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Depletion of SAM leading to loss of heterochromatin drives muscle stem cell ageing

Abstract

The global loss of heterochromatin during ageing has been observed in eukaryotes from yeast to humans, and this has been proposed as one of the causes of ageing. 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 muscle stem cells (MuSCs) as a consequence of the depletion of the methyl donor S-adenosylmethionine (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 restores intracellular SAM content and heterochromatin formation, leading to improvements in aged MuSC function and regenerative capacity in male and female mice. Together, our studies demonstrate a direct causal link between polyamine metabolism and epigenetic dysregulation during murine MuSC ageing.

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Fig. 1: The loss of heterochromatin occurs in MuSCs with age.
Fig. 2: Restoration of SAM promotes heterochromatin formation and rescues dysfunctions of aged MuSCs.
Fig. 3: Excessive spermidine synthesis depletes SAM and elicits heterochromatin loss in aged MuSCs.
Fig. 4: Heterochromatin re-formation by restoration of SAM ameliorates susceptibility to genotoxic stress and cell death in aged MuSCs.
Fig. 5: Restoration of heterochromatin improves the in vivo potency of aged MuSCs to regenerate new muscle.
Fig. 6: SAM restoration is associated with improved muscle regeneration of aged mice.

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Data availability

RNA-seq data have been deposited in the NCBI Gene Expression Omnibus (GEO) with accession no. GSE229853. ATAC-seq data have been deposited in the GEO with accession no. GSE229851. The GRCm39 genome is available in the NCBI RefSeq (GCF_000001635.27-RS_2023_04). All related source data are made available to readers as part of the manuscript files. Source data are provided with this paper.

Code availability

This paper does not report any original code.

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Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (2019R1A6A3A03031423) to J.K., and by funding from the Glenn Foundation for Medical Research and the National Institutes of Health (NIH) (grant nos. P01AG036695, R01 AG068667, R01 AR073248 and AG071783) to T.A.R. Work using the TEM described in this study was supported, in part, by an NIH S10 Award (1S10OD028536-01), titled ‘OneView 4kX4k sCMOS camera for TEM applications’ from the Office of Research Infrastructure Programs. The RNA-seq data were generated with instrumentation purchased with NIH funds (S10OD025212 and 1S10OD021763). LC–MS-based metabolite quantification was supported by the Vincent Coates Foundation Mass Spectrometry Laboratory, Stanford University Mass Spectrometry. It used the Waters Xevo TQ-XS mass spectrometer system (RRID: SCR_018510), which was purchased with funding from an NIH Shared Instrumentation grant no. S10OD026962. Human biopsy samples were obtained from DNW. We thank the cooperation of DNW and all tissue donors and their families, for giving the gift of life and the gift of knowledge, by their generous donation. Schematic images in the figures were created with BioRender.com.

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Contributions

J.K. designed the studies and carried out the experiments with assistance from D.I.B., S.K., J.S.S., G.D., R.L., A.G. and J.O.B. with guidance from T.A.R. throughout. J.K. interpreted the results with guidance and input from T.A.R. and L.L. J.K. and T.A.R. wrote the paper and assembled the data with assistance from D.I.B.

Corresponding author

Correspondence to Thomas A. Rando.

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Nature Metabolism thanks Jason Locasale and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Ashley Castellanos-Jankiewicz, in collaboration with the Nature Metabolism team.

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Extended data

Extended Data Fig. 1 Differential chromatin accessibility and gene expression in young and old MuSCs.

a, Gating strategy for fluorescence-activated cell sorting (FACS) isolation of muscle stem cells (MuSCs). Purity of isolated MuSCs is > 98% as assessed by staining for Pax7 of cells fixed one hour after plating. (Scale bar, 50 μm) b-d, Representative confocal immunofluorescence images of MuSCs of young and old female mice (Scale bar, 5 μm). RFUs of H3K9me3 (b), H3K9me2 (c), or HP1α (d) were normalized to fluorescence intensities of total H3 (n = 4). e, Pearson correlation of ATAC-seq profiles from young and old MuSCs (n = 2). A total of about 60,000 signal summits were found in each sample. f, Representative heat maps of ATAC-seq tag intensity 3 kb around transcription start sites (TSSs) in young and old MuSCs. g, Read density within 3 kb upstream of TSSs and 3 kb downstream of TESs. h, Hierarchical clustering of peak enrichment patterns between young and old MuSCs. i, PCA plot of RNAseq data from freshly isolated young and old MuSCs. j, Volcano plots of differentially expressed genes (DEGs) between young and old MuSCs. Dashed lines indicate fold-change (log2FC > 0.5) and p-value cut-offs (Padj < 0.05). Total 1,641 DEGs were found. Among the DEGs, 866 genes were highly expressed in old MuSCs, and 775 genes were highly expressed in young MuSCs. k, Normalized DESeq read counts of methyltransferases for H3K9 (Suv39h1, Ehmt1, Ehmt2, Setdb1, and Setdb2). (n = 4) l, Normalized DESeq read counts of demethylases for H3K9 (Kdm3a, Kdm3b, Kdm4a, Kdm4b, Kdm4c, and Kdm7b). (n = 4) DESeq normalization was conducted by using median-of-ratio method as described in the Methods. Data are shown as median and quartiles (b-d) and as mean ± SD (k, l). P values were calculated by two-sided unpaired Student’s t-tests (b-d, k, l). *P < 0.05; **P < 0.01; ***P < 0.001. Statistical details are provided in Source Data.

Source data

Extended Data Fig. 2 Restoration of intracellular SAM promotes heterochromatin formation and reduces susceptibility to DNA damage and cell death of old MuSCs.

a, Intracellular SAM content of MuSCs measured by SAM ELISA (n = 4). b-d, Representative confocal immunofluorescence images of MuSCs (Scale bar, 5 μm). RFUs of H3K9me3 (b), H3K9me2 (c), or HP1α (d) were normalized to RFUs of total H3 (n = 4). e-f, Representative confocal immunofluorescence images of old MuSCs treated as indicated (Scale bar, 5 μm). RFUs of H3K9me3 (e) or HP1α (f) were normalized to RFUs of total H3 (n = 4). g, Representative heat maps of ATAC-seq tag intensity 3 kb around TSSs in vehicle- or SAM-treated old MuSCs. h, Read density within 3 kb upstream of TSSs and 3 kb downstream of the TESs. i, Hierarchical clustering of peak enrichment patterns. j, (Top) Representative confocal immunofluorescence images of γ-H2AX foci (Scale bar, 5 μm). (Bottom) Quantification of the number of γ-H2AX foci per cell (n = 3). k, (Left) Representative confocal images of γ-H2AX foci. (Scale bar, 5 μm) (Right) The ratio of γ-H2AX foci per cell was quantified from old MuSCs treated as indicated (n = 4). l, FACS analysis of propidium iodide (PI)-positive MuSCs. A day after the administration of 7 Gy γ-irradiation to the hind limbs, MuSCs were isolated from young, old, or old mice treated with SAM, and subsequently cultured for 2 days. m, The ratio of apoptotic cells over total cells was quantified by TUNEL assay and plotted (n = 4). n-p, Relative fluorescence units (RFUs) of H3K9me3 (n), H3K9me2 (o), and HP1α (p) were normalized to RFUs of total H3 (n = 3). q, Human MuSCs were treated with SAM for 2 days, and 0.75 μM of doxorubicin was added and incubated for another day. TUNEL positive cells were quantified (n = 3). Data are shown as mean ± SD (a, j, k, m, q) and as median and quartiles (b-f, n-p). P values were calculated by two-sided unpaired Student’s t-tests (a-f, j, k, m-q). *P < 0.05; **P < 0.01; ***P < 0.001.

Source data

Extended Data Fig. 3 Inhibiting spermidine synthesis promotes heterochromatin re-formation and reduces DNA damage in old MuSCs.

a, Diagram of spermidine metabolism. The synthesis of spermidine and spermine requires dcSAM as aminopropyl donor, which is produced from SAM by AMD1. b, Representative Western Blots of SRM, PAOX, SAT1, and α-tubulin in MuSCs which were freshly isolated form young and old mice (n = 3). c-e, Band intensities of SRM (c), PAOX (d), and SAT1 (e) were normalized to the level of α-tubulin (n = 3). f, Quantification of RFUs of intracellular spermidine in MuSCs from vehicle- or MCHA-treated old mice (n = 4). g, Intracellular SAM content of MuSCs measured by SAM ELISA (n = 4). h-j, Representative confocal immunofluorescence images of MuSCs of old female mice treated with vehicle or MCHA (Scale bar, 5 μm). RFUs of H3K9me3 (h), H3K9me2 (i), or HP1α (j) were normalized to RFUs of total H3 (n = 4). k, Representative western blots of H3K9me3, H3K9me2, H3K9me1, HP1α, and total H3 of MuSCs (n = 3). l-o, Band intensities of H3K9me3 (l), H3K9me2 (m), HP1α (n), and H3K9me1 (o) were normalized to the band intensities of total H3 (n = 3). p, (Left) Representative transmission electron microscopy images of MuSCs on EDL sections. (Right) The percentage of heterochromatin cross sectional area over total cross-sectional area of nucleus is quantified (n = 15 cells examined over 3 independent young mice treated with vehicle, n = 14 cells examined over 3 independent young mice treated with MCHA). The box represents the interquartile range, with the lower and upper hinges indicating the 25th and 75th percentiles, respectively. The horizontal line inside the box marks the median score. The whiskers extend to the minimum and maximum values. q, Representative heat maps of ATAC-seq tag intensity 3 kb around TSSs. r, Read density within 3 kb upstream of TSSs and 3 kb downstream of the TESs. s, Hierarchical clustering of peak enrichment patterns. t, (Left) Representative confocal immunofluorescence images of γ-H2AX foci. (Right) Quantification of the number of γ-H2AX foci per cell (n = 3). Data are shown as mean ± SD (c-g, l-o, t) and as median and quartiles (h-j, p). P values were calculated by two-sided unpaired Student’s t-tests (c-j, l-p, t). *P < 0.05; **P < 0.01; ***P < 0.001.

Source data

Extended Data Fig. 4 Heterochromatin re-formation induced by restoration of SAM reduces susceptibility to genotoxic stress and cell death of old MuSCs.

a, Quantification of RFUs of intracellular SAM (n = 4). b, Quantification of RFUs of intracellular putrescine. Freshly isolated old MuSCs were treated with vehicle, putrescine, or ornithine for 48 hr (n = 4). c, After treating old MuSCs as described in (b), doxorubicin was added to the media, and the cells were cultured for another day. (Left) Representative confocal images of γ-H2AX foci. (Scale bar, 5 μm) (Right) The ratio of γ-H2AX foci per cell (n = 4). d, Old MuSCs were treated as described in (c). The ratio of apoptotic cells over total cells was quantified by TUNEL assay (n = 4). e, Quantification of mean RFUs of intracellular spermidine (n = 4). f, Quantification of intracellular SAM measured by SAM ELISA (n = 4). g, Representative western blots of SRM and α-tubulin in old MuSCs (n = 3). h, Band intensity of SRM was normalized to band intensity of α-tubulin (n = 3). i-j, Old MuSCs transfected with siControl or siSrm were stained with antibodies against SAM (i) or spermidine (j). Mean RFUs of intracellular SAM or spermidine in each group of cells were quantified (n = 3 for SAM staining, n = 4 for spermidine staining). k, Old MuSCs were transfected with siControl or siSrm and treated with vehicle, UNC0642, or Chaetocin for 24 hr as indicated. (Left) Representative confocal immunofluorescence images of γ-H2AX foci (Scale bar, 5 μm). (Right) Quantification of the number of γ-H2AX foci per cell (n = 4). l, Quantification of the ratio of TUNEL positive cells over total cells (n = 4). m-n, Freshly isolated human MuSCs were treated as indicated for 48 hr. RFUs of H3K9me3 (m) or HP1α (n) were normalized to RFUs of total H3 (n = 3). o, Human MuSCs treated as described in (m-n) were additionally treated with doxorubicin for 24 hr. (Left) Representative confocal images (Scale bar, 5 μm). (Right) Quantification of the number of γ-H2AX foci per cell (n = 3). p, The human MuSCs treated as described in (o) were subjected to TUNEL assay. The ratio of TUNEL positive cells over total cells was quantified (n = 3). Data are shown as mean ± SD (a-d, f, h-l, o, p) and as median and quartiles (e, m, n). P values were calculated by two-sided unpaired Student’s t-tests (a-f, h-p). *P < 0.05; **P < 0.01; ***P < 0.001.

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Extended Data Fig. 5 Inhibiting spermidine synthesis does not impair autophagy of aged MuSCs.

a-b, Quantification of intracellular spermidine levels. Violin plots showing RFUs of spermidine in old MuSCs treated with each dose of spermidine (a) or MCHA (b) as indicated (n = 3). c-d, (Left) Representative confocal immunofluorescence images of old MuSCs treated with vehicle or each dose of spermidine (c) or MCHA (d) for 48 hr. In each case, cells were treated with or without 50 μM of chloroquine (CQ) for the last 3 hr of incubation. (Right) Quantification of number of LC3B foci per cell. (Scale bar, 10 μM) Data are shown as median and quartiles (a, b) and mean ± SD (c, d). P values were calculated by two-sided unpaired Student’s t-tests (a-d). *P < 0.05; **P < 0.01; ***P < 0.001.

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Extended Data Fig. 6 Restoration of SAM and H3K9 methylation improves in vivo potency of old MuSCs to regenerate new muscle.

a-b, Representative images of bioluminescence of vehicle- or SAM-treated MuSCs (a) and vehicle- or MCHA-treated old MuSCs (b) which had been transplanted into muscles of NSG mice. A day after transplantation is Day 1 (n = 6). c, Representative FACS plots for analysis of RFP-positive MuSCs isolated from recipient NSG mice. Purity of isolated RFP-positive MuSCs is > 98% as assessed by staining for Pax7 of cells (Scale bar, 50 μm). d, Old MuSCs were transduced with shRNA containing lentiviruses as indicated. After 48 hr of culture, the cells were subjected to western blotting (n = 4). e, Band intensity of Srm was normalized to the band intensity of α-tubulin (n = 4). f-g, Old MuSCs transduced with shControl or shSrm were stained with antibodies against SAM (f) or spermidine (g). Mean RFUs of intracellular SAM or spermidine were quantified (n = 3). h, Representative BLI images taken from Day 1 to Day 15. MuSCs transduced with shControl or shSrm were transplanted into muscles of NSG mice (n = 6). i, Paired comparisons of relative bioluminescence flux measured from the right and left muscles of each mouse eight days after transplantation. j, Western blot analysis of SRM and SUV39H1 in MuSCs. Old MuSCs were transduced with shRNA containing lentiviruses as indicated. After 48 hr of transduction, the cells were subjected to western blotting (n = 3). k, Representative BLI images of MuSCs transduced with shSrm or shSrm with shSuv39h1 which had been transplanted into muscles of NSG mice (n = 8). l, Paired comparisons of relative bioluminescence flux measured from the right and left muscles of each mouse ten days after transplantation. m, Schematic of transplantation. n, (Left) Quantified results of BLI measured at different time points from 60 to 71 days following transplantation (n = 6). (Right) Representative images of bioluminescence captured 71 days after transplantation. Data are shown as mean ± SD (e-g). P values were calculated by two-sided unpaired Student’s t-tests (e-g) and by one-sided student’s t test, pairwise between groups (i, l, n). *P < 0.05; **P < 0.01; ***P < 0.001. Schematic images in Fig. 6m were created with BioRender.com.

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Extended Data Fig. 7 Restoration of SAM is associated with improved muscle regeneration and function of aged mice.

a, MuSCs were isolated from the injured muscles of female mice at 2 days post injury (dpi). (Left) Representative confocal images. (Scale bar, 5 μm) (Right) The ratio of γ-H2AX foci per cell was quantified (n = 4 for young mice, and old mice treated with vehicle or SAM, n = 3 for old mice treated with MCHA). b, (Left) Representative TUNEL assay images of female MuSCs isolated at 2dpi (Scale bar, 50 μm). (Right) The ratio of apoptotic cells over total cells was quantified (n = 4 for young mice, and old mice treated with vehicle or SAM, n = 3 for old mice treated with MCHA). c, (Left) The percentage of PI positive female MuSCs isolated at 2dpi (n = 4 for young mice, and old mice treated with vehicle or SAM, n = 3 for old mice treated with MCHA). (Right) The representative FACS plot. d-e, Frequency histogram of cross-sectional areas (CSAs) of centrally nucleated myofibers in TA muscle sections. TA muscles were harvested from old mice treated with vehicle, MCHA (d) or SAM (e) at 7 dpi (n = 3). f, (Left) Representative immunostaining of regenerating muscle fibers from female mice (Scale bar, 100 μm). (Right) Quantification of the mean CSAs of myofibers with centrally located nuclei in TA muscle sections (n = 4 for young and old female mice treated with vehicle, n = 3 for old female mice treated with MCHA or SAM). g, (Left) Representative immunostaining of non-regenerating muscle fibers (Scale bar, 100 μm). (Right) Quantification of the mean CSAs of myofibers in TA muscle sections (n = 3). h, (Top) Representative immunostaining of regenerating muscle fibers (Scale bar, 100 μm). (Bottom) Quantification of the mean CSAs of myofibers with centrally located nuclei in TA muscle sections (n = 3). i, Representative tissue immunostaining images of TA muscle sections from young mice collected at 7 dpi (n = 4, Scale bar, 100 μm). j, Quantification of the number of Pax7-positive MuSCs per each tissue section (n = 4). k, Old mice were given vehicle or MTA in drinking water, and MuSCs from each mouse were subjected to SAM ELISA (n = 4). l, (Top) Representative immunostaining of regenerating muscle fibers from old mice treated with vehicle or MTA (Scale bar, 100 μm). (Bottom) Quantification of the mean CSAs of myofibers with centrally located nuclei in TA muscle sections (n = 3). m, Grip force measured without injury (n = 4). n-o, Stance time (n) and stride time (o) were measured at 14 dpi (n = 8 for young mice, n = 7 for old mice treated with vehicle or SAM, n = 6 for old mice treated with MCHA). Data are shown as mean ± SD (a-c, f-h, j-o). P values were calculated by two-sided unpaired Student’s t-tests (e-g) and by one-sided student’s t test, pairwise between groups (a-c, f-h, j-o). *P < 0.05; **P < 0.01; ***P < 0.001.

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Kang, J., Benjamin, D.I., Kim, S. et al. Depletion of SAM leading to loss of heterochromatin drives muscle stem cell ageing. Nat Metab 6, 153–168 (2024). https://doi.org/10.1038/s42255-023-00955-z

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