If researchers have a good catalog of the biochemical differences between old and young stem cells, such as the ways in which the cell surface markers are different, then they can start act on those differences: measure the effects of rejuvenation therapies; attempt to change stem cell state to restore youthful appearance and activity; and so forth. This work is a step in the direction of that catalog:
A chemical code scrawled on histones - the protein husks that coat DNA in every animal or plant cell - determines which genes in that cell are turned on and which are turned off. [Researchers have now] identified characteristic differences in "histone signatures" between stem cells from the muscles of young mice and old mice. The team also distinguished histone-signature differences between quiescent and active stem cells in the muscles of young mice.
Stem cells in several tissues of older mice, including muscle, seemed to act younger after continued exposure to younger mice's blood. Their capacity to divide, differentiate and repopulate tissues, which typically declines with an organism's advancing age, resembled those of their stem-cell counterparts in younger animals. This naturally led to curiosity about exactly what is happening inside a cell to rejuvenate it. [One] likely place to look for an answer was histones, to see if changes in the patterns of the chemical marks on them might reveal any secrets, at the cellular level, of the aging process we all experience - and, perhaps, whether there might be anything we can do about it.
The differences between quiescent and activated cells [are] mirrored by those between young and old quiescent satellite cells. "With age, there's an uptick in repressive markers. A lot more genes are locked in the 'off' position. In a division-capable cell, as opposed to the nondividing, differentiated muscle cells that activated satellite cells may someday become, it may be important to maintain a high level of repression with age. Maybe this increase in repression is a kind of tumor-suppression mechanism, keeping aging satellite cells - which could have accumulated some dangerous mutations over the passing months and years - in check."
The description of the histone-code differences between young and old cells constitutes a yardstick allowing investigators to ask which of these differences are important in aging and in rejuvenation. "We don't have the answers yet. But now that we know what kinds of changes occur as these cells age, we can ask which of these changes reverse themselves when an old cell goes back to becoming a young cell" - as appeared to be the case when tissues of older mice were exposed to blood from younger mice.