Skeletal Muscle Mitochondrial Bioenergetics in a Model of Primary Osteoarthritis after Treatment with Rapamycin and/or Metformin

Abstract

Impaired mitochondrial function is associated with the loss of skeletal muscle function during aging and age-related osteoarthritis (OA). Emerging evidence suggests Rapamycin (Rap), metformin (Met) or the combination of rapamycin plus metformin (Rap+Met) can extend lifespan and preserve skeletal muscle mass and function during aging. However, the mechanisms of action are not completely understood and have not been studied in specific models of age-related disease. The goal of this study was to test to test the hypothesis that Rap, Met, or Rap+Met would improve skeletal muscle mitochondrial function in an animal model of naturally occurring, age-related OA. We chose the Dunkin Hartley guinea pig because they are an outbred strain that develops naturally occurring OA by 5 months of age with a progression and pathology that highly resembles aging humans. After 5 months of age, Dunkin Hartley guinea pigs also have an age-related decline in skeletal muscle mitochondrial protein synthesis rates. Therefore, 5-month old Dunkin Hartley guinea pigs were fed a standard diet (Con; n=8), or a diet enriched with Rap (14ppm; n=8), Met (1000ppm, n=8) or Rap+Met (14+1000ppm; n=7) for 12-weeks. Mitochondrial respiration and hydrogen peroxide (H2O2) emissions were evaluated in permeabilized muscle fibers from the soleus via high-resolution respirometry and fluorometry using either a saturating bolus or titrating doses of ADP. Michaelis-Menten kinetics was used to evaluate Vmax and ADP sensitivity. All comparisons were made to control group. In the current study, Met had no effect while Rap and Rap+Met modified skeletal muscle mitochondrial bioenergetics only when ADP was titrated but not with a saturating bolus. Specifically, Rap (P<0.05) and Rap+Met (P=0.07) increased ADP sensitivity but decreased (P<0.05) maximal complex-I (CI) linked respiration. Rap and Rap+Met also tended to decrease mitochondrial H2O2 emissions but this trend was no longer apparent when mitochondrial H2O2 emissions were expressed relative to respiration. The decrease in CI-linked respiration and H2O2 emissions was attributed to both a lower CI protein abundance and a decreased index of intrinsic mitochondrial function. This is the first inquiry into how lifespan extending treatments can influence skeletal muscle mitochondria in a model of age-related OA. Further research is needed to understand if increased mitochondrial ADP sensitivity at the cost of CI-linked maximal capacity by Rap and Rap+Met is a beneficial strategy to maintain cellular energy homeostasis during age-related OA.