Upregulation and co-localization of connexin43 and cellular adhesion molecules in inflammatory renal disease

Assessment of the impact of glyphosate and 2,4-D herbicides on the kidney injury and transcriptome changes in obese mice fed a Western diet

The development of chronic kidney disease has been associated with comorbidities resulting from the consumption of Westernized dietary (WD) patterns, including obesity and other metabolic dysfunctions. Kidneys also have a crucial role in the metabolism and excretion of xenobiotics, including herbicides. There is limited knowledge regarding the simultaneous exposure to WD and glyphosate (glypho) and 2,4-D, the most used herbicides globally. Thus, this study examined whether exposure to glypho and/or 2,4-D, either individually or in mixed, could impact the early effects of WD intake on kidney histology and gene expression in a rodent model. Male C57BL6J mice were fed a WD containing 20% lard, 0.2% cholesterol, 20% sucrose, and high sugar solution with 23.1 and 18.9 g/L of D-fructose and D-glucose for six months. During this period, the mice also received glypho (0.05 or 5 mg/kg/day), 2,4-D (0.02 or 2 mg/kg/day), or a mixture of both (0.05 +0.02, 5 +2 mg/kg/day) via intragastric administration five times per week. The doses were within or below the established regulatory limits. While single or mixed exposures did not alter WD-induced obesity, tubular lipid vacuolation, or increased serum creatinine levels; the exposure to higher doses of the mixture (5 +2) reduced the mesangial matrix area and tubular cell proliferation, while increasing the density of F4/80 macrophages in the renal interstitium. In terms of transcriptomic analysis, the herbicide mixture altered the expression of 415 genes in the kidney, which were found to be associated with immune response processes, particularly those related to phagocyte activity. While discrete, findings indicate that herbicide mixtures, rather than single exposures, might induce minor deleterious effects on the kidneys of obese mice under WD intake.

Renoprotection by Inhibiting Connexin 43 Expression in a Mouse Model of Obesity-Related Renal Injury

Introduction

Our previous study conducted in an obesity-related renal injury rat model have established a connection between increased connexin 43 (Cx43) expression and renal injury. In this study, we investigated whether inhibiting Cx43 expression could provide renoprotection in a mouse model of obesity-related renal injury.

Methods

Five-week-old C57BL/6J mice were fed with a high-fat diet for 12 weeks to establish an obesity-related renal injury model, then they were treated with Cx43 antisense oligodeoxynucleotide (AS) or scrambled oligodeoxynucleotide (SCR) by an implanted osmotic pump for 4 weeks. Finally, the glomerular filtration function, the histological change in the glomeruli, and the markers of podocyte injury (WT-1, Nephrin) and inflammatory infiltration of renal tissue (CD68, F4/80 and VCAM-1) were examined respectively.

Results

The results showed that inhibiting Cx43 expression by AS in this mouse model of obesity-related renal injury can effectively improve glomerular filtration function, alleviate glomerular expansion and podocyte injury, and attenuate the inflammatory infiltration of renal tissue.

Conclusion

Our results demonstrated that inhibiting Cx43 expression by AS could provide renoprotection for the mouse model of obesity-related renal injury.

The Role of Connexin 43 in Renal Disease: Insights from In Vivo Models of Experimental Nephropathy

Renal disease is a major public health challenge since its prevalence has continuously increased over the last decades. At the end stage, extrarenal replacement therapy and transplantation remain the only treatments currently available. To understand how the disease progresses, further knowledge of its pathophysiology is needed. For this purpose, experimental models, using mainly rodents, have been developed to unravel the mechanisms involved in the initiation and progression of renal disease, as well as to identify potential targets for therapy. The gap junction protein connexin 43 has recently been identified as a novel player in the development of kidney disease. Its expression has been found to be altered in many types of human renal pathologies, as well as in different animal models, contributing to the activation of inflammatory and fibrotic processes that lead to renal damage. Furthermore, Cx43 genetic, pharmacogenetic, or pharmacological inhibition preserved renal function and structure. This review summarizes the existing advances on the role of this protein in renal diseases, based mainly on different in vivo animal models of acute and chronic renal diseases.

Connexin 43 Expression in Cutaneous Biopsies of Lupus Erythematosus

INTRODUCTION

Gap junctions are channels between adjacent cells formed by connexins (Cxs). Cxs also form hemichannels that connect the cell with its extracellular milieu. These channels allow the transport of ions, metabolites, and small molecules; therefore, Cxs, and more specifically, connexin (Cx) 43 has been demonstrated to be in control of several crucial events such as inflammation and cell death.

MATERIAL AND METHODS

We examined the immunostaining of Cx43 in the endothelia of the cutaneous blood vessels of biopsies from 28 patients with several variants of lupus erythematosus.

RESULTS

In 19 cases (67.86%), staining of more than half of the dermal vessels including both vessels of the papillary and of the reticular dermis was identified. Only in 4 cases (14.28%), less than 25% of the vessels in the biopsy showed expression of the marker.

CONCLUSIONS

Our results suggest a role of Cx43 in regulating the endothelial activity in lupus erythematosus, which also opens a door for targeted therapeutic options.

Connexin-mediated cell communication in the kidney: A potential therapeutic target for future intervention of diabetic kidney disease?: Joan Mott Prize Lecture

The ability of cells to communicate and synchronise their activity is essential for the maintenance of tissue structure, integrity and function. A family of membrane-bound proteins called connexins are largely responsible for mediating the local transfer of information between cells. Assembled in the cell membrane as a hexameric connexon, they either function as a conduit for paracrine signalling, forming a transmembrane hemi-channel, or, if aligned with connexons on neighbouring cells, form a continuous aqueous pore or gap junction, which allows for the direct transmission of metabolic and electrical signals. Regulation of connexin synthesis and activity is critical to cellular function, and a number of diseases are attributed to changes in the expression and/or function of these important proteins. A link between hyperglycaemia, connexin expression, altered nucleotide concentrations and impaired function highlights a potential role for connexin-mediated cell communication in complications of diabetes. In the diabetic kidney, glycaemic injury is the leading cause of end-stage renal failure, reflecting multiple aetiologies including glomerular hyperfiltration, albuminuria, increased deposition of extracellular matrix and tubulointerstitial fibrosis. Loss of connexin-mediated cell-to-cell communication in diabetic nephropathy may represent an early sign of disease progression, but our understanding of the process remains severely limited. This review focuses on recent evidence demonstrating that glucose-evoked changes in connexin-mediated cell communication and associated purinergic signalling may contribute to the pathogenesis of kidney disease in diabetes, highlighting the tantalising potential of targeting these proteins as a novel therapeutic intervention.

Connexin43 Contributes to Inflammasome Activation and Lipopolysaccharide-Initiated Acute Renal Injury via Modulation of Intracellular Oxidative Status

Aims: Inflammasome activation plays a pivotal role in many inflammatory diseases. Given that connexin (Cx) channels regulate numerous cellular events leading to inflammasome activation, we determined whether and how connexin affected inflammasome activation and inflammatory cell injury. Results: Exposure of mouse peritoneal macrophages (PMs) to lipopolysaccharide (LPS) plus ATP caused NLRP3 inflammasome activation, together with an increased connexin43 (Cx43). Inhibition of Cx43 blunted inflammasome activation. Consistently, PMs from the Cx43 heterozygous mouse (Cx43^+/-) exhibited weak inflammasome activation, in comparison with those from the Cx43^+/+ mouse. Further analysis revealed that inflammasome activation was preceded by an increased reactive oxygen species (ROS) production, nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) oxidase 2 (NOX2), protein carbonylation, and mitogen-activated protein kinase (MAPK) activation. Suppression of ROS with antioxidant, downregulation of NOX2 with small interfering RNA (siRNA), or inhibition of NADPH oxidase or MAPKs with inhibitors blocked Cx43 elevation and inflammasome activation. Intriguingly, suppression of Cx43 also blunted NOX2 expression, protein carbonylation, p38 phosphorylation, and inflammasome activation. In a model of acute renal injury induced by LPS, the Cx43^+/- mouse exhibited a significantly lower level of blood interleukin-1β (IL-1β), blood urea nitrogen, and urinary protein, together with milder renal pathological changes and renal expression of NLRP3 and NOX4, as compared with the Cx43^+/+ mouse. Moreover, inhibition of gap junctions suppressed IL-1β- and tumor necrosis factor-α-induced expression of NOX4 in glomerular podocytes and tubular epithelial cells. Innovation and Conclusion: Our study indicates that Cx43 contributes to inflammasome activation and the progression of renal inflammatory cell injury through modulation of intracellular redox status. Cx43 could be a novel target for the treatment of certain inflammatory diseases.

Impaired Regeneration Potential in Urinary Stem Cells Diagnosed from the Patients with Diabetic Nephropathy

Stem cells present in urine possess regenerative capacity to repair kidney injury. However, the unique characteristics of urinary stem cells (USC) from patients with diabetic nephropathy (d-USC) are unknown. The goal of this study was to investigate stemness properties in cell phenotype and regenerative potential of d-USC, compared to USC from healthy individuals. Methods: Thirty-six urine samples collected from patients (n=12, age range 60-75 years) with diabetic nephropathy (stages 3-4 stage chronic kidney disease [CKD]) were compared with 30 urine samples from healthy age-matched donors (n=10, age range 60-74 years). Results: There were approximately six times as many cells in urine samples from patients with diabetic nephropathy, including twice as many USC clones as healthy donors. However, approximately 70% of d-USC had weaker regenerative capacity as assessed by cell proliferation, less secretion of paracrine factors, weaker telomerase activity, and lower renal tubular epithelial differentiation potential compared to healthy controls. In addition, the levels of inflammatory factors (IL-1β and Cx43) and apoptotic markers (Caspase-3, and TUNEL) were significantly increased in d-USC compared to USC (p<0.01). Protein levels of autophagy marker (LC3-II) and mTOR signaling molecules (p-mTOR/mTOR, p-Raptor/Raptor and p-S6K1) were significantly lower in patient with diabetic nephropathy (p<0.01). Nevertheless, up to 30% of d-USC possessed similar regenerative capacity as USC from healthy donors. Conclusions: Regenerative performance of most d-USC was significantly lower than normal controls. Understanding the specific changes in d-USC regeneration capability will help elucidate the pathobiology of diabetic nephropathy and lead to prevent USC from diabetic insults, recover the stemness function and also identify novel biomarkers to predict progression of this chronic kidney disease.

Novel Targets for Therapy of Renal Fibrosis

Renal fibrosis is an important component of chronic kidney disease, an incurable pathology with increasing prevalence worldwide. With a lack of available therapeutic options, end-stage renal disease is currently treated with renal replacement therapy through dialysis or transplantation. In recent years, many efforts have been made to identify novel targets for therapy of renal diseases, with special focus on the characterization of unknown mediators and pathways participating in renal fibrosis development. Using experimental models of renal disease and patient biopsies, we identified four novel mediators of renal fibrosis with potential to constitute future therapeutic targets against kidney disease: discoidin domain receptor 1, periostin, connexin 43, and cannabinoid receptor 1. The four candidates were highly upregulated in different models of renal disease and were localized at the sites of injury. Subsequent studies showed that they are centrally involved in the underlying mechanisms of renal fibrosis progression. Interestingly, inhibition of either of these proteins by different strategies, including gene deletion, antisense administration, or specific blockers, delayed the progression of renal disease and preserved renal structure and function, even when the inhibition started after initiation of the disease. This review will summarize the current findings on these candidates emphasizing on their potential to constitute future targets of therapy.

Hydrogen Sulfide Protects Hyperhomocysteinemia-Induced Renal Damage by Modulation of Caveolin and eNOS Interaction

The accumulation of homocysteine (Hcy) during chronic kidney failure (CKD) can exert toxic effects on the glomeruli and tubulo-interstitial region. Among the potential mechanisms, the formation of highly reactive metabolite, Hcy thiolactone, is known to modify proteins by N-homocysteinylation, leading to protein degradation, stress and impaired function. Previous studies documented impaired nitric oxide production and altered caveolin expression in hyperhomocysteinemia (HHcy), leading to endothelial dysfunction. The aim of this study was to determine whether Hhcy homocysteinylates endothelial nitric oxide synthase (eNOS) and alters caveolin-1 expression to decrease nitric oxide bioavailability, causing hypertension and renal dysfunction. We also examined whether hydrogen sulfide (H₂S) could dehomocysteinylate eNOS to protect the kidney. WT and Cystathionine β-Synthase deficient (CBS+/-) mice representing HHcy were treated without or with sodium hydrogen sulfide (NaHS), a H₂S donor (30 µM), in drinking water for 8 weeks. Hhcy mice (CBS+/-) showed low levels of plasma H₂S, elevated systolic blood pressure (SBP) and renal dysfunction. H₂S treatment reduced SBP and improved renal function. Hhcy was associated with homocysteinylation of eNOS, reduced enzyme activity and upregulation of caveolin-1 expression. Further, Hhcy increased extracellular matrix (ECM) protein deposition and disruption of gap junction proteins, connexins. H₂S treatment reversed the changes above and transfection of triple genes producing H₂S (CBS, CSE and 3MST) showed reduction of vascular smooth muscle cell proliferation. We conclude that during Hhcy, homocysteinylation of eNOS and disruption of caveolin-mediated regulation leads to ECM remodeling and hypertension, and H₂S treatment attenuates renovascular damage.

Alteration of Connexin43 expression in a rat model of obesity-related glomerulopathy

It is accepted that alteration of connexin43 (Cx43) expression in glomeruli is a common pathological response in several forms of kidney diseases. To date, however the change of the Cx43 expression in obesity-related glomerulopathy (ORG) has not been reported. In this study, the alteration of Cx43 expression in the glomeruli of rat with ORG was defined. Five-week-old rats were fed with high-fat diet for 18weeks to establish the ORG model, then the histological change of glomeruli, the foot process effacement of podocyte, the markers for podocyte injury (nephrin,podocin and WT1) and Cx43 expression in glomeruli were examined respectively. The results demonstrated metabolic disorder, hyperinsulinemia, systemic inflammation and microalbuminuria in ORG rats. There was significant hypertrophy, glomerular expansion and inflammatory cell infiltration in the kidney of ORG rats compared to the control group. Significant foot process effacement of the podocyte in the glomeruli, nephrin loss and density reduction were shown in the ORG rats, and Cx43 expression was significant upregulated in glomeruli of ORG rats compared to the control group. The results indicate the correlation of overexpressed Cx43 with the obesity related renal inflammation and suggest that Cx43 might be a potential target in the development of obesity related glomerulopathy.

Decreased Expression of Connexin 43 Blunts the Progression of Experimental GN

GN refers to a variety of renal pathologies that often progress to ESRD, but the molecular mechanisms underlying this progression remain incompletely characterized. Here, we determined whether dysregulated expression of the gap junction protein connexin 43, which has been observed in the progression of renal disease, contributes to GN progression. Immunostaining revealed de novo expression of connexin 43 in damaged glomeruli in patients with glomerular diseases as well as in mice after induction of experimental GN. Notably, 2 weeks after the induction of GN with nephrotoxic serum, mice with a heterozygous deletion of the connexin 43 gene (connexin 43+/-) had proteinuria, BUN, and serum creatinine levels significantly lower than those of wild-type animals. Additionally, the connexin 43+/- mice showed less crescent formation, tubular dilation, monocyte infiltration, and interstitial renal fibrosis. Treatment of cultured podocytes with connexin 43-specific blocking peptides attenuated TGF-β-induced cytoskeletal and morphologic changes and apoptosis as did treatment with the purinergic blocker suramin. Finally, therapeutic treatment of GN mice with connexin 43-specific antisense oligodeoxynucleotide improved functional and structural renal parameters. These findings suggest that crosstalk between connexin 43 and purinergic signaling contributes to podocyte damage in GN. Given that this protein is highly induced in individuals with glomerular diseases, connexin 43 may be a novel target for therapeutic treatment of GN.

Testing Danegaptide Effects on Kidney Function after Ischemia/Reperfusion Injury in a New Porcine Two Week Model

INTRODUCTION

Ischemia/reperfusion injury (I/R-I) is a leading cause of acute kidney injury (AKI) and is associated with increased mortality. Danegaptide is a selective modifier of the gap junction protein connexion 43. It has cytoprotective as well as anti-arrhythmic properties and has been shown to reduce the size of myocardial infarct in pigs. The aim of this study was to investigate the ischemia-protective effect of Danegaptide in a porcine renal I/R-I model with two weeks follow up.

METHODS

Unilateral renal I/R-I was induced in pigs by clamping the left renal artery over a two hour period. The model allowed examination of renal blood flow by magnetic resonance imaging (MRI) and the measurement of single kidney GFR two weeks after injury. Eleven animals were randomized to Danegaptide-infusion while nine animals received placebo. Kidney histology and urinary neutrophil gelatinase-associated lipocalin (NGAL) excretion were included as markers of AKI.

RESULTS

Unilateral kidney I/R-I resulted in an immediate ~50% GFR reduction, associated with a four-fold increase in urinary NGAL-excretion. Fourteen days after I/R-I, the total GFR was ~75% of baseline with a significantly lower GFR in the injured left kidney compared to the right kidney. No differences in GFR were observed between the treated and non-treated animals immediately after I/R-I or at Day 14. Furthermore, no differences were observed in the urinary excretion of NGAL, renal blood flow or other markers of renal function.

CONCLUSIONS

As expected this porcine renal I/R-I model was associated with reduced GFR two weeks after injury. Danegaptide did not improve renal function after I/R-I.

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.

Functional roles of connexins and pannexins in the kidney

Kidneys are highly complex organs, playing a crucial role in human physiopathology, as they are implicated in vital processes, such as fluid filtration and vasomotor tone regulation. There is growing evidence that gap junctions are major determinants of renal physiopathology. It has been demonstrated that their expression or channel activity may vary depending on physiological and pathological situations within distinct renal compartments. While some studies have focused on the role of connexins in renal physiology, our knowledge regarding the functional relevance of pannexins is still very limited. In this paper, we provide an overview of the involvement of connexins, pannexins and their channels in various physiological processes related to different renal compartments.

Mind the gap: connexins and cell-cell communication in the diabetic kidney

Connexins, assembled as a hexameric connexon, form a transmembrane hemichannel that provides a conduit for paracrine signalling of small molecules and ions to regulate the activity and function of adjacent cells. When hemichannels align and associate with similar channels on opposing cells, they form a continuous aqueous pore or gap junction, allowing the direct transmission of metabolic and electrical signals between coupled cells. Regulation of gap junction synthesis and channel activity is critical for cell function, and a number of diseases can be attributed to changes in the expression/function of these important proteins. Diabetic nephropathy is associated with several complex metabolic and inflammatory responses characterised by defects at the molecular, cellular and tissue level. In both type 1 and type 2 diabetes, glycaemic injury of the kidney is the leading cause of end-stage renal failure, a consequence of multiple aetiologies, including increased deposition of extracellular matrix, glomerular hyperfiltration, albuminuria and tubulointerstitial fibrosis. In diabetic nephropathy, loss of connexin mediated cell-cell communication within the nephron may represent an early sign of disease; however, our current knowledge of the role of connexins in the diabetic kidney is sparse. This review highlights recent evidence demonstrating that maintenance of connexin-mediated cell-cell communication could benefit region-specific renal function in diabetic nephropathy and suggests that these proteins should be viewed as a tantalising novel target for therapeutic intervention.

Targeting connexin 43 protects against the progression of experimental chronic kidney disease in mice

Excessive recruitment of monocytes and progression of fibrosis are hallmarks of chronic kidney disease (CKD). Recently we reported that the expression of connexin 43 (Cx43) was upregulated in the kidney during experimental nephropathy. To investigate the role of Cx43 in the progression of CKD, we interbred RenTg mice, a genetic model of hypertension-induced CKD, with Cx43+/- mice. The renal cortex of 5-month-old RenTgCx43+/- mice showed a marked decrease of cell adhesion markers leading to reduced monocyte infiltration and interstitial renal fibrosis compared with their littermates. In addition, functional and histological parameters such as albuminuria and glomerulosclerosis were ameliorated in RenTgCx43+/- mice. Interestingly, treatment with Cx43 antisense produced remarkable improvement of renal function and structure in 1-year-old RenTg mice. Similar results were found in Cx43+/- or wild-type mice treated with Cx43 antisense after obstructive nephropathy. Furthermore, in these mice, Cx43 antisense attenuated E-cadherin downregulation and phosphorylation of the transcription factor Sp1 by the ERK pathway resulting in decreased transcription of type I collagen gene. Interestingly, Cx43-specific blocking peptide inhibited monocyte adhesion in activated endothelium and profibrotic pathways in tubular cells. Cx43 was highly increased in biopsies of patients with CKD. Thus, Cx43 may represent a new therapeutic target against the progression of CKD.

Importance of Connexin-43 based gap junction in cirrhosis and acute-on-chronic liver failure

BACKGROUND & AIMS

In cirrhosis, superimposed inflammation often culminates in acute-on-chronic liver failure (ACLF) but the mechanism underlying this increased sensitivity is not clear. Cx43 is a ubiquitous gap junction protein that allows transmission of signals between cells at a much higher rate than the constitutively expressed gap junctions. The aims of the study were to test the hypothesis that inflammation drives the increased expression of hepatic Cx43 and to determine its role by Cx43 inhibition.

METHODS

Four weeks after bile-duct ligation (BDL) or sham operation, rats were treated with an anti-TNF antibody, or saline; with or without LPS (1mg/kg); given 3h prior to termination. Biochemistry and cytokines were measured in the plasma and hepatic protein expression (NFkB, TNFα, iNOS, 4HNE, Cx26, 32, and 43) and confocal microscopy (Cx26, 32, and 43) were performed. The effect of a Cx43-specific inhibitory peptide was studied in a mouse BDL model.

RESULTS

BDL animals administered LPS developed typical features of ACLF but animals administered infliximab were relatively protected. Cx26/32 expression was significantly decreased in BDL animals while Cx43 was significantly increased and increased further following LPS. Infliximab treatment prevented this increase. However, inhibiting Cx43 in BDL mice produced detrimental effects with markedly greater hepatocellular necrosis.

CONCLUSIONS

The results of this study show for the first time an increased expression of hepatic Cx43 in cirrhosis and ACLF, which was related to the severity of inflammation. This increased Cx43 expression is likely to be an adaptive protective response of the liver to allow better cell-to-cell communication.

Urinary vascular cell adhesion molecule, but not neutrophil gelatinase-associated lipocalin, is associated with lupus nephritis

OBJECTIVE

Vascular cell adhesion molecule-1 (VCAM-1), an adhesion molecule, is involved in the progression of glomerular and tubulointerstitial injury. Neutrophil gelatinase-associated lipocalin (NGAL), a member of the lipocalin superfamily, has been shown to rise in both acute and chronic kidney damage. Both VCAM-1 and NGAL have been found at high levels in the urine of patients with active lupus nephritis. We investigated both as potential biomarkers for lupus nephritis.

METHODS

VCAM-1 and NGAL were measured by ELISA during 1 to 8 clinic visits in 107 patients with systemic lupus erythematosus (SLE; 91% women, 51% black, 36% white, 4% Asian, 4% Hispanic, and 5% others) for a total of 190 visits. Patients' mean age was 41 years. We analyzed the relationship between these potential urine biomarkers and the urine protein/creatinine ratio (urine Pr/Cr), the Systemic Lupus International Collaborating Clinics (SLICC) renal activity score, SLE Disease Activity Index renal descriptors, and other clinical variables.

RESULTS

VCAM-1 levels were strongly associated with the physician's global estimate of disease activity (p = 0.0002), the renal visual analog scale (p < 0.0001), the urine Pr/Cr (p < 0.0001), and SLICC renal activity score (p < 0.0001). VCAM-1 levels were also associated with a urine Pr/Cr ≥ 0.5 (p < 0.0001). NGAL was not associated with any measure of disease activity or with lupus serologies.

CONCLUSION

Urine VCAM-1 had a strong association with measures of disease activity, including multiple renal activity descriptors. In contrast to previous SLE studies, NGAL failed to show any association with lupus nephritis.

Functional gap junctions accumulate at the immunological synapse and contribute to T cell activation

Gap junction (GJ) mediates intercellular communication through linked hemichannels from each of two adjacent cells. Using human and mouse models, we show that connexin 43 (Cx43), the main GJ protein in the immune system, was recruited to the immunological synapse during T cell priming as both GJs and stand-alone hemichannels. Cx43 accumulation at the synapse was Ag specific and time dependent, and required an intact actin cytoskeleton. Fluorescence recovery after photobleaching and Cx43-specific inhibitors were used to prove that intercellular communication between T cells and dendritic cells is bidirectional and specifically mediated by Cx43. Moreover, this intercellular cross talk contributed to T cell activation as silencing of Cx43 with an antisense or inhibition of GJ docking impaired intracellular Ca(2+) responses and cytokine release by T cells. These findings identify Cx43 as an important functional component of the immunological synapse and reveal a crucial role for GJs and hemichannels as coordinators of the dendritic cell-T cell signaling machinery that regulates T cell activation.

Alteration of connexin expression is an early signal for chronic kidney disease

Chronic kidney disease is promoted by a variety of factors that induce chronic inflammation and fibrosis. Inflammation and excessive scaring have been recently associated with disruptions of the gap junction-mediated intercellular communication. Nevertheless, little is known about alterations of the expression of gap junction proteins such as connexin (Cx) 43 and 37 in chronic renal disease. In this study, we investigated the expression of these two Cxs in the hypertensive RenTg mice, the anti-glomerular basement membrane glomerulonephritis, and the unilateral ureteral obstruction models, all leading to the development of chronic kidney disease in mice. Expression of Cx43 was almost negligible in the renal cortex of control mice. In contrast, Cx43 was markedly increased in the endothelium of peritubular and glomerular capillaries of the 3-mo-old RenTg mice, in the glomeruli of mice suffering from glomerulonephritis, and in the tubules after obstructive nephropathy. The Cx43 expression pattern was paralleled closely by that of the adhesion markers such as vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 as well as the inflammatory biomarker monocyte chemoattractant protein-1. In contrast, Cx37 that was abundantly expressed in the renal cortex of healthy mice was markedly decreased in the three experimental models. Interestingly, Cx43+/- mice showed restricted expression of VCAM-1 after 2 wk of obstructive nephropathy. These findings suggest the importance of Cxs as markers of chronic renal disease and indicate that these proteins may participate in the inflammatory process during the development of this pathology.

Connexin32 protects against vascular inflammation by modulating inflammatory cytokine expression by endothelial cells

Gap junctions (GJs) play an important role in vascular function, stability, and homeostasis in endothelial cells (ECs), and GJs are comprised of members of the connexin (Cx) family. GJs of vascular ECs are assembled from Cx37, Cx40, and Cx43, and we showed that ECs also express Cx32. In this study, we investigated a potential role for Cx32 during vascular inflammation. Expression of Cx32 mRNA and protein by human umbilical venous ECs (HUVECs) decreased following treatment with tumor necrosis factor (TNF)-α, but lipopolysaccharide (LPS) and interleukin (IL)-1β did not affect Cx32 expression. Intracellular transfer of an inhibitory anti-Cx32 monoclonal antibody significantly enhanced TNF-α-induced monocyte chemotactic protein (MCP)-1 and IL-6 expression, but overexpression of Cx32 abrogated TNF-α-induced MCP-1 and IL-6 expression. LPS treatment of Cx32 knock-out mice significantly increased the serum concentrations of TNF-α, interferon-γ, IL-6 and MCP-1, compared to wild-type littermate mice. These data suggest that Cx32 protects ECs from inflammation by regulating cytokine expression and plays an important role in the maintenance of vascular function.

Connexins and the kidney

Connexins (Cxs) are widely-expressed proteins that form gap junctions in most organs, including the kidney. In the renal vasculature, Cx37, Cx40, Cx43, and Cx45 are expressed, with predominant expression of Cx40 in the endothelial cells and Cx45 in the vascular smooth muscle cells. In the tubules, there is morphological evidence for the presence of gap junction plaques only in the proximal tubules. In the distal nephron, Cx30, Cx30.3, and Cx37 are expressed, but it is not known whether they form gap junctions connecting neighboring cells or whether they primarily act as hemichannels. As in other systems, the major function of Cxs in the kidney appears to be intercellular communication, although they may also form hemichannels that allow cellular secretion of large signaling molecules. Renal Cxs facilitate vascular conduction, juxtaglomerular apparatus calcium signaling, and tubular purinergic signaling. Accordingly, current evidence points to roles for these Cxs in several important regulatory mechanisms in the kidney, including the renin angiotensin system, tubuloglomerular feedback, and salt and water reabsorption. At the systemic level, renal Cxs may help regulate blood pressure and may be involved in hypertension and diabetes.

Redistribution of connexin43 expression in glomerular podocytes predicts poor renal prognosis in patients with type 2 diabetes and overt nephropathy

BACKGROUND

Significance of podocyte injury in the progression of diabetic nephropathy is not well-understood. In this study, we examined whether alteration of gap junction protein connexin43 (Cx43) expression in podocytes is associated with the progression of overt diabetic nephropathy.

METHODS

We recruited 29 type 2 diabetic patients with overt nephropathy who underwent renal biopsy. Nephrectomized kidney samples obtained from seven subjects with localized neoplasm and biopsy specimens from five patients diagnosed as minor glomerular abnormalities were used as controls. Cx43 staining on paraffin-embedded kidney sections were studied by immunohistochemistry.

RESULTS

In controls, Cx43 was expressed at podocytes in a linear pattern along the glomerular basement membrane. In contrast, downregulation and loss of uniformly linear staining of Cx43 (Cx43 heterogeneity) in podocytes were observed in diabetic nephropathy. Cx43 intensity correlated with current renal function (R = 0.647, P < 0.005), whereas the magnitude of Cx43 heterogeneity correlated well with the degree of future decline in renal function (R = -0.705, P < 0.001).

CONCLUSIONS

Alteration of Cx43 expression in podocytes was closely associated with the progression of overt diabetic nephropathy. These results indicate that change in Cx43 expression at podocytes represents a progressive stage in overt diabetic nephropathy and that it may be a convenient way to predict future decline in renal function.

Glucose-evoked alterations in connexin43-mediated cell-to-cell communication in human collecting duct: a possible role in diabetic nephropathy

Aberrant sodium absorption has been linked to the development of hypertension in both renal disease and diabetes. Efficient absorption depends on coordination of cellular activity across the entire epithelium via cell-to-cell coupling. In the current study we have utilized a model human collecting duct cell line (HCD) to assess the role of connexin43 (Cx43)-mediated gap junctions in the transfer of intracellular Ca(2+) transients within coupled cell clusters. HCD cells express Cx43 mRNA and protein, as well as that for the mechanosensitive transient receptor potential receptor (TRPV4). Mechanical stimulation of individual cells within a cluster evoked a transient rise in cytosolic Ca(2+) concentration ([Ca(2+)](i)) that propagated between cells via a heptanol-sensitive mechanism. The rise in [Ca(2+)](i) was dependent on both store release and Ca(2+)-influx pathways. Lucifer yellow dye transfer and Cx43 knockdown experiments confirmed direct cell-to-cell communication. Application of the Ca(2+) ionophore ionomycin, or an increase in glucose (5 to 25 mM), produced a time-dependent (48 h) increase in Cx43 protein expression. The transmission rate of touch-evoked Ca(2+) transients between coupled cells was accelerated after exposure to high glucose, providing a functional correlate to increased Cx43 expression. These data suggest a pivotal role for Cx43-mediated gap junctions in the synchronization of activity between HCD cells in response to stimuli that mimic osmotic and physical changes. Cx43 expression and cell-to-cell communication increased in response to high glucose and may protect the collecting duct from renal damage associated with more established diabetic nephropathy.

Gap junctional communication in tissue inflammation and repair

Local injury induces a complex orchestrated response to stimulate healing of injured tissues, cellular regeneration and phagocytosis. Practically, inflammation is defined as a defense process whereby fluid and white blood cells accumulate at a site of injury. The balance of cytokines, chemokines, and growth factors is likely to play a key role in regulating important cell functions such as migration, proliferation, and matrix synthesis during the process of inflammation. Hence, the initiation, maintenance, and resolution of innate responses depend upon cellular communication. A process similar to tissue repair and subsequent scarring is found in a variety of fibrotic diseases. This may occur in a single organ such as liver, kidneys, pancreas, lung, skin, and heart, but fibrosis may also have a more generalized distribution such as in atherosclerosis. The purpose of this review is to summarize recent advances on the contribution of gap junction-mediated intercellular communication in the modulation of the inflammatory response and tissue repair.

Gap junctions and connexin-mediated communication in the immune system

Gap junctions and connexins are present in the immune system. In haematopoiesis, connexin 43, the most widely distributed gap junction protein, appears to be a key player in the development of progenitor cells and their communication with stromal cells. Connexin 43 is expressed by macrophages, neutrophils and mast cells. Lymphocytes also express connexin 43, and inhibition of gap junction channels in these cells by using highly specific connexin mimetic reagents has profound effects on immunoglobulin secretion and synthesis of cytokines. Lymphocytes and leukocytes also communicate directly in vitro with endothelial cells via gap junctions. Connexins are implicated in inflammatory reactions in a range of tissues. Their involvement in atherosclerotic plaque formation in the vascular system is also a current growth point in research, and could lead to the development of therapeutic interventions.

TNF-alpha plus IFN-gamma induce connexin43 expression and formation of gap junctions between human monocytes/macrophages that enhance physiological responses

In this work, the effects of bacterial LPS, TNF-alpha, and IFN-gamma on gap junctional communication (dye coupling) and on the expression of connexin43 (immunofluorescence, immunoblotting, and RT-PCR) in monocytes/macrophages were studied. Freshly isolated human monocytes plated at high density and treated either with LPS plus IFN-gamma or TNF-alpha plus IFN-gamma became transiently dye coupled (Lucifer yellow) within 24 h. Cells treated with LPS, TNF-alpha, or IFN-gamma alone remained dye uncoupled. In dye-coupled cells, the spread of Lucifer yellow to neighboring cells was reversibly blocked with 18 alpha-glycyrrhetinic acid, a gap junction blocker, but it was unaffected by oxidized ATP or probenecid, which block ionotropic ATP-activated channels and organic anion transporters, respectively. Abs against TNF-alpha significantly reduced the LPS plus IFN-gamma-induced increase in dye coupling. In dye-coupled monocytes/macrophages, but not in control cells, both connexin43 protein and mRNA were detected, and their levels were higher in cells with an elevated incidence of dye coupling. In dye-coupled cells, the localization of connexin43 immunoreactivity was diffuse at perinuclear regions and thin cell processes. The addition of 18-alpha-glycyrrhetinic acid induced a profound reduction of monocyte/macrophage transmigration across a blood brain barrier model. It also induced a significant reduction in the secretion of metalloproteinase-2 in cells treated with TNF-alpha plus IFN-gamma. We propose that some monocyte/macrophage responses are coordinated by connexin-formed membrane channels expressed transiently at inflammatory sites in which these cells form aggregates.

Gap-junctional coupling between neutrophils and endothelial cells: a novel modulator of transendothelial migration

Communication between leukocytes and endothelial cells is crucial for inflammatory reactions. Paracrine cross-talk and outside-in signaling (via adhesion molecules) have been characterized as communication pathways to date. As leukocytes and endothelial cells express connexins, we considered intercellular communication via gap junctions an intriguing additional concept. We found that gap-junctional coupling between neutrophils and endothelium occurred in a time-dependent, bidirectional manner and was facilitated by adhesion. After blockade of connexins, transmigration of neutrophils through the endothelial layer was enhanced, and the barrier function of cell monolayers was reduced during transmigration. Tumor necrosis factor alpha decreased coupling. In the presence of connexins, transmigration of neutrophils did not alter permeability. Thus, neutrophils couple to endothelium via gap junctions, functionally modulating transmigration and leakiness. Gap-junctional coupling may be a novel way of leukocyte-endothelial communication.

Epithelial cells of nasal mucosa express functional gap junctions of connexin 43

OBJECTIVE

Recent studies have suggested that inositol triphosphate is transmitted through the gap junction of adjacent epithelial cells and communicates the intracellular calcium wave that controls the beating of cilia. Therefore, gap junction-mediated intercellular communication may modulate the metachronal ciliary beating of respiratory mucosa.

MATERIAL AND METHODS

The expression and function of gap junctions were evaluated in nasal mucosa using immunofluorescent staining, Western blotting and Lucifer Yellow dye coupling in both whole nasal mucosa and cultured nasal epithelial cells.

RESULTS

Among connexins (Cxs) 26, 32 and 43, only Cx43 was found in the nasal mucosa, in both the epithelial cells and submucosa tissue. Using Western blotting, Cx43 protein was found in protein extracts of both whole mucosa and cultured epithelial cells. The Cx43 protein content increased twofold between the 3rd and 6th days of culture. Lucifer Yellow dye, microinjected into single cultured epithelial cells, had spread to the adjacent cells.

CONCLUSION

Cx43 is present in nasal mucosa. The expression of Cx43 on epithelial cells increases after culture. The dye-coupling study in epithelial cells indicated that the signal may be transmitted to neighboring cells via a gap junction composed of Cx43. Thus, Cx43 may be involved in the regulation of metachronal ciliary beating in ciliated epithelial cells and growth or differentiation in nasal tissue.

Regulation of hepatic connexins in cholestasis: possible involvement of Kupffer cells and inflammatory mediators

Hepatocyte gap junction proteins, connexins (Cxs) 26 and 32, are downregulated during obstructive cholestasis (OC) and lipopolysaccharide hepatocellular cholestasis (LPS-HC). We investigated rat hepatic Cxs during ethynylestradiol hepatocellular cholestasis (EE-HC) and choledochocaval fistula (CCF) and compared them with OC and LPS-HC. Levels (immunoblotting) and cellular distribution (immunofluorescence) of Cx26, -32, and -43, as well as macrophage infiltration, were studied in livers of rats under each condition. Cx26 and -32 were reduced in LPS-HC, OC, and CCF. However, in EE-HC, Cx26 did not change and Cx32 was increased. Prominent inflammation occurred in LPS-HC, OC, and CCF, which was associated with increased levels of Cx43 in LPS-HC and OC but not CCF. No inflammation nor changes in Cx43 levels occurred during EE-HC. In cultured hepatocytes, dye coupling was reduced by tumor necrosis factor-alpha and interleukins-1beta and -6, whereas reduction induced by LPS required coculture with Kupffer cells. Thus hepatocyte gap junctions are downregulated in forms of cholestasis associated with inflammation, and reduced intercellular communication might be induced in part by proinflammatory mediators.

Clinical implications of inflammatory cytokines in the cochlea: a technical note

HYPOTHESIS

Establishing the presence of critical cellular stress response components in cochlear cells can contribute to a better understanding of cochlear cell biology and pathology.

BACKGROUND

Inflammatory cytokines and related proteins play critical roles in a variety of cellular processes, but to date, little is known about the identity and cellular localization of these compounds within the ear. Cytokines are autocrine, which means that cells that produce them have corresponding surface receptors. The presence of these receptors makes the cells vulnerable to disruption by circulating or local sources of cytokines and related ligands. Such disruptions may explain previously poorly understood cochlear pathologies.

METHODS

The messenger RNA precursors that encode inflammatory cytokines and related proteins are identified in the inner ear by using reverse transcriptase-polymerase chain reaction. Cochlear cells that contain the corresponding proteins are identified by immunostaining.

RESULTS

Messenger RNA for interleukin-1alpha, tumor necrosis factor alpha, NFkappaB P65 and P50, and IkappaBalpha was found in cochlear tissue. Cells that immunostained most conspicuously for cytokine production are Type I fibrocytes and root cells located within the spiral ligament.

CONCLUSION

Production of inflammatory cytokines by the above-mentioned cells indicates that they are vulnerable to disruption by extra-cochlear sources of cytokines and associated ligands. These cells play critical roles in cochlear function, and their disruption could induce hearing loss. These findings suggest that systemic or local production of inflammatory ligands may play roles in a number of causes of deafness, including immune mediated hearing loss, sudden hearing loss, and sensorineural hearing loss associated with otosclerosis, otitis media, and bacterial meningitis.

Specific ablation of Stat3beta distorts the pattern of Stat3-responsive gene expression and impairs recovery from endotoxic shock

Alternative splicing of the gene for Stat3, a transcription factor activated by the IL-6 family of cytokines, produces two isoforms: Stat3alpha and a dominant-negative variant, Stat3beta. Stat3beta-deficient mice were generated by gene targeting. Despite intact expression and phosphorylation of Stat3alpha, overall Stat3 activity was impaired in Stat3beta(-/-) cells. Global comparison of transcription in Stat3beta(+/+) and Stat3beta(-/-) cells revealed stable differences. Stat3beta-deficient mice exhibit diminished recovery from endotoxic shock and hyperresponsiveness of a subset of endotoxin-inducible genes in liver. The hepatic response to endotoxin in wild-type mice is accompanied by a transient increase in the ratio of Stat3beta to Stat3alpha. These findings indicate a critical role for Stat3beta in the control of systemic inflammation.

Interleukin-1beta increases the functional expression of connexin 43 in articular chondrocytes: evidence for a Ca2+-dependent mechanism

Cell-to-cell interactions and gap junctions-dependent communication are crucially involved in chondrogenic differentiation, whereas in adult articular cartilage direct intercellular communication occurs mainly among chondrocytes facing the outer cartilage layer. Chondrocytes extracted from adult articular cartilage and grown in primary culture express connexin 43 (Cx43) and form functional gap junctions capable of sustaining the propagation of intercellular Ca2+ waves. Degradation of articular cartilage is a characteristic feature of arthritic diseases and is associated to increased levels of Interleukin-1 (IL-1) in the synovial fluid. We have examined the effects of IL-1 on gap junctional communication in cultured rabbit articular chondrocytes. Incubation with IL-1 potentiated the transmission of intercellular Ca2+ waves and the intercellular transfer of Lucifer yellow. The stimulatory effect was accompanied by a dose-dependent increase in the expression of Cx43 and by an enhanced Cx43 immunostaining at sites of cell-to-cell contact. IL-1 stimulation induced a dose-dependent increase of cytosolic Ca2+ and activates protein tyrosine phosphorylation. IL-1-dependent up-regulation of Cx43 could be prevented by intracellular Ca2+ chelation but not by inhibitors of protein tyrosine kinases, suggesting a crucial role of cytosolic Ca2+ in regulating the expression of Cx43. IL-1 is one of the most potent cytokines that promotes cartilage catabolism; its modulation of intercellular communication represents a novel mechanism by which proinflammatory mediators regulate the activity of cartilage cells.

Cell coupling modulates the contraction of fibroblast-populated collagen lattices

Cultured dermal fibroblasts become notably elongated when incorporated into a fibroblast-populated collagen lattice (FPCL). With time these fibroblasts reorganize the collagen responsible for reduction in lattice size. In monolayer the microinjection of Lucifer Yellow (LY) into cultured human fibroblasts shows cell coupling through gap junctions. Human fibroblasts residing on the periphery of a FPCL are at high density and the microinjection of LY into one of those fibroblasts demonstrates cell coupling. Cells within the center of an FPCL are at low density and appear to be independent of one another; however, the microinjection of LY into selected fibroblasts again demonstrates cell coupling. Hence the microinjection of cells in both the center and the edge of a FPCL pass dye to numerous neighbors. Does cell coupling influence FPCL contraction? FPCL incubated with heptanol and octanol, aliphatic alcohols that uncouple cells, inhibits lattice contraction, whereas hexanol, an aliphatic alcohol that does not uncouple cells, did not alter lattice contraction. Fibroblasts derived from connexin 43 (a transmembrane protein responsible for gap junction structures) knockout mice were demonstrated to lack gap junctional communications. When incorporated into a FPCL these cells failed to elongate and demonstrated retarded lattice contraction. Hence, gap junctional communications between fibroblasts incorporated into collagen lattices appear to optimize FPCL contraction and suggest a role for gap junctions in the organization of collagen fibers.

Intercellular communication in the immune system: differential expression of connexin40 and 43, and perturbation of gap junction channel functions in peripheral blood and tonsil human lymphocyte subpopulations

The distribution and function of connexins (integral membrane proteins assembled into gap junction intercellular communication channels) were studied in human lymphocyte subpopulations. The expression of mRNA encoding connexins in peripheral blood and tonsil-derived T, B and natural killer (NK) lymphocytes was examined. Connexin43 (Cx43) mRNA was expressed in peripheral blood and tonsil lymphocytes, but Cx40 mRNA expression was confined to tonsil-derived T and B lymphocytes; Cx26, Cx32, Cx37 and Cx45 were not detected by reverse transcription-polymerase chain reaction (RT-PCR). Western blot analysis also demonstrated the presence of Cx40 and Cx43 proteins in T and B lymphocytes in a manner coincidental to the mRNA detection. Stimulation in vitro of T and B lymphocytes with phytohaemagglutinin (PHA) and lipopolysaccharide (LPS), respectively, increased Cx40 and Cx43 protein expression. Flow cytometric analysis, using antibodies to extracellular loop amino acid sequences of connexins, confirmed the surface expression of connexins in all lymphocyte subpopulations. Assembly of connexins into gap junctions providing direct intercellular channels linking attached lymphocytes was demonstrated by using a dye transfer technique. The exchange of dye between lymphocytes was inhibited by a connexin extracellular loop mimetic peptide and alpha-glycyrrhetinic acid, two reagents that restrict intercellular communication across gap junctions. Dye coupling occurred between homologous and heterologous co-cultures of T and B lymphocytes, and was not influenced by their stimulation with PHA and LPS. The connexin mimetic peptide caused a significant decrease in the in vitro synthesis of immunoglobulin M (IgM) by T- and B-lymphocyte co-cultured populations in the presence or absence of stimulation by PHA. The results identify connexins as important cell surface components that modulate immune processes.

Gap junctions in cells of the immune system: structure, regulation and possible functional roles

Gap junction channels are sites of cytoplasmic communication between contacting cells. In vertebrates, they consist of protein subunits denoted connexins (Cxs) which are encoded by a gene family. According to their Cx composition, gap junction channels show different gating and permeability properties that define which ions and small molecules permeate them. Differences in Cx primary sequences suggest that channels composed of different Cxs are regulated differentially by intracellular pathways under specific physiological conditions. Functional roles of gap junction channels could be defined by the relative importance of permeant substances, resulting in coordination of electrical and/or metabolic cellular responses. Cells of the native and specific immune systems establish transient homo- and heterocellular contacts at various steps of the immune response. Morphological and functional studies reported during the last three decades have revealed that many intercellular contacts between cells in the immune response present gap junctions or "gap junction-like" structures. Partial characterization of the molecular composition of some of these plasma membrane structures and regulatory mechanisms that control them have been published recently. Studies designed to elucidate their physiological roles suggest that they might permit coordination of cellular events which favor the effective and timely response of the immune system.

Fatal attraction: adhesion molecules and disease

Knowledge of molecular mechanisms in cell-cell and cell-matrix adhesion has increased rapidly in the past decade. Adhesion mechanisms are of prime importance in both physiology and pathology. With respect to the kidney, expression of adhesion molecules has been studied in a variety of diseases, including various forms of glomerulonephritis. Hitherto, these descriptive studies have merely launched extensive speculation regarding the role of adhesion mechanisms in renal pathology. A logical next step is to correlate adhesion molecule expression to alterations in structures which may possibly be affected by altered adhesion, for example gap junctions. Current studies linking structural to functional adhesion expand our understanding of cell biology in health and disease.