Hypothesis

During sleep, the glymphatic system and neuronal autophagy do not merely clear waste; they cooperate to generate a molecular ‘tag‑and‑decide’ signal that determines which synaptic architectures are retained and which are eliminated. This triage relies on sleep‑enhanced cerebrospinal fluid (CSF) influx delivering complement component C1q to synapses that have been flagged by activity‑dependent calcium influx and subsequent autophagic degradation of damaged mitochondria. When sleep is disrupted, reduced glymphatic flow limits C1q deposition, causing a shift toward inappropriate synaptic retention of maladaptive connections and accumulation of toxic protein aggregates.

Mechanistic Basis

• Glymphatic delivery of complement: Slow‑wave sleep drives ~90 % increase in interstitial space, boosting CSF‑ISF exchange 2. This flow carries soluble C1q from the choroid plexus into the parenchyma, where it can bind to neuronal surfaces.
• Activity‑dependent synaptic tagging: Wake‑associated synaptic activity elevates intracellular calcium, activating calpain‑dependent cleavage of neuronal membrane proteins that expose phosphatidylserine and other ‘eat‑me’ cues. These cues are known substrates for complement binding.
• Autophagy‑mediated mitochondrial quality control: Sleep-upregulated autophagy degrades damaged mitochondria via the PINK1/Parkin pathway 3. Healthy mitochondria sustain ATP production needed for complement synthesis and release; dysfunctional mitochondria increase oxidative stress, which inhibits C1q secretion.
• Integration step: Only synapses that simultaneously exhibit (i) elevated calcium‑tagging, (ii) proximity to autophagy‑cleared mitochondria (indicating recent metabolic stress), and (iii) exposure to glymphatic‑delivered C1q become opsonized for microglial phagocytosis. Synapses lacking one or more of these signals are spared.

Predictions & Experimental Tests

1. Enhancing glymphatic flow during slow‑wave sleep will increase complement deposition at active synapses.
   • Test: Use closed‑loop auditory stimulation to boost delta oscillations in mice, measure CSF influx with intrathecal tracer, and quantify C1q immunoreactivity at excitatory synapses via array tomography. Expect a >30 % rise in C1q signal compared with sham stimulation.
2. Blocking autophagy will uncouple synaptic tagging from glymphatic delivery, leading to indiscriminate complement binding.
   • Test: Administer neuron‑specific Atg5 knockout mice, perform the same auditory‑enhanced sleep protocol, and assess whether C1q accumulates at synapses regardless of calcium‑tagging markers (e.g., pCAMKII). Prediction: C1q staining becomes uniform across synapses, unlike the patchy pattern in controls.
3. Sleep fragmentation will reduce C1q‑mediated synaptic pruning and impair memory consolidation despite intact autophagy.
   • Test: Fragment sleep using gentle handling, quantify microglial phagocytic uptake of synaptic material (using pHrodo‑labeled synaptophysin), and assess performance on a contextual fear‑conditioning task. Expect decreased microglial engraftment and poorer memory retention, which can be rescued by exogenous C1q administration.
4. Pharmacological boost of NAD⁺/SIRT3 signaling during sleep will restore mitochondrial autophagy and rescue glymphatic‑dependent tagging in aged mice.
   • Test: Treat aged mice with NR (nicotinamide riboside) before the rest phase, measure mitochondrial clearance via mito‑Keima, CSF flow via MRI glymphatic imaging, and synaptic C1q levels. Prediction: NR restores youthful levels of mitophagy, enhances CSF influx, and normalizes complement tagging.

Potential Implications

If validated, this hypothesis reframes sleep not as a passive cleanup but as an active decision‑making process where the brain’s vascular and intracellular quality‑control systems jointly evaluate synaptic fitness. It offers a mechanistic link between sleep disorders, complement dysregulation, and early synaptic loss in Alzheimer’s disease, suggesting that interventions targeting glymphatic flow (e.g., acoustic stimulation, AQP4 modulators) or autophagy‑mitochondrial axis (NAD⁺ boosters) could preserve synaptic architecture by ensuring the brain’s nightly triage operates correctly.