The synovium is a specialized connective tissue that encapsulates diarthrodial joints like the knee, maintaining a low-friction environment for the articulating surfaces within. This tissue plays a key role in homeostasis by regulating solute transport in and out of the joint, and secreting lubricating factors into the synovial fluid. The predominant cell type in the synovium is the fibroblast-like synoviocyte (FLS), which resides on the intimal surface of the tissue and produces lubricating molecules such as hyaluronan. Because these cells directly face the synovial fluid and apposing tissue surfaces within the joint, they are exposed to a dynamic environment of mechanical stimuli generated during daily activity. This dissertation addresses the global hypothesis that FLS are mechanosensitive to distinct modes of shear stress generated in the knee during articulation, and that modulation of this sensitivity by chemical and physical factors of the osteoarthritic (OA) environment contributes to disease progression.

Previous work has demonstrated that fluid-induced shear stress, generated as synovial fluid redistributes within the capsule during articulation, is a relevant mechanical stimulus for FLS. Exposure of FLS to fluid shear has been shown to modulate downstream functions such as lubricant secretion and the release of degradative matrix-metalloproteinases as induced by the cytokine interleukin-1 (IL-1), the latter indicating a link between mechanotransduction and the inflammatory environment of OA. The first goal of this dissertation was to further elucidate FLS mechanotransduction by characterizing the upstream response of FLS to fluid shear and determine the influence of IL-1 thereupon. The work presented herein demonstrates for the first time a robust calcium signaling response of FLS to fluid shear, a key upstream event in the mechanotransduction of physical stimuli. Key aspects of this response were significantly altered by pre-exposure to IL1, indicating a pathologic modulation of normal mechanosensing in the OA environment. This effect was observed across bovine and human models and was found to be potentiated by both increasing intercellular communication and modulation of cell primary cilia.

In addition to chemical factors such as cytokines, the degradation of cartilage during OA produces a physical factor that perpetuates disease state in the form of cartilage wear particles (CWP). These particles are released into the synovial fluid and attach directly to the synovium. We have previously shown that CWP induce FLS monolayers to release pro-inflammatory mediators of OA. The second goal of this dissertation was to investigate the effect of CWP on both cell-level function and tissue-level properties. To this end we showed first that CWP modulate the calcium signaling response of FLS to fluid shear in a contact dependent manner, and that inhibition of intercellular communication is a potential mechanism of this effect.

In areas of the articulating capsule where apposing tissues slide in direct contact with each other, contact-induced shear stress provides another relevant physical stimulus to FLS. In this case of direct interaction between surfaces, the tissue-level frictional properties may affect the magnitude of shear stress presented to the cells within the intimal layer and thus influence mechanotransduction. A novel bioreactor was developed to characterize the effect of sliding contact on downstream functions of FLS within explant tissues. An increase in metabolic activity with culture under these conditions suggests that contact shear is a relevant stimulus for FLS. While previous work has characterized synovium friction properties in sliding contact against glass, relatively little is known of synovium tribology in native tissue configurations, or the influence of pathologic conditions such as CWP attachment. This dissertation reports for the first time low friction properties for synovium against other tissues within the joint such as cartilage and demonstrates a significant deleterious effect of CWP on these properties.

The research presented in this dissertation further elucidates the processes of normal synoviocyte mechanotransduction, and by demonstrating that key chemical and physical factors of the OA environment modulate both cell and tissue-level functional properties, sheds light on the mechanisms by which the synovium contributes to disease progression. This sets the foundation for future work into synovium mechanotransduction of distinct physical stimuli and the relationship with tissue-level mechanical properties, and points towards clinical interventions that seek to restore the normal mechanical environment of the joint.