Osteoporosis-decreased extracellular matrix stiffness impairs connexin 43-mediated gap junction intercellular communication in osteocytes

Osteocytes are the main sensitive and responsive cells for mechanical stimuli in bone. The connexin family enables them to communicate with each other via forming functional gap junctions. However, how osteoporosis-impaired extracellular mechanical property modulates gap junction intercellular communication in osteocytes remains elusive. In this study, we established an ovariectomy (OVX)-induced osteoporosis mouse model in vivo and a polydimethylsiloxane (PDMS)-based cell culture substrate model in vitro to explore the influence of extracellular matrix (ECM) stiffness on cell-to-cell communication in osteocytes. Firstly, we established an OVX-induced osteoporosis mouse model by characterizing the changes in radiography, morphology and histochemistry of femurs. Our results showed that osteoporosis decreased the bone matrix stiffness together with the changes including the loss of osteocytes and the decrease of protein markers. Meanwhile, the dendritic process interconnection and channel-forming protein, Cx43, were reduced in osteoporosis mice. Next we mimicked ECM stiffness changes in vitro by using PDMS substrates at ratios 1:5 for normal stiffness and 1:45 for osteoporosis stiffness. Our results showed that the decreased ECM stiffness reduced the number of dendritic processes in a single cell and gap junctions between adjacent osteocytes. We further detected the decreased expression of Cx43, in the substrate with decreased stiffness. Finally, we found that gap junction-based intercellular communication was reduced in living osteocytes in the substrate with decreased stiffness. This study demonstrates the correlation between ECM mechanical property and cell-to-cell communication in osteocytes and might pave the way for further exploration of osteoporosis in terms of biomechanics.