Serum and glucocorticoid inducible kinases functionally regulate ClC-2 channels

Intracellular signaling pathways involved in the regulation of gene expression by pilocarpine

OBJECTIVES

Pilocarpine is commonly used clinically to treat dry mouth. The long-term administration of pilocarpine reportedly improves salivary secretion more effectively than short-term administration. Therefore, we hypothesized that pilocarpine alters gene expression in salivary glands via muscarinic receptor stimulation. This study aimed to investigate the effects of pilocarpine use on gene expression mediated by mitogen-activated protein kinase (MAPK) activity.

METHODS

The effects of pilocarpine on gene expression were investigated in rats and human salivary gland (HSY) cells using several inhibitors of intracellular signaling pathways. Gene expression in the rat submandibular gland and HSY cells was determined using reverse transcription-quantitative polymerase chain reaction analysis of total RNA.

RESULTS

In animal experiments, at 7 days after pilocarpine stimulation, Ctgf and Sgk1 expressions were increased in the submandibular gland. In cell culture experiments, pilocarpine increased Ctgf expression in HSY cells. The mitogen-activated protein kinase kinase inhibitor trametinib, the Src inhibitor PP2, and the muscarinic acetylcholine receptor antagonist atropine suppressed the effect of pilocarpine on gene expression.

CONCLUSIONS

Pilocarpine enhances Ctgf and Sgk1 expressions by activating Src-mediated MAPK activity. Although further studies are required to fully understand the roles of Ctgf and Sgk1, changes in gene expression may play an important role in improving salivary secretions.

Contributions of SGK3 to transporter-related diseases

Serum- and glucocorticoid-induced kinase 3 (SGK3), which is ubiquitously expressed in mammals, is regulated by estrogens and androgens. SGK3 is activated by insulin and growth factors through signaling pathways involving phosphatidylinositol-3-kinase (PI3K), 3-phosphoinositide-dependent kinase-1 (PDK-1), and mammalian target of rapamycin complex 2 (mTORC2). Activated SGK3 can activate ion channels (TRPV5/6, SOC, Kv1.3, Kv1.5, Kv7.1, BKCa, Kir2.1, Kir2.2, ENaC, Nav1.5, ClC-2, and ClC Ka), carriers and receptors (Npt2a, Npt2b, NHE3, GluR1, GluR6, SN1, EAAT1, EAAT2, EAAT4, EAAT5, SGLT1, SLC1A5, SLC6A19, SLC6A8, and NaDC1), and Na+/K+-ATPase, promoting the transportation of calcium, phosphorus, sodium, glucose, and neutral amino acids in the kidney and intestine, the absorption of potassium and neutral amino acids in the renal tubules, the transportation of glutamate and glutamine in the nervous system, and the transportation of creatine. SGK3-sensitive transporters contribute to a variety of physiological and pathophysiological processes, such as maintaining calcium and phosphorus homeostasis, hydro-salinity balance and acid-base balance, cell proliferation, muscle action potential, cardiac and neural electrophysiological disturbances, bone density, intestinal nutrition absorption, immune function, and multiple substance metabolism. These processes are related to kidney stones, hypophosphorous rickets, multiple syndromes, arrhythmia, hypertension, heart failure, epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, glaucoma, ataxia idiopathic deafness, and other diseases.

From Pinocytosis to Methuosis-Fluid Consumption as a Risk Factor for Cell Death

The volumes of a cell [cell volume (CV)] and its organelles are adjusted by osmoregulatory processes. During pinocytosis, extracellular fluid volume equivalent to its CV is incorporated within an hour and membrane area equivalent to the cell's surface within 30 min. Since neither fluid uptake nor membrane consumption leads to swelling or shrinkage, cells must be equipped with potent volume regulatory mechanisms. Normally, cells respond to outwardly or inwardly directed osmotic gradients by a volume decrease and increase, respectively, i.e., they shrink or swell but then try to recover their CV. However, when a cell death (CD) pathway is triggered, CV persistently decreases in isotonic conditions in apoptosis and it increases in necrosis. One type of CD associated with cell swelling is due to a dysfunctional pinocytosis. Methuosis, a non-apoptotic CD phenotype, occurs when cells accumulate too much fluid by macropinocytosis. In contrast to functional pinocytosis, in methuosis, macropinosomes neither recycle nor fuse with lysosomes but with each other to form giant vacuoles, which finally cause rupture of the plasma membrane (PM). Understanding methuosis longs for the understanding of the ionic mechanisms of cell volume regulation (CVR) and vesicular volume regulation (VVR). In nascent macropinosomes, ion channels and transporters are derived from the PM. Along trafficking from the PM to the perinuclear area, the equipment of channels and transporters of the vesicle membrane changes by retrieval, addition, and recycling from and back to the PM, causing profound changes in vesicular ion concentrations, acidification, and-most importantly-shrinkage of the macropinosome, which is indispensable for its proper targeting and cargo processing. In this review, we discuss ion and water transport mechanisms with respect to CVR and VVR and with special emphasis on pinocytosis and methuosis. We describe various aspects of the complex mutual interplay between extracellular and intracellular ions and ion gradients, the PM and vesicular membrane, phosphoinositides, monomeric G proteins and their targets, as well as the submembranous cytoskeleton. Our aim is to highlight important cellular mechanisms, components, and processes that may lead to methuotic CD upon their derangement.

Endocytosis: A Turnover Mechanism Controlling Ion Channel Function

Ion channels (IChs) are transmembrane proteins that selectively drive ions across membranes. The function of IChs partially relies on their abundance and proper location in the cell, fine-tuned by the delicate balance between secretory, endocytic, and degradative pathways. The disruption of this balance is associated with several diseases, such as Liddle's and long QT syndromes. Because of the vital role of these proteins in human health and disease, knowledge of ICh turnover is essential. Clathrin-dependent and -independent mechanisms have been the primary mechanisms identified with ICh endocytosis and degradation. Several molecular determinants recognized by the cellular internalization machinery have been discovered. Moreover, specific conditions can trigger the endocytosis of many IChs, such as the activation of certain receptors, hypokalemia, and some drugs. Ligand-dependent receptor activation primarily results in the posttranslational modification of IChs and the recruitment of important mediators, such as β-arrestins and ubiquitin ligases. However, endocytosis is not a final fate. Once internalized into endosomes, IChs are either sorted to lysosomes for degradation or recycled back to the plasma membrane. Rab proteins are crucial participants during these turnover steps. In this review, we describe the major ICh endocytic pathways, the signaling inputs triggering ICh internalization, and the key mediators of this essential cellular process.

CUL4-DDB1-CRBN E3 Ubiquitin Ligase Regulates Proteostasis of ClC-2 Chloride Channels: Implication for Aldosteronism and Leukodystrophy

Voltage-gated ClC-2 channels are essential for chloride homeostasis. Complete knockout of mouse ClC-2 leads to testicular degeneration and neuronal myelin vacuolation. Gain-of-function and loss-of-function mutations in the ClC-2-encoding human CLCN2 gene are linked to the genetic diseases aldosteronism and leukodystrophy, respectively. The protein homeostasis (proteostasis) mechanism of ClC-2 is currently unclear. Here, we aimed to identify the molecular mechanism of endoplasmic reticulum-associated degradation of ClC-2, and to explore the pathophysiological significance of disease-associated anomalous ClC-2 proteostasis. In both heterologous expression system and native neuronal and testicular cells, ClC-2 is subject to significant regulation by cullin-RING E3 ligase-mediated polyubiquitination and proteasomal degradation. The cullin 4 (CUL4)-damage-specific DNA binding protein 1 (DDB1)-cereblon (CRBN) E3 ubiquitin ligase co-exists in the same complex with and promotes the degradation of ClC-2 channels. The CRBN-targeting immunomodulatory drug lenalidomide and the cullin E3 ligase inhibitor MLN4924 promotes and attenuates, respectively, proteasomal degradation of ClC-2. Analyses of disease-related ClC-2 mutants reveal that aldosteronism and leukodystrophy are associated with opposite alterations in ClC-2 proteostasis. Modifying CUL4 E3 ligase activity with lenalidomide and MLN4924 ameliorates disease-associated ClC-2 proteostasis abnormality. Our results highlight the significant role and therapeutic potential of CUL4 E3 ubiquitin ligase in regulating ClC-2 proteostasis.

CLC Chloride Channels and Transporters: Structure, Function, Physiology, and Disease

CLC anion transporters are found in all phyla and form a gene family of eight members in mammals. Two CLC proteins, each of which completely contains an ion translocation parthway, assemble to homo- or heteromeric dimers that sometimes require accessory β-subunits for function. CLC proteins come in two flavors: anion channels and anion/proton exchangers. Structures of these two CLC protein classes are surprisingly similar. Extensive structure-function analysis identified residues involved in ion permeation, anion-proton coupling and gating and led to attractive biophysical models. In mammals, ClC-1, -2, -Ka/-Kb are plasma membrane Cl−channels, whereas ClC-3 through ClC-7 are 2Cl−/H+-exchangers in endolysosomal membranes. Biological roles of CLCs were mostly studied in mammals, but also in plants and model organisms like yeast and Caenorhabditis elegans. CLC Cl−channels have roles in the control of electrical excitability, extra- and intracellular ion homeostasis, and transepithelial transport, whereas anion/proton exchangers influence vesicular ion composition and impinge on endocytosis and lysosomal function. The surprisingly diverse roles of CLCs are highlighted by human and mouse disorders elicited by mutations in their genes. These pathologies include neurodegeneration, leukodystrophy, mental retardation, deafness, blindness, myotonia, hyperaldosteronism, renal salt loss, proteinuria, kidney stones, male infertility, and osteopetrosis. In this review, emphasis is laid on biophysical structure-function analysis and on the cell biological and organismal roles of mammalian CLCs and their role in disease.

CLC-2 is a positive modulator of oligodendrocyte precursor cell differentiation and myelination

Oligodendrocytes (OLs) are myelin-forming cells that are present within the central nervous system. Impaired oligodendrocyte precursor cell (OPC) differentiation into mature OLs is a major cause of demyelination diseases. Therefore, identifying the underlying molecular mechanisms of OPC differentiation is crucial to understand the processes of myelination and demyelination. It has been acknowledged that various extrinsic and intrinsic factors are involved in the control of OPC differentiation; however, the function of ion channels, particularly the voltage-gated chloride channel (CLC), in OPC differentiation and myelination are not fully understood. The present study demonstrated that CLC-2 may be a positive modulator of OPC differentiation and myelination. Western blotting results revealed that CLC-2 was expressed in both OPCs and OLs. Furthermore, CLC-2 currents (I_CLC-2) were recorded in both types of cells. The inhibition of I_CLC-2 by GaTx2, a blocker of CLC-2, was demonstrated to be higher in OPCs compared with OLs, indicating that CLC-2 may serve a role in OL differentiation. The results of western blotting and immunofluorescence staining also demonstrated that the expression levels of myelin basic protein were reduced following GaTx2 treatment, indicating that the differentiation of OPCs into OLs was inhibited following CLC-2 inhibition. In addition, following western blot analysis, it was also demonstrated that the protein expression of the myelin proteins yin yang 1, myelin regulatory factor, Smad-interacting protein 1 and sex-determining region Y-box 10 were regulated by CLC-2 inhibition. Taken together, the results of the present study indicate that CLC-2 may be a positive regulator of OPC differentiation and able to contribute to myelin formation and repair in myelin-associated diseases by controlling the number and open state of CLC-2 channels.

Research and progress on ClC‑2 (Review)

Chloride channel 2 (ClC-2) is one of the nine mammalian members of the ClC family. The present review discusses the molecular properties of ClC‑2, including CLCN2, ClC‑2 promoter and the structural properties of ClC‑2 protein; physiological properties; functional properties, including the regulation of cell volume. The effects of ClC‑2 on the digestive, respiratory, circulatory, nervous and optical systems are also discussed, in addition to the mechanisms involved in the regulation of ClC‑2. The review then discusses the diseases associated with ClC‑2, including degeneration of the retina, Sjögren's syndrome, age‑related cataracts, degeneration of the testes, azoospermia, lung cancer, constipation, repair of impaired intestinal mucosa barrier, leukemia, cystic fibrosis, leukoencephalopathy, epilepsy and diabetes mellitus. It was concluded that future investigations of ClC‑2 are likely to be focused on developing specific drugs, activators and inhibitors regulating the expression of ClC‑2 to treat diseases associated with ClC‑2. The determination of CLCN2 is required to prevent and treat several diseases associated with ClC‑2.

Activated ClC-2 Inhibits p-Akt to Repress Myelination in GDM Newborn Rats

This study aims to investigate the effect and mechanism of type 2 voltage-gated chloride channel (ClC-2) on myelin development of newborn rats' cerebral white matter with gestational diabetes mellitus (GDM). In this study, GDM model was induced in late pregnant rat model. The alteration of ClC-2 expression in various developmental stages of cerebral white matter with/without being exposed to high glucose was analyzed using RT-PCR, active oxygen detection, TUNEL staining, Western Blot as well as immuno-histochemical staining. Our results showed that ClC-2 mRNA and protein expressions in GDM group were significantly increased in white matter of fetal rats after E18 stage, and elevated the level of TNF-α and iNOS in white matter at P0 and P3 stage of newborn rats. Meanwhile, In GDM group, reactive oxygen species (ROS) levels of the white matter at E18, P0, and P3 stage were significantly higher than control group. Furthermore, the expression level of myelin transcription factor Olig2 at P0 stage and CNPase at P3 stage were strikingly lower than that of the control group. In GDM group, ClC-2 expression in the corpus callosum (CC) and cingulate gyrus (CG) regains, and TUNEL positive cell number were increased at P0 and P3 stage. However, PDGFα positive cell number at P0 stage and CNPase expression at P3 stage were significantly decreased. Caspase-3 was also increased in those white matter regions in GDM group, but p-Akt expression was inhibited. While DIDS (a chloride channel blocker) can reverse these changes. In conclusion, ClC-2 and caspase-3 were induced by GDM, which resulted in apoptosis and myelination inhibition. The effect was caused by repressing PI3K-Akt signaling pathway. Application of ClC-2 inhibitor DIDS showed protective effects on cerebral white matter damage stimulated by high glucose concentration.

Chloride channelopathies of ClC-2

Chloride channels (ClCs) have gained worldwide interest because of their molecular diversity, widespread distribution in mammalian tissues and organs, and their link to various human diseases. Nine different ClCs have been molecularly identified and functionally characterized in mammals. ClC-2 is one of nine mammalian members of the ClC family. It possesses unique biophysical characteristics, pharmacological properties, and molecular features that distinguish it from other ClC family members. ClC-2 has wide organ/tissue distribution and is ubiquitously expressed. Published studies consistently point to a high degree of conservation of ClC-2 function and regulation across various species from nematodes to humans over vast evolutionary time spans. ClC-2 has been intensively and extensively studied over the past two decades, leading to the accumulation of a plethora of information to advance our understanding of its pathophysiological functions; however, many controversies still exist. It is necessary to analyze the research findings, and integrate different views to have a better understanding of ClC-2. This review focuses on ClC-2 only, providing an analytical overview of the available literature. Nearly every aspect of ClC-2 is discussed in the review: molecular features, biophysical characteristics, pharmacological properties, cellular function, regulation of expression and function, and channelopathies.

Regulation of ion channels and transporters by AMP-activated kinase (AMPK)

The energy-sensing AMP-activated kinase AMPK ensures survival of energy-depleted cells by stimulating ATP production and limiting ATP utilization. Both energy production and energy consumption are profoundly influenced by transport processes across the cell membane including channels, carriers and pumps. Accordingly, AMPK is a powerful regulator of transport across the cell membrane. AMPK regulates diverse K(+) channels, Na(+) channels, Ca(2+) release activated Ca(2+) channels, Cl(-) channels, gap junctional channels, glucose carriers, Na(+)/H(+)-exchanger, monocarboxylate-, phosphate-, creatine-, amino acid-, peptide- and osmolyte-transporters, Na(+)/Ca(2+)-exchanger, H(+)-ATPase and Na(+)/K(+)-ATPase. AMPK activates ubiquitin ligase Nedd4-2, which labels several plasma membrane proteins for degradation. AMPK further regulates transport proteins by inhibition of Rab GTPase activating protein (GAP) TBC1D1. It stimulates phosphatidylinositol 3-phosphate 5-kinase PIKfyve and inhibits phosphatase and tensin homolog (PTEN) via glycogen synthase kinase 3β (GSK3β). Moreover, it stabilizes F-actin as well as downregulates transcription factor NF-κB. All those cellular effects serve to regulate transport proteins.

Cell biology and physiology of CLC chloride channels and transporters

Proteins of the CLC gene family assemble to homo- or sometimes heterodimers and either function as Cl(-) channels or as Cl(-)/H(+)-exchangers. CLC proteins are present in all phyla. Detailed structural information is available from crystal structures of bacterial and algal CLCs. Mammals express nine CLC genes, four of which encode Cl(-) channels and five 2Cl(-)/H(+)-exchangers. Two accessory β-subunits are known: (1) barttin and (2) Ostm1. ClC-Ka and ClC-Kb Cl(-) channels need barttin, whereas Ostm1 is required for the function of the lysosomal ClC-7 2Cl(-)/H(+)-exchanger. ClC-1, -2, -Ka and -Kb Cl(-) channels reside in the plasma membrane and function in the control of electrical excitability of muscles or neurons, in extra- and intracellular ion homeostasis, and in transepithelial transport. The mainly endosomal/lysosomal Cl(-)/H(+)-exchangers ClC-3 to ClC-7 may facilitate vesicular acidification by shunting currents of proton pumps and increase vesicular Cl(-) concentration. ClC-3 is also present on synaptic vesicles, whereas ClC-4 and -5 can reach the plasma membrane to some extent. ClC-7/Ostm1 is coinserted with the vesicular H(+)-ATPase into the acid-secreting ruffled border membrane of osteoclasts. Mice or humans lacking ClC-7 or Ostm1 display osteopetrosis and lysosomal storage disease. Disruption of the endosomal ClC-5 Cl(-)/H(+)-exchanger leads to proteinuria and Dent's disease. Mouse models in which ClC-5 or ClC-7 is converted to uncoupled Cl(-) conductors suggest an important role of vesicular Cl(-) accumulation in these pathologies. The important functions of CLC Cl(-) channels were also revealed by human diseases and mouse models, with phenotypes including myotonia, renal loss of salt and water, deafness, blindness, leukodystrophy, and male infertility.

The neuronal serum- and glucocorticoid-regulated kinase 1.1 reduces neuronal excitability and protects against seizures through upregulation of the M-current

The M-current formed by tetramerization of Kv7.2 and Kv7.3 subunits is a neuronal voltage-gated K(+) conductance that controls resting membrane potential and cell excitability. In Xenopus laevis oocytes, an increase in Kv7.2/3 function by the serum- and glucocorticoid-regulated kinase 1 (SGK1) has been reported previously (Schuetz et al., 2008). We now show that the neuronal isoform of this kinase (SGK1.1), with distinct subcellular localization and modulation, upregulates the Kv7.2/3 current in Xenopus oocytes and mammalian human embryonic kidney HEK293 cells. In contrast to the ubiquitously expressed SGK1, the neuronal isoform SGK1.1 interacts with phosphoinositide-phosphatidylinositol 4,5-bisphosphate (PIP(2)) and is distinctly localized to the plasma membrane (Arteaga et al., 2008). An SGK1.1 mutant with disrupted PIP(2) binding sites produced no effect on Kv7.2/3 current amplitude. SGK1.1 failed to modify the voltage dependence of activation and did not change activation or deactivation kinetics of Kv7.2/3 channels. These results suggest that the kinase increases channel membrane abundance, which was confirmed with flow cytometry assays. To evaluate the effect of the kinase in neuronal excitability, we generated a transgenic mouse (Tg.sgk) expressing a constitutively active form of SGK1.1 (S515D). Superior cervical ganglion (SCG) neurons isolated from Tg.sgk mice showed a significant increase in M-current levels, paralleled by reduced excitability and more negative resting potentials. SGK1.1 effect on M-current in Tg.sgk-SCG neurons was counteracted by muscarinic receptor activation. Transgenic mice with increased SGK1.1 activity also showed diminished sensitivity to kainic acid-induced seizures. Altogether, our results unveil a novel role of SGK1.1 as a physiological regulator of the M-current and neuronal excitability.

Dual activation of CFTR and CLCN2 by lubiprostone in murine nasal epithelia

Multiple sodium and chloride channels on the apical surface of nasal epithelial cells contribute to periciliary fluid homeostasis, a function that is disrupted in patients with cystic fibrosis (CF). Among these channels is the chloride channel CLCN2, which has been studied as a potential alternative chloride efflux pathway in the absence of CFTR. The object of the present study was to use the nasal potential difference test (NPD) to quantify CLCN2 function in an epithelial-directed TetOn CLCN2 transgenic mouse model (TGN-K18rtTA-hCLCN2) by using the putative CLCN2 pharmacological agonist lubiprostone and peptide inhibitor GaTx2. Lubiprostone significantly increased chloride transport in the CLCN2-overexpressing mice following activation of the transgene by doxycycline. This response to lubiprostone was significantly inhibited by GaTx2 after CLCN2 activation in TGN-CLCN2 mice. Cftr(-/-) and Clc2(-/-) mice showed hyperpolarization indicative of chloride efflux in response to lubiprostone, which was fully inhibited by GaTx2 and CFTR inhibitor 172 + GlyH-101, respectively. Our study reveals lubiprostone as a pharmacological activator of both CFTR and CLCN2. Overexpression and activation of CLCN2 leads to improved mouse NPD readings, suggesting it is available as an alternative pathway for epithelial chloride secretion in murine airways. The utilization of CLCN2 as an alternative chloride efflux channel could provide clinical benefit to patients with CF, especially if the pharmacological activator is administered as an aerosol.

Regulation of ion channels by the serum- and glucocorticoid-inducible kinase SGK1

The ubiquitously expressed serum- and glucocorticoid-inducible kinase-1 (SGK1) is genomically regulated by cell stress (including cell shrinkage) and several hormones (including gluco- and mineralocorticoids). SGK1 is activated by insulin and growth factors through PI3K and 3-phosphoinositide-dependent kinase PDK1. SGK1 activates a wide variety of ion channels (e.g., ENaC, SCN5A, TRPV4-6, ROMK, Kv1.3, Kv1.5, Kv4.3, KCNE1/KCNQ1, KCNQ4, ASIC1, GluR6, ClCKa/barttin, ClC2, CFTR, and Orai/STIM), which participate in the regulation of transport, hormone release, neuroexcitability, inflammation, cell proliferation, and apoptosis. SGK1-sensitive ion channels participate in the regulation of renal Na(+) retention and K(+) elimination, blood pressure, gastric acid secretion, cardiac action potential, hemostasis, and neuroexcitability. A common (∼3-5% prevalence in Caucasians and ∼10% in Africans) SGK1 gene variant is associated with increased blood pressure and body weight as well as increased prevalence of type II diabetes and stroke. SGK1 further contributes to the pathophysiology of allergy, peptic ulcer, fibrosing disease, ischemia, tumor growth, and neurodegeneration. The effect of SGK1 on channel activity is modest, and the channels do not require SGK1 for basic function. SGK1-dependent ion channel regulation may thus become pathophysiologically relevant primarily after excessive (pathological) expression. Therefore, SGK1 may be considered an attractive therapeutic target despite its broad range of functions.

Second AKT: the rise of SGK in cancer signalling

The serum and glucocorticoid kinase (SGK) family of serine/threonine kinases consists of three isoforms, SGK-1, SGK-2 and SGK-3. This family of kinases is highly homologous to the AKT kinase family, sharing similar upstream activators and downstream targets. SGKs have been implicated in the regulation of cell growth, proliferation, survival and migration: cellular processes that are dysregulated in cancer. Furthermore, SGKs lie downstream of phosphoinositide-3-kinase (PI3Kinase) signalling and interact at various levels with RAS/RAF/ERK signalling, two pathways that are involved in promoting tumorigenesis. Recent evidence suggests that mutant PI3Kinase can induce tumorigenesis through an AKT-independent but SGK3-dependent mechanism, thus implicating SGKs as potential players in malignant transformation. Here, we will review the current state of knowledge on the regulation of the SGKs and their role in normal cell physiology and transformation with a particular focus on SGK3.

Role of the ubiquitin system in regulating ion transport

Ion channels and transporters play a critical role in ion and fluid homeostasis and thus in normal animal physiology and pathology. Tight regulation of these transmembrane proteins is therefore essential. In recent years, many studies have focused their attention on the role of the ubiquitin system in regulating ion channels and transporters, initialed by the discoveries of the role of this system in processing of Cystic Fibrosis Transmembrane Regulator (CFTR), and in regulating endocytosis of the epithelial Na(+) channel (ENaC) by the Nedd4 family of ubiquitin ligases (mainly Nedd4-2). In this review, we discuss the role of the ubiquitin system in ER Associated Degradation (ERAD) of ion channels, and in the regulation of endocytosis and lysosomal sorting of ion channels and transporters, focusing primarily in mammalian cells. We also briefly discuss the role of ubiquitin like molecules (such as SUMO) in such regulation, for which much less is known so far.

Down-regulation of intestinal apical calcium entry channel TRPV6 by ubiquitin E3 ligase Nedd4-2

Nedd4-2 is an archetypal HECT ubiquitin E3 ligase that disposes target proteins for degradation. Because of the proven roles of Nedd4-2 in degradation of membrane proteins, such as epithelial Na(+) channel, we examined the effect of Nedd4-2 on the apical Ca(2+) channel TRPV6, which is involved in transcellular Ca(2+) transport in the intestine using the Xenopus laevis oocyte system. We demonstrated that a significant amount of Nedd4-2 protein was distributed to the absorptive epithelial cells in ileum, cecum, and colon along with TRPV6. When co-expressed in oocytes, Nedd4-2 and, to a lesser extent, Nedd4 down-regulated the protein abundance and Ca(2+) influx of TRPV6 and TRPV5, respectively. TRPV6 ubiquitination was increased, and its stability was decreased by Nedd4-2. The Nedd4-2 inhibitory effects on TRPV6 were partially blocked by proteasome inhibitor MG132 but not by the lysosome inhibitor chloroquine. The rate of TRPV6 internalization was not significantly altered by Nedd4-2. The HECT domain was essential to the inhibitory effect of Nedd4-2 on TRPV6 and to their association. The WW1 and WW2 domains interacted with TRPV6 terminal regions, and a disruption of the interactions by D204H and D376H mutations in the WW1 and WW2 domains increased TRPV6 ubiquitination and degradation. Thus, WW1 and WW2 may serve as a molecular switch to limit the ubiquitination of TRPV6 by the HECT domain. In conclusion, Nedd4-2 may regulate TRPV6 protein abundance in intestinal epithelia by controlling TRPV6 ubiquitination.

Blunted IgE-mediated activation of mast cells in mice lacking the serum- and glucocorticoid-inducible kinase SGK3

Previous studies have shown that pharmacological inhibition of the phosphoinositol-3 (PI3) kinase disrupts the activation of mast cells. Through phosphoinositide-dependent kinase PDK1, PI3 kinase activates the serum- and glucocorticoid-inducible kinase 3 (SGK3). The present study explored the role of SGK3 in mast cell function. Mast cells were isolated and cultured from bone marrow (BMMCs) of gene-targeted mice lacking SGK3 (sgk3(-/-)) and their wild-type littermates (sgk3(+/+)). BMMC numbers in the ear conch were similar in both genotypes. Stimulation with IgE and cognate antigen triggered the release of intracellular Ca(2+) and entry of extracellular Ca(2+). Influx of extracellular Ca(2+) but not Ca(2+) release from intracellular stores was significantly blunted in sgk3(-/-) BMMCs compared with sgk3(+/+) BMMCs. Antigen stimulation further led to a rapid increase of a K(+)-selective conductance in sgk3(+/+) BMMCs, an effect again blunted in sgk3(-/-) BMMCs. In contrast, the Ca(2+) ionophore ionomycin activated K(+) currents to a similar extent in sgk3(-/-) and in sgk3(+/+) BMMCs. β-Hexosaminidase release, triggered by antigen stimulation, was also significantly decreased in sgk3(-/-) BMMCs. IgE-dependent anaphylaxis measured as a sharp decrease in body temperature upon injection of DNP-HSA antigen was again significantly blunted in sgk3(-/-) compared with sgk3(+/+) mice. Serum histamine levels measured 30 min after induction of an anaphylactic reaction were significantly lower in sgk3(-/-) than in sgk3(+/+) mice. In conclusion, both in vitro and in vivo function of BMMCs are impaired in gene targeted mice lacking SGK3. Thus SGK3 is critical for proper mast cell function.

Comparison of substrate specificity of the ubiquitin ligases Nedd4 and Nedd4-2 using proteome arrays

Target recognition by the ubiquitin system is mediated by E3 ubiquitin ligases. Nedd4 family members are E3 ligases comprised of a C2 domain, 2–4 WW domains that bind PY motifs (L/PPxY) and a ubiquitin ligase HECT domain. The nine Nedd4 family proteins in mammals include two close relatives: Nedd4 (Nedd4-1) and Nedd4L (Nedd4-2), but their global substrate recognition or differences in substrate specificity are unknown. We performed in vitro ubiquitylation and binding assays of human Nedd4-1 and Nedd4-2, and rat-Nedd4-1, using protein microarrays spotted with ∼8200 human proteins. Top hits (substrates) for the ubiquitylation and binding assays mostly contain PY motifs. Although several substrates were recognized by both Nedd4-1 and Nedd4-2, others were specific to only one, with several Tyr kinases preferred by Nedd4-1 and some ion channels by Nedd4-2; this was subsequently validated in vivo. Accordingly, Nedd4-1 knockdown or knockout in cells led to sustained signalling via some of its substrate Tyr kinases (e.g. FGFR), suggesting Nedd4-1 suppresses their signalling. These results demonstrate the feasibility of identifying substrates and deciphering substrate specificity of mammalian E3 ligases.

PIKfyve-dependent regulation of the Cl- channel ClC-2

The widely expressed chloride channel ClC-2 is stimulated by the serum and glucocorticoid inducible kinase SGK1. The SGK1-dependent regulation of several carriers involves the mammalian phosphatidylinositol-3-phosphate-5-kinase PIKfyve (PIP5K3). The present experiments explored whether SGK1-dependent regulation of ClC-2 similarly involves PIKfyve. The conductance of Xenopus oocytes is increased more than eightfold by ClC-2 expression. In ClC-2-expressing oocytes, but not in water-injected oocytes, the current was further enhanced by coexpression of either, PIKfyve or constitutively active (S422D)SGK1. Coexpression of the inactive SGK1 mutant (K127N)SGK1 did not significantly alter the current in ClC-2-expressing oocytes and abrogated the stimulation of the current by PIKfyve-coexpression. The stimulating effect of PIKfyve was abolished by replacement of the serine with alanine in the SGK1 consensus sequence ((S318A)PIKfyve). Coexpression of (S318A)PIKfyve significantly blunted the stimulating effect of (S422D)SGK1 on ClC-2-activity. In conclusion, PIKfyve is a potent stimulator of ClC-2-activity and contributes to SGK1-dependent regulation of ClC-2.

Functional regulation of the epithelial Na+ channel by IkappaB kinase-beta occurs via phosphorylation of the ubiquitin ligase Nedd4-2

We have previously shown that IkappaB kinase-beta (IKKbeta) interacts with the epithelial Na+ channel (ENaC) beta-subunit and enhances ENaC activity by increasing its surface expression in Xenopus oocytes. Here, we show that the IKKbeta-ENaC interaction is physiologically relevant in mouse polarized kidney cortical collecting duct (mpkCCDc14) cells, as RNA interference-mediated knockdown of endogenous IKKbeta in these cells by approximately 50% resulted in a similar reduction in transepithelial ENaC-dependent equivalent short circuit current. Although IKKbeta binds to ENaC, there was no detectable phosphorylation of ENaC subunits by IKKbeta in vitro. Because IKKbeta stimulation of ENaC activity occurs through enhanced channel surface expression and the ubiquitin-protein ligase Nedd4-2 has emerged as a central locus for ENaC regulation at the plasma membrane, we tested the role of Nedd4-2 in this regulation. IKKbeta-dependent phosphorylation of Xenopus Nedd4-2 expressed in HEK-293 cells occurred both in vitro and in vivo, suggesting a potential mechanism for regulation of Nedd4-2 and thus ENaC activity. 32P labeling studies utilizing wild-type or mutant forms of Xenopus Nedd4-2 demonstrated that Ser-444, a key SGK1 and protein kinase A-phosphorylated residue, is also an important IKKbeta phosphorylation target. ENaC stimulation by IKKbeta was preserved in oocytes expressing wild-type Nedd4-2 but blocked in oocytes expressing either a dominant-negative (C938S) or phospho-deficient (S444A) Nedd4-2 mutant, suggesting that Nedd4-2 function and phosphorylation by IKKbeta are required for IKKbeta regulation of ENaC. In summary, these results suggest a novel mode of ENaC regulation that occurs through IKKbeta-dependent Nedd4-2 phosphorylation at a recognized SGK1 and protein kinase A target site.

A synthetic prostone activates apical chloride channels in A6 epithelial cells

The bicyclic fatty acid lubiprostone (formerly known as SPI-0211) activates two types of anion channels in A6 cells. Both channel types are rarely, if ever, observed in untreated cells. The first channel type was activated at low concentrations of lubiprostone (<100 nM) in >80% of cell-attached patches and had a unit conductance of approximately 3-4 pS. The second channel type required higher concentrations (>100 nM) of lubiprostone to activate, was observed in approximately 30% of patches, and had a unit conductance of 8-9 pS. The properties of the first type of channel were consistent with ClC-2 and the second with CFTR. ClC-2's unit current strongly inwardly rectified that could be best fit by models of the channel with multiple energy barrier and multiple anion binding sites in the conductance pore. The open probability and mean open time of ClC-2 was voltage dependent, decreasing dramatically as the patches were depolarized. The order of anion selectivity for ClC-2 was Cl > Br > NO(3) > I > SCN, where SCN is thiocyanate. ClC-2 was a "double-barreled" channel favoring even numbers of levels over odd numbers as if the channel protein had two conductance pathways that opened independently of one another. The channel could be, at least, partially blocked by glibenclamide. The properties of the channel in A6 cells were indistinguishable from ClC-2 channels stably transfected in HEK293 cells. CFTR in the patches had a selectivity of Cl > Br >> NO(3) congruent with SCN congruent with I. It outwardly rectified as expected for a single-site anion channel. Because of its properties, ClC-2 is uniquely suitable to promote anion secretion with little anion reabsorption. CFTR, on the other hand, could promote either reabsorption or secretion depending on the anion driving forces.

ATP depletion inhibits the endocytosis of ClC-2

The chloride channel, ClC-2 is expressed ubiquitously and participates in multiple physiological processes. In particular, ClC-2 has been implicated in the regulation of neuronal chloride ion homeostasis and mutations in ClC-2 are associated with idiopathic generalized epilepsy. Despite the physiological and pathophysiological significance of this channel, its regulation remains incompletely understood. The functional expression of ClC-2 at the cell surface has been shown to be enhanced by depletion of cellular ATP, implicating its possible role in cellular energy sensing. In the present study, biochemical assays of cell surface expression suggest that this gain of function reflects, in part, an increase in channel number due to the reduction in ClC-2 internalization by endocytosis. Cell surface expression of the disease-causing mutant: G715E, thought to lack wild-type nucleotide binding affinity, is similarly affected, suggesting that ATP-depletion modifies the function of proteins in the endocytic pathway rather than ClC-2 directly. Using a combination of immunofluorescence and biochemical studies, we confirmed that ClC-2 is internalized via dynamin-dependent endocytosis and that the change in surface expression evoked by ATP depletion is partially mimicked by inhibition of dynamin function using a dynamin dominant-negative mutant (DynK44A). Furthermore, trafficking via the early endosomal compartment occurs in part through rab5-associated vesicles and recycling of ClC-2 to the cell surface occurs through a rab11 dependent pathway. In summary, we have determined that the internalization of ClC-2 by endocytosis is inhibited by metabolic stress, highlighting the importance for understanding the molecular mechanisms mediating the endosomal trafficking of this channel.

EAAT4 phosphorylation at the SGK1 consensus site is required for transport modulation by the kinase

EAAT4 (SLC1A6) is a Purkinje-Cell-specific post-synaptic excitatory amino acid transporter that plays a major role in clearing synaptic glutamate. EAAT4 abundance and function is known to be modulated by the serum and glucocorticoid inducible kinase (SGK) 1 but the precise mechanism of kinase action has not been defined yet. The present work aims to identify the molecular mechanism of EAAT4 modulation by the kinase. The EAAT4 sequence bears two putative SGK1 consensus sites (at Thr40 and Thr504) at the amino and carboxy terminus that are conserved among species. Expression studies in Xenopus oocytes demonstrated that EAAT4-mediated [(3)H] glutamate uptake and cell surface abundance are enhanced by co-expression of SGK1. Disruption of the SGK1 phosphorylation site at threonine 40 ((T40A)EAAT4) or of both phosphorylation sites ((T40AT504A)EAAT4) abrogated the effect of SGK1 on transporter function and expression. SGK1 modulates several transport proteins via inhibition of the ubiquitin ligase Nedd4-2. Co-expression of Nedd4-2 inhibited wild-type EAAT4 but not the (T40AT504A)EAAT4 mutant. Besides, RNA interference-mediated reduction of endogenous Nedd4-2 (xNedd4-2) expression increased the activity of the transporter. In conclusion, maximal glutamate transport modulation by SGK1 is accomplished by direct EAAT4 stimulation and to a lesser extent by inhibition of intrinsic Nedd4-2.

Regulation of the Voltage-gated K+ Channels KCNQ2/3 and KCNQ3/5 by Ubiquitination

The muscarine-sensitive K(+) current (M-current) stabilizes the resting membrane potential in neurons, thus limiting neuronal excitability. The M-current is mediated by heteromeric channels consisting of KCNQ3 subunits in association with either KCNQ2 or KCNQ5 subunits. The role of KCNQ2/3/5 in the regulation of neuronal excitability is well established; however, little is known about the mechanisms that regulate the cell surface expression of these channels. Ubiquitination by the Nedd4/Nedd4-2 ubiquitin ligases is known to regulate a number of membrane ion channels and transporters. In this study, we investigated whether Nedd4/Nedd4-2 could regulate KCNQ2/3/5 channels. We found that the amplitude of the K(+) currents mediated by KCNQ2/3 and KCNQ3/5 were reduced by Nedd4-2 (but not Nedd4) in a Xenopus oocyte expression system. Deletion experiments showed that the C-terminal region of the KCNQ3 subunit is required for the Nedd4-2-mediated regulation of the heteromeric channels. Glutathione S-transferase fusion pulldowns and co-immunoprecipitations demonstrated a direct interaction between KCNQ2/3 and Nedd4-2. Furthermore, Nedd4-2 could ubiquitinate KCNQ2/3 in transfected cells. Taken together, these data suggest that Nedd4-2 is potentially an important regulator of M-current activity in the nervous system.

NEDD4-2 as a potential candidate susceptibility gene for epileptic photosensitivity

Photosensitive seizures occur most commonly in childhood and adolescence, usually as a manifestation of complex idiopathic generalized epilepsies (IGEs). Molecular mechanisms underlying this condition are yet to be determined because no susceptibility genes have been identified. The NEDD4-2 (Neuronally Expressed Developmentally Downregulated 4) gene encodes a ubiquitin protein ligase proposed to regulate cell surface levels of several ion channels, receptors and transporters involved in regulating neuronal excitability, including voltage-gated sodium channels (VGSCs), the most clinically relevant of the epilepsy genes. The regulation of NEDD4-2 in vivo involves complex interactions with accessory proteins in a cell type specific manner. We screened NEDD4-2 for mutations in a cohort of 253 families with IGEs. We identified three NEDD4-2 missense changes in highly conserved residues; S233L, E271A and H515P in families with photosensitive generalized epilepsy. The NEDD4-2 variants were as effective as wild-type NEDD4-2 in downregulating the VGSC subtype Na(v)1.2 when assessed in the Xenopus oocyte heterologous expression system showing that the direct interaction with the ion channel was not altered by these variants. These data raise the possibility that photosensitive epilepsy may arise from defective interaction of NEDD4-2 with as yet unidentified accessory or target proteins.

Upregulation of HERG channels by the serum and glucocorticoid inducible kinase isoform SGK3

Human ether-a-go-go (HERG) channels participate in the repolarization of the cardiac action potential. Loss of function mutations of HERG lead to delayed cardiac repolarization reflected by prolonged QT interval. HERG channels are regulated through a signaling cascade involving phosphatidylinositol 3 (PI3) kinase. Downstream targets of PI3 kinase include the serum and glucocorticoid inducible kinase (SGK) and protein kinase B (PKB) isoforms. The present study has been performed to explore whether SGK1 and SGK3 participate in the regulation of HERG channel activity. HERG was expressed in Xenopus oocytes with or without additional expression of SGK1 or SGK3. Chemiluminescence was employed to determine HERG plasma membrane protein abundance. Coexpression of SGK3 but not of SGK1 in Xenopus oocytes resulted in an increase of steady state current (I(HERG)) and enhanced cell membrane protein abundance without affecting gating kinetics of the channel. Replacement of serine by alanine at the two SGK consensus sites decreased I(HERG) but neither mutation abolished the stimulating effect of SGK3. In conclusion, SGK3 participates in the regulation of HERG by increasing HERG protein abundance in the plasma membrane and may thus modify the duration of the cardiac action potential.

Serum and glucocorticoid-regulated protein kinases: variations on a theme

The phosphatidylinositol 3' kinase (PI3K)-signaling pathway plays a critical role in a variety of cellular responses such as modulation of cell survival, glucose homeostasis, cell division, and cell growth. PI3K generates important lipid second messengers-phosphatidylinositides that are phosphorylated at the 3' position of their inositol ring head-group. These membrane restricted lipids act by binding with high affinity to specific protein domains such as the pleckstrin homology (PH) domain. Effectors of PI3K include molecules that harbor such domains such as phosphoinositide-dependent kinase (PDK1) and protein kinase B (PKB), also termed Akt. The mammalian genome encodes three different PKB genes (alpha, beta, and gamma; Akt1, 2, and 3, respectively) and each is an attractive target for therapeutic intervention in diseases such as glioblastoma and breast cancer. A second family of three protein kinases, termed serum and glucocorticoid-regulated protein kinases (SGKs), is structurally related to the PKB family including regulation by PI3K but lack a PH domain. However, in addition to PH domains, a second class of 3' phosphorylated inositol phospholipid-binding domains exists that is termed Phox homology (PX) domain: this domain is found in one of the SGKs (SGK3). Here, we summarize knowledge of the three SGK isoforms and compare and contrast them to PKB with respect to their possible importance in cellular regulation and potential as therapeutic targets.

Membrane cholesterol content modulates ClC-2 gating and sensitivity to oxidative stress

ClC-2 is a broadly expressed member of the voltage-gated ClC chloride channel family. In this study, we aimed to evaluate the role of the membrane lipid environment in ClC-2 function, and in particular the effect of cholesterol and ClC-2 distribution in membrane microdomains. Detergent-resistant and detergent-soluble microdomains (DSM) were isolated from stably transfected HEK293 cells by a discontinuous OptiPrep gradient. ClC-2 was found concentrated in detergent-insoluble membranes in basal conditions and relocalized to DSM upon cholesterol depletion by methyl-beta-cyclodextrin. As assessed by patch clamp recordings, relocalization was accompanied by acceleration of the activation kinetics of the channel. A similar distribution and activation pattern were obtained when cells were treated with the oxidant tert-butyl hydroperoxide and after ATP depletion. In both cases activation was prevented by cholesterol enrichment of cells. We conclude that the cholesterol environment regulates ClC-2 activity, and we provide evidence that the increase in ClC-2 activity in response to acute oxidative or metabolic stress involves relocalization of this channel to DSM.

(Patho)physiological significance of the serum- and glucocorticoid-inducible kinase isoforms

The serum- and glucocorticoid-inducible kinase-1 (SGK1) is ubiquitously expressed and under genomic control by cell stress (including cell shrinkage) and hormones (including gluco- and mineralocorticoids). Similar to its isoforms SGK2 and SGK3, SGK1 is activated by insulin and growth factors via phosphatidylinositol 3-kinase and the 3-phosphoinositide-dependent kinase PDK1. SGKs activate ion channels (e.g., ENaC, TRPV5, ROMK, Kv1.3, KCNE1/KCNQ1, GluR1, GluR6), carriers (e.g., NHE3, GLUT1, SGLT1, EAAT1-5), and the Na+-K+-ATPase. They regulate the activity of enzymes (e.g., glycogen synthase kinase-3, ubiquitin ligase Nedd4-2, phosphomannose mutase-2) and transcription factors (e.g., forkhead transcription factor FKHRL1, beta-catenin, nuclear factor kappaB). SGKs participate in the regulation of transport, hormone release, neuroexcitability, cell proliferation, and apoptosis. SGK1 contributes to Na+ retention and K+ elimination of the kidney, mineralocorticoid stimulation of salt appetite, glucocorticoid stimulation of intestinal Na+/H+ exchanger and nutrient transport, insulin-dependent salt sensitivity of blood pressure and salt sensitivity of peripheral glucose uptake, memory consolidation, and cardiac repolarization. A common ( approximately 5% prevalence) SGK1 gene variant is associated with increased blood pressure and body weight. SGK1 may thus contribute to metabolic syndrome. SGK1 may further participate in tumor growth, neurodegeneration, fibrosing disease, and the sequelae of ischemia. SGK3 is required for adequate hair growth and maintenance of intestinal nutrient transport and influences locomotive behavior. In conclusion, the SGKs cover a wide variety of physiological functions and may play an active role in a multitude of pathophysiological conditions. There is little doubt that further targets will be identified that are modulated by the SGK isoforms and that further SGK-dependent in vivo physiological functions and pathophysiological conditions will be defined.

The N-terminus of the serum- and glucocorticoid-inducible kinase Sgk1 specifies mitochondrial localization and rapid turnover

The serine/threonine protein kinase Sgk1 (serum- and glucocorticoid-inducible kinase 1) is characterized by a short half-life and has been implicated in the control of a large variety of functions in different subcellular compartments and tissues. Here, we analysed the influence of the N-terminus of Sgk1 on protein turnover and subcellular localization. Using green fluorescent protein-tagged Sgk1 deletion variants, we identified amino acids 17-32 to function as an anchor for the OMM (outer mitochondrial membrane). Subcellular fractionation of mouse tissue revealed a predominant localization of Sgk1 to the mitochondrial fraction. A cytosolic orientation of the kinase at the OMM was determined by in vitro import of Sgk1 and protease protection assays. Pulse-chase experiments showed that half-life and subcellular localization of Sgk1 are inseparable and determined by identical amino acids. Our results provide evidence that Sgk1 is primarily localized to the OMM and shed new light on the role of Sgk1 in the control of cellular function.

Association between Hsp90 and the ClC-2 chloride channel upregulates channel function

The voltage-dependent ClC-2 chloride channel has been implicated in a variety of physiological functions, including fluid transport across specific epithelia. ClC-2 is activated by hyperpolarization, weakly acidic external pH, intracellular Cl−, and cell swelling. To add more insight into the mechanisms involved in ClC-2 regulation, we searched for associated proteins that may influence ClC-2 activity. With the use of immunoprecipitation of ClC-2 from human embryonic kidney-293 cells stably expressing the channel, followed by electrophoretic separation of coimmunoprecipitated proteins and mass spectrometry identification, Hsp70 and Hsp90 were unmasked as possible ClC-2 interacting partners. Association of Hsp90 with ClC-2 was confirmed in mouse brain. Inhibition of Hsp90 by two specific inhibitors, geldanamycin or radicicol, did not affect total amounts of ClC-2 but did reduce plasma membrane channel abundance. Functional experiments using the whole cell configuration of the patch-clamp technique showed that inhibition of Hsp90 reduced ClC-2 current amplitude and impaired the intracellular Cl− concentration [Cl−]-dependent rightward shift of the fractional conductance. Geldanamycin and radicicol increased both the slow and fast activation time constants in a chloride-dependent manner. Heat shock treatment had the opposite effect. These results indicate that association of Hsp90 with ClC-2 results in greater channel activity due to increased cell surface channel expression, facilitation of channel opening, and enhanced channel sensitivity to intracellular [Cl−]. This association may have important pathophysiological consequences, enabling increased ClC-2 activity in response to cellular stresses such as elevated temperature, ischemia, or oxidative reagents.

Role of intramolecular and intermolecular interactions in ClC channel and transporter function

The ClC family of chloride channels and transporters includes several members in which mutations have been associated with human disease. Clearly, an understanding of the structure-function relationships of these proteins will be critical in defining the molecular mechanisms underlying disease pathogenesis. The X-ray crystal structure of prokaryotic ClC proteins provides an exquisite template with which to model molecular aspects of eukaryotic ClC protein function. The dimeric structure of these proteins highlights the pivotal importance of intermolecular interactions in the modulation of channel/transporter activity, while mutagenesis studies implicate a crucial role for intrinsic interdomain interactions in regulated function. In this review, we will initially focus on the channel forming members of this family and discuss interactions within homodimeric channel complexes important for gating. Finally, with regard to both channel and transporter family members, we will discuss the multiple heteromeric interactions which occur with cytosolic proteins, and the putative functional impact of these interactions.

Regulation of the excitatory amino acid transporter EAAT5 by the serum and glucocorticoid dependent kinases SGK1 and SGK3

In the mammalian retina, glutamate re-uptake is mediated by the sodium dependent cotransport systems EAAT1-5 thus terminating neuronal excitation and preventing neuroexcitotoxicity. In retinal amacrine and ganglion cells, EAAT5 is colocalized with the serum and glucocorticoid inducible kinase SGK1, a serine/threonine kinase known to regulate transport. The study explored the possible regulation of EAAT5 by SGK1, its isoform SGK3, and the closely related protein kinase B. EAAT5 was coexpressed in Xenopus laevis oocytes with or without the respective kinases. Transport activity was quantified by electrophysiology and cell surface expression was determined by chemiluminescence. Both EAAT5 mediated currents and EAAT5 protein abundance at the cell surface were increased by a factor of 1.5-2 upon coexpression of SGK1 or SGK3 but not following coexpression of PKB. In conclusion, the kinases SGK1 and SGK3 increase EAAT5 activity by increasing cell surface abundance of the carrier.

Transport regulation by the serum- and glucocorticoid-inducible kinase SGK1

The serum- and glucocorticoid-inducible kinase SGK1 is an ubiquitously expressed kinase with the ability to regulate a variety of transport systems. Recent observations point to a role of SGK1 in the regulation of diverse physiological functions such as epithelial transport and cardiac and neuronal excitability. At least partially through its effect on transport, SGK1 contributes to a number of pathophysiological conditions including metabolic syndrome and fibrosing disease.

14-3-3 proteins modulate the expression of epithelial Na+ channels by phosphorylation-dependent interaction with Nedd4-2 ubiquitin ligase

The ubiquitin E3 protein ligase Nedd4-2 is a physiological regulator of the epithelial sodium channel ENaC, which is essential for transepithelial Na+ transport and is linked to Liddle's syndrome, an autosomal dominant disorder of human salt-sensitive hypertension. Nedd4-2 function is negatively regulated by phosphorylation via a serum- and glucocorticoid-inducible protein kinase (Sgk1), which serves as a mechanism to inhibit the ubiquitination-dependent degradation of ENaC. We report here that 14-3-3 proteins participate in this regulatory process through a direct interaction with a phosphorylated form of human Nedd4-2 (a human gene product of KIAA0439, termed hNedd4-2). The interaction is dependent on Sgk1-catalyzed phosphorylation of hNedd4-2 at Ser-468. We found that this interaction preserved the activity of the Sgk1-stimulated ENaC-dependent Na+ current while disrupting the interaction decreased ENaC density on the Xenopus laevis oocytes surface possibly by enhancing Nedd4-2-mediated ubiquitination that leads to ENaC degradation. Our findings suggest that 14-3-3 proteins modulate the cell surface density of ENaC cooperatively with Sgk1 kinase by maintaining hNedd4-2 in an inactive phosphorylated state.

Nedd4-2 Functionally Interacts with ClC-5

Constitutive albumin uptake by the proximal tubule is achieved by a receptor-mediated process in which the Cl(-) channel, ClC-5, plays an obligate role. Here we investigated the functional interaction between ClC-5 and ubiquitin ligases Nedd4 and Nedd4-2 and their role in albumin uptake in opossum kidney proximal tubule (OK) cells. In vivo immunoprecipitation using an anti-HECT antibody demonstrated that ClC-5 bound to ubiquitin ligases, whereas glutathione S-transferase pull-downs confirmed that the C terminus of ClC-5 bound both Nedd4 and Nedd4-2. Nedd4-2 alone was able to alter ClC-5 currents in Xenopus oocytes by decreasing cell surface expression of ClC-5. In OK cells, a physiological concentration of albumin (10 mug/ml) rapidly increased cell surface expression of ClC-5, which was also accompanied by the ubiquitination of ClC-5. Albumin uptake was reduced by inhibiting either the lysosome or proteasome. Total levels of Nedd4-2 and proteasome activity also increased rapidly in response to albumin. Overexpression of ligase defective Nedd4-2 or knockdown of endogenous Nedd4-2 with small interfering RNA resulted in significant decreases in albumin uptake. In contrast, pathophysiological concentrations of albumin (100 and 1000 mug/ml) reduced the levels of ClC-5 and Nedd4-2 and the activity of the proteasome to the levels seen in the absence of albumin. These data demonstrate that normal constitutive uptake of albumin by the proximal tubule requires Nedd4-2, which may act via ubiquitination to shunt ClC-5 into the endocytic pathway.