Cellular and subcellular impact of low-intensity ultrasound: stimulus or stress

Abstract

The therapeutic applicability of ultrasound is perhaps well demonstrated with the advent of High-Intensity Focused Ultrasound (HIFU) that works by rapidly heating tissues to induce ablation. Ultrasound also holds tremendous therapeutic potential at low intensities that are near or below the clinical diagnosis limit (720 mW/cm2 spatial-peak time-averaged intensity, as set forth by FDA), presumably acting through a mechanical interaction pathway. However, unlike HIFU where the biological outcome is typically instant cell death, low-intensity ultrasound may induce numerous short-term and long-term outcomes that may be suppressive or proliferative depending on the acoustic exposure parameters. In order to unravel various therapeutic effects that may be induced by low-intensity ultrasound, it is essential to pursue cellular-level investigations across different cell types and under various acoustic settings. In this talk, we shall present the latest findings from a series of low-intensity ultrasound biophysics studies conducted by our research group. First to be discussed is our work on using ultrasound as a repulsive cue for modulating neuronal development dynamics in-vitro. By monitoring neuronal cell morphology in real-time during pulsed ultrasound exposure, we observed that they would undergo neurite retraction and cell body shrinkage. Such an ultrasound-neuron interaction was found to be mediated by a mechanotransduction mechanism, as determined from our ion-channel blockage experiments and confocal microscopy observations. Similar morphological findings were also obtained for other cell types including fibroblasts and stem cells. Interestingly, we discovered that, after the end of ultrasound exposure, proliferation of the treated cells was significantly enhanced. This suggests that transient low-intensity ultrasound exposure is after all non-destructive and may stimulate cellular growth in the long run. The wave-matter interactions of low-intensity ultrasound become more complex (yet more intriguing) when microbubbles are introduced as agents to induce acoustic cavitation. Temporary membrane perforation may be readily achieved in this case (often referred to as sonoporation), and it has been tipped as an emerging paradigm for drug/gene delivery. However, we discovered that this way of permeating the cellular membrane would inadvertently pose stress to living cells even after membrane resealing has occurred. In particular, sonoporated cells were found to exhibit membrane shrinkage and intracellular lipid accumulation. Also, as compared to normal cells, their DNA synthesis kinetics was found to be significantly lengthened, and the onset of cell-cycle arrest was evident. In some instances, programmed cell death (i.e. apoptosis) may even take place. This prompts the need to refine and optimize sonoporation for drug/gene delivery purposes in order to maintain cellular viability. On the other hand, this may represent another way of inducing cell death in contrast to HIFU-based thermal ablation.