Gene Expression Changes in Response to Severe Burn Injury in Rat Achilles Tendon

IMPORTANCE: Severe burn injury, occurring at a global rate of 5/100,000 per year, can result in serious sequelae such as infection, shock, and direct organ damage. It has been shown that severe burn also results in both systemic inflammation and distant inflammatory effects, specifically in skeletal muscle and bone. However, we are unaware of any work that investigates similar effects in the tendon, which connects muscle and bone. OBJECTIVE: To determine whether, after distant severe burn injury, changes occur in gene expression, protein synthesis, and biomechanical properties in rat Achilles tendon. METHODS: Rats were subjected to thoracolumbar full-thickness severe burn injury under anesthesia before harvesting the Achilles tendon at time points of 6h, 1d, 3d, 7d, and 14d. Unburned rats were used for control. RNA expression of collagen I (Col1), collagen III (Col3), MMP9, MMP13, IL-1β, IL-6, TNF-α, and tenomodulin was measured using qPCR. Quantified Western blots were performed to assess relative protein content for Col1, Col3, and MMP9. Finally, biomechanical testing was performed on a separate group of rats to assess biomechanical changes at 14d versus control (n = 8). RESULTS: The MMP9 gene was upregulated on the order of 20-fold at 14d (p = 0.0016) and 10-fold at 3d (p = 0.0255) and MMP13 showed a trend toward 12-fold increase at 3d (p = 0.079). A significant increase in expression of IL-1β and a trend toward significance IL-6 were also observed at 3d (p < 0.01, and p = 0.059, respectively). Differences in expression of Col1, Col3, TNF-α, and tenomodulin were not significant compared to control. Preliminary data from Western blots showed 5-fold decrease of collagen 1 at 7d (p < 0.01) and 2-fold increase of collagen 3 at 14d (p = 0.016). Significant changes were not found in MMP9. Preliminary biomechanical data shows a trend toward a 4-fold decrease in stiffness in the burn group. All p values are by t-test. CONCLUSIONS: These results newly confirm the existence of inflammation in tendon distant from the burn site after severe burn injury. Specifically, these changes could indicate initial matrix remodeling as carried out by the MMPs beginning at 3d, followed by collagen deposition with a decreased Col1:Col3 ratio, resulting in decreased tissue stiffness. These findings are consistent with the repair processes known to occur in other tissues after inflammation. Additionally, IL-6 and IL-1β may have a more significant role in post-burn acute-phase inflammation than other acute-phase reactants like TNF-α in the Achilles tendon.