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Abstract
The ability of muscle to maintain healthy function after injury and with increasing age largely depends on the health of the mitochondrial network within the muscle fibers. Autophagy is a cellular recycling mechanism that degrades damaged organelles and proteins and plays an important role in maintaining mitochondrial network integrity in muscle. Ulk1 is an autophagy-initiating kinase, however, its contribution to mitochondrial maintenance and its necessity for proper muscle function is unknown in response to muscle injury and with aging. I designed and executed three studies to investigate the role of Ulk1-mediated autophagy in skeletal muscle. Study 1 tested the extent to which muscle injury activates an autophagy response. Injured muscles had a greater expression of autophagy-related proteins LC3II, p62, and Ulk1 which confirmed that muscle injury activates autophagy. Study 2 specifically determined if Ulk1-mediated autophagy is required for the recovery of muscle metabolic and contractile function after injury. Skeletal muscle from muscle-specific Ulk1 knockouts had reduced contractile recovery and satellite cell function compared to Cre negative littermates. This data suggest that Ulk1-mediated autophagy plays an important role in functional muscle recovery following injury and established that Ulk1-mediated autophagy may produce signaling cues that regulate satellite cell activity as Ulk1 was present in the satellite cells which exhibited impaired function. Additionally, I used a fluorescently tagged plasmid to compare autophagy flux in injured and uninjured muscle and discovered that autophagosome degradation is not upregulated sufficiently to match autophagosome assembly after muscle injury which leads to an autophagy bottleneck. Study 3 tested the extent to which basal Ulk1-mediated autophagy is required for the healthy aging of skeletal muscle. Muscles from aged Ulk1 knockout mice had an accumulation of damaged mitochondria which affected type II muscle fibers resulting in weaker EDL muscles. I concluded from this data that Ulk1-mediated autophagy is required for healthy aging of muscle in terms of both metabolic and contractile function. Based on the studies described herein, it is evident that Ulk1-mediated autophagy plays an essential role in maintaining muscle function after injury and throughout life. Furthermore, Ulk1 may provide a powerful therapeutic target to improve muscle function.