Choline acetyltransferase. The absence of multiple forms and purification from mouse brain

Peripheral type of choline acetyltransferase: biological and evolutionary implications for novel mechanisms in cholinergic system

The peripheral type of choline acetyltransferase (pChAT) is an isoform of the well-studied common type of choline acetyltransferase (cChAT), the synthesizing enzyme of acetylcholine. Since pChAT arises by exons skipping, its amino acid sequence is similar to that of cChAT, except the lack of a continuous peptide sequence encoded by all the four exons from 6 to 9. While cChAT expression has been observed in both the central and peripheral nervous systems, pChAT is preferentially expressed in the peripheral nervous system. pChAT appears to be a reliable marker for the visualization of peripheral cholinergic neurons and their processes, whereas other conventional markers including cChAT have not been used successfully for it. In mammals like rodents, pChAT immunoreactivity has been observed in most, if not all, physiologically identified peripheral cholinergic structures such as all parasympathetic postganglionic neurons and most neurons of the enteric nervous system. In addition, pChAT has been found in many peripheral neurons that are derived from the neural crest. These include sensory neurons of the trigeminal ganglion and the dorsal root ganglion, and sympathetic postganglionic neurons. Recent studies moreover indicate that pChAT, as well as cChAT, appears ubiquitously expressed among various species not only of vertebrate mammals but also of invertebrate mollusks. This finding implies that the alternative splicing mechanism to generate pChAT and cChAT has been preserved during evolution, probably for some functional benefits.

Synaptosomal "membrane-bound" choline acetyltransferase is most sensitive to inhibition by choline mustard

The objectives of the present study were to validate the presence of cytoplasmic and membrane-associated pools of choline acetyltransferase (ChAT) in rat brain synaptosomes, and to evaluate inhibition of these different forms of the enzyme by the nitrogen mustard analogue of choline, choline mustard aziridinium ion (ChM Az). The relative distribution of ChAT and lactate dehydrogenase (LDH) was followed in subfractions of synaptosomes to establish whether ChAT activity associated with salt-washed presynaptic membranes represents membrane-bound protein rather than cytosolic enzyme trapped within undisrupted synaptosomes or revesiculated membrane fragments. The percentage of total synaptosomal ChAT activity (14%) recovered in the final membrane pellet always exceeded that of LDH (6%), lending support to the hypothesis that much of the ChAT associated with the membranes was a membrane bound form of the enzyme. Incubation of purified synaptosomes with ChM Az led to irreversible inhibition of ChAT activity; this loss of enzyme activity could not be accounted for by lysis of nerve terminals during incubation in the presence of the mustard analogue. Subfractionation of the ChM Az-treated nerve terminals revealed that the membrane-bound form of ChAT was inhibited to the greatest extent, followed by the ionically membrane-associated enzyme, with the activity of the water-solubilized enzyme not differing significantly from control. Preparation of the synaptosomal ChAT subfractions from untreated nerve terminals prior to incubation with varying concentrations of ChM Az or naphthylvinylpyridine revealed that under these conditions water-solubilized, ionically membrane-associated, and detergent-solubilized membrane-bound pools of ChAT were not differentially inhibited by either compound.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular biology and neurobiology of choline acetyltransferase

In the 45 years since the first description of choline acetyltransferase (ChAT; EC 2.3.1.6.), significant progress has been made in characterizing the molecular properties of this important neurotransmitter synthetic enzyme. We are now on the verge of understanding its genetic regulation and biological function(s). The Drosophila cDNA has been cloned, sequenced, and expressed in both a eucaryotic and a procaryotic system. The levels of ChAT specific mRNA have been determined during Drosophila development. Monoclonal and polyclonal antibodies have been produced to the enzyme from a variety of sources and used for biochemical and immunocytochemical studies. Two well characterized genetic systems have identified the ChAT gene and described a series of useful alleles. As a nervous system specific protein expressed only in the subset of neurons using acetylcholine as a neurotransmitter, ChAT is a good model for uncovering the processes and factors responsible for regulating genes involved in neurotransmitter phenotype selection and maintenance. Recent studies have described the purification of a cholinergic factor from muscle conditioned medium and indicated the potential importance of nerve growth factor (NGF) for regulating ChAT expression in the central nervous system. These factors, or ones remaining to be discovered, may be involved in the etiology or disease process of neurodegenerative nervous system disorders such as Alzheimer's disease.

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.

Immunochemical and immunohistochemical studies on the specificity of a monoclonal antibody to choline acetyltransferase of rat brain

Choline acetyltransferase (ChAT) preparations partially purified from brains of rat, mouse and cat were electrophoresed in polyacrylamide gels and blotted on nitrocellulose sheets. A monoclonal antibody, Ab-57, bound specifically to one protein band which coincided with the peak of ChAT activity in each species. Using Ab-57, ChAT was demonstrated immunohistochemically in cholinergic structures in rat, mouse and cat brains. Thus, the specificity and cross-species reactivity of the monoclonal antibody Ab-57 have been established.

Choline and cholinergic neurons

Mammalian neurons can synthesize choline by methylating phosphatidylethanolamine and hydrolyzing the resulting phosphatidylcholine. This process is stimulated by catecholamines. The phosphatidylethanolamine is synthesized in part from phosphatidylserine; hence the amino acids methionine (acting after conversion to S-adenosylmethionine) and serine can be the ultimate precursors of choline. Brain choline concentrations are generally higher than plasma concentrations, but depend on plasma concentrations because of the kinetic characteristics of the blood-brain-barrier transport system. When cholinergic neurons are activated, acetylcholine release can be enhanced by treatments that increase plasma choline (for example, consumption of certain foods).

Purification and immunochemical properties of choline acetyltransferase from human brain

Choline acetyltransferase (CAT) was purified to homogeneity from 363 g of human neostriatum by means of ammonium sulfate and protamine sulfate fractionation, followed by chromatography on DEAE-Sephadex, hydroxyapatite, phosphocellulose, and agarose-hexane-Co A columns. The final product migrated as a single component on 7.5% gels with or without SDS. It had a molecular weight of 66,000 daltons and a specific activity of 7.3 mumol acetylcholine formed per milligram protein per minute. Antibodies prepared in rabbits gave single precipitin lines against this protein on Ouchterlony immunodiffusion and immunoelectrophoresis plates. The CAT-anti-CAT IgG complex migrated as a single band on gel electrophoresis, establishing the monospecificity of the antibodies. Strong cross-reactivity to the IgG was obtained with CAT from rat, rabbit, and guinea pig, but only weak reactivity with chicken. Fab fragments were prepared from the rabbit IgG and were used to stain CAT-containing neurons in the spinal cord and nerve endings at the neuromuscular junction using the PAP technique.

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.