Calcium response in single osteocytes to locally applied mechanical stimulus: Differences in cell process and cell body

Abstract

It is proposed that osteocytes embedded in the bone matrix have the ability to sense deformation and/or damage to the matrix and to feed these mechanical signals back to the adaptive bone remodeling process. When osteoblasts differentiate into osteocytes during the bone formation process, they change their morphology to a stellate form with many slender processes. This characteristic cell shape may underlie the differences in mechanosensitivity between the cell processes and cell body. To elucidate the mechanism of cellular response to mechanical stimulus in osteocytes, we investigated the site-dependent response to quantitatively controlled local mechanical stimulus in single osteocytes isolated from chick embryos, using the technique of calcium imaging. A mechanical stimulus was applied to a single osteocyte using a glass microneedle targeting a microparticle adhered to the cell membrane by modification with a monoclonal antibody OB7.3. Application of the local deformation induced calcium transients in the vicinity of the stimulated point and caused diffusive wave propagation of the calcium transient to the entire intracellular region. The rate of cell response to the stimulus was higher when applied to the cell processes than when applied to the cell body. In addition, a large deformation was necessary at the cell body to induce calcium transients, whereas a relatively small deformation was sufficient at the cell processes, suggesting that the mechanosensitivity of the cell processes was higher than that of the cell body. These results suggest that the cell shape with slender processes contributes to the site-dependent mechanosensitivity in osteocytes.

Dendritic processes of osteocytes are mechanotransducers that induce the opening of hemichannels

“…However, stimulation of dye uptake of the cell body from dendritic sides suggests that dendrites are mechanical transducer/sensors that lead to the opening of hemichannels. Consistently, a recent report shows that the osteocyte cell bodies and dendritic processes are differentially stimulated and the stimulated osteocyte dendritic processes have higher intracellular calcium response when compared to cell bodies (27). Because the measure for opening of hemichannels used in our experiments is through LY dye uptake, we cannot rule out the possibility that the dye uptake observed could be due to other integrinassociated channels.…”

Multiscale fluid–structure interaction modelling to determine the mechanical stimulation of bone cells in a tissue engineered scaffold

“…For these bridged cells, our models predicted that it was at the cellular extensions (which form the attachment with the scaffold) that cells experience much higher levels of stimulation (177,200µε) compared to the cell body or nucleus regions. Interestingly, an in vitro study on osteocyte cells in 2D culture (Adachi et al 2009) has shown that osteocyte cell processes are much more mechanosensitive than the cell body.…”

Matrix-dependent adhesion mediates network responses to physiological stimulation of the osteocyte cell process

“…Opening of membrane channels for the passage of signaling molecules initiated by pN-level forces is detectable by whole-cell patch-clamp electrophysiology with high temporal resolution. Previous culture studies have shown that networks of osteocytes respond to forces imposed by fluid shear stress and substrate stretching, as well as to cell indentation of a much higher magnitude, with the release of nucleotides, prostaglandin E2 (PGE2), and nitric oxide (NO) into the extracellular space and elevated intercellular Ca 2+ levels owing to receptor activation (20)(21)(22)(23)(24)(25). The osteocyte mechanotransducer is likely a protein complex that includes integrins, pannexin, or connexin hemichannels (21,(26)(27)(28)(29) in addition to ligand-gated receptors, particularly purinergic receptors (30,31) and hormone receptors (32).…”

“…Reports of force-induced mobilization and action of such molecules as PGE 2 (41,42), NO (25,43), VEGF (20), RANKL, OPG (44), ATP (30,45), and Ca 2+ (18,22) affirm autocrine/paracrine signaling in MLO-Y4 cells. Our evaluation of P2R signaling dynamics in MLO-Y4 cells after incubation with the broad-spectrum P2 receptor antagonist suramin demonstrated suppressed effector responses of MLO-Y4 cells, consistent with previously reported suramin-induced interference with P2Y and P2X receptor binding sites that blocked the action of ATP on purinergic receptors and subsequent signaling cascades (46,47).…”