Impact of sleep disturbances on neurodegeneration: Insight from studies in animal models.

Chronic short sleep or extended wake periods are commonly observed in most industrialized countries. Previously neurobehavioral impairment following sleep loss was considered to be a readily reversible occurrence, normalized upon recovery sleep. Recent clinical studies suggest that chronic short sleep and sleep disruption may be risk factors for neurodegeneration. Animal models have been instrumental in determining whether disturbed sleep can injure the brain. We now understand that repeated periods of extended wakefulness across the typical sleep period and/or sleep fragmentation can have lasting effects on neurogenesis and select populations of neurons and glia. Here we provide a comprehensive overview of the advancements made using animal models of sleep loss to understand the extent and mechanisms of chronic short sleep induced neural injury.

Uptake and Toxicity of Hemin and Iron in Cultured Mouse Astrocytes.

Hemin is a breakdown product of the blood protein, hemoglobin and is responsible for much of the secondary damage caused following a hemorrhagic stroke. Hemin is toxic to cultured astrocytes and it is thought that this toxicity is due to iron that is liberated when hemin is degraded. However, free iron applied to astrocytes is not toxic and the reason for this discrepancy is unknown. The present study exposed primary astrocyte cultures from neonatal mice to hemin-iron (25 µM hemin) or non-hemin iron (25 µM ferric ammonium citrate; FAC) for 12 or 24 h. Perls' and Turnbull's staining, as well as measures of cell viability and iron accumulation, were used to assess the valency, solubility and distribution of iron within cells. While cells accumulated similar amounts of iron from both sources, hemin was shown to be highly toxic to astrocytes, whereas FAC was not. Iron released by the degradation of hemin was present in both valencies (Fe(2+) and Fe(3+)), was mostly soluble and did not induce ferritin expression in most cells, whereas non-hemin iron (from FAC) was present in astrocytes almost exclusively as insoluble Fe(3+) and it induced widespread ferritin expression. These results show that the cellular mechanisms for processing hemin-iron and non-hemin iron are very different. The data suggest that hemin-iron has a greater potential to damage astrocytes by participating in unregulated redox reactions.

Phenanthrolines protect astrocytes from hemin without chelating iron.

Hemin, the degradation product of hemoglobin, contributes to the neurodegeneration that occurs in the weeks following a hemorrhagic stroke. The breakdown of hemin in cells releases redox-active iron that can facilitate the production of toxic hydroxyl radicals. The present study used 3-week old primary cultures of mouse astrocytes to compare the toxicity of 33 µM hemin in the presence of the iron chelator 1,10-phenanthroline or its non-chelating analogue, 4,7-phenanthroline. This concentration of hemin killed approximately 75 % of astrocytes within 24 h. Both isoforms of phenanthroline significantly decreased the toxicity of hemin, with the non-chelating analogue providing complete protection at concentrations of 33 µM and above. The decrease in toxicity was associated with less cellular accumulation of hemin. Approximately 90 % of the hemin accumulated was not degraded, irrespective of treatment condition. These observations indicate that chelatable iron is not the cause of hemin toxicity. Cell-free experiments demonstrated that hemin can inactivate a molar excess of hydrogen peroxide (H2O2), and that the rate of inactivation is halved in the presence of either isoform of phenanthroline. We conclude that phenanthrolines may protect astrocytes by limiting hemin uptake and by impairing the capacity of intact hemin to interact with endogenous H2O2.

Late-in-life neurodegeneration after chronic sleep loss in young adult mice.

Chronic short sleep (CSS) is prevalent in modern societies and has been proposed as a risk factor for Alzheimer's disease (AD). In support, short-term sleep loss acutely increases levels of amyloid ß (Aß) and tau in wild type (WT) mice and humans, and sleep disturbances predict cognitive decline in older adults. We have shown that CSS induces injury to and loss of locus coeruleus neurons (LCn), neurons with heightened susceptibility in AD. Yet whether CSS during young adulthood drives lasting Aß and/or tau changes and/or neural injury later in life in the absence of genetic risk for AD has not been established. Here, we examined the impact of CSS exposure in young adult WT mice on late-in-life Aß and tau changes and neural responses in two AD-vulnerable neuronal groups, LCn and hippocampal CA1 neurons. Twelve months following CSS exposure, CSS-exposed mice evidenced reductions in CA1 neuron counts and volume, spatial memory deficits, CA1 glial activation, and loss of LCn. Aß 42 and hyperphosphorylated tau were increased in the CA1; however, amyloid plaques and tau tangles were not observed. Collectively the findings demonstrate that CSS exposure in the young adult mouse imparts late-in-life neurodegeneration and persistent derangements in amyloid and tau homeostasis. These findings occur in the absence of a genetic predisposition to neurodegeneration and demonstrate for the first time that CSS can induce lasting, significant neural injury consistent with some, but not all, features of late-onset AD.

Quantitative analysis of size and regional distribution of corpora amylacea in the hippocampal formation of obstructive sleep apnoea patients.

Corpora amylacea (CoA) are spherical aggregates of glucose polymers and proteins within the periventricular, perivascular and subpial regions of the cerebral cortex and the hippocampal cornu ammonis (CA) subfields. The present study quantified the distribution of CoA in autopsied hippocampi of patients with obstructive sleep apnoea (OSA) using ethanolamine-induced fluorescence. CoA were observed in 29 of 30 patients (96.7%). They were most abundant in periventricular regions (wall of lateral ventricle, alveus, fimbria and CA4), rarely found in the CA3 and CA1, and undetectable in the CA2 or subiculum. A spatiotemporal sequence of CoA deposition was postulated, beginning in the fimbria and progressively spreading around the subpial layer until they extended medially to the wall of the lateral ventricle and laterally to the collateral sulcus. This ranked CoA sequence was positively correlated with CoA packing density (count and area fraction) and negatively correlated with CoA minimum diameters (p < 0.05). Although this sequence was not correlated with age or body mass index (BMI), age was positively correlated with the mean and maximum diameters of CoA. These findings support the view that the spatiotemporal sequence of CoA deposition is independent of age, and that CoA become larger due to the accretion of new material over time.

Alzheimer's disease neuropathology in the hippocampus and brainstem of people with obstructive sleep apnea.

Obstructive sleep apnea (OSA) involves intermittent cessations of breathing during sleep. People with OSA can experience memory deficits and have reduced hippocampal volume; these features are also characteristic of Alzheimer's disease (AD), where they are accompanied by neurofibrillary tangles (NFTs) and amyloid beta (Aß) plaques in the hippocampus and brainstem. We have recently shown reduced hippocampal volume to be related to OSA severity, and although OSA may be a risk factor for AD, the hippocampus and brainstems of clinically verified OSA cases have not yet been examined for NFTs and Aß plaques. The present study used quantitative immunohistochemistry to investigate postmortem hippocampi of 34 people with OSA (18 females, 16 males; mean age 67 years) and brainstems of 24 people with OSA for the presence of NFTs and Aß plaques. OSA severity was a significant predictor of Aß plaque burden in the hippocampus after controlling for age, sex, body mass index (BMI), and continuous positive airway pressure (CPAP) use. OSA severity also predicted NFT burden in the hippocampus, but not after controlling for age. Although 71% of brainstems contained NFTs and 21% contained Aß plaques, their burdens were not correlated with OSA severity. These results indicate that OSA accounts for some of the "cognitively normal" individuals who have been found to have substantial Aß burdens, and are currently considered to be at a prodromal stage of AD.

Neuropathological investigation of cell layer thickness and myelination in the hippocampus of people with obstructive sleep apnea.

Obstructive sleep apnea (OSA) is commonly associated with memory impairments. Although MRI studies have found volumetric differences in the hippocampus of people with OSA compared with controls, MRI lacks the spatial resolution to detect changes in the specific regions of the hippocampus that process different types of memory. The present study performed histopathological investigations on autopsy brain tissue from 32 people with OSA (17 females and 15 males) to examine whether the thickness and myelination of the hippocampus and entorhinal cortex (EC) vary as a function of OSA severity. Increasing OSA severity was found to be related to cortical thinning in the molecular layer of the dentate gyrus (r2 = 0.136, p = 0.038), the CA1 (overall, r2 = 0.135, p = 0.039; layer 1, r2 = 0.157, p = 0.025; layer 2, r2 = 0.255, p = 0.003; and layer 3, r2 = 0.185, p = 0.014) and in some layers of the EC (layer 1, r2 = 0.186, p = 0.028; trend in layer 3, r2 = 0.124, p = 0.078). OSA severity was also related to decreased myelin in the deep layers but not the superficial layers of the EC (layer 6, r2 = 0.282, p = 0.006; deep white matter, r2 = 0.390, p = 0.001). Patients known to have used continuous positive airway pressure (CPAP) treatment showed no significant reductions in cortical thickness when compared with controls, suggesting that CPAP had a protective effect. However, CPAP did not protect against myelin loss. The regions of decreased cortical thickness and demyelination are locations of synaptic connections in both the polysynaptic (episodic and spatial) and direct (semantic) memory pathways and may underpin the impairments observed in episodic, semantic, and spatial memory in people with OSA.