557 – Investigating PRG4 Re-expression as a Treatment for TMJ Degenerative Diseases

Abstract

Lubricin (encoded by the gene PRG4) is the principal boundary lubricant in cartilage, and congenital missing of lubricin leads to progressive joint failure in the long bone. This study aims to characterize the roles of Lubricin (encoded by the gene PRG4) in temporomandibular joint (TMJ) and determine whether restoring the PRG4 expression would delay or rescue the degeneration of TMJ cartilage. 3-week-old PRG4 gene-trap (GT/GT) mice and wild-type (WT) controls were used to characterize the phenotype in TMJ cartilage of PRG4-deficient mice. Subsequently, PRG4 expression was restored in the GT/GT mice through induction of Cre-mediated recombination with tamoxifen injection at 3 weeks of age (i.e., GTR3wk/GTR3wk). The effects of RPG4 re-expression were assessed by comparing GTR3wk/GTR3wk mice and GT/GT mice at the age of 2 months and 6 months. The condyles were examined with micro-CT analysis, histomorphometric analyses, immunostaining, cell apoptosis. In addition, the cartilage stiffness of the mandibular condylar cartilage was measured by an automated indentation technique using a Biomomentum Mach-1 v500csst. At 3 weeks of age, PRG4 GT/GT mice exhibited a significantly lower total volume of condylar head, along with a sharp reduction in cartilage stiffness, when compared to WT mice. Histology and immunostaining revealed a degenerative phenotype in mandibular condylar cartilage (MCC) of 3-week-old GT/GT mice with increased cell apoptosis and decreased SOX-9, Col X, Aggrecan, and BMP2 expression. Osteoclastic activity in subchondral bone was drastically increased in 3-week-old GT/GT mice when compared to WT mice. At 2 months and 6 months of age, the GTR3wk/GTR3wk mice with PRG4 re-expression exhibited improved integrity of MCC, along with a significant increased tissue mineral density and bone volume fraction, when compared to PRG4-deficient GT/GT mice. Also, the condylar cartilage stiffness was significant higher after PRG4 re-expression in the 2-month group. Our results suggest that missing PRG4 leads to condylar cartilage degeneration and PRG4 restoration at 3 weeks of age will slow or prevent degeneration of the TMJ.