title: Impact of Leucine and HMB on Acute Muscle Damage: Focus on Repair and Regeneration in vivo and in vitro
Creators: name: Brown, A
Publisher: name: Liverpool John Moores University
Year: 2021

description: Introduction: Skeletal muscle is an amalgamation of multiple terminally differentiated myofibers which are capable of regeneration after injury. The satellite cell is the driving factor for muscle regeneration, influenced by their internal and external environment. The majority of these processes and factors are impaired with age, in particular, the function of the satellite cell. Interventions including leucine and HMB supplementation are consumed with the aim of improving muscle mass. Currently, there is a lack of understanding around muscle repair and regeneration from injury and specifically, the influence of cellular ageing. In addition, the impact of leucine of HMB on these processes is limited. Therefore, the aim was to utilise amino acid supplementation as an intervention to reduce damage and/or facilitate growth and repair in skeletal muscle using in vivo and in vitro models. Methods: Three models were used to investigate these processes: 1) strenuous eccentric exercise to establish physiological and immune responses in young males and females, in the absence and presence of supplements; 2) replicatively ageing myoblast models to investigate the impact of age on the migration and fusion potential of myoblasts in the absence or presence of leucine or HMB supplementation and finally; 3) dynamic proteome profiling techniques using control and replicatively aged myoblasts and myotubes as models. Results: Post eccentric exercise, at 48 h there was a significant (both P &lt; 0.05) increase in muscle soreness and CK in young males. In females, there was a significant (P &lt; 0.05) increase in muscle soreness, with no effect on CK. Nutritional supplementation was without effect on functional measures. There were, however, significant (P &lt; 0.05) increases in IL-7, with non-significant increases in IL-8 in males vs. females, basally. With leucine, female IL-7 was significantly (P &lt; 0.05) increased vs. males, with TNFR1- also non-significantly increased. Replicatively aged myoblasts migrated significantly (P &lt; 0.05) faster than control myoblasts, with the inhibition of Akt significantly (P &lt; 0.05) blocking migration. Both models responded to supplements through increased rates of migration, with the control cells attaining rates similar to aged cells post-supplementation. Replicatively aged myoblasts did not fuse, in the absence or presence of leucine or HMB. Akt, mTOR and ERK activities were significantly (all P &lt; 0.05) suppressed along with myogenin, IGF-I and IGF-II gene expression. However, aged cells amino acid transporters responded to supplements with increased Akt activation, suggesting that they are not compromised per se, but rather fusion is blunted at the expense of a different cellular response. The proteome of replicatively aged myoblasts contained significantly (P &lt; 0.01) less abundant ribosomal proteins in myoblasts, and myofibrillar proteins in myotubes, with significantly (P &lt; 0.01) greater metabolic enzymes vs. control. Finally, replicatively aged myoblasts and myotubes had significantly (P &lt; 0.01) less fractional synthesis rates compared to control. Conclusion: Taken together, supplementation was with little effect on functional capability in humans following an eccentric exercise protocol. Replicatively aged cells migrate efficiently, but do not fuse, with multiple mechanisms suppressed. Aged cells do however, respond to leucine and HMB with relevant signalling pathways being activated, however, fusion is not the output measure. Proteomic analyses suggest altered metabolism in the ageing cells with reductions in ribosomal and contractile proteins underpin the differences and warrant further investigation.