Low-intensity pulsed ultrasound/nanomechanical force generators enhance osteogenesis of bone marrow mesenchymal stem cells through microfilaments and transient receptor potential melastatin 7 activation

Abstract:

Background: Low-intensity pulsed ultrasound (LIPUS) has been reported to accelerate fresh fracture healing, but its mechanism of action is unclear and its efficacy needs to be further optimized. Ultrasound in combination with functionalized microbubbles has been shown to induce local shear forces and controllable mechanical stress in cells, amplifying the mechanical effects of LIPUS. Compared with microbubbles, nanoscale lipid bubbles (nanobubbles) have high stability and good biosafety. However, the effect of LIPUS combined with functionalized nanobubbles on osteogenesis has rarely been studied.

Results: In this study, we report cyclic arginine-glycine-aspartic acid-modified nanobubbles (cRGD-NBs), with a particle size of ~500 nm, able to actively target integrin receptors. cRGD-NBs can be attached to bone marrow mesenchymal stem cells (BMSCs) in large quantities and act as nanomechanical force generators on the cell membrane, exerting mechanical forces on the cell membrane and intracellular region under the action of LIPUS, activating the mechanosensitive transient receptor potential melastatin 7 (TRPM7) ion channel to mediate extracellular calcium influx; in addition, the actin cytoskeleton responds to the incoming mechanical force, showing enhanced polymerization, which further promotes osteogenic differentiation. Moreover, TRPM7 and actin may regulate each other during this process.

Conclusions: Compared with LIPUS alone, the combination of LIPUS and nanomechanical force generators further promotes osteogenesis. TRPM7, actin cytoskeleton, and intracellular calcium oscillations are all involved in this process. This study provides new directions and ideas for optimizing the efficacy of LIPUS for fracture healing, and provides a theoretical basis for the further application and development of LIPUS in clinical practice.