Hippocampal neurogenesis response: What can we expect from two different models of hypertension?

Obstructive sleep apnea affects cognition: dual effects of intermittent hypoxia on neurons

Obstructive sleep apnea (OSA) is a common respiratory disorder. Multiple organs, especially the central nervous system (CNS), are damaged, and dysfunctional when intermittent hypoxia (IH) occurs during sleep for a long time. The quality of life of individuals with OSA is significantly impacted by cognitive decline, which also escalates the financial strain on their families. Consequently, the development of novel therapies becomes imperative. IH induces oxidative stress, endoplasmic reticulum stress, iron deposition, and neuroinflammation in neurons. Synaptic dysfunction, reactive gliosis, apoptosis, neuroinflammation, and inhibition of neurogenesis can lead to learning and long-term memory impairment. In addition to nerve injury, the role of IH in neuroprotection was also explored. While causing neuron damage, IH activates the neuronal self-repairing mechanism by regulating antioxidant capacity and preventing toxic protein deposition. By stimulating the proliferation and differentiation of neural stem cells (NSCs), IH has the potential to enhance the ratio of neonatal neurons and counteract the decline in neuron numbers. This review emphasizes the perspectives and opportunities for the neuroprotective effects of IH and informs novel insights and therapeutic strategies in OSA.

Role of Ero1α in cognitive impairment induced by chronic hypoxia

Recent reports suggested the endoplasmic reticulum stress (ERS)-associated pathway is involved with cognitive impairment in hypoxia condition. ERO1-like protein alpha (Ero1α), an endoplasmic reticulum membrane-bound N-glycoprotein, has been reported to promote oxidative protein folding. However, no studies have reported whether the Ero1α is trapped in hypoxia-induced neuronal loss through the ERS-associated pathways. In our study, this effect of Ero1α was investigated using C57BL/6J mice, the HT22 cells and primary rat neurons. C57BL/6J mice were modeled in a hypoxic chamber for 4 weeks. Behavioral tests were then carried out to test cognitive functions, including the Morris water maze and fear conditioning test. Proteomics showed that Ero1α distinctly upregulated compared with normoxia group and verified using western blotting. Flow cytometry and immunofluorescence were used to analyze the neuroprotective effect of inhibitor EN460 of Ero1α in the HT22 cells. In C57BL/6J mice, hypoxia significantly caused cognitive decline. Brain slice staining results were also used to confirm this effect. Western blot analysis demonstrated that Ero1α, ERS-associated proteins and apoptosis-associated proteins significantly increased in the hypoxia treated groups, further proliferation-related marker protein decreased. EN460, a selective endoplasmic reticulum oxidation 1 (ERO1) inhibitor, counteracted neuronal apoptosis and ameliorated neuronal cell proliferation in the HT22 cells. Taken together, our data indicate that hypoxia induces cognitive impairment, at least in part, by upregulating Ero1α which contributes to neuronal apoptosis through ERS signaling pathway, providing preliminary experimental evidence that the Ero1α is a promising therapeutic target in hypoxia-induced cognitive deficits.

Intermittent Hypoxia and Effects on Early Learning/Memory: Exploring the Hippocampal Cellular Effects of Pediatric Obstructive Sleep Apnea

This review provides an update on the neurocognitive phenotype of pediatric obstructive sleep apnea (OSA). Pediatric OSA is associated with neurocognitive deficits involving memory, learning, and executive functioning. Adenotonsillectomy (AT) is presently accepted as the first-line surgical treatment for pediatric OSA, but the executive function deficits do not resolve postsurgery, and the timeline for recovery remains unknown. This finding suggests that pediatric OSA potentially causes irreversible damage to multiple areas of the brain. The focus of this review is the hippocampus, 1 of the 2 major sites of postnatal neurogenesis, where new neurons are formed and integrated into existing circuitry and the mammalian center of learning/memory functions. Here, we review the clinical phenotype of pediatric OSA, and then discuss existing studies of OSA on different cell types in the hippocampus during critical periods of development. This will set the stage for future study using preclinical models to understand the pathogenesis of persistent neurocognitive dysfunction in pediatric OSA.

Stage-dependent effects of intermittent hypoxia influence the outcome of hippocampal adult neurogenesis

Over one billion adults worldwide are estimated to suffer from sleep apnea, a condition with wide-reaching effects on brain health. Sleep apnea causes cognitive decline and is a risk factor for neurodegenerative conditions such as Alzheimer’s disease. Rodents exposed to intermittent hypoxia (IH), a hallmark of sleep apnea, exhibit spatial memory deficits associated with impaired hippocampal neurophysiology and dysregulated adult neurogenesis. We demonstrate that IH creates a pro-oxidant condition that reduces the Tbr2⁺ neural progenitor pool early in the process, while also suppressing terminal differentiation of adult born neurons during late adult neurogenesis. We further show that IH-dependent cell-autonomous hypoxia inducible factor 1-alpha (HIF1a) signaling is activated in early neuroprogenitors and enhances the generation of adult born neurons upon termination of IH. Our findings indicate that oscillations in oxygen homeostasis, such as those found in sleep apnea, have complex stage-dependent influence over hippocampal adult neurogenesis.

Hypertension and Its Impact on Stroke Recovery: From a Vascular to a Parenchymal Overview

Hypertension is the first modifiable vascular risk factor accounting for 10.4 million deaths worldwide; it is strongly and independently associated with the risk of stroke and is related to worse prognosis. In addition, hypertension seems to be a key player in the implementation of vascular cognitive impairment. Long-term hypertension, complicated or not by the occurrence of ischemic stroke, is often reviewed on its vascular side, and parenchymal consequences are put aside. Here, we sought to review the impact of isolated hypertension or hypertension associated to stroke on brain atrophy, neuron connectivity and neurogenesis, and phenotype modification of microglia and astrocytes. Finally, we discuss the impact of antihypertensive therapies on cell responses to hypertension and functional recovery. This attractive topic remains a focus of continued investigation and stresses the relevance of including this vascular risk factor in preclinical investigations of stroke outcome.

Reduction in sympathetic tone in patients with obstructive sleep apnoea: is fixed CPAP more effective than APAP? A randomised, parallel trial protocol

INTRODUCTION

Obstructive sleep apnoea (OSA) is a prevalent disease associated with cardiovascular events. Hypertension is one of the major intermediary mechanisms leading to long-term cardiovascular adverse events. Intermittent hypoxia and hypercapnia associated with nocturnal respiratory events stimulate chemoreflexes, resulting in sympathetic overactivity and blood pressure (BP) elevation. Continuous positive airway pressure (CPAP) is the primary treatment for OSA and induces a small but significant reduction in BP. The use of auto-adjusting positive airway pressure (APAP) has increased in the last years and studies showed different ranges of BP reduction when comparing both modalities. However, the pathophysiological mechanisms implicated are not fully elucidated. Variations in pressure through the night inherent to APAP may induce persistent respiratory efforts and sleep fragmentation that might impair sympathovagal balance during sleep and result in smaller decreases in BP. Therefore, this double-blind randomised controlled trial aims to compare muscle sympathetic nerve activity (MSNA) assessed by microneurography (reference method for measuring sympathetic activity) after 1 month of APAP versus fixed CPAP in treatment-naive OSA patients. This present manuscript describes the design of our study, no results are presented herein. and is registered under the below reference number.

METHODS AND ANALYSIS

Adult subjects with newly diagnosed OSA (Apnoea-Hypopnoea Index >20/hour) will be randomised for treatment with APAP or fixed CPAP. Measurements of sympathetic activity by MSNA, heart rate variability and catecholamines will be obtained at baseline and after 30 days. The primary composite outcome will be the change in sympathetic tone measured by MSNA in bursts/min and bursts/100 heartbeats. Sample size calculation was performed with bilateral assumption. We will use the Student's t-test to compare changes in sympathetic tone between groups.

ETHICS AND DISSEMINATION

The protocol was approved by The French Regional Ethics Committee. The study started in March 2018 with primary completion expected to March 2019. Dissemination plans of the results include presentations at conferences and publication in peer-reviewed journals.

TRIAL REGISTRATION NUMBER

NCT03428516; Pre-results.

Intermittent Hypoxia Disrupts Adult Neurogenesis and Synaptic Plasticity in the Dentate Gyrus

Individuals with sleep apnea often exhibit changes in cognitive behaviors consistent with alterations in the hippocampus. It is hypothesized that adult neurogenesis in the dentate gyrus is an ongoing process that maintains normal hippocampal function in many mammalian species, including humans. However, the impact of chronic intermittent hypoxia (IH), a principal consequence of sleep apnea, on hippocampal adult neurogenesis remains unclear. Using a murine model, we examined the impact of 30 d of IH (IH₃₀) on adult neurogenesis and synaptic plasticity in the dentate gyrus. Although IH₃₀ did not affect paired-pulse facilitation, IH₃₀ suppressed long-term potentiation (LTP). Immunohistochemical experiments also indicate that IH perturbs multiple aspects of adult neurogenesis. IH₃₀ increased the number of proliferating Sox2⁺ neural progenitor cells in the subgranular zone yet reduced the number of doublecortin-positive neurons. Consistent with these findings, cell lineage tracing revealed that IH₃₀ increased the proportion of radial glial cells in the subgranular zone, yet decreased the proportion of adult-born neurons in the dentate gyrus. While administration of a superoxide anion scavenger during IH did not prevent neural progenitor cell proliferation, it mitigated the IH-dependent suppression of LTP and prevented adult-born neuron loss. These data demonstrate that IH causes both reactive oxygen species-dependent and reactive oxygen species-independent effects on adult neurogenesis and synaptic plasticity in the dentate gyrus. Our findings identify cellular and neurophysiological changes in the hippocampus that may contribute to cognitive and behavioral deficits occurring in sleep apnea.SIGNIFICANCE STATEMENT Individuals with sleep apnea experience periods of intermittent hypoxia (IH) that can negatively impact many aspects of brain function. Neurons are continually generated throughout adulthood to support hippocampal physiology and behavior. This study demonstrates that IH exposure attenuates hippocampal long-term potentiation and reduces adult neurogenesis. Antioxidant treatment mitigates these effects indicating that oxidative signaling caused by IH is a significant factor that impairs synaptic plasticity and reduces adult neurogenesis in the hippocampus.

The Hippocampal Neuro-Glio-Vascular Network: Metabolic Vulnerability and Potential Neurogenic Regeneration in Disease

Brain metabolism is a fragile balance between nutrient/oxygen supply provided by the blood and neuronal/glial demand. Small perturbations in these parameters are necessary for proper homeostatic functioning and information processing, but can also cause significant damage and cell death if dysregulated. During embryonic and early post-natal development, massive neurogenesis occurs, a process that continues at a limited rate in adulthood in two neurogenic niches, one in the lateral ventricle and the other in the hippocampal dentate gyrus. When metabolic demand does not correspond with supply, which can occur dramatically in the case of hypoxia or ischemia, or more subtly in the case of neuropsychiatric or neurodegenerative disorders, both of these neurogenic niches can respond—either in a beneficial manner, to regenerate damaged or lost tissue, or in a detrimental fashion—creating aberrant synaptic connections. In this review, we focus on the complex relationship that exists between the cerebral vasculature and neurogenesis across development and in disease states including hypoxic-ischemic injury, hypertension, diabetes mellitus, and Alzheimer’s disease. Although there is still much to be elucidated, we are beginning to appreciate how neurogenesis may help or harm the metabolically-injured brain, in the hopes that these insights can be used to tailor novel therapeutics to regenerate damaged tissue after injury.

Novel candidate genes important for asthma and hypertension comorbidity revealed from associative gene networks

BACKGROUND

Hypertension and bronchial asthma are a major issue for people's health. As of 2014, approximately one billion adults, or ~ 22% of the world population, have had hypertension. As of 2011, 235-330 million people globally have been affected by asthma and approximately 250,000-345,000 people have died each year from the disease. The development of the effective treatment therapies against these diseases is complicated by their comorbidity features. This is often a major problem in diagnosis and their treatment. Hence, in this study the bioinformatical methodology for the analysis of the comorbidity of these two diseases have been developed. As such, the search for candidate genes related to the comorbid conditions of asthma and hypertension can help in elucidating the molecular mechanisms underlying the comorbid condition of these two diseases, and can also be useful for genotyping and identifying new drug targets.

RESULTS

Using ANDSystem, the reconstruction and analysis of gene networks associated with asthma and hypertension was carried out. The gene network of asthma included 755 genes/proteins and 62,603 interactions, while the gene network of hypertension - 713 genes/proteins and 45,479 interactions. Two hundred and five genes/proteins and 9638 interactions were shared between asthma and hypertension. An approach for ranking genes implicated in the comorbid condition of two diseases was proposed. The approach is based on nine criteria for ranking genes by their importance, including standard methods of gene prioritization (Endeavor, ToppGene) as well as original criteria that take into account the characteristics of an associative gene network and the presence of known polymorphisms in the analysed genes. According to the proposed approach, the genes IL10, TLR4, and CAT had the highest priority in the development of comorbidity of these two diseases. Additionally, it was revealed that the list of top genes is enriched with apoptotic genes and genes involved in biological processes related to the functioning of central nervous system.

CONCLUSIONS

The application of methods of reconstruction and analysis of gene networks is a productive tool for studying the molecular mechanisms of comorbid conditions. The method put forth to rank genes by their importance to the comorbid condition of asthma and hypertension was employed that resulted in prediction of 10 genes, playing the key role in the development of the comorbid condition. The results can be utilised to plan experiments for identification of novel candidate genes along with searching for novel pharmacological targets.

High salt induced hypertension leads to cognitive defect

Although increasing evidences suggest a relationship between hypertension and brain function for years, it is still unclear whether hypertension constitutes a risk factor for cognitive decline and its underlying mechanism. In the present study, an experimental animal model of hypertension simply by feeding rats with high salt diet was employed. We found that long-term high salt intake caused a marked increase of systolic blood pressure linked to a declined regional cerebral blood flow. Fear conditioning and morris water maze behavioral test revealed that high salt diet induced hippocampal dependent spatial reference memory deficits, while a decreased synaptogenesis without neuronal loss in hippocampus was observed in high salt treated rats. Furthermore, we found that high salt induced a decrease of intracellular calcium, which inactivated CaMK II and resulted in dephosphorylation of CREB at Ser133. These findings suggest a novel etiopathogenic mechanism of cognitive deficit induced by hypertension, which is initiated by high salt diet.

A novel adjustable automated system for inducing chronic intermittent hypoxia in mice

Background

Sleep apnea is a chronic, widely underdiagnosed condition characterized by disruption of sleep architecture and intermittent hypoxia due to short cessations of breathing. It is a major independent risk factor for myocardial infarction, congestive heart failure and stroke as well as one of the rare modifiable risk factors for Alzheimer’s Dementia. Reliable animal disease models are needed to understand the link between sleep apnea and the various clinically linked disorders.

New method

An automated system for inducing hypoxia was developed, in which the major improvement was the possibility to efficiently adjust the length and intensity of hypoxia in two different periods. The chamber used a small volume of gas allowing for fast exchanges of different oxygen levels. The mice were kept in their cages adapted with the system on the cage lid. As a proof of principle, they were exposed to a three week period of intermittent hypoxia for 8 hours a day, with 90 s intervals of 5, 7% and 21% oxygen to validate the model. Treated (n = 8) and control mice (no hypoxia, n = 7) were handled in the same manner and their hippocampal brain regions compared by histology.

Results

The chamber provided a fast, reliable and precise intermittent hypoxia, without inducing noticeable side effects to the animals. The validation experiment showed that apoptotic neurons in the hippocampus were more numerous in the mice exposed to intermittent hypoxia than in the control group, in all tested hippocampal regions (cornu ammonis 1 (CA1) P <0.001; cornu ammonis 3 (CA3) P <0.001; and dentate gyrus (DG) P = 0.023). In both, control and hypoxic conditions, there was a significantly higher number of apoptotic neurons in the DG compared to the CA1 and CA3 subfields (P <0.001).

Conclusion

The new design of a hypoxic chamber provides a fast, adjustable and reliable model of obstructive sleep apnea, which was validated by apoptosis of hippocampal neurons.

Neonatal reflexes and behavior in hypertensive rats of ISIAH strain

Hypertension is one of the most common diseases in humans, and there is a special concern on the consequences of maternal hypertensive conditions for the health of newborns. An inherited stress-induced arterial hypertension (ISIAH) rat strain has been selected but only a few studies have addressed behavior in these rats. Body weight, neurodevelopmental reflexes, and neuronal density in the hippocampus were compared in ISIAH and normotensive WAG rats during their suckling period. Systolic and diastolic blood pressure (SBP, DBP), adult rat performance in the open field (OF), elevated plus maze (EPM), and novel object recognition (NOR) tests were evaluated at the age of 12-14weeks old. Body weight in pups did not differ significantly during the suckling period, while adult ISIAH rats were heavier than age-matched WAG rats and possessed the increased SBP and DBP. ISIAH pups were developmentally more advanced than WAG as indicated by grasp reflex and negative geotaxis reaction scores. This was associated with higher neuronal density in CA1 and CA3 hippocampal areas in ISIAH pups on postnatal day 6 as compared to WAG rats. Adult ISIAH rats demonstrated an increased locomotor and exploratory activity in the OF and EPM tests as well as low levels of anxiety. The NOR test revealed no significant difference in recognition but confirmed higher exploratory activity in ISIAH rats compared to WAG rats. The results indicate that hypertensive ISIAH rats feature accelerated development during their suckling period, and as adults, they are more active and less anxious than normotensive WAG rats.