Therapeutic Ultrasound Effects on Human Induced Pluripotent Stem Cell Cardiomyocytes Measured Optically and with Spectral Ultrasound

Abstract

To the best of our knowledge, therapeutic ultrasound (TUS) is thus far an unexplored means of delivering mechanical stimulation to cardiomyocyte cultures, which is necessary to engineer a more mature cardiomyocyte phenotype in vitro. Spectral ultrasound (SUS) may provide a way to non-invasively, non-disruptively and inexpensively monitor growth and change in cell cultures over long periods. Compared with other measurement methods, SUS as an acoustic measurement tool will not be affected by an acoustic therapy, unlike electrical measurement methods, in which motion caused by acoustic therapy can affect measurements. Further SUS has the potential to provide functional as well as morphological information in cell cultures. Human induced pluripotent stem cell cardiomyocytes (iPS-CMs) were imaged with calcium fluorescence microscopy while TUS was being applied. TUS was applied at 600 kHz and 1, 3.4 and 6 W/cm² for a continuous 1 s pulse. Measures of the instantaneous beat frequency, repolarization rate and calcium spike amplitude were calculated from the fluorescence data. At 600 kHz, TUS at 1 and 6 W/cm² had significant effects on the shortening of both the repolarization rate and instantaneous beat rate of the iPS-CMs (p < 0.05), while TUS at 3.4 and 6 W/cm² had significant effects on the shortening of the calcium spike amplitude (p < 0.05). Three SUS measures and one gray-level measure were captured from the iPS-CM monolayers while they were simultaneously being imaged with calcium-labeled confocal microscopy. The gray-level measure performed the best of all SUS measures; however, it was not reliable enough to produce a consistent determination of the beat rate of the cell. Finally, SUS measures were captured using three different transducers while simultaneously applying TUS. A center-of-mass (COM) measure calculated from the wavelet transform scalogram of the time-averaged radiofrequency data revealed that SUS was able to detect a change in the frequency content of the reflected ultrasound at 1 and 6 W/cm² before and after ultrasound application (p < 0.05), showing promise for the ability of SUS to measure changes in the beating behavior of iPS-CMs. Overall, SUS is promising as a method for constant monitoring of dynamic cell and tissue culture and growth.

Keywords: Cardiomyocytes; Induced pluripotent stem cell cardiomyocytes; Spectral ultrasound; Therapeutic ultrasound.