Mechanical strain induces Cx43 expression in spinal ligament fibroblasts derived from patients presenting ossification of the posterior longitudinal ligament

Ossification of the posterior longitudinal ligament (OPLL) is characterized by ectopic bone formation in spinal ligaments. Some evidence indicates that mechanical strain can lead to the development of OPLL, although the signaling mechanism is not fully understood. Connexin43 (Cx43), a gap-junction protein, has been shown to be of particular importance in bone formation. We hypothesized that Cx43 may play an important role in the signal transmission induced by mechanical strain during the development of OPLL. To explore this possibility, we cultured fibroblasts from spinal ligaments of OPLL and non-OPLL patients and preloaded mechanical stretch onto the cells via a Flexercell 4000 Tension Plus system. We evaluated expression changes in osteocalcin (OCN), alkaline phosphatase (ALP), type I collagen (COL I) and Cx43 via semi-quantitative RT-PCR and western blotting at 12 and 24 h after mechanical strain application in contrast to static conditions. We observed a significant gene up-regulation of OCN, ALP and COL I and Cx43 protein in OPLL cells after mechanical strain application, but no changes in non-OPLL cells. Notably, after RNA interference targeting Cx43 was performed in OPLL cells, we found that there were no significant changes in the expressions of OCN, ALP, COL I and Cx43 after the mechanical strain was applied for 24 h. Thus, we propose that the increase in Cx43 expression induced by mechanical strain in OPLL cells plays an important role in the progression of OPLL.