Phosphatases maintain low catalytic activity of SGK1: DNA damage resets the balance in favor of phosphorylation.

The serum- and glucocorticoid-induced kinase 1 (SGK1) promotes cell survival under stress conditions and facilitates the emergence of drug resistance in cancer. The underlying mechanisms of these observations are not fully understood. In this study, we found that SGK1 activity is suppressed by the action of the S/T phosphatases PP5 and PP2A, which constantly dephosphorylate SGK1.

Lipid droplets and autophagosomes together with chaperones fine-tune expression of SGK1.

Serum-glucocorticoid-induced kinase-1 (SGK1) regulates ion homeostasis and promotes survival under stress conditions. The expression of SGK1 is under transcriptional and post-translational regulations that are frequently altered in cancer and immune disorders. We report that an N-terminal amphipathic alpha-helix determines SGK1 expression levels through two distinct mechanisms.

A flexible GAS belt responds to pore mutations changing the ion selectivity of proton-gated channels.

Proton-gated ion channels conduct mainly Na+ to induce postsynaptic membrane depolarization. Finding the determinants of ion selectivity requires knowledge of the pore structure in the open conformation, but such information is not yet available. Here, the open conformation of the hASIC1a channel was computationally modeled, and functional effects of pore mutations were analyzed in light of the predicted structures.

Structure and analysis of nanobody binding to the human ASIC1a ion channel.

ASIC1a is a proton-gated sodium channel involved in modulation of pain, fear, addiction, and ischemia-induced neuronal injury. We report isolation and characterization of alpaca-derived nanobodies (Nbs) that specifically target human ASIC1a. Cryo-electron microscopy of the human ASIC1a channel at pH 7.

An arginine residue in the outer segment of hASIC1a TM1 affects both proton affinity and channel desensitization.

Acid-sensing ion channels (ASICs) respond to changes in pH in the central and peripheral nervous systems and participate in synaptic plasticity and pain perception. Understanding the proton-mediated gating mechanism remains elusive despite the of their structures in various conformational states. We report here that R64, an arginine located in the outer segment of the first transmembrane domain of all three isoforms of mammalian ASICs, markedly impacts the apparent proton affinity of activation and the degree of desensitization from the open and preopen states.

A valve-like mechanism controls desensitization of functional mammalian isoforms of acid-sensing ion channels.

ASICs are proton-gated sodium channels expressed in neurons. Structures of chicken ASIC1 in three conformations have advanced understanding of proton-mediated gating; however, a molecular mechanism describing desensitization from open and pre-open states (steady-state desensitization or SSD) remains elusive. A distinct feature of the desensitized state is an 180 rotation of residues L415 and N416 in the β11- β12 linker that was proposed to mediate desensitization; whether and how it translates into desensitization has not been explored yet.

The neuronal-specific SGK1.1 (SGK1_v2) kinase as a transcriptional modulator of BAG4, Brox, and PPP1CB genes expression.

The Serum- and Glucocorticoid-induced Kinase 1, SGK1, exhibits a broad range of cellular functions that include regulation of the number of ion channels in plasma membrane and modulation of signaling pathways of cell survival. This diversity of functions is made possible by various regulatory processes acting upon the SGK1 gene, giving rise to various isoforms: SGK1_v1-5, each with distinct properties and distinct aminotermini that serve to target proteins to different subcellular compartments. Among cellular effects of SGK1 expression is to indirectly modulate gene transcription by phosphorylating transcriptional factors of the FOXO family.

A method for activation of endogenous acid-sensing ion channel 1a (ASIC1a) in the nervous system with high spatial and temporal precision.

Protons activate acid-sensing ion channel 1a (ASIC1a) in the central nervous system (CNS) although the impact of such activation on brain outputs remains elusive. Progress elucidating the functional roles of ASIC1a in the CNS has been hindered by technical difficulties of achieving acidification with spatial and temporal precision. We have implemented a method to control optically the opening of ASIC1a in brain slices and also in awake animals.

Heterogeneous nuclear ribonucleoprotein A2/B1 is a tissue-specific aldosterone target gene with prominent induction in the rat distal colon.

The steroid hormone aldosterone enhances transepithelial Na(+) reabsorption in tight epithelia and is crucial to achieve extracellular volume homeostasis and control of blood pressure. One of the main transport pathways regulated by aldosterone involves the epithelial Na(+) channel (ENaC), which constitutes the rate-limiting step of Na(+) reabsorption in parts of the distal nephron and the collecting duct, the distal colon, and sweat and salivary glands. Although these epithelial tissues share the same receptor for aldosterone (mineralocorticoid receptor, MR), and the same transport system (ENaC), it has become clear that the molecular mechanisms involved in the modulation of channel activity are tissue-specific.

Impact of recovery from desensitization on acid-sensing ion channel-1a (ASIC1a) current and response to high frequency stimulation.

Background: Consecutive proton stimulation reduces ASIC1a peak currents leading to silencing of channels.

Results: Kinetic analysis using a fast perfusion system shows that human ASIC1a has two desensitized states with markedly different stabilities.

Conclusion: High frequency trains of short stimuli prevent desensitization.

Outlines of the pore in open and closed conformations describe the gating mechanism of ASIC1.

The proton-activated sodium channel ASIC1 belongs to the ENaC/Degenerins family of ion channels. Little is known about gating of the pore in any member of this class. Here we outline the shape of the ion pathway of ASIC1 in the open and closed conformations by measuring apparent rates of cysteine modification by thiol-specific reagents in the two transmembrane helices that form the pore (TM1 and TM2).

Asp433 in the closing gate of ASIC1 determines stability of the open state without changing properties of the selectivity filter or Ca2+ block.

A constriction formed by the crossing of the second transmembrane domains of ASIC1, residues G432 to G436, forms the narrowest segment of the pore in the crystal structure of chicken ASIC1, presumably in the desensitized state, suggesting that it constitutes the "desensitization gate" and the "selectivity filter." Residues Gly-432 and Asp-433 occlude the pore, preventing the passage of ions from the extracellular side. Here, we examined the role of Asp-433 and Gly-432 in channel kinetics, ion selectivity, conductance, and Ca(2+) block in lamprey ASIC1 that is a channel with little intrinsic desensitization in the pH range of maximal activity, pH 7.

Asn415 in the beta11-beta12 linker decreases proton-dependent desensitization of ASIC1.

Neurons of the mammalian nervous system express the proton-sensing ion channel ASIC1. Low concentrations of protons in the normal range of extracellular pH, pH 7.4-7.

Leu85 in the beta1-beta2 linker of ASIC1 slows activation and decreases the apparent proton affinity by stabilizing a closed conformation.

Acid-sensing ion channels (ASICs) are proton-activated channels expressed in neurons of the central and peripheral nervous systems where they modulate neuronal activity in response to external increases in proton concentration. The size of ASIC1 currents evoked by a given local acidification is determined by the number of channels in the plasma membrane and by the apparent proton affinities for activation and steady-state desensitization of the channel. Thus, the magnitude of the pH drop and the value of the baseline pH both are functionally important.

Two residues in the extracellular domain convert a nonfunctional ASIC1 into a proton-activated channel.

Acid-sensing ion channels (ASICs) are proton-activated sodium channels of the nervous system. Mammals express four ASICs, and orthologs of these genes have been found in all chordates examined to date. Despite a high degree of sequence conservation of all ASICs across species, the response to a given increase in external proton concentration varies markedly: from large and slowly inactivating inward currents to no detectable currents.

Interaction of the aromatics Tyr-72/Trp-288 in the interface of the extracellular and transmembrane domains is essential for proton gating of acid-sensing ion channels.

Acid-sensing ion channels are proton-activated ion channels expressed in the nervous system. They belong to the family of ENaC/Degenerins whose members share a conserved structure but are activated by widely diverse stimuli. We show that interaction of two aromatic residues, Tyr-72, located immediately after the first transmembrane segment, and Trp-288, located at the tip of a loop of the extracellular domain directed toward the first transmembrane segment, is essential for proton activation of the acid-sensing ion channels.

A brain-specific SGK1 splice isoform regulates expression of ASIC1 in neurons.

Neurodegenerative diseases and noxious stimuli to the brain enhance transcription of serum- and glucocorticoid-induced kinase-1 (SGK1). Here, we report that the SGK1 gene encodes a brain-specific additional isoform, SGK1.1, which exhibits distinct regulation, properties, and functional effects.

Simple chordates exhibit a proton-independent function of acid-sensing ion channels.

Acid-sensing ion channels (ASICs) constitute a family of neuron-specific voltage-insensitive sodium channels gated by extracellular protons. Functions of ASICs in mammals include nociception, mechanosensation, and modulation of synaptic transmission. However, the role protons play in mediating the effects of ASICs remains elusive.

Multiple translational isoforms give functional specificity to serum- and glucocorticoid-induced kinase 1.

Serum- and glucocorticoid-induced kinase 1 is a ubiquitous kinase that regulates diverse processes such as ion transport and cell survival. We report that a single SGK1 mRNA produces isoforms with different N-termini owing to alternative translation initiation. The long isoforms, 49 and 47 kDa, are the most abundant, localize to the ER membrane, exhibit rapid turnover, their expression is decreased by ER stress, activate the epithelial sodium channel (ENaC) and translocate FoxO3a transcriptional factors from the nucleus to the cytoplasm.

An amphipathic helix targets serum and glucocorticoid-induced kinase 1 to the endoplasmic reticulum-associated ubiquitin-conjugation machinery.

Serum- and glucocorticoid-induced kinase 1 (Sgk1) regulates many ion channels and transporters in epithelial cells and promotes cell survival under stress conditions. In this study we demonstrate that Sgk1 is a short-lived protein regulated by the endoplasmic reticulum (ER)-associated degradation system and subcellular localization to the ER. We identified a hydrophobic motif (residues 18-30) as the signal for ER localization and rapid degradation by the ubiquitin (Ub)/proteasome pathway in both yeast and mammalian cells.

Gating of acid-sensitive ion channel-1: release of Ca2+ block vs. allosteric mechanism.

The acid-sensitive ion channels (ASICs) are a family of voltage-insensitive sodium channels activated by external protons. A previous study proposed that the mechanism underlying activation of ASIC consists of the removal of a Ca2+ ion from the channel pore (Immke and McCleskey, 2003). In this work we have revisited this issue by examining single channel recordings of ASIC1 from toadfish (fASIC1).

WNK3, a kinase related to genes mutated in hereditary hypertension with hyperkalaemia, regulates the K+ channel ROMK1 (Kir1.1).

The serine-threonine kinase WNK3 modulates Cl- transport into and out of cells through its regulation of SLC12A cation/Cl- cotransporters, implicating it as (one of) the long-sought Cl-/volume-sensitive kinase(s). Integrators in homeostatic systems regulate structurally diverse but functionally coupled elements. For example, the related kinase WNK4 regulates the Na-Cl co-transporter (NCC), paracellular Cl- flux, and the K+ channel ROMK1 (Kir1.

SGK1 activates Na+-K+-ATPase in amphibian renal epithelial cells.

Serum- and glucocorticoid-induced kinase 1 (SGK1) is thought to be an important regulator of Na(+) reabsorption in the kidney. It has been proposed that SGK1 mediates the effects of aldosterone on transepithelial Na(+) transport. Previous studies have shown that SGK1 increases Na(+) transport and epithelial Na(+) channel (ENaC) activity in the apical membrane of renal epithelial cells.

Proton sensitivity of ASIC1 appeared with the rise of fishes by changes of residues in the region that follows TM1 in the ectodomain of the channel.

The acid-sensitive ion channel 1 (ASIC1) is a neuronal Na+ channel insensitive to changes in membrane potential but is gated by external protons. Proton sensitivity is believed to be essential for the role of ASIC1 in modulating synaptic transmission and nociception in the mammalian nervous system. To examine the structural determinants that confer proton sensitivity, we cloned and functionally characterized ASIC1 from different species of the chordate lineage: lamprey, shark, toadfish and chicken.