, the simultaneous assessment of calcium cycling, action potential characteristics, contractile kinetics, and force generation has been challenging. Current methodologies geared at assessing myocardial contractility of adult CMs typically rely on edge detection techniques that directly assess CM shortening (Chen et al., 2014) or the change in sarcomere striations over time (Bub et al., 2010). These approaches require cells with clearly defined cellular borders and distinct striations, limiting their applicability to less mature cell types (Gherghiceanu et al., 2011). Likewise, current TFM approaches for the measurement of contractile force of single myocardial cells require the use of fluorescent microspheres, limiting their capacity for use in conjunction with other fluorescence-based physiological assays. Other approaches, including optical flow analysis, yield a unit-less or dualpeaked curve rather than standard fractional shortening and force generation curves (Ahola et al., 2014;Hayakawa et al., 2014;Maddah et al., 2015).
Here we present a methodology that provides a flexible tool for the study of contraction kinetics of single myocardial cells at different stages of maturation cultured on sub-strates with tunable stiffness. Our approach is based on comparing the similarity of frames in a movie of CMs contracting on well characterized flexible substrates and yields a standard contraction curve of hPSC-CMs contracting against strain. Critically, this allows the generation of fractional shortening curves and the calculation of contraction and relaxation velocities similar to those used in many studies of adult cardiomyocytes. We then incorporate the mechanical properties of the growth substrate to quantify the kinetics of force generation at single-cell resolution and in myocardial clusters. We demonstrate that our approach is ideally suited for rod-like cells and myocardial clusters and has a high degree of correlation with standard TFM, which requires the use of fluorescent microspheres. Because we do not use fluorescence for the assessment of force generation, our approach is readily suitable for the concurrent assessment of the kinetics of force generation along with calcium cycling and action potential at single-cell resolution. An important advantage of the simultaneous assessment of force generation and electrophysiological characteristics is the ability to unequivocally determine whether a pathophysiological process results in electromechanical dissociation. Because our approach yields fractional shortening curves, it should also allow an easier comparison of the contractile phenotype between hPSC-CMs and adult CMs in maturation studies, disease modeling, and pharmacological experiments. Importantly, our method is economical and widely applicable because it only requires movie microscopy without the need for highly specialized hardware or genetically engineered cell lines. Although our approach does not currently offer real-time analysis, it can be automated readily to allow for this in the future.
In proof-of-principle experiments, we examined the response of hESC-derived CMs to verapamil and isoproterenol, two drugs commonly used in cardiovascular research. Single CMs exhibited a significantly reduced calcium transient amplitude associated with a negative inotropic response to treatment with the L-type Ca 2+ channel blocker verapamil. These results are in accordance with previous reports from hPSC-CM-based tissue-engineered heart tissues (Turnbull et al., 2014), embryoid bodies (EBs; Dolnikov et al., 2006), and hPSC-CM monolayers (Himmel, 2013). Likewise, we observed a dose-dependent positive inotropic response to isoproterenol. Prior studies using hPSC-CMs have found a variable inotropic response to isoproterenol. Although some studies show a significant inotropic response in EBs and single hPSC-CMs (Hayakawa et al., 2014;Reppel et al., 2004), other studies show a transient or no response in EBs and engineered heart tissue (Brito-Martins et al., 2008;Turnbull et al., 2014).
Over the past decades, numerous drug development projects have been halted by concerns over cardiotoxicity. Because significant differences exist between humans and rodents, hPSC-CMs have been suggested as a promising alternative to rodent adult CMs. Several studies using hPSC-CMs for cardiotoxicity screening have been performed with different readouts of toxicity, such as increased action potential duration and the occurrence of early afterdepolarizations (Braam et al., 2010;Liang et al., 2013). We demonstrate our ability to rapidly detect druginduced cardiac toxicity by assessing contractility and action potential simultaneously. Our finding that excitation-contraction coupling may be perturbed under toxic conditions suggests that simultaneously assessing action potential characteristics and contractile behavior may improve the sensitivity of drug-induced cardiac toxicity screening.
The treatment of heart failure represents a major unmet clinical need. Despite the recent advances in regenerative medicine, at this time, drug-based therapy remains the cornerstone of treatment for heart failure. Our methodology should have broad applications in the study of normal and diseased myocardial contractile physiology and should be suitable for drug discovery and toxicology testing using normal or diseased stem cell-derived myocardial cells.

Generation of Flexible Culture Substrates
Flexible substrates were generated by adding 50 ml of Sylgard 527 PDMS, parts A and B mixed at a 1:1 ratio, onto a Fluorodish (WPI) and curing at 60 C for 6 hr. (Palchesko et al., 2012) Subsequently the Fluorodishes were UV-sterilized and coated with Matrigel (BD Biosciences) prior to cell seeding.

Human Embryonic Stem Cell Maintenance and Cardiac Differentiation
hESCs from the lines HUES-9 and HES-3 NKX2-5 eGFP/w were cultured with mTESR-1 growth medium (STEMCELL Technologies) in polystyrene plates coated with Matrigel (BD Biosciences) as described previously. Cardiac differentiation of hESCs was performed using a protocol established previously .

Fluorescence-Activated Cell Sorting
The simultaneous DIC imaging with fluorescent calcium imaging experiment was performed with GFP + HES-3 NKX2-5 eGFP/w cells. Cells were isolated with a fluorescence-activated cell sorting (FACS) protocol as described previously (Elliott et al., 2011).

Dissociation and Plating
Beating CMs were dissociated into single-cell suspensions after 15-20 days of differentiation as described previously (Atmanli et al., 2014). Single hESC-CMs were seeded on Matrigel-coated Fluorodishes at 2,000-6,000 cells/cm 2 . Single contracting CMs and CM clusters growing on PDMS substrates for more than 2 weeks were chosen for drug treatment and image acquisition. To obtain hESC-CM clusters in suspension, beating CMs were treated with 0.05% Trypsin (Life Technologies) for 5 min, after which the reaction was neutralized with 10% fetal bovine serum (Life Technologies).

Confocal Imaging and Drug Treatment
Single contracting CMs and CM clusters cultured on flexible substrates, CM clusters in suspension, and CM monolayers on polystyrene were imaged in CM culture medium using a Nikon A1R confocal microscope atop an anti-vibration table (TMC). Cells were placed inside a climate control chamber at 37 C with 5% CO 2 (Pathology Devices) 30 min prior to the start of the experiments. For the verapamil and isoproterenol experiments, the cells were field-stimulated with 10 V at 1.2 Hz using the RC-37WS perfusion insert with electrodes (Warner Instruments) connected to a C-Pace EP stimulator (Ion Optix). Movie recording of CM contraction cycles was performed using a Nikon A1R confocal microscope with DIC microscopy using a 488-or 647-nm laser. Movies were acquired with at least 100 fps unless described otherwise. Movies of CMs displaying at least five contraction cycles were recorded at baseline and then 5 min after each treatment. Cells were treated with verapamil (Sigma), isoproterenol (Sigma), or dofetilide (Sigma) as described in the Results. For calcium imaging, 0.5 mM of Fluo-4 AM (Life Technologies) was added to the CM culture medium 10 min prior to image acquisition. For action potential imaging, a FluoVolt membrane potential kit (Life Technologies) was used at a dilution of 1:1,000 according to the manufacturer's protocol. Both fluorescence and DIC channels were recorded concurrently for multiple contraction cycles.

Data Analysis and Display
The Nikon Elements software package and Fiji were used for movie file conversions. (Schindelin et al., 2012). Movies were cropped using Fiji to isolate the contracting CM from neighboring cells and cellular debris. Movies were loaded into Visible software (Reify) (Garakani et al., 2015;Drubin et al., 2006;Garakani, 2008;Hack et al., 2004;C.A. MacRae et al., 2005, International Conference on Systems Biology, conference presentation) to generate a similarity matrix of n rows by n columns, where n is the number of frames in the movie. Analysis of the similarity matrix was performed in Excel (Microsoft). For analysis of contraction curves, at least five contraction curves from a single video were averaged by matching the start of the contractions. Curve-fitting was applied to this averaged curve to obtain accurate measurements of maximum shortening and relengthening velocities and time to 50% shortening and 90% relengthening, respectively. The three-dimensional matrices were generated using GNU Octave and MATLAB (MathWorks).

Traction Force Microscopy with Fluorescent Microspheres
We used a well established fluorescent microsphere-based method to calculate the contractile force of CMs McCain et al., 2014). Briefly, fluorescent microspheres (Invitrogen) were coated onto PDMS substrates. The substrates were then UVsterilized and coated with Matrigel (BD Biosciences). CMs were seeded onto the substrates and cultured for 14 days. Movies of single CMs and CM clusters contracting on the substrates were recorded in both fluorescent and DIC channels. The software package TractionForAll (Marinkovi c et al., 2012) was used to analyze images from the movies and calculate the force generated by CMs.

Traction Force Microscopy without Fluorescent Microspheres
Contractile forces generated by shortening CMs were calculated in MATLAB based on the contractility curve generated with BASiC, the dimensions of the CM, and the mechanical properties of the PDMS substrate (Palchesko et al., 2012). Variable input for the MATLAB script is the width of the cell in centimeters, the length of the cell (in the direction of contraction) in centimeters, and the amount of cell shortening in centimeters. The Young's modulus of PDMS substrates was 5 kPa for all experiments. The MATLAB script and related mechanical model are based on several assumptions, described in detail in the Supplemental Experimental Procedures.

Statistical Analysis
All data were analyzed using GraphPad Prism 6 software (Graph-Pad Software) and presented as means ± SE from the number of cells indicated in the text and figure legends. Raw data were analyzed for normal distribution using the Kolmogorov-Smirnov test. Contraction kinetics and force generation data were compared using repeated-measures ANOVA followed by Bonferroni post-test analysis for multiple comparisons. Coefficient of variation data were compared using paired t test. Force generation data after calculation with two different TFM methods before and after application of isoproterenol were compared using repeated-measures two-way ANOVA. Differences were considered significant at p < 0.05.