Hypothesis

Low‑molecular‑weight hyaluronan (HA) fragments generated in aged skin act as danger signals that trigger autophagy not as a bulk‑recycling cleanup but as a siege‑mode rationing system. Specifically, HA oligosaccharides in the 50‑500 kDa range engage TLR4 and/or CD44 on dermal fibroblasts, activating AMPK and inhibiting mTORC1, which shifts the cell into a nutrient‑conserving state where intracellular components are selectively degraded to sustain essential functions under perceived matrix stress.

Mechanistic Rationale

• Age‑related dermal fibroblast dysfunction features reduced HAS2‑driven high‑MW HA synthesis and increased MMP‑1–mediated collagen breakdown, creating a milieu rich in HA fragments 1 2.
• Collagen fragments suppress ERK1/2 nuclear accumulation and ELK‑1 phosphorylation, further lowering HAS2 expression 3.
• This establishes a self‑reinforcing loop of matrix fragmentation and HA loss.
• We propose that the resulting HA fragments are sensed by pattern‑recognition receptors (TLR4/CD44), leading to downstream activation of the LKB1‑AMPK axis and concomitant suppression of mTORC1 signaling. AMPK activation phosphorylates ULK1, initiating autophagy that preferentially degrades long‑lived proteins and organelles, thereby conserving ATP and amino acids—a classic rationing response.
• In contrast, canonical starvation‑induced autophagy relies on systemic nutrient deprivation signals (e.g., low glucose, low amino acids) and may differ in kinetics and cargo selectivity.

Experimental Design

1. Define fragment activity

• Prepare defined HA size fractions (≤10 kDa, 10‑50 kDa, 50‑200 kDa, 200‑500 kDa, >500 kDa) via size‑exclusion chromatography.
• Treat primary human dermal fibroblasts from young (<30 y) and aged (>65 y) donors with each fraction at equimolar HA concentrations (100 µg/mL) for 6‑24 h.

2. Measure autophagy flux

• Assess LC3‑II/I ratio and p62 degradation by western blot in the presence and absence of lysosomal inhibitor bafilomycin A1.
• Use tandem mRFP‑GFP‑LC3 reporter to quantify autophagosome vs autolysosome formation by microscopy.

3. Interrogate signaling nodes

• Probe phospho‑AMPK (Thr172) and phospho‑mTOR (Ser2448) levels.
• Apply specific inhibitors: TLR4 antagonist (TAK‑242), CD44 blocking antibody, AMPK inhibitor (Compound C), and siRNA against LKB1.
• Determine whether blockade abolishes fragment‑induced LC3‑II conversion and p62 loss.

4. Functional readouts

• Measure cellular ATP, NAD+/NADH ratio, and secretion of SASP factors (IL‑6, IL‑8).
• Assess cell viability and senescence (β‑galactosidase staining) after prolonged fragment exposure.

5. Rescue with high‑MW HA

• Co‑administer >1 MDa HA (as reported to be protective in naked mole‑rat models) 4 to test competition for receptor binding and suppression of the autophagy response.

Predicted Outcomes and Falsifiability

• If the hypothesis is correct, 50‑500 kDa HA fractions will significantly increase AMPK phosphorylation, decrease mTOR activity, and elevate autophagy flux in aged fibroblasts, effects that will be blocked by TLR4/CD44 inhibition or AMPK knockdown. High‑MW HA co‑treatment will attenuate these responses.
• If the hypothesis is false, HA fragments will fail to modulate AMPK/mTOR signaling, autophagy induction will be absent or indistinguishable from baseline, and receptor blockade will not alter any observed changes.

This framework directly tests whether specific HA fragment sizes act as siege‑like cues that re‑program fibroblast metabolism through a defined autophagy‑activating axis, providing a mechanistic bridge between matrix dysregulation and cellular rationing in aging.