Hypobaric hypoxia affects rat behavior and immediate early gene expression in the brain: the corrective effect of preconditioning

Hypoxia in CNS Pathologies: Emerging Role of miRNA-Based Neurotherapeutics and Yoga Based Alternative Therapies

Cellular respiration is a vital process for the existence of life. Any condition that results in deprivation of oxygen (also termed as hypoxia) may eventually lead to deleterious effects on the functioning of tissues. Brain being the highest consumer of oxygen is prone to increased risk of hypoxia-induced neurological insults. This in turn has been associated with many diseases of central nervous system (CNS) such as stroke, Alzheimer's, encephalopathy etc. Although several studies have investigated the pathophysiological mechanisms underlying ischemic/hypoxic CNS diseases, the knowledge about protective therapeutic strategies to ameliorate the affected neuronal cells is meager. This has augmented the need to improve our understanding of the hypoxic and ischemic events occurring in the brain and identify novel and alternate treatment modalities for such insults. MicroRNA (miRNAs), small non-coding RNA molecules, have recently emerged as potential neuroprotective agents as well as targets, under hypoxic conditions. These 18-22 nucleotide long RNA molecules are profusely present in brain and other organs and function as gene regulators by cleaving and silencing the gene expression. In brain, these are known to be involved in neuronal differentiation and plasticity. Therefore, targeting miRNA expression represents a novel therapeutic approach to intercede against hypoxic and ischemic brain injury. In the first part of this review, we will discuss the neurophysiological changes caused as a result of hypoxia, followed by the contribution of hypoxia in the neurodegenerative diseases. Secondly, we will provide recent updates and insights into the roles of miRNA in the regulation of genes in oxygen and glucose deprived brain in association with circadian rhythms and how these can be targeted as neuroprotective agents for CNS injuries. Finally, we will emphasize on alternate breathing or yogic interventions to overcome the hypoxia associated anomalies that could ultimately lead to improvement in cerebral perfusion.

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.

RNA-seq-based transcriptome profiling reveals differential gene expression in the lungs of Sprague-Dawley rats during early-phase acute hypobaric hypoxia

Individuals subjected to hypobaric hypoxia at high altitudes may exhibit differential physiological responses in terms of susceptibility and tolerance to the development of hypoxia-related disorders. We studied early-phase gene expression in the lungs of Sprague-Dawley rats exhibiting such differential physiological responses after exposure to acute hypobaric hypoxia for 1 h at a simulated altitude of 9144 m. RNA-seq transcriptome profiling of lung tissues revealed differential gene expression in tolerant and susceptible groups, subsequently validated by qRT-PCR for ten selected differentially expressed genes. The gene expression pattern indicated hypometabolism and negative regulation of vasoconstriction in all groups except susceptible rats, coupled with altered MAPK, p53 and JAK-STAT signaling. Upregulation of early-phase response genes including Dusp1 (dual specificity phosphatase), Cdkn1a (cyclin-dependent kinase inhibitor 1A), Txnip (thioredoxin-interacting protein), Rgs1 (regulator of G-protein signaling 1) and Rgs2 (regulator of G-protein signaling 2) in susceptible rats indicated a progression toward growth arrest and apoptosis. Enhanced expression of cell adhesion molecules, wound healing and repair bioprocesses was observed in tolerant males. Upregulated Kcnj15 (potassium inwardly rectifying channel subfamily j membrane 15) and Vsig4 (V-set and Ig domain containing 4) variants in tolerant females suggested adaptation to hypoxia possibly by fluid reabsorption to avoid edematous conditions and suppression of T cell proliferation to avoid acute lung inflammation. Our study might help in understanding the molecular-physiological mechanisms associated with progressive damage in the lung tissues of susceptible and tissue-protective measures in tolerant rats during acute hypobaric hypoxia.

Preconditioning enhances the expression of mitochondrial antioxidant thioredoxin-2 in the forebrain of rats exposed to severe hypobaric hypoxia

The impact of severe hypoxia and preconditioning on the expression of the mitochondrial antioxidant thioredoxin-2 (Trx-2) in rat hippocampus (CA1, CA2, CA3 fields, and dentate gyrus) and neocortex was studied by immunocytochemistry. The preconditioning consisted of three trials of mild hypobaric hypoxia (360 Torr, 2 hr) spaced at 24 hr. The last trial was followed by severe hypobaric hypoxia (180 Torr, 3 hr) 24 hr later. Both in hippocampus and in neocortex, severe hypobaric hypoxia resulted in enhanced Trx-2 expression at 3 hr, followed by a slight decline in Trx-2 levels, which nevertheless remained increased at 24 hr elsewhere except for the CA1 region. The preconditioning considerably augmented severe hypoxia-induced Trx-2 immunoreactivity, affecting both the number of immunoreactive cells and the intensity of immunostaining. The findings suggest a role for Trx-2 in the formation of brain hypoxic/ischemic tolerance accomplished by the preconditioning.

Mild preconditioning hypoxia modifies nerve growth factor-induced gene A messenger RNA expression in the rat brain induced by severe hypoxia

The effect of preconditioning (PC) on the changes of nerve growth factor-induced gene A (NGFI-A) expression induced by severe hypobaric hypoxia was studied by in situ hybridization. A PC consisted of three trials of mild hypobaric hypoxia (360 Torr, 2 h) spaced at 24 h. The last trial was followed by severe hypoxia (SH; 180 Torr, 3 h) 24 h later. The PC hypoxia prevented the NGFI-A messenger RNA (mRNA) increase in the cortex, neostriatum, piriform cortex, amygdala and hippocampus detected 3 h after SH. The preconditioned SH caused a peak in NGFI-A mRNA expression at the 24 h time-point and thus abolished the dramatic decrease of the mRNA in vulnerable areas seen by 24 and 72 h after SH. The findings suggest a role of brain NGFI-A in the protective effect of hypoxic/ischemic PC.