Mechanical load stimulates expression of novel genes in vivo and in vitro in avian flexor tendon cells

Objective: Our experiments were designed to test the hypothesis that tendon cells might respond differently to applied strain in vitro than in vivo. Design: We tested cells in whole tendons from exercised chickens and from isolated surface (TSC) and internal tendon (TIF) in vitro that were subjected to mechanical strain. We hypothesized that tendon cells differentially express genes in response to mechanical loading in vivo and in vitro. Methods: We utilized an in-vivo exercise model in which chickens were run on a treadmill in an acute loading regime for 1 h 45 min with the balance of time at rest to 6 h total time. Gene expression was analyzed by a differential display technique. In addition, isolated avian flexor digitorum profundus TSC and TIF cells were subjected to cyclic stretching at 1 Hz, 5% average elongation for 6 h, ± PDGF-BB, IGF-I, TGF-β1, PTH, estrogen, PGE₂, or no drug and/or no load. mRNA was then collected and samples were subjected to differential display analysis. Conclusions: Load with or without growth factor and hormone treatments induced expression of novel genes as well as some known genes that were novel to tendon cells. We conclude that the study of gene expression in mechanically loaded cells in vivo and in vitro will lead to the discovery of novel and important marker proteins that may yield clues to positive and negative cell strain responses that are protective under one set of conditions and destructive under another.