Hypothesis

Chronic low‑grade translocation of gut‑derived lipopolysaccharide (LPS) activates endothelial Toll‑like receptor 4 (TLR4), which uncouples endothelial nitric oxide synthase (eNOS) by increasing NADPH oxidase‑derived reactive oxygen species (ROS) and depleting tetrahydrobiopterin (BH4). This mechanism reduces nitric oxide (NO) bioavailability, elevates asymmetric dimethylarginine (ADMA)‑mediated eNOS inhibition, and accelerates arterial stiffening as measured by pulse wave velocity (PWV). Consequently, increased PWV exacerbates perfusion‑dependent nerve fiber damage in the brain, raising neurofilament light chain (NfL) levels and precipitating early memory decline. Restoring gut barrier integrity or blocking endothelial TLR4 will normalize eNOS coupling, improve NO-mediated vasodilation, lower PWV, and attenuate NfL rise, thereby preserving cognitive function independent of traditional cardiovascular risk factors.

Mechanistic Rationale

1. LPS‑TLR4 signaling in endothelium – LPS binds TLR4 on endothelial cells, triggering MyD88‑dependent NF‑κB activation and up‑regulating NOX2/NOX4 NADPH oxidase isoforms. The resulting superoxide (O₂⁻) reacts with NO to form peroxynitrite, further oxidizing BH4 to BH2, promoting eNOS uncoupling (producing O₂⁻ instead of NO) [2].
2. BH4 depletion and ADMA rise – Uncoupled eNOS exacerbates oxidative stress, which stimulates arginase I and dimethylarginine dimethylaminohydrolase (DDAH) inhibition, lowering L‑arginine availability and increasing ADMA, a competitive eNOS inhibitor [3].
3. Arterial stiffening cascade – Reduced NO diminishes cGMP‑mediated vascular smooth muscle relaxation, promotes collagen cross‑linking, and stimulates elastin fragmentation via matrix metalloproteinase activation, raising carotid‑femoral PWV [4](https://pmc.ncbi.nlm.nih.gov/articles/PMC11198378/). Elevated PWV transmits heightened pulsatile energy to the cerebral microvasculature, damaging axons and elevating NfL [5]
4. Cognitive impact – Increased NfL correlates with worse episodic memory in cognitively unimpaired adults, suggesting a direct link between arterial stiffness and early neuro‑degeneration [5]

Testable Predictions

• Human observational cohort: Participants with elevated plasma LPS‑binding protein (LBP) will show higher PWV, lower plasma nitrite/nitrate (NOx) ratios, increased ADMA, and higher serum NfL, independent of age, blood pressure, and lipid profile.
• Intervention trial: A 12‑week regimen of a gut‑barrier‑targeting prebiotic (e.g., galacto‑oligosaccharides) or a selective endothelial TLR4 antagonist will: (a) decrease circulating LBP and endotoxin activity, (b) restore BH4/BH2 ratio in peripheral endothelial progenitors, (c) increase NOx, (d) reduce PWV by ≥0.5 m/s, and (e) lower serum NfL by ≥15 % compared with placebo.
• Animal proof‑of‑concept: Endothelial‑specific TLR4 knockout mice fed a high‑fat diet will exhibit preserved eNOS coupling, normal PWV, and attenuated NfL elevation despite comparable gut permeability to wild‑type controls.

Falsifiability

If endothelial TLR4 activation does not contribute to eNOS uncoupling—i.e., TLR4 blockade fails to improve BH4 bioavailability, NOx, or PWV despite reducing gut permeability—the hypothesis would be refuted. Similarly, if PWV improvement does not correlate with NfL reduction, the proposed mechanistic link to cognitive decline would be unsupported.

References

• Endothelial dysfunction can occur in individuals without traditional cardiovascular risk factors [1]
• NO bioavailability stems from decreased eNOS activity, cofactor limitations, and increased ROS [2]
• Inflammation independently contributes by increasing oxidative stress and reducing NO [3]
• Arterial stiffness measured by PWV is a key biomarker for pathological aging [4]
• Stiff arteries amplify early memory decline by exacerbating brain nerve fiber damage [5]