Hypothesis

Sleep-dependent arterial pulsations generate mechanical shear stress that directly activates the Hsp70‑BAG3 chaperone complex in neurons, triggering chaperone‑assisted selective autophagy (CASA). This mechanotransduction step couples extracellular glymphatic CSF influx with intracellular protein quality control, allowing the brain to triage which synaptic proteins and aggregates survive to the next wake period.

Mechanistic Rationale

• Glymphatic influx peaks during NREM sleep, driven by arterial pulsation‑mediated CSF flow 1. AQP4‑rich astrocytic endfeet facilitate water movement, amplifying shear forces on perivascular neurons 2.
• In cardiomyocytes, stretch activates autophagy via AT1‑receptor‑p38 MAPK signaling, independent of angiotensin II 3. A parallel pathway likely exists in neurons where integrin‑β1/focal adhesion kinase (FAK) senses extracellular matrix deformation and phosphorylates p38 MAPK.
• Activated p38 MAPK phosphorylates Hsp70, increasing its affinity for BAG3 and promoting the Hsp70‑BAG3 complex that targets misfolded proteins to CASA 4.
• Autophagy flux decline with age is a regulatory defect, not lysosomal loss 5. Reduced mechanical stimulation from impaired sleep or vascular stiffness could thus be the upstream cause.

Novel Insight

We propose that the glymphatic system does not merely flush waste; its pulsatile flow provides a mechanical cue that licenses neuronal BAG3‑CASA. When this cue is missing, the brain cannot execute its nightly “autopsy,” leading to accumulation of specific phospho‑tau species and amyloid‑β oligomers that escape lysosomal degradation despite intact glymphatic clearance.

Testable Predictions

1. Pharmacological block of BAG3 (e.g., with BAG3‑binding peptide) during sleep will abolish the sleep‑dependent reduction in neuronal insoluble tau, even when CSF influx is unchanged (measured by intrathecal tracer).
2. Enhancing arterial pulsatility during wakefulness using low‑intensity focused ultrasound (LIFU) will increase p38 MAPK phosphorylation and BAG3‑Hsp70 interaction in cortical neurons, rescuing CASA flux in sleep‑deprived mice.
3. Aging mice with impaired vascular pulsatility will show attenuated sleep‑induced p38 MAPK activation and BAG3 recruitment, correlating with higher insoluble amyloid‑β despite normal glymphatic influx measured by MRI‑based CSF velocity.

Experimental Design (brief)

• Use transgenic tau‑P301S mice; record EEG to confirm NREM sleep.
• Administer BAG3 inhibitor or vehicle via intracerebroventricular infusion during sleep windows.
• Quantify tracer (CF‑647) influx via two‑photon imaging to ensure glymphatic function is intact.
• Assess neuronal insoluble tau by sucrose gradient fractionation and Western blot; measure p38 MAPK‑pT180/Y182 and BAG3‑Hsp70 co‑immunoprecipitation.
• For LIFU rescue, apply 0.5 MHz, 0.5 MPa pulses to the skull during wake periods in sleep‑deprived mice; repeat biochemical readouts.
• Correlate MRI arterial spin labeling pulsatility indices with biochemical markers across young, middle‑aged, and aged cohorts.

Falsifiability

If BAG3 inhibition does not alter sleep‑dependent changes in insoluble tau or if LIFU fails to activate p38 MAPK/BAG3 in neurons, the hypothesis that glymphatic‑derived mechanical forces directly trigger neuronal BAG3‑CASA would be refuted. Conversely, positive results would support a unified model where sleep’s autophagic triage is mechanistically gated by vascular pulsation.