Stromal “activation” markers do not confer pathogenic activity in tendinopathy

Abstract

Tendinopathy is a highly prevalent musculoskeletal pathology associated with incremental injury as result of repetitive microtrauma. We sought to explore the physiological significance of stromal “activation” signatures in a human model of tendinopathy. Torn supraspinatus tendon and matched intact subscapularis tendon biopsies were collected from patients undergoing shoulder surgery while healthy tendon was collected from patients undergoing anterior cruciate ligament (ACL) reconstruction. Expression of stromal activation markers was analyzed at transcript/protein level using qRT-PCR and immunohistochemistry. Gene expression of stromal activation markers was silenced by siRNA-mediated knockdown or induced by IL-1β stimulation. Expression of “activation” markers podoplanin, VCAM-1 and CD248 was identified in human tendon tissue. Podoplanin and VCAM-1 expression were significantly increased in tendinopathic tissue. Knockdown of podoplanin and VCAM-1 in normal and tendinopathic tenocytes did not have any significant effect on expression of matrix genes COL1A1, COL3A1, TNC, or DCN. Similarly, no changes in release of inflammatory mediators IL-6, IL-8, and CCL2 were observed in podoplanin/VCAM-1 knockdown cultures. Our data suggest that silencing expression of stromal “activation” markers does not affect the intrinsic inflammatory profile or matrix regulatory behavior of tenocytes. We propose that the term “activation” is more appropriately reflected by alterations in tenocyte behavior that induce changes in the stromal microenvironment and overall tissue architecture rather than identification through potentially arbitrary phenotypic traits.