Articular Cartilage Tissue Engineering with Human Pluripotent Stem Cells

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ABSTRACT Impairment of articular cartilage function after injury and disease like osteoarthritis (OA), remains a major health problem. One of the major drawbacks of tissue engineering-based therapies for damaged joint articular cartilage is that the cartilaginous repair tissue formed by implanted mesenchymal stromal cells or endogenous progenitors does not resemble articular cartilage, likely due to fibrochondrogenesis and the endochondral ossification process. Joint cartilage is generated during embryogenesis by specialized GDF5+ cells called ‘interzone’ cells or ‘joint progenitors’. They are distinct from progenitors that give rise to growth plate chondrocytes. In this proposal, we aim to define the molecular targets that control articular-like permanent chondrocyte formation versus growth plate-like transient chondrocyte formation, by using novel GDF5+ mesenchymal cells developed from human pluripotent stem cells (hPSCs). During in vitro chondrogenesis, such cells express signs of primitive (or embryonic) articular chondrocytes but not of chondrocyte hypertrophy. Significantly, after transplantation of the cartilage they develop, no mineralization was observed for 8 weeks (i.e., permanent cartilage). Therefore, the hPSC-derived GDF5+ cells may share the activity of joint progenitors, although genome-wide RNA-sequencing (seq) analyses suggested association with developing tenocytes or ligamentocytes. In contrast, alternative hPSC-derived chondroprogenitors, SOX9+ cells, generated cartilage that readily underwent complete mineralization, mimicking growth-plate chondroprogenitors. Interestingly, when mixed with GDF5+ cells, the SOX9+ cell-derived cartilage behaved as permanent cartilage in a GDF5+ cell-dose-dependent manner, suggesting the involvement of a non-cell autonomous mechanism. Therefore, we first propose to test if the GDF5+ cells have a joint progenitor-like activity, characterize the (permanent) cartilage developed from them, and shed lights on how the cells generate permanent cartilage (Aim 1). Second, we plan to identify genes potentially involved in permanent cartilage formation from GDF5+ cells through comparative RNA-seq analyses of cartilage pellets developed from the GDF5+ and SOX9+ cells (Aim 2). We will then functionally validate the candidate genes (and their encoded proteins, inhibitors and activators if commercially available) for their ability to enable SOX9+ cells to generate articular-like permanent chondrocytes using gene knockout and overexpression techniques (Aim 3). We will then examine whether GDF5+ cells and such gene-modified SOX9+ cells induce more sustained repair of damaged joint cartilage than SOX9+ cells (Aim 4). Lastly, any of the genes defined in these studies will be manipulated similarly in therapeutically relevant adult mesenchymal stromal cells to confirm that targeting the same mechanisms will convert the adult stem cells to articular cartilage-forming cells (Aim 4). Thus, success of the proposed research will provide mechanistic insights into how articular-like permanent cartilage can be selectively formed from various chondrogenic cells, potentially leading to novel therapeutic strategies for effective, sustained repair of damaged cartilage.