Real-Time Imaging of Sarcomere Dynamics in the Mouse Heart In Vivo

Abstract

Active force in cardiac muscle is highly dependent on sarcomere length (SL), which is known as the Frank-Starling mechanism of the heart. Indeed, a change of merely ∼100 nm in SL causes a dramatic change in cardiac contractile performance under partial activation states, highlighting the need for real-time SL imaging at high spatial and temporal resolution in vivo. In the present study, we expressed AcGFP at sarcomeric Z-disks (α-actinin) by means of an adenovirus vector system in adult mice, and applied nanometry for the measurement of SL displacement (i.e., SL nanometry) at 100 fps in the myocytes in the center of the left ventricle in anesthetized open-chest mice under a fluorescence microscope (combined with a spinning disk confocal unit and an EMCCD camera). First, we found that SL changed in the lengths of ∼1.92 and ∼1.63 μm during diastole and systole, respectively, and varied by ∼300 nm in both phases even in the same myocyte. Second, we found that sarcomere contraction started to occur at the T-wave endpoint on the electrocardiogram, followed by a rapid increase in left ventricular pressure (LVP). Similarly a linear relationship existed between the magnitude of the change in SL and that in LVP, demonstrating for the first time direct evidence of the Frank-Starling mechanism at the sarcomere level in vivo. Finally, we reconstructed the Z-sectioning images (at Z = 1 μm) of single sarcomeres by using the piezoelectric actuator, and successfully observed sarcomeric motions during the entire cardiac cycle. The present experimental system has a broad range of application possibilities for unveiling single sarcomere dynamics during excitation-contraction coupling in cardiomyocytes in the beating heart in vivo. At the meeting, we will discuss the organization of sarcomeric contraction in the beating heart in vivo.