Apolipoprotein E-Deficient Mice Exhibit Increased Vulnerability to Intermittent Hypoxia-Induced Spatial Learning Deficits

Modifiable Innate Biology within the Gut–Brain Axis for Alzheimer’s Disease

Alzheimer’s disease (AD) is a prototypical inflammation-associated loss of cognitive function, with approximately 90% of the AD burden associated with invading myeloid cells controlling the function of the resident microglia. This indicates that the immune microenvironment has a pivotal role in the pathogenesis of the disease. Multiple peripheral stimuli, conditioned by complex and varied interactions between signals that stem at the intestinal level and neuroimmune processes, are involved in the progression and severity of AD. Conceivably, the targeting of critical innate immune signals and cells is achievable, influencing immune and metabolic health within the gut–brain axis. Considerable progress has been made, modulating many different metabolic and immune alterations that can drive AD development. However, non-pharmacological strategies targeting immunometabolic processes affecting neuroinflammation in AD treatment remain general and, at this point, are applied to all patients regardless of disease features. Despite these possibilities, improved knowledge of the relative contribution of the different innate immune cells and molecules comprising the chronically inflamed brain network to AD pathogenesis, and elucidation of the network hierarchy, are needed for planning potent preventive and/or therapeutic interventions. Moreover, an integrative perspective addressing transdisciplinary fields can significantly contribute to molecular pathological epidemiology, improving the health and quality of life of AD patients. This review is intended to gather modifiable immunometabolic processes based on their importance in the prevention and management of AD.

Chronic Intermittent Hypoxia-Induced Aberrant Neural Activities in the Hippocampus of Male Rats Revealed by Long-Term in vivo Recording

Chronic intermittent hypoxia (CIH) occurs in obstructive sleep apnea (OSA), a common sleep-disordered breathing associated with malfunctions in multiple organs including the brain. How OSA-associated CIH impacts on brain activities and functions leading to neurocognitive impairment is virtually unknown. Here, by means of in vivo electrophysiological recordings via chronically implanted multi-electrode arrays in male rat model of OSA, we found that both putative pyramidal neurons and putative interneurons in the hippocampal CA1 subfield were hyper-excitable during the first week of CIH treatment and followed by progressive suppression of neural firing in the longer term. Partial recovery of the neuronal activities was found after normoxia treatment but only in putative pyramidal neurons. These findings correlated well to abnormalities in dendritic spine morphogenesis of these neurons. The results reveal that hippocampal neurons respond to CIH in a complex biphasic and bidirectional manner eventually leading to suppression of firing activities. Importantly, these changes are attributed to a larger extent to impaired functions of putative interneurons than putative pyramidal neurons. Our findings therefore revealed functional and structural damages in central neurons in OSA subjects.

Brain Activity after Intermittent Hypoxic Brain Condition in Rats

Hypoxic brain injury is accompanied by a decrease in various functions. It is also known that obstructive sleep apnea (OSA) can cause hypoxic brain injury. This study aimed to produce a model of an intermittent hypoxic brain condition in rats and determine the activity of the brain according to the duration of hypoxic exposure. Forty male Sprague–Dawley rats were divided into four groups: the control group (n = 10), the 2 h per day hypoxia exposure group (n = 10), the 4 h per day hypoxia exposure group (n = 10), and the 8 h per day hypoxia exposure group (n = 10). All rats were exposed to a hypoxic chamber containing 10% oxygen for five days. Positron emission tomography–computed tomography (PET-CT) brain images were acquired using a preclinical PET-CT scanner to evaluate the activity of brain metabolism. All the rats were subjected to normal conditions. After five days, PET-CT was performed to evaluate the recovery of brain metabolism. Western blot analysis and immunohistochemistry were performed with vascular endothelial growth factor (VEGF) and brain-derived neurotrophic factor (BDNF). The mean SUV was elevated in the 2 h per day and 4 h per day groups, and all brain regions showed increased metabolism except the amygdala on the left side, the auditory cortex on the right side, the frontal association cortex on the right side, the parietal association cortex on the right side, and the somatosensory cortex on the right side immediately after hypoxic exposure. However, there was no difference between 5 days rest after hypoxic exposure and control group. Western blot analysis revealed the most significant immunoreactivity for VEGF in the 2, 4, and 8 h per day groups compared with the control group and quantification of VEGF immunohistochemistry showed more expression in 2 and 4 h per day groups compared with the control group. However, there was no significant difference in immunoreactivity for BDNF among the groups. The duration of exposure to hypoxia may affect the activity of the brain due to angiogenesis after intermittent hypoxic brain conditions in rats.

Alzheimer's Disease, Sleep Disordered Breathing, and Microglia: Puzzling out a Common Link

Sleep Disordered Breathing (SDB) and Alzheimer's Disease (AD) are strongly associated clinically, but it is unknown if they are mechanistically associated. Here, we review data covering both the cellular and molecular responses in SDB and AD with an emphasis on the overlapping neuroimmune responses in both diseases. We extensively discuss the use of animal models of both diseases and their relative utilities in modeling human disease. Data presented here from mice exposed to intermittent hypoxia indicate that microglia become more activated following exposure to hypoxia. This also supports the idea that intermittent hypoxia can activate the neuroimmune system in a manner like that seen in AD. Finally, we highlight similarities in the cellular and neuroimmune responses between SDB and AD and propose that these similarities may lead to a pathological synergy between SDB and AD.

Associations Between the Apnea-Hypopnea Index During REM and NREM Sleep and Cognitive Functioning in a Cohort of Middle-Aged Adults

STUDY OBJECTIVES

Prior research has linked obstructive sleep apnea (OSA) to varied cognitive deficits. Additionally, OSA in rapid eye movement (REM) versus non-rapid eye movement (NREM) sleep has been shown to be a stronger predictor of outcomes such as hypertension. The present study aimed to investigate whether OSA-as characterized by the apnea-hypopnea index (AHI)-during REM and NREM sleep is associated with performance on a range of cognitive tasks. We also investigated whether the presence/absence of the apolipoprotein E4 allele (APOE4) modifies the associations between AHI during REM and NREM sleep and cognitive performance.

METHODS

A cross-sectional sample of 1,250 observations from 755 community-dwelling adults (mean [standard deviation] age, 62.3 [8.2] years) participating in the Wisconsin Sleep Cohort study was carried out by means of overnight polysomnography, paper-and-pencil cognitive tasks, and genetic data. Linear mixed effects models with repeated measures estimated associations of AHI during REM and NREM sleep with cognitive outcomes, stratified by APOE4 status (carrier versus noncarrier).

RESULTS

No significant associations were found between REM AHI and cognitive outcomes for either APOE4 carriers and non-carriers. Higher NREM AHI was associated with worse memory retention among APOE4 carriers; among noncarriers of APOE4, higher NREM AHI was associated with worse performance on a test of psychomotor speed, but better performance on two tests of executive function.

CONCLUSIONS

Sleep state-specific (REM, NREM) OSA may be differentially associated with varying dimensions of cognitive deficits in middle-aged to older adults, and such associations are likely to be modified by genetic factors, include APOE polymorphisms.

Related biomarkers of neurocognitive impairment in children with obstructive sleep apnea

OBJECTIVES

Different experiment approaches have demonstrated that children with obstructive sleep apnea (OSA) exhibit neurocognitive and behavioral deficits. This review summarized the potential biomarkers of OSA-associated neurocognitive impairment in children.

METHODS

A scoping review of studies on children with OSA that evaluated the potential value of different markers in identifying neurocognitive impairment was undertaken. Additionally, the biomarkers were categorized according to the different research methods, including brain imaging studies, serological indicators and urine markers.

RESULTS

Majority of the studies that evaluated blood biomarkers, plasma insulin growth factor-1 (IGF-1) and Alzheimer's disease (AD)-related biomarkers appeared to exhibit a favorable profile, and could discriminate between OSA children with or without neurocognitive impairments. Brain imaging studies and urinary neurotransmitters could also be helpful for screening OSA cognitive morbidity in children.

CONCLUSION

Due to limited research methods available in children, the cognitive susceptibility of children with OSA has been rarely studied. The main reason for this may be the limited research methods in children. Numerous study populations of children and complex psychological tests are required, which involve major labor and costs.Multi-center prospective studies are needed to identify suitable biomarkers for the timely prediction and effective intervention to prevent neurocognitive impairment in children with OSA and to explore further opportunities in this arena.

Chronic Intermittent Hypoxia Induces Robust Astrogliosis in an Alzheimer's Disease-Relevant Mouse Model

Sleep disturbances are a common early symptom of neurodegenerative diseases, including Alzheimer's disease (AD) and other age-related dementias, and emerging evidence suggests that poor sleep may be an important contributor to development of amyloid pathology. Of the causes of sleep disturbances, it is estimated that 10-20% of adults in the United States have sleep-disordered breathing (SDB) disorder, with obstructive sleep apnea accounting for the majority of the SBD cases. The clinical and epidemiological data clearly support a link between sleep apnea and AD; yet, almost no experimental research is available exploring the mechanisms associated with this correlative link. Therefore, we exposed an AD-relevant mouse model (APP/PS1 KI) to chronic intermittent hypoxia (IH) (an experimental model of sleep apnea) to begin to describe one of the potential mechanisms by which SDB could increase the risk of dementia. Previous studies have found that astrogliosis is a contributor to neuropathology in models of chronic IH and AD; therefore, we hypothesized that a reactive astrocyte response might be a contributing mechanism in the neuroinflammation associated with sleep apnea. To test this hypothesis, 10-11-month-old wild-type (WT) and APP/PS1 KI mice were exposed to 10 hours of IH, daily for four weeks. At the end of four weeks brains were analyzed from amyloid burden and astrogliosis. No effect was found for chronic IH exposure on amyloid-beta levels or plaque load in the APP/PS1 KI mice. A significant increase in GFAP staining was found in the APP/PS1 KI mice following chronic IH exposure, but not in the WT mice. Profiling of genes associated with different phenotypes of astrocyte activation identified GFAP, CXCL10, and Ggta1 as significant responses activated in the APP/PS1 KI mice exposed to chronic IH.

DNA binding protein HMGB1 secreted by activated microglia promotes the apoptosis of hippocampal neurons in diabetes complicated with OSA

Type 2 diabetes mellitus (T2DM) complicated with obstructive sleep apnea (OSA) may cause neuronal apoptosis and cognitive deficits, but the underlying mechanisms remain unclear. We aimed to determine the relationship between the activation of microglia and the apoptosis of hippocampal neurons, specifically in terms of high mobility group box-1 (HMGB1), after high glucose (HG) and intermittent hypoxia (IH) exposure. Diabetic KK-Ay mice and non-diabetic C57BL/6J mice (C57 mice) underwent IH or normoxia (control) exposure for 4 weeks. Cognitive function, microglial activation and hippocampal neuronal apoptosis were assessed after IH or normoxia exposure. Compared with C57 control mice, KK-Ay control mice exhibited increased cognitive dysfunction, microglial activation and hippocampal neuronal apoptosis. There were no differences between untreated KK-Ay control mice and C57 mice that had been exposed to IH. The abovementioned responses were aggravated in IH-exposed KK-Ay mice compared with control KK-Ay mice. In vitro, a cellular co-culture experiment showed that HG combined with IH could activate BV2 microglia, leading to the release of neuroinflammatory factors (ROS, TNF-α, IL-1β) and mediating the apoptosis of HT22 cells via the PI3K/Akt/GSK-3β signaling pathway. Meanwhile, HMGB1 was actively secreted into the extracellular environment from activated BV2 microglia. As a proinflammatory factor, it was able to sustain microglial activation by directly acting on those cells. The activation promoted positive feedback and aggravated neuronal damage further. In a cellular monoculture or co-culture system, HMGB1 siRNA was able to alleviate the activation of BV2 cells and the apoptosis of HT22 cells induced by HG combined with IH. Our object is to show that inhibition of HMGB1 may break the vicious cycle to prevent or treat neuroinflammation and hippocampal neuronal apoptosis caused by T2DM complicated with OSA.

Dysfunction of Nrf2-ARE Signaling Pathway: Potential Pathogenesis in the Development of Neurocognitive Impairment in Patients with Moderate to Severe Obstructive Sleep Apnea-Hypopnea Syndrome

The present study investigated the nuclear factor erythroid 2-related factor 2- (Nrf2-) antioxidant response element (ARE) signaling pathway in patients with moderate to severe obstructive sleep apnea-hypopnea syndrome (OSAHS). Their correlation with neurocognitive impairment metrics was investigated to explore potential pathogenesis in OSAHS. Forty-eight patients with OSAHS and 28 controls underwent testing with the Epworth Sleep Scale (ESS), MATRICS Consensus Cognitive Battery (MCCB), Stroop Color and Word Test, polysomnography (PSG), and measurements of the concentration of plasma superoxide dismutase (SOD) and thioredoxin (Trx). Further, 20 pairs of matched patients with OSAHS and controls were selected for measurement of the expression (protein and mRNA) of Nrf2 and of its downstream antioxidase, heme oxygenase-1 (HO-1), in peripheral mononuclear cells (PBMCs). Finally, correlations between neurocognitive impairment and the above metrics were analyzed. Expression of Nrf2 and HO-1 mRNA and protein in the PBMCs, as well as plasma SOD and Trx levels, were significantly reduced in patients with OSAHS. After adjusting for education, sex, age, and smoking index, the expression of Nrf2-ARE signaling pathway proteins (or mRNA) was closely correlated with sleep respiratory parameters. An inverse relationship was demonstrated between the expression of nuclear Nrf2 in PBMCs, concentration of plasma SOD and Trx, and apnea-hypopnea index (AHI) in patients with OSAHS. Trx, nuclear Nrf2 protein, and HO-1 protein were also negatively correlated with the percent of time that SaO₂ was less than 90% (TSat90). Total Nrf2 protein level was positively correlated with AHI and TSat90 and negatively correlated with minimum SaO₂ (LSaO₂), while nuclear Nrf2 protein and HO-1 protein were positively correlated with LSaO₂. Moreover, significant positive correlations were found between maze scores and expression of nuclear Nrf2 protein, HO-1 protein, and SOD and Trx levels. Furthermore, inverse relationships between total Nrf2 protein in PBMCs and HVLT-R and maze scores were found. Multiple linear regression showed plasma Trx concentration as a potential predictor of maze and BVMT-R scores. In conclusion, the expression of Nrf2-ARE molecules and related antioxidases is significantly decreased in patients with OSAHS and is correlated with neurocognitive dysfunction. The Nrf2-ARE signaling pathway may play a crucial role in neurocognitive impairment in patients with moderate to severe OSAHS. Further studies are needed to explore the exact mechanisms and potential treatment interventions.

Brain-derived erythropoietin protects from intermittent hypoxia-induced cardiorespiratory dysfunction and oxidative stress in mice

Study Objectives

Based on the fact that erythropoietin (Epo) administration in rodents protects against spatial learning and cognitive deficits induced by chronic intermittent hypoxia (CIH)-mediated oxidative damage, here we tested the hypothesis that Epo in the brain protects against cardiorespiratory disorders and oxidative stress induced by CIH in adult mice.

Methods

Adult control and transgenic mice overexpressing Epo in the brain only (Tg21) were exposed to CIH (21%-10% O2-10 cycles/hour-8 hours/day-7 days) or room air. After CIH exposure, we used the tail cuff method to measure arterial pressure, and whole-body plethysmography to assess the frequency of apneic episodes at rest, minute ventilation, and ventilatory responses to hypoxia and hypercapnia. Finally, the activity of pro-oxidant (XO-xanthine oxidase, and NADPH) and antioxidant (super oxide dismutase) enzymes was evaluated in the cerebral cortex and brainstem.

Results

Exposure of control mice to CIH significantly increased the heart rate and arterial pressure, the number of apneic events, and the ventilatory response to hypoxia and hypercapnia. Furthermore, CIH increased the ratio of pro-oxidant to antioxidant enzymes in cortex and brainstem tissues. Both physiological and molecular changes induced by CIH were prevented in transgenic Tg21 mice.

Conclusions

We conclude that the neuroprotective effect of Epo prevents oxidative damage in the brain and cardiorespiratory disorders induced by CIH. Considering that Epo is used in clinics to treat chronic kidney disease and stroke, our data show convincing evidence suggesting that Epo may be a promising alternative drug to treat sleep-disorder breathing.

Cognitive Deficits Are Attenuated in Neuroglobin Overexpressing Mice Exposed to a Model of Obstructive Sleep Apnea

Background: Obstructive sleep apnea (OSA) is a highly prevalent disease manifesting as intermittent hypoxia during sleep (IH) and is increasingly recognized as being independently associated with neurobehavioral deficits. These deficits may be due to increased apoptosis in the hippocampus and cerebral cortex, as well as increased oxidative stress and inflammation. It has been reported that neuroglobin (Ngb) is upregulated in response to hypoxia-ischemia insults and exhibits a protective role in ischemia-reperfusion brain injury. We hypothesized that transgenic overexpression of Ngb would attenuate spatial learning deficits in a murine model of OSA.

Methods:Wild-type mice and Ngb overexpressing male mice (Ngb-TG) were randomly assigned to either IH or room air (RA) exposures. The effects of IH during the light period on performance in a water maze spatial task were assessed, as well as anxiety and depressive-like behaviors using elevated plus maze (EPM) and forced swim tests. Cortical tissues from all the mice were extracted for biochemical studies for lipid peroxidation.

Results:Ngb TG mice exhibited increased Ngb immunoreactivity in brain tissues and IH did not elicit significant changes in Ngb expression in either Ngb-TG mice or WT mice. On a standard place training task in the water maze, Ngb-TG mice displayed preserved spatial learning, and were protected from the reduced spatial learning performances observed in WT mice exposed to IH. Furthermore, anxiety and depression levels were enhanced in WT mice exposed to IH as compared to RA controls, while alterations emerged in Ngb-TG mice exposed to IH. Furthermore, WT mice, but not Ngb-TG mice had significantly elevated levels of malondialdehyde in cortical lysates following IH exposures.

Conclusions:In a murine model of OSA, oxidative stress responses and neurocognitive and behavioral impairments induced by IH during sleep are attenuated by the neuroprotective effects of Ngb.

Late gestational intermittent hypoxia induces metabolic and epigenetic changes in male adult offspring mice

KEY POINTS

Late gestation during pregnancy has been associated with a relatively high prevalence of obstructive sleep apnoea (OSA). Intermittent hypoxia, a hallmark of OSA, could impose significant long-term effects on somatic growth, energy homeostasis and metabolic function in offspring. Here we show that late gestation intermittent hypoxia induces metabolic dysfunction as reflected by increased body weight and adiposity index in adult male offspring that is paralleled by epigenomic alterations and inflammation in visceral white adipose tissue. Fetal perturbations by OSA during pregnancy impose long-term detrimental effects manifesting as metabolic dysfunction in adult male offspring.

ABSTRACT

Pregnancy, particularly late gestation (LG), has been associated with a relatively high prevalence of obstructive sleep apnoea (OSA). Intermittent hypoxia (IH), a hallmark of OSA, could impose significant long-term effects on somatic growth, energy homeostasis, and metabolic function in offspring. We hypothesized that IH during late pregnancy (LG-IH) may increase the propensity for metabolic dysregulation and obesity in adult offspring via epigenetic modifications. Time-pregnant female C57BL/6 mice were exposed to LG-IH or room air (LG-RA) during days 13-18 of gestation. At 24 weeks, blood samples were collected from offspring mice for lipid profiles and insulin resistance, indirect calorimetry was performed and visceral white adipose tissues (VWAT) were assessed for inflammatory cells as well as for differentially methylated gene regions (DMRs) using a methylated DNA immunoprecipitation on chip (MeDIP-chip). Body weight, food intake, adiposity index, fasting insulin, triglycerides and cholesterol levels were all significantly higher in LG-IH male but not female offspring. LG-IH also altered metabolic expenditure and locomotor activities in male offspring, and increased number of pro-inflammatory macrophages emerged in VWAT along with 1520 DMRs (P < 0.0001), associated with 693 genes. Pathway analyses showed that genes affected by LG-IH were mainly associated with molecular processes related to metabolic regulation and inflammation. LG-IH induces metabolic dysfunction as reflected by increased body weight and adiposity index in adult male offspring that is paralleled by epigenomic alterations and inflammation in VWAT. Thus, perturbations to fetal environment by OSA during pregnancy can have long-term detrimental effects on the fetus, and lead to persistent metabolic dysfunction in adulthood.

Intermittent hypoxia causes NOX2-dependent remodeling of atrial connexins

BACKGROUND

Obstructive sleep apnea has been linked to the development of heart disease and arrhythmias, including atrial fibrillation. Since altered conduction through gap junction channels can contribute to the pathogenesis of such arrhythmias, we examined the abundance and distributions of the major cardiac gap junction proteins, connexin40 (Cx40) and connexin43 (Cx43) in mice treated with sleep fragmentation or intermittent hypoxia (IH) as animal models of the components of obstructive sleep apnea.

RESULTS

Wild type C57BL/6 mice or mice lacking NADPH 2 (NOX2) oxidase activity (gp91phox(-/Y)) were exposed to room air or to SF or IH for 6 weeks. Then, the mice were sacrificed, and atria and ventricles were immediately dissected. The abundances of Cx40 or Cx43 in atria and ventricles were unaffected by SF. In contrast, immunoblots showed that the abundance of atrial Cx40 and Cx43 and ventricular Cx43 were reduced in mice exposed to IH. qRT-PCR demonstrated significant reductions of atrial Cx40 and Cx43 mRNAs. Immunofluorescence microscopy revealed that the abundance and size of gap junctions containing Cx40 or Cx43 were reduced in atria by IH treatment of mice. However, no changes of connexin abundance or gap junction size/abundance were observed in IH-treated NOX2-null mice.

CONCLUSIONS

These results demonstrate that intermittent hypoxia (but not sleep fragmentation) causes reductions and remodeling of atrial Cx40 and Cx43. These alterations may contribute to the substrate for atrial fibrillation that develops in response to obstructive sleep apnea. Moreover, these connexin changes are likely generated in response to reactive oxygen species generated by NOX2.

Role of Oxidative Stress in the Neurocognitive Dysfunction of Obstructive Sleep Apnea Syndrome

Obstructive sleep apnea syndrome (OSAS) is characterized by chronic nocturnal intermittent hypoxia and sleep fragmentations. Neurocognitive dysfunction, a significant and extraordinary complication of OSAS, influences patients' career, family, and social life and reduces quality of life to some extent. Previous researches revealed that repetitive hypoxia and reoxygenation caused mitochondria and endoplasmic reticulum dysfunction, overactivated NADPH oxidase, xanthine oxidase, and uncoupling nitric oxide synthase, induced an imbalance between prooxidants and antioxidants, and then got rise to a series of oxidative stress (OS) responses, such as protein oxidation, lipid peroxidation, and DNA oxidation along with inflammatory reaction. OS in brain could trigger neuron injury especially in the hippocampus and cerebral cortex regions. Those two regions are fairly susceptible to hypoxia and oxidative stress production which could consequently result in cognitive dysfunction. Apart from continuous positive airway pressure (CPAP), antioxidant may be a promising therapeutic method to improve partially reversible neurocognitive function. Understanding the role that OS played in the cognitive deficits is crucial for future research and therapeutic strategy development. In this paper, recent important literature concerning the relationship between oxidative stress and cognitive impairment in OSAS will be summarized and the results can provide a rewarding overview for future breakthrough in this field.

Biomarkers of Alzheimer Disease in Children with Obstructive Sleep Apnea: Effect of Adenotonsillectomy

STUDY OBJECTIVE

Obese children are at increased risk for developing obstructive sleep apnea (OSA), and both of these conditions are associated with an increased risk for end-organ morbidities. Both OSA and obesity (OB) have been associated with increased risk for Alzheimer disease (AD). This study aimed to assess whether OSA and OB lead to increased plasma levels of 2 AD markers amyloid β protein 42 (Aβ42) and pre-senilin 1 (PS1).

METHODS

Fasting morning plasma samples from otherwise healthy children with a diagnosis of OB, OSA, or both (OSA+OB), and controls, and in a subset of children with OSA after adenotonsillectomy (T&A) were assayed for Aβ42 and PS1 levels using commercial enzyme-linked immunosorbent assay kits.

RESULTS

286 children (mean age of 7.2 ± 2.7 y) were evaluated. Compared to control subjects, OB children had similar Aβ42 (108.3 ± 31.7 pg/mL versus 83.6 ± 14.6 pg/mL) and PS1 levels (0.89 ± 0.44 ng/mL versus 0.80 ± 0.29 pg/mL). However, OSA children (Aβ42: 186.2 ± 66.7 pg/mL; P < 0.001; PS1: 3.42 ± 1.46 ng/mL; P < 0.001), and particularly OSA+OB children had significant elevations in both Aβ42 (349.4 ± 112.9 pg/mL; P < 0.001) and PS1 (PS1: 4.54 ± 1.16 ng/mL; P < 0.001) circulating concentrations. In a subset of 24 children, T&A resulted in significant reductions of Aβ42 (352.0 ± 145.2 versus 151.9 ± 81.4 pg/mL; P < 0.0001) and PS1 (4.82 ± 1.09 versus 2.02 ± 1.18 ng/mL; P < 0.0001).

CONCLUSIONS

Thus, OSA, and particularly OSA+OB, are associated with increased plasma levels of AD biomarkers, which decline upon treatment of OSA in a representative, yet not all- encompassing subset of patients, suggesting that OSA may accelerate AD-related processes even in early childhood. However, the cognitive and overall health-related implications of these findings remain to be defined.

Chronic intermittent hypoxia and obstructive sleep apnea: an experimental and clinical approach

Obstructive sleep apnea (OSA) is a prevalent sleep disorder considered as an independent risk factor for cardiovascular consequences, such as systemic arterial hypertension, ischemic heart disease, cardiac arrhythmias, metabolic disorders, and cognitive dysfunction. The pathogenesis of OSA-related consequence is assumed to be chronic intermittent hypoxia (IH) inducing alterations at the molecular level, oxidative stress, persistent systemic inflammation, oxygen sensor activation, and increase of sympathetic activity. Overall, these mechanisms have an effect on vessel permeability and are considered to be important factors for explaining vascular, metabolic, and cognitive OSA-related consequences. The present review attempts to examine together the research paradigms and clinical studies on the effect of acute and chronic IH and the potential link with OSA. We firstly describe the literature data on the mechanisms activated by acute and chronic IH at the experimental level, which are very helpful and beneficial to explaining OSA consequences. Then, we describe in detail the effect of IH in patients with OSA that we can consider "the human model" of chronic IH. In this way, we can better understand the specific pathophysiological mechanisms proposed to explain the consequences of IH in OSA.

GRK5 deficiency leads to susceptibility to intermittent hypoxia-induced cognitive impairment

Obstructive sleep apnea (OSA) leads to cognitive impairment in about 25% patients, though it remains elusive what makes one more susceptible than the other to be cognitively impaired. G protein-coupled receptor kinase-5 (GRK5) deficiency is recently found to render subjects more susceptible to cognitive impairment triggered by over-expression of Swedish mutant ß-amyloid precursor protein. This study is to determine whether GRK5 deficiency also renders subjects more susceptible to the OSA-triggered cognitive impairment. Both wild type (WT) and GRK5 knockout (KO) mice were placed in conditions absence and presence of intermittent hypoxia (IH) with 8%/21% O2 90-s cycle for 8h a day for a month, and then followed by behavioral assessments with battery of tasks. We found that the selected IH condition only induced marginally abnormal behavior (slightly elevated anxiety with most others unchanged) in the WT mice but it caused significantly more behavioral deficits in the KO mice, ranging from elevated anxiety, impaired balancing coordination, and impaired short-term spatial memory. These results suggest that GRK5 deficiency indeed makes the mice more susceptible to wide range of behavioral impairments, including cognitive impairments.

Apolipoprotein E ɛ2/ɛ3/ɛ4 variant in association with obstructive sleep apnoea and lipid profile: A meta-analysis

Objective

A meta-analysis of the association between haplotypical variants of the apolipoprotein E (APOE) gene (ɛ2/ɛ3/ɛ4) and obstructive sleep apnoea (OSA) risk and changes in lipid profile.

Methods

Electronic databases were searched to retrieve articles that provided data on APOE gene ɛ2/ɛ3/ɛ4 variants in patients with OSA and healthy controls. Data were extracted from eligible articles and statistical analyses were performed.

Results

The meta-analysis included 14 articles involving 19 study populations (3198 patients and 6031 controls). There was no significant association between the presence of the ɛ4 allele and OSA risk. The presence of ɛ4 was associated with significantly increased total cholesterol and decreased high-density lipoprotein cholesterol, compared with ɛ4 allele negative individuals. There was a low probability of publication bias but significant heterogeneity.

Conclusions

There was no association between APOE ɛ2/ɛ3/ɛ4 and OSA susceptibility. The presence of APOE ɛ4 was associated with changes in lipid profile.

Resveratrol attenuates intermittent hypoxia-induced macrophage migration to visceral white adipose tissue and insulin resistance in male mice

Chronic intermittent hypoxia during sleep (IH), as occurs in sleep apnea, promotes systemic insulin resistance. Resveratrol (Resv) has been reported to ameliorate high-fat diet-induced obesity, inflammation, and insulin resistance. To examine the effect of Resv on IH-induced metabolic dysfunction, male mice were subjected to IH or room air conditions for 8 weeks and treated with either Resv or vehicle (Veh). Fasting plasma levels of glucose, insulin, and leptin were obtained, homeostatic model assessment of insulin resistance index levels were calculated, and insulin sensitivity tests (phosphorylated AKT [also known as protein kinase B]/total AKT) were performed in 2 visceral white adipose tissue (VWAT) depots (epididymal [Epi] and mesenteric [Mes]) along with flow cytometry assessments for VWAT macrophages and phenotypes (M1 and M2). IH-Veh and IH-Resv mice showed initial reductions in food intake with later recovery, with resultant lower body weights after 8 weeks but with IH-Resv showing better increases in body weight vs IH-Veh. IH-Veh and IH-Resv mice exhibited lower fasting glucose levels, but only IH-Veh had increased homeostatic model assessment of insulin resistance index vs all 3 other groups. Leptin levels were preserved in IH-Veh but were significantly lower in IH-Resv. Reduced VWAT phosphorylated-AKT/AKT responses to insulin emerged in both Mes and Epi in IH-Veh but normalized in IH-Resv. Increases total macrophage counts and in M1 to M2 ratios occurred in IH-Veh Mes and Epi compared all other 3 groups. Thus, Resv ameliorates food intake and weight gain during IH exposures and markedly attenuates VWAT inflammation and insulin resistance, thereby providing a potentially useful adjunctive therapy for metabolic morbidity in the context of sleep apnea.

The promise of translational and personalised approaches for paediatric obstructive sleep apnoea: an 'Omics' perspective

Obstructive sleep apnoea (OSA) can result in significant morbidities including the cardiovascular, metabolic and neurocognitive systems. These effects are purportedly mediated via activation of inflammatory cascades and the induction of oxidative stress, ultimately resulting in cellular injury and dysfunction. While great advances have been made in sleep medicine research in the past decades, there are still wide gaps in our knowledge concerning the exact underlying pathophysiological mechanisms of OSA and consequences. Without resolving these issues, the reasons why patients with a similar severity of OSA can have markedly different clinical presentation and end-organ morbidity, that is, phenotype, will continue to remain elusive. This review aims to highlight the recent exciting discoveries in genotype-phenotype interactions, epigenetics, genomics and proteomics related to OSA. Just as PCR revolutionised the field of genetics, the potential power of 'Omics' promises to transform the field of sleep medicine, and provide critical insights into the downstream pathological cascades inherent to OSA, thereby enabling personalised diagnosis and management for this highly prevalent sleep disorder.

Effects of chronic sleep fragmentation on wake-active neurons and the hypercapnic arousal response

STUDY OBJECTIVES

Delayed hypercapnic arousals may occur in obstructive sleep apnea. The impaired arousal response is expected to promote more pronounced oxyhemoglobin desaturations. We hypothesized that long-term sleep fragmentation (SF) results in injury to or dysfunction of wake-active neurons that manifests, in part, as a delayed hypercapnic arousal response.

DESIGN

Adult male mice were implanted for behavioral state recordings and randomly assigned to 4 weeks of either orbital platform SF (SF4wk, 30 events/h) or control conditions (Ct4wk) prior to behavioral, histological, and locus coeruleus (LC) whole cell electrophysiological evaluations.

MEASUREMENTS AND RESULTS

SF was successfully achieved across the 4 week study, as evidenced by a persistently increased arousal index, P < 0.01 and shortened sleep bouts, P < 0.05, while total sleep/wake times and plasma corticosterone levels were unaffected. A multiple sleep latency test performed at the onset of the dark period showed a reduced latency to sleep in SF4wk mice (P < 0.05). The hypercapnic arousal latency was increased, Ct4wk 64 ± 5 sec vs. SF4wk 154 ± 6 sec, P < 0.001, and remained elevated after a 2 week recovery (101 ± 4 sec, P < 0.001). C-fos activation in noradrenergic, orexinergic, histaminergic, and cholinergic wake-active neurons was reduced in response to hypercapnia (P < 0.05-0.001). Catecholaminergic and orexinergic projections into the cingulate cortex were also reduced in SF4wk (P < 0.01). In addition, SF4wk resulted in impaired LC neuron excitability (P < 0.01).

CONCLUSIONS

Four weeks of sleep fragmentation (SF4wk) impairs arousal responses to hypercapnia, reduces wake neuron projections and locus coeruleus neuronal excitability, supporting the concepts that some effects of sleep fragmentation may contribute to impaired arousal responses in sleep apnea, which may not reverse immediately with therapy.

Pathological consequences of intermittent hypoxia in the central nervous system

Intermittent hypoxia (IH) is a frequent occurrence in clinical settings. In the last decades, evidence has emerged implicating the gas exchange alterations and sleep disruption associated with those disorders in the high prevalence of cognitive and behavioral deficits afflicting these patients. In an effort to better characterize the role of IH, and to identify potential mechanisms of IH-induced central nervous system (CNS) dysfunction, a large number of rodent models have been recently developed. The cumulative evidence confirms that IH indeed induces a heterotopic pattern of injury in the brain, particularly affecting cortical, subcortical, and hippocampal regions, ultimately leading to neuronal apoptosis and activation of microglia. These IH-induced deleterious processes exhibit substantial variability across the lifespan, are under substantial modulatory influences of diet, physical or intellectual activity, and genetic factors, and preferentially recruit oxidative stress and inflammatory pathways.

Association of sleep disordered breathing and cognitive deficit in APOE ε4 carriers

STUDY OBJECTIVES

The aim of the study was to determine whether apolipoprotein E epsilon 4 genotype (APOE4) modifies the association of sleep disordered breathing (SDB) with cognitive function in a middle-aged population.

DESIGN

Cross-sectional analysis of a community-dwelling cohort.

SETTINGS

Sleep laboratory at the Clinical Research Unit of the University of Wisconsin Hospitals and Clinics.

PARTICIPANTS

There were 755 adults from the Wisconsin Sleep Cohort who provided a total of 1,843 polysomnography and cognitive evaluations (most participants were assessed multiple times at approximately 4-y intervals); 56% males, average age 53.9 years (range 30-81 years).

INTERVENTIONS

None.

MEASUREMENT AND RESULTS

In-laboratory overnight polysomnography was used to assess SDB. Cognition was evaluated by a battery of six neurocognitive tests assessing memory and learning, attention, executive function, and psychomotor efficiency. The APOE4 genotype (ε3/ε4 or ε4/ ε4) was identified in 200 participants. Data were analyzed using linear mixed-effects models, accounting for multiple observations per participant. Cognitive test scores were regressed on SDB categories (AHI < 5, 5 ≤ AHI < 15, AHI ≥ 15); APOE4 and their interaction; and age, education, sex, and body mass index. There was no statistically significant association between SDB and cognitive performance among APOE4-negative individuals. However, in APOE4-positive individuals, those with AHI ≥ 15 had significantly worse performance on the Auditory Verbal Learning Test and the Controlled Oral Word Association Test.

CONCLUSIONS

In APOE4-positive individuals, moderate to severe sleep disordered breathing (AHI ≥ 15) was associated with poorer performance on cognitive tests that require both memory and executive function engagement.

Obstructive sleep apnea and delirium: exploring possible mechanisms

INTRODUCTION

Obstructive sleep apnea (OSA) is a medical disorder strongly associated with multiple comorbidities and postoperative complications. Current evidence suggests that OSA disturbs fundamental biochemical processes, leading to low-grade systemic inflammation and oxidative stress. Animal models have shown that OSA may lead to apoptosis of central neurons. In clinical studies, oxygen desaturation index and sleep fragmentation have been shown to be independently associated with cognitive dysfunction. Moreover, in several studies, patients with OSA were shown to have decreased brain activation in multiple brain areas.

OSA AND DELIRIUM

The possibility of an association between OSA and delirium has been highlighted in several case reports. The first prospective study of the possible link between apnea and delirium showed that the presence of OSA was independently associated with the occurrence of delirium after knee replacement surgery.

CONCLUSIONS

Therefore, we suggest that OSA should be considered as a risk factor for delirium, and clinicians should assess patients for OSA and related risk factors prior to surgery. However, further research is required to shed light on the mechanisms connecting these disorders and on whether the treatment of OSA affects the incidence of delirium.

Genotype-phenotype interactions in pediatric obstructive sleep apnea

Pediatric sleep disordered breathing (PSDB) is not only a very frequent condition affecting 2-4% of all children, but is also associated with an increased risk for a variety of manifestations underlying end-organ injury and dysfunction that impose both immediate and potentially long-term morbidities and corresponding inherent elevations in healthcare costs. One of the major problems with the creation of valid algorithms aiming to stratify diagnostic and treatment prioritization lies in our current inability to predict and identify those children who are most at-risk for PSDB-induced adverse consequences. Thus, improved our understanding of the mechanisms governing phenotype variance in PSDB is essential. Here, we examine some of the potential underpinnings of phenotypic variability in PSDB, and further propose a conceptual framework aimed at facilitating the process of advancing knowledge in this frequent disorder.

Adverse cognitive effects of high-fat diet in a murine model of sleep apnea are mediated by NADPH oxidase activity

Intermittent hypoxia (IH) during sleep, such as occurs in sleep apnea (SA), induces increased NADPH oxidase activation and deficits in hippocampal learning and memory. Similar to IH, high fat-refined carbohydrate diet (HFD), a frequent occurrence in patients with SA, can also induce similar oxidative stress and cognitive deficits under normoxic conditions, suggesting that excessive NADPH oxidase activity may underlie CNS dysfunction in both conditions. The effect of HFD and IH during the light period on two forms of spatial learning in the water maze as well as on markers of oxidative stress was assessed in male mice lacking NADPH oxidase activity (gp91phox⁻/Y) and wild-type littermates fed on HFD. On a standard place training task, gp91phox⁻/Y displayed normal learning, and was protected from the spatial learning deficits observed in wild-type littermates exposed to IH. Moreover, anxiety levels were increased in wild-type mice exposed to HFD and IH as compared to controls, while no changes emerged in gp91phox⁻/Y mice. Additionally, wild-type mice, but not gp91phox⁻/Y mice, had significantly elevated levels of malondialdehyde (MDA) and 8-hydroxydeoxyguanosine (8-OHdG) in hippocampal lysates following IH-HFD exposures. The cognitive deficits of obesity and westernized diets and those of sleep disorders that are characterized by IH during sleep are both mediated, at least in part, by excessive NADPH oxidase activity.

Exogenous erythropoietin administration attenuates intermittent hypoxia-induced cognitive deficits in a murine model of sleep apnea

Background

In rodents, exposure to intermittent hypoxia (IH), a hallmark of obstructive sleep apnea (OSA), is associated with neurobehavioral impairments, increased apoptosis in the hippocampus and cortex, as well as increased oxidant stress and inflammation. Such findings are markedly attenuated in rodents exposed to sustained hypoxia 9SH) of similar magnitude. The hypoxia-sensitive gene erythropoietin (EPO) has emerged as a major endogenous neuroprotectant, and could be involved in IH-induced neuronal dysfunction.

Methods and Results

IH induced only transiently increased expression of EPO mRNA in hippocampus, which was continued in (SH)-exposed mice. IH, but not SH, adversely affected two forms of spatial learning in the water maze, and increased markers of oxidative stress. However, on a standard place training task, mice treated with exogenously administered EPO displayed normal learning, and were protected from the spatial learning deficits observed in vehicle-treated (C) littermates exposed to IH. Moreover, anxiety levels were increased in IH as compared to normoxia, while no changes in anxiety emerged in EPO-treated mice. Additionally, C mice, but not EPO-treated IH-exposed mice had significantly elevated levels of NADPH oxidase expression, as well as increased MDA and 8-OHDG levels in cortical and hippocampal lysates.

Conclusions

The oxidative stress responses and neurobehavioral impairments induced by IH during sleep are mediated, at least in part, by imbalances between EPO expression and increased NADPH oxidase activity, and thus pharmacological agents targeting EPO expression in CNS may provide a therapeutic strategy in sleep-disordered breathing.

Human apolipoprotein E4 targeted replacement in mice reveals increased susceptibility to sleep disruption and intermittent hypoxia

Intermittent hypoxia (IH) and sleep fragmentation (SF) are major manifestations of sleep apnea, a frequent condition in aging humans. Sleep perturbations are frequent in Alzheimer's disease (AD) and may underlie the progression of disease. We hypothesized that acute short-term IH, SF, and their combination (IH+SF) may reveal unique susceptibility in sleep integrity in a murine model of AD. The effects of acute IH, SF, and IH+SF on sleep architecture, delta power, sleep latency, and core body temperature were assessed in adult male human ApoE4-targeted replacement mice (hApoE4) and wild-type (WT) controls. Slow wave sleep (SWS) was significantly reduced, and rapid eye movement (REM) sleep was almost abolished during acute exposure to IH alone and IH+SF for 6 h in hApoE4, with milder effects in WT controls. Decreased delta power during SWS did not show postexposure rebound in hApoE4 unlike WT controls. IH and IH+SF induced hypothermia, which was more prominent in hApoE4 than WT controls. Mice subjected to SF also showed sleep deficits but without hypothermia. hApoE4 mice, unlike WT controls, exhibited increased sleep propensity, especially following IH and IH+SF, suggesting limited ability for sleep recovery in hApoE4 mice. These findings substantiate the potential impact of IH and SF in modulating sleep architecture and sleep homeostasis including maintenance of body temperature. Furthermore, the increased susceptibility and limited recovery ability of hApoE4 mice to sleep apnea suggests that early recognition and treatment of the latter in AD patients may restrict the progression and clinical manifestations of this frequent neurodegenerative disorder.

Chronic intermittent hypoxia-induced deficits in synaptic plasticity and neurocognitive functions: a role for brain-derived neurotrophic factor

Obstructive sleep apnea (OSA) is well known for its metabolic as well as neurobehavioral consequences. Chronic intermittent hypoxia (IH) is a major component of OSA. In recent years, substantial advances have been made in elucidating the cellular and molecular mechanisms underlying the effect of chronic IH on neurocognitive functions, many of which are based on studies in animal models. A number of hypotheses have been put forward to explain chronic IH-induced neurological dysfunctions. Among these, the roles of oxidative stress and apoptosis-related neural injury are widely accepted. Here, focusing on results derived from animal studies, we highlight a possible role of reduced expression of brain-derived neurotrophic factor (BDNF) in causing impairment in long-term synaptic plasticity and neurocognitive functions during chronic IH. The possible relationship between BDNF and previous findings on this subject will be elucidated.

Mouse intermittent hypoxia mimicking apnoea of prematurity: effects on myelinogenesis and axonal maturation

Premature babies are at high risk for both infantile apnoea and long-term neurobehavioural deficits. Recent studies suggest that diffuse structural changes in brain white matter are a positive predictor of poor cognitive outcomes. Since oligodendrocyte maturation, myelination, axon development, and synapse formation mainly occur in the third trimester of gestation and first postnatal year, infantile apnoea could lead to and/or exaggerate white matter impairments in preterm neonates. Therefore, we investigated oligodendroglia and axon development in a neonatal mouse model of intermittent hypoxia between postnatal days 2 and 10. During critical phases of central nervous system development, intermittent hypoxia induced hypomyelination in the corpus callosum, striatum, fornix, and cerebellum, but not in the pons or spinal cord. Intermittent hypoxia-elicited alterations in myelin-forming processes were reflected by decreased expression of myelin proteins, including MBP, PLP, MAG, and CNPase, possibly due to arrested maturation of oligodendrocytes. Ultrastructural abnormalities were apparent in the myelin sheath and axon. Immature oligodendrocytes were more vulnerable to neonatal intermittent hypoxia exposures than developing axons, suggesting that hypomyelination may contribute, at least partially, to axonal deficits. Insufficient neurofilament synthesis with anomalous components of neurofilament subunits, β-tubulin, and MAP2 isoforms indicated immaturity of axons in intermittent hypoxia-exposed mouse brains. In addition, down-regulation of synapsin I, synaptophysin, and Gap-43 phosphorylation suggested a potential stunt in axonogenesis and synaptogenesis. The region-selective and complex impairment in brain white matter induced by intermittent hypoxia was further associated with electrophysiological changes that may underlie long-term neurobehavioural sequelae.

Exogenous growth hormone attenuates cognitive deficits induced by intermittent hypoxia in rats

Sleep disordered breathing (SDB), which is characterized by intermittent hypoxia (IH) during sleep, causes substantial cardiovascular and neurocognitive complications and has become a growing public health problem. SDB is associated with suppression of growth hormone (GH) secretion, the latter being integrally involved in the growth, development, and function of the CNS. Since GH treatment is able to attenuate neurocognitive deficits in a hypoxic-ischemic stroke model, GH, GH receptor (GHR) mRNA expression, and GH protein expression were assessed in rat hippocampus after exposures to chronic sustained hypoxia (CH, 10% O(2)) or IH (10% O(2) alternating with 21% O(2) every 90 s). In addition, the effect of GH treatment (50 μg/kg daily s.c. injection) on erythropoietin (EPO), vascular endothelial growth factor (VEGF), heme oxygenase-1 (HO-1), and GLUT-1 mRNA expression and neurobehavioral function was assessed. CH significantly increased GH mRNA and protein expression, as well as insulin-like growth factor-1 (IGF-1). In contrast, IH only induced a moderate increase in GH mRNA and a slight elevation in GH protein at day 1, but no increases in IGF-1. CH, but not IH, up-regulated GHR mRNA in the hippocampus. IH induced marked neurocognitive deficits compared with CH or room air (RA). Furthermore, exogenous GH administration increased hippocampal mRNA expression of IGF-1, EPO, and VEGF, and not only reduced IH-induced hippocampal injury, but also attenuated IH-induced cognitive deficits. Thus, exogenous GH may provide a viable therapeutic intervention to protect IH-vulnerable brain regions from SDB-associated neuronal loss and associated neurocognitive dysfunction.

Intermittent hypoxia-induced cognitive deficits are mediated by NADPH oxidase activity in a murine model of sleep apnea

BACKGROUND

In rodents, exposure to intermittent hypoxia (IH), a hallmark of obstructive sleep apnea (OSA), is associated with neurobehavioral impairments, increased apoptosis in the hippocampus and cortex, as well as increased oxidant stress and inflammation. Excessive NADPH oxidase activity may play a role in IH-induced CNS dysfunction.

METHODS AND FINDINGS

The effect of IH during light period on two forms of spatial learning in the water maze and well as markers of oxidative stress was assessed in mice lacking NADPH oxidase activity (gp91phox(_/Y)) and wild-type littermates. On a standard place training task, gp91phox(_/Y) displayed normal learning, and were protected from the spatial learning deficits observed in wild-type littermates exposed to IH. Moreover, anxiety levels were increased in wild-type mice exposed to IH as compared to room air (RA) controls, while no changes emerged in gp91phox(_/Y) mice. Additionally, wild-type mice, but not gp91phox(_/Y) mice had significantly elevated levels of NADPH oxidase expression and activity, as well as MDA and 8-OHDG in cortical and hippocampal lysates following IH exposures.

CONCLUSIONS

The oxidative stress responses and neurobehavioral impairments induced by IH during sleep are mediated, at least in part, by excessive NADPH oxidase activity, and thus pharmacological agents targeting NADPH oxidase may provide a therapeutic strategy in sleep-disordered breathing.

Inflammatory pathways in children with insufficient or disordered sleep

Sleep is not only an essential physiological function, but also serves important roles in promoting growth, maturation, and overall health of children and adolescents. There is increasing interest regarding the impact of sleep and its disorders on the regulation of inflammatory processes and end-organ morbidities, particularly in the context of metabolic and cardiovascular diseases (CVD) and their complications. Obstructive sleep apnea syndrome (OSAS) is an increasingly common health problem in children, and in the last decade, the emergence of increasing obesity rates has further led to remarkable increases in the prevalence of OSAS, along with more prominent neurocognitive, behavioral, cardiovascular and metabolic morbidities. Although the underlying mechanisms leading to OSAS-induced morbidities are likely multi-factorial, and remain to be fully elucidated, activation of inflammatory pathways by OSAS has emerged as an important pathophysiological component of the end-organ injury associated with this disorder. To this effect, it would appear that OSAS could be viewed as a chronic, low-grade inflammatory disorder. Furthermore, the concurrent presence of obesity and OSAS poses a theoretically increased risk of OSAS-related complications. In this review, we will critically review the current state of research regarding the impact of insufficient and disrupted sleep and OSAS on the immune processes and inflammatory pathways that underlie childhood OSAS as a distinctive systemic inflammatory condition in children, and will explore potential interactions between OSAS and obesity.

Reactive oxygen species and the brain in sleep apnea

Rodents exposed to intermittent hypoxia (IH), a model of obstructive sleep apnea (OSA), manifest impaired learning and memory and somnolence. Increased levels of reactive oxygen species (ROS), oxidative tissue damage, and apoptotic neuronal cell death are associated with the presence of IH-induced CNS dysfunction. Furthermore, treatment with antioxidants or overexpression of antioxidant enzymes is neuroprotective during IH. These findings mimic clinical cases of OSA and suggest that ROS may play a key causal role in OSA-induced neuropathology. Controlled production of ROS occurs in multiple subcellular compartments of normal cells and de-regulation of such processes may result in excessive ROS production. The mitochondrial electron transport chain, especially complexes I and III, and the NADPH oxidase in the cellular membrane are the two main sources of ROS in brain cells, although other systems, including xanthine oxidase, phospholipase A2, lipoxygenase, cyclooxygenase, and cytochrome P450, may all play a role. The initial evidence for NADPH oxidase and mitochondrial involvement in IH-induced ROS production and neuronal injury unquestionably warrants future research efforts.

Brain-derived neurotrophic factor rescues and prevents chronic intermittent hypoxia-induced impairment of hippocampal long-term synaptic plasticity

Obstructive sleep apnea (OSA) is a common sleep and breathing disorder characterized by repeated episodes of hypoxemia. OSA causes neurocognitive deficits including perception and memory impairment but the underlying mechanisms are unknown. Here we show that in a mouse model of OSA, chronic intermittent hypoxia treatment impairs both early- and late-phase long-term potentiation (LTP) in the hippocampus. In intermittent hypoxia-treated mice the excitability of CA1 neurons was reduced and hippocampal brain-derived neurotrophic factor (BDNF) was down-regulated. We further showed that exogenous application of BDNF restored the magnitude of LTP in hippocampal slices from hypoxia-treated mice. In addition, microinjection of BDNF into the brain of the hypoxic mice prevented the impairment in LTP. These data suggest that intermittent hypoxia impairs hippocampal neuronal excitability and reduces the expression of BDNF leading to deficits in LTP and memory formation. Thus, BDNF level may be a novel therapeutic target for alleviating OSA-induced neurocognitive deficits.

Lack of association of the APOE epsilon 4 allele with the risk of obstructive sleep apnea: meta-analysis and meta-regression

STUDY OBJECTIVES

Reports on the association of polymorphisms in the gene encoding apolipoprotein E (APOE)--a vital macromolecule in cholesterol metabolism--with obstructive sleep apnea (OSA) have provided conflicting results. Our objective was to meta-analytically synthesize the existing evidence for the association of the APOE epsilon4 allele with the risk of OSA.

DESIGN

Random effects meta-analysis and meta-regression.

SETTING

Genetic epidemiological studies reporting the association of APOE epsilon4 allele with OSA susceptibility.

PATIENTS OR PARTICIPANTS

Synthesis of APOE epsilon4 allele data from 6,508 subjects including 1,901 cases of OSA and 4,607 controls.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Eight studies were included in the random effects meta-analysis; the summary effect size measured as odds ratio (OR) for association of the APOE epsilon4 allele with the risk of OSA was found to be 1.13 (95% confidence interval 0.86-1.47). There was a statistically significant heterogeneity (I2 = 72%, P = 0.001) across study results that was not explained by the mean age, proportion of males, or the proportion possessing the APOE epsilon4 allele or when grouped based on the geographic location of the study.

CONCLUSIONS

The hypothesis that the APOE epsilon4 allele may be causally associated with OSA cannot be supported on the basis of published literature.

Vascular effects of intermittent hypoxia

Obstructive sleep apnea is characterized by repeated upper airway obstruction during sleep and affects between 5% and 20% of the population. Epidemiological studies reveal that sleep apnea and associated intermittent hypoxemia increase the risk for hypertension and vascular disease but the mechanisms underlying these effects are incompletely understood. This review reports the results of rodent models of intermittent hypoxia (IH) and relates them to the observed hemodynamic and vascular consequences of sleep apnea. These animal studies have demonstrated that IH exposure in the absence of any other comorbidity causes hypertension, endothelial dysfunction, and augmented constrictor sensitivity, all due at least in part to increased vascular oxidative stress. Animal studies have used a variety of exposure paradigms to study intermittent hypoxia and these different exposure protocols can cause hypocapnia or hypercapnia-or maintain eucapnia-with accompanying alterations in plasma pH. It appears that these different profiles of arterial blood gases can lead to divergent results but the impact of these differences is still being investigated. Overall, the studies in rodents have clearly demonstrated that the vascular and hemodynamic impact of intermittent hypoxia provides a strong rationale for treating clinical sleep apnea to prevent the resulting cardiovascular morbidity and mortality.

Spatial learning and memory deficits following exposure to 24 h of sleep fragmentation or intermittent hypoxia in a rat model of obstructive sleep apnea

Obstructive sleep apnea is primarily characterized by hypoxemia due to frequent apneic episodes and fragmentation of sleep due to the brief arousals that terminate the apneic episodes. Though neurobehavioral deficits frequently accompany sleep apnea, the relative roles of hypoxia versus sleep fragmentation are difficult to separate in apneic patients. Here, we assessed cognitive function as measured by water maze in the Fischer/Brown Norway (FBN) rat, comparing 24 h of sleep interruption (SI) to 24 h of intermittent hypoxia (IH), in order to dissociate their relative contributions to cognitive impairment. For SI, automated treadmills were used to induce brief ambulation in rats every 2 min, either prior to, or after, initial water maze acquisition training. IH was simulated by cycling environmental oxygen levels between 6% and 19% every 2 min, again either prior to, or after, acquisition. Twenty-four hours of IH exposure had no significant effect on either acquisition or retention, irrespective of whether IH occurred prior to, or after, acquisition. To replicate previous work, another group of rats, exposed to 3 days of IH (10 h/day) prior to acquisition, had impaired performance during acquisition. A comparison of the 24 h IH and 3 day IH findings suggests that a minimum amount of IH exposure is necessary to produce detectable spatial memory impairments. Although SI before acquisition had no effect on acquisition or later retention of the hidden platform location, SI after acquisition robustly impaired retention, indicating that spatial memory consolidation is more susceptible to the effects of sleep disruption than is the acquisition (learning) of spatial information.

Obstructive sleep apnea and cardiovascular disease: a perspective and future directions

Data from animal and human studies provide a biological plausibility to the notion that obstructive sleep apnea activates pathways that lead to insulin resistance, atherosclerosis and hypertension. Sleep apnea thus activates the same pathways as does obesity. That obstructive sleep apnea is a risk factor for cardiovascular disease is supported by epidemiological association studies. Longitudinal cohort studies also provide evidence that patients with untreated severe sleep apnea have an increased rate of cardiovascular events. But these studies, while highly suggestive, do not provide the evidence needed to convince the skeptic. This would only be obtained by randomized treatment trials with hard cardiovascular endpoints such as cardiac events and deaths. While such studies are in the planning stages, they will be challenging. There are issues about randomizing individuals with severe sleep apnea and excessive sleepiness into no therapy, since they are at known increased risk for car crashes. Thus, lack of therapy puts others on the road at risk as well as the subject with sleep apnea. There is, moreover, the concern that treating obstructive sleep apnea in very obese individuals will have little impact, since any effect of therapy for OSA will be overwhelmed by the effects of obesity itself. Data from randomized treatment trials for cardiovascular endpoints will likely not be available for many years. In the interim, physicians need to consider how to treat such patients. It is proposed that given that CPAP treatment for obstructive sleep apnea is highly effective and essentially totally safe, and that the evidence is suggestive that sleep apnea is a risk factor for cardiovascular disease, then we propose all patients with severe sleep apnea should be treated to reduce cardiovascular risk.

A pilot study of sleep, cognition, and respiration under 4 weeks of intermittent nocturnal hypoxia in adult humans

STUDY OBJECTIVES

A pilot study to examine the effects of intermittent nocturnal hypoxia on sleep, respiration and cognition in healthy adult humans.

METHODS

Participants were eight healthy, non-smoking subjects (four male, four female), mean age of 26.4+/-5.2 years, and BMI 22.3+/-2.6 kg/m(2), exposed to 9h of intermittent hypoxia between the hours of 10 P.M. and 7 A.M. for 28 consecutive nights. At a simulated altitude of 13,000 feet (FIO(2) 0.13), intermittent hypoxia was achieved by administering nasal nitrogen, alternating with brief (approximately 5s) boluses of nasal oxygen. Pre- and post-exposure assessments included polysomnography, attention (20-min Psychomotor Vigilance Test), working memory (10-min verbal 2 and 3-back), Multiple Sleep Latency Test, and the Rey Auditory Verbal Learning Test. Obstructive and non-obstructive respiratory events were scored.

RESULTS

Overall sleep quality showed worsening trends but no statistically significant change following exposure. There was no difference after hypoxia in sleepiness, encoding, attention or working memory. Hyperoxic central apneas and post-hyperoxic respiratory instability were noted as special features of disturbed respiratory control induced by intermittent nocturnal hypoxia.

CONCLUSIONS

In this model, exposure to nocturnal intermittent hypoxia for 4 weeks caused no significant deficits in subjective or objective alertness, vigilance, or working memory.

The multiple challenges of obstructive sleep apnea in children: morbidity and treatment

PURPOSE OF REVIEW

To delineate some of the major morbid phenotypes that have emerged in pediatric obstructive sleep apnea (OSA), address new concepts in our understanding of OSA-associated morbidities, and elaborate on innovative therapeutic schemes that may improve outcomes for this condition. In addition, the conceptual framework whereby a childhood condition such as OSA can be linked to specific adult diseases will be presented.

RECENT FINDINGS

OSA in children is a frequent condition that affects up to 3% of nonobese, otherwise healthy children. In recent years, increased awareness of OSA and changes in obesity rates in children have contributed to significant changes in disease prevalence and clinical presentation, such that distinct morbidity-related phenotypes have become apparent. Furthermore, oxidative stress and systemic inflammatory pathways are mechanistically involved in the pathophysiology of OSA-associated morbidity. Adenotonsillectomy, the treatment of choice for pediatric OSA, may not be as efficacious as previously thought. Alternative nonsurgical therapies have started to emerge and may become an essential component of treatment.

SUMMARY

Pediatric OSA, particularly when obesity is concurrently present, is associated with substantial end-organ morbidities that primarily but not exclusively affect central nervous and cardiovascular systems. These morbidities are pathophysiologically mediated by inflammatory and free radical mediators. Although adenotonsillectomy remains the first line of treatment, more critical assessment of its role is needed, and incorporation of nonsurgical approaches to pediatric OSA seems warranted.