TGF-β1 promotes gap junctions formation in chondrocytes via Smad3/Smad4 signalling

Objectives Connexin-mediated functional gap junction intercellular communication (GJIC) has a vital role in development, homeostasis and pathology. Transforming growth factor-beta 1 (TGF-beta 1), as one of the most vital factors in chondrocytes, promotes cartilage precursor cell differentiation and chondrocyte proliferation, migration and metabolism. However, how TGF-beta 1 mediates GJIC in chondrocytes remains unclear. This study aims to determine the influence of TGF-beta 1 on GJIC in mouse chondrocytes and its underlying mechanism. Methods qPCR and mRNA microarray were used to verify the expression of genes in the TGF-beta and connexin families in cartilage and chondrocytes. A scrape loading/dye transfer assay was performed to explore GJIC. Western blot analysis was used to detect connexin43 (Cx43) and Smad signalling components. Immunofluorescence staining was performed to characterize protein distribution. Results The TGF-beta 1 mRNA was the highest expressed member of the TGF beta super family in cartilage. TGF-beta 1 promoted functional GJIC through increased expression of Cx43. TGF-beta 1-mediated GJIC required the participation of TGF-beta type I receptor. TGF-beta 1 activated Smad3 and Smad4 signalling to facilitate their nuclear translocation. The Smad3 and Smad4 signalling proteins bound to the promoter of Gja1 and thus initiated Cx43 gene expression. Conclusions For the first time, these results revealed a vital role of TGF-beta 1 in cell-cell communication in chondrocytes via gap junction formation. We describe the regulatory mechanism, the involvement of TGF-beta type I receptor and the nuclear translocation of Smad3/4.