Regulation of the voltage-gated K+ channels KCNQ2/3 and KCNQ3/5 by serum- and glucocorticoid-regulated kinase-1

NEDD4-2 and the CLC-2 channel regulate neuronal excitability in the pathogenesis of mesial temporal lobe epilepsy

An increasing number of studies have focused on the role of NEDD4-2 in regulating neuronal excitability and the mechanism of epilepsy. However, the exact mechanism has not yet been elucidated. Here, we explored the roles of NEDD4-2 and the CLC-2 channel in regulating neuronal excitability and mesial temporal lobe epilepsy (MTLE) pathogenesis. First, chronic MTLE models were induced by lithium-pilocarpine in developmental rats. Coimmunoprecipitation analysis revealed that the interaction between CLC-2 and NEDD4-2. Western blot analyses indicated that NEDD4-2 expression was downregulated, while phosphorylated (P-) NEDD4-2 and CLC-2 expression was upregulated in adult MTLE rats. Then, the primary hippocampal neuronal cells were isolated and cultured, and the NEDD4-2 was knocked down by shRNA vector, resulting in decreased protein levels of CLC-2. While CLC-2 absence caused increased NEDD4-2 in cells. Next, in an epileptic cell model induced by a Mg2+-free culture, whole-cell current-clamp recording demonstrated that NEDD4-2 deficiency inhibited the spontaneous action potentials of cells, and CLC-2 absence caused more significant decrease in the spontaneous action potentials of cells. In conclusion, we herein revealed that NEDD4-2 regulates the expression of CLC-2, which is involved in neuronal excitability, and participates in the pathogenesis of MTLE.

Involvement of Ca2+ in Signaling Mechanisms Mediating Muscarinic Inhibition of M Currents in Sympathetic Neurons

Acetylcholine can excite neurons by suppressing M-type (KCNQ) potassium channels. This effect is mediated by M1 muscarinic receptors coupled to the Gq protein. Although PIP2 depletion and PKC activation have been strongly suggested to contribute to muscarinic inhibition of M currents (IM), direct evidence is lacking. We investigated the mechanism involved in muscarinic inhibition of IM with Ca2+ measurement and electrophysiological studies in both neuronal (rat sympathetic neurons) and heterologous (HEK cells expressing KCNQ2/KCNQ3) preparations. We found that muscarinic inhibition of IM was not blocked either by PIP2 or by calphostin C, a PKC inhibitor. We then examined whether muscarinic inhibition of IM uses multiple signaling pathways by blocking both PIP2 depletion and PKC activation. This maneuver, however, did not block muscarinic inhibition of IM. Additionally, muscarinic inhibition of IM was not prevented either by sequestering of G-protein βγ subunits from Gα-transducin or anti-Gβγ antibody or by preventing intracellular trafficking of channel proteins with blebbistatin, a class-II myosin inhibitor. Finally, we re-examined the role of Ca2+ signals in muscarinic inhibition of IM. Ca2+ measurements showed that muscarinic stimulation increased intracellular Ca2+ and was comparable to the Ca2+ mobilizing effect of bradykinin. Accordingly, 20-mM of BAPTA significantly suppressed muscarinic inhibition of IM. In contrast, muscarinic inhibition of IM was completely insensitive to 20-mM EGTA. Taken together, these data suggest a role of Ca2+ signaling in muscarinic modulation of IM. The differential effects of EGTA and BAPTA imply that Ca2+ microdomains or spatially local Ca2+ signals contribute to inhibition of IM.

AMPA Receptor Antagonists Facilitate NEDD4-2-Mediated GRIA1 Ubiquitination by Regulating PP2B-ERK1/2-SGK1 Pathway in Chronic Epilepsy Rats

The neural precursor cell expressed by developmentally downregulated gene 4-2 (NEDD4-2) is a ubiquitin E3 ligase that has a high affinity toward binding and ubiquitinating glutamate ionotropic receptor α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type subunit 1 (GRIA1, also referred to GluR1 or GluA1). Since dysregulation of GRIA1 surface expression is relevant to the responsiveness to AMPA receptor (AMPAR) antagonists (perampanel and GYKI 52466) in chronic epilepsy rats, it is likely that NEDD4-2 may be involved in the pathogenesis of intractable epilepsy. However, the role of NEDD4-2-mediated GRIA1 ubiquitination in refractory seizures to AMPAR antagonists is still unknown. In the present study, both AMPAR antagonists recovered the impaired GRIA1 ubiquitination by regulating protein phosphatase 2B (PP2B)-extracellular signal-regulated kinase 1/2 (ERK1/2)-serum and glucocorticoid-regulated kinase 1 (SGK1)-NEDD4-2 signaling pathway in responders (whose seizure activities are responsive to AMPAR), but not non-responders (whose seizure activities were uncontrolled by AMPAR antagonists). In addition, cyclosporin A (CsA, a PP2B inhibitor) co-treatment improved the effects of AMPAR antagonists in non-responders, independent of AKT signaling pathway. Therefore, our findings suggest that dysregulation of PP2B-ERK1/2-SGK1-NEDD4-2-mediated GRIA1 ubiquitination may be responsible for refractory seizures and that this pathway may be a potential therapeutic target for improving the treatment of intractable epilepsy in response to AMPAR antagonists.

Infantile spasms-linked Nedd4-2 mediates hippocampal plasticity and learning via cofilin signaling

Individuals affected by infantile spasms (IS), such as those carrying mutations in an IS-linked gene, neural precursor cell expressed developmentally downregulated gene 4-like (Nedd4-2), exhibit developmental delays and learning disabilities, but the underlying mechanism is unknown. Using conditional Nedd4-2 knockout mice, we uncover that Nedd4-2 functions to maintain the excitatory synapses in hippocampal neurons and allows for late-phase long-term synaptic potentiation (L-LTP) at Schaffer collateral synapses in the hippocampus. We also find that Nedd4-2 is required for multiple forms of hippocampus-dependent learning and memory. Mechanistically, we show that loss of Nedd4-2 leads to a decrease in actin polymerization caused by reduced phosphorylation of the actin depolymerizing protein cofilin. A cell-permeable peptide promoting phosphorylation of endogenous cofilin in Nedd4-2 knockout neurons restores the number of hippocampal excitatory synapses and hippocampal L-LTP and partially restores hippocampus-dependent learning in mice. Taken together, our results reveal a novel mechanism underlying IS-associated learning disabilities and may provide information for future therapeutic strategies for IS.

Nedd4-2 haploinsufficiency in mice causes increased seizure susceptibility and impaired Kir4.1 ubiquitination

Neural precursor cell expressed developmentally down-regulated gene 4-like (NEDD4-2) encodes a ubiquitin E3 ligase that is involved in epileptogenesis with mechanisms needing further investigation. We constructed a novel Nedd4-2^+/- mouse model with half level of both Nedd4-2 long and short isoforms in the brain. Nedd4-2 haploinsufficiency caused increased susceptibility and severity of pentylenetetrazole (PTZ)-induced seizures. Of the 3379 proteins identified by the hippocampal proteomic analysis, 55 were considered altered in Nedd4-2^+/- mice compared with wild-type control, among which the inwardly rectifying K⁺ channel Kir4.1 was up-regulated by 1.83-fold. Kir4.1 was subsequently confirmed to be less ubiquitinated in response to comprised Nedd4-2 in mouse brains and C6 cells. Kir4.1 associated with Nedd4-2 through the threonine³¹²-proline motif in the intracellular domain by target mutagenesis. Adaptor protein 14-3-3 facilitated Nedd4-2-mediated ubiquitination of Kir4.1. Our data consolidate the detailed molecular mechanism of Nedd4-2-mediated Kir4.1 ubiquitination, and provide a possible relationship between increased seizure susceptibility and impaired Kir4.1 ubiquitination in the brain.

C2-lacking isoform of Nedd4-2 regulates excitatory synaptic strength through GluA1 ubiquitination-independent mechanisms

Neural precursor cell expressed developmentally downregulated gene 4-like (Nedd4-2) is an epilepsy-associated gene, which encodes a ubiquitin E3 ligase that is highly expressed in the brain. Nedd4-2's substrates include many ion channels and receptors because its N-terminal C2 domain guides Nedd4-2 to the cell membrane. We previously found that Nedd4-2 ubiquitinates the glutamate receptor subunit 1 (GluA1) subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor, which leads to reduction of neuronal excitability and seizures in mice. However, despite awareness of a Nedd4-2 isoform with no C2 domain, the functions of this isoform remain elusive. In this study, we showed that the C2-lacking Nedd4-2 has reduced membrane distribution and exhibits reduced affinity toward ubiquitinating GluA1. However, when expressed in primary cortical neurons, we found that the C2-lacking Nedd4-2 exhibits a similar activity toward reducing excitatory synaptic strength as does the C2-containing Nedd4-2. In an attempt to identify novel Nedd4-2 substrates that could mediate excitatory synaptic strength, we used unbiased proteomic screening and found multiple synaptic regulators that were up-regulated in the brain of conditional Nedd4-2 knockout mice, including protein phosphatase 3 catalytic subunit-α (PPP3CA; alternately called calcineurin A-α). We confirmed PPP3CA as a substrate of the C2-lacking Nedd4-2 and showed that all three epilepsy-associated missense mutations of Nedd4-2 disrupted PPP3CA ubiquitination. Altogether, our results revealed novel potential Nedd4-2 substrates and suggest that the C2-lacking Nedd4-2 represses excitatory synaptic strength most likely through GluA1 ubiquitination-independent mechanisms. These findings provide novel information to further our knowledge about Nedd4-2-dependent neuronal excitability homeostasis and pathological hyperexcitability when Nedd4-2 is compromised.

PLPP/CIN-mediated NEDD4-2 S448 dephosphorylation regulates neuronal excitability via GluA1 ubiquitination

Neuronal precursor cell expressed developmentally downregulated 4-2 (NEDD4-2) is an E3 ubiquitin ligase to regulate ion transport by controlling cellular trafficking/endocytosis and lysosomal degradation of ion channels and transporters. Thus, NEDD4-2 is relevant to neuronal excitability and epileptic encephalopathies in human patients. However, the regulatory molecules for NEDD4-2 dephosphorylation have been still elusive. Here, we demonstrate that pyridoxal-5′-phosphate phosphatase/chronophin (PLPP/CIN) specifically dephosphorylated NEDD4-2 serine (S) 448 site. PLPP/CIN deletion inhibited NEDD4-2 ubiquitination, and diminished the responsiveness of α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid receptor (AMPAR) by facilitating NEDD4-2-mediated ubiquitination of GluA1 subunit under physiological condition. PLPP/CIN overexpression reversed these effects. These PLPP/CIN-mediated processes were required for the increased seizure severity and its progression in response to kainic acid (KA). Therefore, we suggest the novel function of PLPP/CIN as a NEDD4-2 phosphatase, which may be a potential therapeutic target for NEDD4-2-associated diseases as well as various neurological and psychiatric disorders, including epilepsy.

Retigabine ameliorates acute stress-induced impairment of spatial memory retrieval through regulating USP2 signaling pathways in hippocampal CA1 area

Acute stress could trigger maladaptive changes associated with stress-related cognitive and emotional deficits. Dysfunction of ion channel or receptor in the hippocampal area has been linked to the cognitive deficits induced by stress. It is known that Kv7 channel openers, including FDA-approved drug retigabine, show cognitive protective efficacy. However, the underlying molecular mechanisms remain elusive. Here we showed that exposing adult male rats to acute stress significantly impaired the spatial memory, a cognitive process controlled by the hippocampus. Concomitantly, significantly reduced AMPA receptor expression was found in hippocampal CA1 area from acute stressed rats. This effect relied on the down-regulation of deubiquitinating enzyme USP2 and its upstream regulators (PGC-1α and β-catenin), and the subsequent enhancement of mTOR-related autophagy which is regulated by USP2. These findings suggested that acute stress dampened AMPA receptor expression by controlling USP2-related signaling, which caused the detrimental effect on hippocampus-dependent cognitive processes. We also found that retigabine alleviated acute stress-induced spatial memory retrieval impairment through adjusting the aberrance of USP2, its upstream regulators (PGC-1α, E4BP4 and β-catenin) and its downstream targets (mTOR, autophagy and GluA1). Our results have identified USP2 as a key molecule that mediates stress-induced spatial memory retrieval impairment, which provides a framework for new druggable targets to conceptually treat stress-associated cognitive deficits.

Epilepsy-associated gene Nedd4-2 mediates neuronal activity and seizure susceptibility through AMPA receptors

The neural precursor cell expressed developmentally down-regulated gene 4–2, Nedd4-2, is an epilepsy-associated gene with at least three missense mutations identified in epileptic patients. Nedd4-2 encodes a ubiquitin E3 ligase that has high affinity toward binding and ubiquitinating membrane proteins. It is currently unknown how Nedd4-2 mediates neuronal circuit activity and how its dysfunction leads to seizures or epilepsies. In this study, we provide evidence to show that Nedd4-2 mediates neuronal activity and seizure susceptibility through ubiquitination of GluA1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor, (AMPAR). Using a mouse model, termed Nedd4-2^andi, in which one of the major forms of Nedd4-2 in the brain is selectively deficient, we found that the spontaneous neuronal activity in Nedd4-2^andi cortical neuron cultures, measured by a multiunit extracellular electrophysiology system, was basally elevated, less responsive to AMPAR activation, and much more sensitive to AMPAR blockade when compared with wild-type cultures. When performing kainic acid-induced seizures in vivo, we showed that elevated seizure susceptibility in Nedd4-2^andi mice was normalized when GluA1 is genetically reduced. Furthermore, when studying epilepsy-associated missense mutations of Nedd4-2, we found that all three mutations disrupt the ubiquitination of GluA1 and fail to reduce surface GluA1 and spontaneous neuronal activity when compared with wild-type Nedd4-2. Collectively, our data suggest that impaired GluA1 ubiquitination contributes to Nedd4-2-dependent neuronal hyperactivity and seizures. Our findings provide critical information to the future development of therapeutic strategies for patients who carry mutations of Nedd4-2.

Many patients with neurological disorders suffer from an imbalance in neuronal and circuit excitability and present with seizure or epilepsy as the common comorbidity. Human genetic studies have identified many epilepsy-associated genes, but the pathways by which those genes are connected to brain circuit excitability are largely unknown. Our study focused on one of the epilepsy-associated genes, Nedd4-2, and aimed to dissect the molecular mechanism underlying Nedd4-2-associated epilepsy. Nedd4-2 encodes a ubiquitin E3 ligase. Several neuronal ion channels have been identified as its substrates, including the GluA1 subunit of AMPAR. Our results first demonstrate up-regulation of spontaneous neuronal activity and seizure susceptibility when Nedd4-2 is reduced in a mouse model. These deficits can be corrected when GluA1/AMPAR is pharmacologically or genetically inhibited. In addition, we found that three epilepsy-associated missense mutations of Nedd4-2 inhibit the ubiquitination of GluA1 and fail to reduce GluA1 surface expression or spontaneous neuronal activity when compared to wild-type Nedd4-2. These findings suggest the reduction of GluA1 ubiquitination as a crucial deficit underlying insufficient function of Nedd4-2 and provide critical information to the development of therapies for patients who carry mutations of Nedd4-2.

Ubiquitin-specific Protease 36 (USP36) Controls Neuronal Precursor Cell-expressed Developmentally Down-regulated 4-2 (Nedd4-2) Actions over the Neurotrophin Receptor TrkA and Potassium Voltage-gated Channels 7.2/3 (Kv7.2/3)

Ubiquitination of the TrkA neurotrophin receptor in response to NGF is critical in the regulation of TrkA activation and functions. TrkA is ubiquitinated, among other E3 ubiquitin ligases, by Nedd4-2. To understand mechanistically how TrkA ubiquitination is regulated, we performed a siRNA screening to identify deubiquitinating enzymes and found that USP36 acts as an important regulator of TrkA activation kinetics and ubiquitination. However, USP36 action on TrkA was indirect because it does not deubiquitinate TrkA. Instead, USP36 binds to Nedd4-2 and regulates the association of TrkA and Nedd4-2. In addition, depletion of USP36 increases TrkA·Nedd4-2 complex formation, whereas USP36 expression disrupts the complex, resulting in an enhancement or impairment of Nedd4-2-dependent TrkA ubiquitination, respectively. Moreover, USP36 depletion leads to enhanced total and surface TrkA expression that results in increased NGF-mediated TrkA activation and signaling that augments PC12 cell differentiation. USP36 actions extend beyond TrkA because the presence of USP36 interferes with Nedd4-2-dependent Kv7.2/3 channel regulation. Our results demonstrate that USP36 binds to and regulates the actions of Nedd4-2 over different substrates affecting their expression and functions.

Activity-dependent downregulation of M-Type (Kv7) K⁺ channels surface expression requires the activation of iGluRs/Ca²⁺/PKC signaling pathway in hippocampal neuron

M-type (Kv7) K(+) channels, encoded by KCNQ2-KCNQ5 genes, play a pivotal role in controlling neuronal excitability. However, precisely how neuronal activity regulates Kv7 channel translocation has not yet been fully defined. Here we reported activity-dependent changes in Kv7 channel subunits Kv7.2 and Kv7.3 surface expression by glutamate (glu). In the present study, we found that treatment with glutamate rapidly caused a specific decrease in M-current as well as Kv7 channel surface expression in primary cultured hippocampal neurons. The glutamate effects were mimicked by NMDA and AMPA. The glutamate effects on Kv7 channels were partially attenuated by pre-treatment of NMDA receptors antagonist d,l-APV or AMPA-KA receptors antagonist CNQX. The signal required Ca(2+) influx through L-type Ca(2+) channel and intracellular Ca(2+) elevations. PKC activation was involved in the glutamate-induced reduction of Kv7 channel surface expression. Moreover, a significant reduction of Kv7 channel surface expression occurred following glycine-induced "chem"-LTP in vitro and hippocampus-dependent behavioral learning training in vivo. These results demonstrated that activity-dependent reduction of Kv7 channel surface expression through activation of ionotropic glutamate receptors (iGluRs)/Ca(2+)/PKC signaling pathway might be an important molecular mechanism for regulation of neuronal excitability and synaptic plasticity.

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

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

Endocytic regulation of alkali metal transport proteins in mammals, yeast and plants

The relative concentrations of ions and solutes inside cells are actively maintained by several classes of transport proteins, in many cases against their concentration gradient. These transport processes, which consume a large portion of cellular energy, must be constantly regulated. Many structurally distinct families of channels, carriers, and pumps have been characterized in considerable detail during the past decades and defects in the function of some of these proteins have been linked to a growing list of human diseases. The dynamic regulation of the transport proteins present at the cell surface is vital for both normal cellular function and for the successful adaptation to changing environments. The composition of proteins present at the cell surface is controlled on both the transcriptional and post-translational level. Post-translational regulation involves highly conserved mechanisms of phosphorylation- and ubiquitylation-dependent signal transduction routes used to modify the cohort of receptors and transport proteins present under any given circumstances. In this review, we will summarize what is currently known about one facet of this regulatory process: the endocytic regulation of alkali metal transport proteins. The physiological relevance, major contributors, parallels and missing pieces of the puzzle in mammals, yeast and plants will be discussed.

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

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

Nedd4 and Nedd4-2: ubiquitin ligases at work in the neuron

Ubiquitination of proteins by the Nedd4 family of ubiquitin ligases is a significant mechanism in protein trafficking and degradation and provides for tight spatiotemporal regulation. Ubiquitination is gaining increasing recognition as a central mechanism underpinning the regulation of neuronal development and homeostasis in the brain. This review will focus on the Nedd4 and Nedd4-2 E3 ubiquitin ligases that are implicated in an increasing number of neuronal protein-protein interactions. Understanding of the contribution of Nedd4 and Nedd4-2 in regulating key functions in the brain is shedding new light on the ubiquitination signal not only in orchestrating degradation events but also in protein trafficking. Furthermore, the description of several novel Nedd4/4-2 targets in neurons is changing the way we conceptualize how neurons maintain normal function and how this is altered in disease.

Downregulation of Kv1.5 K channels by the AMP-activated protein kinase

BACKGROUND

The voltage gated K(+) channel Kv1.5 participates in the repolarization of a wide variety of cell types. Kv1.5 is downregulated during hypoxia, which is known to stimulate the energy-sensing AMP-activated serine/threonine protein kinase (AMPK). AMPK is a powerful regulator of nutrient transport and metabolism. Moreover, AMPK is known to downregulate several ion channels, an effect at least in part due to stimulation of the ubiquitin ligase Nedd4- 2. The present study explored whether AMPK regulates Kv1.5.

METHODS

cRNA encoding Kv1.5 was injected into Xenopus oocytes with and without additional injection of wild-type AMPK (α1 β 1γ1), of constitutively active (γR70Q)AMPK (α1 β 1γ1(R70Q)), of inactive mutant (αK45R)AMPK (α1(K45R)β1γ1), or of Nedd4-2. Kv1.5 activity was determined by two-electrode voltage-clamp. Moreover, Kv1.5 protein abundance in the cell membrane was determined by chemiluminescence and immunostaining with subsequent confocal microscopy.

RESULTS

Coexpression of wild-type AMPK(WT) and constitutively active AMPK(γR70Q), but not of inactive AMPK(αK45R) significantly reduced Kv1.5-mediated currents. Coexpression of constitutively active AMPKγR70Q further reduced Kv1.5 K(+) channel protein abundance in the cell membrane. Co-expression of Nedd4-2 similarly downregulated Kv1.5-mediated currents.

CONCLUSION

AMPK is a potent regulator of Kv1.5. AMPK inhibits Kv1.5 presumably in part by activation of Nedd4- 2 with subsequent clearance of channel protein from the cell membrane.

Mechanisms underlying rapid aldosterone effects in the kidney

The steroid hormone aldosterone is a key regulator of electrolyte transport in the kidney and contributes to both homeostatic whole-body electrolyte balance and the development of renal and cardiovascular pathologies. Aldosterone exerts its action principally through the mineralocorticoid receptor (MR), which acts as a ligand-dependent transcription factor in target tissues. Aldosterone also stimulates the activation of protein kinases and secondary messenger signaling cascades that act independently on specific molecular targets in the cell membrane and also modulate the transcriptional action of aldosterone through MR. This review describes current knowledge regarding the mechanisms and targets of rapid aldosterone action in the nephron and how aldosterone integrates these responses into the regulation of renal physiology.

Role of the ubiquitin system in regulating ion transport

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

Ion homeostasis in the ear: mechanisms, maladies, and management

PURPOSE OF REVIEW

To describe ion and water homeostatic mechanisms in the inner ear, how they are compromised in hearing disorders, and what treatments are employed to restore auditory function.

RECENT FINDINGS

The ion and water transport functions in the inner ear help maintain the proper endolymph K concentration required for hair cell function. Gene defects and idiopathic alterations in these transport functions cause hearing loss, but often the underlying cause is unknown. Current therapies largely involve glucocorticoid treatment, although the mechanisms of restoration are often undeterminable. Recent studies of these ion homeostatic functions in the ear are characterizing their cellular and molecular control. It is anticipated that future management of these hearing disorders will be more targeted to the cellular processes involved and improve the likelihood of hearing recovery.

SUMMARY

A better understanding of the ion homeostatic processes in the ear will permit more effective management of their associated hearing disorders. Sufficient insight into many homeostatic hearing disorders has now been attained to usher in a new era of better therapies and improved clinical outcomes.

Low dose combination steroids control autoimmune mouse hearing loss

The severe side effects of glucocorticoids prevent long term management of hearing loss. Alternative steroid treatments that minimize or eliminate these effects would significantly benefit therapeutic control of hearing disorders. A steroid treatment study of autoimmune mouse hearing loss was conducted to determine the efficacy of combining aldosterone and prednisolone at low doses. An assessment also was made of low dose fludrocortisone, a synthetic mineralocorticoid that also has a slight glucocorticoid effect. MRL/MpJ-Fas(lpr) mice were tested for baseline ABR thresholds at 3 months of age and then treated with aldosterone (3.0 μg/kg) or prednisolone (1.0 mg/kg) to determine the lowest effective dose of each. Other mice were given the two steroids in combination at doses of Pred 0.5 mg+Aldo 1.5 μg; Pred 1.0 mg+Aldo 3.0 μg; or Pred 1.5 mg+Aldo 5.0 μg. Mice were retested with ABR at 1 and 2 months to determine the efficacy of the different steroid treatments in controlling hearing loss. Another series of mice were given the synthetic mineralocorticoid fludrocortisone at low (2.8 μg/kg) or high (10 μg/kg) doses and retested at monthly intervals for 3 months. Autoimmune mouse hearing loss developed in untreated controls. This threshold elevation was not prevented by prednisolone at 1 mg/kg or by aldosterone at 3 μg/kg when each was given alone. However, the two steroids combined at these doses effectively controlled hearing loss. The fludrocortisone treatments also were effective at low doses in preventing or reversing the autoimmune mouse hearing loss. This efficacy of combined steroids at low doses suggests the potential for reducing the side effects of glucocorticoids in the therapeutic control of hearing disorders.

Significance of SGK1 in the regulation of neuronal function

The present brief review highlights the putative role of the serum- and glucocorticoid-inducible-kinase-1 (SGK1) in the regulation of neuronal function. SGK1 is genomically upregulated by cell shrinkage and by a variety of hormones including mineralocorticoids and glucocorticoids. The kinase is activated by insulin and growth factors via phosphatidylinositide-3-kinase (PI3-kinase), phosphoinositide-dependent kinase PDK1 and mammalian target of rapamycin mTORC2. SGK1 upregulates ion channels (e.g. SCN5A, ENaC, ASIC1, TRPV5,6, ROMK, Kv1.1-5, KCNEx/KCNQ1-5, GluR6, VSOAC, ClC2, CFTR), carriers (e.g. NHE3, NKCC2, NCC, NaPiIIb, SMIT, GLUT1,4, SGLT1, NaDC, EAAT1-5, SN1, ASCT2, 4F2/LAT, PepT2), and the Na(+)/K(+)-ATPase. SGK1 regulates enzymes (e.g. glycogen-synthase-kinase-3, ubiquitin-ligase Nedd4-2, phosphomannose-mutase-2), and transcription factors (e.g. forkhead transcription factor Foxo3a, β-catenin, nuclear factor-kappa-B (NFB)). SGK1 participates in the regulation of transport, hormone release, neuroexcitability, inflammation, coagulation, cell proliferation and apoptosis. SGK1 contributes to regulation of renal Na(+) retention, renal K(+) elimination, salt appetite, gastric acid secretion, intestinal Na(+)/H(+) exchange and nutrient transport, insulin-dependent salt sensitivity of blood pressure, salt sensitivity of peripheral glucose uptake, cardiac repolarization and memory consolidation. Presumably, SGK1 contributes to the regulation of diverse cerebral functions (e.g. memory consolidation, fear retention) and the pathophysiology of several cerebral diseases (e.g. Parkinson's disease, schizophrenia, depression, Alzheimer's disease). Despite multiple SGK1 functions, the phenotype of the SGK1 knockout mouse is mild and becomes only apparent under challenging conditions.

New tricks for old dogs: KCNQ expression and role in smooth muscle

Ion channels encoded by the KCNQ gene family (K(v)7.1-7.5) are major determinants of neuronal membrane potential and the cardiac action potential. This key physiological role is highlighted by the existence of a number of hereditary disorders caused by mutations to KCNQ genes. Recently, KCNQ gene expression has been identified in vascular and non-vascular smooth muscles. In addition, experiments with an array of pharmacological modulators of KCNQ channels have supported a crucial role for these channels in regulating smooth muscle contractility. This article will provide an overview of present understanding in this nascent area of KCNQ research and will offer guidance as to future directions.