NADPH oxidase 4 deficiency reduces aquaporin-2 mRNA expression in cultured renal collecting duct principal cells via increased PDE3 and PDE4 activity

Poly(ADP-ribose) polymerase-1 affects vasopressin-mediated AQP2 expression in collecting duct cells of the kidney

Poly(ADP-ribosyl)ation (PARylation), as a posttranslational modification mediated by poly(ADP-ribose) polymerases (PARPs) catalyzing the transfer of ADP-ribose from NAD+ molecules to acceptor proteins, involves a number of cellular processes. As mice lacking the PARP-1 gene (Parp1) produce more urine, we investigated the role of PARP-1, the most prevalent member of the PARP family, in the vasopressin-responsive expression of aquaporin-2 (AQP2). In biotin-conjugated nicotinamide adenine dinucleotide (biotin-NAD+) pulldown and immunoprecipitation assays of poly(ADP)-ribose in mpkCCDc14 cells, immunoblots demonstrated that 1-deamino-8-D-arginine vasopressin (dDAVP) induced the PARylation of total proteins, associated with an increase in the cleavage of PARP-1 and cleaved caspase-3 expression. By inhibiting PARP-1 with siRNA, the abundance of dDAVP-induced AQP2 mRNA and protein was significantly diminished. In contrast, despite a substantial decrease in PARylation, the PARP-1 inhibitor (PJ34) had no effect on the dDAVP-induced regulation of AQP2 expression. The findings suggest that PARP-1 protein expression itself, and not PARP-1-mediated PARylation, is necessary for dDAVP-regulated AQP2 expression. Bioinformatic analysis revealed that 408 proteins interact with PARP-1 in the collecting duct (CD) cells of the kidney. Among them, the signaling pathway of the vasopressin V2 receptor was identified for 49 proteins. In particular, β-catenin, which is phosphorylated at Ser552 by dDAVP, was identified as the PARP-1-interacting protein. A significant decrease of β-catenin phosphorylation (Ser552) in response to dDAVP was associated with siRNA-mediated PARP-1 knockdown. Taken together, PARP-1 is likely to play a role in vasopressin-induced AQP2 expression by interacting with β-catenin in renal CD cells.NEW & NOTEWORTHY The poly(ADP-ribose) polymerase (PARP) family catalyzes poly(ADP-ribosylation) (PARylation), which is one of the posttranslational modifications of largely undetermined physiological significance. This study investigated the role of PARP-1, the most prevalent member of the PARP family, in the vasopressin-responsive expression of aquaporin-2 (AQP2). The results demonstrated that PARP-1 protein expression itself, and not PARP-1-mediated PARylation, is necessary for dDAVP-regulated AQP2 expression. β-Catenin, which is phosphorylated at Ser552 by dDAVP, was identified as the PARP-1-interacting protein.

AQP2 Promotes Astrocyte Activation by Modulating the TLR4/NFκB-p65 Pathway Following Intracerebral Hemorrhage

Microglial and astrocyte activation and related cytokine secretion play key roles in secondary brain injury following intracerebral hemorrhage (ICH). We assessed the role of aquaporin (AQP)2 in immune response after ICH. We prospectively collected data from 33 patients with ICH and analyzed the serum AQP2 levels in these patients and age-matched healthy controls. A correlation analysis was also performed between patient serum AQP2 levels and clinical factors. In the rat ICH model, double-fluorescence staining for glial fibrillary acidic protein (GFAP) and AQP2 was performed to investigate the relationship between astrocytes and AQP2. Relative mRNA expression levels of GFAP and AQP2 were also measured. In the rat astrocyte cell line CTX-TNA2, toll-like receptor (TLR)4/nuclear factor kappa B (NFκB)-p65 pathway activation and GFAP levels were measured. The indirect influence of AQP2 on microglial polarization was assessed following exposure to the medium of astrocytes treated with AQP2-overexpression plasmid or silencing RNA. We found that the serum AQP2 expression was lower in patients with ICH. Sex and blood neutrophil count influenced serum AQP2 concentrations in patients with ICH on admission. Lower serum AQP2 levels were inversely correlated with 90-day Modified Rankin Scale scores after ICH, but were not correlated with National Institute of Health stroke scale (NIHSS) scores on admission. AQP2 overexpression and localization in GFAP-labeled astrocytes were observed in rats. AQP2 overexpression induced astrocyte activation with GFAP upregulation via TLR/NFκB-p65 signaling pathway activation in the rat astrocyte cell line CTX-TNA2. Astrocyte activation promoted interleukin-1β secretion. The medium of AQP2-overexpression astrocytes promoted the pro-inflammatory M1 phenotype in the immortal rat (HAPI) microglial cell line. Therefore, serum AQP2 is negatively correlated with post-ICH prognosis and may be a marker of inflammation in early-stage ICH. AQP2 overexpression promotes astrocyte activation and pro-inflammatory secretion, affects astrocyte-microglia crosstalk, and indirectly induces microglial polarization, which may augment inflammation after ICH.

"ADPKD-omics": determinants of cyclic AMP levels in renal epithelial cells

The regulation of cyclic adenosine monophosphate (cAMP) levels in kidney epithelial cells is important in at least 2 groups of disorders, namely water balance disorders and autosomal dominant polycystic kidney disease. Focusing on the latter, we review genes that code for proteins that are determinants of cAMP levels in cells. We identify which of these determinants are expressed in the 14 kidney tubule segments using recently published RNA-sequencing and protein mass spectrometry data ("autosomal dominant polycystic kidney disease-omics"). This includes G protein-coupled receptors, adenylyl cyclases, cyclic nucleotide phosphodiesterases, cAMP transporters, cAMP-binding proteins, regulator of G protein-signaling proteins, G protein-coupled receptor kinases, arrestins, calcium transporters, and calcium-binding proteins. In addition, compartmentalized cAMP signaling in the primary cilium is discussed, and a specialized database of the proteome of the primary cilium of cultured "IMCD3" cells is provided as an online resource (https://esbl.nhlbi.nih.gov/Databases/CiliumProteome/). Overall, this article provides a general resource in the form of a curated list of proteins likely to play roles in determination of cAMP levels in kidney epithelial cells and, therefore, likely to be determinants of progression of autosomal dominant polycystic kidney disease.

NOX Inhibitors: From Bench to Naxibs to Bedside

Reactive oxygen species (ROS) are ubiquitous metabolic products and important cellular signaling molecules that contribute to several biological functions. Pathophysiology arises when ROS are generated either in excess or in cell types or subcellular locations that normally do not produce ROS or when non-physiological types of ROS (e.g., superoxide instead of hydrogen peroxide) are formed. In the latter scenario, antioxidants were considered as the apparent remedy but, clinically, have consistently failed and even sometimes induced harm. The obvious reason for that is the non-selective ROS scavenging effects of antioxidants which interfere with both qualities of ROS, physiological and pathological. Therefore, it is essential to overcome this "antidote or neutralizer" strategy. We here review the most promising alternative approach by identifying the disease-relevant enzymatic sources of ROS, target these selectively, but leave physiological ROS signaling through other sources intact. Among all ROS sources, NADPH oxidases (NOX1-5 and DUOX1-2) stand out as their sole function is to produce ROS, whereas most other enzymatic sources only produce ROS as a by-product or upon biochemical uncoupling or damage. This qualifies NOXs as the main potential drug-target candidates in diseases associated with dysfunction in ROS signaling. As a reflection of this, the development of several NOX inhibitors has taken place. Recently, the WHO approved a new stem, "naxib," which refers to NADPH oxidase inhibitors, and thereby recognized NOX inhibitors as a new therapeutic class. This has been announced while clinical trials with the first-in-class compound, setanaxib (initially known as GKT137831) had been initiated. We also review the differences between the seven NOX family members in terms of structure and function in health and disease and then focus on the most advanced NOX inhibitors with an exclusive focus on clinically relevant validations and applications. Therapeutically relevant NADPH oxidase isoforms type 1, 2, 4, and 5 (NOX1, NOX2, NOX4, NOX5). Of note, NOX5 is not present in mice and rats and thus pre-clinically less studied. NOX2, formerly termed gp91^phox, has been correlated with many, too many, diseases and is rather relevant as genetic deficiency in chronic granulomatous disease (CGD), treated by gene therapy. Overproduction of ROS through NOX1, NOX4, and NOX5 leads to the indicated diseases states including atherosclerosis (red), a condition where NOX4 is surprisingly protective.

NOX4-dependent regulation of ENaC in hypertension and diabetic kidney disease

NADPH oxidase 4 (NOX4) is the most abundant NOX isoform in the kidney; however, its importance for renal function has only recently emerged. The NOX4-dependent pathway regulates many factors essential for proper sodium handling in the distal nephron. However, the functional significance of this pathway in the control of sodium reabsorption during the initiation of chronic kidney disease is not established. The goal of this study was to test Nox4-dependent ENaC regulation in two models: SS hypertension and STZ-induced type 1 diabetes. First, we showed that genetic ablation of Nox4 in Dahl salt-sensitive (SS) rat attenuated a high-salt (HS)-induced increase in epithelial Na⁺ channel (ENaC) activity in the cortical collecting duct. We also found that H₂ O₂ upregulated ENaC activity, and H₂ O₂ production was reduced in both the renal cortex and medulla in SS^Nox4-/- rats fed an HS diet. Second, in the streptozotocin model of hyperglycemia-induced renal injury ENaC activity in hyperglycemic animals was elevated in SS but not SS^Nox4-/- rats. NaCl cotransporter (NCC) expression was increased compared to healthy controls, while expression values between SS and SS^Nox4-/- groups were similar. These data emphasize a critical contribution of the NOX4-mediated pathway in maladaptive upregulation of ENaC-mediated sodium reabsorption in the distal nephron in the conditions of HS- and hyperglycemia-induced kidney injury.

Potential benefits and harms of NADPH oxidase type 4 in the kidneys and cardiovascular system

The main function of NADPH oxidases is to catalyse the formation of reactive oxygen species (ROS). NADPH oxidase 4 (NOX4) is expressed at high levels in kidney tubular cells, and at lower levels in endothelial cells, cardiomyocytes and other cell types under physiological conditions. NOX4 is constitutively active producing hydrogen peroxide (H2O2) as the prevalent ROS detected, whereas other NOX isoforms present in the renal and cardiovascular systems (i.e. NOX1, NOX2 and NOX5) generate superoxide radical anions as main products. Pharmacological inhibition of NOX4 has received enormous attention for its potential therapeutic benefit in fibrotic disease and nephropathologies. Ongoing clinical trials are testing this approach in humans. Diabetes elevates NOX4 expression in podocytes and mesangial cells, which was shown to damage glomeruli leading to podocyte loss, mesangial cell hypertrophy and matrix accumulation. Consequently, NOX4 represents an interesting therapeutic target in diabetic nephropathy. On the contrary, experiments using NOX4-deficient mice have shown that NOX4 is cytoprotective in tubular cells, cardiomyocytes, endothelial cells and vascular smooth muscle cells, and has a metabolism-regulating role when these cells are subjected to injury. Mice with systemic NOX4 deletion are more susceptible to acute and chronic tubular injury, heart failure and atherosclerosis. Overall, the current literature suggests a detrimental role of increased NOX4 expression in mesangial cells and podocytes during diabetic nephropathy, but a cytoprotective role of this enzyme in other cellular types where it is expressed endogenously. We review here the recent evidence on the role of NOX4 in the kidneys and cardiovascular system. With the emergence of pharmacological NOX4 inhibitors in clinical trials, caution should be taken in identifying potential side effects in patients prone to acute kidney injury and cardiovascular disease.

Aquaporin Membrane Channels in Oxidative Stress, Cell Signaling, and Aging: Recent Advances and Research Trends

Reactive oxygen species (ROS) are produced as a result of aerobic metabolism and as by-products through numerous physiological and biochemical processes. While ROS-dependent modifications are fundamental in transducing intracellular signals controlling pleiotropic functions, imbalanced ROS can cause oxidative damage, eventually leading to many chronic diseases. Moreover, increased ROS and reduced nitric oxide (NO) bioavailability are main key factors in dysfunctions underlying aging, frailty, hypertension, and atherosclerosis. Extensive investigation aims to elucidate the beneficial effects of ROS and NO, providing novel insights into the current medical treatment of oxidative stress-related diseases of high epidemiological impact. This review focuses on emerging topics encompassing the functional involvement of aquaporin channel proteins (AQPs) and membrane transport systems, also allowing permeation of NO and hydrogen peroxide, a major ROS, in oxidative stress physiology and pathophysiology. The most recent advances regarding the modulation exerted by food phytocompounds with antioxidant action on AQPs are also reviewed.

Roflumilast and aquaporin-2 regulation in rat renal inner medullary collecting duct

Roflumilast is a cyclic nucleotide phosphodiesterase inhibitor that is FDA‐approved for treatment of chronic obstructive pulmonary disease. With a view toward possible use for treatment of patients with X‐linked nephrogenic diabetes insipidus (NDI) due to hemizygous mutations in the V2 vasopressin receptor, this study sought to determine the effect of roflumilast on aquaporin‐2 (AQP2) phosphorylation, AQP2 trafficking, and water permeability in the rat inner medullary collecting duct (IMCD). In the presence of the vasopressin analog dDAVP (0.1 nmol/L), both roflumilast and its active metabolite roflumilast N‐oxide (RNO) significantly increased phosphorylation at S256, S264, and S269, and decreased phosphorylation at S261 (immunoblotting) in IMCD suspensions in a dose‐dependent manner (3–3000 nmol/L). Another commonly used phosphodiesterase inhibitor, IBMX, affected phosphorylation only at the highest concentration in this range. However, neither roflumilast nor RNO had an effect on AQP2 phosphorylation in the absence of vasopressin. Furthermore, roflumilast alone did not increase AQP2 trafficking to the plasma membrane (immunofluorescence) or increase water permeability in freshly microdissected perfused IMCD segments. We conclude that roflumilast can be used to enhance vasopressin's action on AQP2 activity in the renal collecting duct, but has no detectable effect in the absence of vasopressin. These findings suggest that roflumilast may not have a beneficial effect in X‐linked NDI, but could find useful application in acquired NDI.

Angiotensin-(1-7) in Paraventricular Nucleus Contributes to the Enhanced Cardiac Sympathetic Afferent Reflex and Sympathetic Activity in Chronic Heart Failure Rats

Background/Aims

Cardiac sympathetic afferent reflex (CSAR) enhancement contributes to exaggerated sympathetic activation in chronic heart failure (CHF). The current study aimed to investigate the roles of angiotensin (Ang)-(1-7) in CSAR modulation and sympathetic activation and Ang-(1-7) signaling pathway in paraventricular nucleus of CHF rats.

Methods

CHF was induced by coronary artery ligation. Responses of renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) to epicardial application of capsaicin were used to evaluate CSAR in rats with anesthesia.

Results

Ang-(1-7) increased RSNA, MAP, CSAR activity, cAMP level, NAD(P)H oxidase activity and superoxide anion level more significantly in CHF than in sham-operated rats, while Mas receptor antagonist A-779 had the opposite effects. Moreover, Ang-(1-7) augmented effects of Ang II in CHF rats. The effects of Ang-(1-7) were blocked by A-779, adenylyl cyclase inhibitor SQ22536, protein kinase A inhibitor Rp-cAMP, superoxide anion scavenger tempol and NAD(P)H oxidase inhibitor apocynin. Mas and AT₁ receptor protein expressions, Ang-(1-7) and Ang II levels in CHF increased.

Conclusions

These results indicate that Ang-(1-7) in paraventricular nucleus enhances CSAR and sympathetic output not only by exerting its own effects but also by augmenting the effects of Ang II through Mas receptor in CHF. Endogenous Ang-(1-7)/Mas receptor activity contributes to CSAR enhancement and sympathetic activation in CHF, and NAD(P)H oxidase-derived superoxide anions and the cAMP-PKA signaling pathway are involved in mediating the effects of Ang-(1-7) in CHF.

Role for reactive oxygen species in flow-stimulated inner medullary collecting duct endothelin-1 production

Inner medullary collecting duct (IMCD)-derived endothelin-1 (ET-1) is stimulated by volume expansion, in part through augmented luminal flow, whereupon it can elicit natriuresis and diuresis. Since flow can alter nitric oxide (NO) and reactive oxygen species (ROS), both of which can affect collecting duct salt transport, we asked whether NO and/or ROS mediate flow-stimulated IMCD ET-1. Mouse IMCD3 cells were exposed to flow, and ET-1/GAPDH mRNA was assessed. A shear stress of 10 dyn/cm² for 1 h increased ET-1 mRNA by fourfold compared with no flow (ET-1 flow response). Global NO synthase (NOS) inhibition [N^G-nitro-l-arginine methyl ester (l-NAME)] reduced the ET-1 flow response; however, pharmacological inhibition of NOS1 or NOS2, inhibition of NOS3 siRNA, inhibition of arginase inhibition, removal of media l-Arg, or inhibition of NO-dependent signaling pathways (PKG, guanylyl cyclase, or NF-κB) did not affect the ET-1 flow response. Tempol reduced the ET-1 flow response; no further inhibition occurred with l-NAME. Superoxide dismutase, but not catalase, reduced the ET-1 flow response. Inhibition of NAPDH oxidase (NOX) (apocynin), pharmacological inhibition of NOX1/4, or NOX4 siRNA reduced the ET-1 flow response. Finally, flow increased IMCD3 ROS production and this was inhibited by apocynin, NOX1/4 inhibition, and, to a small extent, by l-NAME. Taken together, these data suggest that NOX4-derived ROS in general, and possibly superoxide in particular, are involved in flow-stimulated IMCD ET-1 production. To our knowledge, this is the first report of flow-stimulated ROS production by the CD, as well as the first report of such flow-stimulated CD ROS exerting a biological effect.

Effects of Reactive Oxygen Species on Tubular Transport along the Nephron

Reactive oxygen species (ROS) are oxygen-containing molecules naturally occurring in both inorganic and biological chemical systems. Due to their high reactivity and potentially damaging effects to biomolecules, cells express a battery of enzymes to rapidly metabolize them to innocuous intermediaries. Initially, ROS were considered by biologists as dangerous byproducts of respiration capable of causing oxidative stress, a condition in which overproduction of ROS leads to a reduction in protective molecules and enzymes and consequent damage to lipids, proteins, and DNA. In fact, ROS are used by immune systems to kill virus and bacteria, causing inflammation and local tissue damage. Today, we know that the functions of ROS are not so limited, and that they also act as signaling molecules mediating processes as diverse as gene expression, mechanosensation, and epithelial transport. In the kidney, ROS such as nitric oxide (NO), superoxide (O₂^-), and their derivative molecules hydrogen peroxide (H₂O₂) and peroxynitrite (ONO₂^-) regulate solute and water reabsorption, which is vital to maintain electrolyte homeostasis and extracellular fluid volume. This article reviews the effects of NO, O₂^-, ONO₂^-, and H₂O₂ on water and electrolyte reabsorption in proximal tubules, thick ascending limbs, and collecting ducts, and the effects of NO and O₂^- in the macula densa on tubuloglomerular feedback.

Two Phase Modulation of [Formula: see text] Entry and Cl-/[Formula: see text] Exchanger in Submandibular Glands Cells by Dexmedetomidine

Dexmedetomidine (Dex), a highly selective α2-adrenoceptor agonist, attenuates inflammatory responses induced by lipopolysaccharide (LPS) and induces sedative and analgesic effects. Administration of Dex also reduces salivary secretion in human subjects and inhibits osmotic water permeability in rat cortical collecting ducts. However, little is known about the mechanisms underlying the effects of Dex on salivary glands fluid secretion. We demonstrated the α2-adrenoceptor expression in the basolateral membrane of mouse submandibular glands (SMG). To investigate fluid secretion upon treatment with Dex, we studied the effects of Dex on the activity of Na+-K+-2Cl- cotransporter1 (NKCC1) and Cl-/[Formula: see text] exchange (CBE), and on downstream pro-inflammatory cytokine expression in isolated primary mouse SMG cells. Dex acutely increased CBE activity and NKCC1-mediated and independent [Formula: see text] entry in SMG duct cells, and enhanced ductal fluid secretion in a sealed duct system. Dex showed differential effects on cholinergic/adrenergic stimulations and inflammatory mediators, histamine, and LPS, stimulations-induced Ca2+ in mouse SMG cells. Both, histamine- and LPS-induced intracellular Ca2+ increases were inhibited by Dex, whereas carbachol-stimulated Ca2+ signals were not. Long-lasting (2 h) treatment with Dex reduced CBE activity in SMG and in human submandibular glands (HSG) cells. Moreover, when isolated SMG cells were stimulated with Dex for 2 h, phosphodiesterase 4D (PDE4D) expression was enhanced. These results confirm the anti-inflammatory properties of Dex on LPS-mediated signaling. Further, Dex also inhibited mRNA expression of interleukin-6 and NADPH oxidase 4. The present study also showed that α2-adrenoceptor activation by Dex reduces salivary glands fluid secretion by increasing PDE4D expression, and subsequently reducing the concentration of cAMP. These findings reveal an interaction between the α2-adrenoceptor and PDE4D, which should be considered when using α2-adrenoceptor agonists as sedative or analgesics.

Renal Carcinogenesis, Tumor Heterogeneity, and Reactive Oxygen Species: Tactics Evolved

SIGNIFICANCE

The number of kidney cancers is growing 3-5% each year due to unknown etiologies. Intra- and inter-tumor mediators increase oxidative stress and drive tumor heterogeneity. Recent Advances: Technology advancement in state-of-the-art instrumentation and methodologies allows researchers to detect and characterize global landscaping modifications in genes, proteins, and pathophysiology patterns at the single-cell level.

CRITICAL ISSUES

We postulate that the sources of reactive oxygen species (ROS) and their activation within subcellular compartments will change over a timeline of tumor evolvement and contribute to tumor heterogeneity. Therefore, the complexity of intracellular changes within a tumor and ROS-induced tumor heterogeneity coupled to the advancement of detecting these events globally are limited at the level of data collection, organization, and interpretation using software algorithms and bioinformatics.

FUTURE DIRECTIONS

Integrative and collaborative research, combining the power of numbers with careful experimental design, protocol development, and data interpretation, will translate cancer biology and therapeutics to a heightened level or leave the abundant raw data as stagnant and underutilized. Antioxid. Redox Signal. 25, 685-701.

Diabetes and Kidney Disease: Role of Oxidative Stress

Intrarenal oxidative stress plays a critical role in the initiation and progression of diabetic kidney disease (DKD). Enhanced oxidative stress results from overproduction of reactive oxygen species (ROS) in the context of concomitant, insufficient antioxidant pathways. Renal ROS production in diabetes is predominantly mediated by various NADPH oxidases (NOXs), but a defective antioxidant system as well as mitochondrial dysfunction may also contribute. Recent Advances: Effective agents targeting the source of ROS generation hold the promise to rescue the kidney from oxidative damage and prevent subsequent progression of DKD. Critical Issues and Future Directions: In the present review, we summarize and critically analyze molecular and cellular mechanisms that have been demonstrated to be involved in NOX-induced renal injury in diabetes, with particular focus on the role of increased glomerular injury, the development of albuminuria, and tubulointerstitial fibrosis, as well as mitochondrial dysfunction. Furthermore, novel agents targeting NOX isoforms are discussed. Antioxid. Redox Signal. 25, 657-684.

Evaluating the Oxidative Stress in Renal Diseases: What Is the Role for S-Glutathionylation?

SIGNIFICANCE

Reactive oxygen species (ROS) have long been considered as toxic derivatives of aerobic metabolism displaying a harmful effect to living cells. Deregulation of redox homeostasis and production of excessive free radicals may contribute to the pathogenesis of kidney diseases. In line, oxidative stress increases in patients with renal dysfunctions due to a general increase of ROS paralleled by impaired antioxidant ability.

RECENT ADVANCES

Emerging evidence revealed that physiologically, ROS can act as signaling molecules interplaying with several transduction pathways such as proliferation, differentiation, and apoptosis. ROS can exert signaling functions by modulating, at different layers, protein oxidation since proteins have "cysteine switches" that can be reversibly reduced or oxidized, supporting the dynamic signaling regulation function. In this scenario, S-glutathionylation is a posttranslational modification involved in oxidative cellular response.

CRITICAL ISSUES

Although it is widely accepted that renal dysfunctions are often associated with altered redox signaling, the relative role of S-glutathionylation on the pathogenesis of specific renal diseases remains unclear and needs further investigations. In this review, we discuss the impact of ROS in renal health and diseases and the role of selective S-glutathionylation proteins potentially relevant to renal physiology.

FUTURE DIRECTIONS

The paucity of studies linking the reversible protein glutathionylation with specific renal disorders remains unmet. The growing number of S-glutathionylated proteins indicates that this is a fascinating area of research. In this respect, further studies on the association of reversible glutathionylation with renal diseases, characterized by oxidative stress, may be useful to develop new pharmacological molecules targeting protein S-glutathionylation. Antioxid. Redox Signal. 25, 147-164.

Different effects of ZO-1, ZO-2 and ZO-3 silencing on kidney collecting duct principal cell proliferation and adhesion

Coordinated cell proliferation and ability to form intercellular seals are essential features of epithelial tissue function. Tight junctions (TJs) classically act as paracellular diffusion barriers. More recently, their role in regulating epithelial cell proliferation in conjunction with scaffolding zonula occludens (ZO) proteins has come to light. The kidney collecting duct (CD) is a model of tight epithelium that displays intense proliferation during embryogenesis followed by very low cell turnover in the adult kidney. Here, we examined the influence of each ZO protein (ZO-1, -2 and -3) on CD cell proliferation. We show that all 3 ZO proteins are strongly expressed in native CD and are present at both intercellular junctions and nuclei of cultured CD principal cells (mCCDcl1). Suppression of either ZO-1 or ZO-2 resulted in increased G0/G1 retention in mCCDcl1 cells. ZO-2 suppression decreased cyclin D1 abundance while ZO-1 suppression was accompanied by increased nuclear p21 localization, the depletion of which restored cell cycle progression. Contrary to ZO-1 and ZO-2, ZO-3 expression at intercellular junctions dramatically increased with cell density and relied on the presence of ZO-1. ZO-3 depletion did not affect cell cycle progression but increased cell detachment. This latter event partly relied on increased nuclear cyclin D1 abundance and was associated with altered β1-integrin subcellular distribution and decreased occludin expression at intercellular junctions. These data reveal diverging, but interconnected, roles for each ZO protein in mCCDcl1 proliferation. While ZO-1 and ZO-2 participate in cell cycle progression, ZO-3 is an important component of cell adhesion.

Nox and renal disease

Since the first demonstration of Nox enzyme expression in the kidney in the early 1990s and the subsequent identification of Nox4, or RENOX, a decade later, it has become apparent that the Nox family of reactive oxygen species (ROS) generating enzymes plays an integral role in the normal physiological function of the kidney. As our knowledge of Nox expression patterns and functions in various structures and specialized cell types within the kidney grows, so does the realization that Nox-derived oxidative stress contributes significantly to a wide variety of renal pathologies through their ability to modify lipids and proteins, damage DNA and activate transcriptional programmes. Diverse studies demonstrate key roles for Nox-derived ROS in kidney fibrosis, particularly in settings of chronic renal disease such as diabetic nephropathy. As the most abundant Nox family member in the kidney, much emphasis has been placed on the role of Nox4 in this setting. However, an ever growing body of work continues to uncover key roles for other Nox family members, not only in diabetic kidney disease, but in a diverse array of renal pathological conditions. The objective of the present review is to highlight the latest novel developments in renal Nox biology with an emphasis not only on diabetic nephropathy but many of the other renal disease contexts where oxidative stress is implicated.

Epithelial sodium channel abundance is decreased by an unfolded protein response induced by hyperosmolality

Large shifts of osmolality occur in the kidney medulla as part of the urine concentrating mechanism. Hyperosmotic stress profoundly challenges cellular homeostasis and induces endoplasmic reticulum (ER) stress. Here, we examined the unfolded protein response (UPR) in hyperosmotically-challenged principal cells of the kidney collecting duct (CD) and show its relevance in controlling epithelial sodium channel (ENaC) abundance, responsible for the final adjustment of Na(+) excretion. Dehydration increases medullary but not cortical osmolality. Q-PCR analysis of microdissected CD of water-deprived mice revealed increased aquaporin-2 (AQP2) expression in outer medullary and cortical CD while ENaC abundance decreased in outer medullary but not cortical CD. Immunoblotting, Q-PCR and immunofluorescence revealed that hyperosmolality induced a transient ER stress-like response both ex vivo and in cultured CD principal cells and increased activity of the canonical UPR mediators PERK and ATF6. Both hyperosmolality and chemical induction of ER stress decreased ENaC expression in vitro. ENaC depletion by either stimulus was abolished by transcriptional inhibition and by the chemical chaperone 4-phenylbutyric acid and was partly abrogated by either PERK or ATF6 silencing. Our data suggest that induction of the UPR by hyperosmolality may help preserve body fluid homeostasis under conditions of dehydration by uncoupling AQP2 and ENaC abundance in outer medullary CD.

Glutathionylation of the aquaporin-2 water channel: a novel post-translational modification modulated by the oxidative stress

Aquaporin-2 (AQP2) is the vasopressin-regulated water channel that controls renal water reabsorption and urine concentration. AQP2 undergoes different regulated post-translational modifications, including phosphorylation and ubiquitylation, which are fundamental for controlling AQP2 cellular localization, stability, and function. The relationship between AQP2 and S-glutathionylation is of potential interest because reactive oxygen species (ROS), produced under renal failure or nephrotoxic drugs, may influence renal function as well as the expression and the activity of different transporters and channels, including aquaporins. Here, we show for the first time that AQP2 is subjected to S-glutathionylation in kidney and in HEK-293 cells stably expressing AQP2. S-Glutathionylation is a redox-dependent post-translational modification controlling several signal transduction pathways and displaying an acute effect on free cytosolic calcium concentration. Interestingly, we found that in fresh kidney slices, the increased AQP2 S-glutathionylation correlated with tert-butyl hydroperoxide-induced ROS generation. Moreover, we also found that cells expressing wild-type human calcium-sensing receptor (hCaSR-wt) and its gain of function (hCaSR-R990G; hCaSR-N124K) had a significant decrease in AQP2 S-glutathionylation secondary to reduced ROS levels and reduced basal intracellular calcium concentration compared with mock cells. Together, these new findings provide fundamental insight into cell biological aspects of AQP2 function and may be relevant to better understand and explain pathological states characterized by an oxidative stress and AQP2-dependent water reabsorption disturbs.

Renal response to L-arginine in diabetic rats. A possible link between nitric oxide system and aquaporin-2

The aim of this study was to evaluate whether L-Arginine (L-Arg) supplementation modifies nitric oxide (NO) system and consequently aquaporin-2 (AQP2) expression in the renal outer medulla of streptozotocin-diabetic rats at an early time point after induction of diabetes. Male Wistar rats were divided in four groups: Control, Diabetic, Diabetic treated with L-Arginine and Control treated with L-Arginine. Nitric oxide synthase (NOS) activity was estimated by [¹⁴C] L-citrulline production in homogenates of the renal outer medulla and by NADPH-diaphorase staining in renal outer medullary tubules. Western blot was used to detect the expression of AQP2 and NOS types I and III; real time PCR was used to quantify AQP2 mRNA. The expression of both NOS isoforms, NOS I and NOS III, was decreased in the renal outer medulla of diabetic rats and L-Arg failed to prevent these decreases. However, L-Arg improved NO production, NADPH-diaphorase activity in collecting ducts and other tubular structures, and NOS activity in renal homogenates from diabetic rats. AQP2 protein and mRNA were decreased in the renal outer medulla of diabetic rats and L-Arg administration prevented these decreases. These results suggest that the decreased NOS activity in collecting ducts of the renal outer medulla may cause, at least in part, the decreased expression of AQP2 in this model of diabetes and constitute additional evidence supporting a role for NO in contributing to renal water reabsorption through the modulation of AQP2 expression in this pathological condition. However, we cannot discard that another pathway different from NOS also exists that links L-Arg to AQP2 expression.