Role of Hemichannels in CNS Inflammation and the Inflammasome Pathway

GDNF's Role in Mitigating Intestinal Reactive Gliosis and Inflammation to Improve Constipation and Depressive Behavior in Rats with Parkinson's disease

Constipation is a common symptom in patients with Parkinson's disease (PD) and is often associated with depression. Enteric glial cells (EGCs) are crucial for regulating intestinal inflammation and colon motility, and their activation can lead to the death of intestinal neurons. Glial cell line-derived neurotrophic factor (GDNF) has been recognized for its neuroprotective properties in various neurological disorders, including PD. This study explores the potential of GDNF in alleviating intestinal reactive gliosis and inflammation, thereby improving constipation and depressive behavior in a rat model of PD. A PD model was established via unilateral stereotaxic injection of 6-hydroxydopamine (6-OHDA). Five weeks post-injury, AAV5-GDNF (2 ~ 5 × 10^11) was intraperitoneally injected into experimental and control rats. Fecal moisture percentage (FMP) and colonic propulsion rate (CPPR) were used to evaluate colon motility. Colon-related inflammation and colonic epithelial morphology were assessed, and depressive behavior was analyzed one week before sampling. PD rats exhibited reduced colonic motility and GDNF expression, along with increased EGC reactivity and elevated levels of pro-inflammatory cytokines IL-1, IL-6, and TNF-α. Additionally, there was an up-regulation of CX43 and a decrease in PGP 9.5 expression. The intraperitoneal injection of AAV-GDNF significantly protected colonic neurons by inhibiting EGC activation and down-regulating CX43. This treatment also led to a notable reduction in depressive-like symptoms in PD rats with constipation. GDNF effectively reduces markers of reactive gliosis and inflammation, and promotes the survival of colonic neurons, and improves colonic motility in PD rats by regulating CX43 activity. Furthermore, GDNF treatment alleviates depressive behavior, suggesting that GDNF or its agonists could be promising therapeutic agents for managing gastrointestinal and neuropsychiatric symptoms associated with PD.

Connexin43 reduces LPS-induced inflammation in hDPCs through TLR4-NF-κB pathway via hemichannels

OBJECTIVES

Connexin43 (Cx43) is involved in the inflammation of many tissue types. Dental caries is infectious disease resulting from mineralized tissue dissolution by a specific bacterial population, causing pulp inflammation. However, Cx43's role in dental pulp remains unclear. Here, we investigated the function of Cx43 during pulp inflammation.

MATERIALS AND METHODS

We constructed a dentin injury model in Sprague-Dawley rats to investigate changes in Cx43 expression during pulp inflammation. Cx43 was inhibited in human dental pulp cells (hDPCs) that had been stimulated with lipopolysaccharide (LPS) to investigate the effect of Cx43 on inflammatory response. Promotion of TLR4-NF-κB pathway activity and special Cx43 channel inhibitors were used to clarify the function of Cx43 in hDPCs.

RESULTS

Dentin injury led to low-level inflammation in dental pulp. Following dentin injury, Cx43 expression initially decreased before gradually recovering to normal levels. Cx43 inhibition reduced LPS-induced expression of inflammatory cytokines and NF-κB pathway activity. Promotion of NF-κB pathway activity counteracted the effect of Cx43 in hDPCs. Furthermore, inhibition of Cx43 hemichannels reduced LPS-induced inflammatory cytokine expression.

CONCLUSIONS

Cx43 is involved in inflammation of dental pulp, while its inhibition reduced LPS-induced inflammation in hDPCs through NF-κB pathway via blockage of hemichannels.

Inflammation and Connexin 43 profiles in the prefrontal cortex are relevant to stress susceptibility and resilience in mice

Depression is a major chronic mental illness worldwide, characterized by anhedonia and pessimism. Exposed to the same stressful stimuli, some people behave normally, while others exhibit negative behaviors and psychology. The exact molecular mechanisms linking stress-induced depressive susceptibility and resilience remain unclear. Connexin 43 (Cx43) forms gap junction channels between the astrocytes, acting as a crucial role in the pathogenesis of depression. Cx43 dysfunction could lead to depressive behaviors, and depression down-regulates the expression of Cx43 in the prefrontal cortex (PFC). Besides, accumulating evidence indicates that inflammation is one of the most common pathological features of the central nervous system dysfunction. However, the roles of Cx43 and peripheral inflammation in stress-susceptible and stress-resilient individuals have rarely been investigated. Thus, animals were classified into the chronic unpredictable stress (CUS)-susceptible group and the CUS-resilient group based on the performance of behavioral tests following the CUS protocol in this study. The protein expression of Cx43 in the PFC, the Cx43 functional changes in the PFC, and the expression levels including interleukin (IL)-1β, tumor necrosis factor-α, IL-6, IL-2, IL-10, and IL-18 in the peripheral serum were detected. Here, we found that stress exposure triggered a significant reduction in Cx43 protein expression in the CUS-susceptible mice but not in the CUS-resilient mice accompanied by various Cx43 phosphorylation expression and the changes of inflammatory signals. Stress resilience is associated with Cx43 in the PFC and fluctuation in inflammatory signaling, showing that therapeutic targeting of these pathways might promote stress resilience.

Fascin-1 limits myosin activity in microglia to control mechanical characterization of the injured spinal cord

BACKGROUND

Mechanical softening of the glial scar region regulates axonal regeneration to impede neurological recovery in central nervous system (CNS) injury. Microglia, a crucial cellular component of the glial scar, facilitate neuronal survival and neurological recovery after spinal cord injury (SCI). However, the critical mechanical characterization of injured spinal cord that harmonizes neuroprotective function of microglia remains poorly understood.

METHODS

Spinal cord tissue stiffness was assessed using atomic force microscopy (AFM) in a mouse model of crush injury. Pharmacological depletion of microglia using PLX5622 was used to explore the effect of microglia on mechanical characterization. Conditional knockout of Fascin-1 in microglia (Fascin-1 CKO) alone or in combination with inhibition of myosin activity was performed to delve into relevant mechanisms of microglia regulating mechanical signal. Immunofluorescence staining was performed to evaluate the related protein levels, inflammatory cells, and neuron survival after SCI. The Basso mouse scale score was calculated to assess functional recovery.

RESULTS

Spinal cord tissue significantly softens after SCI. Microglia depletion or Fascin-1 knockout in microglia limits tissue softening and alters mechanical characterization, which leads to increased tissue pathology and impaired functional recovery. Mechanistically, Fascin-1 inhibits myosin activation to promote microglial migration and control mechanical characterization after SCI.

CONCLUSIONS

We reveal that Fascin-1 limits myosin activity to regulate mechanical characterization after SCI, and this mechanical signal should be considered in future approaches for the treatment of CNS diseases.

Out of the core: the impact of focal ischemia in regions beyond the penumbra

The changes in the necrotic core and the penumbra following induction of focal ischemia have been the focus of attention for some time. However, evidence shows, that ischemic injury is not confined to the primarily affected structures and may influence the remote areas as well. Yet many studies fail to probe into the structures beyond the penumbra, and possibly do not even find any significant results due to their short-term design, as secondary damage occurs later. This slower reaction can be perceived as a therapeutic opportunity, in contrast to the ischemic core defined as irreversibly damaged tissue, where the window for salvation is comparatively short. The pathologies in remote structures occur relatively frequently and are clearly linked to the post-stroke neurological outcome. In order to develop efficient therapies, a deeper understanding of what exactly happens in the exo-focal regions is necessary. The mechanisms of glia contribution to the ischemic damage in core/penumbra are relatively well described and include impaired ion homeostasis, excessive cell swelling, glutamate excitotoxic mechanism, release of pro-inflammatory cytokines and phagocytosis or damage propagation via astrocytic syncytia. However, little is known about glia involvement in post-ischemic processes in remote areas. In this literature review, we discuss the definitions of the terms "ischemic core", "penumbra" and "remote areas." Furthermore, we present evidence showing the array of structural and functional changes in the more remote regions from the primary site of focal ischemia, with a special focus on glia and the extracellular matrix. The collected information is compared with the processes commonly occurring in the ischemic core or in the penumbra. Moreover, the possible causes of this phenomenon and the approaches for investigation are described, and finally, we evaluate the efficacy of therapies, which have been studied for their anti-ischemic effect in remote areas in recent years.

Tonabersat Significantly Reduces Disease Progression in an Experimental Mouse Model of Multiple Sclerosis

Multiple sclerosis (MS) is a neurodegenerative disease marked by chronic neuroinflammation thought to be mediated by the inflammasome pathway. Connexin 43 (Cx43) hemichannels contribute to the activation of the inflammasome through the release of adenosine triphosphate (ATP) inflammasome activation signals. The objective of the study was to evaluate if the Cx43 hemichannel blocker, tonabersat, is effective in modulating the inflammatory response and reducing disability in the myelin oligodendrocyte glycoprotein 35-55-induced experimental autoimmune encephalomyelitis (MOG35-55 EAE) model of MS. Here, we show that the Cx43 hemichannel blocking drug, tonabersat, significantly reduced expression of neuroinflammatory markers for microglial activation (ionized calcium-binding adapter molecule 1 (Iba1)) and astrogliosis (glial fibrillary acidic protein (GFAP)) while preserving myelin basic protein (MBP) expression levels in the corpus callosum, motor cortex, and striatum regions of the brain in MOG35-55 EAE mice. Reduced NOD-like receptor protein 3 (NLRP3) inflammasome complex assembly and Caspase-1 activation confirmed the drug's mode of action. MOG35-55 EAE mice showed clinical signs of MS, but MOG35-55 EAE mice treated with tonabersat retained behavior closer to normal. These data suggest that clinical trial phase IIb-ready tonabersat may merit further investigation as a promising candidate for MS treatment.

Astroglial Connexin 43-Mediated Gap Junctions and Hemichannels: Potential Antidepressant Mechanisms and the Link to Neuroinflammation

Major depression disorder (MDD) is a neuropsychiatric disorder associated with a high suicide rate and a higher disability rate than any other disease. Evidence suggests that the pathological mechanism of MDD is related to astrocyte dysfunction. Depression is mainly associated with the expression of connexin 43 (Cx43) and the function of Cx43-mediated gap junctions and hemichannels in astrocytes. Moreover, neuroinflammation has been a hotspot in research on the pathology of depression, and Cx43-mediated functions are thought to be involved in neuroinflammation-related depression. However, the specific mechanism of Cx43-mediated functions in neuroinflammation-related depression pathology remains unclear. Therefore, this review summarizes and discusses Cx43 expression, the role of gap junction intercellular communication, and its relationship with neuroinflammation in depression. This review also focuses on the effects of antidepressant drugs (e.g., monoamine antidepressants, psychotropic drugs, and N-methyl-D-aspartate receptor antagonists) on Cx43-mediated function and provides evidence for Cx43 as a novel target for the treatment of MDD. The pathogenesis of MDD is related to astrocyte dysfunction, with reduced Cx43 expression, GJ dysfunction, decreased GJIC and reduced BDNF expression in the depressed brain. The effect of Cx43 on neuroinflammation-related depression involving inflammatory cytokines, glutamate excitotoxicity, and HPA axis dysregulation. Antidepressant drugs targeting Cx43 can effectively relieve depressive symptoms.

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.

Inhibition of Connexin43 Improves the Recovery of Spinal Cord Injury Against Ferroptosis via the SLC7A11/GPX4 Pathway

Ferroptosis plays a key role in the process of spinal cord injury (SCI). As a signal amplifier, connexin 43 (CX43) participates in cell death signal transduction and aggravates the propagation of injury. However, it remains unclear whether CX43 plays a regulatory role in ferroptosis after SCI. The SCI rat model was established by an Infinite Vertical Impactor to investigate the role of CX43 in SCI-induced ferroptosis. Ferrostatin-1 (Fer-1), an inhibitor of ferroptosis, and a CX43-specific inhibitor (Gap27) were administered by intraperitoneal injection. Behavioral analysis was assessed according to the Basso-Beattie-Bresnahan (BBB) Motor Rating Scale and the inclined plate test. The levels of ferroptosis-related proteins were estimated by qRT-PCR and western blotting, while the histopathology of neuronal injury induced by SCI was evaluated by immunofluorescence, Nissl, FJB and Perl's Blue staining. Meanwhile, transmission electron microscopy was used to observe the ultrastructural changes characteristic of ferroptosis. Gap27 strongly inhibited ferroptosis and therefore improved the functional recovery of SCI, which was similar to the treatment of Fer-1. Notably, the inhibition of CX43 decreased P-mTOR/mTOR expression and reversed the decrease in SLC7A11 induced by SCI. As a result, the levels of GPX4 and glutathione (GSH) increased, while the levels of the lipid peroxidation products 4-hydroxynonenal (4-HNE) and malondialdehyde (MDA) decreased. Together, inhibition of CX43 could alleviate ferroptosis after SCI. These findings reveal a potential mechanism of the neuroprotective role of CX43 after SCI and provide a new theoretical basis for clinical transformation and application.

Divergence between Hemichannel and Gap Junction Permeabilities of Connexin 30 and 26

Cx30 has been proposed to play physiological functions in the kidney and cochlea, and this has often been associated with its hemichannel role (deafness mutants frequently affecting hemichannels more than gap junctions), implicated in ATP release. Here, we used heterologous expression systems (Xenopus oocytes and N2A cells) to describe the properties of Cx30 hemichannels, with the objective of better understanding their physiological functions. As previously observed, Cx30 hemichannels gated in response to transmembrane voltage (V0) and extracellular [Ca2+] (pK[Ca2+] of 1.9 μM in the absence of Mg++). They show minimal charge selectivity with respect to small ions (ratio of Na+: K+: Cl- of 1: 0.4: 0.6) and an MW cut-off for Alexa Dyes between 643 (Alex 488) and 820 Da (Alexa 594). However, while cations follow the expected drop in conductance with size (Na+ to TEA+ is 1: 0.3), anions showed an increase, with a ratio of Cl- to gluconate conductance of 1:1.4, suggesting favorable interactions between larger anions and the pore. This was further explored by comparing the permeabilities of both hemichannels and gap junctions to the natural anion (ATP), the release of which has been implicated in Ca++ signaling through hemichannels. We extended this analysis to two closely related connexins co-expressed in the cochlear, Cx26 and Cx30. Cx30 and 26 hemichannels displayed similar permeabilities to ATP, but surprisingly Cx26 gap junctions were six times more permeable than their hemichannels and four times more permeable than Cx30 gap junctions. This suggests a significant physiological difference in the functions of Cx26 and Cx30 gap junctions in organs where they are co-expressed, at least with regard to the distribution of energy resources of the cells. It also demonstrates that the permeability characteristics of hemichannels can significantly diverge from that of their gap junctions for some connexins but not others.

Connexin 43: insights into candidate pathological mechanisms of depression and its implications in antidepressant therapy

Major depressive disorder (MDD), a chronic and recurrent disease characterized by anhedonia, pessimism or even suicidal thought, remains a major chronic mental concern worldwide. Connexin 43 (Cx43) is the most abundant connexin expressed in astrocytes and forms the gap junction channels (GJCs) between astrocytes, the most abundant and functional glial cells in the brain. Astrocytes regulate neurons' synaptic strength and function by expressing receptors and regulating various neurotransmitters. Astrocyte dysfunction causes synaptic abnormalities, which are related to various mood disorders, e.g., depression. Increasing evidence suggests a crucial role of Cx43 in the pathogenesis of depression. Depression down-regulates Cx43 expression in humans and rats, and dysfunction of Cx43 also induces depressive behaviors in rats and mice. Recently Cx43 has received considerable critical attention and is highly implicated in the onset of depression. However, the pathological mechanisms of depression-like behavior associated with Cx43 still remain ambiguous. In this review we summarize the recent progress regarding the underlying mechanisms of Cx43 in the etiology of depression-like behaviors including gliotransmission, metabolic disorders, and neuroinflammation. We also discuss the effects of antidepressants (monoamine antidepressants and ketamine) on Cx43. The clarity of the candidate pathological mechanisms of depression-like behaviors associated with Cx43 and its potential pharmacological roles for antidepressants will benefit the exploration of a novel antidepressant target.

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.

Connexins Signatures of the Neurovascular Unit and Their Physio-Pathological Functions

Central nervous system (CNS) homeostasis is closely linked to the delicate balance of the microenvironment in which different cellular components of the neurovascular unit (NVU) coexist. Intercellular communication plays a pivotal role in exchanges of signaling molecules and mediators essential for survival functions, as well as in the removal of disturbing elements that can lead to related pathologies. The specific signatures of connexins (Cxs), proteins which form either gap junctions (GJs) or hemichannels (HCs), represent the biological substrate of the pathophysiological balance. Connexin 43 (Cx43) is undoubtedly one of the most important factors in glia–neuro–vascular crosstalk. Herein, Cxs signatures of every NVU component are highlighted and their critical influence on functional processes in healthy and pathological conditions of nervous microenvironment is reviewed.

The Effects of NLRP3 Inflammasome Inhibition in Experimental Acute Pancreatitis: A Systematic Review and Meta-Analysis

ABSTRACT

Acute pancreatitis (AP) is an inflammatory disease, and NLRP3 inflammasome activation is involved in the pathogenesis of AP. Previous research showed that inhibition of NLRP3 inflammasome may exert protective effects on animal models of AP and reduces disease severity. The aim of this systematic review and meta-analysis is to evaluate the effects of drug treatment of NLRP3 inflammasome on the outcomes of experimental AP. PubMed, Embase, Medline, and Web of Science databases were searched for relevant articles without language restrictions. The main outcomes for this study included local pancreatic injury, the incidence of systemic inflammatory responses, and the incidence of organ failure. Twenty-eight animal studies including 556 animals with AP were included in the meta-analysis. Compared with controls, inhibition of NLRP3 inflammasome significantly reduced the pancreatic histopathological scores, serum amylase, and lipase levels. In addition, inhibition of NLRP3 inflammasome reduced the levels of circulating inflammatory cytokines, as well as mitigating severity of AP-associated acute lung injury and acute intestinal injury. To conclude, inhibition of NLRP3 inflammasome has protective effects on AP by mitigating organ injury and systemic inflammation in animal studies, indicating that NLRP3 inflammasome holds promise as a target for specific AP therapy.

Gap Junctions and Hemichannels Composed of Connexins and Pannexins Mediate the Secondary Brain Injury Following Intracerebral Hemorrhage

Simple Summary

Intracerebral hemorrhage (ICH) is a leading medical problem without effective treatment options. The poor prognosis is attributed to the primary brain injury of the mechanical compression caused by hematoma, and secondary brain injury (SBI) that includes inflammation, glutamate excitotoxicity, oxidative stress and disruption of the blood brain barrier (BBB). Evidences suggests that gap junctions and hemichannels composed of connexins and pannexins regulate the inflammation and excitotoxicity insult in the pathological process of central nervous system disease, such as cerebral ischemia and neurodegeneration disease. In this manuscript, we discuss the fact that connexins- and pannexins-based channels could be involved in secondary brain injury of ICH, particularly through mediating inflammation, oxidative stress, BBB disruption and cell death. The details provided in this manuscript may help develop potential targets for therapeutic intervention of ICH.

Abstract

Intracerebral hemorrhage (ICH) is a devastating disease with high mortality and morbidity; the mortality rate ranges from 40% at 1 month to 54% at 1 year; only 12–39% achieve good outcomes and functional independence. ICH affects nearly 2 million patients worldwide annually. In ICH development, the blood leakage from ruptured vessels generates sequelae of secondary brain injury (SBI). This mechanism involves activated astrocytes and microglia, generation of reactive oxygen species (ROS), the release of reactive nitrogen species (RNS), and disrupted blood brain barrier (BBB). In addition, inflammatory cytokines and chemokines, heme compounds, and products of hematoma are accumulated in the extracellular spaces, thereby resulting in the death of brain cells. Recent evidence indicates that connexins regulate microglial activation and their phenotypic transformation. Moreover, communications between neurons and glia via gap junctions have crucial roles in neuroinflammation and cell death. A growing body of evidence suggests that, in addition to gap junctions, hemichannels (composed of connexins and pannexins) play a key role in ICH pathogenesis. However, the precise connection between connexin and pannexin channels and ICH remains to be resolved. This review discusses the pathological roles of gap junctions and hemichannels in SBI following ICH, with the intent of discovering effective therapeutic options of strategies to treat ICH.

Over-activated hemichannels: A possible therapeutic target for human diseases

In our body, all the cells are constantly sharing chemical and electrical information with other cells. This intercellular communication allows them to respond in a concerted way to changes in the extracellular milieu. Connexins are transmembrane proteins that have the particularity of forming two types of channels; hemichannels and gap junction channels. Under normal conditions, hemichannels allow the controlled release of signaling molecules to the extracellular milieu. However, under certain pathological conditions, over-activated hemichannels can induce and/or exacerbate symptoms. In the last decade, great efforts have been put into developing new tools that can modulate these over-activated hemichannels. Small molecules, antibodies and mimetic peptides have shown a potential for the treatment of human diseases. In this review, we summarize recent findings in the field of hemichannel modulation via specific tools, and how these tools could improve patient outcome in certain pathological conditions.

Xentry-Gap19 inhibits Connexin43 hemichannel opening especially during hypoxic injury

Hypoxic injury results in cell death, tissue damage and activation of inflammatory pathways. This is mediated by pathological Connexin43 (Cx43) hemichannel (HC) opening resulting in osmotic and ionic imbalances as well as cytokine production perpetuating the inflammatory environment. Gap19 is an intracellularly acting Cx43 mimetic peptide that blocks HC opening and thus promotes cell survival. However, native Gap19, which must enter the cell in order to function, exhibits low cell permeability. In this study, Gap19 was conjugated to the cell-penetrating peptide, Xentry, to investigate if cellular uptake could be improved while maintaining peptide function. Cellular uptake of Xentry-Gap19 (XG19) was much greater than that of native Gap19 even under normal cell culture conditions. Peptide function was maintained post uptake as shown by reduced ethidium homodimer influx and ATP release due to Cx43 HC block. While XG19 blocked pathologic HC opening though, normal gap junction communication required for cell repair and survival mechanisms was not affected as shown in a dye scrape-load assay. Under hypoxic conditions, increased expression of Syndecan-4, a plasma membrane proteoglycan targeted by Xentry, enabled even greater XG19 uptake leading to higher inhibition of ATP release and greater cell survival. This suggests that XG19, which is targeted specifically to hypoxic cells, can efficiently and safely block Cx43 HC and could therefore be a novel treatment for hypoxic and inflammatory diseases.

Astroglial Connexins in Neurological and Neuropsychological Disorders and Radiation Exposure

Radiotherapy is a common treatment for brain and spinal cord tumors and also a risk factor for neuropathological changes in the brain leading to different neurological and neuropsychological disorders. Astroglial connexins are involved in brain inflammation, development of Alzheimer's Disease (AD), depressive, epilepsy, and amyotrophic lateral sclerosis, and are affected by radiation exposure. Therefore, it is speculated that radiation-induced changes of astroglial connexins may be related to the brain neuropathology and development of neurological and neuropsychological disorders. In this paper, we review the functional expression and regulation of astroglial connexins expressed between astrocytes and different types of brain cells (including oligodendrocytes, microglia, neurons and endothelial cells). The roles of these connexins in the development of AD, depressive, epilepsy, amyotrophic lateral sclerosis and brain inflammation have also been summarized. The radiation-induced astroglial connexins changes and development of different neurological and neuropsychological disorders are then discussed. Based on currently available data, we propose that radiation-induced astroglial connexins changes may be involved in the genesis of different neurological and neuropsychological disorders which depends on the age, brain regions, and radiation doses/dose rates. The abnormal astroglial connexins may be novel therapeutic targets for the prevention of radiation-induced cognitive impairment, neurological and neuropsychological disorders.

Inflammatory response in epilepsy is mediated by glial cell gap junction pathway (Review)

Epilepsy is a common neurological disease that affects more than 50 million people worldwide. Neuroinflammation plays an important role in epilepsy. Activation of the immune system and an excessive inflammatory response can increase the frequency of seizures and increase the susceptibility to epilepsy. Therefore, anti-inflammatory therapies may have antiepileptic effects. Connexin 43 (Cx43) is a major component of astroglial hemichannels and gap junctions. Gap junctions are important for the direct exchange of substances and information between cells, as well as regulating the neuroinflammatory response, changing neuronal excitability, neuronal apoptosis, and synaptic remodeling. Cx43-mediated gap junction pathway can be crucial in epilepsy-induced neuroinflammatory cascades. Further, pro-inflammatory cytokines may in turn directly affect the expression of the Cx43 protein in astrocytes. Therefore, examining the association between neuroinflammation and epilepsy can be instrumental in uncovering the pathogenesis of epilepsy, which can lead to the development of novel and more effective antiepileptic drugs.

Anti-Inflammatory Therapies for Treatment of Inflammation-Related Preterm Brain Injury

Despite the prevalence of preterm brain injury, there are no established neuroprotective strategies to prevent or alleviate mild-to-moderate inflammation-related brain injury. Perinatal infection and inflammation have been shown to trigger acute neuroinflammation, including proinflammatory cytokine release and gliosis, which are associated with acute and chronic disturbances in brain cell survival and maturation. These findings suggest the hypothesis that the inhibition of peripheral immune responses following infection or nonspecific inflammation may be a therapeutic strategy to reduce the associated brain injury and neurobehavioral deficits. This review provides an overview of the neonatal immunity, neuroinflammation, and mechanisms of inflammation-related brain injury in preterm infants and explores the safety and efficacy of anti-inflammatory agents as potentially neurotherapeutics.

Inhibition of astrocyte hemichannel improves recovery from spinal cord injury

Spinal cord injury (SCI) causes severe disability, and the current inability to restore function to the damaged spinal cord leads to lasting detrimental consequences to patients. One strategy to reduce SCI morbidity involves limiting the spread of secondary damage after injury. Previous studies have shown that connexin 43 (Cx43), a gap junction protein richly expressed in spinal cord astrocytes, is a potential mediator of secondary damage. Here, we developed a specific inhibitory antibody, mouse-human chimeric MHC1 antibody (MHC1), that inhibited Cx43 hemichannels, but not gap junctions, and reduced secondary damage in 2 incomplete SCI mouse models. MHC1 inhibited the activation of Cx43 hemichannels in both primary spinal astrocytes and astrocytes in situ. In both SCI mouse models, administration of MHC1 after SCI significantly improved hind limb locomotion function. Remarkably, a single administration of MHC1 30 minutes after injury improved the recovery up to 8 weeks post-SCI. Moreover, MHC1 treatment decreased gliosis and lesion sizes, increased white and gray matter sparing, and improved neuronal survival. Together, these results suggest that inhibition of Cx43 hemichannel function after traumatic SCI reduces secondary damage, limits perilesional gliosis, and improves functional recovery. By targeting hemichannels specifically with an antibody, this study provides a potentially new, innovative therapeutic approach in treating SCI.

Connexin43 Expression and Associated Chronic Inflammation Presages the Development of Cerebral Radiation Necrosis

Cerebral radiation necrosis (CRN) is a delayed complication of radiosurgery that can result in severe neurological deficits. The biological changes leading to necrotic damage may identify therapeutic targets for this complication. Connexin43 expression associated with chronic inflammation may presage the development of CRN. A mouse model of delayed CRN was used. The left hemispheres of adult female mice were irradiated with single-fraction, high-dose radiation using a Leksell Gamma Knife. The brains were collected 1 and 4 days, and 1-3 weeks after the radiation. The expression of connexin43, interleukin-1β (IL-1β), GFAP, isolectin B-4, and fibrinogen was evaluated using immunohistochemical staining and image analysis. Compared with the baseline, the area of connexin43 and IL-1β staining was increased in ipsilateral hemispheres 4 days after radiation. Over the following 3 weeks, the density of connexin43 gradually increased in parallel with progressive increases in GFAP, isolectin B-4, and fibrinogen labeling. The overexpression of connexin43 in parallel with IL-1β spread into the affected brain regions first. Further intensified upregulation of connexin43 was associated with escalated astrocytosis, microgliosis, and blood-brain barrier breach. Connexin43-mediated inflammation may underlie radiation necrosis and further investigation of connexin43 hemichannel blockage is merited for the treatment of CRN.

Synthesis and biological evaluation of S-lipidated lipopeptides of a connexin 43 channel inhibitory peptide

The synthesis and biological activity of 42 novel S-lipidated analogues of a connexin 43 channel inhibitory Peptide5 is described. Unmodified Peptide5 moderates hemichannels and gap junctions that are both implicated in the progression of neurological disease. Peptide5 was site-specifically modified with a cysteine residue, which then underwent thiol-ene mediated S-lipidation to afford S-lipidated Peptide5 analogues containing straight-chain, branched, or aromatic lipids. The modified peptides were assessed for their effect on hemichannel opening and the most promising candidates were evaluated in serum stability studies.

Formation of functional innexin hemichannels, as well as gap junctional channels, in an insect cell line, NIAs-AeAl-2, derived from Asian tiger mosquito Aedes albopictus (Diptera: Culicidae): A partial but significant contribution of innexin 2

In vertebrates, gap junctions and hemichannels consisting of connexins are important cell surface structures for communication with neighboring cells and for the regulation of various cell functions. To date, various gap-junction-related proteins have been found, including innexins in invertebrates and pannexins in vertebrates. Significant contributions of gap junctions by innexins and (hemi-)channels by pannexins to numerous functions have been reported. Verification of the presence and functional significance of innexin hemichannels, however, remains a gap in our knowledge in innexin physiology. In this study, we revealed the localization of an innexin protein (innexin 2) on the cell surface in mosquito tissues and cultured cells. Furthermore, we demonstrated the presence of functional hemichannels, as well as gap junctions, in mosquito cells using dye transfer assays. The inward uptake of fluorescent dye was inhibited by anti-innexin 2 antibody. These results suggest that innexin hemichannels are formed to function in cultured mosquito cells, in at least a partially innexin 2-dependent manner. Although only a few studies on insect hemichannels have been published, innexin-based hemichannels, as well as innexin gap junctions, could also significantly contribute to insect intercellular signal transduction.

Development of a Cx46 Targeting Strategy for Cancer Stem Cells

Gap-junction-mediated cell-cell communication enables tumor cells to synchronize complex processes. We previously found that glioblastoma cancer stem cells (CSCs) express higher levels of the gap junction protein Cx46 compared to non-stem tumor cells (non-CSCs) and that this was necessary and sufficient for CSC maintenance. To understand the mechanism underlying this requirement, we use point mutants to disrupt specific functions of Cx46 and find that Cx46-mediated gap-junction coupling is critical for CSCs. To develop a Cx46 targeting strategy, we screen a clinically relevant small molecule library and identify clofazimine as an inhibitor of Cx46-specific cell-cell communication. Clofazimine attenuates proliferation, self-renewal, and tumor growth and synergizes with temozolomide to induce apoptosis. Although clofazimine does not cross the blood-brain barrier, the combination of clofazimine derivatives optimized for brain penetrance with standard-of-care therapies may target glioblastoma CSCs. Furthermore, these results demonstrate the importance of targeting cell-cell communication as an anti-cancer therapy.

Multi-walled carbon nanotubes induce IL-1β secretion by activating hemichannels-mediated ATP release in THP-1 macrophages

Multi-walled carbon nanotubes (MWCNTs) are known to induce pulmonary inflammatory effects through stimulating pro-inflammatory cytokine secretion from alveolar macrophages. Despite extensive studies on MWCNTs' pro-inflammatory reactivity, the understanding of molecular mechanisms involved is still incomplete. In this study, we investigated hemichannel's involvement in MWCNTs-induced macrophage IL-1β release. Our results showed that the unmodified and COOH MWCNTs could induce ATP release and ATP-P2X₇R axis-dependent IL-1β secretion from THP-1 macrophages. By using various inhibitors, we confirmed that the MWCNTs-induced ATP release was primarily through hemichannels. EtBr dye uptake assay detected significant hemichannels opening in MWCNTs exposed THP-1 macrophages. Inhibition of hemichannels by CBX, ⁴³Gap27, or ¹⁰Panx1 pretreatment results in decreased ATP and IL-1β release. The addition of ATP restored the reduced IL-1β secretion level from hemichannel inhibition. We also confirmed with five other types of MWCNTs that the induction of hemichannels by MWCNTs strongly correlates with their capacity to induce IL-1β secretion. Taken together, we conclude that hemichannels-mediated ATP release and subsequent NLRP3 inflammasome activation through P2X₇R may be one mechanism by which MWCNTs induce macrophage IL-1β secretion. Our findings may provide a novel molecular mechanism for MWCNTs induced IL-1β secretion.

Interferon-γ and high glucose-induced opening of Cx43 hemichannels causes endothelial cell dysfunction and damage

Both IFN-γ or high glucose have been linked to systemic inflammatory imbalance with serious repercussions not only for endothelial function but also for the formation of the atherosclerotic plaque. Although the uncontrolled opening of connexin hemichannels underpins the progression of various diseases, whether they are implicated in endothelial cell dysfunction and damage evoked by IFN-γ plus high glucose remains to be fully elucidated. In this study, by using live cell imaging and biochemical approaches, we demonstrate that IFN-γ plus high glucose augment endothelial connexin43 hemichannel activity, resulting in the increase of ATP release, ATP-mediated Ca²⁺ dynamics and production of nitric oxide and superoxide anion, as well as impaired insulin-mediated uptake and intercellular diffusion of glucose and cell survival. Based on our results, we propose that connexin 43 hemichannel inhibition could serve as a new approach for tackling the activation of detrimental signaling resulting in endothelial cell dysfunction and death caused by inflammatory mediators during atherosclerosis secondary to diabetes mellitus.

Connexin-43 reduction prevents muscle defects in a mouse model of manifesting Duchenne muscular dystrophy female carriers

Duchenne muscular dystrophy (DMD) is a severe X-linked neuromuscular disorder that affects males. However, 8% of female carriers are symptomatic and underrepresented in research due to the lack of animal models. We generated a symptomatic mouse model of DMD carriers via injection of mdx (murine DMD) embryonic stem cells (ESCs) into wild-type (WT) blastocysts (mdx/WT chimera). mdx/WT chimeras developed cardiomyopathic features and dystrophic skeletal muscle phenotypes including elevated mononuclear invasion, central nucleation, fibrosis and declined forelimb grip strength. The disease was accompanied by connexin-43 (Cx43) aberrantly enhanced in both cardiac and skeletal muscles and remodeled in the heart. Genetic reduction of Cx43-copy number in mdx/WT-Cx43(+/-) chimeras protected them from both cardiac and skeletal muscle fiber damage. In dystrophic skeletal muscle, Cx43 expression was not seen in the fibers but in adjacent F4/80+ mononuclear cells. Ethidium Bromide uptake in purified F4/80+/CD11b+ mdx macrophages revealed functional activity of Cx43, which was inhibited by administration of Gap19 peptide mimetic, a Cx43 hemichannel-specific inhibitor. Thus, we suggest that Cx43 reduction in symptomatic DMD carrier mice leads to prevention of Cx43 remodeling in the heart and prevention of aberrant Cx43 hemichannel activity in the skeletal muscle macrophages neighboring Cx43 non-expressing fibers.

Connexins in oligodendrocytes and astrocytes: Possible factors for demyelination in multiple sclerosis

Increasing evidences support that glial connexins are involved in the demyelination pathology of multiple sclerosis (MS), a chronic inflammatory demyelinating disorder. Here, we review the data from patients with MS and animal models of MS that implicate connexins in demyelination. Connexins expressed in oligodendrocytes and astrocytes show diverse changes at the different phases of MS. Loss of oligodendrocyte or astrocyte connexins contributes to demyelination and exaggerates the pathology of MS. Channel-dependent and -independent connexins are involved in the pathology of demyelination, which is related with myelin integrity, metabolic homeostasis, the brain-blood barrier, the immune cell infiltration, and the inflammatory response. A comprehensive understanding of connexin function in demyelination may provide new therapeutic targets for MS.

Connexin 43 deletion in astrocytes promotes CNS remyelination by modulating local inflammation

As the most abundant gap junction protein in the central nervous system (CNS), astrocytic connexin 43 (Cx43) maintains astrocyte network homeostasis, affects oligodendroglial development and participates in CNS pathologies as well as injury progression. However, its role in remyelination is not yet fully understood. To address this issue, we used astrocyte-specific Cx43 conditional knockout (Cx43 cKO) mice generated through the use of a hGFAP-cre promoter, in combination with mice carrying a floxed Cx43 allele that were subjected to lysolecithin so as to induce demyelination. We found no significant difference in the demyelination of the corpus callosum between Cx43 cKO mice and their non-cre littermate controls, while the remyelination process in Cx43 cKO mice was accelerated. Moreover, an increased number of mature oligodendrocytes and an unaltered number of oligodendroglial lineage cells were found in Cx43 cKO mouse lesions. This indicates that oligodendrocyte precursor cell (OPC) differentiation was facilitated by astroglial Cx43 depletion as remyelination progressed. Underlying the latter, there was a down-regulated glial activation and modulated local inflammation as well as a reduction of myelin debris in Cx43 cKO mice. Importantly, 2 weeks of orally administrating boldine, a natural alkaloid that blocks Cx hemichannel activity in astrocytes without affecting gap junctional communication, obviously modulated local inflammation and promoted remyelination. Together, the data suggest that the astrocytic Cx43 hemichannel is negatively involved in the remyelination process by favoring local inflammation. Consequently, inhibiting Cx43 hemichannel functionality may be a potential therapeutic approach for demyelinating diseases in the CNS.

Connexin 26 and 43 play a role in regulating proinflammatory events in the epidermis

Dysregulation of Connexin (CX) expression and function is associated with a range of chronic inflammatory conditions including psoriasis and nonhealing wounds. To mimic a proinflammatory environment, HaCaT cells, a model human keratinocyte cell line, were challenged with 10 µg/ml peptidoglycan (PGN) isolated from Staphylococcus aureus for 15 min to 24 hr in the presence or absence of CX blockers and/or following CX26, CX43, PANX1 and TLR2 small interfering RNA (siRNA) knockdown (KD). Expression levels of IL-6, IL-8, CX26, CX43, PANX1, TLR2 and Ki67 were assessed by quantitative real-time polymerase chain reaction, western blot analysis and/or immunocytochemistry. Nuclear factor kappa β (NF-κβ) was blocked with BAY 11-7082, CX-channel function was determined by adenosine 5'-triphosphate (ATP) release assays. Enzyme-linked immunosorbent assay monitored IL6 release following PGN challenge in the presence or absence of siRNA or blockers of CX or purinergic signalling. Exposure to PGN induced IL-6, IL-8, CX26 and TLR2 gene expression but it did not influence CX43, PANX1 or Ki67 messenger RNA expression levels. CX43 protein levels were reduced following 24 hr PGN exposure. PGN-induced CX26 and IL-6 expression were also aborted by TLR2-KD and inhibition of NF-κβ. ATP and IL-6 release were stimulated following 15 min and 1-24 hr challenge with PGN, respectively. Release of both agents was inhibited by coincubation with CX-channel blockers, CX26-, CX43- and TLR2-KD. The IL-6 response was also reduced by purinergic blockers. CX-signalling plays a role in the innate immune response in the epidermis. PGN is detected by TLR2, which via NF-κβ, directly activates CX26 and IL-6 expression. CX43 and CX26 maintain proinflammatory signalling by permitting ATP release, however, PANX1 does not participate.

Blood-retinal barrier protection against high glucose damage: The role of P2X7 receptor

Blood retinal barrier (BRB) breakdown is a hallmark of diabetic retinopathy, whose occurrence in early or later phases of the disease has not yet been completely clarified. Recent evidence suggests that hyperglycemia induces activation of the P2X7 receptor (P2X7R) leading to pericyte cell death. We herein investigated the role of P2X7R on retinal endothelial cells viability and expression of tight- and adherens-junctions following high glucose (HG) exposure. We found that HG elicited P2X7R activation and expression and release of the pro-inflammatory cytokine IL-1β in human retinal endothelial cells (HRECs). Furthermore, HG exposure caused a decrease in HRECs viability and a damage of the BRB. JNJ47965567, a P2X7R antagonist, protected HRECs from HG-induced damage (LDH release) and preserved the BRB, as shown by transendothelial electrical resistance and cell junction morphology (ZO-1, claudin-5 and VE-cadherin). Moreover, JNJ47965567 treatment significantly decreased IL-1β expression and release, elicited by HG. These data indicate that P2X7R plays an important role to regulate BRB integrity, in particular the block of this receptor was useful to counteract the damage elicited by HG in HRECs, and warranting further clinical evaluation of P2X7R antagonists for the treatment of diabetic macular edema.

Heavy Alcohol Exposure Activates Astroglial Hemichannels and Pannexons in the Hippocampus of Adolescent Rats: Effects on Neuroinflammation and Astrocyte Arborization

A mounting body of evidence indicates that adolescents are specially more susceptible to alcohol influence than adults. However, the mechanisms underlying this phenomenon remain poorly understood. Astrocyte-mediated gliotransmission is crucial for hippocampal plasticity and recently, the opening of hemichannels and pannexons has been found to participate in both processes. Here, we evaluated whether adolescent rats exposed to ethanol exhibit changes in the activity of astrocyte hemichannels and pannexons in the hippocampus, as well as alterations in astrocyte arborization and cytokine levels. Adolescent rats were subjected to ethanol (3.0 g/kg) for two successive days at 48-h periods over 14 days. The opening of hemichannels and pannexons was examined in hippocampal slices by dye uptake, whereas hippocampal cytokine levels and astroglial arborization were determined by ELISA and Sholl analysis, respectively. We found that adolescent ethanol exposure increased the opening of connexin 43 (Cx43) hemichannels and pannexin-1 (Panx1) channels in astrocytes. Blockade of p38 mitogen-activated protein kinase (MAPK), inducible nitric oxide synthase (iNOS) and cyclooxygenases (COXs), as well as chelation of intracellular Ca²⁺, drastically reduced the ethanol-induced channel opening in astrocytes. Importantly, ethanol-induced Cx43 hemichannel and Panx1 channel activity was correlated with increased levels of interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), IL-6 in the hippocampus, as well as with profound alterations in astrocyte arbor complexity. Thus, we propose that uncontrolled opening of astrocyte hemichannels and pannexons may contribute not only to the glial dysfunction and neurotoxicity caused by adolescent alcohol consumption, but also to the pathogenesis of alcohol use disorders in the adulthood.

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.

Inhibition of Connexin 43 Hemichannels Alleviates Cerebral Ischemia/Reperfusion Injury via the TLR4 Signaling Pathway

Connexin 43 (Cx43) widely exists in all components of the neurovascular unit (NVU) and is a constituent of gap junctions and hemichannels. In physiological states, gap junctions are open for regular intercellular communication, and the hemichannels present low open probability in astrocytes. After cerebral ischemia, a large number of hemichannels are unusually opened, leading to cell swelling and even death. Most known hemichannel blockers also inhibit gap junctions and sequentially obstruct normal electrical cell-cell communication. In this study, we tested the hypothesis that Gap19, a selective Cx43-hemichannel inhibitor, exhibited neuroprotective effects on cerebral ischemia/reperfusion (I/R). An obvious improvement in neurological scores and infarct volume reduction were observed in Gap19-treated mice after brain ischemia induced by middle cerebral artery occlusion (MCAO). Gap19 treatment attenuated white matter damage. Moreover, Gap19 treatment suppressed the expression of Cx43 and Toll-like receptor 4 (TLR4) pathway-relevant proteins and prevented the overexpression of tumour necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). To further explore downstream signaling, we established an in vitro model-oxygen glucose deprivation (OGD) to simulate ischemic conditions. Immunofluorescence staining showed that Cx43 co-existed with TLR4 in astrocytes. The hemichannel activity was increased after OGD and Gap19 could inhibit this effect on astrocytes. Gap19 substantially improved relative cell vitality and decreased the expression of Cx43, TLR4 and inflammatory cytokines in vitro. In addition, in the lipopolysaccharide (LPS) stimulation OGD model, Gap19 also exhibited a protective effect via inhibiting TLR4 pathway activation. In summary, our results showed that Gap19 exerted a neuroprotective effect after stroke via inhibition of the TLR4-mediated signaling pathway.

In Vitro Wounding Models Using the Electric Cell-Substrate Impedance Sensing (ECIS)-Zθ Technology

Electric Cell-Substrate Impedance Sensing (ECIS) can produce reproducible wounding models by mechanically disrupting a cell monolayer. This study compared in vitro wound-healing using human cerebral microvascular endothelial cells (hCMVEC) with both single electrode (8W1E) and multiple electrodes (8W10E+) arrays. Measurements of hCMVEC migration and barrier functions were conducted, revealing variable levels of barrier disruption could be achieved by altering the duration and magnitude of the applied current. In all scenarios, the barrier (Rb) did not recover the strength observed prior to injury. Localization of junctional proteins following wounding were analyzed by immunocytochemistry. Following wounding, cell migration was generally faster on the 8W10E+ than the 8W1E array. Immunohistochemical analysis revealed non-viable cells remained on the 8W1E electrodes but not the 8W10E+ electrodes. However, viable cells partially remained on the 8W10E+ electrodes following wounding. In addition, the 8W10E+ electrodes demonstrated variation in cell loss across electrodes within the same well. This suggests the type of wounding is different on the two array types. However, our data show both arrays can be used to model incomplete barrier recovery and therefore both have potential for testing of drugs to improve endothelial barrier function. This is the first time that the possibility of using the 8W10E+ array as a wounding model is addressed. We highlight the differences in wounding produced between the two arrays, and can be used to study the underlying causes for impaired barrier function following CNS injuries.

Connexin 43 Hemichannel Activity Promoted by Pro-Inflammatory Cytokines and High Glucose Alters Endothelial Cell Function

The present work was done to elucidate whether hemichannels of a cell line derived from endothelial cells are affected by pro-inflammatory conditions (high glucose and IL-1β/TNF-α) known to lead to vascular dysfunction. We used EAhy 926 cells treated with high glucose and IL-1β/TNF-α. The hemichannel activity was evaluated with the dye uptake method and was abrogated with selective inhibitors or knocking down of hemichannel protein subunits with siRNA. Western blot analysis, cell surface biotinylation, and confocal microscopy were used to evaluate total and plasma membrane amounts of specific proteins and their cellular distribution, respectively. Changes in intracellular Ca²⁺ and nitric oxide (NO) signals were estimated by measuring FURA-2 and DAF-FM probes, respectively. High glucose concentration was found to elevate dye uptake, a response that was enhanced by IL-1β/TNF-α. High glucose plus IL-1β/TNF-α-induced dye uptake was abrogated by connexin 43 (Cx43) but not pannexin1 knockdown. Furthermore, Cx43 hemichannel activity was associated with enhanced ATP release and activation of p38 MAPK, inducible NO synthase, COX₂, PGE₂ receptor EP₁, and P2X₇/P2Y₁ receptors. Inhibition of the above pathways prevented completely the increase in Cx43 hemichannel activity of cells treated high glucose and IL-1β/TNF-α. Both synthetic and endogenous cannabinoids (CBs) also prevented the increment in Cx43 hemichannel opening, as well as the subsequent generation and release of ATP and NO induced by pro-inflammatory conditions. The counteracting action of CBs also was extended to other endothelial alterations evoked by IL-1β/TNF-α and high glucose, including increased ATP-dependent Ca²⁺ dynamics and insulin-induced NO production. Finally, inhibition of Cx43 hemichannels also prevented the ATP release from endothelial cells treated with IL-1β/TNF-α and high glucose. Therefore, we propose that reduction of hemichannel activity could represent a strategy against the activation of deleterious pathways that lead to endothelial dysfunction and possibly cell damage evoked by high glucose and pro-inflammatory conditions during cardiovascular diseases.

Sustained Connexin43 Mimetic Peptide Release From Loaded Nanoparticles Reduces Retinal and Choroidal Photodamage

Purpose

To evaluate the long-term effect on inflammation and inflammasome activation of intravitreally delivered connexin43 mimetic peptide (Cx43MP) in saline or incorporated within nanoparticles (NPs) for the treatment of the light-damaged rat eye.

Methods

Light-induced damage to the retina was created by exposure of adult albino Sprague-Dawley rats to intense light for 24 hours. A single dose of Cx43MP, Cx43MP-NPs, or saline was injected intravitreally at 2 hours after onset of light damage. Fluorescein isothiocyanate (FITC)-labelled Cx43MP-NPs were intravitreally injected to confirm delivery into the retina. Electroretinogram (ERG) recordings were performed at 24 hours, 1 week, and 2 weeks post cessation of light damage. The retinal and choroidal layers were analyzed in vivo using optical coherence tomography (OCT) and immunohistochemistry was performed on harvested tissues using glial fibrillary acidic protein (GFAP), leukocyte common antigen (CD45), and Cx43 antibodies.

Results

FITC was visualized 30 minutes after injection in the ganglion cell layer and in the choroid. Cx43MP and Cx43MP-NP treatments improved a-wave and b-wave function of the ERG compared with saline-injected eyes at 1 week and 2 weeks post treatment, and prevented photoreceptor loss by 2 weeks post treatment. Inflammation was also reduced and this was in parallel with downregulation of Cx43 expression.

Conclusions

The slow release of Cx43MP incorporated into NPs is more effective at treating retinal injury than a single dose of native Cx43MP in solution by reducing inflammation and maintaining both retinal structure and function. This NP preparation has clinical relevance as it reduces possible ocular complications associated with repeated intravitreal injections.

Connexins and Pannexins in Vascular Function and Disease

Connexins (Cxs) and pannexins (Panxs) are ubiquitous membrane channel forming proteins that are critically involved in many aspects of vascular physiology and pathology. The permeation of ions and small metabolites through Panx channels, Cx hemichannels and gap junction channels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. This review provides an overview of current knowledge with respect to the pathophysiological role of these channels in large arteries, the microcirculation, veins, the lymphatic system and platelet function. The essential nature of these membrane proteins in vascular homeostasis is further emphasized by the pathologies that are linked to mutations and polymorphisms in Cx and Panx genes.

Connexin-Mediated Functional and Metabolic Coupling Between Astrocytes and Neurons

The central nervous system (CNS) requires sophisticated regulation of neuronal activity. This modulation is partly accomplished by non-neuronal cells, characterized by the presence of transmembrane gap junctions (GJs) and hemichannels (HCs). This allows small molecule diffusion to guarantee neuronal synaptic activity and plasticity. Astrocytes are metabolically and functionally coupled to neurons by the uptake, binding and recycling of neurotransmitters. In addition, astrocytes release metabolites, such as glutamate, glutamine, D-serine, adenosine triphosphate (ATP) and lactate, regulating synaptic activity and plasticity by pre- and postsynaptic mechanisms. Uncoupling neuroglial communication leads to alterations in synaptic transmission that can be detrimental to neuronal circuit function and behavior. Therefore, understanding the pathways and mechanisms involved in this intercellular communication is fundamental for the search of new targets that can be used for several neurological disease treatments. This review will focus on molecular mechanisms mediating physiological and pathological coupling between astrocytes and neurons through GJs and HCs.

The Neuroglial Dialog Between Cannabinoids and Hemichannels

The formation of gap junctions was initially thought to be the central role of connexins, however, recent evidence had brought to light the high relevance of unopposed hemichannels as an independent mechanism for the selective release of biomolecules during physiological and pathological conditions. In the healthy brain, the physiological opening of astrocyte hemichannels modulates basal excitatory synaptic transmission. At the other end, the release of potentially neurotoxic compounds through astroglial hemichannels and pannexons has been insinuated as one of the functional alterations that negatively affect the progression of multiple brain diseases. Recent insights in this matter have suggested encannabinoids (eCBs) as molecules that could regulate the opening of these channels during diverse conditions. In this review, we discuss and hypothesize the possible interplay between the eCB system and the hemichannel/pannexon-mediated signaling in the inflamed brain and during event of synaptic plasticity. Most findings indicate that eCBs seem to counteract the activation of major neuroinflammatory pathways that lead to glia-mediated production of TNF-α and IL-1β, both well-known triggers of astroglial hemichannel opening. In contrast to the latter, in the normal brain, eCBs apparently elicit the Ca²⁺-activation of astrocyte hemichannels, which could have significant consequences on eCB-dependent synaptic plasticity.

Monitoring gap junctional communication in astrocytes from acute adult mouse brain slices using the gap-FRAP technique

Intercellular communication through gap junction channels plays a key role in cellular homeostasis and in synchronizing physiological functions, a feature that is modified in number of pathological situations. In the brain, astrocytes are the cell population that expresses the highest amount of gap junction proteins, named connexins. Several techniques have been used to assess the level of gap junctional communication in astrocytes, but so far they remain very difficult to apply in adult brain tissue. Here, using specific loading of astrocytes with sulforhodamine 101, we adapted the gap-FRAP (Fluorescence Recovery After Photobleaching) to acute hippocampal slices from 9 month-old adult mice. We show that gap junctional communication monitored in astrocytes with this technique was inhibited either by pharmacological treatment with a gap junctional blocker or in mice lacking the two main astroglial connexins, while a partial inhibition was measured when only one connexin was knocked-out. We validate this approach using a mathematical model of sulforhodamine 101 diffusion in an elementary astroglial network and a quantitative analysis of the exponential fits to the fluorescence recovery curves. Consequently, we consider that the adaptation of the gap-FRAP technique to acute brain slices from adult mice provides an easy going and valuable approach that allows overpassing this age-dependent obstacle and will facilitate the investigation of gap junctional communication in adult healthy or pathological brain.

Wound healing in the eye: Therapeutic prospects

In order to maintain a smooth optical surface the corneal epithelium has to continuously renew itself so as to maintain its function as a barrier to fluctuating external surroundings and various environmental insults. After trauma, the cornea typically re-epithelializes promptly thereby minimizing the risk of infection, opacification or perforation. A persistent epithelial defect (PED) is usually referred to as a non-healing epithelial lesion after approximately two weeks of treatment with standard therapies to no avail. They occur following exposure to toxic agents, mechanical injury, and ocular surface infections and are associated with significant clinical morbidity in patients, resulting in discomfort or visual loss. In the case of deeper corneal injury and corneal pathology the wound healing cascade can also extend to the corneal stroma, the layer below the epithelium. Although significant progress has been made in recent years, pharmaco-therapeutic agents that promote corneal healing remain limited. This article serves as a review of current standard therapies, recently introduced alternative therapies gaining in popularity, and a look into the newest developments into ocular wound healing.

Glia and hemichannels: key mediators of perinatal encephalopathy

Perinatal encephalopathy remains a major cause of disability, such as cerebral palsy. Therapeutic hypothermia is now well established to partially reduce risk of disability in late preterm/term infants. However, new and complementary therapeutic targets are needed to further improve outcomes. There is increasing evidence that glia play a key role in neural damage after hypoxia-ischemia and infection/inflammation. In this review, we discuss the role of astrocytic gap junction (connexin) hemichannels in the spread of neural injury after hypoxia-ischemia and/or infection/inflammation. Potential mechanisms of hemichannel mediated injury likely involve impaired intracellular calcium handling, loss of blood-brain barrier integrity and release of adenosine triphosphate (ATP) resulting in over-activation of purinergic receptors. We propose the hypothesis that inflammation-induced opening of connexin hemichannels is a key regulating event that initiates a vicious cycle of excessive ATP release, which in turn propagates activation of purinergic receptors on microglia and astrocytes. This suggests that developing new neuroprotective strategies for preterm infants will benefit from a detailed understanding of glial and connexin hemichannel responses.

Immunohistochemical Characterization of Connexin43 Expression in a Mouse Model of Diabetic Retinopathy and in Human Donor Retinas

Diabetic retinopathy (DR) develops due to hyperglycemia and inflammation-induced vascular disruptions in the retina with connexin43 expression patterns in the disease still debated. Here, the effects of hyperglycemia and inflammation on connexin43 expression in vitro in a mouse model of DR and in human donor tissues were evaluated. Primary human retinal microvascular endothelial cells (hRMECs) were exposed to high glucose (HG; 25 mM) or pro-inflammatory cytokines IL-1β and TNF-α (10 ng/mL each) or both before assessing connexin43 expression. Additionally, connexin43, glial fibrillary acidic protein (GFAP), and plasmalemma vesicular associated protein (PLVAP) were labeled in wild-type (C57BL/6), Akita (diabetic), and Akimba (DR) mouse retinas. Finally, connexin43 and GFAP expression in donor retinas with confirmed DR was compared to age-matched controls. Co-application of HG and cytokines increased connexin43 expression in hRMECs in line with results seen in mice, with no significant difference in connexin43 or GFAP expression in Akita but higher expression in Akimba compared to wild-type mice. On PLVAP-positive vessels, connexin43 was higher in Akimba but unchanged in Akita compared to wild-type mice. Connexin43 expression appeared higher in donor retinas with confirmed DR compared to age-matched controls, similar to the distribution seen in Akimba mice and correlating with the in vitro results. Although connexin43 expression seems reduced in diabetes, hyperglycemia and inflammation present in the pathology of DR seem to increase connexin43 expression, suggesting a causal role of connexin43 channels in the disease progression.

Connexin43- and Pannexin-Based Channels in Neuroinflammation and Cerebral Neuropathies

Connexins (Cx) are largely represented in the central nervous system (CNS) with 11 Cx isoforms forming intercellular channels. Moreover, in the CNS, Cx43 can form hemichannels (HCs) at non-junctional membrane as does the related channel-forming Pannexin1 (Panx1) and Panx2. Opening of Panx1 channels and Cx43 HCs appears to be involved in inflammation and has been documented in various CNS pathologies. Over recent years, evidence has accumulated supporting a link between inflammation and cerebral neuropathies (migraine, Alzheimer’s disease (AD), Parkinson’s disease (PD), major depressive disorder, autism spectrum disorder (ASD), epilepsy, schizophrenia, bipolar disorder). Involvement of Panx channels and Cx43 HCs has been also proposed in pathophysiology of neurological diseases and psychiatric disorders. Other studies showed that following inflammatory injury of the CNS, Panx1 activators are released and prolonged opening of Panx1 channels triggers neuronal death. In neuropsychiatric diseases, comorbidities are frequently present and can aggravate the symptoms and make therapeutic management more complex. The high comorbidity between some neuropathies can be partially understood by the fact that these diseases share a common etiology involving inflammatory pathways and Panx1 channels or Cx43 HCs. Thus, anti-inflammatory therapy opens perspectives of targets for new treatments and could have real potential in controlling a cerebral neuropathy and some of its comorbidities. The purpose of this mini review is to provide information of our knowledge on the link between Cx43- and Panx-based channels, inflammation and cerebral neuropathies.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.