Intercellular communication between renin expressing As4.1 cells, endothelial cells and smooth muscle cells

Connexin Hemichannels Contribute to the Activation of cAMP Signaling Pathway and Renin Production

Connexin hemichannels play an important role in the control of cellular signaling and behaviors. Given that lowering extracellular Ca²⁺, a condition that activates hemichannels, is a well-characterized stimulator of renin in juxtaglomerular cells, we, therefore, tested a potential implication of hemichannels in the regulation of renin in As4.1 renin-secreting cells. Lowering extracellular Ca²⁺ induced hemichannel opening, which was associated with cAMP signaling pathway activation and increased renin production. Blockade of hemichannels with inhibitors or downregulation of Cxs with siRNAs abrogated the activation of cAMP pathway and the elevation of renin. Further analysis revealed that cAMP pathway activation was blocked by adenylyl cyclase inhibitor SQ 22536, suggesting an implication of adenyl cyclase. Furthermore, the participation of hemichannels in the activation of the cAMP signaling pathway was also observed in a renal tubular epithelial cell line NRK. Collectively, our results characterized the hemichannel opening as a presently unrecognized molecular event involved in low Ca²⁺-elicited activation of cAMP pathway and renin production. Our findings thus provide novel mechanistic insights into the low Ca²⁺-initiated cell responses. Given the importance of cAMP signaling pathway in the control of multiple cellular functions, our findings also highlight the importance of Cx-forming channels in various pathophysiological situations.

Association of connexin gene polymorphism with essential hypertension in Kazak and Han Chinese in Xinjiang, China

Essential hypertension (EH) is affected by both genetic and environmental factors. The polymorphism of connexin (Cx) genes is found associated with the development of hypertension. However, the association of the polymorphism of Cxs with EH has not been investigated. This study aimed to investigate the association of the polymorphism of connexin (Cx) genes Cx37, Cx40, and Cx43 with EH in Kazak and Han Chinese in Xinjiang, China. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) were used to analyze the polymorphism of Cx genes in Kazak and Han EH patients as well as their normotensive controls. The results showed that there were no significant differences in the frequencies of different three genotypes (A/A, A/G, and G/G) and A and G alleles of Cx40 rs35594137 and rs11552588 between EH patients and normotensive controls. However, in Kazak EH patients, the frequencies of three genotypes (A/A, A/G, and G/G) of Cx37 rs1630310 were 24.8%, 47.2% and 28.0%, respectively, which were significantly different from those in Han EH patients. In Han EH patients, the frequencies of the three genotypes (C/C, C/G and G/G) of Cx43 rs1925223 were 6.4%, 35.6% and 58.0%, respectively. Frequencies of the other four genotypes had no statistical differences among Kazak and Han EH patients and their normotensive controls. These results suggest polymorphisms of Cx37 rs1630310 and Cx43 rs1925223 genes may be associated with the pathogenesis of EH. Carrying Cx37 rs1630310-A or Cx43 rs1925223-G genotypes may protect against the development of EH.

Renal connexins and blood pressure

The kidneys are centrally involved in the regulation of blood pressure. Kidney function requires the coordinated actions of a number of different vascular and tubular cell types in the renal vasculature and in the renal tubular system. The intrarenal coordination of these actions is not well understood. Since gap junctions have been identified in the kidneys, possible pathways involved in this context could be direct intercellular communication via gap junctions or via connexin hemichannels. In this context nine different connexins have been found to be expressed in the kidney, either localized to the vasculature or to the tubular system. Evidence is arising that malfunctions of certain connexins have an impact on the capability of the kidney to maintain blood pressure homeostasis. Findings reported in this context will be outlined and discussed in this review. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Connexin-dependent signaling in neuro-hormonal systems

The advent of multicellular organisms was accompanied by the development of short- and long-range chemical signalling systems, including those provided by the nervous and endocrine systems. In turn, the cells of these two systems have developed mechanisms for interacting with both adjacent and distant cells. With evolution, such mechanisms have diversified to become integrated in a complex regulatory network, whereby individual endocrine and neuro-endocrine cells sense the state of activity of their neighbors and, accordingly, regulate their own level of functioning. A consistent feature of this network is the expression of connexin-made channels between the (neuro)hormone-producing cells of all endocrine glands and secretory regions of the central nervous system so far investigated in vertebrates. This review summarizes the distribution of connexins in the mammalian (neuro)endocrine systems, and what we know about the participation of these proteins on hormone secretion, the life of the producing cells, and the action of (neuro)hormones on specific targets. The data gathered since the last reviews on the topic are summarized, with particular emphasis on the roles of Cx36 in the function of the insulin-producing beta cells of the endocrine pancreas, and of Cx40 in that of the renin-producing juxta-glomerular epithelioid cells of the kidney cortex. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

Regulation of renin release by connexin 43 in As 4.1 cell line

Gap junction channels facilitate chemical and electrical communication between adjacent cells. Gap junction protein, connexin (Cx), is expressed in the endothelial cells of vessels, glomerulus, and renin-secreting cells of the kidney. The purpose of this study was to investigate the role of Cx in renin release using Cx-overexpressing As 4.1 cells. The adenovirus-induced Cx overexpression was conducted by using recombinant adenovirus containing the cDNA encoding Cx37, Cx40, Cx43 (Ad-Cx), and beta-galactosidase (Ad-beta-gal). In 40-overexpressing cells, basal renin release increased in a time-dependent manner but it was significantly lower than that in Ad-beta-gal-treated cells. In Cx37- and Cx43-overexpressing cells, basal renin release was increased in a time-dependent manner, which was not different from control cells. 18-beta glycyrrhetinic acid (GA), a gap junction blocker, stimulated renin release dose-dependently and increased intracellular Ca(2+) in both Cx43-overexpressing cells and control cells. However, no significant differences were observed. An increase in renin release by 3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester, a putative antagonist of Ca(2+) release from intracellular sequestration sites, was also similar between two groups. These results suggest that Cx43 may unlikely alter the regulation of renin release and intracellular Ca(2+) by gap junction blocker in As 4.1 cells.

Gap junctional intercellular communication in the juxtaglomerular apparatus

The juxtaglomerular apparatus (JGA) is a specialized contact region between the glomerulus and the cortical thick ascending limb that plays an active role in the maintenance of ion homeostasis and control of blood pressure. The JGA accommodates several different cell types, including vascular smooth muscle cells, endothelial cells, mesangial cells, macula densa cells, and renin-secreting juxtaglomerular granular cells. These cells, with the exception of the macular densa cells, are tightly coupled by gap junctions. Gap junction-mediated intercellular communication in the JGA provides a pathway for signal transduction and coordination of multicellular functions. Disruption of cell-to-cell communication in the JGA results in altered preglomerular vascular tone and renin secretion. This review summarizes recent data about the roles of gap junctions in the JGA and illustrates how gap junction-mediated intercellular Ca(2+) signals determine physiological responses in the JGA.

Renin Release

The aspartyl-protease renin is the key regulator of the renin-angiotensin-aldosterone system, which is critically involved in salt, volume, and blood pressure homeostasis of the body. Renin is mainly produced and released into circulation by the so-called juxtaglomerular epithelioid cells, located in the walls of renal afferent arterioles at the entrance of the glomerular capillary network. It has been known for a long time that renin synthesis and secretion are stimulated by the sympathetic nerves and the prostaglandins and are inhibited in negative feedback loops by angiotensin II, high blood pressure, salt, and volume overload. In contrast, the events controlling the function of renin-secreting cells at the organ and cellular level are markedly less clear and remain mysterious in certain aspects. The unravelling of these mysteries has led to new and interesting insights into the process of renin release.

Downregulation of connexin 43 expression by high glucose induces senescence in glomerular mesangial cells

Connexin 43 (Cx43) plays an important role in cell differentiation and growth control, but whether it can be regulated by high glucose and whether it can mediate in glomerular mesangial cells (GMC) the phenotype alterations that are induced by high glucose still remain to be explored. In this study, RNA interference and gene transfer techniques were used to knock down and overexpress Cx43 gene in rat GMC to determine the contribution of Cx43 to GMC senescence that was induced by high glucose. The results show that high glucose (30 mM) not only downregulated Cx43 mRNA and protein expression (P<0.05) but also increased the percentage of senescence-associated beta-galactosidase (SA-beta-gal) stained cells and expression of p21cip1 and p27kip1 (P<0.05), indicating that high glucose promoted rat GMC senescence. Knocking down Cx43 gene expression significantly increased the percentage of SA-beta-gal stained cells and p27kip1 and p21cip1 expression in GMC (P<0.05), whereas overexpression of Cx43 significantly decreased the percentage of SA-beta-gal stained cells (P<0.05). These results demonstrate for the first time that downregulation of Cx43 expression by high glucose promotes the senescence of GMC, which may be involved in the pathogenesis of diabetic nephropathy.

Polymorphisms in human connexin40 gene promoter are associated with increased risk of hypertension in men

OBJECTIVE

Gap junctions, formed by connexins (Cx), are important in the regulation of vascular tone. Previously, we reported two closely linked polymorphisms (-44G --> A and +71A --> G) within regulatory regions of the gene for Cx40, a major connexin in the vascular wall and the kidney. In the present study, we examined the hypothesis that these polymorphic variants are associated with hypertension and that they interact with blood pressure in healthy individuals.

METHODS

Cx40 genotypes were determined in 191 subjects with essential hypertension, 198 normotensive individuals, and a healthy control population (178 twin pairs, 108 monozygotic, 70 dizygotic).

RESULTS

We found a significant contribution of the minor Cx40 allele or genotype (-44AA/+71GG) to the risk of hypertension in men (P = 0.013 or P = 0.035; odds ratio, 1.87 or 2.10, respectively), but not in women. Moreover, in the healthy control population a significant effect of Cx40 genotype and sex on systolic blood pressure was found (P < 0.05 and P < 0.0001, respectively). Women carrying the minor Cx40 genotype had significantly higher systolic blood pressure compared with non-carriers (P < 0.05). In men, systolic blood pressure in carriers of the minor Cx40 genotype was not significantly different from the other two genotypes, possibly because of the small number of men in this group. However, men carrying the -44GA/+71AG genotype had higher standing systolic blood pressure compared with the more common Cx40 genotype (-44GG; P = 0.033).

CONCLUSION

These findings suggest that the Cx40 polymorphisms may form a genetic susceptibility factor for essential hypertension in men.

Connexin43-dependent mechanism modulates renin secretion and hypertension

To investigate the function of Cx43 during hypertension, we studied the mouse line Cx43KI32 (KI32), in which the coding region of Cx32 replaces that of Cx43. Within the kidneys of homozygous KI32 mice, Cx32 was expressed in cortical and medullary tubules, as well as in some extra- and intraglomerular vessels, i.e., at sites where Cx32 and Cx43 are found in WT mice. Under such conditions, renin expression was much reduced compared with that observed in the kidneys of WT and heterozygous KI32 littermates. After exposure to a high-salt diet, all mice retained a normal blood pressure. However, whereas the levels of renin were significantly reduced in the kidneys of WT and heterozygous KI32 mice, reaching levels comparable to those observed in homozygous littermates, they were not further affected in the latter animals. Four weeks after the clipping of a renal artery (the 2-kidney, 1-clip [2K1C] model), 2K1C WT and heterozygous mice showed an increase in blood pressure and in the circulating levels of renin, whereas 2K1C homozygous littermates remained normotensive and showed unchanged plasma renin activity. Hypertensive, but not normotensive, mice also developed cardiac hypertrophy. The data indicate that replacement of Cx43 by Cx32 is associated with decreased expression and secretion of renin, thus preventing the renin-dependent hypertension that is normally induced in the 2K1C model.

Differential connexin expression in preglomerular and postglomerular vasculature: accentuation during diabetes

BACKGROUND

Gap junctions may play an important role in regulating renal blood flow and glomerular responses. We have therefore made a comprehensive analysis of connexin expression in the renal vasculature of control and diabetic mice since elevated glucose has been reported to down-regulate connexin 43 in vascular cells in vitro.

METHODS

Connexin distribution was determined with immunohistochemistry using subtype-specific and cell type-specific antibodies. Diabetes was induced with streptozotocin (120/80 mg/kg, intraperitoneally) in C57BL/6 mice.

RESULTS

Connexins 37, 40, and 43 were expressed in endothelial cells of the renal, lobar, arcuate, and interlobular arteries and afferent arterioles, although connexin 43 was weak in the renal and arcuate arteries. Connexin 37 was detected in the media of arcuate, interlobular arteries and afferent arterioles and connexins 37 and 40 were found in renin-secreting cells. Both connexins 37 and 40 were expressed in extraglomerular mesangial cells, connexin 40 was abundantly expressed in intraglomerular mesangial cells, but connexin 37 was limited to mesangial cells at the vascular pole. In contrast, only connexin 43 was detected in endothelial cells of efferent arterioles and there was no connexin staining in the media. In diabetes, connexin 40 was expressed in smooth muscle cells along afferent arterioles, glomerular connexin staining was more extensive and connexin 43 was detected in renin-secreting cells. In contrast connexin 43 expression in endothelial cells of efferent arterioles was markedly reduced.

CONCLUSION

The renal vasculature and mesangial cells are well coupled on the preglomerular side but there is little evidence that the coupling extends into the efferent arteriole. This pattern of cell coupling is accentuated during diabetes.

Enhancer-dependent inhibition of mouse renin transcription by inflammatory cytokines

Inflammatory cytokines have been shown to inhibit renin gene expression in the kidney in vivo and the kidney tumor-derived As4.1 cell line. In this report, we show that cytokines oncostatin M (OSM), IL-6, and IL-1beta inhibit transcriptional activity associated with 4.1 kb of the mouse renin 5'-flanking sequence in As4.1 cells. The 242-bp enhancer (-2866 to -2625 bp) is sufficient to mediate the observed inhibitory effects. Sequences within the enhancer required for inhibition by each of these cytokines have been determined by deletional and mutational analysis. Results indicate that a 39-bp region (CEC) containing a cAMP-responsive element, an E-box, and a steroid receptor-binding site, previously identified as the most critical elements for enhancer activity, is sufficient for the inhibition induced by IL-1beta. However, mutation of each of the three component sites does not abolish the inhibition by IL-1beta, suggesting that the target(s) of cytokine action may not be the transcription factors binding directly to these sites. This CEC region is also critical, but not sufficient, for the inhibition mediated by OSM and IL-6. These data suggest that the direct target of the associated cytokines may be coactivators interacting with transcription factors binding at the enhancer. Finally, we show that OSM treatment caused a 17-fold increase in promoter activity when only 2,625 bp of the Ren-1(c) flanking sequence were tested, in which the enhancer is not present. Three regions including -2625 to -1217 bp, the HOX.PBX binding site at -60 bp, and -59 to +6 bp have been found to contribute to this induction.