Blocking connexin43 expression reduces inflammation and improves functional recovery after spinal cord injury

Gap Junctions or Hemichannel-Dependent and Independent Roles of Connexins in Fibrosis, Epithelial-Mesenchymal Transitions, and Wound Healing

Fibrosis initially appears as a normal response to damage, where activated fibroblasts produce large amounts of the extracellular matrix (ECM) during the wound healing process to assist in the repair of injured tissue. However, the excessive accumulation of the ECM, unresolved by remodeling mechanisms, leads to organ dysfunction. Connexins, a family of transmembrane channel proteins, are widely recognized for their major roles in fibrosis, the epithelial-mesenchymal transition (EMT), and wound healing. Efforts have been made in recent years to identify novel mediators and targets for this regulation. Connexins form gap junctions and hemichannels, mediating communications between neighboring cells and inside and outside of cells, respectively. Recent evidence suggests that connexins, beyond forming channels, possess channel-independent functions in fibrosis, the EMT, and wound healing. One crucial channel-independent function is their role as the primary functional component for cell adhesion. Other channel-independent functions of connexins involve their roles in mitochondria and exosomes. This review summarizes the latest advances in the channel-dependent and independent roles of connexins in fibrosis, the EMT, and wound healing, with a particular focus on eye diseases, emphasizing their potential as novel, promising therapeutic targets.

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.

The emerging science of Glioception: Contribution of glia in sensing, transduction, circuit integration of interoception

Interoception is the process by which the nervous system regulates internal functions to achieve homeostasis. The role of neurons in interoception has received considerable recent attention, but glial cells also contribute. Glial cells can sense and transduce signals including osmotic, chemical, and mechanical status of extracellular milieu. Their ability to dynamically communicate "listening" and "talking" to neurons is necessary to monitor and regulate homeostasis and information integration in the nervous system. This review introduces the concept of "Glioception" and focuses on the process by which glial cells sense, interpret and integrate information about the inner state of the organism. Glial cells are ideally positioned to act as sensors and integrators of diverse interoceptive signals and can trigger regulatory responses via modulation of the activity of neuronal networks, both in physiological and pathological conditions. We believe that understanding and manipulating glioceptive processes and underlying molecular mechanisms provide a key path to develop new therapies for the prevention and alleviation of devastating interoceptive dysfunctions, among which pain is emphasized here with more focused details.

Targeting connexin 43 expression via scaffold mediated delivery of antisense oligodeoxynucleotide preserves neurons, enhances axonal extension, reduces astrocyte and microglial activation after spinal cord injury

Injury to the central nervous system (CNS) provokes an inflammatory reaction and secondary damage that result in further tissue damage and destruction of neurons away from the injury site. Upon injury, expression of connexin 43 (Cx43), a gap junction protein, upregulates and is responsible for the spread and amplification of cell death signals through these gap junctions. In this study, we hypothesise that the downregulation of Cx43 by scaffold-mediated controlled delivery of antisense oligodeoxynucleotide (asODN), would minimise secondary injuries and cell death, and thereby support tissue regeneration after nerve injuries. Specifically, using spinal cord injury (SCI) as a proof-of-principle, we utilised a fibre-hydrogel scaffold for sustained delivery of Cx43asODN, while providing synergistic topographical cues to guide axonal ingrowth. Correspondingly, scaffolds loaded with Cx43asODN, in the presence of NT-3, suppressed Cx43 up-regulation after complete transection SCI in rats. These scaffolds facilitated the sustained release of Cx43asODN for up to 25 days. Importantly, asODN treatment preserved neurons around the injury site, promoted axonal extension, decreased glial scarring, and reduced microglial activation after SCI. Our results suggest that implantation of such scaffold-mediated asODN delivery platform could serve as an effective alternative SCI therapeutic approach.

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.

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.

Targeting gliovascular connexins prevents inflammatory blood-brain barrier leakage and astrogliosis

The blood-brain barrier is formed by capillary endothelial cells expressing Cx37, Cx40 and Cx43, and is joined by closely apposed astrocytes expressing Cx43 and Cx30. We investigated whether connexin-targeting peptides could limit barrier leakage triggered by LPS-induced systemic inflammation in mice. Intraperitoneal LPS increased endothelial and astrocytic Cx43 expression, elevated TNFα, IL1β, IFNγ and IL6 in plasma and IL6 in the brain, and induced barrier leakage recorded over 24h. Barrier leakage was largely prevented by global Cx43 knockdown and Cx43/Cx30 double-knockout in astrocytes, slightly diminished by endothelial Cx43 knockout and not protected by global Cx30 knockout. Intravenous administration of Gap27 or Tat-Gap19 just before LPS also prevented barrier leakage, and intravenous BAPTA-AM to chelate intracellular calcium was equally effective. Patch-clamp experiments demonstrated LPS-induced Cx43 hemichannel opening in endothelial cells, which was suppressed by Gap27, Gap19 and BAPTA. LPS additionally triggered astrogliosis that was prevented by intravenous Tat-Gap19 or BAPTA-AM. Cortically applied Tat-Gap19 or BAPTA-AM to primarily target astrocytes, also strongly diminished barrier leakage. In vivo dye uptake and in vitro patch-clamp showed Cx43 hemichannel opening in astrocytes that was induced by IL6 in a calcium-dependent manner. We conclude that targeting endothelial and astrocytic connexins is a powerful approach to limit barrier failure and astrogliosis.

Connexin and Pannexin Large-Pore Channels in Microcirculation and Neurovascular Coupling Function

Microcirculation homeostasis depends on several channels permeable to ions and/or small molecules that facilitate the regulation of the vasomotor tone, hyperpermeability, the blood–brain barrier, and the neurovascular coupling function. Connexin (Cxs) and Pannexin (Panxs) large-pore channel proteins are implicated in several aspects of vascular physiology. The permeation of ions (i.e., Ca²⁺) and key metabolites (ATP, prostaglandins, D-serine, etc.) through Cxs (i.e., gap junction channels or hemichannels) and Panxs proteins plays a vital role in intercellular communication and maintaining vascular homeostasis. Therefore, dysregulation or genetic pathologies associated with these channels promote deleterious tissue consequences. This review provides an overview of current knowledge concerning the physiological role of these large-pore molecule channels in microcirculation (arterioles, capillaries, venules) and in the neurovascular coupling function.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Connexins, Pannexins and Gap Junctions in Perinatal Brain Injury

Perinatal brain injury secondary to hypoxia-ischemia and/or infection/inflammation remains a major cause of disability. Therapeutic hypothermia significantly improves outcomes, but in randomized controlled trials nearly half of infants still died or survived with disability, showing that additional interventions are needed. There is growing evidence that brain injury spreads over time from injured to previously uninjured regions of the brain. At least in part, this spread is related to opening of connexin hemichannels and pannexin channels, both of which are large conductance membrane channels found in many brain cells. Opening of these membrane channels releases adenosine triphosphate (ATP), and other neuroactive molecules, into the extracellular space. ATP has an important role in normal signaling, but pathologically can trigger the assembly of the multi-protein inflammasome complex. The inflammasome complex promotes activation of inflammatory caspases, and release of inflammatory cytokines. Overall, the connexin hemichannel appears to play a primary role in propagation of injury and chronic disease, and connexin hemichannel blockade has been shown to be neuroprotective in multiple animal models. Thus, there is potential for some blockers of connexin or pannexin channels to be developed into targeted interventions that could be used in conjunction with or separate to therapeutic hypothermia.

Mu and Delta Opioid Receptor Targeting Reduces Connexin 43-Based Heterocellular Coupling during Neuropathic Pain

Chronic neuropathic pain emerges from either central or peripheral lesions inducing spontaneous or amplified responses to non-noxious stimuli. Despite different pharmacological approaches to treat such a chronic disease, neuropathic pain still represents an unmet clinical need, due to long-term therapeutic regimens and severe side effects that limit application of currently available drugs. A critical phenomenon involved in central sensitization is the exchange of signalling molecules and cytokines, between glia and neurons, driving the chronicization process. Herein, using a chronic constriction injury (CCI) model of neuropathic pain, we evaluated the efficacy of the mu (M-) and delta (D-) opioid receptor (-OR) targeting agent LP2 in modulating connexin-based heterocellular coupling and cytokine levels. We found that long-term efficacy of LP2 is consequent to MOR-DOR targeting resulting in the reduction of CCI-induced astrocyte-to-microglia heterocellular coupling mediated by connexin 43. We also found that single targeting of DOR reduces TNF and IL-6 levels in the chronic phase of the disease, but the peripheral and central discharge as the primary source of excitotoxic stimulation in the spinal cord requires a simultaneous MOR-DOR targeting to reduce CCI-induced neuropathic pain.

The mechanisms through which auricular vagus nerve stimulation protects against cerebral ischemia/reperfusion injury

Previous studies have shown that vagus nerve stimulation can improve patients' locomotor function. The stimulation of the auricular vagus nerve, which is the only superficial branch of the vagus nerve, may have similar effects to vagus nerve stimulation. However, the precise mechanisms remain poorly understood. In this study, rat models of cerebral ischemia/reperfusion injury were established by modified Longa ligation. Twenty-four hours later, 7-day auricular vagus nerve stimulation was performed. The results showed that auricular vagus nerve stimulation promoted the secretion of acetylcholine, inhibited the secretion of interleukin-1β, interleukin-6, and tumor necrosis factor-α, and reduced connexin 43 phosphorylation in the ischemic penumbra and motor cortex, promoting locomotor function recovery in rats with cerebral ischemia/reperfusion injury. These findings suggested that auricular vagus nerve stimulation promotes the recovery of locomotor function in rats with cerebral ischemia/reperfusion injury by altering the secretion of acetylcholine and inflammatory factors and the phosphorylation of connexin 43. This study was approved by the Animal Use and Management Committee of Shanghai University of Traditional Chinese Medicine on November 8, 2019 (approval No. PZSHUTCM191108014).

Mechanisms of ATP release in pain: role of pannexin and connexin channels

Pain is a physiological response to bodily damage and serves as a warning of potential threat. Pain can also transform from an acute response to noxious stimuli to a chronic condition with notable emotional and psychological components that requires treatment. Indeed, the management of chronic pain is currently an important unmet societal need. Several reports have implicated the release of the neurotransmitter adenosine triphosphate (ATP) and subsequent activation of purinergic receptors in distinct pain etiologies. Purinergic receptors are broadly expressed in peripheral neurons and the spinal cord; thus, purinergic signaling in sensory neurons or in spinal circuits may be critical for pain processing. Nevertheless, an outstanding question remains: what are the mechanisms of ATP release that initiate nociceptive signaling? Connexin and pannexin channels are established conduits of ATP release and have been suggested to play important roles in a variety of pathologies, including several models of pain. As such, these large-pore channels represent a new and exciting putative pharmacological target for pain treatment. Herein, we will review the current evidence for a role of connexin and pannexin channels in ATP release during nociceptive signaling, such as neuropathic and inflammatory pain. Collectively, these studies provide compelling evidence for an important role of connexins and pannexins in pain processing.

Mechanisms implicated in the contralateral effect in the central nervous system after unilateral injury: focus on the visual system

The retina, as part of the central nervous system is an ideal model to study the response of neurons to injury and disease and to test new treatments. During the last decade is becoming clear that unilateral lesions in bilateral areas of the central nervous system trigger an inflammatory response in the contralateral uninjured site. This effect has been better studied in the visual system where, as a rule, one retina is used as experimental and the other as control. Contralateral retinas in unilateral models of retinal injury show neuronal degeneration and glial activation. The mechanisms by which this adverse response in the central nervous system occurs are discussed in this review, focusing primarily on the visual system.

Cell transdifferentiation in ocular disease: Potential role for connexin channels

Cell transdifferentiation is the conversion of a cell type to another without requiring passage through a pluripotent cell state, and encompasses epithelial- and endothelial-mesenchymal transition (EMT and EndMT). EMT and EndMT are well defined processes characterized by a loss of epithelial/endothelial phenotype and gain in mesenchymal spindle shaped morphology, which results in increased cell migration and decreased apoptosis and cellular senescence. Such cells often develop invasive properties. Physiologically, these processes may occur during embryonic development and can resurface, for example, to promote wound healing in later life. However, they can also be a pathological process. In the eye, EMT, EndMT and cell transdifferentiation have all been implicated in development, homeostasis, and multiple diseases affecting different parts of the eye. Connexins, constituents of connexin hemichannels and intercellular gap junctions, have been implicated in many of these processes. In this review, we firstly provide an overview of the molecular mechanisms induced by transdifferentiation (including EMT and EndMT) and its involvement in eye diseases. We then review the literature for the role of connexins in transdifferentiation in the eye and eye diseases. The evidence presented in this review supports the need for more studies into the therapeutic potential for connexin modulators in prevention and treatment of transdifferentiation related eye diseases, but does indicate that connexin channel modulation may be an upstream and unifying approach for regulating these otherwise complex processes.

The role of gap junctions in cell death and neuromodulation in the retina

Vision altering diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration, myopia, retinal vascular disease, traumatic brain injuries and others cripple many lives and are projected to continue to cause anguish in the foreseeable future. Gap junctions serve as an emerging target for neuromodulation and possible regeneration as they directly connect healthy and/or diseased cells, thereby playing a crucial role in pathophysiology. Since they are permeable for macromolecules, able to cross the cellular barriers, they show duality in illness as a cause and as a therapeutic target. In this review, we take recent advancements in gap junction neuromodulation (pharmacological blockade, gene therapy, electrical and light stimulation) into account, to show the gap junction's role in neuronal cell death and the possible routes of rescuing neuronal and glial cells in the retina succeeding illness or injury.

The Role of Microglia in Modulating Neuroinflammation after Spinal Cord Injury

The pathobiology of traumatic and nontraumatic spinal cord injury (SCI), including degenerative myelopathy, is influenced by neuroinflammation. The neuroinflammatory response is initiated by a multitude of injury signals emanating from necrotic and apoptotic cells at the lesion site, recruiting local and infiltrating immune cells that modulate inflammatory cascades to aid in the protection of the lesion site and encourage regenerative processes. While peripheral immune cells are involved, microglia, the resident immune cells of the central nervous system (CNS), are known to play a central role in modulating this response. Microglia are armed with numerous cell surface receptors that interact with neurons, astrocytes, infiltrating monocytes, and endothelial cells to facilitate a dynamic, multi-faceted injury response. While their origin and essential nature are understood, their mechanisms of action and spatial and temporal profiles warrant extensive additional research. In this review, we describe the role of microglia and the cellular network in SCI, discuss tools for their investigation, outline their spatiotemporal profile, and propose translationally-relevant therapeutic targets to modulate neuroinflammation in the setting of SCI.

Metformin alleviates hydrogen peroxide-induced inflammation and oxidative stress via inhibiting P2X7R signaling in spinal cord tissue cells neurons

Metformin, the first medication that is often prescribed for the treatment of type 2 diabetes mellitus, was recently found to be neuroprotective. To study the mechanism underlying the neuroprotective effect of metformin, we pretreated primary spinal cord neurons with 50 µM or 100 µM metformin for 2 h prior to treatment with hydrogen peroxide (H2O2) for up to 48 h. Our results showed that H2O2 increased the expression of purinergic receptor P2X7 (P2X7R) in spinal cord neurons, which promoted the downstream pro-inflammatory cytokines release and oxidative stress. We found that metformin could reverse these pro-inflammatory and pro-oxidative effects of H2O2. Besides, P2X7R knockdown by siRNA suppressed H2O2-induced pro-inflammatory cytokine release and oxidative stress response. In conclusion, our results show that metformin can alleviate H2O2-induced inflammation and oxidative stress via modulating the P2X7R signaling pathway.

Blocking the inflammasome: A novel approach to treat uveitis

Uveitis is a complex ocular inflammatory disease often accompanied by bacterial or viral infections (infectious uveitis) or underlying autoimmune diseases (non-infectious uveitis). Treatment of the underlying infection along with corticosteroid-mediated suppression of acute inflammation usually resolves infectious uveitis. However, to develop more effective therapies for non-infectious uveitis and to better address acute inflammation in infectious disease, an improved understanding of the underlying inflammatory pathways is needed. In this review, we discuss the disease aetiology, preclinical in vitro and in vivo uveitis models, the role of inflammatory pathways, as well as current and future therapies. In particular, we highlight the involvement of the inflammasome in the development of non-infectious uveitis and how it could be a future target for effective treatment of the disease.

Propofol Mediated Protection of the Brain From Ischemia/Reperfusion Injury Through the Regulation of Microglial Connexin 43

Cerebral ischemia/reperfusion (I/R) injury is a serious condition that leads to increased apoptosis of microglial and neurons in the brain. In this study, we identified that Cx43 expression level is significantly increased in the microglial cells during I/R injury. Using an in vitro model (hypoxia/reoxygenation-H/R injury), we observed that H/R injury leads to an increase in activation of microglial cells and increase in levels of pro-inflammatory markers such as IL-1β, IL-6, and TNF-α. Additionally, we could also observe significant increase in phosphorylation of Cx43 and Cav3.2 levels. To assess the role of H/R injured microglial cells on neuronal population, we cultured the neurons with conditioned media (MCS) from H/R injured microglial cells. Interestingly, we observed that microglial H/R injury significantly decreased Map2 expression and affected neuronal morphology. Further, we aimed to assess the effects of propofol on cerebral H/R injury, and observed that 40 μM propofol significantly decreased Cx43, Cx43 phosphorylation, and CaV3.2 levels. Additionally, propofol decreased apoptosis and increased Map2 expression levels in H/R injured neurons. Using silencing experiments, we confirmed that siCx43 could significantly improve the propofol's rescue after H/R injury in both microglia and neurons. We further developed an in vivo MCAO (middle cerebral artery occlusion) rat model to understand the effect of propofol in I/R injury. Interestingly, propofol treatment and downregulation of Cx43 significantly decreased the infract volume and apoptosis in these MCAO rats. Thus, this study clearly establishes that propofol protects the brain against I/R injury through the downregulation of Cx43 in microglial cells.

Astrocyte Networks as Therapeutic Targets in Glaucomatous Neurodegeneration

Astrocytes are intimately involved in the response to neurodegenerative stress and have become an attractive target for the development of neuroprotective therapies. However, studies often focus on astrocytes as single-cell units. Astrocytes are densely interconnected by gap junctions that are composed primarily of the protein connexin-43 (Cx43) and can function as a broader network of cells. Such networks contribute to a number of important processes, including metabolite distribution and extracellular ionic buffering, and are likely to play an important role in the progression of neurodegenerative disease. This review will focus on the pro-degenerative and pro-survival influence of astrocyte Cx43 in disease progression, with a focus on the roles of gap junctions and hemichannels in the spread of degenerative stress. Finally, we will highlight the specific evidence for targeting these networks in the treatment of glaucomatous neurodegeneration and other optic neuropathies.

Role of pyroptosis in diabetic retinopathy and its therapeutic implications

Pyroptosis has recently been established as a term of programmed-inflammatory cell death. Pyroptosis is mainly divided into two molecular signaling pathways, including caspase-1-dependent canonical and caspase-4/5/11-dependent non-canonical inflammasome pathways. Extensive investigations have reported inflammasome activation facilitates the maturation and secretion of the inflammatory factors interleukin-1β/18 (IL-1β/18), cleavage of gasdermin D (GSDMD), and leading to the stimulation of pyroptosis-mediated cell death. Furthermore, accumulating studies report NLRP3 inflammasome activation plays a significant role in triggering the pyroptosis-mediated cell death and promotes the pathogenesis of diabetic retinopathy (DR). Our current review elaborates on the molecular mechanisms of pyroptosis-signaling pathways and their potential roles in the pathogenesis and impact of DR development. We also emphasize several investigational molecules regulating key steps in pyroptotic-cell death to create new comprehensions and findings to explore the pathogenesis of DR advancement. Our narrative review concisely suggests these potential pharmacological agents could be promising therapies to treat and manage DR in the future.

Characterisation of an ischemia reperfusion model for the formation of a stage I pressure ulcer in mouse skin

Pairs of magnets were applied to the loose skin on the backs of mice in order to cause ischemia for periods of 1.5, 2, 2.5 and 3 h followed by reperfusion. We found 1.5 h of ischemia resulted in the most reliable outcome of blanched skin but no redness or skin breakdown. Histological analysis at 4 h of reperfusion showed, in the centre of the insult, condensed nuclei in the epidermis and sebaceous glands with a build up of neutrophils in the blood vessels, and a reduction in the number of fibroblasts. At 24 h, spongiosis was seen in the epidermis and pockets of neutrophils began to accumulate under it, as well as being scatted through the dermis. In the centre of the insult there was a loss of sebaceous gland nuclei and fibroblasts. Four days after the insult, spongiosis was reduced in the epidermis at the edge of the insult but enhanced in the centre and in hair follicles. Leukocytes were seen throughout the central dermis. At 8 days, spongiosis and epidermal thickness had reduced and fibroblasts were reappearing. However, blood vessels still had leukocytes lining the lumen. The gap junction protein connexin 43 was significantly elevated in the epidermis at 4 h and 24 h reperfusion. Ischemia of 1.5 h generates a sterile inflammatory reaction causing the loss of some cell types but leaving the epidermis intact reminiscent of a stage I pressure ulcer.

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.

Changes in connexin 43 in inflammatory skin disorders: Eczema, psoriasis, and Steven-Johnson syndrome/toxic epidermal necrolysis

BACKGROUND

Connexin 43 (Cx43) plays a central role in the inflammatory response and wound healing. Targeting Cx43 expression reduces inflammation in a variety of injuries. The expression pattern of Cx43 has not been described for many inflammatory skin diseases.

OBJECTIVES

To describe the expression patterns of Cx43 in eczema, psoriasis, Steven-Johnson syndrome/toxic epidermal necrolysis.

METHODS

Archival skin biopsies from patients with eczema, psoriasis, and Steven-Johnson syndrome/toxic epidermal necrosis were identified and examined, with sister sections stained for Cx43 and imaged by confocal microscopy. All samples were compared to age and site-matched normal skin controls.

RESULTS

Epidermal Cx43 is reduced in acute eczema, absent in regions of spongiosis, and is highly elevated in subacute and chronic eczema. In plaque psoriasis, Cx43 is overexpressed in areas with psoriasiform hyperplasia with a fish-scale-like appearance but is lost in regions surrounding neutrophil microabscesses. Cx43 staining is strong in the neutrophils within these microabscesses. In SJS/TEN, Cx43 expression is elevated in areas bordering normal tissue but is rapidly lost in areas of keratinocyte necrosis.

CONCLUSIONS

Dynamic changes in Cx43 levels are seen in inflammatory skin diseases and may represent future potential therapeutic targets.

The role of connexin proteins and their channels in radiation-induced atherosclerosis

Radiotherapy is an effective treatment for breast cancer and other thoracic tumors. However, while high-energy radiotherapy treatment successfully kills cancer cells, radiation exposure of the heart and large arteries cannot always be avoided, resulting in secondary cardiovascular disease in cancer survivors. Radiation-induced changes in the cardiac vasculature may thereby lead to coronary artery atherosclerosis, which is a major cardiovascular complication nowadays in thoracic radiotherapy-treated patients. The underlying biological and molecular mechanisms of radiation-induced atherosclerosis are complex and still not fully understood, resulting in potentially improper radiation protection. Ionizing radiation (IR) exposure may damage the vascular endothelium by inducing DNA damage, oxidative stress, premature cellular senescence, cell death and inflammation, which act to promote the atherosclerotic process. Intercellular communication mediated by connexin (Cx)-based gap junctions and hemichannels may modulate IR-induced responses and thereby the atherosclerotic process. However, the role of endothelial Cxs and their channels in atherosclerotic development after IR exposure is still poorly defined. A better understanding of the underlying biological pathways involved in secondary cardiovascular toxicity after radiotherapy would facilitate the development of effective strategies that prevent or mitigate these adverse effects. Here, we review the possible roles of intercellular Cx driven signaling and communication in radiation-induced atherosclerosis.

The Multifaceted Role of Astrocyte Connexin 43 in Ischemic Stroke Through Forming Hemichannels and Gap Junctions

Ischemic stroke is a multi-factorial cerebrovascular disease with high worldwide morbidity and mortality. In the past few years, multiple studies have revealed the underlying mechanism of ischemia/reperfusion injury, including calcium overload, amino acid toxicity, oxidative stress, and inflammation. Connexin 43 (Cx43), the predominant connexin protein in astrocytes, has been recently proven to display non-substitutable roles in the pathology of ischemic stroke development and progression through forming gap junctions and hemichannels. Under normal conditions, astrocytic Cx43 could be found in hemichannels or in the coupling with other hemichannels on astrocytes, neurons, or oligodendrocytes to form the neuro-glial syncytium, which is involved in metabolites exchange between communicated cells, thus maintaining the homeostasis of the CNS environment. In ischemic stroke, the phosphorylation of Cx43 might cause the degradation of gap junctions and the opening of hemichannels, contributing to the release of inflammatory mediators. However, the remaining gap junctions could facilitate the exchange of protective and harmful metabolites between healthy and injured cells, protecting the injured cells to some extent or damaging the healthy cells depending on the balance of the exchange of protective and harmful metabolites. In this study, we review the changes in astrocytic Cx43 expression and distribution as well as the influence of these changes on the function of astrocytes and other cells in the CNS, providing new insight into the pathology of ischemic stroke injury; we also discuss the potential of astrocytic Cx43 as a target for the treatment of ischemic stroke.

The role of oligodendrocyte gap junctions in neuroinflammation

Gap junctions (GJs) provide channels for direct cell-to-cell connectivity serving the homeostasis in several organs of vertebrates including the central (CNS) and peripheral (PNS) nervous systems. GJs are composed of connexins (Cx), which show a highly distinct cellular and subcellular expression pattern. Oligodendrocytes, the myelinating cells of the CNS, are characterized by extensive GJ connectivity with each other as well as with astrocytes. The main oligodendrocyte connexins forming these GJ channels are Cx47 and Cx32. The importance of these channels has been highlighted by the discovery of human diseases caused by mutations in oligodendrocyte connexins, manifesting with leukodystrophy or transient encephalopathy. Experimental models have provided further evidence that oligodendrocyte GJs are essential for CNS myelination and homeostasis, while a strong inflammatory component has been recognized in the absence of oligodendrocyte connexins. Further studies revealed that connexins are also disrupted in multiple sclerosis (MS) brain, and in experimental models of induced inflammatory demyelination. Moreover, induced demyelination was more severe and associated with higher degree of CNS inflammation in models with oligodendrocyte GJ deficiency, suggesting that disrupted connexin expression in oligodendrocytes is not only a consequence but can also drive a pro-inflammatory environment in acquired demyelinating disorders such as MS. In this review, we summarize the current insights from human disorders as well as from genetic and acquired models of demyelination related to oligodendrocyte connexins, with the remaining challenges and perspectives.

Intercellular communication and ion channels in neuropathic pain chronicization

BACKGROUND

Neuropathic pain is caused by primary lesion or dysfunction of either peripheral or central nervous system. Due to its complex pathogenesis, often related to a number of comorbidities, such as cancer, neurodegenerative and neurovascular diseases, neuropathic pain still represents an unmet clinical need, lacking long-term effective treatment and complex case-by-case approach.

AIM AND METHODS

We analyzed the recent literature on the role of selective voltage-sensitive sodium, calcium and potassium permeable channels and non-selective gap junctions (GJs) and hemichannels (HCs) in establishing and maintaining chronic neuropathic conditions. We finally focussed our review on the role of extracellular microenvironment modifications induced by resident glial cells and on the recent advances in cell-to-cell and cell-to-extracellular environment communication in chronic neuropathies.

CONCLUSION

In this review, we provide an update on the current knowledge of neuropathy chronicization processes with a focus on both neuronal and glial ion channels, as well as on channel-mediated intercellular communication.

Connexin 43 upregulation in burns promotes burn conversion through spread of apoptotic death signals

BACKGROUND

Burn wounds continue to worsen after initial injury in a process known as burn conversion, which lasts about 3-5 days. It causes burn wounds to enlarge and deepen, leading to greater morbidity. Apoptosis is one of the factors contributing to the conversion of the zone of stasis into the zone of coagulation. Suppression of apoptosis has been associated with reducing burn conversion. Connexin 43 (Cx43) gap junctions facilitate the spread of apoptotic signals from dying cells to healthy neighbouring cells in injured tissues through the bystander effect.

OBJECTIVES

The study is to understand the role of Cx43 in burn conversion.

METHODS

In our study, 15 burn tissue samples were arranged into three groups as early (beginning of burn conversion), intermediate (extensive burn conversion) and late (established burn conversion) burns.

RESULTS

We found a striking increase in the amount of Cx43 protein expressed in the dermal fibroblasts (identified with heat shock protein 47 (HSP47) staining) in the zone of stasis in early and intermediate burns. These dermal fibroblasts also express high levels of cleaved-Caspase 3 indicating on-going apoptosis.

CONCLUSIONS

Our findings suggest that elevation of Cx43 may play an active role in burn conversion spreading apoptosis in the early and intermediate burn wound.

Neonatal nutritional programming induces gliosis and alters the expression of T-cell protein tyrosine phosphatase and connexins in male rats

Changes to neonatal nutrition result in long-lasting impairments in energy balance, which may be described as metabolic programing. Astrocytes, which are interconnected by gap junctions, have emerged as important players in the hypothalamic control of food intake. In order to study the effects of nutritional programming on glial morphology and protein expression, cross-fostered male Wistar rats at postnatal day 3 were assigned to three groups based on litter size: small litter (3 pups per dam, SL), normal litter (10 pups per dam, NL), and large litter (16 pups per dam, LL). Rats from the SL group exhibited higher body weight throughout the study and hyperphagia after weaning. LL animals exhibited hyperphagia, high energy efficiency and catch-up of body weight after weaning. Both the SL and LL groups at postnatal day 60 (PN60) exhibited increased levels of plasma leptin, the Lee index (as an index of obesity), adiposity content, immunoreactivity toward T-cell protein tyrosine phosphatase (TCPTP), and glial fibrillary acidic protein (GFAP) in the arcuate nucleus (ARC) of the hypothalamus. Astrocyte morphology was altered in the ARC of SL and LL animals, and this effect occurred in parallel with a reduction in immunoreactivity toward connexin 30 (CX30). The data obtained demonstrate that both neonatal over- and underfeeding promote not only alterations in the metabolic status but also morphological changes in glial cells in parallel with increasing TCPTP and changes in connexin expression.

Connexin43 Hemichannel Targeting With TAT-Gap19 Alleviates Radiation-Induced Endothelial Cell Damage

BACKGROUND

Emerging evidence indicates an excess risk of late occurring cardiovascular diseases, especially atherosclerosis, after thoracic cancer radiotherapy. Ionizing radiation (IR) induces cellular effects which may induce endothelial cell dysfunction, an early marker for atherosclerosis. In addition, intercellular communication through channels composed of transmembrane connexin proteins (Cxs), i.e. Gap junctions (direct cell-cell coupling) and hemichannels (paracrine release/uptake pathway) can modulate radiation-induced responses and therefore the atherosclerotic process. However, the role of endothelial hemichannel in IR-induced atherosclerosis has never been described before.

MATERIALS AND METHODS

Telomerase-immortalized human Coronary Artery/Microvascular Endothelial cells (TICAE/TIME) were exposed to X-rays (0.1 and 5 Gy). Production of reactive oxygen species (ROS), DNA damage, cell death, inflammatory responses, and senescence were assessed with or without applying a Cx43 hemichannel blocker (TAT-Gap19).

RESULTS

We report here that IR induces an increase in oxidative stress, cell death, inflammatory responses (IL-8, IL-1β, VCAM-1, MCP-1, and Endothelin-1) and premature cellular senescence in TICAE and TIME cells. These effects are significantly reduced in the presence of the Cx43 hemichannel-targeting peptide TAT-Gap19.

CONCLUSION

Our findings suggest that endothelial Cx43 hemichannels contribute to various IR-induced processes, such as ROS, cell death, inflammation, and senescence, resulting in an increase in endothelial cell damage, which could be protected by blocking these hemichannels. Thus, targeting Cx43 hemichannels may potentially exert radioprotective effects.

Connexin Signaling Is Involved in the Reactivation of a Latent Stem Cell Niche after Spinal Cord Injury

The ependyma of the adult spinal cord is a latent stem cell niche that is reactivated by spinal cord injury contributing new cells to the glial scar. The cellular events taking place in the early stages of the reaction of the ependyma to injury remain little understood. Ependymal cells are functionally heterogeneous with a mitotically active subpopulation lining the lateral domains of the central canal (CC) that are coupled via gap junctions. Gap junctions and connexin hemichannels are key regulators of the biology of neural progenitors during development and in adult neurogenic niches. Thus, we hypothesized that communication via connexins in the CC is developmentally regulated and may play a part in the reactivation of this latent stem cell niche after injury. To test these possibilities, we combined patch-clamp recordings of ependymal cells with immunohistochemistry for various connexins in the neonatal and the adult (P > 90) normal and injured spinal cord of male and female mice. We find that coupling among ependymal cells is downregulated as postnatal development proceeds but increases after injury, resembling the immature CC. The increase in gap junction coupling in the adult CC was paralleled by upregulation of connexin 26, which correlated with the resumption of proliferation and a reduction of connexin hemichannel activity. Connexin blockade reduced the injury-induced proliferation of ependymal cells. Our findings suggest that connexins are involved in the early reaction of ependymal cells to injury, representing a potential target to improve the contribution of the CC stem cell niche to repair.SIGNIFICANCE STATEMENT Ependymal cells in the adult spinal cord are latent progenitors that react to injury to support some degree of endogenous repair. Understanding the mechanisms by which these progenitor-like cells are regulated in the aftermath of spinal cord injury is critical to design future manipulations aimed at improving healing and functional recovery. Gap junctions and connexin hemichannels are key regulators of the biology of neural progenitors during development and in adult neurogenic niches. We find here that connexin signaling in the ependyma changes after injury of the adult spinal cord, functionally resembling the immature active-stem cell niche of neonatal animals. Our findings suggest that connexins in ependymal cells are potential targets to improve self-repair of the spinal cord.

Oligodendroglial connexin 47 regulates neuroinflammation upon autoimmune demyelination in a novel mouse model of multiple sclerosis

In multiple sclerosis plaques, oligodendroglial connexin (Cx) 47 constituting main gap junction channels with astroglial Cx43 is persistently lost. As mice with Cx47 single knockout exhibit no demyelination, the roles of Cx47 remain undefined. We aimed to clarify the effects of oligodendroglia-specific Cx47 inducible conditional knockout (icKO) on experimental autoimmune encephalomyelitis (EAE) induced by myelin oligodendrocyte glycoprotein peptide (MOG35-55) in PLP/CreERT;Cx47fl/fl mice at 14 d after tamoxifen injection. Cx47 icKO mice demonstrated exacerbation of acute and chronic relapsing EAE with more pronounced demyelination than Cx47 flox (fl)/fl littermates. CD3+ T cells more abundantly infiltrated the spinal cord in Cx47 icKO than in Cx47 fl/fl mice throughout the acute to chronic phases. CXCR3-CCR6+CD4+ and IL17+IFNγ-CD4+ helper T (Th) 17 cells isolated from spinal cord and brain tissues were significantly increased in Cx47 icKO mice compared with Cx47 fl/fl mice, while MOG35-55-specific proliferation and proinflammatory cytokine production of splenocytes were unaltered. Microarray analysis of isolated microglia revealed stronger microglial activation toward proinflammatory and injury-response phenotypes with increased expressions of chemokines that can attract Th17 cells, including Ccl2, Ccl3, Ccl4, Ccl7, and Ccl8, in Cx47 icKO mice compared with Cx47 fl/fl mice. In Cx47 icKO mice, NOS2+ and MHC class II+ microglia were more enriched immunohistochemically, and A1-specific astroglial gene expressions and astroglia immunostained for C3, a representative A1 astrocyte marker, were significantly increased at the acute phase, compared with Cx47 fl/fl mice. These findings suggest that oligodendroglia-specific Cx47 ablation induces severe inflammation upon autoimmune demyelination, underscoring a critical role for Cx47 in regulating neuroinflammation.

RNA interference in glial cells for nerve injury treatment

Drivers of RNA interference are potent for manipulating gene and protein levels, which enable the restoration of dysregulated mRNA expression that is commonly associated with injuries and diseases. This review summarizes the potential of targeting neuroglial cells, using RNA interference, to treat nerve injuries sustained in the central nervous system. In addition, the various methods of delivering these RNA interference effectors will be discussed.

Astrocytes in chronic pain and itch

Astrocytes are critical for maintaining the homeostasis of the CNS. Increasing evidence suggests that a number of neurological and neuropsychiatric disorders, including chronic pain, may result from astrocyte 'gliopathy'. Indeed, in recent years there has been substantial progress in our understanding of how astrocytes can regulate nociceptive synaptic transmission via neuronal-glial and glial-glial cell interactions, as well as the involvement of spinal and supraspinal astrocytes in the modulation of pain signalling and the maintenance of neuropathic pain. A role of astrocytes in the pathogenesis of chronic itch is also emerging. These developments suggest that targeting the specific pathways that are responsible for astrogliopathy may represent a novel approach to develop therapies for chronic pain and chronic itch.

Simultaneous Activation of Mu and Delta Opioid Receptors Reduces Allodynia and Astrocytic Connexin 43 in an Animal Model of Neuropathic Pain

Neuropathic pain is a chronic condition triggered by lesions to the somatosensory nervous system in which pain stimuli occur spontaneously or as pathologically amplified responses. In this scenario, the exchange of signaling molecules throughout cell-to-cell and cell-to-extracellular environment communications plays a key role in the transition from acute to chronic pain. As such, connexin 43 (Cx43), the core glial gap junction and hemichannel-forming protein, is considered a triggering factor for disease chronicization in the central nervous system (CNS). Drugs targeting μ opioid receptors (MOR) are currently used for moderate to severe pain conditions, but their use in chronic pain is limited by the tolerability profile. δ opioid receptors (DOR) have become attractive targets for the treatment of persistent pain and have been associated with the inhibition of pain-sustaining factors. Moreover, it has been shown that simultaneous targeting of MOR and DOR leads to an improved pharmacological fingerprint. Herein, we aimed to study the effects of the benzomorphan ligand LP2, a multitarget MOR/DOR agonist, in an experimental model of neuropathic pain induced by the unilateral sciatic nerve chronic constriction injury (CCI) on male Sprague-Dawley rats. Results showed that LP2 significantly ameliorated mechanical allodynia from the early phase of treatment up to 21 days post-ligatures. We additionally showed that LP2 prevented CCI-induced Cx43 alterations and pro-apoptotic signaling in the CNS. These findings increase the knowledge of neuropathic pain development and the role of spinal astrocytic Cx43, suggesting new approaches for the treatment of neuropathic pain.

Inhibition of MSK1 Promotes Inflammation and Apoptosis and Inhibits Functional Recovery After Spinal Cord Injury

Mitogen- and stress-activated kinase (MSK) 1 is a nuclear serine/threonine kinase. In the central nervous system, it plays an important role in regulating cell proliferation and neuronal survival; it is also involved in astrocyte inflammation and the inhibition of inflammatory cytokine production. However, its specific role in spinal cord injury is not clear. Here, we aimed to elucidate this role using an in vivo animal model. In this study, we found that MSK1 is gradually decreased, starting 1 day after spinal cord injury and to its lowest level 3 days post-injury, after which it gradually increased. To further investigate the possible function of MSK1 in spinal cord injury, we interfered with its expression by utilizing a small interfering RNA (siRNA)-encoding lentivirus, which was injected into the injured spinal cord to inhibit local expression. After MSK1 inhibition, we found that the expression of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-1β were increased. Moreover, the expression of IL-10 was decreased. In addition, neuronal apoptotic cells were increased significantly and expression of the apoptosis-related protein caspase-3 was also increased. Ultrastructural analysis of nerve cells also revealed typical neuronal apoptosis and severe neuronal damage. Finally, we found that hindlimb motor function decreased significantly with MSK1 knockdown. Therefore, our findings suggest that the inhibition of this protein promotes inflammatory responses and apoptosis and suppresses functional recovery after spinal cord injury. MSK1 might thus play an important role in repair after spinal cord injury by regulating inflammation and apoptosis.

A novel serum free primary astrocyte culture method that mimic quiescent astrocyte phenotype

BACKGROUND

Primary astrocyte cultures have been used for decades to study astrocyte functions in health and disease. The current primary astrocyte cultures are mostly maintained in serum-containing medium which produces astrocytes with a reactive phenotype as compared to in vivo quiescent astrocytes. The aim of this study was to establish a serum-free astrocyte culture medium that maintains primary astrocytes in a quiescent state.

NEW METHOD

Serum free astrocyte base medium (ABM) supplemented with basic fibroblast growth factor 2 (FGF2) and epidermal growth factor (EGF) (ABM-FGF2-EGF) or serum supplemented DMEM (MD-10%FBS) was used to culture primary astrocytes isolated from cerebral cortex of postnatal day 1 C57BL/6 mice.

RESULTS

Compared to astrocytes cultured in MD-10%FBS medium, astrocytes in ABM-FGF2-EGF had higher process bearing morphologies similar to in vivo astrocytes. Western blot, immunostaining, quantitative polymerase chain reaction and metabolic assays revealed that astrocytes maintained in ABM-FGF2-EGF had enhanced glycolytic metabolism, higher glycogen content, lower GFAP expression, increased glutamine synthase, and glutamate transporter-1 mRNA levels as compared to astrocytes cultured in MD-10% FBS medium.

COMPARISON TO EXISTING METHODS

These observations suggest that astrocytes cultured in ABM-FGF2-EGF media compared to the usual FBS media promote quiescent and biosynthetic phenotype similar to in vivo astrocytes.

CONCLUSION

This media provides a novel method for studying astrocytes functions in vitro under physiological and pathological conditions.

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

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

Resveratrol alleviates lipopolysaccharide-induced inflammation in PC-12 cells and in rat model

Background

Spinal cord injury (SCI) remains a huge medical problem nowadays as there is no hospital providing the versatile strategies for repairing central nervous system and restoring function. Herein, we focused on PC-12 cells as an important research tool and studied the potential role of resveratrol (RSV) in inflammation induced by LPS.

Results

RSV improved inflammatory injury and functional recovery in rat model of SCI. RSV inhibited LPS-induced inflammatory injury in PC-12 cells via increasing viability, decreasing apoptosis, and suppressing IL-1β, IL-6, and TNF-α expression. miR-132 was down-regulated after LPS treatment but up-regulated after RSV administration. miR-132 silence curbed the protective effect of RSV. The results including increase of cell growth, suppression of inflammatory response, and blocking of NF-κB and p38MAPK pathways produced by RSV were all reversed by miR-132 silence.

Conclusion

RSV could up-regulate miR-132 and further ameliorate inflammatory response in PC-12 cells by inhibiting NF-κB and p38MAPK pathways.

Connexin 30 Deficiency Attenuates Chronic but Not Acute Phases of Experimental Autoimmune Encephalomyelitis Through Induction of Neuroprotective Microglia

Glial connexins (Cxs) form gap junction channels through which a pan-glial network plays key roles in maintaining homeostasis of the central nervous system (CNS). In multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), expression of astrocytic Cx43 is lost in acute lesions but upregulated in chronic plaques, while astrocytic Cx30 is very low in normal white matter and changes in its expression have not been convincingly shown. In Cx30 or Cx43 single knockout (KO) mice and even in Cx30/Cx43 double KO mice, acute EAE is unaltered. However, the effects of Cx30/Cx43 deficiency on chronic EAE remains to be elucidated. We aimed to clarify the roles of Cx30 in chronic neuroinflammation by studying EAE induced by myelin oligodendrocyte glycoprotein peptide 35–55 in Cx30 KO mice. We found that Cx30 deficiency improved the clinical symptoms and demyelination of chronic but not acute EAE without influencing CD3⁺ T cell infiltration. Furthermore, increased ramified microglia in the naïve state and induced earlier and stronger microglial activation in the acute and chronic phases of EAE was observed. These activated microglia had an anti-inflammatory phenotype, as shown by the upregulation of arginase-1 and brain-derived neurotrophic factor and the downregulation of nitric oxide synthase 2. In the naïve state, Cx30 deficiency induced modest enlargement of astrocytic processes in the spinal cord gray matter and a partial reduction of Cx43 expression in the spinal cord white matter. These astrocytes in Cx30 KO mice showed earlier and stronger activation during the acute phase of EAE, with upregulated A2 astrocyte markers and a significant decrease in Cx43 in the chronic phases. Spinal cord neurons and axons were more preserved in Cx30 KO mice than in littermates in the chronic phase of EAE. These findings suggest that Cx30 deficiency increased ramified microglia in the CNS in the naïve state and improved chronic EAE through redirecting microglia toward an anti-inflammatory phenotype, suggesting a hitherto unknown critical role of astrocytic Cx30 in regulating microglial number and functional state.

Therapeutic strategies targeting connexins

The connexin family of channel-forming proteins is present in every tissue type in the human anatomy. Connexins are best known for forming clustered intercellular channels, structurally known as gap junctions, where they serve to exchange members of the metabolome between adjacent cells. In their single-membrane hemichannel form, connexins can act as conduits for the passage of small molecules in autocrine and paracrine signalling. Here, we review the roles of connexins in health and disease, focusing on the potential of connexins as therapeutic targets in acquired and inherited diseases as well as wound repair, while highlighting the associated clinical challenges.

Intracellular oligonucleotide delivery using the cell penetrating peptide Xentry

The current study investigated the use of two cationic peptides, Xentry-KALA (XK) and Xentry-Protamine (XP), for intracellular delivery of Connexin43 antisense oligonucleotides (Cx43AsODN). The charge and size of Cx43AsODN:XK and Cx43AsODN:XP complexes was determined by Zetasizer analysis. The earliest positive zeta potential reading was obtained at a 1:2 and 1:1.2 charge ratio of Cx43AsODN:XK and Cx43AsODN:XP respectively, with Cx43AsODN:XK resulting in overall larger complexes than Cx43AsODN:XP. Gel shift mobility assays revealed complete complex formation at a 1:2.5 and 1:2.2 charge ratio of Cx43AsODN:XK and Cx43AsODN:XP, respectively. Cellular uptake studies were carried out in ARPE-19 cells. While both complexes were able to enter the cells, Cx43AsODN:XK uptake appeared punctate and circular indicative of endosomal containment. Cx43AsODN:XP uptake, in contrast, resulted in diffuse appearance inside the cell suggesting endosomal escape of the cargo. Finally, western blot analysis confirmed that Cx43AsODN:XP was able to knockdown Cx43 expression in these cells under normal and hypoxic conditions.

Function of Connexins in the Interaction between Glial and Vascular Cells in the Central Nervous System and Related Neurological Diseases

Neuronal signaling together with synapse activity in the central nervous system requires a precisely regulated microenvironment. Recently, the blood-brain barrier is considered as a “neuro-glia-vascular unit,” a structural and functional compound composed of capillary endothelial cells, glial cells, pericytes, and neurons, which plays a pivotal role in maintaining the balance of the microenvironment in and out of the brain. Tight junctions and adherens junctions, which function as barriers of the blood-brain barrier, are two well-known kinds in the endothelial cell junctions. In this review, we focus on the less-concerned contribution of gap junction proteins, connexins in blood-brain barrier integrity under physio-/pathology conditions. In the neuro-glia-vascular unit, connexins are expressed in the capillary endothelial cells and prominent in astrocyte endfeet around and associated with maturation and function of the blood-brain barrier through a unique signaling pathway and an interaction with tight junction proteins. Connexin hemichannels and connexin gap junction channels contribute to the physiological or pathological progress of the blood-brain barrier; in addition, the interaction with other cell-cell-adhesive proteins is also associated with the maintenance of the blood-brain barrier. Lastly, we explore the connexins and connexin channels involved in the blood-brain barrier in neurological diseases and any programme for drug discovery or delivery to target or avoid the blood-brain barrier.

Connexin Communication Compartments and Wound Repair in Epithelial Tissue

Epithelial tissues line the lumen of tracts and ducts connecting to the external environment. They are critical in forming an interface between the internal and external environment and, following assault from environmental factors and pathogens, they must rapidly repair to maintain cellular homeostasis. These tissue networks, that range from a single cell layer, such as in airway epithelium, to highly stratified and differentiated epithelial surfaces, such as the epidermis, are held together by a junctional nexus of proteins including adherens, tight and gap junctions, often forming unique and localised communication compartments activated for localised tissue repair. This review focuses on the dynamic changes that occur in connexins, the constituent proteins of the intercellular gap junction channel, during wound-healing processes and in localised inflammation, with an emphasis on the lung and skin. Current developments in targeting connexins as corrective therapies to improve wound closure and resolve localised inflammation are also discussed. Finally, we consider the emergence of the zebrafish as a concerted whole-animal model to study, visualise and track the events of wound repair and regeneration in real-time living model systems.

Conflicting Roles of Connexin43 in Tumor Invasion and Growth in the Central Nervous System

The tumor microenvironment is known to have increased levels of cytokines and metabolites, such as glutamate, due to their release from the surrounding cells. A normal cell around the tumor that responds to the inflammatory environment is likely to be subsequently altered. We discuss how these abnormalities will support tumor survival via the actions of gap junctions (GJs) and hemichannels (HCs) which are composed of hexamer of connexin43 (Cx43) protein. In particular, we discuss how GJ intercellular communication (GJIC) in glioma cells, the primary brain tumor, is a regulatory factor and its attenuation leads to tumor invasion. In contrast, the astrocytes, which are normal cells around the glioma, are “hijacked” by tumor cells, either by receiving the transmission of malignant substances from the cancer cells via GJIC, or perhaps via astrocytic HC activity through the paracrine signaling which enable the delivery of these substances to the distal astrocytes. This astrocytic signaling would promote tumor expansion in the brain. In addition, brain metastasis from peripheral tissues has also been known to be facilitated by GJs formed between cerebral vascular endothelial cells and cancer cells. Astrocytes and microglia are generally thought to eliminate cancer cells at the blood–brain barrier. In contrast, some reports suggest they facilitate tumor progression as tumor cells take advantage of the normal functions of astrocytes that support the survival of the neurons by exchanging nutrients and metabolites. In summary, GJIC is essential for the normal physiological function of growth and allowing the diffusion of physiological substances. Therefore, whether GJIC is cancer promoting or suppressing may be dependent on what permeates through GJs, when it is active, and to which cells. The nature of GJs, which has been ambiguous in brain tumor progression, needs to be revisited and understood together with new findings on Cx proteins and HC activities.

The Potential for Connexin Hemichannels to Drive Breast Cancer Progression through Regulation of the Inflammatory Response

Over the past few decades, connexin hemichannels have become recognized as major players in modulating the inflammatory response. Chronic inflammation is documented to promote tumorigenesis and is a critical component of tumor progression. Furthermore, inflammation is strongly linked to angiogenesis, immunotolerance, invasiveness, metastasis, and resistance in breast cancers. In this review, the literature on the role of connexin hemichannels in inflammation is summarized, and the potential role for hemichannel-mediated inflammation in driving breast cancer progression is discussed. Lastly, the potential for connexin-based therapeutics to modulate the inflammatory component of the tumor microenvironment as an avenue for the treatment of breast cancer is also discussed.

Roles of astrocytic connexin-43, hemichannels, and gap junctions in oxygen-glucose deprivation/reperfusion injury induced neuroinflammation and the possible regulatory mechanisms of salvianolic acid B and carbenoxolone

Background

Glia-mediated neuroinflammation is related to brain injury exacerbation after cerebral ischemia/reperfusion (I/R) injury. Astrocytic hemichannels or gap junctions, which were mainly formed by connexin-43, have been implicated in I/R damage. However, the exact roles of astrocytic hemichannels and gap junction in neuroinflammatory responses induced by I/R injury remain unknown.

Methods

Primary cultured astrocytes were subjected to OGD/R injury, an in vitro model of I/R injury. Salvianolic acid B (SalB) or carbenoxolone (CBX) were applied for those astrocytes. Besides, Cx43 mimetic peptides Gap19 or Gap26 were also applied during OGD/R injury; Cx43 protein levels were determined by western blot and cytoimmunofluorescene staining, hemichannel activities by Ethidium bromide uptake and ATP concentration detection, and gap junction intercellular communication (GJIC) permeability by parachute assay. Further, astrocyte-conditioned medium (ACM) was collected and incubated with microglia. Meanwhile, ATP or apyrase were applied to explore the role of ATP during OGD/R injury. Microglial activation, M1/M2 phenotypes, and M1/M2-related cytokines were detected. Also, microglia-conditioned medium (MEM) was collected and incubated with astrocytes to further investigate its influence on astrocytic hemichannel activity and GJIC permeability. Lastly, effects of ACM and MCM on neuronal viability were detected by flow cytometry.

Results

We found that OGD/R induced abnormally opened hemichannels with increased ATP release and EtBr uptake but reduced GJIC permeability. WB tests showed decreased astrocytic plasma membrane’s Cx43, while showing an increase in cytoplasma. Treating OGD/R-injured microglia with ATP or OGD/R-ACM induced further microglial activation and secondary pro-inflammatory cytokine release, with the M1 phenotype predominating. Conversely, astrocytes incubated with OGD/R-MCM exhibited increased hemichannel opening but reduced GJIC coupling. Both SalB and CBX inhibited abnormal astrocytic hemichannel opening and ATP release and switched the activated microglial phenotype from M1 to M2, thus providing effective neuroprotection. Application of Gap19 or Gap26 showed similar results with CBX. We also found that OGD/R injury caused both plasma membrane p-Cx43(Ser265) and p-Src(Tyr416) significantly upregulated; application of SalB may be inhibiting Src kinase and attenuating Cx43 internalization. Meanwhile, CBX treatment induced obviously downregulation of p-Cx43(Ser368) and p-PKC(Ser729) protein levels in plasma membrane.

Conclusions

We propose a vicious cycle exists between astrocytic hemichannel and microglial activation after OGD/R injury, which would aggravate neuroinflammatory responses and neuronal damage. Astrocytic Cx43, hemichannels, and GJIC play critical roles in OGD/R injury-induced neuroinflammatory responses; treatment differentially targeting astrocytic Cx43, hemichannels, and GJIC may provide novel avenues for therapeutics during cerebral I/R injury.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1127-3) contains supplementary material, which is available to authorized users.