Hypothesis

During sleep, the glymphatic system does not merely flush metabolites; it actively surveys the cerebrospinal fluid (CSF) for extracellular nucleic‑acid‑bound proteins that serve as "damage tags" released from neurons with unrepaired DNA breaks. These tags recruit microglial phagocytic synapses, linking nightly DNA repair outcomes to synaptic pruning. Consequently, sleep deprivation prevents the generation or clearance of these tags, leading to the persistence of damaged synapses and accelerated network dysfunction.

Mechanistic Reasoning

1. DNA damage releases chromatin‑bound peptides – Neuronal double‑strand breaks activate PARP1, which poly‑ADP‑ribosylates histones and promotes the release of ubiquitin‑modified histone fragments into the nucleoplasm. A fraction of these fragments is exported via nuclear pores and secreted into the interstitial space (see PARP1 senses DNA damage and DNA repair during sleep).
2. Glymphatic inflow captures damage‑associated molecular patterns (DAMPs) – The perivascular CSF influx during sleep carries aquaporin‑4‑dependent flow that can capture extracellular histone‑Ub peptides. Recent work shows CSF can transport nucleosome‑sized particles (glymphatic efficiency declines with age).
3. Microglial surveillance via TREM2‑dependent phagocytosis – Microglia express scavenger receptors that bind ubiquitin‑histone complexes, triggering a phagocytic program that preferentially targets synapses apposed to neurons with high extracellular DAMP levels (cf. ATM/CHK2 activation after sleep loss).
4. Feedback loop to DNA repair – Synaptic elimination reduces metabolic load on the affected neuron, lowering oxidative stress and allowing residual repair enzymes (ERCC1, OGG1, XRCC1) to finish unfinished work during the next sleep bout.

Testable Predictions

• Prediction 1: In mice subjected to chronic sleep restriction, extracellular histone‑Ub levels in the CSF will be elevated compared to well‑rested controls, measurable via ELISA of CSF samples (sleep deprivation impairs repair genes).
• Prediction 2: Pharmacological blockade of aquaporin‑4 (e.g., with TGN-020) will attenuate the sleep‑dependent clearance of histone‑Ub from the interstitial space, visualized by two‑photon imaging of fluorescently labeled ubiquitin peptides.
• Prediction 3: Microglia lacking TREM2 will show reduced synaptic engulfment of neurons marked by γH2AX (a DNA‑damage marker) after sleep, quantifiable by colocalization analysis of Iba1, TREM2, synaptophysin, and γH2AX.
• Prediction 4: Restoring glymphatic flow via intranasal CSF infusion in sleep‑deprived animals will rescue the normal rate of synaptic pruning and improve performance on hippocampus‑dependent memory tasks.

Falsifiability

If any of the following observations hold, the hypothesis is weakened:

• Sleep deprivation does not increase extracellular histone‑Ub in CSF.
• Blocking glymphatic inflow fails to alter histone‑Ub clearance despite confirmed inhibition of CSF influx.
• Microglial phagocytosis of damaged synapses proceeds independently of TREM2 or ubiquitin‑histone binding.
• Enhancing glymphatic flow does not ameliorate synaptic loss or cognitive deficits caused by sleep loss.

Significance

This hypothesis reframes sleep as a selective editing phase where the glymphatic system acts as a molecular "quality‑control" checkpoint, coupling nuclear DNA integrity to circuit‑level homeostasis. It offers a concrete mechanistic bridge between the observed accumulation of copy‑number variations in aging brains (CNVs increase with age) and the synaptic dysregulation seen in neurodegenerative disorders, suggesting that therapeutic strategies aimed at boosting glymphatic transport or modulating microglial DAMP sensing could mitigate sleep‑loss‑induced brain aging.