Socrates: A Novel N-Ethyl-N-nitrosourea-Induced Mouse Mutant with Audiogenic Epilepsy

Epilepsy is one of the common neurological diseases that affects not only adults but also infants and children. Because epilepsy has been studied for a long time, there are several pharmacologically effective anticonvulsants, which, however, are not suitable as therapy for all patients. The genesis of epilepsy has been extensively investigated in terms of its occurrence after injury and as a concomitant disease with various brain diseases, such as tumors, ischemic events, etc. However, in the last decades, there are multiple reports that both genetic and epigenetic factors play an important role in epileptogenesis. Therefore, there is a need for further identification of genes and loci that can be associated with higher susceptibility to epileptic seizures. Use of mouse knockout models of epileptogenesis is very informative, but it has its limitations. One of them is due to the fact that complete deletion of a gene is not, in many cases, similar to human epilepsy-associated syndromes. Another approach to generating mouse models of epilepsy is N-Ethyl-N-nitrosourea (ENU)-directed mutagenesis. Recently, using this approach, we generated a novel mouse strain, soc (socrates, formerly s8-3), with epileptiform activity. Using molecular biology methods, calcium neuroimaging, and immunocytochemistry, we were able to characterize the strain. Neurons isolated from soc mutant brains retain the ability to differentiate in vitro and form a network. However, soc mutant neurons are characterized by increased spontaneous excitation activity. They also demonstrate a high degree of Ca2+ activity compared to WT neurons. Additionally, they show increased expression of NMDA receptors, decreased expression of the Ca2+-conducting GluA2 subunit of AMPA receptors, suppressed expression of phosphoinositol 3-kinase, and BK channels of the cytoplasmic membrane involved in protection against epileptogenesis. During embryonic and postnatal development, the expression of several genes encoding ion channels is downregulated in vivo, as well. Our data indicate that soc mutation causes a disruption of the excitation-inhibition balance in the brain, and it can serve as a mouse model of epilepsy.

Features of the cytoprotective effect of selenium nanoparticles on primary cortical neurons and astrocytes during oxygen-glucose deprivation and reoxygenation

The study is aimed at elucidating the effect of selenium nanoparticles (SeNPs) on the death of cells in the primary culture of mouse cerebral cortex during oxygen and glucose deprivation (OGD). A primary cell culture of the cerebral cortex containing neurons and astrocytes was subjected to OGD and reoxygenation to simulate cerebral ischemia-like conditions in vitro. To evaluate the neuroprotective effect of SeNPs, cortical astrocytes and neurons were incubated for 24 h with SeNPs, and then subjected to 2-h OGD, followed by 24-h reoxygenation. Vitality tests, fluorescence microscopy, and real-time PCR have shown that incubation of primary cultured neurons and astrocytes with SeNPs at concentrations of 2.5–10 µg/ml under physiological conditions has its own characteristics depending on the type of cells (astrocytes or neurons) and leads to a dose-dependent increase in apoptosis. At low concentration SeNPs (0.5 µg/ml), on the contrary, almost completely suppressed the processes of basic necrosis and apoptosis. Both high (5 µg/ml) and low (0.5 µg/ml) concentrations of SeNPs, added for 24 h to the cells of cerebral cortex, led to an increase in the expression level of genes Bcl-2, Bcl-xL, Socs3, while the expression of Bax was suppressed. Incubation of the cells with 0.5 µg/ml SeNPs led to a decrease in the expression of SelK and SelT. On the contrary, 5 µg/ml SeNPs caused an increase in the expression of SelK, SelN, SelT, SelP. In the ischemic model, after OGD/R, there was a significant death of brain cells by the type of necrosis and apoptosis. OGD/R also led to an increase in mRNA expression of the Bax, SelK, SelN, and SelT genes and suppression of the Bcl-2, Bcl-xL, Socs3, SelP genes. Pre-incubation of cell cultures with 0.5 and 2.5 µg/ml SeNPs led to almost complete inhibition of OGD/R-induced necrosis and greatly reduced apoptosis. Simultaneously with these processes we observed suppression of caspase-3 activation. We hypothesize that the mechanisms of the protective action of SeNPs involve the activation of signaling cascades recruiting nuclear factors Nrf2 and SOCS3/STAT3, as well as the activation of adaptive pathways of ESR signaling of stress arising during OGD and involving selenoproteins SelK and SelT, proteins of the Bcl-2 family ultimately leading to inactivation of caspase-3 and inhibition of apoptosis. Thus, our results demonstrate that SeNPs can act as neuroprotective agents in the treatment of ischemic brain injuries.

Neuroinflammation in Cerebral Ischemia and Ischemia/Reperfusion Injuries: From Pathophysiology to Therapeutic Strategies

Its increasing incidence has led stroke to be the second leading cause of death worldwide. Despite significant advances in recanalization strategies, patients are still at risk for ischemia/reperfusion injuries in this pathophysiology, in which neuroinflammation is significantly involved. Research has shown that in the acute phase, neuroinflammatory cascades lead to apoptosis, disruption of the blood-brain barrier, cerebral edema, and hemorrhagic transformation, while in later stages, these pathways support tissue repair and functional recovery. The present review discusses the various cell types and the mechanisms through which neuroinflammation contributes to parenchymal injury and tissue repair, as well as therapeutic attempts made in vitro, in animal experiments, and in clinical trials which target neuroinflammation, highlighting future therapeutic perspectives.

Properties of GABAergic Neurons Containing Calcium-Permeable Kainate and AMPA-Receptors

Calcium-permeable kainate and AMPA receptors (CP-KARs and CP-AMPARs), as well as NMDARs, play a pivotal role in plasticity and in regulating neurotransmitter release. Here we visualized in the mature hippocampal neuroglial cultures the neurons expressing CP-AMPARs and CP-KARs. These neurons were visualized by a characteristic fast sustained [Ca²⁺]_i increase in response to the agonist of these receptors, domoic acid (DoA), and a selective agonist of GluK1-containing KARs, ATPA. Neurons from both subpopulations are GABAergic. The subpopulation of neurons expressing CP-AMPARs includes a larger percentage of calbindin-positive neurons (39.4 ± 6.0%) than the subpopulation of neurons expressing CP-KARs (14.2 ± 7.5% of CB⁺ neurons). In addition, we have shown for the first time that NH₄Cl-induced depolarization faster induces an [Ca²⁺]_i elevation in GABAergic neurons expressing CP-KARs and CP-AMPARs than in most glutamatergic neurons. CP-AMPARs antagonist, NASPM, increased the amplitude of the DoA-induced Ca²⁺ response in GABAergic neurons expressing CP-KARs, indicating that neurons expressing CP-AMPARs innervate GABAergic neurons expressing CP-KARs. We assume that CP-KARs in inhibitory neurons are involved in the mechanism of outstripping GABA release upon hyperexcitation.

Mechanisms Underlying the Protective Effect of the Peroxiredoxin-6 Are Mediated via the Protection of Astrocytes during Ischemia/Reoxygenation

Ischemia-like conditions reflect almost the entire spectrum of events that occur during cerebral ischemia, including the induction of oxidative stress, Ca²⁺ overload, glutamate excitotoxicity, and activation of necrosis and apoptosis in brain cells. Mechanisms for the protective effects of the antioxidant enzyme peroxiredoxin-6 (Prx-6) on hippocampal cells during oxygen-glucose deprivation/reoxygenation (OGD/R) were investigated. Using the methods of fluorescence microscopy, inhibitory analysis, vitality tests and PCR, it was shown that 24-h incubation of mixed hippocampal cell cultures with Prx-6 does not affect the generation of a reversible phase of a OGD-induced rise in Ca²⁺ ions in cytosol ([Ca²⁺]_i), but inhibits a global increase in [Ca²⁺]_i in astrocytes completely and in neurons by 70%. In addition, after 40 min of OGD, cell necrosis is suppressed, especially in the astrocyte population. This effect is associated with the complex action of Prx-6 on neuroglial networks. As an antioxidant, Prx-6 has a more pronounced and astrocyte-directed effect, compared to the exogenous antioxidant vitamin E (Vit E). Prx-6 inhibits ROS production in mitochondria by increasing the antioxidant capacity of cells and altering the expression of genes encoding redox status proteins. Due to the close bond between [Ca²⁺]_i and intracellular ROS, this effect of Prx-6 is one of its protective mechanisms. Moreover, Prx-6 effectively suppresses not only necrosis, but also apoptosis during OGD and reoxygenation. Incubation with Prx-6 leads to activation of the basic expression of genes encoding protective kinases—PI3K, CaMKII, PKC, anti-apoptotic proteins—Stat3 and Bcl-2, while inhibiting the expression of signaling kinases and factors involved in apoptosis activation—Ikk, Src, NF-κb, Caspase-3, p53, Fas, etc. This effect on the basic expression of the genome leads to the cell preconditions, which is expressed in the inhibition of caspase-3 during OGD/reoxygenation. A significant effect of Prx-6 is directed on suppression of the level of pro-inflammatory cytokine IL-1β and factor TNFα, as well as genes encoding NMDA- and kainate receptor subunits, which was established for the first time for this antioxidant enzyme. The protective effect of Prx-6 is due to its antioxidant properties, since mutant Prx-6 (mutPrx-6, Prx6-C47S) leads to polar opposite effects, contributing to oxidative stress, activation of apoptosis and cell death through receptor action on TLR4.

Mechanisms of ammonium-induced neurotoxicity. Neuroprotective effect of alpha-2 adrenergic agonists

Here we report the effects of ammonium on the main biophysical features of neurons and astrocytes during the first minutes of exposure. We found that ammonium causes the depolarization of neurons, which leads to the generation of high-frequency action potentials (APs). The initial alkalization and subsequent acidification of the intracellular medium in neurons occur along with the generation of calcium oscillations. Moreover, although the kinetics of calcium response of neurons and astrocytes is different, the dynamics of changes in the intracellular pH (pH_i) is similar. The rate of superoxide production and mitochondrial membrane potential do not change in most neurons and astrocytes during ammonium exposure. At the same time, we observed an increased superoxide production and a decrease in the mitochondrial potential in some neurons in response to ammonium application. However, in both cases, the amplitude of the calcium response in these neurons is significantly higher compared to other neurons. Application of UK 14,304, an agonist of alpha-2 adrenergic receptors (A-2ARs), decreased the frequency of APs upon ammonium-induced high-frequency spike activity. Moreover, we also observed periods of hyperpolarization occurred in individual neurons. We suppose that this hyperpolarization contributes to the suppression of activity and can be mediated by astrocytic GABA release, which is stimulated upon activation of A-2ARs. Thus, our findings reveal a new possible mechanism of the protective action of alpha-2 adrenergic agonists against ammonium-induced hyperexcitation and demonstrate the correlation between intracellular calcium concentration, mitochondrial membrane potential, pH_i, the intensity of superoxide production in hippocampal cells under acute hyperammonemia.

BDNF Overexpression Enhances the Preconditioning Effect of Brief Episodes of Hypoxia, Promoting Survival of GABAergic Neurons

Hypoxia causes depression of synaptic plasticity, hyperexcitation of neuronal networks, and the death of specific populations of neurons. However, brief episodes of hypoxia can promote the adaptation of cells. Hypoxic preconditioning is well manifested in glutamatergic neurons, while this adaptive mechanism is virtually suppressed in GABAergic neurons. Here, we show that brain-derived neurotrophic factor (BDNF) overexpression in neurons enhances the preconditioning effect of brief episodes of hypoxia. The amplitudes of the NMDAR- and AMPAR-mediated Ca²⁺ responses of glutamatergic and GABAergic neurons gradually decreased after repetitive brief hypoxia/reoxygenation cycles in cell cultures transduced with the (AAV)-Syn-BDNF-EGFP virus construct. In contrast, the amplitudes of the responses of GABAergic neurons increased in non-transduced cultures after preconditioning. The decrease of the amplitudes in GABAergic neurons indicated the activation of mechanisms of hypoxic preconditioning. Preconditioning suppressed apoptotic or necrotic cell death. This effect was most pronounced in cultures with BDNF overexpression. Knockdown of BDNF abolished the effect of preconditioning and promoted the death of GABAergic neurons. Moreover, the expression of the anti-apoptotic genes Stat3, Socs3, and Bcl-xl substantially increased 24 h after hypoxic episodes in the transduced cultures compared to controls. The expression of genes encoding the pro-inflammatory cytokines IL-10 and IL-6 also increased. In turn, the expression of pro-apoptotic (Bax, Casp-3, and Fas) and pro-inflammatory (IL-1β and TNFα) genes decreased after hypoxic episodes in cultures with BDNF overexpression. Inhibition of vesicular BDNF release abolished its protective action targeting inhibition of the oxygen-glucose deprivation (OGD)-induced [Ca²⁺]_i increase in GABAergic and glutamatergic neurons, thus promoting their death. Bafilomycin A1, Brefeldin A, and tetanus toxin suppressed vesicular release (including BDNF) and shifted the gene expression profile towards excitotoxicity, inflammation, and apoptosis. These inhibitors of vesicular release abolished the protective effects of hypoxic preconditioning in glutamatergic neurons 24 h after hypoxia/reoxygenation cycles. This finding indicates a significant contribution of vesicular BDNF release to the development of the mechanisms of hypoxic preconditioning. Thus, our results demonstrate that BDNF plays a pivotal role in the activation and enhancement of the preconditioning effect of brief episodes of hypoxia and promotes tolerance of the most vulnerable populations of GABAergic neurons to hypoxia/ischemia.

20 Years of Secretagogin: Exocytosis and Beyond

Calcium is one of the most important signaling factors in mammalian cells. Specific temporal and spatial calcium signals underlie fundamental processes such as cell growth, development, circadian rhythms, neurotransmission, hormonal actions and apoptosis. In order to translate calcium signals into cellular processes a vast number of proteins bind this ion with affinities from the nanomolar to millimolar range. Using classical biochemical methods an impressing number of calcium binding proteins (CBPs) have been discovered since the late 1960s, some of which are expressed ubiquitously, others are more restricted to specific cell types. In the nervous system expression patterns of different CBPs have been used to discern different neuronal cell populations, especially before advanced methods like single-cell transcriptomics and activity recording were available to define neuronal identity. However, understanding CBPs and their interacting proteins is still of central interest. The post-genomic era has coined the term “calciomics,” to describe a whole new research field, that engages in the identification and characterization of CBPs and their interactome. Secretagogin is a CBP, that was discovered 20 years ago in the pancreas. Consecutively it was found also in other organs including the nervous system, with characteristic expression patterns mostly forming cell clusters. Its regional expression and subcellular location together with the identification of protein interaction partners implicated, that secretagogin has a central role in hormone secretion. Meanwhile, with the help of modern proteomics a large number of actual and putative interacting proteins has been identified, that allow to anticipate a much more complex role of secretagogin in developing and adult neuronal cells. Here, we review recent findings that appear like puzzle stones of a greater picture.