Importance of Autophagy in the Recovery of Muscle Function After Injury in an Ovariectomized Model

Abstract

The lack of ovarian hormones accentuates the loss of muscle contractility after muscle injury. Mitochondria are required to meet the energetic demands of muscle contractility, but whether mitochondrial function is affected by muscle injury and impacts repair is unclear in the context of ovarian hormone depletion. PURPOSE: To test mitochondrial dysfunction after muscle injury in the context of ovarian hormone depletion and to investigate autophagy, a cellular process for degrading damaged and dysfunctional mitochondria, as a mechanism of mitochondrial remodeling during regeneration. METHODS: We subjected sham surgery wildtype (WT) and ovariectomized (OVX) mice to traumatic muscle injury and assessed the recovery of in vivo muscle strength (i.e. ankle dorsiflexion) and state 3 respiration from permeabilized muscle fibers at 7 and 14 days post-injury. To investigate autophagy, expression of autophagy-related proteins Beclin1 (Atg6) and LC3 were assessed. To determine if an interaction exists between ovarian hormones and autophagy, muscle strength and state 3 respiration were assessed 14 days post-injury following sham or OVX surgeries on Ulk1 deficient mice, a necessary protein for mitochondrial-specific autophagy, and littermate controls. RESULTS: OVX resulted in a 10% reduction in muscle strength (p=0.045) pre-injury compared to sham. This was exacerbated by muscle injury (14 days post-injury: OVX 25% vs. sham 35% of pre-injury, p=0.038). For state 3 respiration, there was a main effect of injury demonstrating a substantial reduction in mitochondrial function at 7 and 14 days post-injury (p<0.0001), independent of ovarian hormones. There was a large induction of autophagy as indicated by greater Beclin1 and LC3 expression at 7 and 14 days post-injury, independent of ovarian hormones (p=0.001, p=0.014 respectfully). Interestingly, OVX-Ulk1-deficient mice demonstrated less recovery of muscle strength at 14 days post-injury compared to OVX-LM (p= p=0.016). CONCLUSIONS: After muscle injury a robust autophagic response is required to recover muscle function in a timely manner and this occurs in the presence and absence of ovarian hormones. However, decreased strength recovery in OVX-Ulk deficient mice suggests an interaction between ovarian hormones and autophagy during muscle regeneration.