All three WW domains of murine Nedd4 are involved in the regulation of epithelial sodium channels by intracellular Na+

Potential roles of NEDD4 and NEDD4L and their utility as therapeutic targets in high‑incidence adult male cancers (Review)

The term 'cancer' refers to >100 disorders that progressively manifest over time and are characterized by uncontrolled cell division. Although malignant growth can occur in virtually any human tissue, the underlying mechanisms underlying all forms of cancer are consistent. The International Agency for Research on Cancer's annual GLOBOCAN 2020 report provided an update on the global cancer incidence and mortality. Excluding non-melanoma skin cancer, the report predicts that there will be 19.3 million new cancer cases and >10 million cancer-related fatalities in 2023. Lung, prostate, and colon cancers are the most prevalent and lethal cancers in males. It was recognized that post-translational modifications (PTMs) of proteins are necessary for almost all cellular biological processes, as well as in cancer development and metastasis to other bodily organs. Thus, PTMs have a considerable impact on how proteins behave. Various PTMs may have harmful roles by affecting the hallmarks of cancer, metabolism and the regulation of the tumor microenvironment. PTMs and genetic changes/mutations are essential in carcinogenesis and cancer development. A pivotal PTM mechanism is protein ubiquitination. Of note, the rate-limiting stage of the protein ubiquitination cascade is hypothesized to be E3-ligase-mediated ubiquitination. Numerous studies revealed that the neural precursor cell expressed developmentally downregulated protein 4 (NEDD4) E3 ligase is among the E3 ubiquitin ligases that have essential roles in cellular processes. It regulates protein degradation and substrate ubiquitination. In addition, it has been shown that NEDD4 primarily functions as an oncogene in various malignancies but can also act as a tumor suppressor in certain types of tumor. In the present review, the roles of NEDD4 as an anticancer protein in various high-incidence male malignancies and the significance of NEDD4 as a potential cancer therapeutic target are discussed. In addition, the targeting of NEDD4 as a therapeutic strategy for the treatment of human malignancies is explored.

Nedd4-2 binding to 14-3-3 modulates the accessibility of its catalytic site and WW domains

Neural precursor cells expressed developmentally downregulated protein 4-2 (Nedd4-2), a homologous to the E6-AP carboxyl terminus (HECT) ubiquitin ligase, triggers the endocytosis and degradation of its downstream target molecules by regulating signal transduction through interactions with other targets, including 14-3-3 proteins. In our previous study, we found that 14-3-3 binding induces a structural rearrangement of Nedd4-2 by inhibiting interactions between its structured domains. Here, we used time-resolved fluorescence intensity and anisotropy decay measurements, together with fluorescence quenching and mass spectrometry, to further characterize interactions between Nedd4-2 and 14-3-3 proteins. The results showed that 14-3-3 binding affects the emission properties of AEDANS-labeled WW3, WW4, and, to a lesser extent, WW2 domains, and reduces their mobility, but not those of the WW1 domain, which remains mobile. In contrast, 14-3-3 binding has the opposite effect on the active site of the HECT domain, which is more solvent exposed and mobile in the complexed form than in the apo form of Nedd4-2. Overall, our results suggest that steric hindrance of the WW3 and WW4 domains combined with conformational changes in the catalytic domain may account for the 14-3-3 binding-mediated regulation of Nedd4-2.

14-3-3-protein regulates Nedd4-2 by modulating interactions between HECT and WW domains

Neural precursor cell expressed developmentally down-regulated 4 ligase (Nedd4-2) is an E3 ubiquitin ligase that targets proteins for ubiquitination and endocytosis, thereby regulating numerous ion channels, membrane receptors and tumor suppressors. Nedd4-2 activity is regulated by autoinhibition, calcium binding, oxidative stress, substrate binding, phosphorylation and 14-3-3 protein binding. However, the structural basis of 14-3-3-mediated Nedd4-2 regulation remains poorly understood. Here, we combined several techniques of integrative structural biology to characterize Nedd4-2 and its complex with 14-3-3. We demonstrate that phosphorylated Ser³⁴² and Ser⁴⁴⁸ are the key residues that facilitate 14-3-3 protein binding to Nedd4-2 and that 14-3-3 protein binding induces a structural rearrangement of Nedd4-2 by inhibiting interactions between its structured domains. Overall, our findings provide the structural glimpse into the 14-3-3-mediated Nedd4-2 regulation and highlight the potential of the Nedd4-2:14-3-3 complex as a pharmacological target for Nedd4-2-associated diseases such as hypertension, epilepsy, kidney disease and cancer.

Pohl et al. investigated the structural basis of Nedd4-2 regulation by 14-3-3 and found that phosphorylated Ser342 and Ser448 are the main residues that facilitate 14-3-3 binding to Nedd4-2. The authors propose that the Nedd4-2:14-3-3 complex then stimulates a structural rearrangement of Nedd4-2 through inhibiting interaction of its structured domains.

E3 Ubiquitin ligase NEDD4 family‑regulatory network in cardiovascular disease

Protein ubiquitination represents a critical modification occurring after translation. E3 ligase catalyzes the covalent binding of ubiquitin to the protein substrate, which could be degraded. Ubiquitination as an important protein post-translational modification is closely related to cardiovascular disease. The NEDD4 family, belonging to HECT class of E3 ubiquitin ligases can recognize different substrate proteins, including PTEN, ENaC, Nav1.5, SMAD2, PARP1, Septin4, ALK1, SERCA2a, TGFβR3 and so on, via the WW domain to catalyze ubiquitination, thus participating in multiple cardiovascular-related disease such as hypertension, arrhythmia, myocardial infarction, heart failure, cardiotoxicity, cardiac hypertrophy, myocardial fibrosis, cardiac remodeling, atherosclerosis, pulmonary hypertension and heart valve disease. However, there is currently no review comprehensively clarifying the important role of NEDD4 family proteins in the cardiovascular system. Therefore, the present review summarized recent studies about NEDD4 family members in cardiovascular disease, providing novel insights into the prevention and treatment of cardiovascular disease. In addition, assessing transgenic animals and performing gene silencing would further identify the ubiquitination targets of NEDD4. NEDD4 quantification in clinical samples would also constitute an important method for determining NEDD4 significance in cardiovascular disease.

NEDD4 E3 ligase: Functions and mechanism in human cancer

A growing amount of evidence indicates that the neuronally expressed developmentally downregulated 4 (NEDD4, also known as NEDD4-1) E3 ligase plays a critical role in a variety of cellular processes via the ubiquitination-mediated degradation of multiple substrates. The abnormal regulation of NEDD4 protein has been implicated in cancer development and progression. In this review article, we briefly delineate the downstream substrates and upstream regulators of NEDD4, which are involved in carcinogenesis. Moreover, we succinctly elucidate the functions of NEDD4 protein in tumorigenesis and progression, including cell proliferation, apoptosis, cell cycle, migration, invasion, epithelial mesenchymal transition (EMT), cancer stem cells, and drug resistance. The findings regarding NEDD4 functions are further supported by knockout mouse models and human tumor tissue studies. This review could provide a promising and optimum anticancer therapeutic strategy via targeting the NEDD4 protein.

NEDD4L limits cAMP signaling through ubiquitination of CREB-regulated transcription coactivator 3

The transcription factor cAMP-responsive element-binding protein (CREB) is involved in a variety of physiologic processes. Although its activity appears to be largely correlated with its phosphorylation status, cAMP-mediated dephosphorylation and the subsequent nuclear migration of the CREB-regulated transcription factors (CRTCs) are required to stimulate CREB transcriptional activity. Among the 3 identified mammalian homologs of CRTCs, CRTC3 has been shown to be expressed predominantly in adipose tissues in response to catecholamine signals that regulate lipid metabolism. Here, we show that prolonged cAMP signaling down-regulates CRTC3 in a proteasome-dependent manner and that neural precursor cell-expressed developmentally down-regulated gene 4-like (NEDD4L), a specific ubiquitin ligase for CRTC3, is responsible for this process. By recognizing the PY motif of CRTC3, NEDD4L interacts with CRTC3 and promotes its polyubiquitination. Interaction between NEDD4L and CRTC3 is further boosted by cAMP signaling, and this enhanced interaction appears to be dependent on the cAMP-mediated phosphorylation of NEDD4L at the Ser448 site. Furthermore, we show that food withdrawal stimulates NEDD4L phosphorylation in mice, which then show a decrease of adipose tissue CRTC3 protein levels. Together, these results suggest that NEDD4L plays a key role in the feedback regulation of cAMP signaling by limiting CRTC3 protein levels.-Kim, Y.-H., Yoo, H., Hong, A.-R., Kwon, M., Kang, S.-W., Kim, K., Song, Y. NEDD4L limits cAMP signaling through ubiquitination of CREB-regulated transcription coactivator 3.

PKC/Nedd4-2 Signaling Pathway Regulates the Cell Surface Expression of Drug Transporter hOAT1

Human organic anion transporter-1 (hOAT1) regulates the absorption, distribution, and excretion of a wide range of clinically important drugs. Our previous work demonstrated that hOAT1 is a dynamic membrane transporter, constitutively internalizing from and recycling back to the cell plasma membrane. Short-term activation (<30 minutes) of protein kinase C (PKC) promotes the attachment of a lysine 48-linked polyubiquitin chain to hOAT1, a process catalyzed by ubiquitin ligase neural precursor cell expressed developmentally down-regulated 4-2 (Nedd4-2). The ubiquitination of hOAT1 then triggers an accelerated endocytosis of the transporter from plasma membrane, which results in reduced hOAT1 expression at the cell surface and decreased hOAT1 transport activity. In the present study, we investigated the long-term effect of PKC on hOAT1. We showed that long-term activation (>2 hours) of PKC significantly enhanced hOAT1 degradation, and such action was partially blocked by ubiquitin mutant Ub-K48R, which has its lysine (K) 48 mutated to arginine (R) and is incapable of forming a K48-linked polyubiquitin chain. The ubiquitin ligase Nedd4-2 was also found to augment hOAT1 degradation. These results suggest that PKC-regulated and Nedd4-2-catalyzed attachment of a lysine 48-linked polyubiquitin chain to hOAT1 is important for hOAT1 stability. We further showed through coimmunoprecipitation experiments that there was a direct association between hOAT1 and Nedd4-2, and such interaction was weakened when the WW3 and WW4 domains of the ligase were mutated. Mutating WW3 and WW4 domains of the ligase also impaired its ability to ubiquitinate hOAT1. Therefore, WW3 and WW4 domains of Nedd4-2 are critical for its association with and modulation of the transporter.

NEDD4 is involved in acquisition of epithelial-mesenchymal transition in cisplatin-resistant nasopharyngeal carcinoma cells

Nasopharyngeal carcinoma (NPC) is a highly invasive head-neck cancer derived from the nasopharyngeal epithelium, mainly prevalent in southern China and Southeast Asia. Radiotherapy and adjuvant cisplatin (DDP) chemotherapy are standard administrations applied in the treatment of NPC. However, resistance to chemotherapeutic drugs has recently become more common, resulting in worse treatment outcome for NPC therapy. To elucidate the underlying molecular basis of drug resistance to DDP in NPC cells, we examined the morphocytology, cell motility and molecular changes in DDP-resistant NPC cells with respect to epithelial-mesenchymal transition (EMT) features. We found that EMT is closely associated with DDP-induced drug resistance in NPC cells, as DDP-resistant cells displayed morphological and molecular markers changes consistent with EMT. Wound healing and Transwell Boyden chamber assays revealed an enhanced migration and invasion potential in DDP-resistant NPC cells. Mechanistically, upregulation of NEDD4 was observed to relate to EMT in DDP-resistant cells. More importantly, depletion of NEDD4 in resistant cells led to a partial reversion of EMT phenotypes to MET characteristics. These data suggest that NEDD4 is largely involved in EMT features and chemoresistance of NPC cancer cells. NEDD4 could be a novel therapeutic target to overcome drug resistance in successful administrations of NPC.

Fine-tuning of ULK1 mRNA and protein levels is required for autophagy oscillation

ULK1 is a key kinase in autophagy initiation. Nazio et al. demonstrate that the E3 ubiquitin ligase NEDD4L targets ULK1 for degradation soon after autophagy induction, whereas a simultaneous ULK1 mRNA transcription is needed for priming subsequent rounds of autophagy.

Autophagy is an intracellular degradation pathway whose levels are tightly controlled to secure cell homeostasis. Unc-51–like kinase 1 (ULK1) is a conserved serine–threonine kinase that plays a central role in the initiation of autophagy. Here, we report that upon autophagy progression, ULK1 protein levels are specifically down-regulated by the E3 ligase NEDD4L, which ubiquitylates ULK1 for degradation by the proteasome. However, whereas ULK1 protein is degraded, ULK1 mRNA is actively transcribed. Upon reactivation of mTOR-dependent protein synthesis, basal levels of ULK1 are promptly restored, but the activity of newly synthesized ULK1 is inhibited by mTOR. This prepares the cell for a new possible round of autophagy stimulation. Our results thus place NEDD4L and ULK1 in a key position to control oscillatory activation of autophagy during prolonged stress to keep the levels of this process under a safe and physiological threshold.

An isoform of Nedd4-2 is critically involved in the renal adaptation to high salt intake in mice

Epithelial sodium channels (ENaCs) play critical roles in the maintenance of fluid and electrolyte homeostasis, and their genetic abnormalities cause one type of hereditary salt-sensitive hypertension, Liddle syndrome. As we reported previously, both human and rodent Nedd4L/Nedd4-2 showed molecular diversity, with and without a C2 domain in their N-terminal. Nedd4L/Nedd4-2 isoforms with a C2 domain are hypothesized to be related closely to ubiquitination of ENaCs. We generated Nedd4-2 C2 domain knockout mice. We demonstrate here that loss of Nedd4-2 C2 isoform causes salt-sensitive hypertension under conditions of a high dietary salt intake in vivo. The knockout mice had reduced urinary sodium excretion, osmotic pressure and increased water intake and urine volume with marked dilatation of cortical tubules while receiving a high salt diet. To the contrary, there was no difference in metabolic data between wild-type and knockout mice receiving a normal control diet. In the absence of Nedd4-2 C2 domain, a high salt intake accelerated ENaC expression. Coimmunoprecipitation studies revealed suppressed ubiquitination for ENaC with a high salt intake. Taken together, our findings demonstrate that during a high oral salt intake the Nedd4-2 C2 protein plays a pivotal role in maintaining adaptive salt handling in the kidney.

The role of Nedd4-1 WW domains in binding and regulating human organic anion transporter 1

Human organic anion transporter 1 (hOAT1), expressed at the basolateral membrane of kidney proximal tubule cells, mediates the active renal secretion of a diverse array of clinically important drugs, including anti-human immunodeficiency virus therapeutics, antitumor drugs, antibiotics, antihypertensives, and anti-inflammatories. We have previously demonstrated that posttranslational modification of hOAT1 by ubiquitination is an important mechanism for the regulation of this transporter. The present study aimed at identifying the ubiquitin ligase for hOAT1 and its mechanism of action. We showed that overexpression of neural precursor cell expressed, developmentally downregulated (Nedd)4-1, an E3 ubiquitin ligase, enhanced hOAT1 ubiquitination, decreased hOAT1 expression at the cell surface, and inhibited hOAT1 transport activity. In contrast, overexpression of the ubiquitin ligase-dead mutant Nedd4-1/C867S was without effects on hOAT1. Furthermore, knockdown of endogenously expressed Nedd4-1 by Nedd4-1-specific small interfering RNA reduced hOAT1 ubiquitination. Immunoprecipitation experiments in cultured cells and rat kidney slices and immunofluorescence experiments in rat kidney slices showed that there was a physical interaction between OAT1 and Nedd4-1. Nedd4-1 contains four protein-protein interacting WW domains. When these WW domains were inactivated by mutating two amino acid residues in each of the four WW domains (Mut-WW1: V210W/H212G, Mut-WW2: V367W/H369G, Mut-WW3: I440W/H442G, and Mut-WW4: I492W/H494G, respectively), only Mut-WW2 and Mut-WW3 significantly lost their ability to bind and to ubiquitinate hOAT1. As a result, Mut-WW2 and Mut-WW3 were unable to suppress hOAT1-mediated transport as effectively as wild-type Nedd4-1. In conclusion, this is the first demonstration that Nedd4-1 regulates hOAT1 ubiquitination, expression, and transport activity through its WW2 and WW3 domains.

Feedback inhibition of ENaC: acute and chronic mechanisms

Intracellular [Na(+)] ([Na(+)]i) modulates the activity of the epithelial Na channel (ENaC) to help prevent cell swelling and regulate epithelial Na(+) transport, but the underlying mechanisms remain unclear. We show here that short-term (60-80 min) incubation of ENaC-expressing oocytes in high Na(+) results in a 75% decrease in channel activity. When the β subunit was truncated, corresponding to a gain-of-function mutation found in Liddle's syndrome, the same maneuver reduced activity by 45% despite a larger increase in [Na(+)]i. In both cases the inhibition occurred with little to no change in cell-surface expression of γENaC. Long-term incubation (18 hours) in high Na(+) reduced activity by 92% and 75% in wild-type channels and Liddle's mutant, respectively, with concomitant 70% and 52% decreases in cell-surface γENaC. In the presence of Brefeldin A to inhibit forward protein trafficking, high-Na(+) incubation decreased wt ENaC activity by 52% and 88% after 4 and 8 hour incubations, respectively. Cleaved γENaC at the cell surface had lifetimes at the surface of 6 hrs in low Na(+) and 4 hrs in high Na(+), suggesting that [Na(+)]i increased the rate of retrieval of cleaved γ ENaC by 50%. This implies that enhanced retrieval of ENaC channels at the cell surface accounts for part, but not all, of the downregulation of ENaC activity shown with chronic increases in [Na(+)]i.

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.

Ubiquitylation of voltage-gated sodium channels

Ion channel proteins are regulated by different types of posttranslational modifications. The focus of this review is the regulation of voltage-gated sodium channels (Navs) upon their ubiquitylation. The amiloride-sensitive epithelial sodium channel (ENaC) was the first ion channel shown to be regulated upon ubiquitylation. This modification results from the binding of ubiquitin ligase from the Nedd4 family to a protein-protein interaction domain, known as the PY motif, in the ENaC subunits. Many of the Navs have similar PY motifs, which have been demonstrated to be targets of Nedd4-dependent ubiquitylation, tagging them for internalization from the cell surface. The role of Nedd4-dependent regulation of the Nav membrane density in physiology and disease remains poorly understood. Two recent studies have provided evidence that Nedd4-2 is downregulated in dorsal root ganglion (DRG) neurons in both rat and mouse models of nerve injury-induced neuropathic pain. Using two different mouse models, one with a specific knockout of Nedd4-2 in sensory neurons and another where Nedd4-2 was overexpressed with the use of viral vectors, it was demonstrated that the neuropathy-linked neuronal hyperexcitability was the result of Nav1.7 and Nav1.8 overexpression due to Nedd4-2 downregulation. These studies provided the first in vivo evidence of the role of Nedd4-2-dependent regulation of Nav channels in a disease state. This ubiquitylation pathway may be involved in the development of symptoms and diseases linked to Nav-dependent hyperexcitability, such as pain, cardiac arrhythmias, epilepsy, migraine, and myotonias.

The role of ENaC in vascular endothelium

Once upon a time, the expression of the epithelial sodium channel (ENaC) was mainly assigned to the kidneys, colon and sweat glands where it was considered to be the main determinant of sodium homeostasis. Recent, though indirect, evidence for the possible existence of ENaC in a non-epithelial tissue was derived from the observation that the vascular endothelium is a target for aldosterone. Inhibitory actions of the intracellular aldosterone receptors by spironolactone and, more directly, by ENaC blockers such as amiloride supported this view. Shortly after, direct data on the expression of ENaC in vascular endothelium could be demonstrated. There, endothelial ENaC (EnNaC) could be defined as a major regulator of cellular mechanics which is a critical parameter in differentiating between vascular function and dysfunction. Foremost, the mechanical stiffness of the endothelial cell cortex, a layer 50-200 nm beneath the plasma membrane, has been shown to play a crucial role as it controls the production of the endothelium-derived vasodilator nitric oxide (NO) which directly affects the tone of the vascular smooth muscle cells. In contrast to soft endothelial cells, stiff endothelial cells release reduced amounts of NO, the hallmark of endothelial dysfunction. Thus, the combination of endothelial stiffness and myogenic tone might increase the peripheral vascular resistance. An elevation of arterial blood pressure is supposed to be the consequence of such functional changes. In this review, EnNaC is discussed as an aldosterone-regulated plasma membrane protein of the vascular endothelium that could significantly contribute to maintaining of an appropriate arterial blood pressure but, if overexpressed, could participate in the pathogenesis of arterial hypertension.

Neuronal expression of the ubiquitin ligase Nedd4-2 in rat dorsal root ganglia: modulation in the spared nerve injury model of neuropathic pain

Neuronal hyperexcitability following peripheral nerve lesions may stem from altered activity of voltage-gated sodium channels (VGSCs), which gives rise to allodynia or hyperalgesia. In vitro, the ubiquitin ligase Nedd4-2 is a negative regulator of VGSC α-subunits (Na(v)), in particular Na(v)1.7, a key actor in nociceptor excitability. We therefore studied Nedd4-2 in rat nociceptors, its co-expression with Na(v)1.7 and Na(v)1.8, and its regulation in pathology. Adult rats were submitted to the spared nerve injury (SNI) model of neuropathic pain or injected with complete Freund's adjuvant (CFA), a model of inflammatory pain. L4 dorsal root ganglia (DRG) were analyzed in sham-operated animals, seven days after SNI and 48 h after CFA with immunofluorescence and Western blot. We observed Nedd4-2 expression in almost 50% of DRG neurons, mostly small and medium-sized. A preponderant localization is found in the non-peptidergic sub-population. Additionally, 55.7 ± 2.7% and 55.0 ± 3.6% of Nedd4-2-positive cells are co-labeled with Na(v)1.7 and Na(v)1.8 respectively. SNI significantly decreases the proportion of Nedd4-2-positive neurons from 45.9 ± 1.9% to 33.5 ± 0.7% (p<0.01) and the total Nedd4-2 protein to 44% ± 0.13% of its basal level (p<0.01, n=4 animals in each group, mean ± SEM). In contrast, no change in Nedd4-2 was found after peripheral inflammation induced by CFA. These results indicate that Nedd4-2 is present in nociceptive neurons, is downregulated after peripheral nerve injury, and might therefore contribute to the dysregulation of Na(v)s involved in the hyperexcitability associated with peripheral nerve injuries.

Nedd4-mediated AMPA receptor ubiquitination regulates receptor turnover and trafficking

α-Amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid receptors (AMPARs) are the primary mediators of excitatory synaptic transmission in the brain. Alterations in AMPAR localization and turnover have been considered critical mechanisms underpinning synaptic plasticity and higher brain functions, but the molecular processes that control AMPAR trafficking and stability are still not fully understood. Here, we report that mammalian AMPARs are subject to ubiquitination in neurons and in transfected heterologous cells. Ubiquitination facilitates AMPAR endocytosis, leading to a reduction in AMPAR cell-surface localization and total receptor abundance. Mutation of lysine residues to arginine residues at the glutamate receptor subunit 1 (GluA1) C-terminus dramatically reduces GluA1 ubiquitination and abolishes ubiquitin-dependent GluA1 internalization and degradation, indicating that the lysine residues, particularly K868, are sites of ubiquitination. We also find that the E3 ligase neural precursor cell expressed, developmentally down-regulated 4 (Nedd4) is enriched in synaptosomes and co-localizes and associates with AMPARs in neurons. Nedd4 expression leads to AMPAR ubiquitination, leading to reduced AMPAR surface expression and suppressed excitatory synaptic transmission. Conversely, knockdown of Nedd4 by specific siRNAs abolishes AMPAR ubiquitination. These data indicate that Nedd4 is the E3 ubiquitin ligase responsible for AMPAR ubiquitination, a modification that regulates multiple aspects of AMPAR molecular biology including trafficking, localization and stability.

WW domain-mediated interaction with Wbp2 is important for the oncogenic property of TAZ

The transcriptional co-activators YAP and TAZ are downstream targets inhibited by the Hippo tumor suppressor pathway. YAP and TAZ both possess WW domains, which are important protein–protein interaction modules that mediate interaction with proline-rich motifs, most commonly PPXY. The WW domains of YAP have complex regulatory roles as exemplified by recent reports showing that they can positively or negatively influence YAP activity in a cell and context-specific manner. In this study, we show that the WW domain of TAZ is important for it to transform both MCF10A and NIH3T3 cells and to activate transcription of ITGB2 but not CTGF, as introducing point mutations into the WW domain of TAZ (WWm) abolished its transforming and transcription-promoting ability. Using a proteomic approach, we discovered potential regulatory proteins that interact with TAZ WW domain and identified Wbp2. The interaction of Wbp2 with TAZ is dependent on the WW domain of TAZ and the PPXY-containing C-terminal region of Wbp2. Knockdown of endogenous Wbp2 suppresses, whereas overexpression of Wbp2 enhances, TAZ-driven transformation. Forced interaction of WWm with Wbp2 by direct C-terminal fusion of full-length Wbp2 or its TAZ-interacting C-terminal domain restored the transforming and transcription-promoting ability of TAZ. These results suggest that the WW domain-mediated interaction with Wbp2 promotes the transforming ability of TAZ.

A direct interaction with NEDD1 regulates gamma-tubulin recruitment to the centrosome

The centrosome is the primary microtubule organizing centre of the cell. γ-tubulin is a core component of the centrosome and is required for microtubule nucleation and centrosome function. The recruitment of γ-tubulin to centrosomes is mediated by its interaction with NEDD1, a WD40-repeat containing protein. Here we demonstrate that NEDD1 is likely to be oligomeric in vivo and binds directly to γ-tubulin through a small region of just 62 residues at the carboxyl-terminus of the protein. This carboxyl-terminal domain that binds γ-tubulin has a helical structure and is a stable tetramer in solution. Mutation of residues in NEDD1 that disrupt binding to γ-tubulin result in a mis-localization of γ-tubulin away from the centrosome. Hence, this study defines the binding site on NEDD1 that is required for its interaction with γ-tubulin, and shows that this interaction is required for the correct localization of γ-tubulin.

Nedd4 and Nedd4-2: closely related ubiquitin-protein ligases with distinct physiological functions

The Nedd4 (neural precursor cell-expressed developmentally downregulated gene 4) family of ubiquitin ligases (E3s) is characterized by a distinct modular domain architecture, with each member consisting of a C2 domain, 2-4 WW domains, and a HECT-type ligase domain. Of the nine mammalian members of this family, Nedd4 and its close relative, Nedd4-2, represent the ancestral ligases with strong similarity to the yeast, Rsp5. In Saccharomyces cerevisiae Rsp5 has a key role in regulating the trafficking, sorting, and degradation of a large number of proteins in multiple cellular compartments. However, in mammals the Nedd4 family members, including Nedd4 and Nedd4-2, appear to have distinct functions, thereby suggesting that these E3s target specific proteins for ubiquitylation. In this article we focus on the biology and emerging functions of Nedd4 and Nedd4-2, and review recent in vivo studies on these E3s.

An essential function for the centrosomal protein NEDD1 in zebrafish development

The centrosome is the primary microtubule organising centre of the cell. It is composed of many proteins, some of which make up the core of the centrosome, whereas others are used for specific functions. Although the cellular roles of many centrosomal proteins are well defined, much less is known about their functions and the role of the centrosome in development. In this study we investigated the function of NEDD1, a critical component of the centrosome essential for microtubule nucleation, in zebrafish (Danio rerio) development. The zebrafish homologue of NEDD1 (zNEDD1) was cloned and found to have a similar localisation and function to mammalian NEDD1. We show that zNEDD1 is essential for survival, as a high level of knockdown was embryonic lethal. Partial knockdown of zNEDD1 caused abnormalities including an increase in mitotic and apoptotic cells. Pronounced phenotypic defects were seen in the brain, with a lack of defined brain structures, incomplete neural tube formation and a disorganisation of neurons. In addition, we show that a reduction in zNEDD1 resulted in the loss of gamma-tubulin at the centrosome. Our data thus demonstrate that zNEDD1 is critical for the recruitment of gamma-tubulin to the centrosome, and is essential for the proper development of zebrafish.

Na,K-ATPase activity regulates AMPA receptor turnover through proteasome-mediated proteolysis

Neuronal activity largely depends on two key components on the membrane: the Na,K-ATPase (NKA) that maintains the ion gradients and sets the foundation of excitability, and the ionotropic glutamatergic AMPA receptors (AMPARs) through which sodium influx forms the driving force for excitation. Because the frequent sodium transients from glutamate receptor activity need to be efficiently extruded, a functional coupling between NKA and AMPARs should be a necessary cellular device for synapse physiology. We show that NKA is enriched at synapses and associates with AMPARs. NKA dysfunction induces a rapid reduction in AMPAR cell-surface expression as well as total protein abundance, leading to a long-lasting depression in synaptic transmission. AMPAR proteolysis requires sodium influx, proteasomal activity and receptor internalization. These data elucidate a novel mechanism by which NKA regulates AMPAR turnover and thereby synaptic strength and brain function.

Abnormal development of the neuromuscular junction in Nedd4-deficient mice

Nedd4 (neural precursor cell expressed developmentally down-regulated gene 4) is an E3 ubiquitin ligase highly conserved from yeast to humans. The expression of Nedd4 is developmentally down-regulated in the mammalian nervous system, but the role of Nedd4 in mammalian neural development remains poorly understood. Here we show that a null mutation of Nedd4 in mice leads to perinatal lethality: mutant mice were stillborn and many of them died in utero before birth (between E15.5-E18.5). In Nedd4 mutant embryos, skeletal muscle fiber sizes and motoneuron numbers are significantly reduced. Surviving motoneurons project axons to their target muscles on schedule, but motor nerves defasciculate upon reaching the muscle surface, suggesting that Nedd4 plays a critical role in fine-tuning the interaction between the nerve and the muscle. Electrophysiological analyses of the neuromuscular junction (NMJ) demonstrate an increased spontaneous miniature endplate potential (mEPP) frequency in Nedd4 mutants. However, the mutant neuromuscular synapses are less responsive to membrane depolarization, compared to the wildtypes. Ultrastructural analyses further reveal that the pre-synaptic nerve terminal branches at the NMJs of Nedd4 mutants are increased in number, but decreased in diameter compared to the wildtypes. These ultrastructural changes are consistent with functional alternation of the NMJs in Nedd4 mutants. Unexpectedly, Nedd4 is not expressed in motoneurons, but is highly expressed in skeletal muscles and Schwann cells. Together, these results demonstrate that Nedd4 is involved in regulating the formation and function of the NMJs through non-cell autonomous mechanisms.

Role of the UPS in Liddle syndrome

Hypertension is a serious medical problem affecting a large population worldwide. Liddle syndrome is a hereditary form of early onset hypertension caused by mutations in the epithelial Na⁺ channel (ENaC). The mutated region, called the PY (Pro-Pro-x-Tyr) motif, serves as a binding site for Nedd4-2, an E3 ubiquitin ligase from the HECT family. Nedd4-2 binds the ENaC PY motif via its WW domains, normally leading to ENaC ubiquitylation and endocytosis, reducing the number of active channels at the plasma membrane. In Liddle syndrome, this endocytosis is impaired due to the inability of the mutated PY motif in ENaC to properly bind Nedd4-2. This leads to accumulation of active channels at the cell surface and increased Na⁺ (and fluid) absorption in the distal nephron, resulting in elevated blood volume and blood pressure. Small molecules/compounds that destabilize cell surface ENaC, or enhance Nedd4-2 activity in the kidney, could potentially serve to alleviate hypertension.

Republished from Current BioData's Targeted Proteins database (TPdb; ).

A feedback loop between the androgen receptor and a NEDD4-binding protein, PMEPA1, in prostate cancer cells

PMEPA1 was identified originally as a highly androgen-inducible gene with prostate-abundant expression that was restricted to prostatic epithelial cells. PMEPA1 protein is a NEDD4 (ubiquitin-protein isopeptide ligase)-binding protein, which negatively regulates prostate cancer cell growth. In this study we establish that PMEPA1 is a direct transcriptional target of the androgen receptor (AR). We also demonstrate that PMEPA1 negatively regulates AR protein levels in different cell culture models. Transient expression of PMEPA1 down-regulates AR protein levels and AR transcriptional targets in prostate cancer cells. Conversely, knockdown of PMEPA1 leads to elevated levels of AR protein, AR transcriptional targets (prostate-specific antigen), and increased cell cycle S phase. We define that the PMEPA1-dependent down-regulation of AR is because of AR ubiquitination and proteasome-mediated degradation. The mutant PMEPA1 (PY1/2 motif mutation) that is impaired in NEDD4 recruitment shows attenuated AR ubiquitination and AR protein down-regulation. These data support the hypothesis that PMEPA1 negatively regulates the stability of AR protein by enhancing AR ubiquitination and proteasome-mediated degradation through NEDD4. The effect of PMEPA1 on AR ubiquitination and degradation appears to be MDM2-independent. Thus, the PMEPA1-AR degradation pathway may represent a new androgen-dependent mechanism for regulating AR levels in prostate epithelial cells. These findings underscore that the decreased PMEPA1 expression frequently noted in prostate cancers may lead to increased AR functions and strengthen the biological role of PMEPA1 in prostate cancers.

The HECT family of E3 ubiquitin ligases: multiple players in cancer development

The involvement of the homologous to E6-AP carboxyl terminus (HECT)-type E3s in crucial signaling pathways implicated in tumorigenesis is presently an area of intense research and extensive scientific interest. This review highlights recent discoveries on the ubiquitin-mediated degradation of crucial tumor suppressor molecules catalyzed by the HECT-type E3s. By providing a portrait of their protein targets, we intend to link the substrate specificity of HECT-type E3s with their contribution to tumorigenesis. Moreover, we discuss the relevance of targeting the HECT E3s, through the development of small-molecule inhibitors, as an anticancer therapeutic strategy.

Regulation of epithelial Na+ channels by aldosterone: role of Sgk1

1. The epithelial sodium channel (ENaC) is tightly regulated by hormonal and humoral factors, including cytosolic ion concentration and glucocorticoid and mineralocorticoid hormones. Many of these regulators of ENaC control its activity by regulating its surface expression via neural precursor cell-expressed developmentally downregulated (gene 4) protein (Nedd4-2). 2. During the early phase of aldosterone action, Nedd4-2-dependent downregulation of ENaC is inhibited by the serum- and glucocorticoid-induced kinase 1 (Sgk1). 3. Sgk1 phosphorylates Nedd4-2. Subsequently, phosphorylated Nedd4-2 binds to the 14-3-3 protein and, hence, reduces binding of Nedd4-2 to ENaC. 4. Nedd4-2 is also phosphorylated by protein kinase B (Akt1). Both Sgk1 and Akt1 are part of the insulin signalling pathway that increases transepithelial Na(+) absorption by inhibiting Nedd4-2 and activating ENaC.

HECT E3s and human disease

In a simplified view, members of the HECT E3 family have a modular structure consisting of the C-terminal HECT domain, which is catalytically involved in the attachment of ubiquitin to substrate proteins, and N-terminal extensions of variable length and sequence that mediate the substrate specificity of the respective HECT E3. Although the physiologically relevant substrates of most HECT E3s have remained elusive, it is becoming increasingly clear that HECT E3s play an important role in sporadic and hereditary human diseases including cancer, cardiovascular (Liddle's syndrome) and neurological (Angelman syndrome) disorders, and/or in disease-relevant processes including bone homeostasis, immune response and retroviral budding. Thus, molecular approaches to target the activity of distinct HECT E3s, regulators thereof, and/or of HECT E3 substrates could prove valuable in the treatment of the respective diseases.

Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; ).

(NDRG2) Stimulates Amiloride-sensitive Na+ Currents in Xenopus laevis Oocytes and Fisher Rat Thyroid Cells

Regulation of the epithelial sodium channel (ENaC) is highly complex and may involve several aldosterone-induced regulatory proteins. The N-Myc downstream-regulated gene 2 (NDRG2) has been identified as an early aldosterone-induced gene. Therefore, we hypothesized that NDRG2 may affect ENaC function. To test this hypothesis we measured the amiloride-sensitive (2 microm) whole cell current (DeltaI(ami)) in Xenopus laevis oocytes expressing ENaC alone or co-expressing ENaC and NDRG2. Co-expression of NDRG2 significantly increased DeltaI(ami) in some, but not, all batches of oocytes tested. An inhibitory effect of NDRG2 was never observed. Using a chemiluminescence assay we demonstrated that the NDRG2-induced increase in ENaC currents was accompanied by a similar increase in channel surface expression. The stimulatory effect of NDRG2 was preserved in oocytes maintained in a low sodium bath solution to prevent sodium feedback inhibition. These findings suggest that the stimulatory effect of NDRG2 is independent of sodium feedback regulation. Furthermore, the stimulatory effect of NDRG2 on ENaC was at least in part additive to that of Sgk1. A short isoform of NDRG2 also stimulated DeltaI(ami). Overexpression of NDRG2 and ENaC in Fisher rat thyroid cells confirmed the stimulatory effect of NDRG2 on ENaC-mediated short-circuit current (I(SC-ami)). In addition, small interference RNA against NDRG2 largely reduced I(SC-ami) in Fisher rat thyroid cells. Our results indicate that NDRG2 is a likely candidate to contribute to aldosterone-mediated ENaC regulation.

Specificity and autoregulation of Notch binding by tandem WW domains in suppressor of Deltex

WW domains target proline-tyrosine (PY) motifs and frequently function as tandem pairs. When studied in isolation, single WW domains are notably promiscuous and regulatory mechanisms are undoubtedly required to ensure selective interactions. Here, we show that the fourth WW domain (WW4) of Suppressor of Deltex, a modular Nedd4-like protein that down-regulates the Notch receptor, is the primary mediator of a direct interaction with a Notch-PY motif. A natural Trp to Phe substitution in WW4 reduces its affinity for general PY sequences and enhances selective interaction with the Notch-PY motif via compensatory specificity-determining interactions with PY-flanking residues. When WW4 is paired with WW3, domain-domain association, impeding proper folding, competes with Notch-PY binding to WW4. This novel mode of autoinhibition is relieved by binding of another ligand to WW3. Such cooperativity may facilitate the transient regulatory interactions observed in vivo between Su(dx) and Notch in the endocytic pathway. The highly conserved tandem arrangement of WW domains in Nedd4 proteins, and similar arrangements in more diverse proteins, suggests domain-domain communication may be integral to regulation of their associated cellular activities.

Epithelial sodium channel (ENaC) is multi-ubiquitinated at the cell surface

The human ENaC (epithelial sodium channel), a complex of three subunits, provides the rate-limiting step for sodium uptake in the distal nephron, and therefore plays a key role in salt homoeostasis and in regulating blood pressure. The number of active sodium channel complexes present at the plasma membrane appears to be tightly controlled. In Liddle's syndrome, a form of hypertension caused by an increase in the number of active sodium channels at the cell membrane, the betaENaC or gammaENaC subunit gene contains a mutation that disrupts the binding site for the Nedd4 (neuronal precursor cell expressed developmentally down-regulated gene 4) family of ubiquitin-protein ligases. Therefore ubiquitination of channel subunits may be involved in altering cell surface ENaC. Here, we provide evidence that the ENaC subunits located at the cell surface are modified with multiple mono-ubiquitins (multi-ubiquitination) and that Nedd4-2 modulates this ubiquitination. We confirm that ENaC is associated with the mu2 subunit of the AP-2 (adaptor protein 2) clathrin adaptor. Since mono- or multi-ubiquitination of other membrane proteins is a signal for their internalization by clathrin-mediated endocytosis and subsequent trafficking, our results support a model whereby ubiquitin and clathrin adaptor binding sites act in concert to remove ENaC from the cell surface.

The Nedd4-binding partner 1 (N4BP1) protein is an inhibitor of the E3 ligase Itch

Nedd4-binding partner-1 (N4BP1) has been identified as a protein interactor and a substrate of the homologous to E6AP C terminus (HECT) domain-containing E3 ubiquitin-protein ligase (E3), Nedd4. Here, we describe a previously unrecognized functional interaction between N4BP1 and Itch, a Nedd4 structurally related E3, which contains four WW domains, conferring substrate-binding activity. We show that N4BP1 association with the second WW domain (WW2) of Itch interferes with E3 binding to its substrates. In particular, we found that N4BP1 and p73 alpha, a target of Itch-mediated ubiquitin/proteasome proteolysis, share the same binding site. By competing with p73 alpha for binding to the WW2 domain, N4BP1 reduces the ability of Itch to recruit and ubiquitylate p73 alpha and inhibits Itch autoubiquitylation activity both in in vitro and in vivo ubiquitylation assays. Similarly, both c-Jun and p63 polyubiquitylation by Itch are inhibited by N4BP1. As a consequence, genetic and RNAi knockdown of N4BP1 diminish the steady-state protein levels and significantly impair the transcriptional activity of Itch substrates. Notably, stress-induced induction of c-Jun was impaired in N4BP1(-/-) cells. These results demonstrate that N4BP1 functions as a negative regulator of Itch. In addition, because inhibition of Itch by N4BP1 results in the stabilization of crucial cell death regulators such as p73 alpha and c-Jun, it is conceivable that N4BP1 may have a role in regulating tumor progression and the response of cancer cells to chemotherapy.

GRK2 interacts with and phosphorylates Nedd4 and Nedd4-2

Epithelial Na(+) channels (ENaC) mediate the transport of sodium (Na) across epithelia in the kidney, gut, and lungs and are required for blood pressure regulation. They are inhibited by ubiquitin protein ligases, such as Nedd4 and Nedd4-2, which bind to proline-rich motifs (PY motifs) present in the C-termini of ENaC subunits. Loss of inhibition leads to hypertension. ENaC channels are maintained in the active state by G-protein-coupled receptor kinase 2 (GRK2), an enzyme implicated in the development of essential hypertension. Here, we report that GRK2 interacts not only with ENaC, but also with both Nedd4 and Nedd4-2. Additionally, GRK2 is capable of phosphorylating both Nedd4 and Nedd4-2 at multiple sites. Of possible significance is the phosphorylation of the threonine at position 466 in Nedd4, which is located in the area of the ww3 domain that binds ENaC. These results support and extend the role of GRK2 in sodium transport regulation.

Ndfip1 protein promotes the function of itch ubiquitin ligase to prevent T cell activation and T helper 2 cell-mediated inflammation

Nedd4 family interacting protein-1 (Ndfip1) is a protein whose only known function is that it binds Nedd4, a HECT-type E3 ubiquitin ligase. Here we show that mice lacking Ndfip1 developed severe inflammation of the skin and lung and died prematurely. This condition was due to a defect in Ndfip1(-/-) T cells. Ndfip1(-/-) T cells were activated, and they proliferated and adopted a T helper 2 (Th2) phenotype more readily than did their Ndfip1(+/+) counterparts. This phenotype resembled that of Itchy mutant mice, suggesting that Ndfip1 might affect the function of Itch, an E3 ubiquitin ligase. We show that T cell activation promoted both Ndfip1 expression and its association with Itch. In the absence of Ndfip1, JunB half-life was prolonged after T cell activation. Thus, in the absence of Ndfip1, Itch is inactive and JunB accumulates. As a result, T cells produce Th2 cytokines and promote Th2-mediated inflammatory disease.

Stimulation of the epithelial sodium channel (ENaC) by the serum- and glucocorticoid-inducible kinase (Sgk) involves the PY motifs of the channel but is independent of sodium feedback inhibition

The epithelial sodium channel (ENaC) is the major mediator of sodium transport across the apical membranes of the distal nephron, the distal colon, the respiratory tract and the ducts of exocrine glands. It is subject to feedback inhibition by increased intracellular Na+, a regulatory system wherein the ubiquitin protein ligases, Nedd4 and Nedd4-2, bind to conserved PY motifs in the C-termini of ENaC and inactivate the channel. It has been proposed recently that the kinase Sgk activates the channel as a consequence of phosphorylating Nedd4-2, thus preventing it from inhibiting the channels. This proposal predicts that Sgk should interfere with Na+ feedback regulation of ENaC. We have tested this prediction in Xenopus laevis oocytes and in mouse salivary duct cells and found that in neither system did increased activity of Sgk interrupt Na+ feedback inhibition of ENaC. We found, however, that Sgk stimulation was largely abolished in oocytes expressing ENaC channels with C-terminal truncations or mutated PY motifs. We were also unable to confirm that Sgk directly interacts with Nedd4-2 in vitro. We conclude that the stimulatory effect of Sgk on ENaC requires the presence of the channel's PY motifs, but it is not due to the interruption of Na+ feedback regulation.

Flagellin of Pseudomonas aeruginosa inhibits Na+ transport in airway epithelia

Pseudomonas aeruginosa causes severe life-threatening airway infections that are a frequent cause for hospitalization of cystic fibrosis (CF) patients. These Gram-negative pathogens possess flagella that contain the protein flagellin as a major structural component. Flagellin binds to the host cell glycolipid asialoGM1 (ASGM1), which appears enriched in luminal membranes of respiratory epithelial cells. We demonstrate that in mouse airways, luminal exposure to flagellin leads to inhibition of Na+ absorption by the epithelial Na+ channel ENaC, but does not directly induce a secretory response. Inhibition of ENaC was observed in tracheas of wild-type mice and was attenuated in mice homozygous for the frequent cystic fibrosis conductance regulator (CFTR) mutation G551D. Similar to flagellin, anti-ASGM1 antibody also inhibited ENaC. The inhibitory effects of flagellin on ENaC were attenuated by blockers of the purinergic signaling pathway, although an increase in the intracellular Ca2+ concentration by recombinant or purified flagellin or whole flagella was not observed. Because an inhibitor of the mitogen-activated protein kinase (MAPK) pathway also attenuated the effects of flagellin on Na+ absorption, we conclude that flagellin exclusively inhibits ENaC, probably due to release of ATP and activation of purinergic receptors of the P2Y subtype. Stimulation of these receptors activates the MAPK pathway, thereby leading to inhibition of ENaC. Thus, P. aeruginosa reduces Na+ absorption, which could enhance local mucociliary clearance, a mechanism that seem to be attenuated in CF.

Regulation of functional diversity within the Nedd4 family by accessory and adaptor proteins

Ubiquitination is essential in mediating diverse cellular functions including protein degradation and trafficking. Ubiquitin-protein (E3) ligases determine the substrate specificity of the ubiquitination process. The Nedd4 family of E3 ligases is an evolutionarily conserved family of proteins required for the ubiquitination of a large number of cellular targets. As a result, this family regulates a wide variety of cellular processes including transcription, stability and trafficking of plasma membrane proteins, and the degradation of misfolded proteins. The modular architecture of the proteins, comprising a C2 domain, multiple WW domains and a catalytic domain, enables diverse intermolecular interactions and recruitment to various subcellular locations. The WW domains commonly mediate interaction with substrate proteins; however, an increasing number of Nedd4 targets do not contain obvious WW domain-interaction motifs suggesting the involvement of accessory proteins. This review discusses recent insights into how accessory and adaptor proteins modulate the activities of Nedd4 family members, including recruitment of novel substrates, alteration of subcellular localisation and effects on ubiquitination.

Transcriptional diversity and expression of NEDD4L gene in distal nephron

The ubiquitin ligase NEDD4L participates in plasma volume and blood pressure regulation by controlling expression of the epithelial sodium channel (ENaC). Genetic impairment of EnaC-Nedd4L-Proteasome system caused a rare mendelian hereditary human hypertension, Liddle syndrome. This finding suggested that Nedd4L is playing an important role in pathogenesis for hypertensive disorders. This prompted us to test a possible involvement of NEDD4L for the development of sodium-sensitive hypertension in Dahl salt-sensitive (DS) rats and its normotensive littermate Dahl salt-resistant (DR) rats. First, we analyzed the transcriptional diversity of rat Nedd4L gene and observed several isoforms with and without calcium-dependent membrane binding (C2) domain at the N-terminal of the protein as we found in human and mouse before. Then, we analyzed the expression of rat NEDD4L in the kidney of both DS and DR under high and low sodium regimens. NEDD4L expression examined by quantitative PCR technique revealed lower expression of NEDD4L transcripts in DS rats under either diet compared to DR animals; additionally, NEDD4L expression was significantly increased with sodium loading. Using in situ hybridization experiments, rat NEDD4L was predominantly expressed in distal nephron in a manner dependent on both sodium regimen and genetic background. A similar histological distribution pattern was observed in human kidney. The expression of NEDD4L in distal nephron and its response to chronic sodium loading suggest that it participates in the functioning of this segment in sodium reabsorption. This response was impaired in genetically sodium-sensitive animals. These findings suggested that Nedd4L gene products were involved in the development of salt-sensitive hypertension.

Minireview: regulation of epithelial Na+ channel trafficking

The epithelial Na(+) channel (ENaC) is a pathway for Na(+) transport across epithelia, including the kidney collecting duct, lung, and distal colon. ENaC is critical for Na(+) homeostasis and blood pressure control; defects in ENaC function and regulation are responsible for inherited forms of hypertension and hypotension and may contribute to the pathogenesis of cystic fibrosis and other lung diseases. An emerging theme is that epithelial Na(+) transport is regulated in large part through trafficking mechanisms that control ENaC expression at the cell surface. ENaC trafficking is regulated at multiple steps. Delivery of channels to the cell surface is regulated by aldosterone (and corticosteroids) and vasopressin, which increase ENaC synthesis and exocytosis, respectively. Conversely, endocytosis and degradation is controlled by a sequence located in the C terminus of alpha, beta, and gammaENaC (PPPXYXXL). This sequence functions as an endocytosis motif and as a binding site for Nedd4-2, an E3 ubiquitin protein ligase that targets ENaC for degradation. Mutations that delete or disrupt this motif cause accumulation of channels at the cell surface, resulting in Liddle's syndrome, an inherited form of hypertension. Nedd4-2 is a central convergence point for ENaC regulation by aldosterone and vasopressin; both induce phosphorylation of a common set of three Nedd4-2 residues, which blocks Nedd4-2 binding to ENaC. Thus, aldosterone and vasopressin regulate epithelial Na(+) transport in part by altering ENaC trafficking to and from the cell surface.

The ubiquitin-protein ligases Nedd4 and Nedd4-2 show similar ubiquitin-conjugating enzyme specificities

Nedd4 and Nedd4-2 are closely related HECT-type ubiquitin-protein ligases (E3) implicated in the regulation of a number of proteins and pathways. Given the close homology between these E3 enzymes it would be predicted that a conserved ubiquitin-conjugating enzyme (E2) specificity exists between the two proteins. However, E2 specificities for Nedd4 and Nedd4-2 are not well established. In the present studies we aimed at clarifying the E2-specificities of Nedd4 and Nedd4-2 using in vitro ubiquitination assays. We demonstrate strong substrate ubiquitination in the presence of UbcH5b by both Nedd4 and Nedd4-2. We also found that Ube2e3, an E2 previously shown to be used by Nedd4-2, is used less efficiently than UbcH5b. Our results suggest that for optimal ubiquitination Nedd4 and Nedd4-2 require the same E2 enzymes.

ClC-5: a chloride channel with multiple roles in renal tubular albumin uptake

ClC-5 is a chloride (Cl(-)) channel expressed in renal tubules and is critical for normal tubular function. Loss of function nonsense or missense mutations in ClC-5 are associated with Dent's disease, a condition in which patients present with low molecular weight (LMW) proteinuria (including albuminuria), hypercalciuria and nephrolithiasis. Several key studies in ClC-5 knockout mice have shown that the proteinuria results from defective tubular reabsorption of proteins. ClC-5 is typically regarded as an intracellular Cl(-) channel and thus the defect in this receptor-mediated uptake pathway was initially attributed to the failure of the early endosomes to acidify correctly. ClC-5 was postulated to play a key role in transporting the Cl(-) ions required to compensate for the movement of H(+) during endosomal acidification. However, more recent studies suggest additional roles for ClC-5 in the endocytosis of albumin. ClC-5 is now known to be expressed at low levels at the cell surface and appears to be a key component in the assembly of the macromolecular complex involved in protein endocytosis. Furthermore, mutations in ClC-5 affect the trafficking of v-H(+)-ATPase and result in decreased expression of the albumin receptor megalin/cubulin. Thus, the expression of ClC-5 at the cell surface as well as its presence in endosomes appears to be essential for normal protein uptake by the renal proximal tubule.

Impact of Nedd4 proteins and serum and glucocorticoid-induced kinases on epithelial Na+ transport in the distal nephron

The precise control of BP occurs via Na(+) homeostasis and involves the precise regulation of the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. This has been corroborated by the linkage of mutations in the genes encoding ENaC subunits and Liddle's syndrome, a heritable form of human hypertension. Mapping of these mutations on ENaC indicated that inactivation of PY motifs is responsible and leads to the proposition that the channel interacts via its PY motifs with the WW domains of the Nedd4/Nedd4-like ubiquitin-protein ligase family. It is now well established that the cell surface expression of ENaC is controlled via ubiquitylation by this protein family and that this ubiquitylation is regulated by the aldosterone-induced protein serum and glucocorticoid induced kinase 1.

WW domains provide a platform for the assembly of multiprotein networks

WW domains are protein modules that mediate protein-protein interactions through recognition of proline-rich peptide motifs and phosphorylated serine/threonine-proline sites. To pursue the functional properties of WW domains, we employed mass spectrometry to identify 148 proteins that associate with 10 human WW domains. Many of these proteins represent novel WW domain-binding partners and are components of multiprotein complexes involved in molecular processes, such as transcription, RNA processing, and cytoskeletal regulation. We validated one complex in detail, showing that WW domains of the AIP4 E3 protein-ubiquitin ligase bind directly to a PPXY motif in the p68 subunit of pre-mRNA cleavage and polyadenylation factor Im in a manner that promotes p68 ubiquitylation. The tested WW domains fall into three broad groups on the basis of hierarchical clustering with respect to their associated proteins; each such cluster of bound proteins displayed a distinct set of WW domain-binding motifs. We also found that separate WW domains from the same protein or closely related proteins can have different specificities for protein ligands and also demonstrated that a single polypeptide can bind multiple classes of WW domains through separate proline-rich motifs. These data suggest that WW domains provide a versatile platform to link individual proteins into physiologically important networks.

New insights into epithelial sodium channel function in the kidney: site of action, regulation by ubiquitin ligases, serum- and glucocorticoid-inducible kinase and proteolysis

PURPOSE OF REVIEW

The epithelial sodium channel (ENaC) sets the rate of Na+ reabsorption in the collecting duct. This review describes recent advances in our understanding of ENaC function.

RECENT FINDINGS

First, collecting duct-specific deletion of alphaENaC does not cause Na wasting in mice, suggesting that other regions can compensate. Second, Nedd4 and Nedd4-2 are ubiquitin ligases that reduce surface expression of ENaC and inhibit Na+ transport. Nedd4-2, but not Nedd4, is negatively regulated by serum- and glucocorticoid-inducible kinase 1, an aldosterone-induced kinase, providing an attractive mechanism for the stimulatory effect of aldosterone on Na+ transport. However, mice with germline ablation of serum- and glucocorticoid-inducible kinase 1 show only modest hypotension and are able to decrease Na+ excretion rates substantially. Third, maturation of ENaC is associated with processing at consensus furin cleavage sites and this cleavage is critical for channel activity. A separate class of serine proteases, the channel-activating proteases, also stimulates ENaC activity.

SUMMARY

The connecting tubule of the kidney has abundant ENaC and Na(+)- and K(+)-transport capacity and may provide much of ENaC-mediated Na+ transport in the kidney. Aldosterone may increase Na transport, in part, by serum- and glucocorticoid-inducible kinase 1-mediated inhibition of Nedd4-2 but this has not been demonstrated in the native collecting duct or connecting tubule. The mild phenotype of the serum- and glucocorticoid-inducible kinase 1-knockout mouse points to serum- and glucocorticoid-inducible kinase 1-independent mechanisms that regulate Na+ transport. Two separate classes of protease appear to regulate Na+ transport: one is furin or furin-like and cleaves ENaC subunits to stimulate transport; the other, the channel-activating proteases, may act on ENaC or a regulatory molecule.

NEDD4-2 (neural precursor cell expressed, developmentally down-regulated 4-2) negatively regulates TGF-beta (transforming growth factor-beta) signalling by inducing ubiquitin-mediated degradation of Smad2 and TGF-beta type I receptor

Inhibitory Smad, Smad7, is a potent inhibitor of TGF-beta (transforming growth factor-beta) superfamily signalling. By binding to activated type I receptors, it prevents the activation of R-Smads (receptor-regulated Smads). To identify new components of the Smad pathway, we performed yeast two-hybrid screening using Smad7 as bait, and identified NEDD4-2 (neural precursor cell expressed, developmentally down-regulated 4-2) as a direct binding partner of Smad7. NEDD4-2 is structurally similar to Smurfs (Smad ubiquitin regulatory factors) 1 and 2, which were identified previously as E3 ubiquitin ligases for R-Smads and TGF-beta superfamily receptors. NEDD4-2 functions like Smurfs 1 and 2 in that it associates with TGF-beta type I receptor via Smad7, and induces its ubiquitin-dependent degradation. Moreover, NEDD4-2 bound to TGF-beta-specific R-Smads, Smads 2 and 3, in a ligand-dependent manner, and induced degradation of Smad2, but not Smad3. However, in contrast with Smurf2, NEDD4-2 failed to induce ubiquitination of SnoN (Ski-related novel protein N), although NEDD4-2 bound to SnoN via Smad2 more strongly than Smurf2. We showed further that overexpressed NEDD4-2 prevents transcriptional activity induced by TGF-beta and BMP, whereas silencing of the NEDD4-2 gene by siRNA (small interfering RNA) resulted in enhancement of the responsiveness to TGF-beta superfamily cytokines. These data suggest that NEDD4-2 is a member of the Smurf-like C2-WW-HECT (WW is Trp-Trp and HECT is homologous to the E6-accessory protein) type E3 ubiquitin ligases, which negatively regulate TGF-beta superfamily signalling through similar, but not identical, mechanisms to those used by Smurfs.