Wheel Running Decreases LINE‐1 Gene Expression in Rodent Skeletal Muscle
Authors: Romero Matthew A. , Mumford Petey W. , Osburn Shelby C. , Roberson Paul A. , Toedebusch Ryan G. , Childs Thomas E. , Booth Frank W. , Roberts Michael D. 
Published: June 2021

PurposeTransposable elements (TEs) or “jumping genes” are genetic sequences that make up ~50% of the mammalian genome. The TE LINE‐1 (L1) has the ability to move within the genome via a “copy and paste” mechanism termed retrotransposition, which ultimately results in amplification of L1. Retrotransposition has shown to be advantageous in certain context, namely by introducing regulatory elements as well as adding genetic diversity both between and within species. These occurrences, although evolutionarily advantageous, have the ability to cause damage through increased mutagenesis. Indeed, L1 insertions have been the subject of intense investigation with respect to cancer causing mutations, genome instability, and specific cases of Duchenne's muscular dystrophy. Recent research has revealed that L1 gene expression and DNA content is increased in aged tissue and is attenuated with caloric restriction. These findings have been attributed to epigenetic modifications at L1 repeats such as DNA methylation and histone modifications. The above observations suggest that L1 gene regulation is affected by environmental factors. With the current study, we sought to investigate if long‐term voluntary wheel running alters skeletal muscle L1 activity.MethodsRats selectively bred for high voluntary wheel running (EX) and sedentary age‐ and strain‐matched controls (SED) were compared for L1 mRNA expression, L1 DNA content, L1 5′‐UTR DNA methylation, heterochromatic L1 content via nuclease digestion, and gene expression of endogenous inhibitors of L1 activity. EX animals were housed with running wheels for 22 weeks, and the gastrocnemius was harvested from both groups at 27 weeks of age. Due to the rich evolutionary history of L1, multiple subfamilies exist within the L1 gene family. For this reason, a qPCR primer set was designed for the most active form of L1 along with primers that encompass non‐functional L1 elements, termed L1.3 and L1.tot, respectively.ResultsNeither L1.3 (p=0.52) or L1.tot (p=0.51) gDNA was different between groups. Both L1.3 (p=0.01) and L1.tot (p<0.01) mRNA expression showed a significant decrease in EX rats compared to SED rats. Methylated DNA at the L1 promoter was significantly higher in EX rats for both L1.3 (p=0.02) and L1.tot (p<0.01) when compared to SED rats. Gene expression for purported L1 inhibitors was not different between groups for TREX1 (p=0.55) or PIWIL2 (p=0.88). L1.3 within heterochromatin was not different in EX versus SED rats (p=0.22), while L1.tot was significantly greater in EX versus SED rats (p=0.03). Interestingly, there was an inverse correlation with citrate synthase activity and L1 gene expression for the active L1.3 primer set (r = −0.52, p=0.01).ConclusionsThe data herein provide evidence that skeletal muscle L1 activity can be regulated by voluntary wheel running in rodents. Given that there are now multiple studies suggesting exercise affects skeletal muscle L1 activity, one can speculate that exercise may serve a larger role than expected in genome regulation.Support or Funding InformationM.A.R. is funded by the William Townsend Porter Predoctoral Fellowship from the American Physiological Society.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.