Xie et al: Sleep drives metabolite clearance

Question 1

Background

Answer 1

● Sleep is important for health and brain function (hopefully you know
this from experience)
● The brain produces tons of waste
○ Cerebrospinal fluid (CSF) goes around clearing this waste in the interstitial fluid
(.ISF), in lieu of a lymphatic system
● CSF influx around arteries, ISF exit along veins
● Hypothesis: Amyloid beta (Aβ) is cleared via the glymphatic system
during sleep

Question 2

Methods

Answer 2

● 2-photon light microscopy
      ○ Good tissue penetration, nice 
      precise illumination
● ECoG + EMG to monitor brain 
activity
● CSF fluorescent tracers

Question 3

Figure 1 - Wakefulness suppresses influx of CSF tracers.

Answer 3

● CSF enters the brain during natural sleep
● In awake mice, there’s not much CSF influx
● When mice are artificially put to sleep, similar effects to natural sleep
are seen
● Increases in CSF influx over time following the onset of sleep

Question 4

Figure 2 - Real-time TMA+ iontophoretic quantification of the volume of
the extracellular space in cortex.

Answer 4

● TMA+ electrophoresis used to measure size/morphology of interstitial
space
○ Because it’s difficult to measure the actual size of the interstitial space, use
concentration as a proxy
● Waking up decreases the size of interstitial space, without affecting
‘tortuosity’
● When sleep is induced, interstitial space opens up, without affecting
tortuosity
● Lines up with the results in the past experiment

Question 5

Figure 3 - Sleep improves clearance of Aß.

Answer 5

● Aβ is cleared from the brain faster in sleeping mice than awake mice
● These results are recapitulated when using an inactive tracer of
blood-brain barrier transport

Question 6

Figure 4 - Adrenergic inhibition increases CSF influx in awake mice.

Answer 6

● Adrenergic receptor antagonists -> increased CSF tracer influx
○ Mimics sleep
● NE levels can’t be explained by head restraint alone
● Adrenergic antagonists mimic the effects of sleep
○ Increase in interstitial space, without changes in tortuosity
● E.g. adrenergic modulation is ‘sufficient’ to recapitulate the effects of
sleep on CSF

Question 7

Discussion

Answer 7

The restorative function of sleep may be a consequence of the enhanced
removal of potentially neurotoxic waste products that accumulate in the awake central nervous
system.