Hypothesis

In aged mesenchymal stem cells (MSCs), progressive epigenetic derepression of posterior HOX genes reshapes the transcriptional hierarchy of selective autophagy receptors, causing a predictable, stepwise loss of mitophagy, ER‑phagy, and ribophagy capacity. This ordered collapse—not a global autophagy deficit—drives the accumulation of damaged organelles that underlies MSC senescence.

Mechanistic Rationale

1. HOX‑driven transcriptional program – Posterior HOX proteins (e.g., HOXA9, HOXC8) act as transcriptional repressors of specific autophagy receptors. In young MSCs, HOX chromatin is kept compact by H3K27me3, limiting HOX expression and allowing balanced receptor levels (p62≈NBR1>OPTN≈NDP52). With age, loss of H3K27me3 and gain of H3K4me3 at posterior HOX loci increases their transcriptional noise (measured as Shannon entropy of HOX cluster expression). Elevated HOX protein then differentially suppresses receptor genes: p62/SQSTM1 is most sensitive, followed by NBR1, then OPTN, while NDP52 remains relatively stable.

2. Selective autophagy hierarchy – The canonical order of organelle clearance in MSCs is mitophagy (p62‑dependent) → ER‑phagy (NBR1‑dependent) → ribophagy (OPTN‑dependent). When the hierarchy collapses, mitophagy fails first, leading to mitochondrial ROS accumulation, which further destabilizes HOX chromatin, creating a feed‑forward loop.

3. Predicted phenotypic read‑out – Early passage MSCs will show a graded decrease in p62 protein, then NBR1, then OPTN, while total LC3‑II levels remain unchanged. Mitochondrial mass (MitoTracker) rises before ER stress markers (CHOP) and ribosomal stress signs (RPS6 phosphorylation).

Testable Predictions

• Prediction 1: Single‑cell multiome (scRNA‑seq + scATAC‑seq) of MSCs from young vs. old donors will reveal increased expression variance (entropy) of posterior HOX clusters correlating with decreased p62 mRNA, followed by NBR1, then OPTN across pseudotime.

• Prediction 2: CRISPRi‑mediated repression of HOXA9 in aged MSCs will restore p62 levels without altering global autophagy flux (measured by bafilomycin‑A1 LC3‑II accumulation), rescuing mitophagy (mt‑Keima assay) and delaying senescence markers (SA‑β‑gal, p16^INK4a^).

• Prediction 3: Pharmacological induction of mitophagy (Urolithin A) in aged MSCs will reduce mitochondrial ROS and lower HOX cluster entropy, breaking the feedback loop.

Falsification Criteria

If aged MSCs show a uniform decline of all selective autophagy receptors proportional to total autophagy loss, or if HOX perturbation fails to rescue receptor order and organelle clearance, the hypothesis is refuted.

Broader Impact

Linking HOX epigenetic drift to a programmed autophagy hierarchy reframes aging not as random wear but as a loss of developmental‑stage‑specific quality control, opening therapeutic avenues that target HOX chromatin state to reinstate cellular cannibalism order.

References

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC5415306/ [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC10404871/ [3] https://doi.org/10.1101/2024.06.06.597781/ [4] https://doi.org/10.1101/gr.240093.118/