Nathalie Strutz-Seebohm

Molecular Mechanisms of Schizophrenia

Schizophrenia is a complex disorder, where family, twin and adoption studies have been demonstrating a high heritability of the disease and that this disease is not simply defined by several major genes but rather evolves from addition or potentiation of a specific cluster of genes, which subsequently determines the genetic vulnerability of an individual. Linkage and association studies suggest that a genetic vulnerablility, is not forcefully leading to the disease since triggering factors and environmental influences, i.e. birth complications, drug abuse, urban background or time of birth have been identified. This has lead to the assumption that schizophrenia is not only a genetically defined static disorder but a dynamic process leading to dysregulation of multiple pathways. There are several different hypothesis based on several facets of the disease, some of them due to the relatively well-known mechanisms of therapeutic agents. The most widely considered neurodevelopmental hypothesis of schizophrenia integrates environmental influences and causative genes. The dopamine hypothesis of schizophrenia is based on the fact that all common treatments involve antidopaminergic mechanisms and genes such as DRD2, DRD3, DARPP-32, BDNF or COMT are closely related to dopaminergic system functioning. The glutamatergic hypothesis of schizophrenia lead recently to a first successful mGlu2/3 receptor agonistic drug and is underpinned by significant findings in genes regulating the glutamatergic system (SLC1A6, SLC1A2 GRIN1, GRIN2A, GRIA1, NRG1, ErbB4, DTNBP1, DAAO, G72/30, GRM3). Correspondingly, GABA has been proposed to modulate the pathophysiology of the disease which is represented by the involvement of genes like GABRA1, GABRP, GABRA6 and Reelin. Moreover, several genes implicating immune, signaling and networking deficits have been reported to be involved in the disease, i.e. DISC1, RGS4, PRODH, DGCR6, ZDHHC8, DGCR2, Akt, CREB, IL-1B, IL-1RN, IL-10, IL-1B. However, molecular findings suggest that a complex interplay between receptors, kinases, proteins and hormones is involved in schizophrenia. In a unifying hypothesis, different cascades merge into another that ultimately lead to the development of symptoms adherent to schizophrenic disorders.

Regulation of Endocytic Recycling of KCNQ1/KCNE1 Potassium Channels

Stress-dependent regulation of cardiac action potential duration is mediated by the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis. It is accompanied by an increased magnitude of the slow outward potassium ion current, IKs. KCNQ1 and KCNE1 subunits coassemble to form the IKs channel. Mutations in either subunit cause long QT syndrome, an inherited cardiac arrhythmia associated with an increased risk of sudden cardiac death. Here we demonstrate that exocytosis of KCNQ1 proteins to the plasma membrane requires the small GTPase RAB11, whereas endocytosis is dependent on RAB5. We further demonstrate that RAB-dependent KCNQ1/KCNE1 exocytosis is enhanced by the serum- and glucocorticoid-inducible kinase 1, and requires phosphorylation and activation of phosphoinositide 3-phosphate 5-kinase and the generation of PI(3,5)P2. Identification of KCNQ1/KCNE1 recycling and its modulation by serum- and glucocorticoid-inducible kinase 1-phosphoinositide 3-phosphate 5-kinase -PI(3,5)P2 provides a mechanistic insight into stress-induced acceleration of cardiac repolarization.

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 (NFκB)). 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. Parkinsons disease, schizophrenia, depression, Alzheimers disease). Despite multiple SGK1 functions, the phenotype of the SGK1 knockout mouse is mild and becomes only apparent under challenging conditions.

Chromanol 293B Binding in KCNQ1 (Kv7.1) Channels Involves Electrostatic Interactions with a Potassium Ion in the Selectivity Filter

The chromanol 293B (293B, trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dimethyl-chroman) is a lead compound of potential class III antiarrhythmics that inhibit cardiac IKs potassium channels. These channels are formed by the coassembly of KCNQ1 (Kv7.1, KvLQT1) and KCNE1 subunits. Although homomeric KCNQ1 channels are the principal molecular targets, entry of KCNE1 to the channel complex enhances the chromanol block. Because closely related neuronal KCNQ2 potassium channels are insensitive to the drug, we used KCNQ1/KCNQ2 chimeras to identify the binding site of the inhibitor. We localized the putative drug receptor to the H5 selectivity filter and the S6 transmembrane segment. Single residues affecting 293B inhibition were subsequently identified through systematic exchange of amino acids that were either different in KCNQ1 and KCNQ2 or predicted by a docking model of 293B in the open and closed conformation of KCNQ1. Mutant channel proteins T312S, I337V, and F340Y displayed dramatically lowered sensitivity to chromanol block. The predicted drug binding receptor lies in the inner pore vestibule containing the lower part of the selectivity filter, and the S6 transmembrane domain also reported to be important for binding of benzodiazepines. We propose that the block of the ion permeation pathway involves hydrophobic interactions with the S6 transmembrane residues Ile337 and Phe340, and stabilization of chromanol 293B binding through electrostatic interactions of its oxygen atoms with the most internal potassium ion within the selectivity filter.

Regulation of the Glutamate Transporter EAAT4 by PIKfyve

The excitatory amino-acid transporter EAAT4 (SLC1A6), a Na+,glutamate cotransporter expressed mainly in Purkinje cells, serves to clear glutamate from the synaptic cleft. EAAT4 activity is stimulated by the serum and glucocorticoid inducible kinase SGK1. SGK1-dependent regulation of the Na+,glucose transporter SGLT1 (SLC5A1) and the creatine transporter CreaT (SLC6A8) has recently been shown to involve the mammalian phosphatidylinositol-3-phosphate-5-kinase PIKfyve (PIP5K3). The present experiments thus explored whether SGK1-dependent EAAT4-regulation similarly involves PIKfyve. In Xenopus oocytes expressing EAAT4, but not in water injected oocytes, glutamate induced a current which was significantly enhanced by coexpression of PIKfyve and SGK1. The glutamate induced current in Xenopus oocytes coexpressing EAAT4 and both, PIKfyve and SGK1, was significantly larger than the current in Xenopus oocytes expressing EAAT4 together with either kinase alone. Coexpression of the inactive SGK1 mutant K127NSGK1 did not significantly alter glutamate induced current in EAAT4-expressing Xenopus oocytes and abolished the stimulation of glutamate induced current by coexpression of PIKfyve. The stimulating effect of PIKfyve was abrogated by replacement of the serine with alanine in the SGK consensus sequence (S318APIKfyve). Furthermore, coexpression of S318APIKfyve significantly blunted the stimulating effect of SGK1 on EAAT4 activity. The observations disclose that PIKfyve indeed participates in the regulation of EAAT4.

Long QT Syndrome–Associated Mutations in KCNQ1 and KCNE1 Subunits Disrupt Normal Endosomal Recycling of IKs Channels

Physical and emotional stress is accompanied by release of stress hormones such as the glucocorticoid cortisol. This hormone upregulates the serum- and glucocorticoid-inducible kinase (SGK)1, which in turn stimulates IKs, a slow delayed rectifier potassium current that mediates cardiac action potential repolarization. Mutations in IKs channel &agr; (KCNQ1, KvLQT1, Kv7.1) or &bgr; (KCNE1, IsK, minK) subunits cause long QT syndrome (LQTS), an inherited cardiac arrhythmia associated with increased risk of sudden death. Together with the GTPases RAB5 and RAB11, SGK1 facilitates membrane recycling of KCNQ1 channels. Here, we show altered SGK1-dependent regulation of LQTS-associated mutant IKs channels. Whereas some mutant KCNQ1 channels had reduced basal activity but were still activated by SGK1, currents mediated by KCNQ1(Y111C) or KCNQ1(L114P) were paradoxically reduced by SGK1. Heteromeric channels coassembled of wild-type KCNQ1 and the LQTS-associated KCNE1(D76N) mutant were similarly downregulated by SGK1 because of a disrupted RAB11-dependent recycling. Mutagenesis experiments indicate that stimulation of IKs channels by SGK1 depends on residues H73, N75, D76, and P77 in KCNE1. Identification of the IKs recycling pathway and its modulation by stress-stimulated SGK1 provides novel mechanistic insight into potentially fatal cardiac arrhythmias triggered by physical or psychological stress.

PIKfyve in the SGK1 Mediated Regulation of the Creatine Transporter SLC6A8

The Na2+,Cl--,creatine transporter CreaT (SLC6A8) mediates concentrative cellular uptake of creatine into a wide variety of cells. Previous observations disclosed that SLC6A8 transport activity is enhanced by mammalian target of rapamycin (mTOR) at least partially through the serum and glucocorticoid inducible kinase isoforms SGK1 and SGK3. As SLC6A8 does not contain a putative SGK consensus motif, the mechanism linking SGK1 with SLC6A8 activity remained elusive. A candidate kinase is the mammalian phosphatidylinositol-3-phosphate-5-kinase PIKfyve (PIP5K3), which has previously been shown to regulate the glucose transporter GLUT4. The present experiments explored the possibility that SLC6A8 is regulated by PIKfyve. In Xenopus oocytes expressing SLC6A8 but not in water injected oocytes creatine induced a current which was significantly enhanced by coexpression of PIKfyve. The effect of PIKfyve on SLC6A8 was blunted by additional coexpression of the inactive mutant of the serum and glucocorticoid inducible kinase K127NSGK1. The stimulating effect of PIKfyve was abrogated by replacement of the serine in the SGK consensus sequence by alanine (S318APIKfyve). Moreover, coexpression of S318APIKfyve blunted the effect of SGK1 on SLC6A8 activity. The observations suggest that SGK1 regulates the creatine transporter SLC6A8 at least partially through phosphorylation and activation of PIKfyve and subsequent formation of PI(3,5)P2.

GluN3 subunit-containing NMDA receptors: not just one-trick ponies

The two GluN3 subunits were the last NMDA receptor subunits to be cloned some 15 years ago. Strikingly, despite the steadily growing interest in their function, their physiological role remains elusive. The original billing as dominant-negative modulators of classical NMDA receptors composed of GluN1 and GluN2 subunits has given way to proposals of much more complex functions, including roles in synaptogenesis and synaptic plasticity. In addition, GluN3 subunits in the absence of GluN2 surprisingly assemble with GluN1 into excitatory glycine receptors. This review provides an overview of the unique spatial and temporal expression patterns of the GluN3 subunits, discusses proposed functions and physiological roles for receptors comprising these subunits, and briefly summarizes their putative involvement in several neural diseases.

Structural basis of slow activation gating in the cardiac I Ks channel complex.

Accessory β-subunits of the KCNE gene family modulate the function of various cation channel α-subunits by the formation of heteromultimers. Among the most dramatic changes of biophysical properties of a voltage-gated channel by KCNEs are the effects of KCNE1 on KCNQ1 channels. KCNQ1 and KCNE1 are believed to form nativeIKs channels. Here, we characterize molecular determinants of KCNE1 interaction with KCNQ1 channels by scanning mutagenesis, double mutant cycle analysis, and molecular dynamics simulations. Our findings suggest that KCNE1 binds to the outer face of the KCNQ1 channel pore domain, modifies interactions between voltage sensor, S4-S5 linker and the pore domain, leading to structural modifications of the selectivity filter and voltage sensor domain. Molecular dynamics simulations suggest a stable interaction of the KCNE1 transmembrane α-helix with the pore domain S5/S6 and part of the voltage sensor domain S4 of KCNQ1 in a putative pre-open channel state. Formation of this state may induce slow activation gating, the pivotal characteristic of native cardiac IKs channels. This new KCNQ1-KCNE1 model may become useful for dynamic modeling of disease-associated mutant IKs channels.

Overlapping Cardiac Phenotype Associated with a Familial Mutation in the Voltage Sensor of the KCNQ1 Channel

Background: Cardiac action potential repolarisation is determined by K+ currents including IKs. IKs channels are heteromeric channels composed of KCNQ1 and KCNE E-subunits. Mutations in KCNQ1 are associated with sinus bradycardia, familial atrial fibrillation (fAF) and/or short QT syndrome as a result of gain-of-function, and long QT syndrome (LQTS) due to loss-of-function in the ventricles. Here, we report that the missense mutation R231C located in S4 voltage sensor domain is associated with a combined clinical phenotype of sinus bradycardia, fAF and LQTS. We aim to understand the molecular basis of the complex clinical phenotype. Methods: We expressed and functionally analyzed the respective channels kinetics in Xenopus laevis oocytes. The molecular nature of the residue R231 was studied by homology modeling and molecular dynamics simulation. Results: As a result, the mutation reduced voltage sensitivity of channels, possibly due to neutralization of the positive charge of the arginine side chain substituted by cysteine. Modeling suggested that the charge carrying side chain of R231 is positioned suitably to transfer transmembrane voltages into conformational energy. Further, the mutation altered the functional interactions with KCNE subunits. Conclusion: The mutation acted in a E-subunit dependent manner, suggesting IKs function altered by the presence of different KCNE subunits in sinus node, atria and ventricles as the molecular basis of sinus bradycardia, fAF and LQTS in mutation carriers.