Conclusion
MMP-3, MMP-9, and MMP-13 contribute to the remodeling of the ECM of the MCC and subchondral bone during aging by degrading ECM proteins at specific times and sites under the regulation of their production and secretion.
Methods
Mandibular condyles were collected from 10- and 50-week-old mice. Total RNA was extracted from the samples and then analyzed using cap analysis of gene expression (CAGE) to identify age-related DEGs. Gene ontology (GO) enrichment analysis was performed to determine which biological processes were most affected by aging in terms of gene expression using Metascape. The mandibular condyle samples were processed for histology to investigate morphological changes caused by aging and for immunohistochemistry to localize the protein expression encoded by age-related genes identified with CAGE. Semi-quantitative immunohistochemistry was performed to assess age-related extracellular matrix (ECM) protein levels in the MCC. The histological sections were also used for Alcian blue histochemistry to detect glycosaminoglycans (GAGs).
Results
GO enrichment analysis revealed that the genes related to "extracellular matrix organization," including Acan, Col1a1, Col1a2, Col2a1, Mmp3, Mmp9, and Mmp13, were most differentially expressed in the aged mandibular condyle. Among these seven genes, Mmp3 was upregulated, and the others were downregulated with aging. Histological examination showed the age-related morphological and histological changes in the MCC. Immunohistochemical investigation showed the localization of matrix metalloproteinases (MMPs)-3, -9, and -13 and their substrate proteins, aggrecan, type I collagen, and type II collagen, in the mandibular condyle at 10 and 50 weeks, indicating different localizations between the young and the aged. In the aged MCC, semi-quantitative immunohistochemistry showed a significant decrease in the aggrecan protein level, and Alcian blue histochemistry showed a decrease in GAGs.