Hypothesis: During slow-wave sleep, AMPK activation phosphorylates astrocytic aquaporin-4 (AQP4) at serine residues, increasing its water channel activity and biasing glymphatic flow toward clearance of phosphorylated tau species while sparing amyloid-beta. This selective clearance depends on the conformational state of tau that exposes phosphorylation-dependent epitopes, which are preferentially dragged into perivascular CSF conduits when AQP4-mediated convective flux is heightened. Disruption of this AMPK‑AQP4 axis—via genetic knock‑in of non‑phosphorylatable AQP4 or pharmacological AMPK inhibition—should decouple glymphatic influx from autophagic tau degradation, leading to tau accumulation despite normal amyloid-beta clearance. We propose to test this in mice expressing human P301S tau by measuring CSF tau isoforms, AQP4 phosphorylation (p‑S321), and glymphatic influx (using intrathecal fluorescent tracer) under three conditions: (1) normal sleep, (2) sleep deprivation, and (3) sleep deprivation rescued by AMPK activator (AICAR) or astrocyte‑specific AQP4‑S321D phosphomimetic. Predictions: (a) Normal sleep shows increased AQP4‑pS321, elevated CSF tau clearance, and unchanged amyloid-beta levels; (b) Sleep deprivation reduces AQP4‑pS321, diminishes tau clearance but leaves amyloid-beta flux unchanged; (c) AMPK activation or AQP4‑S321D restores tau clearance without affecting amyloid-beta. Failure to observe these patterns would falsify the hypothesis that AMPK‑driven AQP4 phosphorylation mediates a tau‑selective glymphatic window during slow-wave sleep.

Supporting evidence: The glymphatic system’s clearance efficiency doubles during sleep by expanding interstitial space and driving CSF influx via AQP4 water channels glymphatic efficiency increases 2-fold during sleep. Slow-wave sleep’s delta waves mechanically pump CSF into the parenchyma, a process that depends on AQP4 localization to astrocytic endfeet SWS maximizes glymphatic efficiency through rhythmic delta waves driving CSF influx. AQP4 deletion halves glymphatic influx and exacerbates amyloid-beta and tau accumulation glymphatic system's function depends on aquaporin-4 water channels on astrocytes. AMPK activation during sleep’s quiescent phases inhibits mTORC1, triggering TFEB/TFE3‑mediated autophagy that degrades damaged organelles and protein aggregates Autophagy is rhythmically activated during sleep via TFEB/TFE3 when mTORC1 inactivates. AMPK also directly phosphorylates AQP4 at S321 in peripheral tissues, enhancing water permeability AMPK regulates AQP4-mediated water transport in astrocytes. Finally, peripheral immune cells are recruited during sleep to clear glial lipids, showing that sleep orchestrates multiple clearance pathways Peripheral immune cells recruited during sleep to clear metabolic waste like glial lipids.

If AMPK‑AQP4 coupling is essential for tau‑selective clearance, then enhancing this axis should mitigate tauopathy phenotypes without altering amyloid-beta load, offering a mechanistic link between metabolic state (e.g., berberine‑induced AMPK activation) and sleep‑dependent neurodegeneration prevention.