Connexin Hemichannels in Astrocytes: Role in CNS Disorders

Dysregulation of Astrocytic ATP/Adenosine Release in the Hippocampus Cause Cognitive and Affective Disorders: Molecular Mechanisms, Diagnosis, and Therapy

The gliotransmitter adenosine 5'-triphosphate (ATP) and its enzymatic degradation product adenosine play a major role in orchestrating in the hippocampus cognitive and affective functions via P2 purinoceptors (P2X, P2Y) and P1 adenosine receptors (A1, A2A). Although numerous reviews exist on purinoceptors that modulate these functions, there is an apparent gap relating to the involvement of astrocyte-derived extracellular ATP. Our review focuses on the following issues: An impeded release of ATP from hippocampal astrocytes through vesicular mechanisms or connexin hemichannels and pannexin channels interferes with spatial working memory in rodents. The pharmacological blockade of P2Y1 receptors (P2Y1Rs) reverses the deficits in learning/memory performance in mouse models of familial Alzheimer's disease (AD). Similarly, in mouse models of major depressive disorder (MDD), based on acute or chronic stress-induced development of depressive-like behavior, a reduced exocytotic/channel-mediated ATP release from hippocampal astrocytes results in the deterioration of these behavioral responses. However, on the opposite, the increased stimulation of the microglial/astrocytic P2X7R-channel by ATP causes neuroinflammation and in consequence depressive-like behavior. In conclusion, there is strong evidence for the assumption that gliotransmitter ATP is intimately involved in the pathophysiology of cognitive and affective neuron/astrocyte-based human illnesses opening new diagnostic and therapeutic vistas for AD and MDD.

Response of Astrocytes and Their Interaction with Surrounding Brain Cells after Acute Ischemia-Reperfusion Analyzed by Single-Cell Transcriptome Sequencing

Astrocytes play a key role in the occurrence and development of of ischemic stroke. However, reactive astrocytes have both detrimental and protective roles in ischemic stroke. Regrettably, the stimulation signals associated with the transformation of astrocytes into different subclusters lack systemic analysis, and the mechanism by which astrocytes produce multiple effects is not entirely clear. We investigated the heterogeneity of mouse astrocytes 12h after cerebral ischemia-reperfusion via Single-cell RNA sequencing and verified gene expressions by reverse transcription-polymerase chain reaction. We acquired astrocyte subclusters' transcriptional characteristics involved in diversified functions. To explore what stimulus signals cause astrocyte heterogeneity, we present a blueprint for cellular communication between astrocyte subclusters and other surrounding brain cells 12h after ischemia-reperfusion, and identified 9 genes which are potential and promising for being therapeutic targets and 6 genes were specific to astrocyte subcluster 2 that tend to resist ischemia-reperfusion injury. At 12h after ischemia-reperfusion, each subcluster of astrocytes is characteristic in terms of function and communication with surrounding cells, which is based on the activation genes and transcription molecules that we have revealed with subcluster characteristics. Our results provide a basis for revealing the anti-injury response of astrocytes to cerebral ischemia-reperfusion, which involves coordination of different subclusters and the coordination of astrocytes with surrounding brain cells.

Astrocytes in Rodent Anxiety-Related Behavior: Role of Calcium and Beyond

Anxiety is a physiological, emotional response that anticipates distal threats. When kept under control, anxiety is a beneficial response, helping animals to maintain heightened attention in environments with potential dangers. However, an overestimation of potential threats can lead to an excessive expression of anxiety that, in humans, may evolve into anxiety disorders. Pharmacological treatments show variable efficacy among patients, highlighting the need for more efforts to better understand the pathogenesis of anxiety disorders. Mounting evidence suggests that astrocytes, a type of glial cells, are active partners of neurons in brain circuits and in the regulation of behaviors under both physiological and pathological conditions. In this review, I summarize the current literature on the role of astrocytes from different brain regions in modulating anxious states, with the goal of exploring novel cerebral mechanisms to identify potential innovative therapeutic targets for the treatment of anxiety disorders.

Astrocyte alterations during Osmotic Demyelination Syndrome: intermediate filaments, aggresomes, proteasomes, and glycogen storages

INTRODUCTION

A murine model mimicking the human osmotic demyelination syndrome (ODS) revealed with histology demyelinated alterations in the relay posterolateral (VPL) and ventral posteromedial (VPM) thalamic nuclei 12 h and 48 h after chronic hyponatremia due to a fast reinstatement of osmolality. Abnormal expression astrocyte markers ALDHL1 and GFAP with immunohistochemistry in these ODS altered zones, prompted aims to verify in both protoplasmic and fibrillar astrocytes with ultrastructure those changes and other associated subcellular modifications.

METHOD

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 those four groups of mice, with LM and ultrastructure microscopy, the thalamic zones included NN (n = 2), HN (n = 2), ODS12h (n = 3) and ODS48h (n = 3) samples. There, comparisons between astrocytes included organelles, GFAP, and glycogen content changes.

RESULTS

Thalamic ODS epicenter damages comprised both protoplasmic (PA) and fibrillar (FA) astrocyte necroses along with those of neuropil destructions and neuron Wallerian demyelinated injuries surrounded by a centrifugal region gradient revealing worse to mild destructions. Ultrastructure aspects of resilient HN and ODS12h PAs disclosed altered mitochondria and accumulations of beta- to alpha-glycogen granules that became eventually captured into phagophores as glycophagosomes in ODS48h. HN and ODS12h time lapse FAs accumulated ribonucleoproteins, cytoskeletal aggresomes, and proteasomes but distant and resilient ODS48h FAs maintained GFAP fibrils along with typical mitochondria and dispersed β-glycogen, including in their neuropil surroundings. Thus, ODS triggered astrocyte injuries that involved both post-transcriptional and post-translational modifications such that astrocytes were unable to use glycogen and metabolites due to their own mitochondria defects while accumulated stalled ribonucleoproteins, cytoskeletal aggresomes were associated with proteasomes and GFAP ablation. Resilient but distant astrocytes revealed restitution of amphibolism where typical carbohydrate storages were revealed along with GFAP, as tripartite extensions supply for restored nerve axon initial segments, neural Ranvier's junctions, and oligodendrocyte -neuron junctional contacts.

CONCLUSION

ODS caused astrocyte damage associated with adjacent neuropil destruction that included a regional demyelination caused by a loss of dispatched energetic and metabolic exchanges within the injured region, bearing proportional and collateral centrifugal injuries, which involved reactive repairs time after rebalanced osmolarity.

In Vivo Neurodynamics Mapping via High-Speed Two-Photon Fluorescence Lifetime Volumetric Projection Microscopy

Monitoring the morphological and biochemical information of neurons and glial cells at high temporal resolution in three-dimensional (3D) volumes of in vivo is pivotal for understanding their structure and function, and quantifying the brain microenvironment. Conventional two-photon fluorescence lifetime volumetric imaging speed faces the acquisition speed challenges of slow serial focal tomographic scanning, complex post-processing procedures for lifetime images, and inherent trade-offs among contrast, signal-to-noise ratio, and speed. This study presents a two-photon fluorescence lifetime volumetric projection microscopy using an axially elongated Bessel focus and instant frequency-domain fluorescence lifetime technique, and integrating with a convolutional network to enhance the imaging speed for in vivo neurodynamics mapping. The proposed method is validated by monitoring intracellular Ca2+ concentration throughout whole volume, tracking microglia movement and microenvironmental changes following thermal injury in the zebrafish brain, analyzing structural and functional variations of gap junctions in astrocyte networks, and measuring the Ca2+ concentration in neurons in mouse brains. This innovative methodology enables quantitative in vivo visualization of neurodynamics and the cellular processes and interactions in the brain.

Physiology of neuroglia of the central nervous system

Neuroglia of the central nervous system (CNS) are a diverse and highly heterogeneous population of cells of ectodermal, neuroepithelial origin (macroglia, that includes astroglia and oligodendroglia) and mesodermal, myeloid origin (microglia). Neuroglia are primary homeostatic cells of the CNS, responsible for the support, defense, and protection of the nervous tissue. The extended class of astroglia (which includes numerous parenchymal astrocytes, such as protoplasmic, fibrous, velate, marginal, etc., radial astrocytes such as Bergmann glia, Muller glia, etc., and ependymoglia lining the walls of brain ventricles and central canal of the spinal cord) is primarily responsible for overall homeostasis of the nervous tissue. Astroglial cells control homeostasis of ions, neurotransmitters, hormones, metabolites, and are responsible for neuroprotection and defense of the CNS. Oligodendroglia provide for myelination of axons, hence supporting and sustaining CNS connectome. Microglia are tissue macrophages adapted to the CNS environment which contribute to the host of physiologic functions including regulation of synaptic connectivity through synaptic pruning, regulation of neurogenesis, and even modifying neuronal excitability. Neuroglial cells express numerous receptors, transporters, and channels that allow neuroglia to perceive and follow neuronal activity. Activation of these receptors triggers intracellular ionic signals that govern various homeostatic cascades underlying glial supportive and defensive capabilities. Ionic signaling therefore represents the substrate of glial excitability.

Exploring the Potent Roles of an Internally Translated Truncated Connexin-43 Isoform

Connexins are a family of transmembrane proteins that form membrane channels [...].

Changes in Cx43 and AQP4 Proteins, and the Capture of 3 kDa Dextran in Subpial Astrocytes of the Rat Medial Prefrontal Cortex after Both Sham Surgery and Sciatic Nerve Injury

A subpopulation of astrocytes on the brain's surface, known as subpial astrocytes, constitutes the "glia limitans superficialis" (GLS), which is an interface between the brain parenchyma and the cerebrospinal fluid (CSF) in the subpial space. Changes in connexin-43 (Cx43) and aquaporin-4 (AQP4) proteins in subpial astrocytes were examined in the medial prefrontal cortex at postoperative day 1, 3, 7, 14, and 21 after sham operation and sciatic nerve compression (SNC). In addition, we tested the altered uptake of TRITC-conjugated 3 kDa dextran by reactive subpial astrocytes. Cellular immunofluorescence (IF) detection and image analysis were used to examine changes in Cx43 and AQP4 protein levels, as well as TRITC-conjugated 3 kDa dextran, in subpial astrocytes. The intensity of Cx43-IF was significantly increased, but AQP4-IF decreased in subpial astrocytes of sham- and SNC-operated rats during all survival periods compared to naïve controls. Similarly, the uptake of 3 kDa dextran in the GLS was reduced following both sham and SNC operations. The results suggest that both sciatic nerve injury and peripheral tissue injury alone can induce changes in subpial astrocytes related to the spread of their reactivity across the cortical surface mediated by increased amounts of gap junctions. At the same time, water transport and solute uptake were impaired in subpial astrocytes.

Connexin 43 hemichannels and related diseases

Connexin 43 (Cx43) protein forms hemichannels (connexons) and gap junctions, with hemichannels consisting of six Cx43 molecules and gap junctions formed by two hemichannels. While gap junctions are prevalent in organs like the heart and liver, hemichannels are found in specific cell types, such as astrocytes and osteocytes. They allow the passage of small molecules (<1.5 kDa) between the cytoplasm and extracellular matrix. Cx43 hemichannels have emerged as potential therapeutic targets in various diseases, including central nervous system disorders, bone-related diseases, diabetic complications, wound healing, and cancers. Aberrant hemichannel opening can worsen conditions by releasing inflammatory elements, such as causing gliosis in neuronal cells. Conversely, functional hemichannels may inhibit cancer cell growth and metastasis. Recent studies are revealing new mechanisms of Cx43 hemichannels, broadening their therapeutic applications and highlighting the importance of regulating their activity for improved disease outcomes.

Diabetic Neuropathic Pain and Circadian Rhythm: A Future Direction Worthy of Study

More than half of people with diabetes experience neuropathic pain. Previous research has shown that diabetes patients' neuropathic pain exhibits a circadian cycle, which is characterized by increased pain sensitivity at night. Additional clinical research has revealed that the standard opioid drugs are ineffective at relieving pain and do not change the circadian rhythm. This article describes diabetic neuropathic pain and circadian rhythms separately, with a comprehensive focus on circadian rhythms. It is hoped that this characteristic of diabetic neuropathic pain can be utilized in the future to obtain more effective treatments for it.

Pathophysiological role of connexin and pannexin hemichannels in neuromuscular disorders

A growing body of research has provided evidence that de novo expression of connexin hemichannels and upregulation of pannexin hemichannels (Cx HCs and Panx HCs, respectively) in the cytoplasmic membrane of skeletal muscle (sarcolemma) are critical steps in the pathogenesis of muscle dysfunction of many genetic and acquired muscle diseases. This review provides an overview of the current understanding of the molecular mechanisms regulating the expression of Cx and Panx HCs in skeletal muscle, as well as their roles in both muscle physiology and pathologies. Additionally, it addresses existing gaps in knowledge and outlines future challenges in the field.

The angiotensin II receptors type 1 and 2 modulate astrocytes and their crosstalk with microglia and neurons in an in vitro model of ischemic stroke

BACKGROUND

Astrocytes are the most abundant cell type of the central nervous system and are fundamentally involved in homeostasis, neuroprotection, and synaptic plasticity. This regulatory function of astrocytes on their neighboring cells in the healthy brain is subject of current research. In the ischemic brain we assume disease specific differences in astrocytic acting. The renin-angiotensin-aldosterone system regulates arterial blood pressure through endothelial cells and perivascular musculature. Moreover, astrocytes express angiotensin II type 1 and 2 receptors. However, their role in astrocytic function has not yet been fully elucidated. We hypothesized that the angiotensin II receptors impact astrocyte function as revealed in an in vitro system mimicking cerebral ischemia. Astrocytes derived from neonatal wistar rats were exposed to telmisartan (angiotensin II type 1 receptor-blocker) or PD123319 (angiotensin II type 2 receptor-blocker) under normal conditions (control) or deprivation from oxygen and glucose. Conditioned medium (CM) of astrocytes was harvested to elucidate astrocyte-mediated indirect effects on microglia and cortical neurons.

RESULT

The blockade of angiotensin II type 1 receptor by telmisartan increased the survival of astrocytes during ischemic conditions in vitro without affecting their proliferation rate or disturbing their expression of S100A10, a marker of activation. The inhibition of the angiotensin II type 2 receptor pathway by PD123319 resulted in both increased expression of S100A10 and proliferation rate. The CM of telmisartan-treated astrocytes reduced the expression of pro-inflammatory mediators with simultaneous increase of anti-inflammatory markers in microglia. Increased neuronal activity was observed after treatment of neurons with CM of telmisartan- as well as PD123319-stimulated astrocytes.

CONCLUSION

Data show that angiotensin II receptors have functional relevance for astrocytes that differs in healthy and ischemic conditions and effects surrounding microglia and neuronal activity via secretory signals. Above that, this work emphasizes the strong interference of the different cells in the CNS and that targeting astrocytes might serve as a therapeutic strategy to influence the acting of glia-neuronal network in de- and regenerative context.

Perspective and Therapeutic Potential of the Noncoding RNA-Connexin Axis

Noncoding RNAs (ncRNAs) are a class of nucleotide sequences that cannot be translated into peptides. ncRNAs can function post-transcriptionally by splicing complementary sequences of mRNAs or other ncRNAs or by directly engaging in protein interactions. Over the past few decades, the pervasiveness of ncRNAs in cell physiology and their pivotal roles in various diseases have been identified. One target regulated by ncRNAs is connexin (Cx), a protein that forms gap junctions and hemichannels and facilitates intercellular molecule exchange. The aberrant expression and misdistribution of connexins have been implicated in central nervous system diseases, cardiovascular diseases, bone diseases, and cancer. Current databases and technologies have enabled researchers to identify the direct or indirect relationships between ncRNAs and connexins, thereby elucidating their correlation with diseases. In this review, we selected the literature published in the past five years concerning disorders regulated by ncRNAs via corresponding connexins. Among it, microRNAs that regulate the expression of Cx43 play a crucial role in disease development and are predominantly reviewed. The distinctive perspective of the ncRNA-Cx axis interprets pathology in an epigenetic manner and is expected to motivate research for the development of biomarkers and therapeutics.

Crosstalk Between Microglia and Müller Glia in the Age-Related Macular Degeneration: Role and Therapeutic Value of Neuroinflammation

Age-related macular degeneration (AMD) is a progressive neurodegeneration disease that causes photoreceptor demise and vision impairments. In AMD pathogenesis, the primary death of retinal neurons always leads to the activation of resident microglia. The migration of activated microglia to the ongoing retinal lesion and their morphological transformation from branching to ameboid-like are recognized as hallmarks of AMD pathogenesis. Activated microglia send signals to Müller cells and promote them to react correspondingly to damaging stimulus. Müller cells are a type of neuroglia cells that maintain the normal function of retinal neurons, modulating innate inflammatory responses, and stabilize retinal structure. Activated Müller cells can accelerate the progression of AMD by damaging neurons and blood vessels. Therefore, the crosstalk between microglia and Müller cells plays a homeostatic role in maintaining the retinal environment, and this interaction is complicatedly modulated. In particular, the mechanism of mutual regulation between the two glia populations is complex under pathological conditions. This paper reviews recent findings on the crosstalk between microglia and Müller glia during AMD pathology process, with special emphasis on its therapeutic potentials.

ALSUntangled #72: Insulin

ALSUntangled reviews alternative and off-label treatments for people living with amyotrophic lateral sclerosis (PALS). Here we review insulin, which has at least one plausible mechanism for slowing ALS progression. However, pre-clinical studies are limited and there have been no trials in PALS yet. Insulin use in patients without a metabolic need may cause very serious and potentially lethal side effects. While further studies to evaluate potential benefits may be warranted, at this time we cannot endorse insulin treatment to slow ALS progression.

Control of astrocytic Ca2+ signaling by nitric oxide-dependent S-nitrosylation of Ca2+ homeostasis modulator 1 channels

BACKGROUND

Astrocytes Ca2+ signaling play a central role in the modulation of neuronal function. Activation of metabotropic glutamate receptors (mGluR) by glutamate released during an increase in synaptic activity triggers coordinated Ca2+ signals in astrocytes. Importantly, astrocytes express the Ca2+-dependent nitric oxide (NO)-synthetizing enzymes eNOS and nNOS, which might contribute to the Ca2+ signals by triggering Ca2+ influx or ATP release through the activation of connexin 43 (Cx43) hemichannels, pannexin-1 (Panx-1) channels or Ca2+ homeostasis modulator 1 (CALHM1) channels. Hence, we aim to evaluate the participation of NO in the astrocytic Ca2+ signaling initiated by stimulation of mGluR in primary cultures of astrocytes from rat brain cortex.

RESULTS

Astrocytes were stimulated with glutamate or t-ACPD and NO-dependent changes in [Ca2+]i and ATP release were evaluated. In addition, the activity of Cx43 hemichannels, Panx-1 channels and CALHM1 channels was also analyzed. The expression of Cx43, Panx-1 and CALHM1 in astrocytes was confirmed by immunofluorescence analysis and both glutamate and t-ACPD induced NO-mediated activation of CALHM1 channels via direct S-nitrosylation, which was further confirmed by assessing CALHM1-mediated current using the two-electrode voltage clamp technique in Xenopus oocytes. Pharmacological blockade or siRNA-mediated inhibition of CALHM1 expression revealed that the opening of these channels provides a pathway for ATP release and the subsequent purinergic receptor-dependent activation of Cx43 hemichannels and Panx-1 channels, which further contributes to the astrocytic Ca2+ signaling.

CONCLUSIONS

Our findings demonstrate that activation of CALHM1 channels through NO-mediated S-nitrosylation in astrocytes in vitro is critical for the generation of glutamate-initiated astrocytic Ca2+ signaling.

Cx43 hemichannels and panx1 channels contribute to ethanol-induced astrocyte dysfunction and damage

BACKGROUND

Alcohol, a widely abused drug, significantly diminishes life quality, causing chronic diseases and psychiatric issues, with severe health, societal, and economic repercussions. Previously, we demonstrated that non-voluntary alcohol consumption increases the opening of Cx43 hemichannels and Panx1 channels in astrocytes from adolescent rats. However, whether ethanol directly affects astroglial hemichannels and, if so, how this impacts the function and survival of astrocytes remains to be elucidated.

RESULTS

Clinically relevant concentrations of ethanol boost the opening of Cx43 hemichannels and Panx1 channels in mouse cortical astrocytes, resulting in the release of ATP and glutamate. The activation of these large-pore channels is dependent on Toll-like receptor 4, P2X7 receptors, IL-1β and TNF-α signaling, p38 mitogen-activated protein kinase, and inducible nitric oxide (NO) synthase. Notably, the ethanol-induced opening of Cx43 hemichannels and Panx1 channels leads to alterations in cytokine secretion, NO production, gliotransmitter release, and astrocyte reactivity, ultimately impacting survival.

CONCLUSION

Our study reveals a new mechanism by which ethanol impairs astrocyte function, involving the sequential stimulation of inflammatory pathways that further increase the opening of Cx43 hemichannels and Panx1 channels. We hypothesize that targeting astroglial hemichannels could be a promising pharmacological approach to preserve astrocyte function and synaptic plasticity during the progression of various alcohol use disorders.

Essential Role of Astrocytes in Learning and Memory

One of the most biologically relevant functions of astrocytes within the CNS is the regulation of synaptic transmission, i.e., the physiological basis for information transmission between neurons. Changes in the strength of synaptic connections are indeed thought to be the cellular basis of learning and memory. Importantly, astrocytes have been demonstrated to tightly regulate these processes via the release of several gliotransmitters linked to astrocytic calcium activity as well as astrocyte-neuron metabolic coupling. Therefore, astrocytes seem to be integrators of and actors upon learning- and memory-relevant information. In this review, we focus on the role of astrocytes in learning and memory processes. We delineate the recognized inputs and outputs of astrocytes and explore the influence of manipulating astrocytes on behaviour across diverse learning paradigms. We conclude that astrocytes influence learning and memory in various manners. Appropriate astrocytic Ca2+ dynamics are being increasingly identified as central contributors to memory formation and retrieval. In addition, astrocytes regulate brain rhythms essential for cognition, and astrocyte-neuron metabolic cooperation is required for memory consolidation.

The Diverse Roles of Reactive Astrocytes in the Pathogenesis of Amyotrophic Lateral Sclerosis

Astrocytes displaying reactive phenotypes are characterized by their ability to remodel morphologically, molecularly, and functionally in response to pathological stimuli. This process results in the loss of their typical astrocyte functions and the acquisition of neurotoxic or neuroprotective roles. A growing body of research indicates that these reactive astrocytes play a pivotal role in the pathogenesis of amyotrophic lateral sclerosis (ALS), involving calcium homeostasis imbalance, mitochondrial dysfunction, abnormal lipid and lactate metabolism, glutamate excitotoxicity, etc. This review summarizes the characteristics of reactive astrocytes, their role in the pathogenesis of ALS, and recent advancements in astrocyte-targeting strategies.

Non-neoplastic astrocytes: key players for brain tumor progression

Astrocytes are highly plastic cells whose activity is essential to maintain the cerebral homeostasis, regulating synaptogenesis and synaptic transmission, vascular and metabolic functions, ions, neuro- and gliotransmitters concentrations. In pathological conditions, astrocytes may undergo transient or long-lasting molecular and functional changes that contribute to disease resolution or exacerbation. In recent years, many studies demonstrated that non-neoplastic astrocytes are key cells of the tumor microenvironment that contribute to the pathogenesis of glioblastoma, the most common primary malignant brain tumor and of secondary metastatic brain tumors. This Mini Review covers the recent development of research on non-neoplastic astrocytes as tumor-modulators. Their double-edged capability to promote cancer progression or to represent potential tools to counteract brain tumors will be discussed.

Autonomic Nervous System Dysfunction Is Related to Chronic Prostatitis/Chronic Pelvic Pain Syndrome

Chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) is a common and non-lethal urological condition with painful symptoms. The complexity of CP/CPPS's pathogenesis and lack of efficient etiological diagnosis results in incomplete treatment and recurrent episodes, causing long-term mental and psychological suffering in patients. Recent findings indicate that the autonomic nervous system involves in CP/CPPS, including sensory, sympathetic, parasympathetic, and central nervous systems. Neuro-inflammation and sensitization of sensory nerves lead to persistent inflammation and pain. Sympathetic and parasympathetic alterations affect the cardiovascular and reproductive systems and the development of prostatitis. Central sensitization lowers pain thresholds and increases pelvic pain perception in chronic prostatitis. Therefore, this review summarized the detailed processes and mechanisms of the critical role of the autonomic nervous system in developing CP/CPPS. Furthermore, it describes the neurologically relevant substances and channels or receptors involved in this process, which provides new perspectives for new therapeutic approaches to CP/CPPS.

Expression, Purification, and Nanodisc Reconstitution of Connexin-43 Hemichannels for Structural Characterization by Cryo-Electron Microscopy

Connexins are polytopic domain membrane proteins that form hexameric hemichannels (HCs) which can assemble into gap junction channels (GJCs) at the interface of two neighboring cells. The HCs may be involved in ion and small-molecule transport across the cellular plasma membrane in response to various stimuli. Despite their importance, relatively few structures of connexin HCs are available to date, compared to the structures of the GJCs. Here, we describe a protocol for expression, purification, and nanodisc reconstitution of connexin-43 (Cx43) HCs, which we have recently structurally characterized using cryo-EM analysis. Application of similar protocols to other connexin family members will lead to breakthroughs in the understanding of the structure and function of connexin HCs.

Astrocytes are involved in the formation of corpora amylacea-like structures from neuronal debris in the CA1 region of the rat hippocampus after ischemia

Recently, we demonstrated that the corpora amylacea (CA), a glycoprotein-rich aggregate frequently found in aged brains, accumulates in the ischemic hippocampus and that osteopontin (OPN) mediates the entire process of CA formation. Therefore, this study aimed to elucidate the mechanisms by which astrocytes and microglia participate in CA formation during the late phase (4-12 weeks) of brain ischemia. Based on various morphological analyses, including immunohistochemistry, in situ hybridization, immunoelectron microscopy, and correlative light and electron microscopy, we propose that astrocytes are the primary cells responsible for CA formation after ischemia. During the subacute phase after ischemia, astrocytes, rather than microglia, express Opn messenger ribonucleic acid and OPN protein, a surrogate marker and key component of CA. Furthermore, the specific localization of OPN in the Golgi complex suggests that it is synthesized and secreted by astrocytes. Astrocytes were in close proximity to type I OPN deposits, which accumulated in the mitochondria of degenerating neurons before fully forming the CA (type III OPN deposits). Throughout CA formation, astrocytes remained closely attached to OPN deposits, with their processes exhibiting well-developed gap junctions. Astrocytic cytoplasmic protein S100β, a calcium-binding protein, was detected within the fully formed CA. Additionally, ultrastructural analysis revealed direct contact between astroglial fibrils and the forming facets of the CA. Overall, we demonstrated that astrocytes play a central role in mediating CA formation from the initial stages of OPN deposit accumulation to the evolution of fully formed CA following transient ischemia in the hippocampus.

Fluoxetine enhances the antitumor effect of olfactory ensheathing cell-thymidine kinase/ganciclovir gene therapy in human glioblastoma multiforme cells through upregulation of Connexin43 levels

Glioblastoma multiforme (GBM) is the most invasive form of primary brain astrocytoma, resulting in poor clinical outcomes. Herpes simplex virus thymidine kinase/ganciclovir (HSV-TK/GCV) gene therapy is considered a promising strategy for GBM treatment. Since Connexin43 (Cx43) expression is reduced in GBM cells, increasing Cx43 levels could enhance the effectiveness of gene therapy. The present study aims to examine the impact of fluoxetine on HSV-TK/GCV gene therapy in human GBM cells using human olfactory ensheathing cells (OECs) as vectors. The effect of fluoxetine on Cx43 levels was assessed using the western blot technique. GBM-derived astrocytes and OECs-TK were Cocultured, and the effect of fluoxetine on the Antitumor effect of OEC-TK/GCV gene therapy was evaluated using MTT assay and flow cytometry. Our results showed that fluoxetine increased Cx43 levels in OECs and GBM cells and augmented the killing effect of OECs-TK on GBM cells. Western blot data revealed that fluoxetine enhanced the Bax/Bcl2 ratio and the levels of cleaved caspase-3 in the coculture of OECs-TK and GBM cells. Moreover, flow cytometry data indicated that fluoxetine increased the percentage of apoptotic cells in the coculture system. This study suggests that fluoxetine, by upregulating Cx43 levels, could strengthen the Antitumor effect of OEC-TK/GCV gene therapy on GBM cells.

Genetically encoded barcodes for correlative volume electron microscopy

While genetically encoded reporters are common for fluorescence microscopy, equivalent multiplexable gene reporters for electron microscopy (EM) are still scarce. Here, by installing a variable number of fixation-stable metal-interacting moieties in the lumen of encapsulin nanocompartments of different sizes, we developed a suite of spherically symmetric and concentric barcodes (EMcapsulins) that are readable by standard EM techniques. Six classes of EMcapsulins could be automatically segmented and differentiated. The coding capacity was further increased by arranging several EMcapsulins into distinct patterns via a set of rigid spacers of variable length. Fluorescent EMcapsulins were expressed to monitor subcellular structures in light and EM. Neuronal expression in Drosophila and mouse brains enabled the automatic identification of genetically defined cells in EM. EMcapsulins are compatible with transmission EM, scanning EM and focused ion beam scanning EM. The expandable palette of genetically controlled EM-readable barcodes can augment anatomical EM images with multiplexed gene expression maps.

MLKL regulates Cx43 ubiquitinational degradation and mediates neuronal necroptosis in ipsilateral thalamus after focal cortical infarction

Necroptosis is known to play an important role in the pathophysiology of cerebral ischemia; however, its role in the occurrence of secondary thalamic injury after focal cerebral infarction and the mechanism about how mixed lineage kinase domain-like (MLKL) executes necroptosis in this pathophysiology are still unclear. In this study, Sprague-Dawley rats were subjected to distal branch of middle cerebral artery occlusion (dMCAO). The expression of MLKL, connexin 43 (Cx43) and Von Hippel-Lindau (VHL) in vitro and in vivo were assessed by Western blot. Bioinformatic methods were used to predict the potential binding sites where MLKL interacted with Cx43, and the ubiquitination degradation of Cx43 regulated by VHL. The interactions among MLKL, Cx43, VHL, and Ubiquitin were assessed by immunoprecipitation. Dye uptake assay were used to examine the Cx43 hemichannels. Intracellular Ca2+ concentration was measured using Fluo-4 AM. Overexpression and site-directed mutagenesis studies were used to study the mechanisms by which MLKL regulates Cx43 ubiquitinational degradation to mediate neuronal necroptosis. We found that MLKL and Cx43 were upregulated in the ventral posterolateral nucleus (VPN) of the ipsilateral thalamus after dMCAO. In the in vitro experiments MLKL and Cx43 were upregulated after TSZ-mediated necroptosis in SH-SY5Y cells. The interaction between MLKL and Cx43 inhibited the K48-linked ubiquitination of Cx43 in necroptotic SH-SY5Y cells. VHL is an E3 ubiquitin ligase for Cx43, and MLKL competes with VHL for binding to Cx43. Interaction of MLKL Ser454 with Cx43 can trigger the opening of Cx43 hemichannels, causing increased intracellular Ca2+, and cell necroptosis. This innovative study at animal models, cellular, and molecular levels is anticipated to clarify the roles of MLKL and Cx43 in thalamic damage after focal cortical infarction. Our findings may help identify novel targets for neurological recovery after cortical infarction.

SARS-CoV-2 spike protein S1 activates Cx43 hemichannels and disturbs intracellular Ca2+ dynamics

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes the ongoing coronavirus disease 2019 (COVID-19). An aspect of high uncertainty is whether the SARS-CoV-2 per se or the systemic inflammation induced by viral infection directly affects cellular function and survival in different tissues. It has been postulated that tissue dysfunction and damage observed in COVID-19 patients may rely on the direct effects of SARS-CoV-2 viral proteins. Previous evidence indicates that the human immunodeficiency virus and its envelope protein gp120 increase the activity of connexin 43 (Cx43) hemichannels with negative repercussions for cellular function and survival. Here, we evaluated whether the spike protein S1 of SARS-CoV-2 could impact the activity of Cx43 hemichannels.

RESULTS

We found that spike S1 time and dose-dependently increased the activity of Cx43 hemichannels in HeLa-Cx43 cells, as measured by dye uptake experiments. These responses were potentiated when the angiotensin-converting enzyme 2 (ACE2) was expressed in HeLa-Cx43 cells. Patch clamp experiments revealed that spike S1 increased unitary current events with conductances compatible with Cx43 hemichannels. In addition, Cx43 hemichannel opening evoked by spike S1 triggered the release of ATP and increased the [Ca2+]i dynamics elicited by ATP.

CONCLUSIONS

We hypothesize that Cx43 hemichannels could represent potential pharmacological targets for developing therapies to counteract SARS-CoV-2 infection and their long-term consequences.

The Key Role of Astrocytes in Amyotrophic Lateral Sclerosis and Their Commitment to Glutamate Excitotoxicity

In the last two decades, there has been increasing evidence supporting non-neuronal cells as active contributors to neurodegenerative disorders. Among glial cells, astrocytes play a pivotal role in driving amyotrophic lateral sclerosis (ALS) progression, leading the scientific community to focus on the "astrocytic signature" in ALS. Here, we summarized the main pathological mechanisms characterizing astrocyte contribution to MN damage and ALS progression, such as neuroinflammation, mitochondrial dysfunction, oxidative stress, energy metabolism impairment, miRNAs and extracellular vesicles contribution, autophagy dysfunction, protein misfolding, and altered neurotrophic factor release. Since glutamate excitotoxicity is one of the most relevant ALS features, we focused on the specific contribution of ALS astrocytes in this aspect, highlighting the known or potential molecular mechanisms by which astrocytes participate in increasing the extracellular glutamate level in ALS and, conversely, undergo the toxic effect of the excessive glutamate. In this scenario, astrocytes can behave as "producers" and "targets" of the high extracellular glutamate levels, going through changes that can affect themselves and, in turn, the neuronal and non-neuronal surrounding cells, thus actively impacting the ALS course. Moreover, this review aims to point out knowledge gaps that deserve further investigation.

Ion Channels and Ionotropic Receptors in Astrocytes: Physiological Functions and Alterations in Alzheimer's Disease and Glioblastoma

The human brain is composed of nearly one hundred billion neurons and an equal number of glial cells, including macroglia, i.e., astrocytes and oligodendrocytes, and microglia, the resident immune cells of the brain. In the last few decades, compelling evidence has revealed that glial cells are far more active and complex than previously thought. In particular, astrocytes, the most abundant glial cell population, not only take part in brain development, metabolism, and defense against pathogens and insults, but they also affect sensory, motor, and cognitive functions by constantly modulating synaptic activity. Not surprisingly, astrocytes are actively involved in neurodegenerative diseases (NDs) and other neurological disorders like brain tumors, in which they rapidly become reactive and mediate neuroinflammation. Reactive astrocytes acquire or lose specific functions that differently modulate disease progression and symptoms, including cognitive impairments. Astrocytes express several types of ion channels, including K+, Na+, and Ca2+ channels, transient receptor potential channels (TRP), aquaporins, mechanoreceptors, and anion channels, whose properties and functions are only partially understood, particularly in small processes that contact synapses. In addition, astrocytes express ionotropic receptors for several neurotransmitters. Here, we provide an extensive and up-to-date review of the roles of ion channels and ionotropic receptors in astrocyte physiology and pathology. As examples of two different brain pathologies, we focus on Alzheimer's disease (AD), one of the most diffuse neurodegenerative disorders, and glioblastoma (GBM), the most common brain tumor. Understanding how ion channels and ionotropic receptors in astrocytes participate in NDs and tumors is necessary for developing new therapeutic tools for these increasingly common neurological conditions.

The connexin hemichannel inhibitor D4 produces rapid antidepressant-like effects in mice

Depression is a common mood disorder characterized by a range of clinical symptoms, including prolonged low mood and diminished interest. Although many clinical and animal studies have provided significant insights into the pathophysiology of depression, current treatment strategies are not sufficient to manage this disorder. It has been suggested that connexin (Cx)-based hemichannels are candidates for depression intervention by modifying the state of neuroinflammation. In this study, we investigated the antidepressant-like effect of a recently discovered selective Cx hemichannel inhibitor, a small organic molecule called D4. We first showed that D4 reduced hemichannel activity following systemic inflammation after LPS injections. Next, we found that D4 treatment prevented LPS-induced inflammatory response and depressive-like behaviors. These behavioral effects were accompanied by reduced astrocytic activation and hemichannel activity in depressive-like mice induced by repeated low-dose LPS challenges. D4 treatment also reverses depressive-like symptoms in mice subjected to chronic restraint stress (CRS). To test whether D4 broadly affected neural activity, we measured c-Fos expression in depression-related brain regions and found a reduction in c-Fos+ cells in different brain regions. D4 significantly normalized CRS-induced hypoactivation in several brain regions, including the hippocampus, entorhinal cortex, and lateral septum. Together, these results indicate that blocking Cx hemichannels using D4 can normalize neuronal activity and reduce depressive-like symptoms in mice by reducing neuroinflammation. Our work provides evidence of the antidepressant-like effect of D4 and supports glial Cx hemichannels as potential therapeutic targets for depression.

Distinct myeloid population phenotypes dependent on TREM2 expression levels shape the pathology of traumatic versus demyelinating CNS disorders

Triggering receptor expressed on myeloid cell 2 (TREM2) signaling often drives opposing effects in traumatic versus demyelinating CNS disorders. Here, we identify two distinct phenotypes of microglia and infiltrating myeloid populations dependent on TREM2 expression levels at the acute stage and elucidate how they mediate the opposing effects of TREM2 in spinal cord injury (SCI) versus multiple sclerosis animal models (experimental autoimmune encephalomyelitis [EAE]). High TREM2 levels sustain phagocytic microglia and infiltrating macrophages after SCI. In contrast, moderate TREM2 levels sustain immunomodulatory microglia and infiltrating monocytes in EAE. TREM2-ablated microglia (purine-sensing phenotype in SCI and reduced immunomodulatory phenotype in EAE) drive transient protection at the acute stage of both disorders, whereas reduced phagocytic macrophages and lysosome-activated monocytes lead to contrasting neuroprotective and demyelinating effects in SCI versus EAE, respectively. Our study provides comprehensive insights into the complex roles of TREM2 in myeloid populations across diverse CNS disorders, which has crucial implications in devising TREM2-targeting therapeutics.

Acute and Chronic Ethanol Effects during Adolescence on Neuroimmune Responses: Consequences and Potential Pharmacologic Interventions

Heavy ethanol consumption during adolescence has been linked to neuroimmune response dysregulation and cognitive deficits in the developing adolescent brain. During adolescence, the brain is particularly susceptible to the pharmacological effects of ethanol that are induced by acute and chronic bouts of exposure. Numerous preclinical rodent model studies have used different ethanol administration techniques, such as intragastric gavage, self-administration, vapor, intraperitoneal, and free access, and while most models indicated proinflammatory neuroimmune responses in the adolescent brain, there are various factors that appear to influence this observation. This review synthesizes the most recent findings of the effects of adolescent alcohol use on toll-like receptors, cytokines, and chemokines, as well as the activation of astrocytes and microglia with an emphasis on differences associated with the duration of ethanol exposure (acute vs. chronic), the amount of exposure (e.g., dose or blood ethanol concentrations), sex differences, and the timing of the neuroimmune observation (immediate vs. persistent). Finally, this review discusses new therapeutics and interventions that may ameliorate the dysregulation of neuroimmune maladaptations after ethanol exposure.

Excitatory and inhibitory neuronal signaling in inflammatory and diabetic neuropathic pain

Pain, although unpleasant, is an essential warning mechanism against injury and damage of the organism. An intricate network of specialised sensors and transmission systems contributes to reception, transmission and central sensitization of pain. Here, we briefly introduce some of the main aspects of pain signal transmission, including nociceptors and nociceptive signals, mechanisms of inflammatory and neuropathic pain, and the situation of diabetes-associated neuropathic pain. The role of glia-astrocytes, microglia, satellite glia cells-and their specific channels, transporters and signaling pathways is described. A focus is on the contribution of inhibitory synaptic signaling to nociception and a possible role of glycine receptors in glucose-mediated analgesia and treatment-induced diabetic neuropathy. Inhibitory receptors such as GABA_A- and glycine receptors are important contributors to nociceptive signaling; their contribution to altered pain sensation in diabetes may be of clinical relevance, and they could be promising therapeutic targets towards the development of novel analgesics.

Mirror image pain mediated by D2 receptor regulation of astrocytic Cx43 phosphorylation and channel opening

Mirror image pain (MIP), a clinical syndrome of contralateral pain hypersensitivity caused by unilateral injury, has been identified in various neuropathological conditions. Gap junctional protein Connexin 43 (Cx43), its phosphorylation levels and dopamine D2 receptor (DRD2) play key integrating roles in pain processing. We presume D2DR activity may affect Cx43 hemichannel opening via Cx43 phosphorylation levels to regulate MIP. This study shows that spinal astrocytic Cx43 directly interacts with DRD2 to mediate MIP. DRD2 and Cx43 expression levels were asymmetrically elevated in bilateral spinal during MIP, and DRD2 modulated the opening of primary astrocytic Cx43 hemichannels. Furthermore, Cx43 phosphorylation at Ser373 was increased during MIP, but decreased in DRD2 knockout (KO) mice. Finally, activation of spinal protein kinase A (PKA) altered the expression of Cx43 and its phosphorylation bilaterally, thus reversing the analgesic effect in DRD2 KO mice. Together, these data reveal that spinal Cx43 phosphorylation and channel opening are regulated by DRD2 via PKA activation, and that spinal Cx43 and DRD2 are key molecular sensors mediating mirror image pain.

The Similar and Distinct Roles of Satellite Glial Cells and Spinal Astrocytes in Neuropathic Pain

Preclinical studies have identified glial cells as pivotal players in the genesis and maintenance of neuropathic pain after nerve injury associated with diabetes, chemotherapy, major surgeries, and virus infections. Satellite glial cells (SGCs) in the dorsal root and trigeminal ganglia of the peripheral nervous system (PNS) and astrocytes in the central nervous system (CNS) express similar molecular markers and are protective under physiological conditions. They also serve similar functions in the genesis and maintenance of neuropathic pain, downregulating some of their homeostatic functions and driving pro-inflammatory neuro-glial interactions in the PNS and CNS, i.e., "gliopathy". However, the role of SGCs in neuropathic pain is not simply as "peripheral astrocytes". We delineate how these peripheral and central glia participate in neuropathic pain by producing different mediators, engaging different parts of neurons, and becoming active at different stages following nerve injury. Finally, we highlight the recent findings that SGCs are enriched with proteins related to fatty acid metabolism and signaling such as Apo-E, FABP7, and LPAR1. Targeting SGCs and astrocytes may lead to novel therapeutics for the treatment of neuropathic pain.

Alzheimer’s Disease from the Amyloidogenic Theory to the Puzzling Crossroads between Vascular, Metabolic and Energetic Maladaptive Plasticity

Alzheimer’s disease (AD) is a progressive and degenerative disease producing the most common type of dementia worldwide. The main pathogenetic hypothesis in recent decades has been the well-known amyloidogenic hypothesis based on the involvement of two proteins in AD pathogenesis: amyloid β (Aβ) and tau. Amyloid deposition reported in all AD patients is nowadays considered an independent risk factor for cognitive decline. Vascular damage and blood–brain barrier (BBB) failure in AD is considered a pivotal mechanism for brain injury, with increased deposition of both immunoglobulins and fibrin. Furthermore, BBB dysfunction could be an early sign of cognitive decline and the early stages of clinical AD. Vascular damage generates hypoperfusion and relative hypoxia in areas with high energy demand. Long-term hypoxia and the accumulation within the brain parenchyma of neurotoxic molecules could be seeds of a self-sustaining pathological progression. Cellular dysfunction comprises all the elements of the neurovascular unit (NVU) and neuronal loss, which could be the result of energy failure and mitochondrial impairment. Brain glucose metabolism is compromised, showing a specific region distribution. This energy deficit worsens throughout aging. Mild cognitive impairment has been reported to be associated with a glucose deficit in the entorhinal cortex and in the parietal lobes. The current aim is to understand the complex interactions between amyloid β (Aβ) and tau and elements of the BBB and NVU in the brain. This new approach aimed at the study of metabolic mechanisms and energy insufficiency due to mitochondrial impairment would allow us to define therapies aimed at predicting and slowing down the progression of AD.

Gap Junctions and Connexins in Microglia-Related Oxidative Stress and Neuroinflammation: Perspectives for Drug Discovery

Microglia represent the immune system of the brain. Their role is central in two phenomena, neuroinflammation and oxidative stress, which are at the roots of different pathologies related to the central nervous system (CNS). In order to maintain the homeostasis of the brain and re-establish the equilibrium after a threatening imbalance, microglia communicate with each other and other cells within the CNS by receiving specific signals through membrane-bound receptors and then releasing neurotrophic factors into either the extracellular milieu or directly into the cytoplasm of nearby cells, such as astrocytes and neurons. These last two mechanisms rely on the activity of protein structures that enable the formation of channels in the membrane, namely, connexins and pannexins, that group and form gap junctions, hemichannels, and pannexons. These channels allow the release of gliotransmitters, such as adenosine triphosphate (ATP) and glutamate, together with calcium ion (Ca2+), that seem to play a pivotal role in inter-cellular communication. The aim of the present review is focused on the physiology of channel protein complexes and their contribution to neuroinflammatory and oxidative stress-related phenomena, which play a central role in neurodegenerative disorders. We will then discuss how pharmacological modulation of these channels can impact neuroinflammatory phenomena and hypothesize that currently available nutraceuticals, such as carnosine and N-acetylcysteine, can modulate the activity of connexins and pannexins in microglial cells and reduce oxidative stress in neurodegenerative disorders.

Conformational changes in the human Cx43/GJA1 gap junction channel visualized using cryo-EM

Connexin family proteins assemble into hexameric hemichannels in the cell membrane. The hemichannels dock together between two adjacent membranes to form gap junction intercellular channels (GJIChs). We report the cryo-electron microscopy structures of Cx43 GJICh, revealing the dynamic equilibrium state of various channel conformations in detergents and lipid nanodiscs. We identify three different N-terminal helix conformations of Cx43-gate-covering (GCN), pore-lining (PLN), and flexible intermediate (FIN)-that are randomly distributed in purified GJICh particles. The conformational equilibrium shifts to GCN by cholesteryl hemisuccinates and to PLN by C-terminal truncations and at varying pH. While GJIChs that mainly comprise GCN protomers are occluded by lipids, those containing conformationally heterogeneous protomers show markedly different pore sizes. We observe an α-to-π-helix transition in the first transmembrane helix, which creates a side opening to the membrane in the FIN and PLN conformations. This study provides basic structural information to understand the mechanisms of action and regulation of Cx43 GJICh.

Adherens, tight, and gap junctions in ependymal cells: A systematic review of their contribution to CSF-brain barrier

Introduction

The movement of fluids and solutes across the ependymal barrier, and their changes in physiologic and disease states are poorly understood. This gap in knowledge contributes strongly to treatment failures and complications in various neurological disorders.

Methods

We systematically searched and reviewed original research articles treating ependymal intercellular junctions on PubMed. Reviews, opinion papers, and abstracts were excluded. Research conducted on tissue samples, cell lines, CSF, and animal models was considered.

Results

A total of 45 novel articles treating tight, adherens and gap junctions of the ependyma were included in our review, spanning from 1960 to 2022. The findings of this review point toward a central and not yet fully characterized role of the ependymal lining ultrastructure in fluid flow interactions in the brain. In particular, tight junctions circumferentially line the apical equator of ependymal cells, changing between embryonal and adult life in several rodent models, shaping fluid and solute transit in this location. Further, adherens and gap junctions appear to have a pivotal role in several forms of congenital hydrocephalus.

Conclusions

These findings may provide an opportunity for medical management of CSF disorders, potentially allowing for tuning of CSF secretion and absorption. Beyond hydrocephalus, stroke, trauma, this information has relevance for metabolite clearance and drug delivery, with potential to affect many patients with a variety of neurological disorders. This critical look at intercellular junctions in ependyma and the surrounding interstitial spaces is meant to inspire future research on a central and rather unknown component of the CSF-brain interface.

The Relationship of Astrocytes and Microglia with Different Stages of Ischemic Stroke

Ischemic stroke is the predominant cause of severe morbidity and mortality worldwide. Post-stroke neuroinflammation has recently received increasing attention with the aim of providing a new effective treatment strategy for ischemic stroke. Microglia and astrocytes are major components of the innate immune system of the central nervous system. They can be involved in all phases of ischemic stroke, from the early stage, contributing to the first wave of neuronal cell death, to the late stage involving phagocytosis and repair. In the early stage of ischemic stroke, a vicious cycle exists between the activation of microglia and astrocytes (through astrocytic connexin 43 hemichannels), aggravating neuroinflammatory injury post-stroke. However, in the late stage of ischemic stroke, repeatedly activated microglia can induce the formation of glial scars by triggering reactive astrogliosis in the peri-infarct regions, which may limit the movement of activated microglia in reverse and restrict the diffusion of inflammation to healthy brain tissues, alleviating the neuroinflammatory injury poststroke. In this review, we elucidated the various roles of astrocytes and microglia and summarized their relationship with neuroinflammation. We also examined how astrocytes and microglia influence each other at different stages of ischemic stroke. Several potential therapeutic approaches targeting astrocytes and microglia in ischemic stroke have been reviewed. Understanding the details of astrocytemicroglia interaction processes will contribute to a better understanding of the mechanisms underlying ischemic stroke, contributing to the identification of new therapeutic interventions.

Blocking Cx43 alleviates neuropathic pain in rats with chronic constriction injury via the P2X4 and P38/ERK-P65 pathways

Neuropathic pain is a growing concern in the medical community, and studies on new analgesic targets for neuropathic pain have become a new hot spot. Whether Connexin43 (Cx43) has a key role in neuropathic pain mediated by the purinergic 2X4 (P2X4) receptor in rats with chronic constriction injury (CCI) was explored in this study. Our experimental results show that blockade of Cx43 could attenuate neuropathic pain in rats suffering from CCI via the P2X4, p38, ERK, and NF-kB signalling pathways. These results suggest that Cx43 may be a promising therapeutic target for the development of novel pharmacological agents in the management of neuropathic pain.

Novel Antidepressant Mechanism of Ginsenoside Rg1 in Regulating the Dysfunction of the Glutamatergic System in Astrocytes

Ginsenoside Rg1, a traditional Chinese medicine monomer, has been shown to have antidepressant effects. We previously found that Rg1 exerts antidepressant effects by improving the gap junction channels (GJCs) dysfunction; however, the downstream mechanisms through which Rg1 ameliorates GJC dysfunction remain unclear. Since hemichannels directly release glutamate, GJC dysfunction decreases the expression levels of glutamate transporters in astrocytes, and glutamatergic system dysfunction plays an essential role in the pathogenesis of depression. The glutamatergic system may be a potential downstream target of Rg1 that exerts antidepressant effects. Therefore, in this study, we aimed to determine the downstream mechanisms by which Rg1 ameliorated GJC dysfunction and exerted its antidepressant effects. Corticosterone (CORT) is used to mimic high glucocorticoid levels in patients with depression in vitro. Primary cortical astrocytes were isolated and phosphorylation of connexin43 (Cx43) as well as the functions of hemichannels, GJCs, and the glutamatergic system were evaluated after drug treatment. Rg1 pretreatment reversed the anomalous activation of Cx43 phosphorylation as well as the dysfunction of hemichannels, GJCs, and the glutamatergic system induced by CORT. These results suggest that Rg1 can ameliorate CORT-induced dysfunction of the glutamatergic system in astrocytes by potentially reducing Cx43 phosphorylation and inhibiting opening of hemichannels, thereby improving GJC dysfunction.

Connexin 30 Deficiency Ameliorates Disease Progression at the Early Phase in a Mouse Model of Amyotrophic Lateral Sclerosis by Suppressing Glial Inflammation

Connexin 30 (Cx30), which forms gap junctions between astrocytes, regulates cell adhesion and migration, and modulates glutamate transport. Cx30 is upregulated on activated astroglia in central nervous system inflammatory lesions, including spinal cord lesions in mutant superoxide dismutase 1 (mSOD1) transgenic amyotrophic lateral sclerosis (ALS) model mice. Here, we investigated the role of Cx30 in mSOD1 mice. Cx30 was highly expressed in the pre-onset stage in mSOD1 mice. mSOD1 mice with knockout (KO) of the Cx30 gene (Cx30KO-mSOD1 mice) showed delayed disease onset and tended to have an extended survival period (log-rank, p = 0.09). At the progressive and end stages of the disease, anterior horn cells were significantly preserved in Cx30KO-mSOD1 mice. In lesions of these mice, glial fibrillary acidic protein/C3-positive inflammatory astroglia were decreased. Additionally, the activation of astrocytes in Cx30KO-mSOD1 mice was reduced compared with mSOD1 mice by gene expression microarray. Furthermore, expression of connexin 43 at the pre-onset stage was downregulated in Cx30KO-mSOD1 mice. These findings suggest that reduced expression of astroglial Cx30 at the early disease stage in ALS model mice protects neurons by attenuating astroglial inflammation.

Glial Gap Junction Pathology in the Spinal Cord of the 5xFAD Mouse Model of Early-Onset Alzheimer's Disease

Gap junctions (GJs) are specialized transmembrane channels assembled by two hemi-channels of six connexin (Cx) proteins that facilitate neuroglial crosstalk in the central nervous system (CNS). Previous studies confirmed the crucial role of glial GJs in neurodegenerative disorders with dementia or motor dysfunction including Alzheimer's disease (AD). The aim of this study was to examine the alterations in astrocyte and related oligodendrocyte GJs in association with Aβ plaques in the spinal cord of the 5xFAD mouse model of AD. Our analysis revealed abundant Aβ plaque deposition, activated microglia, and astrogliosis in 12-month-old (12M) 5xFAD mice, with significant impairment of motor performance starting from 3-months (3M) of age. Additionally, 12M 5xFAD mice displayed increased immunoreactivity of astroglial Cx43 and Cx30 surrounding Aβ plaques and higher protein levels, indicating upregulated astrocyte-to-astrocyte GJ connectivity. In addition, they demonstrated increased numbers of mature CC1-positive and precursor oligodendrocytes (OPCs) with higher immunoreactivity of Cx47-positive GJs in individual cells. Moreover, total Cx47 protein levels were significantly elevated in 12M 5xFAD, reflecting increased oligodendrocyte-to-oligodendrocyte Cx47-Cx47 GJ connectivity. In contrast, we observed a marked reduction in Cx32 protein levels in 12M 5xFAD spinal cords compared with controls, while qRT-PCR analysis revealed a significant upregulation in Cx32 mRNA levels. Finally, myelin deficits were found focally in the areas occupied by Aβ plaques, whereas axons themselves remained preserved. Overall, our data provide novel insights into the altered glial GJ expression in the spinal cord of the 5xFAD model of AD and the implicated role of GJ pathology in neurodegeneration. Further investigation to understand the functional consequences of these extensive alterations in oligodendrocyte-astrocyte (O/A) GJ connectivity is warranted.

A Microfluidic Model of AQP4 Polarization Dynamics and Fluid Transport in the Healthy and Inflamed Human Brain: The First Step Towards Glymphatics-on-a-Chip

Dysfunction of the aquaporin-4 (AQP4)-dependent glymphatic waste clearance pathway has recently been implicated in the pathogenesis of several neurodegenerative diseases. However, it is difficult to unravel the causative relationship between glymphatic dysfunction, AQP4 depolarization, protein aggregation, and inflammation in neurodegeneration using animal models alone. There is currently a clear, unmet need for in vitro models of the brain's waterscape, and the first steps towards a bona fide "glymphatics-on-a-chip" are taken in the present study. It is demonstrated that chronic exposure to lipopolysaccharide (LPS), amyloid-β(1-42) oligomers, and an AQP4 inhibitor impairs the drainage of fluid and amyloid-β(1-40) tracer in a gliovascular unit (GVU)-on-a-chip model containing human astrocytes and brain microvascular endothelial cells. The LPS-induced drainage impairment is partially retained following cell lysis, indicating that neuroinflammation induces parallel changes in cell-dependent and matrisome-dependent fluid transport pathways in GVU-on-a-chip. Additionally, AQP4 depolarization is observed following LPS treatment, suggesting that LPS-induced drainage impairments on-chip may be driven in part by changes in AQP4-dependent fluid dynamics.

Upregulation of retinal VEGF and connexin 43 in murine nonarteritic anterior ischemic optic neuropathy induced with 577 nm laser

Nonarteritic anterior ischemic optic neuropathy (NAION) is a common acute optic neuropathy and cause of irreversible vision loss in those older than 50 years of age. There is currently no effective treatment for NAION and the biological mechanisms leading to neuronal loss are not fully understood. Promising novel targets include glial cells activation and intercellular communication mediated by molecules such as gap junction protein Connexin 43 (Cx43), which modulate neuronal fate in central nervous system disorders. In this study, we investigated retinal glial changes and neuronal loss following a novel NAION animal model using a 577 nm yellow laser. We induced unilateral photochemical thrombosis using rose bengal at the optic nerve head vasculature in adult C57BL/6 mice using a 577 nm laser and performed morphometric analysis of the retinal structure using serial in vivo optical coherence tomography (OCT) and histology for glial and neuronal markers. One day after experimental NAION, in acute phase, OCT imaging revealed peripapillary thickening of the retinal ganglion cell complex (GCC, baseline: 79.5 ± 1.0 μm, n = 8; NAION: 93.0 ± 2.5 μm, n = 8, P < 0.01) and total retina (baseline: 202.9 ± 2.4 μm, n = 8; NAION: 228.1 ± 6.8 μm, n = 8, P < 0.01). Twenty-one days after ischemia, at a chronic phase, there was significant GCC thinning (baseline 78.3 ± 2.1 μm, n = 6; NAION: 72.2 ± 1.9 μm, n = 5, P < 0.05), mimicking human disease. Examination of molecular changes in the retina one day after ischemia revealed that NAION induced a significant increase in retinal VEGF levels (control: 2319 ± 195, n = 5; NAION: 4549 ± 683 gray mean value, n = 5, P < 0.05), which highly correlated with retinal thickness (r = 0.89, P < 0.05). NAION also led to significant increase in mRNA level for Cx43 (Gj1a) at day 1 (control: 1.291 ± 0.38; NAION: 3.360 ± 0.58 puncta/mm2, n = 5, P < 0.05), but not of glial fibrillary acidic protein (Gfap) at the same time (control: 2,800 ± 0.59; NAION: 4,690 ± 0.90 puncta/mm2 n = 5, P = 0.19). Retinal ganglion cell loss at day 21 was confirmed by a 30% decrease in Brn3a+ cells (control: 2,844 ± 235; NAION: 2,001 ± 264 cells/mm2, n = 4, P < 0.05). We described a novel protocol of NAION induction by photochemical thrombosis using a 577 nm laser, leading to retinal edema and VEGF increase at day 1 and RGCs loss at day 21 after injury, consistent with the pathophysiology of human NAION. Early changes in glial cells intercommunication revealed by increased Cx43+ gap junctions are consistent with a retinal glial role in mediating cell-to-cell signaling after an ischemic insult. Our study demonstrates an early glial response in a novel NAION animal model and reveals glial intercommunication molecules such as Cx43 as a promising therapeutic target in acute NAION.

Bridging the gap of axonal regeneration in the central nervous system: A state of the art review on central axonal regeneration

Neuronal regeneration in the central nervous system (CNS) is an important field of research with relevance to all types of neuronal injuries, including neurodegenerative diseases. The glial scar is a result of the astrocyte response to CNS injury. It is made up of many components creating a complex environment in which astrocytes play various key roles. The glial scar is heterogeneous, diverse and its composition depends upon the injury type and location. The heterogeneity of the glial scar observed in different situations of CNS damage and the consequent implications for axon regeneration have not been reviewed in depth. The gap in this knowledge will be addressed in this review which will also focus on our current understanding of central axonal regeneration and the molecular mechanisms involved. The multifactorial context of CNS regeneration is discussed, and we review newly identified roles for components previously thought to solely play an inhibitory role in central regeneration: astrocytes and p75NTR and discuss their potential and relevance for deciding therapeutic interventions. The article ends with a comprehensive review of promising new therapeutic targets identified for axonal regeneration in CNS and a discussion of novel ways of looking at therapeutic interventions for several brain diseases and injuries.

Heterogeneity and Molecular Markers for CNS Glial Cells Revealed by Single-Cell Transcriptomics

Glial cells, including astrocytes, oligodendrocytes, and microglia, are the major components in the central nervous system (CNS). Studies have revealed the heterogeneity of each glial cell type and that they each may play distinct roles in physiological processes and/or neurological diseases. Single-cell sequencing (scRNA-seq) technology developed in recent years has extended our understanding of glial cell heterogeneity from the perspective of transcriptome profiling. This review summarizes the marker genes of major glial cells in the CNS and reveals their heterogeneity in different species, CNS regions, developmental stages, and pathological states (Alzheimer's disease and spinal cord injury), expanding our knowledge of glial cell heterogeneity on both molecular and functional levels.

Targetability of the neurovascular unit in inflammatory diseases of the central nervous system

The blood-brain barrier (BBB) is a selectively permeable barrier separating the periphery from the central nervous system (CNS). The BBB restricts the flow of most material into and out of the CNS, including many drugs that could be used as potent therapies. BBB permeability is modulated by several cells that are collectively called the neurovascular unit (NVU). The NVU consists of specialized CNS endothelial cells (ECs), pericytes, astrocytes, microglia, and neurons. CNS ECs maintain a complex "seal" via tight junctions, forming the BBB; breakdown of these tight junctions leads to BBB disruption. Pericytes control the vascular flow within capillaries and help maintain the basal lamina. Astrocytes control much of the flow of material that has moved beyond the CNS EC layer and can form a secondary barrier under inflammatory conditions. Microglia survey the border of the NVU for noxious material. Neuronal activity also plays a role in the maintenance of the BBB. Since astrocytes, pericytes, microglia, and neurons are all able to modulate the permeability of the BBB, understating the complex contributions of each member of the NVU will potentially uncover novel and effective methods for delivery of neurotherapies to the CNS.