Renal expression of exchange protein directly activated by cAMP (Epac) 1 and 2

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

Epac induces ryanodine receptor-dependent intracellular and inter-organellar calcium mobilization in mpkCCD cells

Arginine vasopressin (AVP) induces an increase in intracellular Ca2+ concentration ([Ca2+]i) with an oscillatory pattern in isolated perfused kidney inner medullary collecting duct (IMCD). The AVP-induced Ca2+ mobilization in inner medullary collecting ducts is essential for apical exocytosis and is mediated by the exchange protein directly activated by cyclic adenosine monophosphate (Epac). Murine principal kidney cortical collecting duct cells (mpkCCD) is the cell model used for transcriptomic and phosphoproteomic studies of AVP signaling in kidney collecting duct. The present study examined the characteristics of Ca2+ mobilization in mpkCCD cells, and utilized mpkCCD as a model to investigate the Epac-induced intracellular and intra-organellar Ca2+ mobilization. Ca2+ mobilization in cytosol, endoplasmic reticulum lumen, and mitochondrial matrix were monitored with a Ca2+ sensitive fluorescent probe and site-specific Ca2+ sensitive biosensors. Fluorescence images of mpkCCD cells and isolated perfused inner medullary duct were collected with confocal microscopy. Cell permeant ligands of ryanodine receptors (RyRs) and inositol 1,4,5 trisphosphate receptors (IP3Rs) both triggered increase of [Ca2+]i and Ca2+ oscillations in mpkCCD cells as reported previously in IMCD. The cell permeant Epac-specific cAMP analog Me-cAMP/AM also caused a robust Ca2+ mobilization and oscillations in mpkCCD cells. Using biosensors to monitor endoplasmic reticulum (ER) luminal Ca2+ and mitochondrial matrix Ca2+, Me-cAMP/AM not only triggered Ca2+ release from ER into cytoplasm, but also shuttled Ca2+ from ER into mitochondria. The Epac-agonist induced synchronized Ca2+ spikes in cytosol and mitochondrial matrix, with concomitant declines in ER luminal Ca2+. Me-cAMP/AM also effectively triggered store-operated Ca2+ entry (SOCE), suggesting that Epac-agonist is capable of depleting ER Ca2+ stores. These Epac-induced intracellular and inter-organelle Ca2+ signals were mimicked by the RyR agonist 4-CMC, but they were distinctly different from IP3R activation. The present study hence demonstrated that mpkCCD cells retain all reported features of Ca2+ mobilization observed in isolated perfused IMCD. It further revealed information on the dynamics of Epac-induced RyR-dependent Ca2+ signaling and ER-mitochondrial Ca2+ transfer. ER-mitochondrial Ca2+ coupling may play a key role in the regulation of ATP and reactive oxygen species (ROS) production in the mitochondria along the nephron. Our data suggest that mpkCCD cells can serve as a renal cell model to address novel questions of how mitochondrial Ca2+ regulates cytosolic Ca2+ signals, inter-organellar Ca2+ signaling, and renal tubular functions.

Epac1-/- and Epac2-/- mice exhibit deficient epithelial Na+ channel regulation and impaired urinary Na+ conservation

Exchange proteins directly activated by cAMP (Epacs) are abundantly expressed in the renal tubules. We used genetic and pharmacological tools in combination with balance, electrophysiological, and biochemical approaches to examine the role of Epac1 and Epac2 in renal sodium handling. We demonstrate that Epac1^–/– and Epac2^–/– mice exhibit a delayed anti-natriuresis to dietary sodium restriction despite augmented aldosterone levels. This was associated with a significantly lower response to the epithelial Na⁺ channel (ENaC) blocker amiloride, reduced ENaC activity in split-opened collecting ducts, and defective posttranslational processing of α and γENaC subunits in the KO mice fed with a Na⁺-deficient diet. Concomitant deletion of both isoforms led to a marginally greater natriuresis but further increased aldosterone levels. Epac2 blocker ESI-05 and Epac1&2 blocker ESI-09 decreased ENaC activity in Epac WT mice kept on the Na⁺-deficient diet but not on the regular diet. ESI-09 injections led to natriuresis in Epac WT mice on the Na⁺-deficient diet, which was caused by ENaC inhibition. In summary, our results demonstrate similar but nonredundant actions of Epac1 and Epac2 in stimulation of ENaC activity during variations in dietary salt intake. We speculate that inhibition of Epac signaling could be instrumental in treatment of hypertensive states associated with ENaC overactivation.

Identification of Molecular Markers Associated With the Pathophysiology and Treatment of Lupus Nephritis Based on Integrated Transcriptome Analysis

Lupus nephritis (LN) is a well-known complication of systemic lupus erythematosus and is its leading cause of morbidity and mortality. Our study aimed to identify the molecular markers associated with the pathophysiology and treatment of LN. The renal tissue gene expression profiles of LN patients in the GSE32591 dataset were downloaded as a discovery cohort from the Gene Expression Omnibus. Differentially expressed genes (DEGs) were identified; weighted gene co-expression network analysis (WGCNA) was used to identify the co-expression modules of DEGs; and gene function enrichment analysis, molecular crosstalk analysis, and immune cell infiltration analysis were performed to explore the pathophysiological changes in glomeruli and tubulointerstitia of LN patients. The crosstalk genes were validated in another RNA-sequencing cohort. DEGs common in RNA-sequencing dataset and GSE32591 were uploaded to the Connectivity Map (CMap) database to find prospective LN-related drugs. Molecular docking was used to verify the targeting association between candidate small molecular compounds and the potential target. In all, 420 DEGs were identified; five modules and two modules associated with LN were extracted in glomeruli and tubulointerstitia, respectively. Functional enrichment analysis showed that type I interferon (IFN) response was highly active, and some biological processes such as metabolism, detoxification, and ion transport were impaired in LN. Gene transcription in glomeruli and tubulointerstitia might affect each other, and some crosstalk genes, such as IRF7, HLA-DRA, ISG15, PSMB8, and IFITM3, play important roles in this process. Immune cell infiltration analysis revealed that monocytes and macrophages were increased in glomeruli and tubulointerstitia, respectively. CMap analysis identified proscillaridin as a possible drug to treat LN. Molecular docking showed proscillaridin forms four hydrogen bonds with the SH2 domain of signal transducer and activator of transcription 1 (STAT1). The findings of our study may shed light on the pathophysiology of LN and provide potential therapeutic targets for LN.

Patent highlights April-May 2020

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research and development.

Epac1 null mice have nephrogenic diabetes insipidus with deficient corticopapillary osmotic gradient and weaker collecting duct tight junctions

AIM

The cAMP-mediator Epac1 (RapGef3) has high renal expression. Preliminary observations revealed increased diuresis in Epac1^-/- mice. We hypothesized that Epac1 could restrict diuresis by promoting transcellular collecting duct (CD) water and urea transport or by stabilizing CD paracellular junctions to reduce osmolyte loss from the renal papillary interstitium.

METHODS

In Epac1^-/- and Wt C57BL/6J mice, renal papillae, dissected from snap-frozen kidneys, were assayed for the content of key osmolytes. Cell junctions were analysed by transmission electron microscopy. Urea transport integrity was evaluated by urea loading with 40% protein diet, endogenous vasopressin production was manipulated by intragastric water loading and moderate dehydration and vasopressin type 2 receptors were stimulated selectively by i.p.-injected desmopressin (dDAVP). Glomerular filtration rate (GFR) was estimated as [¹⁴ C]inulin clearance. The glomerular filtration barrier was evaluated by urinary albumin excretion and microvascular leakage by the renal content of time-spaced intravenously injected ¹²⁵ I- and ¹³¹ I-labelled albumin.

RESULTS

Epac1^-/- mice had increased diuresis and increased free water clearance under antidiuretic conditions. They had shorter and less dense CD tight junction (TJs) and attenuated corticomedullary osmotic gradient. Epac1^-/- mice had no increased protein diet-induced urea-dependent osmotic diuresis, and expressed Wt levels of aquaporin-2 (AQP-2) and urea transporter A1/3 (UT-A1/3). Epac1^-/- mice had no urinary albumin leakage and unaltered renal microvascular albumin extravasation. Their GFR was moderately increased, unless when treated with furosemide.

CONCLUSION

Our results conform to the hypothesis that Epac1-dependent mechanisms protect against diabetes insipidus by maintaining renal papillary osmolarity and the integrity of CD TJs.

Targeting the Trafficking of Kidney Water Channels for Therapeutic Benefit

The ability to regulate water movement is vital for the survival of cells and organisms. In addition to passively crossing lipid bilayers by diffusion, water transport is also driven across cell membranes by osmotic gradients through aquaporin water channels. There are 13 aquaporins in human tissues, and of these, aquaporin-2 (AQP2) is the most highly regulated water channel in the kidney: The expression and trafficking of AQP2 respond to body volume status and plasma osmolality via the antidiuretic hormone, vasopressin (VP). Dysfunctional VP signaling in renal epithelial cells contributes to disorders of water balance, and research initially focused on regulating the major cAMP/PKA pathway to normalize urine concentrating ability. With the discovery of novel and more complex signaling networks that regulate AQP2 trafficking, promising therapeutic targets have since been identified. Several strategies based on data from preclinical studies may ultimately translate to the care of patients with defective water homeostasis.

A peek into Epac physiology in the kidney

cAMP is a critical second messenger of numerous endocrine signals affecting water-electrolyte transport in the renal tubule. Exchange protein directly activated by cAMP (Epac) is a relatively recently discovered downstream effector of cAMP, having the same affinity to the second messenger as protein kinase A, the classical cAMP target. Two Epac isoforms, Epac1 and Epac2, are abundantly expressed in the renal epithelium, where they are thought to regulate water and electrolyte transport, particularly in the proximal tubule and collecting duct. Recent characterization of renal phenotype in mice lacking Epac1 and Epac2 revealed a critical role of the Epac signaling cascade in urinary concentration as well as in Na⁺ and urea excretion. In this review, we aim to critically summarize current knowledge of Epac relevance for renal function and to discuss the applicability of Epac-based strategies in the regulation of systemic water-electrolyte homeostasis.

Urinary concentrating defect in mice lacking Epac1 or Epac2

cAMP is a universal second messenger regulating a plethora of processes in the kidney. Two downstream effectors of cAMP are PKA and exchange protein directly activated by cAMP (Epac), which, unlike PKA, is often linked to elevation of [Ca2+]i. While both Epac isoforms (Epac1 and Epac2) are expressed along the nephron, their relevance in the kidney remains obscure. We combined ratiometric calcium imaging with quantitative immunoblotting, immunofluorescent confocal microscopy, and balance studies in mice lacking Epac1 or Epac2 to determine the role of Epac in renal water-solute handling. Epac1-/- and Epac2-/- mice developed polyuria despite elevated arginine vasopressin levels. We did not detect major deficiencies in arginine vasopressin [Ca2+]i signaling in split-opened collecting ducts or decreases in aquaporin water channel type 2 levels. Instead, sodium-hydrogen exchanger type 3 levels in the proximal tubule were dramatically reduced in Epac1-/- and Epac2-/- mice. Water deprivation revealed persisting polyuria, impaired urinary concentration ability, and augmented urinary excretion of Na+ and urea in both mutant mice. In summary, we report a nonredundant contribution of Epac isoforms to renal function. Deletion of Epac1 and Epac2 decreases sodium-hydrogen exchanger type 3 expression in the proximal tubule, leading to polyuria and osmotic diuresis.-Cherezova, A., Tomilin, V., Buncha, V., Zaika, O., Ortiz, P. A., Mei, F., Cheng, X., Mamenko, M., Pochynyuk, O. Urinary concentrating defect in mice lacking Epac1 or Epac2.

PDE/cAMP/Epac/C/EBP-β Signaling Cascade Regulates Mitochondria Biogenesis of Tubular Epithelial Cells in Renal Fibrosis

AIMS

Cyclic adenosine 3'5'-monophosphate (cAMP) is a universal second messenger that plays an important role in intracellular signal transduction. cAMP is synthesized by adenylate cyclases from adenosine triphosphate and terminated by the phosphodiesterases (PDEs). In the present study, we investigated the role of the cAMP pathway in tubular epithelial cell mitochondrial biogenesis in the pathogenesis of renal fibrosis.

RESULTS

We found that the cAMP levels were decreased in fibrotic kidney tissues, and replenishing cAMP could ameliorate tubular atrophy and extracellular matrix deposition. The downregulation of cAMP was mainly attributed to the increased PDE4 expression in tubular epithelial cells. The inhibition of PDE4 by PDE4 siRNA or the specific inhibitor, rolipram, attenuated unilateral ureteral obstruction-induced renal interstitial fibrosis and transforming growth factor (TGF)-β1-stimulated primary tubular epithelial cell (PTC) damage. The Epac1/Rap1 pathway contributed to the main effect of cAMP on renal fibrosis. Rolipram could restore C/EBP-β and PGC-1α expression and protect the mitochondrial function and structure of PTCs under TGF-β1 stimulation. The antifibrotic role of rolipram in renal fibrosis relies on C/EBP-β and PGC-1α expression in tubular epithelial cells. Innovation and Conclusion: The results of the present study indicate that cAMP signaling regulates the mitochondrial biogenesis of tubular epithelial cells in renal fibrosis. Restoring cAMP by the PDE4 inhibitor rolipram may ameliorate renal fibrosis by targeting C/EBP-β/PGC1-α and mitochondrial biogenesis. Antioxid. Redox Signal. 29, 637-652.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Activation of AQP2 water channels without vasopressin: therapeutic strategies for congenital nephrogenic diabetes insipidus

Congenital nephrogenic diabetes insipidus (NDI) is characterized by defective urine concentrating ability. Symptomatic polyuria is present from birth, even with normal release of the antidiuretic hormone vasopressin by the pituitary. Over the last two decades, the aquaporin-2 (AQP2) gene has been cloned and the molecular mechanisms of urine concentration have been gradually elucidated. Vasopressin binds to the vasopressin type II receptor (V2R) in the renal collecting ducts and then activates AQP2 phosphorylation and trafficking to increase water reabsorption from urine. Most cases of congenital NDI are caused by loss-of-function mutations to V2R, resulting in unresponsiveness to vasopressin. In this article, we provide an overview of novel therapeutic molecules of congenital NDI that can activate AQP2 by bypassing defective V2R signaling with a particular focus on the activators of the calcium and cAMP signaling pathways.

Insights into exchange factor directly activated by cAMP (EPAC) as potential target for cancer treatment

Cancer remains a global health problem and approximately 1.7 million new cancer cases are diagnosed every year worldwide. Although diverse molecules are currently being explored as targets for cancer therapy the tumor treatment and therapy is highly tricky. Secondary messengers are important for hormone-mediated signaling pathway. Cyclic AMP (cAMP), a secondary messenger responsible for various physiological processes regulates cell metabolism by activating Protein kinase A (PKA) and by targeting exchange protein directly activated by cAMP (EPAC). EPAC is present in two isoforms EPAC1 and EPAC2, which exhibit different tissue distribution and is involved in GDP/GTP exchange along with activating Rap1- and Rap2-mediated signaling pathways. EPAC is also known for its dual role in cancer as pro- and anti-proliferative in addition to metastasis. Results after perturbing EPAC activity suggests its involvement in cancer cell migration, proliferation, and cytoskeleton remodeling which makes it a potential therapeutic target for cancer treatments.

Exchange protein directly activated by cAMP encoded by the mammalian rapgef3 gene: Structure, function and therapeutics

Mammalian exchange protein directly activated by cAMP isoform 1 (EPAC1), encoded by the RAPGEF3 gene, is one of the two-membered family of cAMP sensors that mediate the intracellular functions of cAMP by acting as guanine nucleotide exchange factors for the Ras-like Rap small GTPases. Extensive studies have revealed that EPAC1-mediated cAMP signaling is highly coordinated spatiotemporally through the formation of dynamic signalosomes by interacting with a diverse array of cellular partners. Recent functional analyses of genetically engineered mouse models further suggest that EPAC1 functions as an important stress response switch and is involved in pathophysiological conditions of cardiac stresses, chronic pain, cancer and infectious diseases. These findings, coupled with the development of EPAC specific small molecule modulators, validate EPAC1 as a promising target for therapeutic interventions.

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Human gene RAPGEF3 encodes for EPAC1 protein.

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Along with PKA, CNG & HCN, EPAC is an important cAMP sensor.

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Selective modulators of EPAC1 have been developed for use as pharmacological probes.

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Formation of EPAC1 signalosomes allows spatiotemporal control of cAMP signaling.

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EPAC1 is implicated in major pathophysiological conditions and is an attractive therapeutic target.

Vasopressin regulation of multisite phosphorylation of UT-A1 in the inner medullary collecting duct

Vasopressin signaling is critical for the regulation of urea transport in the inner medullary collecting duct (IMCD). Increased urea permeability is driven by a vasopressin-mediated elevation of cAMP that results in the direct phosphorylation of urea transporter (UT)-A1. The identification of cAMP-sensitive phosphorylation sites, Ser(486) and Ser(499), in the rat UT-A1 sequence was the first step in understanding the mechanism of vasopressin action on the phosphorylation-dependent modulation of urea transport. To investigate the significance of multisite phosphorylation of UT-A1 in response to elevated cAMP, we used highly specific and sensitive phosphosite antibodies to Ser(486) and Ser(499) to determine cAMP action at each phosphorylation site. We found that phosphorylation at both sites was rapid and sustained. Furthermore, the rate of phosphorylation of the two sites was similar in both mIMCD3 cells and rat inner medullary tissue. UT-A1 localized to the apical membrane in response to vasopressin was phosphorylated at Ser(486) and Ser(499). We confirmed that elevated cAMP resulted in increased phosphorylation of both sites by PKA but not through the vasopressin-sensitive exchange protein activated by cAMP pathway. These results elucidate the multisite phosphorylation of UT-A1 in response to cAMP, thus providing the beginning of understanding the intracellular factors underlying vasopressin stimulation of urea transport in the IMCD.

Angiotensin II type 2 receptor regulates ROMK-like K⁺ channel activity in the renal cortical collecting duct during high dietary K⁺ adaptation

The kidney adjusts K⁺ excretion to match intake in part by regulation of the activity of apical K⁺ secretory channels, including renal outer medullary K⁺ (ROMK)-like K⁺ channels, in the cortical collecting duct (CCD). ANG II inhibits ROMK channels via the ANG II type 1 receptor (AT1R) during dietary K⁺ restriction. Because AT1Rs and ANG II type 2 receptors (AT2Rs) generally function in an antagonistic manner, we sought to characterize the regulation of ROMK channels by the AT2R. Patch-clamp experiments revealed that ANG II increased ROMK channel activity in CCDs isolated from high-K⁺ (HK)-fed but not normal K⁺ (NK)-fed rats. This response was blocked by PD-123319, an AT2R antagonist, but not by losartan, an AT1R antagonist, and was mimicked by the AT2R agonist CGP-42112. Nitric oxide (NO) synthase is present in CCD cells that express ROMK channels. Blockade of NO synthase with N-nitro-l-arginine methyl ester and free NO with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt completely abolished ANG II-stimulated ROMK channel activity. NO enhances the synthesis of cGMP, which inhibits phosphodiesterases (PDEs) that normally degrade cAMP; cAMP increases ROMK channel activity. Pretreatment of CCDs with IBMX, a broad-spectrum PDE inhibitor, or cilostamide, a PDE3 inhibitor, abolished the stimulatory effect of ANG II on ROMK channels. Furthermore, PKA inhibitor peptide, but not an activator of the exchange protein directly activated by cAMP (Epac), also prevented the stimulatory effect of ANG II. We conclude that ANG II acts at the AT2R to stimulate ROMK channel activity in CCDs from HK-fed rats, a response opposite to that mediated by the AT1R in dietary K⁺-restricted animals, via a NO/cGMP pathway linked to a cAMP-PKA pathway.

cAMP signaling prevents podocyte apoptosis via activation of protein kinase A and mitochondrial fusion

Our previous in vitro studies suggested that cyclic AMP (cAMP) signaling prevents adriamycin (ADR) and puromycin aminonucleoside (PAN)-induced apoptosis in podocytes. As cAMP is an important second messenger and plays a key role in cell proliferation, differentiation and cytoskeleton formation via protein kinase A (PKA) or exchange protein directly activated by cAMP (Epac) pathways, we sought to determine the role of PKA or Epac signaling in cAMP-mediated protection of podocytes. In the ADR nephrosis model, we found that forskolin, a selective activator of adenylate cyclase, attenuated albuminuria and improved the expression of podocyte marker WT-1. When podocytes were treated with pCPT-cAMP (a selective cAMP/PKA activator), PKA activation was increased in a time-dependent manner and prevented PAN-induced podocyte loss and caspase 3 activation, as well as a reduction in mitochondrial membrane potential. We found that PAN and ADR resulted in a decrease in Mfn1 expression and mitochondrial fission in podocytes. pCPT-cAMP restored Mfn1 expression in puromycin or ADR-treated podocytes and induced Drp1 phosphorylation, as well as mitochondrial fusion. Treating podocytes with arachidonic acid resulted in mitochondrial fission, podocyte loss and cleaved caspase 3 production. Arachidonic acid abolished the protective effects of pCPT-cAMP on PAN-treated podocytes. Mdivi, a mitochondrial division inhibitor, prevented PAN-induced cleaved caspase 3 production in podocytes. We conclude that activation of cAMP alleviated murine podocyte caused by ADR. PKA signaling resulted in mitochondrial fusion in podocytes, which at least partially mediated the effects of cAMP.

Biochemical properties of urea transporters

Urea and urea transporters (UT) are critical to the production of concentrated urine and hence in maintaining body fluid balance. The UT-A1 urea transporter is the major and most important UT isoform in the kidney. Native UT-A1, expressed in the terminal inner medullary collecting duct (IMCD) epithelial cells, is a glycosylated protein with two glycoforms of 117 and 97 kDa. Vasopressin is the major hormone in vivo that rapidly increases urea permeability in the IMCD through increases in phosphorylation and apical plasma-membrane accumulation of UT-A1. The cell signaling pathway for vasopressin-mediated UT-A1 phosphorylation and activity involves two cAMP-dependent signaling pathways: protein kinase A (PKA) and exchange protein activated by cAMP (Epac). In this chapter, we will discuss UT-A1 regulation by phosphorylation, ubiquitination, and glycosylation.

Expression of urea transporters and their regulation

UT-A and UT-B families of urea transporters consist of multiple isoforms that are subject to regulation of both acutely and by long-term measures. This chapter provides a brief overview of the expression of the urea transporter forms and their locations in the kidney. Rapid regulation of UT-A1 results from the combination of phosphorylation and membrane accumulation. Phosphorylation of UT-A1 has been linked to vasopressin and hyperosmolality, although through different kinases. Other acute influences on urea transporter activity are ubiquitination and glycosylation, both of which influence the membrane association of the urea transporter, again through different mechanisms. Long-term regulation of urea transport is most closely associated with the environment that the kidney experiences. Low-protein diets may influence the amount of urea transporter available. Conditions of osmotic diuresis, where urea concentrations are low, will prompt an increase in urea transporter abundance. Although adrenal steroids affect urea transporter abundance, conflicting reports make conclusions tenuous. Urea transporters are upregulated when P2Y2 purinergic receptors are decreased, suggesting a role for these receptors in UT regulation. Hypercalcemia and hypokalemia both cause urine concentration deficiencies. Urea transporter abundances are reduced in aging animals and animals with angiotensin-converting enzyme deficiencies. This chapter will provide information about both rapid and long-term regulation of urea transporters and provide an introduction into the literature.

Recent advances in the discovery of small molecules targeting exchange proteins directly activated by cAMP (EPAC)

3',5'-Cyclic adenosine monophosphate (cAMP) is a pivotal second messenger that regulates numerous biological processes under physiological and pathological conditions, including cancer, diabetes, heart failure, inflammation, and neurological disorders. In the past, all effects of cAMP were initially believed to be mediated by protein kinase A (PKA) and cyclic nucleotide-regulated ion channels. Since the discovery of exchange proteins directly activated by cyclic adenosine 5'-monophosphate (EPACs) in 1998, accumulating evidence has demonstrated that the net cellular effects of cAMP are also regulated by EPAC. The pursuit of the biological functions of EPAC has benefited from the development and applications of a growing number of pharmacological probes targeting EPACs. In this review, we seek to provide a concise update on recent advances in the development of chemical entities including various membrane-permeable analogues of cAMP and newly discovered EPAC-specific ligands from high throughput assays and hit-to-lead optimizations.

Role of guanine-nucleotide exchange factor Epac in renal physiology and pathophysiology

Exchange proteins directly activated by cAMP [Epac(s)] were discovered more than a decade ago as new sensors for the second messenger cAMP. The Epac family members, including Epac1 and Epac2, are guanine nucleotide exchange factors for the Ras-like small GTPases Rap1 and Rap2, and they function independently of protein kinase A. Given the importance of cAMP in kidney homeostasis, several molecular and cellular studies using specific Epac agonists have analyzed the role and regulation of Epac proteins in renal physiology and pathophysiology. The specificity of the functions of Epac proteins may depend upon their expression and localization in the kidney as well as their abundance in the microcellular environment. This review discusses recent literature data concerning the involvement of Epac in renal tubular transport physiology and renal glomerular cells where various signaling pathways are known to be operative. In addition, the potential role of Epac in kidney disorders, such as diabetic kidney disease and ischemic kidney injury, is discussed.

Adiponectin attenuates angiotensin II-induced oxidative stress in renal tubular cells through AMPK and cAMP-Epac signal transduction pathways

Obesity is a risk factor for chronic kidney disease (CKD) progression. Circulating levels of adiponectin, an adipokine, decrease with obesity and play a protective role in the cardiovascular system. We hypothesized that adiponectin might also protect the kidney. Because activation of the renin-angiotensin system (RAS) is a contributor to CKD progression, we tested our hypothesis by studying the interactions between adiponectin and angiotensin II (ANG II) in renal tubular cells. Primary human renal proximal tubule cells expressed both adiponectin receptor 1 and 2 (adipoR1 and R2). ANG II-induced NADPH oxidase activation and oxidative stress were attenuated by adiponectin and dependent on adipoR1. Activation of AMPK with 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) mimicked, while inhibition of AMPK with compound C abrogated, the effect of adiponectin on ANG II-induced activation of NADPH oxidase. Similarly, the effect of adiponectin was recapitulated by the stable cAMP analogs 4-chlorophenylthio (pCPT)-cAMP and dibutyryl (db)-cAMP and blocked by the adenylate cyclase inhibitor SQ22536. Adiponectin did not activate PKA in renal tubular cells, and the specific PKA inhibitor myristoylated PKI (14-22) amide failed to block the inhibitory effect of adiponectin on ANG II-induced NADPH oxidase activation. In contrast, the specific Epac activator 8-(4-chlorophenylthio)-2'-O-methyl (8-CPT-2-O-Me)-cAMP blocked ANG II-induced activation of NADPH oxidase, an effect that was reversed by coincubation with the AMPK inhibitor compound C. Finally, adiponectin attenuated ANG II-induced NF-κB activation and fibronectin protein expression. These in vitro findings support the hypothesis that adiponectin may attenuate the deleterious effects of ANG II in the kidney and play a protective role in CKD.

Exchange protein directly activated by cAMP (epac): a multidomain cAMP mediator in the regulation of diverse biological functions

Since the discovery nearly 60 years ago, cAMP is envisioned as one of the most universal and versatile second messengers. The tremendous feature of cAMP to tightly control highly diverse physiologic processes, including calcium homeostasis, metabolism, secretion, muscle contraction, cell fate, and gene transcription, is reflected by the award of five Nobel prizes. The discovery of Epac (exchange protein directly activated by cAMP) has ignited a new surge of cAMP-related research and has depicted novel cAMP properties independent of protein kinase A and cyclic nucleotide-gated channels. The multidomain architecture of Epac determines its activity state and allows cell-type specific protein-protein and protein-lipid interactions that control fine-tuning of pivotal biologic responses through the "old" second messenger cAMP. Compartmentalization of cAMP in space and time, maintained by A-kinase anchoring proteins, phosphodiesterases, and β-arrestins, contributes to the Epac signalosome of small GTPases, phospholipases, mitogen- and lipid-activated kinases, and transcription factors. These novel cAMP sensors seem to implement certain unexpected signaling properties of cAMP and thereby to permit delicate adaptations of biologic responses. Agonists and antagonists selective for Epac are developed and will support further studies on the biologic net outcome of the activation of Epac. This will increase our current knowledge on the pathophysiology of devastating diseases, such as diabetes, cognitive impairment, renal and heart failure, (pulmonary) hypertension, asthma, and chronic obstructive pulmonary disease. Further insights into the cAMP dynamics executed by the Epac signalosome will help to optimize the pharmacological treatment of these diseases.

Adenylyl cyclase VI mediates vasopressin-stimulated ENaC activity

Vasopressin modulates sodium reabsorption in the collecting duct through adenylyl cyclase-stimulated cyclic AMP, which exists as multiple isoforms; the specific isoform involved in vasopressin-stimulated sodium transport is unknown. To assess this, we studied mice deficient in adenylyl cyclase type VI specifically in the principal cells of the collecting duct. Knockout mice had increased urine volume and reduced urine sodium concentration, but regardless of the level of sodium intake, they did not exhibit significant alterations in urinary sodium excretion, arterial pressure, or pulse rate. Plasma renin concentration was elevated in knockout mice, however, suggesting a compensatory response. Valsartan significantly reduced arterial pressure in knockout mice but not in controls. Knockout mice had decreased renal cortical mRNA content of all three epithelial sodium channel (ENaC) isoforms, and total cell sodium channel isoforms α and γ were reduced in these animals. Patch-clamp analysis of split-open cortical collecting ducts revealed no difference in baseline activity of sodium channels, but knockout mice had abolished vasopressin-stimulated ENaC open probability and apical membrane channel number. In summary, these data suggest that adenylyl cyclase VI mediates vasopressin-stimulated ENaC activity in the kidney.

Slp2-a controls renal epithelial cell size through regulation of Rap–ezrin signaling independently of Rab27

Slp2-a is a Rab27 effector protein that regulates transport of Rab27-bearing vesicles/organelles via its N-terminal Rab27-binding domain and a phospholipid-binding C2A domain. Here we demonstrate a Rab27-independent function of Slp2-a in the control of renal cell size via a previously uncharacterized C2B domain. We found that by recruiting Rap1GAPs to the plasma membrane of MDCK II cells via the C2B domain Slp2-a inactivates Rap signaling and modulates the size of the cells. Functional ablation of Slp2-a resulted in an increase in the size of MDCK II cells. Drosophila Slp bitesize was found to compensate for the function of Slp2-a in MDCK II cells, thereby indicating that the mechanism of the cell size control by Slps has been evolutionarily conserved. Interestingly, blockade of the activity of ezrin, a downstream target of Rap, with the glucosylceramide synthase inhibitor miglustat effectively inhibited cell spreading of Slp2-a-knockdown cells. We also discovered aberrant expression of Slp2-a and increased activity of ezrin in pcy mice, a model of polycystic kidney disease that is characterized by renal cell spreading. Our findings indicate that Slp2-a controls renal cell size through regulation of Rap–ezrin signaling independently of Rab27.

In mpkCCD cells, long-term regulation of aquaporin-2 by vasopressin occurs independent of protein kinase A and CREB but may involve Epac

Urine concentration involves the hormone vasopressin (AVP), which stimulates cAMP production in renal principal cells, resulting in translocation and transcription of aquaporin-2 (AQP2) water channels, greatly increasing the water permeability, leading to a concentrated urine. As cAMP levels decrease shortly after AVP addition, whereas AQP2 levels still increase and are maintained for days, we investigated in the present study the mechanism responsible for the AQP2 increase after long-term 1-desamino-8-d-arginine vasopressin (dDAVP) application using mouse collecting duct (mpkCCD) cells. While 30 min of dDAVP incubation strongly increased cAMP, cAMP was lower with 1 day and was even further reduced with 4 days of dDAVP, although still significantly higher than in control cells. One day of dDAVP incubation increased AQP2 promoter-dependent transcription, which was blocked by the protein kinase A (PKA) inhibitor H89. Moreover, phosphorylation of the cAMP-responsive element binding protein (CREB) and CRE-dependent transcription was observed after short-term dDAVP stimulation. With 4 days of dDAVP, AQP2 transcription remained elevated, but this was not blocked by H89, and CRE-dependent transcription and CREB phosphorylation were not increased. Exchange factor directly activated by cAMP (Epac) 1 and 2 were found to be endogenously expressed in mpkCCD cells. Application of dDAVP increased the expression of Epac1, while Epac2 was reduced. Incubation with a specific Epac activator after dDAVP pretreatment increased both AQP2 abundance and transcription compared with cells left unstimulated the last day. In conclusion, the PKA-CRE pathway is involved in the initial rise in AQP2 levels after dDAVP stimulation but not in the long-term effect of dDAVP. Instead, long-term regulation of AQP2 may involve the activation of Epac.

cAMP signalling protects proximal tubular epithelial cells from cisplatin-induced apoptosis via activation of Epac

BACKGROUND AND PURPOSE

Nephrotoxicity is the principal dose-limiting factor for cisplatin chemotherapy and is primarily associated with proximal tubular epithelial cells, including disruption of cell adhesions and induction of apoptosis. Cell adhesion and survival is regulated by, amongst other factors, the small GTPase Rap and its activator, the exchange protein directly activated by cAMP (Epac). Epac is particularly enriched in renal tubule epithelium. This study investigates the cytoprotective effects of cAMP-Epac-Rap signalling in a model of cisplatin-induced renal cell injury.

EXPERIMENTAL APPROACH

The Epac-selective cAMP analogue 8-pCPT-2'-O-Me-cAMP was used to activate the Epac-Rap signalling pathway in proximal tubular epithelial cells. Cells were exposed to cisplatin, in the presence or absence of 8-pCPT-2'-O-Me-cAMP, and nephrotoxicity was determined by monitoring cell-cell junctions and cell apoptosis.

KEY RESULTS

Activation of Epac-Rap signalling preserves cell-cell junctions and protects against cell apoptosis of mouse proximal tubular cells during cisplatin treatment. Activation with the Epac-selective cAMP analogue 8-pCPT-2'-O-Me-cAMP or receptor-mediated induction of cAMP both induced cytoprotection against cisplatin, whereas a PKA-selective cAMP analogue was not cytoprotective. 8-pCPT-2'-O-Me-cAMP mediated cytoprotection was blocked by RNAi-mediated silencing of Epac-Rap signalling in these cells. In contrast, 8-pCPT-2'-O-Me-cAMP did not protect against cisplatin-induced cell death of cancer cells that lacked Epac1 expression.

CONCLUSIONS AND IMPLICATIONS

Our study identifies activation of Epac-Rap signalling as a potential strategy for reducing the nephrotoxicity associated with cisplatin treatments and, as a result, broadens the therapeutic window of this chemotherapeutic agent.

Regulation of SGLT expression and localization through Epac/PKA-dependent caveolin-1 and F-actin activation in renal proximal tubule cells

This study demonstrated that exchange proteins directly activated by cAMP (Epac) and protein kinase A (PKA) by 8-bromo (8-Br)-adenosine 3',5'-cyclic monophosphate (cAMP) stimulated [(14)C]-α-methyl-D-glucopyranoside (α-MG) uptake through increased sodium-glucose cotransporters (SGLTs) expression and translocation to lipid rafts in renal proximal tubule cells (PTCs). In PTCs, SGLTs were colocalized with lipid raft caveolin-1 (cav-1), disrupted by methyl-β-cyclodextrin (MβCD). Selective activators of Epac or PKA, 8-Br-cAMP, and forskolin stimulated expressions of SGLTs and α-MG uptake in PTCs. In addition, 8-Br-cAMP-induced PKA and Epac activation increased phosphorylation of extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAPK), and nuclear factor kappa B (NF-κB), which were involved in expressions of SGLTs. Furthermore, 8-Br-cAMP stimulated SGLTs translocation to lipid rafts via filamentous actin (F-actin) organization, which was blocked by cytochalasin D. In addition, cav-1 and SGLTs stimulated by 8-Br-cAMP were detected in lipid rafts, which were blocked by cytochalasin D. Furthermore, 8-Br-cAMP-induced SGLTs translocation and α-MG uptake were attenuated by inhibition of cav-1 activation with cav-1 small interfering RNA (siRNA) and inhibition of F-actin organization with TRIO and F-actin binding protein (TRIOBP). In conclusion, 8-Br-cAMP stimulated α-MG uptake via Epac and PKA-dependent SGLTs expression and trafficking through cav-1 and F-actin in PTCs.

Epac1-mediated, high glucose-induced renal proximal tubular cells hypertrophy via the Akt/p21 pathway

The mechanisms involved in tubular hypertrophy in diabetic nephropathy are unclear. We investigated the role of exchange protein activated by cAMP 1(Epac1), which activates Rap-family G proteins in cellular hypertrophy. Epac1 is expressed in heart, renal tubules, and in the HK-2 cell line. In diabetic mice, increased Epac1 expression was observed, and under high glucose ambience (HGA), HK-2 cells also exhibited increased Epac1 expression. We isolated a 1614-bp DNA fragment upstream of the initiation codon of Epac1 gene, inclusive of glucose response elements (GREs). HK-2 or COS7 cells transfected with the Epac1 promoter revealed a dose-dependent increase in its activity under HGA. Mutations in GRE motifs resulted in decreased promoter activity. HK-2 cells exhibited a hypertrophic response and increased protein synthesis under HGA, which was reduced by Epac1-siRNA or -mutants, whereas the use of a protein kinase A inhibitor had minimal effect. Epac1 transfection led to cellular hypertrophy and increased protein synthesis, which was accentuated by HGA. HGA increased the proportion of cells in the G0/G1 cell-cycle phase, and the expression of pAkt and the cyclin-dependent kinase inhibitors p21 and p27 was increased while the activity of cyclin-dependent kinase 4 decreased. These effects were reversed following transfection of cells with Epac1-siRNA or -mutants. These data suggest that HGA increases GRE-dependent Epac1 transcription, leading to cell cycle arrest and instigation of cellular hypertrophy.

Localization of canine, feline, and mouse renal membrane proteins

Immunohistochemistry allows the localization of proteins to specific regions of the nephron. This article reports the identification and localization of proteins in situ within normal canine, feline, and mouse kidney by immunohistochemistry; maps their distribution; and compares results to previously reported findings in other species. The proteins investigated are aquaporin 1, aquaporin 2, calbindin D-28k, glutathione S-transferase-α, and Tamm-Horsfall protein. Aquaporins are integral membrane proteins involved in water transport across cell membranes. Calbindin D-28k is involved in renal calcium metabolism. Glutathione S-transferase-α is a protein that aids in detoxification and drug metabolism. The role of Tamm-Horsfall protein is not fully understood. Proposed functions include inhibition of calcium crystallization and reduction of bacterial urinary tract infection. The authors' findings in the dog are similar to those in other species: Specifically, the authors localize aquaporin 1 to the proximal convoluted tubule epithelium, vasa recta endothelium, and descending thin limbs; aquaporin 2 to collecting duct epithelium; and calbindin D-28k within distal convoluted tubule epithelium. Glutathione S-transferase-α has variable expression and is found in only the renal transitional epithelium in some individuals, in only the proximal straight tubules in others, or in both locations in others. Tamm-Horsfall protein localizes to thick ascending limb epithelium. These findings are similar in the cat, with the exception that aquaporin 1 is located in glomerular podocytes, in addition to proximal convoluted tubule epithelium, and glutathione S-transferase-α is found solely within the proximal convoluted tubule within all kidney samples examined. The mouse kidney is almost identical to the dog but expresses glutathione S-transferase-α in the glomeruli only.

Epac-Rap signaling reduces cellular stress and ischemia-induced kidney failure

Renal ischemia-reperfusion injury is associated with the loss of tubular epithelial cell-cell and cell-matrix interactions which contribute to renal failure. The Epac-Rap signaling pathway is a potent regulator of cell-cell and cell-matrix adhesion. The cyclic AMP analogue 8-pCPT-2'-O-Me-cAMP has been shown to selectively activate Epac, whereas the addition of an acetoxymethyl (AM) ester to 8-pCPT-2'-O-Me-cAMP enhanced in vitro cellular uptake. Here we demonstrate that pharmacological activation of Epac-Rap signaling using acetoxymethyl-8-pCPT-2'-O-Me-cAMP preserves cell adhesions during hypoxia in vitro, maintaining the barrier function of the epithelial monolayer. Intrarenal administration in vivo of 8-pCPT-2'-O-Me-cAMP also reduced renal failure in a mouse model for ischemia-reperfusion injury. This was accompanied by decreased expression of the tubular cell stress marker clusterin-α, and lateral expression of β-catenin after ischemia indicative of sustained tubular barrier function. Our study emphasizes the undervalued importance of maintaining tubular epithelial cell adhesion in renal ischemia and demonstrates the potential of pharmacological modulation of cell adhesion as a new therapeutic strategy to reduce the extent of injury in kidney disease and transplantation.

Urea transport in the kidney

Urea transport proteins were initially proposed to exist in the kidney in the late 1980s when studies of urea permeability revealed values in excess of those predicted by simple lipid-phase diffusion and paracellular transport. Less than a decade later, the first urea transporter was cloned. Currently, the SLC14A family of urea transporters contains two major subgroups: SLC14A1, the UT-B urea transporter originally isolated from erythrocytes; and SLC14A2, the UT-A group with six distinct isoforms described to date. In the kidney, UT-A1 and UT-A3 are found in the inner medullary collecting duct; UT-A2 is located in the thin descending limb, and UT-B is located primarily in the descending vasa recta; all are glycoproteins. These transporters are crucial to the kidney's ability to concentrate urine. UT-A1 and UT-A3 are acutely regulated by vasopressin. UT-A1 has also been shown to be regulated by hypertonicity, angiotensin II, and oxytocin. Acute regulation of these transporters is through phosphorylation. Both UT-A1 and UT-A3 rapidly accumulate in the plasma membrane in response to stimulation by vasopressin or hypertonicity. Long-term regulation involves altering protein abundance in response to changes in hydration status, low protein diets, adrenal steroids, sustained diuresis, or antidiuresis. Urea transporters have been studied using animal models of disease including diabetes mellitus, lithium intoxication, hypertension, and nephrotoxic drug responses. Exciting new animal models are being developed to study these transporters and search for active urea transporters. Here we introduce urea and describe the current knowledge of the urea transporter proteins, their regulation, and their role in the kidney.

Epac: defining a new mechanism for cAMP action

cAMP is a second messenger that is essential for relaying hormonal responses in many biological processes. The discovery of the cAMP target Epac explained various effects of cAMP that could not be attributed to the established targets PKA and cyclic nucleotide-gated ion channels. Epac1 and Epac2 function as guanine nucleotide exchange factors for the small G protein Rap. cAMP analogs that selectively activate Epac have helped to reveal a role for Epac in processes ranging from insulin secretion to cardiac contraction and vascular permeability. Advances in the understanding of the activation mechanism of Epac and its regulation by diverse anchoring mechanisms have helped to elucidate the means by which cAMP fulfills these functions via Epac.

The role of Epac proteins, novel cAMP mediators, in the regulation of immune, lung and neuronal function

Chronic degenerative inflammatory diseases, such as chronic obstructive pulmonary disease and Alzheimer's dementia, afflict millions of people around the world, causing death and debilitation. Despite the global impact of these diseases, there have been few innovative breakthroughs into their cause, treatment or cure. As with many debilitating disorders, chronic degenerative inflammatory diseases may be associated with defective or dysfunctional responses to second messengers, such as cyclic adenosinemonophosphate (cAMP). The identification of the cAMP-activated guanine nucleotide exchange factors for Ras-like GTPases, Epac1 (also known as cAMP-GEF-I) and Epac2 (also known as cAMP-GEF-II), profoundly altered the prevailing assumptions concerning cAMP signalling, which until then had been solely associated with protein kinase A (PKA). Studies of the molecular mechanisms of Epac-related signalling have demonstrated that these novel cAMP sensors regulate many physiological processes either alone and/or in concert with PKA. These include calcium handling, cardiac and smooth muscle contraction, learning and memory, cell proliferation and differentiation, apoptosis, and inflammation. The diverse signalling properties of cAMP might be explained by spatio-temporal compartmentalization, as well as A-kinase anchoring proteins, which seem to coordinate Epac signalling networks. Future research should focus on the Epac-regulated dynamics of cAMP, and, hopefully, the development of compounds that specifically interfere with the Epac signalling system in order to determine the precise significance of Epac proteins in chronic degenerative inflammatory disorders.

Epac regulates UT-A1 to increase urea transport in inner medullary collecting ducts

Urea plays a critical role in the concentration of urine, thereby regulating water balance. Vasopressin, acting through cAMP, stimulates urea transport across rat terminal inner medullary collecting ducts (IMCD) by increasing the phosphorylation and accumulation at the apical plasma membrane of UT-A1. In addition to acting through protein kinase A (PKA), cAMP also activates Epac (exchange protein activated by cAMP). In this study, we tested whether the regulation of urea transport and UT-A1 transporter activity involve Epac in rat IMCD. Functional analysis showed that an Epac activator significantly increased urea permeability in isolated, perfused rat terminal IMCD. Similarly, stimulating Epac by adding forskolin and an inhibitor of PKA significantly increased urea permeability. Incubation of rat IMCD suspensions with the Epac activator significantly increased UT-A1 phosphorylation and its accumulation in the plasma membrane. Furthermore, forskolin-stimulated cAMP significantly increased ERK 1/2 phosphorylation, which was not prevented by inhibiting PKA, indicating that Epac mediated this phosphorylation of ERK 1/2. Inhibition of MEK 1/2 phosphorylation decreased the forskolin-stimulated UT-A1 phosphorylation. Taken together, activation of Epac increases urea transport, accumulation of UT-A1 at the plasma membrane, and UT-A1 phosphorylation, the latter of which is mediated by the MEK-ERK pathway.

cAMP stimulates apical exocytosis of the renal Na(+)-K(+)-2Cl(-) cotransporter NKCC2 in the thick ascending limb: role of protein kinase A

The apical renal Na(+)-K(+)-2Cl(-) cotransporter NKCC2 mediates NaCl absorption by the thick ascending limb (TAL) of Henle's loop. cAMP stimulates NKCC2 by enhancing steady-state apical membrane levels of this protein; however, the trafficking and signaling mechanisms by which this occurs have not been studied. Here, we report that stimulation of endogenous cAMP levels with either forskolin/3-isobutyl-1-methylxanthine (IBMX) or the V2 receptor agonist [deamino-Cys(1),d-Arg(8)]vasopressin increases steady-state surface NKCC2 and that the protein kinase A (PKA) inhibitor H-89 blocks this effect. Confocal imaging of apical surface NKCC2 in isolated perfused TALs confirmed a stimulatory effect of cAMP on apical trafficking that was blocked by PKA inhibition. Selective stimulation of PKA with the agonist N(6)-benzoyl-cAMP (500 microm) stimulated steady-state surface NKCC2, whereas the Epac-selective agonist 8-p-chlorophenylthio-2'-O-methyl-cAMP (100 and 250 microm) had no effect. To explore the trafficking mechanism by which cAMP increases apical NKCC2, we measured cumulative apical membrane exocytosis and NKCC2 exocytic insertion in TALs. By monitoring apical FM1-43 fluorescence, we observed rapid stimulation of apical exocytosis (2 min) by forskolin/IBMX. We also found constitutive exocytic insertion of NKCC2 in TALs over time, which was increased by 3-fold in the presence of forskolin/IBMX. PKA inhibition blunted cAMP-stimulated exocytic insertion but did not affect the rate of constitutive exocytosis. We conclude that cAMP stimulates steady-state apical surface NKCC2 by stimulating exocytic insertion and that this process is highly dependent on PKA but not Epac.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.