Depletion of SAM leading to loss of heterochromatin drives muscle stem cell ageing
The global loss of heterochromatin during aging has been observed in eukaryotes from yeast to humans, and this has been proposed to be one of the causes of aging. However, the cause of this age-associated loss of heterochromatin has remained enigmatic. Here we show that heterochromatin markers, including histone H3K9 di-/tri-methylation and HP1, decrease with age in murine muscle stem cells (MuSCs) as a consequence of the depletion of the methyl donor SAM. We find that restoration of intracellular SAM in aged MuSCs restores heterochromatin content to youthful levels and rejuvenates age-associated features including DNA damage accumulation, increased cell death, and defective muscle regeneration. SAM is not only a methyl group donor for transmethylation but it is also an aminopropyl donor for polyamine synthesis.  Excessive consumption of SAM in polyamine synthesis may reduce its availability for transmethylation. Consistent with this premise, we observe that perturbation of increased polyamine synthesis by inhibiting spermidine synthase (Srm) restores the intracellular SAM content as well as heterochromatin formation, leading to improvements in aged MuSC function and regenerative capacity. Together, our studies demonstrate a direct causal link between polyamine metabolism and epigenetic dysregulation during MuSC aging. Overall design: Chromatin accessibility analysis of ATAC-seq data for FACS-isolated muscle stem cells from young (3-month-old) mice, old (20-month-old) mice treated with vehicle, MCHA, or SAM.