Neuronal gap junction coupling is regulated by glutamate and plays critical role in cell death during neuronal injury

Gap-junction-mediated bioelectric signaling required for slow muscle development and function in zebrafish

Bioelectric signaling, intercellular communication facilitated by membrane potential and electrochemical coupling, is emerging as a key regulator of animal development. Gap junction (GJ) channels can mediate bioelectric signaling by creating a fast, direct pathway between cells for the movement of ions and other small molecules. In vertebrates, GJ channels are formed by a highly conserved transmembrane protein family called the connexins. The connexin gene family is large and complex, creating challenges in identifying specific connexins that create channels within developing and mature tissues. Using the embryonic zebrafish neuromuscular system as a model, we identify a connexin conserved across vertebrate lineages, gjd4, which encodes the Cx46.8 protein, that mediates bioelectric signaling required for slow muscle development and function. Through mutant analysis and in vivo imaging, we show that gjd4/Cx46.8 creates GJ channels specifically in developing slow muscle cells. Using genetics, pharmacology, and calcium imaging, we find that spinal-cord-generated neural activity is transmitted to developing slow muscle cells, and synchronized activity spreads via gjd4/Cx46.8 GJ channels. Finally, we show that bioelectrical signal propagation within the developing neuromuscular system is required for appropriate myofiber organization and that disruption leads to defects in behavior. Our work reveals a molecular basis for GJ communication among developing muscle cells and reveals how perturbations to bioelectric signaling in the neuromuscular system may contribute to developmental myopathies. Moreover, this work underscores a critical motif of signal propagation between organ systems and highlights the pivotal role of GJ communication in coordinating bioelectric signaling during development.

Retinoic acid reduces the formation of, and acutely modulates, invertebrate electrical synapses

Communication between cells in the nervous system is dependent on both chemical and electrical synapses. Factors that can affect chemical synapses have been well studied, but less is known about factors that influence electrical synapses. Retinoic acid, the vitamin A metabolite, is a known regulator of chemical synapses, but few studies have examined its capacity to regulate electrical synapses. In this study, we determine that retinoic acid is capable of rapidly altering the strength of electrical synapses in an isomer- and cell-dependent manner. Furthermore, we provide evidence that this acute effect might be independent of either the retinoid receptors or the activation of a protein kinase. In addition to the rapid modulatory effects of retinoic acid, we provide data to suggest that retinoic acid is also capable of regulating the formation of electrical synapses. Long-term exposure to both all-trans-retinoic acid or 9-cis-retinoic acid reduced the proportion of cell pairs forming electrical synapses, as well as reduced the strength of electrical synapses that did form. In summary, this study provides insights into the role that retinoids might play in both the formation and modulation of electrical synapses in the central nervous system.NEW & NOTEWORTHY Retinoids are known modulators of chemical synapses and mediate synaptic plasticity in the nervous system, but little is known of their effects on electrical synapses. Here, we show that retinoids selectively reduce electrical synapses in a cell- and isomer-dependent manner. This modulatory action on existing electrical synapses was rapid and nongenomic in nature. We also showed for the first time that longer retinoid exposures inhibit the formation of electrical synapses.

Neuroprotective gap-junction-mediated bystander transformations in the adult zebrafish spinal cord after injury

The adult zebrafish spinal cord displays an impressive innate ability to regenerate after traumatic insults, yet the underlying adaptive cellular mechanisms remain elusive. Here, we show that while the cellular and tissue responses after injury are largely conserved among vertebrates, the large-size fast spinal zebrafish motoneurons are remarkably resilient by remaining viable and functional. We also reveal the dynamic changes in motoneuron glutamatergic input, excitability, and calcium signaling, and we underscore the critical role of calretinin (CR) in binding and buffering the intracellular calcium after injury. Importantly, we demonstrate the presence and the dynamics of a neuron-to-neuron bystander neuroprotective biochemical cooperation mediated through gap junction channels. Our findings support a model in which the intimate and dynamic interplay between glutamate signaling, calcium buffering, gap junction channels, and intercellular cooperation upholds cell survival and promotes the initiation of regeneration.

Protective Effects of Hinokitiol on Neuronal Ferroptosis by Activating the Keap1/Nrf2/HO-1 Pathway in Traumatic Brain Injury

In this study, we investigated the effects of hinokitiol, a small-molecule natural compound, against neuronal ferroptosis after traumatic brain injury (TBI). A controlled cortical impact (CCI) mouse model and excess glutamate-treated HT-22 cells were used to study the effects of hinokitiol on TBI. Hinokitiol mitigated TBI brain tissue lesions and significantly improved neurological function. Neuron loss and iron deposition were ameliorated after hinokitiol administration. Hinokitiol alleviated excessive glutamate-induced intracellular reactive oxygen species (ROS), lipid peroxidation, and Fe2+ accumulation in HT-22. Mechanistically, hinokitiol upregulated heme oxygenase-1 (HO-1) expression, promoted nuclear factor-erythroid factor 2-related factor 2 (Nrf2) nuclear translocation, and inhibited the activation of microglia and astrocyte after TBI. These results suggest that hinokitiol has neuroprotective effects on rescuing cells from TBI-induced neuronal ferroptosis. In summary, hinokitiol is a potential therapeutic candidate for TBI by activating the Nrf2/Keap1/HO-1 signaling pathway.

Identification of the Candidate mGlu2 Allosteric Modulator THRX-195518 through In Silico Method and Evaluation of Its Neuroprotective Potential against Glutamate-Induced Neurotoxicity in SH-SY5Y Cell Line

Glutamate (Glu) toxicity has been an important research topic in toxicology and neuroscience studies. In vitro and in vivo studies have shown that Group II metabotropic Glu2 (mGlu2) activators have cell viability effects. This study aims to determine a candidate ligand with high mGlu2 allosteric region activity among cytotoxicity-safe molecules using the in silico positioning method and to evaluate its cell viability effect in vitro. We investigated the candidate molecule's cell viability effect on the SH-SY5Y human neuroblastoma cell line by MTT analysis. In the study, LY 379268 (agonist) and JNJ-46281222 (positive allosteric modulator; PAM) were used as control reference molecules. Drug bank screening yielded THRX-195518 (docking score being -12.4 kcal/mol) as a potential novel drug candidate that has a high docking score and has not been mentioned in the literature so far. The orthosteric agonist LY 379268 exhibited a robust protective effect in our study. Additionally, our findings demonstrate that JNJ-46281222 and THRX-195518, identified as activating the mGlu2 allosteric region through in silico methods, preserve cell viability against Glu toxicity. Therefore, our study not only emphasizes the positive effects of this compound on cell viability against Glu toxicity but also sheds light on the potential of THRX-195518, acting as a mGlu2 PAM, based on in silico absorption, distribution, metabolism, excretion, and toxicity (ADMET) data, as a candidate drug molecule. These findings underscore the potential utility of THRX-195518 against both neurotoxicity and Central Nervous System (CNS) disorders, providing valuable insights.

Gap junction mediated bioelectric coordination is required for slow muscle development, organization, and function

Bioelectrical signaling, intercellular communication facilitated by membrane potential and electrochemical coupling, is emerging as a key regulator of animal development. Gap junction (GJ) channels can mediate bioelectric signaling by creating a fast, direct pathway between cells for the movement of ions and other small molecules. In vertebrates, GJ channels are formed by a highly conserved transmembrane protein family called the Connexins. The connexin gene family is large and complex, presenting a challenge in identifying the specific Connexins that create channels within developing and mature tissues. Using the embryonic zebrafish neuromuscular system as a model, we identify a connexin conserved across vertebrate lineages, gjd4, which encodes the Cx46.8 protein, that mediates bioelectric signaling required for appropriate slow muscle development and function. Through a combination of mutant analysis and in vivo imaging we show that gjd4/Cx46.8 creates GJ channels specifically in developing slow muscle cells. Using genetics, pharmacology, and calcium imaging we find that spinal cord generated neural activity is transmitted to developing slow muscle cells and synchronized activity spreads via gjd4/Cx46.8 GJ channels. Finally, we show that bioelectrical signal propagation within the developing neuromuscular system is required for appropriate myofiber organization, and that disruption leads to defects in behavior. Our work reveals the molecular basis for GJ communication among developing muscle cells and reveals how perturbations to bioelectric signaling in the neuromuscular system_may contribute to developmental myopathies. Moreover, this work underscores a critical motif of signal propagation between organ systems and highlights the pivotal role played by GJ communication in coordinating bioelectric signaling during development.

Localization of PDE4D, HCN1 channels, and mGluR3 in rhesus macaque entorhinal cortex may confer vulnerability in Alzheimer's disease

Alzheimer's disease cortical tau pathology initiates in the layer II cell clusters of entorhinal cortex, but it is not known why these specific neurons are so vulnerable. Aging macaques exhibit the same qualitative pattern of tau pathology as humans, including initial pathology in layer II entorhinal cortex clusters, and thus can inform etiological factors driving selective vulnerability. Macaque data have already shown that susceptible neurons in dorsolateral prefrontal cortex express a "signature of flexibility" near glutamate synapses on spines, where cAMP-PKA magnification of calcium signaling opens nearby potassium and hyperpolarization-activated cyclic nucleotide-gated channels to dynamically alter synapse strength. This process is regulated by PDE4A/D, mGluR3, and calbindin, to prevent toxic calcium actions; regulatory actions that are lost with age/inflammation, leading to tau phosphorylation. The current study examined whether a similar "signature of flexibility" expresses in layer II entorhinal cortex, investigating the localization of PDE4D, mGluR3, and HCN1 channels. Results showed a similar pattern to dorsolateral prefrontal cortex, with PDE4D and mGluR3 positioned to regulate internal calcium release near glutamate synapses, and HCN1 channels concentrated on spines. As layer II entorhinal cortex stellate cells do not express calbindin, even when young, they may be particularly vulnerable to magnified calcium actions and ensuing tau pathology.

Antiepileptogenic and neuroprotective effect of mefloquine after experimental status epilepticus

Acquired temporal lobe epilepsy (TLE) characterized by spontaneous recurrent seizures (SRS) and hippocampal inhibitory neuron dysfunction is often refractory to current therapies. Gap junctional or electrical coupling between inhibitory neurons has been proposed to facilitate network synchrony and intercellular molecular exchange suggesting a role in both seizures and neurodegeneration. While gap junction blockers can limit acute seizures, whether blocking neuronal gap junctions can modify development of chronic epilepsy has not been examined. This study examined whether mefloquine, a selective blocker of Connexin 36 gap junctions which are well characterized in inhibitory neurons, can limit epileptogenesis and related cellular and behavioral pathology in a model of acquired TLE. A single, systemic dose of mefloquine administered early after pilocarpine-induced status epilepticus (SE) in rat reduced both development of SRS and behavioral co-morbidities. Immunostaining for interneuron subtypes identified that mefloquine treatment likely reduced delayed inhibitory neuronal loss after SE. Uniquely, parvalbumin expressing neurons in the hippocampal dentate gyrus appeared relatively resistant to early cell loss after SE. Functionally, whole cell patch clamp recordings revealed that mefloquine treatment preserved inhibitory synaptic drive to projection neurons one week and one month after SE. These results demonstrate that mefloquine, a drug already approved for malaria prophylaxis, is potentially antiepileptogenic and can protect against progressive interneuron loss and behavioral co-morbidities of epilepsy.

Electrical Coupling between Parvalbumin Basket Cells is Reduced after Experimental Status Epilepticus

Acquired epilepsies, characterized by abnormal increase in hypersynchronous network activity, can be precipitated by various factors including brain injuries which cause neuronal loss and increases in network excitability. Electrical coupling between neurons, mediated by gap junctions, has been shown to enhance synchronous neuronal activity and promote excitotoxic neurodegeneration. Consequently, neuronal gap junctional coupling has been proposed to contribute to development of epilepsy. Parvalbumin expressing interneurons (PV-INs), noted for their roles in powerful perisomatic inhibition and network oscillations, have gap junctions formed exclusively by connexin 36 subunits which show changes in expression following seizures, and in human and experimental epilepsy. However, only a fraction of the connexin hemichannels form functional connections, leaving open the critical question of whether functional gap junctional coupling between neurons is altered during development of epilepsy. Using a pilocarpine induced status epilepticus (SE) model of acquired temporal lobe epilepsy in rat, this study examined changes in electrical coupling between PV-INs in the hippocampal dentate gyrus one week after SE. Contrary to expectations, SE selectively reduced the probability of electrical coupling between PV-INs without altering coupling coefficient. Both coupling frequency and coupling coefficient between non-parvalbumin interneurons remained unchanged after SE. The early and selective decrease in functional electrical coupling between dentate PV-INs after SE may represent a compensatory mechanism to limit excitotoxic damage of fast-spiking interneurons and network synchrony during epileptogenesis.

Connexin 43 Phosphorylation: Implications in Multiple Diseases

Connexin 43 (Cx43) is most widely distributed in mammals, especially in the cardiovascular and nervous systems. Its phosphorylation state has been found to be regulated by the action of more than ten kinases and phosphatases, including mitogen-activated protein kinase/extracellular signaling and regulating kinase signaling. In addition, the phosphorylation status of different phosphorylation sites affects its own synthesis and assembly and the function of the gap junctions (GJs) to varying degrees. The phosphorylation of Cx43 can affect the permeability, electrical conductivity, and gating properties of GJs, thereby having various effects on intercellular communication and affecting physiological or pathological processes in vitro and in vivo. Therefore, clarifying the relationship between Cx43 phosphorylation and specific disease processes will help us better understand the disease. Based on the above clinical and preclinical findings, we present in this review the functional significance of Cx43 phosphorylation in multiple diseases and discuss the potential of Cx43 as a drug target in Cx43-related disease pathophysiology, with an emphasis on the importance of connexin 43 as an emerging therapeutic target in cardiac and neuroprotection.

Functional investigation of SLC1A2 variants associated with epilepsy

Epilepsy is a common neurological disorder and glutamate excitotoxicity plays a key role in epileptic pathogenesis. Astrocytic glutamate transporter GLT-1 is responsible for preventing excitotoxicity via clearing extracellular accumulated glutamate. Previously, three variants (G82R, L85P, and P289R) in SLC1A2 (encoding GLT-1) have been clinically reported to be associated with epilepsy. However, the functional validation and underlying mechanism of these GLT-1 variants in epilepsy remain undetermined. In this study, we reported that these disease-linked mutants significantly decrease glutamate uptake, cell membrane expression of the glutamate transporter, and glutamate-elicited current. Additionally, we found that these variants may disturbed stromal-interacting molecule 1 (STIM1)/Orai1-mediated store-operated Ca²⁺ entry (SOCE) machinery in the endoplasmic reticulum (ER), in which GLT-1 may be a new partner of SOCE. Furthermore, knock-in mice with disease-associated variants showed a hyperactive phenotype accompanied by reduced glutamate transporter expression. Therefore, GLT-1 is a promising and reliable therapeutic target for epilepsy interventions.

Asymmetric Dimethylarginine Protects Neurons from Oxygen Glucose Deprivation Insult by Modulating Connexin-36 Expression

Background

Asymmetric dimethylarginine (ADMA) is a nonselective nitric oxide synthase inhibitor. ADMA is thought to inhibit the production of nitric oxide (NO) by neurons after oxygen-glucose deprivation (OGD). The gap junction protein Connexin-36 (cx-36) is involved in the pathophysiology of stroke. We investigated whether ADMA could protect neurons from OGD insults by regulating the expression of cx-36.

Methods

Cultured rat cortical neuronal cells were used. Neurons were treated with OGD with or without ADMA pretreatment. The lactate dehydrogenase (LDH) release rate was used to assess neuronal injury. Intracellular NO levels were determined using 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate. Western blotting was performed to detect cx-36 expression.

Results

The LDH release rate increased in the supernatant of neurons after the OGD insult, whereas ADMA treatment reduced the LDH release rate. Intracellular NO levels increased following OGD treatment, and this increase was not inhibited by ADMA treatment. Expression of cx-36 was upregulated in neurons under OGD conditions, and treatment with ADMA downregulated the expression of cx-36.

Conclusions

ADMA protects neurons from OGD insult, and cx-36 downregulation may be a possible pathway involved in ADMA-mediated neuronal protection.

Crosstalk of Astrocytes and Other Cells during Ischemic Stroke

Stroke is a leading cause of death and long-term disability worldwide. Astrocytes structurally compose tripartite synapses, blood–brain barrier, and the neurovascular unit and perform multiple functions through cell-to-cell signaling of neurons, glial cells, and vasculature. The crosstalk of astrocytes and other cells is complicated and incompletely understood. Here we review the role of astrocytes in response to ischemic stroke, both beneficial and detrimental, from a cell–cell interaction perspective. Reactive astrocytes provide neuroprotection through antioxidation and antiexcitatory effects and metabolic support; they also contribute to neurorestoration involving neurogenesis, synaptogenesis, angiogenesis, and oligodendrogenesis by crosstalk with stem cells and cell lineage. In the meantime, reactive astrocytes also play a vital role in neuroinflammation and brain edema. Glial scar formation in the chronic phase hinders functional recovery. We further discuss astrocyte enriched microRNAs and exosomes in the regulation of ischemic stroke. In addition, the latest notion of reactive astrocyte subsets and astrocytic activity revealed by optogenetics is mentioned. This review discusses the current understanding of the intimate molecular conversation between astrocytes and other cells and outlines its potential implications after ischemic stroke. “Neurocentric” strategies may not be sufficient for neurological protection and recovery; future therapeutic strategies could target reactive astrocytes.

Glutamate Metabotropic Receptor Type 3 (mGlu3) Localization in the Rat Prelimbic Medial Prefrontal Cortex

Metabotropic glutamate receptors type 3 (mGlu3, encoded by GRM3) are increasingly related to cognitive functioning, including the working memory operations of the prefrontal cortex (PFC). In rhesus monkeys, mGlu3 are most commonly expressed on glia (36%), but are also very prominent on layer III dendritic spines (23%) in the dorsolateral PFC (dlPFC) where they enhance working memory-related neuronal firing. In contrast, mGlu2 are predominately presynaptic in layer III of macaque dlPFC, indicating a pre- vs. post-synaptic dissociation by receptor subtype. The current study examined the cellular and subcellular localizations of mGlu3 in the rat prelimbic medial PFC (PL mPFC), a region needed for spatial working memory performance in rodents. Multiple label immunofluorescence demonstrated mGlu3 expression in neurons and astrocytes, with rare labeling in microglia. Immunoelectron microscopy of layers III and V found that the predominant location for mGlu3 was on axons (layer III: 35.9%; layer V: 44.1%), with labeling especially prominent within the intervaricose segments distant from axon terminals. mGlu3 were also found on glia (likely astrocytes), throughout the glial membrane (layer III: 28.2%; layer V: 29.5%). Importantly, mGlu3 could be seen on dendritic spines, especially in layer III (layer III: 15.6%; layer V: 8.2%), with minor labeling on dendrites. These data show that there are some similarities between mGlu3 expression in rat PL mPFC and macaque dlPFC, but the spine expression enriches and differentiates in the more recently evolved primate dlPFC.

The role of astrocytes in epileptic disorders

Epilepsy affects about 1% of the population and approximately 30% of epileptic patients are resistant to current antiepileptic drugs. As a hallmark in epileptic tissue, many of the epileptic patients show changes in glia morphology and function. There are characteristic changes in different types of glia in different epilepsy models. Some of these changes such as astrogliosis are enough to provoke epileptic seizures. Astrogliosis is well known in mesial temporal lobe epilepsy (MTLE), the most common form of refractory epilepsy. A better understanding of astrocytes alterations could lead to novel and efficient pharmacological approaches for epilepsy. In this review, we present the alterations of astrocyte morphology and function and present some instances of targeting astrocytes in seizure and epilepsy.

Glutamate Carboxypeptidase II in Aging Rat Prefrontal Cortex Impairs Working Memory Performance

Glutamate carboxypeptidase II (GCPII) expression in brain is increased by inflammation, and reduces NAAG (N-acetyl aspartyl glutamate) stimulation of mGluR3 signaling. Genetic insults in this signaling cascade are increasingly linked to cognitive disorders in humans, where increased GCPII and or decreased NAAG-mGluR3 are associated with impaired prefrontal cortical (PFC) activation and cognitive impairment. As aging is associated with increased inflammation and PFC cognitive deficits, the current study examined GCPII and mGluR3 expression in the aging rat medial PFC, and tested whether GCPII inhibition with 2-(3-mercaptopropyl) pentanedioic acid (2-MPPA) would improve working memory performance. We found that GCPII protein was expressed on astrocytes and some microglia as expected from previous studies, but was also prominently expressed on neurons, and showed increased levels with advancing age. Systemic administration of the GCPII inhibitor, 2-MPPA, improved working memory performance in young and aged rats, and also improved performance after local infusion into the medial PFC. As GCPII inhibitors are well-tolerated, they may provide an important new direction for treatment of cognitive disorders associated with aging and/or inflammation.

GAP junctions: multifaceted regulators of neuronal differentiation

Gap junctions are intercellular membrane channels consisting of connexin proteins, which contribute to direct cytoplasmic exchange of small molecules, substrates and metabolites between adjacent cells. These channels play important roles in neuronal differentiation, maintenance, survival and function. Gap junctions regulate differentiation of neurons from embryonic, neural and induced pluripotent stem cells. In addition, they control transdifferentiation of neurons from mesenchymal stem cells. The expression and levels of several connexins correlate with cell cycle changes and different stages of neurogenesis. Connexins such as Cx36, Cx45, and Cx26, play a crucial role in neuronal function. Several connexin knockout mice display lethal or severely impaired phenotypes. Aberrations in connexin expression is frequently associated with various neurodegenerative disorders. Gap junctions also act as promising therapeutic targets for neuronal regenerative medicine, because of their role in neural stem cell integration, injury and remyelination.

Regional Variation of Gap Junctional Connections in the Mammalian Inner Retina

The retinas of many species show regional specialisations that are evident in the differences in the processing of visual input from different parts of the visual field. Regional specialisation is thought to reflect an adaptation to the natural visual environment, optical constraints, and lifestyle of the species. Yet, little is known about regional differences in synaptic circuitry. Here, we were interested in the topographical distribution of connexin-36 (Cx36), the major constituent of electrical synapses in the retina. We compared the retinas of mice, rats, and cats to include species with different patterns of regional specialisations in the analysis. First, we used the density of Prox1-immunoreactive amacrine cells as a marker of any regional specialisation, with higher cell density signifying more central regions. Double-labelling experiments showed that Prox1 is expressed in AII amacrine cells in all three species. Interestingly, large Cx36 plaques were attached to about 8-10% of Prox1-positive amacrine cell somata, suggesting the strong electrical coupling of pairs or small clusters of cell bodies. When analysing the regional changes in the volumetric density of Cx36-immunoreactive plaques, we found a tight correlation with the density of Prox1-expressing amacrine cells in the ON, but not in the OFF sublamina in all three species. The results suggest that the relative contribution of electrical synapses to the ON- and OFF-pathways of the retina changes with retinal location, which may contribute to functional ON/OFF asymmetries across the visual field.

Optogenetically Controlled Activity Pattern Determines Survival Rate of Developing Neocortical Neurons

A substantial proportion of neurons undergoes programmed cell death (apoptosis) during early development. This process is attenuated by increased levels of neuronal activity and enhanced by suppression of activity. To uncover whether the mere level of activity or also the temporal structure of electrical activity affects neuronal death rates, we optogenetically controlled spontaneous activity of synaptically-isolated neurons in developing cortical cultures. Our results demonstrate that action potential firing of primary cortical neurons promotes neuronal survival throughout development. Chronic patterned optogenetic stimulation allowed to effectively modulate the firing pattern of single neurons in the absence of synaptic inputs while maintaining stable overall activity levels. Replacing the burst firing pattern with a non-physiological, single pulse pattern significantly increased cell death rates as compared to physiological burst stimulation. Furthermore, physiological burst stimulation led to an elevated peak in intracellular calcium and an increase in the expression level of classical activity-dependent targets but also decreased Bax/BCL-2 expression ratio and reduced caspase 3/7 activity. In summary, these results demonstrate at the single-cell level that the temporal pattern of action potentials is critical for neuronal survival versus cell death fate during cortical development, besides the pro-survival effect of action potential firing per se.

PKA-RIIβ autophosphorylation modulates PKA activity and seizure phenotypes in mice

Temporal lobe epilepsy (TLE) is one of the most common and intractable neurological disorders in adults. Dysfunctional PKA signaling is causally linked to the TLE. However, the mechanism underlying PKA involves in epileptogenesis is still poorly understood. In the present study, we found the autophosphorylation level at serine 114 site (serine 112 site in mice) of PKA-RIIβ subunit was robustly decreased in the epileptic foci obtained from both surgical specimens of TLE patients and seizure model mice. The p-RIIβ level was negatively correlated with the activities of PKA. Notably, by using a P-site mutant that cannot be autophosphorylated and thus results in the released catalytic subunit to exert persistent phosphorylation, an increase in PKA activities through transduction with AAV-RIIβ-S112A in hippocampal DG granule cells decreased mIPSC frequency but not mEPSC, enhanced neuronal intrinsic excitability and seizure susceptibility. In contrast, a reduction of PKA activities by RIIβ knockout led to an increased mIPSC frequency, a reduction in neuronal excitability, and mice less prone to experimental seizure onset. Collectively, our data demonstrated that the autophosphorylation of RIIβ subunit plays a critical role in controlling neuronal and network excitabilities by regulating the activities of PKA, providing a potential therapeutic target for TLE.

The role of connexins and pannexins in orofacial pain

Trigeminal neuralgia is characterized by extensive spreading of pain, referred to as ectopic pain, which describes the phenomenon of the pain passing from the injured regions to uninjured regions. Patients with orofacial pain often show no response to commonly used analgesics, and the exact mechanism of ectopic pain remains unclear, which restricts the development of specific drugs. The present review aims to summarize the contribution of the two families of transmembrane proteins, connexins (Cxs) and pannexins (Panxs), to the induction and spreading of orofacial pain and to provide potential targets for orofacial pain treatment. Cxs and Panxs have recently been shown to play essential roles in intercellular signal propagation in sensory ganglia, and previous studies have provided evidence for the contribution of several subtypes of Cxs and Panxs in various orofacial pain models. Upregulation of the expression of Cxs and Panxs in the trigeminal ganglia is observed in most cases after trigeminal injury, and regulating their expression or activity can improve pain-like behaviors in animals. It is speculated that after trigeminal injury, pain-related signals are transmitted to adjacent neurons and satellite glial cells in the trigeminal ganglia directly through gap junctions and simultaneously through hemichannels and pannexons through both autocrine and paracrine mechanisms. This review highlights recent discoveries in the regulation of Cxs and Panxs in different orofacial pain models and presents a hypothetical mechanism of ectopic pain in trigeminal neuralgia. In addition, the existing problems in current research are discussed.

Revisiting Traumatic Brain Injury: From Molecular Mechanisms to Therapeutic Interventions

Studying the complex molecular mechanisms involved in traumatic brain injury (TBI) is crucial for developing new therapies for TBI. Current treatments for TBI are primarily focused on patient stabilization and symptom mitigation. However, the field lacks defined therapies to prevent cell death, oxidative stress, and inflammatory cascades which lead to chronic pathology. Little can be done to treat the mechanical damage that occurs during the primary insult of a TBI; however, secondary injury mechanisms, such as inflammation, blood-brain barrier (BBB) breakdown, edema formation, excitotoxicity, oxidative stress, and cell death, can be targeted by therapeutic interventions. Elucidating the many mechanisms underlying secondary injury and studying targets of neuroprotective therapeutic agents is critical for developing new treatments. Therefore, we present a review on the molecular events following TBI from inflammation to programmed cell death and discuss current research and the latest therapeutic strategies to help understand TBI-mediated secondary injury.

Regulatory Roles of Metabotropic Glutamate Receptors on Synaptic Communication Mediated by Gap Junctions

Variations of synaptic strength are thought to underlie forms of learning and can functionally reshape neural circuits. Metabotropic glutamate receptors play key roles in regulating the strength of chemical synapses. However, information within neural circuits is also conveyed via a second modality of transmission: gap junction-mediated synapses. We review here evidence indicating that metabotropic glutamate receptors also play important roles in the regulation of synaptic communication mediated by neuronal gap junctions, also known as 'electrical synapses'. Activity-driven interactions between metabotropic glutamate receptors and neuronal gap junctions can lead to long-term changes in the strength of electrical synapses. Further, the regulatory action of metabotropic glutamate receptors on neuronal gap junctions is not restricted to adulthood but is also of critical relevance during brain development and contributes to the pathological mechanisms that follow brain injury.

Role of Connexins in Chronic Pain and Their Potential as Therapeutic Targets for Next-Generation Analgesics

Chronic pain, including inflammatory, neuropathic pain, is a serious clinical issue. There are increasing numbers of patients with chronic pain due to the growing number of elderly and it is estimated that about 25% of the global population will develop chronic pain. Chronic pain patients are refractory to medications used to treat acute pain such as opioids and non-steroidal anti-inflammatory drugs. Furthermore, the complexity and diversity of chronic pain mechanisms hinder the development of new analgesics. Thus, a better understanding of the mechanism of chronic pain is needed, which would facilitate the development of novel analgesics based on novel mechanisms. With this goal, connexins (Cxs) could be targeted for the development of new analgesics. Connexins are proteins with 20 subtypes, and function as channels, gap junctions between cells, and hemichannels that sample the extracellular space and release molecules such as neurotransmitters. Furthermore, Cxs could have functions independent of channel activity. Recent studies have shown that Cxs could be crucial in the induction and maintenance of chronic pain, and modulation of the activity or the expression of Cxs ameliorates nociceptive hypersensitivity in multiple chronic pain models. This review will cite novel findings on the role of of Cxs in the nociceptive transduction pathway under the chronic pain state and antinociceptive effects of various molecules modulating activity or expression of Cxs. Also, the potential of Cx modulation as a therapeutic strategy for intractable chronic pain will be discussed.

Regenerative Adaptation to Electrochemical Perturbation in Planaria: A Molecular Analysis of Physiological Plasticity

Anatomical homeostasis results from dynamic interactions between gene expression, physiology, and the external environment. Owing to its complexity, this cellular and organism-level phenotypic plasticity is still poorly understood. We establish planarian regeneration as a model for acquired tolerance to environments that alter endogenous physiology. Exposure to barium chloride (BaCl₂) results in a rapid degeneration of anterior tissue in Dugesia japonica. Remarkably, continued exposure to fresh solution of BaCl₂ results in regeneration of heads that are insensitive to BaCl₂. RNA-seq revealed transcriptional changes in BaCl₂-adapted heads that suggests a model of adaptation to excitotoxicity. Loss-of-function experiments confirmed several predictions: blockage of chloride and calcium channels allowed heads to survive initial BaCl₂ exposure, inducing adaptation without prior exposure, whereas blockade of TRPM channels reversed adaptation. Such highly adaptive plasticity may represent an attractive target for biomedical strategies in a wide range of applications beyond its immediate relevance to excitotoxicity preconditioning.

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Exposure to BaCl₂ causes the heads of Dugesia japonica to degenerate

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Prolonged exposure to BaCl₂ results in regeneration of a BaCl₂-insensitive head

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Ion channel expression is altered in the head to compensate for excitotoxic stress

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TRPMa is upregulated in BaCl₂-treated animals; blocking TRPM prevents adaptation

Dispelling myths about connexins, pannexins and P2X7 in hypoxic-ischemic central nervous system

In membrane physiology, as in other fields, myths or speculations may be repeated so often and so widely that they are perceived as facts. To some extent, this has occurred with regard to gap junctions, hemichannels, pannexin channels and P2X7 (ionotropic receptors), especially concerning the interpretation of the individual role of these channels in hypoxic-ischemic CNS since these channels may be closed by the same pharmacological blockers. Significance of existing controversial data are highlighted and contradictory views from different groups are critically discussed herein.

Genetic inhibition of PKCε attenuates neurodegeneration after global cerebral ischemia in male mice

Global cerebral ischemia that accompanies cardiac arrest is a major cause of morbidity and mortality. Protein Kinase C epsilon (PKCε) is a member of the novel PKC subfamily and plays a vital role in ischemic preconditioning. Pharmacological activation of PKCε before cerebral ischemia confers neuroprotection. The role of endogenous PKCε after cerebral ischemia remains elusive. Here we used male PKCε-null mice to assess the effects of PKCε deficiency on neurodegeneration after transient global cerebral ischemia (tGCI). We found that the cerebral vasculature, blood flow, and the expression of other PKC isozymes were not altered in the PKCε-null mice. Spatial learning and memory was impaired after tGCI, but the impairment was attenuated in male PKCε-null mice as compared to male wild-type controls. A significant reduction in Fluoro-Jade C labeling and mitochondrial release of cytochrome C in the hippocampus was found in male PKCε-null mice after tGCI. Male PKCε-null mice expressed increased levels of PKCδ in the mitochondria, which may prevent the translocation of PKCδ from the cytosol to the mitochondria after tGCI. Our results demonstrate the neuroprotective effects of PKCε deficiency on neurodegeneration after tGCI, and suggest that reduced mitochondrial translocation of PKCδ may contribute to the neuroprotective action in male PKCε-null mice.

Downregulation of Neuronal and Dendritic Connexin36-Made Electrical Synapses Without Glutamatergic Axon Terminals in Spinal Anterior Horn Cells From the Early Stage of Amyotrophic Lateral Sclerosis

Connexin36 (Cx36) forms gap junctions between neurons, which are called electrical synapses, enabling adjacent neurons to communicate directly. The participation of chemical synapses in neurodegeneration in amyotrophic lateral sclerosis (ALS) has long been indicated, but it remains unclear whether electrical synapses are involved in the pathogenesis of ALS. We performed extensive immunopathological analyses using mutant superoxide dismutase 1 (SOD1^G93A) transgenic mice and their littermates to investigate whether Cx36-made electrical synapses are affected in motor neuron diseases. We found that in the lamina IX of the lumbar spinal cord from wild type mice, about half of the Cx36 puncta existed independently of chemical synapse markers, while the rest coexisted with chemical synapse markers, such as vesicular glutamate transporter 1 (VGLUT1), which is a glutamatergic axon terminal marker, and/or glutamate decarboxylase 65 (GAD65), which is a GABAergic axon terminal marker. Cx36 single or Cx36/GAD65 double positive puncta, but not VGLUT1-containing puncta, were preferentially decreased on neuronal and dendritic surfaces of the anterior horn cells in the early stage of SOD1^G93A ALS mice. Moreover, in five human autopsied sporadic ALS cases with bulbar or upper limb onset, Cx36 immunoreactivity was diminished in the proximal dendrites and neuropils of well-preserved large motor neurons in the lumbar anterior horns. These findings suggest that downregulation of neuronal and dendritic Cx36 in the spinal anterior horns commonly occurs from the early stage of hereditary and sporadic ALS. Cx36-made electrical synapses without glutamatergic signaling appear to be more vulnerable than other chemical synapses and electrical synapses with glutamatergic signaling in the early stage of motor neuron degeneration, suggesting involvement of Cx36-made electrical synapses in the pathogenesis of human ALS.

Targeting gap junction in epilepsy: Perspectives and challenges

Gap junctions (GJs) are multiple cellular intercellular connections that allow ions to pass directly into the cytoplasm of neighboring cells. Electrical coupling mediated by GJs plays a role in the generation of highly synchronous electrical activity. Accumulative investigations show that GJs in the brain are involved in the generation, synchronization and maintenance of seizure events. At the same time, GJ blockers exert potent curative potential on epilepsy in vivo or in vitro. This review aims to shed light on the role of GJs in epileptogenesis. Targeting GJs is likely to be served as a novel therapeutic approach on epileptic patients.

The Effects of Ginsenoside Compound K Against Epilepsy by Enhancing the γ-Aminobutyric Acid Signaling Pathway

The imbalance between the GABA-mediated inhibition and the glutamate-mediated excitation is the primary pathological mechanism of epilepsy. GABAergic and glutamatergic neurotransmission have become the most important targets for controlling epilepsy. Ginsenoside compound K (GCK) is a main metabolic production of the ginsenoside Rb1, Rb2, and Rc in the intestinal microbiota. Previous studies show that GCK promoted the release of GABA from the hippocampal neurons and enhanced the activity of GABA_A receptors. GCK is shown to reduce the expression of NMDAR and to attenuate the function of the NMDA receptors in the brain. The anti-seizure effects of GCK have not been reported so far. Therefore, this study aimed to investigate the effects of GCK on epilepsy and its potential mechanism. The rat model of seizure or status epilepticus (SE) was established with either Pentylenetetrazole or Lithium chloride-pilocarpine. The Racine's scale was used to evaluate seizure activity. The levels of the amino acid neurotransmitters were detected in the pilocarpine-induced epileptic rats. The expression levels of GABA_ARα1, NMDAR1, KCC2, and NKCC1 protein in the hippocampus were determined via western blot or immunohistochemistry after SE. We found that GCK had deceased seizure intensity and prolonged the latency of seizures. GCK increased the contents of GABA, while the contents of glutamate remained unchanged. GCK enhanced the expression of GABA_ARα1 in the brain and exhibited a tendency to decrease the expression of NMDAR1 protein in the hippocampus. The expression of KCC2 protein was elevated by the treatment of GCK after SE, while the expression of NKCC1 protein was reversely down-regulated. These findings suggested that GCK exerted anti-epileptic effects by promoting the hippocampal GABA release and enhancing the GABA_AR-mediated inhibitory synaptic transmission.

Neuroprotective effects of leonurine against oxygen-glucose deprivation by targeting Cx36/CaMKII in PC12 cells

Leonurine has been reported to play an important role in ameliorating cognitive dysfunction, inhibiting ischemic stroke, and attenuating perihematomal edema and neuroinflammation in intracerebral hemorrhage. However, the exact mechanism and potential molecular targets of this effect remain unclear. Thus, in this study we investigated the neuroprotective effects of leonurine on hypoxia ischemia injury and explored the underlying mechanisms. An in vitro model of oxygen-glucose deprivation (OGD)-induced PC12 cells was established to mimic ischemic-like conditions. Cell viability, apoptosis, Cx36 and pCaMKII/CaMKII expression levels were evaluated after treatment with leonurine. The Cx36-selective antagonist mefloquine and CaMKII Inhibitor KN-93 were used to investigate the neuroprotective effect of leonurine on and the involvement of Cx36/CaMKII in this process. The results revealed that cell viability decreased and cell apoptosis and the protein expression of Cx36 and pCaMKII/CaMKII increased in the OGD-induced PC12 cells. Leonurine significantly increased cell viability and decreased cell apoptosis and the protein expression of Cx36 and pCaMKII/CaMKII in the OGD-induced PC12 cells. The specific inhibitor of Cx36 and CaMKII displayed similar protective effects. Moreover, the inhibition of Cx36 reduced pCaMKII levels and the ratio of pCaMKII/CaMKII in the OGD-induced PC12 cells, and vice versa. Taken together, these results suggest that leonurine might have a protective effect on OGD-induced PC12 cells through targeting the Cx36/CaMKII pathway. Thus, leonurine appears to have potential as a preventive or therapeutic drug against ischemic-induced neuronal injury.

A potential role for neuronal connexin 36 in the pathogenesis of amyotrophic lateral sclerosis

Neuronal gap junctional protein connexin 36 (Cx36) contributes to neuronal death following a range of acute brain insults such as ischemia, traumatic brain injury and epilepsy. Whether Cx36 contributes to neuronal death and pathological outcomes in chronic neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), is not known. We show here that the expression of Cx36 is significantly decreased in lumbar segments of the spinal cord of both human ALS subjects and SOD1G93A mice as compared to healthy human and wild-type mouse controls, respectively. In purified neuronal cultures prepared from the spinal cord of wild-type mice, knockdown of Cx36 reduces neuronal death caused by overexpression of the mutant human SOD1-G93A protein. Taken together, these data suggest a possible contribution of Cx36 to ALS pathogenesis. A perspective for the use of blockers of Cx36 gap junction channels for ALS therapy is discussed.

Arguments against the role of cortical spreading depression in migraine

Cortical spreading depression (CSD) is a wave of increased electrocortical activity and vasodilation, followed by sustained decreased activity and prolonged vasoconstriction. Although the discovery of CSD has been ascribed to Leão, rather than vasoconstriction, he only observed a depression of neural activity combined with vasodilation, with much weaker stimulation than used by his followers. There is a longstanding belief that CSD underlies migraine aura, with its positive symptoms such as mosaic patterns and its negative symptoms such as scotoma, and a similar propagation speed and vasoreaction pattern. However, there are many arguments against this theory. CSD is difficult to evoke in man, and electroencephalography (EEG) readings are not flattened during migraine (as opposed to EEG during CSD). Moreover, in contrast to CSD, migraine can occur bilaterally, and is not accompanied by a disrupted blood-brain barrier, increased cerebral metabolism, or cerebral cell swelling. Calcitonin gene-related peptide, which is thought to be characteristic of migraine pain, is increased in the blood from the external jugular vein during migraine in humans, but not during CSD in cats or rats. Moreover, CSD does not explain the appearance of premonitory symptoms or allodynia, long before the actual onset of aura. In addition, there is a variation in the pain mechanisms of migraine and CSD, and in their reaction to transcranial magnetic stimulation and several pharmacologic interventions. Finally, the origin of putative CSD in migraine is currently unknown.

Design principles of electrical synaptic plasticity

Essentially all animals with nervous systems utilize electrical synapses as a core element of communication. Electrical synapses, formed by gap junctions between neurons, provide rapid, bidirectional communication that accomplishes tasks distinct from and complementary to chemical synapses. These include coordination of neuron activity, suppression of voltage noise, establishment of electrical pathways that define circuits, and modulation of high order network behavior. In keeping with the omnipresent demand to alter neural network function in order to respond to environmental cues and perform tasks, electrical synapses exhibit extensive plasticity. In some networks, this plasticity can have dramatic effects that completely remodel circuits or remove the influence of certain cell types from networks. Electrical synaptic plasticity occurs on three distinct time scales, ranging from milliseconds to days, with different mechanisms accounting for each. This essay highlights principles that dictate the properties of electrical coupling within networks and the plasticity of the electrical synapses, drawing examples extensively from retinal networks.

Correlation between connexin and traumatic brain injury in patients

BACKGROUND

Identification of molecular alterations of damaged tissue in patients with neurological disorders can provide novel insight and potential therapeutic target for treatment of the diseases. It has been suggested by animal studies that connexins (CXs), a family of gap junction proteins, could contribute to neuronal cell death and associate with neurological deficits during trauma-induced damage. Nevertheless, whether specific CXs are involved in traumatic brain injury (TBI) has remained unexplored in human patients.

METHODS

In a clinical setting, we performed a correlation study of 131 TBI patients who received brain surgery. CXs (including CX40, CX43, and CX45) were examined in the harvested brain tissues for studying the relationships with the Glasgow Coma Scale scores of the patients.

RESULTS

Specifically, the protein levels of CX43 (negatively) and CX40 (positively) are associated with the extent of disease severity. Meanwhile, the phosphorylation status of CX43 was strongly associated with the severe TBI patients who contain relatively high kinase activities of PKC (protein kinase C) and MAPK (mitogen-activated protein kinase), two possible activators for CX43 phosphorylation.

CONCLUSION

These data highlight that a cluster of connexin family gap junction proteins not previously studied in humans is significantly correlated with the disease progression of TBI.

Gap junctions and hemichannels: communicating cell death in neurodevelopment and disease

Gap junctions are unique membrane channels that play a significant role in intercellular communication in the developing and mature central nervous system (CNS). These channels are composed of connexin proteins that oligomerize into hexamers to form connexons or hemichannels. Many different connexins are expressed in the CNS, with some specificity with regard to the cell types in which distinct connexins are found, as well as the timepoints when they are expressed in the developing and mature CNS. Both the main neuronal Cx36 and glial Cx43 play critical roles in neurodevelopment. These connexins also mediate distinct aspects of the CNS response to pathological conditions. An imbalance in the expression, translation, trafficking and turnover of connexins, as well as mutations of connexins, can impact their function in the context of cell death in neurodevelopment and disease. With the ever-increasing understanding of connexins in the brain, therapeutic strategies could be developed to target these membrane channels in various neurological disorders.

Prolonged Neuropsychiatric Symptoms in a Military Service Member Exposed to Mefloquine

A 32-year-old male developed neuropsychiatric symptoms 2 weeks after starting mefloquine 250 mg/week for malaria prophylaxis. He continued to take the medication for the next 4 months. Initial symptoms included vivid dreams and anxiety, as well as balance problems. These symptoms persisted and progressed over the next 4 years to include vertigo, emotional lability, and poor short-term memory, which have greatly affected his personal and professional life. An extensive evaluation revealed objective evidence supporting a central vestibulopathy. These symptoms have been unresponsive to pharmacologic therapy and psychotherapy. A Naranjo assessment score of 6 was obtained for his initial symptoms, indicating a probable adverse drug reaction to mefloquine given the relationship between the clinical picture and drug exposure.

Characteristics and plasticity of electrical synaptic transmission

Electrical synapses are an omnipresent feature of nervous systems, from the simple nerve nets of cnidarians to complex brains of mammals. Formed by gap junction channels between neurons, electrical synapses allow direct transmission of voltage signals between coupled cells. The relative simplicity of this arrangement belies the sophistication of these synapses. Coupling via electrical synapses can be regulated by a variety of mechanisms on times scales ranging from milliseconds to days, and active properties of the coupled neurons can impart emergent properties such as signal amplification, phase shifts and frequency-selective transmission. This article reviews the biophysical characteristics of electrical synapses and some of the core mechanisms that control their plasticity in the vertebrate central nervous system.

Two-color fluorescent analysis of connexin 36 turnover: relationship to functional plasticity

Gap junctions formed of connexin 36 (Cx36, also known as Gjd2) show tremendous functional plasticity on several time scales. Changes in connexin phosphorylation modify coupling in minutes through an order of magnitude, but recent studies also imply involvement of connexin turnover in regulating cell-cell communication. We utilized Cx36 with an internal HaloTag to study Cx36 turnover and trafficking in cultured cells. Irreversible, covalent pulse-chase labeling with fluorescent HaloTag ligands allowed clear discrimination of newly formed and pre-existing Cx36. Cx36 in junctional plaques turned over with a half-life of 3.1 h, and the turnover rate was unchanged by manipulations of protein kinase A (PKA) activity. In contrast, changes in PKA activity altered coupling within 20 min. New Cx36 in cargo vesicles was added directly to existing gap junctions and newly made Cx36 was not confined to points of addition, but diffused throughout existing gap junctions. Existing connexins also diffused into photobleached areas with a half-time of less than 2 s. In conclusion, studies of Cx36-HaloTag revealed novel features of connexin trafficking and demonstrated that phosphorylation-based changes in coupling occur on a different time scale than turnover.

Baclofen mediates neuroprotection on hippocampal CA1 pyramidal cells through the regulation of autophagy under chronic cerebral hypoperfusion

GABA receptors play an important role in ischemic brain injury. Studies have indicated that autophagy is closely related to neurodegenerative diseases. However, during chronic cerebral hypoperfusion, the changes of autophagy in the hippocampal CA1 area, the correlation between GABA receptors and autophagy, and their influences on hippocampal neuronal apoptosis have not been well established. Here, we found that chronic cerebral hypoperfusion resulted in rat hippocampal atrophy, neuronal apoptosis, enhancement and redistribution of autophagy, down-regulation of Bcl-2/Bax ratio, elevation of cleaved caspase-3 levels, reduction of surface expression of GABAA receptor α1 subunit and an increase in surface and mitochondrial expression of connexin 43 (CX43) and CX36. Chronic administration of GABAB receptors agonist baclofen significantly alleviated neuronal damage. Meanwhile, baclofen could up-regulate the ratio of Bcl-2/Bax and increase the activation of Akt, GSK-3β and ERK which suppressed cytodestructive autophagy. The study also provided evidence that baclofen could attenuate the decrease in surface expression of GABAA receptor α1 subunit, and down-regulate surface and mitochondrial expression of CX43 and CX36, which might enhance protective autophagy. The current findings suggested that, under chronic cerebral hypoperfusion, the effects of GABAB receptors activation on autophagy regulation could reverse neuronal damage.

Connexins in neurons and glia: targets for intervention in disease and injury

Both neurons and glia throughout the central nervous system are organized into networks by gap junctions. Among glia, gap junctions facilitate metabolic homeostasis and intercellular communication. Among neurons, gap junctions form electrical synapses that function primarily for communication. However, in neurodegenerative states due to disease or injury gap junctions may be detrimental to survival. Electrical synapses may facilitate hyperactivity and bystander killing among neurons, while gap junction hemichannels in glia may facilitate inflammatory signaling and scar formation. Advances in understanding mechanisms of plasticity of electrical synapses and development of molecular therapeutics to target glial gap junctions and hemichannels offer new hope to pharmacologically limit neuronal degeneration and enhance recovery.

Hippocampal GABAergic interneurons coexpressing alpha7-nicotinic receptors and connexin-36 are able to improve neuronal viability under oxygen-glucose deprivation

The hippocampal interneurons are very diverse by chemical profiles and rather inconsistent by sensitivity to CI. Some hippocampal GABAergic interneurons survive certain time after ischemia while ischemia-sensitive interneurons and pyramidal neurons are damaged. GABAergic signaling, nicotinic receptors expressing α7-subunit (α7nAChRs(+)) and connexin-36 (Cx36(+), electrotonic gapjunctions protein) contradictory modulate post-ischemic environment. We hypothesized that hippocampal ischemia-resistant GABAergic interneurons coexpressing glutamate decarboxylase-67 isoform (GAD67(+)), α7nAChRs(+), Cx36(+) are able to enhance neuronal viability. To check this hypothesis the histochemical and electrophysiological investigations have been performed using rat hippocampal organotypic culture in the condition of 30-min oxygen-glucose deprivation (OGD). Post-OGD reoxygenation (4h) revealed in CA1 pyramidal layer numerous damaged cells, decreased population spike amplitude and increased pair-pulse depression. In these conditions GAD67(+) interneurons displayed the OGD-resistance and significant increase of GABA synthesis/metabolism (GAD67-immunofluorescence, mitochondrial activity). The α7nAChRs(+) and Cx36(+) co-localizations were revealed in resistant GAD67(+) interneurons. Under OGD: GABAA-receptors (GABAARs) blockade increased cell damage and exacerbated the pair-pulse depression in CA1 pyramidal layer; α7nAChRs and Cx36-channels separate blockades sufficiently decreased cell damage while interneuronal GAD67-immunofluorescence and mitochondrial activity were similar to the control. Thus, hippocampal GABAergic interneurons co-expressing α7nAChRs and Cx36 remained resistant certain time after OGD and were able to modulate CA1 neuron survival through GABAARs, α7nAChRs and Cx36-channels activity. The enhancements of the neuronal viability together with GABA synthesis/metabolism normalization suggest cooperative neuroprotective mechanism that could be used for increase in efficiency of therapeutic strategies against post-ischemic pathology.

Connexin and pannexin signaling pathways, an architectural blueprint for CNS physiology and pathology?

The central nervous system (CNS) is composed of a highly heterogeneous population of cells. Dynamic interactions between different compartments (neuronal, glial, and vascular systems) drive CNS function and allow to integrate and process information as well as to respond accordingly. Communication within this functional unit, coined the neuro-glio-vascular unit (NGVU), typically relies on two main mechanisms: direct cell-cell coupling via gap junction channels (GJCs) and paracrine communication via the extracellular compartment, two routes to which channels composed of transmembrane connexin (Cx) or pannexin (Panx) proteins can contribute. Multiple isoforms of both protein families are present in the CNS and each CNS cell type is characterized by a unique Cx/Panx portfolio. Over the last two decades, research has uncovered a multilevel platform via which Cxs and Panxs can influence different cellular functions within a tissue: (1) Cx GJCs enable a direct cell-cell communication of small molecules, (2) Cx hemichannels and Panx channels can contribute to autocrine/paracrine signaling pathways, and (3) different structural domains of these proteins allow for channel-independent functions, such as cell-cell adhesion, interactions with the cytoskeleton, and the activation of intracellular signaling pathways. In this paper, we discuss current knowledge on their multifaceted contribution to brain development and to specific processes in the NGVU, including synaptic transmission and plasticity, glial signaling, vasomotor control, and blood-brain barrier integrity in the mature CNS. By highlighting both physiological and pathological conditions, it becomes evident that Cxs and Panxs can play a dual role in the CNS and that an accurate fine-tuning of each signaling mechanism is crucial for normal CNS physiology.

Connexin: a potential novel target for protecting the central nervous system?

Connexin subunits are proteins that form gap junction channels, and play an important role in communication between adjacent cells. This review article discusses the function of connexins/hemichannels/gap junctions under physiological conditions, and summarizes the findings regarding the role of connexins/hemichannels/gap junctions in the physiological and pathological mechanisms underlying central nervous system diseases such as brain ischemia, traumatic brain and spinal cord injury, epilepsy, brain and spinal cord tumor, migraine, neuroautoimmune disease, Alzheimer's disease, Parkinson's disease, X-linked Charcot-Marie-Tooth disease, Pelizaeus-Merzbacher-like disease, spastic paraplegia and maxillofacial dysplasia. Connexins are considered to be a potential novel target for protecting the central nervous system.

Death of Neurons following Injury Requires Conductive Neuronal Gap Junction Channels but Not a Specific Connexin

Pharmacological blockade or genetic knockout of neuronal connexin 36 (Cx36)-containing gap junctions reduces neuronal death caused by ischemia, traumatic brain injury and NMDA receptor (NMDAR)-mediated excitotoxicity. However, whether Cx36 gap junctions contribute to neuronal death via channel-dependent or channel-independent mechanism remains an open question. To address this, we manipulated connexin protein expression via lentiviral transduction of mouse neuronal cortical cultures and analyzed neuronal death twenty-four hours following administration of NMDA (a model of NMDAR excitotoxicity) or oxygen-glucose deprivation (a model of ischemic injury). In cultures prepared from wild-type mice, over-expression and knockdown of Cx36-containing gap junctions augmented and prevented, respectively, neuronal death from NMDAR-mediated excitotoxicity and ischemia. In cultures obtained form from Cx36 knockout mice, re-expression of functional gap junction channels, containing either neuronal Cx36 or non-neuronal Cx43 or Cx31, resulted in increased neuronal death following insult. In contrast, the expression of communication-deficient gap junctions (containing mutated connexins) did not have this effect. Finally, the absence of ethidium bromide uptake in non-transduced wild-type neurons two hours following NMDAR excitotoxicity or ischemia suggested the absence of active endogenous hemichannels in those neurons. Taken together, these results suggest a role for neuronal gap junctions in cell death via a connexin type-independent mechanism that likely relies on channel activities of gap junctional complexes among neurons. A possible contribution of gap junction channel-permeable death signals in neuronal death is discussed.

Downregulation of connexin36 in mouse spinal dorsal horn neurons leads to mechanical allodynia

Connexin36 (Cx36), a component of neuronal gap junctions, is crucial for interneuronal communication and regulation. Gap junction dysfunction underlies neurological disorders, including chronic pain. Following a peripheral nerve injury, Cx36 expression in the ipsilateral spinal dorsal horn was markedly decreased over time, which paralleled the time course of hind paw tactile allodynia. Intrathecal (i.t.) injection of Cx36 siRNA (1 and 5 pg) significantly reduced the expression of Cx36 protein in the lumbar spinal cord, peaking 3 days after the injection, which corresponded with the onset of hind paw tactile allodynia. It is possible that some of the tactile allodynia resulting from Cx36 downregulation could be mediated through excitatory neuromodulators, such as glutamate and substance P. The Cx36 knockdown-evoked tactile allodynia was significantly attenuated by i.t. treatment with the N-methyl-D-aspartate glutamate receptor antagonist MK-801 but not the substance P receptor antagonist CP96345. Immunohistochemistry showed that Cx36 was colocalized with glycine transporter-2, a marker for inhibitory glycinergic spinal interneurons, but not with glutamate decarboxylase 67, a marker for inhibitory GABAergic spinal interneurons. The results indicate that spinal inhibition through glycinergic interneurons is reduced, leading to increased glutamatergic neurotransmission, as a result of Cx36 downregulation. The current data suggest that gap junction dysfunction underlies neuropathic pain and further suggest a novel target for the development of analgesics.