Transmission of Mechanical Information by Purinergic Signaling

Abstract

The skeleton constantly interacts and adapts to the physical world. We have previously reported that physiologically relevant mechanical forces lead to small repairable membrane injuries in bone-forming osteoblasts, resulting in release of ATP and stimulation of purinergic (P2) calcium responses in neighboring cells. The goal of this study was to develop a theoretical model describing injury-related ATP and ADP release, their extracellular diffusion and degradation, and purinergic responses in neighboring cells. After validation using experimental data for intracellular free calcium elevations, ATP, and vesicular release after mechanical stimulation of a single osteoblast, the model was scaled to a tissue-level injury to investigate how purinergic signaling communicates information about injuries with varying geometries. We found that total ATP released, peak extracellular ATP concentration, and the ADP-mediated signaling component contributed complementary information regarding the mechanical stimulation event. The total amount of ATP released governed spatial factors, such as the maximal distance from the injury at which purinergic responses were stimulated. The peak ATP concentration reflected the severity of an individual cell injury, allowing to discriminate between minor and severe injuries that released similar amounts of ATP because of differences in injury repair, and determined temporal aspects of the response, such as signal propagation velocity. ADP-mediated signaling became relevant only in larger tissue-level injuries, conveying information about the distance to the injury site and its geometry. Thus, we identified specific features of extracellular ATP and ADP spatiotemporal signals that depend on tissue mechanoresilience and encode the severity, scope, and proximity of the mechanical stimulus.

Decreased pericellular matrix production and selection for enhanced cell membrane repair may impair osteocyte responses to mechanical loading in the aging skeleton

“…Accelerating PMD repair rates, while beneficial for enhancing postwounding cell survival (Hagan et al, ), can blunt downstream mechanotransduction: Fast repair rates limit the amount of ATP released from the wounded cell to initiate mechanotransduction in nonwounded neighboring cells (Mikolajewicz et al, , ; Yu et al, ). To test whether the accelerated PMD repair rate observed in aged osteocytes affected mechanotransduction, we monitored calcium signaling initiated by a laser‐induced PMD, as previously described (Yu et al, ).…”

“…We acknowledge that while statistically significant, the absolute effect of age on calcium wave propagation was mild. It is possible that other metrics of mechanotransduction propagation not measured here, such as ATP release by the wounded cell, could show greater absolute changes with aging (Mikolajewicz et al, , ); this prospect will be investigated in future studies. Still, following the logic supported by selective survival of fast‐repairing osteocytes and poor calcium wave propagation by those same cells, the impaired adaptation of aged bone to mechanical loading may be driven in part by surviving osteocytes possessing rapid PMD repair rate which blunts their ability to properly communicate mechanotransduction signals.…”

“…It has been suggested that fluid drag forces on the osteocyte pericellular matrix (i.e., glycocalyx) are important for osteocyte mechanotransduction, and that fiber volume fraction in the osteocyte pericellular matrix is decreased in aged as compared to younger bone (Wang et al, ), but a direct relationship between osteocyte pericellular matrix and impaired mechanosensation with aging has not yet been demonstrated. We and others have shown that transient plasma membrane disruptions (PMD) develop in osteocytes with mechanical loading both in vitro (fluid flow) and in vivo (exercise), and that these PMD initiate mechanotransduction (Hagan et al, ; Mikolajewicz, Sehayek, Wiseman, & Komarova, ; Mikolajewicz, Zimmermann, Willie, & Komarova, ; Yu et al, ), suggesting that osteocytes may use PMD to detect mechanical loads and direct the activity of remodeling cells to properly adapt the matrix as needed. The effects of aging on osteocyte PMD formation, however, have not been reported.…”

Interactions between Muscle and Bone—Where Physics Meets Biology

“…Evolution has provided multiple pathways for the mutual interaction of muscle and bone in order to adapt the stability of the skeleton and muscle to the forces needed for locomotion and environmental challenges. Recent information indicates that the reciprocity of mechanochemical interaction is extremely dynamic, including the cellular, the tissue, and the organismic level [1][2][3][4]. Intrinsic as well as extrinsic forces such as gravity generate adaptive changes of tissues, which modulate muscle power and fracture resistance.…”

Role of UDP-Sugar Receptor P2Y14 in Murine Osteoblasts

“…The amplitude of mechanically-stimulated [Ca 2+ ] i elevations in Y14 KO C2-Ob cells was significantly higher than in Wt cells (Figure 3c,d). In addition, [Ca 2+ ] i responses in neighbouring non-stimulated C2-Ob cells, which are mediated by ATP and ADP released from the mechanically-stimulated cell [19,20,59], tended to have more oscillatory peaks (p = 0.09) with greater magnitudes (p = 0.09) (Figure 3e-g). When ATP and ADP were applied directly to Fura2-loaded Wt or Y14 KO cells, calcium responses to both agonists were significantly potentiated in Y14 KO cultures ( Figure 3h).…”