Decreased myo-inositol uptake is associated with reduced bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol content in cultured neuroblastoma cells exposed to L-fucose

Applications of Magnetic Resonance Spectroscopy to Psychiatry

The inaccessibility of the human brain to biochemical studies has historically challenged the ability of in vestigators to elucidate the pathophysiology of psychiatric syndromes. Magnetic resonance spectroscopy (MRS) now provides a noninvasive means of assessing neurochemistry in vivo. Since the first application of the technique to the study of the human brain, many new advances have been made. This new technology broadens the applications of the MRS. The major principles of the technique and compounds currently available for study are discussed in this article. A brief review of current and future applications of the technology to the field of psychiatry are discussed. NEUROSCIENTIST 5:192-196, 1999

Dehydroepiandrosterone inhibits intracellular calcium release in beta-cells by a plasma membrane-dependent mechanism

Both dehydroepiandrosterone (DHEA) and DHEA sulfate (DHEAS) affect glucose stimulated insulin secretion, though their cellular mechanisms of action are not well characterized. We tested the hypothesis that human physiological concentrations of DHEA alter insulin secretion by an action initiated at the plasma membrane of beta-cells. DHEA alone had no effect on intracellular calcium concentration ([Ca(2+)](i)) in a rat beta-cell line (INS-1). However, it caused an immediate and dose-dependent inhibition of carbachol-induced Ca(2+) release from intracellular stores, with a 25% inhibition at zero. One nanometer DHEA. DHEA also inhibited the Ca(2+) mobilizing effect of bombesin (29% decrease), but did not inhibit the influx of extracellular Ca(2+) evoked by glyburide (100 microM) or glucose (15 mM). The steroids (androstenedione, 17-alpha-hydroxypregnenolone, and DHEAS) had no inhibitory effect on carbachol-induced intracellular Ca(2+) release. The action of DHEA depended on a signal initiated at the plasma membrane, since membrane impermeant DHEA-BSA complexes also inhibited the carbachol effect on [Ca(2+)](i) (39% decrease). The inhibition of carbachol-induced Ca(2+) release by DHEA was blocked by pertussis toxin (PTX). DHEA also inhibited the carbachol induction of phosphoinositide generation, with a maximal inhibition at 0.1 nM DHEA. Furthermore, DHEA inhibited insulin secretion induced by carbachol in INS-1 cells by 25%, and in human pancreatic islets by 53%. Taken together, this is the first report showing that human physiological concentrations of DHEA decrease agonist-induced Ca(2+) release by a rapid, non-genomic mechanism in INS-1 cells. Furthermore, these data provide evidence consistent with the existence of a specific plasma membrane DHEA receptor, mediating this signal transduction pathway by pertussis toxin-sensitive G-proteins.

Regulation of KCNQ2/KCNQ3 current by G protein cycling: the kinetics of receptor-mediated signaling by Gq

Receptor-mediated modulation of KCNQ channels regulates neuronal excitability. This study concerns the kinetics and mechanism of M₁ muscarinic receptor–mediated regulation of the cloned neuronal M channel, KCNQ2/KCNQ3 (Kv7.2/Kv7.3). Receptors, channels, various mutated G-protein subunits, and an optical probe for phosphatidylinositol 4,5-bisphosphate (PIP₂) were coexpressed by transfection in tsA-201 cells, and the cells were studied by whole-cell patch clamp and by confocal microscopy. Constitutively active forms of Gα_q and Gα₁₁, but not Gα₁₃, caused a loss of the plasma membrane PIP₂ and a total tonic inhibition of the KCNQ current. There were no further changes upon addition of the muscarinic agonist oxotremorine-M (oxo-M). Expression of the regulator of G-protein signaling, RGS2, blocked PIP₂ hydrolysis and current suppression by muscarinic stimulation, confirming that the G_q family of G-proteins is necessary. Dialysis with the competitive inhibitor GDPβS (1 mM) lengthened the time constant of inhibition sixfold, decreased the suppression of current, and decreased agonist sensitivity. Removal of intracellular Mg²⁺ slowed both the development and the recovery from muscarinic suppression. When combined with GDPβS, low intracellular Mg²⁺ nearly eliminated muscarinic inhibition. With nonhydrolyzable GTP analogs, current suppression developed spontaneously and muscarinic inhibition was enhanced. Such spontaneous suppression was antagonized by GDPβS or GTP or by expression of RGS2. These observations were successfully described by a kinetic model representing biochemical steps of the signaling cascade using published rate constants where available. The model supports the following sequence of events for this G_q-coupled signaling: A classical G-protein cycle, including competition for nucleotide-free G-protein by all nucleotide forms and an activation step requiring Mg²⁺, followed by G-protein–stimulated phospholipase C and hydrolysis of PIP₂, and finally PIP₂ dissociation from binding sites for inositol lipid on the channels so that KCNQ current was suppressed. Further experiments will be needed to refine some untested assumptions.

Activation of phospholipase C increases intramembrane electric fields in N1E-115 neuroblastoma cells

We imaged the intramembrane potential (a combination of transmembrane, surface, and dipole potential) on N1E-115 neuroblastoma cells with a voltage-sensitive dye. After activation of the B(2) bradykinin receptor, the electric field sensed by the dye increased by an amount equivalent to a depolarization of 83 mV. The increase in intramembrane potential was blocked by the phospholipase C (PLC) inhibitors U-73122 and neomycin, and was invariably accompanied by a transient rise of [Ca(2+)](i). A depolarized inner surface potential, as the membrane loses negative charges via phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis, and an increase in the dipole potential, as PIP(2) is hydrolyzed to 1,2-diacylglycerol (DAG), can each account for a small portion of the change in intramembrane potential. The primary contribution to the measured change in intramembrane potential may arise from an increased dipole potential, as DAG molecules are generated from hydrolysis of other phospholipids. We found bradykinin produced an inhibition of a M-type voltage-dependent K(+) current (I(K(M))). This inhibition was also blocked by the PLC inhibitors and had similar kinetics as the bradykinin-induced modulation of intramembrane potential. Our results suggest that the change in the local intramembrane potential induced by bradykinin may play a role in mediating the I(K(M)) inhibition.

Kinetic analysis of receptor-activated phosphoinositide turnover

We studied the bradykinin-induced changes in phosphoinositide composition of N1E-115 neuroblastoma cells using a combination of biochemistry, microscope imaging, and mathematical modeling. Phosphatidylinositol-4,5-bisphosphate (PIP2) decreased over the first 30 s, and then recovered over the following 2-3 min. However, the rate and amount of inositol-1,4,5-trisphosphate (InsP3) production were much greater than the rate or amount of PIP2 decline. A mathematical model of phosphoinositide turnover based on this data predicted that PIP2 synthesis is also stimulated by bradykinin, causing an early transient increase in its concentration. This was subsequently confirmed experimentally. Then, we used single-cell microscopy to further examine phosphoinositide turnover by following the translocation of the pleckstrin homology domain of PLCdelta1 fused to green fluorescent protein (PH-GFP). The observed time course could be simulated by incorporating binding of PIP2 and InsP3 to PH-GFP into the model that had been used to analyze the biochemistry. Furthermore, this analysis could help to resolve a controversy over whether the translocation of PH-GFP from membrane to cytosol is due to a decrease in PIP2 on the membrane or an increase in InsP3 in cytosol; by computationally clamping the concentrations of each of these compounds, the model shows how both contribute to the dynamics of probe translocation.

The central role of magnesium deficiency in Tourette's syndrome: causal relationships between magnesium deficiency, altered biochemical pathways and symptoms relating to Tourette's syndrome and several reported comorbid conditions

Prior studies have suggested a common etiology involved in Tourette's syndrome and several comorbid conditions and symptomatology. Reportedly, current medications used in Tourette's syndrome have intolerable side-effects or are ineffective for many patients. After thoroughly researching the literature, I hypothesize that magnesium deficiency may be the central precipitating event and common pathway for the subsequent biochemical effects on substance P, kynurenine, NMDA receptors, and vitamin B6 that may result in the symptomatology of Tourette's syndrome and several reported comorbid conditions. These comorbid conditions and symptomatology include allergy, asthma, autism, attention deficit hyperactivity disorder, obsessive compulsive disorder, coprolalia, copropraxia, anxiety, depression, restless leg syndrome, migraine, self-injurious behavior, autoimmunity, rage, bruxism, seizure, heart arrhythmia, heightened sensitivity to sensory stimuli, and an exaggerated startle response. Common possible environmental and genetic factors are discussed, as well as biochemical mechanisms. Clinical studies to determine the medical efficacy for a comprehensive magnesium treatment option for Tourette's syndrome need to be conducted to make this relatively safe, low side-effect treatment option available to doctors and their patients.

Nordidemnin potently inhibits inositol uptake in cultured astrocytes and dose-dependently augments lithium's proconvulsant effect in vivo

It has been suggested that inositol uptake across the cell membrane is of importance for maintenance of the inositol pool involved in lithium's therapeutic effect in bipolar disease and in the lithium-pilocarpine seizure test in freely moving rats (measuring the latency of a normally subconvulsive concentration of pilocarpine to seizure induction in the additional presence of lithium). We have tested this hypothesis by: 1) demonstrating an extremely high potency of nordidemnin as an inhibitor of myo-inositol uptake in primary cultures of mouse astrocytes; and 2) determining the dose-response correlation of a nordidemnin-induced decrease in the latency before appearance of seizures in the lithium-pilocarpine test after intracerebroventricular injection of minute samples (10 microl) of virtually isotonic saline solution.

Correction of Leukocyte Adhesion Deficiency Type II With Oral Fucose

We describe a simple, noninvasive, and effective therapy for leukocyte adhesion deficiency type II (LAD II), a rare inherited disorder of fucose metabolism. This disorder leads to an immunodeficiency caused by the absence of carbohydrate-based selectin ligands on the surface of neutrophils as well as to severe psychomotor and mental retardation. The fucosylation defect in LAD II fibroblasts can be corrected by addition of L-fucose to the culture medium. This prompted us to initiate dietary fucose therapy on a patient with LAD II. Oral supplementation of fucose in this patient induced the expression of fucosylated selectin ligands on neutrophils and core fucosylation of serum glycoproteins. During 9 months of treatment, infections and fever disappeared, elevated neutrophil counts returned to normal, and psychomotor capabilities improved.

Correction of Leukocyte Adhesion Deficiency Type II With Oral Fucose

We describe a simple, noninvasive, and effective therapy for leukocyte adhesion deficiency type II (LAD II), a rare inherited disorder of fucose metabolism. This disorder leads to an immunodeficiency caused by the absence of carbohydrate-based selectin ligands on the surface of neutrophils as well as to severe psychomotor and mental retardation. The fucosylation defect in LAD II fibroblasts can be corrected by addition of L-fucose to the culture medium. This prompted us to initiate dietary fucose therapy on a patient with LAD II. Oral supplementation of fucose in this patient induced the expression of fucosylated selectin ligands on neutrophils and core fucosylation of serum glycoproteins. During 9 months of treatment, infections and fever disappeared, elevated neutrophil counts returned to normal, and psychomotor capabilities improved.

Differentiated human NT2-N neurons possess a high intracellular content of myo-inositol

myo-Inositol plays a key role in signal transduction and osmotic regulation events in the CNS. Despite the known high concentrations of inositol in the human CNS, relatively little is known about its distribution within the different cell types. In this report, inositol homeostasis was studied in NT2-N cells, a unique cell culture model of human CNS neurons. Differentiation of precursor NT2 teratocarcinoma cells into NT2-N neurons by means of retinoic acid treatment resulted in an increase in inositol concentration from 24 to 195 nmol/mg of protein. After measurement of intracellular water spaces, inositol concentrations of 1.6 and 17.4 mM were calculated for NT2 and NT2-N cells, respectively. The high concentrations of inositol in NT2-N neurons could be explained by (1) an increased uptake of inositol (3.7 vs. 1.6 nmol/mg of protein/h, for NT2-N and NT2 cells, respectively) and (2) a decreased efflux of inositol (1.7%/h for NT2-N neurons vs. 9.0%/h for NT2 cells). Activity of inositol synthase, which mediates de novo synthesis of inositol, was not detected in either cell type. The observation that CNS neurons maintain a high intracellular concentration of inositol may be relevant to the regulation of both phosphoinositide signaling and osmotic stress events in the CNS.

Endothelin-stimulated Ca2+ mobilization by 3T3-L1 adipocytes is suppressed by tumor necrosis factor-alpha

The cytokine tumor necrosis factor-alpha (TNFalpha) contributes to metabolic changes in disease states such as insulin resistance. However, the mechanism by which TNFalpha alters cellular function in these conditions is poorly understood. Because changes in intracellular calcium concentration plays a critical role in hormone action we investigated the effect of TNFalpha on calcium homeostasis in 3T3-L1 adipocytes. In these studies we show that TNFalpha causes a concentration- and time-dependent decrease in Na+/myo-inositol cotransporter (SMIT) mRNA levels and myo-inositol accumulation as well as a decrease in myo-inositol incorporation into phosphoinositides. These changes coincided with a decrease in endothelin-1-induced phosphatidylinositol (PI) cycle activity in 3T3-L1 adipocytes chronically exposed to TNFalpha. Endothelin-1-induced mobilization of calcium from intracellular stores was also diminished by TNFalpha. The effect of TNFalpha on endothelin-1-induced PI cycle activity and calcium mobilization was not due to a decrease in endothelin receptors. However, TNFalpha did cause a moderate decrease in phosphatidylinositol 4,5-bisphosphate (PIP2)-specific phospholipase C (PLC) activity in 3T3-L1 adipocytes. Combined, a decrease in phosphoinositide production and PIP2-specific PLC activity could be responsible for altering PI cycle activity and the generation of the second messenger myo-inositol 1,4,5-trisphosphate, thereby reducing calcium mobilization. Such changes in intracellular signaling may contribute to the pathophysiology of insulin resistance associated with TNFalpha.

Elevated levels of glucose and L-fucose reduce 22Na+ uptake and whole cell Na+ current in cultured neuroblastoma cells

Na+ flux was studied in cultured neuroblastoma cells grown in medium containing increased glucose or L-fucose concentrations. Chronic exposure of neuroblastoma cells to 30 mM glucose or 30 mM L-fucose caused a decrease in ouabain-sensitive and veratridine-stimulated 22Na+ uptake compared with cells cultured in unsupplemented medium. The Na+ current, determined by using whole-cell configuration of the patch clamp, was also decreased in these cells. Tetrodotoxin (3 microM), which blocked whole cell Na+ currents, also blocked veratridine-stimulated 22Na+ accumulation. Culturing cells in medium containing 30 mM fructose as an osmotic control had no effect on Na+ flux. Specific [3H]saxitoxin binding was not affected by 30 mM glucose or 30 mM L-fucose compared with cells grown in unsupplemented medium, suggesting that the number of Na+ channels was not decreased. These studies suggest that exposing cultured neuronal cells to conditions that occur in the diabetic milieu alters Na+ transport and Na(+)-channel activity.