Fear memory is impaired in hypobaric hypoxia: Role of synaptic plasticity and neuro-modulators in limbic region

Decreased cold-inducible RNA-binding protein (CIRP) binding to GluRl on neuronal membranes mediates memory impairment resulting from prolonged hypobaric hypoxia exposure

AIM

To investigate the molecular mechanisms underlying memory impairment induced by high-altitude (HA) hypoxia, specifically focusing on the role of cold-inducible RNA-binding protein (CIRP) in regulating the AMPA receptor subunit GluR1 and its potential as a therapeutic target.

METHODS

A mouse model was exposed to 14 days of hypobaric hypoxia (HH), simulating conditions at an altitude of 6000 m. Behavioral tests were conducted to evaluate memory function. The expression, distribution, and interaction of CIRP with GluR1 in neuronal cells were analyzed. The binding of CIRP to GluR1 mRNA and its impact on GluR1 protein expression were examined. Additionally, the role of CIRP in GluR1 regulation was assessed using Cirp knockout mice. The efficacy of the Tat-C16 peptide, which consists of the Tat sequence combined with the CIRP 110-125 amino acid sequence, was also tested for its ability to mitigate HH-induced memory decline.

RESULTS

CIRP was primarily localized in neurons, with its expression significantly reduced following HH exposure. This reduction was associated with decreased GluR1 protein expression on the cell membrane and increased localization in the cytoplasm. The interaction between CIRP and GluR1 was diminished under HH conditions, leading to reduced GluR1 stability on the cell membrane and increased cytoplasmic relocation. These changes resulted in a decreased number of synapses and dendritic spines, impairing learning and memory functions. Administration of the Tat-C16 peptide effectively ameliorated these impairments by modulating GluR1 expression and distribution in HH-exposed mice.

CONCLUSION

CIRP plays a critical role in maintaining synaptic integrity under hypoxic conditions by regulating GluR1 expression and distribution. The Tat-C16 peptide shows promise as a therapeutic strategy for alleviating cognitive decline associated with HA hypoxia.

Shedding light on subiculum's role in human brain disorders

Subiculum is a pivotal output component of the hippocampal formation, a structure often overlooked in neuroscientific research. Here, this review aims to explore the role of the subiculum in various brain disorders, shedding light on its significance within the functional-neuroanatomical perspective on neurological diseases. The subiculum's involvement in multiple brain disorders was thoroughly examined. In Alzheimer's disease, subiculum alterations precede cognitive decline, while in epilepsy, the subiculum plays a critical role in seizure initiation. Stress involves the subiculum's impact on the hypothalamic-pituitary-adrenocortical axis. Moreover, the subiculum exhibits structural and functional changes in anxiety, schizophrenia, and Parkinson's disease, contributing to cognitive deficits. Bipolar disorder is linked to subiculum structural abnormalities, while autism spectrum disorder reveals an alteration of inward deformation in the subiculum. Lastly, frontotemporal dementia shows volumetric differences in the subiculum, emphasizing its contribution to the disorder's complexity. Taken together, this review consolidates existing knowledge on the subiculum's role in brain disorders, and may facilitate future research, diagnostic strategies, and therapeutic interventions for various neurological conditions.

Kynurenines Dynamics in the Periphery and Central Nervous System Steers Behavioral Deficits in Rats under Hypobaric Hypoxia

People travel to high-altitude regions as tourists, workers, and military personnel on duty. Despite the consistent 21% oxygen content in the atmosphere, ascending to higher altitudes results in a decrease in the partial pressure of oxygen, inducing a state known as hypobaric hypoxia (HH). HH is an environmental stress that is responsible for neuroinflammation and behavioral deficits (anxiety, depression, mood disturbance, etc.), but little is known about its metabolic pathways. The kynurenine pathway (KP) is a promising candidate to uncover the mysteries of HH stress, as it is an important regulator of the immune system and is associated with behavioral deficits. To investigate the role of KP under HH, the levels of KP metabolites in the serum, cerebrospinal fluid (CSF), and brain tissue (prefrontal cortex-PFC, neocortex, and hippocampus) of male Sprague-Dawley rats exposed to HH at 7620 m for 1, 3, and 7 days were estimated utilizing high-performance liquid chromatography (HPLC). The behavioral analogs for anxiety-like and depression-like behavior were assessed using the open field test and forced swim test, respectively. Upon HH exposure, crosstalk between the periphery and central nervous system and KP metabolite region-dependent differential expression in the brain were observed. KP metabolites showed a positive correlation with behavioral parameters. The results of our study are indicative that KP can be proposed as the etiology of behavioral deficits, and KP metabolite levels in serum or CSF can be used as plausible markers for anxiety-like and depression-like behaviors under HH stress with a scope of targeted therapeutic interventions.

Identification of hypoxia- and immune-related biomarkers in patients with ischemic stroke

Background

The immune microenvironment and hypoxia play crucial roles in the pathophysiology of ischemic stroke (IS). Hence, in this study, we aimed to identify hypoxia- and immune-related biomarkers in IS.

Methods

The IS microarray dataset GSE16561 was examined to determine differentially expressed genes (DEGs) utilizing bioinformatics-based analysis. The intersection of hypoxia-related genes and DEGs was conducted to identify differentially expressed hypoxia-related genes (DEHRGs). Then, using weighted correlation network analysis (WGCNA), all of the genes in GSE16561 dataset were examined to create a co-expression network, and module-clinical trait correlations were examined for the purpose of examining the genes linked to immune cells. The immune-related DEHRGs were submitted to gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses. A protein-protein interaction (PPI) network was constructed by Cytoscape plugin MCODE, in order to extract hub genes. The miRNet was used to predict hub gene-related transcription factors (TFs) and miRNAs. Finally, a diagnostic model was developed by least absolute shrinkage and selection operator (LASSO) logistic regression.

Results

Between the control and IS samples, 4171 DEGs were found. Thereafter, the intersection of hypoxia-related genes and DEGs was conducted to obtain 45 DEHRGs. Ten significantly differentially infiltrated immune cells were found-namely, CD56dim natural killer cells, activated CD8 T cells, activated dendritic cells, activated B cells, central memory CD8 T cells, effector memory CD8 T cells, natural killer cells, gamma delta T cells, plasmacytoid dendritic cells, and neutrophils-between IS and control samples. Subsequently, we identified 27 immune-related DEHRGs through the intersection of DEHRGs and genes in important modules of WGCNA. The immune-related DEHRGs were primarily enriched in response to hypoxia, cellular polysaccharide metabolic process, response to decreased oxygen levels, polysaccharide metabolic process, lipid and atherosclerosis, and HIF-1 signaling pathway H. Using MCODE, FOS, DDIT3, DUSP1, and NFIL3 were found to be hub genes. In the validation cohort and training set, the AUC values of the diagnostic model were 0.9188034 and 0.9395085, respectively.

Conclusion

In brief, we identified and validated four hub genes-FOS, DDIT3, DUSP1, and NFIL3-which might be involved in the pathological development of IS, potentially providing novel perspectives for the diagnosis and treatment of IS.

HIF-1α promotes astrocytic production of macrophage migration inhibitory factor following spinal cord injury

BACKGROUND

Macrophage migration inhibitory factor (MIF) is an important mediator of neuropathology in various central nervous system (CNS) diseases. However, little is known about its inducers for production from the nerve cells, as well as the underlying regulatory mechanism. Injury-induced HIF-1α has been shown to exacerbate neuroinflammation by activating multiple downstream target molecules. It is postulated that HIF-1α is involved in the regulation of MIF following spinal cord injury (SCI).

METHODS

SCI model of Sprague-Dawley rats was established by cord contusion at T8-T10. The dynamic changes of HIF-1α and MIF protein levels at lesion site of rat spinal cord were determined by Western blot. The specific cell types of HIF-1α and MIF expression were examined by immunostaining. Primary astrocytes were isolated from the spinal cord, cultured and stimulated with various agonist or inhibitor of HIF-1α for analysis of HIF-1α-mediated expression of MIF. Luciferase report assay was used to determine the relationship between HIF-1α and MIF. The Basso, Beattie, and Bresnahan (BBB) locomotor scale was used to assess the locomotor function following SCI.

RESULTS

The protein levels of HIF-1α and MIF at lesion site were significantly elevated by SCI. Immunofluorescence demonstrated that both HIF-1α and MIF were abundantly expressed in the astrocytes of the spinal cord. By using various agonists or inhibitors of HIF-1α, it was shown that HIF-1α sufficiently induced astrocytic production of MIF. Mechanistically, HIF-1α promoted MIF expression through interaction with MIF promoter. Inhibition of HIF-1α activity using specific inhibitor markedly reduced the protein levels of MIF at lesion site following SCI, which in turn favored for the functional recovery.

CONCLUSION

SCI-induced activation of HIF-1α is able to promote MIF production from astrocytes. Our results have provided new clues for SCI-induced production of DAMPs, which may be helpful for clinical treatment of neuroinflammation.

Insulin enhances contextual fear memory independently of its effect in increasing plasma adrenaline

AIMS

Adrenaline enhances contextual fear memory consolidation possibly by activating liver β₂-adrenoceptors causing transient hyperglycaemia. Contrastingly, insulin-induced hypoglycaemia may culminate in blood adrenaline increment, hidering the separation of each hormone's action in contextual fear memory. Therefore, an Ad-deficient mouse model was used aiming to investigate if contextual fear memory consolidation following insulin administration requires or not subsequent increases in plasma adrenaline, which occurs in response to insulin-induced hypoglycemia.

MAIN METHODS

Fear conditioning was performed in wild-type (WT) and adrenaline-deficient (Pnmt-KO) male mice (129 × 1/SvJ) treated with insulin (2 U/kg, intraperitoneal (i.p.)) or vehicle (0.9 % NaCl (i.p.)). Blood glucose was quantified. Catecholamines were quantified using HPLC with electrochemical detection. Quantitative real-time polymerase chain reaction was used to assess mRNA expression of hippocampal Nr4a1, Nr4a2, Nr4a3, and Bdnf genes.

KEY FINDINGS

Insulin-treated WT mice showed increased freezing behaviour when compared to vehicle-treated WT mice. Also, plasma dopamine, noradrenaline, and adrenaline increased in this group. Insulin-treated Pnmt-KO animals showed increased freezing behaviour when compared with respective vehicle. However, no changes in plasma or tissue catecholamines were identified in insulin-treated Pnmt-KO mice when compared with respective vehicle. Furthermore, insulin-treated Pnmt-KO mice presented increased Bdnf mRNA expression when compared to vehicle-treated Pnmt-KO mice.

SIGNIFICANCE

Concluding, enhanced freezing behaviour after insulin treatment, even in adrenaline absence, may indicate a key role of insulin in contextual fear memory. Insulin may cause central molecular changes promoting contextual fear memory formation and/or retrieval. This work may indicate a further role of insulin in the process of contextual fear memory modulation.

Hypoxia-induced upregulation of matrix metalloproteinase 9 increases basement membrane degradation by downregulating collagen type IV alpha 1 chain

Hypoxia can cause basement membrane (BM) degradation in tissues. Matrix metalloproteinase 9 (MMP-9) is involved in various human cancers as well as BM degradation by downregulating type IV collagen (COL4). This study investigated the role of MMP-9 in hypoxia-mediated BM degradation in rat bone marrow based on its regulation of collagen type IV alpha 1 chain (COL4A1). Eighty male rats were randomly divided into four groups based on exposure to hypoxic conditions at a simulated altitude of 7,000 m, control (normoxia) and 3, 7, and 10 days of hypoxia exposure. BM degradation in bone marrow was determined by transmission electron microscopy. MMP-9 levels were assessed by western blot and real-time PCR, and COL4A1 levels were assessed by western blot and immunohistochemistry. Microvessels BMs in bone marrow exposed to acute hypoxia were observed by electron microscopy. MMP-9 expression increased, COL4A1 protein expression decreased, and BM degradation occurred in the 10-, 7-, and 3-day hypoxia groups compared with that in the control group (all P < 0.05). Hypoxia increased MMP-9 levels, which in turn downregulated COL4A1, thereby increasing BM degradation. MMP-9 upregulation significantly promoted BM degradation and COL4A1 downregulation. Our results suggest that MMP-9 is related to acute hypoxia-induced BM degradation in bone marrow by regulating COL4A1.

Differential synaptic mechanism underlying the neuronal modulation of prefrontal cortex, amygdala, and hippocampus in response to chronic postsurgical pain with or without cognitive deficits in rats

Chronic Postsurgical Pain (CPSP) is well recognized to impair cognition, particularly memory. Mounting evidence suggests anatomic and mechanistic overlap between pain and cognition on several levels. Interestingly, the drugs currently used for treating chronic pain, including opioids, gabapentin, and NMDAR (N-methyl-D-aspartate receptor) antagonists, are also known to impair cognition. So whether pain-related cognitive deficits have different synaptic mechanisms as those underlying pain remains to be elucidated. In this context, the synaptic transmission in the unsusceptible group (cognitively normal pain rats) was isolated from that in the susceptible group (cognitively compromised pain rats). It was revealed that nearly two-thirds of the CPSP rats suffered cognitive impairment. The whole-cell voltage-clamp recordings revealed that the neuronal excitability and synaptic transmission in the prefrontal cortex and amygdala neurons were enhanced in the unsusceptible group, while these parameters remained the same in the susceptible group. Moreover, the neuronal excitability and synaptic transmission in hippocampus neurons demonstrated the opposite trend. Correspondingly, the levels of synaptic transmission-related proteins demonstrated a tendency similar to that of the excitatory and inhibitory synaptic transmission. Furthermore, morphologically, the synapse ultrastructure varied in the postsynaptic density (PSD) between the CPSP rats with and without cognitive deficits. Together, these observations indicated that basal excitatory and inhibitory synaptic transmission changes were strikingly different between the CPSP rats with and without cognitive deficits.

Neuroligin-1 plays an important role in methamphetamine-induced hippocampal synaptic plasticity

The neurotoxic effects of methamphetamine (METH) include not only neuronal apoptosis and autophagy, but also lead to substance use disorder and have become increasingly prominent. Studies suggest that synaptic plasticity may be the structural basis of METH-induced neurological impairment. Neuroligins are postsynaptic adhesion molecules involved in the regulation of synaptic organization and function. Animal studies have shown that neuroligin (NLG)- 1 is involved in memory formation; however, its role in METH-induced neurotoxicity is not clear. In the present study, we used 1 mM METH in vitro; mice in the acute and subacute exposure groups received intraperitoneal injections of 30 mg/kg METH (1 injection) or 15 mg/kg METH (8 separate injections at 12-h intervals). We found that the expression of NLG-1, Synapsin-1, and postsynaptic density-95 were increased after METH exposure. We further observed that METH-induced inhibition of long-term potentiation and spatial memory loss could be alleviated when mice were pretreated with NLG-1 small interfering RNA. Therefore, our study provides evidence that NLG-1 is involved in METH-induced hippocampal synaptic plasticity and may be a potential target for the treatment of METH-induced neurotoxicity.

Hypobaric Hypoxia Induces Deficits in Adult Neurogenesis and Social Interaction via Cyclooxygenase-1/ EP1 Receptor Pathway Activating NLRP3 Inflammasome

Low oxygen environments, like hypobaric hypoxia (HH), are common nodes in a number of diseases characterized by neuroinflammation, which is detrimental to the structural and functional aspects of hippocampal circuitry. Hypoxic conditions lead to elevation of inflammasome-mediated inflammation that may contribute to cognitive deficits. However, a systematic investigation of the impact of inflammasome-mediated neuroinflammation on the components of neurogenic niche during HH remains to be elusive. Cerebral hypoxia was induced in adult male Sprague Dawley rats via decreasing partial pressure of oxygen. The effect of HH (1, 3, and 7 days at 25,000 ft) on social memory, anxiety, adult neurogenesis, and NLRP3- (NLR family pyrin domain containing 3) mediated neuroinflammation in the dentate gyrus (DG) was explored in detail. Furthermore, we explored the therapeutic efficacy of cyclooxygenase-1 inhibitor (valeryl salicylate, 5 mg/kg/day, i.p.) and EP1 receptor (EP1R) antagonist (SC19220, 1 mg/kg/day, i.p.) on HH-induced deficits. Seven days of HH exposure induced alteration in social and anxiety-like behavior along with perturbation in adult neurogenesis. Elevation in NLRP3, caspase-1, and IL-1β levels was observed during HH from day 1. A notable increase in the COX-1/EP1R pathway in activated glial cells in DG was evident during HH. COX-1 inhibitor and EP1R antagonist mitigated the detrimental effects of HH on social memory, adult neurogenesis via blunting NLRP3-mediated inflammation. Our data showed induction of the COX-1/EP1R pathway in the glial cells, which is detrimental to neurogenesis and social memory, opening up the possibility that the COX-1/EP1R pathway is a plausible target for inflammasome-related neurogenesis impairments.

Hypoxia-Inducible Factor (HIF) in Ischemic Stroke and Neurodegenerative Disease

Hypoxia is one of the most common pathological conditions, which can be induced by multiple events, including ischemic injury, trauma, inflammation, tumors, etc. The body's adaptation to hypoxia is a highly important phenomenon in both health and disease. Most cellular responses to hypoxia are associated with a family of transcription factors called hypoxia-inducible factors (HIFs), which induce the expression of a wide range of genes that help cells adapt to a hypoxic environment. Basic mechanisms of adaptation to hypoxia, and particularly HIF functions, have being extensively studied over recent decades, leading to the 2019 Nobel Prize in Physiology or Medicine. Based on their pivotal physiological importance, HIFs are attracting increasing attention as a new potential target for treating a large number of hypoxia-associated diseases. Most of the experimental work related to HIFs has focused on roles in the liver and kidney. However, increasing evidence clearly demonstrates that HIF-based responses represent an universal adaptation mechanism in all tissue types, including the central nervous system (CNS). In the CNS, HIFs are critically involved in the regulation of neurogenesis, nerve cell differentiation, and neuronal apoptosis. In this mini-review, we provide an overview of the complex role of HIF-1 in the adaptation of neurons and glia cells to hypoxia, with a focus on its potential involvement into various neuronal pathologies and on its possible role as a novel therapeutic target.

Association of Peripheral Plasma Neurotransmitters with Cognitive Performance in Chronic High-altitude Exposure

Long-term living at high altitude causes significant impairment of cognitive function. Central neurotransmitters are potential mediators of cognitive performance. We aimed to determine whether there were significant associations between select peripheral plasma neurotransmitters and cognitive performance in humans with chronic high-altitude (HA) exposure and to determine the association between peripheral plasma neurotransmitters and brain neurotransmitters in rats after chronic hypobaric hypoxia (HH) exposure. We demonstrated that 3,4-dihydroxy-L-phenylalanine (DOPA), dopamine, serotonin, 5-hydroxyindole-3-acetic acid (5-HIAA) and GABA in the peripheral plasma were associated with cognitive performance in humans with HA exposure. Consistent with this result, peripheral plasma DOPA, dopamine, serotonin, 5-HIAA and glutamate were associated with brain neurotransmitter levels after chronic HH exposure in rats. These results provide experimental data indicating that neurotransmitter levels and cognitive performance are modified in chronic high-altitude exposure, with a possible causal effect.

Repeated exposure to propofol in the neonatal period impairs hippocampal synaptic plasticity and the recognition function of rats in adulthood

Anesthesia of neonates with propofol induces persistent behavioral abnormalities in adulthood. Although propofol-triggered apoptosis of neurons in the developing brain may contribute to the development of cognitive deficits, the mechanism of neurotoxicity induced by neonatal exposure to propofol remains unclear. In this study, the effects of neonatal propofol anesthesia on synaptic plasticity and neurocognitive function were investigated. Postnatal day 7 (PND-7) Sprague-Dawley rats were intraperitoneally injected with fat emulsion or 20, 40 or 60 mg/kg propofol for three consecutive days. The expression of brain-derived neurotrophic factor (BDNF), tropomyosin-related kinase B (TrkB) and postsynaptic density protein 95 (PSD-95) in the rat hippocampus at PND-10 and PND-12 was measured by Western blotting. The number of dendritic branches, total dendritic length and dendritic spine density were observed by Golgi-Cox staining 24 h and 72 h after the last propofol administration. Long-term potentiation (LTP) was measured electrophysiologically in hippocampus of PND-60 rats to evaluate the synaptic function. The learning and memory abilities of rats were evaluated by Morris water maze (MWM) experiments, Novel object recognition test (NORT) and Object location test (OLT) at PND-60. Our results showed that neonatal exposure to propofol significantly inhibited the expression of BDNF, TrkB and PSD-95 in the rat hippocampus. The number of dendritic branches, total dendritic length and dendritic spine density of neurons in the rat hippocampus were markedly reduced after neonatal propofol anesthesia. LTP was significantly diminished in hippocampus of PND-60 rats after repeated exposure to propofol in the neonatal period. Morris water maze experiments showed that repeated neonatal exposure to propofol significantly prolonged the escape latency and decreased the time spent in the target quadrant and the number of platform crossings. NORT and OLT showed that repeated neonatal exposure to propofol markedly reduced the Investigation Time for novel object or location. All of the results above indicate that repeated exposure to propofol in the neonatal period can impair hippocampal synaptic plasticity and the recognition function of rats in adulthood.