Gap Junctional Communication in Osteocytes Is Amplified by Low Intensity Vibrations In Vitro

Abstract

The physical mechanism by which cells sense high-frequency mechanical signals of small magnitude is unknown. During exposure to vibrations, cell populations within a bone are subjected not only to acceleratory motions but also to fluid shear as a result of fluid-cell interactions. We explored displacements of the cell nucleus during exposure to vibrations with a finite element (FE) model and tested in vitro whether vibrations can affect osteocyte communication independent of fluid shear. Osteocyte like MLO-Y4 cells were subjected to vibrations at acceleration magnitudes of 0.15 g and 1 g and frequencies of 30 Hz and 100 Hz. Gap junctional intracellular communication (GJIC) in response to these four individual vibration regimes was investigated. The FE model demonstrated that vibration induced dynamic accelerations caused larger relative nuclear displacement than fluid shear. Across the four regimes, vibrations significantly increased GJIC between osteocytes by 25%. Enhanced GJIC was independent of vibration induced fluid shear; there were no differences in GJIC between the four different vibration regimes even though differences in fluid shear generated by the four regimes varied 23-fold. Vibration induced increases in GJIC were not associated with altered connexin 43 (Cx43) mRNA or protein levels, but were dependent on Akt activation. Combined, the in silico and in vitro experiments suggest that externally applied vibrations caused nuclear motions and that large differences in fluid shear did not influence nuclear motion (<1%) or GJIC, perhaps indicating that vibration induced nuclear motions may directly increase GJIC. Whether the increase in GJIC is instrumental in modulating anabolic and anti-catabolic processes associated with the application of vibrations remains to be determined.

Effects of Frequency and Acceleration Amplitude on Osteoblast Mechanical Vibration Responses: A Finite Element Study

“…The density ratio 0.4 : 1 : 1.2 (600 kg/m 3  : 1500 kg/m 3  : 1800 kg/m 3 ) of membrane, cytoplasm, and nucleus was assumed [26]. Young's modulus of membrane, cytoplasm, and nucleus was chosen at 1 kPa, 1.5 kPa, and 6 kPa, respectively, and Poisson's ratio was 0.3, 0.37, and 0.37 for membrane, cytoplasm, and nucleus, respectively.…”

“…Previously, Young's modulus of 6.5 kPa and Poisson's ratio of 0.5 were assigned to the cytoplasm of an osteoblastic cell [22]. Elastic modulus of cytoplasm of osteocyte-like MLO-Y4 cell was set at 1.5 kPa [26]. Similarly, based on the measurements by atomic force microscopy, Young's modulus of 6 kPa was assigned to the osteoblast nucleus [30].…”

“…Similarly, based on the measurements by atomic force microscopy, Young's modulus of 6 kPa was assigned to the osteoblast nucleus [30]. The elastic modulus of 1 kPa and Poisson's ratio of 0.3 were selected for the membrane of the adherent eukaryotic cell [16, 26]. …”

Clinical applications of vibration therapy in orthopaedic practice

“…Vibration promotes osteogenic differentiation 64 , cell communication 65 , while reduces osteoclast formation 66 and expression of osteoclastforming RANKL in osteocytes 67 , which is increased during unloading 68 . Table 2.…”

The Efficacy of Low-intensity Vibration to Improve Bone Health in Patients with End-stage Renal Disease Is Highly Dependent on Compliance and Muscle Response

“…Another hypothesis is that mechanical stimulation signals are amplified within the bone tissue by stress-generated increase in fluid flow resulting from direct bone stimulation, thereby activating osteocytes, which act as mechanosensors to mediate the skeleton’s response (26,27). However, there is also considerable evidence that suggests the vibrations cause larger displacements in cell nuclei than fluid shear, indicating the mechanism of action is more likely due to the mechanical coupling between these oscillating cell nuclei and the cytoskeleton, which ultimately induces actin remodeling and reduces bone resorption (6,28,29). …”