Neuronal gap junctions: making and breaking connections during development and injury

Targeting connexins: possible game changer in managing neuropathic pain?

Neuropathic pain is a chronic debilitating condition caused by nerve injury or a variety of diseases. At the core of neuropathic pain lies the aberrant neuronal excitability in the peripheral and/or central nervous system (PNS and CNS). Enhanced connexin expression and abnormal activation of connexin-assembled gap junctional channels are prominent in neuropathic pain along with reactive gliosis, contributing to neuronal hypersensitivity and hyperexcitability. In this review, we delve into the current understanding of how connexin expression and function contribute to the pathogenesis and pathophysiology of neuropathic pain and argue for connexins as potential therapeutic targets for neuropathic pain management.

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.

Connexin Gap Junction Channels and Hemichannels: Insights from High-Resolution Structures

Connexins (Cxs) are a family of integral membrane proteins, which function as both hexameric hemichannels (HCs) and dodecameric gap junction channels (GJCs), behaving as conduits for the electrical and molecular communication between cells and between cells and the extracellular environment, respectively. Their proper functioning is crucial for many processes, including development, physiology, and response to disease and trauma. Abnormal GJC and HC communication can lead to numerous pathological states including inflammation, skin diseases, deafness, nervous system disorders, and cardiac arrhythmias. Over the last 15 years, high-resolution X-ray and electron cryomicroscopy (cryoEM) structures for seven Cx isoforms have revealed conservation in the four-helix transmembrane (TM) bundle of each subunit; an αβ fold in the disulfide-bonded extracellular loops and inter-subunit hydrogen bonding across the extracellular gap that mediates end-to-end docking to form a tight seal between hexamers in the GJC. Tissue injury is associated with cellular Ca2+ overload. Surprisingly, the binding of 12 Ca2+ ions in the Cx26 GJC results in a novel electrostatic gating mechanism that blocks cation permeation. In contrast, acidic pH during tissue injury elicits association of the N-terminal (NT) domains that sterically blocks the pore in a "ball-and-chain" fashion. The NT domains under physiologic conditions display multiple conformational states, stabilized by protein-protein and protein-lipid interactions, which may relate to gating mechanisms. The cryoEM maps also revealed putative lipid densities within the pore, intercalated among transmembrane α-helices and between protomers, the functions of which are unknown. For the future, time-resolved cryoEM of isolated Cx channels as well as cryotomography of GJCs and HCs in cells and tissues will yield a deeper insight into the mechanisms for channel regulation. The cytoplasmic loop (CL) and C-terminal (CT) domains are divergent in sequence and length, are likely involved in channel regulation, but are not visualized in the high-resolution X-ray and cryoEM maps presumably due to conformational flexibility. We expect that the integrated use of synergistic physicochemical, spectroscopic, biophysical, and computational methods will reveal conformational dynamics relevant to functional states. We anticipate that such a wealth of results under different pathologic conditions will accelerate drug discovery related to Cx channel modulation.

Disruption of Cholinergic Retinal Waves Alters Visual Cortex Development and Function

Retinal waves represent an early form of patterned spontaneous neural activity in the visual system. These waves originate in the retina before eye-opening and propagate throughout the visual system, influencing the assembly and maturation of subcortical visual brain regions. However, because it is technically challenging to ablate retina-derived cortical waves without inducing compensatory activity, the role these waves play in the development of the visual cortex remains unclear. To address this question, we used targeted conditional genetics to disrupt cholinergic retinal waves and their propagation to select regions of primary visual cortex, which largely prevented compensatory patterned activity. We find that loss of cholinergic retinal waves without compensation impaired the molecular and synaptic maturation of excitatory neurons located in the input layers of visual cortex, as well as layer 1 interneurons. These perinatal molecular and synaptic deficits also relate to functional changes observed at later ages. We find that the loss of perinatal cholinergic retinal waves causes abnormal visual cortex retinotopy, mirroring changes in the retinotopic organization of gene expression, and additionally impairs the processing of visual information. We further show that retinal waves are necessary for higher order processing of sensory information by impacting the state-dependent activity of layer 1 interneurons, a neuronal type that shapes neocortical state-modulation, as well as for state-dependent gain modulation of visual responses of excitatory neurons. Together, these results demonstrate that a brief targeted perinatal disruption of patterned spontaneous activity alters early cortical gene expression as well as synaptic and physiological development, and compromises both fundamental and, notably, higher-order functions of visual cortex after eye-opening.

Cellular junction dynamics and Alzheimer's disease: a comprehensive review

Alzheimer's disease (AD) is a prevalent neurodegenerative disorder characterized by progressive neuronal damage and cognitive decline. Recent studies have shed light on the involvement of not only the blood-brain barrier (BBB) dysfunction but also significant alterations in cellular junctions in AD pathogenesis. In this review article, we explore the role of the BBB and cellular junctions in AD pathology, with a specific focus on the hippocampus. The BBB acts as a crucial protective barrier between the bloodstream and the brain, maintaining brain homeostasis and regulating molecular transport. Preservation of BBB integrity relies on various junctions, including gap junctions formed by connexins, tight junctions composed of proteins such as claudins, occludin, and ZO-1, as well as adherence junctions involving molecules like vascular endothelial (VE) cadherin, Nectins, and Nectin-like molecules (Necls). Abnormalities in these junctions and junctional components contribute to impaired neuronal signaling and increased cerebrovascular permeability, which are closely associated with AD advancement. By elucidating the underlying molecular mechanisms governing BBB and cellular junction dysfunctions within the context of AD, this review offers valuable insights into the pathogenesis of AD and identifies potential therapeutic targets for intervention.

Time-dependent homeostatic mechanisms underlie brain-derived neurotrophic factor action on neural circuitry

Plasticity and homeostatic mechanisms allow neural networks to maintain proper function while responding to physiological challenges. Despite previous work investigating morphological and synaptic effects of brain-derived neurotrophic factor (BDNF), the most prevalent growth factor in the central nervous system, how exposure to BDNF manifests at the network level remains unknown. Here we report that BDNF treatment affects rodent hippocampal network dynamics during development and recovery from glutamate-induced excitotoxicity in culture. Importantly, these effects are not obvious when traditional activity metrics are used, so we delve more deeply into network organization, functional analyses, and in silico simulations. We demonstrate that BDNF partially restores homeostasis by promoting recovery of weak and medium connections after injury. Imaging and computational analyses suggest these effects are caused by changes to inhibitory neurons and connections. From our in silico simulations, we find that BDNF remodels the network by indirectly strengthening weak excitatory synapses after injury. Ultimately, our findings may explain the difficulties encountered in preclinical and clinical trials with BDNF and also offer information for future trials to consider.

Single-neuron mechanical perturbation evokes calcium plateaus that excite and modulate the network

Mechanical stimulation is a promising means to non-invasively excite and modulate neuronal networks with a high spatial resolution. Despite the thorough characterization of the initiation mechanism, whether or how mechanical responses disperse into non-target areas remains to be discovered. Our in vitro study demonstrates that a single-neuron deformation evokes responses that propagate to about a third of the untouched neighbors. The responses develop via calcium influx through mechanosensitive channels and regeneratively propagate through the neuronal ensemble via gap junctions. Although independent of action potentials and synapses, mechanical responses reliably evoke membrane depolarizations capable of inducing action potentials both in the target and neighbors. Finally, we show that mechanical stimulation transiently potentiates the responding assembly for further inputs, as both gain and excitability are transiently increased exclusively in neurons that respond to a neighbor's mechanical stimulation. The findings indicate a biological component affecting the spatial resolution of mechanostimulation and point to a cross-talk in broad-network mechanical stimulations. Since giga-seal formation in patch-clamp produces a similar mechanical stimulus on the neuron, our findings inform which neuroscientific questions could be reliably tackled with patch-clamp and what recovery post-gigaseal formation is necessary.

Loss of Ephaptic Contacts in the Murine Thalamus during Osmotic Demyelination Syndrome

BACKGROUND AND AIM

A murine model mimicking osmotic demyelination syndrome (ODS) revealed with histology in the relay posterolateral (VPL) and ventral posteromedial (VPM) thalamic nuclei adjoined nerve cell bodies in chronic hyponatremia, amongst the damaged 12 h and 48 h after reinstatement of osmolality. This report aims to verify and complement with ultrastructure other neurophysiology, immunohistochemistry, and molecular biochemistry data to assess the connexin-36 protein, as part of those hinted close contacts.This ODS investigation included four groups of mice: Sham (NN; n = 13), hyponatremic (HN; n = 11), those sacrificed 12 h after a fast restoration of normal natremia (ODS12h; n = 6) and mice sacrificed 48 h afterward, or ODS48 h (n = 9). Out of these, thalamic zones samples included NN (n = 2), HN (n = 2), ODS12h (n = 3) and ODS48h (n = 3).

RESULTS

Ultrastructure illustrated junctions between nerve cell bodies that were immunolabeled with connexin36 (Cx36) with light microscopy and Western blots. These cell's junctions were reminiscent of low resistance junctions characterized in other regions of the CNS with electrophysiology. Contiguous neurons showed neurolemma contacts in intact and damaged tissues according to their location in the ODS zones, at 12 h and 48 h post correction along with other demyelinating alterations. Neurons and ephaptic contact measurements indicated the highest alterations, including nerve cell necrosis in the ODS epicenter and damages decreased toward the outskirts of the demyelinated zone.

CONCLUSION

Ephapses contained C × 36between intact or ODS injured neurons in the thalamus appeared to be resilient beyond the core degraded tissue injuries. These could maintain intercellular ionic and metabolite exchanges between these lesser injured regions and, thus, would partake to some brain plasticity repairs.

Gap junctions regulate the development of neural circuits in the neocortex

Gap junctions between cells are ubiquitously expressed in the developing brain. They are involved in major steps of neocortical development, including neurogenesis, cell migration, synaptogenesis, and neural circuit formation, and have been implicated in cortical column formation. Dysfunctional gap junctions can contribute to or even cause a variety of brain diseases. Although the role of gap junctions in neocortical development is better known, a comprehensive understanding of their functions is far from complete. Here we explore several critical open questions surrounding gap junctions and their involvement in neural circuit development. Addressing them will greatly impact our understanding of the fundamental mechanisms of neocortical structure and function as well as the etiology of brain disease.

Neurotransmitters in Type 2 Diabetes and the Control of Systemic and Central Energy Balance

Efficient signal transduction is important in maintaining the function of the nervous system across tissues. An intact neurotransmission process can regulate energy balance through proper communication between neurons and peripheral organs. This ensures that the right neural circuits are activated in the brain to modulate cellular energy homeostasis and systemic metabolic function. Alterations in neurotransmitters secretion can lead to imbalances in appetite, glucose metabolism, sleep, and thermogenesis. Dysregulation in dietary intake is also associated with disruption in neurotransmission and can trigger the onset of type 2 diabetes (T2D) and obesity. In this review, we highlight the various roles of neurotransmitters in regulating energy balance at the systemic level and in the central nervous system. We also address the link between neurotransmission imbalance and the development of T2D as well as perspectives across the fields of neuroscience and metabolism research.

Gap junction Delta-2b (gjd2b/Cx35.1) depletion causes hyperopia and visual-motor deficiencies in the zebrafish

The zebrafish is a powerful model to investigate the developmental roles of electrical synapses because many signaling pathways that regulate the development of the nervous system are highly conserved from fish to humans. Here, we provide evidence linking the mammalian connexin-36 (Cx36) ortholog gjd2b/Cx35.1, a major component of electrical synapses in the zebrafish, with a refractive error in the context of morphological, molecular, and behavioral changes of zebrafish larvae. Two abnormalities were identified. The optical coherence tomography analysis of the adult retina confirmed changes to the refractive properties caused by eye axial length reduction, leading to hyperopic shifts. The gjd2b/Cx35.1 depletion was also correlated with morphological changes to the head and body ratios in larvae. The differential expression of Wnt/ß-catenin signaling genes, connexins, and dopamine receptors suggested a contribution to the observed phenotypic differences. The alteration of visual-motor behavioral responses to abrupt light transitions was aggravated in larvae, providing evidence that cone photoreceptor cell activity was enhanced when gjd2b/Cx35.1 was depleted. The visual disturbances were reversed under low light conditions in gjd2b ^-/- /Cx35.1^-/- larvae. Since qRT-PCR data demonstrated that two rhodopsin genes were downregulated, we speculated that rod photoreceptor cells in gjd2b/Cx35.1^-/- larvae were less sensitive to bright light transitions, thus providing additional evidence that a cone-mediated process caused the VMR light-ON hyperactivity after losing Cx35.1 expression. Together, this study provides evidence for the role of gjd2b/Cx35.1 in the development of the visual system and visually guided behaviors.

A new path to mental disorders: Through gap junction channels and hemichannels

Behavioral disturbances related to emotional regulation, reward processing, cognition, sleep-wake regulation and activity/movement represent core symptoms of most common mental disorders. Increasing empirical and theoretical evidence suggests that normal functioning of these behavioral domains relies on fine graded coordination of neural and glial networks which are maintained and modulated by intercellular gap junction channels and unapposed pannexin or connexin hemichannels. Dysfunctions in these networks might contribute to the development and maintenance of psychopathological and neurobiological features associated with mental disorders. Here we review and discuss the evidence indicating a prominent role of gap junction channel and hemichannel dysfunction in core symptoms of mental disorders. We further discuss how the increasing knowledge on intercellular gap junction channels and unapposed pannexin or connexin hemichannels in the brain might lead to deeper mechanistic insight in common mental disorders and to the development of novel treatment approaches. We further attempt to exemplify what type of future research on this topic could be integrated into multidimensional approaches to understand and cure mental disorders.

Mechanisms underlying the role of ankyrin-B in cardiac and neurological health and disease

The ANK2 gene encodes for ankyrin-B (ANKB), one of 3 members of the ankyrin family of proteins, whose name is derived from the Greek word for anchor. ANKB was originally identified in the brain (B denotes “brain”) but has become most widely known for its role in cardiomyocytes as a scaffolding protein for ion channels and transporters, as well as an interacting protein for structural and signaling proteins. Certain loss-of-function ANK2 variants are associated with a primarily cardiac-presenting autosomal-dominant condition with incomplete penetrance and variable expressivity characterized by a predisposition to supraventricular and ventricular arrhythmias, arrhythmogenic cardiomyopathy, congenital and adult-onset structural heart disease, and sudden death. Another independent group of ANK2 variants are associated with increased risk for distinct neurological phenotypes, including epilepsy and autism spectrum disorders. The mechanisms underlying ANKB's roles in cells in health and disease are not fully understood; however, several clues from a range of molecular and cell biological studies have emerged. Notably, ANKB exhibits several isoforms that have different cell-type–, tissue–, and developmental stage– expression profiles. Given the conservation within ankyrins across evolution, model organism studies have enabled the discovery of several ankyrin roles that could shed important light on ANKB protein-protein interactions in heart and brain cells related to the regulation of cellular polarity, organization, calcium homeostasis, and glucose and fat metabolism. Along with this accumulation of evidence suggesting a diversity of important ANKB cellular functions, there is an on-going debate on the role of ANKB in disease. We currently have limited understanding of how these cellular functions link to disease risk. To this end, this review will examine evidence for the cellular roles of ANKB and the potential contribution of ANKB functional variants to disease risk and presentation. This contribution will highlight the impact of ANKB dysfunction on cardiac and neuronal cells and the significance of understanding the role of ANKB variants in disease.

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.

Absence of Connexin 43 results in smaller retinas and arrested, depolarized retinal progenitor cells in human retinal organoids

The development of the vertebrate retina relies on complex regulatory mechanisms to achieve its characteristic layered morphology containing multiple neuronal cell types. While connexin 43 (CX43) is not expressed by mature retinal neurons mutations in its gene GJA1 are associated with microphthalmia and low vision in patients. To delineate how lack of CX43 affects retinal development, GJA1 was disrupted in human induced pluripotent stem cells (hiPSCs) (GJA1-/-) using CRISPR/Cas9 editing, and these were subsequently differentiated into retinal organoids. GJA1-/- hiPSCs do not display defects in self-renewal and pluripotency, but the resulting organoids are smaller with a thinner neural retina and decreased abundance of many retinal cell types. CX43-deficient organoids express lower levels of the neural marker PAX6 and the retinal progenitor cell (RPC) markers PAX6, SIX3, and SIX6. Conversely, expression of the early neuroectoderm markers SOX1 and SOX2 remains high in GJA1-/- organoids throughout their development. Lack of CX43 results in an increased population of CHX10-positive RPCs that are smaller, disorganized, do not become polarized, and possess a limited ability to commit to retinal fate specification. Our data indicate that lack of CX43 causes a developmental arrest in RPCs that subsequently leads to pan-retinal defects and stunted ocular growth.

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.

Transmembrane H+ fluxes and the regulation of neural induction in Xenopus laevis

It has previously been reported that in ex vivo planar explants prepared from Xenopus laevis embryos, the intracellular pH (pHi) increases in cells of the dorsal ectoderm from stage 10.5 to 11.5 (i.e. 11-12.5 hpf). It was proposed that such increases (potentially due to H+ being extruded, sequestered, or buffered in some manner), play a role in regulating neural induction. Here, we used an extracellular ion-selective electrode to non-invasively measure H+ fluxes at eight locations around the equatorial circumference of intact X. laevis embryos between stages 9-12 (˜7-13.25 hpf). We showed that at stages 9-11, there was a small H+ efflux recorded from all the measuring positions. At stage 12 there was a small, but significant, increase in the efflux of H+ from most locations, but the efflux from the dorsal side of the embryo was significantly greater than from the other positions. Embryos were also treated from stages 9-12 with bafilomycin A1, to block the activity of the ATP-driven H+ pump. By stage 22 (24 hpf), these embryos displayed retarded development, arresting before the end of gastrulation and therefore did not display the usual anterior and neural structures, which were observed in the solvent-control embryos. In addition, expression of the early neural gene, Zic3, was absent in treated embryos compared with the solvent controls. Together, our new in vivo data corroborated and extended the earlier explant-derived report describing changes in pHi that were suggested to play a role during neural induction in X. laevis embryos.

Excitatory synapses and gap junctions cooperate to improve Pv neuronal burst firing and cortical social cognition in Shank2-mutant mice

NMDA receptor (NMDAR) and GABA neuronal dysfunctions are observed in animal models of autism spectrum disorders, but how these dysfunctions impair social cognition and behavior remains unclear. We report here that NMDARs in cortical parvalbumin (Pv)-positive interneurons cooperate with gap junctions to promote high-frequency (>80 Hz) Pv neuronal burst firing and social cognition. Shank2^–/– mice, displaying improved sociability upon NMDAR activation, show impaired cortical social representation and inhibitory neuronal burst firing. Cortical Shank2^–/– Pv neurons show decreased NMDAR activity, which suppresses the cooperation between NMDARs and gap junctions (GJs) for normal burst firing. Shank2^–/– Pv neurons show compensatory increases in GJ activity that are not sufficient for social rescue. However, optogenetic boosting of Pv neuronal bursts, requiring GJs, rescues cortical social cognition in Shank2^–/– mice, similar to the NMDAR-dependent social rescue. Therefore, NMDARs and gap junctions cooperate to promote cortical Pv neuronal bursts and social cognition.

How NMDAR and GABA neuronal dysfunctions result in impaired social behaviour is unclear. Here, the authors show that NMDARs and gap junctions in cortical PV interneurons modulate burst firing, affecting social behaviour.

Brain and testis: more alike than previously thought?

Several strands of evidence indicate the presence of marked similarities between human brain and testis. Understanding these similarities and their implications has become a topic of interest among the scientific community. Indeed, an association of intelligence with some semen quality parameters has been reported and a relation between dysfunctions of the human brain and testis has also been evident. Numerous common molecular features are evident when these tissues are compared, which is reflected in the huge number of common proteins. At the functional level, human neurons and sperm share a number of characteristics, including the importance of the exocytotic process and the presence of similar receptors and signalling pathways. The common proteins are mainly involved in exocytosis, tissue development and neuron/brain-associated biological processes. With this analysis, we conclude that human brain and testis share several biochemical characteristics which, in addition to their involvement in the speciation process, could, at least in part, be responsible for the expression of a huge number of common proteins. Nonetheless, this is an underexplored topic, and the connection between these tissues needs to be clarified, which could help to understand the dysfunctions affecting brain and testis, as well as to develop improved therapeutic strategies.

The potential antiepileptogenic effect of neuronal Cx36 gap junction channel blockage

Epilepsy is one of the most prevalent neurological disorders and can result in neuronal injury and degeneration. Consequently, research into new antiepileptic drugs capable of providing protection against neuronal injury and degeneration is extremely important. Neuronal Cx36 gap junction channels have been found to play an important role in epilepsy; thus, pharmacological interference using Cx36 gap junction channel blockers may be a promising strategy for disrupting the synchronization of neurons during seizure activity and protecting neurons. Based on these promising findings, several in vivo and in vitro studies are ongoing and the first encouraging results have been published. The results bring hope that neurons can be protected from injury and degeneration in patients with epilepsy, which is currently impossible.

Effects of Constitutive and Acute Connexin 36 Deficiency on Brain-Wide Susceptibility to PTZ-Induced Neuronal Hyperactivity

Connexins are transmembrane proteins that form hemichannels allowing the exchange of molecules between the extracellular space and the cell interior. Two hemichannels from adjacent cells dock and form a continuous gap junction pore, thereby permitting direct intercellular communication. Connexin 36 (Cx36), expressed primarily in neurons, is involved in the synchronous activity of neurons and may play a role in aberrant synchronous firing, as seen in seizures. To understand the reciprocal interactions between Cx36 and seizure-like neural activity, we examined three questions: (a) does Cx36 deficiency affect seizure susceptibility, (b) does seizure-like activity affect Cx36 expression patterns, and (c) does acute blockade of Cx36 conductance increase seizure susceptibility. We utilize the zebrafish pentylenetetrazol [PTZ; a GABA(A) receptor antagonist] induced seizure model, taking advantage of the compact size and optical translucency of the larval zebrafish brain to assess how PTZ affects brain-wide neuronal activity and Cx36 protein expression. We exposed wild-type and genetic Cx36-deficient (cx35.5-/-) zebrafish larvae to PTZ and subsequently mapped neuronal activity across the whole brain, using phosphorylated extracellular-signal-regulated kinase (pERK) as a proxy for neuronal activity. We found that cx35.5-/- fish exhibited region-specific susceptibility and resistance to PTZ-induced hyperactivity compared to wild-type controls, suggesting that genetic Cx36 deficiency may affect seizure susceptibility in a region-specific manner. Regions that showed increased PTZ sensitivity include the dorsal telencephalon, which is implicated in human epilepsy, and the lateral hypothalamus, which has been underexplored. We also found that PTZ-induced neuronal hyperactivity resulted in a rapid reduction of Cx36 protein levels within 30 min. This Cx36 reduction persists after 1-h of recovery but recovered after 3–6 h. This acute downregulation of Cx36 by PTZ is likely maladaptive, as acute pharmacological blockade of Cx36 by mefloquine results in increased susceptibility to PTZ-induced neuronal hyperactivity. Together, these results demonstrate a reciprocal relationship between Cx36 and seizure-associated neuronal hyperactivity: Cx36 deficiency contributes region-specific susceptibility to neuronal hyperactivity, while neuronal hyperactivity-induced downregulation of Cx36 may increase the risk of future epileptic events.

Connexin36 localization along axon initial segments in the mammalian CNS

Electrical synapses formed by gap junctions occur at a variety of neuronal subcellular sites in the mammalian central nervous system (CNS), including at somatic, dendritic and axon terminal compartments. Numerous electrophysiological studies using mice and rats, as well as computer modelling approaches, have predicted the additional occurrence of electrical synapses between axons near their emergence from neuronal somata. Here, we used immunofluorescence methods to search for localization of the neuronal gap junction-forming protein connexin36 (Cx36) along axon initial segments (AISs) labelled for the AIS marker ankyrinG. Immunofluorescent Cx36-puncta were found to be associated with AISs in several CNS regions of mice, including the spinal cord, inferior olive and cerebral cortex. Localization of Cx36-puncta at AISs was confirmed by confocal single scan and 3D imaging, immunofluorescence intensity profiling and high resolution structured illumination microscopy (SIM). AISs measuring up to 30 µm in length displayed typically a single Cx36-punctum and the incidence of these long AISs displaying Cx36-puncta ranged from 3% to 7% in the inferior olive and in various layers of the cerebral cortex. In the inferior olive, the gap junction associated protein zonula occludens-1 (ZO-1) was found to be co-localized with Cx36-puncta on AISs, indicating that these puncta have some of the molecular constituents of gap junctions. Our results add to the neuronal subcellular locations at which Cx36 is deployed, and raise possibilities for its involvement in novel functions in the AIS compartment.

Expression of Connexins 37, 43 and 45 in Developing Human Spinal Cord and Ganglia

Direct intercellular communication via gap junctions has an important role in the development of the nervous system, ranging from cell migration and neuronal differentiation to the formation of neuronal activity patterns. This study characterized and compared the specific spatio-temporal expression patterns of connexins (Cxs) 37, 43 and 45 during early human developmental stages (since the 5th until the 10th developmental week) in the spinal cord (SC) and dorsal root ganglia (DRG) using double immunofluorescence and transmission electron microscopy. We found the expression of all three investigated Cxs during early human development in all the areas of interest, in the SC, DRG, developing paravertebral ganglia of the sympathetic trunk, notochord and all three meningeal layers, with predominant expression of Cx37. Comparing the expression of different Cxs between distinct developmental periods, we did not find significant differences. Specific spatio-temporal pattern of Cxs expression might reflect their relevance in the development of all areas of interest via cellular interconnectivity and synchronization during the late embryonic and early fetal period of human development.

Neuromorphic Engineering: From Biological to Spike-Based Hardware Nervous Systems

The human brain is a sophisticated, high-performance biocomputer that processes multiple complex tasks in parallel with high efficiency and remarkably low power consumption. Scientists have long been pursuing an artificial intelligence (AI) that can rival the human brain. Spiking neural networks based on neuromorphic computing platforms simulate the architecture and information processing of the intelligent brain, providing new insights for building AIs. The rapid development of materials engineering, device physics, chip integration, and neuroscience has led to exciting progress in neuromorphic computing with the goal of overcoming the von Neumann bottleneck. Herein, fundamental knowledge related to the structures and working principles of neurons and synapses of the biological nervous system is reviewed. An overview is then provided on the development of neuromorphic hardware systems, from artificial synapses and neurons to spike-based neuromorphic computing platforms. It is hoped that this review will shed new light on the evolution of brain-like computing.

Mind the gap: A mechanobiological hypothesis for the role of gap junctions in the mechanical properties of injured brain tissue

Despite more than half a century of work on the brain biomechanics, there are still significant unknowns about this tissue. Since the brain is highly susceptible to injury, damage biomechanics has been one of the main areas of interest to the researchers in the field of brain biomechanics. In many previous studies, mechanical properties of brain tissue under sub-injury and injury level loading conditions have been addressed; however, to the best of our knowledge, the role of cell-cell interactions in the mechanical behavior of brain tissue has not been well examined yet. This note introduces the hypothesis that gap junctions as the major type of cell-cell junctions in the brain tissue play a pivotal role in the mechanical properties of the tissue and their failure during injury leads to changes in brain's material properties. According to this hypothesis, during an injury, the gap junctions are damaged, leading to a decrease in tissue stiffness, whereas following the injury, new junction proteins are expressed, leading to an increase in tissue stiffness. We suggest that considering the mechanobiological effect of gap junctions in the material properties of brain tissue may help better understand the brain injury mechanism.

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.

Fusogen-mediated neuron-neuron fusion disrupts neural circuit connectivity and alters animal behavior

Ramón y Cajal’s neuron doctrine, which states that neurons are individual cells that do not share any membrane or cytoplasmic continuity between them, has underpinned our view of modern neuroscience. However, there is considerable evidence that fusogens, specialized proteins essential and sufficient for the fusion of cells in other tissues, are expressed in the nervous system of several species in response to viral infection, stress conditions, and neurological disease. By manipulating the expression of fusogens in the chemosensory neurons of Caenorhabditis elegans, our results provide conclusive evidence that deregulation of fusogen expression causes neuronal fusion and can have deleterious effects on neural circuitry and behavioral outputs, revealing a possible novel underlying cause of neurological disorders.

The 100-y-old neuron doctrine from Ramón y Cajal states that neurons are individual cells, rejecting the process of cell−cell fusion in the normal development and function of the nervous system. However, fusogens—specialized molecules essential and sufficient for the fusion of cells—are expressed in the nervous system of different species under conditions of viral infection, stress, or disease. Despite these findings, whether the expression of fusogens in neurons leads to cell−cell fusion, and, if so, whether this affects neuronal fate, function, and animal behavior, has not been explored. Here, using Caenorhabditis elegans chemosensory neurons as a model system, we provide proof-of-principle that aberrant expression of fusogens in neurons results in neuron−neuron fusion and behavioral impairments. We demonstrate that fusion between chemoattractive neurons does not affect the response to odorants, whereas fusion between chemoattractive and chemorepulsive neurons compromises chemosensation. Moreover, we provide evidence that fused neurons are viable and retain their original specific neuronal fate markers. Finally, analysis of calcium transients reveals that fused neurons become electrically coupled, thereby compromising neural circuit connectivity. Thus, we propose that aberrant expression of fusogens in the nervous system disrupts neuronal individuality, which, in turn, leads to a change in neural circuit connectivity and disruption of normal behavior. Our results expose a previously uncharacterized basis of circuit malfunction, and a possible underlying cause of neurological diseases.

Channels to consciousness: a possible role of gap junctions in consciousness

The neurophysiological basis of consciousness is still unknown and one of the most challenging questions in the field of neuroscience and related disciplines. We propose that consciousness is characterized by the maintenance of mental representations of internal and external stimuli for the execution of cognitive operations. Consciousness cannot exist without working memory, and it is likely that consciousness and working memory share the same neural substrates. Here, we present a novel psychological and neurophysiological framework that explains the role of consciousness for cognition, adaptive behavior, and everyday life. A hypothetical architecture of consciousness is presented that is organized as a system of operation and storage units named platforms that are controlled by a consciousness center (central executive/online platform). Platforms maintain mental representations or contents, are entrusted with different executive functions, and operate at different levels of consciousness. The model includes conscious-mode central executive/online and mental time travel platforms and semiconscious steady-state and preconscious standby platforms. Mental representations or contents are represented by neural circuits and their support cells (astrocytes, oligodendrocytes, etc.) and become conscious when neural circuits reverberate, that is, fire sequentially and continuously with relative synchronicity. Reverberatory activity in neural circuits may be initiated and maintained by pacemaker cells/neural circuit pulsars, enhanced electronic coupling via gap junctions, and unapposed hemichannel opening. The central executive/online platform controls which mental representations or contents should become conscious by recruiting pacemaker cells/neural network pulsars, the opening of hemichannels, and promoting enhanced neural circuit coupling via gap junctions.

Dextran-coated iron oxide nanoparticle-induced nanotoxicity in neuron cultures

Recent technological advances have introduced diverse engineered nanoparticles (ENPs) into our air, water, medicine, cosmetics, clothing, and food. However, the health and environmental effects of these increasingly common ENPs are still not well understood. In particular, potential neurological effects are one of the most poorly understood areas of nanoparticle toxicology (nanotoxicology), in that low-to-moderate neurotoxicity can be subtle and difficult to measure. Culturing primary neuron explants on planar microelectrode arrays (MEAs) has emerged as one of the most promising in vitro techniques with which to study neuro-nanotoxicology, as MEAs enable the fluorescent tracking of nanoparticles together with neuronal electrical activity recording at the submillisecond time scale, enabling the resolution of individual action potentials. Here we examine the dose-dependent neurotoxicity of dextran-coated iron oxide nanoparticles (dIONPs), a common type of functionalized ENP used in biomedical applications, on cultured primary neurons harvested from postnatal day 0-1 mouse brains. A range of dIONP concentrations (5-40 µg/ml) were added to neuron cultures, and cells were plated either onto well plates for live cell, fluorescent reactive oxidative species (ROS) and viability observations, or onto planar microelectrode arrays (MEAs) for electrophysiological measurements. Below 10 µg/ml, there were no dose-dependent cellular ROS increases or effects in MEA bursting behavior at sub-lethal dosages. However, above 20 µg/ml, cell death was obvious and widespread. Our findings demonstrate a significant dIONP toxicity in cultured neurons at concentrations previously reported to be safe for stem cells and other non-neuronal cell types.

Evolutionary origins of chemical synapses

Synaptic transmission is a fundamental neurobiological process by which neurons interact with each other and non-neuronal cells. It involves release of active substances from the presynaptic neuron onto receptive elements of postsynaptic cells, inducing waves of spreading electrochemical response. While much has been learned about the cellular and molecular mechanisms driving and governing transmitter release and sensing, the evolutionary origin of synaptic connections remains obscure. Herein, we review emerging evidence and concepts suggesting that key components of chemical synapse arose independently from neurons, in different functional and biological contexts, before the rise of multicellular living forms. We argue that throughout evolution, distinct synaptic constituents have been co-opted from ancestral forms for a new role in early metazoan, leading to the rise of chemical synapses and neurotransmission. Such a mosaic model of the origin of chemical synapses agrees with and supports the pluralistic hypothesis of evolutionary change.

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.

Neuronal Glutamatergic Network Electrically Wired with Silent But Activatable Gap Junctions

It is widely assumed that electrical synapses in the mammalian brain, especially between interneurons, underlie neuronal synchrony. In the hippocampus, principal cells also establish electrical synapses with each other and have also been implicated in network oscillations, whereby the origin of fast electrical activity has been attributed to ectopic spikelets and dendro-dendritic or axo-axonal gap junctions. However, if electrical synapses were in axo-dendritic connections, where chemical synapses occur, the synaptic events would be mixed, having an electrical component preceding the chemical one. This type of communication is less well studied, mainly because it is not easily detected. Moreover, a possible scenario could be that an electrical synapse coexisted with a chemical one, but in a nonconductive state; hence, it would be considered inexistent. Could chemical synapses have a quiescent electrical component? If so, can silent electrical synapses be activated to be detected? We addressed this possibility, and we here report that, indeed, the connexin-36-containing glutamatergic mossy fiber synapses of the rat hippocampus express previously unrecognized electrical synapses, which are normally silent. We reveal that these synapses are pH sensitive, actuate in vitro and in vivo, and that the electrical signaling is bidirectional. With the simultaneous recording of hundreds of cells, we could reveal the existence of an electrical circuit in the hippocampus of adult rats of either sex consisting of principal cells where the nodes are interregional glutamatergic synapses containing silent but ready-to-use gap junctions.SIGNIFICANCE STATEMENT In this work, we present a series of experiments, both in vitro and in vivo, that reveal previously unrecognized silent pH-sensitive electrical synapses coexisting in one of the best studied glutamatergic synapses of the brain, the mossy fiber synapse of the hippocampus. This type of connectivity underlies an "electrical circuit" between two substructures of the adult rat hippocampus consisting of principal cells where the nodes are glutamatergic synapses containing silent but ready-to-use gap junctions. Its identification will allow us to explore the participation of such a circuit in physiological and pathophysiological functions and will provide valuable conceptual tools to understanding computational and regulatory mechanisms that may underlie network activity.

Role of Connexins 30, 36, and 43 in Brain Tumors, Neurodegenerative Diseases, and Neuroprotection

Gap junction (GJ) channels and their connexins (Cxs) are complex proteins that have essential functions in cell communication processes in the central nervous system (CNS). Neurons, astrocytes, oligodendrocytes, and microglial cells express an extraordinary repertory of Cxs that are important for cell to cell communication and diffusion of metabolites, ions, neurotransmitters, and gliotransmitters. GJs and Cxs not only contribute to the normal function of the CNS but also the pathological progress of several diseases, such as cancer and neurodegenerative diseases. Besides, they have important roles in mediating neuroprotection by internal or external molecules. However, regulation of Cx expression by epigenetic mechanisms has not been fully elucidated. In this review, we provide an overview of the known mechanisms that regulate the expression of the most abundant Cxs in the central nervous system, Cx30, Cx36, and Cx43, and their role in brain cancer, CNS disorders, and neuroprotection. Initially, we focus on describing the Cx gene structure and how this is regulated by epigenetic mechanisms. Then, the posttranslational modifications that mediate the activity and stability of Cxs are reviewed. Finally, the role of GJs and Cxs in glioblastoma, Alzheimer's, Parkinson's, and Huntington's diseases, and neuroprotection are analyzed with the aim of shedding light in the possibility of using Cx regulators as potential therapeutic molecules.

A Genetic Model of the Connectome

The connectomes of organisms of the same species show remarkable architectural and often local wiring similarity, raising the question: where and how is neuronal connectivity encoded? Here, we start from the hypothesis that the genetic identity of neurons guides synapse and gap-junction formation and show that such genetically driven wiring predicts the existence of specific biclique motifs in the connectome. We identify a family of large, statistically significant biclique subgraphs in the connectomes of three species and show that within many of the observed bicliques the neurons share statistically significant expression patterns and morphological characteristics, supporting our expectation of common genetic factors that drive the synapse formation within these subgraphs. The proposed connectome model offers a self-consistent framework to link the genetics of an organism to the reproducible architecture of its connectome, offering experimentally falsifiable predictions on the genetic factors that drive the formation of individual neuronal circuits.

Characterizing signal encoding and transmission in class I and class II neurons via ordinal time-series analysis

Neurons encode and transmit information in spike sequences. However, despite the effort devoted to understand the encoding and transmission of information, the mechanisms underlying the neuronal encoding are not yet fully understood. Here, we use a nonlinear method of time-series analysis (known as ordinal analysis) to compare the statistics of spike sequences generated by applying an input signal to the neuronal model of Morris-Lecar. In particular, we consider two different regimes for the neurons which lead to two classes of excitability: class I, where the frequency-current curve is continuous and class II, where the frequency-current curve is discontinuous. By applying ordinal analysis to sequences of inter-spike-intervals (ISIs) our goals are (1) to investigate if different neuron types can generate spike sequences which have similar symbolic properties; (2) to get deeper understanding on the effects that electrical (diffusive) and excitatory chemical (i.e., excitatory synapse) couplings have; and (3) to compare, when a small-amplitude periodic signal is applied to one of the neurons, how the signal features (amplitude and frequency) are encoded and transmitted in the generated ISI sequences for both class I and class II type neurons and electrical or chemical couplings. We find that depending on the frequency, specific combinations of neuron/class and coupling-type allow a more effective encoding, or a more effective transmission of the signal.

Memristive plasticity in artificial electrical synapses via geometrically reconfigurable, gramicidin-doped biomembranes

It is now known that mammalian brains leverage plasticity of both chemical and electrical synapses (ES) for collocating memory and processing. Unlike chemical synapses, ES join neurons via gap junction ion channels that permit fast, threshold-independent, and bidirectional ion transport. Like chemical synapses, ES exhibit activity-dependent plasticity, which modulates the ionic conductance between neurons and, thereby, enables adaptive synchronization of action potentials. Many types of adaptive computing devices that display discrete, threshold-dependent changes in conductance have been developed, yet far less effort has been devoted to emulating the continuously variable conductance and activity-dependent plasticity of ES. Here, we describe an artificial electrical synapse (AES) that exhibits voltage-dependent, analog changes in ionic conductance at biologically relevant voltages. AES plasticity is achieved at the nanoscale by linking dynamical geometrical changes of a host lipid bilayer to ion transport via gramicidin transmembrane ion channels. As a result, the AES uniquely mimics the composition, biophysical properties, bidirectional and threshold-independent ion transport, and plasticity of ES. Through experiments and modeling, we classify our AES as a volatile memristor, where the voltage-controlled conductance is governed by reversible changes in membrane geometry and gramicidin channel density. Simulations show that AES plasticity can adaptively synchronize Hodgkin-Huxley neurons. Finally, by modulating the molecular constituents of the AES, we show that the amplitude, direction, and speed of conductance changes can be tuned. This work motivates the development and integration of ES-inspired computing devices for achieving more capable neuromorphic hardware.

Dynamic postnatal development of the cellular and circuit properties of striatal D1 and D2 spiny projection neurons

KEY POINTS

Imbalances in the activity of the D1-expressing direct pathway and D2-expressing indirect pathway striatal projection neurons (SPNs) are thought to contribute to many basal ganglia disorders, including early-onset neurodevelopmental disorders such as obsessive-compulsive disorder, attention deficit hyperactivity disorder and Tourette's syndrome. This study provides the first detailed quantitative investigation of development of D1 and D2 SPNs, including their cellular properties and connectivity within neural circuits, during the first postnatal weeks. This period is highly dynamic with many properties changing, but it is possible to make three main observations: many aspects of D1 and D2 SPNs progressively mature in parallel; there are notable exceptions when they diverge; and many of the defining properties of mature striatal SPNs and circuits are already established by the first and second postnatal weeks, suggesting guidance through intrinsic developmental programmes. These findings provide an experimental framework for future studies of striatal development in both health and disease.

ABSTRACT

Many basal ganglia neurodevelopmental disorders are thought to result from imbalances in the activity of the D1-expressing direct pathway and D2-expressing indirect pathway striatal projection neurons (SPNs). Insight into these disorders is reliant on our understanding of normal D1 and D2 SPN development. Here we provide the first detailed study and quantification of the striatal cellular and circuit changes occurring for both D1 and D2 SPNs in the first postnatal weeks using in vitro whole-cell patch-clamp electrophysiology. Characterization of their intrinsic electrophysiological and morphological properties, the excitatory long-range inputs coming from cortex and thalamus, as well their local gap junction and inhibitory synaptic connections reveals this period to be highly dynamic with numerous properties changing. However it is possible to make three main observations. Firstly, many aspects of SPNs mature in parallel, including intrinsic membrane properties, increases in dendritic arbours and spine densities, general synaptic inputs and expression of specific glutamate receptors. Secondly, there are notable exceptions, including a transient stronger thalamic innervation of D2 SPNs and stronger cortical NMDA receptor-mediated inputs to D1 SPNs, both in the second postnatal week. Thirdly, many of the defining properties of mature D1 and D2 SPNs and striatal circuits are already established by the first and second postnatal weeks, including different electrophysiological properties as well as biased local inhibitory connections between SPNs, suggesting this is guided through intrinsic developmental programmes. Together these findings provide an experimental framework for future studies of D1 and D2 SPN development in health and disease.

Could electrical coupling contribute to the formation of cell assemblies?

Cell assemblies and central pattern generators (CPGs) are related types of neuronal networks: both consist of interacting groups of neurons whose collective activities lead to defined functional outputs. In the case of a cell assembly, the functional output may be interpreted as a representation of something in the world, external or internal; for a CPG, the output ‘drives’ an observable (i.e. motor) behavior. Electrical coupling, via gap junctions, is critical for the development of CPGs, as well as for their actual operation in the adult animal. Electrical coupling is also known to be important in the development of hippocampal and neocortical principal cell networks. We here argue that electrical coupling – in addition to chemical synapses – may therefore contribute to the formation of at least some cell assemblies in adult animals.

How Early Can a Seizure Happen? Pathophysiological Considerations of Extremely Premature Infant Brain Development

Seizures in neonates represent a neurologic emergency requiring prompt recognition, determination of etiology, and treatment. Yet, the definition and identification of neonatal seizures remain challenging and controversial, in part due to the unique physiology of brain development at this life stage. These issues are compounded when considering seizures in premature infants, in whom the complexities of brain development may engender different clinical and electrographic seizure features at different points in neuronal maturation. In extremely premature infants (< 28 weeks gestational age), seizure pathophysiology has not been explored in detail. This review discusses the physiological and structural development of the brain in this developmental window, focusing on factors that may lead to seizures and their consequences at this early time point. We hypothesize that the clinical and electrographic phenomenology of seizures in extremely preterm infants reflects the specific pathophysiology of brain development in that age window.

Quantitative Systems Pharmacological Analysis of Drugs of Abuse Reveals the Pleiotropy of Their Targets and the Effector Role of mTORC1

Existing treatments against drug addiction are often ineffective due to the complexity of the networks of protein-drug and protein-protein interactions (PPIs) that mediate the development of drug addiction and related neurobiological disorders. There is an urgent need for understanding the molecular mechanisms that underlie drug addiction toward designing novel preventive or therapeutic strategies. The rapidly accumulating data on addictive drugs and their targets as well as advances in machine learning methods and computing technology now present an opportunity to systematically mine existing data and draw inferences on potential new strategies. To this aim, we carried out a comprehensive analysis of cellular pathways implicated in a diverse set of 50 drugs of abuse using quantitative systems pharmacology methods. The analysis of the drug/ligand-target interactions compiled in DrugBank and STITCH databases revealed 142 known and 48 newly predicted targets, which have been further analyzed to identify the KEGG pathways enriched at different stages of drug addiction cycle, as well as those implicated in cell signaling and regulation events associated with drug abuse. Apart from synaptic neurotransmission pathways detected as upstream signaling modules that “sense” the early effects of drugs of abuse, pathways involved in neuroplasticity are distinguished as determinants of neuronal morphological changes. Notably, many signaling pathways converge on important targets such as mTORC1. The latter emerges as a universal effector of the persistent restructuring of neurons in response to continued use of drugs of abuse.

Dye-coupling between neonatal spinal motoneurons and interneurons revealed by prolonged back-filling of a ventral root with a low molecular weight tracer in the mouse

We investigated dye-coupling between motoneurons in the L6 segment of the neonatal mouse spinal cord that contains limb-innervating motoneurons and sexually dimorphic motor nuclei. Using an isolated spinal cord preparation, we back-filled the cut, L6 ventral root with the small molecule Neurobiotin, or the much larger dextran-conjugated fluorophores for 16–24 hours. Motoneurons and parasympathetic preganglionic neurons were filled with both markers, but dye-coupling was only seen with Neurobiotin fills. Following a neurobiotin fill, fluorescence was observed in contralateral motoneurons, in motoneurons innervating adjacent ventral roots, and in ChAT-negative, putative interneurons outside of the motoneuron pools in addition to the directly back-labeled L6 motoneurons. It is known that the gap junction protein connexin-36 is widely expressed in the sexually dimorphic motoneurons of the L6 segment, suggesting that the dye-coupling is mediated by gap junctions. The technique has revealed previously unknown connections of motoneurons in the neonatal mouse cord that are likely to play important roles in the development and function of spinal circuits.

PARIS, an optogenetic method for functionally mapping gap junctions

Cell-cell communication via gap junctions regulates a wide range of physiological processes by enabling the direct intercellular electrical and chemical coupling. However, the in vivo distribution and function of gap junctions remain poorly understood, partly due to the lack of non-invasive tools with both cell-type specificity and high spatiotemporal resolution. Here, we developed PARIS (pairing actuators and receivers to optically isolate gap junctions), a new fully genetically encoded tool for measuring the cell-specific gap junctional coupling (GJC). PARIS successfully enabled monitoring of GJC in several cultured cell lines under physiologically relevant conditions and in distinct genetically defined neurons in Drosophila brain, with ~10 s temporal resolution and sub-cellular spatial resolution. These results demonstrate that PARIS is a robust, highly sensitive tool for mapping functional gap junctions and study their regulation in both health and disease.

For the tissues and organs of our bodies to work properly, the cells within them need to communicate with each other. One important part of cellular communication is the movement of signals – usually small molecules or ions – directly from one cell to another. This happens via structures called gap junctions, a type of sealed ‘channel’ that connects two cells.

Gap junctions are found throughout the body, but investigating their precise roles in health and disease has been difficult. This is due to problems with the tools available to detect and monitor gap junctions. Some are simply harmful to cells, while others cannot be restricted to specific cell populations within a tissue. This lack of specificity makes it difficult to study gap junctions in the brain, where it is important to understand the connectivity patterns between distinct types of nerve cells. Wu et al. wanted to develop a new, non-harmful method to track gap junctions in distinct groups of cells within living tissues.

To do this, Wu et al. devised PARIS, a two-part, genetically encoded system. The first part comprises a light-sensitive molecular ‘pump’, which can only be turned on by shining a laser onto the cell of interest. When the pump is active, it transports hydrogen ions out of the cell. The second part of the system is a fluorescent sensor, present inside ‘receiving’ cells, which responds to the outcoming hydrogen ions (small enough to pass through gap junctions). If an illuminated ‘signaling’ cell is connected via gap junctions to cells containing the fluorescent sensor, they will light up within seconds, but other cells not connected through gap junctions will not.

The researchers first tested PARIS in cultured human and rat cells that had been genetically engineered to produce both components of the system. The experiments confirmed that PARIS could both detect networks of gap junctions in healthy cells and reveal when these networks had been disrupted, for instance by drugs or genetic mutations. Experiments using fruit flies demonstrated that PARIS was stable in living tissue and could also map the gap junctions connecting specific groups of nerve cells.

PARIS is a valuable addition to the toolbox available to study cell communication. In the future, it could help increase our understanding of diseases characterized by defective gap junctions, such as seizures, cardiac irregularities, and even some cancers.

Synaptic Functions of Hemichannels and Pannexons: A Double-Edged Sword

The classical view of synapses as the functional contact between presynaptic and postsynaptic neurons has been challenged in recent years by the emerging regulatory role of glial cells. Astrocytes, traditionally considered merely supportive elements are now recognized as active modulators of synaptic transmission and plasticity at the now so-called "tripartite synapse." In addition, an increasing body of evidence indicates that beyond immune functions microglia also participate in various processes aimed to shape synaptic plasticity. Release of neuroactive compounds of glial origin, -process known as gliotransmission-, constitute a widespread mechanism through which glial cells can either potentiate or reduce the synaptic strength. The prevailing vision states that gliotransmission depends on an intracellular Ca²⁺/exocytotic-mediated release; notwithstanding, growing evidence is pointing at hemichannels (connexons) and pannexin channels (pannexons) as alternative non-vesicular routes for gliotransmitters efflux. In concurrence with this novel concept, both hemichannels and pannexons are known to mediate the transfer of ions and signaling molecules -such as ATP and glutamate- between the cytoplasm and the extracellular milieu. Importantly, recent reports show that glial hemichannels and pannexons are capable to perceive synaptic activity and to respond to it through changes in their functional state. In this article, we will review the current information supporting the "double edge sword" role of hemichannels and pannexons in the function of central and peripheral synapses. At one end, available data support the idea that these channels are chief components of a feedback control mechanism through which gliotransmitters adjust the synaptic gain in either resting or stimulated conditions. At the other end, we will discuss how the excitotoxic release of gliotransmitters and [Ca²⁺]_i overload linked to the opening of hemichannels/pannexons might impact cell function and survival in the nervous system.

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.

Newly Identified Electrically Coupled Neurons Support Development of the Drosophila Giant Fiber Model Circuit

The Drosophila giant fiber (GF) escape circuit is an extensively studied model for neuron connectivity and function. Researchers have long taken advantage of the simple linear neuronal pathway, which begins at peripheral sensory modalities, travels through the central GF interneuron (GFI) to motor neurons, and terminates on wing/leg muscles. This circuit is more complex than it seems, however, as there exists a complex web of coupled neurons connected to the GFI that widely innervates the thoracic ganglion. Here, we define four new neuron clusters dye coupled to the central GFI, which we name GF coupled (GFC) 1-4. We identify new transgenic Gal4 drivers that express specifically in these neurons, and map both neuronal architecture and synaptic polarity. GFC1-4 share a central site of GFI connectivity, the inframedial bridge, where the neurons each form electrical synapses. Targeted apoptotic ablation of GFC1 reveals a key role for the proper development of the GF circuit, including the maintenance of GFI connectivity with upstream and downstream synaptic partners. GFC1 ablation frequently results in the loss of one GFI, which is always compensated for by contralateral innervation from a branch of the persisting GFI axon. Overall, this work reveals extensively coupled interconnectivity within the GF circuit, and the requirement of coupled neurons for circuit development. Identification of this large population of electrically coupled neurons in this classic model, and the ability to genetically manipulate these electrically synapsed neurons, expands the GF system capabilities for the nuanced, sophisticated circuit dissection necessary for deeper investigations into brain formation.

Plasticity of Retinal Gap Junctions: Roles in Synaptic Physiology and Disease

Electrical synaptic transmission via gap junctions underlies direct and rapid neuronal communication in the central nervous system. The diversity of functional roles played by electrical synapses is perhaps best exemplified in the vertebrate retina, in which gap junctions are expressed by each of the five major neuronal types. These junctions are highly plastic; they are dynamically regulated by ambient illumination and circadian rhythms acting through light-activated neuromodulators. The networks formed by electrically coupled neurons provide plastic, reconfigurable circuits positioned to play key and diverse roles in the transmission and processing of visual information at every retinal level. Recent work indicates gap junctions also play a role in the progressive cell death and aberrant activity seen in various pathological conditions of the retina. Gap junctions thus form potential targets for novel neuroprotective therapies in the treatment of neurodegenerative retinal diseases such as glaucoma and ischemic retinopathies.

Combined Use of Emodin and Ginsenoside Rb1 Exerts Synergistic Neuroprotection in Cerebral Ischemia/Reperfusion Rats

Acute ischemic stroke (AIS) generally causes neurological dysfunction and poses a serious threat to public health. Here, we aimed to assess the independent and combined effects of ginsenoside Rb1 (GRb1) and Emodin on neuroprotection through regulating Connexin 43 (Cx43) and Aquaporin 4 (AQP4) expression in cerebral ischemia/reperfusion (I/R) model rats. Adult male Sprague-Dawley (SD) rats were randomly divided into five groups: sham group, I/R group, Emodin group, GRb1 group and Emodin+GRb1 group. They were further allocated to four subgroups according to the 6h, 1d, 3d, and 7d time points except the sham group. Based on the modified Longa suture method, the focal cerebral I/R model was established by middle cerebral artery occlusion (MCAO). The neurological deficit scores (NDS), blood brain barrier (BBB) permeability and cerebral infarction area were assessed at each corresponding time point. Cx43 and AQP4 levels were assessed by Real-time PCR and Immunofluorescence. Compared with I/R group, both the independent and combined use of GRb1 and Emodin could alleviate NDS, reduce the BBB permeability, reduce the infarction area and down-regulate Cx43 and AQP4 expression at 6h, 1d, 3d, and 7d after I/R (P < 0.05). The Emodin+GRb1 group had more significant effects than Emodin group and GRb1 group (P < 0.05). In conclusion, the combination of Emodin and GRb1 exerts synergistically neuroprotective functions through regulating AQP4 and Cx43 after I/R.

The interplay between electrical and chemical synaptogenesis

Neurons communicate with each other via electrical or chemical synaptic connections. The pattern and strength of connections between neurons are critical for generating appropriate output. What mechanisms govern the formation of electrical and/or chemical synapses between two neurons? Recent studies indicate that common molecular players could regulate the formation of both of these classes of synapses. In addition, electrical and chemical synapses can mutually coregulate each other’s formation. Electrical activity, generated spontaneously by the nervous system or initiated from sensory experience, plays an important role in this process, leading to the selection of appropriate connections and the elimination of inappropriate ones. In this review, we discuss recent studies that shed light on the formation and developmental interactions of chemical and electrical synapses.