The regulation of mTORC1 signaling in immobilized rat hindlimb skeletal muscle

Open Access
Author:
Kelleher, Andrew Ryan
Graduate Program:
Physiology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 13, 2014
Committee Members:
  • Leonard Shelton Jefferson Jr., Dissertation Advisor
  • Scot R Kimball, Committee Member
  • Christopher Martin Yengo, Committee Member
  • Lisa M Shantz, Committee Member
  • Ian Alexander Simpson, Committee Member
  • Donald Leon Gill, Special Member
Keywords:
  • disuse atrophy
  • anabolic resistance
  • casting
  • REDD1
  • REDD2
  • Ddit4
  • Ddit4l
  • p70S6K1
  • muscle wasting
  • fixed muscle length
Abstract:
Limb immobilization, limb suspension, and bed rest cause substantial loss of skeletal muscle mass, a phenomenon termed disuse atrophy. Disuse atrophy is attributed to a depression in the rates of protein synthesis in a fasted state, and a resistance to stimulation by nutrients and other anabolic stimuli. Skeletal muscle protein synthesis is modulated by mechanistic target of rapamycin complex 1 (mTORC1) signaling, which is repressed during hindlimb immobilization. The overall goal of this research was to understand the molecular mechanisms responsible for repression of mTORC1 signaling in immobilized rat hindlimb soleus muscle. The overall hypothesis was that mTORC1 signaling is repressed in immobilized rat hindlimb skeletal muscle due to induction in the mRNA expression of two repressors of mTORC1 signaling, regulated in development and DNA damage responses (REDD) 1 and REDD2. The studies show that REDD1 and REDD2 mRNA expression are induced in association with repression of mTORC1 signaling after 1-3 days of hindlimb immobilization. Hindlimb immobilization repressed mTORC1 signaling in a fasted state and blunted the stimulation of mTORC1 signaling in response to a bolus of leucine, a potent nutrient stimulator of mTORC1 signaling. Fixed muscle length was identified as a physiological trigger for the expression of genes associated with disuse atrophy, particularly REDD1 and REDD2. These genes were induced in soleus muscle immobilized for 3 days in a shortened position, but not in a stretched position. Aging was also associated with repression of mTORC1 signaling and induction in the mRNA expression of REDD2, while 7 days of remobilization was associated with augmented mTORC1 signaling and repression in the mRNA expression of REDD2. Collectively, these findings indicate that the mRNA expression of REDD1, and primarily REDD2 at time points longer than 3 days, is associated with changes in mTORC1 signaling under conditions of hindlimb immobilization, aging, and remobilization. REDD1 and REDD2 act as repressors and governors of the capacity for mTORC1 signaling in fasted and fed states. Furthermore, impaired PDK1 signaling to 70-kDa ribosomal protein S6 kinase 1 (p70S6K1) and induction of REDD1 and REDD2 are associated with resistance to anabolic stimulation of mTORC1 signaling. The results support the overall hypothesis that mTORC1 signaling is repressed in immobilized rat hindlimb skeletal muscle due to induction of REDD1 and REDD2 mRNA expression.