Neuroadaptive Biochemical Mechanisms of Remote Ischemic Conditioning.

This review summarizes the currently known biochemical neuroadaptive mechanisms of remote ischemic conditioning. In particular, it focuses on the significance of the pro-adaptive effects of remote ischemic conditioning which allow for the prevention of the neurological and cognitive impairments associated with hippocampal dysregulation after brain damage. The neuroimmunohumoral pathway transmitting a conditioning stimulus, as well as the molecular basis of the early and delayed phases of neuroprotection, including anti-apoptotic, anti-oxidant, and anti-inflammatory components, are also outlined. Based on the close interplay between the effects of ischemia, especially those mediated by interaction of hypoxia-inducible factors (HIFs) and steroid hormones, the involvement of the hypothalamic-pituitary-adrenocortical system in remote ischemic conditioning is also discussed.

Mechanisms underlying the health benefits of intermittent hypoxia conditioning.

Intermittent hypoxia (IH) is commonly associated with pathological conditions, particularly obstructive sleep apnoea. However, IH is also increasingly used to enhance health and performance and is emerging as a potent non-pharmacological intervention against numerous diseases. Whether IH is detrimental or beneficial for health is largely determined by the intensity, duration, number and frequency of the hypoxic exposures and by the specific responses they engender. Adaptive responses to hypoxia protect from future hypoxic or ischaemic insults, improve cellular resilience and functions, and boost mental and physical performance. The cellular and systemic mechanisms producing these benefits are highly complex, and the failure of different components can shift long-term adaptation to maladaptation and the development of pathologies. Rather than discussing in detail the well-characterized individual responses and adaptations to IH, we here aim to summarize and integrate hypoxia-activated mechanisms into a holistic picture of the body's adaptive responses to hypoxia and specifically IH, and demonstrate how these mechanisms might be mobilized for their health benefits while minimizing the risks of hypoxia exposure.

Molecular Mechanisms of Adaptation to Hypoxia.

Oxygen is one of the most important elements, ensuring the vital activity of the body [...].

Differences in the Autophagy Response to Hypoxia in the Hippocampus and Neocortex of Rats.

Autophagy is a regulated mechanism of degradation of misfolded proteins and organelles in the cell. Neurons are highly differentiated cells with extended projections, and therefore, their functioning largely depends on the mechanisms of autophagy. For the first time in an animal model using immunohistochemistry, dot analysis, and qRT-PCR, the autophagy (macroautophagy) activity in neurons of two brain regions (hippocampus and neocortex) under normoxia and after exposure to hypoxia was studied. It was found that under normoxia, the autophagic activity was higher in the hippocampal neurons than in the neocortex of rats. In the hippocampus, the exposure of rats to hypoxia resulted in a decrease in the content of autophagy markers LC3 and p62, which was followed by activation of the autophagy-related gene expression. In the neocortex, no changes in these marker proteins were observed after the exposure to hypoxia. These data indicate that the neurons in the hippocampus and neocortex differ in the autophagy response to hypoxia, which may reflect the physiological and functional differences of the pyramidal cells of these brain regions and may to some extent account for the extreme vulnerability of the CA1 hippocampal neurons and relatively high resistance of the neocortical neurons to hypoxia.

Intermittent Hypoxic Training as an Effective Tool for Increasing the Adaptive Potential, Endurance and Working Capacity of the Brain.

This review is devoted to the phenomenon of intermittent hypoxic training and is aimed at drawing the attention of researchers to the necessity of studying the mechanisms mediating the positive, particularly neuroprotective, effects of hypoxic training at the molecular level. The review briefly describes the historical aspects of studying the beneficial effects of mild hypoxia, as well as the use of hypoxic training in medicine and sports. The physiological mechanisms of hypoxic adaptation, models of hypoxic training and their effectiveness are summarized, giving examples of their beneficial effects in various organs including the brain. The review emphasizes a high, far from being realized at present, potential of hypoxic training in preventive and clinical medicine especially in the area of neurodegeneration and age-related cognitive decline.

Glucocorticoid-Dependent Mechanisms of Brain Tolerance to Hypoxia.

Adaptation of organisms to stressors is coordinated by the hypothalamic-pituitary-adrenal axis (HPA), which involves glucocorticoids (GCs) and glucocorticoid receptors (GRs). Although the effects of GCs are well characterized, their impact on brain adaptation to hypoxia/ischemia is still understudied. The brain is not only the most susceptible to hypoxic injury, but also vulnerable to GC-induced damage, which makes studying the mechanisms of brain hypoxic tolerance and resistance to stress-related elevation of GCs of great importance. Cross-talk between the molecular mechanisms activated in neuronal cells by hypoxia and GCs provides a platform for developing the most effective and safe means for prevention and treatment of hypoxia-induced brain damage, including hypoxic pre- and post-conditioning. Taking into account that hypoxia- and GC-induced reprogramming significantly affects the development of organisms during embryogenesis, studies of the effects of prenatal and neonatal hypoxia on health in later life are of particular interest. This mini review discusses the accumulated data on the dynamics of the HPA activation in injurious and non-injurious hypoxia, the role of the brain GRs in these processes, interaction of GCs and hypoxia-inducible factor HIF-1, as well as cross-talk between GC and hypoxic signaling. It also identifies underdeveloped areas and suggests directions for further prospective studies.

Comparative Analysis of Pathobiochemical Changes in Major Depression and Post-Traumatic Stress Disorder.

Comparative analysis of available literature data on the pathogenetic neuroendocrine mechanisms of depression and post-traumatic stress disorder (PTSD) is provided in this review to identify their common features and differences. We discuss the multidirectional modifications of the activity of cortical and subcortical structures of the brain, levels of neurotransmitters and their receptors, and functions of the hypothalamic-pituitary-adrenocortical axis in depression and PTSD. The analysis shows that these disorders are examples of opposite failures in the system of adaptive stress response of the body to stressful psychotraumatic events. On this basis, it is concluded that the currently widespread use of similar approaches to treat these disorders is not justified, despite the significant similarity of their anxiety-depressive symptoms; development of differential therapeutic strategies is required.

Long-Term Effects of Prenatal Severe Hypoxia on Central and Peripheral Components of the Glucocorticoid System in Rats.

INTRODUCTION: Prenatal hypoxia is a risk factor for the development of numerous neurological disorders. It is known that the maternal stress response to hypoxia determines the epigenetic impairment of the perinatal expression of glucocorticoid receptors (GR) in the hippocampus of the progeny, but so far no detailed study of how this affects the functional state of the glucocorticoid system during further ontogenesis has been performed. OBJECTIVE: The goal of the present study was to examine the long-term effects of the prenatal hypoxia on the functioning of the glucocorticoid system throughout life. METHODS: Prenatal severe hypobaric hypoxia (PSH) was induced in the critical period of embryonic hippocampal formation on days 14-16 of gestation in a hypobaric chamber (180 Torr, 5% oxygen, 3 h). The activity of central (hippocampus) and peripheral (liver) components of the glucocorticoid system was assessed in 1-day-old (newborn), 2-week-old (juvenile), 3-month-old (adult), and 18-month-old (aged) male rats. RESULTS: The PSH resulted in continuously elevated baseline corticosterone blood levels in the adult and aged rats. The chronic elevation of the corticosterone levels was accompanied by a progressive deficit of the GR expression in the liver, increased hepatic glycogen content, dysregulated glucose-6-phosphatase activity, and eventually hypoglycemia. Elevated corticosterone appears to result from the impairment of the mechanisms of glucocorticoid negative feedback since a substantial decrease in both the total number of GR and their nuclear localization was observed already in the hippocampus of newborn rat pups and persisted throughout life. Corresponding stable hippocampal downregulation of GR-dependent genes was observed as well. Suppression of the maternal glucocorticoid stress response to hypoxia by metyrapone injection to pregnant rats prior to each hypoxic challenge considerably reduced corticosterone over-response to hypoxia and prevented reduced hippocampal GR. CONCLUSIONS: Our findings demonstrate that in progeny a deficit of hippocampal GR resulting from maternal glucocorticoid response to hypoxia remains stable throughout life and is accompanied by severe disturbances of baseline glucocorticoid levels and its peripheral reception. Negative consequences of PSH can be prevented by injection with an inhibitor of corticosterone synthesis (metyrapone) to pregnant females undergoing hypoxia.

Pharmacological HIF1 Inhibition Eliminates Downregulation of the Pentose Phosphate Pathway and Prevents Neuronal Apoptosis in Rat Hippocampus Caused by Severe Hypoxia.

The pentose phosphate pathway (PPP) of glucose metabolism in the brain serves as a primary source of NADPH which in turn plays a crucial role in multiple cellular processes, including maintenance of redox homeostasis and antioxidant defense. In our model of protective mild hypobaric hypoxia in rats (3MHH), an inverse correlation between hypoxia-inducible factor-1 (HIF1) activity and mRNA levels of glucose-6-phosphate dehydrogenase (G6PD), the key enzyme of PPP, was observed. In the present study, it was demonstrated that severe hypobaric hypoxia (SH) induced short-term upregulation of HIF1 alpha-subunit (HIF1α) in the hippocampal CA1 subfield and decreased the activity of G6PD. The levels of NADPH were also reduced, promoting oxidative stress, triggering apoptosis, and neuronal loss. Injection of a HIF1 inhibitor (HIF1i), topotecan hydrochloride (5 mg/kg, i.p.), before SH prevented the upregulation of HIF1α and normalized G6PD activity. In addition, HIF1i injection caused an increase in NADPH levels, normalization of total glutathione levels and of the cellular redox status as well as suppression of free-radical and apoptotic processes. These results demonstrate a new molecular mechanism of post-hypoxic cerebral pathology development which involves HIF1-dependent PPP depletion and support a recently suggested injurious role of HIF1 activation in the acute phase of cerebral hypoxia/ischemia. Application of PPP stimulators in early post-hypoxic/ischemic period might represent a promising neuroprotective strategy. Graphical abstract HIF1-dependent down-regulation of the pentose phosphate pathway contributes to the hypoxia-induced oxidative stress and neuronal apoptosis in the rat hippocampus.

Neuroprotective action of PHD inhibitors is predominantly HIF-1-independent: An Editorial for 'Sex differences in neonatal mouse brain injury after hypoxia-ischemia and adaptaquin treatment' on page 759.

Hypoxia-inducible factor (HIF-1) as the primary factor mediating gene-dependent cellular responses to hypoxia represents an attractive target for the therapeutic interventions. The current Editorial comments on an as yet underestimated facet of HIF-1-related research. The activity of HIF-1 is being regulated by the availability of its α-subunit HIF-1α, which undergoes quick degradation. The process of degradation is initiated by prolyl 4-hydroxylase (PHD). PHD is an oxygen-dependent enzyme and therefore is inactivated in hypoxia, in turn resulting in HIF-1α stabilization, its dimerization with HIF-1ß subunit thereby producing the transcriptionally active factor. It has been suggested that pharmacological inhibition of PHD activity might give the same results. Indeed, a large body of evidence on beneficial effects of PHD inhibitors has been accumulated in multiple laboratory and clinical trials. In addition to them, a paper by Li and colleagues published in this issue of Journal of Neurochemistry also reports that inhibition of PHD by adaptaquin reduces hypoxic-ischemic brain injury in a neonatal mouse model. When dissecting the underlying molecular mechanisms, Li and colleagues surprisingly found that the observed effects appear to be independent of HIF-1. These findings draw attention back to the question about possible HIF-1 effects independent of PHD inhibitors, which has been raised several years ago but has not received sufficient attention so far, and is being discussed in this Editorial. One of the possible mechanisms might be ascribed to the ferroptosis pathway affected by PHD inhibitors but this question needs further careful studies, as well as clarification of other mechanisms possibly involved. Even if they represent a prospective therapeutic strategy, the lack of current knowledge about endogenous targets of PHD inhibitors, except for PHD, calls for a careful and balanced approach toward their clinical use.

Neuroprotective Mechanism of Hypoxic Post-conditioning Involves HIF1-Associated Regulation of the Pentose Phosphate Pathway in Rat Brain.

Post-conditioning is exposure of an injured organism to the same harmful factors but of milder intensity which mobilizes endogenous protective mechanisms. Recently, we have developed a novel noninvasive post-conditioning (PostC) protocol involving three sequential episodes of mild hypobaric hypoxia which exerts pronounced neuroprotective action. In particular, it prevents development of pathological cascades caused by severe hypobaric hypoxia (SH) such as cellular loss, lipid peroxidation, abnormal neuroendocrine responses and behavioural deficit in experimental animals. Development of these post-hypoxic pathological effects has been associated with the delayed reduction of hypoxia-inducible factor 1 (HIF1) regulatory α-subunit levels in rat hippocampus, whereas PostC up-regulated it. The present study has been aimed at experimental examination of the hypothesis that intrinsic mechanisms underlying the neuroprotective and antioxidant effects of PostC involves HIF1-dependent stimulation of the pentose phosphate pathway (PPP). We have observed that SH leads to a decrease of glucose-6-phosphate dehydrogenase (G6PD) activity in the hippocampus and neocortex of rats as well as to a reduction in NADPH and total glutathione levels. This depletion of the antioxidant defense system together with excessive lipid peroxidation during the reoxygenation phase resulted in increased oxidative stress and massive cellular death observed after SH. In contrast, PostC led to normalization of G6PD activity, stabilization of the NADPH and total glutathione levels and thereby resulted in recovery of the cellular redox state and prevention of neuronal death. Our data suggest that stabilization of the antioxidant system via HIF1-associated PPP regulation represents an important neuroprotective mechanism enabled by PostC.

Brain, antibiotics, and microbiota - how do they interplay?: An Editorial for 'Antibiotics-induced modulation of large intestinal microbiota altered aromatic amino acid profile and expression of neurotransmitters in the hypothalamus of piglets' on page 219.

The microbiome and its cross-talk with the brain have drawn increasing attention lately, since imbalances in the gut microbiota's composition may result in pathogenic dysfunctions affecting brain functioning up to development of neurodegenerative and mental diseases. The current Editorial discusses a study by Gao and coworkers in the current issue of the Journal of Neurochemistry in which the authors use a model of antibiotic-induced dysbiosis - targeted infusion of antibiotics into the gut - to assess if microbiotic metabolites exert effects on local neurotransmitter expression or contribute to the gut-brain axis. The authors mechanistically link distal ileal infusion of antibiotics with a change in the levels of microbial metabolites that affect the expression of neurotransmitters in the brain and thereby can participate in the fine-tuning of the hypothalamic functions, including regulation of visceral and neuroendocrine processes, stress responses, mood and anxiety. Their study thus represents an important step towards our understanding of the brain-gut axis, with the potential to advance therapeutics.

Neuroprotective effect of hypobaric hypoxic postconditioning is accompanied by dna protection and lipid peroxidation changes in rat hippocampus.

The present study was performed to explore the effect of severe hypobaric hypoxia (180Torr, 3h) and severe hypoxia followed by hypoxic postconditioning (360Torr, 2h, 3 episodes) on DNA fragmentation and dynamics of lipid peroxidation products in rat hippocampus. The severe hypoxia induced intense DNA fragmentation in the hippocampus. A persistent decrease of thiobarbituric acid reactive substances in the hippocampus was also detected in response to severe hypoxia while the levels of Schiff bases did not significantly change. The postconditioning prevented severe hypoxia-induced DNA fragmentation, returned the levels of thiobarbituric acid reactive substances to the baseline and decreased the levels of Schiff bases. These findings indicate that the neuroprotective effect of hypoxic postconditioning on hippocampal neurons detected as suppression of hypoxia-induced DNA fragmentation is accompanied by the changes in lipid peroxidation processes.

Acetylation of histones in neocortex and hippocampus of rats exposed to different modes of hypobaric hypoxia: Implications for brain hypoxic injury and tolerance.

Acetylation of nucleosome histones results in relaxation of DNA and its availability for the transcriptional regulators, and is generally associated with the enhancement of gene expression. Although it is well known that activation of a variety of pro-adaptive genes represents a key event in the development of brain hypoxic/ischemic tolerance, the role of epigenetic mechanisms, in particular histone acetylation, in this process is still unexplored. The aim of the present study was to investigate changes in acetylation of histones in vulnerable brain neurons using original well-standardized model of hypobaric hypoxia and preconditioning-induced tolerance of the brain. Using quantitative immunohistochemistry and Western blot, effects of severe injurious hypobaric hypoxia (SH, 180mm Hg, 3h) and neuroprotective preconditioning mode (three episodes of 360mm Hg for 2h spaced at 24h) on the levels of the acetylated proteins and acetylated H3 Lys24 (H3K24ac) in the neocortex and hippocampus of rats were studied. SH caused global repression of the acetylation processes in the neocortex (layers II-III, V) and hippocampus (CA1, CA3) by 3-24h, and this effect was prevented by the preconditioning. Moreover, hypoxic preconditioning remarkably increased the acetylation of H3K24 in response to SH in the brain areas examined. The preconditioning hypoxia without subsequent SH also stimulated acetylation processes in the neocortex and hippocampus. The moderately enhanced expression of the acetylated proteins in the preconditioned rats was maintained for 24h, whereas acetylation of H3K24 was intense but transient, peaked at 3h. The novel data obtained in the present study indicate that large activation of the acetylation processes, in particular acetylation of histones might be essential for the development of brain hypoxic tolerance.

Current insights into the molecular mechanisms of hypoxic pre- and postconditioning using hypobaric hypoxia.

Exposure of organisms to repetitive mild hypoxia results in development of brain hypoxic/ischemic tolerance and cross-tolerance to injurious factors of a psycho-emotional nature. Such preconditioning by mild hypobaric hypoxia functions as a "warning" signal which prepares an organism, and in particular the brain, to subsequent more harmful conditions. The endogenous defense processes which are mobilized by hypoxic preconditioning and result in development of brain tolerance are based on evolutionarily acquired gene-determined mechanisms of adaptation and neuroprotection. They involve an activation of intracellular cascades including kinases, transcription factors and changes in expression of multiple regulatory proteins in susceptible areas of the brain. On the other hand they lead to multilevel modifications of the hypothalamic-pituitary-adrenal endocrine axis regulating various functions in the organism. All these components are engaged sequentially in the initiation, induction and expression of hypoxia-induced tolerance. A special role belongs to the epigenetic regulation of gene expression, in particular of histone acetylation leading to changes in chromatin structure which ensure access of pro-adaptive transcription factors activated by preconditioning to the promoters of target genes. Mechanisms of another, relatively novel, neuroprotective phenomenon termed hypoxic postconditioning (an application of mild hypoxic episodes after severe insults) are still largely unknown but according to recent data they involve apoptosis-related proteins, hypoxia-inducible factor and neurotrophins. The fundamental data accumulated to date and discussed in this review open new avenues for elaboration of the effective therapeutic applications of hypoxic pre- and postconditioning.

Neocortical pCREB and BDNF expression under different modes of hypobaric hypoxia: role in brain hypoxic tolerance in rats.

Preconditioning with repetitive mild hypobaric hypoxia is known to increase tolerance of susceptible brain neurons to severe hypoxia, whereas a single trial of mild hypoxia has been ineffective. In the present study, the effects of three-trial and one-trial hypobaric preconditioning on the expression of the protective transcription factor phosphorylated CREB (pCREB) and neurotrophin BDNF, before and after severe hypobaric hypoxia, have been comparatively studied in the neocortex of rats. As revealed by quantitative immunocytochemistry, the severe hypobaric hypoxia (180 Torr, 3h) substantially down-regulated the levels of pCREB and BDNF in cortical neurons assessed 24h after the treatment. One trial of mild hypoxia (360 Torr, 2h) also reduced by half the number of BDNF-expressing cells, but had no effect on pCREB expression in the neocortex. In contrast, the exposure to three trials of mild hypoxia at 24h intervals considerably up-regulated pCREB and BDNF levels in the neocortex of rats. Only preconditioning by three trials of mild hypoxia (360 Torr, 2h, 24h intervals), but not a single trial preconditioning, was neuroprotective significantly enhancing the pCREB and BDNF neuronal expression in response to severe hypoxic challenge. The results of the present study indicate that development of the neuronal hypoxic tolerance induced by the three-trial mild hypoxic preconditioning is apparently associated with activation of CREB and BDNF expression.

Postconditioning by mild hypoxic exposures reduces rat brain injury caused by severe hypoxia.

A potent neuroprotective effect of ischemic postconditioning has previously been described using cerebral artery occlusion but this is not a practical therapeutic option. The present study has been performed to determine whether postconditioning by mild episodes of hypobaric hypoxia (hypoxic postconditioning, HP) can reduce post-hypoxic brain injury in rats. Male Wistar rats were submitted to severe hypobaric hypoxia (180 Torr, 3 h) followed by HP (360 Torr, 2 h, 3 trials spaced at 24 h) starting either 3h (early HP) or 24 h (delayed HP) after severe hypoxia. The structural and functional brain injury was assessed by a complex of histological techniques, behavioral methods, and by testing the functions of the hypothalamic-pituitary-adrenal axis (HPA). It was found that early and delayed HP considerably attenuated post-hypoxic injury, reducing pyknosis, hyperchromatosis, and interstitial brain edema, as well as the rates of neuronal loss in hippocampus and neocortex. Delayed HP produced prominent anxiolytic effect on rat behavior, preventing development of post-hypoxic anxiety. Both modes of HP had beneficial effect on the functioning of HPA, but only delayed HP normalized completely the baseline HPA activity and its reactivity to stress. The results obtained demonstrate that postconditioning by using repetitive episodes of mild hypobaric hypoxia may provide a powerful neuroprotective procedure that can be easily adopted for clinical practice and recommended as a research tool for identification of endogenous mechanisms involved in post-ischemic neuroprotection.