Boosting the expression of the transcription factor NANOG reduces markers of cellular senescence in cultured muscle cells and in live mouse models, according to new research . This holds out the hope that cells could be rejuvenated without reprogramming them to pluripotency, avoiding some of the associated risks such as oncogenicity. However, some of the changes only last while NANOG expression was increased, so further work is needed to understand the pathways involved and identify potential interventions.
Each plays its part
Both aging and cellular senescence result in a decline in the number of progenitor muscle cells and in their reproductive capacity. This is linked with a familiar set of hallmarks: genomic instability, telomere shortening, a loss of proteostasis (leading to larger cells), and mitochondrial dysfunction. Earlier research has shown that senescence can be reversed – and the hallmarks ameliorated – by interventions such as partial reprogramming with the OSKM factors.
Reprogramming cells to pluripotency with OSKM activates a set of transcription factors, including NANOG, OCT4, and SOX2. NANOG has proven essential for maintaining the self-renewal of stem cells, and recent experiments showed that expressing NANOG in senescent muscle precursor cells restored their ability to differentiate and produce muscle cells. These findings prompted a team of researchers in the US to investigate whether and how changing NANOG expression might affect markers of senescence and aging.
Taking center stage
The team began by engineering human muscle precursor cells to express NANOG in an inducible manner. They next cultured the cells until they were senescent and then induced NANOG expression in some of them. Senescent cells normally take longer to double their population, but the doubling time was decreased in those with boosted NANOG, indicating that it helped reduce senescence. The researchers also found that NANOG restored the expression of many genetic pathways that had been impaired by senescence, restored the size and shape of the nucleus, and re-established heterochromatin in senescent cells. However, they note that these epigenetic changes only lasted as long as NANOG expression was induced; when the induction was removed and NANOG levels returned to normal, heterochromatin levels also decreased.
Cells with increased NANOG expression also had improved DNA damage repair as well as increased proteolysis and autophagy. Improvements in proteolysis help restore proteostasis, while increased autophagy could clear out dysfunctional mitochondria to help the cells restore proper energy homeostasis. Taken together, all of these changes represent an amelioration of the hallmarks of senescence and signify that increased NANOG expression reverts the cells to a more youthful state.
Finally, the researchers looked at the effect of boosting NANOG in mice that were engineered to age prematurely. The increased expression was induced by an injection into the muscles of transgenic mice, enabling the team to study immediate local effects rather than dealing with the complexity of a systemic increase in NANOG. Skeletal muscle cells from the injection site showed a decrease in the senescence marker SA-ß-gal and restored the activity of muscle progenitor cells.
Senescence of myogenic progenitors impedes skeletal muscle regeneration. Here, we show that overexpression of the transcription factor NANOG in senescent myoblasts can overcome the effects of cellular senescence and confer a youthful phenotype to senescent cells. NANOG ameliorated primary hallmarks of cellular senescence including genomic instability, loss of proteostasis, and mitochondrial dysfunction. The rejuvenating effects of NANOG included restoration of DNA damage response via up-regulation of DNA repair proteins, recovery of heterochromatin marks via up-regulation of histones, and reactivation of autophagy and mitochondrial energetics via up-regulation of AMP-activated protein kinase (AMPK). Expression of NANOG in the skeletal muscle of a mouse model of premature aging restored the number of myogenic progenitors and induced formation of eMyHC+ myofibers. This work demonstrates the feasibility of reversing the effects of cellular senescence in vitro and in vivo, with no need for reprogramming to the pluripotent state.
These findings pretty clearly demonstrate that NANOG induction is an effective way to alleviate senescence in these cells. However, it’s also clearly not going to be an intervention that anyone can directly use. Nevertheless, showing that NANOG can rejuvenate cells without the need for reprogramming or induced pluripotency opens a path towards understanding the mechanisms involved in this process, which will not only improve our grasp of how rejuvenation works but could also point towards potential therapeutics.
 Shahini, A. et al. Ameliorating the hallmarks of cellular senescence in skeletal muscle myogenic progenitors in vitro and in vivo. Science Advances (2021), doi: 10.1126/sciadv.abe5671