Joint degenerative disorders impose a large burden on lifestyle and the healthcare system. The goals of this thesis were to elucidate pathological mechanisms in osteoarthritis (OA) and juvenile osteochondritis dissecans (JOCD) in order to improve understanding of these diseases, and to provide well-characterized platforms for therapeutic development and testing. OA is the leading cause of disability in the U.S., and it is a disease of the joint that affects multiple tissues. Despite breakthroughs in molecular events (e.g. mechanisms in cartilage degradation), its pathological mechanisms are still largely unknown. There are currently no FDA-approved disease modifying OA drugs (DMOADs), despite promising preclinical data. In order to bridge the gap in knowledge between preclinical and clinical studies, we characterized molecular events that occur in the rat medial meniscus transection (MMT) model of post-traumatic OA as the disease develops and progresses. Our results indicated that pathological events in the articular cartilage and synovium of the MMT model are similar to known human OA development. Our results also suggested feedback interactions between joint tissues during disease progression.

Second, we investigated the mechanisms of action of micronized dehydrated human amnion/chorion membrane (AmnioFix Injectable, MiMedx, USA) in order to elucidate potential disease-modifying mechanisms of this therapeutic. Results showed that AmnioFix does not have a direct influence on the gene expression of articular cartilage or tissue from the osteophyte-forming region of the joint. Instead, AmnioFix acted through the synovial membrane, modulating its microenvironment to a favorable chondro-protective profile. These results further supported the importance of tissue interactions in the MMT model and in OA, and also provided a new view point concerning disease-modifying approaches for OA.

JOCD is an increasingly common disorder that affects children and adolescents with an open physis. JOCD results in the partial or complete fragmentation of a necrotic osteochondral body from the parent bone, which permanently affects the joint and alters its mechanics. Therefore, JOCD patients have a higher probability of developing OA at an early age. JOCD presents a unique challenge, as treatment strategies are limited to surgical interventions at advanced stages. Although there a number of hypotheses about the etiology of JOCD, its pathological mechanisms are yet to be investigated. In this thesis, we established induced pluripotent stem cell (iPSC)-derived models of JOCD chondrogenesis and endochondral ossification in order to study its pathology. Our results demonstrated that cells from JOCD patients have a lower chondrogenic capacity than normal, control cells. Results also showed that although endochondral ossification is successfully accomplished, there may be irregularities in its process. We also established ER-stress induction models in order to dissect mechanistically how JOCD-iPSC-derived mesenchymal stem cells (iMSCs) responded to ER stress. Our results showed that JOCD cells have a different response to ER stress, which could lead to cell death should the ER stressor persist. We propose that this pathological feature could lead to the onset of clinical JOCD.

Taken together, this thesis significantly contributed to the knowledge gap of OA and JOCD pathomechanisms. This work provided new insights into development of joint degeneration in the MMT model and established a well-characterized baseline to evaluate mechanistic effects of potential therapeutic agents in this OA-like model. It also investigated mechanisms of action of AmnioFix, which may be leveraged to develop more specific DMOADs. Most importantly, this thesis presented pioneering work on patient-specific iPSC-based disease modeling. This is the first study to elucidate pathomechanisms of JOCD and to establish JOCD-specific in vitro models for future therapeutic testing.