Short-term stimulation of the thiazide-sensitive Na+-Cl- cotransporter by vasopressin involves phosphorylation and membrane translocation

Navigating the multifaceted intricacies of the Na+-Cl- cotransporter, a highly regulated key effector in the control of hydromineral homeostasis

The Na+-Cl- cotransporter (NCC; SLC12A3) is a highly regulated integral membrane protein that is known to exist as three splice variants in primates. Its primary role in the kidney is to mediate the cosymport of Na+ and Cl- across the apical membrane of the distal convoluted tubule. Through this role and the involvement of other ion transport systems, NCC allows the systemic circulation to reclaim a fraction of the ultrafiltered Na+, K+, Cl-, and Mg+ loads in exchange for Ca2+ and [Formula: see text]. The physiological relevance of the Na+-Cl- cotransport mechanism in humans is illustrated by several abnormalities that result from NCC inactivation through the administration of thiazides or in the setting of hereditary disorders. The purpose of the present review is to discuss the molecular mechanisms and overall roles of Na+-Cl- cotransport as the main topics of interest. On reading the narrative proposed, one will realize that the knowledge gained in regard to these themes will continue to progress unrelentingly no matter how refined it has now become.

Arginine vasopressin regulates the renal Na+-Cl- and Na+-K+-Cl- cotransporters through with-no-lysine kinase 4 and inhibitor 1 phosphorylation

Vasopressin regulates water homeostasis via the V2 receptor in the kidney at least in part through protein kinase A (PKA) activation. Vasopressin, through an unknown pathway, upregulates the activity and phosphorylation of Na+-Cl- cotransporter (NCC) and Na+-K+-2Cl- cotransporter 2 (NKCC2) by Ste20-related proline/alanine-rich kinase (SPAK) and oxidative stress-responsive kinase 1 (OSR1), which are regulated by the with-no-lysine kinase (WNK) family. Phosphorylation of WNK4 at PKA consensus motifs may be involved. Inhibitor 1 (I1), a protein phosphatase 1 (PP1) inhibitor, may also play a role. In human embryonic kidney (HEK)-293 cells, we assessed the phosphorylation of WNK4, SPAK, NCC, or NKCC2 in response to forskolin or desmopressin. WNK4 and cotransporter phosphorylation were studied in desmopressin-infused WNK4-/- mice and in tubule suspensions. In HEK-293 cells, only wild-type WNK4 but not WNK1, WNK3, or a WNK4 mutant lacking PKA phosphorylation motifs could upregulate SPAK or cotransporter phosphorylation in response to forskolin or desmopressin. I1 transfection maximized SPAK phosphorylation in response to forskolin in the presence of WNK4 but not of mutant WNK4 lacking PP1 regulation. We observed direct PP1 regulation of NKCC2 dephosphorylation but not of NCC or SPAK in the absence of WNK4. WNK4-/- mice with desmopressin treatment did not increase SPAK/OSR1, NCC, or NKCC2 phosphorylation. In stimulated tubule suspensions from WNK4-/- mice, upregulation of pNKCC2 was reduced, whereas upregulation of SPAK phosphorylation was absent. These findings suggest that WNK4 is a central node in which kinase and phosphatase signaling converge to connect cAMP signaling to the SPAK/OSR1-NCC/NKCC2 pathway.NEW & NOTEWORTHY With-no-lysine kinases regulate the phosphorylation and activity of the Na+-Cl- and Na+-K+-2Cl- cotransporters. This pathway is modulated by arginine vasopressin (AVP). However, the link between AVP and WNK signaling remains unknown. Here, we show that AVP activates WNK4 through increased phosphorylation at putative protein kinase A-regulated sites and decreases its dephosphorylation by protein phosphatase 1. This work increases our understanding of the signaling pathways mediating AVP actions in the kidney.

Thirty years of the NaCl cotransporter: from cloning to physiology and structure

The primary structure of the thiazide-sensitive NaCl cotransporter (NCC) was resolved 30 years ago by the molecular identification of the cDNA encoding this cotransporter, from the winter's flounder urinary bladder, following a functional expression strategy. This review outlines some aspects of how the knowledge about thiazide diuretics and NCC evolved, the history of the cloning process, and the expansion of the SLC12 family of electroneutral cotransporters. The diseases associated with activation or inactivation of NCC are discussed, as well as the molecular model by which the activity of NCC is regulated. The controversies in the field are discussed as well as recent publication of the three-dimensional model of NCC obtained by cryo-electron microscopy, revealing not only the amino acid residues critical for Na+ and Cl- translocation but also the residues critical for polythiazide binding to the transporter, opening the possibility for a new era in thiazide diuretic therapy.

Alterations in Kidney Structures Caused by Age Vary According to Sex and Dehydration Condition

Aging is a complex biological process, resulting in gradual and progressive decline in structure and function in many organ systems. Our objective is to determine if structural changes produced by aging vary with sex in a stressful situation such as dehydration. The expression of Slc12a3 mRNA in the renal cortex, α-smooth muscle actin (α-SMA), and fibronectin was evaluated in male and female rats, aged 3 and 18 months, submitted and not submitted to water deprivation (WD) for 48 h, respectively. When comparing ages, 18-month-old males showed a lower expression of Slc12a3 mRNA than 3-month-old males, and control and WD 18-month-old male and female rats exhibited a higher expression of α-SMA than the respective 3-month-old rats. Fibronectin was higher in both control and WD 18-month-old males than the respective 3-month-old males. In females, only the control 18-month-old rats showed higher fibronectin than the control 3-month-old rats. When we compared sex, control and WD 3-month-old female rats had a lower expression of Slc12a3 mRNA than the respective males. The WD 18-month-old male rats presented a higher expression of fibronectin and α-SMA than the WD 18-month-old female rats. When we compared hydric conditions, the WD 18-month-old males displayed a lower relative expression of Slc12a3 mRNA and higher α-SMA expression than the control 18-month-old males. Aging, sex, and dehydration lead to alterations in kidney structure.

Activation of the Thiazide-Sensitive Sodium-Chloride Cotransporter by Beta3-Adrenoreceptor in the Distal Convoluted Tubule

We previously showed that the beta-3 adrenergic receptor (BAR3) is expressed in most segments of the nephron where its agonism promotes a potent antidiuretic effect. We localized BAR3 in distal convoluted tubule (DCT) cells expressing the thiazide-sensitive sodium-chloride cotransporter (NCC). Aim of this study is to investigate the possible functional role of BAR3 on NCC modulation in DCT cells. Here, we found that, in mice, the knockout of BAR3 was paralleled by a significant attenuation of NCC phosphorylation, paralleled by reduced expression and activation of STE-20/SPS1-related proline-alanine-rich kinase (SPAK) and WNKs the main kinases involved in NCC activation. Conversely, in BAR1/2 knockout mice, we found reduced NCC abundance with no changes in the phosphorylation state of NCC. Moreover, selective BAR3 agonism promotes both SPAK and NCC activation in wild-type mouse kidney slices. In conclusion, our findings suggest a novel role for BAR3 in the regulation of NCC in DCT.

Local cyclic adenosine monophosphate signalling cascades-Roles and targets in chronic kidney disease

The molecular mechanisms underlying chronic kidney disease (CKD) are poorly understood and treatment options are limited, a situation underpinning the need for elucidating the causative molecular mechanisms and for identifying innovative treatment options. It is emerging that cyclic 3',5'-adenosine monophosphate (cAMP) signalling occurs in defined cellular compartments within nanometre dimensions in processes whose dysregulation is associated with CKD. cAMP compartmentalization is tightly controlled by a specific set of proteins, including A-kinase anchoring proteins (AKAPs) and phosphodiesterases (PDEs). AKAPs such as AKAP18, AKAP220, AKAP-Lbc and STUB1, and PDE4 coordinate arginine-vasopressin (AVP)-induced water reabsorption by collecting duct principal cells. However, hyperactivation of the AVP system is associated with kidney damage and CKD. Podocyte injury involves aberrant AKAP signalling. cAMP signalling in immune cells can be local and slow the progression of inflammatory processes typical for CKD. A major risk factor of CKD is hypertension. cAMP directs the release of the blood pressure regulator, renin, from juxtaglomerular cells, and plays a role in Na⁺ reabsorption through ENaC, NKCC2 and NCC in the kidney. Mutations in the cAMP hydrolysing PDE3A that cause lowering of cAMP lead to hypertension. Another major risk factor of CKD is diabetes mellitus. AKAP18 and AKAP150 and several PDEs are involved in insulin release. Despite the increasing amount of data, an understanding of functions of compartmentalized cAMP signalling with relevance for CKD is fragmentary. Uncovering functions will improve the understanding of physiological processes and identification of disease-relevant aberrations may guide towards new therapeutic concepts for the treatment of CKD.

Salt-sensitive hypertension in chronic kidney disease: distal tubular mechanisms

Chronic kidney disease (CKD) causes salt-sensitive hypertension that is often resistant to treatment and contributes to the progression of kidney injury and cardiovascular disease. A better understanding of the mechanisms contributing to salt-sensitive hypertension in CKD is essential to improve these outcomes. This review critically explores these mechanisms by focusing on how CKD affects distal nephron Na+ reabsorption. CKD causes glomerulotubular imbalance with reduced proximal Na+ reabsorption and increased distal Na+ delivery and reabsorption. Aldosterone secretion further contributes to distal Na+ reabsorption in CKD and is not only mediated by renin and K+ but also by metabolic acidosis, endothelin-1, and vasopressin. CKD also activates the intrarenal renin-angiotensin system, generating intratubular angiotensin II to promote distal Na+ reabsorption. High dietary Na+ intake in CKD contributes to Na+ retention by aldosterone-independent activation of the mineralocorticoid receptor mediated through Rac1. High dietary Na+ also produces an inflammatory response mediated by T helper 17 cells and cytokines increasing distal Na+ transport. CKD is often accompanied by proteinuria, which contains plasmin capable of activating the epithelial Na+ channel. Thus, CKD causes both local and systemic changes that together promote distal nephron Na+ reabsorption and salt-sensitive hypertension. Future studies should address remaining knowledge gaps, including the relative contribution of each mechanism, the influence of sex, differences between stages and etiologies of CKD, and the clinical relevance of experimentally identified mechanisms. Several pathways offer opportunities for intervention, including with dietary Na+ reduction, distal diuretics, renin-angiotensin system inhibitors, mineralocorticoid receptor antagonists, and K+ or H+ binders.

Geniposide in Gardenia jasminoides var. radicans Makino modulates blood pressure via inhibiting WNK pathway mediated by the estrogen receptors

Objectives

To investigate the effects of geniposide in an iridoid found in Gardenia jasminoides var. radicans Makino (GJRM) in spontaneous hypertensive rat (SHR) and explore the possible mechanisms.

Methods

In this study, we detected the content of geniposide in GJRM by high-performance liquid chromatography (HPLC). Then, we used acute diuretic experiments to determine whether geniposide has diuretic effect. Moreover, we carried out experiments on SHR to further study the mechanism of hypertension, while real-time PCR, Western blot and immunohistochemistry were used for the experiments in vivo test. Hypotonic model was used for in vitro test.

Key findings

Our data showed that the content of geniposide in the extract of GJRM is 27.54%. Meanwhile, 50 mg/kg geniposide showed the strongest effect on promoting urine volume. Further study indicated that the extract of GJRM and geniposide could significantly reduce blood pressure and promote the excretion of urine and Na+ in SHR. In addition, geniposide significantly inhibited the activation of the with-no-lysine kinase (WNK) signalling pathway and significantly increases the protein expressions of estrogen receptor α (ERα), estrogen receptor β (ERβ) and G protein-coupled receptor 30 (GPR30) in SHR. In hypotonic model, geniposide significantly inhibits the phosphorylation of NKCC and NCC and could be antagonistic to estrogen receptor antagonists.

Conclusions

Collectively, we would suggest that geniposide may potentially be utilized as an adjunct to existing thiazide and thiazide-like diuretics to control hypertension, mainly through inhibiting the activation of the WNK signalling pathway mediated by the estrogen receptor.

Arginine Vasopressin Modulates Ion and Acid/Base Balance by Regulating Cell Numbers of Sodium Chloride Cotransporter and H+-ATPase Rich Ionocytes

Arginine vasopressin (Avp) is a conserved pleiotropic hormone that is known to regulate both water reabsorption and ion balance; however, many of the mechanisms underlying its effects remain unclear. Here, we used zebrafish embryos to investigate how Avp modulates ion and acid–base homeostasis. After incubating embryos in double-deionized water for 24 h, avp mRNA expression levels were significantly upregulated. Knockdown of Avp protein expression by an antisense morpholino oligonucleotide (MO) reduced the expression of ionocyte-related genes and downregulated whole-body Cl⁻ content and H⁺ secretion, while Na⁺ and Ca²⁺ levels were not affected. Incubation of Avp antagonist SR49059 also downregulated the mRNA expression of sodium chloride cotransporter 2b (ncc2b), which is a transporter responsible for Cl⁻ uptake. Correspondingly, avp morphants showed lower NCC and H⁺-ATPase rich (HR) cell numbers, but Na⁺/K⁺-ATPase rich (NaR) cell numbers remained unchanged. avp MO also downregulated the numbers of foxi3a- and p63-expressing cells. Finally, the mRNA expression levels of calcitonin gene-related peptide (cgrp) and its receptor, calcitonin receptor-like 1 (crlr1), were downregulated in avp morphants, suggesting that Avp might affect Cgrp and Crlr1 for modulating Cl⁻ balance. Together, our results reveal a molecular/cellular pathway through which Avp regulates ion and acid–base balance, providing new insights into its function.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

Hyperkalemia and blood pressure regulation

Hypertension is common in the general population. Management of hypertensive patients at risk of hyperkalemia is challenging due to potential life-threatening complications such as cardiac arrest. Chronic hyperkalemia is often associated with impaired renal ability to excrete excessive potassium ions (K+). This may refer to chronic kidney disease or certain pharmacological interventions, including broadly used renin-angiotensin-aldosterone system and calcineurin inhibitors. Understanding the intrinsic mechanisms permitting kidney adaptations to hyperkalemia is critical for choosing therapeutic strategies. Valuable insights were obtained from the analysis of familial hyperkalemic hypertension (FHHt) syndrome, which became a classic model for coincidence of high blood pressure and hyperkalemia. FHHt can be caused by mutations in several genes, all of them resulting in excessive activity of with-no-lysine kinases (WNKs) in the distal nephron of the kidney. WNKs have been increasingly recognized as key signalling enzymes in the regulation of renal sodium ions (Na+) and K+ handling, enabling adaptive responses to systemic shifts of potassium homoeostasis consequent to variations in dietary potassium intake or disease. The WNK signalling pathway recruits a complex protein network mediating catalytic and non-catalytic effects of distinct WNK isoforms on relevant Na+- or K+-transporting proteins. In this review article, we summarize recent progress in understanding WNK signalling. An update of available models for renal adaptation to hyperkalemic conditions is presented. Consequences for blood pressure regulation are discussed. Pharmacological targeting of WNKs or their substrates offers promising options to manage hypertension while preventing hyperkalemia.

Protein Phosphatase 1 Inhibitor-1 Mediates the cAMP-Dependent Stimulation of the Renal NaCl Cotransporter

BACKGROUND

A number of cAMP-elevating hormones stimulate phosphorylation (and hence activity) of the NaCl cotransporter (NCC) in the distal convoluted tubule (DCT). Evidence suggests that protein phosphatase 1 (PP1) and other protein phosphatases modulate NCC phosphorylation, but little is known about PP1's role and the mechanism regulating its function in the DCT.

METHODS

We used ex vivo mouse kidney preparations to test whether a DCT-enriched inhibitor of PP1, protein phosphatase 1 inhibitor-1 (I1), mediates cAMP's effects on NCC, and conducted yeast two-hybrid and coimmunoprecipitation experiments in NCC-expressing MDCK cells to explore protein interactions.

RESULTS

Treating isolated DCTs with forskolin and IBMX increased NCC phosphorylation via a protein kinase A (PKA)-dependent pathway. Ex vivo incubation of mouse kidney slices with isoproterenol, norepinephrine, and parathyroid hormone similarly increased NCC phosphorylation. The cAMP-induced stimulation of NCC phosphorylation strongly correlated with the phosphorylation of I1 at its PKA consensus phosphorylation site (a threonine residue in position 35). We also found an interaction between NCC and the I1-target PP1. Moreover, PP1 dephosphorylated NCC in vitro, and the PP1 inhibitor calyculin A increased NCC phosphorylation. Studies in kidney slices and isolated perfused kidneys of control and I1-KO mice demonstrated that I1 participates in the cAMP-induced stimulation of NCC.

CONCLUSIONS

Our data suggest a complete signal transduction pathway by which cAMP increases NCC phosphorylation via a PKA-dependent phosphorylation of I1 and subsequent inhibition of PP1. This pathway might be relevant for the physiologic regulation of renal sodium handling by cAMP-elevating hormones, and may contribute to salt-sensitive hypertension in patients with endocrine disorders or sympathetic hyperactivity.

Optical Clearing in the Kidney Reveals Potassium-Mediated Tubule Remodeling

Distal nephron remodeling contributes to the pathophysiology of many clinically relevant scenarios, including diuretic resistance and certain Mendelian disorders of blood pressure. However, constitutive genetic disruptions are likely to have substantial developmental effects in this segment, and whether tubule remodeling upon physiological stimuli is a normal homeostatic mechanism is not known. Since the distal nephron acts as a potassium sensor, we assessed proliferation and tubule length in three dimensions upon dietary or inducible genetic manipulation by using optical clearing of adult mouse kidneys, whole-mount immunolabeling, and advanced light microscopy. We show that dietary potassium restriction leads promptly to proliferation of various nephron segments, including the distal convoluted tubule, whereas disruption of the potassium sensor Kir4.1 causes atrophy, despite ambient hypokalemia. These results provide proof that kidney tubules adapt rapidly to diet and indicate the power of clearing approaches to assess cell number and tubule length in healthy and diseased kidney.

Role of the alternative splice variant of NCC in blood pressure control

The renal thiazide-sensitive sodium-chloride cotransporter (NCC), located in the distal convoluted tubule (DCT) of the kidney, plays an important role in blood pressure regulation by fine-tuning sodium excretion. The human SLC12A3 gene, encoding NCC, gives rise to three isoforms, of which only the third isoform (NCC₃) has been extensively investigated so far. However, recent studies unraveled the importance of the isoforms 1 and 2, collectively referred to as NCC splice variant (NCC_SV), in several (patho)physiological conditions. In the human kidney, NCC_SV localizes to the apical membrane of the DCT and could constitute a functional route for renal sodium-chloride reabsorption. Analysis of urinary extracellular vesicles (uEVs), a non-invasive method for measuring renal responses, demonstrated that NCC_SV abundance changes in response to acute water loading and correlates with patients’ thiazide responsiveness. Furthermore, a novel phosphorylation site at serine 811 (S811), exclusively present in NCC_SV, was shown to play an instrumental role in NCC_SV as well as NCC₃ function. This review aims to summarize these new insights of NCC_SV function in humans that broadens the understanding on NCC regulation in blood pressure control.

Dominant functional role of the novel phosphorylation site S811 in the human renal NaCl cotransporter

The NaCl cotransporter (NCC) is essential for electrolyte homeostasis and control of blood pressure. The human SLC12A3 gene, which encodes NCC, gives rise to 3 isoforms, of which only the shortest isoform [NaCl cotransporter isoform 3 (NCC₃)] has been studied extensively. All NCC isoforms share key phosphorylation sites at T55 and T60 that are essential mediators of NCC function. Recently, a novel phosphorylation site at S811 was identified in isoforms 1 and 2 [NaCl cotransporter splice variant (NCC_SV)], which are only present in humans and higher primates. The aim of the current study, therefore, is to investigate the role of S811 phosphorylation in the regulation of NCC by a combination of biochemical and fluorescent microscopy analyses. We demonstrate that hypotonic low-chloride buffer increases S811 phosphorylation, whereas phosphorylation-deficient S811A mutant hinders phosphorylation at T55 and T60 in NCC_SV and NCC₃. NCC_SV S811A impairs NCC₃ activity in a dominant-negative fashion, although it does not affect plasma membrane abundance. This effect may be explained by the heterodimerization of NCC_SV with NCC₃. Taken together, our study highlights the dominant-negative effect of NCC_SV on T55 and T60 phosphorylation and NCC activity. Here, we reveal a new function of NCC_SV in humans that broadens the understanding on NCC regulation in blood pressure control.-Tutakhel, O. A. Z., Bianchi, F., Smits, D. A., Bindels, R. J. M., Hoenderop, J. G. J., van der Wijst, J. Dominant functional role of the novel phosphorylation site S811 in the human renal NaCl cotransporter.

EHD4 is a novel regulator of urinary water homeostasis

The Eps15-homology domain-containing (EHD) protein family comprises 4 members that regulate endocytic recycling. Although the kidney expresses all 4 EHD proteins, their physiologic roles are largely unknown. This study focused on EHD4, which we found to be expressed differentially across nephron segments with the highest expression in the inner medullary collecting duct. Under baseline conditions, Ehd4^-/- [EHD4-knockout (KO)] mice on a C57Bl/6 background excreted a higher volume of more dilute urine than control C57Bl/6 wild-type (WT) mice while maintaining a similar plasma osmolality. Urine excretion after an acute intraperitoneal water load was significantly increased in EHD4-KO mice compared to WT mice, and although EHD4-KO mice concentrated their urine during 24-h water restriction, urinary osmolality remained significantly lower than in WT mice, suggesting that EHD4 plays a role in renal water handling. Total aquaporin 2 (AQP2) and phospho-S256-AQP2 (pAQP2) protein expression in the inner medulla was similar in the two groups in baseline conditions. However, localization of both AQP2 and pAQP2 in the renal inner medullary principal cells appeared more dispersed, and the intensity of apical membrane staining for AQP2 was reduced significantly (by ∼20%) in EHD4-KO mice compared to WT mice in baseline conditions, suggesting an important role of EHD4 in trafficking of AQP2. Together, these data indicate that EHD4 play important roles in the regulation of water homeostasis.-Rahman, S. S., Moffitt, A. E. J., Trease, A. J., Foster, K. W., Storck, M. D., Band, H., Boesen, E. I. EHD4 is a novel regulator of urinary water homeostasis.

Phosphorylation by PKC and PKA regulate the kinase activity and downstream signaling of WNK4

With-no-lysine kinase 4 (WNK4) regulates electrolyte homeostasis and blood pressure. WNK4 phosphorylates the kinases SPAK (Ste20-related proline alanine-rich kinase) and OSR1 (oxidative stress responsive kinase), which then phosphorylate and activate the renal Na-Cl cotransporter (NCC). WNK4 levels are regulated by binding to Kelch-like 3, targeting WNK4 for ubiquitylation and degradation. Phosphorylation of Kelch-like 3 by PKC or PKA downstream of AngII or vasopressin signaling, respectively, abrogates binding. We tested whether these pathways also affect WNK4 phosphorylation and activity. By tandem mass spectrometry and use of phosphosite-specific antibodies, we identified five WNK4 sites (S47, S64, S1169, S1180, S1196) that are phosphorylated downstream of AngII signaling in cultured cells and in vitro by PKC and PKA. Phosphorylation at S64 and S1196 promoted phosphorylation of the WNK4 kinase T-loop at S332, which is required for kinase activation, and increased phosphorylation of SPAK. Volume depletion induced phosphorylation of these sites in vivo, predominantly in the distal convoluted tubule. Thus, AngII, in addition to increasing WNK4 levels, also modulates WNK4 kinase activity via phosphorylation of sites outside the kinase domain.

Calcineurin inhibitor cyclosporine A activates renal Na-K-Cl cotransporters via local and systemic mechanisms

Calcineurin dephosphorylates nuclear factor of activated T cells transcription factors, thereby facilitating T cell-mediated immune responses. Calcineurin inhibitors are instrumental for immunosuppression after organ transplantation but may cause side effects, including hypertension and electrolyte disorders. Kidneys were recently shown to display activation of the furosemide-sensitive Na-K-2Cl cotransporter (NKCC2) of the thick ascending limb and the thiazide-sensitive Na-Cl cotransporter (NCC) of the distal convoluted tubule upon calcineurin inhibition using cyclosporin A (CsA). An involvement of major hormones like angiotensin II or arginine vasopressin (AVP) has been proposed. To resolve this issue, the effects of CsA treatment in normal Wistar rats, AVP-deficient Brattleboro rats, and cultured renal epithelial cells endogenously expressing either NKCC2 or NCC were studied. Acute administration of CsA to Wistar rats rapidly augmented phosphorylation levels of NKCC2, NCC, and their activating kinases suggesting intraepithelial activating effects. Chronic CsA administration caused salt retention and hypertension, along with stimulation of renin and suppression of renal cyclooxygenase 2, pointing to a contribution of endocrine and paracrine mechanisms at long term. In Brattleboro rats, CsA induced activation of NCC, but not NKCC2, and parallel effects were obtained in cultured cells in the absence of AVP. Stimulation of cultured thick ascending limb cells with AVP agonist restored their responsiveness to CsA. Our results suggest that the direct epithelial action of calcineurin inhibition is sufficient for the activation of NCC, whereas its effect on NKCC2 is more complex and requires concomitant stimulation by AVP.

New insights into sodium transport regulation in the distal nephron: Role of G-protein coupled receptors

The renal handling of Na⁺ balance is a major determinant of the blood pressure (BP) level. The inability of the kidney to excrete the daily load of Na⁺ represents the primary cause of chronic hypertension. Among the different segments that constitute the nephron, those present in the distal part (i.e., the cortical thick ascending limb, the distal convoluted tubule, the connecting and collecting tubules) play a central role in the fine-tuning of renal Na⁺ excretion and are the target of many different regulatory processes that modulate Na⁺ retention more or less efficiently. G-protein coupled receptors (GPCRs) are crucially involved in this regulation and could represent efficient pharmacological targets to control BP levels. In this review, we describe both classical and novel GPCR-dependent regulatory systems that have been shown to modulate renal Na⁺ absorption in the distal nephron. In addition to the multiplicity of the GPCR that regulate Na⁺ excretion, this review also highlights the complexity of these different pathways, and the connections between them.

Alternative splice variant of the thiazide-sensitive NaCl cotransporter: a novel player in renal salt handling

The thiazide-sensitive NaCl cotransporter (NCC) is an important pharmacological target in the treatment of hypertension. The human SLC12A3 gene, encoding NCC, gives rise to three isoforms. Only the third isoform has been extensively investigated. The aim of the present study was, therefore, to establish the abundance and localization of the almost identical isoforms 1 and 2 (NCC1/2) in the human kidney and to determine their functional properties and regulation in physiological conditions. Immunohistochemical analysis of NCC1/2 in the human kidney revealed that NCC1/2 localizes to the apical plasma membrane of the distal convoluted tubule. Importantly, NCC1/2 mRNA constitutes ∼44% of all NCC isoforms in the human kidney. Functional analysis performed in the Xenopus laevis oocyte revealed that thiazide-sensitive 22Na+ transport of NCC1 was significantly increased compared with NCC3. Mimicking a constitutively active phosphorylation site at residue 811 (S811D) in NCC1 further augmented Na+ transport, while a nonphosphorylatable variant (S811A) of NCC1 prevented this enhanced response. Analysis of human urinary exosomes demonstrated that water loading in human subjects significantly reduces the abundance of NCC1/2 in urinary exosomes. The present study highlights that previously underrepresented NCC1/2 is a fully functional thiazide-sensitive NaCl-transporting protein. Being significantly expressed in the kidney, it may constitute a unique route of renal NaCl reabsorption and could, therefore, play an important role in blood pressure regulation.

Mini-review: regulation of the renal NaCl cotransporter by hormones

The renal thiazide-sensitive NaCl cotransporter, NCC, is the major pathway for salt reabsorption in the distal convoluted tubule. The activity of this cotransporter is critical for regulation of several physiological variables such as blood pressure, serum potassium, acid base metabolism, and urinary calcium excretion. Therefore, it is not surprising that numerous hormone-signaling pathways regulate NCC activity to maintain homeostasis. In this review, we will provide an overview of the most recent evidence on NCC modulation by aldosterone, angiotensin II, vasopressin, glucocorticoids, insulin, norepinephrine, estradiol, progesterone, prolactin, and parathyroid hormone.

Clopidogrel attenuates lithium-induced alterations in renal water and sodium channels/transporters in mice

Lithium (Li) administration causes deranged expression and function of renal aquaporins and sodium channels/transporters resulting in nephrogenic diabetes insipidus (NDI). Extracellular nucleotides (ATP/ADP/UTP), via P2 receptors, regulate these transport functions. We tested whether clopidogrel bisulfate (CLPD), an antagonist of ADP-activated P2Y(12) receptor, would affect Li-induced alterations in renal aquaporins and sodium channels/transporters. Adult mice were treated for 14 days with CLPD and/or Li and euthanized. Urine and kidneys were collected for analysis. When administered with Li, CLPD ameliorated polyuria, attenuated the rise in urine prostaglandin E2 (PGE2), and resulted in significantly higher urinary arginine vasopressin (AVP) and aldosterone levels as compared to Li treatment alone. However, urine sodium excretion remained elevated. Semi-quantitative immunoblotting revealed that CLPD alone increased renal aquaporin 2 (AQP2), Na-K-2Cl cotransporter (NKCC2), Na-Cl cotransporter (NCC), and the subunits of the epithelial Na channel (ENaC) in medulla by 25-130 %. When combined with Li, CLPD prevented downregulation of AQP2, Na-K-ATPase, and NKCC2 but was less effective against downregulation of cortical α- or γ-ENaC (70 kDa band). Thus, CLPD primarily attenuated Li-induced downregulation of proteins involved in water conservation (AVP-sensitive), with modest effects on aldosterone-sensitive proteins potentially explaining sustained natriuresis. Confocal immunofluorescence microscopy revealed strong labeling for P2Y(12)-R in proximal tubule brush border and blood vessels in the cortex and less intense labeling in medullary thick ascending limb and the collecting ducts. Therefore, there is the potential for CLPD to be directly acting at the tubule sites to mediate these effects. In conclusion, P2Y(12)-R may represent a novel therapeutic target for Li-induced NDI.

Vasopressin regulation of sodium transport in the distal nephron and collecting duct

Arginine vasopressin (AVP) is released from the posterior pituitary gland during states of hyperosmolality or hypovolemia. AVP is a peptide hormone, with antidiuretic and antinatriuretic properties. It allows the kidneys to increase body water retention predominantly by increasing the cell surface expression of aquaporin water channels in the collecting duct alongside increasing the osmotic driving forces for water reabsorption. The antinatriuretic effects of AVP are mediated by the regulation of sodium transport throughout the distal nephron, from the thick ascending limb through to the collecting duct, which in turn partially facilitates osmotic movement of water. In this review, we will discuss the regulatory role of AVP in sodium transport and summarize the effects of AVP on various molecular targets, including the sodium-potassium-chloride cotransporter NKCC2, the thiazide-sensitive sodium-chloride cotransporter NCC, and the epithelial sodium channel ENaC.

A Systems Level Analysis of Vasopressin-mediated Signaling Networks in Kidney Distal Convoluted Tubule Cells

The kidney distal convoluted tubule (DCT) plays an essential role in maintaining body sodium balance and blood pressure. The major sodium reabsorption pathway in the DCT is the thiazide-sensitive NaCl cotransporter (NCC), whose functions can be modulated by the hormone vasopressin (VP) acting via uncharacterized signaling cascades. Here we use a systems biology approach centered on stable isotope labeling by amino acids in cell culture (SILAC) based quantitative phosphoproteomics of cultured mouse DCT cells to map global changes in protein phosphorylation upon acute treatment with a VP type II receptor agonist 1-desamino-8-D-arginine vasopressin (dDAVP). 6330 unique proteins, containing 12333 different phosphorylation sites were identified. 185 sites were altered in abundance following dDAVP. Basophilic motifs were preferential targets for upregulated sites upon dDAVP stimulation, whereas proline-directed motifs were prominent for downregulated sites. Kinase prediction indicated that dDAVP increased AGC and CAMK kinase families’ activities and decreased activity of CDK and MAPK families. Network analysis implicated phosphatidylinositol-4,5-bisphosphate 3-kinase or CAMKK dependent pathways in VP-mediated signaling; pharmacological inhibition of which significantly reduced dDAVP induced increases in phosphorylated NCC at an activating site. In conclusion, this study identifies unique VP signaling cascades in DCT cells that may be important for regulating blood pressure.

Downregulation of transient receptor potential M6 channels as a cause of hypermagnesiuric hypomagnesemia in obese type 2 diabetic rats

We assessed the expression profile of Mg2+-transporting molecules in obese diabetic rats as a cause of hypermagnesiuric hypomagnesemia, which is involved in the development of insulin resistance, hypertension, and coronary diseases. Kidneys were obtained from male Otsuka Long-Evans Tokushima fatty (OLETF) and Long-Evans Tokushima Otsuka (LETO) obese diabetic rats at the ages of 16, 24, and 34 wk. Expression profiles were studied by real-time PCR and immunohistochemistry together with measurements of urine Mg2+excretion. Urine Mg2+excretion was increased in 24-wk-old OLETF rats and hypomagnesemia was apparent in 34-wk-old OLETF rats but not in LETO rats (urine Mg2+excretion: 0.16 ± 0.01 μg·min−1·g body wt−1in 24-wk-old LETO rats and 0.28 ± 0.01 μg·min−1·g body wt−1in 24-wk-old OLETF rats). Gene expression of transient receptor potential (TRP)M6 was downregulated (85.5 ± 5.6% in 34-wk-old LETO rats and 63.0 ± 3.5% in 34-wk-old OLETF rats) concomitant with Na+-Cl−cotransporter downregulation, whereas the expression of claudin-16 in tight junctions of the thick ascending limb of Henle was not different. The results of the semiquantitative analysis of immunohistochemistry were consistent with these findings (TRPM6: 0.49 ± 0.04% in 16-wk-old LETO rats, 0.10 ± 0.01% in 16-wk-old OLETF rats, 0.52 ± 0.03% in 24-wk-old LETO rats, 0.10 ± 0.01% in 24-wk-old OLETF rats, 0.48 ± 0.02% in 34-wk-old LETO rats, and 0.12 ± 0.02% in 34-wk-old OLETF rats). Gene expression of fibrosis-related proinflammatory cytokines as well as histological changes showed that the hypermagnesiuria-related molecular changes and tubulointerstitial nephropathy developed independently. TRPM6, located principally in distal convoluted tubules, appears to be a susceptible molecule that causes hypermagnesiuric hypomagnesemia as a tubulointerstitial nephropathy-independent altered tubular function in diabetic nephropathy.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Paradoxical activation of the sodium chloride cotransporter (NCC) without hypertension in kidney deficient in a regulatory subunit of Na,K-ATPase, FXYD2

Na,K‐ATPase generates the driving force for sodium reabsorption in the kidney. Na,K‐ATPase functional properties are regulated by small proteins belonging to the FXYD family. In kidney FXYD2 is the most abundant: it is an inhibitory subunit expressed in almost every nephron segment. Its absence should increase sodium pump activity and promote Na⁺ retention, however, no obvious renal phenotype was detected in mice with global deletion of FXYD2 (Arystarkhova et al. 2013). Here, increased total cortical Na,K‐ATPase activity was documented in the Fxyd2^−/− mouse, without increased α1β1 subunit expression. We tested the hypothesis that adaptations occur in distal convoluted tubule (DCT), a major site of sodium adjustments. Na,K‐ATPase immunoreactivity in DCT was unchanged, and there was no DCT hypoplasia. There was a marked activation of thiazide‐sensitive sodium chloride cotransporter (NCC; Slc12a3) in DCT, predicted to increase Na⁺ reabsorption in this segment. Specifically, NCC total increased 30% and NCC phosphorylated at T53 and S71, associated with activation, increased 4‐6 fold. The phosphorylation of the closely related thick ascending limb (TAL) apical NKCC2 (Slc12a1) increased at least twofold. Abundance of the total and cleaved (activated) forms of ENaC α‐subunit was not different between genotypes. Nonetheless, no elevation of blood pressure was evident despite the fact that NCC and NKCC2 are in states permissive for Na⁺ retention. Activation of NCC and NKCC2 may reflect an intracellular linkage to elevated Na,K‐ATPase activity or a compensatory response to Na⁺ loss proximal to the TAL and DCT.

We discovered a substantial activation of renal NCC cotransporter in mice genetically depleted for the regulatory inhibitory subunit of Na,K‐ATPase, FXYD2. Surprisingly, no significant changes in urine output as well as elevation of blood pressure were detected suggesting compensatory adaptation elsewhere in nephron

Electroneutral absorption of NaCl by the aldosterone-sensitive distal nephron: implication for normal electrolytes homeostasis and blood pressure regulation

Sodium absorption by the distal part of the nephron, i.e., the distal convoluted tubule, the connecting tubule, and the collecting duct, plays a major role in the control of homeostasis by the kidney. In this part of the nephron, sodium transport can either be electroneutral or electrogenic. The study of electrogenic Na(+) absorption, which is mediated by the epithelial sodium channel (ENaC), has been the focus of considerable interest because of its implication in sodium, potassium, and acid-base homeostasis. However, recent studies have highlighted the crucial role played by electroneutral NaCl absorption in the regulation of the body content of sodium chloride, which in turn controls extracellular fluid volume and blood pressure. Here, we review the identification and characterization of the NaCl cotransporter (NCC), the molecule accounting for the main part of electroneutral NaCl absorption in the distal nephron, and its regulators. We also discuss recent work describing the identification of a novel "NCC-like" transport system mediated by pendrin and the sodium-driven chloride/bicarbonate exchanger (NDCBE) in the β-intercalated cells of the collecting system.

WNK3 abrogates the NEDD4-2-mediated inhibition of the renal Na+-Cl- cotransporter

The serine/threonine kinase WNK3 and the ubiquitin-protein ligase NEDD4-2 are key regulators of the thiazide-sensitive Na+-Cl- cotransporter (NCC), WNK3 as an activator and NEDD2-4 as an inhibitor. Nedd4-2 was identified as an interacting partner of WNK3 through a glutathione-S-transferase pull-down assay using the N-terminal domain of WNK3, combined with LC-MS/MS analysis. This was validated by coimmunoprecipitation of WNK3 and NEDD4-2 expressed in HEK293 cells. Our data also revealed that the interaction between Nedd4-2 and WNK3 does not involve the PY-like motif found in WNK3. The level of WNK3 ubiquitylation did not change when NEDD4-2 was expressed in HEK293 cells. Moreover, in contrast to SGK1, WNK3 did not phosphorylate NEDD4-2 on S222 or S328. Coimmunoprecipitation assays showed that WNK3 does not regulate the interaction between NCC and NEDD4-2. Interestingly, in Xenopus laevis oocytes, WNK3 was able to recover the SGK1-resistant NEDD4-2 S222A/S328A-mediated inhibition of NCC and further activate NCC. Furthermore, elimination of the SPAK binding site in the kinase domain of WNK3 (WNK3-F242A, which lacks the capacity to bind the serine/threonine kinase SPAK) prevented the WNK3 NCC-activating effect, but not the Nedd4-2-inhibitory effect. Together, these results suggest that a novel role for WNK3 on NCC expression at the plasma membrane, an effect apparently independent of the SPAK kinase and the aldosterone-SGK1 pathway.

Phosphorylation decreases ubiquitylation of the thiazide-sensitive cotransporter NCC and subsequent clathrin-mediated endocytosis

The thiazide-sensitive sodium chloride cotransporter, NCC, is the major NaCl transport protein in the distal convoluted tubule (DCT). The transport activity of NCC can be regulated by phosphorylation, but knowledge of modulation of NCC trafficking by phosphorylation is limited. In this study, we generated novel tetracycline-inducible Madin-Darby canine kidney type I (MDCKI) cell lines expressing NCC to examine the role of NCC phosphorylation and ubiquitylation on NCC endocytosis. In MDCKI-NCC cells, NCC was highly glycosylated at molecular weights consistent with NCC monomers and dimers. NCC constitutively cycles to the apical plasma membrane of MDCKI-NCC cells, with 20-30% of the membrane pool of NCC internalized within 30 min. The use of dynasore, PitStop2, methyl-β-cyclodextrin, nystatin, and filipin (specific inhibitors of either clathrin-dependent or -independent endocytosis) demonstrated that NCC is internalized via a clathrin-mediated pathway. Reduction of endocytosis resulted in greater levels of NCC in the plasma membrane. Immunogold electron microscopy confirmed the association of NCC with the clathrin-mediated internalization pathway in rat DCT cells. Compared with controls, inducing phosphorylation of NCC via low chloride treatment or mimicking phosphorylation by replacing Thr-53, Thr-58, and Ser-71 residues with Asp resulted in increased membrane abundance and reduced rates of NCC internalization. NCC ubiquitylation was lowest in the conditions with greatest NCC phosphorylation, thus providing a mechanism for the reduced endocytosis. In conclusion, our data support a model where NCC is constitutively cycled to the plasma membrane, and upon stimulation, it can be phosphorylated to both increase NCC activity and decrease NCC endocytosis, together increasing NaCl transport in the DCT.

Regulation of nephron water and electrolyte transport by adenylyl cyclases

Adenylyl cyclases (AC) catalyze formation of cAMP, a critical component of G protein-coupled receptor signaling. So far, nine distinct membrane-bound AC isoforms (AC1-9) and one soluble AC (sAC) have been identified and, except for AC8, all of them are expressed in the kidney. While the role of ACs in renal cAMP formation is well established, we are just beginning to understand the function of individual AC isoforms, particularly with regard to hormonal regulation of transporter and channel phosphorylation, membrane abundance, and trafficking. This review focuses on the role of different AC isoforms in regulating renal water and electrolyte transport in health as well as potential pathological implications of disordered AC isoform function. In particular, we focus on modulation of transporter and channel abundance, activity, and phosphorylation, with an emphasis on studies employing genetically modified animals. As will be described, it is now evident that specific AC isoforms can exert unique effects in the kidney that may have important implications in our understanding of normal physiology as well as disease pathogenesis.

Regulation of with-no-lysine kinase signaling by Kelch-like proteins

In 2001, with-no-lysine (WNK) kinases were identified as the genes responsible for the human hereditary hypertensive disease pseudohypoaldosteronism type II (PHAII). It took a further 6 years to clarify that WNK kinases participate in a signaling cascade with oxidative stress-responsive gene 1 (OSR1), Ste20-related proline-alanine-rich kinase (SPAK), and thiazide-sensitive NaCl cotransporter (NCC) in the kidney and the constitutive activation of this signaling cascade is the molecular basis of PHAII. Since this discovery, the WNK-OSR1/SPAK-NCC signaling cascade has been shown to be involved not only in PHAII but also in the regulation of blood pressure under normal and pathogenic conditions, such as hyperinsulinemia. However, the molecular mechanisms of WNK kinase regulation by dietary and hormonal factors and by PHAII-causing mutations remain poorly understood. In 2012, two additional genes responsible for PHAII, Kelch-like 3 (KLHL3) and Cullin3, were identified. At the time of their discovery, the molecular mechanisms underlying the interaction between these genes and their involvement in PHAII were unknown. Here we review the pathophysiological roles of the WNK signaling cascade clarified to date and introduce a new mechanism of WNK kinase regulation by KLHL3 and Cullin3, which provides insight on previously unknown mechanisms of WNK kinase regulation.

The sodium chloride cotransporter SLC12A3: new roles in sodium, potassium, and blood pressure regulation

SLC12A3 encodes the thiazide-sensitive sodium chloride cotransporter (NCC), which is primarily expressed in the kidney, but also in intestine and bone. In the kidney, NCC is located in the apical plasma membrane of epithelial cells in the distal convoluted tubule. Although NCC reabsorbs only 5 to 10% of filtered sodium, it is important for the fine-tuning of renal sodium excretion in response to various hormonal and non-hormonal stimuli. Several new roles for NCC in the regulation of sodium, potassium, and blood pressure have been unraveled recently. For example, the recent discoveries that NCC is activated by angiotensin II but inhibited by dietary potassium shed light on how the kidney handles sodium during hypovolemia (high angiotensin II) and hyperkalemia. The additive effect of angiotensin II and aldosterone maximizes sodium reabsorption during hypovolemia, whereas the inhibitory effect of potassium on NCC increases delivery of sodium to the potassium-secreting portion of the nephron. In addition, great steps have been made in unraveling the molecular machinery that controls NCC. This complex network consists of kinases and ubiquitinases, including WNKs, SGK1, SPAK, Nedd4-2, Cullin-3, and Kelch-like 3. The pathophysiological significance of this network is illustrated by the fact that modification of each individual protein in the network changes NCC activity and results in salt-dependent hypotension or hypertension. This review aims to summarize these new insights in an integrated manner while identifying unanswered questions.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Neurohormonal interactions on the renal oxygen delivery and consumption in haemorrhagic shock-induced acute kidney injury

Haemorrhagic shock is a common cause of acute kidney injury (AKI), which is a major risk factor for developing chronic kidney disease. The mechanism is superficially straightforward. An arterial pressure below the kidney's autoregulatory region leads to a direct reduction in filtration pressure and perfusion, which in turn cause renal failure with reduced glomerular filtration rate and AKI because of hypoxia. However, the kidney's situation is further worsened by the hormonal and neural reactions to reduced perfusion pressure. There are three major systems working to maintain arterial pressure in shock: sympathetic signalling, the renin-angiotensin system and vasopressin. These work to retain electrolytes and water and to increase peripheral resistance and cardiac output. In the kidney, the increased electrolyte reabsorption consumes oxygen. At the same time, at the signalling level seen in shock, all of these hormones reduce renal perfusion and thereby oxygen delivery. This creates an exaggerated hypoxic situation that is liable to worsen the AKI. The present review will examine this mechanistic background and identify a number of areas that require further studies. At this time, the ideal treatment of haemorrhagic shock appears to be slow fluid resuscitation, possibly with hyperosmolar sodium, low chloride and no artificial colloids. From the standpoint of the kidney, renin-angiotensin system inhibitors appear fruitful for further study.

The SLC12 family of electroneutral cation-coupled chloride cotransporters

The SLC12 family encodes electroneutral cation-coupled chloride cotransporters that are critical for several physiological processes including cell volume regulation, modulation of intraneuronal chloride concentration, transepithelial ion movement, and blood pressure regulation. Members of this family are the targets of the most commonly used diuretic drugs, have been shown to be the causative genes for inherited disease such as Gitelman, Bartter and Andermann syndromes, and potentially play a role in polygenic complex diseases like arterial hypertension, epilepsy, osteoporosis, and cancer.

SPAK differentially mediates vasopressin effects on sodium cotransporters

Activation of the Na(+)-K(+)-2Cl(-)-cotransporter (NKCC2) and the Na(+)-Cl(-)-cotransporter (NCC) by vasopressin includes their phosphorylation at defined, conserved N-terminal threonine and serine residues, but the kinase pathways that mediate this action of vasopressin are not well understood. Two homologous Ste20-like kinases, SPS-related proline/alanine-rich kinase (SPAK) and oxidative stress responsive kinase (OSR1), can phosphorylate the cotransporters directly. In this process, a full-length SPAK variant and OSR1 interact with a truncated SPAK variant, which has inhibitory effects. Here, we tested whether SPAK is an essential component of the vasopressin stimulatory pathway. We administered desmopressin, a V2 receptor-specific agonist, to wild-type mice, SPAK-deficient mice, and vasopressin-deficient rats. Desmopressin induced regulatory changes in SPAK variants, but not in OSR1 to the same degree, and activated NKCC2 and NCC. Furthermore, desmopressin modulated both the full-length and truncated SPAK variants to interact with and phosphorylate NKCC2, whereas only full-length SPAK promoted the activation of NCC. In summary, these results suggest that SPAK mediates the effect of vasopressin on sodium reabsorption along the distal nephron.

Molecular physiology of SPAK and OSR1: two Ste20-related protein kinases regulating ion transport

SPAK (Ste20-related proline alanine rich kinase) and OSR1 (oxidative stress responsive kinase) are members of the germinal center kinase VI subfamily of the mammalian Ste20 (Sterile20)-related protein kinase family. Although there are 30 enzymes in this protein kinase family, their conservation across the fungi, plant, and animal kingdom confirms their evolutionary importance. Already, a large volume of work has accumulated on the tissue distribution, binding partners, signaling cascades, and physiological roles of mammalian SPAK and OSR1 in multiple organ systems. After reviewing this basic information, we will examine newer studies that demonstrate the pathophysiological consequences to SPAK and/or OSR1 disruption, discuss the development and analysis of genetically engineered mouse models, and address the possible role these serine/threonine kinases might have in cancer proliferation and migration.

Enhanced phosphorylation of Na(+)-Cl- co-transporter in experimental metabolic syndrome: role of insulin

In the present study, we investigated the activity of the thiazide-sensitive NCC (Na(+)-Cl(-) co-transporter) in experimental metabolic syndrome and the role of insulin in NCC activation. Renal responses to the NCC inhibitor HCTZ (hydrochlorothiazide), as a measure of NCC activity in vivo, were studied in 12-week-old ZO (Zucker obese) rats, a model of the metabolic syndrome, and in ZL (Zucker lean) control animals, together with renal NCC expression and molecular markers of NCC activity, such as localization and phosphorylation. Effects of insulin were studied further in mammalian cell lines with inducible and endogenous expression of this molecule. ZO rats displayed marked hyperinsulinaemia, but no differences in plasma aldosterone, compared with ZL rats. In ZO rats, natriuretic and diuretic responses to NCC inhibition with HCTZ were enhanced compared with ZL rats, and were associated with a decrease in BP (blood pressure). ZO rats displayed enhanced Thr(53) NCC phosphorylation and predominant membrane localization of both total and phosphorylated NCC, together with a different profile in expression of SPAK (Ste20-related proline/alanine-rich kinase) isoforms, and lower expression of WNK4. In vitro, insulin induced NCC phosphorylation, which was blocked by a PI3K (phosphoinositide 3-kinase) inhibitor. Insulin-induced reduction in WNK4 expression was also observed, but delayed compared with the time course of NCC phosphorylation. In summary, we report increased NCC activity in hyperinsulinaemic rodents in conjunction with the SPAK expression profile consistent with NCC activation and reduced WNK4, as well as an ability of insulin to induce NCC stimulatory phosphorylation in vitro. Together, these findings indicate that hyperinsulinaemia is an important driving force of NCC activity in the metabolic syndrome with possible consequences for BP regulation.

Increased expression of renal TRPM6 compensates for Mg(2+) wasting during furosemide treatment

Background

Furosemide is a loop diuretic, which blocks the Na⁺, K⁺, 2Cl⁻ cotransporter (NKCC2) in the thick ascending limb of Henle (TAL). By diminishing sodium (Na⁺) reabsorption, loop diuretics reduce the lumen-positive transepithelial voltage and consequently diminish paracellular transport of magnesium (Mg²⁺) and calcium (Ca²⁺) in TAL. Indeed, furosemide promotes urinary Mg²⁺ excretion; however, it is unclear whether this leads, especially during prolonged treatment, to hypomagnesaemia. The aim of the present study was, therefore, to determine the effect of chronic furosemide application on renal Mg²⁺ handling in mice.

Methods

Two groups of 10 mice received an osmotic minipump subcutaneously for 7 days with vehicle or 30 mg/kg/day furosemide. Serum and urine electrolyte concentrations were determined. Next, renal mRNA levels of the epithelial Mg²⁺ channel (TRPM6), the Na⁺, Cl⁻ cotransporter (NCC), the epithelial Ca²⁺ channel (TRPV5), the cytosolic Ca²⁺-binding protein calbindin-D_28K, as well parvalbumin (PV), claudin-7 (CLDN7) and claudin-8 (CLDN8), the epithelial Na⁺ channel (ENaC) and the Na⁺–H⁺ exchanger 3 (NHE3) were determined by real-time quantitative polymerase chain reaction. Renal protein levels of NCC, TRPV5, calbindin-D_28K and ENaC were also measured using semi-quantitative immunohistochemistry and immunoblotting.

Results

The mice chronically treated with 30 mg/kg/day furosemide displayed a significant polyuria (2.1 ± 0.3 and 1.3 ± 0.2 mL/24 h, furosemide versus control respectively, P < 0.05). Furosemide treatment resulted in increased serum concentrations of Na⁺ [158 ± 3 (treated) and 147 ± 1 mmol/L (control), P < 0.01], whereas serum K⁺, Ca²⁺ and Mg²⁺ values were not significantly altered in mice treated with furosemide. Urinary excretion of Na⁺, K⁺, Ca²⁺ and Mg²⁺ was not affected by chronic furosemide treatment. The present study shows specific renal upregulation of TRPM6, NCC, TRPV5 and calbindin-D_28K.

Conclusions

During chronic furosemide treatment, enhanced active reabsorption of Mg²⁺ via the epithelial channel TRPM6 in DCT compensates for the reduced reabsorption of Mg²⁺ in TAL.

Adenylyl cyclase 6 enhances NKCC2 expression and mediates vasopressin-induced phosphorylation of NKCC2 and NCC

Arginine vasopressin (AVP) affects kidney function via vasopressin V2 receptors that are linked to activation of adenylyl cyclase (AC) and an increase in cyclic adenosine monophosphate formation. AVP/cyclic adenosine monophosphate enhance the phosphorylation of the Na-K-2Cl cotransporter (NKCC2) at serine residue 126 (pS126 NKCC2) and of the Na-Cl cotransporter (NCC) at threonine 58 (pT58 NCC). The isoform(s) of AC involved in these responses, however, were unknown. Phosphorylation of S126 NKCC2 and T58 NCC, induced by the V2 receptor agonist (1-desamino-8-D-arginine vasopressin) in wild-type mice, is lacking in knockout mice for AC isoform 6 (AC6). With regard to NKCC2 phosphorylation, the stimulatory effect of 1-desamino-8-D-AVP and the defect in AC6(-/-) mice seem to be restricted to the medullary portion of the thick ascending limb. AC6 is also a stimulator of total renal NKCC2 protein abundance in medullary and cortical thick ascending limb. Consequently, mice lacking AC6 have lower NKCC2 expression and a mild Bartter syndrome-like phenotype, including lower plasma concentrations of K+ and H+ and compensatory upregulation of NCC. Increased AC6-independent phosphorylation of NKCC2 at S126 might help to stabilize NKCC2 activity in the absence of AC6. Renal AC6 determines total NKCC2 expression and mediates vasopressin-induced NKCC2/NCC phosphorylation. These regulatory mechanisms, which are defective in AC knockout mice, are likely responsible for the observed mild Bartter syndrome.

Effects of ACE inhibition and ANG II stimulation on renal Na-Cl cotransporter distribution, phosphorylation, and membrane complex properties

The renal distal tubule Na-Cl cotransporter (NCC) reabsorbs <10% of the filtered Na(+) but is a key control point for blood pressure regulation by angiotensin II (ANG II), angiotensin-converting enzyme inhibitors (ACEI), and thiazide diuretics. This study aimed to determine whether NCC phosphorylation (NCCp) was regulated by acute (20-30 min) treatment with the ACEI captopril (12 μg/min × 20 min) or by a sub-pressor dose of ANG II (20 ng·kg(-1)·min(-1)) in Inactin-anesthetized rats. By immuno-EM, NCCp was detected exclusively in or adjacent to apical plama membranes (APM) in controls and after ACEI or ANG II treatment, while NCC total was detected in both APM and subapical cytoplasmic vesicles (SCV) in all conditions. In renal homogenates, neither ACEI nor ANG II treatment altered NCCp abundance, assayed by immunoblot. However, by density gradient fractionation we identified a pool of low-density APM in which NCCp decreased 50% in response to captopril and was restored during ANG II infusion, and another pool of higher-density APM that responded reciprocally, indicative of regulated redistribution between two APM pools. In both pools, NCCp was preferentially localized to Triton-soluble membranes. Blue Native gel electrophoresis established that APM NCCp localized to ~700 kDa complexes (containing γ-adducin) while unphosphorylated NCC in intracellular membranes primarily localized to ~400 kDa complexes: there was no evidence for native monomeric or dimeric NCC or NCCp. In summary, this study demonstrates that phosphorylated NCC, localized to multimeric complexes in the APM, redistributes in a regulated manner within the APM in response to ACEI and ANG II.

Regulation of the renal Na+-Cl- cotransporter by phosphorylation and ubiquitylation

The activity of the renal thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule plays a key role in defining arterial blood pressure levels. Increased or decreased activity of the NCC is associated with arterial hypertension or hypotension, respectively. Thus it is of major interest to understand the activity of NCC using in vivo models. Phosphorylation of certain residues of the amino-terminal domain of NCC has been shown to be associated with its activation. The development of phospho-specific antibodies against these sites provides a powerful tool that is helping to increase our understanding of the molecular physiology of NCC. Additionally, NCC expression in the plasma membrane is modulated by ubiquitylation, which represents another major mechanism for regulating protein activity. This work presents a review of our current knowledge of the regulation of NCC activity by phosphorylation and ubiquitylation.

Annexin A1 modulates macula densa function by inhibiting cyclooxygenase 2

Annexin A1 (ANXA1) exerts anti-inflammatory effects through multiple mechanisms including inhibition of prostaglandin synthesis. Once secreted, ANXA1 can bind to G protein-coupled formyl peptide receptors (Fpr) and activate diverse cellular signaling pathways. ANXA1 is known to be expressed in cells of the juxtaglomerular apparatus, but its relation to the expression of cyclooxygenase 2 (COX-2) in thick ascending limb and macula densa cells has not been elucidated. We hypothesized that ANXA1 regulates the biosynthesis of COX-2. ANXA1 abundance in rat kidney macula densa was extensively colocalized with COX-2 (95%). Furosemide, an established stimulus for COX-2 induction, caused enhanced expression of both ANXA1 and COX-2 with maintained colocalization (99%). In ANXA1-deficient mice, COX-2-positive cells were more numerous than in control mice (+107%; normalized to glomerular number; P < 0.05) and renin expression was increased (+566%; normalized to glomerular number; P < 0.05). Cultured macula densa cells transfected with full-length rat ANXA1 revealed downregulation of COX-2 mRNA (−59%; P < 0.05). Similarly, treatment with dexamethasone suppressed COX-2 mRNA in the cells (−49%; P < 0.05), while inducing ANXA1 mRNA (+56%; P < 0.05) and ANXA1 protein secretion. Inhibition of the ANXA-1 receptor Fpr1 with cyclosporin H blunted the effect of dexamethasone on COX-2 expression. These data show that ANXA1 exerts an inhibitory effect on COX-2 expression in the macula densa. ANXA1 may be a novel intrinsic modulator of renal juxtaglomerular regulation by inhibition of PGE2 synthesis.

Characterization of a novel phosphorylation site in the sodium-chloride cotransporter, NCC

The sodium-chloride cotransporter, NCC, is essential for renal electrolyte balance. NCC function can be modulated by protein phosphorylation. In this study, we characterized the role and physiological regulation of a novel phosphorylation site in NCC at Ser124 (S124). Novel phospho-specific antibodies targeting pS124-NCC demonstrated a band of 160 kDa in the kidney cortex, but not medulla, which was preabsorbed by a corresponding phosphorylated peptide. Confocal microscopy with kidney tubule segment-specific markers localized pS124-NCC to all distal convoluted tubule cells. Double immunogold electron microscopy demonstrated that pS124-NCC co-localized with total NCC in the apical plasma membrane of distal convoluted tubule cells and intracellular vesicles. Acute treatment of Munich-Wistar rats or vasopressin-deficient Brattleboro rats with the vasopressin type 2 receptor-specific agonist dDAVP significantly increased pS124-NCC abundance, with no changes in total NCC plasma membrane abundance. pS124-NCC levels also increased in abundance in rats after stimulation of the renin-angiotensin-aldosterone system by dietary low sodium intake. In contrast to other NCC phosphorylation sites, the STE20/SPS1-related proline-alanine-rich kinase and oxidative stress-response kinases (SPAK and OSR1) were not able to phosphorylate NCC at S124. Protein kinase arrays identified multiple kinases that were able to bind to the region surrounding S124. Four of these kinases (IRAK2, CDK6/Cyclin D1, NLK and mTOR/FRAP) showed weak but significant phosphorylation activity at S124. In oocytes, (36)Cl uptake studies combined with biochemical analysis showed decreased activity of plasma membrane-associated NCC when replacing S124 with alanine (A) or aspartic acid (D). In novel tetracycline-inducible MDCKII-NCC cell lines, S124A and S124D mutants were able to traffic to the plasma membrane similarly to wildtype NCC.