Osteocyte calcium signals encode strain magnitude and loading frequency in vivo

Area of Science:

• Biomedical Engineering
• Cell Biology
• Skeletal Biology

Background:

• Osteocytes are key mechanosensory cells in bone, crucial for sensing mechanical forces.
• Understanding in vivo osteocyte response to mechanical loading is limited.
• Previous studies on osteocyte calcium signaling were primarily in vitro.

Purpose of the Study:

• To investigate in vivo intracellular calcium (Ca2+) signaling in osteocytes under mechanical loading.
• To characterize osteocyte responses to varying strain magnitudes and frequencies in living mice.
• To develop and utilize a novel in vivo technique for studying bone cell mechanotransduction.

Main Methods:

• Utilized a three-point bending device to apply controlled mechanical loads to mouse metatarsal bones.
• Employed a genetically encoded fluorescent calcium indicator for real-time monitoring of intracellular Ca2+.
• Used multiphoton fluorescence microscopy to image osteocyte responses in vivo.
• Tested strains from 250 to 3,000 microstrain and frequencies from 0.5 to 2 Hz.

Main Results:

• The number of responding osteocytes significantly increased with higher applied strain magnitudes across all tested frequencies.
• Intracellular Ca2+ intensity within activated osteocytes did not show significant changes with varying physiological loading magnitudes.
• Demonstrated a dose-dependent relationship between mechanical strain and the number of osteocytes activated.

Conclusions:

• Osteocyte activation in vivo is strain-magnitude dependent.
• Calcium signaling intensity in individual osteocytes may not be the primary encoding mechanism for load magnitude.
• This study provides a valuable in vivo model for dissecting bone mechanosensing mechanisms.