Modulation of aggrecan and ADAMTS expression in ovine tendinopathy induced by altered strain.

OBJECTIVE: To evaluate histologic, immunohistochemical, and molecular changes in tendon induced by altered strain in a large-animal model. METHODS: A full-thickness partial-width laceration of the infraspinatus tendon was created in 5 sheep, while 5 sham-operated sheep were used as controls. Sheep were killed after 4 weeks, and 4 differentially stressed tendon regions (tensile or near bone attachment from overstressed or stress-deprived halves) were evaluated for histopathology, proteoglycan (PG) accumulation, and characterization of glycosaminoglycans and aggrecan catabolites. Gene expression of matrix components, enzymes, and inhibitors was analyzed by reverse transcriptase-polymerase chain reaction. RESULTS: Histopathologic changes were detected in both overstressed and stress-deprived tensile tendon, but only in stress-deprived tendon near bone. In overstressed and stress-deprived tensile tendon, levels of keratan sulfate, chondroitin 4-sulfate, and chondroitin 6-sulfate were increased. In overstressed tensile tendon, levels of ADAMTS-generated aggrecan catabolites were increased. There was increased matrix metalloproteinase 13 (MMP-13) and decreased fibromodulin and decorin expression in all regions. Increased MMP-1, MMP-9, MMP-14, and ADAMTS-1 expression, and decreased type II collagen expression were restricted to stress-deprived tendon. In stress-deprived bone-attachment regions, messenger RNA (mRNA) for aggrecan was decreased, and ADAMTS was increased. In overstressed tensile tendon, aggrecan mRNA was increased, and ADAMTS was decreased. CONCLUSION: The distinct molecular changes in adjacent tissue implicate altered strain rather than humoral factors in controlling abnormal tenocyte metabolism, and highlight the importance of regional sampling. Tendon abnormalities induced by increased strain are accompanied by increased aggrecan, decreased ADAMTS, and low PG expression, which may negatively impact the structural integrity of the tissue and predispose to rupture.