Hypothesis

We propose that age‑associated increases in cytoskeletal tension transmitted through the LINC complex alter nuclear mechanics, leading to stereotyped patterns of chromatin accessibility erosion and gain that are modulated by lamin A/C levels and YAP/TAZ activity. This mechanochemical feedback loop explains both the loss of stem‑cell identity signatures and the ectopic opening of lineage‑associated regulatory elements observed in aged HSCs, NSCs, and satellite cells, and predicts that normalizing nuclear tension will remodel the epigenome toward a youthful state irrespective of tissue origin.

Mechanistic Rationale

Aging stem cells exhibit elevated Rho/ROCK activity and increased actomyosin contractility, which we posit enhances force transmission via nesprin‑2 and SUN proteins to the nuclear lamina. Chromatin domains attached to the lamina experience tensile strain, promoting nucleosome sliding or histone eviction at lamina‑associated domains (LADs) – manifesting as accessibility erosion at stem‑cell identity loci. Simultaneously, tension‑induced chromatin decondensation at interior nuclear regions exposes distal enhancers, particularly those bound by mechanosensitive transcription factors such as MEF2C and AP‑1, driving the gain of progenitor‑associated signals. Lamin A/C, whose expression rises with age in many stem cell niches, stiffens the nucleus and amplifies this effect, whereas YAP/TAZ nuclear translocation acts as a tension‑sensitive co‑activator that stabilizes the opened state at MEF2‑motif–enriched regions observed in hippocampal NSCs.

Testable Predictions

1. Pharmacological or genetic reduction of cytoskeletal tension (ROCK inhibitor, blebbistatin, or dominant‑negative RhoA) in aged HSCs, NSCs, or satellite cells will decrease accessibility gains at MEF2/AP‑1 motifs and restore erosion‑lost stem‑cell signatures, as measured by ATAC‑seq and functional assays (migration, neurogenesis, colony‑forming unit).
2. LINC complex disruption (CRISPR‑mediated knockout of nesprin‑2 or SUN1/2) will attenuate both erosion and gain phenotypes, uncoupling mechanical input from chromatin changes.
3. Lamin A/C overexpression in young stem cells will recapitulate the aged accessibility pattern (increased erosion at identity genes, gain at distal mechanosensitive enhancers) and impair function, while lamin A/C knockdown in aged cells will reverse these changes.
4. YAP/TAZ inhibition (verteporfin or TEAD‑blocking peptide) will specifically block the accessibility gain at MEF2‑motif–enriched regions without affecting erosion, predicting a selective rescue of migration/NSC activation phenotypes.
5. Atomic force microscopy or microfluidic stretching of isolated nuclei from aged vs. young stem cells will show increased nuclear stiffness and altered chromatin strain distribution that correlates with ATAC‑seq signal changes.

Experimental Approach

• Isolate aged and young HSCs (Lin‑-Sca1+\cKit+), NSCs (Sox2+\Nestin+), and satellite cells (Pax7+\Integrinα7+) from mouse models.
• Apply ROCK inhibitor (Y‑27632), blebbistatin, verteporfin, or express dominant‑negative constructs via lentiviral transduction; include lamin A/C overexpression or shRNA knockdown.
• Perform ATAC‑seq and RNA‑seq, focusing on LADs, MEF2/AP‑1 motif sites, and distal enhancers.
• Functional readouts: competitive repopulation assay (HSCs), neurosphere formation and migration assay (NSCs), and myotube formation assay (satellite cells).
• Measure nuclear stiffness by AFM and assess LINC complex localization by immunofluorescence.

Falsifiability

If tension reduction fails to normalize accessibility patterns or improve stem cell function across all three lineages, or if lamin A/C manipulation does not produce the predicted epigenetic shifts, the hypothesis would be refuted. Conversely, consistent rescue of both chromatin landscape and functional output would support a central role for mechanotransduction in stem cell aging.

References

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