Muscarinic activation of phosphatidylcholine hydrolysis

Functions and pathophysiological roles of phospholipase D in the brain

Ten years after the isoforms of mammalian phospholipase D (PLD), PLD1 and 2, were cloned, their roles in the brain remain speculative but several lines of evidence now implicate these enzymes in basic cell functions such as vesicular trafficking as well as in brain development. Many mitogenic factors, including neurotransmitters and growth factors, activate PLD in neurons and astrocytes. Activation of PLD downstream of protein kinase C seems to be a required step for astroglial proliferation. The characteristic disruption of the PLD signaling pathway by ethanol probably contributes to the delay of brain growth in fetal alcohol syndrome. The post-natal increase of PLD activities concurs with synapto- and myelinogenesis in the brain and PLD is apparently involved in neurite formation. In the adult and aging brain, PLD activity has antiapoptotic properties suppressing ceramide formation. Increased PLD activities in acute and chronic neurodegeneration as well as in inflammatory processes are evidently due to astrogliosis and may be associated with protective responses of tissue repair and remodeling. ARF-regulated PLD participates in receptor endocytosis as well as in exocytosis of neurotransmitters where PLD seems to favor vesicle fusion by modifications of the shape and charge of lipid membranes. Finally, PLD activities contribute free choline for the synthesis of acetylcholine in the brain. Novel tools such as RNA interference should help to further elucidate the roles of PLD isoforms in brain physiology and pathology.

Challenges and complexities of alpha-synuclein toxicity: new postulates in unfolding the mystery associated with Parkinson's disease

The discovery of two missense mutations in alpha-synuclein gene and the identification of the alpha-synuclein as the major component of Lewy bodies and Lewy neurites have imparted a new direction in understanding Parkinson's disease. Now that alpha-synuclein has been implicated in several neurodegenerative disorders makes it increasingly clear that aggregation of alpha-synuclein is a hallmark feature in neurodegeneration. Although little has been learned about its normal function, alpha-synuclein appears to be associated with membrane phospholipids and may therefore participate in a number of cell signaling pathways. Here, we review the localization, structure, and function of alpha-synuclein and provide a new hypothesis on, (a) the disruption in the membrane binding ability of synuclein which may be the major culprit leading to the alpha-synuclein aggregation and (b) the complexity associated with nuclear localization of alpha-synuclein and its possible binding property to DNA. Further, we postulated the three possible mechanisms of synuclein induced neuronal degeneration in Parkinson's disease.

Dual adrenergic control of in vivo choline levels in the mouse major salivary glands

1. The purpose of this study was to demonstrate that the adrenergic nervous system regulates the in vivo choline levels in the mouse major salivary glands. 2. Methoxamine (alpha1-adrenoceptor agonist, 2.5-20 mg kg-1, s.c.) elevated choline levels dose-dependently and the effect of methoxamine (10 mg kg-1) was completely inhibited by the alpha-adrenoceptor antagonist phentolamine (5 mg kg-1, i.p.) but not by the beta-adrenoceptor antagonist propranolol (3 mg kg-1, i.p.). 3. In contrast, isoprenaline (beta-adrenoceptor agonist 0.25-20 mg kg-1, s.c.) lowered choline levels and the effect of isoprenaline (2 mg kg-1) was inhibited by propranolol, but not by phentolamine. 4 Noradrenaline (1-4 mg kg-1, s.c.) manifested both the alpha- and beta-adrenergic actions depending on its dose. Noradrenaline at 1-2 mg kg-1, lowered choline levels and the effect of noradrenaline (1 mg kg-1) was inhibited by propranolol, but not by phentolamine. On the other hand, noradrenaline (4 mg kg-1) elevated choline levels and the effect was blocked by phentolamine, but not by propranolol. 5. Tyramine (5-80 mg kg-1, s.c.) elicited the release of noradrenaline from sympathetic nerve terminals and induced essentially the same effects on the choline levels as noradrenaline. Tyramine (10 mg kg-1) lowered choline levels and the effect was inhibited by propranolol, but not by phentolamine. However, tyramine (80 mg kg-1) elevated choline levels and the effect was inhibited by phentolamine, but not by propranolol. 6. These results suggest that choline levels in the salivary glands may be under separate alpha- and beta-adrenergic control and suggest a possibility that the neurotransmitter noradrenaline released for sympathetic nerve terminals can manage the dual control of choline levels in some autonomic organs in a characteristic dose-dependent manner.

Muscarinic receptor stimulation induces translocation of an alpha-synuclein oligomer from plasma membrane to a light vesicle fraction in cytoplasm

The close correspondence between the distribution of brain alpha-synuclein and that of muscarinic M1 and M3 receptors suggests a role for this protein in cholinergic transmission. We thus examined the effect of muscarinic stimulation on alpha-synuclein in SH-SY5Y, a human dopaminergic cell line that expresses this protein. Under basal conditions, alpha-synuclein was detected in all subcellular compartments isolated as follows: plasma membrane, cytoplasm, nucleus, and two vesicle fractions. The lipid fractions contained only a 45-kDa alpha-synuclein oligomer, whereas the cytoplasmic and nuclear fractions contained both the oligomer and the monomer. This finding suggests alpha-synuclein exists physiologically as a lipid-bound oligomer and a soluble monomer. Muscarinic stimulation by carbachol reduced the alpha-synuclein oligomer in plasma membrane over a 30-min period, with a concomitant increase of both the oligomer and the monomer in the cytoplasmic fraction. The oligomer was associated with a light vesicle fraction in cytoplasm that contains uncoated endocytotic vesicles. The carbachol-induced alteration of alpha-synuclein was blocked by atropine. Translocation of the alpha-synuclein oligomer in response to carbachol stimulation corresponds closely with the time course of ligand-stimulated muscarinic receptor endocytosis. The data suggest that the muscarine receptor stimulated release of the alpha-synuclein oligomer from plasma membrane, and its subsequent association with the endocytotic vesicle fraction may have a role in muscarine receptor endocytosis. We propose that its function may be a transient release of membrane-bound phospholipase D2 from alpha-synuclein inhibition, thus allowing this lipase to participate in muscarinic receptor endocytosis.

Group I metabotropic glutamate receptors mediate phospholipase D stimulation in rat cultured astrocytes

We have studied the activation of phospholipase D (PLD) by glutamate in rat cultured astrocytes by measuring the PLD-catalyzed formation of [32P]phosphatidylbutanol in [32P]Pi-prelabeled cells, stimulated in the presence of butanol. Glutamate elicited the activation of PLD in cortical astrocytes but not in cortical neurons, whereas similar glutamate activation of phosphoinositide phospholipase C was found in both astrocytes and neurons. The extent of PLD stimulation by glutamate was similar in astrocytes from brain cortex and hippocampus, but no effect was found in cerebellar astrocytes. In cortical astrocytes, the glutamate response was insensitive to antagonists of ionotropic glutamate receptors and was reproduced by agonists of metabotropic glutamate receptors (mGluRs) with a rank order of agonist potency similar to that reported for group I mGluR-mediated phosphoinositide phospholipase activation [quisqualate > (S)-3,5-dihydroxyphenylglycine > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid]. The response to (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid was inhibited by the mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine and, less potently, by 1-aminoindan-1,5-dicarboxylic acid and 4-carboxyphenylglycine, two antagonists of group I mGluRs that display higher potency on mGluR1 than on mGluR5. The mGluR5-selective agonist (RS)-2-chloro-5-hydroxyphenylglycine also activated PLD in astrocytes. These findings indicate the involvement of group I mGluRs, most likely mGluR5, in the glutamate activation of PLD in cultured rat cortical astrocytes.

Phospholipase D in rat myocardium: formation of lipid messengers and synergistic activation by G-protein and protein kinase C

Activation of phospholipase D (PLD) and phosphoinositide-specific phospholipase C (PI-PLC) by fluoride, to stimulate heterotrimeric G-proteins, and by phorbol esters, to stimulate protein kinase C (PKC), was studied in rat atria. Fluoride and 4beta-phorbol-12beta,13alpha-dibutyrate (PDB), in contrast to 4beta-phorbol-13alpha-acetate (PAc), activated PLD, catalyzing the formation of [3H]-phosphatidylethanol ([3H]-PETH), [3H]-phosphatidic acid ([3H]-PA), choline and sn-1,2-diacylglycerol (DAG). Basal PLD activity was resistant to drastic changes in Ca2+ and to Ro 31-8220, a PKC inhibitor, but was decreased by genistein, an inhibitor of tyrosine kinase, and increased by vanadate, a tyrosine phosphatase inhibitor; both effects were, however, very small. Fluoride-evoked PLD activity was resistant to Ro 31-8220 and to genistein, but was Ca2+-dependent. The rate of fluoride-induced PLD activation was maintained for at least 60 min. In contrast, PDB-mediated PLD activity was blocked by Ro 31-8220 and was resistant to extracellular Ca2+-depletion and desensitized within ca. 15 min. PDB markedly potentiated the fluoride-evoked generation of [3H]-phosphatidylethanol and of choline, but inhibited the formation of [3H]-inositol phosphates ([3H]-IP(1-3)). Ethanol (2%) blocked the PDB-evoked generation of both [3H]-phosphatidic acid and of sn-1,2-diacylglycerol, whereas fluoride-evoked responses were reduced only to approximately 50%. In conclusion, the trimeric G-protein-PLD pathway in heart tissue did not enclose PKC activation and was long-lasting and Ca2+-dependent; there was no evidence for an involvement of tyrosine phosphorylation. However, PKC activation modulated G-protein-coupled PLD and PI-PLC activities in opposite directions. PLD activity significantly contributed to the mass production of sn-1,2-diacylglycerol in the heart. The evidence for a pathophysiological role of PLD activation in cardiac hypertrophy and in ischemic preconditioning is discussed.

Brain choline has a typical precursor profile

Choline is product and precursor to both acetylcholine and membrane phospholipids, and, in the brain, is ultimately provided by the circulation. The brain is protected from excess choline and choline deprivation by a refined system of homeostatic mechanisms that maintain a level of extracellular choline that, for its role as precursor, meets saturation criteria under normal conditions. The kinetic and activity profiles of choline are typical for a biosynthetic precursor.