Mechanism of activation of ERK and H-K-ATPase by isoproterenol in rat cortical collecting duct

TRPV4 expression in the renal tubule is necessary for maintaining whole body K+ homeostasis

The Ca2+-permeable transient receptor potential vanilloid type 4 (TRPV4) channel serves as the sensor of tubular flow, thus being well suited to govern mechanosensitive K+ transport in the distal renal tubule. Here, we directly tested whether the TRPV4 function is significant in affecting K+ balance. We used balance metabolic cage experiments and systemic measurements with different K+ feeding regimens [high (5% K+), regular (0.9% K+), and low (<0.01% K+)] in newly created transgenic mice with selective TRPV4 deletion in the renal tubule (TRPV4fl/fl-Pax8Cre) and their littermate controls (TRPV4fl/fl). Deletion was verified by the absence of TRPV4 protein expression and lack of TRPV4-dependent Ca2+ influx. There were no differences in plasma electrolytes, urinary volume, and K+ levels at baseline. In contrast, plasma K+ levels were significantly elevated in TRPV4fl/fl-Pax8Cre mice on high K+ intake. K+-loaded knockout mice exhibited lower urinary K+ levels than TRPV4fl/fl mice, which was accompanied by higher aldosterone levels by day 7. Moreover, TRPV4fl/fl-Pax8Cre mice had more efficient renal K+ conservation and higher plasma K+ levels in the state of dietary K+ deficiency. H+-K+-ATPase levels were significantly increased in TRPV4fl/fl-Pax8Cre mice on a regular diet and especially on a low-K+ diet, pointing to augmented K+ reabsorption in the collecting duct. Consistently, we found a significantly faster intracellular pH recovery after intracellular acidification, as an index of H+-K+-ATPase activity, in split-opened collecting ducts from TRPV4fl/fl-Pax8Cre mice. In summary, our results demonstrate an indispensable prokaliuretic role of TRPV4 in the renal tubule in controlling K+ balance and urinary K+ excretion during variations in dietary K+ intake. NEW & NOTEWORTHY The mechanoactivated transient receptor potential vanilloid type 4 (TRPV4) channel is expressed in distal tubule segments, where it controls flow-dependent K+ transport. Global TRPV4 deficiency causes impaired adaptation to variations in dietary K+ intake. Here, we demonstrate that renal tubule-specific TRPV4 deletion is sufficient to recapitulate the phenotype by causing antikaliuresis and higher plasma K+ levels in both states of K+ load and deficiency.

Drug Repurposing for Cystic Fibrosis: Identification of Drugs That Induce CFTR-Independent Fluid Secretion in Nasal Organoids

Individuals with cystic fibrosis (CF) suffer from severe respiratory disease due to a genetic defect in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which impairs airway epithelial ion and fluid secretion. New CFTR modulators that restore mutant CFTR function have been recently approved for a large group of people with CF (pwCF), but ~19% of pwCF cannot benefit from CFTR modulators Restoration of epithelial fluid secretion through non-CFTR pathways might be an effective treatment for all pwCF. Here, we developed a medium-throughput 384-well screening assay using nasal CF airway epithelial organoids, with the aim to repurpose FDA-approved drugs as modulators of non-CFTR-dependent epithelial fluid secretion. From a ~1400 FDA-approved drug library, we identified and validated 12 FDA-approved drugs that induced CFTR-independent fluid secretion. Among the hits were several cAMP-mediating drugs, including β2-adrenergic agonists. The hits displayed no effects on chloride conductance measured in the Ussing chamber, and fluid secretion was not affected by TMEM16A, as demonstrated by knockout (KO) experiments in primary nasal epithelial cells. Altogether, our results demonstrate the use of primary nasal airway cells for medium-scale drug screening, target validation with a highly efficient protocol for generating CRISPR-Cas9 KO cells and identification of compounds which induce fluid secretion in a CFTR- and TMEM16A-indepent manner.

Airway Surface Liquid pH Regulation in Airway Epithelium Current Understandings and Gaps in Knowledge

Knowledge on the mechanisms of acid and base secretion in airways has progressed recently. The aim of this review is to summarize the known mechanisms of airway surface liquid (ASL) pH regulation and their implication in lung diseases. Normal ASL is slightly acidic relative to the interstitium, and defects in ASL pH regulation are associated with various respiratory diseases, such as cystic fibrosis. Basolateral bicarbonate (HCO3-) entry occurs via the electrogenic, coupled transport of sodium (Na+) and HCO3-, and, together with carbonic anhydrase enzymatic activity, provides HCO3- for apical secretion. The latter mainly involves CFTR, the apical chloride/bicarbonate exchanger pendrin and paracellular transport. Proton (H+) secretion into ASL is crucial to maintain its relative acidity compared to the blood. This is enabled by H+ apical secretion, mainly involving H+/K+ ATPase and vacuolar H+-ATPase that carry H+ against the electrochemical potential gradient. Paracellular HCO3- transport, the direction of which depends on the ASL pH value, acts as an ASL protective buffering mechanism. How the transepithelial transport of H+ and HCO3- is coordinated to tightly regulate ASL pH remains poorly understood, and should be the focus of new studies.

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.

ERK1,2 Signalling Pathway along the Nephron and Its Role in Acid-base and Electrolytes Balance

Mitogen-activated protein kinases (MAPKs) are intracellular molecules regulating a wide range of cellular functions, including proliferation, differentiation, apoptosis, cytoskeleton remodeling and cytokine production. MAPK activity has been shown in normal kidney, and its over-activation has been demonstrated in several renal diseases. The extracellular signal-regulated protein kinases (ERK 1,2) signalling pathway is the first described MAPK signaling. Intensive investigations have demonstrated that it participates in the regulation of ureteric bud branching, a fundamental process in establishing final nephron number; in addition, it is also involved in the differentiation of the nephrogenic mesenchyme, indicating a key role in mammalian kidney embryonic development. In the present manuscript, we show that ERK1,2 signalling mediates several cellular functions also in mature kidney, describing its role along the nephron and demonstrating whether it contributes to the regulation of ion channels and transporters implicated in acid-base and electrolytes homeostasis.

Pyk2 regulates H+-ATPase-mediated proton secretion in the outer medullary collecting duct via an ERK1/2 signaling pathway

Acid-secreting intercalated cells respond to changes in systemic pH through regulation of apical H(+) transporters. Little is known about the mechanism by which these cells sense changes in extracellular pH (pH(o)). Pyk2 is a nonreceptor tyrosine kinase activated by autophosphorylation at Tyr402 by cell-specific stimuli, including decreased pH, and is involved in the regulation of MAPK signaling pathways and transporter activity. We examined whether the Pyk2 and MAPK signaling pathway mediates the response of transport proteins to decreased pH in outer medullary collecting duct cells. Immunoblot analysis of phosphorylated Pyk2 (Tyr402), ERK1/2 (Thr202/Tyr204), and p38 (Thr180/Tyr182) was used to assay protein activation. To examine specificity of kinase activation and its effects, we used Pyk2 small interfering RNA to knockdown Pyk2 expression levels, the Src kinase inhibitor 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]-pyrimidine (PP 1) to inhibit Pyk2 phosphorylation, and the MEK inhibitor U0126 to inhibit ERK1/2 phosphorylation. The pH-sensitive fluorescent probe 2'-7'-bis(carboxyethyl)-5(6)-carboxyfluorescein-acetoxymethyl ester (BCECF-AM) was used to assay H(+) transporter activity. The activity of H(+) transporters was measured as the rate of intracellular pH (pH(i)) recovery after an NH(4)Cl prepulse. We show that Pyk2 is endogenously expressed and activated by acid pH in mouse-derived outer medullary collecting duct (mOMCD1) cells. Incubation of mOMCD1 cells in acid media [extracellular pH (pH(o)) 6.7] increased the phosphorylation of Pyk2, ERK1/2, and p38. Reduction in pH(i) induced by an NH(4)Cl prepulse also increased the phosphorylation of Pyk2, ERK1/2, and p38. Consistent with our previous studies, we found that mOMCD1 cells exhibit H(+)-ATPase and H(+),K(+)-ATPase activity. Pyk2 inhibition by Pyk2 siRNA and PP 1 prevented Pyk2 phosphorylation as well as H(+)-ATPase-mediated recovery in mOMCD1 cells. In addition, ERK1/2 inhibition by U0126 prevented acid-induced ERK1/2 phosphorylation and H(+)-ATPase-mediated pH(i) recovery but not phosphorylation of p38. We conclude that Pyk2 and ERK1/2 are required for increasing H(+)-ATPase, but not H(+),K(+)-ATPase, activity at decreased pH(i) in mOMCD1 cells.

Regulation of pendrin by cAMP: possible involvement in β-adrenergic-dependent NaCl retention

The sodium-independent anion exchanger pendrin is expressed in several tissues including the kidney cortical collecting duct (CCD), where it acts as a chloride/bicarbonate exchanger and has been shown to participate in the regulation of acid-base homeostasis and blood pressure. The renal sympathetic nervous system is known to play a key role in the development of salt-induced hypertension. This study aimed to determine whether pendrin may partly mediate the effects of β adrenergic receptors (β-AR) on renal salt handling. We investigated the regulation of pendrin activity by the cAMP/protein kinase A (PKA) signaling pathway, both in vitro in opossum kidney proximal (OKP) cells stably transfected with pendrin cDNA and ex vivo in isolated microperfused CCDs stimulated by isoproterenol, a β-AR agonist. We found that stimulation of the cAMP/PKA pathway in OKP cells increased the amount of pendrin at the cell surface as well as its transport activity. These effects stemmed from increased exocytosis of pendrin and were associated with its phosphorylation. Furthermore, cAMP effects on the membrane expression and activity of pendrin were abolished by mutating the serine 49 located in the intracellular N-terminal domain of pendrin. Finally, we showed that isoproterenol increases pendrin trafficking to the apical membrane as well as the reabsorption of both Cl(-) and Na(+) in microperfused CCDs. All together, our results strongly suggest that pendrin activation by the cAMP/PKA pathway underlies isoproterenol-induced stimulation of NaCl reabsorption in the kidney collecting duct, a mechanism likely involved in the sodium-retaining effect of β-adrenergic agonists.

A comprehensive analysis of gene expression profiles in distal parts of the mouse renal tubule

The distal parts of the renal tubule play a critical role in maintaining homeostasis of extracellular fluids. In this review, we present an in-depth analysis of microarray-based gene expression profiles available for microdissected mouse distal nephron segments, i.e., the distal convoluted tubule (DCT) and the connecting tubule (CNT), and for the cortical portion of the collecting duct (CCD; Zuber et al., Proc Natl Acad Sci USA 106:16523-16528, 2009). Classification of expressed transcripts in 14 major functional gene categories demonstrated that all principal proteins involved in maintaining the salt and water balance are represented by highly abundant transcripts. However, a significant number of transcripts belonging, for instance, to categories of G-protein-coupled receptors or serine/threonine kinases exhibit high expression levels but remain unassigned to a specific renal function. We also established a list of genes differentially expressed between the DCT/CNT and the CCD. This list is enriched by genes related to segment-specific transport functions and by transcription factors directing the development of the distal nephron or collecting ducts. Collectively, this in silico analysis provides comprehensive information about relative abundance and tissue specificity of the DCT/CNT and the CCD expressed transcripts and identifies new candidate genes for renal homeostasis.

The renal H+-K+-ATPases: physiology, regulation, and structure

The H(+)-K(+)-ATPases are ion pumps that use the energy of ATP hydrolysis to transport protons (H(+)) in exchange for potassium ions (K(+)). These enzymes consist of a catalytic alpha-subunit and a regulatory beta-subunit. There are two catalytic subunits present in the kidney, the gastric or HKalpha(1) isoform and the colonic or HKalpha(2) isoform. In this review we discuss new information on the physiological function, regulation, and structure of the renal H(+)-K(+)-ATPases. Evaluation of enzymatic functions along the nephron and collecting duct and studies in HKalpha(1) and HKalpha(2) knockout mice suggest that the H(+)-K(+)-ATPases may function to transport ions other than protons and potassium. These reports and recent studies in mice lacking both HKalpha(1) and HKalpha(2) suggest important roles for the renal H(+)-K(+)-ATPases in acid/base balance as well as potassium and sodium homeostasis. Molecular modeling studies based on the crystal structure of a related enzyme have made it possible to evaluate the structures of HKalpha(1) and HKalpha(2) and provide a means to study the specific cation transport properties of H(+)-K(+)-ATPases. Studies to characterize the cation specificity of these enzymes under different physiological conditions are necessary to fully understand the role of the H(+)-K(+) ATPases in renal physiology.

H2O2 and Src-dependent transactivation of the EGF receptor mediates the stimulatory effect of leptin on renal ERK and Na+, K+-ATPase

We examined the mechanism through which leptin increases Na(+), K(+)-ATPase activity in the rat kidney. Leptin was infused under anaesthesia into the abdominal aorta proximally to the renal arteries and then Na(+), K(+)-ATPase activity was measured in the renal cortex and medulla. Leptin (1mug/kgmin) increased Na(+), K(+)-ATPase activity after 3h of infusion, which was accompanied by the increase in urinary H(2)O(2) excretion and phosphorylation level of extracellular signal regulated kinase (ERK). The effect of leptin on ERK and Na(+), K(+)-ATPase was abolished by catalase, specific inhibitors of epidermal growth factor (EGF) receptor, AG1478 and PD158780, as well as by ERK inhibitor, PD98059, and was mimicked by both exogenous H(2)O(2) and EGF. The effect of leptin was also prevented by the inhibitor of Src tyrosine kinase, PP2. Leptin and H(2)O(2) increased Src phosphorylation at Tyr(418). We conclude that leptin-induced stimulation of renal Na(+), K(+)-ATPase involves H(2)O(2) generation, Src kinase, transactivation of the EGF receptor, and stimulation of ERK.

Mechanism of isoproterenol-induced RGS2 up-regulation in astrocytes

Regulators of G protein signaling (RGSs) are inducibly expressed in response to various stimuli and the up-regulation of RGSs leads to significant decreases in GPCR responsiveness. Isoproterenol, an adrenergic receptor agonist, stimulated RGS2 mRNA in C6 rat astrocytoma cells. The up-regulation of RGS2 mRNA was abrogated by genistein, a protein tyrosine kinase inhibitor (PTK), and by broad-spectrum protein kinase C (PKC) inhibitors (staurosporine and GF109203X). alpha-Adrenergic antagonist (prazocin), beta-adrenergic antagonist (prazocin), and pertussis toxin only partially blocked the RGS2 up-regulation, suggesting that the RGS2 up-regulation is concomitantly mediated by Galphai, Galphas, and Galphaq. It is interesting to note that SB203580, a potent p38 mitogen-activated protein kinase (MAPK) inhibitor, completely inhibited the isoproterenol-mediated RGS2 expression. In addition, isoproterenol also markedly stimulated RGS2 mRNA in rat primary astrocytes, which were sensitive to SB203580 and staurosporine. Therefore, our data suggest that adrenergic receptor-mediated signaling (induced by isoproterenol) may be involved in the regulation of RGS2 expression in astrocytes via activating PTK, PKC, and p38 MAPK.