Incorporation of exogenous ganglioside GM1 into neuroblastoma membranes: Inhibition by calcium ion and dependence upon membrane protein

Since exogenous gangliosides are known to promote neuritogenesis, the incorporation of exogenous GM1 into neuroblastoma membranes was examined. Neuro-2A cells, synchronized in the G1/G0 phase, were suspended in HEPES buffered saline containing 10^–4 M [³H]GM1, and membrane incorporation was measured as radioactivity remaining with the cell pellet following incubation with serum-containing medium and trypsin. Calcium ion (0.01 to 10 mM) reduced incorporation of exogenous GM1, due to its interaction with GM1 micelles in solution. When cells were treated with proteases prior to incubation with GM1, the inhibitory effect of Ca²⁺ was lost and total incorporation into membranes was lowered by approximately one order of magnitude. Pretreatment of cells with 0.05% trypsin resulted in an inhibition of GM1 incorporation within 5 minutes. When trypsinized cells were resuspended in complete growth medium, the cells recovered the ability to incorporate GM1 with time, and this paralleled labeling of cellular protein with [³H]leucine. The role of membrane protein in the incorporation of exogenous GM1 could not be explained by the lytic release of cytosolic transfer proteins nor the artifactual coating of the cell surface by serum proteins. These results suggest that the incorporation of exogenous gangliosides into cellular membrane lipid bilayers cannot be fully explained by considerations of lipophilicity alone, and leads us to propose that initial recognition by membrane protein(s) is necessary.Abbreviations used GM1 H³NeuAc-GgOse₄Cer - HBS HEPES buffered saline - DMEM Dulbecco's modified Eagle's medium - FCS fetal calf serum     

Phospholipid liposomes and prolactin secretion: Effects on spontaneous and drug-induced hyperprolactinaemia

Bovine brain phospholipid liposomes (BC-PL) reduce plasma prolactin (PRL) levels in humans after acute administration and counteract the metoclopramide- and sulpiride-induced hyperprolactinaemia. However, BC-PL, like nomifensine, a dopaminergic compound, does not influence TRH-induced hyperprolactinaemia. Moreover BC-PL and nomifensine reduce plasma PRL levels in hyperprolactinaemic PCO syndromes but not in PRL secreting pituitary adenomas. The results obtained indicate that BC-PL antagonizes the DA blockade-induced hyperprolactinaemia and that the main site of action of BC-PL seems to be at the hypothalamic level; however a concomitant pituitary effect cannot be ruled out.     

Synergism between lysophosphatidylserine and the phorbol ester tetradecanoylphorbolacetate in rat mast cells

In rat peritoneal mast cells tetradecanoylphorbolacetate (TPA) induced a non cytotoxic histamine release in the absence of extracellular calcium. The addition of calcium prevented the TPA effect but micromolar concentrations of lysophosphatidylserine (lysoPS) converted the calcium-induced inhibition into a stimulation. Other lysophospholipids were inactive. In agreement with a mutual influence between lysoPS and TPA, minimal TPA concentrations enhanced the calcium-dependent histamine release induced by lysoPS in the presence of nerve-growth factor. It is proposed that the calcium-dependent pathway promoted by lysoPS and the activation of protein kinase C by TPA act synergically to induce histamine release from mast cells.     

Activation of phosphoinositide hydrolysis by nerve growth factor and lysophosphatidylserine in rat peritoneal mast cells

Histamine secretion in rat peritoneal mast cells stimulated by nerve growth factor requires a synergistic signal delivered by lysophosphatidylserine. To study the signal-transducing system activated by these compounds, phospholipid metabolism has been investigated in these cells. Phospholipid labeling with 32PO4 reveals a 5-9-fold stimulation of phosphatidic acid, phosphatidylinositol and phosphatidylcholine synthesis. Increased synthesis of phosphatidylinositol is also monitored using [3H]inositol incorporation. When [3H]inositol-labeled mast cells are incubated in the presence of Li+, nerve growth factor and lysophosphatidylserine enhance the accumulation of inositol monophosphate, inositol bisphosphate and inositol trisphosphate. Similar to the induced histamine release, accumulation of inositol phosphates (a) does not occur when the two agonists are added separately; (b) is inhibited when lysophosphatidyl-L-serine is replaced by lysophosphatidyl-D-serine; and (c) is enhanced in the presence of extracellular Ca2+. The data suggest that the interactive stimulus of nerve growth factor and lysophosphatidylserine is transmitted through the polyphosphoinositide-phospholipase C system.     

Activation of (Na+, K+)-ATPase by Nanomolar Concentrations of GM1 Ganglioside

GM1 ganglioside binding to the crude mitochondrial fraction of rat brain and its effect on (Na+, K+)-ATPase were studied, the following results being obtained: (a) the binding process followed a biphasic kinetics with a break at 50 nM-GM1; GM1 at concentrations below the break was stably associated, while over the break it was loosely associated; (b) stably bound GM1 activated (Na+, K+)-ATPase up to a maximum of 43%; (c) the activation was dependent upon the amount of bound GM1 and was highest at the critical concentration of 20 pmol bound GM1 X mg protein-1; (d) loosely bound GM1 suppressed the activating effect on (Na+, K+)-ATPase elicited by firmly bound GM1; (e) GM1-activated (Na+, K+)-ATPase had the same pH optimum and apparent Km (for ATP) as normal (Na+, K+)-ATPase but a greater apparent Vmax; (f) under identical binding conditions (2 h, 37 degrees C, with 40 nM substance) all tested gangliosides (GM1, GD1a, GD1b, GT1b) activated (Na+, K+)-ATPase (from 26-43%); NeuNAc, sodium dodecylsulphate, sulphatide and cerebroside had only a very slight effect. It is suggested that the ganglioside activation of (Na+-K+)-ATPase is a specific phenomenon not related to the amphiphilic and ionic properties of gangliosides, but due to modifications of the membrane lipid environment surrounding the enzyme.     

Evidence for a physiological role of nerve growth factor in the central nervous system of neonatal rats

Forebrain cholinergic neurons have been shown to respond in vivo to administration of nerve growth factor (NGF) with a prominent and selective increase of choline acetyltransferase (ChAT) activity. This has suggested that NGF can act as a trophic factor for these neurons. To test this hypothesis directly, anti-NGF antibodies (and their Fab fragments) were intracerebroventricularly injected into neonatal rats to neutralize endogenously occurring NGF. The anti-NGF antibody administration produced a decrease of ChAT activity in the hippocampus, septal area, cortex, and striatum of rat pups. This finding was substantiated by a concomitant decrease of immunopositive staining for ChAT in the septal area. These effects indicate that the occurrence of endogenous NGF in the CNS is physiologically relevant for regulating the function of forebrain cholinergic neurons.     

EFFECT OF BRAIN CORTEX PHOSPHOLIPIDS ON ADENYLATE-CYCLASE ACTIVITY OF MOUSE BRAIN

Intravenous injection of a sonicated dispersion of bovine brain phospholipids results in a significant change in both NaF-dependent and dopamine dependent adenylate cyclase activity. High dosage of phospholipids inhibits the dopamine dependent, but not the NaF dependent, adenylate cyclase activity. The stimulation of cyclase activity is accompanied by an increased level of cAMP in mice brains. Treatment with haloperidol abolishes the increase in cAMP. Among individual phospholipids, phosphatidylserine is the most active component for inducing the activation of DA-dependent adenylate cyclase activity.     

TSH circadian secretions in aged men and effect of phosphatidylserine treatment

In 20 euthyroid aged men (from 65 to 85 years of age) no significant circadian periodicity of thyrotropin (TSH) secretion has been shown by the population mean cosinor method. At the end of a period of 30 days of hospitalization the cosinor evaluation of TSH secretion showed a restored highly significant (p less than 0.001) circadian rhythmicity in phosphatidylserine (PS) (400 mg/daily) treated group (10 aged subjects). By contrast, hospitalization seems to further deteriorate the periodicity of the hormone secretion in 10 placebo-treated subjects.     

Melanin biosynthesis from dopamine. II. A mass spectrometric and collisional spectroscopic investigation.

Electron impact (EI) and fast atom bombardment (FAB) mass spectrometry together with collisional activation (CA) experiments were applied to the study of the oxidation pathway of dopamine by tyrosinase. In order to prevent attachment of the protein to the highly reactive intermediates, ultrafiltration was employed to remove the enzyme at different reaction times. FAB, privileging molecular species formation, was successfully used for identification of transient intermediates and their relative concentrations with respect to time, directly in the reaction mixture. The presence of isobaric molecular species made chromatographic separation necessary. Further EI mass spectrometry and collision spectroscopy led to structural identification of pure components. Of these, dopamine-o-quinone, leucoaminochrome, and aminochrome semiquinone were characterized for the first time as real intermediates in dopamine melanogenesis, in agreement with previous hypotheses. This approach elucidated the pathway of dopamine melanogenesis.     

Effect of lyso-phosphatidylserine on rat hypothalamic cAMP, in vivo

Lyso-Phosphatidylserine, like its precursor phosphatidylserine, increases the concentration of cAMP in the rat hypothalamus. The effect is dose-dependent and reaches its maximum 10–20 minutes from the injection. Antiadrenergic drugs, such as reserpine, α-methyl-p-tyrosine (α-MpT), haloperidol and chlorpromazine, but not phentolamine or propranolol, prevent the accumulation of hypothalamic cAMP induced by lysophosphati dylserine. Serotoninergic, histaminergic, cholinergic and GABAergic drugs do not modify the lyso-phosphatidylserine effect. The obtained results suggest a primary involvement of the dopaminergic system in the mediation of the pharmacological effect of lyso-phosphatidylserine.