The EP3 receptor regulates water excretion in response to high salt intake

Deletion of the prostaglandin EP4 receptor in the kidney tubule of mature mice alters kidney water handling only in males

Prostaglandin E2 (PGE2) is a lipid mediator modulating several aspects of kidney function. PGE2 acts via four prostanoid receptors, EP1 to EP4. In renal tubules, EP4 has very low expression, yet a role for EP4 in maintaining water balance has been proposed. The major aim of this study was to clarify the role of the EP4 receptor in the kidney tubule of adult mice for body water homeostasis. To examine this, a mouse model with doxycycline-dependent deletion of EP4 along the renal tubule (Pax8Cre system) was developed and phenotyped with respect to water handling. Two weeks after doxycycline treatment, EP4 mRNA expression (RT-qPCR) was reduced by >80% in the medulla of male and female knockout mice (EP4-/-) compared with controls (EP4+/+). With free access to water, there were no detectable differences between genotypes in food intake, body weight, or plasma biochemistries. Male, but not female, EP4-/- mice had a small but significantly higher basal urinary output with decreased osmolality, concomitant with lower urinary Na+, K+, Cl-, urea, and creatinine concentrations. The urea channel UT-A1 was reduced in the medulla, but otherwise no major differences in the levels of proteins involved in water balance were observed between genotypes. There were no differences between genotypes in their ability to concentrate urine during a 14-h water restriction or after treatment with the vasopressin V2 receptor agonist 1-deamino-8-d-arginine-vasopressin. EP4-/- mice of both sexes excreted an acute water load similarly to control mice. In conclusion, EP4 in the renal tubule has a mild role in renal water handling, but only in male mice. This new model provides a novel tool for assessing the role of EP4 in kidney tubule function in various (patho)physiological conditions independently of developmental abnormalities or systemic effects.NEW & NOTEWORTHY Acting through the EP1-EP4 receptors, prostaglandin E2 (PGE2) modulates various aspects of kidney function. Despite extremely low expression in the collecting duct, EP4 has been proposed to play a key role in regulation of Aquaporin-2 and water balance. In this study, we show that EP4 only plays a mild role in kidney water handling in male mice, calling into question the standard concepts presented in physiology textbooks regarding PGE2 actions on the collecting duct.

Inducible deletion of the prostaglandin EP3 receptor in kidney tubules of male and female mice has no major effect on water homeostasis

The prostaglandin E2 (PGE2) receptor EP3 has been detected in the thick ascending limb (TAL) and the collecting duct of the kidney, where its actions are proposed to inhibit water reabsorption. However, EP3 is also expressed in other cell types, including vascular endothelial cells. The aim here was to determine the contribution of EP3 in renal water handling in male and female adult mice by phenotyping a novel mouse model with doxycycline-dependent deletion of EP3 throughout the kidney tubule (EP3-/- mice). RNAscope demonstrated that EP3 was highly expressed in the cortical and medullary TAL of adult mice. Compared with controls EP3 mRNA expression was reduced by >80% in whole kidney (RT-qPCR) and nondetectable (RNAscope) in renal tubules of EP3-/- mice. Under basal conditions, there were no significant differences in control and EP3-/- mice of both sexes in food and water intake, body weight, urinary output, or clinical biochemistries. No differences were detectable between genotypes in handling of an acute water load or in their response to the vasopressin analog 1-deamino-8-d-arginine-vasopressin (dDAVP). No differences in water handling were observed when PGE2 production was enhanced using 1% NaCl load. Expression of proteins involved in kidney water handling was not different between genotypes. This study demonstrates that renal tubular EP3 is not essential for body fluid homeostasis in males or females, even when PGE2 levels are high. The mouse model is a novel tool for examining the role of EP3 in kidney function independently of potential developmental abnormalities or systemic effects.NEW & NOTEWORTHY The prostanoid EP3 receptor is proposed to play a key role in the kidney tubule and antagonize the effects of vasopressin on aquaporin-mediated water reabsorption. Here, we phenotyped a kidney tubule-specific inducible knockout mouse model of the EP3 receptor. Our major finding is that, even under physiological stress, tubular EP3 plays no detectable role in renal water or solute handling. This suggests that other EP receptors must be important for renal salt and water handling.

Associations of E-proteinoid 3 receptor genetic polymorphisms with salt sensitivity, longitudinal blood pressure changes, and hypertension incidence in Chinese adults

The E-proteinoid 3 receptor (PTGER3), a member of the prostaglandin E2 (PGE2) subtype receptor, belongs to the G-protein-coupled superfamily of receptors. Animal studies have demonstrated its involvement in salt sensitivity by regulating sodium reabsorption. This study aimed to investigate the association between genetic variants of PTGER3 and salt sensitivity, longitudinal blood pressure (BP) changes, and the incidence of hypertension in Chinese adults. A chronic salt intake intervention was conducted involving 514 adults from 124 families in the 2004 Baoji Salt-Sensitivity Study Cohort in northern China. These participants followed a 3-day regular baseline diet, followed by a 7-day low-salt diet (3.0 g/d) and a 7-day high-salt diet (18 g/d), and were subsequently followed for 14 years. The findings revealed a significant relationship between the single nucleotide polymorphism (SNP) rs17482751 of PTGER3 and diastolic blood pressure (DBP) response to high salt intervention. Additionally, SNPs rs11209733, rs3765894, and rs2268062 were significantly associated with longitudinal changes in systolic blood pressure (SBP), DBP, and mean arterial pressure (MAP) during the 14-year follow-up period. SNP rs6424414 was significantly associated with longitudinal changes in DBP over 14 years. Finally, SNP rs17482751 showed a significant correlation with the incidence of hypertension over 14 years. These results emphasize the significant role of PTGER3 gene polymorphism in salt sensitivity, longitudinal BP changes, and the development of hypertension in the Chinese population.

TNF inhibits AQP2 expression via a miR137-dependent pathway

As miR-137 is a regulator of aquaporin (AQP)2 expression and tumor necrosis factor (TNF) inhibits the expression of several extrarenal AQPs, we tested the hypothesis that TNF inhibits AQP2 in the kidney via a miR-137-dependent mechanism. AQP2 mRNA and protein expression decreased ∼70% and 53%, respectively, in primary renal inner medullary collecting duct (IMCD) cells transfected with a miRNA mimic of mmu-miR-137, suggesting that miR-137 directly targets AQP2 mRNA in these cells. Exposure of IMCD cells for 2 h to 400 mosmol/kgH2O medium increased mmu-miR-137 mRNA expression about twofold, conditions that also increased TNF production approximately fourfold. To determine if the increase in mmu-miR-137 mRNA expression was related to the concomitant increase in TNF, IMCD cells were transfected with a lentivirus construct to silence TNF. This construct decreased mmu-miR-137 mRNA expression by ∼63%, suggesting that TNF upregulates the expression of miR-137. Levels of miR-137 also increased approximately twofold in IMCD tubules isolated from male mice given 1% NaCl in the drinking water for 3 days. Intrarenal lentivirus silencing of TNF increased AQP2 mRNA levels and protein expression concomitant with a decrease in miR-137 levels in tubules isolated from mice given NaCl. The changes in AQP2 expression levels affected the diluting ability of the kidney, which was assessed by measuring urine osmolality and urine volume, as the decrease in these parameters after renal silencing of TNF was prevented on intrarenal administration of miR-137. The study reveals a novel TNF function via a miR-137-dependent mechanism that regulates AQP2 expression and function.NEW & NOTEWORTHY An emerging intratubular tumor necrosis factor system, functioning during normotensive noninflammatory conditions, acts as a breaking mechanism that attenuates both the increases in Na+-K+-2Cl- cotransporter and aquaporin-2 induced by arginine vasopressin, thereby contributing to the regulation of electrolyte balance and blood pressure. A greater appreciation for the role of cytokines as mediators of immunophysiological responses may help reveal the relationship between the immune system and other physiological systems.

Intact prostaglandin signaling through EP2 and EP4 receptors in stromal progenitor cells is required for normal development of the renal cortex in mice

Cyclooxygenase (Cox) inhibitors are known to have severe side effects during renal development. These consist of reduced renal function, underdeveloped subcapsular glomeruli, interstitial fibrosis, and thinner cortical tissue. Global genetic deletion of Cox-2 mimics the phenotype observed after application of Cox inhibitors. This study aimed to investigate which cell types express Cox-2 and prostaglandin E2 receptors and what functions are mediated through this pathway during renal development. Expression of EP2 and EP4 mRNA was detected by RNAscope mainly in descendants of FoxD1+ stromal progenitors; EP1 and EP3, on the other hand, were expressed in tubules. Cox-2 mRNA was detected in medullary interstitial cells and macula densa cells. Functional investigations were performed with a cell-specific approach to delete Cox-2, EP2, and EP4 in FoxD1+ stromal progenitor cells. Our data show that Cox-2 expression in macula densa cells is sufficient to drive renal development. Deletion of EP2 or EP4 in FoxD1+ cells had no functional effect on renal development. Codeletion of EP2 and EP4 in FoxD1+ stromal cells, however, led to severe glomerular defects and a strong decline of glomerular filtration rate (1.316 ± 69.7 µL/min/100 g body wt in controls vs. 644.1 ± 64.58 µL/min/100 g body wt in FoxD1+/Cre EP2-/- EP4ff mice), similar to global deletion of Cox-2. Furthermore, EP2/EP4-deficient mice showed a significant increase in collagen production with a strong downregulation of renal renin expression. This study shows the distinct localization of EP receptors in mice. Functionally, we could identify EP2 and EP4 receptors in stromal FoxD1+ progenitor cells as essential receptor subtypes for normal renal development.NEW & NOTEWORTHY Cyclooxygenase-2 (Cox-2) produces prostaglandins that are essential for normal renal development. It is unclear in which cells Cox-2 and the receptors for prostaglandin E2 (EP receptors) are expressed during late nephrogenesis. This study identified the expression sites for EP subtypes and Cox-2 in neonatal mouse kidneys. Furthermore, it shows that stromal progenitor cells may require intact prostaglandin E2 signaling through EP2 and EP4 receptors for normal renal development.

Genetic Variants in PGE2 Receptors Modulate the Risk of Nephrosclerosis and Clinical Outcomes in These Patients

Prostaglandin E2 (PGE2) is a major actor mediating renal injury. We aimed to determine genetic variability in the genes coding for its receptors (PTGER1-4) and study associations with nephrosclerosis risk and clinical outcomes. We identified 96 tag-SNPs capturing global variability in PTGER1-4 and screened 1209 nephrosclerosis patients and controls. The effect of these variants was evaluated by multivariate regression analyses. Two PTGER3 SNPs, rs11209730 and rs10399704, remained significant in a backward elimination regression model with other non-genetic variables (OR = 1.45 (1.07-1.95), p = 0.016 and OR = 0.71 (0.51-0.99), p = 0.041, respectively). In the nephrosclerosis patients, a proximal region of PTGER3 was tagged as relevant for eGFR (p values for identified SNPs ranged from 0.0003 to 0.038). Two consecutive PTGER3 SNPs, rs2284362 and rs2284363, significantly decreased systolic (p = 0.005 and p = 0.0005), diastolic (p = 0.039 and p = 0.005), and pulse pressure values (p = 0.038 and 0.014). Patients were followed for a median of 47 months (7-54) to evaluate cardiovascular (CV) risk. Cox regression analysis showed that carriers of the PTGER1rs2241360 T variant had better CV event-free survival than wild-type individuals (p = 0.029). In addition, PTGER3rs7533733 GG carriers had lower event-free survival than AA/AG patients (p = 0.011). Our results indicate that genetic variability in PGE2 receptors, particularly EP3, may be clinically relevant for nephrosclerosis and its associated CV risk.

International Union of Basic and Clinical Pharmacology. CIX. Differences and Similarities between Human and Rodent Prostaglandin E2 Receptors (EP1-4) and Prostacyclin Receptor (IP): Specific Roles in Pathophysiologic Conditions

Prostaglandins are derived from arachidonic acid metabolism through cyclooxygenase activities. Among prostaglandins (PGs), prostacyclin (PGI2) and PGE2 are strongly involved in the regulation of homeostasis and main physiologic functions. In addition, the synthesis of these two prostaglandins is significantly increased during inflammation. PGI2 and PGE2 exert their biologic actions by binding to their respective receptors, namely prostacyclin receptor (IP) and prostaglandin E2 receptor (EP) 1-4, which belong to the family of G-protein-coupled receptors. IP and EP1-4 receptors are widely distributed in the body and thus play various physiologic and pathophysiologic roles. In this review, we discuss the recent advances in studies using pharmacological approaches, genetically modified animals, and genome-wide association studies regarding the roles of IP and EP1-4 receptors in the immune, cardiovascular, nervous, gastrointestinal, respiratory, genitourinary, and musculoskeletal systems. In particular, we highlight similarities and differences between human and rodents in terms of the specific roles of IP and EP1-4 receptors and their downstream signaling pathways, functions, and activities for each biologic system. We also highlight the potential novel therapeutic benefit of targeting IP and EP1-4 receptors in several diseases based on the scientific advances, animal models, and human studies. SIGNIFICANCE STATEMENT: In this review, we present an update of the pathophysiologic role of the prostacyclin receptor, prostaglandin E2 receptor (EP) 1, EP2, EP3, and EP4 receptors when activated by the two main prostaglandins, namely prostacyclin and prostaglandin E2, produced during inflammatory conditions in human and rodents. In addition, this comparison of the published results in each tissue and/or pathology should facilitate the choice of the most appropriate model for the future studies.

Prostaglandin E2 in the Regulation of Water Transport in Renal Collecting Ducts

The kidney plays a central role in the regulation of the body water balance. The process of targeting the water channel aquaporin-2 (AQP2) on the apical plasma membrane of the collecting duct (CD) principal cells is mainly regulated by the antidiuretic peptide hormone arginine vasopressin (AVP), which is responsible for the maintenance of water homeostasis. Recently, much attention has been focused on the local factors modulating renal water reabsorption by AQP2 in the collecting ducts, especially prostaglandin E2 (PGE₂). PGE₂ is a lipid mediator involved in a variety of physiological and pathophysiological processes in the kidney. The biological function of PGE₂ is mainly mediated by four G-protein-coupled receptors, namely EP1-4, which couple to drive separate intracellular signaling pathways. Increasing evidence demonstrates that PGE₂ is essential for renal water transport regulation via multiple mechanisms. Each EP receptor plays a unique role in regulating water reabsorption in renal collecting ducts. This brief review highlights the role of PGE₂ in the regulation of water reabsorption and discusses the involvement of each EP receptor subtype in renal collecting duct. A better understanding of the role of PGE₂ in renal water transport process may improve disease management strategies for water balance disorders, including nephrogenic diabetes insipidus.