DIFFERENTIAL RESPONSES TO LEUCINE SUPPLEMENTATION IN YOUNG AND AGED MICE DURING SKELETAL MUSCLE REGENERATION

Richard A. Perry Jr., Lemuel A. Brown, David E. Lee, Jacob L. Brown, Megan E. Rosa, Nicholas P. Greene & Tyrone A. Washington University of Arkansas, Fayetteville, Arkansas Skeletal muscle regeneration requires the coordination of inflammation, myoblast proliferation and myoblast differentiation. Aging skeletal muscle has a reduced capacity to regenerate from injury. Myogenic regulatory factors (MRFs) such as MyoD and myogenin are necessary for myoblast proliferation and differentiation, respectively, and are highly expressed at the onset of skeletal muscle regeneration. Tumor necrosis factor-α (TNF-α), an inflammatory cytokine, is also highly expressed during skeletal muscle regeneration. Leucine supplementation has been shown to enhance the skeletal muscle regenerative response. The effects of leucine supplementation on regeneration in aged skeletal muscle has not been fully described. PURPOSE: To determine how leucine supplementation affects TNF-α, MyoD and myogenin gene expression in young and aged mice during skeletal muscle regeneration. METHODS: Young (3 months; n = 27) and aged (24 months; n = 27) female C57/BL6 mice were assigned to one of four groups: uninjured, uninjured + leucine, injured, and injured + leucine. To induce muscle damage, bupivacaine was injected into the tibialis anterior (TA) whereas the uninjured groups received PBS injections. Leucine was given in the drinking water ad libitum at a concentration of 1.5%. Three days following injection, the TA was extracted. Quantitative PCR was performed to measure gene expression. RESULTS: In the young + no leucine group, there was a 9% decrease in TA mass to tibia length (2.2 ± 0.1 mg/mm vs. 2.0 ± 0.1 mg/mm, p\u3c 0.05) whereas, in the young + leucine group there was a 19% decrease in TA mass to tibia length (2.1 ± 0.1 mg/mm vs. 1.7 ± 0.0 mg/mm, p\u3c 0.05) 3 days post-bupivacaine injection. In the aged + no leucine group there was an 11% increase TA mass to tibia length (2.0 ± 0.1 mg/mm vs 2.2 ± 0.1 mg/mm, p\u3c 0.05) and no change in the aged + leucine group 3 days post-bupivacaine injection. In the young + no leucine group, TNF-α, MyoD and myogenin gene expression increased (p\u3c 0.05). MyoD and myogenin gene expression also increased in the young + leucine group (p\u3c 0.05), but TNF-α decreased (p\u3c 0.05). In the aged + no leucine group, TNF-α, MyoD, and myogenin increased (p\u3c 0.05) 3 days post-bupivacaine injection. However, the aged + leucine group had a blunted MyoD and myogenin response (p\u3c 0.05) 3 days post-bupivacaine injection. TNF-α remained elevated in the aged + leucine group 3 days post-bupivacaine injection (p\u3c 0.05). CONCLUSION: Leucine supplementation decreases TNF-α in the young but increases it in the aged group during skeletal muscle regeneration. This increase corresponds with a blunting of the MRF response during regeneration in the aged. Project funded by NIH/UAMS Claude Pepper Older Americans Independence Center pilot grant