Picture a muscle's control panel where buttons are removed with age. First to go are the buttons that handle hormone response, then those that kick off recovery after exercise. Researchers studied cell nuclei from vastus lateralis muscle biopsies from 287 donors aged 20 to 79 and saw exactly how these buttons disappear at the level of DNA packaging.
In every cell, DNA is folded into a dense structure called chromatin. Regions of chromatin can be open or closed, and this determines which genes are active. The researchers sequenced RNA and open chromatin in individual muscle fiber nuclei and found that with age, more regions close than open. What closes is mostly enhancers, DNA regions that boost gene activity but don't code for proteins themselves. It's as if the control panel lost not the buttons themselves but their signal amplifiers.
This is especially noticeable in men in fast type 2a muscle fibers. The closing regions turned out to be rich in binding motifs for hormone receptors: glucocorticoid (NR3C1), mineralocorticoid (NR3C2), and androgen (NR3C4). These receptors are like radio receivers that pick up hormone signals. If their 'antennas' are closed, the muscle stops hearing commands to maintain mass and recover after exercise. In women, motifs for factors of the NFI family were more common, pointing to a somewhat different mechanism.
Changes also affected cell composition: with age, the share of nuclei from fast glycolytic type 2x fibers declined, while the share of nuclei from neurons and neuromuscular junctions grew. This fits the known picture: during aging, fast fibers are lost faster, and innervation is disrupted. Now an epigenomic layer has been added to this picture: gene switches that should respond to hormones close, and the muscle loses sensitivity to maintenance signals.
Previously it was unclear which specific chromatin regions change accessibility during muscle aging and whether this differs between fiber types and sexes. Now it's clear that what closes is mainly enhancers and regions with hormone receptor motifs, not promoters of active genes; in men this is more pronounced. These data can serve as a reference for sarcopenia research and for finding epigenomic markers of muscle aging. The muscle's control panel doesn't just lose buttons with age: it loses exactly those that should catch hormone signals.
The chromatin regions that close with age are most often far from genes (more than 5 thousand base pairs from the transcription start site), meaning aging hits the 'control panels,' not the genes themselves.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.