Ex vivo real-time observation of Ca²⁺ signaling in living bone in response to shear stress applied on the bone surface

Bone cells respond to mechanical stimuli by producing a variety of biological signals, and one of the earliest events is intracellular calcium ([Ca²⁺]_i) mobilization. Our recently developed ex vivo live [Ca²⁺]_i imaging system revealed that bone cells in intact bone explants showed autonomous [Ca²⁺]_i oscillations, and osteocytes specifically modulated these oscillations through gap junctions. However, the behavior and connectivity of the [Ca²⁺]_i signaling networks in mechanotransduction have not been investigated in intact bone. We herein introduce a novel fluid-flow platform for probing cellular signaling networks in live intact bone, which allows the application of capillary-driven flow just on the bone explant surface while performing real-time fluorogenic monitoring of the [Ca²⁺]_i changes. In response to the flow, the percentage of responsive cells was increased in both osteoblasts and osteocytes, together with upregulation of c-fos expression in the explants. However, enhancement of the peak relative fluorescence intensity was not evident. Treatment with 18 α-GA, a reversible inhibitor of gap junction, significantly blocked the [Ca²⁺]_i responsiveness in osteocytes without exerting any major effect in osteoblasts. On the contrary, such treatment significantly decreased the flow-activated oscillatory response frequency in both osteoblasts and osteocytes. The stretch-activated membrane channel, when blocked by Gd³⁺, is less affected in the flow-induced [Ca²⁺]_i response. These findings indicated that flow-induced mechanical stimuli accompanied the activation of the autonomous [Ca²⁺]_i oscillations in both osteoblasts and osteocytes via gap junction-mediated cell-cell communication and hemichannel. Although how the bone sense the mechanical stimuli in vivo still needs to be elucidated, the present study suggests that cell-cell signaling via augmented gap junction and hemichannel-mediated [Ca²⁺]_i mobilization could be involved as an early signaling event in mechanotransduction.