Title: Role of the Synovial Membrane in Osteoarthritis Pathogenesis and Cartilage Repair
Description: Osteoarthritis (OA) affects an estimated 250 million people worldwide, representing an enormous economic and social burden across demographic groups. While classically attributed to ‘wear and tear’ of the articular cartilage, there is a growing appreciation that OA is a whole-joint disease with a complex etiology involving the synovium and surrounding tissues. The synovium is a specialized connective tissue membrane that envelops the diarthrodial joint and maintains the synovial fluid environment through molecular secretion as well as bi-directional filtration of these constituents, nutrients, and cellular waste products. Moreover, synovium-derived cells have been directly implicated in both the native repair response as well as degradation of articular cartilage. Much of the existing research of synovium has been conducted in the context of rheumatoid arthritis (RA). And while synovitis is a key feature of both RA and OA, clinical reports have described OA synovium as distinct in its cellular and structural composition, molecular secretion, and chronic onset.

However, literature studies have not adequately addressed the mechanisms by which alterations in synovium structure-function affect joint and cartilage health, particularly the contribution of different cell types within the synovium to solute transport and lubrication. The work described in this dissertation addresses these knowledge gaps in the context of existing and emerging OA therapies, namely glucocorticoids and electrical stimulation. We anticipate that a more comprehensive characterization of changes to the synovium composition, secretion of key metabolic mediators, lubrication properties, as well as its ability to regulate solute transport in and out of the joint space will not only contribute to our basic science understanding of the synovium but also the development and modification of therapeutic strategies aimed at restoring and maintaining joint health. This characterization will be facilitated by our laboratory’s expertise in tissue engineering and explant culture, IL-1 and DEX stimulation, and electrical stimulation of joint tissues.

The approach of using an engineered synovium model is attractive in that quantitative high throughput in vitro mechanistic studies can be performed on tissues that are fabricated from cells derived from normal and OA synovium of patients and corresponding immune cells at defined density and cell type ratios. It also facilitates isolating effects of certain cell types or starting composition that are found in explant specimens. Intra-articular glucocorticoid injections are commonly administered to patients in an effort to control inflammation and pain. And while these high dose injections are known to have significant detrimental local and systemic effects, comparatively low doses of dexamethasone (DEX), a synthetic glucocorticoid, are known to have pro-anabolic and anti-catabolic effects on cartilage cultures. Our laboratory has published extensively on the benefits of DEX stimulation in growth and maintenance of engineered and explanted cartilage as well as chondroprotection from pro-inflammatory cytokines (e.g interleukin-1; IL-1), both in juvenile bovine basic science and adult canine preclinical systems.

However, the concomitant effects of DEX on synovium structure-function have not been elucidated. In Part I, we describe a functional tissue engineered synovium model that was validated against explant behavior. We were able to recapitulate many of the unique structural and functional characteristics of synovium, including protein expression, intimal lining formation, solute transport, and friction coefficient. Additionally, changes in engineered synovium structure-function mirrored that of explants when treated with IL-1 or DEX. The engineered synovium model was then expanded to include resident macrophage-like synoviocytes (MLS), demonstrating the key role that these cells play in structural reorganization of synovium. The model was also translated to human cells, showing the potential of the system for personalized medicine. Finally, motivated by insights into solute transport in the synovium as well as its strong anti-inflammatory response to DEX, we developed a sustained low-dose DEX delivery platform for mitigating synovial inflammation while simultaneously stimulating cartilage growth.

Utilizing a preclinical adult canine model, we showed that extended intra-articular delivery of DEX improved functional outcomes and cartilage tissue quality. In Part II, we evaluated synovium behavior and cartilage repair in response to modes of electrical stimulation. Electrical stimulation of cells and tissues has been a topic of interest for decades, owed in part to the knowledge that endogenous electric field (EF) gradients guide cell behavior during embryogenesis and wound healing. Pulsed electromagnetic fields (PEMFs) have been used in a clinical setting to stimulate bone repair and alleviate pain, however their use for OA and cartilage repair is controversial. Culture studied of PEMFs have shown anti-catabolic and pro-anabolic effects on isolated FLS and cartilage, respectively. And previous work in our laboratory demonstrated directed 2D migration of synoviocytes and chondrocytes in response to direct current (DC) EF stimulation. These modes of electrical stimulation have not been explored in synovium explants, so it is unclear to what extent the observed phenomena translate to the 3D tissue environment.

For the first time, we characterized the biological response of both healthy bovine and OA human synovium explants, showing distinct anti-inflammatory behavior in bovine tissues and a highly variable response in arthritic human tissues, likely due to different inflammatory cell content. Motivated by the potent anti-inflammatory effect seen in normal tissue and previous work showing a pro-anabolic effect on cartilage, the PEMF system was then adapted for use with a preclinical adult canine model of engineered cartilage repair. In this model, PEMFs significantly enhanced functional outcomes and cartilage tissue quality. Finally, we investigated the potential for direct synovial cell-mediated cartilage repair via induced migration with DC EFs. By developing and validating a novel tissue-scale bioreactor capable of applying DC EFs in sterile culture conditions to three-dimensional constructs, we showed increased recruitment of synovial repair cells to the site of a cartilage wound. Taken together, the sum of the work builds on existing therapeutic strategies by developing models to understand the contribution of the synovium to joint maintenance and repair.

By modeling dexamethasone- and electrical- induced changes to composition and function of synovium and cartilage, via complementary explant and engineered approaches, valuable mechanistic insights into osteoarthritis pathogenesis and cartilage repair were gathered. These findings lay the groundwork for more complex and personalized in vitro models of OA and motivate future work to capitalize on knowledge of the functional plasticity of the synovium to develop synovium-targeted strategies for OA treatment and prevention.