Parathyroid Hormone-Related Protein Gradients Affect the Progression of Mesenchymal Stem Cell Chondrogenesis and Hypertrophy

Introduction: Mesenchymal stem cells (MSCs) are considered a promising cell source for cartilage repair strategies due to their chondrogenic differentiation potential. However, their in vitro tendency to progress toward hypertrophy limits their clinical use. This unfavorable result may be due to the fact that MSCs used in tissue engineering approaches are all at the same developmental stage, and have lost crucial spatial and temporal signaling cues. In this study, we sought to investigate the effect of a spatial parathyroid hormone-related protein (PTHrP) signaling gradient on the chondrogenic differentiation of MSCs and progression to hypertrophy. Methods: Human bone marrow-derived MSCs were transduced with adenoviral vectors overexpressing PTHrP and seeded into fibrin-poly(ester-urethane) scaffolds. To investigate the effect of a spatial PTHrP signaling gradient, scaffolds were seeded with PTHrP-overexpressing MSCs positioned on top of the scaffold, with untransduced MSCs seeded evenly within. Scaffolds were cultured with or without 2 ng/mL transforming growth factor (TGF)-β1 for 28 days. Results: PTHrP overexpression increased glycosaminoglycan (GAG) production by MSCs irrespective of TGF-β1 treatment, and exerted differential effects on chondrogenic and hypertrophic gene expression when MSCs were cultured in the presence of a PTHrP signaling gradient. Furthermore, PTHrP-overexpressing MSCs were associated with an increase of endogenous TGF-β1 production and reduced total MMP-13 secretion compared to controls. Conclusion: The presence of a spatial PTHrP signaling gradient may support chondrogenic differentiation of MSCs and promote the formation of a more stable cartilage phenotype in tissue engineering applications.