Skeletal muscle LINE-1 ORF1 mRNA is higher in older humans but decreases with endurance exercise and is negatively associated with higher physical activity

Abstract
The long interspersed nuclear element-1 (L1) is a retrotransposon that constitutes 17% of the human genome and is associated with various diseases and aging. Estimates suggest that ~100 L1 copies are capable of copying and pasting into other regions of the genome. Herein, we examined if skeletal muscle L1 markers are affected by aging or an acute bout of cycling exercise in humans. Apparently healthy younger (23 ± 3 y, n = 15) and older participants (58 ± 8 y, n = 15) donated a vastus lateralis biopsy before 1 h of cycling exercise (PRE) at ~70% of heart rate reserve. Second (2 h) and third (8 h) postexercise muscle biopsies were also obtained. L1 DNA and mRNA expression were quantified using three primer sets [5′ untranslated region (UTR), L1.3, and ORF1]. 5′UTR and L1.3 DNA methylation as well as ORF1 protein expression were also quantified. PRE 5′UTR, ORF1, or L1.3 DNA were not different between age groups (P > 0.05). ORF1 mRNA was greater in older versus younger participants (P = 0.014), and cycling lowered this marker at 2 h versus PRE (P = 0.027). 5′UTR and L1.3 DNA methylation were higher in younger versus older participants (P < 0.05). Accelerometry data collected during a 2-wk period before the exercise bout indicated higher moderate-to-vigorous physical activity (MVPA) levels per day was associated with lower PRE ORF1 mRNA in all participants (r = −0.398, P = 0.032). In summary, skeletal muscle ORF1 mRNA is higher in older apparently healthy humans, which may be related to lower DNA methylation patterns. ORF1 mRNA is also reduced with endurance exercise and is negatively associated with higher daily MVPA levels.