Contraction-induced Damage in Skeletal Muscles of Young and Old Mice: Investigating Reactive Oxygen Species and Myeloid Cells as Contributing Factors.

Abstract

Muscles of aged individuals display high susceptibility to injury and impaired regeneration. Developing strategies to restrict damage or enhance repair for older individuals requires a mechanistic understanding of both the damage and repair processes following injury and the impact of aging on those processes. The overall objective of this dissertation was to address fundamental gaps in our knowledge of cellular and molecular events associated with a common form of muscle injury and to identify age-related changes in key events. We conducted experiments using established models of lengthening contraction-induced injury in young and old mice. We first pursued the question of whether reactive oxygen species (ROS) generated during damaging lengthening contractions contribute to the initiation of the injury. We found that lengthening contractions did not generate more ROS than non-damaging isometric contractions, arguing against ROS as an initiating factor in the injury process. Because neutrophils exacerbate muscle damage while macrophages contribute to repair, we next investigated molecular mechanisms of neutrophil migration into injured muscle and the associations between myeloid cell levels and muscle degeneration and regeneration in old animals. Treatment with blocking antibodies for P- and E-selectin reduced neutrophil levels in injured muscles by half, supporting the importance of these molecules for neutrophil accumulation after lengthening contractions. Despite 50% fewer neutrophils, no reduction in damage was observed, indicating no direct relationship between neutrophil levels and injury. Moreover, 30-50% more neutrophils in muscles of old compared with adult mice was not associated with more severe injury. Despite more neutrophils, impaired regeneration in muscles of old mice was not associated with an inability to clear these cells nor impaired recruitment of macrophages with age. Indeed, for a given level of muscle injury in old mice, we found 20-50% more macrophages, including anti-inflammatory macrophages. Muscles of old mice also showed aberrant expression of macrophage-associated inflammatory mediators including tumor necrosis factor-alpha and interleukin-10, which have the potential to undermine muscle regeneration. In summary, our studies do not support antioxidant or anti-P/E-selectin therapies to mitigate damage in older individuals. Instead, targeting specific myeloid cell functions may represent a superior therapeutic approach.