Muscle memory: are myonuclei ever lost?

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LETTER TO THE EDITOR

Muscle memory: are myonuclei ever lost?

XEinar Eftestøl,¹Niklas Psilander,²Kristoffer Toldnes Cumming,³Inga Juvkam,¹Maria Ekblom,² Kerstin Sunding,⁴Mathias Wernbom,⁵Hans-Christer Holmberg,⁶Björn Ekblom,²Jo C. Bruusgaard,^1,7 XTruls Raastad,³and Kristian Gundersen¹

1Department of Biosciences, University of Oslo, Oslo, Norway;²Department of Sport Performance and Training, The Swedish School of Sport and Health Sciences (GIH), Stockholm, Sweden;³Department of Physical Performance, Norwegian School of Sport Sciences, Oslo, Norway;⁴Stockholm Sport Trauma Research Center, Karolinska Institutet, Stockholm, Sweden;⁵Center for Health and Performance, Department of Food and Nutrition and Sport Science, University of Gothenburg, Gothenburg, Sweden;⁶Swedish Winter Sports Research Centre, Department of Health Sciences, Mid Sweden University, Östersund, Sweden; and⁷Department of Health Sciences, Kristiania University College, Oslo, Norway

Submitted 4 November 2019; accepted in final form 16 December 2019

TO THE EDITOR: A recent Viewpoint article in Journal of Applied Physiology by Murach et al. (14) analyzed raw material underlying our recent article, Psilander et al. (15), on possible muscle memory in humans. Due to the small and variable effects of the training on relevant parameters, we were unable to draw any conclusions one way or the other related to muscle memory in humans.

We have previously suggested that myonuclei acquired from satellite cells during hypertrophy, and subsequently not lost, could serve as a mechanism for muscle memory (3, 6, 11). A critical premise for this hypothesis is that myonuclei added during hypertrophy are retained during subsequent atrophy, e.g., during detraining or other disuse conditions. This we have demonstrated in mice by direct observation (including single fiber time lapse imaging) of myonuclei in vivo (3, 4).

As a critique of our hypothesis, Murach et al. (14) reana- lyzed our data in an attempt to provide evidence that in humans myonuclei are lost during detraining. They did so by selecting the 11 of 19 subjects that had the highest apparent increase in number of nuclei by strength exercise, and calculated that this subgroup seemed to lose nuclei during subsequent detraining.

Since histological sections are prone to several artifacts (see, e.g., 2, 11, 12), the most precise data on myonuclear number are obtained from single fiber observations (11). In the single fiber data from our paper selected by Murach et al., 7 of the 11 subjects showed a nominal decrease in the number of nuclei during detraining when measured this way.

In our opinion, even if taken at face value, this is at best an extremely weak indication that nuclei can be lost during detraining in humans. Moreover, there is a large variability in the material, and biopsy-to-biopsy variation in human experiments is known to be high. Thus, the apparent effect is more likely related to statistical regression toward the mean of the selected material.

We commend Murach et al. (14) for trying to find interesting effects where we did not, but in our opinion their statistical analysis is not prudent. To select high values in a population of biopsies where much of the variability would be related to biopsy-to-biopsy differences (e.g., results from different biop-

sies taken from the same individual at the same time could vary significantly), and then observe lower values in new biopsies taken at a later stage, the resulting material would be likely to display lower values due to a regression toward the mean alone, without any underlying biological effect (i.e., loss of myonuclei due to disuse).

If one, for example, picks the 11 individuals that had the apparent lowest increase in the number of nuclei during exercise from the single fiber data, they on average increased their number of nuclei by 13% during detraining. Are Murach et al.

(14) suggesting that these individuals gained myonuclei during detraining? Our interpretation would be that the result is an example of a regression toward the mean, in this case “from below.”

As we have discussed before (11, 12), there is considerable evidence from animals that when the most precise observation methods are used, myonuclei are not lost during relative atrophy, and this was also confirmed recently by others (13, 16). We are only aware of one paper based on single fiber observations suggesting that myonuclei could be lost during detraining, a recent paper from the Viewpoint group (5).

We would like to add that a selective loss of myonuclei in intact fibers during atrophy is also theoretically problematic, since it would probably require a special form of nuclear apoptosis without cell disintegration. Although such “special”

mechanisms were proposed almost 20 years ago (1), a proof-of principle still seems to be lacking (11, 12).

Murach et al. also touch upon the debate on whether myonuclear accretion is driving or even necessary for hypertrophy in fully mature muscle. As discussed before (7, 8), there is evidence that accretion of myonuclei from satellite cells is required in 3- to 4-mo-old mice (which in most mouse- literature is considered mature), and recently exercise-induced hypertrophy was also shown to be completely dependent on satellite fusion in mice as mature as 8 mo (10). As Murach et al. concede, myonuclear accretion accompanies most hypertrophy conditions studied, and several reports now suggest that preventing the accretion of myonuclei in mature mice abolishes or greatly attenuates hypertrophy (8 –10).

In summary, our own interpretation of the preponderance of the best evidence currently available suggests that de novo muscle hypertrophy in mature mammals is dependent on myo-

Address for reprint requests and other correspondence: K. Gundersen, Dept.

of Biosciences, Univ. of Oslo, Oslo, Norway (e-mail: kgunder@ibv.uio.no).

J Appl Physiol 128: 456–457, 2020;

doi:10.1152/japplphysiol.00761.2019.

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nuclear accretion, and that nuclei are subsequently not lost from intact fibers. The elevated number of nuclei might serve as a substrate for a cellular memory, but we are not excluding that additional mechanisms such as long lasting epigenetic imprinting (17) might influence on the level of protein synthe- sis governed by each nucleus.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

K.G. conceived and designed research; E.E. analyzed data; E.E. and K.G.

interpreted results of experiments; K.G. drafted manuscript; E.E. and K.G.

edited and revised manuscript; E.E., N.P., K.T.C., I.J., M.M.E., K.S., M.W., H.-C.H., B.E., J.C.B., T.R., and K.G. approved final version of manuscript.

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