MiR-26a modulates extracellular matrix homeostasis in cartilage

MicroRNAs (miRNAs) may represent new therapeutic targets for bone and joint diseases. We hypothesized that several cartilage-specific proteins are targeted by a single miRNA and used bioinformatics to identify a miRNA that can modulate extracellular matrix (ECM) homeostasis in cartilage. Bioinformatic analysis of miRNA binding sequences in the 3'-untranslated region (3'-UTR) of target genes was performed to identify a miRNA that could bind to the 3'-UTR of cartilage matrix-related genes. MiRNA expression was studied by quantitative PCR of microdissected growth plate cartilage and binding to the 3'-UTR sequences was analyzed by luciferase interaction studies. Levels of proteins encoded by target genes in cultures of miR-26a mimic- or inhibitor-transfected chondrocytes were determined by FACS or immunoblot analysis. The complementary binding sequence of miR-26a and miR-26b was found in the 3'-UTR of the prehypertrophic/hypertrophic-specific genes Cd200, Col10a1 as well as Col9a1 and Ctgf. Both miRNAs were expressed in cartilage and only miR-26a was downregulated in hypertrophic growth plate cartilage. MiR-26a could interact with the 3'-UTR of Cd200 and Col10a1 in luciferase binding studies, but not with Col9a1 and Ctgf. However, protein expression of target genes and the ECM adaptor genes matrilin-3 and COMP was significantly altered in miR-26a mimic- or inhibitor-transfected chondrocytes, whereas the abundance of the cell surface receptor for insulin was not changed. In conclusion, miR-26a suppresses hypertrophic and ECM adaptor protein production. Dysregulation of miR-26a expression could contribute to ECM changes in cartilage diseases and this miRNA may therefore act as a therapeutic target.