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

Transcriptome Profile of a New Mouse Model of Spinocerebellar Ataxia Type 14 Implies Changes in Cerebellar Development

The autosomal dominant inherited spinocerebellar ataxias (SCAs) are a group of neurodegenerative disorders characterized by cerebellar atrophy and loss of Purkinje neurons. Spinocerebellar ataxia type 14 (SCA14) is a rare variant of SCAs caused by missense mutations or deletions in the PRKCG gene encoding the protein kinase C γ (PKCγ). Although mutated PKCγs are responsible for SCA14, it is still unclear exactly how mutated PKCγs are involved in SCA14 pathogenesis. Therefore, it is important to study how PKCγ signaling is altered in the cerebellum, which genes or signaling pathways are affected, and how this leads to neurological disease. In this study, we used a mouse line carrying a knock-in pseudo-substrate domain mutation in PKCγ (PKCγ-A24E) as an SCA14 model and performed RNA sequencing (RNA-seq) analysis at an early developmental timepoint (postnatal day 15) to investigate changes in the gene profile compared to wildtype mice. We analyzed both heterozygous (Het) PKCγ-A24E mice and homozygous (Homo) PKCγ-A24E mice for transcriptomic changes. The Het PKCγ-A24E mice reflects the situation observed in human SCA14 patient, while Homo PKCγ-A24E mice display stronger phenotypes with respect to Purkinje cell development and behavior. Our findings highlight an abundance of modifications affecting genes involved in developmental processes, suggesting that at least a part of the final phenotype is shaped by altered cerebellar development and is not only caused by changes in mature animals.

Glutamate transporters: Critical components of glutamatergic transmission

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. Once released, it binds to specific membrane receptors and transporters activating a wide variety of signal transduction cascades, as well as its removal from the synaptic cleft in order to avoid its extracellular accumulation and the overstimulation of extra-synaptic receptors that might result in neuronal death through a process known as excitotoxicity. Although neurodegenerative diseases are heterogenous in clinical phenotypes and genetic etiologies, a fundamental mechanism involved in neuronal degeneration is excitotoxicity. Glutamate homeostasis is critical for brain physiology and Glutamate transporters are key players in maintaining low extracellular Glutamate levels. Therefore, the characterization of Glutamate transporters has been an active area of glutamatergic research for the last 40 years. Transporter activity its regulated at different levels: transcriptional and translational control, transporter protein trafficking and membrane mobility, and through extensive post-translational modifications. The elucidation of these mechanisms has emerged as an important piece to shape our current understanding of glutamate actions in the nervous system.

Regulation of glutamate transporter trafficking by Nedd4-2 in a Parkinson's disease model

Glutamate transporters play a key role in glutamate clearance and protect the central nervous system from glutamate excitotoxicity. Dysfunctional glutamate transporters contribute to the pathogenesis of Parkinson's disease (PD); however, the mechanisms that underlie the regulation of glutamate transporters in PD are still not well characterized. Here we report that Nedd4-2 mediates the ubiquitination of glutamate transporters in 1-methyl-4- phenylpyridinium (MPP⁺)-treated astrocytes and in the midbrain of 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP)-constructed PD model mice. Nedd4-2-mediated ubiquitination induces abnormal glutamate transporter trafficking between the membrane and cytoplasm and consequently decreases the expression and function of glutamate transporters in the membrane. Conversely, Nedd4-2 knockdown decreases glutamate transporter ubiquitination, promotes glutamate uptake and increases glutamate transporter expression in vitro and in vivo. We report for the first time that Nedd4-2 knockdown ameliorates movement disorders in PD mice and increases tyrosine hydroxylase expression in the midbrain and striatum of PD mice; Nedd4-2 knockdown also attenuates astrogliosis and reactive microgliosis in the MPTP model that may be associated with glutamate excitotoxicity. Furthermore, the SGK/PKC pathway is regulated downstream of Nedd4-2 in MPTP-treated mice. These findings indicate that Nedd4-2 may serve as a potential therapeutic target for the treatment of PD.

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.

Protein Kinase C Phosphorylates the System N Glutamine Transporter SN1 (Slc38a3) and Regulates Its Membrane Trafficking and Degradation

The system N transporter SN1 (also known as SNAT3) is enriched on perisynaptic astroglial cell membranes. SN1 mediates electroneutral and bidirectional glutamine transport, and regulates the intracellular as well as the extracellular concentrations of glutamine. We hypothesize that SN1 participates in the glutamate/γ-aminobutyric acid (GABA)-glutamine cycle and regulates the amount of glutamine supplied to the neurons for replenishment of the neurotransmitter pools of glutamate and GABA. We also hypothesize that its activity on the plasma membrane is regulated by protein kinase C (PKC)-mediated phosphorylation and that SN1 activity has an impact on synaptic plasticity. This review discusses reports on the regulation of SN1 by PKC and presents a consolidated model for regulation and degradation of SN1 and the subsequent functional implications. As SN1 function is likely also regulated by PKC-mediated phosphorylation in peripheral organs, the same mechanisms may, thus, have impact on e.g., pH regulation in the kidney, urea formation in the liver, and insulin secretion in the pancreas.

Klotho sensitivity of the neuronal excitatory amino acid transporters EAAT3 and EAAT4

Klotho, a transmembrane protein, which can be cleaved off as β-glucuronidase and hormone, is released in both, kidney and choroid plexus and encountered in blood and cerebrospinal fluid. Klotho deficiency leads to early appearance of age-related disorders and premature death. Klotho may modify transport by inhibiting 1,25(OH)₂D₃ formation or by directly affecting channel and carrier proteins. The present study explored whether Klotho influences the activity of the Na⁺-coupled excitatory amino acid transporters EAAT3 and EAAT4, which are expressed in kidney (EAAT3), intestine (EAAT3) and brain (EAAT3 and EAAT4). To this end, cRNA encoding EAAT3 or EAAT4 was injected into Xenopus oocytes with and without additional injection of cRNA encoding Klotho. EAAT expressing Xenopus oocytes were further treated with recombinant human β-Klotho protein with or without β-glucuronidase inhibitor D-saccharic acid 1,4-lactone monohydrate (DSAL). Electrogenic excitatory amino acid transport was determined as L-glutamate-induced current (I_glu) in two electrode voltage clamp experiments. EAAT3 and EAAT4 protein abundance in the Xenopus oocyte cell membrane was visualized by confocal microscopy and quantified utilizing chemiluminescence. As a result, coexpression of Klotho cRNA significantly increased I_glu in both, EAAT3 or EAAT4-expressing Xenopus oocytes. Klotho cRNA coexpression significantly increased the maximal current and cell membrane protein abundance of both EAAT3 and EAAT4. The effect of Klotho coexpression on EAAT3 and EAAT4 activity was mimicked by treating EAAT3 or EAAT4-expressing Xenopus oocytes with recombinant human β-Klotho protein. The effects of Klotho coexpression and of treatment with recombinant human β-Klotho protein were both abrogated in the presence of DSAL (10 µM). In conclusion, Klotho is a novel, powerful regulator of the excitatory amino acid transporters EAAT3 and EAAT4.

The high-affinity choline transporter CHT1 is regulated by the ubiquitin ligase Nedd4-2

The high-affinity choline transporter (CHT1), which is specifically expressed in cholinergic neurons, constitutes a rate-limiting step for acetylcholine synthesis. We have found that the exogenous ubiquitin ligase Nedd4-2 interacts with CHT1 expressed in HEK293 cells decreasing the amount of cell surface CHT1 by approximately 40%, and that small interfering RNA for endogenous Nedd4-2 enhances the choline uptake activity by CHT1 in HEK293 cells. These results indicate that Nedd4-2-mediated ubiquitination regulates the cell surface expression of CHT1 in cultured cells and suggest a possibility that treatments or drugs which inhibit the interaction between CHT1 and Nedd4-2 might be useful for diseases involving decrease in acetylcholine level such as Alzheimer's disease.

Genome-wide association study in bipolar patients stratified by co-morbidity

Background

Bipolar disorder is a severe psychiatric disorder with high heritability. Co-morbid conditions are common and might define latent subgroups of patients that are more homogeneous with respect to genetic risk factors.

Methodology

In the Caucasian GAIN bipolar disorder sample of 1000 cases and 1034 controls, we tested the association of single nucleotide polymorphisms with patient subgroups defined by co-morbidity.

Results

Bipolar disorder with psychosis and/or substance abuse in the absence of alcohol dependence was associated with the rare variant rs1039002 in the vicinity of the gene phosphodiesterase 10A (PDE10A) on chromosome 6q27 (p = 1.7×10⁻⁸). PDE10A has been implicated in the pathophysiology of psychosis. Antagonists to the encoded protein are currently in clinical testing. Another rare variant, rs12563333 (p = 5.9×10⁻⁸) on chromosome 1q41 close to the MAP/microtubule affinity-regulating kinase 1 (MARK1) gene, approached the genome-wide level of significance in this subgroup. Homozygotes for the minor allele were present in cases and absent in controls. Bipolar disorder with alcohol dependence and other co-morbidities was associated with SNP rs2727943 (p = 3.3×10⁻⁸) on chromosome 3p26.3 located between the genes contactin-4 precursor (BIG-2) and contactin 6 (CNTN6). All three associations were found under the recessive genetic model. Bipolar disorder with low probability of co-morbid conditions did not show significant associations.

Conclusion

Conceptualizing bipolar disorder as a heterogeneous disorder with regard to co-morbid conditions might facilitate the identification of genetic risk alleles. Rare variants might contribute to the susceptibility to bipolar disorder.

Stimulation of Ca2+-channel Orai1/STIM1 by serum- and glucocorticoid-inducible kinase 1 (SGK1)

Ca(2+) signaling includes store-operated Ca(2+) entry (SOCE) following depletion of endoplasmic reticulum (ER) Ca(2+) stores. On store depletion, the ER Ca(2+) sensor STIM1 activates Orai1, the pore-forming unit of Ca(2+)-release-activated Ca(2+) (CRAC) channels. Here, we show that Orai1 is regulated by serum- and glucocorticoid-inducible kinase 1 (SGK1), a growth factor-regulated kinase. Membrane Orai1 protein abundance, I(CRAC), and SOCE in human embryonic kidney (HEK293) cells stably expressing Orai1 and transfected with STIM1 were each significantly enhanced by coexpression of constitutively active (S422D)SGK1 (by+81, +378, and+136%, respectively) but not by inactive (K127N)SGK1. Coexpression of the ubiquitin ligase Nedd4-2, an established negatively regulated SGK1 target, down-regulated SOCE (by -48%) and I(CRAC) (by -60%), an effect reversed by expression of (S422D)SGK1 (by +175 and +173%, respectively). Orai1 protein abundance and SOCE were significantly lower in mast cells from SGK1-knockout (sgk1(-/-)) mice (by -37% and -52%, respectively) than in mast cells from wild-type (sgk1(+/+)) littermates. Activation of SOCE by sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase-inhibitor thapsigargin (2 μM) stimulated migration, an effect significantly higher (by +306%) in (S422D)SGK1-expressing than in (K127N)SGK1-expressing HEK293 cells, and also significantly higher (by +108%) in sgk1(+/+) than in sgk1(-/-) mast cells. SGK1 is thus a novel key player in the regulation of SOCE.

Down-regulation of Na+-coupled glutamate transporter EAAT3 and EAAT4 by AMP-activated protein kinase

The glutamate transporters EAAT3 and EAAT4 are expressed in neurons. They contribute to the cellular uptake of glutamate and aspartate and thus to the clearance of the excitatory transmitters from the extracellular space. During ischemia, extracellular accumulation of glutamate may trigger excitotoxicity. Energy depletion leads to activation of the AMP-activated protein kinase (AMPK), a kinase enhancing energy production and limiting energy expenditure. The present study thus explored the possibility that AMPK regulates EAAT3 and/or EAAT4. To this end, EAAT3 or EAAT4 were expressed in Xenopus oocytes with or without AMPK and electrogenic glutamate transport determined by dual electrode voltage clamp. In EAAT3- and in EAAT4-expressing oocytes glutamate generated a current (I(g)), which was half maximal (K(M)) at 74 microM (EAAT3) or at 4 microM (EAAT4) glutamate. Co-expression of constitutively active (gammaR70Q)AMPK or of wild type AMPK did not affect K(M) but significantly decreased the maximal I(g) in both EAAT3- (by 34%) and EAAT4- (by 49%) expressing oocytes. Co-expression of the inactive mutant (alphaK45R)AMPK [alpha1(K45R)beta1gamma1] did not appreciably affect I(g). According to confocal microscopy and chemiluminescence co-expression of (gammaR70Q)AMPK or of wild type AMPK reduced the membrane abundance of EAAT3 and EAAT4. The observations show that AMPK down-regulates Na(+)-coupled glutamate transport.

Targeting SGK1 in diabetes

Compelling evidence is accumulating indicating a pathophysiological role of the serum-and-glucocorticoid-inducible-kinase-1 (SGK1) in the development and complications of diabetes. SGK1 is ubiquitously expressed with exquisitely high transcriptional volatility. Stimulators of SGK1 expression include hyperglycemia, cell shrinkage, ischemia, glucocorticoids and mineralocorticoids. SGK1 is activated by insulin and growth factors via PI3K, 3-phosphoinositide dependent kinase PDK1 and mTOR. SGK1 activates ion channels (including ENaC, TRPV5, ROMK, KCNE1/KCNQ1 and CLCKa/Barttin), carriers (including NCC, NKCC, NHE3, SGLT1 and EAAT3), and the Na(+)/K(+)-ATPase. It regulates the activity of several enzymes (e.g., glycogen-synthase-kinase-3, ubiquitin-ligase Nedd4-2, phosphomannose-mutase-2), and transcription factors (e.g., forkhead-transcription-factor FOXO3a, beta-catenin and NF-kappaB). A common SGK1 gene variant ( approximately 3 - 5% prevalence in Caucasians, approximately 10% in Africans) is associated with increased blood pressure, obesity and type 2 diabetes. In patients suffering from type 2 diabetes, SGK1 presumably contributes to fluid retention and hypertension, enhanced coagulation and increased deposition of matrix proteins leading to tissue fibrosis such as diabetic nephropathy. Accordingly, targeting SGK1 may favourably influence occurrence and course of type 2 diabetes.

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.

The physiological impact of the serum and glucocorticoid-inducible kinase SGK1

PURPOSE OF REVIEW

The role of serum and glucocorticoid-inducible kinase 1 (SGK1) in renal physiology and pathophysiology is reviewed with particular emphasis on recent advances.

RECENT FINDINGS

The mammalian target of rapamycin complex 2 has been shown to phosphorylate SGK1 at Ser422 (the so-called hydrophobic motif). Ser397 and Ser401 are two additional SGK1-phosphorylation sites required for maximal SGK1 activity. A 5' variant alternate transcript of human Sgk1 has been identified that is widely expressed and shows improved stability, enhanced membrane association, and greater stimulation of epithelial Na+ transport. SGK1 is essential for optimal processing of the epithelial sodium channel and also regulates the expression of the Na+-Cl- cotransporter. With regard to pathophysiology, SGK1 participates in the stimulation of renal tubular glucose transport in diabetes, the renal profibrotic effect of both angiotensin II and aldosterone, and in fetal programing of arterial hypertension.

SUMMARY

The outlined recent findings advanced our understanding of the molecular regulation of SGK1 as well as the role of the kinase in renal physiology and the pathophysiology of renal disease and hypertension. Future studies using pharmacological inhibitors of SGK1 will reveal the utility of the kinase as a new therapeutic target.

SGK1 phosphorylation of IkappaB Kinase alpha and p300 Up-regulates NF-kappaB activity and increases N-Methyl-D-aspartate receptor NR2A and NR2B expression

Serum- and glucocorticoid-inducible kinase 1 (SGK1) is a downstream target of phosphatidylinositol 3-kinase signaling, and it regulates various cellular and physiological functions, but the SGK1 substrate proteins and genes regulated by SGK1 are less known. Here we have identified IkappaB kinase alpha (IKKalpha) as a novel substrate of SGK1 by using biochemical and bioinformatic approaches. SGK1 directly phosphorylates IKKalpha at Thr-23 and indirectly activates IKKalpha at Ser-180. Furthermore, SGK1 enhanced nuclear factor kappaB (NF-kappaB) activity and up-regulated N-methyl-d-aspartate receptor NR2A and NR2B expression through activation of IKKalpha at Thr-23 and Ser-180, and these two residues play an equally important role in mediating these effects of SGK1. Although SGK1 does not phosphorylate IKKbeta, IKKbeta activity is still required for IKK complex activation and for SGK1 phosphorylation and activation of NF-kappaB. In addition, SGK1 increased the acetylation of NF-kappaB through phosphorylation of p300 at Ser-1834, and this also leads to NF-kappaB activation and NR2A and NR2B expression. Moreover, an endogenous stimulus of SGK1, insulin, increased IKKalpha and NF-kappaB phosphorylation as well as NF-kappaB acetylation and NF-kappaB activity, but SGK1 small interfering RNA transfection blocked these effects of insulin. In examination of the functional significance of the SGK1-IKKalpha-NF-kappaB signaling pathway, we found that transfection of the IKKalpha double mutant (IKKalphaT23A/S180A) to rat hippocampus antagonized SGK-1-mediated spatial memory facilitation. Our results together demonstrated novel substrate proteins of SGK1 and novel SGK1 signaling pathways. Activation of these signaling pathways enhances NR2A and NR2B expression that is implicated in neuronal plasticity.

Regulation of the epithelial Na+ channel by the protein kinase CK2

CK2 is a ubiquitous, pleiotropic, and constitutively active Ser/Thr protein kinase that controls protein expression, cell signaling, and ion channel activity. Phosphorylation sites for CK2 are located in the C terminus of both β- and γ-subunits of the epithelial Na⁺ channel (ENaC). We examined the role of CK2 on the regulation of both endogenous ENaC in native murine epithelia and in Xenopus oocytes expressing rENaC. In Ussing chamber experiments with mouse airways, colon, and cultured M1-collecting duct cells, amiloride-sensitive Na⁺ transport was inhibited dose-dependently by the selective CK2 inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB). In oocytes, ENaC currents were also inhibited by TBB and by the structurally unrelated inhibitors heparin and poly(E:Y). Expression of a trimeric channel lacking both CK2 sites (αβ_S631Aγ_T599A) produced a largely attenuated amiloride-sensitive whole cell conductance and rendered the mutant channel insensitive to CK2. In Xenopus oocytes, CK2 was translocated to the cell membrane upon expression of wt-ENaC but not of αβ_S631Aγ_T599A-ENaC. Phosphorylation by CK2 is essential for ENaC activation, and to a lesser degree, it also controls membrane expression of αβγ-ENaC. Channels lacking the Nedd4-2 binding motif in β-ENaC (R561X, Y618A) no longer required the CK2 site for channel activity and siRNA-knockdown of Nedd4-2 eliminated the effects of TBB. This implies a role for CK2 in inhibiting the Nedd4-2 pathway. We propose that the C terminus of β-ENaC is targeted by this essential, conserved pleiotropic kinase that directs its constitutive activity toward many cellular protein complexes.

Up-regulation of hypertonicity-activated myo-inositol transporter SMIT1 by the cell volume-sensitive protein kinase SGK1

Mechanisms of regulatory cell volume increase following cell shrinkage include accumulation of organic osmolytes such as betaine, taurine, sorbitol, glycerophosphorylcholine (GPC) and myo-inositol. Myo-inositol is taken up by the sodium-myo-inositol-transporter SMIT1 (SLC5A3) expressed in a wide variety of cell types. Hypertonicity induces the transcription of the SMIT1 gene upon binding of the transcription factor tonicity enhancer binding protein (TonEBP) to tonicity responsive enhancers (TonE) in the SMIT1 promoter region. However, little is known about post-translational regulation of the carrier protein. In this study we show that SMIT1 is modulated by the serum- and glucocorticoid-inducible kinase SGK1, a protein genomically up-regulated by hypertonicity. As demonstrated by two-electrode voltage-clamp in the Xenopus oocyte expression system, SMIT1-mediated myo-inositol-induced currents are up-regulated by coexpression of wild type SGK1 and constitutively active (S422D)SGK1 but not by inactive (K127N)SGK1. The increase in SMIT1 activity is due to an elevated cell surface expression of the carrier while its kinetic properties remain unaffected. According to the decay of SMIT1 activity in the presence of brefeldin A, SGK1 stabilizes the SMIT1 protein in the plasma membrane. The SGK isoforms SGK2, SGK3 and the closely related protein kinase B (PKB) are similarly capable of activating SMIT1 activity. SMIT1-mediated currents are decreased by coexpression of the ubiquitin-ligase Nedd4-2, an effect counteracted by additional coexpression of SGK1. In conclusion, the present observations disclose SGK isoforms and protein kinase B as novel regulators of SMIT1 activity.