The phosphorylation of choline acetyltransferase

Antioxidants Reverse the Changes in the Cholinergic System Caused by L-Tyrosine Administration in Rats

Tyrosinemia type II is an inborn error of metabolism caused by a deficiency in the activity of the enzyme tyrosine aminotransferase, leading to tyrosine accumulation in the body. Although the mechanisms involved are still poorly understood, several studies have showed that higher levels of tyrosine are related to oxidative stress and therefore may affect the cholinergic system. Thus, the aim of this study was to investigate the effects of chronic administration of L-tyrosine on choline acetyltransferase activity (ChAT) and acetylcholinesterase (AChE) in the brain of rats. Moreover, we also examined the effects of one antioxidant treatment (N-acetylcysteine (NAC) + deferoxamine (DFX)) on cholinergic system. Our results showed that the chronic administration of L-tyrosine decreases the ChAT activity in the cerebral cortex, while the AChE activity was increased in the hippocampus, striatum, and cerebral cortex. Moreover, we found that the antioxidant treatment was able to prevent the decrease in the ChAT activity in the cerebral cortex. However, the increase in AChE activity induced by L-tyrosine was partially prevented the in the hippocampus and striatum, but not in the cerebral cortex. Our results also showed no differences in the aversive and spatial memory after chronic administration of L-tyrosine. In conclusion, the results of this study demonstrated an increase in AChE activity in the hippocampus, striatum, and cerebral cortex and an increase of ChAT in the cerebral cortex, without cognitive impairment. Furthermore, the alterations in the cholinergic system were partially prevented by the co-administration of NAC and DFX. Thus, the restored central cholinergic system by antioxidant treatment further supports the view that oxidative stress may be involved in the pathophysiology of tyrosinemia type II.

Alzheimer's disease: Targeting the Cholinergic System

Acetylcholine (ACh) has a crucial role in the peripheral and central nervous systems. The enzyme choline acetyltransferase (ChAT) is responsible for synthesizing ACh from acetyl-CoA and choline in the cytoplasm and the vesicular acetylcholine transporter (VAChT) uptakes the neurotransmitter into synaptic vesicles. Following depolarization, ACh undergoes exocytosis reaching the synaptic cleft, where it can bind its receptors, including muscarinic and nicotinic receptors. ACh present at the synaptic cleft is promptly hydrolyzed by the enzyme acetylcholinesterase (AChE), forming acetate and choline, which is recycled into the presynaptic nerve terminal by the high-affinity choline transporter (CHT1). Cholinergic neurons located in the basal forebrain, including the neurons that form the nucleus basalis of Meynert, are severely lost in Alzheimer’s disease (AD). AD is the most ordinary cause of dementia affecting 25 million people worldwide. The hallmarks of the disease are the accumulation of neurofibrillary tangles and amyloid plaques. However, there is no real correlation between levels of cortical plaques and AD-related cognitive impairment. Nevertheless, synaptic loss is the principal correlate of disease progression and loss of cholinergic neurons contributes to memory and attention deficits. Thus, drugs that act on the cholinergic system represent a promising option to treat AD patients.

Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype-Phenotype Correlations

Choline acetyltransferase catalyzes the synthesis of acetylcholine at cholinergic nerves. Mutations in human CHAT cause a congenital myasthenic syndrome due to impaired synthesis of ACh; this severe variant of the disease is frequently associated with unexpected episodes of potentially fatal apnea. The severity of this condition varies remarkably, and the molecular factors determining this variability are poorly understood. Furthermore, genotype-phenotype correlations have been difficult to establish in patients with biallelic mutations. We analyzed the protein expression of phosphorylated ChAT of seven CHAT mutations, p.Val136Met, p.Arg207His, p.Arg186Trp, p.Val194Leu, p.Pro211Ala, p.Arg566Cys, and p.Ser694Cys, in HEK-293 cells to phosphorylated ChAT, determined their enzyme kinetics and thermal stability, and examined their structural changes. Three mutations, p.Arg207His, p.Arg186Trp, and p.Arg566Cys, are novel, and p.Val136Met and p.Arg207His are homozygous in three families and associated with severe disease. The characterization of mutants showed a decrease in the overall catalytic efficiency of ChAT; in particular, those located near the active-site tunnel produced the most seriously disruptive phenotypic effects. On the other hand, p.Val136Met, which is located far from both active and substrate-binding sites, produced the most drastic reduction of ChAT expression. Overall, CHAT mutations producing low enzyme expression and severe kinetic effects are associated with the most severe phenotypes.

A model for dynamic regulation of choline acetyltransferase by phosphorylation

Choline acetyltransferase (ChAT) synthesizes the neurotransmitter acetylcholine (ACh) and is a phenotypic marker for cholinergic neurons. Cholinergic neurons in brain are involved in cognitive function, attentional processing and motor control, and decreased ChAT activity is found in several neurological disorders including Alzheimer's disease. Dysregulation of ChAT and cholinergic communication is also associated with some spontaneous point-mutations in ChAT that alter its substrate binding kinetics, or by disruption of signaling pathways that could regulate protein kinases for which ChAT is a substrate. It has been identified recently that the catalytic activity and subcellular distribution of ChAT, and its interaction with other cellular proteins, can be modified by phosphorylation of the enzyme by protein kinase-C and Ca2+/calmodulin-dependent protein kinase II; these kinases appear also to mediate some of the effects of beta-amyloid peptides on cholinergic neuron functions, including the effects on ChAT. This review outlines a new model for the regulation of cholinergic transmission at the level of the presynaptic terminal that is mediated by hierarchically-regulated, multi-site phosphorylation of ChAT.

Rat choline acetyltransferase of the peripheral type differs from that of the common type in intracellular translocation

Choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine, has been implicated to involve multiple isoforms of ChAT mRNA in several animals. Since these isoforms are mostly non-coding splice variants, only a homologous ChAT protein of about 68 kDa has been shown to be produced in vivo. Recent evidence indicates the existence of a protein coding splice variant of ChAT mRNA, which lacks exons 6-9 of the rat ChAT gene. The encoded protein was designated ChAT of a peripheral type (pChAT), because of its preferential expression in the peripheral nervous system as confirmed by Western blot and immunohistochemistry. However, functional significance of pChAT is unknown. To obtain a clue to this question, we examined a possible difference in intracellular trafficking between pChAT and the well-known ChAT of the common type (cChAT) using green fluorescent protein (GFP) in living human embryonic kidney cells. Confocal laser scanning microscopy revealed that pChAT-GFP was detectable in the cytoplasm but not in the nucleus, whereas cChAT-GFP was found in both cytoplasm and nucleus. Following treatment with leptomycin B, a nuclear export pathway inhibitor, pChAT-GFP became detectable in both cytoplasm and nucleus, indicating that pChAT can be translocated to the nucleus. In contrast, the leptomycin B treatment did not seem to affect the content of intranuclear cChAT-GFP. After incubation with protein kinase C inhibitors, enhanced accumulation of pChAT-GFP but not cChAT-GFP occurred in the nucleus. These results clearly indicate that pChAT varies from cChAT in intracellular transportation, probably reflecting the difference in physiological roles between pChAT and cChAT.

Two methods for large-scale purification of recombinant human choline acetyltransferase

Choline acetyltransferase (ChAT) catalyzes the transfer of an acetyl group from acetyl-CoA to choline to produce the neurotransmitter acetylcholine (ACh). We have produced large quantities of pure human ChAT using two different bacterial expression systems. In the first, ChAT is fused to a chitin-binding domain via a self-cleavable linker allowing the release of ChAT without the use of proteases. In the second, ChAT is fused to a hexahistidine (His6) tag at the N-terminus with a linker incorporating a TEV protease cleavage site. In both cases, pure ChAT was produced that has a final specific activity of approximately 50 micromol ACh/min/mg and is suitable for structural characterization. Analysis of purified ChAT by Western blots and mass spectrometry revealed that the C-terminal 15 amino acids were slowly removed by endogenous proteolytic activity, to produce a stable 615 residue protein. Furthermore, we show that purified recombinant human ChAT is highly prone to oxidation, leading to the formation of covalent dimers and/or a loss of catalytic activity. Kinetic parameters of our purified proteins were obtained and, when compared to previously published constants for human placental ChAT, we found that recombinant human ChAT displays lower values for Michaelis and inhibition constants for ACh, which may be due to the complete absence of post-translational modifications.

Protein Kinase C Isoforms Differentially Phosphorylate Human Choline Acetyltransferase Regulating Its Catalytic Activity

Choline acetyltransferase (ChAT) synthesizes acetylcholine in cholinergic neurons; regulation of its activity or response to physiological stimuli is poorly understood. We show that ChAT is differentially phosphorylated by protein kinase C (PKC) isoforms on four serines (Ser-440, Ser-346, Ser-347, and Ser-476) and one threonine (Thr-255). This phosphorylation is hierarchical, with phosphorylation at Ser-476 required for phosphorylation at other serines. Phosphorylation at some, but not all, sites regulates basal catalysis and activation. Ser-476 with Ser-440 and Ser-346/347 maintains basal ChAT activity. Ser-440 is targeted by Arg-442 for phosphorylation by PKC. Arg-442 is mutated spontaneously (R442H) in congenital myasthenic syndrome, rendering ChAT inactive and causing neuromuscular failure. This mutation eliminates phosphorylation of Ser-440, and Arg-442, not phosphorylation of Ser-440, appears primarily responsible for ChAT activity, with Ser-440 phosphorylation modulating catalysis. Finally, basal ChAT phosphorylation in neurons is mediated predominantly by PKC at Ser-476, with PKC activation increasing phosphorylation at Ser-440 and enhancing ChAT activity.

Choline acetyltransferase: regulation and coupling with protein kinase and vesicular acetylcholine transporter on synaptic vesicles

Both the membrane-bound choline acetyltransferase (MChAT) and soluble ChAT (SChAT) were found to be activated by ATP-mediated protein phosphorylation. ATP activation of MChAT but not SChAT was found to depend on the integrity of proton gradient of synaptic vesicles because conditions disrupting the proton gradient also abolished the activation of MChAT by ATP. Among the kinases studied, Ca2+/calmodulin kinase II is most effective in activation of MChAT. Transport of ACh into synaptic vesicles by vesicular acetylcholine transporter (VAChT) is also proton gradient-dependent; therefore we proposed that there is a functional coupling between ACh synthesis and its packaging into synaptic vesicles. This notion is supported by the following findings: first, the newly synthesized [3H]-ACh from [3H]-choline was taken up much more efficiently than the pre-existing ACh; second, ATP-activation of MChAT was abolished when VAChT was inhibited by the specific inhibitor vesamicol; third, the activity of ChAT was found to be markedly increased when neurons are under depolarizing conditions.

Choline acetyltransferase structure reveals distribution of mutations that cause motor disorders

Choline acetyltransferase (ChAT) synthesizes acetylcholine in neurons and other cell types. Decreases in ChAT activity are associated with a number of disease states, and mutations in ChAT cause congenital neuromuscular disorders. The crystal structure of ChAT reported here shows the enzyme divided into two domains with the active site in a solvent accessible tunnel at the domain interface. A low-resolution view of the complex with one substrate, coenzyme A, defines its binding site and suggests an additional interaction not found in the related carnitine acetyltransferase. Also, the preference for choline over carnitine as an acetyl acceptor is seen to result from both electrostatic and steric blocks to carnitine binding at the active site. While half of the mutations that cause motor disorders are positioned to affect enzyme activity directly, the remaining changes are surprisingly distant from the active site and must exert indirect effects. The structure indicates how ChAT is regulated by phosphorylation and reveals an unusual pattern of basic surface patches that may mediate membrane association or macromolecular interactions.

Cytosolic choline acetyltransferase binds specifically to cholinergic plasma membrane of rat brain synaptosomes to generate membrane-bound enzyme

Uncovering the way membrane-bound choline acetyltransferase (ChAT) interacts with membranes and with which membrane in cholinergic neurons may help in understanding its role in acetylcholine metabolism. Subfractionation of rat hippocampal synaptosomes aiming to separate synaptic vesicles from plasma membranes shows that membrane-bound ChAT is bound to plasma membrane. Either detergents or urea and alkali can solubilize membrane-bound enzyme. Detergent-solubilized enzyme has a higher sedimentation rate than urea-alkali solubilized or cytosolic ChAT. Once dissociated, membrane-bound ChAT reassociates specifically with cholinergic plasma membranes, a process that was abolished by previous treatment of membranes with trypsin. Cytosolic ChAT behaves similarly. Thus, in cholinergic synaptosomes, ChAT exists as cytosolic and peripheral activity. Cytosolic ChAT generates peripheral enzyme most probably by interacting with a protein of plasma membrane of cholinergic nerve terminals. This "receptor" protein might regulate the amount of membrane-bound ChAT in cholinergic neurons.

Functional regulation of choline acetyltransferase by phosphorylation

Choline acetyltransferase (ChAT) catalyzes synthesis of acetylcholine (ACh) in cholinergic neurons. ACh synthesis is regulated by availability of precursors choline and acetyl coenzyme A or by activity of ChAT; ChAT regulates ACh synthesis under some conditions. Posttranslational phosphorylation is a common mechanism for regulating the function of proteins. Analysis of the primary sequence of 69-kD human ChAT indicates that it has putative phosphorylation consensus sequences for multiple protein kinases. ChAT is phosphorylated on serine-440 and threonine-456 by protein kinase C and CaM kinase II, respectively. These phosphorylation events regulate activity of the enzyme, as well as its binding to plasma membrane and interaction with other cellular proteins. It is relevant to investigate differences in constitutive and inducible patterns of phosphorylation of ChAT under physiological conditions and in response to challenges that cholinergic neurons may be exposed to, and to determine how changes in phosphorylation relate to changes in neurochemical transmission.

Phosphorylation of 69-kDa choline acetyltransferase at threonine 456 in response to amyloid-beta peptide 1-42

Choline acetyltransferase synthesizes acetylcholine in cholinergic neurons. In the brain, these neurons are especially vulnerable to effects of beta-amyloid (A beta) peptides. Choline acetyltransferase is a substrate for several protein kinases. In the present study, we demonstrate that short term exposure of IMR32 neuroblastoma cells expressing human choline acetyltransferase to A beta-(1-42) changes phosphorylation of the enzyme, resulting in increased activity and alterations in its interaction with other cellular proteins. Using mass spectrometry, we identified threonine 456 as a new phosphorylation site in choline acetyltransferase from A beta-(1-42)-treated cells and in purified recombinant ChAT phosphorylated in vitro by calcium/calmodulin-dependent protein kinase II (CaM kinase II). Whereas phosphorylation of choline acetyltransferase by protein kinase C alone caused a 2-fold increase in enzyme activity, phosphorylation by CaM kinase II alone did not alter enzyme activity. A 3-fold increase in choline acetyltransferase activity was found with coordinate phosphorylation of threonine 456 by CaM kinase II and phosphorylation of serine 440 by protein kinase C. This phosphorylation combination was observed in choline acetyltransferase from A beta-(1-42)-treated cells. Treatment of cells with A beta-(1-42) resulted in two phases of activation of choline acetyltransferase, the first within 30 min and associated with phosphorylation by protein kinase C and the second by 10 h and associated with phosphorylation by both CaM kinase II and protein kinase C. We also show that choline acetyltransferase from A beta-(1-42)-treated cells co-immunoprecipitates with valosin-containing protein, and mutation of threonine 456 to alanine abolished the A beta-(1-42)-induced effects. These studies demonstrate that A beta-(1-42) can acutely regulate the function of choline acetyltransferase, thus potentially altering cholinergic neurotransmission.

Regulation of acetylcholine synthesis and storage

Acetylcholine is one of the major modulators of brain functions and it is the main neurotransmitter at the peripheral nervous system. Modulation of acetylcholine release is crucial for nervous system function. Moreover, dysfunction of cholinergic transmission has been linked to a number of pathological conditions. In this manuscript, we review the cellular mechanisms involved with regulation of acetylcholine synthesis and storage. We focus on how phosphorylation of key cholinergic proteins can participate in the physiological regulation of cholinergic nerve-endings.

Effects of aging and amyloid-beta peptides on choline acetyltransferase activity in rat brain

Choline acetyltransferase (ChAT, acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6), involved in the learning and memory processes is responsible for the synthesis of acetylcholine. There are many discrepancies in literature concerning ChAT activity during brain aging and the role of amyloid beta peptides in modulation of this enzyme. The aim of the study was to investigate the mechanism of ChAT regulation and age-related alteration of ChAT activity in different parts of the brain. Moreover the effect of Abeta peptides on ChAT activity in adult and aged brain was investigated. The enzyme activity was determined in the brain cortex, hippocampus and striatum in adult (4-months-old), adult-aged (14-months-old) and aged (24-months-old) animals. The highest ChAT activity was observed in the striatum. We found that inhibitors of protein kinase C, A, G and phosphatase A2 have no effect on ChAT activity and that this enzyme is not dependent on calcium ions. About 70% of the total ChAT activity is present in the cytosol. Arachidonic acid significantly inhibited cytosolic form of this enzyme. In the brain cortex and striatum from aged brain ChAT activity is inhibited by 50% and 37%, respectively. The aggregated form of Abeta 25-35 decreased significantly ChAT activity only in the aged striatum and exerted inhibitory effect on this enzyme in adult, however, statistically insignificant. ChAT activity in the striatum was diminished after exposure to 1 mM H2O2. The results from our study indicate that aging processes play a major role in inhibition of ChAT activity and that this enzyme in striatum is selectively sensitive for amyloid beta peptides.

Cholinergic and serotonergic activities are required in triggering conditioned NK cell response

The purpose of the study was to examine the importance of the cholinergic system in triggering the conditioned NK cell response. The fact that serotonergic system can modulate cholinergic functions suggested that it might be involved in conditioned NK cell response. To evaluate the potential pathways, cholinergic and serotonergic antagonists were applied centrally at either the conditioned association or recall stage, to interfere with the conditioned NK cell response. The results showed that both the cholinergic and serotonergic systems were necessary for eliciting the conditioned enhancement of NK cell activity. Involvements of the two systems were found to be critical for establishing the conditioned association and recall of the conditioned response. The blocks are believed to be receptor mediated. The receptors identified to be involved in the regulation of the conditioned NK cell response were: M(1), M(2) and M(3) muscarinic; nicotinic; 5 HT(1) and 5 HT(2) receptors.

Functional characterization of phosphorylation of 69-kDa human choline acetyltransferase at serine 440 by protein kinase C

Choline acetyltransferase, the enzyme that synthesizes the transmitter acetylcholine in cholinergic neurons, is a substrate for protein kinase C. In the present study, we used mass spectrometry to identify serine 440 in recombinant human 69-kDa choline acetyltransferase as a protein kinase C phosphorylation site, and site-directed mutagenesis to determine that phosphorylation of this residue is involved in regulation of the enzyme's catalytic activity and binding to subcellular membranes. Incubation of HEK293 cells stably expressing wild-type 69-kDa choline acetyltransferase with the protein kinase C activator phorbol 12-myristate 13-acetate showed time- and dose-related increases in specific activity of the enzyme; in control and phorbol ester-treated cells, the enzyme was distributed predominantly in cytoplasm (about 88%) with the remainder (about 12%) bound to cellular membranes. Mutation of serine 440 to alanine resulted in localization of the enzyme entirely in cytoplasm, and this was unchanged by phorbol ester treatment. Furthermore, activation of mutant enzyme in phorbol ester-treated HEK293 cells was about 50% that observed for wild-type enzyme. Incubation of immunoaffinity purified wild-type and mutant choline acetyltransferase with protein kinase C under phosphorylating conditions led to incorporation of [(32)P]phosphate, with radiolabeling of mutant enzyme being about one-half that of wild-type, indicating that another residue is phosphorylated by protein kinase C. Acetylcholine synthesis in HEK293 cells expressing wild-type choline acetyltransferase, but not mutant enzyme, was increased by about 17% by phorbol ester treatment.

Modulation of choline acetyltransferase synthesis by okadaic acid, a phosphatase inhibitor, and KN-62, a CaM kinase inhibitor, in NS-20Y neuroblastoma

Choline-O-acetyltransferase (ChAT) is the enzyme which catalyses the biosynthesis of the neurotransmitter acetylcholine in cholinergic neurons. Here we show that in mouse cholinergic NS-20Y neuroblastoma cells cultured in the presence of either okadaic acid (serine/threonine phosphatases 1 and 2A inhibitor) or KN-62 (CaM kinase inhibitor) ChAT activity and mRNA either increased or decreased as a function of the drug concentration, respectively. After 24 h exposure, okadaic acid exerted a dramatic effect on cell morphology; cells became round and had no more neurites. On the contrary, KN-62 induced a slight morphological differentiation of the cells. The present results suggest that phosphatases 1 and 2A and CaM kinase could mediate regulation of ChAT gene expression.

Expression, purification and characterization of recombinant human choline acetyltransferase: phosphorylation of the enzyme regulates catalytic activity

Choline acetyltransferase synthesizes acetylcholine in cholinergic neurons and, in humans, may be produced in 82- and 69-kDa forms. In this study, recombinant choline acetyltransferase from baculovirus and bacterial expression systems was used to identify protein isoforms by two-dimensional SDS/PAGE and as substrate for protein kinases. Whereas hexa-histidine-tagged 82- and 69-kDa enzymes did not resolve as individual isoforms on two-dimensional gels, separation of wild-type choline acetyltransferase expressed in insect cells revealed at least nine isoforms for the 69-kDa enzyme and at least six isoforms for the 82-kDa enzyme. Non-phosphorylated wild-type choline acetyltransferase expressed in Escherichia coli yielded six (69 kDa) and four isoforms (82 kDa) respectively. Immunofluorescent labelling of insect cells expressing enzyme showed differential subcellular localization with the 69-kDa enzyme localized adjacent to plasma membrane and the 82-kDa enzyme being cytoplasmic at 24 h. By 64 h, the 69-kDa form was in cytoplasm and the 82-kDa form was only present in nucleus. Studies in vitro showed that recombinant 69-kDa enzyme was a substrate for protein kinase C (PKC), casein kinase II (CK2) and alpha-calcium/calmodulin-dependent protein kinase II (alpha-CaM kinase), but not for cAMP-dependent protein kinase (PKA); phosphorylation by PKC and CK2 enhanced enzyme activity. The 82-kDa enzyme was a substrate for PKC and CK2 but not for PKA or alpha-CaM kinase, with only PKC yielding increased enzyme activity. Dephosphorylation of both forms of enzyme by alkaline phosphatase decreased enzymic activity. These studies are of functional significance as they report for the first time that phosphorylation enhances choline acetyltransferase catalytic activity.

Enzyme activity and protein of multiple forms of choline acetyltransferase: effects of calyculin A and okadaic acid

Choline acetyltransferase (ChAT) appears to exist in multiple forms, three of which can be isolated biochemically as cytosolic (cChAT), ionically-membrane bound (ibChAT) and non-ionic membranous (mChAT). In this study, we first examined whether the quantitative distribution of enzyme protein and enzyme activity was the same. Enzyme activity and ChAT protein distributed similarly: the majority of ChAT activity and protein were found in cChAT followed by mChAT and least activity and amount were in ibChAT. Our second objective was to investigate the effects of calyculin A or okadaic acid on the subcellular distribution of ChAT activity and amount from rat hippocampal formation. Calyculin A and okadaic acid decreased significantly (p < 0.01) cytosolic and membranous ChAT activity; ionically-bound ChAT was not significantly (p > 0.67) different from control. Removal of calyculin A or okadaic acid restored cytosolic ChAT activity (p > 0.9 as compared to control), but not membranous enzyme activity (p < 0.05 as compared to control). The immunoreactive cytosolic ChAT was reduced significantly (p < 0.01) by calyculin A and okadaic acid. Enzyme amount of membranous ChAT was decreased significantly by calyculin A (p < 0.01) and okadaic acid (p < 0.001). Enzyme amount of ionically-bound ChAT was not changed (p > 0.99) by either of these two phosphatase inhibitors. This investigation demonstrates that alterations in ChAT activity of each subfraction parallel changes in enzyme amounts in the same fractions.

Inhibition of choline acetyltransferase activity by serum albumin modified with octanoic acid and other fatty acids

In this study, we examined the effect of fatty acids on choline acetyltransferase (ChAT) activity. ChAT is unstable in a solution of low protein concentration, so serum albumin (BSA) is usually added to stabilize the enzyme. However, we found that ChAT from bovine caudate nucleus rapidly lost its activity when diluted with a buffer containing commercial preparations of BSA. This effect was caused by octanoic acid, which was found in the gas chromatography/mass spectrometry system of lipid extract in commercial BSAs. The inhibition of ChAT activity by octanoic acid depended on the concentrations of the octanoic acid and of the albumin. We also found that ChAT activity was decreased by some long-chain fatty acids, arachidonic acid having exhibited the strongest effect. The extent to which arachidonic acid inhibited ChAT activity depended on the molar ratio of arachidonic acid and albumin, rather than upon the concentration of arachidonic acid. The effect of octanoic acid and arachidonic acid on ChAT activity appeared to increase in the presence of albumin.

Inhibitors of serine/threonine phosphatases increase membrane-bound choline acetyltransferase activity and enhance acetylcholine synthesis

The present investigation examines the effects of phosphatase inhibition on short-term regulation of cholinergic function, with particular emphasis on choline acetyltransferase, the enzyme which synthesizes acetylcholine. Rat hippocampal synaptosomes were treated with either okadaic acid (10 nM) or calyculin-A (50 nM) to inhibit protein phosphatases 1 and 2A for 20 min prior to subfractionation of nerve terminals and measurement of choline acetyltransferase activity, or quantification of high-affinity choline transport and acetylcholine synthesis. Inhibition of synaptosomal phosphatases did not alter total or salt-soluble choline acetyltransferase activity, but membrane-bound and water-soluble forms of the enzyme were selectively increased in okadaic acid-treated nerve terminals to 129 +/- 11% and 137 +/- 10% of control, respectively. High-affinity choline transport was reduced to 77 +/- 6% and 76 +/- 7% of control in calyculin-A- and okadaic acid-treated nerve terminals, respectively. Acetylcholine synthesis was reduced to 73 +/- 6% of control in calyculin-A-treated synaptosomes only; acetylcholine synthesis was at control levels in okadaic acid-treated cultures correlating with enhanced choline acetyltransferase activity in the water-soluble and nonionically membrane-bound fractions. These investigations indicate a role for phosphoprotein phosphatases in the regulation of acetylcholine synthesis in the cholinergic nerve terminal. The observed increases in choline acetyltransferase activity in two subcellular fractions appears to compensate for decreased choline precursor availability, allowing acetylcholine synthesis to be maintained at control levels. The uncoupling of choline transport and acetylcholine synthesis in this situation represents a unique functional role for a subfraction of choline acetyltransferase.

Microglia from the developing rat medial septal area can affect cholinergic and GABAergic neuronal differentiation in vitro

The normal development of the central nervous system is regulated by glia. In this regard, we have reported that astrocytes, stimulated by epidermal growth factor or transforming growth factor alpha, suppress the biochemical differentiation of rat medial septal cholinergic neurons in vitro, as evidenced by a decrease in choline acetyltransferase activity. In this study, we found that, in contrast to astrocytes, microglia enhance rather than suppress this aspect of cholinergic cell expression. When in excess, microglia can revert the effects of epidermal growth factor on the septal cholinergic neurons without altering the astroglial proliferative response to this growth factor. In the absence of growth factors or other glial cell types, microglia increase choline acetyltransferase activity above control levels and thus, may be a source of cholinergic differentiating activity. The increase in enzyme activity induced by microglia is rapid in onset, detected as early as 2 h after their addition to the septal neurons and maintained up to six or seven days in vitro. Furthermore, in the absence or presence of other glial cell types, microglia also influence septal GABAergic neurons by significantly increasing glutamate decarboxylase activity. As microglia affect neither septal cholinergic nor GABAergic neuronal cell survival, they appear to enhance the biochemical differentiation of these two neuronal cell types. Specific immunoneutralizing antibodies were used to identify the microglia-derived factors affecting these two neuronal types. In this regard, we found that the microglia-derived cholinergic differentiating activity is significantly suppressed by antibodies raised against interleukin-3. Furthermore, interleukin-3 was detected in both conditioned media and cell homogenates from septal neuronal-microglial co-cultures by western blotting. Finally, although basic fibroblast growth factor and interleukin-3 significantly increase septal glutamate decarboxylase activity, neither appears to be implicated in the GABAergic cell response to the microglia. In conclusion, these results demonstrate that microglia can enhance the biochemical differentiation of developing cholinergic and GABAergic neurons in vitro.

Expression of alpha-, beta-, and gamma-subspecies of protein kinase C in the motor neurons in the embryonic and postnatal rat spinal cord

Using polyclonal antibodies against alpha-, beta- and gamma-subspecies of protein kinase C, developmental changes in expression of these subspecies in the motor neurons in the rat cervical spinal cord were immunohistochemically investigated. On embryonic day-12, the motor neurons began to differentiate from undifferentiated neuroepithelial cells. On embryonic day-13, they began to express weak immunoreactivity for alpha- and beta-protein kinase C and slightly more evident immunoreactivity for gamma-protein kinase C. Immunoreactivity for protein kinase C in these neurons gradually became stronger, as the development progressed. Between embryonic day-18 and postnatal day-7, the motor neurons showed distinct immunoreactivity in the nucleus, perikaryal cytoplasm, axon and dendrites. At these stages, distribution and intensity of immunoreactivity for alpha-, beta- and gamma-protein kinase C were very similar. Thereafter, the expression of this enzyme in the nucleus gradually declined, while in the other structures, expression of each subspecies changed independently. On postnatal day-28 and 35, expression of beta-protein kinase C in the axons was stronger than that of alpha- and gamma-protein kinase C, and immunoreactivity for gamma-protein kinase C in the perikaryal cytoplasm and dendrites was slightly weaker than that for alpha- and beta-protein kinase C. Expression of this enzyme in the motor neurons at these stages was almost the same as in the adult animal. Electron microscopically, immunoreactivity for protein kinase C was randomly distributed in the nucleus, and in the perikaryal cytoplasm, often near the cisterns of the endoplasmic reticulum. Expression of protein kinase C in the growing axons was quite different from that in the mature axons. In the dendrites, immunoreactivity for protein kinase C was distributed randomly in the cytoplasm and at the postsynaptic densities. These findings suggest that protein kinase C might regulate not only the neural functions, but also several aspects of the differentiation process in the motor neurons.

Activation of rat choline acetyltransferase by limited proteolysis

In the past, purification of choline acetyltransferase (ChAT, EC 2.3.1.6.), the enzyme responsible for the biosynthesis of the neurotransmitter acetylcholine, has yielded fragmented species of the enzyme. The nature and possible function of these forms of ChAT are not well understood. Using a bacterial expression system, recombinant rat ChAT in its active form has been purified to homogeneity. The purified enzyme was found to be activated to >25-fold when assayed at low ionic strength and >5-fold when assayed at high ionic strength by limited proteolysis with either trypsin or chymotrypsin, but not with proteinase K. The activated ChAT shows an increased Km for both substrates, diminished sensitivity to salt activation and a pH optimum that is shifted approximately 1 pH unit. On a denaturing SDS-polyacrylamide gel, the activated ChAT is composed of three to four polypeptides; however, it migrates as an intact 68-k-Da protein species on gel filtration. In order to delineate the site of cleavage by proteolysis, the newly generated fragments have been subjected to N-terminal sequencing. By comparing cleavage sites between trypsin and chymotrypsin, the putative activation sites were identified.

Role of neurotrophins in cholinergic-neurone function in the adult and aged CNS

Cholinergic neurones in the CNS undergo complex changes during normal aging. In recent years, considerable attention has focussed on the neurotrophins and, in particular, nerve growth factor, as potential maintenance factor for cholinergic-neurone function, and as therapeutic agents for use in a variety of neurodegenerative disorders including Alzheimer's disease. While brain cholinergic neurones from the neonate to the aged respond to nerve growth factor with enhanced expression of transmitter phenotype, there appears to be an age-related, region-specific decline in responsiveness. This age-related decrement in neurotrophin action might play a role in dysfunction of cholinergic neurones, and cognitive loss, and could limit the use of these factors as therapeutic agents.

Choline acetyltransferase: celebrating its fiftieth year

It is well known that the regulation of choline acetyltransferase (ChAT) activity under physiological and pathological conditions is important for the development and neuronal activities of cholinergic systems involved in many fundamental brain functions. This review focuses on recent progress in understanding the regulation of ChAT at the levels of both the protein and the mRNA. A deficiency in ChAT activity has been reported for neurodegenerative conditions such as Alzheimer's disease, amyotrophic lateral sclerosis, and schizophrenia. Although a major feature of ChAT regulation is likely to involve the spatial and temporal control of transcription, regulation of expression can also be at the level of RNA processing, transport/translocation, turnover, or translation. In addition, there is increasing evidence that ChAT might be regulated at the posttranslational level by compartmentation and/or covalent modification, i.e., phosphorylation, as well as noncovalent modification (protein-protein interaction, etc.). Synaptic activity and the state of neuronal transmission may also involve the regulation of ChAT at different levels via both positive and negative feedback loops, as was demonstrated in the characterization of two ChAT mutant Drosophila strains. Clearly, identification of cholinergic-specific elements and the characterization of the trans-acting factors that bind to them represent an important area of future research. Equally important is research on the mechanisms governing ChAT as an enzymatic entity. The future should be an exciting time during which we look forward to the elucidation of the cholinergic signal and its regulation as well as the determination of the three-dimensional structure of the enzyme.

Hydrophilic and amphiphilic forms of Drosophila choline acetyltransferase are encoded by a single mRNA

We have previously shown that the enzyme choline-O-acetyltransferase (ChAT) exists in a hydrophilic and an amphiphilic form in Drosophila head. A complementary DNA clone of 4.2 kb containing the entire coding region of ChAT was isolated from a cDNA library of Drosophila heads. The cDNA was subcloned in an expression vector and injected into the nucleus of Xenopus oocytes. Injected oocytes expressed high levels of ChAT activity. This activity was inhibited by bromoacetylcholine, a specific inhibitor of the enzyme. In the present study the non-ionic detergent Triton X-114 was used to analyse whether the expression of hydrophilic and amphiphilic ChAT was or was not directed by a single cDNA. The two forms of ChAT were found to be synthesized in injected oocytes. Approximately 9% of the recombinant enzyme partitioned as amphiphilic activity. This value was similar to that found for native amphiphilic ChAT in Drosophila heads. Sedimentation in sucrose gradients of amphiphilic enzyme was found to be influenced by the type of detergent present in the gradient whereas this was not the case for hydrophilic ChAT. Hydrophilic and amphiphilic enzyme activities differed in some of their biochemical properties. Amphiphilic ChAT was less sensitive to inhibition by the product acetylcholine than was hydrophilic ChAT. Moreover, amphiphilic ChAT was found to be more resistant than hydrophilic ChAT to heat inactivation at 45 degrees C. These properties were observed for the native as well as for recombinant ChAT. These results demonstrate that the hydrophilic and amphiphilic forms of ChAT are derived from one mRNA.

Phosphorylation of rat brain choline acetyltransferase and its relationship to enzyme activity

Choline acetyltransferase catalyzes the formation of acetylcholine from choline and acetyl-CoA in cholinergic neurons. The present study examined conditions for modulation of kinase-mediated phosphorylation of this enzyme. By using a monospecific polyclonal rabbit anti-human choline acetyltransferase antibody to immunoprecipitate cytosolic and membrane-associated subcellular pools of enzyme from rat hippocampal synaptosomes, we determined that only the cytosolic fraction of the enzyme (67,000 +/- 730 daltons) was phosphorylated under basal, unstimulated conditions. The quantity of this endogenous phosphoprotein was dependent, in part, upon the level of intracellular calcium, with 32Pi incorporation into the enzyme in nerve terminals incubated in nominally calcium-free medium only 43 +/- 7% of control. The corresponding enzymatic activity of cytosolic choline acetyltransferase did not appear to be altered by lowered cytosolic calcium, whereas membrane-associated choline acetyltransferase activity was decreased to 58 +/- 11% of control. Depolarization of synaptosomes with 50 microM veratridine neither altered the extent of phosphorylation or specific activity of cytosolic choline acetyltransferase, nor induced detectable phosphorylation of membrane-associated choline acetyltransferase, although the specific activity of the membrane-associated enzyme was increased to 132 +/- 5% of control. In summary, phosphorylation of choline acetyltransferase does not appear to regulate cholinergic neurotransmission by a direct action on catalytic activity of the enzyme.

Regulation of expression of cholinergic neuronal phenotypic markers in neuroblastoma LA-N-2

Cholinergic neurons in PNS and CNS are identified by the presence of choline acetyltransferase and the accumulation of choline by a high-affinity, sodium-coupled choline transporter to be used for acetylcholine synthesis. It appears that expression of choline acetyltransferase can be altered by several physiological conditions, including hormones and trophic factors, but little is known about control of expression of the sodium-coupled choline carrier or whether these two phenotypic markers are regulated similarly. In the present study, the cholinergic human neuroblastoma LA-N-2 was used to investigate regulation of expression of choline acetyltransferase and choline uptake activity associated with differentiation and neurite extension. Cells grown in serum-containing basal medium maintained a relatively undifferentiated morphology, expressed low levels of choline acetyltransferase activity, and accumulated choline by a sodium-dependent process followed by conversion to acetylcholine. Transfer of cells to an enriched, serum-free defined medium resulted in morphological and neurochemical differentiation, with an enhancement of cholinergic phenotype. Hemicholinium-sensitive choline uptake activity was increased about sixfold over a 4-day period, with no change in choline acetyltransferase or acetylcholinesterase specific activity. Acetylcholine synthesis was increased in parallel with the changes in choline accumulation; choline metabolism in the differentiated cells differed significantly from that observed in the undifferentiated cells, with proportionally less converted to phosphorylcholine and proportionally more remaining as unmetabolized choline and converted to acetylcholine. The enhanced choline accumulation appeared to be mediated by an increased number of choline carriers, demonstrated by increased binding of the affinity ligand [3H]-choline mustard to the transporter and by an increased Vmax for the uptake process. The increased expression of the transport function appeared to be under transcriptional control, as the enhancement of uptake was blocked by the RNA polymerase II inhibitor alpha-amanitin as well as by the protein synthesis inhibitor cycloheximide. These results show that expression of sodium-coupled choline carriers and choline acetyltransferase may be regulated separately in the differentiating neuroblastoma LA-N-2 and that neurotransmitter synthesis is controlled by provision of precursor rather than at the level of the biosynthetic enzyme.

Increased acetylcholine content induced by adenosine in a sympathetic ganglion and its subsequent mobilization by electrical stimulation

The present study was initiated to examine the effects of ATP on acetylcholine (ACh) synthesis. The exposure of superior cervical ganglia to ATP increased ACh stores by 25%, but this effect was also evident with ADP, AMP, and adenosine, but not with beta gamma-methylene ATP, a nonhdydrolyzable analogue of ATP, or with inosine, the deaminated product of adenosine. Thus, we attribute the enhanced ACh content caused by ATP to the presence of adenosine derived from its hydrolysis by 5'-nucleotidase. The adenosine-induced increase of tissue ACh was not the consequence of an adenosine-induced decrease of ACh release. The extra ACh remained in the tissue for more than 15 min after the removal of adenosine, but it was not apparent when ganglia were exposed to adenosine in a Ca(2+)-free medium. Incorporation of radiolabelled choline into [3H]ACh was also enhanced in the presence of adenosine, suggesting an extracellular source of precursor. Moreover, the synthesis of radiolabelled forms of phosphorylcholine and phospholipid was not reduced in adenosine's presence, suggesting that the extra ACh was not likely derived from choline destined for phospholipid synthesis. Aminophylline did not prevent the adenosine effect to increase ACh content; this effect was blocked by dipyridamole, but not by nitrobenzylthioinosine (NBTI). In addition, two benzodiazepine stereoisomers known to inhibit stereoselectively the NBTI-resistant nucleoside transporter displayed a similar stereoselective ability to block the effect of adenosine. Together, these results argue that adenosine is transported through an NBTI-resistant nucleoside transporter to exert an effect on ACh synthesis. The extra ACh accumulated as a result of adenosine's action was releasable during subsequent preganglionic nerve stimulation, but not in the presence of vesamicol, a vesicular ACh transporter inhibitor. We conclude that the mobilization of ACh is enhanced as a result of adenosine pretreatment.

Transcriptional regulation of choline acetyltransferase gene by cyclic AMP

The effect of cyclic AMP on the gene expression of choline acetyltransferase (ChAT) was studied in NG108-15, mouse neuroblastoma and rat glioma hybrid cell lines. Addition of dibutyryl cyclic AMP to the culture medium increased both the ChAT mRNA level and ChAT activity twofold. Polymerase chain reaction analysis of the ChAT mRNA indicated that, among the multiple mRNA species, M-type mRNA was transcribed most efficiently, with or without the addition of dibutyryl cyclic AMP. The 5' region of the mouse ChAT gene was ligated to the bacterial chloramphenicol acetyltransferase gene, and the expression of chloramphenicol acetyltransferase activity was determined by transfection analysis. Cyclic AMP derivatives enhanced the reporter gene expression in both transiently and stably transfected cells. DNA deletion analysis indicated that the intron region downstream of the M-type exon is necessary for the cyclic AMP responsiveness, and that cyclic AMP derivatives increase ChAT gene transcription mainly from M-type promoter. These results suggest that a cis-acting DNA element that confers the cyclic AMP responsiveness of the ChAT gene is present in the intron downstream of the M-type exon.

Acetylcholine levels, choline acetyltransferase and acetylcholinesterase molecular forms during thyroxine-induced cardiac hypertrophy

The effects of left ventricular hypertrophy induced by hyperthyroidism on three biochemical markers of parasympathetic innervation were investigated. In response to subcutaneous injections of thyroxine (400 micrograms/kg; T4) for 6 days, the left ventricle, but not the right, developed significant hypertrophy (20%). In the enlarged left ventricle, acetylcholine (ACh) content and choline acetyltransferase (ChAT) activity per chamber were elevated approx. 25-30%, although no change in these two markers was evident when the data were expressed per unit wet weight. Immunoblot analysis showed that the relative abundance of ChAT protein increased in the hypertrophied left ventricle in correlation with the increased ChAT activity. No changes in ACh content, ChAT activity and ChAT relative abundance were evident in the right ventricle of T4-treated animals. Although hyperthyroidism did not alter AChE specific activity (per unit wet weight) in the left ventricle, the percent activities of the individual AChE globular forms were affected in this chamber. Specifically, T4-treatment reduced the percent activity of globular (G)4 AChE by 20% and increased that of the combined G1 and G2 AChE pool by 15%. Interestingly, in the hypertrophied left ventricle total AChE activity in its extracellular or functionally-relevant pool was reduced due to a loss of G4 AChE activity. These results show that a compensatory increase in parasympathetic innervation can occur during hyperthyroid-induced left ventricular hypertrophy. However, the reduced activity of the functionally-relevant AChE pool suggests that the clearance of ACh after release may be slowed in the hypertrophied left ventricle.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Localization of subspecies of protein kinase C in the mammalian central nervous system

Activation of protein kinase C (PKC) is regulated by dual second messengers; diacylglycerol (DG) produced by receptor mediated hydrolysis of phosphatidylinositol and Ca2+ which is released by inositol 1,4,5-triphosphate (IP3) from intracellular stores in the endoplasmic reticulum. In the mammalian central nervous system, available evidence suggests that PKC plays a prominent role in the processing of neuronal signals and in the short-term or long-term modulation of synaptic transmission. This enzyme is a member of a family consisting of at least eight subspecies, alpha, beta I, beta II, gamma, delta, epsilon, zeta and eta. The homologous structure of each subspecies makes difficult resolution of the enzymological properties of the enzyme. The distinct functional roles of PKC subspecies in mammalian tissues have been elucidated by defining the localization of each subspecies. We identified alpha-, beta I-, beta II- and gamma-PKC subspecies in the rat brain by in situ hybridization and by light and electron microscopic immunohistochemistry, using antibodies specific for each subspecies. Most immunoreactions of the alpha, beta I, beta II and gamma subspecies were evident in neurons and there were few, if any, in glial cells. In this article, we summarize known cellular and subcellular localizations of PKC subspecies in mammalian CNS and some aspects of current studies in neuronal functions regulated by this enzyme are discussed.

A complementary DNA for human choline acetyltransferase induces two forms of enzyme with different molecular weights in cultured cells

Complementary DNA (cDNA) clones containing the entire coding region of human choline acetyltransferase (ChAT) were isolated from cDNA libraries prepared from the autopsied spinal cord. In the human cDNA, the ATG codon assigned to the putative initiation codon for pig, rat and mouse ChAT cDNAs was replaced by ACG. The human cDNA contained an in-frame ATG codon 324 nucleotides upstream of the ACG codon. Therefore, human ChAT cDNA should code for a 748 amino acid polypeptide of 82.6 kDa. This deduced molecular weight was larger than that of ChAT protein purified from the human brain and placenta (64-70 kDa). The human ChAT cDNA containing the entire coding region was ligated to an expression vector and introduced into African green monkey kidney (COS) cells and Chinese hamster ovary (CHO) cells. The cells expressed high ChAT activity and produced two protein bands immunostained with an antibody to monkey ChAT. The molecular weight of the proteins was estimated to be approximately 70 and 80 kDa by polyacrylamide-SDS gel electrophoresis. When partial cDNAs that lacked the first ATG but contained the replaced ACG codon were introduced into COS cells, the cells expressed moderate ChAT activity and an immunoreactive protein band of 70 kDa. These results indicate that translation of human ChAT mRNA starts at two sites and produces two enzyme proteins with different molecular weights. It might be that the larger form of ChAT molecule is an enzyme precursor for processing or that the N-terminal extrapeptide is needed for subcellular localization of the enzyme.

High-level synthesis and fate of acetylcholine in baculovirus-infected cells: characterization and purification of recombinant rat choline acetyltransferase

Rat choline acetyltransferase (ChAT) has been expressed at a high level in Spodoptera frugiperda Sf9 cells using a baculovirus expression system. A cDNA containing the coding sequence for ChAT was inserted into the transfer vector pAcYM1 to yield the recombinant vector pAcYM1/ChAT. Sf9 cells were then coinfected with pAcYM1/ChAT and the wild-type Autographa californica virus. One recombinant virus particle, containing the cDNA for ChAT, was selected that expressed a protein of 68.5 kDa. Forty hours after infection of cells with the recombinant virus, the specific activity of ChAT in the cytosol was 190 nmol of acetylcholine/min/mg of protein, accounting for approximately 24% of the cell cytosolic proteins as being ChAT. The apparent Km values of the enzyme for choline and acetyl-CoA were 299 and 221 microM, respectively, whereas the respective Vmax values were 10.6 and 11.4 mumol of acetylcholine/min/mg of protein. In addition, analysis of the protein revealed that ChAT is phosphorylated in Sf9 cells. About 0.5 mg of ChAT was obtained from a one-step purification procedure starting with 10(8) infected Sf9 cells. Addition of choline to the incubation medium led to accumulation of high amounts of acetylcholine in the cytosol of the infected cells. The neurotransmitter was not released by Sf9 cells in response to membrane depolarization or on ionophore-mediated calcium entry. Some acetylcholine, which most likely originated from cell death inherent to viral infection, accumulated in the culture medium. The infected insect cells, which synthesize and store neurotransmitter, provide a new and convenient model for analyzing synaptic transmission at the molecular level.

Down-regulation of quantal size at frog neuromuscular junctions: possible roles for elevated intracellular calcium and for protein kinase C

Previous work showed that quantal size can be at least doubled at the frog neuromuscular junction by pretreatment with hormones or hypertonic solutions, primarily by the release of more acetylcholine (ACh) per quantum. Once increased, quantal size slowly declined over hours. Quantal size was measured from miniature end-plate potentials (MEPPs) or currents (MEPCs). In the present experiments, preparations in which quantal size had been increased were exposed to 17-25 mM [K+], quantal size decreased within minutes. Release of comparable numbers of quanta by nerve stimulation did not decrease size. K(+)-solutions did not decrease size if Ca2+ was omitted or replaced with Sr2+. The phosphokinase C (PKC) activators phorbol 12,13-diacetate (PDA) and 1-oleoyl-2-acetyl-rac-glycerol (OAG) also decreased quantal size within minutes when applied in a hypertonic solution that increased the rate of spontaneous release. Phorbol 12,13-dideconate, which does not activate PKC, did not decrease quantal size. The size decrease triggered by K(+)-solutions or PKC activators was blocked by 100 microM 1-(5-isoquinolinyl-sulfonyl)-2-methyl-piperazine (H7), a protein kinase inhibitor. Apparently, increasing [K+] elevated intracellular [Ca2+], which activates PKC, and which leads to the down-regulation of quantal size. During the period in which size is decreasing, there appears to be large and normal subpopulations of MEPP sizes, with normal gradually replacing large. This suggests that large quanta are formed by adding additional ACh to preformed quanta shortly before they are available for release.

Choline acetyltransferase messenger RNA expression in developing and adult rat brain: regulation by nerve growth factor

The polymerase chain reaction (PCR) was used to develop a method for detection and relative quantification of the choline acetyltransferase (ChAT) mRNA in neonatal and adult rat CNS. Oligonucleotide primers derived from a porcine ChAT cDNA sequence were used in coupled reverse transcriptase (RT)-PCR to amplify a cDNA sequence of 206 bp which arises in a cycle- and RNA-dependent manner and which hybridizes with both an internal oligonucleotide and a ChAT cDNA probe. ChAT mRNA was detected in spinal cord, septal area, striatum, cortex and hippocampus but not in cerebellum and cardiac or skeletal muscle. In the septal area, relative quantitative evaluation of ChAT mRNA levels by RT-PCR indicates that this transcript is developmentally regulated and increased following intracerebral administration of nerve growth factor (NGF) to both neonatal and young adult rats. This suggests that the increases of ChAT activity observed in basal forebrain during development or after NGF administration are, at least in part, associated with an increase in corresponding levels of mRNA.

Stimulation of carnitine acetyltransferase in PC12 cells by nerve growth factor: relationship to choline acetyltransferase stimulation

The activity of carnitine acetyltransferase (acetyl-CoA:L-carnitine O-acetyltransferase) was found to be at least 50-fold higher than that of choline acetyltransferase in PC12 cells. Nerve growth factor stimulated both enzymes in a parallel manner with respect to concentration of NGF and culture time. The stimulation of both enzymes was completely inhibited by 10 microM 6-thioguanine, an inhibitor of protein kinase N. Results are discussed with reference to the hypothesis that the two enzymes may be functionally related in neuronal cells.