Ultrasound-activated microbubbles mediate F-actin disruptions and endothelial gap formation during sonoporation

Locally opening up the endothelial barrier in a safe and controlled way is beneficial for drug delivery into the extravascular tissue. Although ultrasound-induced microbubble oscillations can affect endothelial barrier integrity, the mechanism remains unknown. Here we uncover a new role for F-actin in microbubble-mediated endothelial gap formation. Unique simultaneous high-resolution confocal microscopy and ultra-high-speed camera imaging (10 million frames per second) reveal that oscillating microbubbles (radius 1.3-3.8 µm) induce sonoporation in all cells in which F-actin remodeling occurred. F-actin disruption only mainly resulted in tunnel formation (75%) and F-actin stress fiber severing and recoil mainly resulted in cell-cell contact opening within 15 s upon treatment (54%) and tunnel formation (15%). Stress fiber severing occurred when fibers were within reach of the microbubble’s maximum radius during oscillation, requiring normal forces of ≥230 nN. Together, these findings reveal a novel mechanism of microbubble-mediated drug delivery, which associates with the underlying cytoskeletal organization.