The effect of Cl- on choline acetyltransferase kinetic parameters and a proposed role for Cl- in the regulation of acetylcholine synthesis

The Regulatory Effects of Acetyl-CoA Distribution in the Healthy and Diseased Brain

Brain neurons, to support their neurotransmitter functions, require a several times higher supply of glucose than non-excitable cells. Pyruvate, the end product of glycolysis, through pyruvate dehydrogenase complex reaction, is a principal source of acetyl-CoA, which is a direct energy substrate in all brain cells. Several neurodegenerative conditions result in the inhibition of pyruvate dehydrogenase and decrease of acetyl-CoA synthesis in mitochondria. This attenuates metabolic flux through TCA in the mitochondria, yielding energy deficits and inhibition of diverse synthetic acetylation reactions in all neuronal sub-compartments. The acetyl-CoA concentrations in neuronal mitochondrial and cytoplasmic compartments are in the range of 10 and 7 μmol/L, respectively. They appear to be from 2 to 20 times lower than acetyl-CoA Km values for carnitine acetyltransferase, acetyl-CoA carboxylase, aspartate acetyltransferase, choline acetyltransferase, sphingosine kinase 1 acetyltransferase, acetyl-CoA hydrolase, and acetyl-CoA acetyltransferase, respectively. Therefore, alterations in acetyl-CoA levels alone may significantly change the rates of metabolic fluxes through multiple acetylation reactions in brain cells in different physiologic and pathologic conditions. Such substrate-dependent alterations in cytoplasmic, endoplasmic reticulum or nuclear acetylations may directly affect ACh synthesis, protein acetylations, and gene expression. Thereby, acetyl-CoA may regulate the functional and adaptative properties of neuronal and non-neuronal brain cells. The excitotoxicity-evoked intracellular zinc excess hits several intracellular targets, yielding the collapse of energy balance and impairment of the functional and structural integrity of postsynaptic cholinergic neurons. Acute disruption of brain energy homeostasis activates slow accumulation of amyloid-β_1-42 (Aβ). Extra and intracellular oligomeric deposits of Aβ affect diverse transporting and signaling pathways in neuronal cells. It may combine with multiple neurotoxic signals, aggravating their detrimental effects on neuronal cells. This review presents evidences that changes of intraneuronal levels and compartmentation of acetyl-CoA may contribute significantly to neurotoxic pathomechanisms of different neurodegenerative brain disorders.

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.

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.

Basal synthesis of acetylcholine in hippocampal synaptosomes is not dependent upon membrane-bound choline acetyltransferase activity

Choline acetyltransferase, the enzyme which catalyses the formation of acetylcholine within cholinergic nerve terminals, exists in both cytosolic and membrane-associated subcellular pools. In the present study, alteration in nerve terminal Cl- homeostasis was used as an experimental tool to elucidate the role of membrane-bound choline acetyltransferase in regulation of the biosynthesis of acetylcholine in rat hippocampal synaptosomes under basal or resting conditions. Reduction of extracellular Cl- concentration from 131 to 48 mM through iso-osmotic replacement with isethionate ions produced a selective decrease, to approximately 50% of control, of nerve terminal membrane-associated choline acetyltransferase activity. Under these experimental conditions, there were no changes in the activity of cytosolic enzyme or high-affinity choline uptake, or in acetylcholine synthesis. Replacement of medium Cl- with Br- supported maintenance of synaptosomal membrane-bound choline acetyltransferase activity better than did I- or isethionate ions; high-affinity choline uptake activity and acetylcholine synthesis were affected similarly. Incubation of synaptosomes with low concentrations of the Cl- channel blockers 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (50 microM) and niflumic acid (100 microM) selectively decreased activity of the membrane-bound enzyme, with no effect on cytosolic choline acetyltransferase or high-affinity choline uptake activities. Acetylcholine synthesis was unchanged, even though membrane-bound choline acetyltransferase activity was decreased in some samples (250 microM 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid) to about 10% of control. Experimental manipulations designed to alter neuronal Cl- homeostasis resulted in selective changes in membrane-bound choline acetyltransferase activity, thereby allowing the first direct examination of its physiological role in regulation of acetylcholine synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Some properties of frog vestibular choline acetyltransferase and acetylcholinesterase

The amount and some properties of choline acetyltransferase (ChAT) and of acetylcholinesterase (AchE) were investigated in the frog vestibule. Enzyme activities were found to be of the same order of magnitude as in frog nervous tissue and various properties of vestibular ChAT (dependence on pH, chloride and Triton X-100 activation, phosphate sensitivity) and AchE (inhibition by eserine but not by Tetraisopropylpyrophosphoramide) were also similar as those of the homologous central nervous system enzymes. Although the precise localization of ChAT and AchE is not yet certain the efferent neurotransmitter in the vertebrate vestibular sensory periphery is believed to be acetylcholine and thus the enzymes responsible for its synthesis and degradation may participate in regulating inner ear function.

Mechanism of activation of choline acetyltransferase in a human neuroblastoma cell line

In our previous report we have shown that the enzyme choline acetyltransferase (ChAT), responsible for the synthesis of the neurotransmitter acetylcholine, can be regulated in response to treatment by either retinoic acid or sodium butyrate. These responses were dose and time dependent, but the mechanism by which these agents were acting was not understood. We now report the results of studies aimed at elucidating the level at which both sodium butyrate and retinoic acid are able to increase ChAT activity. The effects of these agents on macromolecular synthesis appeared to be limited to small but statistically significant increases in the rate of RNA synthesis. However, inhibition of DNA, RNA and protein synthesis in these cells had no effect on the stimulation of ChAT activity by either sodium butyrate or retinoic acid. Several experiments appeared to rule out a role for cyclic AMP or protein kinase C in the regulation of ChAT activity, even though retinoic acid treatment could increase endogenous levels of cyclic AMP 3- to 4-fold over the time course of ChAT activity stimulation. Experiments performed to determine kinetic parameters of this enzyme demonstrated changes only in the Vmax, but not the Km of ChAT, suggesting that the affinity of enzyme for either of its substrates was not responsible for the increase in specific activity. Taken together, this evidence suggests that the activation of choline acetyltransferase in this human neuroblastoma cell line occurs at the post-translational level.

Neurochemical evidence for afferent GABAergic and efferent cholinergic neurotransmission in the frog vestibule

Glutamate decarboxylase and choline acetyltransferase activities with magnitudes similar to those of their homologous enzymes in frog nervous tissue were found in homogenates of the frog labyrinth. Transection of the vestibular nerve resulted in a gradual diminution of choline acetyltransferase activity until it reached an 88% decrease 6 weeks after surgery. In contrast, glutamate decarboxylase activity did not suffer any alteration at any time after nerve excision. The presence of their enzymes of synthesis is evidence of the neurotransmitter participation of GABA and acetylcholine in the frog vestibule; the observed decrease of choline acetyltransferase following vestibule nerve excision supports the efferent synaptic bouton localization of choline acetyltransferase. The suggestion that glutamate decarboxylase is located in a cell type (or compartment) that may well be the hair cell is supported by the fact that this enzyme does not suffer any modification after surgery. These results are in accordance with an efferent cholinergic neurotransmission and a putative afferent role of GABA in the frog vestibule.

Properties and partial purification of choline acetyltransferase from the nematode Caenorhabditis elegans

We have stabilized and studied choline acetyltransferase from the nematode Caenorhabditis elegans. The enzyme is soluble, and two discrete forms were resolved by gel filtration. The larger of these two forms (MW approximately 154,000) was somewhat unstable and in the presence of 0.5 M NaI was converted to a form indistinguishable from the "native" small form (MW approximately 71,000). We have purified the small form of the enzyme greater than 3,300-fold by a combination of gel filtration, ion-exchange chromatography, and nucleotide affinity chromatography. The purified preparation has a measured specific activity of 3.74 mumol/min/mg protein, and is free of acetylcholinesterase and acetyl-CoA hydrolase activities. The Vmax of the purified enzyme is stimulated by NaCl, with half-maximal stimulation at 80 mM NaCl. The Km for each substrate is also affected by salt, but in different manners from each other and the Vmax; the kinetic parameter Vmax/Km thus changes significantly as a function of the salt concentration.

Possible cholinergic neurotransmission in the cristae ampullares of the chick inner ear

The presence of choline acetyltransferase (ChAT) was investigated in the chick inner ear in order to assess the possible role of acetylcholine (ACh) in neurotransmission within the vestibular labyrinth. ChAT activity found in homogenates of isolated chick vestibular cristae ampullares is of the same order of magnitude and has similar properties as its homologous enzyme in nervous tissue from various regions. These findings are in accord with a probable neurotransmitter role of ACh in the chick vestibular labyrinth.

Homocholine and short-chain N-alkyl choline analogues as substrates for Torpedo choline acetyltransferase

The kinetic parameters, Km and Vmax, for the acetylation of choline and several close analogues were determined by using (a) purified choline acetyltransferase and (b) a hypotonically lysed synaptosomal extract prepared from the electric organ of Torpedo marmorata. Whereas the Km for choline was similar in both cases (0.51 and 0.42 mM), the crude enzyme showed a three- to fivefold greater affinity for its analogues than the purified enzyme, the activity decreasing rapidly with increased N-alkyl substitution. Homocholine was a poor substrate, but was clearly acetylated by both preparations. The effect of salt on analogue acetylation by the crude enzyme was studied by increasing NaCl concentration from zero to 150 mM. There was an increase in both Km and Vmax for all substrates: choline, N,N,N-dimethylmonothylaminoethanol, -monomethyldiethylaminoethanol and -dimethylmonobutylaminoethanol showed the greatest changes, whilst N,N,N-triethylaminoethanol and -dimethylmonopropylaminoethanol and homocholine were much less affected However, in all cases, the kinetic parameter Vmax/Km remained unchanged. The maximal velocities of the different substrates varied more under conditions of high than of low salt. Sodium chloride up to 300 mM had no effect on the amount of enzyme which was bound to membranes in the synaptosomal extract. It is concluded that choline acetyltransferase has a high degree of substrate specificity, which is slightly altered by purification. The effects of salt cannot be explained as a consequence of nonspecific ionic association with membranes.

The role of chloride in acetylcholine metabolism

The chloride dependence of acetylcholine (ACh) synthesis and release and of choline uptake was studied in synaptosomal preparations from rat brain. The substitution of propionate for chloride, in the presence of 35 mM-potassium, lowered the ACh content of the synaptosomes. However, in the presence of 5 mM-potassium, the ACh level in synaptosomes was reduced, but significantly less so. Propionate had no effect on choline acetyltransferase (EC 2.3.1.6) activity when measured in a standard chloride-containing medium. In the presence of propionate, the spontaneous release of ACh was unchanged, but potassium-stimulated release of ACh was markedly reduced as compared with a chloride-containing medium. The synthesis of ACh, as measured by the net increase in the amount of ACh in the synaptosomes and that released to the medium, was reduced with propionate at 5 mM-potassium and was totally inhibited when the potassium concentration was increased to 35 mM. Choline uptake studies revealed that with propionate only a low-affinity component of the choline transport system existed. Further, the Vmax was markedly reduced when the potassium concentration was increased to 35 mM. The results suggest that under certain conditions choline transported by a low-affinity system might provide a substantial source of choline for ACh synthesis.

Evidence for presence of an arginine residue in the coenzyme A binding site of choline acetyltransferase

Choline acetyltransferase (acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6) may be inactivated by arginine-specific reagents such as butanedione, phenylglyoxal, and camphorquinone-10-sulfonic acid. The enantiomers of the latter compound were prepared, but inactivation was not stereospecific. Protection against inactivation by the arginine-specific reagents was provided by CoA and, to a lesser extent, by 3'-dephospho-CoA. No protection was provided by choline, NAD+, NADH, NADP+, or NADPH. Sodium chloride could protect, to some extent, against inactivation by arginine-specific reagents; this protection showed no cation or anion specificity. The data are compatible with the postulate that the salt anion competes with the attachment of the 3'-phospho group of CoA to an active site arginine residue.

Tryptophan and phenylalanine transport in rat cerebral cortex slices as influenced by sodium ions

Tryptophan and phenylalanine transport in rat cerebral cortex slices was studied in sodium-free media and during influx and efflux of sodium ions. Choline as a substitute for sodium in incubation media increased efflux and decreased influx of tryptophan and phenylalanine. Exchange of intracellular [3H]tryptophan and [3H]phenylalanine with extracellular unlabeled histidine, phenylalanine, and tryptophan was sodium-independent. Efflux of sodium ions from the slices had no immediate effects on phenylalanine and tryptophan efflux, but influx decreased. Influx of sodium into the sodium-depleted slices provoked a transient increase in tryptophan and phenylalanine efflux and also enhanced influx. The results are interpreted to indicate that sodium ions may possibly affect the function of the primary transport sites for aromatic amino acids at cerebral membranes by controlling the orientation of their reactive sites towards the intracellular and extracellular sides, rather than by being directly involved in the binding of amino acids to the carriers.

High affinity choline transport and acetylCoA production in brain and their roles in the regulation of acetylcholine synthesis

This review describes recent advances made in the understanding of the regulation of acetylcholine synthesis in brain with regard to the availability of its two precursors, choline and acetylCoA. Choline availability appears to be regulated by the high affinity choline transport system. Investigations of the localization and inhibition of this system are reviewed. Procedures for measuring high affinity choline transport and their shortcomings are described. The kinetics and effects of previous in vivo and in vitro treatments on high affinity choline transport are reviewed. Kinetic and direct coupling of the transport and acetylation of choline are discussed. Recent investigations of the source of acetylCoA used for the synthesis of acetylcholine are reviewed. Three sources of acetylCoA have recently received support: citrate conversion catalyzed by citrate lyase, direct release of acetylCoA from mitochondria following its synthesis from pyruvate catalyzed by pyruvate dehydrogenase, and production of acetylCoA by cytoplasmic pyruvate dehydrogenase. Investigations indicating that acetylCoA availability may limit acetylcholine synthesis are reviewed. A model for the regulation of acetylcholine synthesis which incorporates most of the reviewed material is presented.