Skeletal Muscle Nuclei in Mice are not Post-mitotic

Abstract

The skeletal muscle research field generally accepts that nuclei in skeletal muscle fibers (i.e., myonuclei) are post-mitotic and unable to proliferate. Because our deuterium oxide (D2O) labeling studies showed DNA synthesis in skeletal muscle tissue, we hypothesized that resident myonuclei can replicate in vivo. To test this hypothesis, we used a mouse model that temporally labeled myonuclei with GFP followed by D2O labeling during normal cage activity, functional overload, and with satellite cell ablation. During normal cage activity, we observed deuterium enrichment into myonuclear DNA in 7 out of 7 plantaris (PLA), 6 out of 6 tibialis anterior (TA), 5 out of 7 gastrocnemius (GAST) and 7 out of 7 quadriceps (QUAD). The average fractional synthesis rates (FSR) of DNA in myonuclei were: 0.0202 ± 0.0093 in PLA, 0.0239 ± 0.0040 in TA, 0.0076 ± 0. 0058 in GAST, and 0.0138 ± 0.0039 in QUAD, while there was no replication in myonuclei from EDL. These FSR values were largely reproduced in the overload and satellite cell ablation conditions although there were higher synthesis rates in the overloaded PLA muscle. We further provided evidence that myonuclear replication is through endoreplication that results in polyploidy. These novel findings contradict the dogma that skeletal muscle nuclei are post-mitotic and open potential avenues to harness the intrinsic replicative ability of myonuclei for muscle maintenance and growth.

Cellular senescence and disrupted proteostasis induced by myotube atrophy are prevented with low-dose metformin and leucine cocktail

“…Indeed, some myonuclei within myotubes demonstrated BrdU incorporation (Supplementary Figure 10H), however, this was observed similarly across groups elucidating that it was not a phenomenon driven by SD. These data could be due to late myoblast fusion events but may also agree with recent work by Borowik et al supporting that myonuclei are indeed not post-mitotic [22].…”

Challenging Dogma about Myonuclei Behavior in Skeletal Muscle Cells

“…The work in this issue of Function by Borowik et al, 1 demonstrates increases in DNA synthesis in myonuclei, which would suggest myonuclear replication is occurring. Within the manuscript, the authors provide a synopsis describing a sequence of publications that led them to test if DNA synthesis may be occurring in myonuclei.…”

“…I do not always have to agree with investigators interpretations, but if the results cause me to really think and reconsider my own work then for me it often becomes a memorable paper. The work of Borowik et al 1 definitely met that criterion for me.…”

“….” Collectively these statements are something I always accepted as proven and something I would venture to say that most individuals, who study skeletal muscle would consider dogma. Thus, reading the work of Borowik et al 1 in this issue of Function , caused me to stop and really focus on the data because the ideas challenged these very same concepts. Perhaps, this illustrates the dangers of the word “dogma” in science.…”

The myonuclear domain in adult skeletal muscle fibres: past, present and future

“…The focus regarding satellite cell contributions to the myonuclear domain has understandably been on myonuclei and RNA, but perhaps the delivery of mitochondria (which has its own DNA) is also worth considering (Wang et al., 2022). Modern technology applied to the study of myonuclei could shed new light on dogmatic beliefs in muscle, such as whether mammalian myonuclei are truly post‐mitotic (Stockdale & Holtzer, 1961; Moss & Leblond, 1970; Pullman & Yeoh, 1978; Clegg & Hauschka, 1987), which has recently been challenged (Borowik et al., 2022). The identification of myonucleus‐specific protein markers could help address these questions and others across species; at the current time, a highly specific marker is seemingly lacking (Winje et al., 2018; Viggars, Owens et al., 2022).…”