Modulation of connexin expression and gap junction communication in astrocytes by the gram-positive bacterium S. aureus

Phloretin ameliorates heart function after myocardial infarction via NLRP3/Caspase-1/IL-1β signaling

OBJECTIVES/AIMS

Inflammation is crucial in structural and electrical remodeling after myocardial infarction (MI), affecting cardiac pump function and conduction pathways. Phloretin possesses an anti-inflammation role by inhibiting the NLRP3/Caspase-1/IL-1β pathway. However, the effects of Phloretin on cardiac contractile and electrical conduction function after MI remained unclear. Therefore, we aimed to investigate the potential role of Phloretin in a rat model of MI.

METHODS

Rats were assigned into four groups: Sham, Sham+Phloretin, MI and MI+Phloretin, with ad libitum food and water. In the MI and MI+Phloretin groups, the left anterior descending coronary artery was occluded for 4 weeks, while the Sham and Sham+Phloretin groups received sham operation. The Sham+Phloretin group and the MI+Phloretin group received oral administration of Phloretin. In vitro, H9c2 cells were subjected to hypoxic conditions to simulate an MI model, with Phloretin for 24 h. Cardiac electrophysiological properties were assessed following MI, including the effective refractory period (ERP), action potential duration (APD)90 and ventricular fibrillation (VF) incidence. Echocardiography evaluated left ventricular ejection fraction (LVEF), left ventricular fraction shortening (LVFS), left ventricular internal diameter at end-diastole (LVIDd), left ventricular internal diameter at end-systole (LVIDs), left ventricular end-systolic volume (LVESV) and left ventricular end-diastolic volume (LVEDV) to assess cardiac function. Serum type B natriuretic peptide (BNP) level was applied to evaluate the degree of Heart failure (HF). The fibrosis area and severity were assessed by Masson staining and protein expression levels of collagen 3, collagen 1, TGF-β and α-SMA. Western blot analysis estimated the protein expression levels of NLRP3, Pro Caspase-1, Caspase-1, ASC, IL-18, IL-1β, pp38, p38, and Connexin43(Cx43) to elucidate the influence of inflammation on electrical remodeling after MI.

RESULTS

Our findings demonstrate that Phloretin inhibits the NLRP3/Caspase-1/IL-1β pathway, leading to the upregulation of Cx43 by limiting p38 phosphorylation, which further decreases susceptibility to ventricular arrhythmias (VAs). Additionally, Phloretin attenuated fibrosis by inhibiting inflammation to prevent HF. In vitro experiments also provided strong evidence supporting the inhibitory effects of Phloretin on the NLRP3/Caspase-1/IL-1β pathway.

CONCLUSION

Our results suggest that Phloretin could suppress the NLRP3/Caspase-1/IL-1β pathway to reverse structural and electrical remodeling after MI to prevent the occurrence of VAs and HF.

The caspase-1 inhibitor VX765 upregulates connexin 43 expression and improves cell-cell communication after myocardial infarction via suppressing the IL-1β/p38 MAPK pathway

Connexin 43 (Cx43) is the most important protein in the gap junction channel between cardiomyocytes. Abnormalities of Cx43 change the conduction velocity and direction of cardiomyocytes, leading to reentry and conduction block of the myocardium, thereby causing arrhythmia. It has been shown that IL-1β reduces the expression of Cx43 in astrocytes and cardiomyocytes in vitro. However, whether caspase-1 and IL-1β affect connexin 43 after myocardial infarction (MI) is uncertain. In this study we investigated the effects of VX765, a caspase-1 inhibitor, on the expression of Cx43 and cell-to-cell communication after MI. Rats were treated with VX765 (16 mg/kg, i.v.) 1 h before the left anterior descending artery (LAD) ligation, and then once daily for 7 days. The ischemic heart was collected for histochemical analysis and Western blot analysis. We showed that VX765 treatment significantly decreased the infarct area, and alleviated cardiac dysfunction and remodeling by suppressing the NLRP3 inflammasome/caspase-1/IL-1β expression in the heart after MI. In addition, VX765 treatment markedly raised Cx43 levels in the heart after MI. In vitro experiments were conducted in rat cardiac myocytes (RCMs) stimulated with the supernatant from LPS/ATP-treated rat cardiac fibroblasts (RCFs). Pretreatment of the RCFs with VX765 (25 μM) reversed the downregulation of Cx43 expression in RCMs and significantly improved intercellular communication detected using a scrape-loading/dye transfer assay. We revealed that VX765 suppressed the activation of p38 MAPK signaling in the heart tissue after MI as well as in RCMs stimulated with the supernatant from LPS/ATP-treated RCFs. Taken together, these data show that the caspase-1 inhibitor VX765 upregulates Cx43 expression and improves cell-to-cell communication in rat heart after MI via suppressing the IL-1β/p38 MAPK pathway.

Uptake, recognition and responses to peptidoglycan in the mammalian host

Microbiota, and the plethora of signalling molecules that they generate, are a major driving force that underlies a striking range of inter-individual physioanatomic and behavioural consequences for the host organism. Among the bacterial effectors, one finds peptidoglycan, the major constituent of the bacterial cell surface. In the steady-state, fragments of peptidoglycan are constitutively liberated from bacterial members of the gut microbiota, cross the gut epithelial barrier and enter the host system. The fate of these peptidoglycan fragments, and the outcome for the host, depends on the molecular nature of the peptidoglycan, as well the cellular profile of the recipient tissue, mechanism of cell entry, the expression of specific processing and recognition mechanisms by the cell, and the local immune context. At the target level, physiological processes modulated by peptidoglycan are extremely diverse, ranging from immune activation to small molecule metabolism, autophagy and apoptosis. In this review, we bring together a fragmented body of literature on the kinetics and dynamics of peptidoglycan interactions with the mammalian host, explaining how peptidoglycan functions as a signalling molecule in the host under physiological conditions, how it disseminates within the host, and the cellular responses to peptidoglycan.

Connexins and the Epithelial Tissue Barrier: A Focus on Connexin 26

Simple Summary

Tissues that face the external environment are known as ‘epithelial tissue’ and form barriers between different body compartments. This includes the outer layer of the skin, linings of the intestine and airways that project into the lumen connecting with the external environment, and the cornea of the eye. These tissues do not have a direct blood supply and are dependent on exchange of regulatory molecules between cells to ensure co-ordination of tissue events. Proteins known as connexins form channels linking cells directly and permit exchange of small regulatory signals. A range of environmental stimuli can dysregulate the level of connexin proteins and or protein function within the epithelia, leading to pathologies including non-healing wounds. Mutations in these proteins are linked with hearing loss, skin and eye disorders of differing severity. As such, connexins emerge as prime therapeutic targets with several agents currently in clinical trials. This review outlines the role of connexins in epithelial tissue and how their dysregulation contributes to pathological pathways.

Abstract

Epithelial tissue responds rapidly to environmental triggers and is constantly renewed. This tissue is also highly accessible for therapeutic targeting. This review highlights the role of connexin mediated communication in avascular epithelial tissue. These proteins form communication conduits with the extracellular space (hemichannels) and between neighboring cells (gap junctions). Regulated exchange of small metabolites less than 1kDa aide the co-ordination of cellular activities and in spatial communication compartments segregating tissue networks. Dysregulation of connexin expression and function has profound impact on physiological processes in epithelial tissue including wound healing. Connexin 26, one of the smallest connexins, is expressed in diverse epithelial tissue and mutations in this protein are associated with hearing loss, skin and eye conditions of differing severity. The functional consequences of dysregulated connexin activity is discussed and the development of connexin targeted therapeutic strategies highlighted.

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.

Astroglia in Sepsis Associated Encephalopathy

Cellular pathophysiology of sepsis associated encephalopathy (SAE) remains poorly characterised. Brain pathology in SAE, which is manifested by impaired perception, consciousness and cognition, results from multifactorial events, including high levels of systemic cytokines, microbial components and endotoxins, which all damage the brain barriers, instigate neuroinflammation and cause homeostatic failure. Astrocytes, being the principal homeostatic cells of the central nervous system contribute to the brain defence against infection. Forming multifunctional anatomical barriers, astroglial cells maintain brain-systemic interfaces and restrict the damage to the nervous tissue. Astrocytes detect, produce and integrate inflammatory signals between immune cells and cells of brain parenchyma, thus regulating brain immune response. In SAE astrocytes are present in both reactive and astrogliopathic states; balance between these states define evolution of pathology and neurological outcomes. In humans pathophysiology of SAE is complicated by frequent presence of comorbidities, as well as age-related remodelling of the brain tissue with senescence of astroglia; these confounding factors further impact upon SAE progression and neurological deficits.

General Pathophysiology of Astroglia

Astroglial cells are involved in most if not in all pathologies of the brain. These cells can change the morpho-functional properties in response to pathology or innate changes of these cells can lead to pathologies. Overall pathological changes in astroglia are complex and diverse and often vary with different disease stages. We classify astrogliopathologies into reactive astrogliosis, astrodegeneration with astroglial atrophy and loss of function, and pathological remodelling of astrocytes. Such changes can occur in neurological, neurodevelopmental, metabolic and psychiatric disorders as well as in infection and toxic insults. Mutation in astrocyte-specific genes leads to specific pathologies, such as Alexander disease, which is a leukodystrophy. We discuss changes in astroglia in the pathological context and identify some molecular entities underlying pathology. These entities within astroglia may repent targets for novel therapeutic intervention in the management of brain pathologies.

Astrocytes in juvenile neuronal ceroid lipofuscinosis (CLN3) display metabolic and calcium signaling abnormalities

Juvenile neuronal ceroid lipofuscinosis (JNCL) is a lysosomal storage disease caused by autosomal recessive mutations in ceroid lipofuscinosis 3 (CLN3). Children with JNCL experience progressive visual, cognitive, and motor deterioration with a decreased life expectancy (late teens-early 20s). Neuronal loss is thought to occur, in part, via glutamate excitotoxicity; however, little is known about astrocyte glutamate regulation in JNCL. Spontaneous Ca²⁺ oscillations were reduced in murine Cln3^Δex7/8 astrocytes, which were also observed following glutamate or cytokine exposure. Astrocyte glutamate transport is an energy-demanding process and disruptions in metabolic pathways could influence glutamate homeostasis in Cln3^Δex7/8 astrocytes. Indeed, basal mitochondrial respiration and ATP production were significantly reduced in Cln3^Δex7/8 astrocytes. These changes were not attributable to reduced mitochondria, since mitochondrial DNA levels were similar between wild type and Cln3^Δex7/8 astrocytes. Interestingly, despite these functional deficits in Cln3^Δex7/8 astrocytes, glutamate transporter expression and glutamate uptake were not dramatically affected. Concurrent with impaired astrocyte metabolism and Ca²⁺ signaling, murine Cln3^Δex7/8 neurons were hyper-responsive to glutamate, as reflected by heightened and prolonged Ca²⁺ signals. These findings identify intrinsic metabolic and Ca²⁺ signaling defects in Cln3^Δex7/8 astrocytes that may contribute to neuronal dysfunction in CLN3 disease. This article is part of the Special Issue "Lysosomal Storage Disorders".

Astrogliopathology in the infectious insults of the brain

Astroglia, a heterogeneous type of neuroglia, play key homeostatic functions in the central nervous system (CNS) and represent an important defence system. Impaired homeostatic capacity of astrocytes manifests in diseases and this is mirrored in various astrocyte-based pathological features including reactive astrogliosis, astrodegeneration with astroglial atrophy and pathological remodelling of astrocytes. All of these manifestations are most prominently associated with infectious insults, mediated by bacteria, protozoa and viruses. Here we focus onto neurotropic viruses such as tick-borne encephalitis (TBEV) and Zika virus (ZIKV), both belonging to Flaviviridae and both causing severe neurological impairments. We argue that astrocytes provide a route through which neurotropic infectious agents attack the CNS, since they are anatomically associated with the blood-brain barrier and exhibit aerobic glycolysis, a metabolic specialisation of highly morphologically dynamic cells, which may provide a suitable metabolic milieu for proliferation of infectious agents, including viral bodies.

Glucocorticoid receptor in astrocytes regulates midbrain dopamine neurodegeneration through connexin hemichannel activity

The precise contribution of astrocytes in neuroinflammatory process occurring in Parkinson’s disease (PD) is not well characterized. In this study, using GR^Cx30CreERT2 mice that are conditionally inactivated for glucocorticoid receptor (GR) in astrocytes, we have examined the actions of astrocytic GR during dopamine neuron (DN) degeneration triggered by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The results show significantly augmented DN loss in GR^Cx30CreERT2 mutant mice in substantia nigra (SN) compared to controls. Hypertrophy of microglia but not of astrocytes was greatly enhanced in SN of these astrocytic GR mutants intoxicated with MPTP, indicating heightened microglial reactivity compared to similarly-treated control mice. In the SN of GR astrocyte mutants, specific inflammation-associated transcripts ICAM-1, TNF-α and Il-1β as well as TNF-α protein levels were significantly elevated after MPTP neurotoxicity compared to controls. Interestingly, this paralleled increased connexin hemichannel activity and elevated intracellular calcium levels in astrocytes examined in acute midbrain slices from control and mutant mice treated with MPP+ . The increased connexin-43 hemichannel activity was found in vivo in MPTP-intoxicated mice. Importantly, treatment of MPTP-injected GR^Cx30CreERT2 mutant mice with TAT-Gap19 peptide, a specific connexin-43 hemichannel blocker, reverted both DN loss and microglial activation; in wild-type mice there was partial but significant survival effect. In the SN of post-mortem PD patients, a significant decrease in the number of astrocytes expressing nuclear GR was observed, suggesting the participation of astrocytic GR deregulation of inflammatory process in PD. Overall, these data provide mechanistic insights into GR-modulated processes in vivo, specifically in astrocytes, that contribute to a pro-inflammatory state and dopamine neurodegeneration in PD pathology.

Genome-wide association study of sepsis in extremely premature infants

OBJECTIVE

To identify genetic variants associated with sepsis (early-onset and late-onset) using a genome-wide association (GWA) analysis in a cohort of extremely premature infants.

STUDY DESIGN

Previously generated GWA data from the Neonatal Research Network's anonymised genomic database biorepository of extremely premature infants were used for this study. Sepsis was defined as culture-positive early-onset or late-onset sepsis or culture-proven meningitis. Genomic and whole-genome-amplified DNA was genotyped for 1.2 million single-nucleotide polymorphisms (SNPs); 91% of SNPs were successfully genotyped. We imputed 7.2 million additional SNPs. p Values and false discovery rates (FDRs) were calculated from multivariate logistic regression analysis adjusting for gender, gestational age and ancestry. Target statistical value was p<10-5. Secondary analyses assessed associations of SNPs with pathogen type. Pathway analyses were also run on primary and secondary end points.

RESULTS

Data from 757 extremely premature infants were included: 351 infants with sepsis and 406 infants without sepsis. No SNPs reached genome-wide significance levels (5×10-8); two SNPs in proximity to FOXC2 and FOXL1 genes achieved target levels of significance. In secondary analyses, SNPs for ELMO1, IRAK2 (Gram-positive sepsis), RALA, IMMP2L (Gram-negative sepsis) and PIEZO2 (fungal sepsis) met target significance levels. Pathways associated with sepsis and Gram-negative sepsis included gap junctions, fibroblast growth factor receptors, regulators of cell division and interleukin-1-associated receptor kinase 2 (p values<0.001 and FDR<20%).

CONCLUSIONS

No SNPs met genome-wide significance in this cohort of extremely low birthweight infants; however, areas of potential association and pathways meriting further study were identified.

Translational potential of astrocytes in brain disorders

Fundamentally, all brain disorders can be broadly defined as the homeostatic failure of this organ. As the brain is composed of many different cells types, including but not limited to neurons and glia, it is only logical that all the cell types/constituents could play a role in health and disease. Yet, for a long time the sole conceptualization of brain pathology was focused on the well-being of neurons. Here, we challenge this neuron-centric view and present neuroglia as a key element in neuropathology, a process that has a toll on astrocytes, which undergo complex morpho-functional changes that can in turn affect the course of the disorder. Such changes can be grossly identified as reactivity, atrophy with loss of function and pathological remodeling. We outline the pathogenic potential of astrocytes in variety of disorders, ranging from neurotrauma, infection, toxic damage, stroke, epilepsy, neurodevelopmental, neurodegenerative and psychiatric disorders, Alexander disease to neoplastic changes seen in gliomas. We hope that in near future we would witness glial-based translational medicine with generation of deliverables for the containment and cure of disorders. We point out that such as a task will require a holistic and multi-disciplinary approach that will take in consideration the concerted operation of all the cell types in the brain.

Connexins and their channels in inflammation

Inflammation may be caused by a variety of factors and is a hallmark of a plethora of acute and chronic diseases. The purpose of inflammation is to eliminate the initial cell injury trigger, to clear out dead cells from damaged tissue and to initiate tissue regeneration. Despite the wealth of knowledge regarding the involvement of cellular communication in inflammation, studies on the role of connexin-based channels in this process have only begun to emerge in the last few years. In this paper, a state-of-the-art overview of the effects of inflammation on connexin signaling is provided. Vice versa, the involvement of connexins and their channels in inflammation will be discussed by relying on studies that use a variety of experimental tools, such as genetically modified animals, small interfering RNA and connexin-based channel blockers. A better understanding of the importance of connexin signaling in inflammation may open up towards clinical perspectives.

Regulation of gap junction channels by infectious agents and inflammation in the CNS

Gap junctions (GJs) are conglomerates of intercellular channels that connect the cytoplasm of two or more cells, and facilitate the transfer of ions and small molecules, including second messengers, resulting in metabolic and electrical coordination. In general, loss of gap junctional communication (GJC) has been associated with cellular damage and inflammation resulting in compromise of physiological functions. Recently, it has become evident that GJ channels also play a critical role in the pathogenesis of infectious diseases and associated inflammation. Several pathogens use the transfer of intracellular signals through GJ channels to spread infection and toxic signals that amplify inflammation to neighboring cells. Thus, identification of the mechanisms by which several infectious agents alter GJC could result in new potential therapeutic approaches to reduce inflammation and their pathogenesis.

Evidence for aberrant astrocyte hemichannel activity in Juvenile Neuronal Ceroid Lipofuscinosis (JNCL)

Juvenile Neuronal Ceroid Lipofuscinosis (JNCL) is a lysosomal storage disease caused by an autosomal recessive mutation in CLN3 that leads to vision loss, progressive cognitive and motor decline, and premature death. Morphological evidence of astrocyte activation occurs early in the disease process and coincides with regions where neuronal loss eventually ensues. However, the consequences of CLN3 mutation on astrocyte function remain relatively ill-defined. Astrocytes play a critical role in CNS homeostasis, in part, by their ability to regulate the extracellular milieu via the formation of extensive syncytial networks coupled by gap junction (GJ) channels. In contrast, unopposed hemichannels (HCs) have been implicated in CNS pathology by allowing the non-discriminant passage of molecules between the intracellular and extracellular milieus. Here we examined acute brain slices from CLN3 mutant mice (CLN3^Δex7/8) to determine whether CLN3 loss alters the balance of GJ and HC activity. CLN3^Δex7/8 mice displayed transient increases in astrocyte HC opening at postnatal day 30 in numerous brain regions, compared to wild type (WT) animals; however, HC activity steadily decreased at postnatal days 60 and 90 in CLN3^Δex7/8 astrocytes to reach levels lower than WT cells. This suggested a progressive decline in astrocyte function, which was supported by significant reductions in glutamine synthetase, GLAST, and connexin expression in CLN3^Δex7/8 mice compared to WT animals. Based on the early increase in astrocyte HC activity, CLN3^Δex7/8 mice were treated with the novel carbenoxolone derivative INI-0602 to inhibit HCs. Administration of INI-0602 for a one month period significantly reduced lysosomal ceroid inclusions in the brains of CLN3^Δex7/8 mice compared to WT animals, which coincided with significant increases in astrocyte GJ communication and normalization of astrocyte resting membrane potential to WT levels. Collectively, these findings suggest that alterations in astrocyte communication may impact the progression of JNCL and could offer a potential therapeutic target.

Implanted neural progenitor cells regulate glial reaction to brain injury and establish gap junctions with host glial cells

Transplantation of neural stem/progenitor cells (NPCs) in the lesioned brain is able to restore morphological and physiological alterations induced by different injuries. The local microenvironment created at the site of grafting and the communication between grafted and host cells are crucial in the beneficial effects attributed to the NPC implants. We have previously described that NPC transplantation in an animal model of central axotomy restores firing properties and synaptic coverage of lesioned neurons and modulates their trophic factor content. In this study, we aim to explore anatomical relationships between implanted NPCs and host glia that might account for the implant-induced neuroprotective effects. Postnatal rat subventricular zone NPCs were isolated and grafted in adult rats after transection of the medial longitudinal fascicle. Brains were removed and analyzed eight weeks later. Immunohistochemistry for different glial markers revealed that NPC-grafted animals displayed significantly greater microglial activation than animals that received only vehicle injections. Implanted NPCs were located in close apposition to activated microglia and reactive astrocytes. The gap junction protein connexin43 was present in NPCs and glial cells at the lesion site and was often found interposed within adjacent implanted and glial cells. Gap junctions were identified between implanted NPCs and host astrocytes and less frequently between NPCs and microglia. Our results show that implanted NPCs modulate the glial reaction to lesion and establish the possibility of communication through gap junctions between grafted and host glial cells which might be involved in the restorative effects of NPC implants.

Role of connexin/pannexin containing channels in infectious diseases

In recent years it has become evident that gap junctions and hemichannels, in concert with extracellular ATP and purinergic receptors, play key roles in several physiological processes and pathological conditions. However, only recently has their importance in infectious diseases been explored, likely because early reports indicated that connexin containing channels were completely inactivated under inflammatory conditions, and therefore no further research was performed. However, recent evidence indicates that several infectious agents take advantage of these communication systems to enhance inflammation and apoptosis, as well as to participate in the infectious cycle of several pathogens. In the current review, we will discuss the role of these channels/receptors in the pathogenesis of several infectious diseases and the possibilities of generating novel therapeutic approaches to reduce or prevent these diseases.

Lipopolysaccharide induces degradation of connexin43 in rat astrocytes via the ubiquitin-proteasome proteolytic pathway

The astrocytic syncytium plays a critical role in maintaining the homeostasis of the brain through the regulation of gap junction intercellular communication (GJIC). Changes to GJIC in response to inflammatory stimuli in astrocytes may have serious effects on the brain. We have previously shown that lipopolysaccharide (LPS) reduces connexin43 (Cx43) expression and GJIC in cultured rat astrocytes via a toll-like receptor 4-mediated signaling pathway. In the present study, treatment of astrocytes with LPS resulted in a significant increase in levels of the phosphorylated forms of stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) -1, -2, and -3 for up to 18 h. An increase in nuclear transcription factor NF-κB levels was also observed after 8 h of LPS treatment and was sustained for up to 18 h. The LPS-induced decrease in Cx43 protein levels and inhibition of GJIC were blocked by the SAPK/JNK inhibitor SP600125, but not by the NF-κB inhibitor BAY11-7082. Following blockade of de novo protein synthesis by cycloheximide, LPS accelerated Cx43 degradation. Moreover, the LPS-induced downregulation of Cx43 was blocked following inhibition of 26S proteasome activity using the reversible proteasome inhibitor MG132 or the irreversible proteasome inhibitor lactacystin. Immunoprecipitation analyses revealed an increased association of Cx43 with both ubiquitin and E3 ubiquitin ligase Nedd4 in astrocytes after LPS stimulation for 6 h and this effect was prevented by SP600125. Taken together, these results suggest that LPS stimulation leads to downregulation of Cx43 expression and GJIC in rat astrocytes by activation of SAPK/JNK and the ubiquitin-proteasome proteolytic pathway.

Brain abscess: Current management

Brain abscess (BA) is defined as a focal infection within the brain parenchyma, which starts as a localized area of cerebritis, which is subsequently converted into a collection of pus within a well-vascularized capsule. BA must be differentiated from parameningeal infections, including epidural abscess and subdural empyema. The BA is a challenge for the neurosurgeon because it is needed good clinical, pharmacological, and surgical skills for providing good clinical outcomes and prognosis to BA patients. Considered an infrequent brain infection, BA could be a devastator entity that easily left the patient into dead. The aim of this work is to review the current concepts regarding epidemiology, pathophysiology, etiology, clinical presentation, diagnosis, and management of BA.

The unfolded protein response to endoplasmic reticulum stress in cultured astrocytes and rat brain during experimental diabetes

Oxidative-nitrosative stress and inflammatory responses are associated with endoplasmic reticulum (ER) stress in diabetic retinopathy, raising the possibility that disturbances in ER protein processing may contribute to CNS dysfunction in diabetics. Upregulation of the unfolded protein response (UPR) is a homeostatic response to accumulation of abnormal proteins in the ER, and the present study tested the hypothesis that the UPR is upregulated in two models for diabetes, cultured astrocytes grown in 25mmol/L glucose for up to 4weeks and brain of streptozotocin (STZ)-treated rats with diabetes for 1-7months. Markers associated with translational blockade (phospho-eIF2α and apoptosis (CHOP), inflammatory response (inducible nitric oxide synthase, iNOS), and nitrosative stress (nuclear translocation of glyceraldehyde-3-phosphate dehydrogenase, GAPDH) were not detected in either model. Nrf2 was present in nuclei of low- and high-glucose cultures, consistent with oxidative stress. Astrocytic ATF4 expression was not altered by culture glucose concentration, whereas phospho-IRE and ATF6 levels were higher in low- compared with high-glucose cultures. The glucose-regulated chaperones, GRP78 and GRP94, were also expressed at higher levels in low- than high-glucose cultures, probably due to recurrent glucose depletion between feeding cycles. In STZ-rat cerebral cortex, ATF4 level was transiently reduced at 4months, and p-IRE levels were transiently elevated at 3months. However, GRP78 and GRP94 expression was not upregulated, and iNOS, amyloid-β, and nuclear accumulation of GAPDH were not evident in STZ-diabetic brain. High-glucose cultured astrocytes and STZ-diabetic brain are relatively resistant to diabetes-induced ER stress, in sharp contrast with cultured retinal Müller cells and diabetic rodent retina.

Connexin43 mimetic peptide is neuroprotective and improves function following spinal cord injury

Connexin43 (Cx43) is a gap junction protein up-regulated after spinal cord injury and is involved in the on-going spread of secondary tissue damage. To test whether a connexin43 mimetic peptide (Peptide5) reduces inflammation and tissue damage and improves function in an in vivo model of spinal cord injury, rats were subjected to a 10g, 12.5mm weight drop injury at the vertebral level T10 using a MASCIS impactor. Vehicle or connexin43 mimetic peptide was delivered directly to the lesion via intrathecal catheter and osmotic mini-pump for up to 24h after injury. Treatment with Peptide5 led to significant improvements in hindlimb function as assessed using the Basso-Beattie-Bresnahan scale. Peptide5 caused a reduction in Cx43 protein, increased Cx43 phosphorylation and decreased levels of TNF-α and IL-1β as assessed by Western blotting. Immunohistochemistry of tissue sections 5 weeks after injury showed reductions in astrocytosis and activated microglia as well as an increase in motor neuron survival. These results show that administration of a connexin mimetic peptide reduces secondary tissue damage after spinal cord injury by reducing gliosis and cytokine release and indicate the clinical potential for mimetic peptides in the treatment of spinal cord patients.

Loss of astrocyte connexins 43 and 30 does not significantly alter susceptibility or severity of acute experimental autoimmune encephalomyelitis in mice

We showed previously that mice deficient in astrocyte gap junctions Cx43 and Cx30 exhibit white matter vacuolation and hypomyelination. In this study we tested the hypothesis that loss of astrocytic gap junction proteins leads to exacerbation of the primary demyelinating diseases, using experimental autoimmune encephalomyelitis (EAE) as a model system. To test for this, Cx43 floxed mice were crossed with GFAP:Cre, Cx30 null mice to generate mice lacking astrocytic expression of both Cx43 and Cx30 (dKO). EAE was induced using myelin oligodendrocyte glycoprotein (MOG(35-55)) peptide, and mice were monitored for acute expression of disease. No statistically significant difference in clinical or pathological expression of EAE was observed. Lesion load and susceptibility of different areas of the CNS to inflammation were similar in all genotypes. Moreover, no differences were noted in blood-brain barrier (BBB) permeability, tissue wet weight, axonal pathology, gliosis or demyelination during acute disease. These data show that loss of the astrocytic connexins, Cx43 and Cx30, and the white matter pathology observed in these mice does not statistically affect clinical or pathological expression of EAE and show that astrocyte gap junctions do not regulate autoimmune inflammation and associated BBB disruption in acute EAE.

Microglia are required for astroglial Toll-like receptor 4 response and for optimal TLR2 and TLR3 response

Within the central nervous system, astrocytes and microglia are the primary responders to endogenous ligands released upon injury and stress, as well as to infectious pathogens. Toll-like receptors (TLRs) are implicated in recognition of both types of stimulus. Whether astrocytes respond as strongly as microglia to TLR agonists remains contentious. In this study, we have rigorously purified astrocytes to determine their capacity for autonomous TLR response, in absence of microglia. We used flow cytometry and differential adhesion as well as a myeloid lineage-specific suicide gene to purify astrocytes from mixed glial cultures and measured their response to TLR agonists. Our results show that the response of astrocytes to TLR2 and TLR3 agonists is greatly enhanced by, and response to TLR4 agonists is completely dependent on, the presence of functional microglia. In the case of the TLR4 response to lipopolysaccharide, microglia exert their effect on astrocytes at least partially through release of soluble mediators that directly activate or facilitate astrocyte responses. Our findings underline the contribution of glial crosstalk in CNS responses to injury or inflammation.

Comparison of Helicobacter bilis-Associated Protein Expression in Huh7 Cells Harbouring HCV Replicon and in Replicon-Cured Cells

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related deaths worldwide. Hepatitis B or C infections are the main causes of HCC with hepatitis C being the major risk factor for liver cancer in the developed countries. Recently, complications with bacteria of the genus Helicobacter have been associated with HCV-induced HCC. To further understand the mechanisms leading to the development of HCC in the presence of HCV and/or Helicobacter spp., investigation of the differential protein expression in Huh7 cells harbouring HCV-replicon, and replicon cured-Huh7 cells cocultured with H. bilis was done employing two-dimensional gel electrophoresis and mass spectrometry. In the transfected-Huh7 cells exposed to sublethal inoculum densities of H. bilis, 53 different proteins were identified comprising of 28 upregulated and 16 downregulated proteins including 9 potential protein isoforms; in the cured Huh7 cells, 45 different proteins were identified including 33 upregulated, 8 downregulated and, 9 potential protein isoforms. H. bilis affected the modulation of proteins involved in different pathways of Huh7-derived cells physiology including proteins involved in the progression from dysplasia to neoplasm. The result also indicated that the response of the Huh7-derived cells to the presence of H. bilis depended on whether or not HCV replicon was present.

Modulation of connexin signaling by bacterial pathogens and their toxins

Inherent to their pivotal tasks in the maintenance of cellular homeostasis, gap junctions, connexin hemichannels, and pannexin hemichannels are frequently involved in the dysregulation of this critical balance. The present paper specifically focuses on their roles in bacterial infection and disease. In particular, the reported biological outcome of clinically important bacteria including Escherichia coli, Shigella flexneri, Yersinia enterocolitica, Helicobacter pylori, Bordetella pertussis, Aggregatibacter actinomycetemcomitans, Pseudomonas aeruginosa, Citrobacter rodentium, Clostridium species, Streptococcus pneumoniae, and Staphylococcus aureus and their toxic products on connexin- and pannexin-related signaling in host cells is reviewed. Particular attention is paid to the underlying molecular mechanisms of these effects as well as to the actual biological relevance of these findings.

Microglia and Astrocyte Activation by Toll-Like Receptor Ligands: Modulation by PPAR-gamma Agonists

Microglia and astrocytes express numerous members of the Toll-like receptor (TLR) family that are pivotal for recognizing conserved microbial motifs expressed by a wide array of pathogens. Despite the critical role for TLRs in pathogen recognition, when dysregulated these pathways can also exacerbate CNS tissue destruction. Therefore, a critical balance must be achieved to elicit sufficient immunity to combat CNS infectious insults and downregulate these responses to avoid pathological tissue damage. We performed a comprehensive survey on the efficacy of various PPAR-γ agonists to modulate proinflammatory mediator release from primary microglia and astrocytes in response to numerous TLR ligands relevant to CNS infectious diseases. The results demonstrated differential abilities of select PPAR-γ agonists to modulate glial activation. For example, 15d-PGJ₂ and pioglitazone were both effective at reducing IL-12 p40 release by TLR ligand-activated glia, whereas CXCL2 expression was either augmented or inhibited by 15d-PGJ₂, effects that were dependent on the TLR ligand examined. Pioglitazone and troglitazone demonstrated opposing actions on microglial CCL2 production that were TLR ligand-dependent. Collectively, this information may be exploited to modulate the host immune response during CNS infections to maximize host immunity while minimizing inappropriate bystander tissue damage that is often characteristic of such diseases.

Reduced gap junctional communication among astrocytes in experimental diabetes: contributions of altered connexin protein levels and oxidative-nitrosative modifications

Experimental diabetes increases production of reactive oxygen-nitrogen species and inhibits astrocytic gap junctional communication in tissue culture and brain slices from streptozotocin (STZ)-diabetic rats by unidentified mechanisms. Relative connexin (Cx) protein levels were assessed by Western blotting using extracts from cultured astrocytes grown in high (25 mmol/liter) or low (5.5 mmol/liter) glucose for 2-3 weeks and STZ-diabetic rat brain. Chemiluminescent signals for diabetic samples were normalized to those of controls on the same blot and same protein load. Growth in high glucose did not alter relative Cx26 level, whereas Cx30 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were reduced by ∼30%, and Cx43 increased ∼1.9-fold. In the inferior colliculus of STZ-diabetic rats, Cx30 and Cx43 levels in three of four rats were half those of controls, whereas GAPDH and actin were unaffected. Diabetes did not affect levels of Cx30, Cx43, or GAPDH in cerebral cortex, but actin level rose 24%. Cx43 was predominantly phosphorylated in control and diabetic samples, so the reduced dye transfer is not due to overall dephosphorylation of Cx43. Astrocytic growth in high glucose reduced the dye-labeled area by 75%, but 10 min of treatment with dithiothreitol restored normal dye transfer. In contrast, nitric oxide donors inhibited dye transfer among astrocytes grown in low glucose by 50-65% within 1 hr. Thus, modifications arising from oxidative-nitrosative stress, not altered connexin levels, may underlie the reduced dye transfer among severely hyperglycemic cultured astrocytes, whereas both oxidative-nitrosative stress and regionally selective down-regulation of connexin protein content may affect gap junctional communication in the brains of STZ-diabetic rats.

Neuroinflammation leads to region-dependent alterations in astrocyte gap junction communication and hemichannel activity

Inflammation attenuates gap junction (GJ) communication in cultured astrocytes. Here we used a well-characterized model of experimental brain abscess as a tool to query effects of the CNS inflammatory milieu on astrocyte GJ communication and electrophysiological properties. Whole-cell patch-clamp recordings were performed on green fluorescent protein (GFP)-positive astrocytes in acute brain slices from glial fibrillary acidic protein-GFP mice at 3 or 7 d after Staphylococcus aureus infection in the striatum. Astrocyte GJ communication was significantly attenuated in regions immediately surrounding the abscess margins and progressively increased to levels typical of uninfected brain with increasing distance from the abscess proper. Conversely, astrocytes bordering the abscess demonstrated hemichannel activity as evident by enhanced ethidium bromide (EtBr) uptake that could be blocked by several pharmacological inhibitors, including the connexin 43 (Cx43) mimetic peptide Gap26, carbenoxolone, the pannexin1 (Panx1) mimetic peptide (10)Panx1, and probenecid. However, hemichannel opening was transient with astrocytic EtBr uptake observed near the abscess at day 3 but not day 7 after infection. The region-dependent pattern of hemichannel activity at day 3 directly correlated with increases in Cx43, Cx30, Panx1, and glutamate transporter expression (glial L-glutamate transporter and L-glutamate/L-aspartate transporter) along the abscess margins. Changes in astrocyte resting membrane potential and input conductance correlated with the observed changes in GJ communication and hemichannel activity. Collectively, these findings indicate that astrocyte coupling and electrical properties are most dramatically affected near the primary inflammatory site and reveal an opposing relationship between the open states of GJ channels versus hemichannels during acute infection. This relationship may extend to other CNS diseases typified with an inflammatory component.

Peptidoglycan derived from Staphylococcus epidermidis induces Connexin43 hemichannel activity with consequences on the innate immune response in endothelial cells

Gram-positive bacterial cell wall components including PGN (peptidoglycan) elicit a potent pro-inflammatory response in diverse cell types, including endothelial cells, by activating TLR2 (Toll-like receptor 2) signalling. The functional integrity of the endothelium is under the influence of a network of gap junction intercellular communication channels composed of Cxs (connexins) that also form hemichannels, signalling conduits that are implicated in ATP release and purinergic signalling. PGN modulates Cx expression in a variety of cell types, yet effects in endothelial cells remain unresolved. Using the endothelial cell line b.End5, a 6 h challenge with PGN induced IL-6 (interleukin 6), TLR2 and Cx43 mRNA expression that was associated with enhanced Cx43 protein expression and gap junction coupling. Cx43 hemichannel activity, measured by ATP release from the cells, was induced following 15 min of exposure to PGN. Inhibition of hemichannel activity with carbenoxolone or apyrase prevented induction of IL-6 and TLR2 mRNA expression by PGN, but had no effect on Cx43 mRNA expression levels. In contrast, knockdown of TLR2 expression had no effect on PGN-induced hemichannel activity, but reduced the level of TLR2 and Cx43 mRNA expression following 6 h of PGN challenge. PGN also acutely induced hemichannel activity in HeLa cells transfected to express Cx43, but had no effect in Cx43-deficient HeLa OHIO cells. All ATP responses were blocked with Cx-specific channel blockers. We conclude that acute Cx43 hemichannel signalling plays a role in the initiation of early innate immune responses in the endothelium.

Loss of connexin36 in rat hippocampus and cerebellar cortex in persistent Borna disease virus infection

Neonatal Borna disease virus (BDV) infection of the Lewis rat leads to progressive degeneration of dentate gyrus granule cells, and cerebellar Purkinje neurons. Our aim here was to clarify whether BDV interfered with the formation of electrical synapses, and we, therefore, analysed expression of the neuronal gap junction protein connexin36 (Cx36) in the Lewis rat hippocampal formation, and cerebellar cortex, 4 and 8 weeks after neonatal infection. Semiquantitative RT-PCR, revealed a BDV-dependent decrease in Cx36 mRNA in the hippocampal formation 4 and 8 weeks post-infection (p.i.), and in the cerebellar cortex 8 weeks p.i. Correspondingly, immunofluorescent staining revealed reduced Cx36 immunoreactivity in both dentate gyrus, and ammons horn CA3 region, 4 and 8 weeks post-infection. In the cerebellar cortex, Cx36 immunoreactivity was detected only 8 weeks post-infection in the molecular layer, where it was down regulated by BDV. Our findings demonstrate, for the first time, distinct BDV-dependent reductions in Cx36 mRNA and protein in the rat hippocampal formation and cerebellar cortex, suggesting altered neuronal network properties to be an important feature of persistent viral brain infections.

Modulation of brain hemichannels and gap junction channels by pro-inflammatory agents and their possible role in neurodegeneration

In normal brain, neurons, astrocytes, and oligodendrocytes, the most abundant and active cells express pannexins and connexins, protein subunits of two families forming membrane channels. Most available evidence indicates that in mammals endogenously expressed pannexins form only hemichannels and connexins form both gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activity, hemichannels communicate the intra- and extracellular compartments and serve as a diffusional pathway for ions and small molecules. A subthreshold stimulation by acute pathological threatening conditions (e.g., global ischemia subthreshold for cell death) enhances neuronal Cx36 and glial Cx43 hemichannel activity, favoring ATP release and generation of preconditioning. If the stimulus is sufficiently deleterious, microglia become overactivated and release bioactive molecules that increase the activity of hemichannels and reduce gap junctional communication in astroglial networks, depriving neurons of astrocytic protective functions, and further reducing neuronal viability. Continuous glial activation triggered by low levels of anomalous proteins expressed in several neurodegenerative diseases induce glial hemichannel and gap junction channel disorders similar to those of acute inflammatory responses triggered by ischemia or infectious diseases. These changes are likely to occur in diverse cell types of the CNS and contribute to neurodegeneration during inflammatory process.

Layer specific changes of astroglial gap junctions in the rat cerebellar cortex by persistent Borna Disease Virus infection

Neonatal Borna Disease Virus (BDV) infection of the Lewis rat brain, leads to Purkinje cell degeneration, in association with astroglial activation. Since astroglial gap junctions (GJ) are known to influence neuronal degeneration, we investigated BDV dependent changes in astroglial GJ connexins (Cx) Cx43, and Cx30 in the Lewis rat cerebellum, 4, and 8 weeks after neonatal infection. On the mRNA level, RT-PCR demonstrated a BDV dependent increase in cerebellar Cx43, and a decrease in Cx30, 8, but not 4 weeks p.i. On the protein level, Western blot analysis revealed no overall upregulation of Cx43, but an increase of its phosphorylated forms, 8 weeks p.i. Cx30 protein was downregulated. Immunohistochemistry revealed a BDV dependent reduction of Cx43 in the granular layer (GL), 4 weeks p.i. 8 weeks p.i., Cx43 immunoreactivity recovered in the GL, and was induced in the molecular layer (ML). Cx30 revealed a BDV dependent decrease in the GL, both 4, and 8 weeks p.i. Changes in astroglial Cxs correlated not with expression of the astrogliotic marker GFAP, which was upregulated in radial glia. With regard to functional coupling, primary cerebellar astroglial cultures, revealed a BDV dependent increase of Cx43, and Cx30 immunoreactivity and in spreading of the GJ permeant dye Lucifer Yellow. These results demonstrate a massive, BDV dependent reorganization of astroglial Cx expression, and of functional GJ coupling in the cerebellar cortex, which might be of importance for the BDV dependent neurodegeneration in this brain region.

Glial connexins and gap junctions in CNS inflammation and disease

Gap junctions facilitate direct cytoplasmic communication between neighboring cells, facilitating the transfer of small molecular weight molecules involved in cell signaling and metabolism. Gap junction channels are formed by the joining of two hemichannels from adjacent cells, each composed of six oligomeric protein subunits called connexins. Of paramount importance to CNS homeostasis are astrocyte networks formed by gap junctions, which play a critical role in maintaining the homeostatic regulation of extracellular pH, K+, and glutamate levels. Inflammation is a hallmark of several diseases afflicting the CNS. Within the past several years, the number of publications reporting effects of cytokines and pathogenic stimuli on glial gap junction communication has increased dramatically. The purpose of this review is to discuss recent observations characterizing the consequences of inflammatory stimuli on homocellular gap junction coupling in astrocytes and microglia as well as changes in connexin expression during various CNS inflammatory conditions.

TGF-beta3 inhibits chondrogenesis of cultured chick leg bud mesenchymal cells via downregulation of connexin 43 and integrin beta4

Transforming growth factor beta (TGF-beta) is a multifunctional cytokine that regulates a number of biological responses including chemotaxis, cell cycle progression, differentiation, and apoptosis of cells. Even though temporal and spatial expression of TGF-beta3 suggests its role in chick limb development, it is not well characterized how TGF-beta3 regulates chondrogenic differentiation of limb bud mesenchymal cells. In this study, differential display polymerase chain reaction (DD-PCR) screening and reverse transcription PCR analysis revealed that the mRNA expression of the gap junction protein, connexin 43 (Cx43), was significantly decreased during the first treatment of TGF-beta3 for 24 h in cultured chick leg bud mesenchymal cells. Treatment of these cells with lindane, a general gap junction blocker, or expression of dominant negative Cx43 increased apoptotic cell death and decreased the level of integrin beta4 protein, in a manner similar to that observed when these cells were exposed to TGF-beta3. Similarly, exposure of cultured leg chondroblasts to a functional blocking antibody against integrin-beta4 induced an increase in apoptosis. Treatment of cells with TGF-beta3 decreased the membrane translocation of PKC-alpha, leading to activation of ERK. The increase in apoptotic cell death triggered by TGF-beta3 and dominant negative Cx43 was blocked by inhibition of ERK but increased by inhibition of PKC. Collectively, these data indicate that, in cultured chick leg bud mesenchyme cells, TGF-beta3 treatment downregulates Cx43 and induces apoptotic cell death via downregulation of integrin beta4, activation of ERK and suppression of PKC-alpha activation.

Persistent Borna Disease Virus infection changes expression and function of astroglial gap junctions in vivo and in vitro

Neonatal Borna Disease Virus (BDV) infection of the Lewis rat brain leads to dentate gyrus (DG) degeneration, underlying mechanisms are not fully understood. Since astroglial gap junction (GJ) coupling is known to influence neurodegenerative processes, the question arose whether persistent BDV infection influences astroglial connexins (Cx) Cx43 and Cx30 in the hippocampal formation (HiF) of Lewis rats. RT-PCR and Western blot analysis of forebrain (FB) samples revealed a virus dependent reduction of both Cx types 8 but not 4 weeks post infection (p.i.). Immunohistochemistry revealed an increase of Cx43 in the DG and a decrease in the CA3 region 4 and 8 weeks p.i. Cx30, which was detectable only 8 weeks p.i., revealed a BDV dependent increase in DG and CA3 regions. BDV dependent astrogliosis as revealed by immunodetection of glial fibrillary acidic protein (GFAP) correlated not with astroglial connexin expression. With regard to functional coupling as revealed by scrape loading, BDV infection resulted in increased spreading of the GJ permeant dye Lucifer yellow in primary hippocampal astroglial cultures, and in increased expression of Cx43 and Cx30 as revealed by immunocytochemistry. In conclusion, persistent BDV infection of the Lewis rat brain leads to changes in astroglial Cx expression both in vivo and in vitro and of functional coupling in vitro. Distribution and time course of these changes suggest them to be a direct result of neurodegeneration in the DG and an indirect effect of neuronal deafferentiation in the CA3 region.