Connexin43-dependent mechanism modulates renin secretion and hypertension

Activation of Pannexin-1 channels causes cell dysfunction and damage in mesangial cells derived from angiotensin II-exposed mice

Chronic kidney disease (CKD) is a prevalent health concern associated with various pathological conditions, including hypertensive nephropathy. Mesangial cells are crucial in maintaining glomerular function, yet their involvement in CKD pathogenesis remains poorly understood. Recent evidence indicates that overactivation of Pannexin-1 (Panx1) channels could contribute to the pathogenesis and progression of various diseases. Although Panx1 is expressed in the kidney, its contribution to the dysfunction of renal cells during pathological conditions remains to be elucidated. This study aimed to investigate the impact of Panx1 channels on mesangial cell function in the context of hypertensive nephropathy. Using an Ang II-infused mouse model and primary mesangial cell cultures, we demonstrated that in vivo exposure to Ang II sensitizes cultured mesangial cells to show increased alterations when they are subjected to subsequent in vitro exposure to Ang II. Particularly, mesangial cell cultures treated with Ang II showed elevated activity of Panx1 channels and increased release of ATP. The latter was associated with enhanced basal intracellular Ca2+ ([Ca2+]i) and increased ATP-mediated [Ca2+]i responses. These effects were accompanied by increased lipid peroxidation and reduced cell viability. Crucially, all the adverse impacts evoked by Ang II were prevented by the blockade of Panx1 channels, underscoring their critical role in mediating cellular dysfunction in mesangial cells. By elucidating the mechanisms by which Ang II negatively impacts mesangial cell function, this study provides valuable insights into the pathogenesis of renal damage in hypertensive nephropathy.

Dietary Potassium Supplementation Reduces Chronic Kidney Lesions Independent of Blood Pressure in Deoxycorticosterone-Acetate and High Sodium Chloride-Treated Mice

We have previously shown that an excess of deoxycorticosterone acetate and high sodium chloride intake (DOCA/salt) in one-renin gene mice induces a high urinary Na/K ratio, hypokalemia, and cardiac and renal hypertrophy in the absence of hypertension. Dietary potassium supplementation prevents DOCA/salt-induced pathological processes. In the present study, we further study whether DOCA/salt-treated mice progressively develop chronic inflammation and fibrosis in the kidney and whether dietary potassium supplementation can reduce the DOCA/salt-induced renal pathological process. Results showed that (1) long-term DOCA/salt-treated one-renin gene mice developed severe kidney injuries including tubular/vascular hypertrophy, mesangial/interstitial/perivascular fibrosis, inflammation (lymphocyte's immigration), proteinuria, and high serum creatinine in the absence of hypertension; (2) there were over-expressed mRNAs of plasminogen activator inhibitor-1 (PAI-1), fibronectin, collagen type I and III, interferon-inducible protein-10 (IP-10), monocyte chemotactic protein-1 (MCP1), transforming growth factor-β (TGF-β), tumor necrosis factor-alpha (TNF-α), osteopontin, Nuclear factor kappa B (NF-κB)/P65, and intercellular adhesion molecule (ICAM)-1; and (3) dietary potassium supplementation normalized urinary Na/K ratio, hypokalemia, proteinuria, and serum creatinine, reduced renal hypertrophy, inflammations, and fibrosis, and down-regulated mRNA expression of fibronectin, Col-I and III, TGF-β, TNF-α, osteopontin, and ICAM without changes in the blood pressure. The results provide new evidence that potassium and sodium may modulate proinflammatory and fibrotic genes, leading to chronic renal lesions independent of blood pressure.

Hypertensive Nephropathy: Unveiling the Possible Involvement of Hemichannels and Pannexons

Hypertension is one of the most common risk factors for developing chronic cardiovascular diseases, including hypertensive nephropathy. Within the glomerulus, hypertension causes damage and activation of mesangial cells (MCs), eliciting the production of large amounts of vasoactive and proinflammatory agents. Accordingly, the activation of AT1 receptors by the vasoactive molecule angiotensin II (AngII) contributes to the pathogenesis of renal damage, which is mediated mostly by the dysfunction of intracellular Ca²⁺ ([Ca²⁺]_i) signaling. Similarly, inflammation entails complex processes, where [Ca²⁺]_i also play crucial roles. Deregulation of this second messenger increases cell damage and promotes fibrosis, reduces renal blood flow, and impairs the glomerular filtration barrier. In vertebrates, [Ca²⁺]_i signaling depends, in part, on the activity of two families of large-pore channels: hemichannels and pannexons. Interestingly, the opening of these channels depends on [Ca²⁺]_i signaling. In this review, we propose that the opening of channels formed by connexins and/or pannexins mediated by AngII induces the ATP release to the extracellular media, with the subsequent activation of purinergic receptors. This process could elicit Ca²⁺ overload and constitute a feed-forward mechanism, leading to kidney damage.

TNF-α Plus IL-1β Induces Opposite Regulation of Cx43 Hemichannels and Gap Junctions in Mesangial Cells through a RhoA/ROCK-Dependent Pathway

Connexin 43 (Cx43) is expressed in kidney tissue where it forms hemichannels and gap junction channels. However, the possible functional relationship between these membrane channels and their role in damaged renal cells remains unknown. Here, analysis of ethidium uptake and thiobarbituric acid reactive species revealed that treatment with TNF-α plus IL-1β increases Cx43 hemichannel activity and oxidative stress in MES-13 cells (a cell line derived from mesangial cells), and in primary mesangial cells. The latter was also accompanied by a reduction in gap junctional communication, whereas Western blotting assays showed a progressive increase in phosphorylated MYPT (a target of RhoA/ROCK) and Cx43 upon TNF-α/IL-1β treatment. Additionally, inhibition of RhoA/ROCK strongly antagonized the TNF-α/IL-1β-induced activation of Cx43 hemichannels and reduction in gap junctional coupling. We propose that activation of Cx43 hemichannels and inhibition of cell-cell coupling during pro-inflammatory conditions could contribute to oxidative stress and damage of mesangial cells via the RhoA/ROCK pathway.

Endothelial Connexins in Developmental and Pathological Angiogenesis

Connexins (Cxs) constitute a large family of transmembrane proteins that form gap junction channels, which enable the direct transfer of small signaling molecules from cell to cell. In blood vessels, Cx channels allow the endothelial cells (ECs) to respond to external and internal cues as a whole and, thus, contribute to the maintenance of vascular homeostasis. While the role of Cxs has been extensively studied in large arteries, a growing body of evidence suggests that they also play a role in the formation of microvascular networks. Since the formation of new blood vessels requires the coordinated response of ECs to external stimuli, endothelial Cxs may play an important role there. Recent studies in developmental and pathologic models reveal that EC Cxs regulate physiological and pathological angiogenesis through canonical and noncanonical functions, making these proteins potential therapeutic targets for the development of new strategies aimed at a better control of angiogenesis.

A role of connexin 43 on the drug-induced liver, kidney, and gastrointestinal tract toxicity with associated signaling pathways

Drug-induced organ toxicity/injury, especially in the liver, kidney, and gastrointestinal tract, is a systematic disorder that causes oxidative stress formation and inflammation resulting in cell death and organ failure. Current therapies target reactive oxygen species (ROS) scavenging and inhibit inflammatory factors in organ injury to restore the functions and temporary relief. Organ cell function and tissue homeostasis are maintained through gap junction intercellular communication, regulating connexin hemichannels. Mis-regulation of such connexin, especially connexin (Cx) 43, affects a comprehensive process, including cell differentiation, inflammation, and cell death. Aim to describe knowledge about the importance of connexin role and insights therapeutic targeting. Cx43 misregulation has been implicated in recent decades in various diseases. Moreover, in recent years there is increasing evidence that Cx43 is involved in the toxicity process, including hepatic, renal, and gastrointestinal disorders. Cx43 has the potential to initiate the immune system to cause cell death, which has been activated in the acceleration of apoptosis, necroptosis, and autophagy signaling pathway. So far, therapies targeting Cx43 have been under inspection and are subjected to clinical trial phases. This review elucidates the role of Cx43 in drug-induced vital organ injury, and recent reports compromise its function in the major signaling pathways.

Connexin 43 prevents the progression of diabetic renal tubulointerstitial fibrosis by regulating the SIRT1-HIF-1α signaling pathway

Hyperglycemia-induced renal epithelial-to-mesenchymal transition (EMT) is a key pathological factor in diabetic renal tubulointerstitial fibrosis (RIF). Our previous studies have shown that connexin 43 (Cx43) activation attenuated the development of diabetic renal fibrosis. However, whether Cx43 regulates the EMT of renal tubular epithelial cells (TECs) and the pathological process of RIF under the diabetic conditions remains to be elucidated. In the present study, we identified that Cx43 protein expression was down-regulated in the kidney tissues of db/db mice as well as in high glucose (HG)-induced NRK-52E cells. Overexpression of Cx43 improved renal function in db/db spontaneous diabetic model mice, increased SIRT1 levels, decreased hypoxia-inducible factor (HIF)-1α expression, and reduced production of EMT markers and extracellular matrix (ECM) components. Additionally, Cx43 overexpression inhibited the EMT process and reduced the expression of ECM components such as fibronectin (FN), Collagen I, and Collagen IV in HG-induced NRK-52E cells, whereas Cx43 deficiency had the opposite effects. Mechanistically, Cx43 in a carboxyl-terminal signal transduction-dependent manner could up-regulate SIRT1 expression and enhance SIRT1-dependent deacetylation of HIF-1α to reduce HIF-1α activity, which eventually ameliorated renal EMT and diabetic RIF. Our study indicates the essential role of Cx43 in regulating renal EMT and diabetic RIF via regulating the SIRT1-HIF-1α signaling pathway and provides an experimental basis for Cx43 as a potential target for diabetic nephropathy (DN).

Connexin43 is differentially distributed within renal vasculature and mediates profibrotic differentiation in medullary fibroblasts

Connexins (Cxs) form gap junctions for intercellular exchange of inorganic ions and messenger molecules. In the kidney, Cxs play essential roles within its compartments, but data on the precise cellular localization and cell type-related function of their isoforms are scarce. We tested whether Cx43 distribution is restricted to vascular and interstitial cells and whether medullary fibroblasts express Cx43 to coordinate profibrotic signaling. Confocal immunofluorescence techniques, ultrastructural labeling, and functional experiments in cell culture were performed. Cx43 was chiefly expressed in the vasculature but was absent from tubular epithelia. All arterial, arteriolar, and lymphatic endothelia showed continuous Cx43 signal along their borders. In the inner medulla, only the interstitium showed Cx43 signals, which were assigned to fibroblasts and their processes. Cultured Cx43-expressing medullary fibroblasts served to study the role of gap junctions in a profibrotic context. In a dye spreading assay, Cx43-sensitive diffusion of Lucifer yellow was dependent on gap junctional passage. The addition of transforming growth factor-β1 (5 ng/mL for 48 h) activated Cx43 biosynthesis and caused Cx43-sensitive transformation of the fibroblasts into a myofibroblast phenotype. This suggested that Cx43 gap junctional channels enable the coordination of profibrotic signaling between cells of the medullary interstitium. In summary, we demonstrate the presence of Cx43-expressing gap junctions within the two major renal compartments, the vasculature and interstitium. Endothelial Cx43 likely provides functions of an earlier-defined "electrical syncytium" within the vascular wall. Additionally, Cx43 facilitates profibrotic signaling between medullary interstitial fibroblasts.

Genetic deletion of connexin 37 causes polyuria and polydipsia

The connexin 37 (Cx37) channel is clustered at gap junctions between cells in the renal vasculature or the renal tubule where it is abundant in basolateral cell interdigitations and infoldings of epithelial cells in the proximal tubule, thick ascending limb, distal convoluted tubule and collecting duct; however, physiological data regarding its role are limited. In this study, we investigated the role of Cx37 in fluid homeostasis using mice with a global deletion of Cx37 (Cx37-/- mice). Under baseline conditions, Cx37-/- had ~40% higher fluid intake associated with ~40% lower urine osmolality compared to wild-type (WT) mice. No differences were observed between genotypes in urinary adenosine triphosphate or prostaglandin E2, paracrine factors that alter renal water handling. After 18-hours of water deprivation, plasma aldosterone and urine osmolality increased significantly in Cx37-/- and WT mice; however, the latter remained ~375 mmol/kg lower in Cx37-/- mice, an effect associated with a more pronounced body weight loss despite higher urinary AVP/creatinine ratios compared to WT mice. Consistent with this, fluid intake in the first 3 hours after water deprivation was 37% greater in Cx37-/- vs WT mice. Cx37-/- mice showed significantly lower renal AQP2 abundance and AQP2 phosphorylation at serine 256 than WT mice in response to vehicle or dDAVP, suggesting a partial contribution of the kidney to the lower urine osmolality. The abundance and responses of the vasopressin V2 receptor, AQP3, NHE3, NKCC2, NCC, H+-ATPase, αENaC, γENaC or Na+/K+-ATPase were not significantly different between genotypes. In summary, these results demonstrate that Cx37 is important for body water handling.

Polydatin attenuates renal fibrosis in diabetic mice through regulating the Cx32-Nox4 signaling pathway

We previously found that polydatin could attenuate renal oxidative stress in diabetic mice and improve renal fibrosis. Recent evidence shows that NADPH oxidase 4 (Nox4)-derived reactive oxygen species (ROS) contribute to inflammatory and fibrotic processes in diabetic kidneys. In this study we investigated whether polydatin attenuated renal fibrosis by regulating Nox4 in vitro and in vivo. In high glucose-treated rat glomerular mesangial cells, polydatin significantly decreased the protein levels of Nox4 by promoting its K48-linked polyubiquitination, thus inhibited the production of ROS, and eventually decreasing the expression of fibronectin (FN) and intercellular adhesion molecule-1 (ICAM-1), the main factors that exacerbate diabetic renal fibrosis. Overexpression of Nox4 abolished the inhibitory effects of polydatin on FN and ICAM-1 expression. In addition, the expression of Connexin32 (Cx32) was significantly decreased, which was restored by polydatin treatment. Cx32 interacted with Nox4 and reduced its protein levels. Knockdown of Cx32 abolished the inhibitory effects of polydatin on the expression of FN and ICAM-1. In the kidneys of streptozocin-induced diabetic mice, administration of polydatin (100 mg·kg-1·d-1, ig, 6 days a week for 12 weeks) increased Cx32 expression and reduced Nox4 expression, decreased renal oxidative stress levels and the expression of fibrotic factors, eventually attenuating renal injury and fibrosis. In conclusion, polydatin promotes K48-linked polyubiquitination and degradation of Nox4 by restoring Cx32 expression, thereby decreasing renal oxidative stress levels and ultimately ameliorating the pathological progress of diabetic renal fibrosis. Thus, polydatin reduces renal oxidative stress levels and attenuates diabetic renal fibrosis through regulating the Cx32-Nox4 signaling pathway.

Connexin Signaling in the Juxtaglomerular Apparatus (JGA) of Developing, Postnatal Healthy and Nephrotic Human Kidneys

Our study analyzed the expression pattern of different connexins (Cxs) and renin positive cells in the juxtaglomerular apparatus (JGA) of developing, postnatal healthy human kidneys and in nephrotic syndrome of the Finnish type (CNF), by using double immunofluorescence, electron microscopy and statistical measuring. The JGA contained several cell types connected by Cxs, and consisting of macula densa, extraglomerular mesangium (EM) and juxtaglomerular cells (JC), which release renin involved in renin-angiotensin- aldosteron system (RAS) of arterial blood pressure control. During JGA development, strong Cx40 expression gradually decreased, while expression of Cx37, Cx43 and Cx45 increased, postnatally showing more equalized expression patterning. In parallel, initially dispersed renin cells localized to JGA, and greatly increased expression in postnatal kidneys. In CNF kidneys, increased levels of Cx43, Cx37 and Cx45 co-localized with accumulations of renin cells in JGA. Additionally, they reappeared in extraglomerular mesangial cells, indicating association between return to embryonic Cxs patterning and pathologically changed kidney tissue. Based on the described Cxs and renin expression patterning, we suggest involvement of Cx40 primarily in the formation of JGA in developing kidneys, while Cx37, Cx43 and Cx45 might participate in JGA signal transfer important for postnatal maintenance of kidney function and blood pressure control.

Regulation of connexins genes expression contributes to reestablishes tissue homeostasis in a renovascular hypertension model

Connexins (Cx) are essential for cardiovascular regulation and maintenance of cardio-renal response involving the natriuretic peptide family. Changes in the expression of connexins promote intercellular communication dysfunction and may induce hypertension, atherosclerosis, and several other vascular diseases. This study analyzed the expression of the genes involved in the renin-angiotensin system (RAS) and the relation of the connexins gene expression with the renovascular hypertension 2K1C in different tissues. The insertion of a silver clip induced renovascular hypertension 2K1C into the left renal artery. Biochemical measurements were made using commercial kits. Gene expression was evaluated in the liver, heart, and kidneys by RT-PCR. The genes investigated were LDLr, Hmgcr, Agt, Ren, Ace, Agtr1a, Anp, Bnp, Npr1, Cx26, Cx32, Cx37, Cx40 and Cx43. All genes involved in the RAS presented increased transcriptional levels in the 2K1C group, except hepatic Agt. The natriuretic peptides (Anp; Bnp) and the receptor genes (Npr1) appeared to increase in the heart, however, Npr1 decreased in the kidneys. In hepatic tissue, hypertension promoted increased expression of Cx32, Cx37, and Cx40 genes however, Cx26 and Cx43 genes were not influenced. Expression was upregulated for Cx37 and Cx43 in cardiac tissue in the 2K1C group, but Cx40 did not demonstrate any difference between groups. The stenotic kidney showed an upregulated expression for Cx37 vs Sham and contralateral kidney, although Cx40 and Cx43 were downregulated. Hypertension did not modify the transcriptional expression of Cx26 and Cx32. Therefore, this study indicated that RAS and cardiac response were regulated transcriptionally by renovascular hypertension 2K1C. Moreover, the results of connexin gene expression demonstrated differential transcriptional regulation in different tissues studied and suggest a relationship between cardiac and renal physiological changes as an adaptive mechanism to the hypertensive state.

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.

Role of a RhoA/ROCK-Dependent Pathway on Renal Connexin43 Regulation in the Angiotensin II-Induced Renal Damage

In various models of chronic kidney disease, the amount and localization of Cx43 in the nephron is known to increase, but the intracellular pathways that regulate these changes have not been identified. Therefore, we proposed that: "In the model of renal damage induced by infusion of angiotensin II (AngII), a RhoA/ROCK-dependent pathway, is activated and regulates the abundance of renal Cx43". In rats, we evaluated: 1) the time-point where the renal damage induced by AngII is no longer reversible; and 2) the involvement of a RhoA/ROCK-dependent pathway and its relationship with the amount of Cx43 in this irreversible stage. Systolic blood pressure (SBP) and renal function (urinary protein/urinary creatinine: Uprot/UCrea) were evaluated as systemic and organ outcomes, respectively. In kidney tissue, we also evaluated: 1) oxidative stress (amount of thiobarbituric acid reactive species), 2) inflammation (immunoperoxidase detection of the inflammatory markers ED-1 and IL-1β), 3) fibrosis (immune detection of type III collagen; Col III) and 4) activity of RhoA/ROCK (amount of phosphorylated MYPT1; p-MYPT1). The ratio Uprot/UCrea, SBP, oxidative stress, inflammation, amount of Cx43 and p-MYPT1 remained high 2 weeks after suspending AngII treatment in rats treated for 4 weeks with AngII. These responses were not observed in rats treated with AngII for less than 4 weeks, in which all measurements returned spontaneously close to the control values after suspending AngII treatment. Rats treated with AngII for 6 weeks and co-treated for the last 4 weeks with Fasudil, an inhibitor of ROCK, showed high SBP but did not present renal damage or increased amount of renal Cx43. Therefore, renal damage induced by AngII correlates with the activation of RhoA/ROCK and the increase in Cx43 amounts and can be prevented by inhibitors of this pathway.

Connexin37-Dependent Mechanisms Selectively Contribute to Modulate Angiotensin II -Mediated Hypertension

Background Gap junction channels made of Connexin37 (Cx37) are expressed by aortic endothelial and smooth muscle cells of hypertensive mice, as well as by the renin-secreting cells of kidneys. Methods and Results To decipher whether Cx37 has any role in hypertension, angiotensin II (Ang II ) was infused in normotensive wild-type and Cx37-deficient mice (Cx37-/-). After 2 to 4 weeks, the resulting increase in blood pressure was lower in Cx37-/- than in wild-type mice, suggesting an alteration in the Ang II response. To investigate this possibility, mice were submitted to a 2-kidney, 1-clip procedure, a renin-dependent model of hypertension. Two weeks after this clipping, Cx37-/- mice were less hypertensive than wild-type mice and, 2 weeks later, their blood pressure had returned to control values, in spite of abnormally high plasma renin levels. In contrast, Cx37-/- and wild-type mice that received N-nitro-l-arginine-methyl-ester, a renin-independent model of hypertension, featured a similar and sustained increase in blood pressure. The data indicate that loss of Cx37 selectively altered the Ang II -dependent pathways. Consistent with this conclusion, aortas of Cx37-/- mice featured an increased basal expression of the Ang II type 2 receptors ( AT 2R), and increased transcripts levels of downstream signaling proteins, such as Cnksr1 and Ptpn6 ( SHP -1). Accordingly, the response of Cx37-/- mice aortas to an ex vivo Ang II exposure was altered, since phosphorylation levels of several proteins of the Ang II pathway ( MLC 2, ERK , and AKT ) remained unchanged. Conclusions These findings provide evidence that Cx37 selectively influences Ang II signaling, mostly via a modulation of the expression of the Ang II type 2 receptor.

The Functional Implications of Endothelial Gap Junctions and Cellular Mechanics in Vascular Angiogenesis

Angiogenesis—the sprouting and growth of new blood vessels from the existing vasculature—is an important contributor to tumor development, since it facilitates the supply of oxygen and nutrients to cancer cells. Endothelial cells are critically affected during the angiogenic process as their proliferation, motility, and morphology are modulated by pro-angiogenic and environmental factors associated with tumor tissues and cancer cells. Recent in vivo and in vitro studies have revealed that the gap junctions of endothelial cells also participate in the promotion of angiogenesis. Pro-angiogenic factors modulate gap junction function and connexin expression in endothelial cells, whereas endothelial connexins are involved in angiogenic tube formation and in the cell migration of endothelial cells. Several mechanisms, including gap junction function-dependent or -independent pathways, have been proposed. In particular, connexins might have the potential to regulate cell mechanics such as cell morphology, cell migration, and cellular stiffness that are dynamically changed during the angiogenic processes. Here, we review the implication for endothelial gap junctions and cellular mechanics in vascular angiogenesis.

Lack of connexin 40 decreases the calcium sensitivity of renin-secreting juxtaglomerular cells

The so-called calcium paradoxon of renin describes the phenomenon that exocytosis of renin from juxtaglomerular cells of the kidney is stimulated by lowering of the extracellular calcium concentration. The yet poorly understood effect of extracellular calcium on renin secretion appears to depend on the function of the gap junction protein connexin 40 (Cx40) in renin-producing cells. This study aimed to elucidate the role of Cx40 for the calcium dependency of renin secretion in more detail by investigating if Cx40 function is really essential for the influence of extracellular calcium on renin secretion, if and how Cx40 affects intracellular calcium dynamics in renin-secreting cells and if Cx40-mediated gap junctional coupling of renin-secreting cells with the mesangial cell area is relevant for the influence of extracellular calcium on renin secretion. Renin secretion was studied in isolated perfused mouse kidneys. Calcium measurements were performed in renin-producing cells of microdissected glomeruli. The ultrastructure of renin-secreting cells was examined by electron microscopy. We found that Cx40 was not essential for stimulation of renin secretion by lowering of the extracellular calcium concentration. Instead, Cx40 increased the sensitivity of renin secretion response towards lowering of the extracellular calcium concentration. In line, the sensitivity and dynamics of intracellular calcium in response to lowering of extracellular calcium were dampened when renin-secreting cells lacked Cx40. Disruption of gap junctional coupling of renin-secreting cells by selective deletion of Cx40 from mesangial cells, however, did not change the stimulation of renin secretion by lowering of the extracellular calcium concentration. Deletion of Cx40 from renin cells but not from mesangial cells was associated with a shift of renin expression from perivascular cells of afferent arterioles to extraglomerular mesangial cells. Our findings suggest that Cx40 is not directly involved in the regulation of renin secretion by extracellular calcium. Instead, it appears that in renin-secreting cells of the kidney lacking Cx40, intracellular calcium dynamics and therefore also renin secretion are desensitized towards changes of extracellular calcium. Whether the dampened calcium response of renin-secreting cells lacking Cx40 function results from a direct involvement of Cx40 in intracellular calcium regulation or from the cell type shift of renin expression from perivascular to mesangial cells remains to be clarified. In any case, Cx40-mediated gap junctional coupling between renin and mesangial cells is not relevant for the calcium paradoxon of renin secretion.

Angiotensin II-Induced Mesangial Cell Damaged Is Preceded by Cell Membrane Permeabilization Due to Upregulation of Non-Selective Channels

Connexin43 (Cx43), pannexin1 (Panx1) and P2X₇ receptor (P2X₇R) are expressed in kidneys and are known to constitute a feedforward mechanism leading to inflammation in other tissues. However, the possible functional relationship between these membrane channels and their role in damaged renal cells remain unknown. In the present work, we found that MES-13 cells, from a cell line derived from mesangial cells, stimulated with angiotensin II (AngII) developed oxidative stress (OS, thiobarbituric acid reactive species (TBARS) and generated pro-inflammatory cytokines (ELISA; IL-1β and TNF-α). The membrane permeability increased progressively several hours before the latter outcome, which was a response prevented by Losartan, indicating the involvement of AT1 receptors. Western blot analysis showed that the amount of phosphorylated MYPT (a substrate of RhoA/ROCK) and Cx43 increased progressively and in parallel in cells treated with AngII, a response followed by an increase in the amount in Panx1 and P2X₇R. Greater membrane permeability was partially explained by opening of Cx43 hemichannels (Cx43 HCs) and Panx1 channels (Panx1 Chs), as well as P2X₇Rs activation by extracellular ATP, which was presumably released via Cx HCs and Panx1 Chs. Additionally, inhibition of RhoA/ROCK blocked the progressive increase in membrane permeability, and the remaining response was explained by the other non-selective channels. The rise of activity in the RhoA/ROCK-dependent pathway, as well as in Cx HCs, P2X₇R, and to a minor extent in Panx1 Chs led to higher amounts of TBARS and pro-inflammatory cytokines. We propose that AngII-induced mesangial cell damage could be effectively inhibited by concomitantly inhibiting the RhoA/ROCK-dependent pathway and one or more non-selective channel(s) activated through this pathway.

Gap junction-mediated regulation of endothelial cellular stiffness

Endothelial monolayers have shown the ability to signal each other through gap junctions. Gap junction-mediated cell-cell interactions have been implicated in the modulation of endothelial cell functions during vascular inflammation. Inflammatory mediators alter the mechanical properties of endothelial cells, although the exact role of gap junctions in this process remains unclear. Here, we sought to study the role of gap junctions in the regulation of endothelial stiffness, an important physical feature that is associated with many vascular pathologies. The endothelial cellular stiffness of living endothelial cells was determined by using atomic force microscopy. We found that tumor necrosis factor-α transiently increased endothelial cellular stiffness, which is regulated by cytoskeletal rearrangement and cell-cell interactions. We explored the role of gap junctions in endothelial cellular stiffening by utilizing gap junction blockers, carbenoxolone, inhibitory anti-connexin 32 antibody or anti-connexin 43 antibody. Blockade of gap junctions induced the cellular stiffening associated with focal adhesion formation and cytoskeletal rearrangement, and prolonged tumor necrosis factor-α-induced endothelial cellular stiffening. These results suggest that gap junction-mediated cell-cell interactions play an important role in the regulation of endothelial cellular stiffness.

Gap junction proteins are key drivers of endocrine function

It has long been known that the main secretory cells of exocrine and endocrine glands are connected by gap junctions, made by a variety of connexin species that ensure their electrical and metabolic coupling. Experiments in culture systems and animal models have since provided increasing evidence that connexin signaling contributes to control the biosynthesis and release of secretory products, as well as to the life and death of secretory cells. More recently, genetic studies have further provided the first lines of evidence that connexins also control the function of human glands, which are central to the pathogenesis of major endocrine diseases. Here, we summarize the recent information gathered on connexin signaling in these systems, since the last reviews on the topic, with particular regard to the pancreatic beta cells which produce insulin, and the renal cells which produce renin. These cells are keys to the development of various forms of diabetes and hypertension, respectively, and combine to account for the exploding, worldwide prevalence of the metabolic syndrome. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Alterations in Fibronectin Type III Domain Containing 1 Protein Gene Are Associated with Hypertension

Multiple quantitative trait loci (QTLs) for blood pressure (BP) have been detected in rat models of human polygenic hypertension. Great challenges confronting us include molecular identifications of individual QTLs. We first defined the chromosome region harboring C1QTL1 to a segment of 1.9 megabases that carries 9 genes. Among them, we identified the gene encoding the fibronectin type III domain containing 1 protein (Fndc1)/activator of G protein signaling 8 (Ags8) to be the strongest candidate for C1QTL1, since numerous non-synonymous mutations are found. Moreover, the 5’ Fndc1/Ags8 putative promoter contains numerous mutations that can account for its differential expression in kidneys and the heart, prominent organs in modulating BP, although the Fndc1/Ags8 protein was not detectable in these organs under our experimental conditions. This work has provided the premier evidence that Fndc1/Ags8 is a novel and strongest candidate gene for C1QTL1 without completely excluding other 8 genes in the C1QTL1-residing interval. If proven true by future in vivo function studies such as single-gene Fndc1/Ags8 congenics, transgenesis or targeted-gene modifications, it might represent a part of the BP genetic architecture that operates in the upstream position distant from the end-phase physiology of BP control, since it activates a Gbetagamma component in a signaling pathway. Its functional role could validate the concept that a QTL in itself can influence BP ‘indirectly’ by regulating other genes downstream in a pathway. The elucidation of the mechanisms initiated by Fndc/Ags8 variations will reveal novel insights into the BP modulation via a regulatory hierarchy.

Flow stimulated endocytosis in the proximal tubule

PURPOSE OF REVIEW

The proximal tubule plays a critical role in the reabsorption of ions, solutes and low molecular weight proteins from the glomerular filtrate. Although the proximal tubule has long been known to acutely modulate ion reabsorption in response to changes in flow rates of the glomerular filtrate, it has only recently been discovered that proximal tubule cells can similarly adjust endocytic capacity in response to flow. This review synthesizes our current understanding of mechanosensitive regulation of endocytic capacity in proximal tubule epithelia and highlights areas of opportunity for future investigations.

RECENT FINDINGS

Recent studies have reported that the endocytic capacity of proximal tubule cells is dramatically increased upon exposure to flow and the accompanying fluid shear stress. Modulation of this pathway is dependent on increases in intracellular calcium initiated by bending of the primary cilium, and also requires purinergic receptor activation that is mediated by release of extracellular ATP. This article summarizes what is currently known about the signaling cascade that transduces changes in flow into alterations in endocytosis. We discuss the implications of this newly described regulatory pathway with respect to our understanding of protein retrieval by the kidney under normal conditions, and in diseases that present with low molecular weight proteinuria.

SUMMARY

Primary cilia act as mechanotransducers that modulate apical endocytic capacity in proximal tubule cells in response to changes in fluid shear stress.

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.

HIF-driven SF3B1 induces KHK-C to enforce fructolysis and heart disease

Fructose is a major component of dietary sugar and its overconsumption exacerbates key pathological features of metabolic syndrome. The central fructose-metabolising enzyme is ketohexokinase (KHK), which exists in two isoforms: KHK-A and KHK-C, generated through mutually exclusive alternative splicing of KHK pre-mRNAs. KHK-C displays superior affinity for fructose compared with KHK-A and is produced primarily in the liver, thus restricting fructose metabolism almost exclusively to this organ. Here we show that myocardial hypoxia actuates fructose metabolism in human and mouse models of pathological cardiac hypertrophy through hypoxia-inducible factor 1α (HIF1α) activation of SF3B1 and SF3B1-mediated splice switching of KHK-A to KHK-C. Heart-specific depletion of SF3B1 or genetic ablation of Khk, but not Khk-A alone, in mice, suppresses pathological stress-induced fructose metabolism, growth and contractile dysfunction, thus defining signalling components and molecular underpinnings of a fructose metabolism regulatory system crucial for pathological growth.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

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.

Endothelial connexin 32 regulates tissue factor expression induced by inflammatory stimulation and direct cell-cell interaction with activated cells

OBJECTIVE

Endothelial cell (EC) interacts with adjacent EC through gap junction, and abnormal expression or function of Cxs is associated with cardiovascular diseases. In patients with endothelial dysfunction, the up-regulation of tissue factor (TF) expression promotes the pathogenic activation of blood coagulation, however the relationship between gap junctions and TF expression in ECs remains uncharacterized. ECs express the gap junction (GJ) proteins connexin32 (Cx32), Cx37, Cx40 and Cx43. We investigated the role of endothelial gap junctions, particularly Cx32, in modulating TF expression during vascular inflammation.

METHODS AND RESULTS

Human umbilical vein endothelial cells (HUVECs) were stimulated with tumor necrosis factor-α (TNF-α) and TF activity was assessed in the presence of GJ blockers and an inhibitory anti-Cx32 monoclonal antibody. Treatment with GJ blockers and anti-Cx32 monoclonal antibody enhanced the TNF-α-induced TF activity and mRNA expression in HUVECs. TNF-α-activated effector HUVECs or mouse MS-1 cells were co-cultured with non-stimulated acceptor HUVECs and TF expression in acceptor HUVECs was detected. Effector EC induced TF expression in adjacent acceptor HUVECs through direct cell-cell interaction. Cell-cell interaction induced TF expression was reduced by anti-intercellular adhesion molecule-1 (ICAM1) monoclonal antibody. Soluble ICAM1-Fc fusion protein promotes TF expression. GJ blockers and anti-Cx32 monoclonal antibody enhanced TF expression induced by cell-cell interaction and ICAM1-Fc treatment.

CONCLUSION

Blockade of endothelial Cx32 increased TF expression induced by TNF-α stimulation and cell-cell interaction which was at least partly dependent upon ICAM1. These results suggest that direct Cx32-mediated interaction modulates TF expression in ECs during vascular inflammation.

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.

Connexin 43 is not essential for the control of renin synthesis and secretion

The juxtaglomerular areas of mammalian kidneys express the gap junction proteins connexin 37, 40, 43, and 45. Among these, Cx40 plays a major role for the function of juxtaglomerular renin-expressing cells, while Cx37 and Cx45 appear to be less relevant in this context. Since the role of the remaining Cx43 for the function of renin expression is not well understood, this study aimed to systematically characterize the direct role of Cx43 for renin expression and secretion. For this aim, we generated mice with endothelium and with renin cell-specific deletions of Cx43, and we characterized the regulation of renin expression and renin secretion in the kidneys of these mice on normal salt diet and during chronic challenge of the renin system by pretreatment of mice with a low-salt diet in combination with an angiotensin I-converting enzyme inhibitor. We found that renal renin mRNA abundance, plasma renin concentration, and systolic blood pressure did not differ between wild-type, Cx43(fl/fl) Ren1d(+/Cre) mice as well as Cx43(fl/fl) Tie-2(+/Cre) mice under basal conditions nor under chronic stimulation by salt depletion. The localization of renin-expressing cells was also regular in kidneys of all genotypes, and moreover, regulation of renin secretion by beta-adrenergic stimulation and renal perfusion pressure measured in isolated perfused kidneys of Cx43(fl/fl) Ren1d(+/Cre) and Cx43(fl/fl) Tie-2(+/Cre) mice was not different from control. We infer from these results that Cx43 plays if at all only a minor role for the functional control of renin-producing cells in the kidney.

Cell-cell communication in the kidney microcirculation

In the renal vasculature of humans, rats, and mice, at least four isoforms of Cx, Cxs 37, 40, 43, and 45 are expressed. In the ECs, Cx40 is the predominantly expressed Cx, whereas Cx45 is suggested to be expressed in the VSMCs. The preglomerular vasculature has a higher expression of Cxs than the postglomerular vasculature. Cxs form gap junctions between neighboring cells, and as in other organ systems, the major function of Cxs in the kidney appears to be mediation of intercellular communication. Cxs may also form hemichannels that allow cellular secretion of signaling molecules like ATP, and thereby mediate paracrine signaling. Renal Cxs facilitate vascular conduction, juxtaglomerlar apparatus calcium signaling, and enable ECs and VSMCs to communicate. Thus, current research suggests multiple roles for Cxs in important regulatory mechanisms within the kidney, including the renin-angiotensin system, TGF, and salt and water homeostasis. Interestingly, changes in the activity of the renin-angiotensin system or changes in blood pressure seem to affect the expression of the renal vascular Cxs. At the systemic level, renal Cxs may be involved in blood pressure regulation, and possibly in the pathogenesis of hypertension and diabetes.

Lipopolysaccharide effects on the proliferation of NRK52E cells via alternations in gap-junction function

BACKGROUND

Gap junctions regulate proper kidney function by facilitating intercellular communication, vascular conduction, and tubular purinergic signaling. However, no clear relationship has been described between gap-junction function and acute kidney injury induced by the endotoxin lipopolysaccharide (LPS).

METHODS

Normal rat kidney epithelial cells (NRK52E cells) were seeded at high and low densities to promote or impede gap-junction formation, respectively, and establish distinctive levels of intercellular communication in culture. Cells were then challenged with LPS at various concentrations (10-1,000 ng/mL). LPS-induced formation and function of gap junctions were assessed by measuring changes in cell proliferation and colony-forming rates, fluorescent dye transmission to adjacent cells, expression levels of connexin43, and repositioning of confluent cells in response to the gap junction inhibitor oleamide or agonist retinoic acid.

RESULTS

The cell proliferation rate and colony-forming rate of high- and low-density NRK52E cells were decreased upon LPS challenge, in a dose-dependent manner. The colony-forming rate of confluent high-density cells was significantly lower than that of low-density cells. Oleamide treatment raised the LPS-induced colony-forming rate of high-density cells, whereas retinoic acid decreased the rate. Neither oleamide nor retinoic acid significantly affected the LPS-induced colony-forming rate of low-density cells. Fluorescence transmission of high-density cells was reduced by LPS challenge, in a dose-dependent manner, but inclusion of retinoic acid increased the LPS-induced transmission of fluorescence. LPS challenge of either high- or low-density NRK52E cells resulted in down-regulated connexin43 expression.

CONCLUSION

Gap-junction function plays an important role in concentration-dependent cytotoxic effect of LPS on normal rat kidney cells in vitro.

Biological role of connexin intercellular channels and hemichannels

Gap junctions (GJ) and hemichannels (HC) formed from the protein subunits called connexins are transmembrane conduits for the exchange of small molecules and ions. Connexins and another group of HC-forming proteins, pannexins comprise the two families of transmembrane proteins ubiquitously distributed in vertebrates. Most cell types express more than one connexin or pannexin. While connexin expression and channel activity may vary as a function of physiological and pathological states of the cell and tissue, only a few studies suggest the involvement of pannexin HC in acquired pathological conditions. Importantly, genetic mutations in connexin appear to interfere with GJ and HC function which results in several diseases. Thus connexins could serve as potential drug target for therapeutic intervention. Growing evidence suggests that diseases resulting from HC dysfunction might open a new direction for development of specific HC reagents. This review provides a comprehensive overview of the current studies of GJ and HC formed by connexins and pannexins in various tissue and organ systems including heart, central nervous system, kidney, mammary glands, ovary, testis, lens, retina, inner ear, bone, cartilage, lung and liver. In addition, present knowledge of the role of GJ and HC in cell cycle progression, carcinogenesis and stem cell development is also discussed.

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.

Dual Effects of Bilirubin on the Proliferation of Rat Renal NRK52E Cells and ITS Association with Gap Junctions

OBJECTIVE

The effect of bilirubin on renal pathophysiology is controversial. This study aimed to observe the effects of bilirubin on the proliferation of normal rat renal tubular epithelial cell line (NRK52E) and its potential interplay with gap junction function.

METHODS

Cultured NRK52E cells, seeded respectively at high- or low- densities, were treated with varying concentrations of bilirubin for 24 hours. Cell injury was assessed by measuring cell viability and proliferation, and gap junction function was assessed by Parachute dye-coupling assay. Connexin 43 protein was assessed by Western blotting.

RESULTS

At doses from 17.1 to 513μmol/L, bilirubin dose-dependently enhanced cell viability and colony-formation rates when cells were seeded at either high- or low- densities (all p<0.05 vs. solvent group) accompanied with enhanced intercellular fluorescence transmission and increased Cx43 protein expression in high-density cells. However, the above effects of BR were gradually reversed when its concentration increased from 684 to 1026μmol/L. In high-density cells, gap junction inhibitor 12-O-tetradecanoylphorbol 13-acetate attenuated bilirubin-induced enhancement of colony-formation and fluorescence transmission. However, in the presence of high concentration bilirubin (1026μmol/L), activation of gap junction with retinoid acid decreased colony-formation rates.

CONCLUSION

Bilirubin can confer biphasic effects on renal NRK52E cell proliferation potentially by differentially affecting gap junction functions.

Inhibition of renin activity slows down the progression of HIV-associated nephropathy

In the present study, we evaluated the effect of inhibition of renin activity (aliskiren) on the progression of renal lesions in two different mouse models (Vpr and Tg26) of human immunodeficiency virus (HIV)-associated nephropathy (HIVAN). In protocol A, Vpr mice were fed either water (C-VprA) or doxycycline [Doxy (D-VprA)] in their drinking water for 6 wk. In protocols B and C, Vpr mice received either normal saline (C-VprB/C), Doxy + normal saline (D-VprB/C), or Doxy + aliskiren (AD-VprB/C) for 6 wk (protocol B) or 12 wk (protocol C). In protocols D and E, Vpr mice were fed Doxy for 6 wk followed by kidney biopsy. Subsequently, half of the mice were administered either normal saline (D-VprD/E) or aliskiren (AD-VprD/E) for 4 wk (protocol D) or 8 (protocol E) wk. All D-VprA mice showed renal lesions in the form of focal segmental glomerular sclerosis and dilatation of tubules. In protocols B and C, aliskiren diminished both progression of renal lesions and proteinuria. In protocol C, aliskiren also diminished (P < 0.01) the rise in blood urea. In all groups, Doxy-treated mice displayed increased serum ANG I levels (the product of plasma renin activity); on the other hand, all aliskiren-treated mice displayed diminished serum ANG I levels. Renal tissues of D-VprC displayed increased ANG II content; however, aliskiren attenuated renal tissue ANG II production in AD-VprC. In protocol D, AD-VprD showed a 24.2% increase in the number of sclerosed glomeruli compared with 139.2% increase in sclerosed glomeruli in D-VprD (P < 0.01) from their baseline. The attenuating effect of aliskiren on the progression of renal lesions continued in AD-VprE. Aliskiren also diminished blood pressure, proteinuria, and progression of renal lesions in Tg26 mice. These findings indicate that inhibition of renin activity has a potential to slow down the progression of HIVAN.

Functional redundancy and compensation among members of gap junction protein families?

Gap junctions are intercellular conduits for small molecules made up by protein subunits called connexins. A large number of connexin genes were found in mouse and man, and most cell types express several connexins, lending support to the view that redundancy and compensation among family members exist. This review gives an overview of the current knowledge on redundancy and functional compensation - or lack thereof. It takes into account the different properties of connexin subunits which comprise gap junctional intercellular channels, but also the compatibility of connexins in gap junctions. Most insight has been gained by the investigation of mice deficient for one or more connexins and transgenic mice with functional replacement of one connexin gene by another. Most single deficient mice show phenotypical alterations limited to critical developmental time points or to specific organs and tissues, while mice doubly deficient for connexins expressed in the same cell type usually show more severe phenotypical alterations. Replacement of a connexin by another connexin in some cases gave rise to rescue of phenotypical alterations of connexin deficiencies, which were restricted to specific tissues. In many tissues, connexin substitution did not restore phenotypical alterations of connexin deficiencies, indicating that connexins are specialized in function. In some cases, fatal consequences arose from the replacement. The current consensus gained from such studies is that redundancy and compensation among connexins exists at least to a limited extent. 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.

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.

Assessment of human islet labeling with clinical grade iron nanoparticles prior to transplantation for graft monitoring by MRI

Ex vivo labeling of islets with superparamagnetic iron oxide (SPIO) nanoparticles allows posttransplant MRI imaging of the graft. In the present study, we compare two clinical grade SPIOs (ferucarbotran and ferumoxide) in terms of toxicity, islet cellular uptake, and MRI imaging. Human islets (80-90% purity) were incubated for 24 h with various concentrations of SPIOs (14-280 μg/ml of iron). Static incubations were performed, comparing insulin response to basal (2.8 mM) or high glucose stimulation (16.7 mM), with or without cAMP stimulation. Insulin and Perl's (assessment of iron content) staining were performed. Electronic microscopy analysis was performed. Labeled islets were used for in vitro or in vivo imaging in MRI 1.5T. Liver section after organ removal was performed in the same plane as MRI imaging to get a correlation between histology and radiology. Postlabeling islet viability (80 ± 10%) and function (in vitro static incubation and in vivo engraftment of human islets in nude mice) were similar in both groups. Iron uptake assessed by electron microscopy showed iron inclusions within the islets with ferucarbotran, but not with ferumoxide. MRI imaging (1.5T) of phantoms and of human islets transplanted in rats, demonstrated a strong signal with ferucarbotran, but only a weak signal with ferumoxide. Signal persisted for >8 weeks in the absence of rejection. An excellent correlation was observed between radiologic images and histology. The hepatic clearance of intraportally injected ferucarbotran was faster than that of ferumoxide, generating less background. A rapid signal decrease was observed in rejecting xenogeneic islets. According to the present data, ferucarbotran is the most appropriate of available clinical grade SPIOs for human islet imaging.

An angiotensin II- and NF-kappaB-dependent mechanism increases connexin 43 in murine arteries targeted by renin-dependent hypertension

AIMS

Connexins (Cxs) play a role in the contractility of the aorta wall. We investigated how connexins of the endothelial cells (ECs; Cx37, Cx40) and smooth muscle cells (SMCs; Cx43, Cx45) of the aorta change during renin-dependent and -independent hypertension.

METHODS AND RESULTS

We subjected both wild-type (WT) mice and mice lacking Cx40 (Cx40(-/-)), to either a two-kidney, one-clip procedure or to N-nitro-l-arginine-methyl-ester treatment, which induce renin-dependent and -independent hypertension, respectively. All hypertensive mice featured a thickened aortic wall, increased levels of Cx37 and Cx45 in SMC, and of Cx40 in EC (except in Cx40(-/-) mice). Cx43 was up-regulated, with no effect on its S368 phosphorylation, only in the SMCs of renin-dependent models of hypertension. Blockade of the renin-angiotensin system of Cx40(-/-) mice normalized blood pressure and prevented both aortic thickening and Cx alterations. Ex vivo exposure of WT aortas, carotids, and mesenteric arteries to physiologically relevant levels of angiotensin II (AngII) increased the levels of Cx43, but not of other Cx. In the aortic SMC line of A7r5 cells, AngII activated kinase-dependent pathways and induced binding of the nuclear factor-kappa B (NF-kappaB) to the Cx43 gene promoter, increasing Cx43 expression.

CONCLUSION

In both large and small arteries, hypertension differently regulates Cx expression in SMC and EC layers. Cx43 is selectively increased in renin-dependent hypertension via an AngII activation of the extracellular signal-regulated kinase and NF-kappaB pathways.

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.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

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.

The role of calcium in the regulation of renin secretion

Renin is the enzyme which is the rate-limiting step in the formation of the hormone angiotensin II. Therefore, the regulation of renin secretion is critical in understanding the control of the renin-angiotensin-aldosterone system and its many biological and pathological actions. Renin is synthesized, stored in, and released from the juxtaglomerular (JG) cells of the kidney. While renin secretion is positively regulated by the "second messenger" cAMP, unlike most secretory cells, renin secretion from the JG cell is inversely related to the extracellular and intracellular calcium concentrations. This novel relationship is referred to as the "calcium paradox." This review will address observations made over the past 30 years regarding calcium and the regulation of renin secretion, and focus on recent observations which address this scientific conundrum. These include 1) receptor-mediated pathways for changing intracellular calcium; 2) the discovery of a calcium-inhibitable isoform of adenylyl cyclase associated with renin in the JG cells; 3) calcium-sensing receptors in the JG cells; 4) calcium-calmodulin-mediated signals; 5) the role of phosphodiesterases; and 6) connexins, gap junctions, calcium waves, and the cortical extracellular calcium environment. While cAMP is the dominant second messenger for renin secretion, calcium appears to modulate the integrated activities of the enzymes, which balance cAMP synthesis and degradation. Thus this review concludes that calcium modifies the amplitude of cAMP-mediated renin-signaling pathways. While calcium does not directly control renin secretion, increased calcium inhibits and decreased calcium amplifies cAMP-stimulated renin secretion.

Connexin 37 is localized in renal epithelia and responds to changes in dietary salt intake

Connexins are the main components of gap junction channels, which are important for intercellular communication. In the kidney, several members of the connexin (Cx) family have been identified. Renal vascular expression and hemodynamic impacts have so far been shown for Cx37, Cx40, and Cx43. Additionally, Cx30, Cx30.3, and Cx43 have been identified to be part of tubular epithelial gap junctions and/or hemichannels. However, the localization and role of other Cx family members in renal epithelial structures remain undetermined. We aimed to localize Cx37 in the kidney to obtain information on its epithelial expression and potential functions. Immunohistochemistry in rodent kidney showed characteristic punctate patterns in the vasculature and along the nephron. Strong basolateral expression was found in the thick ascending limb and distal convoluted tubule. Weaker abundances were found in the proximal tubule and the collecting duct also at the basolateral side. In situ hybridization and real-time PCR of isolated nephron segments confirmed this distribution at the mRNA level. Ultrastructurally, Cx37 immunostaining was confined to basolateral cell interdigitations and infoldings. As a functional approach, rats were fed low- or high-salt diets. Compared with control and high-salt diets, rats treated with low-salt diet showed significantly increased Cx37 mRNA and protein levels. This may be indicative of an adaptive tubular response to changes in sodium reabsorption. In summary, renal epithelia express Cx37 in their basolateral membranes. Here, the formation of Cx37 gap junctions may be involved in cellular communication and adjustments of vectorial epithelial transport.

Consortin, a trans-Golgi network cargo receptor for the plasma membrane targeting and recycling of connexins

Targeting of numerous transmembrane proteins to the cell surface is thought to depend on their recognition by cargo receptors that interact with the adaptor machinery for anterograde traffic at the distal end of the Golgi complex. We report here on consortin, a novel integral membrane protein that is predicted to be intrinsically disordered, i.e. that contains large segments whose native state is unstructured. We identified consortin as a binding partner of connexins, the building blocks of gap junctions. Consortin is located at the trans-Golgi network (TGN), in tubulovesicular transport organelles, and at the plasma membrane. It directly interacts with the TGN clathrin adaptors GGA1 and GGA2, and disruption of this interaction by expression of a consortin mutant lacking the acidic cluster-dileucine (DXXLL) GGA interaction motif causes an intracellular accumulation of several connexins. RNA interference-mediated silencing of consortin expression in HeLa cells blocks the cell surface targeting of these connexins, which accumulate intracellularly, whereas partial depletion and redistribution of the consortin pool slows down the intracellular degradation of gap junction plaques. Altogether, our results show that, by studying connexin trafficking, we have identified the first TGN cargo receptor for the targeting of transmembrane proteins to the plasma membrane. The identification of consortin provides in addition a potential target for therapies aimed at diseases in which connexin traffic is altered, including cardiac ischemia, peripheral neuropathies, cataracts and hearing impairment. Sequence accession numbers. GenBank: Human CNST cDNA, NM_152609; mouse Cnst cDNA, NM_146105.