Effects of Non-thermal Ultrasound on a Fibroblast Monolayer Culture: Influence of Pulse Number and Pulse Repetition Frequency

Despite the use of therapeutic ultrasound in the treatment of soft tissue pathologies, there remains some controversy regarding its efficacy. In order to develop new treatment protocols, it is a common practice to carry out in vitro studies in cell cultures before conducting animal tests. The lack of reproducibility of the experimental results observed in the literature concerning in vitro experiments motivated us to establish a methodology for characterizing the acoustic field in culture plate wells. In this work, such acoustic fields are fully characterized in a real experimental configuration, with the transducer being placed in contact with the surface of a standard 12-well culture plate. To study the non-thermal effects of ultrasound on fibroblasts, two different treatment protocols are proposed: long pulse (200 cycles) signals, which give rise to a standing wave in the well with the presence of cavitation (I_SPTP max = 19.25 W/cm²), and a short pulse (five cycles) of high acoustic pressure, which produces a number of echoes in the cavity (I_SPTP = 33.1 W/cm², with P_max = 1.01 MPa). The influence of the acoustic intensity, the number of pulses, and the pulse repetition frequency was studied. We further analyzed the correlation of these acoustic parameters with cell viability, population, occupied surface, and cell morphology. Lytic effects when cavitation was present, as well as mechanotransduction reactions, were observed.

Engineering Biomaterials and Approaches for Mechanical Stretching of Cells in Three Dimensions

Mechanical stretch is widely experienced by cells of different tissues in the human body and plays critical roles in regulating their behaviors. Numerous studies have been devoted to investigating the responses of cells to mechanical stretch, providing us with fruitful findings. However, these findings have been mostly observed from two-dimensional studies and increasing evidence suggests that cells in three dimensions may behave more closely to their in vivo behaviors. While significant efforts and progresses have been made in the engineering of biomaterials and approaches for mechanical stretching of cells in three dimensions, much work remains to be done. Here, we briefly review the state-of-the-art researches in this area, with focus on discussing biomaterial considerations and stretching approaches. We envision that with the development of advanced biomaterials, actuators and microengineering technologies, more versatile and predictive three-dimensional cell stretching models would be available soon for extensive applications in such fields as mechanobiology, tissue engineering, and drug screening.

Functionalizable Antifouling Coatings as Tunable Platforms for the Stress-Driven Manipulation of Living Cell Machinery

Cells are continuously sensing their microenvironment and subsequently respond to different physicochemical cues by the activation or inhibition of different signaling pathways. To study a very complex cellular response, it is necessary to diminish background environmental influences and highlight the particular event. However, surface-driven nonspecific interactions of the abundant biomolecules from the environment influence the targeted cell response significantly. Yes-associated protein (YAP) translocation may serve as a marker of human hepatocellular carcinoma (Huh7) cell responses to the extracellular matrix and surface-mediated stresses. Here, we propose a platform of tunable functionable antifouling poly(carboxybetain) (pCB)-based brushes to achieve a molecularly clean background for studying arginine, glycine, and aspartic acid (RGD)-induced YAP-connected mechanotransduction. Using two different sets of RGD-functionalized zwitterionic antifouling coatings with varying compositions of the antifouling layer, a clear correlation of YAP distribution with RGD functionalization concentrations was observed. On the other hand, commonly used surface passivation by the oligo(ethylene glycol)-based self-assembled monolayer (SAM) shows no potential to induce dependency of the YAP distribution on RGD concentrations. The results indicate that the antifouling background is a crucial component of surface-based cellular response studies, and pCB-based zwitterionic antifouling brush architectures may serve as a potential next-generation easily functionable surface platform for the monitoring and quantification of cellular processes.

Cell Mechanotransduction With Piconewton Forces Applied by Optical Tweezers

Mechanical stresses are always present in the cellular environment and mechanotransduction occurs in all cells. Although many experimental approaches have been developed to investigate mechanotransduction, the physical properties of the mechanical stimulus have yet to be accurately characterized. Here, we propose a mechanical stimulation method employing an oscillatory optical trap to apply piconewton forces perpendicularly to the cell membrane, for short instants. We show that this stimulation produces membrane indentation and induces cellular calcium transients in mouse neuroblastoma NG108-15 cells dependent of the stimulus strength and the number of force pulses.

TRPM7 Mediates Mechanosensitivity in Adult Rat Odontoblasts

Odontoblasts, with their strategic arrangement along the outermost compartment of the dentin-pulp complex, have been suggested to have sensory function. In addition to their primary role in dentin formation, growing evidence shows that odontoblasts are capable of sensing mechanical stimulation. Previously, we found that most odontoblasts express TRPM7, the nonselective mechanosensitive ion channel reported to be critical in Mg²⁺ homeostasis and dentin mineralization. In line with this finding, we sought to elucidate the functional expression of TRPM7 in odontoblasts by pharmacological approaches and mechanical stimulation. Naltriben, a TRPM7-specific agonist, induced calcium transient in the majority of odontoblasts, which was blocked by TRPM7 blockers such as extracellular Mg²⁺ and FTY720 in a dose-dependent manner. Mechanical stretch of the odontoblastic membrane with hypotonic solution also induced calcium transient, which was blocked by Gd³⁺, a nonselective mechanosensitive channel blocker. Calcium transient induced by hypotonic solution was also blocked by high extracellular Mg²⁺ or FTY720. When TRPM7-mediated calcium transients in odontoblasts were analyzed on the subcellular level, remarkably larger transients were detected in the distal odontoblastic process compared with the soma, which was further verified with comparable immunocytochemical analysis. Our results demonstrate that TRPM7 in odontoblasts can serve as a mechanical sensor, with its distribution to facilitate intracellular Ca²⁺ signaling in the odontoblastic process. These findings suggest TRPM7 as a mechanical transducer in odontoblasts to mediate intracellular calcium dynamics under diverse pathophysiological conditions of the dentin.

Integrated strain array for cellular mechanobiology studies

We have developed an integrated strain array for cell culture enabling high-throughput mechano-transduction studies. Biocompatible cell culture chambers were integrated with an acrylic pneumatic compartment and microprocessor-based control system. Each element of the array consists of a deformable membrane supported by a cylindrical pillar within a well. For user-prescribed waveforms, the annular region of the deformable membrane is pulled into the well around the pillar under vacuum, causing the pillar-supported region with cultured cells to be stretched biaxially. The optically clear device and pillar-based mechanism of operation enables imaging on standard laboratory microscopes. Straightforward fabrication utilizes off-the-shelf components, soft lithography techniques in polydimethylsiloxane, and laser ablation of acrylic sheets. Proof of compatibility with basic biological assays and standard imaging equipment were accomplished by straining C2C12 skeletal myoblast cells on the device for 6 hours. At higher strains, cells and actin stress fibers realign with a circumferential preference.