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Date of Award
Doctor of Philosophy (PhD)
Tendon injuries are the cause of 1 in 16 physician visits, while rotator cuff tendon injury affects over 17 million Americans annually. Chronic tendon injuries are associated with repetitive mechanical strain and loading, often without macroscopic damage at the tendon level. Acute tendon injuries are associated with sudden eccentric movement that causes the tendon to tear or rupture from the bone. Non-steroidal anti-inflammatory drugs (NSAIDs) and corticosteroids are often part of clinical intervention. Although excessive inflammation can contribute to tendon injury, an inflammatory response is necessary for tendon healing. The objective of this dissertation was to evaluate the role of inflammation in tendon degeneration, injury, and healing while identifying specific molecular targets for clinical intervention.
Two novel transgenic mouse lines with targeted deletion of IKKβ and activation of IKKβ in tendon fibroblasts were developed. Tendon-specific modification of IKKβ, a key component of the IKK complex in the canonical NF-κB pathway, was achieved with the use of cre-recombinase under the scleraxis promoter. No phenotypic changes were observed with partial deletion of IKKβ, but activation of IKKβ in tendon resulted in increased cellularity, increased secretion of chemoattractants, reduced bone quality at the tendon-to-bone attachment, and decreased tendon mechanical properties.
Three tendinopathy models were used in this study to determine the effects of IKKβ modification in tendon responses to inflammatory stimuli. First, an in vitro model of inflammation consisting of exogenous addition of the cytokine IL-1β demonstrated increased gene and protein expression of inflammatory cytokines. Partial deletion of IKKβ led to muted responses to IL-1β, while activation of IKKβ exacerbated responses to IL-1β. Second, an in vivo model of overuse-induced rotator cuff tendinopathy was then developed to determine the effect of IKKβ modification on tendon degeneration. Overuse activity led to reduced bone volume, bone density, and mechanical properties at the supraspinatus tendon enthesis in wild type mice. Deletion of IKKβ in tendon protected the tissue from the harmful effects of overuse activity. Third, an acute tendon injury and repair model was used to determine the effect of IKKβ modification on tendon healing. Deletion of IKKβ led to better mechanical properties following tendon injury and repair compared to wild type mice and mice with activation of IKKβ. In both in vivo models, partial deletion of IKKβ masked the harmful effects of injury.
In conclusion, activation of IKKβ in tendon was detrimental to tendon properties under normal and tendinopathic conditions. Deletion of IKKβ protected the tendon from the harmful effects of inflammatory stimuli in vitro and with most outcomes in vivo. Notably, only one allele in transgenic mice was scleraxis-cre positive, resulting in partial reduction/activation of IKKβ. This level of inhibition of IKKβ activity is likely feasible with IKKβ inhibitors that are clinically available. The dissertation work identified the importance of NF-κB in tendinopathy and identified IKKβ as a therapeutic target that may benefit patients with chronic or acute tendinopathy.
Matthew Silva, Lori Setton, Kristen Naegle, Yousef Abu-Amer
Available for download on Tuesday, December 15, 2116