Hypothesis

Aging is accompanied by a shift in the gut microbiota toward increased expression of bacterial hyaluronidases that degrade host‑synthesized hyaluronic acid (HA) in the intestinal glycocalyx. This loss of high‑molecular‑weight (HMW) HA generates low‑molecular‑weight (LMW) HA fragments that engage CD44 on vagal afferent terminals, triggering a pro‑inflammatory cascade (NF‑κB activation, A2aR downregulation) while simultaneously diminishing vagal anti‑inflammatory tone. The resulting signal propagates to the CNS, amplifying systemic inflammaging. We propose that the microbial HA‑degrading load, not host senescence per se, sets the pace of inflammaging.

Mechanistic Chain

1. Microbiota‑derived hyaluronidase up‑regulation – Metagenomic aging studies show enrichment of Bacteroides and Clostridium hyaluronidase genes (hyalA, hyalB) in centenarians vs young adults. Increased enzyme activity reduces the HA synthesis‑degradation balance in the mucus layer.
2. Glycocalyx HA fragmentation – Host epithelial HAS2 continues to produce HMW HA, but microbial hyaluronidases cleave it into LMW HA (<200 kDa). As shown, LMW HA binds CD44 and downregulates the anti‑inflammatory adenosine A2aR receptor Low‑MW HA fragments downregulate A2aR via CD44.
3. Vagal afferent CD44 activation – LMW HA in the lamina propria reaches the tips of intestinal villi where vagal afferent endings express CD44. Engagement initiates Src‑family kinase signaling, leading to IKKβ‑mediated NF‑κB translocation and cytokine release (IL‑6, TNF‑α) locally and via vagal efferents to the spleen (the cholinergic anti‑inflammatory pathway is blunted).
4. Loss of vagal glycocalyx protection – Vagal stimulation normally preserves the enterocyte glycocalyx and reduces permeability Vagal stimulation protects glycocalyx. When HA is degraded, this protective loop fails, increasing bacterial translocation and further feeding microbial hyaluronidase production – a positive feedback loop.
5. Collagen‑fragment synergy – Age‑related ECM breakdown yields collagen fragments that suppress epithelial HAS2 via ERK1/2 inhibition Collagen fragments suppress HAS2 via ERK1/2, amplifying HA depletion.
6. CNS inflammaging – Persistent vagal afferent firing drives microglial priming and elevated CNS IL‑1β, establishing a feed‑forward loop of neuroinflammation that accelerates systemic aging phenotypes.

Testable Predictions

• Prediction 1: Older mice (24 mo) will show higher fecal hyaluronidase activity and lower intestinal HA molecular weight than young mice (3 mo). Treatment with a broad‑spectrum hyaluronidase inhibitor (e.g., apigenin) will restore HMW HA in the glycocalyx and reduce vagal CD44 phosphorylation.
• Prediction 2: Vagotomized mice will be resistant to inflammaging induced by microbiota transfer from aged donors, despite exhibiting similar luminal HA degradation.
• Prediction 3: Supplementation with a HA‑synthesizing probiotic engineered to overexpress HAS2 will rescue glycocalyx thickness, lower serum IL‑6, and improve cognitive performance in aged mice.
• Prediction 4: Single‑cell RNA‑seq of vagal afferents from aged mice will reveal increased CD44‑associated NF‑κB target genes and decreased A2aR expression compared with young controls.

Falsifiability

If hyaluronidase inhibition does not alter intestinal HA size, vagal CD44 signaling, or systemic inflammatory markers in aged animals, the hypothesis is falsified. Likewise, if transferring aged microbiota to germ‑free young recipients fails to accelerate inflammaging when vagal signaling is intact, the microbial‑HA‑vagal axis is not sufficient.

Broader Implications

This framework positions the gut microbiome’s enzymatic repertoire as a tunable "clock" that can be modulated by diet, prebiotics, or targeted biologics to decouple microbial aging from host inflammaging. It also suggests that measuring specific microbial hyaluronidase abundance could serve as a biomarker for biological age beyond telomere length or epigenetic clocks.