Hypothesis

Persistent low‑grade type I interferon signaling in aged nasopharyngeal lymphatic endothelial cells depletes cellular ATP and contractile proteins, rendering the glymphatic system unable to perform its nightly “autopsy” during sleep. This metabolic exhaustion, not JAK‑STAT pathway desensitization, underlies the observed decline in cerebrospinal fluid outflow and contributes to the accumulation of neurotoxic waste in aging brains.

Mechanistic Rationale

• JAK‑STAT feedback via SOCS proteins and phosphatases maintains signal amplitude but does not limit the cumulative transcriptional load of interferon‑stimulated genes (ISGs) [1] [3]
• Chronic ISG expression forces sustained protein synthesis, protein‑folding demand, and antioxidant responses, consuming ATP and NADPH [4]
• Lymphatic endothelial cells in the nasopharyngeal plexus lack contractile smooth muscle; their pumping capacity relies on actin‑myosin cortical tension and mitochondrial‑derived ATP [[synthetic_research_questions_feedback.md]]
• We propose that chronic ISG‑induced mitochondrial stress reduces oxidative phosphorylation efficiency, lowering the ATP reserve needed for the high‑frequency, low‑amplitude contractions that drive CSF influx during slow‑wave sleep.
• Consequently, the glymphatic “audit” fails, allowing misfolded proteins and damaged synapses to persist, which in turn fuels cGAS‑STING activation via extracellular nucleic acids, creating a vicious loop.

Testable Predictions

1. Aged mice with elevated nasopharyngeal ISG signatures will show reduced ATP levels and lower mitochondrial membrane potential specifically in lymphatic endothelial cells isolated during the sleep phase, compared with young controls or wake‑phase samples.
2. Pharmacological boosting of endothelial NAD+ (e.g., with NR or NMN) will restore ATP, increase lymphatic contraction frequency, and rescue glymphatic tracer clearance in aged mice without altering JAK‑STAT phosphorylation status.
3. Genetic knockdown of a representative ISG such as IFITM3 in lymphatic endothelium will ameliorate the metabolic deficit and improve CSF outflow, even when upstream interferon signaling remains active.
4. Artificially inducing metabolic exhaustion (e.g., with low‑dose oligomycin) in young mice will mimic the aged glymphatic phenotype, independent of interferon levels.

Experimental Approach

• Cellular Metabolism: Sort CD31⁺Lyve‑1⁺ nasopharyngeal lymphatic endothelial cells from young (3 mo) and aged (20 mo) mice at ZT2 (peak sleep) and ZT14 (wake). Measure Seahorse OCR/ECAR, ATP luminescence, and mitochondrial ROS.
• Functional Clearance: Inject fluorescent CSF tracer (e.g., Alexa‑647‑labeled albumin) intrathecally before sleep onset; quantify brain parenchyma accumulation via confocal imaging and whole‑body lymphatic drainage via near‑infrared fluorescence.
• Interventions: Treat aged mice with NR (400 mg/kg/day) for 4 weeks or deliver lymphatic‑specific shRNA against IFITM3 via AAV‑Lyve1. Assess whether tracer clearance improves while phospho‑STAT1 levels remain unchanged.
• Causality Test: Apply endothelial‑targeted oligomycin (via nanoparticle) to young mice during sleep and evaluate whether glymphatic flux drops to aged‑like levels despite low interferon signaling.

Falsifiability

If boosting endothelial ATP or reducing ISG burden fails to improve glymphatic clearance in aged animals, or if metabolic manipulation does not reproduce the clearance defect, the hypothesis that interferon‑driven metabolic exhaustion limits sleep‑dependent brain editing would be refuted. Conversely, a positive outcome would support the model that chronic interferon signaling imposes a bioenergetic ceiling on the brain’s nightly repair process.