Uniaxial cyclic stretch induces osteogenic differentiation and synthesis of bone morphogenetic proteins of spinal ligament cells derived from patients with ossification of the posterior longitudinal ligaments

Abstract 

Ossification of the posterior longitudinal ligament of the spine (OPLL) is characterized by ectopic bone formation in the spinal ligaments. Mechanical stress, which acts on the posterior ligaments, is thought to be an important factor in the progression of OPLL. To elucidate this mechanism, we investigated the effects of in vitro sinusoidal cyclic stretch (120% peak to peak, at 1 Hz) on cultured spinal ligament cells derived from OPLL and non-OPLL patients. The mRNA expressions of alkaline phosphatase (ALP), osteopontin, bone morphogenetic protein (BMP)-2, BMP-4, and BMP receptors as well as ALP activity in cell layers and production of BMPs into the conditioned medium were significantly increased by cyclic stretch in OPLL cells, whereas no change was observed in non-OPLL cells. A stretch-activated Ca²⁺ channel blocker, Gd³⁺, the voltage-dependent L-type Ca²⁺ channel blockers diltiazem and nifedipine, and Ca²⁺-free medium suppressed stretch-induced ALP activity, which suggests a role of Ca²⁺ influx in the signal transduction of mechanical stress to the osteogenic response of OPLL cells. Our study provides first evidences that mechanical stress plays a key role in the progression of OPLL through the induction of osteogenic differentiation in spinal ligament cells and the promotion of the autocrine/paracrine mechanism of BMPs in this lesion.

Keywords:  Ossification of the posterior longitudinal ligament, Mechanical stress, Bone morphogenetic protein, Calcium channel, Alkaline phosphatase