Candidate mechanisms underlying the association between sleep-wake disruptions and Alzheimer's disease

Abstract

During wakefulness, extracellular levels of metabolites in the brain increase. These include amyloid beta (Aβ), which contributes to the pathogenesis of Alzheimer's disease (AD). Counterbalancing their accumulation in the brain, sleep facilitates the removal of these metabolites from the extracellular space by convective flow of the interstitial fluid from the para-arterial to the para-venous space. However, when the sleep-wake cycle is disrupted (characterized by increased brain levels of the wake-promoting neuropeptide orexin and increased neural activity), the central nervous system (CNS) clearance of extracellular metabolites is diminished. Disruptions to the sleep-wake cycle have furthermore been linked to increased neuronal oxidative stress and impaired blood-brain barrier function - conditions that have also been proposed to play a role in the development and progression of AD. Notably, recent human and transgenic animal studies have demonstrated that AD-related pathophysiological processes that occur long before the clinical onset of AD, such as Aβ deposition in the brain, disrupt sleep and circadian rhythms. Collectively, as proposed in this review, these findings suggest the existence of a mechanistic interplay between AD pathogenesis and disrupted sleep-wake cycles, which is able to accelerate the development and progression of this disease.

Keywords: Aging; Amyloid beta; Blood brain barrier; Circadian misalignment; Neurodegeneration; Orexin; Oxidative stress; Sleep disruption; Slow-wave sleep; Tau.

Fig. 1

Temporal association between homeostatic sleep pressure and CSF concentrations of amyloid beta. The propensity to sleep is considered to be regulated by two interacting mechanisms: a circadian process (C) and a homeostatic process (S) [110]. Process C drives arousal and helps time the onset of normal sleep (driven by e.g., environmental light changes and meal patterns), whereas Process S drives sleep pressure and increases as wakefulness continues and decreases during slow-wave sleep (SWS), a sleep stage that predominates during the first 1/3 of the night. In humans, in a study where samples were collected via an indwelling lumbar catheter, both CSF Aβ40 and Aβ42 fluctuated by 25% with a diurnal pattern (labeled as Aβ in the figure) (higher during wakefulness and lower during sleep), with the lowest Aβ42 levels at around 10:00 h [2]. This corresponds to approximately 04:00 h in sleep time as there is a 6-h lag for brain Aβ to reach the lumbar space [12,57]. Abbreviations: Aβ, amyloid beta; Aβ40, amyloid beta peptide 1–40; Aβ42, amyloid beta peptide 1–42 CSF: cerebrospinal fluid.

Fig. 2

Overview of proposed mechanisms through which disruptions to the sleep-wake cycle form a positive feedback loop with AD pathogenesis in humans. Abbreviations: Aβ, amyloid beta; AD, Alzheimer disease; CNS, central nervous system; BBB, blood–brain barrier; EE, energy expenditure; NFTs, neurofibrillary tangles.

Fig. 3

Scheme illustrating the glymphatic system (primarily based on [37]). Akin to other cells in the body, brain cells are surrounded by interstitial fluid (ISF), which contains nutrients, proteins and other solutes essential for brain cell survival, but also includes extracellular waste molecules that may be neurotoxic if not cleared properly (e.g., amyloid beta, Aβ). By utilizing real-time assessments of tetramethylammonium diffusion and two-photon imaging in mice, a brain-specific system with a similar function as lymph vessels, for removing ISF from the brain, was discovered in 2012 [62], and termed the “glymphatic” system as it depends on glial cell functioning. This system promotes clearanceof soluble metabolites from the brain [37,62,68,111]. Following entry of CSF through the para-arterial space that surrounds penetrating arteries in the brain, CSF exchanges with parenchymal ISF, moving across the parenchyma. ISF and interstitial solutes are then cleared via exit into the para-venous space surrounding large-caliber cerebral veins [62]. This system was found to depend on the function of astroglial cells that express the protein aquaporin-4 (AQP4; a water channel), in a highly polarized manner along the cells’ perivascular endfeet, thus ensheathing the cerebral vasculature. When AQP4 was deleted, CSF influx decreased, coupled with a 70% reduction in ISF solute clearance. This suggests that the system is involved in clearance of substances such as Aβ, the clearance of which was also markedly reduced following deletion of AQP4 [62]. Abbreviations: Aβ, amyloid beta; AD, Alzheimer disease; AQP4, aquaporin-4; CSF, cerebrospinal fluid; ISF, interstitial fluid.