Neuropsychiatric disease relevance of circulating anti-NMDA receptor autoantibodies depends on blood-brain barrier integrity

In 2007, a multifaceted syndrome, associated with anti-NMDA receptor autoantibodies (NMDAR-AB) of immunoglobulin-G isotype, has been described, which variably consists of psychosis, epilepsy, cognitive decline and extrapyramidal symptoms. Prevalence and significance of NMDAR-AB in complex neuropsychiatric disease versus health, however, have remained unclear. We tested sera of 2817 subjects (1325 healthy, 1081 schizophrenic, 263 Parkinson and 148 affective-disorder subjects) for presence of NMDAR-AB, conducted a genome-wide genetic association study, comparing AB carriers versus non-carriers, and assessed their influenza AB status. For mechanistic insight and documentation of AB functionality, in vivo experiments involving mice with deficient blood-brain barrier (ApoE(-/-)) and in vitro endocytosis assays in primary cortical neurons were performed. In 10.5% of subjects, NMDAR-AB (NR1 subunit) of any immunoglobulin isotype were detected, with no difference in seroprevalence, titer or in vitro functionality between patients and healthy controls. Administration of extracted human serum to mice influenced basal and MK-801-induced activity in the open field only in ApoE(-/-) mice injected with NMDAR-AB-positive serum but not in respective controls. Seropositive schizophrenic patients with a history of neurotrauma or birth complications, indicating an at least temporarily compromised blood-brain barrier, had more neurological abnormalities than seronegative patients with comparable history. A common genetic variant (rs524991, P=6.15E-08) as well as past influenza A (P=0.024) or B (P=0.006) infection were identified as predisposing factors for NMDAR-AB seropositivity. The >10% overall seroprevalence of NMDAR-AB of both healthy individuals and patients is unexpectedly high. Clinical significance, however, apparently depends on association with past or present perturbations of blood-brain barrier function.

Eag1 potassium channel immunohistochemistry in the CNS of adult rat and selected regions of human brain

Eag1 (K(V)10.1) is the founding member of an evolutionarily conserved superfamily of voltage-gated K(+) channels. In rats and humans Eag1 is preferentially expressed in adult brain but its regional distribution has only been studied at mRNA level and only in the rat at high resolution. The main aim of the present study is to describe the distribution of Eag1 protein in adult rat brain in comparison to selected regions of the human adult brain. The distribution of Eag1 protein was assessed using alkaline-phosphatase based immunohistochemistry. Eag1 immunoreactivity was widespread, although selective, throughout rat brain, especially noticeable in the perinuclear space of cells and proximal regions of the extensions, both in rat and human brain. To relate the results to the relative abundance of Eag1 transcripts in different regions of rat brain a reverse-transcription coupled to quantitative polymerase chain reaction (real time PCR) was performed. This real time PCR analysis showed high Eag1 expression in the olfactory bulb, cerebral cortex, hippocampus, hypothalamus, and cerebellum. The results indicate that Eag1 protein expression greatly overlaps with mRNA distribution in rats and humans. The physiological relevance of potassium channels in the different regions expressing Eag1 protein is discussed.

Role of voltage-gated potassium channels in cancer

Ion channels are being associated with a growing number of diseases including cancer. This overview summarizes data on voltage-gated potassium channels (VGKCs) that exhibit oncogenic properties: ether-à-go-go type 1 (Eag1). Normally, Eag1 is expressed almost exclusively in tissue of neural origin, but its ectopic expression leads to uncontrolled proliferation, while inhibition of Eag1 expression produces a concomitant reduction in proliferation. Specific monoclonal antibodies against Eag1 recognize an epitope in over 80% of human tumors of diverse origins, endowing it with diagnostic and therapeutic potential. Eag1 also possesses unique electrophysiological properties that simplify its identification. This is particularly important, as specific blockers of Eag1 currents are not available. Molecular imaging of Eag1 in live tumor models has been accomplished with dye-tagged antibodies using 3-D imaging techniques in the near-infrared spectral range.

Cytoskeletal interactions determine the electrophysiological properties of human EAG potassium channels

The electrophysiological properties of ether a go-go (EAG) potassium channels are modified during the cell cycle when they are expressed in heterologous systems. In Chinese hamster ovary (CHO) mammalian somatic cells we found that the cell-cycle-dependent modulation of human EAG (hEAG) channels occurs during the M phase. This modulation has three components: reduction in current density, increased sensitivity to block by intracellular sodium, and increased selectivity for potassium ions. In this work, these three properties have been used to define the mitotic phenotype of EAG currents. The signaling pathway leading to such changes of channel properties is unknown. We tested the hypothesis that cytoskeletal interactions might affect the electrophysiological changes observed during the cell cycle. The disruption of actin filaments induces a significant increase in current density, without inducing the cell-cycle-related phenotype. In contrast, disturbance of the microtubules, achieved by pharmacological means or by mechanical excision of the membrane patch, does induce the cell-cycle-related phenotype. Our results demonstrate that hEAG channels establish complex interactions with cytoskeletal elements, and that these interactions strongly influence the properties of the channels. We also conclude that the electrophysiological changes observed during the cell cycle are most likely due to reorganization of the cytoskeleton during the G2/M transition.

Oncogenic potential of EAG K(+) channels

We have investigated the possible implication of the cell cycle-regulated K(+) channel ether à go-go (EAG) in cell proliferation and transformation. We show that transfection of EAG into mammalian cells confers a transformed phenotype. In addition, human EAG mRNA is detected in several somatic cancer cell lines, despite being preferentially expressed in brain among normal tissues. Inhibition of EAG expression in several of these cancer cell lines causes a significant reduction of cell proliferation. Moreover, the expression of EAG favours tumour progression when transfected cells are injected into immune-depressed mice. These data provide evidence for the oncogenic potential of EAG.

Cell cycle-related changes in the conducting properties of r-eag K+ channels

Release from arrest in G2 phase of the cell cycle causes profound changes in rat ether-à-go-go (r-eag) K+ channels heterologously expressed in Xenopus oocytes. The most evident consequence of the onset of maturation is the appearance of rectification in the r-eag current. The trigger for these changes is located downstream of the activation of mitosis-promoting factor (MPF). We demonstrate here that the rectification is due to a voltage-dependent block by intracellular Na+ ions. Manipulation of the intracellular Na+ concentration indicates that the site of Na+ block is located approximately 45% into the electrical distance of the pore and is only present in oocytes undergoing maturation. Since the currents through excised patches from immature oocytes exhibited a fast rundown, we studied CHO-K1 cells permanently transfected with r-eag. These cells displayed currents with a variable degree of block by Na+ and variable permeability to Cs+. Partial synchronization of the cultures in G0/G1 or M phases of the cell cycle greatly reduced the variability. The combined data obtained from mammalian cells and oocytes strongly suggest that the permeability properties of r-eag K+ channels are modulated during cell cycle-related processes.

Demonstration of an inwardly rectifying K+ current component modulated by thyrotropin-releasing hormone and caffeine in GH3 rat anterior pituitary cells

Reduction of an inwardly rectifying K+ current by thyrotropin-releasing hormone (TRH) and caffeine has been considered to be an important determinant of electrical activity increases in GH3 rat anterior pituitary cells. However, the existence of an inwardly rectifying K+ current component was recently regarded as a misidentification of an M-like outward current, proposed to be the TRH target in pituitary cells, including GH3 cells. In this report, an inwardly rectifying component of K+ current is indeed demonstrated in perforated-patch voltage-clamped GH3 cells. The degree of rectification varied from cell to cell, but both TRH and caffeine specifically blocked a fraction of current with strong rectification in the hyperpolarizing direction. Use of ramp pulses to continuously modify the membrane potential demonstrated a prominent blockade even in cells with no current reduction at voltages at which M-currents are active. Depolarization steps to positive voltages at the maximum of the inward current induced a caffeine-sensitive instantaneous outward current followed by a single exponential decay. The magnitude of this current was modified in a biphasic way according to the duration of the previous hyperpolarization step. The kinetic characteristics of the current are compatible with the possibility that removal from inactivation of a fast-inactivating delayed rectifier causes the hyperpolarization-induced current. Furthermore, the inwardly rectifying current was blocked by astemizole, a potent and selective inhibitor of human ether-á-go-go -related gene (HERG) K+ channels. Along with other pharmacological and kinetic evidence, this indicates that the secretagogue-regulated current is probably mediated by a HERG-like K+ channel. Addition of astemizole to current-clamped cells induced clear increases in the frequency of action potential production. Thus, an inwardly-rectifying K+ current and not an M-like outward current seems to be involved in TRH and caffeine modulation of electrical activity in GH3 cells.

Molecular basis for different pore properties of potassium channels from the rat brain Kv1 gene family

Members of the rat brain Kv1 family of cloned potassium channels are structurally highly homologous, but have diverse conductance and pharmacological characteristics. Here we present data on the effects of mutating residues K533 in the P-region and H471 in the S4-S5 linker of Kv1.4 to their equivalent residues in Kv1.1 and Kv1.6 on single-channel conductance and sensitivity to external tetraethylammonium cations (TEA+) and internal Mg2+. Exchange of residue K533 for its equivalent residue (Y) in Kv1.1 and Kv1.6 increased the single-channel conductance at both negative and positive potentials. This mutation is known to reduce the IC50 for external TEA+ from > 100 mM to 0.6 mM, almost identical to that for Kv1.1 (0.53 mM). We have now found that the additional exchange of residue H471 for the equivalent residue (K) in Kv1.6 increased the IC50 for external TEA+ from 0.6 mM (Kv1.4K533Y) to 2.39 mM; this is very close to that for wild-type Kv1.6 channels (2.84 mM). The mutation H471K alone was ineffective. We thus provide evidence that the S4-S5 linker does contribute to the channel's inner-pore region. Data on the block of Kv1 channels by internal Mg2+ indicate that while the binding site is probably situated within the deep-pore region, its exact location may be channel specific.

Altered ligand dissociation rates in thyrotropin-releasing hormone receptors mutated in glutamine 105 of transmembrane helix III

Glutamine 105 in the third transmembrane helix of the thyrotropin-releasing hormone receptor (TRH-R) occupies a position equivalent to a conserved negatively charged residue in receptors for biogenic amines where it acts as counterion interacting with the cationic amine moiety of the ligand. Maximum levels of response to TRH in oocytes expressing wild-type TRH-Rs were indistinguishable from those of oocytes expressing receptors mutated to Glu, Asn, or Asp in position 105. However, the EC50 values for activation of oocyte responses increased more than 500 times in oocytes expressing mutant Glu105 receptors, in which the amido group of Gln105 has been removed by site-directed mutagenesis. Charge effects do not seem to be involved in the huge effect of mutating Gln105 to Glu, since mutation of Gln105 to Asp induces only a 15-fold increase in EC50. Furthermore, no change in EC50 is observed after mutation of Asn110 to Asp. The affinity shift (identified by changes in EC50 values for systems of comparable efficacy) in Glu105 mutant receptors was partially recovered in oocytes expressing Asn105 mutant receptors. These results and those obtained after substitution of Lys, Leu, Tyr, and Ser for Gln105 suggest that the presence and the correct position of the Gln hydrogen bond-donor amido group are important for normal functionality of the receptor. In wild type or Asp105 mutant receptors showing the same maximal responses, decreases in affinity with TRH and methyl-histidyl-TRH correlated with increased dissociation rates of hormone from the receptor. Rapid dilution experiments following subsecond stimulation indicate that the TRH-R is converted rapidly from a form showing fast dissociation kinetics to a form from which the hormone dissociates slowly. Mutation of residue 105 impairs the receptor shift between these two forms. This effect was demonstrated in a direct way by comparing [3H]methyl-histidyl-TRH dissociation rates in COS-7 cells transfected with either wild type or Asp105 mutant TRH-Rs. Thus, residues located in transmembrane helix III positions equivalent to those of the counterions for biogenic amines, regulate hormone-receptor interactions in the TRH receptor (and perhaps other receptors). Furthermore, the nature of the amino acid in these positions may also play a role, directly or indirectly, in conformational changes leading to receptor activation, and hence to signal transduction.

Mitosis-promoting factor-mediated suppression of a cloned delayed rectifier potassium channel expressed in Xenopus oocytes

The cell cycle is the crucial process that leads to mitosis in all cell types. The dramatic redirectioning of many cellular processes during the cycle is known to involve ion channels, either changing their level of expression or their voltage dependence, as in the case of inward rectifiers. Here we describe the specific inhibition of heterologously expressed ionic channels at the onset of maturation in Xenopus oocytes. In cells expressing rat eag (R-eag) potassium channels, maturation induces a dramatic reduction in the current amplitude, which is almost complete in most cases. The key molecule in oocyte maturation, the mitosis-promoting factor (a complex of cyclin B and p34cdc2), is able to induce similar changes when injected into the oocytes.

Caffeine enhancement of electrical activity through direct blockade of inward rectifying K+ currents in GH3 rat anterior pituitary cells

Treatment of rat anterior pituitary GH3 cells with caffeine causes a reversible enhancement of electrical activity superimposed over a depolarization of the plasma membrane potential. Similar results are obtained with theophylline, but not with isobutylmethylxanthine or forskolin. The effects of caffeine are not related to Ca2+ liberation from intracellular stores since they are not affected by incubation of the cells with ryanodine or thapsigargin. Furthermore, caffeine-induced hyperpolarization of the membrane is not detectable even in cells in which Ca2+ liberation from inositol 1,4,5-trisphosphate-sensitive compartments produces a prominent transient hyperpolarization in response to thyrotropin-releasing hormone. Reductions of Ca2+-dependent K+ currents caused by partial block of L-type Ca2+ channels by caffeine are not sufficient to explain the effects of the xanthine, since the results obtained with caffeine are not mimicked by direct blockade of Ca2+ channels with nisoldipine. GH3 cell inwardly rectifying K+ currents are inhibited by caffeine. Studies on the voltage dependence of the caffeine-induced effects indicate a close correlation between alterations of electrical parameters and reported values of steady-state voltage dependence of inactivation of these currents. We conclude that, as previously shown for thyrotropin-releasing hormone, modulation of inwardly rectifying K+ currents plays a major role determining the firing rate of GH3 cells and its enhancement by caffeine.

Gs couples thyrotropin-releasing hormone receptors expressed in Xenopus oocytes to phospholipase C

Coupling of thyrotropin-releasing hormone (TRH) receptors to individual G-proteins has been studied in Xenopus oocytes injected with receptor cRNA and antisense oligonucleotides to mRNA encoding different G-protein alpha- and beta-subunits. Injection of antisenses which target mRNA sequences shared by several G-protein alpha or beta gamma polypeptides effectively blocked Ca(2+)-dependent Cl- currents induced by TRH through activation of phospholipase C. Three different alpha s-specific antisense oligonucleotides complementary to sequences located in different positions along the coding region of the alpha s protein mRNA were highly effective in inhibiting TRH-induced responses. Anti-alpha o, -alpha q, -alpha i, or -alpha z oligonucleotides were not able to modify the TRH-evoked response. In contrast, anti-alpha o, but not anti-alpha s, oligonucleotides blocked the response to serotonin in oocytes injected with serotonin 5-HT1c receptor cRNA. Cholera toxin catalyzed the [32P]ADP-ribosylation of 40-42- and 50-52-kDa proteins in GH3 cell plasma membranes. [32P]ADP-ribosylation of oocyte membranes with the toxin labeled several proteins. These include a single 50-55-kDa substrate, which is clearly diminished in membranes from anti-alpha s-injected oocytes. Amplification of oocyte RNA in a polymerase chain reaction system and sequencing of the amplified products demonstrated that anti-alpha s oligonucleotides selectively recognize the message for the Xenopus alpha s polypeptide. It is concluded that Gs, but not Go, Gq, Gi, or Gz, couples TRH receptors expressed in oocytes to activation of phospholipase C and subsequent inositol 1,4,5-trisphosphate-dependent stimulation of Ca(2+)-dependent Cl- currents.

Functional expression of a rat homologue of the voltage gated either á go-go potassium channel reveals differences in selectivity and activation kinetics between the Drosophila channel and its mammalian counterpart

We have cloned a mammalian (rat) homologue of Drosophila ether á go-go (eag) cDNA, which encodes a distinct type of voltage activated potassium (K) channel. The derived Drosophila and rat eag polypeptides share > 670 amino acids, with a sequence identity of 61%, exhibiting a high degree of similarity at the N-terminus, the hydrophobic core including the pore forming P region and a potential cyclic nucleotide binding site. Rat eag mRNA is specifically expressed in the central nervous system. In the Xenopus oocyte expression system rat eag mRNA gives rise to voltage activated K channels which have distinct properties in comparison with Drosophila eag channels and other voltage activated K channels. Thus, the rat eag channel further extends the known diversity of K channels. Most notably, the kinetics of rat eag channel activation depend strongly on holding membrane potential. Hyperpolarization slows down the kinetics of activation; conversely depolarization accelerates the kinetics of activation. This novel K channel property may have important implications in neural signal transduction allowing neurons to tune their repolarizing properties in response to membrane hyperpolarization.

Ether-à-go-go encodes a voltage-gated channel permeable to K+ and Ca2+ and modulated by cAMP

The Drosophila ether-à-go-go (eag) mutant is responsible for altered potassium currents in excitable tissue. These mutants exhibit spontaneous, repetitive firing of action potentials in the motor axons of larval neuromuscular junctions. The eag gene encodes a polypeptide that shares sequence similarities with several different ionic channel proteins, including voltage-gated potassium channels, an inward rectifier as well as cyclic-nucleotide-gated channels. These formal similarities in the derived primary sequences indicate that eag polypeptides might express a new type of ion channel. Here we report the expression by eag RNA in Xenopus oocytes of such a channel which incorporates properties of both voltage- and ligand-gated channels. The permeability of these eag channels to potassium and calcium is dependent on voltage and cyclic AMP. The ability to mediate potassium-outward and calcium-inward currents endows this channel with properties likely to be important in the modulation of synaptic efficiency in both central and peripheral nervous systems.

Glucose activation of adenylate cyclase in Saccharomyces cerevisiae mutants lacking glucose-phosphorylating enzymes

The "in vitro" activation by glucose of the RAS-adenylate cyclase system in membranes from a strain of Saccharomyces cerevisiae lacking any functional glucose kinase activity presents similar features to those of the wild type. However, this triple mutant appears to be unable to produce the glucose-induced increase of cAMP levels "in vivo". The results obtained in vitro indicate that the signal transduction mechanism is active in the mutant cells and suggest that the absence of intracellular acidification in vivo might be responsible for the lack of response to glucose.

Activation of adenylate cyclase in cdc25 mutants of Saccharomyces cerevisiae

The activation of adenylate cyclase by guanine nucleotides and 6-deoxyglucose was studied in membrane preparations from S. cerevisiae mutants lacking the CDC25 gene product. Adenylate cyclase from cdc25 ts membranes was activated by GTP and GppNHp in membranes from cells collected after glucose was exhausted from the medium. The activation was also observed in membranes from repressed cells at 2.5 mM Mg2+. It is also shown that 6-deoxyglucose can activate adenylate cyclase in the absence of CDC25 gene product. The relative amount of membrane-bound adenylate cyclase was drastically reduced in cdc25 ts membranes when subjected to the restrictive temperature, while no significant change was observed in the wild type. These data suggest that Cdc25 might not be required in certain conditions for the guanine nucleotide exchange reaction in Ras and that it might be implicated in anchoring the Ras/adenylate cyclase system to the plasma membrane.

Cloning and functional characterization of the rat stomach fundus serotonin receptor

A DNA segment homologous to the third exons of the serotonin 1C and 2 receptor genes was isolated from a mouse genomic library. The positions of the introns flanking these exons were conserved in the three genes. To examine whether the new fragment was part of an active gene, we used a quantitative PCR protocol to analyse rat RNAs from different tissues and ages. The gene was expressed in stomach fundus at an abundance of 1 x 10(5) mRNA molecules. This tissue contracts in response to serotonin via a receptor that has previously resisted classification. We constructed a cDNA library from rat stomach fundus and isolated clones containing 2020 bp inserts with open reading frames of 465 amino acids comprising seven putative membrane-spanning regions. The protein was transiently expressed in COS cells and binding of serotonergic ligands to the membranes was analysed. The pharmacological profile resembled that described for the serotonin-stimulated contraction of the stomach fundus. After expression of this receptor in Xenopus oocytes, the application of serotonin triggered the typical chloride current which presumably results from the activation of phospholipase C. The coupling to this response system was less efficient than that of the 5-HT1C or 5-HT2 receptors.

Extracellular K+ specifically modulates a rat brain K+ channel

Extracellular potassium concentration is actively maintained within narrow limits in all higher organisms. Slight variations in extracellular potassium levels can induce major alterations of essential physiological functions in excitable tissues. Here we describe that superfusion of cultured rat hippocampal neurones with potassium-free medium leads to a decrease of a specific outward potassium current, probably carried by RCK4-type channels (RCK4 are potassium channels found in rat brain). This is confirmed by heterologous expression of these channels in Xenopus oocytes. In this system, variations of extracellular potassium in the physiological concentration range induce significant differences in current amplitude. Moreover, the current is completely suppressed in the absence of extracellular potassium. The potassium dependence of macroscopic conductance in RCK4 channels was related by site-directed mutagenesis to that lysine residue in the extracellular loop between the transmembrane segments S5 and S6 of RCK4 protein that confers resistance to extracellular blockage by tetraethylammonium. It is shown that extracellular potassium affects the number of available RCK4 channels, but not the single-channel conductance, the mean open time, or the gating charge displacement upon depolarization.

In vitro activation of the Saccharomyces cerevisiae Ras/adenylate cyclase system by glucose and some of its analogues

Using crude membrane preparations of Saccharomyces cerevisiae, we have demonstrated that glucose and glucose analogues which are not efficiently phosphorylated activate the guanine nucleotide-dependent adenylate cyclase in vitro. The activation appears to be mediated by the Ras proteins. Moreover, data are presented indicating that glucose and its analogues activate adenylate cyclase by stimulating the exchange of guanine nucleotides at its regulatory component. Thus, it has been possible to show the action of a physiological effector on the nucleotide exchange reaction in a member of the ras superfamily.

The mechanism of intracellular acidification induced by glucose in Saccharomyces cerevisiae

Addition of glucose or fructose to cells of Saccharomyces cerevisiae adapted to grow in the absence of glucose induced an acidification of the intracellular medium. This acidification appeared to be due to the phosphorylation of the sugar since: (i) glucose analogues which are not efficiently phosphorylated did not induce internal acidification; (ii) glucose addition did not cause internal acidification in a mutant deficient in all the three sugar-phosphorylating enzymes; (iii) fructose did not affect the intracellular pH in a double mutant having only glucokinase activity; (iv) glucose was as effective as fructose in inducing the internal pH drop in a mutant deficient in phosphoglucose isomerase activity; and (v) in strains deficient in two of the three sugar-phosphorylating activities, there was a good correlation between the specific glucose- or fructose-phosphorylating activity of cell extracts and the sugar-induced internal acidification. In addition, in whole cells any of the three yeast sugar kinases were capable of mediating the internal acidification described. Glucose-induced internal acidification was observed even when yeast cells were suspended in growth medium and in cells suspended in buffer containing K+, which supports the possible signalling function of the glucose-induced internal acidification. Evaluation of internal pH by following fluorescence changes of fluorescein-loaded cells indicated that the change in intracellular pH occurred immediately after addition of sugar. The apparent Km for glucose in this process was 2 mM. Changes in both the internal and external pH were determined and it was found that the internal acidification induced by glucose was followed by a partial alkalinization coincident with the initiation of H+ efflux. This reversal of acidification could be due to the activity of the H+-ATPase, since it was inhibited by diethylstilboestrol. Coincidence between internal alkalinization and the H+ efflux was also observed after addition of ethanol.