The innate ability of cells to respond to a wide range of internal and external stimuli is of great significance in tissue engineering and regenerative medicine. The mechanical cues could be in the form of active cell stimulation by imposing various forms of stress and strain or passive substrate cell interactions like variations in the local microenvironment. By applying various external stimuli, it is possible to direct stem fate to achieve controlled targeted differentiation effectively. In addition, mechanotransduction processes have also drawn attention for potential application in cellular differentiation, given their critical roles in the primary functions of a cell and therapeutic potential for tissue engineering.
Externally-driven dynamic mechanical stimuli have been extensively studied. The stimuli are mainly induced by vibrational excitation or cyclic pressure/stretching or laminar shear on the cells. Nevertheless, most of these studies have been limited to low frequencies (Hz-kHz), forcing extended treatment duration, typically above 7 days, often with biochemical stimulants. This is because low frequencies are characteristic physiological frequencies related to the motion of cells in their local environments.
Up to then, studies have however reported little osteogenic upregulation beyond 1 kHz, asserting little benefit of applying high frequencies beyond 1 kHz. Given however that MHz-order frequencies have recently provided new avenues for effective materials and fluids manipulation beyond conventional sonochemical and ultrasonic processes, it stands to reason that there is no compelling reason why high-frequency mechanostimulation at these frequencies might not be able to influence stem cell fate specification and regulation.
On this account, PhD candidate Lizebona August, Dr. Amy Gelmi and led by Professor Leslie Yeo from RMIT University studied the effects of 10 MHz surface reflected bulk waves (SRBWs) vibration excitation on human mesenchymal stem cells (MSCs) and its implication for osteogenic modulation. Compared with kHz-order mechanostimulation in the previous studies, the 10 nanometer amplitude high MHz-order frequency vibration excitation used in this study was not only physically different in nature but also induced a different mechanism for triggering the cell response. The work is currently published in the journal, Small.
The research team showed that short duration (a few minutes daily for 5 days) of high-frequency excitation of human MSCs resulted in early and long-term osteogenic lineage commitment in the bone marrow derived MSCs without chemical stimuli when applied at basal conditions, contrary to the reports in the previous studies. In addition to triggering the upregulation in early osteogenic markers, the high frequency-induced rapid treatment also sustained an increase in the late markers. Similar responses were observed in human MSCs derived from adipose and umbilical cord blood, confirming the efficacy of the proposed stimulation treatment for osteogenic differentiating. These observations were mainly attributed to the role of surface reflected bulk waves in triggering a perpetual shift in the human MSCs by activating the mechanosensitive ion channels as well as the modulation of the signaling pathway of RhoA protein kinase.
In summary, the study is the first to report the possibility of inducing and preserving the viability of osteogenetic differentiation in human MSCs via high-frequency mechanostimulation of four magnitudes higher than that used in previous studies. Given the low costs, simplicity and miniaturizability advantages of the devices and experimental setup, there is a huge possibility of upscaling the platform for practical bioreactors or integrating it with existing standard tissue cultureware to facilitate facile pre-treatment of human MSCs from multiple tissues sources before transplantation. In a statement to Advances in Engineering, Professor Leslie Yeo the lead and corresponding author said that beyond studying the intriguing fundamental mechanisms on how cells feel and respond to the high frequency stimuli, their findings would contribute to developing more efficient stem cell differentiation technologies for tissue engineering and regenerative medicine.
Ambattu, L., Gelmi, A., & Yeo, L. (2022). Short‐Duration High Frequency MegaHertz‐Order Nanomechanostimulation Drives Early and Persistent Osteogenic Differentiation in Mesenchymal Stem Cells. Small, 18(8), 2106823.