Have you noticed that as you age, your body finds it harder to recover from injuries and loses vitality?
This is largely because the cells responsible for repair and regeneration in our bodies usually stay in a quiescent state (conserving energy) and wait to be activated when needed—such as during injury or infection. However, with aging, these cells become increasingly difficult to wake up.
Why is this? A recent study published in Cell reveals an unexpected mechanism [1]: Macroautophagy (a major autophagic process) in quiescent cells may damage lysosomes—the cell’s "waste disposal system"—restricting cell reactivation and leaving cells trapped in quiescence.
1. Quiescence and Reactivation
Take stem cells as an example: They typically remain in the quiescent G0 phase [2]. During this period, they still retain the ability to divide and differentiate. Upon receiving signals (e.g., tissue damage), they can be quickly activated to participate in tissue repair and regeneration.
Figure Note: Cell cycle (G0 = Resting phase; G1 = Cell growth; S = DNA synthesis; G2 = Preparation for mitosis; M = Mitosis).
Even in quiescence, the cell interior is not inactive. Over time, cells inevitably produce "defective proteins" (misfolded or dysfunctional proteins). If not cleared promptly, these proteins accumulate like garbage, form aggregates, and accelerate aging.
Figure Note: Detection of misfolded/aggregated proteins using Proteostat showed that long-quiescent cells accumulate significant amounts of such "protein garbage" (middle panel of the right figure).
To address this accumulation, cells activate macroautophagy: The key transmembrane protein ATG-9 wraps abnormal protein aggregates into autophagosomes, which are then transported to lysosomes for complete degradation and recycling.
Figure Note: Left: Green lysosomes (SCAV-3::GFP) covered with magenta aggregates (showing white/purple overlap in the merged image). Right: In normal cells, a high proportion (≈50%–85%) of lysosomes are associated with these aggregates.
Up to this point, macroautophagy acts perfectly as a "garbage collector," and everything seems to go as planned.
2. Unexpected Side Effects of Macroautophagy
However, experiments in C. elegans (roundworms) gave scientists a surprising twist: In long-quiescent worms, excessive macroautophagy became a problem. The continuously overloaded macroautophagy process kept transporting large amounts of protein garbage to lysosomes, eventually causing lysosomal membrane damage, functional disorder, and harm to the cell itself.
Figure Note: Larvae in the arrested L1 stage (initial life cycle stage) showed lysosomal damage.
In arrested L1 larvae, lysosomal damage markers formed distinct punctate structures—clearly indicating lysosomal dysfunction. To confirm this, scientists used Galectin-3 (a lysosomal damage marker) and found that these Galectin-3 puncta perfectly colocalized with lysosomes—confirming lysosomal damage.
So, how does macroautophagy relate to this lysosomal damage? In fact, the damage is largely caused by macroautophagy. In mutants with defective macroautophagy, the number of Galectin-3-labeled lysosomes (a sign of damage) decreased visibly, and lysosomal damage was alleviated.
Figure Note: Colocalization analysis of Galectin-3 at the single-lysosome level (red overlap indicates lysosomal damage; Z1–Z7 = different planes of a single lysosome). ATG-9 mutation (autophagy inhibition) reduced damage compared to controls.
Moreover, this autophagy-induced lysosomal damage was not unique to C. elegans—it was also confirmed in lab-cultured human and mouse cells.
Figure Note: Cells tested included 3T3 (mouse), C3H/10T1/2 (mouse), and ARPE-19 (human). P = Proliferative state; Q = Quiescent state. More blue indicates stronger lysosomal damage.
This closes the evidence loop: Macroautophagy is the culprit behind lysosomal damage in quiescent cells.
3. Repair and Synergy
Damaged lysosomes not only lose efficiency in waste disposal but also impair the cell’s ability to sense nutrients and regulate key growth signals (e.g., the mTORC1 pathway)—stalling the recovery process. Do cells have repair mechanisms to counter this? Yes.
3.1 Repair via the IRE-1/XBP-1 Signaling Pathway
The IRE-1/XBP-1 pathway is key to helping cells recover from long-term quiescence. It reactivates the mTORC1 pathway (critical for nutrient and growth factor sensing) and restores cell growth.
Figure Note: Worms lacking IRE-1 or XBP-1 could barely restart development after refeeding—unlike normal worms. The y-axis shows the percentage of animals that recovered from L1 arrest and continued development.
Beyond awakening quiescent cells, IRE-1/XBP-1 also repairs damaged lysosomes. Studies showed that after awakening from long quiescence, normal worms used the IRE-1 pathway to effectively clear lysosomal damage markers (Galectin-3 puncta) and accumulated protein garbage.
More importantly, IRE-1 ensures the proper synthesis and maturation of lysosomal membrane proteins—such as CPL-1 (a key lysosomal digestive protein). In contrast, worms lacking IRE-1 had obvious defects in repair and regeneration, leaving their lysosomes unable to recover after long-term quiescence stress.
Figure Note: IRE-1 mutant worms failed to restore lysosomal function after refeeding.
3.2 Synergistic Effect: "1+1 > 2"
Scientists found that single interventions had limited effects:
- Inhibiting macroautophagy alone (ATG-9 mutation)
- Enhancing lysosomal function alone (overexpressing HLH-30)
Both only partially restored cell reactivation (blue and green lines in the figure below).
However, combining the two interventions—moderately inhibiting macroautophagy while enhancing lysosomal function—produced a dramatic, near-complete rescue. The worms’ reactivation ability and developmental recovery speed far exceeded any single intervention, almost reaching the level of normal wild-type worms (purple line).
Figure Note: Y-axis = Percentage of animals that recovered from L1 arrest and continued development; X-axis = Days in arrest.
Conclusion
The ability of quiescent cells to reactivate (re-enter the cell cycle) is limited by a built-in barrier: Macroautophagy itself causes lysosomal damage, making cells hard to "awaken." To overcome this barrier, cells rely on the IRE-1/XBP-1 signaling pathway to repair damaged lysosomes, restore their function, and smoothly transition to a proliferative state.
References
[1] Murley, A., Popovici, A. C., Hu, X. S., et al. (2025). Quiescent cell re-entry is limited by macroautophagy-induced lysosomal damage. Cell. https://doi.org/10.1016/j.cell.2025.03.009[2] Yaglova, N. V., Timokhina, E. P., Obernikhin, S. S., & Yaglov, V. V. (2023). Emerging Role of Deuterium/Protium Disbalance in Cell Cycle and Apoptosis. International Journal of Molecular Sciences, 24(4), 3107. https://doi.org/10.3390/ijms24043107