Epigenetic Priming Enhances Chondrogenic Potential of Expanded Chondrocytes for Cartilage Repair

Expansion of chondrocytes presents a major obstacle in the cartilage regeneration procedure matrix-induced autologous chondrocyte implantation (MACI). Dedifferentiation of chondrocytes during the expansion process leads to the emergence of a fibrotic (chondrofibrotic) phenotype that decreases the chondrogenic potential of the implanted cells. We aim to 1) determine the extent that chromatin architecture of H3K27me3 and H3K9me3 remodels during dedifferentiation and persists when expanded chondrocytes are transferred to a 3D culture; and 2) to prevent this persistent remodeling to enhance the chondrogenic potential of expanded chondrocytes. Chromatin architecture remodeling of H3K27me3 and H3K9me3 was observed at 0, 8 and 16 population doublings in a two-dimensional (2D) culture and after encapsulation of the expanded chondrocytes in a three-dimensional (3D) hydrogel culture. Chondrocytes were treated with inhibitors of epigenetic modifiers (epigenetic priming) for 16 population doublings and then encapsulated in 3D hydrogels. Chromatin architecture of chondrocytes and gene expression were evaluated before and after encapsulation. We observed a change in chromatin architecture of epigenetic modifications H3K27me3 and H3K9me3 during chondrocyte dedifferentiation. Although inhibiting enzymes that modify H3K27me3 and H3K9me3 did not alter the dedifferentiation process in 2D culture, applying these treatments during the 2D expansion did increase the expression of select chondrogenic genes and protein deposition of type II collagen when transferred to a 3D environment. Overall, we found that epigenetic priming of expanded chondrocytes alters the cell fate when chondrocytes are later encapsulated into a 3D environment, providing a potential method to enhance the success of cartilage regeneration procedures.