Prostaglandin E2 EP2 and EP4 receptor activation mediates cAMP-dependent hyperpolarization and exocytosis of renin in juxtaglomerular cells

Physiological role of hydrogen sulfide in the kidney and its therapeutic implications for kidney diseases

For over three centuries, hydrogen sulfide (H2S) has been known as a toxic and deadly gas at high concentrations, with a distinctive smell of rotten eggs. However, studies over the past two decades have shown that H2S has risen above its historically notorious label and has now received significant scientific attention as an endogenously produced gaseous signaling molecule that participates in cellular homeostasis and influences a myriad of physiological and pathological processes at low concentrations. Its endogenous production is enzymatically regulated, and when dysregulated, contributes to pathogenesis of renal diseases. In addition, exogenous H2S administration has been reported to exhibit important therapeutic characteristics that target multiple molecular pathways in common renal pathologies in which reduced levels of renal and plasma H2S were observed. This review highlights functional anatomy of the kidney and renal production of H2S. The review also discusses current understanding of H2S in renal physiology and seeks to lay the foundation as a new targeted therapeutic agent for renal pathologies such as hypertensive nephropathy, diabetic kidney disease and water balance disorders.

A novel CLCNKB variant in a Chinese family with classic Bartter syndrome and prenatal genetic diagnosis

Background

Type III Bartter syndrome (BS), often known as classic Bartter syndrome is caused by variants in CLCNKB gene, which encoding the basolateral chloride channel protein ClC‐Kb, and is characterized by renal salt wasting, hypokalemia, metabolic alkalosis, increased renin, and aldosterone levels.

Methods

A 2‐year‐old boy presented severe malnutrition, severe metabolic alkalosis and severe hypokalemia and was clinically diagnosed with BS. The trio exome sequencing (ES) was performed to discover the genetic cause of this patient, followed by validation using Sanger sequencing and quantitative polymerase chain reaction subsequently.

Results

The genetic analysis indicated that this patient with a compound heterozygous variants of CLCNKB gene including a novel nonsense variant c.876 T > A and a whole‐gene deletion. The two variants were inherited from his parents, respectively. Subsequently, target sequencing of CLCNKB gene was performed for next pregnancy, and prenatal genetic diagnosis was provided for the family.

Conclusions

The results of current study identified the compound heterozygous variants in a patient with classic BS. The novel variant expands the spectrum of CLCNKB variants in BS. Our study also indicates that ES is an alternative tool to simultaneously detect single‐nucleotide variants and copy‐number variants.

Na+-Retaining Action of COX-2 (Cyclooxygenase-2)/EP1 Pathway in the Collecting Duct via Activation of Intrarenal Renin-Angiotensin-Aldosterone System and Epithelial Sodium Channel

BACKGROUND

The collecting duct (CD) is a major site of both biosynthesis and action of prostaglandin E₂ as highlighted by the predominant expression of COX-2 (cyclooxygenase-2) and some E-prostanoid (EP) subtypes at this nephron site. The purpose of this study was to determine the relevance and mechanism of CD COX-2/prostaglandin E₂/EP₁ signaling for the regulation of Na⁺ hemostasis during Na⁺ depletion.

METHODS

Mice with Aqp2Cre-driven deletion of COX-2 (COX-2^fl/flAqp2Cre⁺) or the EP₁ subtype (EP₁^fl/flAqp2Cre⁺) were generated and the Na⁺-wasting phenotype of these mice during low-salt (LS) intake was examined. EP subtypes responsible for prostaglandin E₂-induced local renin response were analyzed in primary cultured mouse inner medullary CD cells.

RESULTS

Following 28-day LS intake, COX-2^fl/flAqp2Cre⁺ mice exhibited a higher urinary Na⁺ excretion and lower cumulative Na⁺ balance, accompanied with suppressed intrarenal renin, AngII (angiotensin II), and aldosterone, expression of CYP11B2 (cytochrome P450 family 11 subfamily B member 2), and blunted expression of epithelial sodium channel subunits compared to floxed controls (COX-2^fl/flAqp2Cre^-), whereas no differences were observed for indices of systemic renin-angiotensin-aldosterone system. In cultured CD cells, exposure to prostaglandin E₂ stimulated release of soluble (pro)renin receptor, prorenin/renin and aldosterone and the stimulation was more sensitive to antagonism of EP₁ as compared other EP subtypes. Subsequently, EP₁^fl/flAqp2Cre⁺ mice largely recapitulated Na⁺-wasting phenotype seen in COX-2^fl/flAqp2Cre⁺ mice.

CONCLUSIONS

The study for the first time reports that CD COX-2/EP₁ pathway might play a key role in maintenance of Na⁺ homeostasis in the face of Na⁺ depletion, at least in part, through activation of intrarenal renin-angiotensin-aldosterone-system and epithelial sodium channel.

Importance of the renal ion channel TRPM6 in the circadian secretion of renin to raise blood pressure

Blood pressure has a daily pattern, with higher values in the active period. Its elevation at the onset of the active period substantially increases the risk of fatal cardiovascular events. Renin secretion stimulated by renal sympathetic neurons is considered essential to this process; however, its regulatory mechanism remains largely unknown. Here, we show the importance of transient receptor potential melastatin-related 6 (TRPM6), a Mg²⁺-permeable cation channel, in augmenting renin secretion in the active period. TRPM6 expression is significantly reduced in the distal convoluted tubule of hypotensive Cnnm2-deficient mice. We generate kidney-specific Trpm6-deficient mice and observe a decrease in blood pressure and a disappearance of its circadian variation. Consistently, renin secretion is not augmented in the active period. Furthermore, renin secretion after pharmacological activation of β-adrenoreceptor, the target of neuronal stimulation, is abrogated, and the receptor expression is decreased in renin-secreting cells. These results indicate crucial roles of TRPM6 in the circadian regulation of blood pressure.

Circadian variation of blood pressure, with higher values in the active period, is associated with the risk of fatal cardiovascular events. Here, we show the importance of renal TRPM6, a Magnesium-permeable cation channel, in raising blood pressure by stimulating renin secretion.

Delayed blockage of prostaglandin EP4 receptors can reduce dedifferentiation, epithelial-to-mesenchymal transition and fibrosis following acute kidney injury

Dedifferentiation of tubular epithelial cells is involved in both regeneration and fibrosis following acute kidney injury (AKI). Prostaglandin E2 receptor 4 (EP4 ) antagonist can inhibit the dedifferentiation of renal tubular cells. The present study investigated whether the time of blockage of EP4 receptors, using grapiprant, could affect the tubular regeneration or interstitial fibrosis in AKI. Cisplatin was used to induce AKI in 72 C57BL/6 adult female mice. Animals were assigned to four groups; control, cisplatin-treated, cisplatin-treated with early grapiprant intervention and cisplatin-treated with late grapiprant intervention. AKI was assessed by kidney function tests and histopathology. Fibrosis was evaluated by Masson's trichrome and alpha smooth muscle actin (α-SMA) expression. Markers of dedifferentiation, CD133, and epithelial to mesenchymal transition (EMT), vimentin were assessed. Early intervention with grapiprant significantly ameliorated AKI more efficiently than late intervention. However, even late intervention was useful in reducing the overall fibrosis as demonstrated by Masson's trichrome and α-SMA expression. In both grapiprant-treated groups, a parallel reduction of dedifferentiation (CD133) and EMT (vimentin) was evident. It seems that the progressive fibrotic changes that follow AKI could still be reduced possibly by targeting dedifferentiation and/or EMT.

Renin Cells, the Kidney, and Hypertension

Renin cells are essential for survival perfected throughout evolution to ensure normal development and defend the organism against a variety of homeostatic threats. During embryonic and early postnatal life, they are progenitors that participate in the morphogenesis of the renal arterial tree. In adult life, they are capable of regenerating injured glomeruli, control blood pressure, fluid-electrolyte balance, tissue perfusion, and in turn, the delivery of oxygen and nutrients to cells. Throughout life, renin cell descendants retain the plasticity or memory to regain the renin phenotype when homeostasis is threatened. To perform all of these functions and maintain well-being, renin cells must regulate their identity and fate. Here, we review the major mechanisms that control the differentiation and fate of renin cells, the chromatin events that control the memory of the renin phenotype, and the major pathways that determine their plasticity. We also examine how chronic stimulation of renin cells alters their fate leading to the development of a severe and concentric hypertrophy of the intrarenal arteries and arterioles. Lastly, we provide examples of additional changes in renin cell fate that contribute to equally severe kidney disorders.

Agonism of Prostaglandin E2 Receptor 4 Ameliorates Tubulointerstitial Injury in Nephrotoxic Serum Nephritis in Mice

Selectively targeting the E-type prostanoid receptor 4 (EP4) might be a new therapeutic option in the treatment of glomerulonephritis (GN), since the EP4 receptor is expressed on different immune cells, resident kidney cells, and endothelial cells, which are all involved in the pathogenesis of immune-complex GN. This study aimed to evaluate the therapeutic potential and to understand the mode of action of EP4 agonist in immune-complex GN using the murine model of nephrotoxic serum nephritis (NTS). In vivo, NTS mice were treated two times daily with two different doses of an EP4 agonist ONO AE1-329 or vehicle for 14 days total. The effect of PGE2 and EP4 agonism and antagonism was tested on murine distal convoluted tubular epithelial cells (DCT) in vitro. In vivo, the higher dose of the EP4 agonist led to an improved NTS phenotype, including a reduced tubular injury score and reduced neutrophil gelatinase-associated lipocalin (NGAL) and blood urea nitrogen (BUN) levels. EP4 agonist treatment caused decreased CD4+ T cell infiltration into the kidney and increased proliferative capacity of tubular cells. Injection of the EP4 agonist resulted in dose-dependent vasodilation and hypotensive episodes. The low-dose EP4 agonist treatment resulted in less pronounced episodes of hypotension. In vitro, EP4 agonism resulted in cAMP production and increased distal convoluted tubular (DCT) proliferation. Taken together, EP4 agonism improved the NTS phenotype by various mechanisms, including reduced blood pressure, decreased CD4+ T cell infiltration, and a direct effect on tubular cells leading to increased proliferation probably by increasing cAMP levels.

The interaction partners of (pro)renin receptor in the distal nephron

The (pro)renin receptor (PRR), a key regulator of intrarenal renin-angiotensin system (RAS), is predominantly presented in podocytes, proximal tubules, distal convoluted tubules, and the apical membrane of collecting duct A-type intercalated cells, and plays a crucial role in hypertension, cardiovascular disease, kidney disease, and fluid homeostasis. In addition to its well-known renin-regulatory function, increasing evidence suggests PRR can also act in a variety of intracellular signaling cascades independently of RAS in the renal medulla, including Wnt/β-catenin signaling, cyclooxygenase-2 (COX-2)/prostaglandin E₂ (PGE₂ ) signaling, and the apelinergic system, and work as a component of the vacuolar H⁺ -ATPase. PRR and these pathways regulate the expression/activity of each other that controlling blood pressure and renal functions. In this review, we highlight recent findings regarding the antagonistic interaction between PRR and ELABELA/apelin, the mutually stimulatory relationship between PRR and COX-2/PGE₂ or Wnt/β-catenin signaling in the renal medulla, and their involvement in the regulation of intrarenal RAS thereby control blood pressure, renal injury, and urine concentrating ability in health and patho-physiological conditions. We also highlight the latest progress in the involvement of PRR for the vacuolar H⁺ -ATPase activity.

Bartter syndrome with long-term follow-up: a case report

Bartter syndrome is a rare inherited disease caused by CLCNKB mutation, which results in inactivation of the chloride channel Kb protein. Bartter syndrome is characterized by extreme hypokalemia, hypochloremia, metabolic alkalosis, hyperrenin-induced angiotensinemia, hyperaldosteronemia, and normal blood pressure. We herein report a case of Bartter syndrome that manifested as vomiting, hypokalemia, metabolic alkalosis, normal blood pressure, and significant hyperrenin-induced angiotensinemia. The patient, a 5-month-old girl, carried two known heterozygous pathogenic mutations: c.88C > T (p.Arg30*), which she had inherited from her father, and c.1313G > A (p.Arg438His), which she had inherited from her mother. Treatment with indomethacin, a nonsteroidal anti-inflammatory drug, led to rapid improvement of the hypokalemia, and treatment was continued for 14 years. The indomethacin also induced a sustainable reduction in the hypokalemia and metabolic alkalosis.

Beyond the Paradigm: Novel Functions of Renin-Producing Cells

The juxtaglomerular renin-producing cells (RPC) of the kidney are referred to as the major source of circulating renin. Renin is the limiting factor in renin-angiotensin system (RAS), which represents a proteolytic cascade in blood plasma that plays a central role in the regulation of blood pressure. Further cells disseminated in the entire organism express renin at a low level as part of tissue RASs, which are thought to locally modulate the effects of systemic RAS. In recent years, it became increasingly clear that the renal RPC are involved in developmental, physiological, and pathophysiological processes outside RAS. Based on recent experimental evidence, a novel concept emerges postulating that next to their traditional role, the RPC have non-canonical RAS-independent progenitor and renoprotective functions. Moreover, the RPC are part of a widespread renin lineage population, which may act as a global stem cell pool coordinating homeostatic, stress, and regenerative responses throughout the organism. This review focuses on the RAS-unrelated functions of RPC - a dynamic research area that increasingly attracts attention.

A novel mutation associated with Type III Bartter syndrome: A report of five cases

The clinical, biochemical and mutation spectra of Chinese patients with Type III Bartter syndrome (type III BS), a rare autosomal recessive disorder, were investigated. A total of five unrelated Chinese patients aged 8 months to 24 years were diagnosed with type III BS via analysis of biochemical markers, including chloride, potassium and calcium, and genetic sequencing. The levels of insulin‑like growth factor‑1 (IGF‑1) were evaluated via ELISA and a mutation study of cultured amniocytes was conducted for prenatal diagnosis. The child patients were admitted for polydipsia, polyuria, myasthenia and developmental delay, whereas the adult patients were hospitalized for limb numbness, polydipsia and polyuria. Nine variants in the chloride voltage‑gated channel Kb (CLCNKB) gene were detected, including eight sequence variants and one whole CLCNKB gene deletion. One sequence variant (c.1967T>C) was novel, whereas the remaining variants (c.595G>T, c.908A>C, c.1004T>C, c.1312C>T, c.1334_1335delCT and c.1718C>A) and the whole gene deletion had been previously reported. The whole gene deletion was frequently observed in patients with early‑onset type III BS in the present study. Two patients showed IGF‑1 deficiency with normal growth hormone level. All patients were treated with potassium supplementation and indometacin. The mother of one patient underwent amniocentesis during her second pregnancy; the fetus was not affected by type III BS based on screening for sequence variants, and normal development and blood electrolyte analysis following birth confirmed the diagnosis. In conclusion, five cases of type III BS in patients from mainland China were reported. Large deletions were frequently detected, particularly in early‑onset patients; isolated IGF‑1 deficiency was found, one novel sequence variant was identified. Prenatal diagnosis was successfully established using genetic analysis of cultured amniocytes, and may facilitate the prevention of congenital defect of type III BS in the next pregnancy.

Bartter syndrome: causes, diagnosis, and treatment

Bartter syndrome is an inherited renal tubular disorder caused by a defective salt reabsorption in the thick ascending limb of loop of Henle, resulting in salt wasting, hypokalemia, and metabolic alkalosis. Mutations of several genes encoding the transporters and channels involved in salt reabsorption in the thick ascending limb cause different types of Bartter syndrome. A poor phenotype-genotype relationship due to the interaction with other cotransporters and different degrees of compensation through alternative pathways is currently reported. However, phenotypic identification still remains the first step to guide the suspicion of Bartter syndrome. Given the rarity of the syndrome, and the lack of genetic characterization in most cases, limited clinical evidence for treatment is available and the therapy is based mainly on the comprehension of renal physiology and relies on the physician's personal experiences. A better understanding of the mutated channels and transporters could possibly generate targets for specific treatment in the future, also encompassing drugs aiming to correct deficiencies in folding or plasma membrane expression of the mutated proteins.

COX-2-derived PGE2 triggers hyperplastic renin expression and hyperreninemia in aldosterone synthase-deficient mice

Pharmacological inhibition or genetic loss of function defects of the renin angiotensin aldosterone system (RAAS) causes compensatory renin cell hyperplasia and hyperreninemia. The triggers for the compensatory stimulation of renin synthesis and secretion in this situation may be multimodal. Since cyclooxygenase-2 (COX-2) expression in the macula densa is frequently increased in states of a defective RAAS, we have investigated a potential role of COX-2 and its derived prostaglandins for renin expression and secretion in aldosterone synthase-deficient mice (AS^−/−) as a model for a genetic defect of the RAAS. In comparison with wild-type mice (WT), AS^−/− mice had 9-fold and 30-fold increases of renin mRNA and of plasma renin concentrations (PRC), respectively. Renin immunoreactivity in the kidney cortex of AS^−/− mice was 10-fold higher than in WT. Macula densa COX-2 expression was 5-fold increased in AS^−/− kidneys relative to WT kidneys. Treatment of AS^−/− mice with the COX-2 inhibitor SC-236 for 1 week lowered both renal renin mRNA and PRC by 70%. Hyperplastic renin cells in AS^−/− kidneys were found to express the prostaglandin E₂ receptors EP2 and EP4. Global deletion of EP2 receptors did not alter renin mRNA nor PRC values in AS^−/− mice. Renin cell-specific inducible deletion of the EP4 receptor lowered renin mRNA and PRC by 25% in AS^−/− mice. Renin cell-specific inducible deletion of the EP4 receptor in combination with global deletion of the EP2 receptor lowered renin mRNA and PRC by 70–75% in AS^−/− mice. Lineage tracing of renin-expressing cells revealed that deletion of EP2 and EP4 leads to a preferential downregulation of perivascular renin expression. Our findings suggest that increased macula densa COX-2 activity in AS^−/− mice triggers perivascular renin expression and secretion via prostaglandin E₂.

Prostaglandin E2 receptor EP4 is involved in the cell growth and invasion of prostate cancer via the cAMP‑PKA/PI3K‑Akt signaling pathway

Prostate cancer (PCa) is one of the most prevalent diagnosed malignancies globally. Previous studies have demonstrated that prostaglandin E2 (PGE2) is closely associated with the tumorigenesis and progression of PCa. However, the underlying molecular mechanisms remain unclear and require further investigation. Matrix metalloproteinases (MMPs), receptor activator of nuclear factor‑κB ligand (RANKL) and runt‑related transcription factor 2 (RUNX2), which are involved in cell growth and bone metastasis, are frequently activated or overexpressed in various types of cancer, including PCa. The present study was designed to investigate the associations between PGE2 and the PGE2 receptor EP4, and MMPs, RANKL and RUNX2 in PCa, and to define their roles in PCa cell proliferation and invasion in addition to understanding the molecular mechanisms. The results of western blotting and reverse transcription‑quantitative polymerase chain reaction demonstrated that the protein and the mRNA expression levels of MMP‑2, MMP‑9, RANKL and RUNX2 in PC‑3 cells were significantly upregulated by treatment with PGE2, respectively, and knockdown of these proteins blocked PGE2‑induced cell proliferation and invasion in PC‑3 cells, as determined by Cell Counting Kit‑8 and Matrigel invasion assays, respectively. The effect of PGE2 on the protein and mRNA expression levels was primarily regulated via the EP4 receptor. EP4 receptor signaling activates the cyclic (c)AMP‑protein kinase A (PKA) signaling pathway, and forskolin, an activator of adenylate cyclase (AC), exhibited similar effects to an EP4 receptor agonist on the protein expression, while SQ22536, an inhibitor of AC, inhibited the protein expression. These results confirmed that the AC/cAMP pathway may be involved in EP4 receptor‑mediated upregulation of protein expression. By using a specific inhibitor of PKA, it was also demonstrated that cAMP/PKA was also involved in the EP4 receptor‑mediated upregulation of protein expression. In addition to the signaling pathway involving PKA, the EP4 receptor also exerts activities through activation of Akt kinase. The results in the present study confirmed the hypothesis that EP4 receptor‑mediated protein expression in PCa cells that were pretreated with a specific inhibitor of phosphatidylinositol 3‑kinase (PI3K) was significantly inhibited. In conclusion, the results of the present study indicate that PGE2 significantly upregulated the mRNA and protein expression levels of the MMP‑2, MMP‑9, RANKL and RUNX2, and the EP4 receptor was involved in the cell proliferation and invasion of PCa via the cAMP‑PKA/PI3K‑Akt signaling pathway. These results may provide novel insight into potential therapeutic strategies for the prevention and treatment of PCa.

Analysis of the calcium paradox of renin secretion

The secretion of the protease renin from renal juxtaglomerular cells is enhanced by subnormal extracellular calcium concentrations. The mechanisms underlying this atypical effect of calcium have not yet been unraveled. We therefore aimed to characterize the effect of extracellular calcium concentration on calcium handling of juxtaglomerular cells and on renin secretion in more detail. For this purpose, we used a combination of experiments with isolated perfused mouse kidneys and direct calcium measurements in renin-secreting cells in situ. We found that lowering of the extracellular calcium concentration led to a sustained elevation of renin secretion. Electron-microscopical analysis of renin-secreting cells exposed to subnormal extracellular calcium concentrations revealed big omega-shaped structures resulting from the intracellular fusion and subsequent emptying of renin storage vesicles. The calcium concentration dependencies as well as the kinetics of changes were rather similar for renin secretion and for renovascular resistance. Since vascular resistance is fundamentally influenced by myosin light chain kinase (MLCK), myosin light chain phosphatase (MLCP), and Rho-associated protein kinase (Rho-K) activities, we examined the effects of MLCK-, MLCP-, and Rho-K inhibitors on renin secretion. Only MLCK inhibition stimulated renin secretion. Conversely, inhibition of MCLP activity lowered perfusate flow and strongly inhibited renin secretion, which could not be reversed by lowering of the extracellular calcium concentration. Renin-secreting cells and smooth muscle cells of afferent arterioles showed immunoreactivity of MLCK. These findings suggest that the inhibitory effect of calcium on renin secretion could be explained by phosphorylation-dependent processes under control of the MLCK.

Angiotensin II-AT1-receptor signaling is necessary for cyclooxygenase-2-dependent postnatal nephron generation

Deletion of cyclooxygenase-2 (COX-2) causes impairment of postnatal kidney development. Here we tested whether the renin angiotensin system contributes to COX-2-dependent nephrogenesis in mice after birth and whether a rescue of impaired renal development and function in COX-2^-/- mice was achievable. Plasma renin concentration in mouse pups showed a birth peak and a second peak around day P8 during the first 10 days post birth. Administration of the angiotensin II receptor AT1 antagonist telmisartan from day P1 to P3 did not result in cortical damage. However, telmisartan treatment from day P3 to P8, the critical time frame of renal COX-2 expression, led to hypoplastic glomeruli, a thinned subcapsular cortex and maturational arrest of superficial glomeruli quite similar to that observed in COX-2^-/- mice. In contrast, AT2 receptor antagonist PD123319 was without any effect on renal development. Inhibition of the renin angiotensin system by aliskiren and enalapril caused similar glomerular defects as telmisartan. Administration of the AT1 receptor agonist L162313 to COX-2^-/- pups improved kidney growth, ameliorated renal defects, but had no beneficial effect on reduced cortical mass. L162313 rescued impaired renal function by reducing serum urea and creatinine and mitigated pathologic albumin excretion. Moreover, glomerulosclerosis in the kidneys of COX-2^-/- mice was reduced. Thus, angiotensin II-AT1-receptor signaling is necessary for COX-2-dependent normal postnatal nephrogenesis and maturation.

Vascular Smooth Muscle-Specific EP4 Receptor Deletion in Mice Exacerbates Angiotensin II-Induced Renal Injury

AIMS

Cyclooxygenase inhibition by non-steroidal anti-inflammatory drugs is contraindicated in hypertension, as it may reduce glomerular filtration rate (GFR) and renal blood flow. However, the identity of the specific eicosanoid and receptor underlying these effects is not known. We hypothesized that vascular smooth muscle prostaglandin E2 (PGE2) E-prostanoid 4 (EP4) receptor deletion predisposes to renal injury via unchecked vasoconstrictive actions of angiotensin II (AngII) in a hypertension model. Mice with inducible vascular smooth muscle cell (VSMC)-specific EP4 receptor deletion were generated and subjected to AngII-induced hypertension.

RESULTS

EP4 deletion was verified by PCR of aorta and renal vessels, as well as functionally by loss of PGE2-mediated mesenteric artery relaxation. Both AngII-treated groups became similarly hypertensive, whereas albuminuria, foot process effacement, and renal hypertrophy were exacerbated in AngII-treated EP4^VSMC-/- but not in EP4^VSMC+/+ mice and were associated with glomerular scarring, tubulointerstitial injury, and reduced GFR. AngII-treated EP4^VSMC-/- mice exhibited capillary damage and reduced renal perfusion as measured by fluorescent bead microangiography and magnetic resonance imaging, respectively. NADPH oxidase 2 (Nox2) expression was significantly elevated in AngII-treated EP4^-/- mice. EP4-receptor silencing in primary VSMCs abolished PGE2 inhibition of AngII-induced Nox2 mRNA and superoxide production.

INNOVATION

These data suggest that vascular EP4 receptors buffer the actions of AngII on renal hemodynamics and oxidative injury.

CONCLUSION

EP4 agonists may, therefore, protect against hypertension-associated kidney damage. Antioxid. Redox Signal. 25, 642-656.

Analysis of mutations of two Gitelman syndrome family SLC12A3 genes and proposed treatments using Chinese medicine

OBJECTIVE

To determine the gene location of two Gitelman syndrome (GS) family SLC12A3 genes and explore treatments using Chinese medicine (CM) prescriptions.

METHODS

In order to locate the two GS mutations, samples were collected from 11 people from two different pedigrees for direct genetic sequencing and comparison of the 26 exons of SLC12A3. Furthermore, the change of serum potassium was monitored throughout the therapy and those two probands undertook a sequential superposition of Western medicine (including potassium, Panangin and potassium-sparing diuretics) with CM prescription based on Buyang Huanwu Decoction () and Sijunzi Decoction (). The treatment included three stages, oral potassium chloride for the first 2 weeks (stage 1), potassium-sparing diuretic and Panangin with potassium chloride for the next 2 weeks (stage 2), CM along with the medicine in stage 2 for the final 2 weeks (stage 3).

RESULTS

The three mutations occurring in proband 1 from pedigree I were Thr60Met, 965-1_976del13ins12 (small indels mutation) and Ala122Ala (homozygous silent mutation). Likewise, three mutations, Asn359Lys, Thr382Met and Arg913Gln, appeared in the proband 2 from pedigree II. The serum potassium levels increasing from baseline to sequential stages were 1.63 mmol/L (baseline), 2.5 mmol/L (stage 1), 3.1 mmol/L (stage 2) and 3.9 mmol/L (stage 3) in the proband 1, and 2.8 mmol/L (baseline), 3.1 mmol/L (stage 1), 3.5 mmol/L (stage 2) and 4.3 mmol/L (stage 3) in the proband 2, respectively. The symptoms (numbness of limbs, weakness, palpitations, etc.) of both probands were all alleviated.

CONCLUSIONS

The mutations of both GS pedigrees can be defined as compound heterozygous mutations, most of which are known as missense mutations. Applying CM could be an appropriate choice for future intervention of GS.

The renin-angiotensin-aldosterone system and calcium-regulatory hormones

There is increasing evidence of a clinically relevant interplay between the renin-angiotensin-aldosterone system and calcium-regulatory systems. Classically, the former is considered a key regulator of sodium and volume homeostasis, while the latter is most often associated with skeletal health. However, emerging evidence suggests an overlap in regulatory control. Hyperaldosteronism and hyperparathyroidism represent pathophysiologic conditions that may contribute to or perpetuate each other; aldosterone regulates parathyroid hormone and associates with adverse skeletal complications, and parathyroid hormone regulates aldosterone and associates with adverse cardiovascular complications. As dysregulation in both systems is linked to poor cardiovascular and skeletal health, it is increasingly important to fully characterize how they interact to more precisely understand their impact on human health and potential therapies to modulate these interactions. This review describes the known clinical interactions between these two systems including observational and interventional studies. Specifically, we review studies describing the inhibition of renin activity by calcium and vitamin D, and a potentially bidirectional and stimulatory relationship between aldosterone and parathyroid hormone. Deciphering these relationships might clarify variability in outcomes research, inform the design of future intervention studies and provide insight into the results of prior and ongoing intervention studies. However, before these opportunities can be addressed, more effort must be placed on shifting observational data to the proof of concept phase. This will require reallocation of resources to conduct interventional studies and secure the necessary talent.

Early co-expression of cyclooxygenase-2 and renin in the rat kidney cortex contributes to the development of N(G)-nitro-L-arginine methyl ester induced hypertension

We investigated the involvement of cyclooxygenase-2 (COX-2) and the renin-angiotensin system in N(G)-nitro-L-arginine methyl ester (L-NAME)-induced hypertension. Male Wistar rats were treated with L-NAME (75.0 mg·(kg body mass)(-1)·day(-1), in their drinking water) for different durations (1-33 days). COX-2 and renin mRNA were measured using real-time PCR in the renal cortex, and prostanoids were assessed in the renal perfusate, whereas angiotensin II (Ang II) and Ang (1-7) were quantified in plasma. In some rats, nitric oxide synthase inhibition was carried out in conjunction with oral administration of captopril (30.0 mg·kg(-1)·day(-1)) or celecoxib (1.0 mg·kg(-1)·day(-1)) for 2 or 19 days. We found a parallel increase in renocortical COX-2 and renin mRNA starting at day 2 of treatment with L-NAME, and both peaked at 19-25 days. In addition, L-NAME increased renal 6-Keto-PGF(1α) (prostacyclin (PGI2) metabolite) and plasma Ang II from day 2, but reduced plasma Ang (1-7) at day 19. Captopril prevented the increase in blood pressure, which was associated with lower plasma Ang II and increased COX-2-derived 6-Keto-PGF(1α) at day 2 and plasma Ang (1-7) at day 19. Celecoxib partially prevented the increase in blood pressure; this effect was associated with a reduction in plasma Ang II. These findings indicate that renal COX-2 expression increased in parallel with renin expression, renal PGI2 synthesis, and plasma Ang II in L-NAME-induced hypertension.

Cyclooxygenase metabolites in the kidney

In the mammalian kidney, prostaglandins (PGs) are important mediators of physiologic processes, including modulation of vascular tone and salt and water. PGs arise from enzymatic metabolism of free arachidonic acid (AA), which is cleaved from membrane phospholipids by phospholipase A2 activity. The cyclooxygenase (COX) enzyme system is a major pathway for metabolism of AA in the kidney. COX are the enzymes responsible for the initial conversion of AA to PGG2 and subsequently to PGH2, which serves as the precursor for subsequent metabolism by PG and thromboxane synthases. In addition to high levels of expression of the "constitutive" rate-limiting enzyme responsible for prostanoid production, COX-1, the "inducible" isoform of cyclooxygenase, COX-2, is also constitutively expressed in the kidney and is highly regulated in response to alterations in intravascular volume. PGs and thromboxane A2 exert their biological functions predominantly through activation of specific 7-transmembrane G-protein-coupled receptors. COX metabolites have been shown to exert important physiologic functions in maintenance of renal blood flow, mediation of renin release and regulation of sodium excretion. In addition to physiologic regulation of prostanoid production in the kidney, increases in prostanoid production are also seen in a variety of inflammatory renal injuries, and COX metabolites may serve as mediators of inflammatory injury in renal disease.

Putative interaction of brush cells with bicarbonate secreting cells in the proximal corpus mucosa

The gastric epithelium is protected from the highly acidic luminal content by alkaline mucus which is secreted from specialized epithelial cells. In the stomach of mice strong secretion of alkaline fluid was observed at the “gastric groove,” the border between corpus and fundus mucosa. Since this region is characterized by numerous brush cells it was proposed that these cells might secrete alkaline solution as suggested for brush cells in the bile duct. In fact, it was found that in this region multiple cells express elements which are relevant for the secretion of bicarbonate, including carbonic anhydrase (CAII), the cystic fibrosis transmembrane conductance regulator (CFTR) and the Na⁺/H⁺ exchanger (NHE1). However, this cell population was distinct from brush cells which express the TRP-channel TRPM5 and are considered as putative sensory cells. The location of both cell populations in close proximity implies the possibility for a paracrine interaction. This view was substantiated by the finding that brush cells express prostaglandin synthase-1 (COX-1) and the neighboring cells a specific receptor type for prostaglandins. The notion that brush cells may be able to sense a local acidification was supported by the observation that they express the channel PKD1L3 which contributes to the acid responsiveness of gustatory sensory cells. The results support the concept that brush cells may sense the luminal content and influence via prostaglandins the secretion of alkaline solution.

E-type prostanoid receptor 4 (EP4) in disease and therapy

The large variety of biological functions governed by prostaglandin (PG) E2 is mediated by signaling through four distinct E-type prostanoid (EP) receptors. The availability of mouse strains with genetic ablation of each EP receptor subtype and the development of selective EP agonists and antagonists have tremendously advanced our understanding of PGE2 as a physiologically and clinically relevant mediator. Moreover, studies using disease models revealed numerous conditions in which distinct EP receptors might be exploited therapeutically. In this context, the EP4 receptor is currently emerging as most versatile and promising among PGE2 receptors. Anti-inflammatory, anti-thrombotic and vasoprotective effects have been proposed for the EP4 receptor, along with its recently described unfavorable tumor-promoting and pro-angiogenic roles. A possible explanation for the diverse biological functions of EP4 might be the multiple signaling pathways switched on upon EP4 activation. The present review attempts to summarize the EP4 receptor-triggered signaling modules and the possible therapeutic applications of EP4-selective agonists and antagonists.

New roles for renin and prorenin in heart failure and cardiorenal crosstalk

The renin-angiotensin-aldosterone-system (RAAS) plays a central role in the pathophysiology of heart failure and cardiorenal interaction. Drugs interfering in the RAAS form the pillars in treatment of heart failure and cardiorenal syndrome. Although RAAS inhibitors improve prognosis, heart failure–associated morbidity and mortality remain high, especially in the presence of kidney disease. The effect of RAAS blockade may be limited due to the loss of an inhibitory feedback of angiotensin II on renin production. The subsequent increase in prorenin and renin may activate several alternative pathways. These include the recently discovered (pro-) renin receptor, angiotensin II escape via chymase and cathepsin, and the formation of various angiotensin subforms upstream from the blockade, including angiotensin 1–7, angiotensin III, and angiotensin IV. Recently, the direct renin inhibitor aliskiren has been proven effective in reducing plasma renin activity (PRA) and appears to provide additional (tissue) RAAS blockade on top of angiotensin-converting enzyme and angiotensin receptor blockers, underscoring the important role of renin, even (or more so) under adequate RAAS blockade. Reducing PRA however occurs at the expense of an increase plasma renin concentration (PRC). PRC may exert direct effects independent of PRA through the recently discovered (pro-) renin receptor. Additional novel possibilities to interfere in the RAAS, for instance using vitamin D receptor activation, as well as the increased knowledge on alternative pathways, have revived the question on how ideal RAAS-guided therapy should be implemented. Renin and prorenin are pivotal since these are at the base of all of these pathways.

Endothelium-derived nitric oxide supports renin cell recruitment through the nitric oxide-sensitive guanylate cyclase pathway

Chronic challenge of renin-angiotensin causes recruitment of renin-producing cells in the kidney along the media layer of afferent arterioles and hypertrophy of cells in the juxtaglomerular apparatus. This study aimed to define the role of nitric oxide (NO) with regard to the recruitment pattern of renin-producing cells and to the possible pathways along which NO could act. We considered the hypothesis that endothelium-derived NO acts via NO-sensitive guanylate cyclase. Mice were treated with low-salt diet in combination with the angiotensin I-converting enzyme inhibitor enalapril for 3 weeks, which led to a 13-fold increase in renin expression associated with marked recruitment of renin cells in afferent arterioles and hypertrophy of the juxtaglomerular apparatus in wild-type mice. In wild-type mice additionally treated with the nonselective NO synthase inhibitor L-NAME, the recruitment of renin-expressing cells along the afferent arterioles was absent and juxtaglomerular hypertrophy was diminished. An almost identical attenuation of renin cell recruitment as with L-NAME treatment in wild-type mice was found in mice lacking the endothelial isoform of NO synthase. Treatment of mice lacking NO-sensitive guanylate cyclase in renin-expressing cells and preglomerular smooth muscle cells with low-salt diet in combination with the angiotensin I-converting enzyme inhibitor enalapril for 3 weeks produced juxtaglomerular hypertrophy like in wild-type mice, but no recruitment in afferent arterioles. These findings suggest that endothelium-derived NO and concomitant formation of cGMP in preglomerular renin cell precursors supports recruitment of renin-expressing cells along preglomerular vessels, but not in the juxtaglomerular apparatus.

Juxtaglomerular cell CaSR stimulation decreases renin release via activation of the PLC/IP(3) pathway and the ryanodine receptor

The calcium-sensing receptor (CaSR) is a G-coupled protein expressed in renal juxtaglomerular (JG) cells. Its activation stimulates calcium-mediated decreases in cAMP content and inhibits renin release. The postreceptor pathway for the CaSR in JG cells is unknown. In parathyroids, CaSR acts through G(q) and/or G(i). Activation of G(q) stimulates phospholipase C (PLC), and inositol 1,4,5-trisphosphate (IP(3)), releasing calcium from intracellular stores. G(i) stimulation inhibits cAMP formation. In afferent arterioles, the ryanodine receptor (RyR) enhances release of stored calcium. We hypothesized JG cell CaSR activation inhibits renin via the PLC/IP(3) and also RyR activation, increasing intracellular calcium, suppressing cAMP formation, and inhibiting renin release. Renin release from primary cultures of isolated mouse JG cells (n = 10) was measured. The CaSR agonist cinacalcet decreased renin release 56 ± 7% of control (P < 0.001), while the PLC inhibitor U73122 reversed cinacalcet inhibition of renin (104 ± 11% of control). The IP(3) inhibitor 2-APB also reversed inhibition of renin from 56 ± 6 to 104 ± 11% of control (P < 0.001). JG cells were positively labeled for RyR, and blocking RyR reversed CaSR-mediated inhibition of renin from 61 ± 8 to 118 ± 22% of control (P < 0.01). Combining inhibition of IP(3) and RyR was not additive. G(i) inhibition with pertussis toxin plus cinacalcet did not reverse renin inhibition (65 ± 12 to 41 ± 8% of control, P < 0.001). We conclude stimulating JG cell CaSR activates G(q), initiating the PLC/IP(3) pathway, activating RyR, increasing intracellular calcium, and resulting in calcium-mediated renin inhibition.

Restoration of renal function by a novel prostaglandin EP4 receptor-derived peptide in models of acute renal failure

Acute renal failure (ARF) is a serious medical complication characterized by an abrupt and sustained decline in renal function. Despite significant advances in supportive care, there is currently no effective treatment to restore renal function. PGE(2) is a lipid hormone mediator abundantly produced in the kidney, where it acts locally to regulate renal function; several studies suggest that modulating EP(4) receptor activity could improve renal function following kidney injury. An optimized peptidomimetic ligand of EP(4) receptor, THG213.29, was tested for its efficacy to improve renal function (glomerular filtration rate, renal plasma flow, and urine output) and histological changes in a model of ARF induced by either cisplatin or renal artery occlusion in Sprague-Dawley rats. THG213.29 modulated PGE(2)-binding dissociation kinetics, indicative of an allosteric binding mode. Consistently, THG213.29 antagonized EP(4)-mediated relaxation of piglet saphenous vein rings, partially inhibited EP(4)-mediated cAMP production, but did not affect Gα(i) activation or β-arrestin recruitment. In vivo, THG213.29 significantly improved renal function and histological changes in cisplatin- and renal artery occlusion-induced ARF models. THG213.29 increased mRNA expression of heme-oxygenase 1, Bcl2, and FGF-2 in renal cortex; correspondingly, in EP(4)-transfected HEK293 cells, THG213.29 augmented FGF-2 and abrogated EP(4)-dependent overexpression of inflammatory IL-6 and of apoptotic death domain-associated protein and BCL2-associated agonist of cell death. Our results demonstrate that THG213.29 represents a novel class of diuretic agent with noncompetitive allosteric modulator effects on EP(4) receptor, resulting in improved renal function and integrity following acute renal failure.

The PGE(2)-EP4 receptor is necessary for stimulation of the renin-angiotensin-aldosterone system in response to low dietary salt intake in vivo

Increased cyclooxygenase-2 (COX-2) expression and PGE(2) synthesis have been shown to be prerequisites for renal renin release after Na(+) deprivation. To answer the question of whether EP4 receptor type of PGE(2) mediates renin regulation under a low-salt diet, we examined renin regulation in EP4(+/+), EP4(-/-), and in wild-type mice treated with EP4 receptor antagonist. After 2 wk of a low-salt diet (0.02% wt/wt NaCl), EP4(+/+) mice showed diminished Na(+) excretion, unchanged K(+) excretion, and reduced Ca(2+) excretion. Diuresis and plasma electrolytes remained unchanged. EP4(-/-) exhibited a similar attenuation of Na(+) excretion; however, diuresis and K(+) excretion were enhanced, and plasma Na(+) concentration was higher, whereas plasma K(+) concentration was lower compared with control diet. There were no significant differences between EP4(+/+) and EP4(-/-) mice in blood pressure, creatinine clearance, and plasma antidiuretic hormone (ADH) concentration. Following salt restriction, plasma renin and aldosterone concentrations and kidney renin mRNA level rose significantly in EP4(+/+) but not in EP4(-/-) and in wild-type mice treated with EP4 antagonist ONO-AE3-208. In the latter two groups, the low-salt diet caused a significantly greater rise in PGE(2) excretion. Furthermore, mRNA expression for COX-2 and PGE(2) synthetic activity was significantly greater in EP4(-/-) than in EP4(+/+) mice. We conclude that low dietary salt intake induces expression of COX-2 followed by enhanced renal PGE(2) synthesis, which stimulates the renin-angiotensin-aldosterone system by activation of EP4 receptor. Most likely, defects at the step of EP4 receptor block negative feedback mechanisms on the renal COX system, leading to persistently high PGE(2) levels, diuresis, and K(+) loss.

Temporal expression of the PGE2 synthetic system in the kidney is associated with the time frame of renal developmental vulnerability to cyclooxygenase-2 inhibition

Pharmacological blockade of cyclooxygenase-2 (COX-2) causes impairment of kidney development. The present study was aimed at determining temporal expression pattern and activity of the PGE(2) synthetic pathway during postnatal nephrogenesis in mice and its association to the time window sensitive to COX-2 inhibition. During the first 10 days after birth, we observed transient induction of mRNA and protein for microsomal PGE synthase (mPGES)-1 between postnatal days 4 (P4) and P8, but not for mPGES-2 or cytosolic PGE synthase (cPGES). PGE(2) synthetic activity using arachidonic acid and PGH(2) as substrates and also urinary excretion of PGE(2) were enhanced during this time frame. In parallel to the PGE(2) system, COX-2 but not COX-1 expression was also transiently induced. Studying glomerulogenesis in EP receptor knockout mice revealed a reduction in glomerular size in EP1(-/-), EP2(-/-), and EP4(-/-) mice, supporting the developmental role of PGE(2). The most vulnerable time window to COX-2 inhibition by SC-236 was found closely related to the temporal expression of COX-2 and mPGES-1. The strongest effects of COX-2 inhibition were achieved following 8 days of drug administration. Similar developmental damage was caused by application of rofecoxib, but not by the COX-1-selective inhibitor SC-560. COX-2 inhibition starting after P10 has had no effect on the size of glomeruli or on the relative number of superficial glomeruli; however, growth of the renal cortex was significantly diminished, indicating the requirement of COX-2 activity after P10. Effects of COX-2 inhibition on renal cell differentiation and on renal fibrosis needed a prolonged time of exposition of at least 10 days. In conclusion, temporal expression of the PGE(2) synthetic system coincides with the most vulnerable age interval for the induction of irreversible renal abnormalities. We assume that mPGES-1 is coregulated with COX-2 for PGE(2) synthesis to orchestrate postnatal kidney development and growth.

The influence of extracellular and intracellular calcium on the secretion of renin

Changes in plasma, extracellular, and intracellular calcium can affect renin secretion from the renal juxtaglomerular (JG) cells. Elevated intracellular calcium directly inhibits renin release from JG cells by decreasing the dominant second messenger intracellular cyclic adenosine monophosphate (cAMP) via actions on calcium-inhibitable adenylyl cyclases and calcium-activated phosphodiesterases. Increased extracellular calcium also directly inhibits renin release by stimulating the calcium-sensing receptor (CaSR) on JG cells, resulting in parallel changes in the intracellular environment and decreasing intracellular cAMP. In vivo, acutely elevated plasma calcium inhibits plasma renin activity (PRA) via parathyroid hormone-mediated elevations in renal cortical interstitial calcium that stimulate the JG cell CaSR. However, chronically elevated plasma calcium or CaSR activation may actually stimulate PRA. This elevation in PRA may be a compensatory mechanism resulting from calcium-mediated polyuria. Thus, changing the extracellular calcium in vitro or in vivo results in inversely related acute changes in cAMP, and therefore renin release, but chronic changes in calcium may result in more complex interactions dependent upon the duration of changes and the integration of the body's response to these changes.

Synthesis and secretion of renin in mice with induced genetic mutations

The juxtaglomerular (JG) cell product renin is rate limiting in the generation of the bioactive octapeptide angiotensin II. Rates of synthesis and secretion of the aspartyl protease renin by JG cells are controlled by multiple afferent and efferent pathways originating in the CNS, cardiovascular system, and kidneys, and making critical contributions to the maintenance of extracellular fluid volume and arterial blood pressure. Since both excesses and deficits of angiotensin II have deleterious effects, it is not surprising that control of renin is secured by a complex system of feedforward and feedback relationships. Mice with genetic alterations have contributed to a better understanding of the networks controlling renin synthesis and secretion. Essential input for the setting of basal renin generation rates is provided by β-adrenergic receptors acting through cyclic adenosine monophosphate, the primary intracellular activation mechanism for renin mRNA generation. Other major control mechanisms include COX-2 and nNOS affecting renin through PGE2, PGI2, and nitric oxide. Angiotensin II provides strong negative feedback inhibition of renin synthesis, largely an indirect effect mediated by baroreceptor and macula densa inputs. Adenosine appears to be a dominant factor in the inhibitory arms of the baroreceptor and macula densa mechanisms. Targeted gene mutations have also shed light on a number of novel aspects related to renin processing and the regulation of renin synthesis and secretion.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Renin release: sites, mechanisms, and control

In the adult organism, systemically circulating renin almost exclusively originates from the juxtaglomerular cells in the afferent arterioles of the kidneys. These cells share similarities with pericytes and myofibro-blasts. They store renin in a vesicular network and granules and release it in a regulated fashion. The release mode of renin is not understood; in particular, the involvement of SNARE proteins is unknown. Renin release is acutely increased via the cAMP signaling pathway, which is triggered mainly by catecholamines and other G(s)-coupled agonists, and is inhibited by calcium-related pathways that are commonly activated by vasoconstrictors. Renin release from juxtaglomerular cells is directly modulated in an inverse fashion by the blood pressure inside the afferent arterioles and by the chloride content in the tubule fluid at the macula densa segment of the distal tubule. Renin release is stimulated by nitric oxide and by prostanoids released by neighboring endothelial and macula densa cells. Steady-state renin concentrations in the plasma are determined essentially by the number of renin-producing cells in the afferent arterioles, which changes in parallel with challenges to the renin-angiotensin-aldosterone system.

PPARgamma-dependent regulation of adenylate cyclase 6 amplifies the stimulatory effect of cAMP on renin gene expression

The second messenger cAMP plays an important role in the regulation of renin gene expression. Nuclear receptor peroxisome proliferator-activated receptor-γ (PPARγ) is known to stimulate renin gene transcription acting through PPARγ-binding sequences in renin promoter. We show now that activation of PPARγ by unsaturated fatty acids or thiazolidinediones drastically augments the cAMP-dependent increase of renin mRNA in the human renin-producing cell line Calu-6. The underlying mechanism involves potentiation of agonist-induced cAMP increase and up-regulation of adenylate cyclase 6 (AC6) gene expression. We identified a palindromic element with a 3-bp spacer (Pal3) in AC6 intron 1 (AC6Pal3). AC6Pal3 bound PPARγ and mediated trans-activation by PPARγ agonist. AC6 knockdown decreased basal renin mRNA level and attenuated the maximal PPARγ-dependent stimulation of the cAMP-induced renin gene expression. AC6Pal3 decoy oligonucleotide abrogated the PPARγ-dependent potentiation of cAMP-induced renin gene expression. Treatment of mice with PPARγ agonist increased AC6 mRNA kidney levels. Our data suggest that in addition to its direct effect on renin gene transcription, PPARγ "sensitizes" renin gene to cAMP via trans-activation of AC6 gene. AC6 has been identified as PPARγ target gene with a functional Pal3 sequence.

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 ).

Hypotonicity-induced Renin exocytosis from juxtaglomerular cells requires aquaporin-1 and cyclooxygenase-2

The mechanism by which extracellular hypotonicity stimulates release of renin from juxtaglomerular (JG) cells is unknown. We hypothesized that osmotically induced renin release depends on water movement through aquaporin-1 (AQP1) water channels and subsequent prostanoid formation. We recorded membrane capacitance (C(m)) by whole-cell patch clamp in single JG cells as an index of exocytosis. Hypotonicity increased C(m) significantly and enhanced outward current. Indomethacin, PLA(2) inhibition, and an antagonist of prostaglandin transport impaired the C(m) and current responses to hypotonicity. Hypotonicity also increased exocytosis as determined by a decrease in single JG cell quinacrine fluorescence in an indomethacin-sensitive manner. In single JG cells from COX-2(-/ -) and AQP1(-/ -) mice, hypotonicity increased neither C(m) nor outward current, but 0.1-muM PGE(2) increased both in these cells. A reduction in osmolality enhanced cAMP accumulation in JG cells but not in renin-producing As4.1 cells; only the former had detectable AQP1 expression. Inhibition of protein kinase A blocked the hypotonicity-induced C(m) and current response in JG cells. Taken together, our results show that a 5 to 7% decrease in extracellular tonicity leads to AQP1-mediated water influx in JG cells, PLA(2)/COX-2-mediated prostaglandin-dependent formation of cAMP, and activation of PKA, which promotes exocytosis of renin.

Prostaglandin F2alpha elevates blood pressure and promotes atherosclerosis

Little is known about prostaglandin F(2alpha) in cardiovascular homeostasis. Prostaglandin F(2alpha) dose-dependently elevates blood pressure in WT mice via activation of the F prostanoid (FP) receptor. The FP is expressed in preglomerular arterioles, renal collecting ducts, and the hypothalamus. Deletion of the FP reduces blood pressure, coincident with a reduction in plasma renin concentration, angiotensin, and aldosterone, despite a compensatory up-regulation of AT1 receptors and an augmented hypertensive response to infused angiotensin II. Plasma and urinary osmolality are decreased in FP KOs that exhibit mild polyuria and polydipsia. Atherogenesis is retarded by deletion of the FP, despite the absence of detectable receptor expression in aorta or in atherosclerotic lesions in Ldlr KOs. Although vascular TNF(alpha), inducible nitric oxide enzyme and TGF(beta) are reduced and lesional macrophages are depleted in the FP/Ldlr double KOs, this result reflects the reduction in lesion burden, as the FP is not expressed on macrophages and its deletion does not alter macrophage cytokine generation. Blockade of the FP offers an approach to the treatment of hypertension and its attendant systemic vascular disease.

Regulation of renin release by local and systemic factors

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.

Physiological regulation of prostaglandins in the kidney

Cyclooxygenase-derived prostanoids exert complex and diverse functions within the kidney. The biological effect of each prostanoid is controlled at multiple levels, including (a) enzymatic reactions catalyzed sequentially by cyclooxygenase and prostanoid synthase for the synthesis of bioactive prostanoid and (b) the interaction with its receptors that mediate its functions. Cyclooxygenase-derived prostanoids act in an autocrine or a paracrine fashion and can serve as physiological buffers, protecting the kidney from excessive functional changes during physiological stress. Through these actions, prostanoids play important roles in maintaining renal function, body fluid homeostasis, and blood pressure. Renal cortical COX2-derived prostanoids, particularly PGI2 and PGE2, play critical roles in maintaining blood pressure and renal function in volume-contracted states. Renal medullary COX2-derived prostanoids appear to have an antihypertensive effect in individuals challenged with a high-salt diet. Loss of EP2 or IP receptor is associated with salt-sensitive hypertension. COX2 also plays a role in maintaining renal medullary interstitial cell viability in the hypertonic environment of the medulla. Cyclooxygenase-derived prostanoids also are involved in certain pathological processes. The cortical COX2-derived PGI2 participates in the pathogenesis of renal vascular hypertension through stimulating renal renin synthesis and release. COX-derived prostanoids also appear to be involved in the pathogenesis of diabetic nephropathy. COXs, prostanoid synthases, and prostanoid receptors should provide fruitful targets for intervention in the pharmacological treatment of renal disease.

Compartmentalized cAMP signalling in regulated exocytic processes in non-neuronal cells

Cyclic adenosine monophosphate (cAMP) is a central second messenger controlling a plethora of vital functions. Studies of cAMP dynamics in living cells have revealed markedly inhomogeneous concentrations of the second messenger in different compartments. Moreover, cAMP effectors such as cAMP-dependent protein kinase (PKA) and cAMP-activated GTP-exchange factors (Epacs) are tethered to specific cellular sites. Both the tailoring of cAMP concentrations, and the activities of cAMP-dependent signalling systems at specific cellular locations are prerequisites for most, if not all, cAMP-dependent processes. This review focuses on the role of compartmentalized cAMP signalling in exocytic processes in non-neuronal cells. Particularly, the insertion of aquaporin-2 into the plasma membrane of renal principal cells as an example for a cAMP-dependent exocytic process in a non-secretory cell type, renin secretion from juxtaglomerular cells as a cAMP-triggered exocytosis from an endocrine cell, insulin release from pancreatic beta-cells as a Ca2+-mediated and cAMP-potentiated exocytic processes in an endocrine cell, and cAMP- or Ca2+ -triggered H+ secretion from gastric parietal cells as an exocytic process in an exocrine cell are discussed. The selected examples of cAMP-regulated exocytic pathways are reviewed with regard to key proteins involved: adenylyl cyclases, phosphodiesterases, PKA, A kinase anchoring proteins (AKAPs) and Epacs.

Prostaglandins in the kidney: developments since Y2K

There are five major PGs (prostaglandins/prostanoids) produced from arachidonic acid via the COX (cyclo-oxygenase) pathway: PGE(2), PGI(2) (prostacyclin), PGD(2), PGF(2alpha) and TXA(2) (thromboxane A(2)). They exert many biological effects through specific G-protein-coupled membrane receptors, namely EP (PGE(2) receptor), IP (PGI(2) receptor), DP (PGD(2) receptor), FP (PGF(2alpha) receptor) and TP (TXA(2) receptor) respectively. PGs are implicated in physiological and pathological processes in all major organ systems, including cardiovascular function, gastrointestinal responses, reproductive processes, renal effects etc. This review highlights recent insights into the role of each prostanoid in regulating various aspects of renal function, including haemodynamics, renin secretion, growth responses, tubular transport processes and cell fate. A thorough review of the literature since Y2K (year 2000) is provided, with a general overview of PGs and their synthesis enzymes, and then specific considerations of each PG/prostanoid receptor system in the kidney.

Renin Release

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

Physiologic and pathophysiologic roles of lipid mediators in the kidney

Small lipids such as eicosanoids exert diverse and complex functions. In addition to their role in regulating normal kidney function, these lipids also play important roles in the pathogenesis of kidney diseases. Cyclooxygenase (COX)-derived prostanoids play important role in maintaining renal function, body fluid homeostasis, and blood pressure. Renal cortical COX2-derived prostanoids, particularly (PGI2) and PGE2 play critical roles in maintaining blood pressure and renal function in volume contracted states. Renal medullary COX2-derived prostanoids appear to have antihypertensive effect in individuals challenged with a high salt diet. 5-Lipoxygenase (LO)-derived leukotrienes are involved in inflammatory glomerular injury. LO product 12-hydroxyeicosatetraenoic acid (12-HETE) is associated with pathogenesis of hypertension, and may mediate angiotensin II and TGFbeta induced mesengial cell abnormality in diabetic nephropathy. P450 hydroxylase-derived 20-HETE is a potent vasoconstrictor and is involved in the pathogenesis of hypertension. P450 epoxygenase derived epoxyeicosatrienoic acids (EETs) have vasodilator and natriuretic effect. Blockade of EET formation is associated with salt-sensitive hypertension. Ceramide has also been demonstrated to be an important signaling molecule, which is involved in pathogenesis of acute kidney injury caused by ischemia/reperfusion, and toxic insults. Those pathways should provide fruitful targets for intervention in the pharmacologic treatment of renal disease.

Regulation of renin in mice with Cre recombinase-mediated deletion of G protein Gsα in juxtaglomerular cells

By crossing mice with expression of Cre recombinase under control of the endogenous renin promoter (Sequeira Lopez ML, Pentz ES, Nomasa T, Smithies O, Gomez RA. Dev Cell 6: 719–728, 2004) with mice in which exon 1 of the Gnas gene was flanked by loxP sites (Chen M, Gavrilova O, Liu J, Xie T, Deng C, Nguyen AT, Nackers LM, Lorenzo J, Shen L, Weinstein LS. Proc Natl Acad Sci USA), we generated animals with preferential and nearly complete excision of Gsα in juxtaglomerular granular (JG) cells. Compared with wild-type animals, mice with conditional Gsα deficiency had markedly reduced basal levels of renin expression and very low plasma renin concentrations. Furthermore, the acute release responses to furosemide, hydralazine, and isoproterenol were virtually abolished. Consistent with a state of primary renin depletion, Gsα-deficient mice had reduced arterial blood pressure, reduced levels of aldosterone, and a low glomerular filtration rate. Renin content and renin secretion of JG cells in primary culture were drastically reduced, and the stimulatory response to the addition of PGE2or isoproterenol was eliminated. Unexpectedly, Gsα recombination was also observed in the renal medulla, and this was associated with a vasopressin-resistant concentrating defect. Our study shows that Cre recombinase under control of the renin promoter can be used for the excision of floxed targets from JG cells. We conclude that Gsα-mediated signal transduction is essential and nonredundant in the control of renin synthesis and release.