Assessing Cardiac Contractility From Single Molecules to Whole Hearts

Figures

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  Central Illustration. Multiscale Modeling of Cardiac Contractility iPSC = induced pluripotent stem cell.

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  Figure 1. Calcium-Dependent Actin-Myosin Interactions (A) Single myosin motor (purple) bound to an actin filament (orange/yellow) in pre– and post–power stroke states that results in movement of the Z-disc (left, solid black line) toward the M-line (right, dashed black line). (B) In the absence of calcium, tropomyosin (dark/light purple line) and troponin (green box) block binding of myosin to the thin filament. This is referred to as the “blocked state.” (C) In the presence of high calcium, calcium (Ca2+) binds to troponin (green), resulting in movement of tropomyosin (dashed light/dark purple lines), allowing myosin binding and thin filament translocation.

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  Figure 2. 3-Bead Optical Trap Myosin motors are sparsely coated on a fixed pedestal bead (large bead). A single myosin can then bind to and displace an actin filament held between to 2 optically trapped beads (smaller gray beads). When the myosin motor moves the actin filament, a detector senses the displacement of the beads.

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  Figure 3. In Vitro Motility Assay Cartoon showing fluorescently labeled actin gliding on top of a bed of nonfluorescent myosin adsorbed to a glass coverslip. This movement can be measured using a standard fluorescence microscope.

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  Figure 4. Skinned Cardiomyocyte Contractility (A) A commercially available system that can be used to measure the force of single myocytes (Aurora Scientific 1600A). The myocyte is mounted between a length mover and a force transducer. Image courtesy of Chris Rand and Aurora Scientific. (B) Magnified view of the attached skinned cardiomyocyte. Reproduced from Greenman et al288 and licensed under a Creative Commons CC BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Scale bar in red. (C) Cartoon showing force development in buffers containing differing calcium concentrations. Lower calcium concentrations result in less force generated, with no force generated in the low-calcium pCa 6 (10−6 M calcium) buffer. (D) Cartoon showing the sigmoidal relationship between the maximum force generated and the concentration of calcium (pCa). Colored dots correspond to respectively colored pCa concentrations displayed in C.

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  Figure 5. Single-Myofibril Contractility (A) Commercially available system for single myofibril measurements (Aurora Scientific 1700A). The myofibril is mounted between a length mover (left) and cantilever holder (right), which is visualized by an optical periscope (middle). Image courtesy of Chris Rand and Aurora Scientific. (B) Magnified view of attached single myofibril held between a length mover and the stationary force transducer. Approximate scale bar displayed in red. Image used with permission from Aurora Scientific. (C) Cartoon showing the force generated by a single myofibril. The myofibril is initially bathed in relaxing buffer, then bathed in activating buffer, and then relaxing buffer again. Note the difference in the magnitude of forces between a single myofibril shown here and a skinned cardiomyocyte in Figure 4 (nanonewtons vs millinewtons). (D) Cartoon showing the time expanded view of region of interest from C showing transient linear relaxation phase (krel-slow) and the longer exponential relaxation phase (krel-fast).

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  Figure 6. Fluorescent Sarcomere Contractility Tracking in Live iCMs (A) Stem cell–derived cardiomyocytes expressing ACTN2-mEGFP, which demarcates Z-discs. Sarcomeres were identified and tracked using the SarcTrack software package.213 Note that not all sarcomeres are tracked, on the basis of a filtering algorithm in SarcTrack, which removes sarcomeres that are not reliably tracked for the entirety of the recorded video. (B) Representative data generated using SarcTrack showing a mutant (Mut) cell line that has greater contractility compared with the wild-type (WT) line.

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  Figure 7. Traction Force Microscopy of Cardiomyocytes (A) Bright-field image of a patterned stem cell–derived cardiomyocyte on a hydrogel. (B) Fluorescent beads embedded within the hydrogel. (C) Resultant strain map showing forces generated by contraction. This was calculated using the software package Contrax. Note that the strain points toward the center of the cell with maximal strain at opposite poles of the cell.289

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  Figure 8. Force Measurement With Passive Linear EHT System (A) Side-view schematic of a single engineered heart tissue (EHT) (orange) adhered around posts of known stiffness and immersed in media with electric stimulation. (B) Image of a linear EHT courtesy of Lina Greenberg (Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine). (C) Schematic showing post deformation with contraction. The force can be calculated from the stiffness of the posts and the magnitude of the displacement. (D) Sample force trace showing EHT contraction with electric stimulation.

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  Figure 9. Force Measurement With Active Ring Shaped EHT System (A) Side-view schematic of a ring tissue (orange) attached between a force transducer (left) and a length controller (right). The tissue is immersed in media and electrically stimulated. When the tissue contracts, this results in displacement of the force transducer. (B) Partial overhead image of ring tissue attached to force transducer (left, off screen) and length controller (right). (C) Representative force tracing (blue) showing tissue beating before and after a length step (red). The length step results in an increase from the initial passive force (P1) to a new passive force (P2). Active force modestly increases after stretch (A1 vs A2) because of the Frank-Starling mechanism. The tissue shows evidence of viscoelasticity from the increase and subsequent relaxation in total force following the length step. EHT = engineered heart tissue.

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  Figure 10. Drosophila Heart Tube Kymography Adult heart tube (red) with a line of focus drawn through the walls of the heart (green arrow) is recorded at high frame rate to produce a kymograph (right). Diastole and systole can be observed on the basis of the positioning of the anterior and posterior walls, similar to M-line echocardiography. Fly image created using BioRender.com (left). Kymogram (right) courtesy of Anthony Cammarato (Division of Cardiology, Department of Medicine, Johns Hopkins University School of Medicine).

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  Figure 11. Light-Sheet Microscopy of Zebrafish Heart Single-frame captures of light-sheet fluorescence microscopic videos of fluorescently labeled zebrafish hearts that are 96 hours postfertilization. Scale bar represents 40 μm. Image courtesy of Jamison Leid (Center for Cardiovascular Research, Washington University School of Medicine). a = atrium; v = ventricle.