Synthesis of mRNAs for cathepsins L and K during development of the rat mandibular condylar cartilage

Cathepsins are cysteine proteinase family members which are known to degrade proteoglycan and collagen, components of several extracellular matrices. Their functions in the condylar cartilage during skeletal development have not yet been fully clarified. In this study, we investigated the mRNA expression of cathepsins L and K in the growing rat mandibular condylar cartilage, as compared to that in the rat long bone cartilage, by in situ hybridization, reverse transcription polymerase chain reaction (RT-PCR) and Southern blotting analysis. In the condylar cartilage, cathepsin L mRNA expression was widely observed throughout the zones of maturative and hypertrophic chondrocytes at embryonic day (E) 17 and postnatal day (P) 1. The signal was restricted to maturative and upper hypertrophic chondrocytes at P7, and finally it became undetectable by P28. In the long bone cartilage, cathepsin L was not expressed in the chondrocytes at any stage. Cathepsin K mRNA was not, however, detected either in the mandibular condylar cartilage cells or in the long bone cartilage cells but it was selectively detected in osteoclasts in calcified cartilage and bone in both tissues. RT-PCR also showed a similar mRNA expression pattern of cathepsins L and K. These results indicate that cathepsins L and K may be involved in the skeletal development of both the long bone and the mandibular condyle. Furthermore, cathepsin L may play an important role in the degradation of the cartilaginous extracellular matrix in maturative and hypertrophic cell layers during successive developmental stages of the mandibular condyle.