Hypothesis

In aged tissues, accumulating low-molecular-weight hyaluronan (HA) fragments actively suppress autophagy in neighboring cells through two convergent pathways: CD44-dependent activation of RhoA/ROCK that stiffens the cytoskeleton and blocks autophagosome-lysosome fusion, and TLR4-triggered NF-kB signaling that activates mTORC1 and retains TFEB in the cytoplasm, shutting down autophagy gene expression. This dual blockade turns the extracellular matrix into a signaling toxin that preserves a damaged cellular state by preventing the catastrophic loss of components that a full autophagic reset would cause.

Mechanistic Model

1. HA fragment generation – Senescent fibroblasts downregulate HAS2 and increase hyaluronidase activity, producing LMW-HA fragments (Senescent fibroblasts accumulate in aged dermis and drive abnormal ECM remodeling).
2. Cytoskeletal lock – LMW-HA binds CD44 on nearby cells, activating RhoA/ROCK (HAS2 inhibition in chondrocytes activates RhoA/ROCK signaling and disrupts the cytoskeleton). ROCK-mediated actin crosslinking increases cortical tension, impairing the motility of autophagosomes and preventing their fusion with lysosomes.
3. Signaling shut‑off – The same HA fragments engage TLR2/4 on the plasma membrane, triggering MyD88-dependent NF-kB activation (Low-molecular-weight HA fragments induce NF-kB activation via TLR2/4). NF-kB drives transcription of pro-inflammatory cytokines and upregulates mTORC1 activity, which phosphorylates ULK1 (inhibiting initiation) and retains TFEB in the cytosol, blocking lysosomal biogenesis.
4. Feedback loop – Suppressed autophagy reduces clearance of damaged mitochondria and protein aggregates, amplifying oxidative stress and reinforcing the senescent phenotype of the HA-producing fibroblasts. Meanwhile, autophagy-competent senescent cells still rely on basal autophagy to sustain their SASP (autophagy paradoxically sustains the SASP of established senescent cells), creating a spatial split where neighbors are actively inhibited while the senescent source remains partially autophagic.

Testable Predictions

• Prediction 1: In aged mouse skin, pharmacological inhibition of ROCK (e.g., with Y-27632) will restore autophagosome-lysosome colocalization in CD44-positive fibroblasts without altering HA fragment levels.
• Prediction 2: Blocking TLR4 signaling (using TAK-242) in aged tissue will decrease phospho-S6K (mTORC1 readout) and increase nuclear TFEB, even when LMW-HA concentrations remain high.
• Prediction 3: Simultaneous ROCK and TLR4 inhibition will synergistically boost LC3-II conversion and p62 degradation, surpassing the effect of either single treatment.
• Prediction 4: Senescent fibroblast-conditioned medium enriched for LMW-HA will suppress autophagy in naïve fibroblasts; this suppression will be rescued by adding neutralizing anti-CD44 antibodies or soluble TLR4 ectodomain.

Experimental Approach

1. In vivo: Treat aged (24-month) mice with topical ROCK inhibitor, TLR4 antagonist, or both for two weeks. Harvest skin sections and perform immunostaining for LC3, LAMP1, phospho-S6K, and nuclear TFEB. Quantify autophagic flux using chloroquine chase.
2. In vitro: Isolate primary human dermal fibroblasts from young and old donors. Induce senescence with irradiation. Collect conditioned medium, fractionate by size to isolate LMW-HA (<200 kDa). Apply to naïve fibroblasts with or without ROCK/TLR4 inhibitors, CD44-blocking antibody, or TLR4 ectodomain. Measure autophagy flux via mCherry-GFP-LC3 reporter and western blot for LC3-II/I and p62.
3. Rescue: Overexpress a constitutively active RhoA (RhoA-Q63L) or a dominant-negative ROCK (ROCK-KD) to test causality of the cytoskeletal arm; overexpress a non-phosphorylatable TFEB mutant (TFEB-3A) to bypass mTORC1-mediated sequestration.

If the data show that disrupting either CD44-RhoA/ROCK or TLR4-NF-kB-mTORC1 axes restores autophagy despite persistent HA fragments, the hypothesis that aged matrix actively suppresses autophagy via these dual signaling routes will be supported. Conversely, if autophagy remains blocked despite pathway inhibition, the model would be falsified, prompting investigation of alternative matrix-derived inhibitors.

This framework converts a correlative observation into a mechanistic, falsifiable claim: the aged extracellular matrix is not a passive by-product but an active regulator that tunes autophagy to preserve a damaged, senescent-rich tissue niche.