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

The role of SLC12A family of cation-chloride cotransporters and drug discovery methodologies

The solute carrier family 12 (SLC12) of cation-chloride cotransporters (CCCs) comprises potassium chloride cotransporters (KCCs, e.g. KCC1, KCC2, KCC3, and KCC4)-mediated Cl- extrusion, and sodium potassium chloride cotransporters (N[K]CCs, NKCC1, NKCC2, and NCC)-mediated Cl- loading. The CCCs play vital roles in cell volume regulation and ion homeostasis. Gain-of-function or loss-of-function of these ion transporters can cause diseases in many tissues. In recent years, there have been considerable advances in our understanding of CCCs' control mechanisms in cell volume regulations, with many techniques developed in studying the functions and activities of CCCs. Classic approaches to directly measure CCC activity involve assays that measure the transport of potassium substitutes through the CCCs. These techniques include the ammonium pulse technique, radioactive or nonradioactive rubidium ion uptake-assay, and thallium ion-uptake assay. CCCs' activity can also be indirectly observed by measuring γ-aminobutyric acid (GABA) activity with patch-clamp electrophysiology and intracellular chloride concentration with sensitive microelectrodes, radiotracer 36Cl-, and fluorescent dyes. Other techniques include directly looking at kinase regulatory sites phosphorylation, flame photometry, 22Na+ uptake assay, structural biology, molecular modeling, and high-throughput drug screening. This review summarizes the role of CCCs in genetic disorders and cell volume regulation, current methods applied in studying CCCs biology, and compounds developed that directly or indirectly target the CCCs for disease treatments.

In Primary Aldosteronism Acute Potassium Chloride Supplementation Suppresses Abundance and Phosphorylation of the Sodium-Chloride Cotransporter

Background

Elevated abundance of sodium-chloride cotransporter (NCC) and phosphorylated NCC (pNCC) are potential markers of primary aldosteronism (PA), but these effects may be driven by hypokalemia.

Methods

We measured plasma potassium in patients with PA. If potassium was <4.0 mmol/L, patients were given sufficient oral potassium chloride (KCl) over 24 hours to achieve as close to 4.0 mmol/L as possible. Clinical chemistries were assessed, and urinary extracellular vesicles (uEVs) were examined to investigate effects on NCC.

Results

Among 21 patients with PA who received a median total dose of 6.0 g (2.4-16.8 g) of KCl, increases were observed in plasma potassium (from 3.4 to 4.0 mmol/L; P<0.001), aldosterone (from 305 to 558 pmol/L; P=0.01), and renin (from 1.2 to 2.5 mIU/L; P<0.001), whereas decreases were detected in uEV levels of NCC (median fold change_(post/basal) [FC]=0.71 [0.09-1.99]; P=0.02), pT60-NCC (FC=0.84 [0.06-1.66]; P=0.05), and pT55/60-NCC (FC=0.67 [0.08-2.42]; P=0.02). By contrast, in 10 patients with PA who did not receive KCl, there were no apparent changes in plasma potassium, NCC abundance, and phosphorylation status, but increases were observed in plasma aldosterone (from 178 to 418 pmol/L; P=0.006) and renin (from 2.0 to 3.0 mU/L; P=0.009). Plasma potassium correlated inversely with uEV levels of NCC (R ²=0.11; P=0.01), pT60-NCC (R ²=0.11; P=0.01), and pT55/60-NCC (R ²=0.11; P=0.01).

Conclusions

Acute oral KCl loading replenished plasma potassium in patients with PA and suppressed NCC abundance and phosphorylation, despite a significant rise in plasma aldosterone. This supports the view that potassium supplementation in humans with PA overrides the aldosterone stimulatory effect on NCC. The increased plasma aldosterone in patients with PA without KCl supplementation may be due to aldosterone response to posture challenge.

Elevated intracellular Na+ and osmolarity stimulate catalytic activity of the ubiquitin ligase Nedd4-2

Regulation of catalytic activity of E3 ubiquitin ligases is critical for their cellular functions. We identified an unexpected mode of regulation of E3 catalytic activity by ions and osmolarity; enzymatic activity of the HECT family E3 Nedd4-2/Nedd4L is enhanced by increased intracellular Na⁺ ([Na⁺]_i) and by hyperosmolarity. This stimulated activity is mediated by activation of p38-MAPK and is inhibited by WNKs. Moreover, protease (Furin)-mediated activation of the epithelial Na⁺ channel ENaC (a bona fide Nedd4-2 substrate), which leads to increased [Na⁺]_i and osmolarity, results in enhanced Nedd4-2 catalytic activity. This enhancement is inhibited by a Furin inhibitor, by a protease-resistant ENaC mutant, or by treatment with the ENaC inhibitor amiloride. Moreover, WNK inhibition, which stimulates catalytic activity of Nedd4-2, leads to reduced levels of cell-surface ENaC and reduced channel activity. ENaC activity does not affect Nedd4-2:ENaC binding. Therefore, these results demonstrate activation of a ubiquitin ligase by Na⁺ and osmotic changes. Importantly, they reveal a negative feedback loop in which active ENaC leads to stimulation of catalytic activity of its own suppressor, Nedd4-2, to protect cells from excessive Na⁺ loading and hyperosmotic stress and to protect the animal from hypertension.

The European and Japanese eel NaCl cotransporter β exhibit chloride currents and are resistant to thiazide type diuretics

The thiazide-sensitive Na⁺-Cl^- cotransporter (NCC) is the major pathway for salt reabsorption in the mammalian distal convoluted tubule, and the inhibition of its function with thiazides is widely used for the treatment of arterial hypertension. In mammals and teleosts, NCC is present as one ortholog that is mainly expressed in the kidney. One exception, however, is the eel, which has two genes encoding NCC. The eNCCa is located in the kidney and eNCCb, which is present in the apical membrane of the rectum. Interestingly, the European eNCCb functions as a NaCl cotransporter that is nevertheless resistant to thiazides and is not activated by low-chloride hypotonic stress. However, in the Japanese eel rectal sac, a thiazide-sensitive NaCl transport mechanism has been described. The protein sequences between eNCCβ and jNCCβ are 98% identical. Here, by site-directed mutagenesis, we transformed eNCCβ into jNCCβ. Our data showed that jNCCβ, similar to eNCCβ, is resistant to thiazides. In addition, both NCCβ proteins have high transport capacity with respect to their renal NCC orthologs, and in contrast to known NCCs, exhibit electrogenic properties that are reduced when residue I172 is substituted by A, G or M. This is considered a key residue for the chloride ion-binding sites of NKCC and KCC. We conclude that NCCb proteins are not sensitive to thiazides and have electrogenic properties dependent on Cl^-, and site I172 is important for the function of NCCβ.

Regulatory control of the Na-Cl co-transporter NCC and its therapeutic potential for hypertension

Hypertension is the largest risk factor for cardiovascular disease, the leading cause of mortality worldwide. As blood pressure regulation is influenced by multiple physiological systems, hypertension cannot be attributed to a single identifiable etiology. Three decades of research into Mendelian forms of hypertension implicated alterations in the renal tubular sodium handling, particularly the distal convoluted tubule (DCT)-native, thiazide-sensitive Na-Cl cotransporter (NCC). Altered functions of the NCC have shown to have profound effects on blood pressure regulation as illustrated by the over activation and inactivation of the NCC in Gordon's and Gitelman syndromes respectively. Substantial progress has uncovered multiple factors that affect the expression and activity of the NCC. In particular, NCC activity is controlled by phosphorylation/dephosphorylation, and NCC expression is facilitated by glycosylation and negatively regulated by ubiquitination. Studies have even found parvalbumin to be an unexpected regulator of the NCC. In recent years, there have been considerable advances in our understanding of NCC control mechanisms, particularly via the pathway containing the with-no-lysine [K] (WNK) and its downstream target kinases, SPS/Ste20-related proline-alanine-rich kinase (SPAK) and oxidative stress responsive 1 (OSR1), which has led to the discovery of novel inhibitory molecules. This review summarizes the currently reported regulatory mechanisms of the NCC and discusses their potential as therapeutic targets for treating hypertension.

Activated WNK3 induced by intracerebral hemorrhage deteriorates brain injury maybe via WNK3/SPAK/NKCC1 pathway

BACKGROUND

Intracerebral hemorrhage (ICH) is the common brain diseases in middle-aged and elderly people, with high disability and/or mortality rate, and is a serious public health concern. Both WNK3 kinase and the WNK3/SPAK/NKCC1 signaling pathway play an integral role in maintaining normal cell homeostasis. However, their role and underlying mechanisms in ICH-induced secondary brain injury (SBI) have yet to be elucidated.

METHODS

We established an ICH model using male Sprague-Dawley (SD) rats by injecting autologous arterial blood into the unilateral basal ganglia. To establish ICH model in vitro, oxyhemoglobin (OxyHb; 20 μM) and neurons were cultured for 6 h at 37 °C, 5% CO₂ atmosphere. To investigate the role of WNK3 and the WNK3/SPAK/NKCC1 signaling pathway in SBI, after genetic interventions, rotation and water maze test, brain edema and neuroinflammation were detected, and terminal-deoxynucleoitidyl transferase mediated dUTP nick end labeling (TUNEL), Fluoro-Jade C (FJC), and Nissl staining were performed.

RESULTS

Our data showed that WNK3 expression in brain tissue were upregulated after ICH induction. In addition, silencing of WNK3 reduced neuronal apoptosis, and inflammatory responses in rats that underwent ICH. Inhibition of WNK3 expression reduced the damaged blood-brain barrier (BBB), alleviated the impaired degree of cerebral edema, and improved disruptive neurobehavioral cognition caused by ICH. Moreover, overexpression of WNK3 had the opposite effects. Finally, WNK3/SPAK/NKCC1 signaling pathway may be involved in the above-mentioned processes.

CONCLUSIONS

In conclusion, our findings showed that WNK3 and WNK3/SPAK/NKCC1 signaling pathway play a vital biological function in ICH-induced SBI. Depletion of WNK3 attenuated brain injury after ICH both in vivo and in vitro. Thus, WNK3 and WNK3/SPAK/NKCC1 signaling pathway are potential targets for treating SBI after ICH.

Electrolyte transport in the renal collecting duct and its regulation by the renin-angiotensin-aldosterone system

Distal nephron of the kidney plays key roles in fluid volume and electrolyte homeostasis by tightly regulating reabsorption and excretion of Na⁺, K⁺, and Cl^- Studies to date demonstrate the detailed electrolyte transport mechanisms in principal cells of the cortical collecting duct, and their regulation by renin-angiotensin-aldosterone system (RAAS). In recent years, however, accumulating data indicate that intercalated cells, another cell type that is present in the cortical collecting duct, also play active roles in the regulation of blood pressure. Notably, pendrin in β-intercalated cells not only controls acid/base homeostasis, but is also one of the key components controlling salt and K⁺ transport in distal nephron. We have recently shown that pendrin is regulated by the co-ordinated action of angiotensin II (AngII) and aldosterone, and at the downstream of AngII, mammalian target of rapamycin (mTOR) signaling regulates pendrin through inhibiting the kinase unc51-like-kinase 1 and promoting dephosphorylation of mineralocorticoid receptor (MR). In this review, we summarize recent advances in the current knowledge on the salt transport mechanisms in the cortical collecting duct, and their regulation by the RAAS.

The thiazide sensitive sodium chloride co-transporter NCC is modulated by site-specific ubiquitylation

The renal sodium chloride cotransporter, NCC, in the distal convoluted tubule is important for maintaining body Na⁺ and K⁺ homeostasis. Endogenous NCC is highly ubiquitylated, but the role of individual ubiquitylation sites is not established. Here, we assessed the role of 10 ubiquitylation sites for NCC function. Transient transfections of HEK293 cells with human wildtype (WT) NCC or various K to R mutants identified greater membrane abundance for K706R, K828R and K909R mutants. Relative to WT-NCC, stable tetracycline inducible MDCKI cell lines expressing K706R, K828R and K909R mutants had significantly higher total and phosphorylated NCC levels at the apical plasma membrane under basal conditions. Low chloride stimulation increased membrane abundance of all mutants to similar or greater levels than WT-NCC. Under basal conditions K828R and K909R mutants had less ubiquitylated NCC in the plasma membrane, and all mutants displayed reduced NCC ubiquitylation following low chloride stimulation. Thiazide-sensitive sodium-22 uptakes were elevated in the mutants and internalization from the plasma membrane was significantly less than WT-NCC. K909R had increased half-life, whereas chloroquine or MG132 treatment indicated that K706 and K909 play roles in lysosomal and proteasomal NCC degradation, respectively. In conclusion, site-specific ubiquitylation of NCC plays alternative roles for NCC function.

The Ubiquitin Ligase Nedd4L Regulates the Na/K/2Cl Co-transporter NKCC1/SLC12A2 in the Colon

The ubiquitin ligase Nedd4-like (Nedd4L, or Nedd4-2) binds to and regulates stability of the epithelial Na⁺ channel (ENaC) in salt-absorbing epithelia in the kidney, lung, and other tissues. Its role in the distal colon, which also absorbs salt and fluid and expresses ENaC, is unknown. Using a conditional knock-out approach to knock out Nedd4L in mice intestinal epithelium (Nedd4L^f/f ;Vil-Cre^ERT2 ) we show here that Nedd4L depletion leads to a higher steady-state short circuit current (Isc) in mouse distal colon tissue relative to controls. This higher Isc was partially reduced by the addition of apical amiloride and strongly reduced by basolateral bumetanide as well as by depletion of basolateral Cl^-, suggesting that Na⁺/K⁺/2Cl^- (NKCC1/SLC12A2) co-transporter and ENaC are targets of Nedd4L in the colon. In accordance, NKCC1 (and γENaC) protein abundance in the colon of the Nedd4L knock-out animals was increased, indicating that Nedd4L normally suppresses these proteins. However, we did not observe co-immunoprecipitation between Nedd4L and NKCC1, suggesting that Nedd4L indirectly suppresses NKCC1 expression. Low salt diet resulted in a strong increase in β and γ (but not α) ENaC mRNA and protein expression and ENaC activity. Although salt restriction also increased NKCC1 protein and mRNA abundance, it did not lead to its elevated activity (Isc). These results identify NKCC1 as a novel target for Nedd4L-mediated down-regulation in vivo, which modulates ion and fluid transport in the distal colon together with ENaC.

The European Eel NCCβ Gene Encodes a Thiazide-resistant Na-Cl Cotransporter

The thiazide-sensitive Na-Cl cotransporter (NCC) is the major pathway for salt reabsorption in the mammalian distal convoluted tubule. NCC plays a key role in the regulation of blood pressure. Its inhibition with thiazides constitutes the primary baseline therapy for arterial hypertension. However, the thiazide-binding site in NCC is unknown. Mammals have only one gene encoding for NCC. The eel, however, contains a duplicate gene. NCCα is an ortholog of mammalian NCC and is expressed in the kidney. NCCβ is present in the apical membrane of the rectum. Here we cloned and functionally characterized NCCβ from the European eel. The cRNA encodes a 1043-amino acid membrane protein that, when expressed in Xenopus oocytes, functions as an Na-Cl cotransporter with two major characteristics, making it different from other known NCCs. First, eel NCCβ is resistant to thiazides. Single-point mutagenesis supports that the absence of thiazide inhibition is, at least in part, due to the substitution of a conserved serine for a cysteine at position 379. Second, NCCβ is not activated by low-chloride hypotonic stress, although the unique Ste20-related proline alanine-rich kinase (SPAK) binding site in the amino-terminal domain is conserved. Thus, NCCβ exhibits significant functional differences from NCCs that could be helpful in defining several aspects of the structure-function relationship of this important cotransporter.

WNK3 Kinase Enhances the Sodium Chloride Cotransporter Expression via an ERK 1/2 Signaling Pathway

BACKGROUND

WNK kinase is a serine/threonine kinase that plays an important role in normal blood pressure homeostasis. WNK3 was previously found to enhance the activity of sodium chloride cotransporter (NCC) in Xenopus oocyte. However, the mechanism through which it works remains unclear.

METHODS

Using overexpression and siRNA knock-down techniques, the effects of WNK3 on NCC in both Cos-7 and mouse distal convoluted cells were analyzed by Western blot.

RESULTS

We found that WNK3 significantly increased NCC protein expression in a dose-dependent manner. NCC protein expression in Cos-7 cells was markedly decreased after 2 h treatment with protease inhibitor, cycloheximide (CHX) in the NCC alone group, but was significantly decreased after 8 h treatment of CHX in the WNK3 + NCC group. WNK3 significantly increased NCC protein expression in both NCC alone and WNK3 + NCC groups regardless the overnight treatments of bafilomycin A1, a proton pump inhibitor, suggesting that WNK3-mediated increased NCC expression is not dependent on the lysosomal pathway. We further found that WNK3 group had a quicker NCC recovery than the control group using CHX pulse assay, suggesting that WNK3 increases NCC protein synthesis. WNK3 enhanced NCC protein level while reducing ERK 1/2 phosphorylation. In addition, knock-down of ERK 1/2 expression reversed WNK3-mediated increase of NCC expression.

CONCLUSION

These results suggest that WNK3 enhances NCC protein expression by increasing NCC synthesis via an ERK 1/2-dependent signaling pathway.

Hypertension: the missing WNKs

The With no Lysine [K] (WNK) family of enzymes are central in the regulation of blood pressure. WNKs have been implicated in hereditary hypertension disorders, mainly through control of the activity and levels of ion cotransporters and channels. Actions of WNKs in the kidney have been heavily investigated, and recent studies have provided insight into not only the regulation of these enzymes but also how mutations in WNKs and their interacting partners contribute to hypertensive disorders. Defining the roles of WNKs in the cardiovascular system will provide clues about additional mechanisms by which WNKs can regulate blood pressure. This review summarizes recent developments in the regulation of the WNK signaling cascade and its role in regulation of blood pressure.

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.

NEDD4-2 and salt-sensitive hypertension

PURPOSE OF REVIEW

NEDD4-2 is an ubiquitin-protein ligase that was originally identified as an interactor of the epithelial Na+ channel (ENaC); this interaction is defective in Liddle's syndrome, causing elevated ENaC activity and salt-sensitive hypertension. In this review we aim to highlight progress achieved in recent years demonstrating that NEDD4-2 is involved in the control of Na+ transporters that are different from ENaC, but which also play a role in salt-sensitive hypertension.

RECENT FINDINGS

It has been shown that NEDD4-2 interacts with ubiquitylates and negatively regulates the thiazide-sensitive NCC (Na+,Cl- -cotransporter), both in vitro and in vivo in inducible, nephron-specific Nedd4-2 knockout mice. Moreover, evidence has been provided that NEDD4-2 is also involved in the regulation of human NHE3 (Na+,H+-exchanger 3) and NKCC2 (Na+,K+,2Cl- -cotransporter 2).

SUMMARY

The emerging role of NEDD4-2 in the regulation of different Na+ transporters along the nephron and the identification of human polymorphisms in the NEDD4-2 gene (Nedd4L) related to salt-sensitive hypertension makes this ubiquitin-protein ligase an interesting target for the development of antihypertensive drugs.

Alternatively spliced proline-rich cassettes link WNK1 to aldosterone action

The thiazide-sensitive NaCl cotransporter (NCC) is important for renal salt handling and blood-pressure homeostasis. The canonical NCC-activating pathway consists of With-No-Lysine (WNK) kinases and their downstream effector kinases SPAK and OSR1, which phosphorylate NCC directly. The upstream mechanisms that connect physiological stimuli to this system remain obscure. Here, we have shown that aldosterone activates SPAK/OSR1 via WNK1. We identified 2 alternatively spliced exons embedded within a proline-rich region of WNK1 that contain PY motifs, which bind the E3 ubiquitin ligase NEDD4-2. PY motif-containing WNK1 isoforms were expressed in human kidney, and these isoforms were efficiently degraded by the ubiquitin proteasome system, an effect reversed by the aldosterone-induced kinase SGK1. In gene-edited cells, WNK1 deficiency negated regulatory effects of NEDD4-2 and SGK1 on NCC, suggesting that WNK1 mediates aldosterone-dependent activity of the WNK/SPAK/OSR1 pathway. Aldosterone infusion increased proline-rich WNK1 isoform abundance in WT mice but did not alter WNK1 abundance in hypertensive Nedd4-2 KO mice, which exhibit high baseline WNK1 and SPAK/OSR1 activity toward NCC. Conversely, hypotensive Sgk1 KO mice exhibited low WNK1 expression and activity. Together, our findings indicate that the proline-rich exons are modular cassettes that convert WNK1 into a NEDD4-2 substrate, thereby linking aldosterone and other NEDD4-2-suppressing antinatriuretic hormones to NCC phosphorylation status.

Renal NCC is unchanged in the midpregnant rat and decreased in the late pregnant rat despite avid renal Na+ retention

Pregnancy is characterized by plasma volume expansion due to Na(+) retention, driven by aldosterone. The aldosterone-responsive epithelial Na(+) channel is activated in the kidney in pregnancy. In the present study, we investigated the aldosterone-responsive Na(+)-Cl(-) cotransporter (NCC) in mid- and late pregnant rats compared with virgin rats. We determined the abundance of total NCC, phosphorylated NCC (pNCC; pT53, pS71 and pS89), phosphorylated STE20/SPS-1-related proline-alanine-rich protein kinase (pSPAK; pS373), and phosphorylated oxidative stress-related kinase (pOSR1; pS325) in the kidney cortex. We also measured mRNA expression of NCC and members of the SPAK/NCC regulatory kinase network, serum and glucocorticoid-regulated kinase (SGK)1, total with no lysine kinase (WNK)1, WNK3, and WNK4. Additionally, we performed immunohistochemistry for NCC kidneys from virgin and pregnant rats. Total NCC, pNCC, and pSPAK/OSR1 abundance were unchanged in midpregnant versus virgin rats. In late pregnant versus virgin rats, total NCC and pNCC were decreased; however, pSPAK/OSR1 was unchanged. We detected no differences in mRNA expression of NCC, SGK1, total WNK1, WNK3, and WNK4. By immunohistochemistry, NCC was mainly localized to the apical region in virgin rats, and density in the apical region was reduced in late pregnancy. Therefore, despite high circulating aldosterone levels in pregnancy, the aldosterone-responsive transporter NCC is not increased in total or activated (phosphorylated) abundance or in apical localization in midpregnant rats, and all are reduced in late pregnancy. This contrasts to the mineralocorticoid-mediated activation of the epithelial Na(+) channel, which we have previously reported. Why and how NCC escapes aldosterone activation in pregnancy is not clear but may relate to regional differences in aldosterone sensitivity the increased K(+) intake or other undefined mechanisms.

Revisiting the NaCl cotransporter regulation by with-no-lysine kinases

The renal thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) is the salt transporter in the distal convoluted tubule. Its activity is fundamental for defining blood pressure levels. Decreased NCC activity is associated with salt-remediable arterial hypotension with hypokalemia (Gitelman disease), while increased activity results in salt-sensitive arterial hypertension with hyperkalemia (pseudohypoaldosteronism type II; PHAII). The discovery of four different genes causing PHAII revealed a complex multiprotein system that regulates the activity of NCC. Two genes encode for with-no-lysine (K) kinases WNK1 and WNK4, while two encode for kelch-like 3 (KLHL3) and cullin 3 (CUL3) proteins that form a RING type E3 ubiquitin ligase complex. Extensive research has shown that WNK1 and WNK4 are the targets for the KLHL3-CUL3 complex and that WNKs modulate the activity of NCC by means of intermediary Ste20-type kinases known as SPAK or OSR1. The understanding of the effect of WNKs on NCC is a complex issue, but recent evidence discussed in this review suggests that we could be reaching the end of the dark ages regarding this matter.

NEDD4-2 (NEDD4L): the ubiquitin ligase for multiple membrane proteins

NEDD4-2 (also known as NEDD4L, neural precursor cell expressed developmentally down-regulated 4-like) is a ubiquitin protein ligase of the Nedd4 family which is known to bind and regulate a number of membrane proteins to aid in their internalization and turnover. Several of the NEDD4-2 substrates include ion channels, such as the epithelial and voltage-gated sodium channels. Given the critical function of NEDD4-2 in regulating membrane proteins, this ligase is essential for the maintenance of cellular homeostasis. In this article we review the biology and function of this important ubiquitin-protein ligase and discuss its pathophysiological significance.