Cross Talk rebuttal: Schwartz and Gundersen

While we agree with Kirby and Dupont-Versteegden (2022a) that muscles acquire new nuclei via satellite cell fusion to facilitate growth and repair, we differ in our views regarding the fate of preexisting myonuclei. Based on lineage tracing studies in mouse, they suggest that ∼4% of nuclei are newly acquired in homeostatic muscle (Masschelein et al., 2020). However, as the authors of that paper warn, this represents a baseline-value with methodological problems associated with: promoter leakiness, imperfect sorting (see Bengtsen et al., 2021), and diffusible reporters. On the presumption that myonuclear numbers do increase over time, Kirby and Dupont-Versteegden (2022a) reason that some mechanism for nuclear elimination must maintain steady-state numbers. While they caution that published studies only offer a ‘snap shot’ of nuclear numbers, and therefore may miss alterations in myonuclear number, Bruusgaard and Gundersen (2008) performed a detailed timeseries analysis in single muscle fibers in vivo, and demonstrated complete retention of myonuclei over a 3-week period of atrophy. In agreement with the long-term preservation of myonuclei, 14C measurements suggest that they have a half-life of >15 years in mammals (Spalding et al., 2005). In the absence of a demonstrable mechanism for myonuclear loss, Kirby and Dupont-Versteegden (2022a) reason that some yet-to-be-defined process may be at play. One proposed mechanism is EndoG-mediated apoptosis. However, like caspases, EndoG is a diffusible protein, so it is unclear how one nucleus could be destroyed without affecting neighbouring ones. They also speculate that ‘nuclear fallout’ could be involved. This is a very specialized process in Drosophila embryos that takes place before cellularization, at a time when the nuclei are not yet transcriptionally active (Iampietro et al., 2014). During ‘fall out’, damaged nuclei descend from the periphery to preclude incorporation into the developing epithelium. Since vertebrate myonuclei are not mobile in vivo (Bruusgaard et al., 2003), and a mechanism for myonuclear destruction has not been described, it is not clear how this could reduce myonuclear number. Kirby and Dupont-Versteegden (2022b) also suggest that there may be evolutionary advantages to removing old nuclei in favour of new ones.While intriguing, there is more compelling evidence that the retention of preexisting myonuclei during atrophy facilitates rapid re-growth of muscle mass upon re-training, a phenomenon known as ‘muscle memory’ (Egner et al., 2013). In summary, the prevailing data supports the hypothesis that once a muscle fibre acquires a nucleus, it keeps it.