Concerted roles of SGK1 and the Na+/H+ exchanger regulatory factor 2 (NHERF2) in regulation of NHE3

The recycling regulation of sodium-hydrogen exchanger isoform 3(NHE3) in epithelial cells

As the main exchanger of electroneutral NaCl absorption, sodium-hydrogen exchanger isoform 3 (NHE3) circulates in the epithelial brush border (BB) and intracellular compartments in a multi-protein complex. The size of the NHE3 complex changes during rapid regulation events. Recycling regulation of NHE3 in epithelial cells can be roughly divided into three stages. First, when stimulated by Ca²⁺, cGMP, and cAMP-dependent signaling pathways, NHE3 is converted from an immobile complex found at the apical microvilli (MV) into an easily internalized and mobile form that relocates to a compartment near the base of the MV. Second, NHE3 is internalized by clathrin and albumin-dependent pathways into cytoplasmic endosomal compartments, where the complex is reprocessed and reassembled. Finally, NHE3 is translocated from the recycling endosomes (REs) to the apex of epithelial cells, a process that can be stimulated by an increase in sodium-glucose cotransporter 1 (SGLT1) activity, epidermal growth factor receptor (EGFR) signaling, Ca²⁺ signaling, and binding to βPix and SH3 and multiple ankyrin repeat domains 2 (Shank2) proteins. This review describes the molecular steps and protein interactions involved in the recycling movement of NHE3 from the apex of epithelial cells, into vesicles, where it is reprocessed and reassembled, and returned to its original location on the plasma membrane, where it exerts its physiological function.

SGK regulates pH increase and cyclin B-Cdk1 activation to resume meiosis in starfish ovarian oocytes

Tight regulation of intracellular pH (pH_i) is essential for biological processes. Fully grown oocytes, having a large nucleus called the germinal vesicle, arrest at meiotic prophase I. Upon hormonal stimulus, oocytes resume meiosis to become fertilizable. At this time, the pH_i increases via Na⁺/H⁺ exchanger activity, although the regulation and function of this change remain obscure. Here, we show that in starfish oocytes, serum- and glucocorticoid-regulated kinase (SGK) is activated via PI3K/TORC2/PDK1 signaling after hormonal stimulus and that SGK is required for this pH_i increase and cyclin B-Cdk1 activation. When we clamped the pH_i at 6.7, corresponding to the pH_i of unstimulated ovarian oocytes, hormonal stimulation induced cyclin B-Cdk1 activation; thereafter, oocytes failed in actin-dependent chromosome transport and spindle assembly after germinal vesicle breakdown. Thus, this SGK-dependent pH_i increase is likely a prerequisite for these events in ovarian oocytes. We propose a model that SGK drives meiotic resumption via concomitant regulation of the pH_i and cell cycle machinery.

SLC9 Gene Family: Function, Expression, and Regulation

The Slc9 family of Na⁺ /H⁺ exchangers (NHEs) plays a critical role in electroneutral exchange of Na⁺ and H⁺ in the mammalian intestine as well as other absorptive and secretory epithelia of digestive organs. These transport proteins contribute to the transepithelial Na⁺ and water absorption, intracellular pH and cellular volume regulation as well as the electrolyte, acid-base, and fluid volume homeostasis at the systemic level. They also influence the function of other membrane transport mechanisms, affect cellular proliferation and apoptosis as well as cell migration, adherence to the extracellular matrix, and tissue repair. Additionally, they modulate the extracellular milieu to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺ /H⁺ exchange is inhibited in selected gastrointestinal diseases, either by intrinsic factors (e.g., bile acids, inflammatory mediators) or infectious agents and associated bacterial toxins. Disrupted NHE activity may contribute not only to local and systemic electrolyte imbalance but also to the disease severity via multiple mechanisms. In this review, we describe the cation proton antiporter superfamily of Na⁺ /H⁺ exchangers with a particular emphasis on the eight SLC9A isoforms found in the digestive tract, followed by a more integrative description in their roles in each of the digestive organs. We discuss regulatory mechanisms that determine the function of Na⁺ /H⁺ exchangers as pertinent to the digestive tract, their regulation in pathological states of the digestive organs, and reciprocally, the contribution of dysregulated Na⁺ /H⁺ exchange to the disease pathogenesis and progression. © 2018 American Physiological Society. Compr Physiol 8:555-583, 2018.

Phosphorylation of NHE3-S719 regulates NHE3 activity through the formation of multiple signaling complexes

CK2 regulates NHE3 by phosphorylating a single C-terminal amino acid, which, when mutated, reduces basal NHE3 activity and its acute stimulation and inhibition. It also is necessary for binding of proteins throughout the C-terminus, which means that it determines the C-terminal structure.

Casein kinase 2 (CK2) binds to the NHE3 C-terminus and constitutively phosphorylates a downstream site (S719) that accounts for 40% of basal NHE3 activity. The role of CK2 in regulation of NHE3 activity in polarized Caco-2/bbe cells was further examined by mutation of NHE3-S⁷¹⁹ to A (not phosphorylated) or D (phosphomimetic). NHE3-S719A but not -S719D had multiple changes in NHE3 activity: 1) reduced basal NHE3 activity—specifically, inhibition of the PI3K/AKT-dependent component; 2) reduced acute stimulation of NHE3 activity by LPA/LPA₅R stimulation; and 3) reduced acute inhibition of NHE3 activity—specifically, elevated Ca²⁺ related (carbachol/Ca²⁺ ionophore), but there was normal inhibition by forskolin and hyperosmolarity. The S719A mutant had reduced NHE3 complex size, reduced expression in lipid rafts, increased BB mobile fraction, and reduced binding to multiple proteins that bind throughout the NHE3 intracellular C-terminus, including calcineurin homologous protein, the NHERF family and SNX27 (related PDZ domains). These studies show that phosphorylation of the NHE3 at a single amino acid in the distal part of the C-terminus affects multiple aspects of NHE3 complex formation and changes the NHE3 lipid raft distribution, which cause changes in specific aspects of basal as well as acutely stimulated and inhibited Na⁺/H⁺ exchange activity.

Progression of experimental autoimmune encephalomyelitis is associated with up-regulation of major sodium transporters in the mouse kidney cortex under a normal salt diet

Recent demonstrations of exacerbation of experimental autoimmune encephalomyelitis (EAE) by high salt diets prompted us to study whether EAE stimulated Na absorption by the renal cortex, a primary regulatory site for Na balance, even under a normal NaCl diet. We found that as EAE progressed from mild to severe symptoms, there were parallel increases in the protein abundance of NHE3 and αENaC and the Na,K-ATPase activity with an affiliated elevation of its β1-subunit protein. These effects are associated with increases in the protein levels of the well-known regulators SGK1 and scaffold NHERF2, and phosphorylation of ERK1/2. These effects of EAE could not be explained by reduction in water or food intake. We conclude that EAE progression is associated with up-regulation of major Na transporters, which is most likely driven by increased expression of SGK1 and NHERF2 and activation of ERK1/2. These data suggest that EAE progression increases Na absorption by the renal cortex.

Serum and Glucocorticoid Regulated Kinase 1 in Sodium Homeostasis

The ubiquitously expressed serum and glucocorticoid regulated kinase 1 (SGK1) is tightly regulated by osmotic and hormonal signals, including glucocorticoids and mineralocorticoids. Recently, SGK1 has been implicated as a signal hub for the regulation of sodium transport. SGK1 modulates the activities of multiple ion channels and carriers, such as epithelial sodium channel (ENaC), voltage-gated sodium channel (Nav1.5), sodium hydrogen exchangers 1 and 3 (NHE1 and NHE3), sodium-chloride symporter (NCC), and sodium-potassium-chloride cotransporter 2 (NKCC2); as well as the sodium-potassium adenosine triphosphatase (Na⁺/K⁺-ATPase) and type A natriuretic peptide receptor (NPR-A). Accordingly, SGK1 is implicated in the physiology and pathophysiology of Na⁺ homeostasis. Here, we focus particularly on recent findings of SGK1’s involvement in Na⁺ transport in renal sodium reabsorption, hormone-stimulated salt appetite and fluid balance and discuss the abnormal SGK1-mediated Na⁺ reabsorption in hypertension, heart disease, edema with diabetes, and embryo implantation failure.

Na+-sensitive elevation in blood pressure is ENaC independent in diet-induced obesity and insulin resistance

The majority of patients with obesity, insulin resistance, and metabolic syndrome have hypertension, but the mechanisms of hypertension are poorly understood. In these patients, impaired sodium excretion is critical for the genesis of Na(+)-sensitive hypertension, and prior studies have proposed a role for the epithelial Na(+) channel (ENaC) in this syndrome. We characterized high fat-fed mice as a model in which to study the contribution of ENaC-mediated Na(+) reabsorption in obesity and insulin resistance. High fat-fed mice demonstrated impaired Na(+) excretion and elevated blood pressure, which was significantly higher on a high-Na(+) diet compared with low fat-fed control mice. However, high fat-fed mice had no increase in ENaC activity as measured by Na(+) transport across microperfused cortical collecting ducts, electrolyte excretion, or blood pressure. In addition, we found no difference in endogenous urinary aldosterone excretion between groups on a normal or high-Na(+) diet. High fat-fed mice provide a model of metabolic syndrome, recapitulating obesity, insulin resistance, impaired natriuresis, and a Na(+)-sensitive elevation in blood pressure. Surprisingly, in contrast to previous studies, our data demonstrate that high fat feeding of mice impairs natriuresis and produces elevated blood pressure that is independent of ENaC activity and likely caused by increased Na(+) reabsorption upstream of the aldosterone-sensitive distal nephron.

Upregulation of intestinal NHE3 following saline ingestion

BACKGROUND

Little is known about the effect of salt content of ingested fluid on intestinal transport processes. Osmosensitive genes include the serum- and glucocorticoid-inducible kinase SGK1, which is up-regulated by hyperosmolarity and cell shrinkage. SGK1 is in turn a powerful stimulator of the intestinal Na(+)/H(+) exchanger NHE3. The present study was thus performed to elucidate, whether the NaCl content of beverages influences NHE3 activity.

METHODS

Mice were offered access to either plain water or isotonic saline ad libitum. NHE3 transcript levels and protein abundance in intestinal tissue were determined by confocal immunofluorescent microscopy, RT-PCR and western blotting, cytosolic pH (pHi) in intestinal cells from 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) fluorescence and Na(+)/H(+) exchanger activity from the Na(+) dependent realkalinization following an ammonium pulse.

RESULTS

Saline drinking significantly enhanced fluid intake and increased NHE3 transcript levels, NHE3 protein and Na(+)/H(+) exchanger activity.

CONCLUSIONS

Salt content of ingested fluid has a profound effect on intestinal Na(+)/H(+) exchanger expression and activity.

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.

Inducible kidney-specific Sgk1 knockout mice show a salt-losing phenotype

The expression of the serum- and glucocorticoid-regulated kinase 1 (Sgk1) is induced by mineralocorticoids and, in turn, upregulates the renal epithelial Na(+) channel (ENaC). Total inactivation of Sgk1 has been associated with transient urinary Na(+) wasting with a low-Na(+) diet, while the aldosterone-mediated ENaC channel activation was unchanged in the collecting duct. Since Sgk1 is ubiquitously expressed, we aimed to study the role of renal Sgk1 and generated an inducible kidney-specific knockout (KO) mouse. We took advantage of the previously described TetOn/CreLoxP system, in which rtTA is under the control of the Pax8 promotor, allowing inducible inactivation of the floxed Sgk1 allele in the renal tubules (Sgk1fl/fl/Pax8/LC1 mice). We found that under a standard Na(+) diet, renal water and Na(+)/K(+) excretion had a tendency to be higher in doxycycline-treated Sgk1 KO mice compared with control mice. The impaired ability of Sgk1 KO mice to retain Na(+) increased significantly with a low-salt diet despite higher plasma aldosterone levels. On a low-Na(+) diet, the Sgk1 KO mice were also hyperkaliuric and lost body weight. This phenotype was accompanied by a decrease in systolic and diastolic blood pressure. At the protein level, we observed a reduction in phosphorylation of the ubiquitin protein-ligase Nedd4-2 and a decrease in the expression of the Na(+)-Cl(-)-cotransporter (NCC) and to a lesser extent of ENaC.

NHERF1 and NHERF2 are necessary for multiple but usually separate aspects of basal and acute regulation of NHE3 activity

Na(+)/H(+) exchanger 3 (NHE3) is expressed in the brush border (BB) of intestinal epithelial cells and accounts for the majority of neutral NaCl absorption. It has been shown that the Na(+)/H(+) exchanger regulatory factor (NHERF) family members of multi-PDZ domain-containing scaffold proteins bind to the NHE3 COOH terminus and play necessary roles in NHE3 regulation in intestinal epithelial cells. Most studies of NHE3 regulation have been in cell models in which NHERF1 and/or NHERF2 were overexpressed. We have now developed an intestinal Na(+) absorptive cell model in Caco-2/bbe cells by expressing hemagglutinin (HA)-tagged NHE3 with an adenoviral infection system. Roles of NHERF1 and NHERF2 in NHE3 regulation were determined, including inhibition by cAMP, cGMP, and Ca(2+) and stimulation by EGF, with knockdown (KD) approaches with lentivirus (Lenti)-short hairpin RNA (shRNA) and/or adenovirus (Adeno)-small interfering RNA (siRNA). Stable infection of Caco-2/bbe cells by NHERF1 or NHERF2 Lenti-shRNA significantly and specifically reduced NHERF protein expression by >80%. NHERF1 KD reduced basal NHE3 activity, while NHERF2 KD stimulated NHE3 activity. siRNA-mediated (transient) and Lenti-shRNA-mediated (stable) gene silencing of NHERF2 (but not of NHERF1) abolished cGMP- and Ca(2+)-dependent inhibition of NHE3. KD of NHERF1 or NHERF2 alone had no effect on cAMP inhibition of NHE3, but KD of both simultaneously abolished the effect of cAMP. The stimulatory effect of EGF on NHE3 was eliminated in NHERF1-KD but occurred normally in NHERF2-KD cells. These findings show that both NHERF2 and NHERF1 are involved in setting NHE3 activity. NHERF2 is necessary for cGMP-dependent protein kinase (cGK) II- and Ca(2+)-dependent inhibition of NHE3. cAMP-dependent inhibition of NHE3 activity requires either NHERF1 or NHERF2. Stimulation of NHE3 activity by EGF is NHERF1 dependent.

Regulation of intracellular pH by p90Rsk-dependent activation of an Na(+)/H(+) exchanger in starfish oocytes

Starfish oocytes arrest at metaphase of the first meiotic division (MI arrest) in the ovary and resume meiosis after spawning into seawater. MI arrest is maintained by lower intracellular pH (pH(i)) and release from arrest by cellular alkalization. To elucidate pH(i) regulation in oocytes, we cloned the starfish (Asterina pectinifera) Na(+)/H(+) exchanger 3 (ApNHE3) expressed in the plasma membrane of oocytes. The cytoplasmic domain of ApNHE3 contains p90 ribosomal S6 kinase (p90Rsk) phosphorylation sites, and injection of a constitutively active p90Rsk and the upstream regulator Mos to immature oocytes, stimulated an increase in pH(i). This increase was blocked by 5-(N-ethyl-N-isopropyl)-amiloride, a NHE inhibitor, and SL0101, a specific Rsk inhibitor. The MAPK kinase (MEK) inhibitor U0126 blocked the Mos-induced, but not the p90Rsk-induced, pH(i) increase, suggesting that the Mos-MEK-MAPK-p90Rsk pathway promotes ApNHE3 activation. In a cell-free extract, the Mos-MEK-MAPK-p90Rsk pathway phosphorylates ApNHE3 at Ser-590, -606, and -673. When p90Rsk-dependent ApNHE3 phosphorylation was blocked by a dominant-negative C-terminal fragment, or neutralizing antibody, the p90Rsk-induced pH(i) increase was suppressed in immature oocytes. However, ApNHE3 is up-regulated via the upstream phosphatidylinositol 3-kinase pathway before MAPK activation and the active state is maintained until spawning, suggesting that the p90Rsk-dependent ApNHE3 phosphorylation is unlikely to be the primary regulatory mechanism involved in MI arrest exit. After meiosis is completed, unfertilized eggs maintain their elevated pH(i) ( approximately 7.4) until the onset of apoptosis. We suggest that the p90Rsk/ApNHE3-dependent elevation of pH(i) increases fertilization success by delaying apoptosis initiation.

Stimulation of electrogenic intestinal dipeptide transport by the glucocorticoid dexamethasone

According to recent in vitro experiments, the peptide transporter PepT2 is stimulated by the serum- and glucocorticoid-inducible kinase SGK1. The present study explored the contribution of SGK1 to the regulation of electrogenic intestinal peptide transport. Intestinal PepT1 was expressed in Xenopus oocytes, and peptide transport was determined by dual electrode voltage clamping. Peptide transport in intestinal segments was determined utilizing Ussing chamber. Cytosolic pH (pH( i )) was determined by BCECF fluorescence and Na(+)/H(+) exchanger activity was estimated from Na(+)-dependent pH recovery (pH ( i )) following an ammonium pulse. In PepT1-expressing Xenopus oocytes, coexpression of SGK1 enhanced electrogenic peptide transport. Intestinal transport and pH( i ) of untreated mice were similar in SGK1 knockout mice (sgk1 ( -/- )) and their wild-type littermates (sgk1 ( +/+ )). Glucocorticoid treatment (4 days 10 microg/g body weight (bw)/day dexamethasone) increased peptide transport in sgk1 ( +/+ ) but not in sgk1 (-/-) mice. Irrespective of dexamethasone treatment, luminal peptide (5 mM glycyl-glycine) led to a similar early decrease of pH( i ) in sgk1 (-/-) and sgk1 (+/+) mice, but to a more profound and sustained decline of pH( i ) in sgk1 (-/-) than in sgk1 ( +/+ ) mice. In the presence and absence of glycyl-glycine, pH ( i ) was significantly enhanced by dexamethasone treatment in sgk1 ( +/+ ) mice, an effect significantly blunted in sgk1 ( -/- ) mice. During sustained exposure to glycyl-glycine, pH ( i ) was significantly larger in sgk1 (+/+) mice than in sgk1 (-/-) mice, irrespective of dexamethasone treatment. In conclusion, basal intestinal peptide transport does not require stimulation by SGK1. Glucocorticoid treatment stimulates both Na(+)/H(+) exchanger activity and peptide transport, effects partially dependent on SGK1. Moreover, chronic exposure to glycyl-glycine stimulates Na(+)/H(+) exchanger activity, an effect again involving SGK1.

The role of the NHERF family of PDZ scaffolding proteins in the regulation of salt and water transport

The four members of the NHERF (Na(+)/H(+) exchanger regulatory factor) family of PDZ adapter proteins bind to a variety of membrane transporters and receptors and modulate membrane expression, mobility, interaction with other proteins, and the formation of signaling complexes. All four family members are expressed in the intestine. The CFTR (cystic fibrosis transmembrane regulator) anion channel and the Na(+)/H(+) exchanger NHE3 (Na/H exchanger- isoform 3) are two prominent binding partners to this PDZ-adapter family, which are also known key players in the regulation of intestinal electrolyte and fluid transport. Experiments in heterologous expression systems have provided a number of mechanistic models how NHERF protein interactions can affect the function of their targets at the molecular level. Recently, NHERF1, 2, and 3 knockout mice have become available, and this review summarizes the reports on electrolyte and fluid transport regulation in the native intestine of these mice.

Tethering, recycling and activation of the epithelial sodium–proton exchanger, NHE3

NHE3 is a sodium–proton exchanger expressed predominantly in the apical membrane of renal and intestinal epithelia, where it plays a key role in salt and fluid absorption and pH homeostasis. It performs these functions through the exchange of luminal sodium for cytosolic protons. Acute regulation of NHE3 function is mediated by altering the total number of exchangers in the plasma membrane as well as their individual activity. Traffic between endomembrane and plasmalemmal pools of NHE3 dictates the density of exchangers available at the cell surface. The activity of the plasmalemmal pool, however,is not fixed and can be altered by the association with modifier proteins, by post-translational alterations (such as cAMP-mediated phosphorylation) and possibly also via interaction with specific plasmalemmal phospholipids. Interestingly, association with cytoskeletal components affects both levels of regulation, tethering NHE3 molecules at the surface and altering their intrinsic activity. This paper reviews the role of proteins and lipids in the modulation of NHE3 function.

SGK1-sensitive renal tubular glucose reabsorption in diabetes

The hyperglycemia of diabetes mellitus increases the filtered glucose load beyond the maximal tubular transport rate and thus leads to glucosuria. Sustained hyperglycemia, however, may gradually increase the maximal renal tubular transport rate and thereby blunt the increase of urinary glucose excretion. The mechanisms accounting for the increase of renal tubular glucose transport have remained ill-defined. A candidate is the serum- and glucocorticoid-inducible kinase SGK1. The kinase has been shown to stimulate Na(+)-coupled glucose transport in vitro and mediate the stimulation of electrogenic intestinal glucose transport by glucocorticoids in vivo. SGK1 expression is confined to glomerula and distal nephron in intact kidneys but may extend to the proximal tubule in diabetic nephropathy. To explore whether SGK1 modifies glucose transport in diabetic kidneys, Akita mice (akita(+/-)), which develop spontaneous diabetes, have been crossbred with gene-targeted mice lacking SGK1 on one allele (sgk1(+/-)) to eventually generate either akita(+/-)/sgk1(-/-) or akita(+/-)/sgk1(+/+) mice. Both akita(+/-)/sgk1(-/-) and akita(+/-)/sgk1(+/+) mice developed profound hyperglycemia (>20 mM) within approximately 6 wk. Body weight and plasma glucose concentrations were not significantly different between these two genotypes. However, urinary excretion of glucose and urinary excretion of fluid, Na(+), and K(+), as well as plasma aldosterone concentrations, were significantly higher in akita(+/-)/sgk1(-/-) than in akita(+/-)/sgk1(+/+) mice. Studies in isolated perfused proximal tubules revealed that the electrogenic glucose transport was significantly lower in akita(+/-)/sgk1(-/-) than in akita(+/-)/sgk1(+/+) mice. The data provide the first evidence that SGK1 participates in the stimulation of renal tubular glucose transport in diabetic kidneys.

IRBIT, inositol 1,4,5-triphosphate (IP3) receptor-binding protein released with IP3, binds Na+/H+ exchanger NHE3 and activates NHE3 activity in response to calcium

Calcium (Ca2+) is a highly versatile second messenger that regulates various cellular processes. Previous studies showed that elevation of intracellular Ca2+ regulates the activity of Na+/H+ exchanger 3 (NHE3). However, the effect of Ca2+-dependent signaling on NHE3 activity varies depending on cell types. In this study, we report the identification of IP3 receptor-binding protein released with IP3 (IRBIT) as a NHE3 interacting protein and its role in regulation of NHE3 activity. IRBIT bound to the carboxyl-terminal domain of NHE3, which is necessary for acute regulation of NHE3. Ectopic expression of IRBIT resulted in Ca2+-dependent activation of NHE3 activity, whereas silencing of endogenous IRBIT resulted in inhibition of NHE3 activity. Ca2+-dependent stimulation of NHE3 activity was dependent on the binding of IRBIT to NHE3. Previously Ca2+-dependent inhibition of NHE3 was demonstrated in the presence of NHERF2. Co-expression of IRBIT was able to reverse the NHERF2-dependent inhibition of NHE3. We also showed that IRBIT-dependent activation of NHE3 involves exocytic trafficking of NHE3 to the plasma membrane and this activation was blocked by inhibition of calmodulin (CaM) or CaM-dependent kinase II. These results suggest that the overall effect of Ca2+ on NHE3 activity is balanced by IRBIT-dependent activation and NHERF2-dependent inhibition.

Association of SGK1 gene polymorphisms with type 2 diabetes

The serum and glucocorticoid inducible kinase SGK1 is genomically upregulated by glucocorticoids and in turn stimulates a variety of carriers and channels including the renal epithelial Na(+) channel ENaC and the intestinal Na(+) glucose transporter SGLT1. Twin studies disclosed an association of a specific SGK1 haplotype with moderately enhanced blood pressure in individuals who are carrying simultaneously a homozygous genotype for a variant in intron 6 [I6CC] and a homozygous or heterozygous genotype for the C allele of a polymorphism in exon 8 [E8CC/CT] of the SGK1 gene. A subsequent study confirmed the impact of this risk haplotype on blood pressure. SGK1 knockout mice are resistant to the insulin and high salt induced increase of blood pressure, glucocorticoid induced increase of electrogenic glucose transport, and glucocorticoid induced suppression of insulin release. The present study explored whether the I6CC/E8CC/CT haplotype impacts on the prevalence of type 2 diabetes. The prevalence of the I6CC genotype was 3.1% in a healthy German, 2.4 % in a healthy Romanian and 11.6 % in a healthy African population from Ghana (p=0.0006 versus prevalence in Caucasians). Comparison of genotype frequencies between type 2 diabetic patients and the respective control groups revealed significant differences for the intron 6 T>C variant. Carriers of at least one T allele were protected against type 2 diabetes (Romanians: p=0.023; OR 0.29; 95% CI 0.09-0.89; Germans: p=0.01; OR 0.37; 95% CI 0.17-0.81). The SGK1 risk haplotype (I6CC/E8CC/CT) was significantly (p=0.032; OR 4.31, 95% CI 1.19-15.58) more frequent in diabetic patients (7.2 %) than in healthy volunteers from Romania (1.8%). The observations support the view that SGK-1 may participate in the pathogenesis of metabolic syndrome.

Differential regulation of serum- and glucocorticoid-inducible kinase 1 (SGK1) splice variants based on alternative initiation of transcription

The serum- and glucocorticoid-inducible kinase 1 (SGK1) is a key-regulator of transport, cell volume and cell survival. SGK1 transcription is under genomic control of a wide variety of hormones and cell stressors. Little is known, however, about sequence variation in SGK1 transcripts. Thus, we took an in silico approach to determine sequence variations in the N-terminal region of SGK1, which is considered particularly important for subcellular SGK1 localization. Expressed Sequence Tag analysis revealed two novel phylogenetically highly conserved SGK1 mRNAs with different promoter sites based on alternative initiation of transcription at -2981, -850 upstream of the transcription initiation site (+1) of the reference mRNA. RT-PCR in various human cell lines and tissues confirmed the expression of the 3 alternative splice variants, which differed exclusively in their first exons. The two novel variants were devoid of the localization and degradation signal with otherwise unchanged and intact open reading frames. Spatial distribution of transcription factor binding sites among the three promoter sites indicated common responsiveness to glucocorticoids but different responsiveness to hypoxia and cellular differentiation. Differential expression under those conditions was confirmed for all variants in cultured myoblasts and myotubes. p53 and ETF-1 binding sites were overrepresented at the promoter site of the reference sequence variant SGK1(+1). Transcript levels were 4.1-fold [SGK1(+1)] and 3.1-fold [SGK1(-850)] higher in renal clear cell carcinoma than in remote tissue. The transcript levels were 42-fold [SGK1(+1)], 26-fold [SGK1(-850)] and 17-fold [SGK1(-2981)] higher in highly malignant human glioma cells than in non-neoplastic brain tissue. SGK1 transcript levels were differentially increased by differentiation or hypoxia (treatment with CoCl(2)). In conclusion, the present observations disclose the transcription of three distinct SGK1 splice variants, which are all markedly upregulated in tumor tissue but differentially upregulated following differentiation or hypoxia.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Resistance of mice lacking the serum- and glucocorticoid-inducible kinase SGK1 against salt-sensitive hypertension induced by a high-fat diet

Mineralocorticoids enhance expression and insulin stimulates activity of the serum- and glucocorticoid-inducible kinase SGK1, which activates the renal epithelial Na+ channel (ENaC). Under a salt-deficient diet, SGK1 knockout mice ( sgk1−/−) excrete significantly more NaCl than their wild-type littermates ( sgk1 +/+) and become hypotensive. The present experiments explored whether SGK1 participates in the hypertensive effects of a high-fat diet and high-salt intake. Renal SGK1 protein abundance of sgk1 +/+ mice was significantly elevated after a high-fat diet. Under a control diet, fluid intake, blood pressure, urinary flow rate, and urinary Na+, K+, and Cl− excretion were similar in sgk1−/− and sgk1 +/+ mice. Under a standard diet, high salt (1% NaCl in the drinking water for 25 days) increased fluid intake, urinary flow rate, and urinary Na+, K+, and Cl− excretion similarly in sgk1−/− and sgk1 +/+ mice without significantly altering blood pressure. A high-fat diet alone (17 wk) did not significantly alter fluid intake, urinary flow rate, urinary Na+, K+, or Cl− excretion, or plasma aldosterone levels but increased plasma insulin, total cholesterol, triglyceride concentrations, and systolic blood pressure to the same extent in both genotypes. Additional salt intake (1% NaCl in the drinking water for 25 days) on top of a high-fat diet did not affect hyperinsulinemia or hyperlipidemia but increased fluid intake, urinary flow rate, and urinary NaCl excretion significantly more in sgk1−/− than in sgk1 +/+mice. Furthermore, in animals receiving a high-fat diet, additional salt intake increased blood pressure only in sgk1 +/+ mice (to 132 ± 3 mmHg) but not in sgk1−/− mice (120 ± 4 mmHg). Thus lack of SGK1 protects against the hypertensive effects of a combined high-fat/high-salt diet.

(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.

Impaired intestinal NHE3 activity in the PDK1 hypomorphic mouse

In vitro experiments have demonstrated the stimulating effect of serum- and glucocorticoid-inducible kinase (SGK)1 on the activity of the Na+/H+ exchanger (NHE3). SGK1 requires activation by phosphoinositide-dependent kinase (PDK)1, which may thus similarly play a role in the regulation of NHE3-dependent epithelial electrolyte transport. The present study was performed to explore the role of PDK1 in the regulation of NHE3 activity. Because mice completely lacking functional PDK1 are not viable, hypomorphic mice expressing approximately 20% of PDK1 (pdk1(hm)) were compared with their wild-type littermates (pdk1(wt)). NHE3 activity in the intestine and PDK1-overexpressing HEK-293 cells was estimated by utilizing 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein fluorescence for the determination of intracellular pH. NHE activity was reflected by the Na+-dependent pH recovery from an ammonium prepulse (DeltapH(NHE)). The pH changes after an ammonium pulse allowed the calculation of cellular buffer capacity, which was not significantly different between pdk1(hm) and pdk1(wt) mice. DeltapH(NHE) was in pdk1(hm) mice, only 30 +/- 6% of the value obtained in pdk1(wt) mice. Conversely, DeltapH(NHE) was 32 +/- 7% larger in PDK1-overexpressing HEK-293 cells than in HEK-293 cells expressing the empty vector. The difference between pdk1(hm) and pdk1(wt) mice and between PDK1-overexpressing and empty vector-transfected HEK cells, respectively, was completely abolished in the presence of the NHE3 inhibitor S3226 (10 microM). In conclusion, defective PDK1 expression leads to significant impairment of NHE3 activity in the intestine, pointing to a role of PDK1-dependent signaling in the regulation of NHE-mediated electrolyte transport.

Acute activation of NHE3 by dexamethasone correlates with activation of SGK1 and requires a functional glucocorticoid receptor

Glucocorticoids stimulate the intestinal absorption of Na(+) and water partly by regulation of the Na(+)/H(+) exchanger 3 (NHE3). Previous studies have shown both genomic and nongenomic regulation of NHE3 by glucocorticoids. Serum and glucocorticoid-inducible kinase 1 (SGK1) has been shown to be part of this cascade, where phosphorylation of NHE3 by SGK1 initiates the translocation of NHE3 to the cell surface. In the present work, we examined a series of changes in SGK1 and NHE3 induced by glucocorticoids using human colonic Caco-2 and opossum kidney cells. We found that dexamethasone rapidly stimulated SGK1 mRNAs, but a significant change in protein abundance was not detected. Instead, there was an increase in SGK1 kinase activity as early as at 2 h. An increase in NHE3 protein abundance was not detected until 12 h of dexamethasone exposure, although the transport activity was significantly stimulated at 4 h. These data demonstrate that the changes of SGK1 precede those of NHE3. Chronic regulation (24 h) of NHE3 was blocked completely by prevention of protein synthesis with cycloheximide or actinomycin D and by the glucocorticoid receptor blocker RU486. The acute effect of dexamethasone was similarly abrogated by RU486, but was insensitive to cycloheximide and actinomycin D. Similarly, the stimulation of SGK1 activity by dexamethasone was blocked by RU486 but not by actinomycin D. Together, these data show that the acute effect of glucocorticoids on NHE3 is mediated by a glucocorticoid receptor dependent mechanism that activates SGK1 in a nongenomic manner.

Influence of the parathyroid glands on bone metabolism

Bone is a classic target tissue for parathyroid hormone (PTH), whose calciotropic effect is mediated largely via catabolic actions on this tissue. Paradoxically, PTH also exerts anabolic actions, with intermittent injections of PTH or its amino-terminal fragments causing an increase in bone formation and bone mass, actions that form the basis for the use of PTH in the treatment of osteoporosis. Besides vitamin D, PTH is the only other known bone anabolic agent. High-affinity PTH receptors (PTH-1R) have been detected on osteoblasts and osteoclasts (albeit in lower numbers). Bone turnover, which includes activation of osteoclasts and osteoblasts, appears to be best reflected not by absolute concentrations of PTH (which can vary based on the assay and antibody used) but by a balance of circulating full-length PTH-(1-84) and amino-terminally truncated C-PTH fragments. When PTH-(1-84) is predominant, bone turnover is promoted. Among PTH fragments, PTH-(7-84) appears to be the most potent antagonist of PTH-(1-84). The mechanisms involved in these effects are unclear although mediation via unique C-terminal receptors has been suggested. We propose that, within the range of total PTH (100-1000 pg mL(-1)), the ratio of PTH-(1-84)/C-PTH fragment is a valuable tool for diagnosis of bone turnover. Data indicate that at PTH levels < 100-150 pg mL(-1) and > 1000 pg mL(-1), the ratio looses its predictive power. Assay type, patient characteristics (race, underlying renal disease) and treatment attributes (vitamin D, corticosteroids, phosphate binders) have an impact on the PTH ratio, and care should be used in interpreting assay results and making subsequent treatment decisions.

Stimulation of the creatine transporter SLC6A8 by the protein kinases SGK1 and SGK3

Creatine binds phosphate thus serving energy storage. Cellular creatine uptake is accomplished by the Na+,Cl-, creatine transporter CreaT (SLC6A8). The present study explored the regulation of SLC6A8 by the serum and glucocorticoid inducible kinase SGK1, a kinase upregulated during ischemia. In Xenopus oocytes expressing SLC6A8 but not in water injected oocytes creatine induced a current which was significantly enhanced by coexpression of wild type SGK1 and constitutively active (S422D)SGK1, but not inactive (K127N)SGK1. Kinetic analysis revealed that (S422D)SGK1 enhanced maximal current without significantly altering affinity. The effect of SGK1 was mimicked by the constitutively active isoform (S419D)SGK3 but not by inactive (K119N)SGK3, wild type isoform SGK2 or constitutively active related kinase (T308D,S473D)PKB. In conclusion, the kinases SGK1 and SGK3 increase SLC6A8 activity by increasing the maximal transport rate of the carrier. Deranged SGK1 and/or SGK3 dependent regulation of SLC6A8 may affect energy storage particularly in skeletal muscle, heart, and neurons.

Renal function of gene-targeted mice lacking both SGK1 and SGK3

Serum- and glucocorticoid-inducible kinase (SGK) 1 and SGK3 share the ability to upregulate several ion channels, including the epithelial Na(+) channel. Whereas SGK1 is under genomic control of mineralocorticoids and glucocorticoids, SGK3 is constitutively expressed. The SKG1-knockout (sgk1(-/-)) mouse is seemingly normal when it is fed a standard diet, but its ability to retain NaCl is impaired when it is fed a salt-deficient diet. In the SGK3-knockout (sgk3(-/-)) mouse fed standard and salt-deficient diets, hair growth is strikingly delayed but NaCl excretion is normal. Thus the possibility was considered that SGK1 and SGK3 could mutually replace each other, thus preventing severe NaCl loss in sgk1(-/-) and sgk3(-/-) mice. We crossed SGK1- and SGK3-knockout mice and compared renal electrolyte excretion of the double mutants (sgk1(-/-)/sgk3(-/-)) with that of their wild-type littermates (sgk1(+/+)/sgk3(+/+)). Similar to sgk3(-/-) mice, the sgk1(-/-)/sgk3(-/-) mice display delayed hair growth. Blood pressure was slightly, but significantly (P < 0.03), lower in sgk1(-/-)/sgk3(-/-) (102 +/- 4 mmHg) than in sgk1(+/+)/sgk3(+/+) (114 +/- 3 mmHg) mice, a difference that was maintained in mice fed low- and high-salt diets. Plasma aldosterone concentrations were significantly (P < 0.01) higher in sgk1(-/-)/sgk3(-/-) than in sgk1(+/+)sgk3(+/+) mice fed control (511 +/- 143 vs. 143 +/- 32 pg/ml) and low-salt (1,325 +/- 199 vs. 362 +/- 145 pg/ml) diets. During salt depletion, absolute and fractional excretions of Na(+) were significantly (P < 0.01) higher in sgk1(-/-)/sgk3(-/-) (1.2 +/- 0.2 micromol/24 h g body wt, 0.12 +/- 0.03%) than in sgk1(+/+)/sgk3(+/+) (0.4 +/- 0.1 micromol/24 h g body wt, 0.04 +/- 0.01%) mice. The sgk1(-/-)/sgk3(-/-) mice share the delayed hair growth with sgk3(-/-) mice and the modestly impaired renal salt retention with sgk1(-/-) mice. Additional lack of the isoform kinase does not substantially compound the phenotype for either property.

Intestinal function of gene-targeted mice lacking serum- and glucocorticoid-inducible kinase 1

In vitro experiments have revealed the ability of serum- and glucocorticoid-inducible kinase 1 (SGK1) to stimulate intestinal Na(+)-coupled glucose cotransporter 1 (SGLT1) and intestinal Na(+)/H(+) exchanger 3 (NHE3). The present study explored the contribution of SGK1 to the regulation of intestinal transport in vivo. SGK1 transcript levels were determined by real-time PCR and glucose-induced currents (I(g)) reflecting SGLT1 activity by Ussing chamber experiments. BCECF fluorescence was utilized for the determination of Na(+)-dependent pH recovery from an ammonium pulse (DeltapH(NHE)) reflecting NHE activity. As a result, intestinal SGK1 transcript levels were significantly enhanced by a 4-day treatment with 10 microg.mg body wt(-1).day(-1) dexamethasone (Dex). I(g) was, under control conditions, virtually identical in sgk1 knockout mice (sgk1(-/-)) and their wild type littermates (sgk1(+/+)). A 4-day treatment with Dex, however, increased I(g) approximately threefold in sgk1(+/+) mice but not in sgk1(-/-) mice. DeltapH(NHE) was similar in sgk1(-/-) and sgk1(+/+) mice before treatment. Dex increased DeltapH(NHE) approximately threefold in sgk1(+/+) mice and approximately twofold in sgk1(-/-)mice, an effect significantly blunted in the presence of the specific NHE3 blocker S-3226 (10 microM). According to Western blot analysis, Dex significantly enhanced SGLT1 and NHE3 protein abundance in brush-border membranes of sgk1(+/+) mice but not of sgk1(-/-)mice. In conclusion, basic functions of SGLT1 and NHE3 in the intestine do not require stimulation by SGK1. However, the effects of glucocorticoids on SGLT1 are fully, and on NHE3 partially, dependent on SGK1.

Additive regulation of GluR1 by stargazin and serum- and glucocorticoid-inducible kinase isoform SGK3

The serum- and glucocorticoid-inducible kinase isoform 3 (SGK3) and stargazin have both been shown to enhance the synaptic expression level of GluR1. The present study was performed to elucidate whether SGK3 and stargazin interact or are effective through different pathways in the regulation of GluR1. Proteins were expressed in Xenopus oocytes by injection of complementary RNA (cRNA) encoding GluR1, SGK isoforms, and/or stargazin. In oocytes expressing GluR1 6 days after cRNA injection, glutamate induced an inward current (IGlu), which was increased approximately fourfold following coexpression of SGK3. Coexpression of stargazin similarly enhanced IGlu. Coexpression of both SGK3 and stargazin stimulated the current by a factor of 15.5. Replacement of the serine by alanine at the only SGK consensus sequence (RXRXXS/T) in stargazin enhanced the efficacy of stargazin but did not prevent further stimulation of IGlu by additional coexpression of SGK3. Western blotting showed that stargazin accelerated membrane insertion of GluR1 protein leading to enhanced GluR1 plasma membrane protein abundance 2 days, but not 6 days, after cRNA injection, while SGK3 increased plasma membrane protein abundance 6 days after cRNA injection. In conclusion, SGK3 and stargazin regulate GluR1 independently, and thus, their effects on glutamate-induced currents are additive.

Reduced locomotion in the serum and glucocorticoid inducible kinase 3 knock out mouse

The serum and glucocorticoid inducible kinase isoform SGK3 is expressed in the brain including hippocampal neurons. It is activated by phosphoinositide-3 (PI3) kinase and thus a putative target of neurotrophic factors. In vitro experiments pointed to the ability of SGK3 to regulate several transporters and ion channels including the AMPA receptor GluR1. In order to explore the in vivo functional significance of SGK3 in the regulation of spatial learning and exploratory behavior, we assessed the performance of SGK3 knockout mice (SGK3-/-) and their wild type littermates (SGK3+/+) in a place navigation task in the water-maze, radial maze in a battery of forced and free exploration tests, acoustic startle and a test for motoric coordination. According to water-maze and radial maze testing reference and working memory was intact in SGK3-/- mice. However, detailed analysis of swimming patterns of SGK3-/- mice in the water-maze revealed a deficit in precision and goal-directed navigation in space. SGK3-/- mice showed reduced exploratory activity, which was observed in several environments and increased centre field avoidance in the open-field. SGK3-/- mice further showed reduced darting behavior on open surfaces, indicating that the knock out may modify basic patterns of locomotion. In conclusion, lack of SGK3 leads to subtle behavioral defects which may result from deranged neuronal regulation of transporters and ion channels.

SGK1-mediated Fibronectin Formation in Diabetic Nephropathy

The serum and glucocorticoid inducible kinase SGK1 has been shown to be up regulated in fibrosing tissue including diabetic nephropathy. The present study has been performed to determine the time course of SGK1 transcription in mouse kidneys following induction of diabetes by streptozotocin (STZ). Moreover, the study aimed to explore whether SGK1 may play an active role in the stimulation of matrix protein formation during hyperglycemia. The induction of diabetes in 8 weeks old male C57Bl/6 mice was indeed followed by a significant (p< 0.001) increase of SGK1 transcript levels (up to 2.5-fold) and protein abundance (up to 2.8-fold) both peaking 4 weeks after STZ treatment. The SGK1 transcript levels and protein abundance declined thereafter but remained significantly elevated up to 12 weeks (p<0.05). Exposure to high extracellular glucose concentration (25 mM) significantly increased SGK1 transcript levels in human mesangial cells (HMCs). At low extracellular glucose concentration (5.5 mM), transfection with constitutively active (S422D)SGK1 and transdominant inhibitory (K127N)SGK1 did not significantly modify fibronectin formation by HMCs. Exposure to high extracellular glucose concentration stimulated fibronectin formation (by 2.2 fold), an effect abrogated by transfection with inactive (K127N)SGK1 (1.2 fold) and markedly enhanced by transfection with (S422D)SGK1 (4.7 fold). In conclusion, hyperglycemia of diabetes mellitus leads to partially transient increase of SGK1 transcription and translation. SGK1 overexpression alone has little effect on fibronectin formation but potentiates the effect of hyperglycemia. Thus, SGK1 is upregulated in diabetic nephropathy and actively participates in the stimulation of matrix protein deposition in this common deleterious complication of diabetic hyperglycemia.

Blunted hypertensive effect of combined fructose and high-salt diet in gene-targeted mice lacking functional serum- and glucocorticoid-inducible kinase SGK1

Serum- and glucocorticoid-inducible kinase (SGK1) is transcriptionally upregulated by mineralocorticoids and activated by insulin. The kinase stimulates the renal epithelial Na(+) channel and may thus participate in blood pressure regulation. Hyperinsulinemia is triggered by dietary fructose, which sensitizes blood pressure for salt intake. The role of SGK1 in hypertensive effects of combined fructose and high-salt intake was thus explored in SGK1 knockout mice (sgk1(-/-)) and their wild-type littermates (sgk1(+/+)). Renal SGK1 transcript levels of sgk1(+/+) mice were significantly elevated after fructose diet. Under control diet, fluid intake, urinary flow rate, urinary Na(+), K(+), and Cl(-) excretion, and blood pressure were similar in sgk1(-/-) and sgk1(+/+) mice. Addition of 10% fructose to drinking water increased fluid intake and urinary flow rate in both genotypes, and did not significantly alter urinary Na(+), K(+), and Cl(-) output in either genotype. Additional high NaCl diet (4% NaCl) did not significantly alter fluid intake and urine volume but markedly increased urinary output of Na(+) and Cl(-), approaching values significantly (P < 0.05) larger in sgk1(-/-) than in sgk1(+/+) mice (Na(+): 2,572 +/- 462 vs. 1,428 +/- 236; Cl(-): 2,364 +/- 388 vs. 1,379 +/- 225 micromol/24 h). Blood pressure was similar in sgk1(+/+) and sgk1(-/-) mice at control diet or fructose alone but increased only in sgk1(+/+) mice (115 +/- 1 vs. 103 +/- 0.7 mmHg, P < 0.05) after combined fructose and high-salt intake. Acute intravenous insulin infusion (during glucose clamp) caused antinatriuresis in sgk1(+/+) mice, an effect significantly blunted in sgk1(-/-) mice. The observations reveal a pivotal role of SGK1 in insulin-mediated sodium retention and the salt-sensitizing hypertensive effect of high fructose intake.

Postnatal developmental expression of the PDZ scaffolds Na+ -H+ exchanger regulatory factors 1 and 2 in the rat cochlea

Sensory transduction in the mammalian cochlea requires the maintenance of specialized fluid compartments with distinct ionic compositions. This is achieved by the concerted action of diverse ion channels and transporters, some of which can interact with the PDZ scaffolds, Na(+)-H(+) exchanger regulatory factors 1 and 2 (NHERF-1, NHERF-2). Here, we report that NHERF-1 and NHERF-2 are widely expressed in the rat cochlea, and that their expression is developmentally regulated. Reverse transcription/polymerase chain reaction (RT-PCR) and Western blotting initially confirmed the RNA and protein expression of NHERFs. We then performed immunohistochemistry on cochlea during various stages of postnatal development. Prior to the onset of hearing (P8), NHERF-1 immunolabeling was prominently polarized to the apical membrane of cells lining the endolymphatic compartment, including the stereocilia and cuticular plates of the inner and outer hair cells, marginal cells of the stria vascularis, Reissner's epithelia, and tectorial membrane. With maturation (P21, P70), NHERF-1 immunolabeling was reduced in the above structures, whereas labeling increased in the apical membrane of the interdental cells of the spiral limbus and the inner and outer sulcus cells, Hensen's cells, the inner and outer pillar cells, Deiters cells, the inner border cells, spiral ligament fibrocytes, and spiral ganglion neurons (particularly type II). NHERF-1 expression in strial basal and intermediate cells was persistent. NHERF-2 immunolabeling was similar to that for NHERF-1 during postnatal development, with the exception of expression in the synaptic regions beneath the outer hair cells. NHERF-1 and NHERF-2 co-localized with glial fibrillary acidic protein and vimentin in glia. The cochlear localization of NHERF scaffolds suggests that they play important roles in the developmental regulation of ion transport, homeostasis, and auditory neurotransmission.

High glucose transactivates the EGF receptor and up-regulates serum glucocorticoid kinase in the proximal tubule

BACKGROUND

Serum glucocorticoid regulated kinase (SGK-1) is induced in the kidney in diabetes mellitus. However, its role in the proximal tubule is unclear. This study determined the expression and functional role of SGK-1 in PTCs in high glucose conditions. As the epidermal growth factor (EGF) receptor is activated by both EGF and other factors implicated in diabetic nephropathy, the relationship of SGK-1 with EGFR activity was assessed.

METHODS

mRNA and protein expression of SGK-1 and mRNA expression of the sodium hydrogen exchanger NHE3 were measured in human PTCs exposed to 5 mmol/L (control) and 25 mmol/L (high) glucose. The effects of SGK-1 on cell growth, apoptosis, and progression through the cell cycle and NHE3 mRNA were examined following overexpression of SGK-1 in PTCs. The role of EGFR activation in observed changes was assessed by phospho-EGFR expression, and response to the EGFR blocker PKI166. SGK-1 expression was then assessed in vivo in a model of streptozotocin-induced diabetes mellitus type 2.

RESULTS

A total of 25 mmol/L glucose and EGF (10 ng/mL) increased SGK-1 mRNA (P < 0.005 and P < 0.002, respectively) and protein (both P < 0.02) expression. High glucose and overexpression of SGK-1 increased NHE3 mRNA (P < 0.05) and EGFR phosphorylation (P < 0.01), which were reversed by PKI166. SGK-1 overexpression increased PTC growth (P < 0.0001), progression through the cell cycle (P < 0.001), and increased NHE3 mRNA (P < 0.01), which were all reversed with PKI166. Overexpression of SGK-1 also protected against apoptosis induced in the PTCs (P < 0.0001). Up-regulation of tubular SGK-1 mRNA in diabetes mellitus was confirmed in vivo. Oral treatment with PKI166 attenuated this increase by 51%. No EGF protein was detectable in PTCs, suggestive of phosphorylation of the EGFR by high glucose and downstream induction of SGK-1.

CONCLUSION

The effects of high glucose on PTC proliferation, reduced apoptosis and increased NHE3 mRNA levels are mediated by EGFR-dependent up-regulation of SGK-1.

Renal tubular transport and the genetic basis of hypertensive disease

Several monogenic hypertensive disorders are caused by genetic mutations leading to the deranged function and/or regulation of renal tubular NaCl transport, such as mutations of the renal epithelial Na+ channel (ENaC) in Liddle syndrome, of the kinase WNK1 (with no K) in Gordon syndrome, and of the mineralocorticoid receptor, or of 11beta-hydroxysteroid dehydrogenase. Moreover, excessive formation of aldosterone in glucocorticoid-remediable hypertension leads to severe hypertension. Conversely, impaired function of the Na+,K+,2Cl- cotransporter (NKCC2), the renal outer medullary K+ channel (ROMK1), and the renal epithelial Cl- channel ClCKb/Barttin causes Bartter syndrome and defective Na+,Cl+ cotransporter (NCCT) Gitelman syndrome, salt-wasting disorders with hypotension. These monogenic disorders are rare, but illustrate the significance of renal tubular transport in blood pressure regulation. There is little doubt, however, that deranged renal salt reabsorption significantly contributes to essential hypertension polymorphisms of several genes participating in the regulation of renal Na+ transport have been shown to be associated with blood pressure and prevalence of hypertension. Two common genes will be discussed in more detail. The first encodes the renal Cl- channel ClCKb. A gain-of-function mutation of ClCKb, increasing channel activity by 7- to 20-fold is found in approximately 20% of unselected Caucasians and 40% of an unselected African population. The second common gene variant (prevalence, 3%-5% in unselected Caucasians), to be discussed in more detail, affects the serum and glucocorticoid inducible kinase SGK1, a kinase upregulated by mineralocorticoids and enhancing the activity of ENaC, ROMK, and Na+/K+ATPase. Both gene variants are associated with slightly increased blood pressure. SGK1 further stimulates the glucose transporter SGLT1, and the SGK1 gene variant correlates, in addition, with increased body mass index.

Decreased intestinal glucose transport in the sgk3-knockout mouse

Xenopus oocyte coexpression experiments revealed the capacity of the serum- and glucocorticoid-inducible kinase isoform 3 (SGK3) to up-regulate a variety of transport systems including the sodium-dependent glucose transporter SGLT1. The present study explored the functional significance of SGK3-dependent regulation of intestinal transport. To this end, experiments were performed in gene targeted mice lacking functional sgk3 (sgk3(-/-)) and their wild type littermates (sgk3(+/+)). Oral food intake and fecal dry weight were significantly larger in sgk3(-/-) than in sgk3(+/+) mice. Glucose-induced current (I(g)) in Ussing chamber as a measure of Na(+) coupled glucose transport was significantly smaller in sgk3(-/-) than in sgk3(+/+) mouse jejunal segments. Fasting plasma glucose concentrations were significantly lower in sgk3(-/-) than in sgk3(+/+) mice. Intestinal electrogenic transport of phenylalanine, cysteine, glutamine and proline were not significantly different between sgk3(-/-) and sgk3(+/+) mice. In conclusion, SGK3 is required for adequate intestinal Na(+) coupled glucose transport and impaired glucose absorption may contribute to delayed growth and decreased plasma glucose concentrations of SGK3 deficient mice. The hypoglycemia might lead to enhanced food intake to compensate for impaired intestinal absorption.

Activation of NHE3 by dexamethasone requires phosphorylation of NHE3 at Ser663 by SGK1

Glucocorticoids stimulate Na+ absorption by activation of the epithelial Na+/H+ exchanger NHE3 in the kidney and intestine. It has been thought that glucocorticoid-induced activation of NHE3 is solely dependent on transcriptional induction of the NHE3 gene. While the transcriptional regulation remains an essential part of the chronic effect of glucocorticoids, a previous study by us identified the serum- and glucocorticoid-inducible kinase 1 (SGK1) as an important component of the activation of NHE3 by glucocorticoids. In this work, we have demonstrated phosphorylation of NHE3 by SGK1 as the key mechanism for the stimulation of the transport activity by glucocorticoids. By using in vitro SGK1 kinase assay and site-directed mutagenesis, we have identified Ser663 of NHE3 to be the major site of phosphorylation by SGK1. Ser663 is invariantly conserved in all NHE3 proteins from several species, and the mutation of Ser663 to Ala blocks the effect of dexamethasone, demonstrating the importance of phosphorylation at Ser663. We also show that phosphorylation of NHE3 precedes the changes in NHE3 activity, and the increased activity is associated with an increased amount of NHE3 proteins in the surface membrane. These data reveal that dexamethasone activates NHE3 activity by phosphorylating the NHE3 protein, which initiates trafficking of the protein into the plasma membrane.

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.

Regulation of CLC-Ka/barttin by the ubiquitin ligase Nedd4-2 and the serum- and glucocorticoid-dependent kinases

BACKGROUND

ClC-Ka and ClC-Kb, chloride channels participating in renal tubular Cl- transport, require the coexpression of barttin to become functional. Mutations of the barttin gene lead to the Bartter's syndrome variant BSND, characterized by congenital deafness and severe renal salt wasting. Barttin bears a proline-tyrosine motif, a target structure for the ubiquitin ligase Nedd4-2, which mediates the clearance of channel proteins from the cell membrane. Nedd4-2 is, in turn, a target of the serum- and glucocorticoid-inducible kinase SGK1, which phosphorylates and, thus, inactivates the ubiquitin ligase. ClC-Ka also possesses a SGK1 consensus site in its sequence. We hypothesized that ClC-Ka/barttin is stimulated by SGK1, and down-regulated by Nedd4-2, an effect that may be reversed by SGK1 and its isoforms, SGK2 or SGK3.

METHODS

To test this hypothesis, ClC-Ka/barttin was heterologously expressed in Xenopus oocytes with or without the additional expression of Nedd4-2, SGK1, SGK2, SGK3, constitutively active S422DSGK1, or inactive K127NSGK1.

RESULTS

Expression of ClC-Ka/barttin induced a slightly inwardly rectifying current that was significantly decreased upon coexpression of Nedd4-2, but not the catalytically inactive mutant C938SNedd4-2. The coexpression of S422DSGK1, SGK1, or SGK3, but not SGK2 or K127NSGK1 significantly stimulated the current. Moreover, S422DSGK1, SGK1, and SGK3 also phosphorylated Nedd4-2 and thereby inhibited Nedd4-2 binding to its target. The down-regulation of ClC-Ka/barttin by Nedd4-2 was abolished by elimination of the PY motif in barttin.

CONCLUSION

ClC-Ka/barttin channels are regulated by SGK1 and SGK3, which may thus participate in the regulation of transport in kidney and inner ear.

Influence of standard haemodialysis treatment on transcription of human serum- and glucocorticoid-inducible kinase SGK1 and taurine transporter TAUT in blood leukocytes

BACKGROUND

Standard haemodialysis (HD) rapidly alters osmolality and composition of extracellular fluid and, thus, challenges cell volume constancy. Cell volume-sensitive genes upregulated by osmotic cell shrinkage include those encoding for taurine transporter TAUT as well as for serum- and glucocorticoid-inducible kinase SGK1.

METHODS

Six HD patients were haemodialysed for 4 h with high-flux dialysers. Blood was drawn from the arterial section of the fistula immediately prior to start of HD and subsequently after 60, 120 and 240 min of HD treatment and, in addition, 120 min after HD treatment. Taurine plasma concentrations ([taurine]p) and erythrocytic taurine content ([taurine]e) were determined by high-performance liquid chromatography. SGK1 and TAUT transcript levels in leukocytes were quantified by real-time polymerase chain reaction.

RESULTS

The [taurine]p was significantly higher in HD patients before HD treatment when compared with healthy controls and it decreased significantly during 4 h of HD. The ratio of SGK1/GAPDH and of TAUT/GAPDH transcript levels increased significantly by 50% or 27%, respectively, during HD.

CONCLUSIONS

Standard HD treatment decreases plasma taurine concentration and upregulates leukocyte SGK1 and TAUT transcription. As SGK1 is a potent regulator of ion channels and transporters in nervous system, heart muscle and epithelial cells, the deranged regulation of SGK1 may contribute to acute side effects of HD treatment.

Stimulation of the EAAT4 glutamate transporter by SGK protein kinase isoforms and PKB

The serum and glucocorticoid inducible kinase (SGK) 1 is expressed in brain tissue and upregulated by ischemia, neuronal excitation, and dehydration. The present study has been performed to elucidate the expression of SGK1 in cerebellar Purkinje cells and to explore whether it influences the colocalized glutamate transporter EAAT4. Intense SGK1 staining was observed in Purkinje cells following 48h of water deprivation. The kinase activates glutamate induced current (I(GLU)) in Xenopus oocytes heterologously expressing EAAT4, an effect mimicked by its isoforms SGK2, 3 and PKB. I(GLU) was decreased by the ubiquitin ligase Nedd4-2, an effect partially but not completely reversed by additional coexpression of the SGK kinase isoforms or PKB. According to immunohistochemistry EAAT4 protein abundance in the cell membrane was enhanced by SGK1 and decreased by Nedd4-2. In conclusion, SGK1 expression is upregulated by ischemia, excitation, and dehydration in cerebellar Purkinje cells. The upregulation of SGK1 may serve to stimulate EAAT4 and thus to reduce neuroexcitotoxicity.

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.

Requirement of PDZ Domains for the Stimulation of the Epithelial Ca2+ Channel TRPV5 by the NHE Regulating Factor NHERF2 and the Serum and Glucocorticoid Inducible Kinase SGK1

Renal calcium reabsorption involves the epithelial calcium channel ECaC1 (TRPV5) which is tightly regulated by 1,25(OH)2D3. As shown recently, TRPV5 is activated by the serum and glucocorticoid inducible kinase SGK1, a kinase transcriptionally upregulated by 1,25(OH)2D3. This stimulatory effect is due to enhanced TRPV5 abundance in the plasma membrane and requires the presence of the scaffold protein NHERF2 (sodium hydrogen exchanger regulating factor 2). The present study aims to define the molecular requirements for the interaction of TRPV5 with SGK1 and NHERF2. Pull-down experiments and overlay assays revealed that the TRPV5 C-tail interacts in a Ca2+-independent manner with NHERF2. Deletion of the second but not of the first PDZ domain in NHERF2 abrogates the stimulating effect of SGK1/NHERF2 on TRPV5 protein abundance in the plasma membrane as quantified by chemiluminescence and electrophysiology. Thus, the second PDZ domain in NHERF2 is required for stabilization at or TRPV5 targeting to the plasma membrane. The experiments demonstrate the significance of SGK1 and NHERF2 as TRPV5 modulators which are likely to participate in the regulation of calcium homeostasis by 1,25(OH)2D3.

Regulation of epithelial ion transport by aldosterone through changes in gene expression

The year 2003 marks the 50th year since the unfolding of the chemical structures of both aldosterone and DNA. Since the recognition in the early 1960's that aldosterone and its cousin cortisol act through DNA binding proteins that alter gene transcription, research on these corticosteroid hormones and their receptors has attracted fervent attention, both for their importance in endocrine physiology, and as model systems for understanding gene regulation. Recently, aldosterone has emerged as arguably the single most important physiological regulator of extracellular fluid volume and blood pressure in mammals, and has been implicated in a variety of disease states in humans. Moreover, its principal receptor, the mineralocorticoid receptor is increasingly recognized as an important therapeutic target for the treatment of hypertension and congestive heart failure, as well as an important model system for understanding aspects of gene regulation. This increased insight into the functional and pathophysiologic importance of aldosterone has been accompanied by increased insight into its cellular and molecular mechanisms of action. Aldosterone acts in a variety of epithelial and non-epithelial tissues to influence extracellular fluid volume, blood pressure, salt appetite, and can under the appropriate conditions cause cardiac fibrosis. This review will address the current view of aldosterone's molecular mechanism of action in epithelia focusing primarily on the classical MR and on a particular MR target gene, SGK1.

Stimulation of renal Na+ dicarboxylate cotransporter 1 by Na+/H+ exchanger regulating factor 2, serum and glucocorticoid inducible kinase isoforms, and protein kinase B

Renal tubular citrate transport is accomplished by electrogenic Na(+) coupled dicarboxylate transporter NaDC-1, a carrier subjected to regulation by acidosis. Trafficking of the Na(+)/H(+) exchanger NHE3 is controlled by NHE regulating factors NHERF-1 and NHERF-2 and the serum and glucocorticoid inducible kinase SGK1. To test for a possible involvement in NaDC-1 regulation, mRNA encoding NaDC-1 was injected into Xenopus oocytes with or without cRNA encoding NHERF-1, NHERF-2, SGK1, SGK2, SGK3, and/or the constitutively active form of the related protein kinase B ((T308,S473D)PKB). Succinate induced inward currents (I(succ)) were taken as a measure of transport rate. Coexpression of neither NHERF-1 nor NHERF-2 in NaDC-1 expressing oocytes significantly altered I(succ). On the other hand, coexpression of SGK1, SGK3, and (T308,S473D)PKB stimulated I(succ), an effect further stimulated by additional coexpression of NHERF-2 but not of NHERF-1. The action of the kinases and NHERF-2 may link urinary citrate excretion to proximal tubular H(+) secretion.