Hypothesis

Rapamycin prolongs life by mimicking nutrient scarcity, which suppresses high‑molecular‑weight hyaluronan (HMW‑HA) synthesis via mTORC2‑AKT‑FOXO signaling. Unlike natural aging, this reduction occurs without generating low‑molecular‑weight HA (LMW‑HA) fragments that act as damage‑associated molecular patterns (DAMPs). Consequently, rapamycin‑treated cells avoid TLR2/4‑driven NLRP3 inflammasome activation, lowering chronic inflammation and extending lifespan.

Mechanistic Reasoning

• mTORC2 phosphorylates AKT (Ser473), promoting FOXO cytoplasmic retention and allowing HAS2 transcription. Rapamycin disrupts mTORC2 assembly, decreasing p‑AKT(S473) and enabling FOXO nuclear translocation, which represses HAS2 promoter activity.
• In aged dermis, MMP‑cleaved collagen fragments inhibit HAS2 and suppress phospho‑ERK1/2, reducing HMW‑HA synthesis while simultaneously releasing LMW‑HA DAMPs that activate TLR2/4‑NF‑κB and NLRP3 inflammasomes.
• Rapamycin therefore uncouples HMW‑HA loss from DAMP production, yielding a "clean" low‑HA state that does not provoke innate immune sensing.

Testable Predictions

1. Rapamycin treatment will decrease HMW‑HA abundance and increase the HMW‑HA/LMW‑HA ratio in fibroblasts and skin tissue, without elevating LMW‑HA–induced TLR2/4 signaling.
2. Exogenous HMW‑HA supplementation will restore TLR2/4‑NF‑κB activity and NLRP3 inflammasome signaling in rapamycin‑treated cells, reversing the anti‑inflammatory phenotype.
3. In vivo, providing HMW‑HA to rapamycin‑fed mice will diminish lifespan extension and increase age‑related inflammatory markers (IL‑1β, IL‑6, TNF‑α).
4. Genetic rescue of HAS2 expression in rapamycin‑treated mice will reinstate inflammasome activation and blunt longevity benefits.

Experimental Design

In vitro

• Culture primary human dermal fibroblasts; treat groups with (a) vehicle, (b) rapamycin (100 nM), (c) rapamycin + exogenous HMW‑HA (500 kDa, 100 µg/mL), (d) LMW‑HA (10 kDa) as positive inflammasome control.
• Measure: HAS2 mRNA/protein (qPCR, Western), HA size distribution (SEC‑MALS), phospho‑AKT(S473), nuclear FOXO, TLR2/4 surface expression (flow), NF‑κB luciferase activity, NLRP3 inflammasome assembly (ASC speck formation), caspase‑1 cleavage, IL‑1β release (ELISA).

In vivo

• Use C57BL/6 mice (n=15 per group): (1) chow, (2) chow + rapamycin (14 ppm), (3) chow + rapamycin + HM‑HA (200 mg/kg i.p. weekly), (4) chow + HM‑HA alone.
• Monitor survival, frailty index, grip strength, and glucose tolerance.
• At 18 months, harvest skin and spleen for HA fragment analysis, cytokine profiling, and inflammasome readouts.
• Optional: fibroblasts isolated from HAS2‑overexpressing transgenic mice treated with rapamycin to test genetic rescue.

Expected Outcomes

• Rapamycin reduces p‑AKT(S473), increases nuclear FOXO, lowers HAS2 and HMW‑HA, but does not raise LMW‑HA or inflammasome markers.
• Adding HMW‑HA restores TLR2/4 signaling, NF‑κB activity, and caspase‑1 activation despite rapamycin presence.
• Mice receiving rapamycin + HM‑HA show shortened median lifespan (~15 % reduction) and elevated serum IL‑1β compared with rapamycin alone, falsifying the hypothesis if no difference is observed.

Implications

If validated, this work reframes rapamycin‑mediated longevity as a selective suppression of a specific matrix‑derived inflammatory signal rather than a global stress response. It suggests that modulating HA size distribution—boosting HMW‑HA while preventing LMW‑HA generation—could mimic rapamycin’s benefits without direct mTOR inhibition, opening avenues for matrix‑targeted gerotherapeutics.