Connexin40 regulates renin production and blood pressure

Complementary and Inducible creERT2 Mouse Models for Functional Evaluation of Endothelial Cell Subtypes in the Bone Marrow

In the adult bone marrow (BM), endothelial cells (ECs) are an integral component of the hematopoietic stem cell (HSC)-supportive niche, which modulates HSC activity by producing secreted and membrane-bound paracrine signals. Within the BM, distinct vascular arteriole, transitional, and sinusoidal EC subtypes display unique paracrine expression profiles and create anatomically-discrete microenvironments. However, the relative contributions of vascular endothelial subtypes in supporting hematopoiesis is unclear. Moreover, constitutive expression and off-target activity of currently available endothelial-specific and endothelial-subtype-specific murine cre lines potentially confound data analysis and interpretation. To address this, we describe two tamoxifen-inducible cre-expressing lines, Vegfr3-creERT2 and Cx40-creERT2, that efficiently label sinusoidal/transitional and arteriole endothelium respectively in adult marrow, without off-target activity in hematopoietic or perivascular cells. Utilizing an established mouse model in which cre-dependent recombination constitutively-activates MAPK signaling within adult endothelium, we identify arteriole ECs as the driver of MAPK-mediated hematopoietic dysfunction. These results define complementary tamoxifen-inducible creERT2-expressing mouse lines that label functionally-discrete and non-overlapping sinusoidal/transitional and arteriole EC populations in the adult BM, providing a robust toolset to investigate the differential contributions of vascular subtypes in maintaining hematopoietic homeostasis.

Capillary oxygen regulates demand-supply coupling by triggering connexin40-mediated conduction: Rethinking the metabolic hypothesis

Coupling red blood cell (RBC) supply to O2 demand is an intricate process requiring O2 sensing, generation of a stimulus, and signal transduction that alters upstream arteriolar tone. Although actively debated, this process has been theorized to be induced by hypoxia and to involve activation of endothelial inwardly rectifying K+ channels (KIR) 2.1 by elevated extracellular K+ to trigger conducted hyperpolarization via connexin40 (Cx40) gap junctions to upstream resistors. This concept was tested in resting healthy skeletal muscle of Cx40-/- and endothelial KIR2.1-/- mice using state-of-the-art live animal imaging where the local tissue O2 environment was manipulated using a custom gas chamber. Second-by-second capillary RBC flow responses were recorded as O2 was altered. A stepwise drop in PO2 at the muscle surface increased RBC supply in capillaries of control animals while elevated O2 elicited the opposite response; capillaries were confirmed to express Cx40. The RBC flow responses were rapid and tightly coupled to O2; computer simulations did not support hypoxia as a driving factor. In contrast, RBC flow responses were significantly diminished in Cx40-/- mice. Endothelial KIR2.1-/- mice, on the other hand, reacted normally to O2 changes, even when the O2 challenge was targeted to a smaller area of tissue with fewer capillaries. Conclusively, microvascular O2 responses depend on coordinated electrical signaling via Cx40 gap junctions, and endothelial KIR2.1 channels do not initiate the event. These findings reconceptualize the paradigm of blood flow regulation in skeletal muscle and how O2 triggers this process in capillaries independent of extracellular K+.

Pathophysiology and pathogenic mechanisms of pulmonary hypertension: role of membrane receptors, ion channels, and Ca2+ signaling

The pulmonary circulation is a low-resistance, low-pressure, and high-compliance system that allows the lungs to receive the entire cardiac output. Pulmonary arterial pressure is a function of cardiac output and pulmonary vascular resistance, and pulmonary vascular resistance is inversely proportional to the fourth power of the intraluminal radius of the pulmonary artery. Therefore, a very small decrease of the pulmonary vascular lumen diameter results in a significant increase in pulmonary vascular resistance and pulmonary arterial pressure. Pulmonary arterial hypertension is a fatal and progressive disease with poor prognosis. Regardless of the initial pathogenic triggers, sustained pulmonary vasoconstriction, concentric vascular remodeling, occlusive intimal lesions, in situ thrombosis, and vascular wall stiffening are the major and direct causes for elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension and other forms of precapillary pulmonary hypertension. In this review, we aim to discuss the basic principles and physiological mechanisms involved in the regulation of lung vascular hemodynamics and pulmonary vascular function, the changes in the pulmonary vasculature that contribute to the increased vascular resistance and arterial pressure, and the pathogenic mechanisms involved in the development and progression of pulmonary hypertension. We focus on reviewing the pathogenic roles of membrane receptors, ion channels, and intracellular Ca2+ signaling in pulmonary vascular smooth muscle cells in the development and progression of pulmonary hypertension.

Connexin 40-Mediated Regulation of Systemic Circulation and Arterial Blood Pressure

Vascular system is a complex network in which different cell types and vascular segments must work in concert to regulate blood flow distribution and arterial blood pressure. Although paracrine/autocrine signaling is involved in the regulation of vasomotor tone, direct intercellular communication via gap junctions plays a central role in the control and coordination of vascular function in the microvascular network. Gap junctions are made up by connexin (Cx) proteins, and among the four Cxs expressed in the cardiovascular system (Cx37, Cx40, Cx43, and Cx45), Cx40 has emerged as a critical signaling pathway in the vessel wall. This Cx is predominantly found in the endothelium, but it is involved in the development of the cardiovascular system and in the coordination of endothelial and smooth muscle cell function along the length of the vessels. In addition, Cx40 participates in the control of vasomotor tone through the transmission of electrical signals from the endothelium to the underlying smooth muscle and in the regulation of arterial blood pressure by renin-angiotensin system in afferent arterioles. In this review, we discuss the participation of Cx40-formed channels in the development of cardiovascular system, control and coordination of vascular function, and regulation of arterial 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.

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.

Connexin mRNA distribution in adult mouse kidneys

Kidneys are thought to express eight different connexin isoforms (i.e., Cx 26, 30, 32, 37, 40, 43, 45, and 46), which form either hemichannels or gap junctions serving to intercellular communication and functional synchronization. Proper function of connexins has already been shown to be crucial for regulation of renal hemodynamics and renin secretion, and there is also growing evidence for connexins to play a role in pathologic conditions such as renal fibrosis or diabetic nephropathy. Therefore, exact intrarenal localization of the different connexin isoforms gains particular interest. Until now intrarenal expression of connexins has mainly been examined by immunohistochemistry, which in part generated conflicting results depending on antibodies and fixation protocols used. In this work, we used fluorescent RNAscope as an alternative technical approach to localize renal connexin mRNAs in healthy mouse kidneys. Addition of RNAscope probes for cell type specific mRNAs was used to assign connexin mRNA signals to specific cell types. We hereby found Cx26 mRNA strongly expressed in proximal tubules, Cx30 mRNA was selectively detected in the urothelium, and Cx32 mRNA was found in proximal tubules and to a lesser extent also in collecting ducts. Cx37 mRNA was mainly associated with vascular endothelium, Cx40 mRNA was largely found in glomerular mesangial and less in vascular endothelial cells, Cx43 mRNA was sparsely expressed by interstitial cells of all kidney zones, and Cx45 mRNA was predominantly found in smooth muscle cell layers of both blood vessels and ureter as well as in mesangial and interstitial (fibroblastic) cells. Cx46 mRNA could not be detected. In summary our results essentially confirm previous data on connexin expression in the renal vasculature and in glomeruli. In addition, they demonstrate strong connexin gene expression in proximal tubules, and they suggest significant connexin expression in resident tubulointerstitial cells.

Klotho supplementation attenuates blood pressure and albuminuria in murine model of IgA nephropathy

BACKGROUND

Klotho interacts with various membrane proteins, such as transforming growth factor-β (TGFβ) and insulin-like growth factor (IGF) receptors. The renal expression of klotho is diminished in chronic kidney disease.

METHOD

In this study, we assessed the effects of klotho supplementation on a murine model of IgA nephropathy. Twenty-four-week-old hyper serum IgA (HIGA) mice were subcutaneously injected daily with recombinant human klotho protein (20 μg/kg per day) or the vehicle. After 2 months, the mice were killed using an anesthesia overdose and their kidneys were harvested for analysis.

RESULTS

Supplementation of exogenous klotho protein reduced SBP, albuminuria, 8-epi-prostaglandin F2α excretion, glomerular filtration rate, renal angiotensin II concentration, and angiotensinogen expression in HIGA mice. Additionally, it enhanced renal expression of superoxide dismutase (SOD) and renal klotho itself. The findings using laser-manipulated microdissection demonstrated that klotho supplementation reduced the glomerular expression of TGFβ, fibronectin, and IGF, and increased the glomerular expression of connexin (Cx) 40.

CONCLUSION

These results indicate that klotho supplementation reduces blood pressure by suppressing the renin--angiotensin system in HIGA mice. Klotho inhibits IGF signaling to preserve glomerular Cx40 levels, ameliorating albuminuria in HIGA mice. Klotho protein supplementation attenuates mesangial expansion by inhibiting TGFβ signaling in HIGA mice.

Chronic Hypoxia Decreases Endothelial Connexin 40, Attenuates Endothelium-Dependent Hyperpolarization-Mediated Relaxation in Small Distal Pulmonary Arteries, and Leads to Pulmonary Hypertension

Background Abnormal endothelial function in the lungs is implicated in the development of pulmonary hypertension; however, there is little information about the difference of endothelial function between small distal pulmonary artery (PA) and large proximal PA and their contribution to the development of pulmonary hypertension. Herein, we investigate endothelium-dependent relaxation in different orders of PAs and examine the molecular mechanisms by which chronic hypoxia attenuates endothelium-dependent pulmonary vasodilation, leading to pulmonary hypertension. Methods and Results Endothelium-dependent relaxation in large proximal PAs (second order) was primarily caused by releasing NO from the endothelium, whereas endothelium-dependent hyperpolarization (EDH)-mediated vasodilation was prominent in small distal PAs (fourth-fifth order). Chronic hypoxia abolished EDH-mediated relaxation in small distal PAs without affecting smooth muscle-dependent relaxation. RNA-sequencing data revealed that, among genes related to EDH, the levels of Cx37, Cx40, Cx43, and IK were altered in mouse pulmonary endothelial cells isolated from chronically hypoxic mice in comparison to mouse pulmonary endothelial cells from normoxic control mice. The protein levels were significantly lower for connexin 40 (Cx40) and higher for connexin 37 in mouse pulmonary endothelial cells from hypoxic mice than normoxic mice. Cx40 knockout mice exhibited significant attenuation of EDH-mediated relaxation and marked increase in right ventricular systolic pressure. Interestingly, chronic hypoxia led to a further increase in right ventricular systolic pressure in Cx40 knockout mice without altering EDH-mediated relaxation. Furthermore, overexpression of Cx40 significantly decreased right ventricular systolic pressure in chronically hypoxic mice. Conclusions These data suggest that chronic hypoxia-induced downregulation of endothelial Cx40 results in impaired EDH-mediated relaxation in small distal PAs and contributes to the development of pulmonary hypertension.

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.

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.

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.

Gambogic Acid and Its Analogs Inhibit Gap Junctional Intercellular Communication

Gap junctions (GJs) are intercellular channels composed of connexins. Cellular molecules smaller than 1 kDa can diffuse through GJs by a process termed gap junctional intercellular communication (GJIC), which plays essential roles in various pathological and physiological conditions. Gambogic acid (GA), a major component of a natural yellow dye, has been used as traditional medicine and has been reported to have various therapeutic effects, including an anti-cancer effect. In this study, two different GJ assay methods showed that GA and its analogs inhibited GJIC. The inhibition was rapidly reversible and was not mediated by changes in surface expression or S368 phosphorylation of Cx43, cellular calcium concentration, or redox state. We also developed an assay system to measure the intercellular communication induced by Cx40, Cx30, and Cx43. Dihydrogambogic acid (D-GA) potently inhibited GJIC by Cx40 (IC50 = 5.1 μM), whereas the IC50 value of carbenoxolone, which is known as a broad spectrum GJIC inhibitor, was 105.2 μM. Thus, D-GA can act as a pharmacological tool for the inhibition of Cx40.

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.

Targeting Cx40 (Connexin40) Expression or Function Reduces Angiogenesis in the Developing Mouse Retina

OBJECTIVE

Cx40 (Connexin40) forms intercellular channels that coordinate the electric conduction in the heart and the vasomotor tone in large vessels. The protein was shown to regulate tumoral angiogenesis; however, whether Cx40 also contributes to physiological angiogenesis is still unknown.

APPROACH AND RESULTS

Here, we show that Cx40 contributes to physiological angiogenesis. Genetic deletion of Cx40 leads to a reduction in vascular growth and capillary density in the neovascularization model of the mouse neonatal retina. At the angiogenic front, vessel sprouting is reduced, and the mural cells recruited along the sprouts display an altered phenotype. These alterations can be attributed to disturbed endothelial cell functions as selective reexpression of Cx40 in these cells restores normal angiogenesis. In vitro, targeting Cx40 in microvascular endothelial cells, by silencing its expression or by blocking gap junction channels, decreases their proliferation. Moreover, loss of Cx40 in these cells also increases their release of PDGF (platelet-derived growth factor) and promotes the chemoattraction of mural cells. In vivo, an intravitreal injection of a Cx40 inhibitory peptide, phenocopies the loss of Cx40 in the retinal vasculature of wild-type mice.

CONCLUSIONS

Collectively, our data show that endothelial Cx40 contributes to the early stages of physiological angiogenesis in the developing retina, by regulating vessel growth and maturation. Cx40 thus represents a novel therapeutic target for treating pathological ocular angiogenesis.

Protein Interactions at Endothelial Junctions and Signaling Mechanisms Regulating Endothelial Permeability

The monolayer of endothelial cells lining the vessel wall forms a semipermeable barrier (in all tissue except the relatively impermeable blood-brain and inner retinal barriers) that regulates tissue-fluid homeostasis, transport of nutrients, and migration of blood cells across the barrier. Permeability of the endothelial barrier is primarily regulated by a protein complex called adherens junctions. Adherens junctions are not static structures; they are continuously remodeled in response to mechanical and chemical cues in both physiological and pathological settings. Here, we discuss recent insights into the post-translational modifications of junctional proteins and signaling pathways regulating plasticity of adherens junctions and endothelial permeability. We also discuss in the context of what is already known and newly defined signaling pathways that mediate endothelial barrier leakiness (hyperpermeability) that are important in the pathogenesis of cardiovascular and lung diseases and vascular inflammation.

Mitochondrial connexin40 regulates mitochondrial calcium uptake in coronary endothelial cells

Connexins (Cxs) are a group of integral membrane proteins that can form gap junctions between adjacent cells. Recently, it was reported that Cx43 is expressed not only in the plasma membrane but also in the inner mitochondrial membrane and that it regulates mitochondrial functions. Cx40 is predominantly expressed in vascular endothelial cells (ECs) and plays an important role in the electrical propagation between ECs and endothelial/smooth muscle cells. However, it is unknown whether Cx40 is expressed in the mitochondria and what the role of mitochondrial Cx40 is in endothelial functions. We observed in coronary ECs that Cx40 protein was expressed in the mitochondria, as determined by Western blot and immunofluorescence studies. We found that mouse coronary ECs (MCECs) isolated from Cx40 knockout (Cx40 KO) mice exhibited significantly lower resting mitochondrial calcium concentration ([Ca2+]mito) than MCECs from wild-type (WT) mice. After increase in cytosolic Ca2+ concentration ([Ca2+]cyto) with cyclopiazonic acid, calcium uptake into the mitochondria was significantly attenuated in MCECs from Cx40 KO mice compared with WT MCECs. There was no difference in resting [Ca2+]cyto and store-operated calcium entry in MCECs from WT and Cx40 KO mice. We also detected a significant decrease in the concentration of mitochondrial reactive oxygen species (ROS) in Cx40 KO MCECs. Cx40 overexpression in ECs significantly increased resting [Ca2+]mito level and calcium uptake by mitochondria in response to increased [Ca2+]cyto and augmented mitochondrial ROS production. These data suggest that mitochondrial Cx40 contributes to the regulation of mitochondrial calcium homeostasis.

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.

P/Q-type and T-type voltage-gated calcium channels are involved in the contraction of mammary and brain blood vessels from hypertensive patients

AIM

Calcium channel blockers are widely used in cardiovascular diseases. Besides L-type channels, T- and P/Q-type calcium channels are involved in the contraction of human renal blood vessels. It was hypothesized that T- and P/Q-type channels are involved in the contraction of human brain and mammary blood vessels.

METHODS

Internal mammary arteries from bypass surgery patients and cerebral arterioles from patients with brain tumours with and without hypertension were tested in a myograph and perfusion set-up. PCR and immunohistochemistry were performed on isolated blood vessels.

RESULTS

The P/Q-type antagonist ω-agatoxin IVA (10^-8  mol L^-1 ) and the T-type calcium blocker mibefradil (10^-7  mol L^-1 ) inhibited KCl depolarization-induced contraction in mammary arteries from hypertensive patients with no effect on blood vessels from normotensive patients. ω-Agatoxin IVA decreased contraction in cerebral arterioles from hypertensive patients. L-type blocker nifedipine abolished the contraction in mammary arteries. PCR analysis showed expression of P/Q-type (Ca_v 2.1), T-type (Ca_v 3.1 and Ca_v 3.2) and L-type (Ca_v 1.2) calcium channels in mammary and cerebral arteries. Immunohistochemical labelling of mammary and cerebral arteries revealed the presence of Ca_v 2.1 in endothelial and smooth muscle cells. Ca_v 3.1 was also detected in mammary arteries.

CONCLUSION

P/Q- and T-type Ca_v are present in human internal mammary arteries and in cerebral penetrating arterioles. P/Q- and T-type calcium channels are involved in the contraction of mammary arteries from hypertensive patients but not from normotensive patients. Furthermore, in cerebral arterioles P/Q-type channels importance was restricted to hypertensive patients might lead to that T- and P/Q-type channels could be a new target in hypertensive patients.

Enhanced expression of Cx43 and gap junction communication in vascular smooth muscle cells of spontaneously hypertensive rats

Niflumic acid (NFA) is a novel gap junction (GJ) inhibitor. The aim of the present study was to investigate the effect of NFA on GJ communication and the expression of connexin (Cx) in vascular smooth muscle cells (VSMCs) of mesenteric arterioles of spontaneously hypertensive rats (SHR). Whole-cell patch clamp recording demonstrated that NFA at 1×10–4 M significantly inhibited the inward current and its effect was reversible. The time for charging and discharging of cell membrane capacitance (Cinput) reduced from 9.73 to 0.48 ms (P<0.05; n=6). Pressure myograph measurement showed that NFA at 3×10-4 M fully neutralized the contraction caused by phenylephrine. The relaxation responses of normotensive control Wistar Kyoto (WKY) rats were significantly higher, compared with those of the SHRs (P<0.05; n=6). Western blot and reverse transcription-quantitative polymerase chain reaction analyses showed that the mRNA and protein expression levels of Cx43 of the third-level branch of mesenteric arterioles of the SHRs and WKY rats were higher, compared with those of the first-level branch. The mRNA and protein expression levels of Cx43 of the primary and third-level branches of the mesenteric arterioles in the SHRs were higher, compared with those in the WKY rats (P<0.05; n=6). The mRNA levels of Cx43 in the mesenteric arterioles were significantly downregulated by NFA in a concentration-dependent manner (P<0.01; n=6). The protein levels of Cx43 in primary cultured VSMCs isolated from the mesenteric arterioles were also significantly downregulated by NFA in a concentration-dependent manner (P<0.01; n=6). These results showed that the vasorelaxatory effects of GJ inhibitors were reduced in the SHRs, which was associated with a higher protein expression level of Cx43 in the mesenteric arterioles of the SHRs. NFA also relaxed the mesenteric arterioles by reducing the expression of Cx43, which decreased blood pressure. Therefore, regulation of the expression of GJs may be a therapeutic target for the treatment of hypertension.

Communication Through Gap Junctions in the Endothelium

A swarm of fish displays a collective behavior (swarm behavior) and moves "en masse" despite the huge number of individual animals. In analogy, organ function is supported by a huge number of cells that act in an orchestrated fashion and this applies also to vascular cells along the vessel length. It is obvious that communication is required to achieve this vital goal. Gap junctions with their modular bricks, connexins (Cxs), provide channels that interlink the cytosol of adjacent cells by a pore sealed against the extracellular space. This allows the transfer of ions and charge and thereby the travel of membrane potential changes along the vascular wall. The endothelium provides a low-resistance pathway that depends crucially on connexin40 which is required for long-distance conduction of dilator signals in the microcirculation. The experimental evidence for membrane potential changes synchronizing vascular behavior is manifold but the functional verification of a physiologic role is still open. Other molecules may also be exchanged that possibly contribute to the synchronization (eg, Ca(2+)). Recent data suggest that vascular Cxs have more functions than just facilitating communication. As pharmacological tools to modulate gap junctions are lacking, Cx-deficient mice provide currently the standard to unravel their vascular functions. These include arteriolar dilation during functional hyperemia, hypoxic pulmonary vasoconstriction, vascular collateralization after ischemia, and feedback inhibition on renin secretion in the kidney.

Targeting endothelial connexin40 inhibits tumor growth by reducing angiogenesis and improving vessel perfusion

Endothelial connexin40 (Cx40) contributes to regulate the structure and function of vessels. We have examined whether the protein also modulates the altered growth of vessels in tumor models established in control mice (WT), mice lacking Cx40 (Cx40-/-), and mice expressing the protein solely in endothelial cells (Tie2-Cx40). Tumoral angiogenesis and growth were reduced, whereas vessel perfusion, smooth muscle cell (SMC) coverage and animal survival were increased in Cx40-/- but not Tie2-Cx40 mice, revealing a critical involvement of endothelial Cx40 in transformed tissues independently of the hypertensive status of Cx40-/- mice. As a result, Cx40-/- mice bearing tumors survived significantly longer than corresponding controls, including after a cytotoxic administration. Comparable observations were made in WT mice injected with a peptide targeting Cx40, supporting the Cx40 involvement. This involvement was further confirmed in the absence of Cx40 or by peptide-inhibition of this connexin in aorta-sprouting, matrigel plug and SMC migration assays, and associated with a decreased expression of the phosphorylated form of endothelial nitric oxide synthase. The data identify Cx40 as a potential novel target in cancer treatment.

Connexins, Pannexins, and Their Channels in Fibroproliferative Diseases

Cellular and molecular mechanisms of wound healing, tissue repair, and fibrogenesis are established in different organs and are essential for the maintenance of function and tissue integrity after cell injury. These mechanisms are also involved in a plethora of fibroproliferative diseases or organ-specific fibrotic disorders, all of which are associated with the excessive deposition of extracellular matrix components. Fibroblasts, which are key cells in tissue repair and fibrogenesis, rely on communicative cellular networks to ensure efficient control of these processes and to prevent abnormal accumulation of extracellular matrix into the tissue. Despite the significant impact on human health, and thus the epidemiologic relevance, there is still no effective treatment for most fibrosis-related diseases. This paper provides an overview of current concepts and mechanisms involved in the participation of cellular communication via connexin-based pores as well as pannexin-based channels in the processes of tissue repair and fibrogenesis in chronic diseases. Understanding these mechanisms may contribute to the development of new therapeutic strategies to clinically manage fibroproliferative diseases and organ-specific fibrotic disorders.

Role of connexins and pannexins in cardiovascular physiology

Connexins and pannexins form connexons, pannexons and membrane channels, which are critically involved in many aspects of cardiovascular physiology. For that reason, a vast number of studies have addressed the role of connexins and pannexins in the arterial and venous systems as well as in the heart. Moreover, a role for connexins in lymphatics has recently also been suggested. This review provides an overview of the current knowledge regarding the involvement of connexins and pannexins in cardiovascular physiology.

Connexin 40 is dispensable for vascular renin cell recruitment but is indispensable for vascular baroreceptor control of renin secretion

Defects of the gap junction protein connexin 40 (Cx40) in renin-secreting cells (RSCs) of the kidney lead to a shift of the localization of RSCs from the media layer of afferent arterioles to the periglomerular interstitium. The dislocation of RSCs goes in parallel with elevated plasma renin levels, impaired pressure control of renin secretion, and hypertension. The reasons for the extravascular shift of RSCs and the blunted pressure regulation of renin secretion caused by the absence of Cx40 are still unclear. We have therefore addressed the question if Cx40 is essential for the metaplastic transformation of preglomerular vascular smooth muscle cells (SMCs) into RSCs and if Cx40 is essential for the pressure control of renin secretion from RSCs located in the media layer of afferent arterioles. For our study, we used mice lacking the angiotensin II type 1A (AT1A) receptors, which display a prominent and reversible salt-sensitive metaplastic transformation of SMCs into RSCs. This mouse line was crossed with Cx40-deficient mice to obtain AT1A and Cx40 double deleted mice. The kidneys of AT1A (-/-)Cx40(-/-) mice kept on normal salt (0.3 %) displayed RSCs both in the inner media layer of preglomerular vessels and in the periglomerular interstitium. In contrast to hypotensive AT1A (-/-) (mean bp syst 112 mmHg) and hypertensive Cx40(-/-) (mean bp syst 160 mmHg) mice AT1A (-/-)Cx40(-/-) mice were normotensive(mean bp syst 130 mmHg). Pressure regulation of renin secretion from isolated kidneys was normal in AT1A (-/-) mice, but was absent in AT1A (-/-)Cx40(-/-) mice alike in Cx40(-/-) mice. Low-salt diet (0.02 %) increased RSC numbers in the media layer, whilst high-salt diet (4 %) caused disappearance of RSCs in the media layer but not in the periglomerular interstitium. Blood pressure was clearly salt sensitive both in AT1A (-/-) and in AT1A (-/-)Cx40(-/-) mice but was shifted to higher pressure values in the latter genotype. Our data indicate that Cx40 is not a requirement for intramural vascular localization of RSCs nor for reversible metaplastic transformation of SMCs into RSCs. Therefore, the ectopic localization of RSCs in Cx40(-/-) kidneys is more likely due to a disturbed intercellular communication rather than being the result of chronic overactivation of the renin-angiotensin-aldosterone system or hypertension. Moreover, our findings suggest that Cx40 is a requirement for the pressure control of renin secretion irrespective of the localization of RSCs.

Interplay between connexin40 and nitric oxide signaling during hypertension

Connexins (Cxs) and endothelial nitric oxide synthase (eNOS) contribute to the adaptation of endothelial and smooth muscle cells to hemodynamic changes. To decipher the in vivo interplay between these proteins, we studied Cx40-null mice, a model of renin-dependent hypertension which displays an altered endothelium-dependent relaxation of the aorta because of reduced eNOS levels. These mice, which were either untreated or subjected to the 1-kidney, 1-clip (1K1C) procedure, a model of volume-dependent hypertension, were compared with control mice submitted to either the 1K1C or the 2-kidney, 1-clip (2K1C) procedure, a model of renin-dependent hypertension. All operated mice became hypertensive and featured hypertrophy and altered Cx expression of the aorta. The combination of volume- and renin-dependent hypertension in Cx40-/- 1K1C mice raised blood pressure and cardiac weight index. Under these conditions, all aortas showed increased levels of Cx40 in endothelial cells and of both Cx37 and Cx45 in smooth muscle cells. In the wild-type 1K1C mice, the interactions between Cx40 and Cx37 with eNOS were enhanced, resulting in increased NO release. The Cx40-eNOS interaction could not be observed in mice lacking Cx40, which also featured decreased levels of eNOS. In these animals, the volume overload caused by the 1K1C procedure resulted in increased phosphorylation of eNOS and in a higher NO release. The findings provide evidence that Cx40 and Cx37 play an in vivo role in the regulation of eNOS.

P2Y₂ receptor activation decreases blood pressure via intermediate conductance potassium channels and connexin 37

AIMS

Nucleotides are important paracrine regulators of vascular tone. We previously demonstrated that activation of P2Y₂ receptors causes an acute, NO-independent decrease in blood pressure, indicating this signalling pathway requires an endothelial-derived hyperpolarization (EDH) response. To define the mechanisms by which activation of P2Y₂ receptors initiates EDH and vasodilation, we studied intermediate-conductance (KCa3.1, expressed in endothelial cells) and big-conductance potassium channels (KCa1.1, expressed in smooth muscle cells) as well as components of the myoendothelial gap junction, connexins 37 and 40 (Cx37, Cx40), all hypothesized to be part of the EDH response.

METHODS

We compared the effects of a P2Y₂/₄ receptor agonist in wild-type (WT) mice and in mice lacking KCa3.1, KCa1.1, Cx37 or Cx40 under anaesthesia, while monitoring intra-arterial blood pressure and heart rate.

RESULTS

Acute activation of P2Y₂/₄ receptors (0.01-3 mg kg(-1) body weight i.v.) caused a biphasic blood pressure response characterized by a dose-dependent and rapid decrease in blood pressure in WT (maximal response % of baseline at 3 mg kg(-1) : -38 ± 1%) followed by a consecutive increase in blood pressure (+44 ± 11%). The maximal responses in KCa3.1(-/-) and Cx37(-/-) were impaired (-13 ± 5, +17 ± 7 and -27 ± 1, +13 ± 3% respectively), whereas the maximal blood pressure decrease in response to acetylcholine at 3 μg kg(-1) was not significantly different (WT: -53 ± 3%; KCa3.1(-/-) : -52 ± 3; Cx37(-/-) : -53 ± 3%). KCa1.1(-/-) and Cx40(-/-) showed an identical biphasic response to P2Y2/4 receptor activation compared to WT.

CONCLUSIONS

The data suggest that the P2Y2/4 receptor activation elicits blood pressure responses via distinct mechanisms involving KCa3.1 and Cx37.

Inducible deletion of connexin 40 in adult mice causes hypertension and disrupts pressure control of renin secretion

Genetic loss-of-function defects of connexin 40 in renal juxtaglomerular cells are associated with renin-dependent hypertension. The dysregulation of renin secretion results from an intrarenal displacement of renin cells and an interruption of the negative feedback control of renin secretion by blood pressure. It is unknown whether this phenotype is secondary to developmental defects of juxtaglomerular renin cells due to connexin 40 malfunction, or whether acute functional defects of connexin 40 in the normal adult kidney can also lead to a similar dysregulation of renin secretion and hypertension. To address this question, we generated mice with an inducible deletion of connexin 40 in the adult kidney by crossing connexin 40-floxed mice with mice harboring a ubiquitously expressed tamoxifen-inducible Cre recombinase. Tamoxifen treatment in these mice strongly reduced connexin 40 mRNA and protein expression in the kidneys. These mice displayed persistent hypertension with renin expression shifted from the media layer of afferent arterioles to juxtaglomerular periglomerular cells. Control of renin secretion by the perfusion pressure was abolished in vitro, whereas in vivo plasma renin concentrations were increased. Thus, interruption of the connexin 40 gene in the adult kidney produced very similar changes in the renin system as had embryonic deletion. Hence, impairments of connexin 40 function in the normal adult kidney can cause renin-dependent hypertension.

Loss of functional endothelial connexin40 results in exercise-induced hypertension in mice

During activity, coordinated vasodilation of microcirculatory networks with upstream supply vessels increases blood flow to skeletal and cardiac muscles and reduces peripheral resistance. Endothelial dysfunction in humans attenuates activity-dependent vasodilation, resulting in exercise-induced hypertension in otherwise normotensive individuals. Underpinning activity-dependent hyperemia is an ascending vasodilation in which the endothelial gap junction protein, connexin (Cx)40, plays an essential role. Because exercise-induced hypertension is proposed as a forerunner to clinical hypertension, we hypothesized that endothelial disruption of Cx40 function in mice may create an animal model of this condition. To this end, we created mice in which a mutant Cx40T152A was expressed alongside wildtype Cx40 selectively in the endothelium. Expression of the Cx40T152A transgene in Xenopus oocytes and mouse coronary endothelial cells in vitro impaired both electric and chemical conductance and acted as a dominant-negative against wildtype Cx40, Cx43, and Cx45, but not Cx37. Endothelial expression of Cx40T152A in Cx40T152ATg mice attenuated ascending vasodilation, without effect on radial coupling through myoendothelial gap junctions. Using radiotelemetry, Cx40T152ATg mice showed an activity-dependent increase in blood pressure, which was significantly greater than in wildtype mice, but significantly less than in chronically hypertensive, Cx40knockout mice. The increase in heart rate with activity was also greater than in wildtype or Cx40knockout mice. We conclude that the endothelial Cx40T152A mutation attenuates activity-dependent vasodilation, producing a model of exercise-induced hypertension. These data highlight the importance of endothelial coupling through Cx40 in regulating blood pressure during activity.

Functional importance of T-type voltage-gated calcium channels in the cardiovascular and renal system: news from the world of knockout mice

Over the years, it has been discussed whether T-type calcium channels Cav3 play a role in the cardiovascular and renal system. T-type channels have been reported to play an important role in renal hemodynamics, contractility of resistance vessels, and pacemaker activity in the heart. However, the lack of highly specific blockers cast doubt on the conclusions. As new T-type channel antagonists are being designed, the roles of T-type channels in cardiovascular and renal pathology need to be elucidated before T-type blockers can be clinically useful. Two types of T-type channels, Cav3.1 and Cav3.2, are expressed in blood vessels, the kidney, and the heart. Studies with gene-deficient mice have provided a way to investigate the Cav3.1 and Cav3.2 channels and their role in the cardiovascular system. This review discusses the results from these knockout mice. Evaluation of the literature leads to the conclusion that Cav3.1 and Cav3.2 channels have important, but different, functions in mice. T-type Cav3.1 channels affect heart rate, whereas Cav3.2 channels are involved in cardiac hypertrophy. In the vascular system, Cav3.2 activation leads to dilation of blood vessels, whereas Cav3.1 channels are mainly suggested to affect constriction. The Cav3.1 channel is also involved in neointima formation following vascular damage. In the kidney, Cav3.1 regulates plasma flow and Cav3.2 plays a role setting glomerular filtration rate. In conclusion, Cav3.1 and Cav3.2 are new therapeutic targets in several cardiovascular pathologies, but the use of T-type blockers should be specifically directed to the disease and to the channel subtype.

Classical Renin-Angiotensin system in kidney physiology

The renin-angiotensin system has powerful effects in control of the blood pressure and sodium homeostasis. These actions are coordinated through integrated actions in the kidney, cardiovascular system and the central nervous system. Along with its impact on blood pressure, the renin-angiotensin system also influences a range of processes from inflammation and immune responses to longevity. Here, we review the actions of the "classical" renin-angiotensin system, whereby the substrate protein angiotensinogen is processed in a two-step reaction by renin and angiotensin converting enzyme, resulting in the sequential generation of angiotensin I and angiotensin II, the major biologically active renin-angiotensin system peptide, which exerts its actions via type 1 and type 2 angiotensin receptors. In recent years, several new enzymes, peptides, and receptors related to the renin-angiotensin system have been identified, manifesting a complexity that was previously unappreciated. While the functions of these alternative pathways will be reviewed elsewhere in this journal, our focus here is on the physiological role of components of the "classical" renin-angiotensin system, with an emphasis on new developments and modern concepts.

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.

Connexins, renin cell displacement and hypertension

Vascular gap junctions formed by specific connexins proteins Cx37, 40, 43 and 45 are important for proper vascular function. This review outlines that defects of the connexin 40 protein leads to hypertension because of dysfunction of renin secreting cells of the kidney. Thus defects of Cx40 but not of other vascular connexins blunt the negative feedback control of renin secretion by the blood pressure, and moreover, lead to a shift of renin expression from the juxtaglomerular vessels walls into the periglomerular interstitium. Evidence exists to indicate that those findings which were primarily obtained with mice are also relevant for humans.

Mutations in cardiovascular connexin genes

Connexins (Cxs) form a family of transmembrane proteins comprising 21 members in humans. Cxs differ in their expression patterns, biophysical properties and ability to combine into homomeric or heteromeric gap junction channels between neighbouring cells. The permeation of ions and small metabolites through gap junction channels or hemichannels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. Among others, Cx37, Cx40, Cx43, Cx45 and Cx47 are found in heart, blood and lymphatic vessels. Mutations or polymorphisms in the genes coding for these Cxs have not only been implicated in cardiovascular pathologies but also in a variety of other disorders. While mutations in Cx43 are mostly linked to oculodentodigital dysplasia, Cx47 mutations are associated with Pelizaeus-Merzbacher-like disease and lymphoedema. Cx40 mutations are principally linked to atrial fibrillation. Mutations in Cx37 have not yet been described, but polymorphisms in the Cx37 gene have been implicated in the development of arterial disease. This review addresses current knowledge on gene mutations in cardiovascular Cxs systematically and links them to alterations in channel properties and disease.

Restoration of connexin 40 (Cx40) in Renin-producing cells reduces the hypertension of Cx40 null mice

Connexin 40 (Cx40) is expressed by the renin-producing cells (RSCs) of the kidneys and the endothelial cells of blood vessels. Cx40 null mice (Cx40(-/-)) feature a much increased renin synthesis and secretion, which results in chronic hypertension, and also display an altered endothelium-dependent relaxation of the aorta because of reduced eNOS levels and nitric oxide production. To discriminate the effect of Cx40 in renin secretion and vascular signaling, we targeted Cx40 to either the RSCs or the endothelial cells of Cx40 null mice. When compared with Cx40(-/-) controls, the animals expressing Cx40 in RSCs were less hypertensive and featured reduced renin levels, still numerous RSCs outside the wall of the afferent arterioles. In contrast, mice expressing Cx40 in the endothelial cells were as hypertensive as Cx40(-/-) mice, in spite of control levels of Cx37 and eNOS. Our data show that blood pressure is improved by restoration of Cx40 expression in RSCs but not in endothelial cells, stressing the prominent role of renin in the mouse hypertension linked to loss of Cx40.

Control of renin secretion from kidneys with renin cell hyperplasia

In states of loss-of-function mutations of the renin-angiotensin-aldosterone system, kidneys develop a strong hyperplasia of renin-producing cells. Those additional renin cells are located outside the classic juxtaglomerular areas, mainly in the walls of preglomerular vessels and most prominently in multilayers surrounding afferent arterioles. Since the functional behavior of those ectopic renin cells is yet unknown, we aimed to characterize the control of renin secretion from kidneys with renin cell hyperplasia. As a model, we used kidneys from mice lacking aldosterone synthase (AS⁻/⁻ mice), which displayed 10-fold elevations of renin mRNA and plasma renin concentrations. On the absolute level, renin secretion from isolated AS⁻/⁻ kidneys was more than 10-fold increased over wild-type kidneys. On the relative level, the stimulation of renin secretion by the β-adrenergic activator isoproterenol or by lowering of the concentration of extracellular Ca²⁺ was very similar between the two genotypes. In addition, the inhibitory effects of ANG II and of perfusion pressure were similar between the two genotypes. Deletion of connexin40 blunted the pressure dependency of renin secretion and the stimulatory effect of low extracellular Ca²⁺ on renin secretion in the same manner in kidneys of AS⁻/⁻ mice as in wild-type mice. Our findings suggest a high degree of functional similarity between renin cells originating during development and located at different positions in the adult kidney. They also suggest a high similarity in the expression of membrane proteins relevant for the control of renin secretion, such as β₁-adrenergic receptors, ANG II type 1 receptors, and connexin40.

Role of T-type channels in vasomotor function: team player or chameleon?

Low-voltage-activated T-type calcium channels play an important role in regulating cellular excitability and are implicated in conditions, such as epilepsy and neuropathic pain. T-type channels, especially Cav3.1 and Cav3.2, are also expressed in the vasculature, although patch clamp studies of isolated vascular smooth muscle cells have in general failed to demonstrate these low-voltage-activated calcium currents. By contrast, the channels which are blocked by T-type channel antagonists are high-voltage activated but distinguishable from their L-type counterparts by their T-type biophysical properties and small negative shifts in activation and inactivation voltages. These changes in T-channel properties may result from vascular-specific expression of splice variants of Cav3 genes, particularly in exon 25/26 of the III-IV linker region. Recent physiological studies suggest that T-type channels make a small contribution to vascular tone at low intraluminal pressures, although the relevance of this contribution is unclear. By contrast, these channels play a larger role in vascular tone of small arterioles, which would be expected to function at lower intra-vascular pressures. Upregulation of T-type channel function following decrease in nitric oxide bioavailability and increase in oxidative stress, which occurs during cardiovascular disease, suggests that a more important role could be played by these channels in pathophysiological situations. The ability of T-type channels to be rapidly recruited to the plasma membrane, coupled with their subtype-specific localisation in signalling microdomains where they could modulate the function of calcium-dependent ion channels and pathways, provides a mechanism for rapid up- and downregulation of vasoconstriction. Future investigation into the molecules which govern these changes may illuminate novel targets for the treatment of conditions such as therapy-resistant hypertension and vasospasm.

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.

Direct assessment of tubuloglomerular feedback responsiveness in connexin 40-deficient mice

Participation of connexin 40 (Cx40) in the regulation of renin secretion and in the tubuloglomerular feedback (TGF) component of renal autoregulation suggests that gap junctional coupling through Cx40 contributes to the function of the juxtaglomerular apparatus. In the present experiments, we determined the effect of targeted Cx40 deletion in C57BL/6 and FVB mice on TGF responsiveness. In C57BL/6 mice, stop-flow pressure (PSF) fell from 40.3 ± 2 to 34.5 ± 2 mmHg in wild-type (WT) and from 31 ± 1.06 to 26.6 ± 0.98 mmHg in Cx40-/- mice. PSF changes of 5.85 ± 0.67 mmHg in WT and of 4.3 ± 0.55 mmHg in Cx40-/- mice were not significantly different (P = 0.08). In FVB mice, PSF fell from 37.4 ± 1.5 to 31.6 ± 1.5 mmHg in WT and from 28.1 ± 1.6 to 25.4 ± 1.7 mmHg in Cx40-/-, with mean TGF responses being significantly greater in WT than Cx40-/- (5.5 ± 0.55 vs. 2.7 ± 0.84 mmHg; P = 0.002). In both genetic backgrounds, PSF values were significantly lower in Cx40-/- than WT mice at all flow rates. Arterial blood pressure in the animals prepared for micropuncture was not different between WT and Cx40-/- mice. We conclude that the TGF response magnitude in superficial cortical nephrons is reduced by 30-50% in mice without Cx40, but that with the exception of a small number of nephrons, residual TGF activity is maintained. Thus gap junctional coupling appears to modulate TGF, perhaps by determining the kinetics of signal transmission.

Polymorphism in endothelial connexin40 enhances sensitivity to intraluminal pressure and increases arterial stiffness

OBJECTIVE

To determine whether impairment of endothelial connexin40 (Cx40), an effect that can occur in hypertension and aging, contributes to the arterial dysfunction and stiffening in these conditions.

APPROACH AND RESULTS

A new transgenic mouse strain, expressing a mutant Cx40, (Cx40T202S), specifically in the vascular endothelium, has been developed and characterized. This mutation produces nonfunctional hemichannels, whereas gap junctions containing the mutant are electrically, but not chemically, patent. Mesenteric resistance arteries from Cx40T202S mice showed increased sensitivity of the myogenic response to intraluminal pressure in vitro, compared with wild-type mice, whereas transgenic mice overexpressing native Cx40 (Cx40Tg) showed reduced sensitivity. In control and Cx40Tg mice, the sensitivity to pressure of myogenic constriction was modulated by both NO and endothelium-derived hyperpolarization; however, the endothelium-derived hyperpolarization component was absent in Cx40T202S arteries. Analysis of passive mechanical properties revealed that arterial stiffness was enhanced in vessels from Cx40T202S mice, but not in wild-type or Cx40Tg mice.

CONCLUSIONS

Introduction of a mutant form of Cx40 in the endogenous endothelial Cx40 population prevents endothelium-derived hyperpolarization activation during myogenic constriction, enhancing sensitivity to intraluminal pressure and increasing arterial stiffness. We conclude that genetic polymorphisms in endothelial Cx40 can contribute to the pathogenesis of arterial disease.

Regulation of cardiovascular connexins by mechanical forces and junctions

Connexins form a family of transmembrane proteins that consists of 20 members in humans and 21 members in mice. Six connexins assemble into a connexon that can function as a hemichannel or connexon that can dock to a connexon expressed by a neighbouring cell, thereby forming a gap junction channel. Such intercellular channels synchronize responses in multicellular organisms through direct exchange of ions, small metabolites, and other second messenger molecules between the cytoplasms of adjacent cells. Multiple connexins are expressed in the cardiovascular system. These connexins not only experience the different biomechanical forces within this system, but may also act as effector proteins in co-ordinating responses within groups of cells towards these forces. This review discusses recent insights regarding regulation of cardiovascular connexins by mechanical forces and junctions. It specifically addresses effects of (i) shear stress on endothelial connexins, (ii) hypertension on vascular connexins, and (iii) changes in afterload and the composition of myocardial mechanical junctions on cardiac connexins.

Spreading vasodilatation in the murine microcirculation: attenuation by oxidative stress-induced change in electromechanical coupling

Regulation of blood flow in microcirculatory networks depends on spread of local vasodilatation to encompass upstream arteries; a process mediated by endothelial conduction of hyperpolarization. Given that endothelial coupling is reduced in hypertension, we used hypertensive Cx40ko mice, in which endothelial coupling is attenuated, to investigate the contribution of the renin-angiotensin system and reduced endothelial cell coupling to conducted vasodilatation of cremaster arterioles in vivo. When the endothelium was disrupted by light dye treatment, conducted vasodilatation, following ionophoresis of acetylcholine, was abolished beyond the site of endothelial damage. In the absence of Cx40, sparse immunohistochemical staining was found for Cx37 in the endothelium, and endothelial, myoendothelial and smooth muscle gap junctions were identified by electron microscopy. Hyperpolarization decayed more rapidly in arterioles from Cx40ko than wild-type mice. This was accompanied by a shift in the threshold potential defining the linear relationship between voltage and diameter, increased T-type calcium channel expression and increased contribution of T-type (3 μmol l(-1) NNC 55-0396), relative to L-type (1 μmol l(-1) nifedipine), channels to vascular tone. The change in electromechanical coupling was reversed by inhibition of the renin-angiotensin system (candesartan, 1.0 mg kg(-1) day(-1) for 2 weeks) or by acute treatment with the superoxide scavenger tempol (1 mmol l(-1)). Candesartan and tempol treatments also significantly improved conducted vasodilatation. We conclude that conducted vasodilatation in Cx40ko mice requires the endothelium, and attenuation results from both a reduction in endothelial coupling and an angiotensin II-induced increase in oxidative stress. We suggest that during cardiovascular disease, the ability of microvascular networks to maintain tissue integrity may be compromised due to oxidative stress-induced changes in electromechanical coupling.

Compromised regulation of tissue perfusion and arteriogenesis limit, in an AT1R-independent fashion, recovery of ischemic tissue in Cx40(-/-) mice

Recently, we reported that recovery of tissue perfusion in the ischemic hindlimb was reduced, inflammatory response increased, and survival of distal limb tissue compromised in connexin 40 (Cx40)-deficient (Cx40(-/-)) mice. Here we evaluate whether genotype-specific differences in tissue perfusion, native vascular density, arteriogenesis, blood pressure, and chronic ANG II type 1 receptor (AT1R) activation contribute to poor recovery of ischemic hindlimb tissue in Cx40(-/-) mice. Hindlimb ischemia was induced in wild-type (WT), Cx40(-/-), and losartan-treated Cx40(-/-) mice by using surgical procedures that either maintained (mild surgery) or compromised (severe surgery) perfusion of major collateral vessels supplying the distal limb. Pre- and postsurgical hindlimb perfusion was evaluated, and tissue survival, microvascular density, and macrophage infiltration were documented during recovery. Hindlimb perfusion was compromised in presurgical Cx40(-/-) versus WT mice despite comparable native microvascular density. Hindlimb perfusion 24 h postsurgery in Cx40(-/-) and WT mice was comparable after mild surgery (collateral vessels maintained), but compromised arteriogenesis in Cx40(-/-) animals nevertheless limited subsequent recovery of tissue perfusion and compromised tissue survival. Prolonged pre- and postsurgical treatment of Cx40(-/-) mice with losartan (an AT1R antagonist) normalized blood pressure but did not improve tissue perfusion or survival, despite reduced macrophage infiltration. Thus it appears Cx40 is necessary for normal tissue perfusion and for recovery of perfusion, arteriogenesis, and tissue survival in the ischemic hindlimb. Our data suggest that Cx40(-/-) mice are at significantly greater risk for poor recovery from ischemic insult due to compromised regulation of tissue perfusion, vascular remodeling, and prolonged inflammatory response.

Gap junction signalling is a stress-regulated component of adrenal neuroendocrine stimulus-secretion coupling in vivo

Elucidating the mechanisms whereby neuroendocrine tissues coordinate their input and output signals to ensure appropriate hormone secretion is currently a topical issue. In particular, whether a direct communication mediated by gap junctions between neurosecretory cells contributes to hormone release in vivo still remains unknown. Here we address this issue using a microsurgical approach allowing combined monitoring of adrenal catecholamine secretion and splanchnic nerve stimulation in anaesthetised mice. Pharmacological blockade of adrenal gap junctions by the uncoupling agent carbenoxolone reduces nerve stimulation-evoked catecholamine release in control mice and to a larger extent in stressed mice. In parallel, the gap junction-coupled cell network is extended in stressed mice. Altogether, this argues for a significant contribution of adrenomedullary gap junctions to catecholamine secretion in vivo. As such, gap junctional signalling appears to be a substantial component for neuroendocrine function in the adrenal medulla, as it may represent an additional lever regulating hormone release.

Defective Cx40 maintains Cx37 expression but intact Cx40 is crucial for conducted dilations irrespective of hypertension

The gap junction channel protein connexin40 (Cx40) is crucial in vascular and renal physiology, because Cx40-deficient mice exhibit impaired conduction of endothelium-dependent dilations and pronounced hypertension. The latter precludes mechanistic insights into the role of endothelial Cx40, because long-lasting hypertension itself may affect conduction and Cx expression. We aimed to identify endothelial Cx40 functions, their dependency on the conductive capability, and to separate these from hypertension-related alterations. We assessed conduction and Cx expression in mice with cell type-specific deletion of Cx40 and in mice expressing a defective Cx40 (Cx40A96S) identified in humans, which forms nonconducting gap junction channels. Confined arteriolar stimulation with acetylcholine or bradykinin elicited local dilations that conducted upstream without attenuation of the amplitude for distances up to 1.2-mm in controls with a floxed Cx40 gene (Cx40(fl/fl)). Conducted responses in hypertensive animals devoid of Cx40 in renin-producing cells were unaltered but remote dilations were reduced in normotensive animals deficient for Cx40 in endothelial cells (Cx40(fl/fl):Tie2-Cre). Surprisingly, Cx37 expression was undetectable by immunostaining in arteriolar endothelium only in Cx40(fl/fl):Tie2-Cre; however, transcriptional activity of Cx37 in the cremaster was comparable with Cx40(fl/fl) controls. Cx40A96S mice were hypertensive with preserved expression of Cx40 and Cx37. Nevertheless, conducted responses were blunted. We conclude that endothelial Cx40 is necessary to support conducted dilations initiated by endothelial agonists and to locate Cx37 into the plasma membrane. These functions are unaltered by long-lasting hypertension. In the presence of a nonconducting Cx40, Cx37 is present but cannot support the conduction highlighting the importance of endothelial Cx40.

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.

Role of connexin40 in the autoregulatory response of the afferent arteriole

Connexins in renal arterioles affect autoregulation of arteriolar tonus and renal blood flow and are believed to be involved in the transmission of the tubuloglomerular feedback (TGF) response across the cells of the juxtaglomerular apparatus. Connexin40 (Cx40) also plays a significant role in the regulation of renin secretion. We investigated the effect of deleting the Cx40 gene on autoregulation of afferent arteriolar diameter in response to acute changes in renal perfusion pressure. The experiments were performed using the isolated blood perfused juxtamedullary nephron preparation in kidneys obtained from wild-type or Cx40 knockout mice. Renal perfusion pressure was increased in steps from 75 to 155 mmHg, and the response in afferent arteriolar diameter was measured. Hereafter, a papillectomy was performed to inhibit TGF, and the pressure steps were repeated. Conduction of intercellular Ca(2+) changes in response to local electrical stimulation was examined in isolated interlobular arteries and afferent arterioles from wild-type or Cx40 knockout mice. Cx40 knockout mice had an impaired autoregulatory response to acute changes in renal perfusion pressure compared with wild-type mice. Inhibition of TGF by papillectomy significantly reduced autoregulation of afferent arteriolar diameter in wild-type mice. In Cx40 knockout mice, papillectomy did not affect the autoregulatory response, indicating that these mice have no functional TGF. Also, Cx40 knockout mice showed no conduction of intercellular Ca(2+) changes in response to local electrical stimulation of interlobular arteries, whereas the Ca(2+) response to norepinephrine was unaffected. These results suggest that Cx40 plays a significant role in the renal autoregulatory response of preglomerular resistance vessels.