Utilizing Developmental Cues to ‘Engineer’ Native Tissues from Human Induced Pluripotent Stem Cells (iPSC)

The remarkable discovery of induced pluripotent stem cells (iPSC), generated by ‘reprogramming’ of somatic cells by introducing a few transcription factors, has provided an unprecedented source of patient-specific cells for regenerative medicine. Besides pluripotency, somatic cells have also been demonstrated to ‘reprogram’ or ‘transdifferentiate’ directly into another somatic cell type. A repertoire of these reprogramming tools can therefore be utilized to ‘design’ and ‘engineer’ autologous, native tissues for defects and injuries. A challenge with iPSC-derived cells has been the variable developmental maturity of the cells, for example for muscle and heart tissues wherein the iPSC-derived cells are immature and not fully functional as in the native tissues. Developmental immaturity or pliability can however be advantageous for other tissues like cartilage that do not regenerate efficiently in adulthood thereby frequently leading to degenerative diseases like Osteoarthritis (OA). Cell-based therapeutic approaches for repairing focal cartilage defects have utilized autologous adult cartilage cells i.e. chondrocytes or adult mesenchymal stem cells (MSC) but with limited success due to generation of inferior fibrocartilage and paucity of cells. Indeed, recent reports have provided compelling evidence that juvenile chondrocytes (from donors below 13 years of age) are more efficient at generating cartilage tissue as compared to adult chondrocytes. Our goal has been to identify the molecular basis for such a superior regenerative capability and to test whether iPSC-derived chondrocytes mimic the juvenile chondrocytes in a molecular and functional manner. Towards this goal, we firstly developed a defined growth factor based protocol for inducing chondrogenesis in human iPSC with high efficiency (Lee et al, FASEB J, 2015). A major advantage of this method is that the human iPSC are efficiently differentiated into chondrocytes with an articular rather than fibrocartilage-like phenotype. Molecular characterization of the differentiating human iPSC revealed that differentiation proceeds through an intermediate Sox9^low CD44^low mesodermal population leading to a fairly homogenous Sox9^high CD140^high CD44^high hiChondrocyte population expressing high level of the articular genes, Wnt9a and SOSTDC1, chondrogenic transcription factors (Sox 5/6/9) and Col2a1, but not fibrocartilage marker Col1a1 or the hypertrophic marker Col10. The iPSC-derived chondrocytes (hichondrocytes) are stable and comparable to adult chondrocytes in their ability to generate cartilage tissue. In addition, hichondrocytes mimic juvenile chondrocytes in faster cell proliferation than adult chondrocytes, providing an advantage in terms of cell expansion and tissue generation. Additionally, in vivo cartilage generation in mice by the hichondrocytes generated strictly cartilage and not teratomas demonstrating the efficacy and safety of the hiChondrocytes. Whole genome transcriptome analyses revealed a subset of factors conserved between juvenile and iPSC-derived chondrocytes that are likely responsible for their superior regenerative potential. These studies have identified novel factors that enhance stem cell-mediated cartilage regeneration and highlight the importance of studying developmental processes to understand and identify the ideal cells to engineer to realize the generation of a functional native tissue.