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 clarify this mechanism, we investigated the effects of in vitro cyclic stretch (120% peak to peak, at 0.5 Hz) on cultured spinal ligament cells derived from OPLL (OPLL cells) and non-OPLL (non-OPLL cells) patients. The mRNA expressions of Cbfa1 (an osteoblast-specific transcription factor), type I collagen, alkaline phosphatase (ALP), osteocalcin and integrin beta1 (a mechanotransducer) were increased by cyclic stretch in OPLL cells, whereas no change was observed in non-OPLL cells. The effects of cyclic stretch on the spinal ligament tissues derived from OPLL and non-OPLL patients were also analyzed by immunohistochemistry using an antibody against Cbfa1. The expression of Cbfa1 was increased by cyclic stretch at the center of the spinal ligament tissues of OPLL patients, whereas no change was observed in the tissues of non-OPLL patients. Furthermore, U0126, a specific inhibitor of MAPK kinase (MEK), suppressed the stretch-induced mRNA expressions of Cbfa1, ALP and type I collagen in OPLL cells. These results suggest that in OPLL cells, mechanical stress is converted by integrin beta1 into intracellular signaling and that Cbfa1 is activated through the MAP kinase pathway. Therefore, we propose that mechanical stress plays a key role in the progression of OPLL through an increase in Cbfa1 expression.

Purpose Although serum hyaluronan (HA) levels increase in patients with osteoarthritis (OA), the association between OA severity and elevation of serum HA levels is not clear. Our purpose was to investigate the relationship between serum HA levels and OA in various anatomical sites and to detect which joints are strongly correlated with elevated serum HA levels. Methods Seven hundred and ten individuals from the general population who participated in the Iwaki Health Promotion Project in 2008 were involved. Kellgren-Lawrence grade 2 or higher in the knee, hip, lumbar spine, finger and wrist was defined as OA. Serum HA levels were determined on the same day. Spearman's correlation coefficients between serum HA levels and total number of joints affected by OA were calculated. Linear regression was analysed with serum HA levels as the independent variable; age, gender, presence of OA and intake of supplements were used as dependent variables.Results Prevalence of knee OA was 30.7 %, hip 16.8 %, lumbar spine 65.1 %, wrist 9.0 % and finger 22.0 %. Serum HA levels had a positive correlation with the number of involved joints, and the correlation coefficient was 0.410 (p<0.001). Serum HA was significantly affected by age (β=0.382), knee OA (β=0.163) and finger OA (β=0.164). Conclusion Although this biomarker reflects a systemic condition, higher serum HA levels were associated with total number of OA joints. Knee and finger OA were key joints related to increased serum HA levels. These results are valuable in understanding characteristics of serum HA levels as a biomarker for osteoarthritis.

Ossification of the posterior longitudinal ligament (OPLL) of the spine is characterized by progressive ectopic bone formation in the spinal ligament. To identify the genes related to ossification affected by mechanical stress during OPLL, analyses using cDNA microarray were carried out using cultured human spinal ligament cells that had been subjected to uniaxial cyclic stretching. Samples were obtained from a total of 14 patients: seven cervical or thoracic OPLL patients and seven control patients. Spinal ligament cells derived from tissues of OPLL (OPLL cells) and control (non-OPLL cells) patients were subjected to uniaxial sinusoidal cyclic stretching (0.5 Hz, 20% stretch) for various time periods (0-9 hours). cDNA microarrays revealed that ranges of distribution of both up- and downregulated genes evoked by cyclic stretching were significantly wider in OPLL cells than in non-OPLL cells. Increases in the mRNA expression of endothelin-1 (ET-1) as well as various marker genes related to ossification were also observed. mRNA expression of ET-1 and alkaline phosphatase was increased by mechanical stress in a time-dependent manner, while addition of ET-1 to static cultures of OPLL cells increased mRNA expression of alkaline phosphatase in a dose-dependent manner. During 9 hours of cyclic stretching, ET-1 release increased to about sixfold the amount observed in nonstretched cells. In non-OPLL cells, neither cyclic stretching nor ET-1 induced any increase in alkaline phosphatase expression. These results suggest that mechanical stress promotes the progression of ossification in OPLL cells through autocrine and/or paracrine mechanisms of ET-1.

Ossification of the posterior longitudinal ligament of the spine (OPLL) is characterized by ectopic bone formation in the spinal ligaments, and mechanical stress has been suggested to play an important role in the progression of OPLL. To identify the genes that participate in OPLL, the differential display reverse transcription-polymerase chain reaction (RT-PCR) method was used. A 283-base pair cDNA fragment corresponding to prostaglandin I 2 (PGI 2 ) synthase was highly expressed in OPLL cells compared with non-OPLL cells. To examine the effect of mechanical stress on the expression of PGI 2 synthase, cells were subjected to uniaxial cyclic stretch (0.5 Hz, 20% stretch), and PGI 2 synthase mRNA expression was assessed by quantitative RT-PCR. Cyclic stretch induced an increase in PGI 2 synthase in OPLL cells in a time-dependent manner, whereas no change was observed in non-OPLL cells. Cyclic stretch for 9 h also induced a 2.86ϫ increase in PGI 2 production. Beraprost (a stable PGI 2 analog) and dibutyryl cAMP (a membrane-permeable cAMP analog) increased the mRNA expression of alkaline phosphatase (ALP) as a marker for osteogenic differentiation up to 240 and 200%, respectively, in OPLL cells, whereas no change was observed in non-OPLL cells. The increases in ALP mRNA induced by beraprost and cyclic stretch were both inhibited by SQ22536, a potent adenylate cyclase inhibitor. These data suggest that the increase in PGI 2 synthase induced by mechanical stress plays a key role in the progression of OPLL, at least in part through the induction of osteogenic differentiation in spinal ligament cells via the PGI 2 /cAMP system.