An Engineered Tidemark for Osteochondral Tissue Engineering

Lesions to articular cartilage and underlying subchondral bone eventually lead to osteoarthritis, a debilitating disease with no cure. A successful therapy will need to promote tissue regeneration, support integrative repair, and protect the surrounding tissue from further degeneration. The overarching goal for this project is to develop

a mechanically competent, stem cell-based regenerative approach to treat osteochondral (OC) defects. During the initial funding period, our team developed an OC-mimetic hydrogel with a design that decoupled the load- bearing (i.e., structural) component from the soft cellular biomimetic component. This allowed us to create a

functionally graded, stiff structure with cartilage-matched mechanical stiffness, while creating soft cellular niches that supported mesenchymal stem cell (MSC) differentiation. Building from key in vitro milestones, this renewal aims to translate the OC-mimetic hydrogel in vivo. We will test the hypothesis that the OC-mimetic hydrogel

induces rapid and targeted differentiation of exogeneous MSCs in vivo, enabling their direct participation in OC- tissue regeneration while simultaneously protecting and supporting integration with the surrounding tissue. A new feature of our design is a cement line-mimetic within the structural support that similar to the native

cement line will be impervious to cell migration across the cartilage-bone interface, but pervious to nutrient transport. This will protect the MSCs in the cartilage layer, enabling their rapid differentiation and contribution to regeneration. We will test the overarching hypothesis in three specific aims. In Aim1, we will identify

mechanotransduction pathways that differentially control MSC fate in the OC-mimetic hydrogel, which will allow us to establish a mechanistic understanding of the physiochemical cues that achieve robust MSC differentiation in a dynamic environment with loading. In Aim 2, we will determine MSC fate in vivo within the

OC-mimetic hydrogel after implantation in a rat OC defect model by tracking differentially labeled MSCs isolated from DsRed+ and GFP+ rats. This aim will confirm MSC fate and their direct and indirect contribution to OC-tissue regeneration. In Aim 3, we will create a structural support that undergoes surface degradation to maintain its

mechanical properties. We will evaluate the effectiveness of this fully degradable and mechanically competent OC-mimetic hydrogel using three models of increasing complexity: an OC explant defect model to monitor the health of and integration with articular cartilage adjacent to the defect as the support structure

degrades; a rat OC defect model for longitudinal studies to monitor in vivo degradation of the structure concomitant with tissue regeneration and integrative repair; and, testing in a pre-clinical animal (swine) model. At the conclusion of this project, we expect to have (1) advanced our fundamental understanding of the

mechanotransduction pathways in MSCs and their fate in vivo and (2) established a mechanically competent and degradable OC-mimetic hydrogel that achieves OC-tissue regeneration and integrative repair, while maintaining joint health.